Session M1: Recent Advances in Density Functional Theory VI

The derivative discontinuity of the exchange-correlation functional

The derivative discontinuity is a fundamental feature of the exchange-correlation energy. It is the change in the derivative of the energy as the number of electrons goes through an integer, and it is often expressed as a potential that jumps by a constant on going from $N-\delta$ to $N+\delta$. In this talk we will show manifestations in the total energy of integer systems at fixed $N$. One manifestation is the complete failure of all functionals in the literature to give the energy of {\textbf{both}} infinitely stretched H$_2^+$ and infinitely stretched H$_2$. Another very clear example is the failure to correctly reproduce the density in a two electron H$_2$ like system when changing the charge of one of the protons to be non-integer. More examples in chemistry and physics will be shown, ranging from the behaviour of electrons in the simplest chemical reactions to the gap of the 1D-Hubbard model and electron transport in the Anderson model. To understand the derivative discontinuity in these systems it is important to consider three perspectives, (1) the true behaviour of electrons which can be found by an exact FCI calculation (2) the failures of most currently used approximate functionals, which are all missing the derivative discontinuity (3) investigating new functionals that may have some aspects of the derivative discontinuity. Overall this is a great challenge for $E_{xc}[\rho]$ that must be considered in the development of new functionals.   

Hohenberg-Kohn Theorem Including Electron Spin in the Presence of a Magnetostatic Field

We consider a system of $N$ electrons in the presence of an external electrostatic ${\mathbf{\cal{E}}}({\bf{r}}) = - {\mathbf{\nabla}} v({\bf{r}})$ and magnetostatic ${\bf{B}} ({\bf{r}}) = {\mathbf{\nabla}} \times {\bf{A}} ({\bf{r}})$ fields, and include the interaction of the latter with both the orbital and spin angular momentum. The relationship between the potentials $\{ v ({\bf{r}}), {\bf{A}} ({\bf{r}}) \}$ and the nondegenerate ground state $\Psi$ is many-to-one. Explicitly accounting for this, we prove as in the case\footnote{XYP and VS, IJQC 2013, DOI: 10.1002/qua.24532} when only the orbital interaction is considered, that for $\Psi$ real, there is the one-to-one relationship: $\{ v ({\bf{r}}), {\bf{A}} ({\bf{r}}) \} \leftrightarrow \{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$, where $\rho ({\bf{r}})$ and ${\bf{j}} ({\bf{r}})$ are the corresponding density $\rho ({\bf{r}})$ and physical current density ${\bf{j}} ({\bf{r}})$. Thus, $\{ \rho ({\bf{r}}), {\bf{j}} ({\bf{r}}) \}$ are the basic variables of the system. At present, except for the one electron system, no proof of bijectivity exists for the case of $\Psi$ complex.   

Wigner High Electron Correlation Regime of Nonuniform Electron Density Systems: A Quantal Density Functional Theory (QDFT) Study

We investigate the Wigner regime of the nonuniform electron density system of the Hooke's atom characterized by the ``fat attractor'' profile via QDFT. We determine the quantal sources: the density; the nonlocal Fermi and Coulomb hole charges; and the single-particle and Dirac density matrices. From these sources we obtain, respectively, the Hartree, Pauli, Coulomb, and Correlation-Kinetic fields. The work done in these fields leads to the corresponding components of the local electron-interaction potential of the noninteracting fermions that reproduce the density. The corresponding components of the total energy are determined by the respective integral virial expressions in terms of the fields. We discover that Correlation-Kinetic effects are very significant. We propose that in addition to a high electron-interaction energy, the Wigner regime also be characterized by a high Correlation-Kinetic energy.   

Magnetic Orders of LaTiO$3$ and YTiO$3$ Under Epitaxial Strain: a First-Principles study

Perovskite $R$TiO$_3$ family is a typical Mott-insulator with localized $3$d electrons. In this work, the epitaxial strain effects on the ground magnetic orders of LaTiO$_3$ and YTiO$_3$ films have been studied using the first-principles density-functional theory. For the YTiO$_3$ films, A-type antiferromagnetic order emerges against the original ferromagnetic order under the in-plane compressive strain by LaAlO$_3$ ($001$) substrate, although the A-type antiferromagnetic order does not exist in any $R$TiO$_3$ bulks. Then, for the LaTiO$_3$ films under the compressive strain, e.g. LaTiO$_3$ films grown on LaAlO$_3$, LaGaO$_3$, and SrTiO$_3$ substrates, undergo a phase transition from the original G-type antiferromagnetism to A-type antiferromagnetism. While under the tensile strain, e.g. grown on the BaTiO$_3$ and LaScO$_3$ substrate, LaTiO$_3$ films show a tendency to transit to the C-type antiferromagnetism. Furthermore, our calculations find that the magnetic transitions under epitaxial strain do not change the insulating fact of LaTiO$_3$ and YTiO$_3$.   

Testing the Standard Approach for Density-Functional Transport Calculations

Conductance across a single molecular junction can be calculated via the Landauer formalism. This is the standard approach for density-functional theory calculations of transport, but it requires extremely accurate Kohn-Sham potentials that can only be achieved under certain conditions using accurate functionals. Recent work has shown an example where the standard approach works remarkably well for a site model [1, 2]. In this work, we test the standard approach for one dimension in real space where we can extract numerically exact potentials using density-matrix renormalization group [3]. \\[4pt] [1] J. P. Bergfield, Z.-F. Liu, K. Burke, and C. A. Stafford, ``Bethe ansatz approach to the kondo effect within density-functional theory,'' Phys. Rev. Lett., 108, 066801 (2012).\\[0pt] [2] Z.-F. Liu, J. P. Bergfield, K. Burke, and C. A. Stafford, ``Accuracy of density functionals for molecular electronics: the anderson junction,'' Phys. Rev. B (2012).\\[0pt] [3] E. M. Stoudenmire, L. O. Wagner, S. R. White, and K. Burke, ``One-dimensional continuum electronic structure with the density-matrix renormalization group and its implications for density-functional theory,'' Phys. Rev. Lett., 109, 056402 (2012).   

Quantum oscillations in the kinetic energy density: Gradient corrections from the Airy gas

We show how one can systematically derive exact quantum corrections to the kinetic energy density (KED) in the Thomas-Fermi (TF) limit of the Airy gas (AG). The resulting expression is of second order in the density variation and we demonstrate how it applies universally to a certain class of model systems in the slowly varying regime, for which the accuracy of the gradient corrections of the extended Thomas-Fermi (ETF) model is limited. In particular we study two kinds of related electronic edges, the Hermite gas (HG) and the Mathieu gas (MG), which are both relevant for discussing periodic systems. We also consider two systems with finite integer particle number, namely non-interacting electrons subject to harmonic confinement as well as the hydrogenic potential. Finally we discuss possible implications of our findings mainly related to the field of functional development of the local kinetic energy contribution.   

Nuclear Quantum Effects in Liquid Water: A Highly Accurate \textit{ab initio} Path-Integral Molecular Dynamics Study

In this work, we report highly accurate \textit{ab initio} path-integral molecular dynamics (AI-PIMD) simulations on liquid water at ambient conditions utilizing the recently developed PBE0+vdW(SC) exchange-correlation functional, which accounts for exact exchange and a self-consistent pairwise treatment of van der Waals (vdW) or dispersion interactions, combined with nuclear quantum effects (\textit{via} the colored-noise generalized Langevin equation\footnote{M Ceriotti, DE Manolopoulus Phys. Rev. Lett., \textbf{109}, 100604 (2012).}). The importance of each of these effects in the theoretical prediction of the structure of liquid water will be demonstrated by a detailed comparative analysis of the predicted and experimental oxygen-oxygen (O-O), oxygen-hydrogen (O-H), and hydrogen-hydrogen (H-H) radial distribution functions as well as other structural properties. In addition, we will discuss the theoretically obtained proton momentum distribution, computed using the recently developed Feynman path formulation,\footnote{L Lin, JA Morrone, R Car, M Parrinello Phys. Rev. Lett., \textbf{105}, 110602 (2010).} in light of the experimental deep inelastic neutron scattering (DINS) measurements.   

Gutzwiller density functional theory for solid hydrogen calculations

We have recently proposed a Gutzwiller density functional theory (G-DFT) by innovatively replacing the noninteracting trial wavefunction in Kohn-Sham DFT with the Gutzwiller projected trial wavefunction to explicitly account for correlation effects, which renders a renormalized correlation matrix in the calculation as the key ingredient in our theory. Our approach does not require adjustable Coulomb interaction parameters, nor need of double counting terms present in LDA$+$U and LDA$+$DMFT. Our method has been demonstrated to work well in hydrogen and nitrogen molecule systems. In the presentation we will show its performance on the Hydrogen solid by specifically work out the total energy curves for different phases discussed in the literature, and compare them against the benchmark Quantum Monte Carlo (QMC) calculations.   

Understanding Machine-learned Density Functionals

Recently, some of us applied machine learning (ML) in a completely new approach to approximating density functionals [1,2]. In a proof of principal, kernel ridge regression was used to approximate the kinetic energy of non-interacting fermions confined to a 1d box as a functional of the electron density [1]. In that work, a modified orbital-free DFT scheme was able to produce highly accurate self-consistent densities and energies, which were systemically improvable with additional training data. In this work, we explore the properties of the ML approximated functional derived in [1]. In particular, we investigate the use of various kernels and their properties and compare various cross validation methods. We discuss the issue of functional derivatives in greater detail, explain how a modified constraint to the standard Euler equation enables highly accurate self-consistent densities, and derive a projected gradient descent algorithm using local principal component analysis. Finally, we explore the use of a sparse grid representation of the electron density and its effects on the method.\\[4pt] [1] J. C. Snyder, et al. Phys. Rev. Lett., 108, 253002 (2012).\\[0pt] [2] J. C. Snyder, et al. accepted to J. Chem. Phys. (2013).   

Exorcising Ghost Transmission from Electron Transport Calculations: Refighting Old Battles in New Contexts

First-principles calculations of electron transport aim to understand the dynamics of electrons as they traverse quantum mechanical systems. For instance, how does electric current travel through a molecule? Despite their successes over the years, these calculations are known to be haunted by several numerical artifacts. Ghost transmission is among the most serious of these unphysical results, causing transmission coefficients to show an extreme dependence on the basis set and to be many orders of magnitude too large. In this talk, we discuss electron transport formalisms, uncover the cause of ghost transmission, develop exorcism strategies, and present several numerical examples. In the end, ghost transmission is a ramification of poorly chosen spatial partitions. Instead of choosing partitions with the basis set (in a manner reminiscent of Mulliken or L\"owdin population analyses), the relevant projection operators must be selected without referencing the basis set.   

Green's functions in equilibrium and nonequilibrium from real-time bold-line Monte Carlo

Green's functions for the Anderson impurity model are obtained within a numerically exact formalism. We investigate the limits of analytical continuation for equilibrium systems, and show that with real time methods even sharp high-energy features can be reliably resolved. Continuing to an Anderson impurity in a junction, we evaluate two-time correlation functions, spectral properties, and transport properties, showing how the correspondence between the spectral function and the differential conductance breaks down when nonequilibrium effects are taken into account. Finally, a long-standing dispute regarding this model has involved the voltage splitting of the Kondo peak, an effect which was predicted over a decade ago by approximate analytical methods but never successfully confirmed by numerics. We settle the issue by demonstrating in an unbiased manner that this splitting indeed occurs.   

Session M2: Focus Session: Charge & Energy Transfer I

The optoelectronic properties of in-plane grain boundaries and out-of-plane interfaces in two-dimensional transition metal dichalcogenides

Two-dimensional monolayer transition metal dichacogenides (TMDs) are a promising new class of nanomaterial for energy harvesting systems. Monolayer group 6 TMDs (MX2, M $=$ Mo, W, X $=$ S, Se) are direct bandgap semiconductors with strongly bound excitons giving potential for diverse physical phenomena such as multiple exciton generation, trion formation, spin valley coupling and hot electron extraction. In this talk, we will study the behavior of electrons and excitons along in- and out-of-plane interfaces of monolayer materials. Using chemical vapor deposition, we produce high-quality, large-area monolayer molybdenum disulfide. Using electron microscopy, optical spectroscopy and electrical transport, we show that these monolayers contain in-plane grain boundaries composed of 8-4-4 ring defects in the hexagonal lattice and that that these grain boundaries impact the local optical and electronic properties of the material. We examine the role of interlayer coupling by building heterostructures of similar and dissimilar monolayer materials using ultra-clean transfer techniques. First, we build bilayers of molybdenum disulfide with a well-defined interlayer twist. Using optical spectroscopy, we observe that the layers electronically hybridize to form an indirect optical transition and that we can continuously tune electronic and optical properties of the bilayer with the twist angle. Next, we study the properties of an atomically thin p-n junction formed by a MoS2-WSe2 heterostucture. From photoluminescence and scanned photocurrent measurements, we demonstrate a photovoltaic response in the junction, mediated by charge transfer across the van der Waals interface.   

Photosynthesis Revisited: Optimization of Charge and Energy Transfer in Quantum Materials

The integration of new nano- and molecular-scale quantum materials into ultra-efficient energy harvesting devices presents significant scientific challenges. Of the many challenges, the most difficult is achieving high photon-to-electron conversion efficiency while maintaining broadband absorption. Due to exciton effects, devices composed of quantum materials may allow near-unity optical absorption efficiency yet require the choice of precisely one fundamental energy (HOMO-LUMO gap). To maximize absorption, the simplest device would absorb at the peak of the solar spectrum, which spans the visible wavelengths. If the peak of the solar spectrum spans the visible wavelengths, then why are terrestrial plants green? Here, I discuss a physical model of photosynthetic absorption and photoprotection in which the cell utilizes active feedback to optimize charge and energy transfer, thus maximizing stored energy rather than absorption. This model, which addresses the question of terrestrial greenness, is supported by several recent results that have begun to unravel the details of photoprotection in higher plants. More importantly, this model indicates a novel route for the design of next-generation energy harvesting systems based on nano- and molecular-scale quantum materials.   

Charge and spin transport in porphyrin-based molecular junctions

We study weak-bias charge and spin transport behavior of four metal-porphyrin molecules in molecular junctions, using a combination of break-junction experiments and a self-energy corrected first-principles approach based on density functional theory. Some of the molecules are open-shell, and they are of potential interest to spin filtering and solar energy conversion. Optimally-tuned range-separated hybrid functionals are used in combination with a correction for static polarization effects to yield accurate level alignment between Fermi level and dominating conducting orbital energies in the junction. We find that the conductance can change by up to a factor of two when different metal cations are used. Our calculations of low-bias conductance generate similar trends and are in quantitative agreement with experimental measurements. Implications for spin transport are discussed.   

Two Dimensional White Light Spectroscopy Reveals Energy Transfer Pathways in Semiconducting Carbon Nanotube Thin Films

Carbon nanotubes are promising materials for the active layer in photovoltaic devices because of their tunable bandgaps and large exciton diffusion lengths. We are studying thin films of coupled semiconducting nanotubes to understand the dynamics of exciton transfer. Previous work used transient absorption to follow photoexcitation transfer from large to small bandgap nanotubes, but a comprehensive mechanism could not be obtained, largely due to the wide range of wavelengths over which these films absorb. We have developed two-dimensional white light spectroscopy (2D WL) as a novel probe of these films and many other systems in the solar energy sciences. By studying the evolution (as a function of waiting time) of crosspeaks between the E$_{ii}$ states for different bandgap nanotubes, we are able to map out the energy transfer pathway. The advantage to using 2D WL over traditional 2D electronic spectroscopies is that the spectral bandwidth produced from supercontinuum generation is significantly larger than that accessible from an optical parametric amplifier. Thus, we are able to cover the entire film absorption simultaneously, thereby obtaining a map of the instantaneous exciton distribution. Several surprising results will be reported.   

Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are a new class of 2D semiconductors with the band edge at the corners of the hexagonal Brillouin zone. There has been rapid progress in demonstrating the interesting 2D excitonic properties of TMDs, such as tunable exciton charging effects, large exciton and trion binding energies, and valley exciton polarization and coherence. In this talk, I will discuss the role of excitons in solid--state light emitting devices made from monolayer TMDs, as well as intralayer and interlayer excitonic properties in both TMD bilayers and heterostructure devices. The results are relevant for energy-efficient optoelectronics based on 2D layered materials.   

Probing Transport through Single Molecule Junctions by Electrolytic Gating

Organic field effect transistors made using ionic liquids or electrolyte solutions as gate dielectrics have received significant attention due to their ability to generate huge interfacial capacitances at the nano-scale. We apply this technique to single-molecule junctions created using the scanning tunneling microscope-based break-junction technique. We demonstrate that we can tune the transport characteristics of single-molecule junctions, modulating the conductance of junctions with molecules that are electrochemically inactive, within the gate bias range probed. For molecules that conduct through the highest occupied molecular orbital (HOMO), we see a decreasing conductance while applying a positive electrochemical gate potential while those that conduct though the lowest unoccupied molecular orbital (LUMO) show the opposite trend. Furthermore, we are able to fit the experimental gating data with a Lorentzian transmission function, and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations. This work shows that electrolyte gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transmission characteristics.   

Projection operators - non-equilibrium Green functions approach to quantum transport

We consider projection operator approach to non-equilbrium Green function equation-of-motion (PO-NEGF EOM) method. The technique resolves problem of arbitrariness in truncation of an infinite chain of EOMs, and prevents violation of symmetry relations resulting from the truncation. The approach, originally developed by Tserkovnikov [Theor. Math. Phys. \textbf{118}, 85 (1999)] for equilibrium systems, is reformulated to be applicable to time-dependent non-equilibrium situations. We derived canonical form of EOMs, thus explicitly demonstrating a proper introduction of the non-equilibrium atomic limit in junction problems. A simple practical scheme applicable to quantum transport simulations is formulated. We perform numerical simulations within simple models, and compare results of the approach to other techniques, and where available also to exact results.   

Optical Properties of Two-Dimensional Crystals and Heterostructures

Atomically thin two-dimensional materials, including graphene, boron nitride, metal dichalcogenides and their heterostructures, can exhibit novel optical phenomena that are distinctly different from bulk materials. In this talk, I will present our recent results on tunable optical properties in graphene/boron nitride heterostructures, where the coupling between graphene and boron nitride gives rise to new functionality. I will also discuss new optical behavior observed in metal dichalcogenide materials.   

Session M3: Physics Education

Newton's Bridge Learning Community: Can Student Learning in Introductory Physics and Calculus be a Pathway to Undergraduate Research?

A pathway to undergraduate research for freshman level physics through interdisciplinary pairings of physics and calculus courses is examined. Through ``pairing courses,'' active learning approaches, and jointly constructed inquiry-based course activities, students formulate and investigate a ``research problem.'' Some effects of a student-peer-mentor program is also examined. The use of technology incorporated into ``theme-focused'' activities is outlined. Some of the technological components include the iPad, Vernier sensors with related software, and introductory MATLAB. This presentation analyzes some of the outcomes of the learning community pairing of calculus-based Physics I (Mechanics and Heat) and Math (Calculus II), called a ``A Journey Across Newton's Bridge,'' and also the follow-up course pairing calculus-based Physics II (Electricity and Magnetism) and Multi-variable calculus called ``Multi-Dimensional Experiences'' which are being offered at Montgomery College.   

Observation, Understanding and Belief: guiding students through the great texts of physics and astronomy

Questions such as ``Is Newton's theory of gravity correct?'' and ``How do you know?'' appeal to the innate sense of inquisitiveness and wonder that attracted many students (and professors) to the study of natural science in the first place. Seeking to answer such questions, one must typically acquire a deeper understanding of the technical aspects of the theory. In this way, broadly posed questions can serve as a motivation and guide to understanding scientific theories. During the past decade, I have developed and taught a four-semester introductory physics curriculum to undergraduate students at Wisconsin Lutheran College which is based on the careful reading, analysis and discussion of foundational texts in physics and astronomy---texts such as Newton's \textit{Principia}, Huygens' \textit{Treatise on Light}, and Pascal's \textit{Equilibrium of Liquids}. This curriculum is designed to encourage a critical and circumspect approach to natural science, while at the same time developing a suitable foundation for advanced coursework in physics. In this talk, I will discuss the motivation, organization, unique features, and target audience of an undergraduate physics textbook, recently submitted for publication, which is based on this curriculum.   

Using direct measurement videos to teach physics

The video format has many advantages over written texts. For this reason we have been working to create a library of short, high-quality videos of real situations that allow students to directly analyze and measure the phenomenon. In this talk I will discuss the advantages and disadvantages of video as compared to written material and hands-on labs, and will share highlights from our library. A library of our videos can be found here: http://serc.carleton.edu/sp/library/direct\textunderscore measurement\textunderscore video/video\textunderscore library.html   

Improving Student Learning and Views of Physics in a Large Enrollment Introductory Physics Class

Interactive engagement (IE) strategies can be helpful for students learning introductory physics with small group recitations. Less is known about their impact for large lecture-based courses. This study examined student learning and views of physics in a large enrollment course that included IE but no small-group recitation. The questions addressed were: (a) What do students learn about physics and how does this compare to reports for traditional courses?, (b) How do students' views of physics change and how does this compare to reports for traditional courses?, and (c) Which instructional strategies contribute to student outcomes? Data included pre-post FCI scores, classroom examinations during the term, pre-post CLASS scores, and student work, interviews, and open-ended surveys. Findings include a FCI average normalized gain of 0.32, which is high for students with low pre-test score (30\% for this group) and instructors new to IE methods. Students' views of physics remained relatively unchanged, which is promising given the typical decline for student views. And instructional strategies as a set, not individual strategies, impacted student outcomes. Findings support the recommendation to adopt IE methods in introductory physics classes, particularly when pre-tests are low.   

Re--thinking the Rubric for grading the Colorado Upper-Division Electrostatics Diagnostic

The University of Colorado has developed the Colorado Upper-Division Electrostatics (CUE) Diagnostic to test students' understanding of the content of the first semester of an upper-division electricity and magnetism course. We analyze three CUE problems: a problem involving superposition principle, a problem involving Gauss' Law and a problem involving electric potential. Using data from both Oregon State University and the University of Colorado, we discuss the strengths and limitations of the current rubric and compare results using a different analysis scheme. We discuss the implications for assessing students' understanding.   

Designing for Impact: Recommendations for Curriculum Developers and Change Agents

Many innovations in teaching undergraduate STEM courses have been developed in the past 30 years, but few have been widely adopted. As part of a NSF-funded project designed to increase the impact of STEM education development efforts we have examined this problem from several perspectives. This talk will describe our model of how curriculum developers and change agents can plan for development and dissemination of new instructional strategies and teaching materials in ways that are likely to impact teaching practices. Development of this model was informed by (1) studies of typical development and dissemination practices, (2) studies of instructional strategies and teaching materials that have had a large impact, and (3) review of the related literature.   

A versatile university-grade research lab in a high school setting

Early experiences with physics at the advanced level of active research are feasible in a high school setting. A versatile and modular framework for supporting such experiences across a large school district is located in a free-standing building next to Gateway High School in Aurora, Colorado. Called the Innovation Hyperlab, this facility provides the technical infrastructure of 52 different technologies ranging from materials to electronics to optics to microtechnology. A modular curriculum supports learning ``on demand'' as projects proceed. Elements of this curriculum are also being integrated into mainstream daytime coursework for high school students, including regular physics courses and a new set of courses on biomedical instrumentation. An Innovation Academy provides a weekend venue for students to go beyond normal classwork and pursue active research and technical innovation mentored by teachers and university undergraduates.   

Project SOS: The Science of Sustainability

Project SOS: Making Connections Using The Science Of Sustainability is an Informal Science Education Pathways Project designed to teach the science of sustainability to middle-school aged youth in rural communities of northern ID and eastern WA. The educational focus is the physics of convection, conduction and radiation and how these exist in nature and specifically in the home of the youth. Our goal is to explore the implementation of a cooperative-learning model in which youth become experts in their area of heat transfer using portable exhibits, teach their fellow team-members about those mechanisms, and apply this knowledge as a team to improve the energy efficiency of a model house. We provide simple tools and instructions so that they may apply their new knowledge to their own homes. We analyze audio and video of the interactions of our facilitators with the youth and among the youth, and use pre- and post-surveys to document the increase in understanding of energy transfer mechanisms in their homes and the environment. The tools and techniques developed to accomplish our goals and our current findings regarding the effectiveness of this approach will be discussed.   

In Support of Physics: Redesigning library collections, spaces, and services

In order to improve support for physics learning, teaching, and research at the University of Arkansas, Fayetteville, the Physics Library personnel have implemented important changes over the past three years. Updates in collection-building practices, changes to physical spaces, and developments of new services have all been made so the Physics Library is more useful to students and faculty. In terms of collection management, all patrons -- students, staff, and faculty -- have been encouraged to make suggestions for additions to the library collection. The print collections were rearranged to encourage circulation. Spaces within the library have been designated as either group study or silent study, and teaching assistants are encouraged to use the space for their office hours. Library services have also been taken directly to undergraduate physics lab sections to make library information easily accessible for more students. The Physics Library, along with the other branch libraries on campus, has been highlighted in conjunction with the library campaign promoting subject librarians and introducing undergraduate students to ``their'' librarian. Trends in circulation, research questions, and door count statistics will be presented alongside explanations of the implemented changes.   

Serving Physicists and the STEM Community: What is the Future of the Science Library?

What are the academic work behaviors and needs of physicists and the STEM Community? How are science libraries already used? What assumptions and approaches to information access and control need to be challenged? What does this mean for the future of library support for physics? These are just some of the questions being addressed by research at Florida State University Libraries. Learn how the findings of these studies addresses these questions and what the findings could mean for the future of library support for science research and teaching.   

Sum Rules, Classical and Quantum -- A Pedagogical Approach

Sum rules in the form of integrals over the response of a system to an external probe provide general analytical tools for both experiment and theory. For example, the celebrated $f$-sum rule gives a system's plasma frequency as an integral over the optical-dipole absorption spectrum regardless of the specific spectral distribution. Moreover, this rule underlies Smakula's equation for the number density of absorbers in a sample in terms of the area under their absorption bands. Commonly such rules are derived from quantum-mechanical commutation relations, but many are fundamentally classical (independent of $\hbar )$ and so can be derived from more transparent mechanical models. We have exploited this to illustrate the fundamental role of inertia in the case of optical sum rules. Similar considerations apply to sum rules in many other branches of physics. Thus, the ``attenuation integral theorems'' of ac circuit theory reflect the ``inertial'' effect of Lenz's Law in inductors or the potential energy ``storage'' in capacitors. These considerations are closely related to the fact that the real and imaginary parts of a response function cannot be specified independently, a result that is encapsulated in the Kramers-Kronig relations.   

Algebraic Proof of the Distributive Law for Vector Multiplication

Courses in first year mechanics generally start with an introduction to vector methods which include scalar and vector multiplication$^{\mathrm{1}}$. While the demonstration of the validity of the distributive law for scalar multiplication is straightforward, this is not so for vector multiplication. The latter requires complicated geometrical visualization, so its proof is often skipped$^{\mathrm{1}}$. Neither the commutative nor associative law holds for vector multiplication, so there is no a priori reason that the distributive law should hold. In this paper we present an algebraic approach to the proof that requires no geometric visualization. It is based on two relations: (1) the distributive law for scalar multiplication and (2) a*(bxc)$=$c*(axb)$=$b*(cxa). 1. e.g. C. Kittlel, W.D. Knight, M.A. Ruderman, Mechanics, Berkeley Physics Course Vol. 1, 2$^{\mathrm{nd}}$ ed. McGraw Hill, pp34-39.   

Session M4: Focus Session: Pyrochlore magnets: spin ice and spin liquid

Dy$_{2}$Ti$_{2}$O$_{7}$ Spin Ice Thin-Films

Spin ice[1] illustrates much novel science, including unusual phases, degeneracies, quasiparticles and topology[1-4]. A characteristic feature of spin ice is its apparent violation of the Third Law of thermodynamics. This leads to a number of interesting properties including the emergence of an effective vacuum for `magnetic monopoles' and their currents - `magnetricity'. Here we add a new dimension to the experimental study of spin ice by fabricating thin epitaxial films of Dy$_{2}$Ti$_{2}$O$_{7}$ on an inert substrate. The films show the distinctive characteristics of spin ice at temperatures greater than $2$ ${\rm K}$, but at lower temperature we find evidence of a zero entropy state. This restoration of the third law in spin ice thin films is consistent with a predicted [5] strain-induced ordering. Our results illustrates how the fabrication and study of thin films opens up new possibilities for the control and manipulation of the unusual magnetic properties of spin ice materials and related frustrated magnets. \\[4pt] [1] Harris M.J. et al. PRL 79, 2554(1997) [2] Ramirez A.P. et al. Nature 399, 333(1999) [3] Ryzhkin I.A. J. Exp. and Theor. Phys. 101, 481(2005) [4] Castelnovo C. et al. Nature 451, 42(2008) [5] Jaubert L.D.C. PRL 105, 087201(2010)   

Wien Effect on a Lattice

The Second Wien Effect is an increase of conductivity of Coulomb gas in an external field, driven by enhanced dissociation of Coulombically bound pairs. The importance of the Wien effect for spin ice was suggested previously since spin ice maps to a Coulomb gas of magnetic monopoles. We present simulations of a lattice Coulomb gas and spin ice. The results confirm Onsager's theory of the Wien effect and reveal additional corrections, while allowing access to microscopic dynamics underlying the increase in the charge carrier density. Main extensions of the original theory involve the Debye screening, field dependent mobility and the character of the association constant. We discuss further corrections specific to spin ice due its emergent topological charge and Dirac string network.   

Hubbard Model on the Pyrochlore Lattice: a 3D Quantum Spin Liquid

We demonstrate that the insulating one-band Hubbard model on the pyrochlore lattice contains, for realistic parameters, an extended quantum spin-liquid phase. This is a three-dimensional spin liquid formed from a highly degenerate manifold of dimer-based states, which is a subset of the classical dimer coverings obeying the ice rules. It possesses spinon excitations, which are both massive and deconfined, and on doping it exhibits spin-charge separation. We discuss the realization of this state in effective $S = 1/2$ pyrochlore materials.   

Time Domain Terahertz Spectroscopy Study of Composite Spin Excitations in a Quantum Spin Ice

We report the terahertz transmission spectra of the quantum spin ice material $Yb_{2}Ti_{2}O_{7}$. Several branches of magnetic absorption are observed with applied magnetic field. We compare the experimental results with classical spin wave analysis, and identify signatures of composite spin excitations.   

Sm$_{2}$Ti$_{2}$O$_{7}$: An exchange spin ice candidate?

A phase pure single crystal of Sm$_{2}$Ti$_{2}$O$_{7}$ was grown from phase pure powder synthesized by a standard solid state reaction. A Curie-Weiss fit yielded a Curie constant corresponding to a smaller $\mu_{eff}$ compared to $\mu_{eff,free}$ and a value for $\theta_{CW}$ corresponding to dominant AFM interactions. C$_{p}$ measurements were performed at 0 T and 9 T down to 0.35 K with both yielding a low T anomaly but with the latter being shifted to lower T with an increase in $\Delta$ from a high T expansion of the Schottky anomaly. While the Schottky fit was successful for 0 T, the fit proved unsuccessful for 9 T indicating possible ordering. With the reduced $\mu_{eff}$ and the lack of an LRO state down to 0.35 K, providing an ${f}>>$1, the system is frustrated with its ${\cal H}$ being $J$ dominated. Future work will consist of growing an isotopically pure crystal for neutron scattering, lower T DC $\chi$ measurements to reduce CF effects, AC $\chi$ to yield $\tau$ through degenerate configurations with the objective for providing a comparison with spin ices in literature. Furthermore, additional C$_{p}$ at multiple H$_{o}$ and at lower T will be performed to determine both $\Delta(H_{o}$) and if the 9 T anomaly is indeed a transition that is electronic in origin.   

Far from equilibrium behaviour of spin ice materials

Non-equilibrium physics in spin ice is a novel setting which combines kinematic constraints, emergent topological defects, and magnetic long range Coulomb interactions. In spin ice, magnetic frustration leads to highly degenerate yet locally constrained ground states. Together, they form a highly unusual magnetic state -- a ``Coulomb phase'' -- whose excitations are pointlike defects -- magnetic monopoles -- in the absence of which effectively no dynamics is possible. At low temperatures, the monopoles are sparse and dynamics becomes very slow. These systems are therefore prone to falling out of equilibrium at low temperatures, for instance following comparatively rapid changes in temperature or applied magnetic field. In this regime, a wealth of dynamical phenomena occur, including reaction diffusion behaviour, slow dynamics due to kinematic constraints, as well as behaviour that mimic the deposition of interacting dimers on a lattice. The situation is further complicates by the presence of disorder that, even at small densities, appears to have a sizeable effect on the low-temperature dynamics of these systems. Here we investigate some of these phenomena and we propose how to effectively extend existing theories to to describe spin ice far from equilibrium.   

Engineering entropy: novel phases on the pyrochlore lattice

Frustrated pyrochlores such as Yb$_2$Ti$_2$O$_7$ push our understanding of magnetism to its limits [1, 2]. Here we explore a highly general model for spins on the pyrochlore lattice. We establish a complete phase diagram for the model [3] and are able to identify several previously unstudied limits where classical order breaks down entirely. Here we focus on two limits of special interest: a classical spin liquid and a ``hidden order" spin nematic. These ideas are explored in the context of experiments on the pyrochlore stannates and titanates. [1] J. S. Gardner, M. J. P. Gingras, J. E. Greedan, Rev. Mod. Phys. {\bf 82}, 53, (2010). [2]~K.~A.~Ross, L. Savary, B. D. Gaulin and L. Balents, Phys. Rev. X {\bf 1}, 021002 (2011). [3]~H.~Yan, O.~Benton, L. Jaubert and N.~Shannon, arXiv:1311.3501.   

Quantum effects in a realistic model of spin ice

The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 offer a widely-studied example of a classical spin liquid, complete with magnetic monopole excitations. Here we use exact diagonalization and quantum Monte Carlo simulation and to explore how quantum tunnelling between different ice states changes the ground state of a realistic model of a spin ice. We find that the competition between long-range dipolar interactions and second-neighbour exchange interactions helps to stabilize a quantum spin liquid phase and, for large enough exchange, a ferromagnet ground state. We discuss the implications of these results for the spin ice Dy2Ti2O7.   

Effective electromagnetism in rare-earth pyrochlore oxides

Rare-earth pyrochlore oxides show a fabulously diverse range of different forms of magnetism, including both classical and quantum spin-liquid phases. Here we develop a unified picture of ordered and disordered states in a pyrochlore magnet with anisotropic exchange interactions. We find that the great majority of these phases can be understood in terms of a simple classical field theory, reminiscent of electromagnetism.   

Magnetoelastic spin liquid in Tb2Ti2O7?

In the rare earth pyrochlore Tb$_2$Ti$_2$O$_7$, a three-fold puzzle exists - the mechanism by which Tb$_2$Ti$_2$O$_7$ escapes both magnetic order and/or a structural distortion, and furthermore, the nature of the spin liquid which exists instead, are long standing questions in the field of frustrated magnetism. Recent theories propose that classical spin order is suppressed by virtual crystal field excitations which renormalize the antiferromagnetic exchange, making Tb$_2$Ti$_2$O$_7$ into a type of quantum spin ice [1]; or that an undetected structural distortion leads to a spin-liquid state built of singlets [2]. Using polarized neutron scattering, we have recently shown that, at low temperature, Tb$_2$Ti$_2$O$_7$ has power-law spion correlations, manifested by pinch point scattering, somewhat similar to a spin ice [3]. We have also discovered that an acoustic phonon is coupled to an excited crystal field state, producing a sharp, dispersive mode with both magnetic and phononic character [4]. I will show that the overall structure of the low temperature state of Tb$_2$Ti$_2$O$_7$ should therefore be viewed as a Coulomb phase with propagating spin excitations [4,5]. \\[4pt] [1] Molavian et al., PRL 98, 157204 (2007);\\[0pt] [2] Petit et al., PRB 86, 174403 (2012);\\[0pt] [3] Fennell et al., PRL 109, 017201 (2012);\\[0pt] [4] Fennell et al., arXiv:1305.5405;\\[0pt] [5] Guitteney et al., PRL 111, 087201 (2013)   

Spin freezing in geometrically frustrated magnets

Materials which are believed to be faithfully represented by classical frustrated magnets with macroscopically degenerate groundstates, often exhibit spin-freezing. The latter is a transition to a spin-glass phase. Explaining the mechanism of such freezing is not always a simple task, since conventional ingredients, like randomness of the interactions, is not always present in the systems under study. We present a model, where dilution alone generates frustrating interaction between certain spins in the systems and leads to their freezing. The effective model deals with antiferromagnetically coupled Heisenberg spins in 2D. Both the long-range nature of the interaction and its dependence on the distance are crucial for the existence of the glass phase. We confirm our predictions by performing Monte-Carlo simulation of the effective model.   

Stability of Gapless Quantum Spin-Liquids

Strong correlations can lead to novel quantum phases with striking features, such as, emergent fermions and photons in a bosonic system, or even phases which lack any sharply defined quasiparticle. Given their scarcity, a fundamental question is: when are such `fractionalized' phases stable? In this talk, I will employ the recently discovered results which relate quantum entanglement and the renormalization group, to determine the stability of several gapless quantum spin-liquids. I will also provide a general argument which shows that the phase transitions out of a topological phase necessarily lie beyond Landau-Ginzburg paradigm.   

Spectroscopic signatures of crystal momentum fractionalization

In spin liquids with fractional excitations, the low-energy edge $\Omega(q)$ of the two-spinon continuum carries information about the single-spinon physics. This physics is accessible experimentally in inelastic neutron scattering, for example, in the dynamical spin structure factor. We show that some types of quantum-number fractionalization in gapped, $Z_2$ spin liquids lead to dramatic signatures in $\Omega(q)$. Notably, it may need to repeat within the first Brillouin zone, which is a direct signature of fractional crystal momentum, remarkable in the absence of symmetry-breaking spatial order.   

Session M6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Ferroelectric Effects

Anisotropic conductance at improper ferroelectric domain walls

In ferroelectric oxides natural interfaces spontaneously arise in form of domain walls. Just like artificially constructed interfaces these domain walls are a rich source for fascinating physics resulting from their low symmetry, geometric confinement, electrostatics, and strain. Enhanced electronic transport properties are for instance reported to emerge at domain walls in various ferroelectrics such as BaTiO$_{\mathrm{3}}$, BiFeO$_{\mathrm{3}}$, LiNbO$_{\mathrm{3}}$, or Pb(Zr$_{\mathrm{0.2}}$T$_{\mathrm{i0.8}})$O$_{\mathrm{3}}$. In my talk I will demonstrate and discuss additional degrees of freedom that arise at domain walls in so-called improper ferroelectrics -- systems in which the domain formation is determined by a primary order parameter other than the polarization. Due to the secondary nature of the polarization rather unusual domain wall configurations are stabilized leading to novel functionalities. Here, I will present two examples: Geometrically driven ferroelectric domain walls with anisotropic conductance properties and their magnetic analogue, i.e. hybrid domain walls in magnetically induced ferroelectrics. Results gained by cathode-lens microscopy, scanning-probe microscopy, and nonlinear optics will be shown providing insight to the domain wall physics on nano- to mesoscopic length scales.   

Polaronic nature of competing interfacial ferromagnetic/antiferromagnetic order in a La0.7Ca0.3MnO3/BiFeO3 heterostructure

We reveal the polaronic behavior associated with reduced interfacial ferromagnetic order in a La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/BiFeO$_{3}$ (LCMO/BFO) heterostructure, which is likely the origin of tunable magnetotransport upon switching the ferroelectric polarity in LCMO/BFO. This is discovered through the difference in dynamical spectral weight transfer between LCMO and LCMO/BFO at low temperatures. This polaronic feature in LCMO/BFO decreases in relatively high magnetic fields due to the increased spin alignment, while no discernible change is found in the LCMO film at low temperatures. These results thus shed new light on the intrinsic mechanisms governing magnetoelectric coupling in this heterostructure and potentially offer a new route to enhancing multiferroic functionality.   

Element-Specific Depth Profile of Magnetism and Stoichiometry at the La$_{0.67}$Sr$_{0.33}$MnO$_3$/BiFeO$_3$ Interface

Depth-sensitive magnetic, structural and chemical characterization is important in the understanding and optimization of novel physical phenomena emerging at interfaces of transition metal oxide heterostructures. In this work we have investigated an epitaxial bi-layers of ferromagnetic La$_{0.33}$Sr$_{0.67}$MnO$_3$ (LSMO) / multiferroic BiFeO$_3$. Polarised Neutron Reflectivity measurements conducted at OPAL, Australia; Chalk River, Canada; and FRM-II, Munich provided the absolute magnetic moment at the interface and X-ray Resonant Magnetic Reflectivity measurements performed at BESSY-II, Berlin provided element specific magnetic information. Our measurements indicate a region of depleted magnetization extending into the LSMO at the interface. Additional resonant X-ray reflection measurements indicate a corresponding region with an altered Mn- and O-content as origin of the reduction of the magnetic moment. This will help to systematically tune the interface stoichiometry to achieve a desired property.   

Examination of exchange fields at LaSrMnO$_3$/BiFeO$_3$ interfaces

The complex oxide materials are providing a vast playground of interesting material properties that couples spin, orbital, and charge degrees of freedom. We examine the presence of significant magnetization within the antiferromagnetic layer of BiFeO$_3$ (BFO) between ferromagnetic (FM) LaSrMnO$_3$ layers. Using a classical exchange field to account for orbital reconstruction and possible inter-layer mixing, we quantify the energy scale for the interface exchange based from polarized neutron reflectivity measurements. Furthermore, we estimate the critical layer thickness in which the magnetization will be reduced to zero (or close to zero).   

Spin-phonon coupling and ferroelectricity in magnetoelectric gallium ferrite

Gallium ferrite (GaFeO$_{3}$ or GFO) is a low temperature ferrimagnet and room temperature piezoelectric wherein the magnetic transition temperature (T$_{\mathrm{C}})$ could be tailored to room temperature and above by tuning the stoichiometry and processing conditions. Such tunability of the magnetic transition temperature renders GFO a unique perspective in the research of multiferroics to potentially demonstrate room temperature magnetoelectric effect attractive for futuristic digital memory applications. Recent studies in several transition metal oxides highlight the importance of spin-phonon coupling in designing novel multiferroics by means of strain induced phase transition. In the present work, we have systematically studied the evolution of phonons in good quality samples of GFO across the T$_{\mathrm{C}}$ using Raman spectroscopy. Using the phonon softening behavior and nearest neighbor spin-spin correlation function below T$_{\mathrm{C}}$ we estimated spin-phonon coupling strength in the magnetically ordered state. In the process, we also show, for the first time, the presence of a spin glass phase in GFO where the spin-glass transition has a signature of abrupt change in spin-phonon coupling strength. Though GFO is piezoelectric and crystallizes in polar Pc2$_{1}$n symmetry, its ferroelectric nature remained controversial probably due to the large leakage current in the bulk material. To address this issue, we deposited epitaxial thin film on single crystalline yttria stabilized zirconia (YSZ) substrate using indium tin oxide (ITO) as a bottom conducting layer. We demonstrate clear evidence of room temperature ferroelectricity in the thin films from the 180$^{\mathrm{o}}$ phase shift of the piezoresponse upon switching the electric field. Further, suppression of dielectric anomaly in presence of an external magnetic field clearly reveals a pronounced magneto-dielectric coupling across the magnetic transition temperature. In addition, using first principles calculations we elucidate that Fe ions are not only responsible for ferrimagnetism as observed earlier, but give rise to the observed ferroelectricity also, making GFO an unique single phase multiferroic.   

Intrinsic magnetic properties of multiferroic h-LuFeO3

The discovery of multiferroic materials with large magnetoelectrical couplings would lead to significant advancements in many technologies. Hexagonal LuFeO$_{\mathrm{3}}$ ($h$-LuFeO$_{\mathrm{3}})$ is a multiferroic that has recently been reported to be multiferroic at room temperature; in this work, we grow 200 nm thick $h$-LuFeO$_{\mathrm{3}}$ thin films to determine its intrinsic magnetic properties. We first deposit $h$-LuFeO$_{\mathrm{3}}$ in a composition-spread geometry, creating samples that range from iron rich to lutetium rich. We use x-ray diffraction, atomic force microscopy, scanning transmission electron microscopy, and SQUID magnetometry to determine the region of the sample that is nearest to perfect stoichiometry. After identifying this region, we grow an additional sample with a rotating sample stage to ensure uniform composition throughout the sample. We determine the magnetic properties to be quite different from previously reported findings, most notably with a higher $T_{\mathrm{N}}=$147 K. Our findings show that it is easy for $h$-LuFeO$_{\mathrm{3}}$ to incorporate defects and impurities phases, leading to degraded magnetic properties compared to the stoichiometric phase.   

Low-temperature structure transition in hexagonal LuFeO3

The structural change of h-LuFeO$_3$ films at low temperature has been studied using x-ray diffraction and x-ray absorption experiments. The results are analyzed using the displacements of three phonon modes that are related to the P6$_3$/mmc to P6$_3$cm structural transition. The data indicate that the in-plane motion of the Fe and apex oxygen are responsible for the observed anomaly in both x-ray absorption and diffraction experiments. This subtle structural transition may be an origin of the low temperature magnetic phase transition at $T_R$=130 K.   

Neutron Investigations of Multiferroic LuFe1-xMnxO3

While many new multiferroic materials have surfaced, only BiFeO$_{3}$ has been shown to evince coupling of both order parameters at room temperature. Materials in which the application of an electric field can directly switch the magnetization by 180 degrees have also been elusive. New theoretical predictions suggest that this will be possible in hexagonal LuFeO$_{3}$. Recent measurements of LuFeO$_{3}$ are promising. Bulk LuFeO$_{3}$ crystallizes in the Pbnm space group. However, it can be stabilized in the P6$_{3}$cm space group in thin films. Films are found to be ferroelectric at room temperature with a remanent polarization of 6.5 $\frac{\mu C}{cm^{2}}$along the c-axis and is of a respectable magnitude, evincing long range magnetic order with spins in the plane forming the familiar 120 degree structure. At lower temperatures, it was found that the moments begin to cant. Theoretical predictions suggest that this canted moment can be switched with an electric field. Unfortunately, this canting occurs at 130 K. While the recent work in films is exciting, it is important to understand what is intrinsic to the material. Recently, we have been able to stabilize ceramic samples of LuFeO$_{3}$ in the hexagonal form. During this talk we will discuss the magnetic structure of this compound in the bulk. We will also discuss our inelastic neutron scattering results.   

Origin of room-temperature multiferroism in hexagonal LuFeO$_3$

Combined theoretical and experimental studies are carried out, focusing on the exchange interactions and their couplings with the structural instabilities in hexagonal LuFeO$_3$ (hLFO). We apply an extended Kugel-Khomskii model based on maximally localized Wannier functions generated from band structure calculations. The model clearly shows that the single occupied $d_{z^2}$ orbital in hLFO greatly increases the exchange coupling compared to that of hexagonal LuMnO$_3$ in which $d_{z^2}$ is empty. The interlayer exchange interaction is the key to the spin reorientation (SR) and weak ferromagnetic moment observed below 130K. Our calculations show that SR is strongly coupled to the $K_1$ phonon mode and only weakly dependent on $K_3$ and $\Gamma_2^{-}$ phonons. It indicates that the atomic displacements along positive direction of $K_1$ mode is responsible for the spin reorientation. This scenario is confirmed by our X-ray diffraction and X-ray absorption experiments. In the end, we propose that $T_{\rm SR}$ can be adjusted to be room temperature by structural competition between $K_1$ and $\Gamma_2^-$ modes in hLFO or by interface engineering.   

Interfacial magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$ / BaTiO$_3$ bilayers

Interfaces between the ferromagnetic and ferroelectric oxides may host artificial multiferroic phases with a strong magnetoelectric coupling, which can potentially be utilized for energy-efficient spintronics. Key for potential applications is that the magnetization of the ferromagnet is preserved at the interface, which is not always the case in complex oxide systems. In this work, we have explored the interfacial magnetic properties of ferromagnetic La0.7Sr0.3MnO3 and ferroelectric BaTiO$_3$ bilayers. The samples studied consist 10 nm La$_{0.7}$Sr$_{0.3}$MnO$_3$/ $t$ BaTiO$_3$ (LSMO/BTO) and $t$ BaTiO$_3$ (LSMO/BTO)/10 nm La$_{0.7}$Sr$_{0.3}$MnO$_3$ bilayers grown on SrTiO$_3$ substrates, with $t$ = 1.2, 2.4 and 4.8 nm. Results from X-ray resonant magnetic scattering, X-ray magnetic circular dichroism, and x-ray and polarized neutron reflectometry are combined to provide insights into how the interfacial magnetization of La$_{0.7}$Sr$_{0.3}$MnO$_3$ is influenced by the presence of the adjacent BaTiO$_3$. We find a modified interfacial magnetization in the ferromagnetic manganite layer that is dependent on the thickness and relative position of the ferroelectric layer.   

Concurrency control of the multiferroic transition in tetragonal-like BiFeO$_{3}$

The highly-elongated tetragonal-like BiFeO$_{3}$ (BFO) shows the concurrent transition of antiferromagnetic and ferroelectric order close to room temperature [1]. Despite the \textit{concurrency} indicating strong spin-lattice coupling effect, electric switching of the magnetic state has not been demonstrated so far. In this talk, we will introduce our efforts controlling the multiferroic transition temperature by means of A-site chemical substitution. Structural, ferroelectric, and magnetic states with varying chemical substitution ratio and temperature were systematically investigated through x-ray reciprocal space maps, capacitance measurement, and soft x-ray absorption spectroscopy. Landau phenomenological theory was employed to understand the behavior of multiple order parameters in a proposed phase diagram. Finally, we will discuss a new pathway to the electric switching of the magnetic state. \\[4pt] [1] K.-T. Ko \textit{et al.}, Concurrent transition of ferroelectric and magnetic ordering near room temperature. \textit{Nature Communications} \textbf{2}, 567 (2011).   

Session M7: Focus Session: Molecular Nanomagnets

Spintronic anisotropy: proximity-induced superparamagnetism

Superparamagnetism of molecular magnets, i.e. the preferential alignment of their spins along an easy axis, is a useful effect for nanoscale applications as it prevents undesired spin reversals. In these systems such a stabilization of axial spin states is ensured by the magnetic anisotropy barrier stemming from intrinsic spin-orbit coupling. Here we demonstrate that any spin-isotropic high-spin quantum dot coupled to ferromagnets can in fact acquire such superparamagnetic properties in a spintronic way [1], even though spin-orbit interaction is negligible. We predict a proximity-induced spin-anisotropy barrier, which has hallmarks of a spintronic exchange-field of quadrupolar nature: it is highly localized, electrically controllable, increases with tunnel coupling and spin-polarization. Such a field is a generalization of the dipolar exchange field that relates to a current-induced spin-torque, effect well established in spintronics [1-3].\newline [1] M. Misiorny, M. Hell and M. Wegewijs, Nature Phys. advanced online publication, 6 October 2013.\newline [2] J. Martinek et al., Phys. Rev. Lett. 91, 127203 (2003); Phys. Rev. B 72, 121302 (2005).\newline [3] J. Hauptmann et al., Nature Phys. 4, 373 (2008).\newline [4] M. Gaass et al., Phys. Rev Lett. 107, 176808 (2011)   

Magnetic Relaxation Mechanisms in Lanthanide Single Molecule Magnets

Ab initio investigation of multiplet spectrum of lanthanides in archetypal coordination geometries shows an unexpected regular structure consisting of (i) mirror symmetry of anisotropic magnetic properties of doublet states, (ii) high magnetic axiality of low-lying and high-lying doublets, comparable to complexes with ideal axial symmetry, and (iii) the strong rotation of the anisotropy axes of individual doublets [1]. The obtained high axiality of the ground doublet states explains the SMM behaviour of low-symmetry lanthanide complexes. Ab initio calculations predict that depending on the relative orientation of anisotropy axes in different doublet states, the relaxation can proceed via the first or the second excited state. Here we report new lanthanide cage complexes where two competing relaxation pathways through the first and second excited states are observed, leading to very high energy barriers for loss of magnetisation [2]. \\[4pt] [1] L. Ungur, L.F. Chibotaru, P.C.C.P., 2011, 13, 20086–20090.\\[0pt] [2] R.J. Blagg, L. Ungur, F. Tuna, D. Collison, E.J.L. McInnes L.F. Chibotaru, R.E.P. Winpenny,. Nature Chem., 2013, 5, 673-678.   

Three-fold angular modulation of the tunnel splittings in a trigonal Mn3 single-molecule magnet

We report the results of magnetization studies performed on a Mn3 single-molecule magnet of trigonal site symmetry, which detail Berry phase interference phenomenon that we relate to the geometry of the individual constituent ions. We observe for the first time the three-fold modulation of the quantum tunneling probabilities expected for a system of this symmetry, including tunnel-quenching effects resulting from destructive topological interference at tunneling resonances. These effects are symmetric with respect to a full inversion of the applied field (longitudinal and transverse) as a consequence of the time reversal invariance of the spin Hamiltonian. A multi-spin interaction Hamiltonian representing the three exchange-coupled manganese ions shows good agreement with experimental results. Shifts of the Berry phase minima in the transverse field magnitude and angular modulations of the tunnel probability for the various resonances enable a complete determination of the exact three-dimensional spatial orientations of the single-ion anisotropy tensors.   

Evidence for Geometric-Phase Interference in a Mn$_{12}$-Acetate Single-Molecule Magnet

Recent work by our group has shown evidence for geometric-phase interference between tunneling paths in the Mn$_{12}$ $^t$BuAc single-molecule magnet, the first observation of this effect in a system that has true four-fold rotational symmetry [1]. This effect was not previously observed in the bellwether Mn$_{12}$Acetate molecule, presumably because of solvent disorder inherent to the crystal. Here we report measurements on a crystal of Mn$_{12}$Acetate$\,\cdot\,$MeOH, which crystallizes without solvent disorder and therefore preserves the molecule's four-fold symmetry. The relaxation rate $\Gamma$ as a function of transverse field $H_T$ exhibits structure indicative of interference between tunneling paths similar to that found in [1]. This suggests that the solvent disorder, and not the larger dipole interactions found in Mn$_{12}$Acetate, is the most important factor in suppressing the interference effect. \\ \smallskip [1] S. T. Adams et al., Phys. Rev. Lett., {\bf 110}, 087205 (2013).   

Raman scattering studies of the temperature- and magnetic field-dependent studies of the single molecule magnet Mn$_{12}$-acetate

Single molecule magnets (SMMs) have attracted much interest since they were first reported in 1991. SMMs are a class of metal-organic compounds that show superparamagnetic behavior below a certain blocking temperature at the molecular scale. We present a study of the temperature- and magnetic-field-dependence of the single molecule magnet Mn$_{12}$-acetate using Raman scattering. Temperature-dependent measurements show an anomalous phonon behavior near 200K, indicating a lower crystal symmetry than tetragonal and supporting the inclusion of a second-order rhombic term $E(S^2_x - S^2_y)$ in the Hamiltonian, consistent with previous neutron and X-ray studies. Our field-dependent measurements near 3K show that a magnetic field oriented perpendicular to the Mn$_{12}$ magnetization direction does not affect the phonon vibrational energies. However, when the magnetic field is oriented along the easy-axis direction, there is a clear phonon mode splitting at 540 cm$^{-1}$, indicating a strong spin-phonon coupling associated with this phonon mode and the existence of a fourth-order anisotropy term in the Hamiltonian for Mn$_{12}$ acetate. The field-induced nonzero local transverse term may be responsible for a small tilt of the anisotropy axis and the odd resonance tunneling.   

The Possibility of Quantum Deflagration in the Fe8 Nano-Magnet

We report spatially resolved, time-dependent, magnetization reversal measurements of Fe8 single crystals using a microscopic Hall bar array. We found that in some samples the molecules reverse their spin direction at the resonance field in the form of deflagration. The deflagration front velocity is on the order of 1 m/sec and sensitive to field gradients and sweep rates. We discuss the possibility that this slow deflagration can be explained by flipping rates determined by the tunnel splitting only, with no over-the-barrier motion.   

Pressure tuning of anisotropy barrier in Fe$_8$ SMMs probed using high frequency EPR

Single-molecule magnets (SMMs) are spin systems with large spin ground state where quantum phenomena such as tunneling of magnetization via a considerable anisotropy barrier manifest. One such SMM that has been extensively studied is [Fe$_8$O$_2$(OH)$_{12}$(tacn)$_6$]Br$_8$.9H$_2$O, also known as Fe$_8$, with a giant spin ground state of S=10. The eight Fe atoms bridged by the ligands form a butterfly structure where six Fe atoms have spins up and two spins down in the simplest model. This structure in fact gives rise to geometrical spin frustration effects within the cluster. By varying the interaction between the spins, manipulation of quantum tunneling in SMMs may be achieved. Typically, the manipulation of spin interactions is realized using chemical methods. As an alternative approach, we employ high pressure to induce changes in the ligand-field environment of the Fe atoms. In this presentation, the pressure-dependent changes in the anisotropy barrier in single crystal Fe$_8$ SMMs investigated by high frequency electron paramagnetic resonance measurements will be discussed.   

High-Frequency Electron Paramagnetic Resonance (HFEPR) Studies on Supramolecular Aggregates of Exchange-Biased Single-Molecule Magnets

Single-Molecule magnets (SMMs) have potential applications in molecular memory and spintronics devices. For these applications, coupling two or more SMMs either to each other or to other components of a device is essential. However, the interaction should be relatively weak in order to maintain the intrinsic properties of each SMM. We have performed comprehensive HFEPR studies on supramolecular aggregates of triangular Mn$_{3}$ SMMs: [Mn$^{\mathrm{III}}_{6}$O$_{2}$(O$_{2}$CMe)$_{6}$(dpd)$_{3}$](I$_{3})_{2}$ ([Mn$_{3}$]$_{2})$ and [Mn$_{12}$O$_{4}$(O$_{2}$CMe)$_{12}$(pdpd)$_{6})$](ClO$_{4})_{4}$([Mn$_{3}$]$_{4})$. Single-crystal [Mn$_{3}$]$_{2}$ spectra shows additional spectral features that are lacking in spectra of the isolated Mn$_{3}$ units, which can be attributed to the exchange coupling within the dimers. Furthermore powder and solution spectra are essentially identical, indicating that the supramolecular dimers remain intact in solution and thus their unique properties survive outside of a crystal. Analysis of the powder spectra of [Mn$_{3}$]$_{4}$ suggests that the four Mn$_{3}$ building blocks are too weakly coupled to be detected by EPR. The results on [Mn$_{3}$]$_{4}$ support the DC susceptibility and micro-SQUID measurements [1].\\[4pt] [1] T. N. Nguyen, et al., JACS 133, 20688-20691 (2011).   

Spin Hamiltonian Analysis of the SMM V15 Using High Field ESR

We have studied molecular magnets using high field / high frequency Electron Spin Resonance. Such molecular structures contain many quantum spins linked by exchange interactions and consequently their energy structure is often complex and require a good understanding of the molecular spin Hamiltonian. In particular, we studied the V15 molecule [1], comprised of 15 spins 1/2 and a total spin 1/2, which is a system that recently showed quantum Rabi oscillations of its total quantum spin [2]. This type of molecule is an essential system for advancing molecular structures into quantum computing. We used high frequency characterization techniques (of hundreds of GHz) to gain insight into the exchange anisotropy interactions, crystal field, and anti-symmetric interactions [3] present in this system. We analyzed the data using a detailed numerical analysis of spin interactions and our findings regarding the V15 spin Hamiltonian will be discussed. \\[4pt] [1] I. Chiorescu, W. Wernsdorfer, A. M\"uller, H. B\"ogge, B. Barbara, Phys. Rev. Lett. 84, 3454 (2000).\\[0pt] [2] S. Bertaina, S. Gambarelli, T. Mitra, B. Tsukerblat, Nature 453, 203-U5 (2008).\\[0pt] [3] B. Tsukerblat, A. Tarantul and A. Muller, Physics Letters A 353, 48-59 (2006).   

Spin dynamics of molecular nanomagnets unravelled at atomic scale by four-dimensional inelastic neutron scattering

The application of inelastic neutron scattering (INS) as a microscopic probe of spin dynamics within molecular based magnets (MM) is discussed with focus on results following recent technological developments. It will be shown that recently-developed INS instrumentation enables single crystal studies of MM, yielding the four-dimensional inelastic-neutron scattering function $S(Q_{xyz},E)$ in vast portions of reciprocal space [1]. Such detailed information of neutron momentum transfer enables spin pair correlations within MM to be directly extracted without the need to pass through a model Hamiltonian. INS results for example MM exhibiting interesting physical properties such as magnetic spin frustration [2] and quantum tunnelling will be presented. The potential of four dimensional INS as a new probe of elusive magnetic phenomena present in MM will be explored. For example, the examination of how a quantum fluctuation propagates around a cyclic antiferromagnetic chain is presented and used to test the degree of validity of the N\'{e}el vector tunneling. \\[4pt] [1] M. L. Baker, T. Guidi, S. Carretta, J. Ollivier, H. Mutka, H. U. G\"{u}del, G.A. Timco, E. J. L. McInnes, G. Amoretti, R. E. P. Winpenny and P. Santini., Nature. Phys., 8, 906, (2012).\\[0pt] [2] M. L. Baker, G. A. Timco, S. Piligkos, J. S. Mathieson, H. Mutka, F. Tuna, P. Kozlowski, M. Antkowiak, T. Guidi, T. Gupta, H. Rath, R. J. Woolfson, G. Kamieniarz, R. G. Pritchard, H. Weihe, L. Cronin, G. Rajaraman, D. Collison, E. J. L. McInnes and R. E. P. Winpenny. Proc. Natl. Acad. Sci., 109, 19113, (2012).   

Quantifying the size of linear superpositions in molecular nanomagnets

Molecular nanomagnets are relatively complex spin systems that exhibit quantum mechanical behavior at low temperatures. Exploiting quantum-information theoretic measures we quantify the size of linear superpositions that can be generated within the ground multiplet of high-spin nanomagnets [1]. In particular, we consider the prototypical single-molecule magnets (namely Mn$_{12}$ and Fe$_{8})$, characterized by a ferrimagnetic spin ordering in the ground state. We show that the size of these linear superpositions are comparable to those achievable in mesoscopic systems, and be further enhanced by increasing the asymmetry between the sublattices, and by reducing the competition between exchange interactions within the nanomagnets. The same tools are also applied to the study of low-spin molecules, such as Cr$_{7}$Ni and V$_{15}$, characterized by antiferromagnetic interactions between the constituent spins. The size of the linear superpositions that have been generated within their S$=$1/2 ground doublets is contrasted with that of single s$=$1/2 spins. \\[4pt] [1] F. Troiani and P. Zanardi, Phys. Rev. B 88, 094413 (2013);   

The first radical-based spintronic memristors: Towards resistive RAMs made of organic magnets

Using molecules as building blocks for electronic devices offers ample possibilities for new device functionalities due to a chemical tunability much higher than that of standard inorganic materials, and at the same time offers a decrease in the size of the electronic component down to the single-molecule level. Purely organic molecules containing no metallic centers such as organic radicals can serve as an electronic component with magnetic properties due to the unpaired electron in the radical state. Here we present memristive logic units based on organic radicals of the nitronyl-nitroxide kind. Integrating these purely molecular units as a spin coated layer into crossbar arrays, electrically induced unipolar resistive switching is observed with a change in resistance of up to 100\%. We introduce a model based on filamentary reorganization of molecules of different oxidation state revealing the importance of the molecular nature for the switching properties. The major role of the oxidation state of these paramagnetic molecules introduces a magnetic field dependence to the device functionality, which goes along with magnetoresistive charactistics observed for the material. These are the first steps towards a spintronic implementation of organic radicals in electronic devices.   

Session M8: Focus Session: Spin Hall Effect and Related Phenomenon

Inverse Spin Hall Effect in a Ferromagnetic Metal

Recently, intense attention has been focused on the generation, detection, and exploitation of pure spin current. Only few mechanisms, among them spin Hall effect (SHE) [1], lateral spin valve [2], spin pumping [3] and spin Seebeck effect (SSE) [4], can generate a pure spin current. Once generated, a pure spin current cannot be detected electrically but by the inverse spin Hall effect (ISHE) that converts it back into a charge current. To date, ISHE has been observed only in non-magnetic metals, such as Pt and Au, with a strong spin-orbit coupling. We report the observation of ISHE in a ferromagnetic metal permalloy (Py) on ferromagnetic insulator yttrium iron garnet (YIG) [5]. Through controlling the spin current injection by altering the Py/YIG interface, we have isolated the spin current contribution and demonstrated the ISHE in a ferromagnetic metal, the reciprocal phenomenon of anomalous Hall effect. A large spin Hall angle in Py, determined from Py thin films of different thicknesses, indicates many other ferromagnetic metals may be exploited as superior pure spin current detectors and for applications in spin current.\\[4pt] [1] Hirsch, J. E. Spin Hall Effect. Phys. Rev. Lett., 83, 1834-1837 (1999).\\[0pt] [2] Valenzuela, S. O., Tinkham, M. Direct electronic measurement of the spin Hall effect. Nature, 442, 176-179 (2006).\\[0pt] [3] Saitoh, E., Ueda, M., Miyajima, H., Tatara, G. Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Appl. Phys. Lett., 88, 182509-182503 (2006).\\[0pt] [4] Uchida, K., Xiao, J., Adachi, H., Ohe, J., Takahashi, S., Ieda, J., et al. Spin Seebeck insulator. Nature Mater, 9, 894-897 (2010). \\[0pt] [5] Miao, B. F., Huang, S. Y., Qu, D. R., Chien, C. L. Inverse spin Hall effect in a ferromagnetic metal. Phys. Rev. Lett., 111, 066602 (2013).   

Spin Hall effect tunneling spectroscopy

The spin Hall effect (SHE) has attracted a lot research interest recently. Up to now, almost all of the experimental efforts related to the SHE have been focused on utilizing or characterizing electrons at the Fermi surface (FS). In this talk, we will report a technique with which one can determine the magnitude of the SHE under finite bias voltage. In our study, the spin Hall effect (SHE) was measured by injecting a spin polarized current from a ferromagnet electrode into SHE materials through an insulating tunnel barrier. By applying a combination of DC and AC current across the tunnel barrier, we were able to probe the SHE under finite biases via measuring the generated transverse voltage. Two different materials, Ta and Pt were examined with this technique. Under zero bias, the obtained SH angles agree well with previous results determined through the SHE induced switching and oscillation experiments, while below and above the FS, the SHE in those two materials shows different voltage dependences. The experimentally determined voltage dependence of the SHE can be used to check the validity of various types of band structure calculations and it also provides a guideline on further increasing the magnitude the SHE via electronic structure engineering.   

The spin Hall effect in single-crystal platinum

We have developed a process to grow thin films of single-crystal platinum by DC magnetron sputtering at elevated temperatures with post growth annealing. We have incorporated these films into bilayers with polycrystalline permalloy (Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}})$ for measurements of the spin transfer torque generated by the spin Hall effect in the platinum. We will compare measurements of the strength of the spin Hall effect, the spin diffusion length, and the Pt/permalloy spin mixing conductance between these samples and samples containing a polycrystalline Pt layer. With these studies we hope to understand better how disorder may affect the generation of spin currents by the spin Hall effect and the transmission of spin currents from a spin Hall metal to a ferromagnet.   

The spin Hall effect and spin-orbit torques in SH-metal/normal metal/ferromagnetic trilayers

The spin Hall effect (SHE) in ferromagnet/transition metal bilayer structures has been demonstrated to be a powerful means for producing pure spin currents and for exerting spin-orbit damping-like and field-like torques on the ferromagnetic layer. Large spin Hall angles have been reported for Pt, $\beta$-Ta and $\beta$-W films and have been utilized to achieve magnetic switching of in-plane and out-of-plane magnetized nanomagnets, spin torque auto-oscillators, and the control of high velocity domain wall motion. In general the spin orbit torques and the effective damping are predicted to depend directly on the spin-mixing conductance of the SHE/ferromagnet interface. This opens up the possibility of tuning these properties with the insertion of a very thin layer of another metal between the SH metal and the ferromagnet. Here we will report on experiments with such trilayer structures where we have studied the variation of the effective spin Hall angle and the effective damping constant with the choice and thickness of the insertion layer. Our results indicate that there is considerable opportunity to optimize the effectiveness and energy efficiency of the damping-like torque through engineering of such trilayer structures.   

Determination of the spin diffusion length via spin pumping and spin Hall effects

We present an experimental approach for determining the spin diffusion length of various metals by using spin pumping -- spin Hall effect via a coplanar waveguide ferromagnetic resonance (CPW-FMR) broadband technique. By studying the ratio of two voltage components (anisotropic magnetoresistance and inverse spin Hall effect) as a function of the metal layer thickness, the spin diffusion length of the material can be directly extracted. As examples, we determined spin diffusion lengths for paramagnetic Pt (1.2 nm), Pd (5 nm), Ir (0.5 nm), and antiferromagnetic IrMn (0.75 nm) at room temperature. In addition, temperature-dependent measurements show only weak dependence of these lengths with temperature. This approach for determining the spin diffusion length at any temperatures has the advantage that it does not require knowing the resistivity value of the metal layer, which changes with both thickness and temperature. Finally, the ratio of the two voltage components can also be used to probe the temperature-dependent proximity effect for metals such as Pt.   

Sign change of spin Hall effect due to electron correlation in CuIr alloys

Recently the predominant extrinsic skew scattering mechanism with a positive spin Hall angle (SHA) was experimentally observed in nonmagnetic CuIr alloys [Niimi et al., Phys. Rev. Lett. 106, 126601 (2011)], while the negative SHA was obtained by ab initio simulation if the consistent definition of SHA is used [Fedorov et al., Phys. Rev. B 88, 085116 (2013)]. We reconsider the SHA in CuIr alloys by the quantum Monte Carlo method, where the Coulomb correlation U in 5d orbitals of Ir impurities is properly included. It indicates that the SHA is negative without electron correlation (U=0), and becomes positive when an electron correlation of U=0.5 eV is included, which is consistent with the experiment. It opens a way to control the sign of SHA by electron correlation in novel spintronic devices.   

Manipulation of Magnetic Insulators Using Spin Torque from the Spin Hall Effect

We will report the growth and fabrication of devices incorporating thin films of the magnetic insulators yttrium iron garnet and lutetium iron garnet with thicknesses less than 20 nm. We perform the growth using oxide MBE, achieving high-quality films with magnetic damping parameters for single 5 nm films as small as 0.00036. We use electron beam lithography and ion milling to pattern the films into device structures with sizes ranging from 100 nm to above 1 micron, integrated with Ta contacts so that we can use the spin Hall effect to apply spin-transfer torque to the magnetic materials. We will use these devices to study how the spin Hall torque affects the effective magnetic damping parameter of isolated magnetic insulator devices, and whether spin Hall torque can be used to drive reliable magnetic switching in these materials at low current levels.   

Spin-cluster Formation and Spin-Hall Effect in a Pyrochlore Magnet

Metallic magnets with interacting localized moments and itinerant electrons are known to be the source of various fascinating phenomena. Recent theoretical studies on the spin-charge coupled systems have shown that the presence of frustration further gives rise to nontrivial magnetic states as well as transport phenomena. These situations are expected to be realized in pyrochlore and triangular metallic magnets, which have gained much interest due to their peculiar magnetism and transport properties. In this study, to explore novel phases and transport phenomena that may take place in the frustrated spin-charge coupled systems, we studied a double-exchange model on a pyrochlore lattice with spin-ice type localized moments [1]. By using a Monte Carlo technique, we show that the model shows peculiar spin-cluster formation induced by competition between the double-exchange interaction and the super-exchange interaction between the localized moments. Furthermore, we show that the peculiar intermediate phase accompanies an unconventional spin-Hall effect which originates from noncollinear spin configurations. [1] H. Ishizuka and Y. Motome, PRB 88 100402 (2013).   

Electrical spin manipulation in spin-orbit coupling systems

Generation of spin by applying as electric current in a spin-orbit coupling system has been investigated with much theoretical and experimental attention in spintronics. Although the electronic spin is the well-defined quantity, the spin is not conserved in the presence of spin-orbit interaction and therefore the theoretical definition of spin current is not uniquely given. To resolve this ambiguity in the definition, the non-Abelian gauge theory is one of the possible solutions. By associating the spin-orbit interaction with the non-Abelian vector potential, a proper definition of spin current is given on the basis of the SU(2) gauge invariance and the electronic spin is covariantly conserved. In this context, we present theoretically a general form of spin polarization in terms of an effective Yang-Mills field corresponding to the spin-orbit interaction and usual U(1) Maxwell electromagnetic field. In particular, we focus on a purely electrical spin manipulation, and we find that both of the spin Hall effect and the inverse of the spin galvanic effect arise from the same origin, i.e., the SU(2)$\times$U(1) Hall effect.   

Driving a uniform magnetization to a metastable, mixed state by Spin Hall Effect Spin Torque

Spin Hall effect based magnetic switching and domain wall motion have recently attracted a lot of attention both from a fundamental and an application perspective [1,2,3]. In that context it is important to understand how spin Hall current acts on a uniform magnetization in the absence of external magnetic field. We observe that in Hall bars made from a thin film stack of Ta (10 nm)/CoFeB (1 nm)/MgO (1 nm)/Ta (10 nm) a current pulse of magnitude 5x10$^6$ A/sq. cm. drives a uniformly polarized magnet to a metastable mixed state of up and down polarized domains. We have experimentally confirmed the mixed state through anomalous Hall effect measurement and magneto-optic Kerr effect imaging. The magnet breaks into domains due to nucleation of domain walls followed by free domain wall motion as a result of depinning of the domain walls from defects by the spin Hall torque.\\[4pt] [1] Liu, L. \textit{et al. Science }\textbf{336,} 555-558 (2012).\\[0pt] [2] Bhowmik, D. \textit{et al.} Nature Nanotechnology (2013), DOI:10.1038/nnano.2013.241.\\[0pt] [3] Emori, S. \textit{et al. }Nature Materials \textbf{12}, 611-616 (2013).   

Spin Hall effect excited parametric resonance in permalloy/platinum nanowires

We report measurement of Spin Hall effect excited parametric resonance of spin wave modes in Permalloy (Py) / Platinum (Pt) bilayer nanowires. The excitation of the parametric resonance is assisted by spin torque from direct Spin Hall current generated in the Pt layer, which acts like effective negative magnetic damping on Py magnetization. A saturating in-plane magnetic field is applied perpendicular to the wire axis. We simultaneously apply a direct and a microwave current to the nanowire and measure voltage as a function of magnetic field. At approximately twice the frequency of the spin wave eigenmodes in the Py wire in the field range used in the experiment, peaks in voltage versus field are observed above a threshold value of the ac current. The double frequency of the drive and the threshold character of the excitation demonstrate the parametric nature of the excited resonance. We also measured the dependence of the voltage peak amplitude on the microwave current amplitude measured for several values of the direct current bias applied to the nanowire. It's found that the threshold ac current shifts to lower values at higher dc bias, indicating that dc spin Hall current injected from Pt into Py reduces the effective damping in the Py layer.   

Dependence of the spin Hall effect in platinum / ferromagnet bilayers on the composition of the ferromagnet

The spin Hall effect (SHE) provides a mechanism by which charge current is converted to a pure spin current via spin--orbit interactions. These spin currents can be used to manipulate magnetization via diffusion of the spin current into neighboring magnetic layers, and, conversely, the change in the magnetization in the presence of such spin currents can be used to infer the magnitude and sign of the spin accumulation generated via the SHE. Recently, it has been recognized that large spin currents are generated in platinum layers via the SHE and that these strongly influence the current induced motion of domain walls in coupled magnetic layers. A variety of experimental techniques have been used to measure the SHE in Pt but these give inconsistent results. We have studied the SHE in Pt layers that are coupled to several different ferromagnetic layers, including, permalloy (Py), Ni, Co, and CoFeB alloys. The spin Hall angle is characterized using the spin torque ferromagnetic resonance technique. We find that there is a strong dependence of the spin Hall angle in Pt on the ferromagnetic layer to which it is coupled. The interface between the ferromagnetic layer and Pt plays a central role in determining the magnitude of the SHE which will be the focus of this talk.   

Session M10: FIAP Prize Session: Beyond Academia: Personal Journeys of Successful Physics Careers in Industry

Pake Prize: The Evolving Nature of Industrial Research

The support of basic research in the physical sciences expanded rather quickly in the late 1950s and into the 1960s stimulated, in part, by the successful contribution of the scientific effort to the Second World War. Many in industry took on the challenge of creating research activities that focused on basic research. Many of these activities slowed in the 1970s and 1980s because of events in the economy and the marketplace. The challenges confronting the industrial research complexes were many. Retaining the high standards of quality in the research organization, encouraging partial realignment of programs to be more supportive of product lines and the various pressures arising from legislative measures to increase product efficiencies while reducing unwanted pollutants into the atmosphere, to name a few. Some of the highlights of the past 50 years will be reviewed using automotive industry examples. There will be another talk from Paul Grant for the next 36 minutes. I do not have his abstract yet but it has been requested.   

From Electrons Paired to Electric Power Delivered-- A Personal Journey in Research and Applications of Superconductivity at IBM, EPRI, and Beyond

This talk will reprise a personal journey by the speaker in industrial and applied physics, commencing with his employment by IBM at age 17 in the early 1950s, and continuing through his corporate sponsored undergraduate and graduate years at Clarkson and Harvard Universities, resulting in 1965 in a doctorate in applied physics from the latter. He was subsequently assigned by IBM to its research division in San Jose (now Almaden), where he initially carried out both pure and applied theoretical and experimental investigations encompassing a broad range of company-related product technologies\textellipsis storage, display, printer and data acquisition hardware and software. In 1973, he undertook performing DFT and quantum Monte Carlo calculations in support of group research in the then emerging field of organic and polymer superconductors, a very esoteric pursuit at the time. Following upon several corporate staff assignments involving various product development and sales strategies, in 1982 he was appointed manager of the cooperative phenomena group in the Almaden Research Center, which beginning in early 1987, made significant contributions to both the basic science and applications of high temperature superconductivity (HTSC). In 1993, after a 40-year career, he retired from IBM to accept a Science Fellow position at the Electric Power Research Institute (EPRI) where he funded power application development of superconductivity. In 2004, he retired from his EPRI career to undertake ``due diligence'' consulting services in support of the venture capital community in Silicon Valley. As a ``hobby,'' he currently pursues and publishes DFT studies in hope of discovering the pairing mechanism of HTSC. In summary, the speaker's career in industrial and applied physics demonstrates one can combine publishing a record three PRLs in one month with crawling around underground in substations with utility lineman helping install superconducting cables, along the way publishing 10 patents, conducting numerous interviews with the national media, serving a sabbatical as visiting professor at the National University of Mexico, writing review articles, commentaries and book reviews for Scientific American, Physics World and Nature and, most importantly, having lots of fun at the end of the day!   

Session M12: Invited Session: The Impact of Heterogeneous High Performance Computing Platforms on Computational Physics

Heterogeneous computing. What is it and do we need it?

Heterogeneous computing promises more compute power while consuming less energy. However employing GPUs and Xeon Phi's come with a hefty price tag. Only software that is highly adapted to these new architectures will gain any performance increase. In my talk I will address these questions: \begin{itemize} \item What is heterogeneous computing? \item How can it help producing better results faster? \item Is it worthwhile exploring for the broader audience? \end{itemize} And finally I will try to shed some light on the question whether the future will really be heterogeneous or not.   

Particle-in-Cell Simulations on Emerging Architectures

Emerging High-Performance Computing (HPC) Architectures consist of multiple accelerators with multiple vector processors. Such platforms require programming with multiple levels of parallelism and pose considerable challenges for performing simulations of plasmas. In this talk we focus on lessons learned from our experience with implementations of Particle-in-Cell (PIC) codes on GPUs. We will discuss aspects of mixed shared memory/distributed memory algorithms, streaming, blocking (tiling), and vector , data structures, latency and load balancing. Many of these lessons are of course familiar from other architectures, but will be seen here in a new perspective. We will discuss strategies for the development of algorithms for PIC codes that are expected to work on a variety of emerging HPC architectures.   

Using GPUs in Lattice Chromodynamics

Lattice quantum chromodynamics (QCD) calculations were one of the first applications to demonstrate the potential of GPUs in the area of high-performance computing, because the nature of lattice QCD calculations matches well the GPUs' computational model. In this talk, we will discuss ways to effectively use GPUs for lattice calculations using the overlap operator, a discretization that preserves chiral symmetry even at nonzero lattice spacing and makes possible lattice QCD simulations in the parameter region relevant to Nuclear Physics. We will show that the large memory footprint of these codes requires the use of multiple GPUs in parallel and we will discuss methods used to implement this operator efficiently: mixed-precision for inverters, hybrid CPU/GPU memory use for eigensolvers, and MPI/OpenMP/CUDA parallelization strategies required to take full advantage of both GPU and CPU available resources. We compare the performance of our codes on a GPU cluster and a CPU cluster with similar interconnects. We discuss the strong scaling for problem sizes relevant to current lattice QCD simulations.   

The Beacon Project: Challenges, Solutions, and Lessons Learned

With physical limitations imposing increasingly significant performance limitations on future generations of computing hardware, computer architects are turning to increased parallelism and specialized hardware to accelerate key applications and workloads. As a result, emerging high-performance computing (HPC) systems are much more heterogeneous than their predecessors, leading to both operational challenges and application challenges that must be overcome to effectively utilize the associated architectures. With support from the National Science Foundation, the Application Acceleration Center of Excellence (AACE) at the University of Tennessee is currently exploring the impact of the Intel$^{\textregistered}$ Xeon Phi{\texttrademark} coprocessor on computational science and engineering through the Beacon Project, an ongoing research project that encompasses the deployment and operation of an energy-efficient supercomputer and the coordination of an associated research program allowing project teams across the country to explore the applicability of the associated architecture to a variety of scientific codes and libraries. This talk presents an overview of encountered challenges along with associated solutions, highlights some of the current results of the application project teams, and summarizes many of the lessons learned through the Beacon Project to date.   

Ensembles of AMBER biomolecular simulations on GPUs for assessment and validation of RNA models

Ensembles of molecular dynamics simulations, using methods including multi-dimensional replica exchange on large-scale GPU clusters such as Blue Waters, provide a fast and efficient means to explore the conformational ensembles of biomolecules such as RNA. Enabling exploration of models in days to weeks instead of months to years we are able to better explore, assess, validate and improve the molecular mechanical force fields for RNA. We will describe our experiences modeling RNA on the large-scale resources and also outline the problems with and improvements in the AMBER suite of programs for simulation and analysis of biomolecules on heterogeneous computing platforms.   

Session M13: Focus Session: Fe-based Superconductors-Optical properties

Strong electronic correlations in iron pnictides: Comparison of the optical spectra for BaFe$_{2}$As$_{2}$-related compounds

The role of electronic correlations in iron pnictides is one of the hottest issues in research of iron-based superconductors. Utilizing optical spectroscopy, we quantified the strength of electronic correlations in BaFe$_{2}$As$_{2}$-related compounds. For the parent compound BaFe$_{2}$As$_{2}$, the fraction of the coherent spectral weight in the low-energy optical conductivity spectrum is distinctly small. Such a spectral feature is also observed in KFe$_{2}$As$_{2}$, indicating that the charge dynamics is highly incoherent in iron arsenides. It is found that the strength of electronic correlations significantly changes by chemical substitution, either through changing the electron filling and/or the As-Fe-As bond angle. The present result indicates that superconductivity of the iron pnictides emerges when the materials possess adequate amount of electronic correlations, and that either too weak or too strong correlations are not favorable for high-$T_{\mathrm{c}}$ superconductivity. The degree of electronic correlations in iron arsenides turns out to be comparable to that in the hole-underdoped cuprate superconductors. In this sense, the iron arsenides are classified into strongly correlated systems, probably arising from the Hund's rule coupling. This work was done in collaboration with S. Ishida, K. Kihou, Y. Tomioka, C. H. Lee, A. Iyo, T. Ito, H. Eisaki (AIST), T. Tanaka, T. Kakeshita, S. Uchida (University of Tokyo), T. Saito, H. Fukazawa, and Y. Kohori (Chiba University).   

Optical conductivity of clean-limit superconductor LiFeAs

We present the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral weight loss and yields a penetration depth of 215 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the very clean limit. The normal state optical conductivity can be modeled through a Drude-Lorentz decomposition and allows to determine a quasiparticle scattering rate that evolves linearly with temperature.   

Superfluid and quasiparticle behavior below Tc of strain introduced high-quality epitaxial thin films of Fe(Se,Te)

We succeeded in introducing compressive strain in epitaxial films of FeSe and Fe(Te,Se), leading to high Tcs' (1.5 times higher that in bulk crystals for FeSe)[1]. It is of great interest how the effect of strain shows up in properties in the superconducting state of these thin-film samples. We investigated superfluid- and quaiparticle response at THz frequencies. Structures characteristic of superconductivity was found clearly both in real part and imaginary part of the conductivity spectrum. Increase of quasiparticle scattering time below Tc was observed even in THz frequencies, which is connected with microwave data measured in bulk crystals consistently. Even in these high-quality, high Tc films, development of superfluid density with decreasing temperature is rather gradual, keeping a ``dirty'' feature. This might be related to possible excess Fe characteristic of this material, and further improvement of Tc is expected by additional heat treatment. Alternatively, the contribution of Legget mode is also considered. At present, any anomalous features related to strain have not been observed in these properties. The data at microwave frequencies taken by a dielectric resonator will also be discussed.\\[4pt] [1] F. Nabeshima et al.: APL 103 (2013) 172602.   

Infrared spectroscopy of rare-earth-doped CaFe$_{2}$As$_{2}$

Recently, rare-earth doping in CaFe$_{2}$As$_{2}$ has been used to tune its electronic, magnetic, and structural properties. The substitution of rare-earth ions at the alkaline-earth sites leads to the suppression of the spin-density wave (SDW) phase transition in CaFe$_{2}$As$_{2}$. For example, Pr substitution results in a paramagnetic metal in the tetragonal phase that is susceptible to a low temperature structural transition to a collapsed tetragonal phase. However, La-doped CaFe$_{2}$As$_{2}$ remains in the uncollapsed tetragonal structure down to the lowest measured temperatures. Both the uncollapsed and collapsed tetragonal structures exhibit superconductivity with maximum Tc reaching 47 K, the highest observed in inter-metallics albeit with a small superconducting volume fraction. In this work, we perform ab-plane infrared spectroscopy of rare-earth-doped CaFe$_{2}$As$_{2}$ at different cryogenic temperatures. Our aim is to ascertain the contributions of electron doping and chemical pressure to the charge and lattice dynamics of this iron-arsenide system.   

Spin-density-wave–induced anomalies in the optical conductivity of AFe2As2, (A=Ca, Sr, Ba)

We report the complex dielectric function of high-quality AFe2As2, (A=Ca,Sr,Ba) single crystals with TN=150K, 200K, and 138K, respectively, determined by broadband spectroscopic ellipsometry. In CaFe2As2 we identify the optical spin-density--wave gap $2\Delta_{\mathrm{SDW}}\approx1250\ \textrm{cm}^{-1}$. The $2\Delta_{\mathrm{SDW}}/(k_{\mathrm{B}}T_{\mathrm{N}})$ ratio amounts to 12 in CFA, significantly larger than the corresponding values for the SFA and BFA compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both SFA and CFA and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in CFA at a temperature T*=80K, well below the N\'eel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. Our results single out CFA in the class of 122 iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.   

Negative transport times due to interband scattering

Negative transport times lead to unexpected transport behavior such as negative magnetoresistance, strongly enhanced Hall coefficient, and reduced resistivity. Within a semiclassical Boltzmann approach beyond the relaxation-time approximation, it is demonstrated that negative transport times generically arise due to anisotropic single-particle scattering between electronlike and holelike Fermi surfaces. This mechanism could be responsible for the anomalous transport properties of materials close to an excitonic instability. In particular we discuss the case of one circular hole pocket and two elliptical electron pockets, which is relevant for iron pnictides.   

Low-energy quasiparticle excitations in $A$Fe$_{2}$As$_{2}$, $A=$Rb, Cs revealed by magnetic penetration depth measurements

In superconductors with strong correlations, clarifying the superfluid response in the superconducting state plays a crucial role to determine the symmetry and the structure of superconducting gap. In hole-doped iron-based pnictide superconductor KFe$_{2}$As$_{2}$ without electron pockets, highly unusual nodal structure in the superconducting gap has been reported. How this gap structure changes in related materials is important to understand its pairing mechanism. Here, we report on the magnetic penetration depth measurements in $A$Fe$_{2}$As$_{2}$, $A=$Rb, Cs. We observe strong temperature dependence of penetration depth at low temperatures, evidencing low-energy quasiparticle excitations. From detailed comparisons of the data between K, Rb, Cs systems, the gap structure in these materials will be discussed.   

Superconducting gap structure in over-doped (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ ($x=$0.35, 0.47, 0.56, and 0.64) from London penetration depth measurements

Single crystals of (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ were extensively studied from the optimal doping to the very underdoped regime. However, overdoped regime was out of reach due to various issues with the crystal growth. Here we report London penetration depth measured in high quality single crystals of (Ba$_{1-x}$,K$_x$)Fe$_2$As$_2$ with ($x=$0.35, 0.47, 0.56, and 0.64) that have $T_c=$30 K, 39 K, 32 K and 22 K, respectively. The study of the evolution from the optimally doped composition toward the end member, KFe$_2$As$_2$, is especially important, since the former is clearly a full isotropic gap material, whereas the latter is a d-wave superconductor. Our results suggest a gradual evolution from the full gap to the nodal gap with doping. The results will be discussed in terms of competing s- and d- channels in the general $s_\pm$ framework.   

Effect of electron irradiation on resistivity and London penetration depth of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$

The effect of electron irradiation on the in-plane resistivity and London penetration depth was studied in single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (x = 0.19, 0.24, and 0.34). The irradiation fluence varied between $8.7\times10^{18}$ and $5.2\times10^{19}$ electrons per cm$^{2}$. We found a profound decrease of the critical temperature, $T_c$, by 3 - 10 K depending on doping and the irradiation dose. Expectedly, the residual resistivity increases. The analysis of low-temperature part of London penetration depth shows that the superconducting gap becomes more anisotropic in under-doped (x = 0.19 and 0.24) crystals. Interestingly, however, the full - gap at the optimal doping (x = 0.34) remained at the same $\Delta(0)/T_c$ ratio after $5.2\times10^{19}$ e/cm$^2$ irradiation even though $T_c$ has decreased by almost 10 K (1/4 of the original value). The results will be discussed in a framework of s$_\pm$ pairing with a complex interplay between intra - and inter - band interactions and scattering.   

Flux-flow resistivity and penetration depth measurements of BaFe$_2$(As,P)$_2$: semi-quantitative estimates of gap anisotropy

Flux-flow measured by using a microwave technique is unique method to investigate quasiparticles in the vortex core. By measuring the magnetic-field dependence of the flux-flow resistivity, $\rho_f$, of several Fe-based SCs, we found that $\rho_f(H)$ is expressed as $\rho_f/\rho_n=\alpha H/H_{\rm c2}$ with $\alpha$ strongly depends on materials, suggesting that $\rho_f(H)$ of Fe-based SCs is dominated by the gap structure. By comparing these with the penetration depth data, we also found that $\alpha$ becomes larger when the gap function is more anisotropic [1,2]. To make this gap-anisotropy scenario more convincing, we focused on BaFe$_2$(As,P)$_2$ (P=30, 45\%), and found that the penetration depth increased in proportion to $T^{1.5-1.7}$. The fractional exponent can be understood by assuming that these materials have gaps with lines of nodes and deeply-warped nodeless gaps. As for flux-flow, $\rho_f(H)$ showed a large gradient of $\alpha>2.5$, similar to that of SrFe$_2$(As,P)$_2$ [2], pointing to highly anisotropic gaps in a consistent manner. These results support the gap-anisotropy scenario. [1]T. Okada $et\ al.$, PRB {\bf 86}, 064516 (2012); Physica C {\bf 484}, 27 (2013); $ibid$ {\bf 494}, 109 (2013) [2]H. Takahashi $et\ al.$, PRB {\bf 86}, 144525 (2012).   

Nodal superconducting state in clean single crystals of FeSe

Among iron-based superconductors, the binary ``11'' family offers the possibility to investigate systems consisting of just the iron arsenic/selenium layers without the intermediate layers which are present in the ``111'', ``122'' and ``1111'' families. This simplest iron based superconductor may therefore yield vital information about the origin of superconductivity in the iron pnictides/chalcogenides. Here we measured the penetration depth and thermal conductivity in very clean single crystals of FeSe~[1] with RRR $\textgreater$ 200. Presence of line nodes is evident by the quasi $T$-linear dependence of the penetration depth. Moreover, a large residual thermal conductivity, which is much larger than that expected for $d$-wave symmetry, suggests that nodes are accidental and nearly vanishing. The field dependence of thermal conductivity suggests a possible field induced phase transition in the superconducting state. \\ ~[1] A.E. B\"{o}hmer et al., Phys. Rev. B {\bf 87}, 180505(R) (2013).   

Ultrafast Critical Nematic Fluctuations and Giant Magnetoelastic Coupling in Iron Pnictide

A ubiquitous anisotropy in the normal state properties of many of the iron pnictides presents a crosscutting challenge important for understanding quantum magnetism and high-temperature superconductivity. Although an electronically-driven nematicity has been invoked, distinguishing this from spin and structural orders is challenging because they all couple together to break the same tetragonal symmetry. Here we use femtosecond-resolved polarimetry to reveal critical fluctuations of nematic correlation in unstrained Ba(Fe1-xCox)2As2. The ultrafast anisotropic response, which arises from the two-fold in-plane anisotropy of the refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase, with increasing relaxation time, indicative of critical nematic fluctuations approaching the structural transition temperature. The dynamics reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.   

Coherent A$_{\mathrm{1g}}$ Phonon in thin Film Superconductor FeSe$_{0.5}$Te$_{0.5}$: $\pi $/2 Phase Difference over Superconducting Phase Transition

Coherent $A_{1g}$ phonon mode in a thin film superconductor FeSe$_{0.5}$Te$_{0.5}$ was generated and detected using ultrafast laser pulses. At below and above the transition temperature $T_{\mathrm{c}}$, the coherent lattice oscillation we observed exhibited a $\pi $/2 phase difference, manifesting a ``displacive limit $\sim$ impulsive limit'' transition upon crossing a phase transition within the same sample. We ascribe this $\pi $/2 phase difference to the different lifetimes ($\tau_{\mathrm{c}})$ of excited charge density components that couples to the fully symmetric $A_{1g}$ phonon mode, i.e. the different strength of electron-phonon couplings. In the superconducting and paramagnetic metallic states the lifetimes of such carrier excitations are largely different. Our investigation reveals possible correlation of superconducting electrons with zone-center optical phonons. Our 170nm thin film sample contains tension stress, which leads to enhanced $T_{\mathrm{c}}$ and thus facilitated our measurements.   

Session M14: Invited Session: Physics for Everyone

The Universe: Beginnings, Ending and Our Miserable Future

Beginnings and endings are not only tied together in literature, but in the real world as well. Our Universe is a good example. While storing energy in empty space was probably responsible, early on, due to inflation, for the existence of the large spatial volume in which we live, the current inflationary phase in which we live will make the universe ultimately unlivable, as arguments about energy, information, and quantum mechanics show. I shall also describe how, before the demise of life, future astronomers will come to completely incorrect conclusions about the universe in which they live.   

Attosecond Physics - Probing and Controlling Matter on Its Natural Time Scale

The goal of attosecond physics is to probe and control matter on its natural time scale, which for electronic motion in atoms, molecules, and solids is measured in attoseconds ($=$ 10$^{-18}$ sec). Both single attosecond pulses and attosecond pulse trains can be produced.~ Such pulses have opened new avenues for time-domain studies of multi-electron dynamics in atoms, molecules, and solids on their natural time scale and at dimensions shorter than molecular and even atomic dimensions. They promise a revolution in our microscopic knowledge and understanding of matter. At present the intensities of isolated attosecond pulses are very weak, so that perturbation theory is sufficient to describe the interaction of attosecond pulses with matter. Consequently, they can only be used either to initiate (``pump'') a physical process or to probe a process already under way by other means. Experimental efforts currently aim to increase the intensities of isolated attosecond pulses by orders of magnitude. Intense attosecond pulses will open the regime of nonlinear attosecond physics, in which pump/probe processes with isolated attosecond pulses will become possible and in which the broad bandwidth of isolated few-cycle attosecond pulses will enable significant control over electron motion.   

Quantum Matter Meets Living Matter

Magnetic resonance imaging (MRI) is a powerful tool for biology and medicine, but has limited spatial resolution (about 0.1 millimeters in living creatures) and thus cannot visualize individual living cells or subcellular structures, let alone the constituent molecules and atoms. However, recent developments in quantum science have rapidly and radically changed this story, enabling a new form of optical MRI with nanoscale spatial resolution and applicable to living biological cells. I will describe how special quantum defects in room temperature diamond crystals, known as nitrogen vacancy (NV) color centers, provide a practical means for extending the reach of MRI to the nanoscale and even Angstrom scale, with wide-ranging applications in biology, medicine, and materials science.   

Journey to the Center of the Earth

The center of Earth is at about the temperature of the surface of the Sun (about 6000K) but frozen because of the extreme pressure. I will place the Earth in a more general context of planets (including exoplanets) and explain how it is that the materials deep in Earth can behave differently from the same composition at low pressure.I will describe the sequence of layers and materials and conditions as one travels in a hypothetical probe from the surface to the center, emphasizing the things we do not understand well. I will talk about he extent to which Earth's mantle is imperfectly mixed and may have a bottom layer above the core that is different in composition. I will discuss the Urey number puzzle (what explains Earth's heat flow?). I will focus on the puzzle that Earth's magnetic field presents: How is it generated and how has this worked for billions of years? It seems that we need another energy source. I will talk about how Earth has a memory of how it formed, in particulate the high temperatures resulting from events such as the giant impact that led to our Moon. I will end with a discussion of what to do about the remaining puzzles, in particular the possible value of the geoneutrino experiment and attempts to directly probe the interior.   

Higgs and Beyond

On July 4th 2012, the ATLAS and CMS experiments announced the discovery of a new boson. Following analysis of the full dataset from LHC Run 1, the properties of this particle have been determined to be in agreement with those expected for the long-sought after Higgs boson, completing the particle family in the Standard Model. However, we know from fundamental arguments that a considerable degree of fine tuning is needed to make the particle masses fit what we observe. This talk will address the observation of the Higgs boson, what we have learned since this discovery, why we should consider physics beyond the Standard Model, and what physics may remain to be discovered such as a composite Higgs or Supersymmetry. To perform these searches the detector must deal with extreme conditions and this talk will address some of the experimental challenges to be faced and their possible solutions.   

Session M16: Focus Session: Computational Studies of Thermoelectric Materials

Finding new thermoelectrics: Parabolic bands are (probably) not enough

Thermoelectric performance as characterized by the figure of merit ZT is a counterindicated property of matter. While the electronic structure of common semiconductors is well understood in terms of band models, most commonly the parabolic band model, this type of electronic structure is not the best for finding high thermoelectric performance. Instead high ZT thermoelectrics often have unusual band structure features. Here I discuss some of those features, and their essential aspects in relation to thermoelectric performance and outline strategies for finding more high ZT materials based on them.   

Quantum Resonance Effects to Thermoelectric Property of Organometallic Molecular Materials

Superior long-range electric transport has been observed in several organometallic wires and films. Here, we propose use of organometallic molecules for thermoelectric materials by focusing on the overlapping resonance effect, which enables long-range coherent tunneling and enhancement of Seebeck coefficient. We examine the possibility of high thermoelectric figure of merit (\textit{ZT}) by controlling the quantum resonance based on first principles transport calculations of electron and phonon. [1] We found distinct length and temperature dependences of \textit{ZT} from those of inorganic bulk materials or organic molecules. We will present an alternative approach to obtain high \textit{ZT} by using organometallic molecular materials. \\[4pt] [1] H. Nakamura, T. Ohto, T. Ishida, and Y. Asai, \textit{J. Am. Chem. Soc.} DOI: 10.1021/ja407662m   

First Principles explanation of the positive Seebeck coefficient of lithium

Lithium is one of the simplest metals, with negative charge carriers and a nearly free electron dispersion. Experimentally, however, Li is one of a handful of elements (Cu, Ag, Au) where the sign of the Seebeck coefficient ($S$) is not that of the carrier. We calculate $S$ fully from first-principles, within P.B. Allen's formulation of Boltzmann theory. The constant relaxation time approximation fails and gives a sign for $S$ necessarily identical to the carriers. Our calculated $S$ are in excellent agreement with experimental data. In comparison with Na, we demonstrate that, within the simplest non-trivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) is related to the sign of $S$. The exceptional energy dependence of the DOS is beyond the free-electron model, and distorted by the Brillouin Zone edge, a stronger effect in Li than other Alikis. The electron lifetime dependency on energy is central, but details of the electron-phonon interaction are less important, contrary to what has been believed for several decades. The mechanism exposed here may open the door to new ``ambipolar'' thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible.   

Thermoelectric transport through single molecule junctions

The charge and heat transport properties of single multilayered cyclophane stacks connected to gold electrodes are calculated in the framework of ab initio electronic structure calculations combined with non-equilibrium greens function techniques. The heat transport include both the electronic and phononic contribution, which allows a fully ab initio determination of the thermoelectric transport properties. We investigate the influence of the molecular length and of the molecule-electrode binding motif on the electron and phonon transport characteristics. We find that the power factor is limited due to the fact that the Wiedeman-Franz law remains approximate valid due to the strong off-resonant electron transport. On the other hand the large phonon mismatch between the molecule and the gold electrodes leads to a suppression of the phonon thermal conductance.   

First-principles study of the thermoelectric properties of Cu2S

The mineral chalcocite, or copper sulphide (Cu$_2$S), is of interest as a thermoelectric material due to its abundance and non-toxic nature. Yet, the study of Cu$_2$S is complicated by the disordered phases (hexagonal and face-centered cubic) that it exists in at high temperatures. Here, we discuss our random structure search leading to the most stable structures. Based on these results, we report the thermoelectric properties of hole doped Cu$_2$S using first-principles calculations and Boltzmann transport theory. We show that a high Seebeck coefficient of over 200 $\mu$V/K is achievable with hole doping levels up to $10^{20}$ cm$^{3}$ above 500 K.   

Atomic simulations of nonlinear lattice dynamics in PbTe

PbTe is of great interest as a thermoelectric material and for displaying signs of strong phonon interactions. Inelastic neutron scattering experiments reveal a signature of strong anharmonicity as evidenced in anomalous temperature dependence of the phonon spectra. Here we perform molecular dynamic simulations using a 4th-order interatomic potential deduced from first-principles calculations. The temperature dependent phonon spectra are successfully reproduced from first-principles for the first time. The emergence of a new mode at the zone center is unambiguously shown, as observed in experiment. Furthermore, we confirm that there is not a local spontaneously broken symmetry, clarifying recent controversy among experimental results. Phonon self-energies at different temperatures are computed to show the origin of the phonon anomalies.   

Computational design for low-temperature thermoelectric materials

Thermoelectric materials are usually doped with external impurity atoms which provide the required level of carrier concentration (electrons/holes) for a good electronic performance. These impurity atoms scatter the conduction carriers and limit their mobility. Such limitation can be improved by introducing new doping schemes. For instance, impurity atoms can be substituted by metallic/ semi-metallic nanoparticles, or heavily doped semiconducting grains/nanowires can be embedded inside a host matrix in order to create a three dimensional modulation doping structure. We have recently demonstrated three dimensional modulation doping scheme in nanostructured SiGe materials and observed about 40{\%} enhancement in the carrier mobility compared to uniform doping. The enhancement could be much larger if a complete separation of carriers and ions is achieved e.g. by addition of a spacer layer It is possible to shield the nanoparticles with a coating layer to minimize the conduction carrier scattering and reduce the scattering cross section by 4 orders of magnitudes below the physical cross section to cloak the nanoclusters and to design invisible dopants. Extension of such a design to realistic materials can increase the carrier mobility by orders of magnitude especially at low temperatures, and can potentially increase the thermoelectric performance by two orders of magnitude.   

An ab initio study of the effect of host-guest interaction on thermal transport in Ba$_{8}$Ga$_{16}$Ge$_{30}$

Inorganic clathrate compounds are promising candidates for the next-generation thermoelectric devices because of their low lattice thermal-conductivities. In these materials, rattling vibrations of guest ions inside host cages are considered to play a significant role in reducing the lattice thermal-conductivity. In order to elucidate the microscopic mechanism of the reduction more clearly, we have performed first-principles analyses on a type-I clathrate Ba$_{8}$Ga$_{16}$Ge$_{30}$. Firstly, we calculated harmonic and anharmonic force constants of the material using the direct-method. Then, phonon scattering probabilities are evaluated from the imaginary part of the phonon self-energy. Our analysis shows that host-guest interactions increase the scattering probability of acoustic modes by one order of magnitude, and also cause a 10-fold reduction in the lattice thermal-conductivity. In addition, we observe that phonon mean-free-paths are far larger than the separation of Ba atoms, indicating that Ba atoms cannot be considered as individual scattering centers.   

Spherical Harmonic Expansion Method for Coupled Electron-Phonon Boltzmann Transport

Thermoelectric transport modeling often relies on independent Boltzmann transport equations (BTEs) for electrons and phonons which work best near equilibrium (linearized) and steady-state. Device design relies heavily on this baseline approximation. Monte Carlo methods can allow for complex physical interactions (e.g., anharmonicity) but their stochastic nature has practical limits. Distribution functions with wide disparities in population (e.g., ratios $> 10^8$ between majority and minority carriers.\footnote{The SHE method has treated majority/minority carriers in bipolar transistors, S.-M. Hong, et al, IEEE Trans. Electr. Dev. \textbf{57}, 2390 (2010).}) are a computational challenge. We present a coupled BTE solver based on a k-space spherical harmonic expansion (SHE) of the distribution functions and eigenstates of electrons and phonons. The method is deterministic and allows for detailed treatments of scattering processes, yet ameliorates the issues with population disparity within phase space. We set the formalism and examine the accuracy of the SHE for phonon band structures, calculate scattering rates determined within that representation, and compare our preliminary results for distribution statistics in control examples such as thermal conductivity and drift velocity.   

Temperature dependent anharmonic lattice dynamics

We have developed a thorough and accurate method of determining anharmonic properties in solids, the temperature dependent effective potential technique (TDEP). It is based on ab initio molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The effective Hamiltonian contains implicit temperature dependence, electron phonon coupling and renormalized anharmonicity to arbitrary order, making it suitable for strongly anharmonic systems. We show excellent results for a host of thermoelectric materials (PbTe, SnTe, Bi$_2$Te$_3$, FeSi, ScN), reproducing temperature dependent phonon spectra, thermal conductivity, and phonon self energies.   

Session M17: Fracture and Other Problems in Statistical Physics

Fracture on Curved Surfaces

When an elastic film conforms to a surface with Gaussian curvature, stresses arise in the film. As a result, cracks---typically studied in flat materials---interact with curvature when propagating through the system. Using silicone elastomer sheets that conform to the surface of a Gaussian bump, we find experimental evidence for the deflection of a crack propagating through the material. We interpret our experiments with reference to analytical modeling and simulations of a simplified model system.   

Fracture of Thin Anisotropic Sheets: selection of the fracture path

It is often postulated that quasistatic cracks propagate along the direction allowing fracture for the lowest load. Nevertheless, this statement is debated, in particular for anisotropic materials. We performed tearing experiments in anisotropic brittle thin sheets that validate this principle in the case of weak anisotropy. We also predict the existence of forbidden directions and facets in strongly anisotropic materials, through an analogy with the description of equilibrium shapes in crystals. However, we observe cracks that do not necessarily follow the easiest direction but can select a harder direction, which is only locally more advantageous than neighboring paths. These results challenge the traditional description of fracture propagation, and we suggest a modified, less restrictive criterion compatible with our experimental observations.   

Fracture of a solid with vanishing shear modulus

The dissipative processes -damage, secondary crack openings- taking place ahead of a propagating crack tip in an amorphous solid are not yet completely characterized, despite the ubiquity of these materials in industrial applications. Questions regarding the extent of the process zone and the nature of the plastic events were addressed with models and numerical simulations but have never been confronted with experiments. In order to tackle this problem, we have designed a novel on-chip experiment that enables altogether to check the physical chemistry of the tested soft materials, to grow controlled cracks at a prescribed velocity, and to visualize the crack tip and its surroundings from a macroscopic to a microscopic scale. Although this experiment is aimed at studying crack propagation in model amorphous materials like colloidal glasses or gels, we have first studied fracture of polymeric gels. From the analysis of crack morphology and crack-induced deformation fields, we have disclosed the relevant time and length scales and investigated their evolution in the vicinity of the sol-gel transition, where the shear modulus vanishes.   

Fracture and material geometry

Linear elastic fracture mechanics provides a firm foundation for understanding crack propagation in a continuum material-- how are these predictions modified when a material elastic constant becomes vanishingly small? We study fracture in fragile lattices in experiments by fabricating materials containing voids, thus modifying the ratio of shear to bulk modulus, G/B, such that G/B$\rightarrow$0. We compare these results to simulations on a braced square lattice where rigidity is controlled by varying coordination number [1]. In the quasi-static limit for both experiment and simulation, we observe a crossover as the material becomes more fragile: propagating cracks are progressively superseded by isolated bond-breaking events. This crossover is signaled by the crack width increasing as G/B$\rightarrow$0, until it saturates at the system size, consistent with the random breaking of bonds. We also study dynamic fracture in a material containing a 1D array of voids. We measure the crack velocity, and again find two distinct regimes of behavior governed by material rigidity.\\ $[1]$ B. G. Chen, S. Ulrich, N. Upadhyaya, L. Mahadevan, V.Vitelli, in preparation   

Highly stretchable and transparent electrodes of Au nanomeshes

Metallic nano-networks or nanomeshes may serve as the flexible transparent electrodes (FTEs) for bendable and foldable electronics. Here we present Au nanomeshes made by grain boundary lithography, showing good electrical conductance and transparency comparable to ITO film, but exceptionally high stretchability. The sheet resistance increases only $\sim$ 3 times when stretched to an ultra-large strain of 160{\%}. The Au nanomeshes also exhibit excellent performance under cyclic strain, and work well after exposing to high temperature of up to 500 $^{\circ}$C. In addition, the low surface roughness enables good compatibility with device integration. The ultra-large stretchability of the Au nanomesh FTEs lies in a subtle balance between two roles played by the underlying elastomeric substrate. The vast difference in the elastic moduli of Au and the substrate allows the stretched Au mesh to deflect and twist out of the plane, while the elastomeric substrate stabilizes distributed rupture of Au ligaments. The Au nanomesh may be used as a FTE for bendable and foldable electronics.   

Nanoscale Tribological Properties of Nanodiamond

Due to their rich surface chemistry, excellent mechanical properties, and non-toxic nature, nanodiamond particles have found applications in a wide variety of fields such as filler materials in nanocomposites, biomedicine, tribology and lubrication, targeted drug delivery systems, and surgical implants. This study is focused on nanodiamond particles synthesized using detonation synthesis. We used peak force tapping atomic force microscopy to study adhesion and agglomeration in nanodiamond particles. We find that adhesion force between nanodiamond particles and sharp atomic force microscope tips can range from 0.1 to 2.0 nN depending on purity of particles, size of the probe, and environmental conditions. We observed that these particles can form agglomerates consisting of about 4 to 6 particles, due to interparticle forces.   

Parametrically driven field emission in strongly nonlinear coupled electron-shuttles

The transition of coupled electron shuttles from a stable to a strongly nonlinear response is demonstrated at room temperature. The electron transport is Coulomb-controlled at low voltages but changes to the conventional field emission in this transition. This reversible process forms a well-defined band within a broad frequency range in the parameter space. Both the experimental data and numerical calculations indicate that the source of the nonlinearity is provided by the electromechanical coupling. The increased current in the nonlinear regime has the potential to form the basis for energy harvesting via nanomechanical shuttles.   

Computational and experimental studies of charged particles in a scalable 1D spatial and temporal periodic potential created with twin periodic electrode curtains

A Twin Electric Curtains (TEC) used in our study consists of two parallel planar arrays of linear electrodes. The electrodes are driven by an oscillating two-phase electric potential. The electric potentials applied to two electrodes of the TEC are in phase when the two electrodes are in the same vertical plane whereas the potentials of the neighboring electrodes in each planar array are $180^{\circ}$ out of phase. A linear quadrupole trap is also used to constrain charged particles' motion to a straight line perpendicular and equidistant to the electrodes of the two electric curtain arrays where the component of the electric field generated by the TEC perpendicular to the axis of the quadrupole is zero. Dynamic motion of charged particles under excitation of electric field in the form $f(t)h(x)$ are studied, where $f(t)$ is periodic in time and $h(x)$ is periodic in space. The presentation will be on interesting single and multiple particle dynamic behavior including stable oscillations as well as propagating and chaotic characteristics. They may be considered as simple models related to current research in areas of molecular motors, Hamiltonian and artificial thermal ratchets, and the variety of particle transport phenomena that occur in self excited systems.   

The Plasmoelectric Effect: Optical Control of the Electrochemical State of Plasmonic Absorbers

The plasmonic properties of metallic nanostructures depend strongly on charge carrier density. While researchers have reported shifts of the resonant absorption frequency of plasmonic nanostructures due to electrostatically induced changes of charge density, the converse ---the dependence of charge density and electrostatic potential on optical absorption---has been largely overlooked. Here, we report a theoretical framework and provide experimental evidence for a `plasmoelectric effect', an optically induced electrochemical potential in plasmonic nanostructures. Our thermodynamic model shows that, unlike the more familiar thermoelectric or photovoltaic effects, the magnitude and sign of the plasmoelectric potential depends on the frequency difference between the plasmon resonance and incident radiation. Radiation at higher frequencies induces an increase of electron density in the nanostructure that blue-shifts the plasmon resonance. This response is favored due to increased entropy from increased absorption. Similarly, radiation at lower frequencies decreases electron density. Systematic electrical and optical studies of plasmonic Au nanostructures display clear evidence for the structure-dependent and wavelength-dependent trends consistent with our theoretical framework.   

Bound states and perfect transmission scattering states in $\mathcal{PT}$-symmetric open quantum systems

We study the point spectrum and transmission scattering spectrum in $\mathcal{PT}$-symmetric open quantum systems containing balanced regions of energy amplification and attenuation, using tight-binding chains with matching sink and source sites as prototype models. For a given system geometry, we write the boundary conditions that permit scattering state and bound state solutions with wave functions that likewise satisfy $\mathcal{PT}$ symmetry; we further demonstrate the $\mathcal{PT}$-symmetric scattering states give rise to perfect transmission through the scattering region. We also discuss bound states in continuum and other spectral effects that may be discovered in $\mathcal{PT}$-symmetric open quantum systems. Finally we discuss the potential for experimental realization of our models in systems containing whispering gallery mode resonators with balanced loss and gain.   

The Asymmetric Top Molecule In An Electric Field

The quantum and classical behavior of the asymmetric top has been studied in a variety of different contexts, and is known for its dynamical complexity due to have greater than 2 DOF. In this presentation the focus will be on the classical dynamics of an asymmetric top molecule with a dipole moment in an electric field (a non-integrable system), and how the underlying classical phase space structures, such as resonances, can impact the behavior of its quantum analog. Specifically, we will be presenting results from a classical simulation, which highlights both chaotic and regular regions for small electric fields depending on initial conditions.   

Wigner-function approach to study the radiation of complex electromagnetic sources

In this work, we develop a mathematical framework to predict the radiation of complex electromagnetic sources in free-space. We show how to propagate field-field correlation functions from near- to far-field using the formalism of Wigner-Weyl quantum mechanics. In so doing, the key point is to make a connection between the field-field correlation function in configuration space and a corresponding Wigner function in phase space. We make an analogy between the evolution of waves and the evolution in phase space of the underlying classical trajectories, for which we derive and generalize a Frobenius-Perron Equation. In the context of electromagnetic problems, the Wigner-function approach has been championed by Marcuvitz using the ``quasiparticle'' picture of wave evolution. In the proposed approach, we approximate the propagation of field-field correlation functions by propagating phase-space densities of ray families, which is effectively a lower-dimensional calculation and therefore easier to compute. In particular, we discuss how the Wigner-function approach can be extended to boundary-value problems by using the results of semiclassics and the random matrix theory.   

Steering most probable escape paths by varying relative noise intensities

We demonstrate the possibility to systematically steer the most probable escape paths (MPEPs) by adjusting the relative noise intensities in non-gradient dynamical systems that exhibit escape from a metastable point via a saddle point in the limit of small noise. Based on a geometric formulation of this escape process, an asymptotic theory is developed which is broadly applicable to fast-slow systems of two or more dimensions. In simple systems, our theory permits analytical expressions for the MPEPs and their associated minimum action values as a function of the relative noise intensities. These analytical predictions are in excellent agreement with computed MPEPs obtained using a geometric minimum action method (gMAM) [1], and both of these results are consistent with prehistory probability distributions obtained by direct simulation of the underlying stochastic differential equations. [1] M. Heymann and E. Vanden-Eijnden, Phys. Rev. Lett. $\mathbf{100}$, 140601 (2008).   

Session M18: Rheology & Phases of Complex Colloidal Systems

Random-Walk Trajectories of Probe Particles in Viscoelastic Complex Fluids

Trajectories of tracer spheres in complex fluids can exhibit exotic patterns that have interesting temporal and spatial dependence. In passive particle-tracking microrheology, measured trajectories can often be converted into linear viscoelastic properties of the complex fluids. To better portray the diversity in potentially observable trajectories, we have created a random walk simulation for spheres in viscoelastic complex fluids. In a simple case, for a Maxwell-Voigt fluid, a tracer bead is modeled as a harmonically bound Brownian particle in a potential well that itself diffuses over longer time-scales. We also show trajectories for a complex fluid having a wide distribution of relaxation times, as described by a generalized Maxwell fluid, and for a different complex fluid having a significantly anisotropic viscoelasticity along orthogonal spatial directions. This generalized approach enables us to generate trajectories for a wide range of complex fluids within the limit of linear viscoelasticity, and these trajectories, when viewed at different sampling times and total observation times, provide insight into experimentally measured particle-tracking microrheology measurements.   

Inertial effects in viscoelastic materials and their implication in passive microrheology

We review our recent series of works about inertial effects of soft viscoelastic materials on the particle diffusion in the materials, and their ramifications for one- and two- bead passive microrheology. Firstly we focus on the effects of particle inertia, especially on the oscillation of the particle's mean-square displacement (MSD). This is the resonance oscillation between the inertial motion of the particle and the elastic components of the viscoelastic materials. Secondly we discuss the material inertia, focusing on the so-called Basset force of the viscoelastic bodies. The kinetic energy of the particle is dissipated not only due to the Stokes drag but also through the Basset force as the radiational propagation of the shear wave excited by the particle motion. The resonance oscillation of the MSD tends to decrease due to the Basset force. The Basset force is characterized by the wave length $\Lambda $ and the penetration depth $\Delta $ of the shear wave. At high frequencies, the Basset force becomes important when $\Lambda $ is less than the particle size (for single-bead microrheology) or less than the distance between two particles (for two-bead microrheology). On the other hand, at low frequencies, the Basset force is effective when $\Delta $ is larger than the sample size. Finally we show several examples of microrheological analysis taking account of these inertial effects.   

Nanoparticle Salts: Structure, Rheology and Ion Transport

Above a critical volume fraction associated with nanoscale particle spacing, interactions between tethered molecules (charged or uncharged) significantly affect particle-particle interactions and hence suspension rheology. We report on the structure, rheology, and ion transport of nanoparticles cofunctionalized with tethered salts and neutral molecules. Contrary to uncharged counterparts, charged particles in a low dielectric medium are shown to exhibit soft glassy rheology behaviors at low particle loadings, due to electrostatic bridging of salts. In addition, tethered molecules in a nanochannel created by particle crowding are conceptually similar to entangled polymers in a tube and, as a result, concentrated particle suspensions share a similar plateau modulus with entangled polymer melts. Our findings suggest that particle interactions can be fine-tuned using tethered salts of different charge densities, and geometrical confinement on tethered molecules produces topological constraints analogous to those in entangled polymers.   

Liquid-Gel-Liquid Transition and Shear-Thickening in Mixed Suspensions of Silica Colloid and Hyperbranched Polyethyleneimine

The rheological property of mixed suspensions of silica colloid and hyperbranched polylethyleneimine was studied as functions of particle volume fraction, ratio of polymer to particle, and pH value. A mechanism of liquid-gel-liquid transition for this mixed system was proposed based on the amount and the conformation of polyelectrolyte bridges which were able to self-arrange with solution environments. The equilibrium adsorbed amount ($C_{\mathrm{p}}$*) for a given volume fraction of particles is an important concentration ratio of polymer to particle denoting the transition of irreversible and reversible bridging. For mixed suspensions at equilibrium adsorbed state ($C_{\mathrm{p}}\approx C_{\mathrm{p}}$*), the adsorption-desorption of polymer bridges on the particles can reversibly take place, and shear thickening is observed under a steady shear flow as a result of rapid extension of bridges when the relaxation time scale of extension is shorter than that of desorption.   

Dynamics of Cubic Colloids

There have been significant advances in the synthesis of anisotropic particles, however little is known about how shape and directional interactions influence particle dynamics in a suspension. We address this issue by studying both the bulk rheology and micro-scale particle dynamics in suspensions of hollow, silica cubes. These cubes are particularly well-suited for studying the role of anisotropy since they are mono-disperse, readily dyed and index-matched for confocal imaging, and can be synthesized in bulk quantities. Using confocal microscopy to image dilute, quiescent suspensions of cubes, we find the long-time diffusion coefficient decreases with packing density as $D_\infty/D_0 \simeq 1-3.1\phi$, differing from the standard hard-sphere slope of -2.1. Similarly, small-volume viscometry reveal a higher intrinsic viscosity for the cubic particles, demonstrating that the particle shape has a significant impact on the suspension dynamics. We further investigate these shape-effects using confocal-rheometry to characterize shear-induced diffusion in these cubes. Using depletion, we also investigate the role of attractive, directional interactions, tuning the interaction strength by varying the depletant size and concentration.   

Using colloids to model worm-like micelles

We measure the viscosity of self-assembling chains of colloidal particles using a Zimm viscometer, a custom built Couette apparatus. Our microscopic particles are shaped like contact lenses or bowls, specially fabricated to fit inside one another so that they readily form chains in the presence of a depletant. Careful tuning of the interaction strength in a suspension of particles induces the formation of long chains. Shearing this material can twist, stretch, and break the chains, causing the material to exhibit unique rheological properties. We anticipate that these colloidal chains will model the behavior of worm-like micelles.   

Transient formation of bcc crystal in suspensions of pNIPAM-based microgels

In suspensions of soft and deformable microgel particles, both the colloidal and the polymeric degrees of freedom are relevant for phase behavior at high concentrations. Crystal structures different from those formed by hard spheres (HS) are expected to form at high concentrations. However, our and other group's experimental work have shown that the crystal structure is comparable to that found in HS. Here, we present a small-angle X-ray scattering study of crystal growth in a system of slightly charged and swollen microgels of poly(N-isopropylacrylamide) co-polymerized with acrylic acid. As in HS, we find that random hexagonal close packed crystal grows in all samples and slowly transforms towards the face centered cubic lattice, which appears to be the equilibrium structure, as in HS. However, at intermediate volume fractions, a body centered cubic crystal phase appears, which is not stable and disappears as the samples age. This behavior is expected for fuzzy particles with a steric repulsion [P. Ziherl and R.D. Kamien, Phys. Rev. Lett. 85, 3528 (2000)]. This suggests that our observations could be related to the predictions of this model for fuzzy particles.   

Crystals and liquids of ionic microgel particles: Osmotic pressure and phase coexistence

We quantify the phase behavior of suspensions comprised of swollen, ionic microgels while measuring the system osmotic pressure. Surprisingly, the osmotic pressure of dilute suspensions is much larger than that expected for an ideal gas. Furthermore, we find that the width of the liquid-crystal phase coexistence region increases as the microgels are made softer; this is true in terms of a generalized volume fraction, $\zeta =$\textit{nV}$_{0}$, with $n$ the particle density and $V_{0}$ the dilute microgel volume. We will discuss the role played by the ions in our observations and compare with expectations based on computer simulations.   

Dissipative Particle Dynamics simulation of colloidal suspensions

DPD as a mesoscale method was firstly proposed to study dynamics of suspensions under flow condition. However the proposed method failed to capture shear properties of suspensions because it lacked: first a potential to reproduce lubrication forces and second a clear definition for the colloid surface. Recently we reported a modified DPD method which defines colloidal particles as particles with hard core and a dissipative coat. An additional lubrication force was introduced to include the short-range hydrodynamics that are not captured in original DPD. The model was found to be able to reproduce shear properties of suspensions for a wide range of different systems, from monodisperse to bimodal with different volume fractions, compositions and size ratios. In present work our modified DPD method is employed to study both equilibrium and flow properties of colloidal suspension. Zero shear viscosity of suspension is measured using Green-Kubo expressions and the results are compared to theoretical predictions. Furthermore, structure formation in suspensions is studied in respect to energy landscape of the fluid both at rest and under flow.   

Inertia and dissipation mechanism in jammed soft-particle suspensions

Suspensions of soft particles exhibit a remarkable bifurcation at the random close packing volume fraction, fc. There is a yield stress above fc but not below, and the flow curves at various f have been shown to collapse onto a universal scaling function near this point. Particle-level models take contact deformation into account to model elastic forces and treat the drag forces in the very dense regime where long-range hydrodynamic interactions are thought to be negligible - with varying levels of sophistication: from ``pair-drag'' formulations that apply a lubrication calculation to the film at contact to simple ``mean drag'' approaches where the mobility matrix is diagonal. We show that, in simple shear, these two model give consistent results for the shear modulus, yield stress, and single-particle diffusivity as functions of f but only in the quasi-static regime. They show dramatically different behavior in the rate-dependent regime. In particular, the diffusion constant scales as the shearing rate to a non-trivial power with the power depending on the damping mechanism. Furthermore, we explore a ``granular'' regime where the inertia of the particles is no longer negligible and the finite rate behavior shows a complex interplay between inertial and dissipative timescales.   

Comparison of the Physical Aging Behavior of a Colloidal Glass after Shear-Melting and Concentration Jumps

We have prepared a thermosensitive core-shell PS-PNIPAM/AA particle system and have investigated the aging response of its colloidal dispersions subsequent to both shear-melting and temperature (concentration)-jump perturbations using sequential creep experiments to probe the response of the system. The experiments were performed in the vicinity of the glass concentration or temperature as evidenced by the strongly varying relaxation time with decreasing temperature. The colloidal glass displays aging behavior after both types of perturbation, but the kinetics of the aging are different, demonstrating that the structural changes induced by the mechanical perturbation are different from those induced by the temperature or concentration jump. We find that time-aging time superposition is valid in both cases and that the aging rate, as measured by the double logarithmic slope of the aging time shift factor vs. aging time, decreases with increasing temperature, similar to what is seen in aging of molecular glasses.   

Structural bistability in quasi-hard-discs under adaptive circular confinement

The behaviour of materials under spatial confinement is dramatically different from that in the bulk. The exact nature of behavioural modification in confined systems is strongly dependent on the boundary enclosing the system with soft walls inducing different phenomena than similar hard walls. Here we present a quasi-two-dimensional colloidal model system confined by an adaptive circular boundary defined using holographic optical tweezers. The adaptive boundary is deformable, enabling mechanical measurements of pressure and leading to the observation of a novel structural bistability between concentric particle layering and locally hexagonal configurations at high density. These findings are reproduced in analogous Monte Carlo simulations. Additionally, shearing the confined system drives the this bistability resulting in the observation of a novel oscillatory state characterised by periodically self-similar structural organisation. Under varying conditions, both shear melted and rigid-body-like flow behaviour is observed.   

Session M21: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization IV

First Principles Study of Electronic Properties of MoS$_{2}$/HfO$_{2}$ Interface

Monolayer MoS$_{2}$ is direct band gap two dimensional (2D) semiconductor which has been recently investigated for low-powered field effect transistors and shown promising performance of high on/off current ratio (10$^{8})$ and a carrier mobility $\sim$ 200 cm$^{2}$/Vs with a high-k gate dielectric [1]. For a detailed understanding of the MoS$_{2}$ electronic devices, it is important to examine the detailed atomic and electronic structures of the MoS$_{2}$/HfO$_{2}$ interface. We have developed a lattice matched MoS$_{2}$/HfO$_{2}$ interface model, and investigated the interface atomic structures and the corresponding electronic structures using the density functional theory (DFT) calculations. The model interface was extensively investigated as a function of oxygen and hydrogen incorporation representing different HfO$_{2}$ growth conditions on MoS$_{2}$. The interface formation energies show strong effects of interfacial oxygen content and the valence band offset. \textit{In situ} XPS study of HfO$_{2}$ ALD on MoS$_{2}$ shows that the experimental MoS$_{2}$/HfO$_{2}$ interface properties are consistent with DFT results [2]. These studies can be extended to other TMDs in an effort to identify most promising candidates for electronic device applications. \\[4pt] [1] B. Radisavljevic \textit{et}.al, \textit{Nat. Nanotechnol}. \textbf{6}, 147 (2011). \\[0pt] [2] S. McDonnell \textit{et}. al. \textit{ACS Nano}~\textbf{(}Just Accepted).   

ARPES studies of transition metal dichalcogenides MoS2 and MoSe2

Transition metal dichalcogenides have attracted much attention recently due to their potential applications in nanoelectronics and photonics, as a result of the desirable changes in their electronic band structure upon moving from the bulk limit to few layers and monolayer limit. Here we report our high resolution angle-resolved photoemission spectroscopy study on MoS2 and MoSe2. We, for the first time, resolve the two distinct bands at the Brillouin zone corner of the bulk MoSe2. By depositing potassium on the cleaved surface of MoSe2, we demonstrate the formation of a nearly free 2D electron gas on top of MoSe2. Moreover, the electronic structure of CVD-grown monolayer MoSe2 is carefully examined by ARPES.   

Electronic properties of misoriented bilayer transition metal dichalcogenides

Motivated by a growing interest in vertically stacked van-der-Waal heterostructures, we explore the effect of misorientation in bilayers of MoS2, MoSe2, WS2 and WSe2 on their electronic properties. Mechanical stacking of individual monolayers or chemical and epitaxial growth of this family of layered materials often leads to misoriented interfaces between individual monolayers. Isolated monolayers of MoS2, MoSe2, WS2 and WSe2 exhibit a direct band gap between 1 - 2 eV. The band gap transitions from direct to indirect when the film thickness increases from a monolayer to a bilayer. The question we address is ``Does misorientation between semiconducting TMD bilayers electronically decouple them, as is observed in misoriented bilayers of graphene?'' Using density-functional-theory we investigate the effect of different commensurate rotation angles, stacking order and displacements on the electronic structure of these materials. The effect of these atomic structural variations on the inter-layer coupling, band gaps and effective masses is presented and compared to the equivalent monolayer and bilayer properties for each material.   

Twisted MoS2 Bilayers

Research interest in novel two-dimensional materials has grown rapidly recently because of their potential use for electronic and spintronic applications. Two-dimensional transition metal dichalcogenides are promising compounds for these applications since they possess a bandgap and a strong spin orbit interaction. One of the dichalcogenides that has been studied extensively is MoS2. In this talk I will present the results of our theoretical study of the electronic structure of twisted MoS2 bilayers formed by two single MoS2 layers stacked with a relative twist angle. Our results suggest that the twist angle can be used effectively to tune the electronic properties of MoS2.   

Dimensionality-dependent Electronic and Optical Properties of MoS$_{2}$

We present theoretical calculations based on Density-Functional Theory (DFT) for MoS$_{2}$, a layered material which can be shaped into single-layer and several other nanostructures with unique catalytic, mechanical, electronic and optical properties. We consider ribbons, single-layer, bilayer and bulk structures, at equilibrium and under hydrostatic strain. We calculate the electronic band structure and use linear-response theory to obtain the imaginary part of the dielectric function. Other optical properties, such as absorption and reflectivity, are also calculated. Strain changes dramatically the electronic structure, as it induces changes in the location of both the conduction band minimum and the valence band maximum. Single layer MoS2 becomes an indirect-gap semiconductor while a direct gap is observed at zero strain. The results of the simulations are in good agreement with experimental measurements of energy-dependent reflectivity and photoluminescence spectra. We compare the dielectric properties of bulk (3D), single-layer (2D) and ribbons (1D) of MoS2 and discuss general trends of the macroscopic dielectric constant as a function of dimensionality. Some closely related dichalcogenides will also be discussed.   

Direct observation of the indirect to direct band gap transition in epitaxial monolayer MoSe$_{2}$ film

As a class of graphene-like two-dimensional materials, the layered metal dichalcogenides MX$_{2}$ (M $=$ Mo, W; X $=$ S, Se, Te) have gained significant interest due to the indirect to direct band gap transition in monolayer. Because of this direct band gap, monolayer MX$_{2}$ is favorable for optoelectronic applications. Here we report the direct observation such band gap transition by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe$_{2}$, with variable thickness from monolayer to 8 monolayer, grown by molecular beam epitaxy. The experimental band structure indicates a stronger tendency of monolayer MoSe$_{2}$ towards direct band gap, and with larger gap size, than theoretical prediction. Moreover, we observed a significant band splitting of $\sim$ 180 meV at valence band maximum of a monolayer MoSe$_{2}$, which was theoretically predicted to be 100{\%} spin-polarized. This spin signature gives the layered MoSe$_{2}$ great application potential in spintronic devices, as well as a new playground to investigate spin-obit physics beyond the topological insulators.   

Optical pump-THz probe measurements of self-assembled h-BN/G heterostructures

Two dimensional materials have attracted significant interest in recent times due to properties like large electron mobility, extreme thermal conductivity and high young's modulus. The potential of combining different two-dimensional materials to form new heterostructures with new functionality offers intriguing possibilities. Here, we study the opto-electronic properties of new types of solids consisting of randomly stacked layers of hexagonal boron nitride (h-BN) and graphene (G). We prepare these artificially stacked h-BN/G solids with different ratios of h-BN and G by mixing dispersions of exfoliated h-BN layers and graphene in different concentrations and allowing the exfoliated flakes to form the h-BN/G solids via van der Waals interaction. We study the ultrafast photocarrier dynamics in these solids by pumping with femtosecond visible-near-infrared pulses of light, and probing the transient photo-conductivity with sub-picosecond Terahertz pulses. As we tune the ratio of h-BN and G in the new h-BN/G solids, we not only observe opto-electronic properties that tune from the insulating h-BN phase to semi-metallic G phase, but we also see unique behavior, distinct from either phase, for certain h-BN/G ratios in between the two extreme phases.   

Electronic and Optical Properties of Atomically Thin PbI$_{2}$ Crystals

Layered materials with weak inter-layer van der Waals bonds such as PbI$_{2}$ are of interest for the novel properties they can exhibit as their thickness is reduced to the monolayer limit. We present the results of a joint experimental and theoretical study of the optical and electronic properties of atomically thin samples of PbI$_{2}$. First-principles calculations based on density functional and many-body perturbation theory were performed for the electronic, excitonic, and optical properties of mono and few-layer structures. These results are compared with emission data from photoluminescence experiments performed on mechanically exfoliated samples ranging from bulk to a few monolayers. Our results show that despite a significant increase in the electronic band gap due to quantum confinement in ultrathin samples, the optical gap, defined by excitonic effects, remains unaffected by quantum confinement until its dimensions are reduced to one monolayer. Computational resources were provided by the DOE NERSC facility.   

Coupling of MoS$_{2}$ thin films with different substrates probed by temperature dependent Raman Spectroscopy

Few-layer MoS$_{2}$ is emerging as a new 2-D material beyond graphene, showing a number of interesting properties that could lead to applications in optoelectronics. In most if not all real applications, the MoS$_{2}$ films are expected to be supported by substrates, thus the film-substrate coupling is inevitable. In this work, we study the temperature-dependent Raman shifts of both in-plane (E$_{\mathrm{2g}}^{1})$ and out-of-plane (A$_{\mathrm{1g}})$ phonon modes for single-layer and bi-layer MoS$_{2}$ films on different substrates in a temperature range of 25 -- 500 $^{\circ}$C. By investigating the temperature dependence of Raman scattering, we show that, with increasing temperature, the chemical bonding between film and substrate introduces a damping to E$_{\mathrm{2g}}^{1}$ Raman temperature shift for the MoS$_{2}$ thin-film grown on sapphire by CVD, while the changes in the film morphology leads to significant nonlinear effects for the A$_{\mathrm{1g}}$ mode, such as nonlinear sometimes even non-monotonic temperature shift of Raman frequency and temperature dependence of Raman linewidth, for the transferred MoS$_{2}$ thin-film on SiO$_{2}$/Si substrate.   

Tuning excitons in monolayer and few-layer MoS$_{2}$

Our recent ab initio GW-BSE calculations showed that monolayer MoS$_{2}$ is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS$_{2}$ has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K$'$ valleys in the Brillouin zone and a higher energy series localized in a ring around the $\Gamma$ point. There is very little hybridization between these two exciton series in monolayer MoS$_{2}$, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS$_{2}$. We find that layering and straining MoS$_{2}$ systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near $\Gamma$.   

Control of Two-Dimensional Excitonic Light Emission via Photonic Crystal

Monolayers of transition metal dichalcogenides (TMDCs) exhibit many novel and outstanding photonics and optoelectronic behaviors in the two dimensional (2D) limit, such as rich spin-valley interplays, tunable excitonic effects, and strong light-matter interactions. Excitonic light emission is essential for many of these novelties and potential applications. However, the manipulation of its light emission is still undeveloped. Here we demonstrate the control of excitonic light emission from monolayer tungsten diselenide (WSe2) in an integrated photonic structure, achieved by transferring one monolayer onto a photonic crystal (PhC) with nanocavity. A greatly enhanced ($\sim$ 60 times) photoluminescence of WSe2 and an effectively coupled cavity-mode emission is observed in such systems. More importantly, we are able to redistribute the emitted photons in both polar and azimuthal directions in the far field through designing PhC structures. A 2D optical antenna is thus constructed in our hybids. Our work suggests a new way of manipulating photons in hybrid 2D photonics, important for future energy efficient optoelectronics and 2D nano-lasers.   

Electron-Hole Asymmetry in WS$_2$ Revealed by Scanning Photocurrent Microscopy under Ionic-Liquid Gating

We perform scanning photocurrent microscopy on WS$_2$-based ambipolar ionic liquid-gated field effect transistors with almost ideal transport characteristics. Both in the electron- and the hole-doping regimes, the photocurrent decays exponentially as a function of the distance between electrical contacts and the illumination spot, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on the same sample. Interestingly, the diffusion length of the minority electrons is several times larger than the one of the minority holes at the same doping concentration, which points to a strong intrinsic electron-hole asymmetry.   

Electron-Phonon Coupling and Photoluminescence in monolayer MoS$_{2}$

We have carried out first principles calculations of the photoluminescent properties of monolayer MoS$_{2}$ using density functional theory. In particular, we have analyzed the role of electron-phonon interactions in the photoluminescence process. Phonon dispersion curves calculated using density functional perturbation theory served as the basis for the evaluation of the system electron-phonon coupling, which in turn was used to calculate electron self-energy and the electron spectral function within the Eliashberg approach. We find that the resulting photoemission spectrum is in good agreement with experimental data. We pay special attention to the ultrafast relaxation of the electron system as manifested by the electron-phonon coupling and evaluate the ultrafast photoluminescence of the excited system by using the two-temperature model. It is shown that similar to graphene, MoS$_{2}$ may demonstrate significant ultrafast photoluminescence.   

Novel Exciton States in Monolayer MoS$_2$: Unconventional Effective Hamiltonian

Recent well-converged ab inito GW-BSE calculations show that monolayer MoS$_2$ has a large number of strongly bound excitons with varying characters. We show that these excitonic states cannot be even qualitatively described by an effective mass hydrogenic model without a detailed understanding of the 2D screening. Additionally, we identify and analyze one exciton series having an unusually high binding energy, which originates around the $\Gamma$ point of the Brillouin zone. We show that this excitonic series arises from a fundamentally different effective Hamiltonian with a kinetic energy term resembling a Mexican hat in momentum space, which is responsible for the unusual ordering of the energy levels and distribution of oscillator strength. This work was supported by NSF grant No. DMR10-1006184 and the U.S. DOE under Contract No. DE-AC02-05CH11231.   

Wavefunction Properties and Electronic Band Structures of High-Mobility Semiconductor Nanosheet MoS$_{2}$

Molybdenum disulfide (MoS$_{2})$ nanosheet is regarded as one of the most promising alternatives to the current semiconductors due to its significant band-gap and electron-mobility enhancement upon exfoliating. To elucidate such thickness-dependent properties, we have studied the electronic band structures of bulk and monolayer MoS$_{2}$ by using the first-principles density-functional method as implemented in the SIESTA code. Based on the wavefunction analyses at the conduction band minimum (CBM) points, we have investigated possible origins of mobility difference between bulk and monolayer MoS$_{2}$. We provide formation energies of substitutional impurities at the Mo and S sites, and discuss feasible electron sources which may induce a significant difference in the carrier lifetime. This work was supported by NRF of Korea (Grant Nos. 2009-0079462 and 2011-0018306), Nano-Material Technology Development Program (2012M3a7B4034985), and KISTI supercomputing center (Project No. KSC-2013-C3-008).   

Session M22: Hydrogen Storage, Transportation & Novel PV

Electrification of the transportation sector offers limited country-wide greenhouse gas reductions

Compared with conventional propulsion, plugin and hybrid vehicles may offer reductions in greenhouse gas (GHG) emissions, regional air/noise pollution, petroleum dependence, and ownership cost. Comparing only plugins and hybrids amongst themselves, and focusing on GHG, relative merits of different options have been shown to be more nuanced, depending on grid-carbon-intensity, range and thus battery manufacturing and weight, and trip patterns. We present a life-cycle framework to compare GHG emissions for three drivetrains (plugin-electricity-only, gasoline-only-hybrid, and plugin-hybrid) across driving ranges and grid-carbon-intensities, for passenger cars, vans, buses, or trucks (well-to-wheel plus storage manufacturing). Parameter and model uncertainties are quantified via sensitivity analyses. We find that owing to the interplay of range, GHG/km, and portions of country-wide kms accessible to electrification, GHG reductions achievable from plugins (whether electricity-only or hybrids) are limited even when assuming low-carbon future grids. Furthermore, for policy makers considering GHG from electricity and transportation sectors combined, plugin technology may in fact increase GHG compared to gasoline-only-hybrids, regardless of grid-carbon-intensity.   

High-pressure, ambient temperature hydrogen storage in metal-organic frameworks and porous carbon

We investigated hydrogen storage in micro-porous adsorbents at ambient temperature and pressures up to 320 bar. We measured three benchmark adsorbents: two metal-organic frameworks, Cu$_{\mathrm{3}}$(1,3,5-benzenetricarboxylate)$_{\mathrm{2}}$ [Cu$_{\mathrm{3}}$(btc)$_{\mathrm{2}}$; HKUST-1] and Zn$_{\mathrm{4}}$O(1,3,5-benzenetribenzoate)$_{\mathrm{2}}$ [Zn$_{\mathrm{4}}$O(btb)$_{\mathrm{2}}$; MOF-177], and the activated carbon MSC-30. In this talk, we focus on adsorption enthalpy calculations using a single adsorption isotherm. We use the differential form of the Claussius-Clapeyron equation applied to the Dubinin-Astakhov adsorption model to calculate adsorption enthalpies. Calculation of the adsorption enthalpy in this way gives a temperature independent enthalpy of 5-7 kJ/mol at the lowest coverage for the three materials investigated. Additionally, we discuss the assumptions and corrections that must be made when calculating adsorption isotherms at high-pressure and adsorption enthalpies.   

Liquid-like hydrogen densities in engineered carbon nanospaces

High surface area materials, such as those engineered from synthetic carbon compounds, have narrow pore sizes resulting in exceptionally high stored densities for hydrogen. Stored density is a measurement of the average hydrogen density within a pore. At supercritical temperatures and high pressures, these materials can achieve stored densities 20{\%} higher than liquid hydrogen at 1 bar and 20 K. At 77 K and 200 bar, we have achieved stored densities of up to 85 g/L. We can show, depending on the pore structure, a maximum of gravimetric hydrogen excess adsorption at 100 bar and 296 K and binding energies of 8-10 kJ/mol. The occurrence of a maximum of gravimetric excess adsorption at relatively low pressures, indicating a high binding energy, is due to the overlapping adsorption potentials in narrow pores.   

Hydrogen spillover mechanism on covalent organic frameworks as investigated by DFT

The hydrogen spillover mechanism, including the H chemisorption, diffusion, and H$_{2}$ associative desorption on the surface of COFs and H atoms migration from metal catalyst to COFs, have been studied via DFT. The results described herein show that each sp$^{2}$ C atom on COFs' surface can adsorb one H atom with the bond length d$_{C-H}$ between 1.11 and 1.14 {\AA}, and the up-down arrangement of the adsorbed H atoms is the most stable configuration. By counting the chemisorptions binding sites for these COFs, we predict the saturation storage densities. High hydrogen storage densities can be found that the gravimetric uptakes of COFs are in the range of 5.13 $\sim$ 6.06 wt{\%}. The CI-NEB calculations reveal that one H atom diffuse along C-C path on HHTP surface should overcome 1.41 $\sim$ 2.16 eV energy barrier. We choose tetrahedral Pt$_{4}$ cluster and HHTP as the representative catalyst and substrate, respectively, to study the H migration from metal cluster to COFs.   

Adsorption Enthalpies of Hydrogen on Chemically Enhanced Carbon Nanospaces

Chemical functionalization of carbon nanopore spaces has been shown to significantly increase the differential enthalpy of adsorption of hydrogen (ca. 9kJ/mol). This improved surface interaction corresponds to an increased density of the adsorbed film. Functionalized carbon samples have been produced through KOH activation, deposition of decaborane, and high temperature annealing. Hydrogen sorption measurements have shown significant improvements to stored film densities and binding energies. In this talk, a systematic study of the effect that boron concentration has on the samples' pore structures, binding energies, surface excess concentrations, and volumetric storage capacities is presented.   

Comparative analysis of density functional theory for hydrogen adsorption on metalloporphyrin incorporated graphenes

Porphyrins are often found in nature. The center of the molecules is chemically highly active, thus they can accommodate various metals with high structural stability. A recent study using density functional theory (DFT) suggests that metal incorporated porphyrins can store H$_{\mathrm{2}}$ in significant amounts at the uniformly distributed metal centers, indicating their great potential as a material for hydrogen storage. In this work, we evaluate the performance of DFT exchange-correlation (XC) functionals to describe the properties of hydrogen adsorption on porphyrin-incorporated graphenes (PIG). We studied PIGs doped with different metals (Mg, Ca, Zn, Ti, V) using various XC functionals, ranging from LDA to van der Waals corrected functionals. Metals interacting with hydrogen through chemical binding, dominated by the Cubas mechanism, have a hydrogen binding strength with much stronger dependence on the XC functional than van der Waals systems. The specific shape of the hydrogen energy potential near metal centers is important in determining the thermodynamic stabilities of the hydrogen adsorption and desorption mechanism. The insights obtained in this work should be useful also when applying DFT methods to more generalized adsorption systems.   

High Density Methane Storage in Nanoporous Carbon

Development of low-pressure, high-capacity adsorbent based storage technology for natural gas (NG) as fuel for advanced transportation (flat-panel tank for NG vehicles) is necessary in order to address the temperature, pressure, weight, and volume constraints present in conventional storage methods (CNG {\&} LNG.) Subcritical nitrogen adsorption experiments show that our nanoporous carbon hosts extended narrow channels which generate a high surface area and strong Van der Waals forces capable of increasing the density of NG into a high-density fluid. This improvement in storage density over compressed natural gas without an adsorbent occurs at ambient temperature and pressures ranging from 0-260 bar (3600 psi.) The temperature, pressure, and storage capacity of a 40 L flat-panel adsorbed NG tank filled with 20 kg of nanoporous carbon will be featured.   

Investigation of Morphology and Hydrogen Adsorption Capacity of Disordered Carbons

We have applied small angle neutron scattering (SANS) technique to study the morphologies and hydrogen adsorption capabilities of wood-based ultramicroporous carbon and poly(furfuryl alcohol) derived carbon. The Polydispersed Spherical model and chord length analysis of the scattering profiles were performed to obtain morphological parameters such as average pore size and pore size distribution of the dry carbons, which agreed reasonably well with the independent gas sorption measurements. The hydrogen physisorbed in these two carbons at room temperature and moderate pressures was investigated by In-situ SANS measurements. The experimental data analyzed using a modified Kalliat model for decoupling scattering contributions from pores with different sizes indicates that the molecular hydrogen condenses preferentially in narrow micropores at all measured pressures, which supports the theoretical prediction by quantum mechanical and thermodynamical models.   

Determination of Hydrogen absorption capacity of different nanomaterials using a Quartz Crystal Microbalance

Hydrogen has become an alternative energy source and a key gas for the fuel cell technology. For these reasons, there is a growing need of developing more efficient materials for hydrogen storage in a safer way and to develop hydrogen sensors for hydrogen detection. We studied hydrogen absorption properties of different nanomaterials-assembled systems using a Quartz Crystal Microbalance. The nanomaterials inspected include palladium-based thin films, metal oxides, polymer-metal composites as well as carbon nanoparticles.   

Infrared absorption enhancement at nickel-silicide/silicon interfaces

Nanoparticle embedded thin films are of interest because they have been predicted to enhance absorption and improve thin-film photovoltaic devices. For the case of nickel silicide nanoparticles embedded in amorphous silicon there is experimental observation of absorption enhancement, especially in the infrared where the solar spectrum is strong, but silicon absorption is weak. However, it is not known whether this enhancement is due to effects at the silicide/silicon interface that can actually be applied to photovoltaic devices or simply bulk absorption into the metal. To study these effects, we created theoretical supercells of the interface between nickel di-silicide and silicon, and calculate the optical properties using density functional theory. The supercells show a strong absorption enhancement peak in the red/near-IR, which is the optimal region for absorption. An analysis of the DOS reveals that shifts in the dominant nickel d-orbitals create interface transitions in the IR that are unavailable in the bulk.   

Using surfaces, ligands, and dimensionality to obtain desired nanostructure properties

Nanostructured materials are intensively investigated to obtain material properties different from their bulk counterparts. It has been demonstrated that nanoscaled semiconductor can have interesting size, shape and morphology dependent optoelectronic properties. But the effect of surfaces, ligands and dimensionality (0D quantum dots to 2D nanosheets) has been largely unexplored. Here, we will show how tuning the surface and dimensionality can affect the electronic states of the semiconductor, and how these states can play an important role in their fundamental photophysical properties or thermal transport. Using the specific case for silicon, we will show how ``new'' surface states in small uniform can lead to light absorption/emission without phonon assistance, while hindering the phonon-drag of charge carriers leading to low Seebeck coefficient for thermoelectric applications. These measurements will shed light on designing appropriate surface, size, and dimensionality for desired applications of nanostructured films.   

Selective Defects Passivation of GaInNAs Solar Cells by Hydrogenation

While GaInNAs has the potential to be a fourth-junction in multi-junction solar cells it has proved difficult to achieve the optimal alloy composition due to the low solubility of nitrogen in these materials. At this point we investigate the possibility of improving the performance of GaInNAs using hydrogenation to selectively passivate mid-gap defects, while preserving the functionality of substitutional nitrogen. Temperature dependent photoluminescence measurements of the intrinsic region of a GaInNAs p-i-n solar cell show a classic ``s-shape'' associated with localization prior to hydrogenation, while after hydrogenation no sign of the ``s-shape'' is evident. The preliminary investigations of the effect of hydrogenation on the efficiency of carrier transport in the solar cells will also be presented. Amethyst Research Inc.'s photon-assisted defect mitigation-hydrogenation technique is usually a low temperature process; however, the annealing effects will be de-convoluted from that of hydrogenation.   

Study of LO-phonon decay in semiconductors for hot carrier solar cell

Knowledge of phonon decay is of crucial importance when studying basic properties of semiconductors, since they are closely related to Raman linewidth and non-equilibrium-hot-carriers cooling. The latter indeed cools down to the bottom of the conduction band within a picosecond range because of electron-phonon interaction. The eventual emitted hot phonons then decay in few picoseconds. The hot carriers cooling can be slowed down by considering the decay rate dependence of phonon on conservation rules, whose tuning may reduce the allowed two-phonon final states density. This is of direct interest for the third generation photovoltaic devices that are Hot Carrier Solar Cells (HCSC), in which the photoexcited carriers are extracted at an energy higher than thermal equilibrium. One of the HCSC main challenges then is to find an absorber material in which the hot phonons has a relaxation time longer than the carriers cooling time, so that we can expect the electron to ``reabsorb'' a phonon, slowing down the electronic cooling. HCSC yield is ultimately limited by LO phonon decay, though. In this work, we present theoretical results obtained from ab initio calculations of phonon lifetime in III-V and IV-IV semiconductors through a three-phonon process. Common approximations in the literature are questioned. In particular, we show that the usual ``zone-center approximation'' is not valid in some specific semiconductors. The analysis allows to correctly investigate phonon decay mechanisms in bulk and nanostructured materials.   

Session M23: Invited Session: Industrial Physics Forum: Advances in Measurement Technology

Advances in Measurement Technology at NIST's Physical Measurement Laboratory

The NIST mission is to promote U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology. The Physical Measurement Laboratory (PML) has responsibility for maintaining national standards for two dozen physical quantities needed for international trade; and, importantly, it carries out advanced research at the frontiers of measurement science to enable extending innovation into new realms and new markets. This talk will highlight advances being made across several sectors of technology; and it will describe how PML interacts with its many collaborators and clients in industry, government, and academe.   

Recent Advances in AFM Technology

We will review several recent advances in commercial Atomic Force Microscopy (AFM) technology. The first is the further miniaturization of cantilevers for AFM which has increased their resonant frequencies and decreased their thermal noise, allowing faster, lower noise measurements. When used in an extremely low-noise AFM, these levers have enabled significant improvements in imaging resolution in air and especially in liquids, including the resolution of individual point defects. The second is quantitative mechanical measurements using multifrequency AC techniques. Previously, the contrast in AM-AFM has been difficult to quantify. Recent work has provided an interpretation of the tapping mode observables that allows unambiguous interpretation of material properties. The AM-FM imaging mode combines normal AM mode with the quantitative and high sensitivity of frequency modulated (FM) mode. The mode provides four observables that can be used to solve for parameters, such as sample modulus, in models of the tip-sample interaction. Finally, we will discuss the benefits of photothermal excitation, a new drive mechanism for AC mode techniques that replaces conventional piezo drive. It vastly improves ease-of-use in liquids and provides greatly improved stability in the cantilever drive response relative to piezo drive. Additionally, the drive provides a near-perfect transfer function to the cantilever, enabling more quantitative interpretation in areas like nanomechanical measurement.\\[4pt] In collaboration with Aleks Labuda, Deron Walters, Mario Viani, Sophia Hohlbauch, Irene Revenko, Marta Kocun, and Roger Proksch, Asylum Research, an Oxford Instruments Company.   

Metrology Tools for Semiconductor Manufacturing

The nanoscale dimension of the devices and structures used to fabricate present and future generations of integrated circuits provide numerous challenges for measurement technology. There are two means by which measurement technology advance. First, existing measurement equipment often provides unexpected capability through advances in modeling and new applications. Examples of this include optical models for nanoscale materials for film thickness and X-Ray diffraction X-Ray reciprocal space maps (RSM) for measurement of SiGe/Si thin films and fin stress state and fin pitch. RSMs are sensitive to the key lithography issue of pitch walking. Pitch walking refers to the two pitch that occur when new double patterning lithography processes are used. The other means by which metrology advances is through new measurement equipment. An example of this is the Mueller Matrix spectroscopic ellipsometry equipment that is used for critical dimension measurement. Two examples of this will be shown including 3D shape and CD measurement of fins and measurement of structures fabricated using directed self-assembly of block co-polymers. This talk will cover the physical principles of the examples stated above as well as recent advances and breakthroughs in metrology.   

Recent advances in medical ultrasound

Ultrasound has become one of the most widely used imaging modalities in medicine; yet, before ultrasound-imaging systems became available, high intensity ultrasound was used as early as the 1950s to ablate regions in the brains of human patients. Recently, a variety of novel applications of ultrasound have been developed that include site-specific and ultrasound-mediated drug delivery, acoustocautery, lipoplasty, histotripsy, tissue regeneration, and bloodless surgery, among many others. This lecture will review several new applications of therapeutic ultrasound and address some of the basic scientific questions and future challenges in developing these methods and technologies for general use in our society. We shall particularly emphasize the use of High Intensity Focused Ultrasound (HIFU) in the treatment of benign and malignant tumors.   

High Sensitivity Gravity Measurements in the Adverse Environment of Oil Wells

Bulk density is a primary measurement within oil and gas reservoirs and is the basis of most reserves calculations by oil companies. The measurement is performed with a gamma-ray source and two scintillation gamma-ray detectors from within newly drilled exploration and production wells. This nuclear density measurement, while very precise is also very shallow and is therefore susceptible to errors due to any alteration of the formation and fluids in the vicinity of the borehole caused by the drilling process. Measuring acceleration due to gravity along a well provides a direct measure of bulk density with a very large depth of investigation that makes it practically immune to errors from near-borehole effects. Advances in gravity sensors and associated mechanics and electronics provide an opportunity for routine borehole gravity measurements with comparable density precision to the nuclear density measurement and with sufficient ruggedness to survive the rough handling and high temperatures experienced in oil well logging. We will describe a borehole gravity meter and its use under very realistic conditions in an oil well in Saudi Arabia. The density measurements will be presented.   

Session M24: Focus Session: NanoPV Novel Photophysics and Transport II

Modeling Charge Mobility in Nanoparticle Solar Cells

Nanoparticle (NP) solar cells show the promise to enhance the efficiency of solar cells over the Shockley-Queisser limit due to quantum confinement enhanced charge multiplication processes [1]. A fundamental challenge of NP solar cells, however, is that the very reason that leads to enhanced charge generation also tends to hinder charge transport. To address this challenge, we outline a multi-scale transport modeling scheme based on our previous calculations [2] that involves determining NP parameters from ab-initio and semi-empirical calculations, such as energy level structures, charging energies. This is then embedded in a Kinetic Monte Carlo hopping transport framework to calculate electron and hole mobilities in NP devices as a function composition, disorder, temperature. As a first demonstration, we apply our method to PbSe NP Schottky devices.\\[4pt] Work done in collaboration with Ian Carbone, Physics Department, University of California, Santa Cruz and Marton Voros, Physics Department, University of California, Davis.\\[4pt] [1] Matthew C. Beard et al., Acc. Chem. Res. 46, 1252 (2013).\\[0pt] [2] Ian Carbone, S.A. Carter, G.T. Zimanyi, accepted in J. of Appl. Phys.   

Ultrafast spectroscopy of CuInSeS colloidal quantum dots: Auger recombination, carrier multiplication, and electron transfer

We perform systematic transient absorption and time-resolved photoluminescence measurements on CuInSe$_{\mathrm{x}}$S$_{\mathrm{2-x}}$ quantum dots (QDs), with sizes of 3-5 nm, that have recently been utilized in sensitized solar cells achieving certified efficiencies above 5{\%}. We study QD volume and composition dependence of various excited charge carrier processes, including biexciton Auger recombination, carrier multiplication (CM), and electron transfer (ET) to TiO$_{\mathrm{2}}$. Biexciton decay is similar to that of CdSe QDs of the same volume, which supports the previously reported generality of Auger lifetimes in QDs. CM quantum yields approach 20{\%} indicating that this material could enable photovoltaic efficiencies exceeding the Shockley-Queisser limit. Size-dependent ET (20-40 ns) is fairly slow, which highlights the need for efficient suppression of competing nonradiative processes that can be associated, for example, with the surfaces of poorly passivated QDs. We also demonstrate the importance of having a redox electrolyte (used in sensitized solar cells for hole extraction) present during ET studies in order to prevent charge build-up. Our measurements are critical for understanding the photophysical properties of this new material, and they also suggest general pathways towards improving QD solar cells.   

High-pressure Phase Ge nanoparticles and Si-ZnS nanocomposites: New Paradigms to Improve the Efficiency of MEG Solar Cells

The efficiency of nanoparticle (NP) solar cells may substantially exceed the Shockley-Queisser limit by exploiting multi-exciton generation. However, (i) quantum confinement tends to increase the electronic gap and thus the MEG threshold beyond the solar spectrum and (ii) charge extraction through NP networks may be hindered by facile recombination. Using \emph{ab initio} calculations we found that (i) Ge NPs with exotic core structures such as BC8 exhibit significantly lower gaps and MEG thresholds than particles with diamond cores, and an order of magnitude higher MEG rates. (ii) We also investigated Si NPs embedded in a ZnS host matrix and observed complementary charge transport networks, where electron transport occurs by hopping between NPs and hole transport through the ZnS-matrix. Such complementary pathways may substantially reduce recombination, as was indeed observed in recent experiments. We employed several levels of theory, including DFT with hybrid functionals and GW calculations.   

InAs quantum wells with AlAs$_{0.16}$Sb$_{0.84}$ barriers and GaAs$_{0.09}$Sb$_{0.91}$ absorber for hot carrier and multicarrier generation solar cell

We present an investigation of a series of InAs/AlAs$_{0.16}$Sb$_{0.84}$ superlattice structures tuned to 0.7 eV to facilitate the study of carrier multiplication and hot carrier effects in the narrow gap material. The alloy composition of the barrier materials is designed such that photons of over three times the well energy gap are absorbed in the InAs wells. Three distinct structures are studied: 1) a superlattice composed of multiple wells and barrier material, 2) a hybrid structure composed of a GaAs$_{0.09}$Sb$_{0.91}$ bulk absorber with a superlattice structure, and 3) a bulk heterostructure of GaAs$_{0.09}$Sb$_{0.91}$ with an energy matched to the superlattice, as reference. Power and temperature dependent photoluminescence measurements will be presented to describe the relative (hot) carrier temperatures, and their potential for next generation solar cells. With this in mind, the performance of solar cells based on these designs will also be presented.   

InAs quantum dots in a GaAs$_{1-x}$Sb$_{x}$ matrix for intermediate band solar cell

Self-assembled InAs quantum dots (QDs) were grown by the migration-enhanced epitaxy (MEE) technique in a GaAs$_{1-x}$Sb$_{x}$ matrix material on a GaAs substrate for application as intermediate band single junction solar cells. Initially, a series of InAs QDs structures were studied with a nominal deposition of 1.75 -- 3.5 ML and Sb concentration of $x = 0.13$. The areal density measured by atomic force microscopy was observed to increase with total deposition to a maximum of $\sim$ 4.0x10$^{11}$/cm$^{2}$ after $\sim$ 3 MLs. A high QD density is required to facilitate the formation of an intermediate band (IB) within the band gap of the matrix material. With increasing QD density a simultaneous increase in the optical emission is also observed. The promise in this system is the potential to form a degenerate valence band offset, while forming an IB in the conduction band. As such, a second series of QDs was investigated in which the concentration of Sb in the matrix varied from $x = 0.10$ to $x = 0.18$. The transition from type-I band alignment to type-II is observed. Temperature and power dependent photoluminescence, along with 8 band $k \cdot p$ calculations of the band structure will also be presented.   

Asymmetric tunneling rates for electrons and holes at CdSe quantum dot/carbon nanotube interfaces

Decorating carbon nanotubes with CdSe quantum dots (QDs) is one potential approach for creating high efficiency photovoltaics. Our collaborators at Yale recently produced a ligand-free covalent attachment of CdSe QDs to carbon nanotubes through an organic ligand exchange mechanism. Our prior first principles work described the energetics of the various binding processes and rationalized the experimental growth methodology. After a brief review of the system, we will describe our intriguing finding that excited electrons and holes tunnel with different rates out of the QD and into the carbon nanotubes. The asymmetric tunneling rate itself can, in principle, boost the separation of photo-excited charge at the interface even if there are insufficient band energy differences across the interface. We describe our results for the tunneling rates computed using (i) a brute force approach with increasing simulation cell size to remove periodic effects, and (ii) a Green's function method that directly connects the QD to a thermodynamically large electron reservoir (e.g., a very long pristine nanotube).   

Quantum Dot TiO$_2$-Ge Solar Cells

Colloidal germanium (Ge) quantum dots (CQDs) are attractive solar materials due to their low toxicity compared to Pb- or Cd- based nanocrystals (NC), low cost, and optimal, tunable bandgap for both increased IR response and potential power conversion efficiency ($\eta$) boosts from Multiple Exciton Generation (MEG). We report on the successful fabrication and characterization of spun-cast donor/acceptor type TiO$_2$-Ge CQD solar cells utilizing Ge colloidal quantum dots (CQD) synthesized via a facile microwave method as the active layer. We find that our Ge QD size performance-related trends are similar to other QD systems studied. Additionally, our best heterojunction devices achieved short circuit currents (J$_{SC}$) of 450 $\mu$A and open circuit voltages (V$_{OC}$) of 0.335 V, resulting in $\eta$ = 0.022$\%$. While this represents significant increases over previous Ge CQD PV (85$\%$ over hybrid Ge-P3HT PV, $\>$ 350$\%$ over Ge NC PV), our photocurrents are still much lower than other NC systems. Analysis of intensity-dependent J-V characteristics reveal that our currents are limited by a space-charge region that forms leading to unbalanced charge extraction. We conclude by discussing a variety of film treatments and device structures we have tested to increase J$_{SC}$.   

Investigation on Photoelectric Behavior of Metal-Insulator-Semiconductor Structure Based on Titania Nanotubes Arrays

Titanium dioxide (TiO$_{2})$ has attracted great interest as an inexpensive, earth-abundant and environment-friendly anode material for next generation photovoltaic devices and the metal-insulator-semiconductor (MIS) concept is one of the most promising approaches for improving solar cell cost effectiveness (in {\$}/W). We investigated hybrid MIS structures of semiconducting ordered titania nanotube arrays integrated with insulating iron oxide or copper oxide layers and metallic copper. The morphological and structural properties of the samples were analyzed by scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy with elemental mapping, and X-ray diffraction. The nanotubular morphology represents a step change from the current thin film approach, providing significantly larger surface area while facilitating the charge separation and electron transport. Photoelectric behavior of the new structures was estimated by transient response, quantum efficiency and spectral response, and a solar simulator was used for recording the photovoltaic response.   

Size Effects in Dye-Sensitized TiO$_{2}$ Clusters

The development of solar cells is driven by the need for clean and sustainable energy. Organic and dye sensitized cells are considered as promising technologies, particularly for large area, low cost applications. However, the efficiency of such cells is still far from the theoretical limit. \textit{Ab initio} simulations may be used for computer-aided design of new materials and nano-structures for more efficient solar cells. It is essential to obtain an accurate description of the electronic structure, including the fundamental gaps and energy level alignment at the interfaces in the device active region. This requires going beyond ground-state DFT to the GW approximation. A recently developed GW method [PRB 86, 041110R (2012)] is applied to dye-sensitized TiO$_{2}$ clusters [PRB 84, 245115 (2011)]. The effect of cluster size on the energy level alignment at the dye-TiO$_{2}$ interface is discussed. With the increase in the TiO$_{2}$ cluster size its gap narrows. The gap of the molecule attached to the cluster subsequently narrows due to screening. As a result, the energy level alignment at the interface changes in an unexpected way [Marom, K\"{o}rzd\"{o}rfer, Ren, Tkatchenko, Chelikowsky, \textit{to be published}].   

An integrated approach to realizing high-performance liquid-junction quantum dot sensitized solar cells

Solution processed semiconductor quantum dot solar cells offer a path towards both reduced fabrication cost and higher efficiency enabled by novel processes such as hot-electron extraction and carrier multiplication. Here we use a new class of low-cost, low-toxicity CuInSe$_{x}$S$_{\mathrm{2-x}}$ quantum dots to demonstrate sensitized solar cells with certified efficiencies exceeding 5{\%}. Among other material and device design improvements studied, use of a methanol-based polysulfide electrolyte results in a particularly dramatic enhancement in photocurrent and reduced series resistance. Despite the high vapor pressure of methanol, the solar cells are stable for months under ambient conditions, which is much longer than any previously reported quantum dot sensitized solar cell. A study of electron transfer QD/TiO2 interface reveals the process to be surprisingly slow and confirms that methanol does not act as a sacrificial donor. This study demonstrates the large potential of CuInSe$_{x}$S$_{\mathrm{2-x}}$ quantum dots as active materials for the realization of low-cost, robust, and efficient photovoltaics as well as a platform for investigating various advanced concepts derived from the unique physics of the nanoscale size regime. This work was just accepted to Nature Communications.   

Transient lateral photovoltaic effect in patterned metal-oxide-semiconductor films.

Time dependent transient lateral photovoltaic effect (T-LPE) has been studied in lithographically patterned thin Co films grown over naturally passivated p-type Si (100) substrates. Investigation has been done at room temperature in 21 nm thick, 5 and 10 microns wide and 700 microns long Co films as a function of the position of the laser focused spot with respect to the contacts, pulse frequency (in kHz range) and up to few mW (at wavelength 405 nm or 487 nm) laser power with the spot diameter ranging between 1 and 10 microns. The observed abrupt (faster than in 5 microsecond) change in sign of the T-LPE after the laser is switched off was qualitatively explained by the model which considers redistribution of the life time of non-equilibrium carriers in the electric field due to charged local centres formed during the previous illumination. Exponential relaxation in the inverted T-LPE allows the characterization of the relaxation process as a function of the spot position with respect to the contacts. Numerical simulations satisfactory reproduce the observed unusual time dependence of the T-LPE.   

Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber

Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3-x}}$Cl$_{\mathrm{x}})$ and triiodide (CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}})$ perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of order 100 nanometers. Finally, we fabricated solution-processed thin-film planar heterojunction devices, achieving power conversion efficiencies of over 12{\%} using the mixed halide absorber but only 4{\%} with the triiodide perovskite. Our results show that the long diffusion lengths justify the high efficiency of planar heterojunction perovskite solar cells, and identify a critical parameter to optimize for future perovskite absorber development.   

The role of the methylammonium cation in the structural and electronic properties of 3D organic-inorganic perovskite halides: a DFT analysis including Spin Orbit Coupling

Many papers have been recently published reporting the enhanced photoconversion efficiency (PCE) up to 15{\%} [Nature 501, 395 (2013)] for organic-inorganic solar cells containing sandwiches of perovskite compounds (the light harvester), mesoporous TiO$_{2}$, and a polymeric hole conductor. The usage of these 3D MAPbX$_{3}$ (MA$=$CH$_{3}$NH$_{3}^{+}$;X$=$Cl$^{-}$,Br,I$^{-})$ perovskites, stems by their chemical stability and good transport characteristics in the device. Anyway, these materials with so high applicability in PV and with many undisclosed features still find scarce attention in the theoretical community. Here, two aspects stimulated our work: the Spin Orbit Coupling (SOC) impact previously always speculated but ignored in predicting the electronic properties of these compounds, and the overlooked role played by the organic part. We focused on the electronic properties of MAPbI$_{3}$, on the impact played by SOC, on how hybrid functionals improve the bandgap prediction, and on the role ascribed to MA cation.   

Session M25: Focus Session: Thermoelectrics - Phonons and Heat Conduction I

Phonon and magnon heat transport and drag effects

Thermoelectric generators and coolers constitute today's solid-state energy converters. The two goals in thermoelectrics research are to enhance the thermopower while simultaneously maintaining a high electrical conductivity of the same material, and to minimize its lattice thermal conductivity without affecting its electronic properties. Up to now the lattice thermal conductivity has been minimized by using alloy scattering and, more recently, nanostructuring [1]. In the first part of the talk, a new approach to minimize the lattice thermal conductivity is described that affects phonon scattering much more than electron scattering. This can be done by selecting potential thermoelectric materials that have a very high anharmonicity, because this property governs phonon-phonon interaction probability. Several possible types of chemical bonds will be described that exhibit such high anharmonicity, and particular emphasis will be put on solids with highly-polarizable lone-pair electrons, such as the rock salt I-V-VI2 compounds (e.g. NaSbSe2). The second part of the talk will give an introduction to a completely new class of solid-state thermal energy converters based on spin transport. One configuration for such energy converters is based on the recently discovered spin-Seebeck effect (SSE). This quantity is expressed in the same units as the conventional thermopower, and we have recently shown that it can be of the same order of magnitude. The main advantage of SSE converters is that the problem of optimization is now distributed over two different materials, a ferromagnet in which a flux of magnetization is generated by a thermal gradient, and a normal metal where the flux of magnetization is converted into electrical power. The talk will focus on the basic physics behind the spin-Seebeck effect. Recent developments [2] will then be described based on phonon-drag of spin polarized electrons. This mechanism has made it possible to reach magnitudes of SSE that are comparable to the highest values of classical thermopower measured in semiconductors. This work is supported as part of the Revolutionary Materials for Solid State Energy Conversion (RMSSEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, by AFOSR MURI ``Cryogenic Peltier Cooling'' Contract {\#}FA9550-10-1-0533and by NSF-CBET-1133589. \\[4pt] [1] J. P. Heremans {\&} al., Nature Nanotechnology\textbf{ 8,} 471-473 (2013)\\[0pt] [2] C. M. Jaworski {\&} al., Nature, \textbf{487}, 210-213 (2012)   

Phonon Scattering Mechanisms in Thermoelectrics

Improving our current microscopic understanding of thermal conductivity is needed to design more efficient thermoelectric materials. Thus, establishing a complete picture of phonon dispersions and mean-free-paths is crucial to provide a realistic microscopic characterization of phonon transport, against which theories can be tested. Thanks to recent advances in instrumentation, inelastic neutron scattering can map phonon dispersions and lifetimes across the entire Brillouin zone. As our studies illustrate, such measurements provide key insights about phonon scattering mechanisms, including phonon anharmonicity, electron-phonon coupling, and scattering by point defects or nanostructures. In addition, we perform first-principles simulations of atomic dynamics, including effects of anharmonicity and electron-phonon coupling, to quantitatively model the large experimental datasets. We present results from several studies of important thermoelectric materials [1,2], illustrating how this integrated approach can be used to reach a new level of microscopic understanding of thermal conductivity. [1] O. Delaire, J. Ma, K. Marty, et al. Nature Materials 10, 614 (2011). [2] J. Ma*, O. Delaire*, A. F. May et al. Nature Nanotechnology 8, 445 (2013).   

Anisotropic Deviations from Fourier's Law in Si and MgO and the Importance of Temperature-Profile Extrema

Efforts to engineer thermal conductivity values by alloying, doping, or nanostructuring rely on a fundamental understanding of phonon-phonon and phonon-defect scattering. However, experimentally resolving phonon dynamics remains challenging. Recent studies demonstrate that time-domain thermoreflectance and frequency-domain thermoreflectance are sensitive to the mean-free-paths of heat-carrying phonons. The sensitivity of both techniques relies on the failure of Fourier theory when important length-scales of the temperature-profile become shorter than phonon mean-free-paths. However the correct interpretation of these experiments remains unclear. To address this issue, we characterize the relationship between the failure of Fourier's law, phonon mean-free-paths, important length-scales of the temperature-profile, and interfacial phonon scattering by performing extensive time-domain thermoreflectance experiments on Si, Si$_{\mathrm{0.99}}$Ge$_{\mathrm{0.01}}$, boron doped Si, and MgO crystals between 40 and 300 K. We find the failure of Fourier's law causes anisotropic thermal transport in Si and MgO despite cubic symmetry, and that in situations where Fourier's law fails, interfacial phonon scattering can affect the heat-current away from the interface.   

Reducing the lattice thermal conductivity of the rocksalt I-V-VI$_{2}$ compounds

Reducing the lattice thermal conductivity is a crucial task in the optimization of the thermoelectric figure of merit. Recent theoretical calculations [1] have revealed the presence marginally stable acoustic phonons which have extremely large Gr\"{u}nesien parameters which result in a strong anharmonicity in heat-carrying acoustic phonon branches of select rocksalt I-V-VI2 compounds as a result of lone pair electrons on group V elements. Here, we present a new simple method of mapping Gr\"{u}nesien parameters, using readily available information on the ionization and the hybridization of the chemical bonds involved, and avoiding extensive numerical simulations. Additionally, we present current advances in doping on alkali based compounds which have inexpensive and non-toxic starting constituents. \\[4pt] [1] Michele D. Nielsen, Vidvuds Ozolins and Joseph P. Heremans, Lone pair electrons minimize lattice thermal conductivity, Energy Environ. Sci., 6, 570 -- 578 (2013)   

Exploration of Phonon Behavior in PbTe from Ultrafast Time-Resolved Pump-Probe Measurements

We report femtosecond-resolution measurements of phonon dynamics on photo-excited PbTe, an incipient ferroelectric. PbTe is a leading thermoelectric material with an unusually low thermal conductivity, which has been attributed to strongly anharmonic phonon interactions. In an attempt to understand in detail the nature of these interactions, we perform time-resolved pump-probe measurements using combinations of THz- and optical-based excitation and optical- and x-ray-based probes in variable temperature environments. Several interesting observations are highlighted. In IR pump/IR probe, anomalous oscillations are seen near 1.4 THz and just below 1 THz, with their relative amplitudes varying with temperature and pump fluence. The frequencies fall close to the Raman-forbidden TO phonon mode. Additionally, we use single-cycle THz pulses centered near 1 THz in an attempt to drive the TO mode. Probing with IR results in unexpected modulations with oscillatory behavior that last for a few picoseconds, fluctuate at a rate just below 1.4 THz, and grow in strength with decreasing temperature. This talk will discuss possible explanations for these effects and their impact on further understanding the relationship between anharmonicity and high temperature thermoelectric behavior.   

Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe show a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured energy spectra and their temperature dependence. The contributions to the complex features of the transverse-optic ferroelectric mode from phase-space for three-phonon scattering processes and the lattice instability are compared.   

Fast full-spectrum phonon calculations for large lattices by Bloch mode synthesis

Computation of thermal properties from lattice dynamics models involves integration over all phonon modes in the system. These phonon modes can be found by solving an eigenvalue problem. For periodic nanostructures, the degrees of freedom in the system may number in the thousands or even millions, resulting in a very expensive computational problem. Bloch mode synthesis is a recently developed model reduction technique whereby the size of the eigenvalue problem is greatly decreased. Similar to sub-structuring techniques, this method separates the domain into interface and interior degrees of freedom, and performs a modal reduction on the interior. Meanwhile the interface is represented using a set of static constraint modes. In the current work, this reduction is modified by using dynamic constraint modes so that it can be applied at higher frequency ranges. This effectively breaks up the large eigenvalue problem into many small eigenvalue problems resulting in large computational savings.   

Study on the Lattice Dynamics of the Argyrodite Ag$_{8}$GeTe$_{6}$

Ag$_{8}$GeTe$_{6}$~was initially studied as a super ionic-electronic mixed conductor in the 1970s, and more recently has attracted new interest for its thermoelectric performance. A key to the desirable thermoelectric performance of Ag$_{8}$GeTe$_{6}$ is its exceptionally low lattice thermal conductivity ($\sim$ 0.25W/m*K at 300K), which is intimately related to its structure, consecutive structural instabilities, and unusual lattice dynamics (e.g., anharmonicity). In this work, we have studied Ag$_{8}$GeTe$_{6}$ by means of thermal conductivity, electrical conductivity, Seebeck coefficient, Hall coefficient, magnetic susceptibility, resonant ultrasound spectroscopy (RUS), photoacoustic spectroscopy, and synchrotron x-ray diffraction at low temperatures in order to further understand the coexistence of mixed conduction and high thermoelectric performance at elevated temperatures.   

First-principles study of anharmonic lattice dynamics and thermal conductivity of AgSbTe2

We report on first-principles calculations of anharmonic lattice dynamics and thermal conductivity of AgSbTe2. We study the temperature dependence of phonon scattering and, in particular, examine the mechanism responsible for the low thermal conductivity of AgSbTe2, which holds the key to its potential thermoelectric applications. We perform systematic calculations and analysis to discuss the role of intrinsic anharmonic phonon-phonon scattering and strong phonon-nanodomain scattering in determining the phonon transport process in AgSbTe2.   

Thermal conductivity of nano-structured materials

Manipulating the thermal properties of materials by nano-structuring is new successful route to improve the performance of thermoelectric materials. We present a new parameter free model to predict anharmonic scattering in bulk and nanoscale materials. Velocities and anharmonic scattering rates are calculated from the Gr\"uneisen parameter of the full phonon dispersions and used to calculate the lattice thermal conductivity using the phonon Boltzmann transport equation in the relaxation time approximation. We find good agreement with experiments for a range of materials. Furthermore, we show that our model, as opposed to simple models based on only the acoustic bands, finds the correct trend in the thermal conductivity of Mg2Si, Mg2Ge and Mg2Sn. We also examine thermal transport in more complex materials like Type-I Si clathrates and zinc-antimonides. Finally, discuss how nano-structure and disorder effect the thermal conductivity.   

Phase Transition Enhanced Thermoelectric Performance in Copper Chalcogenides

Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of \textit{zT} in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu$_{\mathrm{2}}$Se over a broad (360-410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of \textit{zT} (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the \textit{zT} of Cu$_{\mathrm{1.97}}$Ag$_{\mathrm{0.03}}$Se (almost 1.0 at 400K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high \textit{zT} and new green applications for thermoelectrics.   

On Minority Carrier Scattering for Thermoelectrics

Most of the past studies on thermoelectric materials have been focused on majority carriers and lattice phonons in heavily doped semiconductors. In this talk I will show that minority carriers, however, could have a significant impact on both electrical and thermal transport, especially at elevated temperatures. I will also describe means of improving thermoelectric performance of heavily doped semiconductors via selective minority carrier scattering. These results offer insights for understanding experimental findings and optimizing thermoelectric properties of narrow band-gap semiconductors.   

Optical Band Gap and the Burstein Moss Shift in Doped PbTe

We will present an analysis of the room temperature Burstein Moss shift in Iodine doped PbTe. The shift explains the phenomena of a measured increase in the optical band gap as the carrier concentration increases. We quantify the magnitude of the effect and extract an estimate of the true band gap---which is observed to decrease with doping level (also known as band gap renormalization). The results imply that care be taken when using measured optical band gaps for comparison to thermoelectric transport data in doped samples because the true band gap can be quite different than the measured one. Temperature dependent optical measurements in IV-VI materials will also be discussed.   

Session M26: Focus Session: Materials in Extremes: High-Strain-Rate Phenomena II

Integrating Simulation and Data for Materials in Extreme Environments

We are using large-scale molecular dynamics (MD) simulations to study the response of nanocrystalline metals such as tantalum to uniaxial (e.g., shock) compression. With modern petascale-class platforms, we are able to model sample sizes with edge lengths over one micrometer, which match the length and time scales experimentally accessible at Argonne's Advanced Photon Source (APS) and SLAC's Linac Coherent Light Source (LCLS). I will describe our simulation predictions and their recent verification at LCLS, as well as outstanding challenges in modeling the response of materials to extreme mechanical and radiation environments, and our efforts to tackle these as part of the multi-institutional, multi-disciplinary Exascale Co-design Center for Materials in Extreme Environments (ExMatEx). ExMatEx has initiated an early and deep collaboration between domain (computational materials) scientists, applied mathematicians, computer scientists, and hardware architects, in order to establish the relationships between algorithms, software stacks, and architectures needed to enable exascale-ready materials science application codes within the next decade. We anticipate that we will be able to exploit hierarchical, heterogeneous architectures to achieve more realistic large-scale simulations with adaptive physics refinement, and are using tractable application scale-bridging proxy application testbeds to assess new approaches and requirements. The current scale-bridging strategies accumulate (or recompute) a distributed response database from fine-scale calculations, in a top-down rather than bottom-up multiscale approach. I will demonstrate this approach and our initial assessments, using the newly emerging capabilities at new 4$^{\mathrm{th}}$ generation synchrotron light sources as an experimental driver.   

Bridging simulations and experiment in shock and ramp induced phenomena

The high pressure materials physics program at Sandia's Z facility includes strong collaboration between theory, simulations and experiments. This multi-disciplinary approach has led to new insights in many cases. Several examples will be discussed to illustrate the benefits of bridging simulations and experiments. Results will be chosen from recent work on the xenon equation of state, phase change in MgO, shock induced chemistry in CO2 and tantalum strength. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

High-speed velocimetry inside imploding cylindrical liners

Dynamic planar compression is conceptually simple but difficult to maintain at extreme pressure ($>$5~Mbar). Higher pressures are attainable with imploding cylindrical liners, using Photonic Doppler velocimetry (PDV) to track the liner interior. PDV measures Doppler shift directly---1~GHz of beat frequency for every 1~km/s of velocity---requiring a special ``leapfrog'' approach for liners traveling in excess of 20~km/s. Single-point and multi-point PDV measurements have been performed in aluminum, beryllium, and tantalum liners under ramp compression, and the technique can readily applied to other implosion experiments. Combined with electrical current diagnostics, these measurements test thermodynamic equations of state at pressures up to 10~MBar and beyond.   

Surface evolution effects observed in velocimetry of materials at high strain rates

According to the accepted model for photon Doppler velocimetry (PDV), a particular probe measures the bulk (or average) motion of a surface moving along its beam axis. Utilizing this model, a surface's velocity vector may be reconstructed via a number of probes, at distinct angles of incidence, all of which view the same region on the surface. However, this approach does not account for localized effects of surface evolution, which may interact with PDV's interferometer in ways that are not yet fully appreciated. Consider, for example, that the material flow of a straining surface occurs tangent to the surface and may project along the beam axes of non-normal probes. We present a recent series of explosive tests, whose results suggest that non-normal PDV probes measure the effects of surface evolution as it projects along their beam axes. We believe that these effects have not been observed before. The implication is that PDV probes are capable of measuring the bulk motion of a surface, as well as measuring discrete events associated with surface evolution and failure.   

Progress on Understanding Structure and Compression of Laser Ramp-Compressed Matter into the Terapascal Pressure Regime

Recent results from x-ray diffraction experiments at the Omega and NIF laser facilities on dynamically-compressed Sn, Ta, MgO, Pb and diamond will be presented. In order to relate laser ramp-compression results to the equilibrium phase diagram, it is necessary to understand the affects of the rapid compression on the phase transitions, microstructure and temperature of a material. To accomplish these measurements, a number of technical challenges must be overcome. The stress profile and history and the temperature of the sample need to be adequately controlled to make a measurement in a highly-compressed solid state, requiring precise laser pulse shaping and timing. Accurate understanding of the spectral emission from a plasma created during the laser ablation process is required in order to filter this emission and produce high contrast diffraction images. Progress towards understanding and resolving these scientific and technical issues will be discussed along with the experimental diffraction results.   

Simulating laser speckle dynamics that result from surface evolution

In Photon Doppler velocimetry (PDV), motion along a laser beam's axis is quantified via frequency shifts in the backscattered light. The local intensity of the backscattered field varies spatially due to interference between coherent reflections. The randomly arranged bright and dark regions that result from this interference are commonly referred to as laser speckles. As a consequence of surface evolution, new microfacets become illuminated and the speckle pattern changes. While strain-induced speckle dynamics have been experimentally observed, little work has been done towards understanding the direct relationships between surface evolution and quantifiable speckle properties. We present a computational model that simulates the scattering of electromagnetic waves off of a rough surface and that simulates conditions inherent to PDV experiments. The results contribute to our understanding of how surface evolution relates to the speckle dynamics measured in PDV.   

Failure of brittle heterogeneous materials: Intermittency, Crackling and Seismicity

The problem of the solid fracture is classically addressed within the framework of continuum mechanics. Still, stress enhancement at crack tips makes the failure behavior observed at the continuum-level scale extremely dependent on the presence of microstructural inhomogeneities. This yields statistical aspects which, by essence, cannot be addressed using the conventional engineering continuum approaches. We designed an experimental setup that allows growing well-controlled tensile cracks in brittle heterogeneous solids of tunable microstructure, over a wide range of loading speed. The crack dynamics and the evolution of stored and released mechanical energy are monitored in real time. In parallel, the acoustic emission is recorded via a series of acoustic transducers, and analyzed in a way similar to that develop by geophysicists to process seismic signals. This experiments allowed us to characterize quantitatively the crackling dynamics of cracks, also to evidence intriguing statistical similarities between the seismicity associated with this simple situation (single crack under tension) and the much more complex situation of multicracking in compressive fracture and in earthquakes.   

Strong plastic deformation and softening of fast colliding Lennard-Jones nanoparticles by Molecular Dynamics simulations

We present a Molecular Dynamics study of the coefficient of restitution $e$ for colliding two equal sized nanoparticles. Nanoparticles often show distinctly different mechanical and dynamical properties than bulk materials. We investigate the collision velocity $v_{\mathrm{coll}}$ and the nanoparticle size dependence of coefficient of restitution. We find that the size dependent yield velocity $v_{Y}$, a sharp crossover point between elastic collision and plastic collision, appears to approach the theoretical constant value for macroscopic spheres as the nanoparticle size grows. We also find that above $v_{Y}$, the coefficient of restitution $e \propto v_{\mathrm{coll}}^{-\alpha}$, where $\alpha \sim 1$, which is distinct from the inelastic macroscopic sphere collision case, $\alpha = 1/4$. It indicates that nanoparticles colliding at high collision velocity are softened. We discuss possible insights of the size dependent yield velocity and the soft nanoparticles above $v_{Y}$.   

Session M27: Focus Session: Petascale Science and Beyond: Applications and Opportunities for Materials Science III

Exciting Quantized Vortex Rings in a Superfluid Unitary Fermi Gas

In a recent article, Yefsah et al., Nature \textbf{499}, 426 (2013) report the observation of an unusual quantum excitation mode in an elongated harmonically trapped unitary Fermi gas. After phase imprinting a domain wall, they observe collective oscillations of the superfluid atomic cloud with a period almost an order of magnitude larger than that predicted by any theory of domain walls, which they interpret as a possible new quantum phenomenon dubbed ``a heavy soliton'' with an inertial mass some 50 times larger than one expected for a domain wall. We present compelling evidence that this ``heavy soliton'' is instead a quantized vortex ring by showing that the main aspects of the experiment can be naturally explained within an extension of the time-dependent density functional theory (TDDFT) to superfluid systems. The numerical simulations required the solution of some 260,000 nonlinear coupled time-dependent 3-dimensional partial differential equations and was implemented on 2048 GPUs on the Cray XK7 supercomputer Titan of the Oak Ridge Leadership Computing Facility.   

Basis Function Sampling for Material Property Computations

Wang--Landau sampling, and the associated class of flat histogram simulation methods, have been particularly successful for free energy calculations in a wide array of physical systems. Practically, the convergence of these calculations to a target free energy surface is hampered by reliance on parameters which are unknown {\it a priori}. We derive and implement a method based on orthogonal (basis) functions which is fast, parameter-free, and geometrically robust. An important feature of this method is its ability to achieve arbitrary levels of description for the free energy. It is thus ideally suited to {\it in silico} measurement of elastic moduli and other quantities related to free energy perturbations. We demonstrate the utility of such applications by applying our method to calculation of the Frank elastic constants of the Lebwohl--Lasher model.   

Performance of replica-exchange Wang-Landau sampling for the study of spin systems

The recently proposed replica-exchange Wang-Landau sampling (REWL)\footnote{Phys. Rev. Lett. \textbf{110}, 210603 (2013)} is a novel, massively parallel Monte Carlo method which allows for the parallelization of Wang-Landau sampling based on a replica-exchange framework. The robustness of the scheme is demonstrated by its broad applicability on a variety of spin systems: from the simplest models with discrete or continuous energy domains, to complex systems captured by large-scale first principles density functional theory calculations. The accuracy of REWL is studied by comparing the thermodynamic properties with exact solutions and results obtained by the original, serial Wang-Landau sampling. The principles for the speed-up, the strong and weak scaling behavior of REWL are also investigated when different parameter settings are employed. We will show, with the aid of selected spin systems, that the method accelerates the simulations significantly with a possible improved accuracy.\footnote{This research was partly sponsored by the Office of Advanced Computing Research of the US Department of Energy. It used resources of the Oak Ridge Leadership Computing Facility at ORNL supported by the Office of Science of the DOE under contract DE-AC05-00OR22725.}   

Magnetic entropy change calculated from first principles based statistical sampling technique: Ni2MnGa

Magnetic entropy change in Magneto-caloric Effect materials is one of the key parameters in choosing materials appropriate for magnetic cooling and offers insight into the coupling between the materials' thermodynamic and magnetic degrees of freedoms. We present computational workflow to calculate the change of magnetic entropy due to a magnetic field using the DFT based statistical sampling of the energy landscape of Ni2MnGa. The statistical density of magnetic states is calculated with Wang-Landau sampling, and energies are calculated with the Locally Self-consistent Multiple Scattering technique. The high computational cost of calculating energies of each state from first principles is tempered by exploiting a model Hamiltonian fitted to the DFT based sampling. The workflow is described and justified. The magnetic adiabatic temperature change calculated from the statistical density of states agrees with the experimentally obtained value in the absence of structural transformation. The study also reveals that the magnetic subsystem alone cannot explain the large MCE observed in Ni2MnGa alloys. This work was performed at the ORNL, which is managed by UT-Batelle for the U.S. DOE. It was sponsored by the Division of Material Sciences and Engineering, OBES. This research used resources of the OLCF at ORNL, which is supported by the Office of Science of the U.S. DOE under Contract DE-AC05-00OR22725.   

Exact enumeration of an Ising model for Ni$_2$MnGa

Exact evaluations of partition functions are generally prohibitively expensive due to exponential growth of phase space with the degrees of freedom. An Ising model with $N$ sites has $2^N$ possible states, requiring the use of better scaling methods, such as importance sampling Monte-Carlo for all but the smallest systems. Yet the ability to obtain exact solutions for large systems can provide important benchmark results and opportunities for unobscured insight into the underlying physics of the system. Here we present an Ising model for the magnetic sublattices of the important magneto-caloric material Ni$_2$MnGa and use an exact enumeration algorithm to calculate the number of states $g(E,M_1,M_2)$ for each energy $E$ and sublattice magnetization $M_1$ and $M_2$. This allows the efficient calculation of the partition function and derived thermodynamic quantities such as specific heat and susceptibility. Utilizing resources at the Oak Ridge Leadership Facility we are able to calculate $g(E,M_1,M_2)$ for systems of up to 48 sites, which provides important insight into the mechanism for the large magnet-caloric effect in Mn$_2$NiGa as well as an important benchmark for Monte-Carlo based calculations (esp. Wang-Landau) of $g(E,M_1,M_2)$.   

First-principles calculation of electronic stopping contributions from core electrons and off-channeling

In order to understand the interaction of projectile atoms with targets under particle radiation in materials, e.g.\ in space applications or nuclear reactors, it is critical to investigate electronic and ionic contributions to stopping power. The goal of such efforts is detailed understanding of radiation damages as well as fundamental effects such as ion-electron interaction. While ionic stopping has been successfully modeled by molecular dynamics in the past, only recently a computational framework came within reach that is capable of accurately describing \emph{electronic} stopping from first principles. Using our large-scale implementation of real-time time-dependent density functional theory in non-adiabatic Ehrenfest molecular dynamics, we are able to gain deep insight into electronic stopping for systems with hundreds of atoms and thousands of electrons, taking into account their quantum-mechanical electron-electron interaction. We discuss distinct contributions of valence and core electrons of aluminum target atoms to electronic stopping, and we study their importance for different projectile (hydrogen and helium atoms) velocities. There is striking influence of the stopping geometry especially for fast projectiles, and we find excellent agreement with experiment.   

Highly parallel implementation of non-adiabatic Ehrenfest molecular dynamics

While the adiabatic Born-Oppenheimer approximation tremendously lowers computational effort, many questions in modern physics, chemistry, and materials science require an explicit description of coupled \emph{non-adiabatic} electron-ion dynamics. Electronic stopping, i.e.\ the energy transfer of a fast projectile atom to the electronic system of the target material, is a notorious example. We recently implemented real-time time-dependent density functional theory based on the plane-wave pseudopotential formalism in the Qbox/qb@ll codes. We demonstrate that explicit integration using a fourth-order Runge-Kutta scheme is very suitable for modern highly parallelized supercomputers. Applying the new implementation to systems with hundreds of atoms and thousands of electrons, we achieved excellent performance and scalability on a large number of nodes both on the BlueGene based ``Sequoia'' system at LLNL as well as the Cray architecture of ``Blue Waters'' at NCSA. As an example, we discuss our work on computing the electronic stopping power of aluminum and gold for hydrogen projectiles, showing an excellent agreement with experiment. These first-principles calculations allow us to gain important insight into the the fundamental physics of electronic stopping.   

Large-scale massively parallel atomistic simulations of short pulse laser interaction with metals

Taking advantage of petascale supercomputing architectures, large-scale massively parallel atomistic simulations (10$^{8}$-10$^{9}$ atoms) are performed to study the microscopic mechanisms of short pulse laser interaction with metals. The results of the simulations reveal a complex picture of highly non-equilibrium processes responsible for material modification and/or ejection. At low laser fluences below the ablation threshold, fast melting and resolidification occur under conditions of extreme heating and cooling rates resulting in surface microstructure modification. At higher laser fluences in the spallation regime, the material is ejected by the relaxation of laser-induced stresses and proceeds through the nucleation, growth and percolation of multiple voids in the sub-surface region of the irradiated target. At a fluence of $\sim$ 2.5 times the spallation threshold, the top part of the target reaches the conditions for an explosive decomposition into vapor and small droplets, marking the transition to the phase explosion regime of laser ablation. The dynamics of plume formation and the characteristics of the ablation plume are obtained from the simulations and compared with the results of time-resolved plume imaging experiments.   

Challenges in Modeling of the Plasma-Material Interface

Recent work with lithium coatings deposited on a variety of metallic and graphitic surfaces, in a number of tokamak fusion machines around the world, has provided evidence of the sensitive dependence plasma behavior has on these ultra-thin deposited films. Our computer simulations, validated by recent experiments, have elucidated roles of lithium in carbon walls to the recycling of the plasma hydrogen [1]. We performed quantum-classical atomistic calculations on many thousands of random trajectories to clarify the interplay of lithium and oxygen in amorphous carbon. We show that the presence of oxygen in the surface plays the key role in the increased uptake chemistry and suppression of erosion, while lithium has a decisive role in achieving high concentrations of oxygen in the upper layers of the surface upon bombardment by deuterium. D atoms preferentially bind with O and C-O. The plasma-facing walls of the next-generation fusion reactors will be exposed to high fluxes of neutrons and plasma-particles and will operate at high temperatures for thermodynamic efficiency. To this end we have been studying the evolution dynamics of vacancies and interstitials to high doses of tungsten surfaces bombarded by self-atoms, using classical molecular dynamics. Results show surprising saturation of the defects upon cumulative irradiation of only 1 DPA, as well as the defects clustering at the tungsten surface. These findings are obtaining validation in recent experiments.   

Beyond Petascale with the HipGISAXS Software Suite

We have developed HipGISAXS, a software suite to analyze GISAXS and SAXS data for structural characterization of materials at the nano scale using X-rays. The software has been developed as a massively-parallel system capable of harnessing the raw computational power offered by clusters and supercomputers built using graphics processors (GPUs), Intel Phi co-processors, or commodity multi-core CPUs. Currently the forward GISAXS simulation is a major component of HipGISAXS, which simulates the X-ray scattering process based on the Distorted Wave Born Approximation (DWBS) theory, for any given nano structures and morphologies with a set of experimental configurations. These simulations are compute-intensive, and have a high degree of parallelism available, making them well-suited for fine-grained parallel computations on highly parallel many core processors like GPUs. Furthermore, a large number of such simulations can be carried out simultaneously for various experimental input parameters. HipGISAXS also includes a Reverse Monte Carlo based modeling tool for SAXS data. With HipGISAXS we have demonstrated a sustained compute performance of over 1 Petaflop on 8000 GPU nodes of the Titan supercomputer at ORNL, and have shown it to be highly scalable.   

Hyperdynamics boost factor achievable with an ideal bias potential

Hyperdynamics has been proven to be very promising for bridging the time scale gap between simulations and experiments. Much effort has been devoted to developing valid bias potentials, however the limiting performance of hyperdynamics is still unknown. In this work, a nearly ``ideal'' bias potential is designed to study the limiting performing of hyperdynamics. This bias potential is constructed based on the minimum energy pathways (MEP) of all the pathways out of the current state. We apply this MEP-based hyperdynamics (MEP-HD) to several metallic surface diffusion systems. By using proper parameters for constructing such ``ideal'' bias potential, both the Kramers recrossings and the branch ratios of different transitions can be reproduced. Since such MEP-based bias potential is directly built on reaction coordinates, in most cases it gives boost factors that are orders of magnitude larger than the best existing bias potentials. Such impressive performance of MEP-HD is believed to be very close to the limiting performance of hyperdynamics, and shows that further development of hyperdynamics could have a significant payoff.   

Discrete Event-based Performance Prediction for Temperature Accelerated Dynamics

We present an example of a new class of tools that we call {\em application simulators}, parameterized fast-running proxies of large-scale scientific applications using parallel discrete event simulation (PDES). We demonstrate our approach with a TADSim {\em application simulator} that models the Temperature Accelerated Dynamics (TAD) method, which is an algorithmically complex member of the Accelerated Molecular Dynamics (AMD) family. The essence of the TAD application is captured without the computational expense and resource usage of the full code. We use TADSim to quickly characterize the runtime performance and algorithmic behavior for the otherwise long-running simulation code. We further extend TADSim to model algorithm extensions to standard TAD, such as speculative spawning of the compute-bound stages of the algorithm, and predict performance improvements without having to implement such a method. Focused parameter scans have allowed us to study algorithm parameter choices over far more scenarios than would be possible with the actual simulation. This has led to interesting performance--related insights into the TAD algorithm behavior and suggested extensions to the TAD method.   

Beyond the Harmonic Approximation: Lattice Dynamics and Thermal Conductivity on Massively Parallel Heterogenous Systems

The theory of lattice dynamics provides the mathematical foundation necessary to solve, exactly, the potential energy and interatomic forces of a system on a lattice, which can easily be written as functions of atomic displacements and so-called force constants. We have used this formalism to develop a highly-parallel algorithm that is capable of calculating the potential energy and interatomic forces across large computational clusters and on graphics processing units (GPUs). The necessary force constants are calculated well beyond the simple harmonic approximation (up to n-order), via a compressive sensing-based algorithm, which are then used in molecular dynamics simulations to study lattice thermal conductivity in a variety of crystal systems.   

Session M28: Superconducting Qubits: Measurement & Photodetection

Efficient Measurement of Superconducting Resonators

S-parameter measurements of high-Q superconducting resonators at single-photon drive powers often require significant averaging with associated long acquisition time. We have developed a procedure for optimizing the frequency sweep-plan of the measurement, and found that an appropriate choice of frequencies has a significant impact on its efficiency. An optimized sweep-plan design offers up to a factor of two reduction in the variance of extracted parameters, in comparison to a linear sweep-plan having the same total acquisition time. We experimentally compare the performance of the optimized and linear sweeps in measurements of high-Q aluminum CPW resonators.   

Fast Multiplexed Readout of Xmon Qubits Part I: Design

Realization of a surface code quantum computer requires fast scalable qubit readout. Previous systems have shown accurate readout in continuous wave mode. This neglects the transient response time which is crucial for the operation of the surface code and for measurement accuracy in the presence of finite qubit T1. We have designed a readout system, based on an integrated band pass filter, which achieves very fast transient response while maintaining long qubit T1. Our design uses separate readout resonators for each qubit. This allows individual qubit readout with frequency multiplexing while preventing correlated measurement errors. By connecting each resonator to a single filter the device requires zero additional on chip area and no extra control lines. We present design considerations, theory of operation, and physical layout of the device. With high fidelity gates this system forms the final element needed for a surface code cell.   

Fast Multiplexed Readout of Xmon Qubits Part II: Results

Fast and scalable qubit readout is an essential part of building a surface code based quantum computer. Here we show single- and multi-qubit frequency multiplexed readout of Xmon qubits with independent readout resonators coupled to a single readout line. We analyze both the CW behavior and the the transient response, finding that the ring-up time of the resonators is a major contribution to total readout time -- an important criterion for scalability in a fault tolerant system. Our bandpass filter design allows fast ring-up without compromising T1. We show single-qubit readout with an intrinsic fidelity of 99\% in 120~ns. Multiple-qubit readout is limited by amplifier saturation and achieves 99\% fidelity on 4 qubits in 200~ns. Correlated errors are a major problem for surface code quantum computing. We measure very low correlated errors and measurement crosstalk, which we attribute to using independent readout resonators.   

Optimizing filtering for fast measurements in circuit QED

Quantum error correction schemes, for example the popular surface code, involve running interleaved gate operations and measurements on a set of physical qubits. For this reason it is important to have fast measurements. In a fast measurement most of the information will be in the transients of the signal. In this talk we present a filtering technique to extract optimal qubit state information from the transient response of the resonator. I will also discuss techniques for rapidly driving the readout resonator to its ground state independent of the qubit state. We acknowledge support from IARPA under contract W911NF-10-1-0324.   

Ultrafast quantum nondemolition measurement based on diamond-shaped artificial atom

We present a theoretical study of a quantum nondemolition readout scheme based on a superconducting artificial atom with two internal degrees of freedom [1]. In comparison with the most widely employed readout scheme for superconducting qubits, the dispersive readout in a circuit quantum electrodynamics architecture, our approach promises a significantly stronger measurement signal. This should allow for a high-fidelity readout in a single shot. Our device consists of two transmons (i.e., small capacitively shunted Josephson junctions) coupled via a large inductance. The resulting circuit exhibits a symmetric and an antisymmetric oscillation which we use as a logical and ancilla qubit, respectively. The Josephson non-linearity leads to a cross-Kerr-like coupling of the two oscillations. This allows us to read out the logical qubit state by measuring the ancilla qubit frequency. To measure the ancilla qubit frequency, we couple it to a superconducting microwave resonator, allowing for a large amplitude and a fast response of the transmitted microwave signal. At the same time, the logical qubit remains weakly coupled and far detuned from the resonator, preventing qubit relaxation due to the Purcell effect. \\[4pt] [1] I. Diniz et al., Phys. Rev. A {\bf 87}, 033837 (2013)   

High quality superconducting resonators for QND Measurements of Qubits and Sensitive Photon Detections

We proposed an approach to implement the QND measurements of qubits by probing the intensity and phase transmissions of driven signals through a dispersively-coupled cavity. With such a technique we foud that the states of the qubits can be high-effectively reconstructed tomographically and Bell's-, Mermin's- and Svetlichny's inequalities for confirming the existences of quantum nonlocal correlations can be tested numerically. We designed and fabricated the half-wavelength- and quarter-wavelength superconducting transmission line resonators with various coupling configurations by sputtering and photolithographic techniques. The measured quality factors of these resonators are 10$^4$ and 10$^6$, respectively, at low-temperature (20mK). We have experimentally demonstrated that the fabricated resonators could be served as the desired sensitive detectors of single photons. Applications of these resonators for experimental solid state quantum information processing are possible.   

Tailoring Multiqubit Measurement Operators Through Dynamic Cavity States (Part 1)

Recent improvements in resonator coherence times in the field of superconducting qubits have allowed access to a rich new toolbox which takes advantage of their large and long-lived Hilbert space. In this talk, I introduce several techniques utilizing the cavity state and protocols built from these techniques. We condition the evolution of a cavity state via dispersive interactions with multiple qubits, and manipulate the system to implement quantum erasure, selectively reducing the space of the resulting entanglement. This can be tailored to create a spectrum of measurement operators including measurements on a selectable subset of the system. This ability is a prerequisite for most approaches to quantum error correction. The following talk will cover the experimental implementation.   

Tailoring multi-qubit measurement operators through dynamic cavity states (Part 2)

Recent improvements in cavity coherence in cQED have allowed high precision manipulation of photonic cavity states, illustrating a powerful toolbox for manipulating and encoding quantum information in either superconducting qubits or cavity states. In order for this architecture to become a viable system for computation, it will be necessary to have the flexibility to probe both global as well as limited properties of a register of qubits. In particular, the ability to tailor measurement operators is a technology that will be required for error correction. Extending the theoretical framework discussed in the previous talk, we will show experimental work toward realizing this goal. Our design consists of a register of qubits coupled to a high Q storage cavity and ancilla qubit enabled fast readout through a low Q cavity.   

Photon-number-dependent Purcell relaxation rate

We analyze the Purcell relaxation rate of a superconducting qubit coupled to a resonator, which is coupled to a transmission line and pumped by an external microwave drive. Considering the typical regime of the qubit measurement, we focus on the case when the qubit frequency is significantly detuned from the resonator frequency. Surprisingly, the Purcell rate decreases when the strength of the microwave drive is increased. This suppression becomes significant in the nonlinear regime. The microwave drive also causes excitation of the qubit; however, the excitation rate is much smaller than the relaxation rate. Our analysis also applies to a more general case of a two-level quantum system coupled to a cavity.   

Low-Power Dispersive Measurements of High-Coherence Flux Qubits

We report on progress towards nondestructive dispersive measurements of a high-coherence flux qubit. A~capacitively shunted flux qubit that incorporates high-Q MBE aluminum will have longer relaxation and dephasing times when compared to a conventional flux qubit, while also maintaining the large anharmonicity necessary for complex gate operations. We numerically investigate the expected measurement fidelity of the improved qubit and present measurements that explore the boundary between destructive and nondestructive dispersive readout. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research {\&} Engineering under Air Force Contract number FA8721-05-C-0002.~ All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government   

Large gain quantum-limited qubit state measurement using a two mode nonlinear cavity

A single nonlinear cavity dispersively coupled to a qubit functions as a large gain detector near a bifurcation, but also has an unavoidable large backaction that prevents quantum-limited measurement at weak couplings [1]. We show theoretically that a modified setup involving two cavities (one linear, one nonlinear) and a dispersively coupled qubit allows for a far more optimal measurement. In particular, operating near a point of bifurcation, one is able to both achieve a large gain as well as a near quantum-limited backaction. The increased system flexibility also enables large measurement rate and smaller nonlinear shot noise dephasing than is possible with single nonlinear cavity setups. We present analytic results for the gain and noise of this detector and a heuristic understanding of the physics, thus presenting a complete description of this new way of performing weak qubit state measurements. The setup we describe can easily be realised in experiments with superconducting circuits involving Josephson junctions [2, 3].\\[4pt] [1] C. LaFlamme, A.A. Clerk, Phys. Rev. A 83, 033803 (2011)\\[0pt] [2] F.R. Ong et al., Phys. Rev. Lett. 106, 167002 (2011)\\[0pt] [3] M. Hatridge et al., Phys. Rev. B 83, 134501 (2011)   

Catching Classical Shaped Microwave Photons in a Superconducing Resonator with 99.4\% Efficiency

Deterministic quantum state transfer requires receivers to transfer quantum states from traveling qubits to fixed logic qubits. Reflections must be minimized to avoid energy loss and phase interference. Here, we classically drive a 6GHz superconducting coplanar resonator with tunable coupling to the drive/readout line while we measure the reflected and captured signals with a HEMT amplifier. Using an exponentially increasing drive pulse, we demonstrate a 99.4\% deterministic single photon absorption efficiency (97.4\% receiver efficiency). We further demonstrate that experimental absorption efficiencies agree with theory within 3\% for various pulse parameters and shapes. With the fidelity now at the error threshold for fault tolerant quantum communication (96\%) and computation (99.4\%) and comparable to fidelities of good logic gates and measurements, new designs may be envisioned for quantum communication and computation systems.   

Qubit coupling to superconducting whispering gallery mode resonator

A protected quantum register composed of a high quality mode coupled to a quantum bit and a fast readout mode promise a hardware efficient and technically realizable module-based quantum network [Science 339, 1169 (2013); PRL 111, 120501 (2013)]. Such a module is designed in an integrated manner by embedding a quantum bit inside the clean environment of a superconducting whispering gallery mode resonator. Its two orthogonal modes can have a large asymmetry in coupling to a microwave transmission line, thus realizing a storage and readout mode.   

Graph theory and nonreciprocity in coupled-mode systems

Coupled-mode systems involving more than 2 interacting modes can break reciprocal symmetry and unidirectional mode conversion can be observed. This is the case, for example, in multiple-pump parametric processes and in superconducting DC-SQUID amplifiers.\footnote{A. Kamal, \textit{et al.} ``Noiseless non-reciprocity in a parametric active device'' \textit{Nature Physics}, 7.4, (2001): 311-315} While reciprocity in dual-mode systems can be broken only in a sequenced coupling scheme, a sequence is not required in systems with more than 2 interacting modes. The analysis of such systems is, however, extremely complex when a high number of coupled modes is involved. In this talk we are going to discuss how graphs can be used to analyse reciprocity in coupled-mode systems and reveal the conditions that need to be satisfied for reciprocity to be broken. In this representation modes are associated to vertices and couplings to edges in an abstract graph. General conditions for reciprocity can be determined from the connectivity of the graph.   

Probe susceptibility of a strongly driven qubit

The characteristic of a quantum system change upon coupling it to a driving field. In a typical circuit QED setup, driving is implemented with microwaves and the driven superconducting qubit is probed via a coupled LC cavity whose transmission or reflection properties are measurable. However, the entanglement between the qubit and the driving field leads to the formation of quasienergy states and, thus, modifications of the probe response. Even if the non-driven qubit-cavity system lies in the dispersive regime, the response of the driven system may be better described by an absorptive approach[1]. We have shown by using the Floquet formalism and a calculation reminiscent of the Fermi's golden rule that the concept of the probe susceptibility can be extended to strongly driven quantum systems[2,3]. Furthermore, we have applied these results to interpret and explain accurately a measurement where a charge qubit is strongly driven via the Josephson nonlinearity and probed via an LC cavity[3]. [1] J. Tuorila et al., Phys. Rev. Lett. 105, 257003 (2010). [2] M. Silveri et al., Phys. Rev. B 87, 134505 (2013). [3] J. Tuorila et al., Supercond. Sci. Technol. 26, 124001 (2013).   

Session M29: Quench dynamics

Quantum quenches of cold-atom gases in optical lattices: the influence of Anderson localization

We consider the following kind of non-equilibrium experiment. An ultracold fluid of fermions is prepared in a potential consisting of three parts: an optical lattice; a short-range-correlated disorder potential of finite strength; and a shallow harmonic trapping potential. After the fluid has equilibrated, the minimum of the harmonic potential is suddenly ``jumped'' to the side by a finite distance, $d$. The observables of interest are the subsequent evolution of the density distribution and phase correlations in the fluid. This kind of experiment is theoretically interesting because it contains two energy-dependent length scales: the localization length of the single-particle orbitals due to the disorder potential, $\xi$; and the ``Bragg localization length'' of the single-particle orbitals due to the combined effect of the harmonic trap and optical lattice, $l_B$. We present numerical results on the evolution of the density distributions and phase correlations in such cases, for a range of strengths of the disorder. In addition, we provide an approximate analytical framework for understanding our results in terms of the relative size of the length scales $\xi$ and $l_B$ at the Fermi energy. Possibilities for further work are also discussed.   

Quantum quenches and work distributions in ultra-low-density systems

In our contribution we present results on quantum quenches in systems with a fixed number of particles in a large volume, the situation accessible in cold atom experiments. We show that the typical differences between local and global quenches present in systems with regular thermodynamic limit are lacking in this low-density limit. In particular, we show that local and global quenches can have power-law work distributions (edge singularities) typically associated with only local quenches for finite-density systems. We show that this regime allows for large edge singularity exponents beyond that allowed by the constraints of the usual thermodynamic limit (e.g., by Anderson's orthogonality catastrophe). This large-exponent singularity has observable consequences in the time evolution, leading to a distinct intermediate power-law regime in time. We demonstrate these results using local quantum quenches in a low-density Kondo-like system, and additionally through global and local quenches in Bose-Hubbard, Aubry-Andre, and hard-core boson systems in the low-density regime.\\[4pt] [1] Y.E. Shchadilova, P. Ribeiro, M. Haque, Quantum quenches and work distributions in ultra low density systems, arXiv:1303.4103, (2013).   

Quantum quench from classical evolution: the fate of a soliton

In a quantum quench, one prepares a system in an eigenstate of a given Hamiltonian, and then lets it evolve after suddenly changing a control parameter of the Hamiltonian. By observing this evolution, one tries to understand whether and how a quantum system reaches a (thermal) equilibrium. Normally, the initial state is taken to be the ground state: we propose a different experimentally feasible protocol, in which the system is prepared in an excited state corresponding to a collective solitonic excitation. If we are interested only in the single particle density, the time evolution can be reduced to the study of a semi-classical non-linear differential equation. We study both integrable and non-integrable systems, in a confining (parabolic) potential and on a ring. The short time dynamics is universal, while the long time configuration depends on the system.   

Correlations after a quantum quench in the Bose Hubbard model

Recent experimental advances that allow for the atomic resolution of dynamics for cold atoms in optical lattices call for theory to describe these dynamics. We use the Schwinger-Keldysh technique to study time and space dependent correlations after a quantum quench in the Bose Hubbard model. We focus on the case of time dependent hopping and use a real-time action that allows for the description of both the superfluid and Mott insulating phases to obtain dynamical equations for these correlations. We relate our results to recent experiments.   

Particle-hole pair excitations in Mott insulator quench dynamics

We investigate the dynamics of strongly interacting bosons in an optical lattice in a quantum quench scenario where we start from a Mott insulator state and suddenly raise the lattice depth. Despite the nature of short-range coherence in the Mott state, we find that the coherence visibility exhibits collapse and revival oscillations which could be observed in experiments. The quasi-momentum distribution oscillates between a maximum occupation at $k=0$ (during revivals) and $k=\pi$ (during collapse). We show that the $k=\pi$ revivals are caused by the presence of particle-hole pair excitations on top of a constant Mott background. We further show that similar effects are found in other lattice models such as with fermions and Bose-Fermi mixtures. We provide a general framework and point to a new avenue to probe strongly correlated many-body states going beyond the superfluid paradigm of collapse and revivals.   

Bloch oscillations and quench dynamics of interacting bosons in an optical lattice

We study the dynamics of interacting superfluid bosons in a one dimensional vertical optical lattice after a sudden increase of the lattice potential depth. We show that this system can be exploited to investigate the effects of strong interactions on Bloch oscillations. We perform theoretical modelling of this system, identify experimental challenges and explore a new regime of Bloch oscillations characterized by interaction-induced matter-wave collapse and revivals. In addition, we study three dephasing mechanisms: effective three-body interactions, finite value of tunneling, and a background harmonic potential. We also find that the center of mass motion in the presence of finite tunneling goes through collapse and revivals, giving an example of quantum transport where interaction-induced revivals are important. We quantify the effects of residual harmonic trapping on the momentum distribution dynamics and show the effects of interactions on the temporal Talbot effect. Finally, we analyze the prospects and challenges exploiting Bloch oscillations of cold atoms in the mean-field regime for precision measurement of the gravitational acceleration $g$.   

Quantum ratchets, the orbital Josephson effect, and chaos in Bose-Einstein condensates

In a system of ac-driven condensed bosons we study a new type of Josephson effect occurring between states sharing the same region of space and the same internal atom structure. We first develop a technique to calculate the long-time dynamics of a driven interacting many-body system. For resonant frequencies, this dynamics can be shown to derive from an effective time-independent Hamiltonian which is expressed in terms of standard creation and annihilation operators. Within the subspace of resonant states, and if the undriven states are plane waves, a locally repulsive interaction between bosons translates into an effective attraction. We apply the method to study the effect of interactions on the coherent ratchet current of an asymmetrically driven boson system. We find a wealth of dynamical regimes which includes Rabi oscillations, self-trapping and chaotic behavior. In the latter case, a full quantum many-body calculation deviates from the mean-field results by predicting large quantum fluctuations of the relative particle number. Moreover, we find that chaos and entanglement, as defined by a variety of widely used and accepted measures, are overlapping but distinct notions.   

Many-body Bloch oscillations

We consider Bloch oscillations of interacting quantum particles in a one-dimensional lattice subject to a linear potential gradient (a tilt). For hard-core bosons and for free fermions, we show perfectly periodic behavior of density and momentum distributions. The oscillations can be predominantly position oscillations, or predominantly width oscillations, depending on the initial state. We show how the periodic behavior is modified for weak and strong interactions.   

Quench dynamics of a strongly interacting resonant Bose gas

We explore the dynamics of a Bose gas following its quench to a strongly interacting regime near a Feshbach resonance. Within a self-consistent Bogoliubov analysis we find that after the initial condensate-quasiparticle Rabi oscillations, at long time scales the gas is characterized by a nonequilibrium steady-state momentum distribution function, with depletion, condensate density and contact that deviate strongly from their corresponding equilibrium values. These are in a qualitative agreement with recent experiments on $^{85}$Rb by Makotyn. Our analysis also suggests that for sufficiently deep quenches close to the resonance the nonequilibrium state undergoes a phase transition to a fully depleted state, characterized by a vanishing condensate density.   

Many-body dynamics of a BEC quenched to unitarity

The dynamics of a dilute BEC quenched to unitarity are studied using a variational ansatz for the many-body quantum state. Despite the resonant atom-atom interactions, the condensate does not deplete instantaneously, and this allows for a self-consistent mean-field-like description of the system at short (but experimentally-accessible) times. At infinite scattering length and zero temperature, the dynamics are found to scale universally with the number density, as reported in the experiment of Makotyn et al, arXiv1308.3696. We predict the time evolution of observables such as the momentum distribution $n_k(t)$, the contact $C(t)$, and the density $n_m(t)$ of Feshbach molecules generated by the interaction quench. We observe a saturation of large-momentum populations on a time scale that is consistent with recent measurements.   

Quenching to unitarity: Quantum dynamics in a 3D Bose gas

We study the dynamics of a zero temperature Bose condensate following a sudden quench of the scattering length from noninteracting to unitarity (infinite scattering length). In this talk we discuss how a qualitative understanding of the dynamics can be built up by understanding few-body physics under the same dynamical scenario. We calculate the coherent evolution of the momentum distribution, particularly focusing on the time dependence of the contact. By comparing the results to a many-body mean-field calculation, we gauge the qualitative and quantitative accuracy of this approach. We then discuss the results of a three-body calculation, in which loss dynamics occurs due to three-body recombination. One the key results of this work indicates that loss dynamics takes place over a much longer timescale than the coherent dynamics. This exciting result supports the idea that meta-stable degenerate unitary Bose gases may be experimentally observable in such a non-equilibrium scenario.   

Equilibrating Dynamics in Quenched Bose Gases

Recent interaction quench experiments in cold bosonic gases are challenging our understanding of out-of-equilibrium dynamics of quantum systems. In particular, the cross-over between short-time (strongly out-of-equilibrium) and long-time equilibration (to a (meta-)stable state) is a complicated problem that needs to be addressed in order to understand the multiple time scales (associated with oscillations, equilibration, etc.), and their momentum dependence in these experiments. In this talk, we present a model that simulates the out-of-equilibrium dynamics of a condensed Bose gas, which importantly allows for the ultimate equilibration of the system via a coupling to a bath [Phys. Rev. A (88) 031601 (2013)]. We show why (as in quench experiments) large k, high energy states equilibrate more rapidly than those at small k. In this context we discuss the implications for calculations and measurements of the Tan contact. We finally address the question of how the intermediate time dynamics can, in principle, reflect the presence or absence of a condensate.   

Dynamics of phase separation and coarsening in binary Bose-Einstein condensates

Cold quantum gases provide an ideal testbed to study the out-of-equilibrium dynamics of quantum systems. We consider the nonequilibrium dynamics of a coupled binary mixture of Bose-Einstein condensates. Depending on the coupling between the two components, the system can exist in either a miscible or a phase-separated ground state, which are separated by a quantum phase transition. We present results on the dynamics of domain formation and coarsening after a quench across this phase boundary.   

Small Quench Dynamics as an Investigative Tool for Cold Atom Systems

Finite one-dimensional systems of bosons or fermions described by the Hubbard model can be realized using cold atoms confined in an optical lattice. The ground states of these systems are often characterized by a coexistence of phases when a non-homogeneous trapping potential is applied. We propose to analyze this phase coexistence by studying the out-of-equlibrium dynamics following a sudden quench. In particular we show that the temporal variance of the local densities is able to spot the boundaries between the different phases. The feasibility of this approach is demonstrated for several Hamiltonians using numerical simulations. We first consider an integrable system, hardcore bosons confined by quadratic or quartic trapping potentials, where Mott and superfluid phases are coexistent. We also analyze a non-integrable system, a $t-VV'$ model which has a charge density wave phase coexisting with a superfluid one when subjected to a quadratic confining potential. We find that the temporal variance is more effective than other standard indicators of phase boundaries such as the local compressibility or density fluctuations.   

Quantum quenches in 1D Bose Gases: Glimmers of Quantum KAM

Using a numerical renormalization group based on exploiting an underlying exactly solvable non-relativistic theory, we study the out-of-equilibrium dynamics of a 1D Bose gas (as described by the Lieb-Liniger model) released from a parabolic trap into a weak cosine potential. The presence of the cosine potential breaks integrability and leads the formerly conserved charges of Lieb-Liniger to be time dependent. We however argue that from these charges we are able to construct approximately conserved quantities despite the presence of the cosine term. How good the time invariance of these quasi-conserved quantities is can be related to the strength of the post-quench cosine potential and the width of this perturbation in Fourier space. This gives then an analog in a quantum example to the classical KAM theorem.   

Session M30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Sensing using 2D Materials

Controlled healing of graphene nanopore

Graphene is often mentioned as a promising material for nanopores applications in DNA sequencing, sensory, biosensoring and molecular detectors. We will present realistic computer simulation studies of regrowth and healing of graphene nanopores of different sizes ranging from $30$ to $5$~\AA. Our simulations clearly point to at least two distinct healing mechanisms of graphene sheet: one so called edge attachment mechanism, where carbons are attached to the edges of graphene sheet and second, the direct insertion mechanism, that involves atom insertion directly into a sheet of graphene even in the absence of the edges. Insertion mechanism is a suprising prediction that points to the growth process that would be operational even in the pristine graphene. We have uncovered an unusual dependence in the speed of nanopores regrowth and structure of ``healed'' areas as function of its size in the wide range of temperatures. Our findings point a significantly more complicated pathways for graphene annealing. They also provide an important enabling step in development of graphene based devices for numerous nanotechnology applications.   

Hybrid graphene nanoribbon-nanopore devices for biomolecule detection and DNA sequencing

We present a study of hybrid graphene nanoribbon-nanopore devices for biomolecule detection and ultimately DNA sequencing. We realized back or side gated devices comprised of nanopores(2$-$10 nm) at the edge or in the center of GNRs with widths of 5-200nm, on SiNx membranes. Electron beam-induced irradiation effects[1] are studied by in situ conductance measurements during nanopore formation inside a 200kV transmission electron microscope (TEM) for different doping levels. Bases on our findings we devise a scanning TEM procedure which prevent the GNR electron induced damage, enabling sensitive biosensors. We finally present the operation of this sensor for biomolecule detection and DNA sequencing. The higher current ($\mu $A) driven through a GNR compared to the ionic current(nA) in nanopore devices leads to a hundredfold increase in the measuring bandwidth(10-100MHz), possibly enabling DNA sequencing without slowing the molecules. [1] Towards sensitive graphene nanoribbon-nanopore devices by preventing electron beam induced damage. M. Puster, J. A. Rodriguez- Manzo, A. Balan, M. Drndic. ACS Nano,10.1021/nn405112m.   

Graphene Trans-Electrode Membranes

We report an electrical study of suspended single-layer graphene membranes separating reservoirs of electrolyte solution. Because the opposing reservoirs are separated only by an atomically thin membrane, the trans-conductance (ionic current response to a voltage across the membrane) is extremely sensitive to nanoscale defects in the membrane. This sensitivity allows the precise examination and characterization of intrinsic defects in graphene membranes, as well as engineered defects for devices. We will discuss methods for creating single nanopores or distributed defects in our graphene membranes, with the applications of nanopore DNA sequencing and water desalination in mind.   

Evidence for Stochastic Switching of Transport through Molecular-Sieving Graphene Membranes

Two dimensional materials represent an emerging class of gas transport membranes capable of ultrahigh fluxes with molecular sieving potential. Herein, we study gas transport through atomically thin, monolayer graphene membranes open with a single (or several) molecularly sized, sub-nm pores by ozone activation and UV etching. We provide the first evidence for stochastic switching of permeance states through such membranes made from monolayer graphene during CO$_{\mathrm{2}}$~transport. This switching is analyzed using a Hidden Markov Model to estimate the activation barriers of switching. Further evidence is provided using gold clusters formed on the surface of the graphene. Such clusters migrate and partially block the pore upon laser heating in vacuum. This work represents the first example of controlling gas phase transport through molecularly sized pores.   

Modification of electrical transport properties of graphene field effect devices due to electron-mediated molecular adsorption

We study graphene field effect transistor devices that have been exposed to electron beam irradiation. Upon irradiation in vacuum, the Dirac point shifted to negative gate voltage, and in-situ electrical transport measurements indicate the emergence of gate voltage hysteresis of the graphene devices. Both the Dirac point and the hysteresis revert towards their pre-irradiation status over the course of a few days when the graphene is maintained under vacuum. Once the irradiated devices are exposed to ambient air, the original Dirac point was recovered within two hours and the hysteresis disappeared. However, transport properties were not fully recovered but instead degraded depending on electron dosage. In addition, as a result of the irradiation the Raman `D' band, which is an indication of defect generation, emerged and its intensity increased with increasing electron dosage. In addition, we investigated the adsorbate on graphene by atomic force microscopy. The state of the adsorbate on graphene was observed to change with electron dosage indicating that redox coupling is a likely cause of both the Raman defect signal as well as the scattering centers that deteriorate the transport properties.   

Ab initio energetics, kinetics, and quantum transport characteristics of graphene nanoribbons as nanosensors for detecting nitrogen dioxide

Molecules adsorption on graphene nanoribbons (GNRs) can be used to engineer and make use of their properties for applications such as energy storage and sensors. We investigate adsorption characteristics by considering nitrogen dioxide as a sample molecule for assessing nanosensor functionality of GNRs. Using ab initio modeling, energetics of various adsorption possibilities are determined and their rate constants are calculated and compared. Nonbonding and weak sp3 adsorptions at the hydrogen-terminated edges are shown to be more feasible than center adsorptions. This shows increased reactivity compared to graphene. Calculated quantum transport responses upon molecules adsorption indicate possibility of sensing extremely low nitrogen dioxide concentrations. Possible approaches for improving gas nanosensor functionality of GNRs are discussed. Reference: RSC Advances, 2013, DOI: 10.1039/c3ra46372a.   

Patterning, Characterization and Chemical Sensing Applications of Graphene Nanoribbon Arrays Down to 5 nm Using Helium Ion Beam Lithography

Bandgap engineering of graphene is an essential step towards employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of \textless 10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization and superior chemical sensing of ultra-narrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO2 gas sensitivity of the 5 nm GNR array devices down to ppb levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications.   

High Performance Chemical Sensing Using Schottky-Contacted CVD Grown Monolayer MoS2 Transistors

Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of chemical vapor deposition grown monolayer MoS2 as high performance chemical sensors with Schottky contacts. The Schottky-contacted MoS2 transistors show current changes by two to three orders of magnitude upon exposure to NO2 and NH3. The MoS2 sensors show clear detection of NO2 down to 20 ppb and NH3 down to 1 ppm, both of which are the best among various monolayer or few-layer MoS2 and other 2D transition metal dichalcogenides materials based chemical sensors reported so far. We attribute the observed high performance to both well known charger transfer mechanism and more importantly, the Schottky barrier modulation upon analyte molecules adsorption, the latter of which is made possible by the Schottky contacts in our transistors and is not identified previously for MoS2 sensors. This study may open up new ways for 2D semiconductors as sensors and also may benefit the fundamental studies of interfacial phenomena and interactions between various chemical species and monolayer semiconductors.   

Electrochemistry and molecular sensing in layered materials

We combine experimental and computational techniques to study electrochemistry in solvated MoS$_2$. We show that MoS$_2$ has a high catalytic activity for important industrial reactions. In a similar way, we investigate electronic transport properties of functionalized graphene with adsorbed molecules. We show that well designed nanostructures can lead to novel detection mechanisms with a very high molecular sensitivity.   

Session M31: Focus Session: Computational Discovery and Design of New Materials IV

Design of Metamaterials for control of electromagnetic waves

Metamaterials are artificial effective media supporting propagating waves that derive their properties form the average response of deliberately designed and arranged, usually resonant scatterers with structural length-scales much smaller than the wavelength inside the material. Electromagnetic metamaterials are the most important implementation of metamaterials, which are made from deeply sub-wavelength electric, magnetic and chiral resonators and can be designed to work from radio frequencies all the way to visible light. Metamaterials have been major new development in physics and materials science over the last decade and are still attracting more interest as they enable us to create materials with unique properties like negative refraction, flat and super lenses, impedance matching eliminating reflection, perfect absorbers, deeply sub-wavelength sized wave guides and cavities, tunability, enhanced non-linearity and gain, chirality and huge optical activity, control of Casimir forces, and spontaneous emission, etc. In this talk, I will discuss the design, numerical simulation, and mathematical modeling of metamaterials. I will survey the current state of the art and discuss challenges, possible solutions and perspectives. In particular, the problem of dissipative loss and their possible compensation by incorporating spatially distributed gain in metamaterials. If the gain sub-system is strongly coupled to the sub-wavelength resonators of the metamaterial loss compensation and undamping of the resonant response of the metamaterials can occur. I will explore new, alternative dielectric low loss resonators for metamaterials as well as the potential of new conducting materials such as Graphene to replace metals as the conducting material in resonant metamaterials. Two dimensional metamaterials or metasurfaces, implementations of effective electromagnetic current sheets in which both electric and magnetic sheet conductivities are controlled by the average response of sub-wavelength local resonators, emerge as simpler implementation of many of the unique properties of metamaterials. I will discuss a few novel examples how these metamaterials can be used for dispersion engineering, and beam shaping.   

Computational nano-materials design of high efficiency photovoltaic materials by spinodal nano-decomposition in Chalcopyrite-type semiconductors

Chalcopyrite-type semiconductor CuInSe2 (CIS) is one of the most promising materials for low cost photovoltaic solar-cells due to its self-regeneration mechanism. However, from the point of resource security, high concentration of In in CIS is serious disadvantage. Recently, Cu2ZnSnS4 (CZTS) attracts much attention to overcome this disadvantage of CIS. This material has already been investigated as a photovoltaic material but the efficiency is not high enough. Based on the first-principles calculations by the KKR-CPA method, we propose how we can enhance the efficiency of CZTS by utilizing the self-organization phenomena caused by spinodal nano-decomposition of Cu \& Cu-vacancy, S \& Se, and Se \& Oxygen [1]. We will compare our design with the available experimental data of STEM-EDX, EELS, Atom Probe Tomography and Raman Scattering data. In addition to the above materials design, we also discuss intermediate band type solar-cells caused by the spinodal nano-decomposition, and propose Fe-doped CuFeS2-CuAlS2 (CFS-CAS), CuFeS2-CuGaS2 (CFS-CGS) and CuFeS2-CuInS2 (CFS-CIS) as promising materials with enhanced conversion efficiency up to 50\%.\\[4pt] [1] Y. Tani et al., Appl. Phys. Express 3 (2010) 101201. Jpn. J. Appl. Phys. 51 (2012) 050202.   

Understanding Electronic, Optical and Thermal Properties of Transition Metal Chalcogenides (TMCs)

The fundamental properties of a material depend on their atomic structure, nature of bonding and elemental/chemical composition. Confinement of electrons in 2 dimensional planar structures leads to realization of several intriguing properties that are not seen in the bulk 3-dimendional counterparts. In this work, we explore the properties of single and few layer MX (M:Transition metal, X: chalcogen atom) both theoretically and experimentally. Using state of art density functional theory (DFT) we carried out a stability analysis through phonon and electronic, magnetic and elastic structure calculations where M$=$Cu, and X$=$S, Se, Te. The stacking of transition metal chalcogenide (TMC) monolayers is of the type MX-M2X2 instead of the usual X-M-X stacking found in TMDs. The differences in geometric structure result in many different stable monolayer forms with different electronic and magnetic properties. Depending on the number of layers, MX structures can be found in 2, 3, 4 and 6 MX layer stable configurations. These dimensionality effects predicted by DFT such as energy band structures and Raman active modes are confirmed by~experiments. Various different monolayers of MX possess a number of properties that make them highly promising materials for future nanoscale applications.   

Inverse Design of Materials by Multi-Objective Differential Evolution($IM^2ODE$)

Inverse design is a new approach in the realm of material science for finding the structure with desired property. We developed a novel algorithm for inverse design named as $IM^{2}ODE$ (Inverse Design of Materials by Multi-Objective Differential Evolution). The target properties of concern include optical and electrical properties of semiconductors, solar absorbers and hardness of materials. $IM^{2}ODE$ can easily predict the atomic configurations with desired properties for crystal structures, interfaces and clusters. This novel method has been applied successfully in predicting new titanium dioxide ($\textrm{TiO}_2$) polymorph with optimal band gap for solar cell application.   

Design of molecular sidechains to enhance thermal conductivity

Higher thermal conductivity polymer composites would provide lighter and cheaper materials for large scale industrial applications as well as improve heat dissipation on the microscopic scale in electronics. Carbon nanotubes and graphene have high intrinsic thermal conductivity, but their large interface thermal resistance prevents their use in polymer composites. We investigate the design of molecular sidechains built from selectively chlorinated and/or fluorinated carbons which have the advantage of a higher linear mass density than an alkane chain and are expected to be quite stiff. We present results of a search for an optimal configuration of a sidechain consisting of a number of chlorinated carbon, fluorinated carbon, and simple hydrogenated carbon units that maximizes heat flow. This search will involve concepts from electron transport generalized to the study of phonon transport.   

Efficiency enhancement due to self-organization of nano-structures in Cd(S, Te) solar cell material

CdTe is one of the most important solar cell materials. Its energy gap is 1.44 eV, which is ideal for solar cell application. So far, conversion efficiency of 18.3 percent has been realized, but it is lower than the Shockley-Queisser limit. In this paper, we propose computational materials design for enhancing conversion efficiency by using self-organization in Cd(Te, S) alloy semiconductor. Firstly, we performed cluster expansion of total energy of the Cd(Te, S) system and simulated self-organization of nano-structures in Cd(Te, S) by using Monte Carlo method. It is found that layered structure becomes stable by applying strain during the crystal growth. The electronic structure of the self-organized layered structure was calculated by using the hybrid method (HSE06) implemented in the VASP code to derive optical absorption coefficient. By using the calculated absorption coefficient the efficiency limit was derived based on the Shockley-Queisser theory. It is shown that the efficiency limit does not change so much due to the nano-structure formation. However, our calculation shows spatial separation between photo-generated electrons and holes. This might enhance the efficiency due to the suppression of recombination.   

Is Orthorhombic C32 Actually a New Metastable Allotropic Form of Carbon?

Carbon hybridizes in different geometries ($sp$, $sp^2$, and $sp^3$) forming a number of well-known allotropic forms such as: cubic diamond, graphite, $C_{60}$, graphene, hexagonal diamond, and amorphous carbon. With the advent of novel computational optimization tools many other candidate allotropic forms for carbon (e.g. monoclinic M-carbon, body-centered tetragonal C4 carbon (bct-C4), orthorhombic W-carbon, and Z-carbon) have been proposed which might be metastable at high pressures. In fact, the M-carbon structure has been experimentally seen with Raman spectroscopy. Recently, Zhang {\it et al.}\footnote{M. Zhang, H. Liu, Y. Du, X. Zhang, Y. Wang, and Q. Li, {\it Phys. Chem. Chem. Phys. {\bf15,}} 14120 (2013).} have reported the theoretical existence of a new allotropic form of carbon which they named: orthorhombic C32. In this work we have discovered that orthorhombic C32 is actually {\bf not} a novel carbon allotropic form of carbon, but it is simply hexagonal diamond decorated with defect planes separated by arbitrary distances. We have used first-principles DFT calculations to compute the energies and phonon spectra for these structures and compared our results with an extensive library of other possible metastable allotropic forms of carbon from the literature.   

Ab-initio study of the electronic structure and optical properties for carbon in the glitter phase

Experimental evidence has showed the existence of a new crystalline phase of carbon. Electron diffraction studies show that this new phase of carbon has the same reflections that diamond but showing additional reflections that are forbidden for diamond. This new carbon has been called n-diamond. Although the results suggest that n-diamond correspond to a cubic phase, the crystal structure remains unclear. Thus, based on theoretical computational studies have been proposed different cubic structures to explain the observed diffraction patterns in n-diamond. However, recently has been proposed that the n-diamond could be explained by a tetragonal structure, which is called glitter. More recently, based on ab-initio calculations has been shown that the glitter structure is vibrationally stable. In this work, we study the electronic structure and optical properties of carbon in the glitter structure by means of first principles calculations. The electronic density of states of that carbon in glitter structure corresponds to a metallic material which is corroborating by the optical conductivity.   

Atomic structures of magic ZnSe clusters from first principles calculation

We report the atomic and electronic structures of {\it magic} (ZnSe)$_n$ ($n$ = 13, 33, and 34) clusters, employing first principles technique based on a pseudopotential approach. These sizes are important as laser ablated plumes of ZnSe have clusters with ($n$) = 6, 13, 19, 23, \& 33 ZnSe molecular units in high abundance suggesting their high stablity and magic behavior. Earlier we had predicted the atomic structures of these clusters to be filled cage structures with a Se centered 3-D structure for $n$ = 13 and a cage/core structure for $n$ = 33 \& 34. In the later two cases, a core of Zn$_5$Se$_5$ and Zn$_6$Se$_6$, respectively, is enclosed by a Zn$_{28}$Se$_{28}$ cage to form a 3-D structure. In contrast to ZnSe clusters, ZnO clusters in this size range have empty cage structures. Therefore, we have performed further calculations using both, GGA-PBE and hybrid HSE06 type of exchange-correlation functionals that suggest that our conclusion for the size $n$ = 13 remains unchanged, but for larger clusters of sizes $n$ = 33 \& 34, hollow cage stuctures made up of 4- and 6-membered rings of ZnSe, are energetically more favourable than the filled cage structures. We shall discuss the trends in the electronic structure, binding enery, and HOMO-LUMO gap, as we vary the ZnSe size.   

Prediction of elastic and vibrational stability for Sc, Ti, Y, Zr, Tc, Ru, Hf, Re, and Os in the fcc structure

The discovery of a metastable phase for a given material is interesting because corresponds to a new bonding and new properties are expected. The calculation of the total-energy along the Bain path is frequently used as a method to find tetragonal metastable states. However, a local minimum in the tetragonal distortion is not a definitive proof of a metastable state, and the elastic and vibrational stability needs to be evaluated. In a previous work, using the elastic stability criteria for a cubic structure, we have shown that the transition metals with hcp ground state; Ti, Zr, and Hf have a fcc metastable phase. That result is interesting since the fcc crystal structure does not appear in the current pressure-temperature phase diagram of these metals, and support the experimental observations of fcc Ti and Zr in thin films. In the present work, we extend the stability study of the fcc structure to the non-magnetic transition metals with hcp ground state; Sc, Ti, Y, Zr, Tc, Ru, Hf, Re, and Os. We find that all the metals involved in this study have a metastable fcc structure, since the phonon band structure shows only positive frequencies. Finally, substrates on which the fcc structure of these metals could be growth epitaxially are predicted.   

Generalized Kanzaki-Krivoglaz model of lattice relaxations in concentrated size-mismatched substitutional alloys applied to Cu-Au and Fe-Pt systems

A generalization of the Kanzaki-Krivoglaz model to concentrated alloys was developed and applied to Cu$_{1-x}$Au$_x$ and Fe$_{1-x}$Pt$_x$ alloys at x = 0.25, 0.5, and 0.75. This model is based on many-body cluster expansions of the configuration-dependent Kanzaki forces and force constants defined with respect to the ideal fcc lattice. The parameters of these expansions were directly fitted to the forces calculated from first-principles for a number of ordered structures at fixed concentration and volume. The Kanzaki forces are dominated by nearest-neighbor terms, which are strongly asymmetric between the atomic species. This asymmetry leads to a non-pairwise effective interaction with a long-range elastic singularity. The ability to capture this singular non-pairwise interaction accurately is a major advantage of the generalized Kanzaki-Krivoglaz model. The comparison of the predicted stable phases and ordering temperatures with experiment is generally favorable; while the prediction for Cu$_{0.25}$Au$_{0.75}$ is wrong due to a known failure of semi-local functionals, the remaining discrepancies for Cu$_{0.5}$Au$_{0.5}$ and Fe$_{0.25}$Pt$_{0.75}$ are attributed to the contributions from the strong tetragonal striction in the L1$_0$ phase and of magnetic disorder, respectively.   

On relationship among composition, electronic structure and reactivity of catalytically active monolayers on metal substrates

Rational design of efficient electrocatalysts requires understanding of the relationship among the surface composition, its electronic structure, reactivity and catalytic activity. In this work by applying the first principle computational approach, we reveal the nature of the substrate effects on the electronic structure and reactivity of active monolayers (AM) deposited on a metal substrate (MS). In particular, we consider the Pt/MS structures (MS$=$Au, Ir, Ru, or Pt substrate). We reveal rationale for the interlayer hybridization to dominate over the strain effect in determining the AE/MS surface reactivity, in contrast to a widely accepted opinion that a strain is the main factor controlling the reactivity. We also find that, if AE is weakly bound to MS, the surface electronic structure does not suffice to characterize the surface reactivity, because of involvement of lattice response to adsorption of a reaction intermediate. We trace surface reactivity to a newly introduced hybridization parameter that reflects important features of the electronic structure of the AE/MS surface, which are not taken into account in the original $d-$band center model.   

Session M32: Invited Session: Semiconductor Qubits

Hybrid Circuit QED with Double Quantum Dots

Cavity quantum electrodynamics explores quantum optics at the most basic level of a single photon interacting with a single atom. We have been able to explore cavity QED in a condensed matter system by placing a double quantum dot (DQD) inside of a high quality factor microwave cavity. Our results show that measurements of the cavity field are sensitive to charge and spin dynamics in the DQD.\footnote{M. D. Schroer, M. Jung, K. D. Petersson, and J. R. Petta, ``Radio frequency charge parity meter,'' Phys. Rev. Lett. \textbf{109}, 166804 (2012).}$^,$\footnote{K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck, and J. R. Petta,``Circuit quantum electrodynamics with a spin qubit,'' Nature (London) \textbf{490}, 380 (2012).} We can explore non-equilibrium physics by applying a finite source-drain bias across the DQD, which results in sequential tunneling. Remarkably, we observe a gain as large as 15 in the cavity transmission when the DQD energy level detuning is matched to the cavity frequency. These results will be discussed in the context of single atom lasing.\footnote{Y.-Y. Liu, K. D. Petersson, J. Stehlik, J. Taylor, and J. R. Petta, ``Photon emission from a cavity-coupled double quantum dot,'' (in preparation).} I will also describe recent progress towards reaching the strong-coupling limit in cavity-coupled Si DQDs.   

High Visibility Coherent Oscillations in a Si/SiGe Quantum Dot Hybrid Qubit

We discuss measurement and manipulation of a quantum dot hybrid qubit [1] formed in a Si/SiGe heterostructure. X-rotations on the Bloch sphere are performed by pulsing a gate voltage so that the detuning of a double quantum dot makes the (1,2) and (2,1) occupation ground states degenerate [2]. The resulting rotation rate is approximately 5 GHz and reveals an experimentally measured visibilty greater than 80 percent. Z-rotations on the Bloch sphere are performed by pulsing a gate voltage away from the (1,2)-(2,1) degeneracy point, resulting in oscillations at a rate of approximately 10 GHz and measured visibility greater than 85 percent. The T2* time at this detuning is greater than 15 ns, many times longer than the 100 ps gate operation time. In part because of the large ratio between the gate time and the dephasing time, improvements in the pulses used in the experiment are expected to enhance the visibility beyond that reported here and to enable high fidelity quantum gates. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. This work was performed in collaboration with Dohun Kim, Zhan Shi, C. B. Simmons, D. R. Ward, J. R. Prance, Xian Wu, R. T. Mohr, Teck Seng Koh, John King Gamble, Ryan Foote, D. E. Savage, M. G. Lagally, Mark Friesen, and S. N. Coppersmith. \\[4pt] [1] Z. Shi, C. B. Simmons, J. R. Prance, John King Gamble, Teck Seng Koh, Yun-Pil Shim, Xuedong Hu, D. E. Savage, M. G. Lagally, M. A. Eriksson, Mark Friesen, and S. N. Coppersmith, Phys. Rev. Lett. 108, 140503 (2012).\\[0pt] [2] Teck Seng Koh, John King Gamble, Mark Friesen, M. A. Eriksson, and S. N. Coppersmith, Phys. Rev. Lett. 109, 250503 (2012).   

Quantum gate-set tomography

Quantum information technology is built on (1) physical qubits and (2) precise, accurate quantum logic gates that transform their states. Developing quantum logic gates requires good characterization -- both in the development phase, where we need to identify a device's flaws so as to fix them, and in the production phase, where we need to make sure that the device works within specs and predict residual error rates and types. This task falls to quantum state and process tomography. But until recently, protocols for tomography relied on a pre-existing and perfectly calibrated reference frame comprising the measurements (and, for process tomography, input states) used to characterize the device. In practice, these measurements are neither independent nor perfectly known -- they are usually implemented via exactly the same gates that we are trying to characterize! In the past year, several partial solutions to this self-consistency problem have been proposed. I will present a framework (gate set tomography, or GST) that addresses and resolves this problem, by self-consistently characterizing an entire set of quantum logic gates on a black-box quantum device. In particular, it contains an explicit closed-form protocol for linear-inversion gate set tomography (LGST), which is immune to both calibration error and technical pathologies like local maxima of the likelihood (which plagued earlier methods). GST also demonstrates significant (multiple orders of magnitude) improvements in efficiency over standard tomography by using data derived from long sequences of gates (much like randomized benchmarking). GST has now been applied to qubit devices in multiple technologies. I will present and discuss results of GST experiments in technologies including a single trapped-ion qubit and a silicon quantum dot qubit.   

Quantum Computing in Silicon with Donor Electron Spins

Extremely long electron and nuclear spin coherence times have recently been demonstrated in isotopically pure Si-28 [1-3] making silicon one of the most promising semiconductor materials for spin based quantum information. The two level spin state of single electrons bound to shallow phosphorus donors in silicon in particular provide well defined, reproducible qubits [4] and represent a promising system for a scalable quantum computer in silicon. An important challenge in these systems is the realisation of an architecture, where we can position donors within a crystalline environment with approx. 20-50nm separation, individually address each donor, manipulate the electron spins using ESR techniques and read-out their spin states. We have developed a unique fabrication strategy for a scalable quantum computer in silicon using scanning tunneling microscope hydrogen lithography to precisely position individual P donors in a Si crystal [5] aligned with nanoscale precision to local control gates [6] necessary to initialize, manipulate, and read-out the spin states [7]. During this talk I will focus on demonstrating electronic transport characteristics and single-shot spin read-out of precisely-positioned P donors in Si. Additionally I will report on our recent progress in performing single spin rotations by locally applying oscillating magnetic fields and initial characterization of transport devices with two and three single donors. The challenges of scaling up to practical 2D architectures will also be discussed. \\[4pt] [1] M. Steger et al., Science 336, 1280 (2012).\\[0pt] [2] A.M. Tyryshkin et al., Nature Materials 11, 143 (2012). \\[0pt] [3] K. Saeedi et al., Science 342, 130 (2013).\\[0pt] [4] B.E. Kane, Nature 393, 133 (1998).\\[0pt] [5] M. Fuechsle et al., Nature Nanotechnology 7, 242 (2012).\\[0pt] [6] B. Weber et al., Science 335, 6064 (2012).\\[0pt] [7] H. Buch et al., Nature Communications 4, 2017 (2011).   

Session M33: Invited Session: Postdocs and the Application Process

Panel Discussion on Postdocs

Participation in the panel discussion, sharing experience on postdoc fellowships inside and outside the USA.   

Session M34: Focus Session: AMO Quantum Information Processing: Ion Trapping Technologies

Reducing Motional Decoherence in Ion Traps with Surface Science Methods

Many trapped ions experiments ask for low motional heating rates while trapping the ions close to trapping electrodes. However, in practice small ion-electrode distances lead to unexpected high heating rates. While the mechanisms for the heating is still unclear, it is now evident that surface contamination of the metallic electrodes is at least partially responsible for the elevated heating rates. I will discuss heating rate measurements in a microfabricated surface trap complemented with basic surface science studies. We monitor the elemental surface composition of the Cu-Al alloy trap with an Auger spectrometer. After bake-out, we find a strong Carbon and Oxygen contamination and heating rates of 200 quanta/s at 1 MHz trap frequency. After removing most of the Carbon and Oxygen with Ar-Ion sputtering, the heating rates drop to 4 quanta/s. Interestingly, we still measure the decreased heating rate even after the surface oxidized from the background gas throughout a 40-day waiting time in UHV.   

Multiparicle Entanglement in one step

In this presentation, I will show how the linear ramp dynamics of phonons in a one-dimensional trapped ion system can be used for both generating multiparticle entangled states and motional state cooling of a string of trapped ions where all the trapped ions are prepared in a state of transverse motional mode. These phonons are well known to be described by an effective Bose Hubbard model where the onsite potential of this model is induced by an optical dipole potential which can be created by an off-resonant standing wave to individual ion. I will present a specific ramping protocol which involves a site specific dynamical tuning of the onsite potential of the model leads to generate entangled state and to achieve transverse motional state cooling without involving electronic states of the ions.\\[4pt] [1] T. Dutta, M. Mukherjee and K. Sengupta, Phys. Rev. Lett {\bf 111},170406 (2013).\\[0pt] [2] T. Dutta, M. Mukherjee and K. Sengupta, Phys. Rev. A {\bf 85}, 063401 (2012).\\[0pt] [3] D. Porras and J.I. Cirac, Phys. Rev. Lett.{\bf 92}, 207901 (2004).   

Ion Motion Control and Heating Measurements in a Y Junction Surface Electrode Trap

Trapped atomic ions have demonstrated all the basic quantum operations necessary to implement quantum computation. Micro-fabricated surface electrode ion traps are a promising tool for implementing the scalable Kielpinski-Monroe-Wineland (KMW) architecture [1]. In the KMW scheme, trapped ions are held in small chains and communication between chains is performed by shuttling ions. Here we present our measurements of shuttling operations on a Sandia Y-junction trap [2] over a two year period. We have measured the ion heating after an adiabatic linear shuttling and after transport through the junction. The low linear shuttling heating is consistent with adiabatic motion. The high heating after crossing the junction indicates that sympathetic cooling will be required to perform high-fidelity two qubit operations. [1] D. Kielpinski, C. Monroe, and D. J. Wineland, Nature \textbf{417}, 709, (2002) [2] D. L. Moehring, C. Highstrete, D. Stick, K. M. Fortier, R. Haltli, C. Tigges, and M. G. Blain, New J. Phys. \textbf{13}, 075018, (2011)   

Microwave quantum logic spectroscopy and control of molecular ions

A general method for rotational microwave spectroscopy and control of polar molecular ions via direct microwave addressing is considered. Our method makes use of spatially varying ac Stark shifts, induced by far off-resonant, focused laser beams to achieve an effective coupling between the rotational state of a molecular ion and the electronic state of an atomic ion. In this setting, the atomic ion is used for read-out of the molecular ion state, in a manner analogous to quantum logic spectroscopy based on Raman transitions. In addition to high-precision spectroscopy, this setting allows for rotational ground state cooling, and can be considered as a candidate for the quantum information processing with polar molecular ions. All elements of our proposal can be realized with currently available technology.   

Entangling spin-spin interactions of ions in individually controlled potential wells

Physical systems that cannot be modeled with classical computers appear in many different branches of science, including condensed-matter physics, statistical mechanics, high-energy physics, atomic physics and quantum chemistry. Despite impressive progress on the control and manipulation of various quantum systems, implementation of scalable devices for quantum simulation remains a formidable challenge. As one approach to scalability in simulation, here we demonstrate an elementary building-block of a configurable quantum simulator based on atomic ions. Two ions are trapped in separate potential wells that can individually be tailored to emulate a number of different spin-spin couplings mediated by the ions' Coulomb interaction together with classical laser and microwave fields. We demonstrate deterministic tuning of this interaction by independent control of the local wells and emulate a particular spin-spin interaction to entangle the internal states of the two ions with 0.81(2) fidelity. Extension of the building-block demonstrated here to a 2D-network, which ion-trap micro-fabrication processes enable, may provide a new quantum simulator architecture with broad flexibility in designing and scaling the arrangement of ions and their mutual interactions.   

A Graphene-Coated Ion Trap for Electric Field Noise Suppression

Trapped ions have proven to be effective quantum bits; but increasing electric field noise as traps are miniaturized limits gate fidelity and progress towards a large-scale quantum computer. Removing contamination from surfaces is important for noise suppression; but cleaning techniques like argon ion bombardment are difficult to integrate with current systems and are too harsh for traps incorporating optical devices. We investigate an alternative solution: a protective coating against surface contamination. We fabricate copper traps with a graphene passivation layer and characterize them with single ions. Surprisingly, we find worse noise performance than for an uncoated metal trap.   

Measurement Scheme with 171Yb+ Chains in a Microfabricated Ion Trap

Trapped ions are promising candidates for implementing a scalable quantum computing system. We consider a quantum information processor implemented in an ion chain, where a multi-qubit gate between ions is executed using the transverse modes of ion motion. Quantum error correction requires that the states of data qubits be maintained during the initialization and readout of ancilla qubits. Such procedures require the ability to collect light from individual fluorescing ions without resonantly exciting other ions in the system. We describe an ion measurement protocol that uses shuttling to separate the ions being detected from the rest of the chain in order to decrease the resonant crosstalk between measured and unmeasured qubits. We will discuss experimental progress towards the implementation of this scheme in a microfabricated surface trap where scattered photons are collected using a high numerical aperture lens, and characterize the impact of resonant scattering from the measured qubits on the remaining qubits in the ion chain. A similar isolation scheme is required for the generation of heralded entanglement between two ion chains.   

Photon extraction and conversion for scalable ion-trap quantum computing

Trapped ions represent one of the most mature and promising systems for quantum information processing. They have high-fidelity one- and two-qubit gates, long coherence times, and their qubit states can be reliably prepared and detected. Taking advantage of these inherent qualities in a system with many ions requires a means of entangling spatially separated ion qubits. One architecture achieves this entanglement through the use of emitted photons to distribute quantum information - a favorable strategy if photon extraction can be made efficient and reliable. Here I present results for photon extraction from an ion in a cavity formed by integrated optics on a surface trap, as well as results in frequency converting extracted photons for long distance transmission or interfering with photons from other types of optically active qubits. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U. S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Operation of a planar-electrode ion trap array with adjustable RF electrodes

One path to scaling-up trapped atomic ions for large-scale quantum computing and simulation is to create a two-dimensional array of ion traps in close proximity to each other. A method to control the interactions between nearest neighboring ions is demonstrated and characterized here using an adjustable radio-frequency (RF) electrode between trapping sites. A printed circuit board planar-electrode ion trap is demonstrated, trapping laser-cooled $^{40}$Ca$^+$\, ions. RF shuttling and secular-frequency adjustment are shown as a function of the power applied to the addressed RF electrode. The trapped ion's heating rate is measured via a fluorescence recooling method.   

Microfabricated Surface Trap and Cavity Integration for High Fidelity State Detection and Photon Collection from Trapped Ions.

Atomic ions trapped in microfabricated traps can provide a useful resource for quantum information processing. Traditional approaches to qubit state detection using state dependent fluorescence utilize refractive lenses or reflective optics to direct scattered photons to the detector. Here we show progress towards a new method which can drastically enhance the fidelity and speed of qubit state detection by using the interaction between a trapped ion and an optical field in a cavity. Our experiment uses a concentric cavity geometry with a surface trap fabricated on a mirror which is highly reflective at UV wavelengths for $^{171}$Yb$^+$ ions. Using this system, we show that it is feasible to reduce the qubit measurement time to that comparable to single qubit gate times (~1$\mu$s), and the measurement errors down to the $10^{-5}$ range. Furthermore, this system can be used for enhanced photon collection and remote ion entanglement. We describe the design and fabrication of the traps used in the cavity system, and report the experimental progress towards the cavity realization.   

Measurement of the magnetic interaction between two electrons

In this talk we will report on the first measurement of the magnetic interaction between two electronic spins. While the dipolar magnetic interactions between different spin systems, such as an electron and its nucleus or several multi-electron spin complexes, were experimentally studied, the magnetic interaction between two isolated electronic spins was never observed. We will explain why columb exchange forces on the one hand, and magnetic field noise on the other hand, make the electron-electron magnetic interaction measurement a challenging one. This challenge was resolved by the use of Quantum Information techniques. In our experiment, we used the ground state valence electrons of two $^{88}Sr^+$ ions, co-trapped in an electric Paul trap and separated by more than two micrometers. We measured a weak, millihertz scale, magnetic interaction between their electronic spins, in the presence of magnetic noise that was six orders of magnitude larger than the respective magnetic fields the electrons apply on each other. Spin dynamics was restricted to a Decoherence Free Subspace where a coherent evolution of 15 s led to spin-entanglement. Finally, by varying the separation between the two ions, we were able to recover the cubic distance dependence of the interaction   

Dissipative production of a maximally entangled steady state

We combine unitary processes with engineered dissipation into a zero-temperature bath to deterministically produce and stabilize an approximate Bell state of two trapped-ion qubits independent of their initial state [arXiv:1307.4443]. We implement the process on a $^9$Be$^+$-$^{24}$Mg$^+$-$^{24}$Mg$^+$-$^9$Be$^+$ four-ion chain in a linear radio-frequency Paul trap. The two $^9$Be$^+$\% ions serve as qubit ions while the two $^{24}$Mg$^+$ ions are used for sympathetic cooling as the zero-temperature bath. We simultaneously apply a combination of a unitary process consists of microwave and laser fields on $^9$Be$^+$ ions, and dissipative processes of optical pumping on $^9$Be$^+$\% ions and sympathetic cooling on $^{24}$Mg$^+$ ions. We realize maximally entangled steady states with a fidelity of F = 0.75(3). We also demonstrate that a sequential stepwise application of unitary and dissipative process can speed up the dynamics of the scheme and achieve a fidelity of F = 0.89(2) after approximately 30 repetitions. In both cases, the errors can be attributed to known experimental imperfections.   

Quantum synchronization of quantum van der Pol oscillators with trapped ions

Van der Pol oscillators are prototypical driven-dissipative oscillators that have been used to study synchronization phenomena in classical systems. We study the van der Pol oscillator in the quantum limit, when the oscillator is near the quantum ground state, and the behavior is sensitive to the quantization of energy levels. We consider four scenarios: one oscillator with and without an external drive, two coupled oscillators, and an infinite number of oscillators with global coupling. We find that phase-locking is much more robust in the quantum model than in the equivalent classical model. Trapped-ion experiments are ideally suited to simulate van der Pol oscillators in the quantum regime via sideband heating and cooling of motional modes. Phys. Rev. Lett. (in press), arXiv:1306.6359   

Session M35: Spin-orbit Coupling and the BEC-BCS Crossover

Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit-Coupled Fermi Gas

We consider the time evolution of the magnetization in a Rashba spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We model the dynamics using a Boltzmann equation, which we solve in the Hartree-Fock approximation. The resulting non-linear system of equations gives rise to three distinct dynamical regimes with qualitatively different asymptotic behaviors of the magnetization at long times. The distinct regimes and the transitions between them are controlled by the interaction strength: for weakly interacting fermions, the magnetization decays to zero. For intermediate interactions, it displays undamped oscillations about zero and for strong interactions, a partially magnetized state is dynamically stabilized. The dynamics we find is a spin analog of interaction induced self-trapping in double-well Bose Einstein condensates. The predicted phenomena can be realized in trapped Fermi gases with synthetic spin-orbit interactions.   

Cavity-Assisted Dynamical Spin-Orbit Coupling in Cold Atoms

In this work, we consider the effect of spin-orbit coupling in ultracold atoms induced by a quantized light field inside an optical cavity, where the back-action from the atom to the cavity light field plays an essential role. The Raman coupling gives rise to effective spin-orbit interaction which couples atom's center-of-mass motion to its pseudospin degrees of freedom. Meanwhile, the cavity photon is dynamically affected by the atom. This system possesses remarkable features. For example, loop structure may emerge in dispersion curves, effective nonlinearity from the atom-photon feedback induces dynamical instability, etc. To understand the intriguing physics, we analytically computed the critical condition for forming loops and performed stability and dynamical analysis. Furthermore, we propose to demonstrate dynamical instability experimentally in terms of counting sudden change of photon number inside the cavity. From a practical point of view, all the ingredients proposed in this work has been demonstrated in various labs. Hence our proposal can be readily tested in experiment.   

Vortices and vortex states of Rashba spin-orbit coupled condensates

The Rashba spin-orbit coupling in two spatial dimensions is captured by a static SU(2) gauge field with a non-zero magnetic Yang-Mills flux. This SU(2) analogue of magnetic field enables two-dimensional topological insulators (TI) reminiscent of integer quantum Hall states. An outstanding question is whether non-Abelian fractional TIs could exist as well. We explore this from the point of view that quantum melting of a vortex lattice can produce fractional incompressible liquids when the number of flux quanta per particle is not small. Physical systems in which an SU(2) vortex lattice melting could perhaps be arranged include two or three-component bosonic cold atoms in optical lattices, as well as solid-state heterostructures with a conventional or Kondo TI quantum well. This talk will discuss the types of vortices and vortex lattices that could exist in these systems as ``parent'' states to fractional quantum liquids. Analytical arguments based on conservation laws reveal several possibilities for vortex states, some of which do not break the time-reversal symmetry. We will present mean-field numerical results that paint certain vortex states as excellent metastable or ground states of a microscopic lattice model.   

The Pairing of Rashba Spin-orbit Coupled Fermi Gas in Optical Lattice

We make an urgent advance using determinant quantum Monte Carlo (DQMC) simulations on Rashba spin-orbit coupled Fermi gases in square optical lattice, which is free of the sign-problem. We show that the Berezinskii-Kosterlitz-Thoules phase transition temperature is firstly enhanced and then suppressed by Rashba spin-orbit coupling at strong attraction region. At weak attraction region, Rashba spin-orbit coupling always suppresses the transition temperature. We also show that the spin susceptibility becomes anisotropic and retain finite at zero temperature.   

Wavefunction Vortex Attachment via Matrix Products: Application to Atomic Fermi Gases in Flat Spin-Orbit Bands

Ultracold atomic gases in the presence of strong spin-orbit coupling present challenging many-body problems. For very strong spin orbit coupling, interaction effects dominate. The resulting many-body problem is non-perturbative but progress can be made with validated wavefunctions that properly account for the location of wavefunction vortices. I will discuss a new method to construct and validate Jastrow-correlated wavefunctions in arbitrary bases. The method implements vortex insertion in terms of matrix products. The approach was tested on a model of a dilute gas of Rashba spin-orbit coupled fermions in the presence of slow rotation. Validated wavefunctions show that vortices in slowly rotating atomic fermi gases with strong spin-orbit coupling cluster near the system center and should therefore be directly visible in time-of-flight imaging.   

Novel magnetic phases for two-component Bose-Hubbard model with synthetic spin-orbit coupling in one dimension

We present a new phase diagram for the two-component Bose-Hubbard model with a synthetic spin-orbit coupling in one dimension by employing the density-matrix renormalization group method. A ferromagnetic long-range order emerges in both the Mott insulator and superfluid phases. It results from the spontaneous breaking of the $Z_2$ symmetry, when the spin-orbit coupling term becomes comparable to the hopping kinetic energy and the inter-component interaction is smaller than the intra-component one. These novel effects are expected to be detectable with the present realization of synthetic spin-orbit coupling in experiments.   

Dilute spin-orbit Fermi gases

We study repulsive Fermi gases with Rashba spin-orbit coupling in two and three dimensions when they are dilute enough that a single branch of the spectrum is occupied in the non-interacting ground state. We develop an effective renormalizable theory for fermions in the lower branch and obtain the energy of the system in three dimensions to second order in the renormalized coupling constant. We then exploit the non-Galilean-relativistic nature of spin-orbit coupled gases. We find that at finite momentum, the two-dimensional Fermi sea is deformed in a non-trivial way. Using mean-field theory to include interactions, we show that the ground-state of the system acquires a finite momentum, and is consequently deformed, when the interaction is stronger than a critical value.   

Spin-Orbit Coupled Dengenerate Fermi Gases with Topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase

The spin-orbit coupled degenerate Fermi gas provides an ideal platform for the search of topological matters and associated topological excitations. Recently, it has been shown that the topological Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, in which the Cooper pairs carry finite center-of-mass momentum, can be realized in the present of in-plane and out-of-plane Zeeman field. In this work, we study the topological phase transition and topological edge modes in the degenerate Fermi gas system. We first show that the in-plane Zeeman field creates an s-wave pairing channel, thus the effective pairing is a $s+p$ wave pairing in the topological FFLO phase regime. Then we study the phase diagram in the parameter space and discuss how these phases can be reached in experiments. At last we study topological excitations in a slab geometry, and discuss their possible experimental measurement issues.   

Unconventional superfluidity in spin-orbit coupled ultracold atomic Fermi gases

Ultracold atoms has been proven to be an ideal table-top system to reveal novel states of quantum matter. The latest development of engineering synthetic spin-orbit coupling in ultracold atoms has created a new frontier that is endowed with a strong interdisciplinary character. This is a system that has a close connection to new functional materials such as topological insulators and has the potential to perform topological quantum computation based on Majorana fermions. Here we report our recent theoretical work on spin-orbit coupled atomic Fermi gases. We predict a new anisotropic state of matter which consists of exotic quasi-particles with anisotropic effective mass. In the superfluid phase, these exotic quasi-particles exhibit salient features in the momentum distribution, single-particle spectral function and spin structure factor. By applying an external Zeeman field, novel states of matter known as topological superfluids or inhomogeneous Fulde-Ferrell superfluids can form. We propose that strong nonmagnetic or magnetic impurity scattering, created by a narrow dimple laser beam, can induce a universal mid-gap bound state in topological superfluids.   

BCS-BEC Crossover and topological phase transition in Fermi Gases driven by Spin-Orbit Coupling and Zeeman field

In this work, we investigate 3D and 2D Fermi gases with uniaxial, Rashba-type and isotropic spin-orbit coupling (SOC). By calculating the chemical potentials and cooper-pair sizes, we find that the increasing Rashba and isotropic SOC can drive a crossover from BCS side to BEC side, while uniaxial SOC will not affect the properties of the many-body system. According to recent experiments, we also consider both a zeeman field and SOC simultaneously appearing in a 3D and 2D Fermi gas. We find the zeeman field can drive the system from the normal state to the topological superfluid states in Rashba SOC case. In 3D Rashba case, there are two topological superfluid phases which have different number of Weyl Fermions. At the same time, our results also show that the zeeman field can drive a converse crossover from BEC side to BCS side.   

Magnetic excitations and spin-gap phenomenon in the BCS-BEC crossover regime of an ultracold Fermi gas

We investigate the uniform spin susceptibility $\chi$ and strong-coupling corrections in the BCS-BEC crossover regime of an ultracold Fermi gas. Within the framework of an extended $T$-matrix theory,\footnote{T.Kashimura, R.Watanabe, and Y.Ohashi, Phys. Rev. A \textbf{86}, 043622 (2012).} we show that $\chi$ exhibits non-monotonic temperature dependence in the normal state, and is suppressed near the superfluid phase transition temperature $T_{\rm c}$. This spin-gap phenomenon is found to be deeply related to the pseudogap phenomenon appearing in the single-particle density of states. To characterize this magnetic phenomenon, we introduce the spin-gap temperature $T_{\rm s}$ as the temperature at which $\chi$ takes a maximum value. Determining $T_{\rm s}$ in the entire BCS-BEC crossover region, we identify the spin-gap regime in the phase diagram of a Fermi gas with respect to the temperature and the strength of a pairing interaction. Since the spin-gap is crucial key phenomenon in high-$T_{\rm c}$ cuprates, our results would be useful for the study of this many-body phenomenon using ultracold Fermi gases, as well as in observing the pseudogap phenomenon through the spin-gap phenomenon.   

Exact calculations of strongly paired fermions in two dimensions

The problem of strongly interacting superfluid Fermi gas has attracted considerable interest, especially in three-dimensions at unitarity. Although the corresponding problem in two-dimensions does not have a unitarity limit per se, it is expected to offer a rich interplay between the interaction strength and density. A quantitative understanding is important, particularly in light of its possible experimental realization with cold atoms. To this end, we have carried out auxiliary-field quantum Monte Carlo simulations on large system sizes. The ground-state energy is obtained for an unpolarized gas with a zero-range attractive interaction. The calculations are exact, and are performed using a BCS trial wave function that greatly reduces the statistical fluctuation. We present the calculated equation of state as a function of $k_F a$, and make comparisons with BCS and other results. Other ground-state observables will also be discussed.   

Theory of BCS-BEC crossover in altracold atomic Fermi gases in the presence of impurities

We present a theory of BCS-BEC crossover in ultracold atomic Fermi gases in the presence of nonmagnetic impurities, for variable impurity strength from the Born to the unitary limit. The particle-particle scattering T-matrix and the impurity scattering T-matrix will both be considered self-consistently at the same time, in either a 3D continuum or an optical lattice. Result of $T_c$, the chemical potential $\mu$ and the excitation gap $\Delta$ as well as the order parameter $\Delta_{SC}$, will be presented as a function of impurity strength and impurity density, and will also be compared with the case of $d$-wave pairing such as in high $T_c$ superconductors. References: Q.J. Chen and J.R. Schrieffer, Phys. Rev. B 66, 014512 (2002).   

Superfluid phase transition and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas: A self-consistent T-matrix theory

We investigate a two-component Fermi gas with mass imbalance ($m_\uparrow\ne m_\downarrow$, where $m_\sigma$ is an atomic mass in the $\sigma$-component) in the BCS-BEC crossover region. Including pairing fluctuations within a self-consistent $T$-matrix theory, we examine how the superfluid instability is affected by the presence of mass imbalance. We determine the superfluid region in the phase diagram of a Fermi gas in terms of the temperature, the strength of a pairing interaction, and the ratio of mass imbalance. The superfluid phase transition is shown to always occur even when $m_\uparrow\ne m_\downarrow$.\footnote{R.Hanai and Y.Ohashi, J. Low Temp. Phys., DOI 10.1007/s10909-013-0909-3.} This behavior of $T_{\rm c}$ is quite different from the previous result in an extended $T$-matrix theory,\footnote{R.Hanai, \textit{et. al.}, Phys. Rev. A (2013) in press.} where $T_{\rm c}$ vanishes at a certain value of $m_\uparrow/m_\downarrow>0$ in the BCS regime. Since Fermi condensates with mass imbalance have been discussed in various systems, such as a cold Fermi gas, an exciton(polariton) condensate, as well as color superconductivity, our results would be useful for further understandings of these novel Fermi superfluids.   

Quasiparticle Berry curvature and Chern numbers in spin-orbit-coupled bosonic Mott insulators

We study the ground-state topology and quasiparticle properties in bosonic Mott insulators with two- dimensional spin-orbit couplings in cold atomic optical lattices. We show that the many-body Chern and spin-Chern number can be expressed as an integral of the quasihole Berry curvatures over the Brillouin zone. Using a strong-coupling perturbation theory, for an experimentally feasible spin-orbit coupling, we compute the Berry curvature and the spin Chern number and find that these quantities can be generated purely by interactions. We also compute the quasiparticle dispersions, spectral weights, and the quasimomentum space distribution of particle and spin density, which can be accessed in cold-atom experiments and used to deduce the Berry curvature and Chern numbers. Physical Review A \textbf{88}, 053631 (2013)   

Session M36: Superconducting Qubits: Fabrication & Materials

Tunable TiN or NbTiN resonators and couplers using nonlinear kinetic inductance for superconducting qubits

Nitride superconductors such as TiN and NbTiN have a nonlinear kinetic inductance when driven at high current. Using this current-tunable reactance, we have designed superconducting devices that are tunable with a DC current without using Josephson junctions. We show that when the DC current is directly coupled to a lumped element resonator, the resonant frequency can be tuned by \textgreater 4{\%} without inducing loss. In other circuits, we can use a DC current to independently tune the coupling of a long microwave transmission line to a standard superconducting resonator from zero to maximum coupling. In addition to characterizing the non-linear current response of these materials, these tunable devices could be used as a tunable coupler in transmon qubits, by adjusting the strength of the cavity's Purcell effect to the qubit as needed. They also have potential to be used as tunable filters or parametric amplifiers in superconducting circuits.   

Superconducting qubits using titanium nitride

Recent results in the community strongly implicate surface loss as a dominant source of decoherence (primarily energy relaxation) for superconducting transmon qubits. Resonators and qubits made of titanium nitride (TiN) showed significant device improvement compared with lift-off aluminum, with quality factors of up to approximately 1 million. We present more detailed characterization results of the TiN films including the substrate-metal interface.   

Superconducting metamaterial transmission line

Left-handed metamaterials are artificial composite structures with unusual properties. Such systems have a wide range of potential applications in photonics. We are developing transmission lines composed of superconducting metamaterials using thin-film lumped circuit elements. Such structures allow for the possibility of generating novel transmission spectra with a high density of modes in some frequency ranges and stop-bands in others. We discuss possible couplings of these lines to superconducting qubits in circuit QED architectures.   

Fabrication of capacitively-shunted superconducting qubits

Improvements in superconducting qubit coherence times and reproducibility have been demonstrated using capacitive shunting. In this study, we present methods for the preparation of both capacitively-shunted charge qubits (transmons) and capacitively-shunted flux qubits. Hybrid fabrication techniques were employed to combine high-quality-factor aluminum capacitive shunts with shadow-evaporated Josephson junctions, and the Josephson junctions were prepared using suspended-bridge germanium masks. We also will describe process testing results that were acquired to assess wafer-to-wafer reproducibility of our fabrication protocols. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government.   

Radially symmetric transmon with long lifetime

We present a radially symmetric design for a large pad transmon qubit. The symmetry reduces the dipole radiation by orders of magnitude relative to axial large pad qubits that are widely used for 3D-circuit QED experiments. The reduction in radiation allows for the use of large area structures that are needed to reduce the effects of interface losses. This enables long qubit lifetimes without the use of a high-Q cavity resonator. Energy relaxation and coherence times of up to 35 microseconds have been measured. The qubit can be implemented in a microstrip geometry. This gives the advantage of removing discontinuous ground planes that can cause stray resonances.~ In addition, this geometry is well suited for implementing and exploring circuits with direct qubit-qubit coupling.   

Josephson Phase Qubit with a Distributed Reactance

We present our characterization of a novel phase qubit design in which the capacitance across the Josephson junction and the inductance of the SQUID are provided by a microstrip resonator instead of lumped circuit elements. The spectroscopic data from this device clearly shows a behavior with respect to applied flux that cannot be accurately described by a simple SQUID model. We present measurements of the devices coherence times and anharmonicity, and compare the spectrum to a theoretical model that treats the phase drop across the length of the resonator as a continuous field.   

Fabrication of superconducting single crystal aluminum resonators on silicon and sapphire

Superconducting Al on sapphire are the mainstay materials used for the current development of superconducting qubit devices. We have grown single crystal Al films using MBE on both sapphire and silicon wafers with different surface preparations. Structural analysis indicates high quality films on both substrates with the twinned single crystal aluminum films abruptly relaxing misfit strain at the substrate interface. Different fabrication recipes for etching quarter-wave resonators and their impact on resonator performance will also be discussed. We have observed internal quality factors at low photon numbers above 600k for resonators on both substrates. Most resonators exhibit a lower than expected power dependence.   

Measurement of superconducting single-crystal Al resonators on Si and sapphire substrates

Al made via molecular beam epitaxy offers improvements to existing superconducting qubit architectures due to decreased loss at the interface between the Al and the substrate [1]. We have studied this loss by measuring the quality factors of a variety of superconducting quarter-wave resonators fabricated under different conditions from single-crystal aluminum on both silicon and sapphire substrates. The resonators, which have resonant frequencies between 4.5 and 6 GHz, were measured at a temperature of 25 mK and from an average stored photon number n$\sim$1 up to 10$^{6}$. At low photon numbers, we consistently observe Q\textgreater 200k on both substrates. We will discuss potential limitations on the measured loss and steps taken to mitigate them. \\[4pt] [1] A. Megrant, et al., App. Phys. Lett., {\bf 100}, 113510 (2012).   

Microscopic Sources of Paramagnetic Noise on $\alpha$-Al2O3 Substrates for Superconducting Qubits

Superconducting qubits (SQs) represent a promising route to achieving a scalable quantum computer. However, the coupling between electro-dynamic qubits and (as yet largely unidentified) ambient parasitic noise sources has so far limited the functionality of current SQs by limiting coherence times of the quantum states below a practical threshold for measurement and manipulation. Further improvement can be enabled by a detailed understanding of the various noise sources afflicting SQs. In this work, first principles density functional theory (DFT) calculations are employed to identify the microscopic origins of magnetic noise sources in SQs on an $\alpha$-Al2O3 substrate. The results indicate that it is unlikely that the existence of intrinsic point defects and defect complexes in the substrate are responsible for low frequency noise in these systems. Rather, a comprehensive analysis of extrinsic defects shows that surface aluminum ions interacting with ambient molecules will form a bath of magnetic moments that can couple to the SQ paramagnetically. The microscopic origin of this magnetic noise source is discussed and strategies for ameliorating the effects of these magnetic defects are proposed.   

Characterization of the temperature dependence of dielectric loss at microwave frequencies in Al$_2$O$_3$ and TiO$_2$ films grown by atomic layer deposition

Low temperature dielectric loss is one of the primary sources of decoherence in superconducting quantum bits and resonators. We performed detailed dielectric loss measurements of Al$_2$O$_3$ and TiO$_2$ thin films grown by atomic layer deposition in the 3-8 GHz frequency range at temperatures ranging from 36mK to 1K. The intrinsic Q-factor is extracted by measuring superconducting Niobium lumped element resonators which contain the dielectric material of thickness ranging from 30-100 nm. We find the temperature dependence of the loss tangent and resonance frequency agree with the tunnelling two-level system model. We also find a systematic dependence of the saturation voltage on temperature and film thickness. We compare the results obtained for Al$_2$O$_3$ films grown by atomic layer deposition with those grown by plasma oxidation. For these two different growth methods, we find similar values of the loss tangent despite different impurity content.   

Engineering Filters for Reducing Spontaneous Emission in cQED

Inserting a notch filter between a qubit and the external environment at the qubit frequency can significantly suppress spontaneous emission mediated by the cavity (``Purcell effect''). In order to realize this filtering in multi-qubit architectures, where space comes at a premium, we will present a filter with minimal space requirements.   

Fabrication and Characterization of Aluminum Airbridges for Superconducting Qubit Circuits

Superconducting circuits based on coplanar waveguides (CPWs) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers. These results pave the way for building airbridge crossovers on more complex qubit circuits.   

Substrate removal near superconducting qubits and resonators in circuit QED processors

We investigate the effect of etching away substrate volumes in the capacitive gaps of transmon qubits and resonators in planar circuit QED quantum processors. We use cryogenic and deep reactive-ion etching techniques to control the etching depth, profile, wall roughness, and passivation chemistries in high-resistivity silicon substrates. Two independent etching steps allow freedom to choose which areas to etch using standard fluorine etch (i.e., feeds and readout resonators), and which volumes to deeply etch (i.e., capacitive gaps in transmons and bus resonators). We study the effects of different etching techniques on losses in superconducting resonators operating in the quantum regime and on relaxation times of transmon qubits.   

Chip Mount Design as a Dissipation-Limiting Factor in High Quality Superconducting Resonators

Superconducting quantum computing technology continues to make progress with regards to both materials quality and circuit complexity. We have found that chip mount design can become a coherence-limiting factor for superconducting coplanar resonators with an internal quality factor above 1 million. Understanding the impact of chip-to-mount coupling will aid in both proper mount design for higher density circuits as well as the further improvement of coherence times. These coplanar resonators provide an ideal test circuit as they are sensitive to a variety of loss mechanisms including radiation, infrared light, and magnetic fields which also affect more complex superconducting circuits. I will present results relating the coherence and performance of resonators to box design, box material, and chip layout.   

Metamaterials for circuit QED: Quantum simulations and other applications

The ability to design periodically structured materials not present in nature provides scientists with new tools, ranging from sub-wavelength imaging to well controlled band structures for wave propagation in photonic crystals. Superconducting metamaterials have been recently proposed to manipulate the density-of-modes of transmission lines [D. J. Egger and F. K. Wilhelm, Phys. Rev. Letters {\bf 111}, 163601 (2013)]. We further build on these ideas and develop a toolbox for environment manipulation based on nano-structured, periodic, lossless, superconducting circuits. In particular we show that high density of low energy states can be achieved using a superlattice arrangement of left-handed circuit elements. Multimode, ultra-strong coupling of superconducing qubits to such engineered environments thus allow for experimental implementation of quantum simulation of interesting new phenomena as well as for complex quantum state engineering.   

Session M37: Focus Session: Graphene on Cu and Other Metal Substrates

Role of catalytic metals on formation process of carbon nanotube and graphene: ab initio molecular dynamics study

The growth mechanism of carbon nanotubes and graphene has been widely discussed from both the experimental and computational points of view. At the present, most of numerical studies focuses on the aggregation of isolate carbon atoms on the catalytic metal surface, whereas the initial dissociation of carbon source molecules should affect the yield and quality of the products [1]. Under such circumstance, we have investigated the dissociation of carbon source molecules on the metal surface using the ab initio molecular dynamics simulation in order to discuss the initial stage of graphene growth via a chemical vapor deposition (CVD) technique [2,3]. In the presentation, we performed the ab initio MD simulations of the dissociation process of methane on Ni(111) surface to discuss initial dissociation process of the graphene formation, and the dissociation process of ethanol on Ni32 cluster to discuss that for the carbon nanotube formation. [1] Y. Shibuta, Diamond and Related Materials, 20 (2011) 334-338. [2] Y. Shibuta, R. Arfin, K. Shimamura, T. Oguri, F. Shimojo, S. Yamaguchi , Chem. Phys. Lett. 565 (2013) 92. [3] T. Oguri, K. Shimamura, Y. Shibuta*, F. Shimojo, S. Yamaguchi, J. Phys. Chem. C 117 (2013) 9983.   

CVD Growth Studies of Graphene on Cu(111)

Because of its unique chemical and physical properties, graphene shows great promise for use in a wide variety of technological applications. However, Industry has not been able to widely implement the use of graphene because of the difficulty in growing low-cost, defect-free, large-area graphene films. One method of producing graphene films with a low defect density is to grow epitaxial films on single crystal substrates. A study of the growth of graphene on the Cu(111) surface in UHV was performed with methane and ethylene. With ethylene, no graphene was formed at 900 $^{\circ}$C with pressures as high as 5 mTorr. By using an Ar overpressure of 50 mTorr, single-domain epitaxial graphene films could be formed. With methane, no graphene could be formed even with an Ar overpressure. This result indicates that methane has a much lower dissociation probability on the Cu(111) surface than ethylene. In addition, the effect of predosing the surface with a chemisorbed oxygen layer was measured. The oxygen predosing was determined to adversely affect the order of the graphene grains with respect to the Cu(111) substrate.   

Influence of Subsurface Hydrogen on the Structural Properties of Graphene Templates Grown on Ru(0001)

Graphene has aroused tremendous interest due to its remarkable electronic and mechanical properties. Graphene's optical properties and conductance make it an ideal candidate for use in nanoelectronic devices and organic photoelectric devices. We will present a STM/LEED/DFT study of the single layer graphene on Ru(0001) system grown via a novel growth mechanism that co-adsorbs atomic hydrogen and carbon vapor to the ruthenium surface while simultaneously segregating carbon from the crystal bulk to the surface. Structural studies show a wide array of moire superlattices sizes ranging from 0.9 to 3.0 nm. DFT calculations help explain the appearance of these graphene reconstructions driven by the H presence at the Ru interface. A LEED I(V) study guided by DFT calculations will accompany the STM investigation to provide insight into the graphene layer thickness. The structural polymorphism displayed by this system is of interest for the study of directed self-assembly. Control over moire superstructure size can aid in future work using graphene as a nanotemplate for self-assembled growth of nanoelectronic and organic photovoltaic devices based on pentacenes and fullerenes. Finally the impact of the structural changes on the electronic properties of the system will be studied.   

Momentum-Space Imaging of the Dirac Band Structure in Molecular Graphene via Quasiparticle Interference

Molecular graphene is a nanoscale artificial lattice composed of carbon monoxide molecules arranged one by one, realizing a dream of exploring exotic quantum materials by design. This assembly is done by atomic manipulation with a scanning tunneling microscope (STM) on a Cu(111) surface. To directly probe the transformation of normal surface state electrons into massless Dirac fermions, we map the momentum space dispersion through the Fourier analysis of quasiparticle scattering maps acquired at different energies with the STM. The Fourier analysis not only bridges the real-space and momentum-space data but also reveals the chiral nature of those quasiparticles, through a set of selection rules of allowed scattering involving the pseudospin and valley degrees of freedom. The graphene-like band structure can be reshaped with simple alterations to the lattice, such as the addition of a strain. We analyze the effect on the momentum space band structure of multiple types of strain on our system.   

Metal Oxide Growth, Characterization and Spin Precession Measurements in CVD Graphene

Thin metal oxide layers deposited on graphene can be utilized as dielectric barriers between metals and graphene to help isolate a metal contact from the graphene channel. This is important for graphene based spintronic devices as dielectric layers between the ferromagnetic electrode and graphene have been shown to increase the spin relaxation time measured utilizing non-local detection and spin precession measurements by avoiding the conductivity mismatch problem. However, simply depositing metal oxide layers such as aluminum oxide on graphene results in non-uniform film lowering the quality of the interface barrier. We will present a systematic study of aluminum oxide layers grown on CVD (chemical vapor deposition) graphene under ultra-high vacuum conditions with and without titanium seed layers. The aluminum oxide layers with the 0.2 nm titanium seed layers showed reduced surface roughness. The chemical and structural composition determined by XPS (X-ray photoelectron spectroscopy) will be also presented that shows full oxidation of the aluminum and partial oxidation of the titanium. The results on the I-V and spin precession measurements in CVD graphene will be also presented.   

Laser induced nanoparticles and crystals and their characterization

Intense nanosecond lasers are used to fabricate nanoparticles by direct laser solid interactions as well as laser produced shock wave induced crystallization in saturated solutions. In particular, laser graphite interactions under liquid nitrogen results in variety of interesting new carbon nanoclusters. In particular, exfoliation of graphite to produce graphene is considered. Laser produced shock wave in unsaturated salt (e.g. NaCl, NaClO$_{3})$ solution immediately produces thousands of tiny crystals. These nonmaterials are examined using Raman spectroscopy under liquid nitrogen, RUN), laser induced fluorescence, plasma spectroscopy, UV-Vis spectroscopy as well as conventional characterization methods such as SEM and HRTEM imaging.   

Metal-catalyzed etching of graphene

We present a comparative investigation on the etching of graphene catalyzed by Fe and Cu. With combined evidence from scanning electron microscopy and Raman spectroscopy, we demonstrate that the strikingly different etching behaviors between Fe and Cu are governed by their distinct interactions with carbon. Due to the strong Fe-C interactions, graphene is severely damaged through not only catalytic carbon hydrogenation but also carbon dissolution into Fe alone. In contrast, due to the weak Cu-C interactions and non-wetting behavior of Cu on graphene, Cu particles etch channels in graphene through carbon hydrogenation and the width of the channel width is much narrower than the diameter of catalytic particle. This work provides unprecedented insights into the metal-catalyzed carbon hydrogenation.   

Fabrication of contamination-free CVD Graphene devices using soak and peel method

Large area graphene-based devices are commonly fabricated by transferring the CVD grown graphene from metal foils to semiconductor substrates. However, during device fabrication, the transfer process involves chemical etching of metal that leads to the degradation of electrical properties of graphene. Recently, a clean transfer of graphene to devices with improved electrical properties, by delamination of graphene from metal substrates by soak and peel using DI-water has been demonstrated [1]. We employed the soak and peel scheme to fabricate graphene transistor arrays on a SiO$_{2}$/Si substrate with a back gate configuration. The source-drain contacts are patterned using Ti/Pt with graphene channel length varying from 2-50um. The graphene is transferred subsequently to the substrate and yields a high quality junction between metal electrodes and graphene. The contact resistance is low and the Dirac peak is observed across the array. The suitability of the graphene transistors for chemical functionalization will be presented. Possible application of this transfer technique for fabricating large area suspended nano-electro mechanical systems will be discussed. \\[4pt] [1] Priti Gupta, et al., arXiv: 1308.1587 [cond-mat.mtrl-sci]   

Direct transfer of graphene onto flexible substrates

We explore the direct transfer via lamination of chemical vapor deposition graphene onto different flexible substrates. The transfer method investigated here is fast, simple, and does not require an intermediate transfer membrane, such as polymethylmethacrylate. Various substrates of general interest in research and industry were studied including polytetrafluoroethylene filter membranes, PVC, cellulose nitrate/cellulose acetate filter membranes, polycarbonate, paraffin, polyethylene terephthalate, paper, and cloth. By comparing the properties of these substrates, two critical factors to ensure a successful transfer on bare substrates were identified: the substrate's hydrophobicity and good contact between the substrate and graphene. For substrates that do not satisfy those requirements, polymethylmethacrylate can be used as a surface modifier or glue to ensure successful transfer. Our results can be applied to facilitate present processes and open up directions for applications of chemical vapor deposition (CVD) graphene on flexible substrates. A broad range of applications of CVD graphene can be envisioned, including fabrication of graphene devices for opto/organic electronics, graphene membranes for gas/liquid separation, and ubiquitous electronics with graphene.   

A novel polymer-free transfer technique for high mobility graphene field effect transistors (FET)

We demonstrate a novel polymer-free method that can routinely transfer large-area graphene to any substrates and preserve the optimal properties of as-grown samples as compared to the graphene transferred with conventional polymer-assisted methods. We have also developed a one-step method that employs plasma-enhanced chemical vapor deposition for rapidly producing superior quality, large-area, monolayer graphene on Cu at low temperature (LT). Combining these two techniques, we find excellent properties of the LT-CVD grown graphene based on studies of Raman spectroscopy, XPS, UPS and STM. We have also investigated the effect of various substrates and PMMA residuals on the performance of the LT-CVD grown graphene FETs by constructing four types of devices (graphene/SiO$_{2}$ FETs, graphene/BN FETs, PMMA residuals/ graphene/SiO$_{2}$ FETs, and PMMA residuals/graphene/BN FETs). The LT-CVD grown graphene combined with the polymer-free transfer technique has achieved an electrical mobility $\sim$ 60,000 cm$^{2}$ V$^{-1}$ s$^{-1}$, which may be further improved to approaching the ideal value of pristine graphene.   

Transfer-free growth of atomically thin hexagonal boron nitride

Recent interest in and hence the opportunities presented by two-dimensional materials and their stacked assemblies have necessitated growth of high quality sheets of hexagonal boron nitride (h-BN). Chemical vapor deposition on transition metals is perhaps the most promising technique for large-scale growth of single or few-layer h-BN films with relatively controllable means to produce predetermined number of layers. In most of the studies till date, it is not very clear as to why the growth is not self-limiting to a monolayer and how multilayer h-BN is grown. In this study we present growth of high quality h-BN on Nil and Co films deposited on oxidized silicon. h-BN films thus produced show excellent optical (E$_{g} =$ 5.85 eV) and electrical insulating properties (breakdown strength $=$ 7.94 MV/cm). We deliberate on the growth mechanism driven by diffusion vs. segregation of B and N, with evidence that the growth occurs via segregation of B and N from the metal films. We discuss solubility of N and B in Ni and Co films. By controlling the growth parameters we show that h-BN segregation can be achieved on both sides of the metal film, thus allowing deposition of such atomic films by a transfer free method on arbitrary substrates.   

Improved synthesis of chemically derived graphene using a thermal processing step

The excellent physical and electronic properties of graphene have fueled the exploration of novel methods for its large-scale production and solution-processability. To this end, thermal or chemical reduction of graphene oxide (GO) represents a promising step. However, the problem of incomplete reduction and the presence of residual oxygen in reduced GO (rGO) sheets continue to persist in current reduction protocols. Here, we present a thermal processing step that improves the reduction efficiency of GO sheets, and results in superior sheet properties of chemically derived graphene. For instance, upon using the additional thermal processing step, the electronic conductivity increased by a factor of 6-8 in the reduced GO samples. Using atomistic calculations, we provide detailed insights into the physical mechanisms resulting in improved reduction. Overall, we show that our processing step can be easily integrated into current thermal and chemical reduction protocols, and could be crucial toward producing large-scale, high quality graphene.   

Synthesis of Graphene Nanoribbons by Covalent Assembly of Monomers

We present bottom up approach for synthesis of graphene nanoribbons on Ag (111) from monomers using scanning tunneling microscopy, photoemission, ultraviolet and Raman spectroscopy. In this study we used N-modified precursor molecules to form graphene nanoribbons by thermal evaporation on Ag (111) under UHV conditions. Of particular interest is the role of substrate temperature, which catalyses the polymerization and de-hydrogenation of the precursor molecules. The catalytic nature of the surface is demonstrated by the fact the polymerization happens only in the first layer monomers while the second layer monomers remain as individuals. The orientation of these ribbons with respect to substrate can be controlled by the structure of the monomers. Instead of lying flat on Ag (111) surface, nanoribbons form $\pi $-stacked networks and they stand up tilted with respect to substrate surface. This type of arrangement is attributed to the replacement of two carbon atoms in the precursor molecules with nitrogen atoms. Our approach not only bolsters previously demonstrated bottom up fabrication of graphene nanoribbons but also provides additional insight into manipulation of their orientation on substrate surface by modifying the edge of precursor monomers.   

Graphene Superlattice Construction by Intercalation of Fullerenes at the Metal-Graphene Interface

The electronic properties of graphene can be modified through the formation of a charge or topographic superlattice, in our study this is achieved by intercalation of fullerene molecules at the interface between copper and graphene. Amorphous and crystalline superlattices can be synthesized and are controlled by annealing T (650 K to 850 K) and time. The crystalline superlattices present a square geometry defined by the Cu(001) facet and the period can be controlled by deposition conditions. The geometric and electronic structure of the superlattice is measured with STM (scanning tunneling microscopy), ST spectroscopy and differential conductivity maps. The intercalation of C60 is confirmed by (i) atomic resolution of graphene on top of molecule, (ii) spectral signature of graphene is modulated with shoulder at 250 meV, (iii) bias voltage dependence of apparent height, and (iv) depth between molecules correlates with intermolecule distance due to mechanical deformation of graphene. The crystalline layer imprints a charge superlattice with 1.5 holes/molecule donated to graphene - while the graphene is nearly neutral in between. The intercalation is a versatile method to control superlattice formation with potential for tuning charge carrier transport.   

Graphene/Ni Nanocomposite Materials from First Principles

Nanocomposite materials made by alternating layers of graphene and nickel offer exciting new possibilities for light-weight, high-strength materials. To better understand these systems, we used density functional theory with dispersion to first study graphene adsorbed onto Ni(111). The results indicate strong binding at the interface and a substantial perturbation of the graphene electronic structure, consistent with previous work. We mapped out the potential energy for sliding graphene on Ni(111) along the C-C bond and find a maximum binding energy that is substantially stronger than the interlayer binding in graphite. Ni 3$d$ to graphene $\pi $* charge transfer causes the strong chemisorption; although occupation of graphene's antibonding states likely affects the C-C bond strength. Next, we studied the bulk composite material with varying Ni layer thickness. Charge density analysis shows graphene sandwiched between Ni layers accepts charge from both layers, which should enhance the binding. The computed elastic coefficients will be presented.   

Session M38: Invited Session: The Keithley Award Session: Submolecular Resolution and Exchange Force Measurements Using Atomic Force Microscopy with Quartz Cantilevers

Joseph F. Keithley Award: Force microscopy with subatomic spatial resolution

For a long time, atomic force microscopy has been inferior to the scanning tunneling microscope (STM) in its spatial resolution, partially because measurements of small forces are more challenging than measurements of small currents. With the introduction of frequency modulation force microscopy, the static deflection measurement of a cantilever under a tip-sample force was replaced by a frequency measurement of an oscillating cantilever induced by an average force gradient. Atomic resolution of the challenging silicon reconstruction by frequency modulation atomic force microscopy has been demonstrated in 1995 using a silicon cantilever with a stiffness of $k=$17 N/m and an oscillation amplitude of $A=$34 nm. In 1996, a quartz cantilever (``qPlus sensor''), originally built from a quartz tuning fork from a wristwatch, has been proposed. At $k=$1800 N/m, this quartz sensor is 100 times stiffer than the original Si cantilever, allowing stable oscillation amplitudes down to fractions of an atomic diameter. It has a high Q factor, simple piezoelectric readout, little frequency variation with temperature and allows to simply mount metal tips as used in STM. The demonstration of high spatial resolution, the detection of very small forces, the capability to perform simultaneous STM and AFM as well as the ease of use of the qPlus sensor has led to its adaptation in leading scanning probe microscopy laboratories worldwide as well as in a growing number of commercial scanning probe instruments.   

Mapping the force-field of a hydrogen bonded assembly

Hydrogen-bonding underpins the structure, properties, and dynamics of a vast array of systems spanning a wide variety of scientific fields. From the striking complexity of the phase diagram of H$_{\mathrm{2}}$O and the elegance of base pair interactions in DNA, to the directionality inherent in supramolecular self-assembly at surfaces, hydrogen bonds play an essential role in directing intermolecular forces. Yet fundamental aspects of the H-bond, including the magnitude of the force and binding energy, force constant, and decay length associated with the interaction, have been vigorously debated for many decades. I will discuss how dynamic force microscopy (DFM) using a qPlus sensor can quantitatively map the tip-sample force-field for naphthalene tetracarboxylic diimide (NTCDI) molecules hydrogen-bonded in 2D assemblies. A comparison of experimental images and force spectra with their simulated counterparts from density functional theory calculations shows that image contrast due to intermolecular hydrogen bonds arises fundamentally from charge density depletion due to strong tip-sample interactions. Interpretation of DFM images of hydrogen bonds therefore necessitates detailed consideration of the coupled tip-molecule system: analyses based on intermolecular charge density in the absence of the tip fail to capture the essential physical chemistry underpinning the imaging mechanism.   

Single Molecules Investigated using atomically functionalized qPlus Sensors

Single organic molecules were investigated using scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM), and Kelvin probe force microscopy (KPFM) employing a qPlus sensor [F. J. Giessibl, \textit{Appl. Phys. Lett.} \textbf{73}, 3956 (1998)]. The resolution was increased due to tip functionalization by atomic manipulation. Using NC-AFM and CO functionalized tips, atomic resolution on molecules [L. Gross \textit{et al. Science} \textbf{325}, 1110 (2009)] and molecular structure identification was demonstrated [L. Gross \textit{et al. Nature Chem.} \textbf{2}, 821 (2010)]. Moreover, the bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes were distinguished [L. Gross \textit{et al.} \textit{Science} \textbf{337}, 1326 (2012)]. Using Xe terminated tips the adsorption height and tilt of individual molecules was determined [B. Schuler \textit{et al. PRL }\textbf{111}, 106103 (2013)]. With KPFM information about the intramolecular charge distribution was gained [F. Mohn \textit{et al.} \textit{Nature Nanotechnol.} \textbf{7}, 227 (2012)].   

Dynamic Force Imaging and Spectroscopy of Individual Molecules

In atomic force microscopy (AFM) the qPlus sensor [1] facilitates the use of metallic tips, which are typically used in scanning tunneling microscopy (STM), and thereby facilitates combined STM and AFM experiments at cryogenic temperatures. The use of CO-functionalized tips as has been introduced recently by Gross and co-workers [2] enabled unprecedented resolution and thereby fostered the rapid recent development of the field. We made use of the complementary information that STM and AFM can provide in different contexts. When applied to STM-based single-molecule synthesis, the combination of these techniques enables a direct quantification of the interplay of geometry and electronic coupling in metal-organic complexes in real space [3]. Further, we combined STM on semiconductors with Kelvin probe force spectroscopy (KPFS) performed simultaneously in the same setup with the very same tip. This combination of tools allows us to experimentally recover the zero point of the energy scale usually being obscured due to so-called tip-induced band bending when measuring on surfaces of semiconductors [4]. Finally, we used KPFS with sub-molecular resolution to image the polarity of individual bonds. \\[4pt] [1] F. J. Giessibl, Appl. Phys. Lett. 76, 1470 (2000);\\[0pt] [2] L. Gross et al., Science 325, 1110 (2009);\\[0pt] [3] F. Albrecht et al., JACS 135, 9200 (2013);\\[0pt] [4] G. M {\"u}nnich et al., Phys. Rev. Lett. 111, 216802 (2013).   

Real-space identification of intermolecular bonding with atomic force microscopy

A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms, whose formation and breaking result in chemical reactions and the production of new substances. Distinct from the covalent bond, the intermolecular interactions are often a vague concept elusive in experimental observations. Nevertheless, intermolecular interactions virtually affect all physical and chemical properties of substances in the condensed phases. The interactions between molecules, particularly the hydrogen bond, are responsible for the structural transformations and functions of biological molecules. Because most of the molecular characterization techniques are more sensitive to the covalent structures of the molecules, it remains a challenge to quantitatively study the weak interactions between molecules despite the tremendous efforts toward this goal. Here we report a real-space identification of the formation of hydrogen bonding between molecules adsorbed on metal substrate using a non-contact atomic force microscope (nc-AFM). The atomically resolved molecular structures with unprecedented details enable a precise determination of the characteristics of the hydrogen bond network, including bonding sites, orientations and lengths. The observed bond contrast was interpreted by ab initio density functional calculations that indicate the electron density contribution from the hybridized electronic state of hydrogen bond. Given the extensively discussion on the nature of hydrogen bonding and the recent redefinition by IUPAC, the observation of hydrogen bonding in real-space may be a stimulating evidence for theoretical chemistry. Meanwhile, the direct identification of local bonding configurations by nc-AFM would advance the understanding of intermolecular interactions in complex molecules with multiple active sites, offering complementary structural information essential for various applications in materials and biological sciences.   

Session M39: Invited Session: SmB6: A Possible Topological Kondo Insulator

Heavy Fermions, Rise of the Topologies

The electrons in Heavy fermion materials are subject to spin-orbit coupling interactions that greatly exceed their Kinetic energy. It has long been known that the spin orbit coupling stablizes new kinds of heavy fermion metals, superconductors and ``Kondo insulators'' against the competing state of magnetism. In this talk I will discuss the new realization that spin orbit copling can influence the ground state, changing its topology and giving rise to Topological Kondo insulators. We'll look at samarium hexaboride, SmB6, ``the worlds oldest topological insulator,'' a Kondo insulator discovered 45 years ago, predicted to be topological in 2011, and tentatively confirmed to be so in a series of hot new experimental studies of the past few months. I'll discuss a simple model for a topological Kondo insulator and introduce the most recent measurements, including ARPES, de Haas van Alphen and weak antilocalization that appear to support the idea that this is a strongly interacting topological insulator in which the surface conductance is carried by electrons on spin-orbit coupled Dirac cones. We'll also discuss the open unanswered questions surrounding this topic.   

Topological surface state in the Kondo insulator Samarium Hexaboride

Topological invariants of electron wave functions in condensed matter reveal many intriguing phenomena. The most exotic one is the topological insulator (TI) characterized by the Z2 group where an insulating bulk coexists with a metallic boundary state. Possible novel quantum states supporting coherent qubits using Majorana fermions with their potential for technological application have led to intense research into Bi based TIs with large band gap. However, the main complication concerning these Bi based materials is their considerable residual conductivity in the bulk, and only experimental techniques distinguishing bulk and surface clearly such as ARPES or STM can be used to explore the surface properties of these materials properly. Theories predict that a Kondo insulator SmB6, which evolves from a Kondo lattice metal to an insulator with a small gap as the temperature is lowered, could be a topological insulator. Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements, these were not able to prove that the metallic surface state is topologically protected. we report careful thickness dependence transport measurements on doped SmB6 which show that magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior supporting that that SmB6 is the first perfect 3D topological insulator with virtually zero residual bulk conductivity. We anticipate our results to be a starting point to explore the details of topological Kondo insulators and their potential applications toward scalable quantum information processing.   

Topological phases in mix valence compounds

In this talk, I will propose that the mix valence phenomena in some of the rare earth compounds will naturally lead to non-trivial topology in band structure.One of the typical example is SmB6, where the intermediate valence of Sm generates band inversion at the X point and the non-trivial Z2 index. Other than SmB6, YbB6 and YbB12 are both mix valence compounds. By applying LDA$+$Gutzwiller to these materials, we find that YbB6 has non-trivial Z2 index, indicating that YbB6 is another three dimensional topological insulator with strong correlation effects. Our calculation also finds that YbB12 is a trivial insulator in the sense of Z2 but it can be classified as topological crystalline insulator with non-zero mirror Chern number. The electronic structure at finite temperature has also been studied using LDA$+$DMFT, indicating YbB6 is still in the mix valence region while YbB12 is quite close to the Kondo limit.   

Topological Kondo Insulator (TKI) and related candidate materials: High-resolution ARPES studies

In this talk, I plan to present ARPES (synchrotron and laser-based) studies of several mix valence and Kondo insulator phenomena in some of the rare earth heavy fermion compounds in connection to their non-trivial topology of band structures. Focus will be on SmB6 which has been predicted to be a TKI recently. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we accessed the surface electronic structure of the anomalous conductivity regime. At low T, we observe in-gap states within a 4 meV energy window of the Fermi level, which lie clearly within the bulk insulating gap. The in-gap states are found to be suppressed and eventually disappear, as the temperature is raised in approaching the coherent Kondo lattice hybridization (30 K), which proves that the in-gap states strongly depend on the existence of Kondo lattice hybridization and the effective Kondo gap, in agreement with their theoretical predicted origin of topological surface states within the Kondo insulating gap . Our Fermi mapping at the energy corresponding to these in-gap states shows distinct Fermi pockets that enclose the three Kramers' points the surface Brillouin zone, which are remarkably consistent with the theoretically predicted topological surface Fermi surface in the topological Kondo insulating phase within the level of energy resolution. The observed Fermi surface topology of the in-gap states, their temperature dependence across the transport anomaly and Kondo lattice hybridization temperatures, as well as their robustness against repeated thermal recycling, collectively not only provide a unique insight illuminating the nature of the residual conductivity anomaly but also serve as a strong experimental evidence to the predicted topological Kondo insulator phase. I also plan to present results on YbB6 and YbB12 both of which are mix valence compounds. This work is in collaboration with \textbf{Madhab Neupane}, N. Alidoust, S.-Y. Xu, T. Kondo, Y. Ishida, D.-J. Kim, Chang Liu, I. Belopolski, T.-R. Chang, H.-T. Jeng, T. Durakiewicz, L. Balicas, H. Lin, A. Bansil, S. Shin and Z. Fisk and primarily supported by U.S. DOE and Princeton University.   

Hybridization Gap, Metallic Surface States and Quantum Transport in SmB$_6$

Topological insulators, with metallic boundary states protected against time-reversal-invariant perturbations, are a promising avenue for realizing exotic quantum states of matter. According to recent theoretical predictions, a topological insulating state can emerge not only from a weakly interacting system with strong spin-orbit coupling, but also in insulators driven by strong electron correlations. The Kondo insulator compound SmB$_6$ is an ideal candidate for realizing this exotic state of matter, with hybridization between itinerant conduction electrons and localized $f$-electrons driving an insulating gap that facilitates the emergence of topological surface states at low temperatures. In this talk I will discuss our point-contact spectroscopy studies of the bulk hybridization gap of SmB$_6$ and its relation to purported metallic surface states. I will also present milliKelvin magnetotransport studies that reveal both weak antilocalization and quantized conductance phenomena that provide strong evidence for topologically non-trivial surface states in SmB$_6$.   

Session M40: Invited Session: Onsager / Lilienfeld / UG Inst / Apker 1 Prize session

Lars Onsager Prize: Topological Defects in Condensed Matter Phases

Circulation quantization in superfluid 4He and superconductors. General principles of classification of topologically stable defects in ordered media. Superfluid phases of 3He. Topology at different scales of length. Superfluids under rotation. Biaxial nematics. Nonabelian disclinations. Half-quantum vortices: 3He-A, Sr2RuO4, exciton-polariton condensates, FFLO, Super Solid.   

Julius Edgar Lilienfeld Prize: Chaotic Dynamics in the Physical Sciences: Some Comments and Examples

Chaos was first discovered by Poincare in his famous 1887 work on the motion of N \textgreater 2 bodies interacting through gravitational attraction. Although steady progress was made by mathematicians following Poincare's work, widespread impact and development of chaos in the physical sciences is only comparatively recent, i.e., approximately starting in the 1970's. This talk will review this history and give some examples illustrating the types of questions, problems and results arising from perspectives resulting from widespread participation of physical scientists.   

Prize to a Faculty Member for Research in an Undergraduate: Chaotic mixing and front propagation

We present results from a series of experiments -- all done with undergraduate students -- on chaotic fluid mixing and the effects of fluid flows on the behavior of reaction systems. Simple, well-ordered laminar fluid flows can give rise to fluid mixing with complexity far beyond that of the underlying flow, with tracers that separate exponentially in time and invariant manifolds that act as barriers to transport. Recently, we have studied how fluid mixing affects the propagation of reaction fronts in a flow. This is an issue with applications to a wide range of systems including microfluidic chemical reactors, blooms of phytoplankton in the oceans, and the spreading of a disease in a moving population. To analyze and predict the behavior of the fronts, we generalize tools developed to describe passive mixing. In particular, the concept of an invariant manifold is expanded to account for reactive burning. ``Burning invariant manifolds'' (BIMs) are predicted and measured experimentally as structures in the flow that act as one-way barriers that block the motion of reaction fronts. We test these ideas experimentally in three fluid flows: (a) and chain of alternating vortices; (b) an extended, spatially-random pattern of vortices; and (c) a time-independent, three-dimensional, nested vortex flow. The reaction fronts are produced chemically with variations of the well-known Belousov-Zhabotinsky reaction.   

Using 3D Printing and Stereoscopic Imaging to Measure the Alignment and Rotation of Anisotropic Particles in Turbulence

We have developed a general methodology to experimentally measure the time-resolved Lagrangian orientation and solid body rotation rate of anisotropic particles with arbitrary aspect ratio from standard stereoscopic video image data. We apply these techniques to particles advected in a $R_\lambda \approx 110$ fluid flow, where turbulence is generated by two grids oscillating in phase. We use 3D printing technology to design and fabricate neutrally buoyant rods, crosses (two perpendicular rods), and jacks (three mutually perpendicular rods) with a largest dimension of 7 times the Kolmogorov length scale, which makes them good approximations to tracer particles. We have measured the mean square rotation rate, $\dot{p}_i \dot{p}_i$, of particles spanning the full range of aspect ratios and obtained results that agree with direct numerical simulations. Our measurements of the full solid-body rotation of jacks, in particular, are of broad experimental relevance because they demonstrate a new and extensible way to directly probe the Lagrangian vorticity of a fluid. Lastly, we will present our direct measurements of the alignment of crosses with the direction of their solid body rotation rate vector, demonstrating how turbulence aligns particles along their longest dimension.   

Session M41: Topological Superconductors: Bulk and Interfaces

Topological Superconductivity and the Strong Coupling Expansion

Due to the nontrivial winding of their order parameter phase, topological superconductors are expected to support Majorana Fermions in their vortex cores and for this reason have been an area of intense interest over the past couple decades. Current proposals for a device that may support Majorana Fermions are based on semiconductor heterostructures, where pairing is driven by proximity to a normal superconductor. We have recently found mean field results supporting the existence of topological superconductivity in a model with spin-orbit coupled electrons and pairing induced by interactions rather than proximity effect. This talk will look at developing methods to treat the opposite limit of this model, that of strongly coupled electrons. Results of a strong coupling expansion and the development of an analogy to the Gutzwiller approximation will be presented as well as data from renormalized mean field theory.   

Topological superconductivity at the edge of transition metal dichalcogenides

Time-reversal breaking topological superconductors are new states of matter which can support Majorana zero modes at the edge. In this paper, we propose a new realization of one-dimensional topological superconductivity and Majorana zero modes. The proposed system consists of a monolayer of transition metal dichalcogenides MX$_2$ (M=Mo, W; X=S, Se) on top of a superconducting substrate. Based on first-principles calculations, we show that a zigzag edge of the monolayer MX$_2$ terminated by metal atom M has edge states with strong spin-orbit coupling and spontaneous magnetization. By proximity coupling with a superconducting substrate, topological superconductivity can be induced at such an edge. We propose NbS$_2$ as a natural choice of substrate, and estimate the proximity induced superconducting gap based on first-principles calculation and low energy effective model. As an experimental consequence of our theory, we predict that Majorana zero modes can be detected at the 120$^\circ$ corner of a MX$_2$ flake in proximity with a superconducting substrate.   

Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a $\textit{d}$-wave high-temperature superconductor

The interplay of superconductivity and topological surface states which are protected by time-reversal symmetry provides a platform for exploring new quantum phenomena, such as Majorana zero modes that may find application in fault-tolerant quantum computation. Here, by growing high-quality topological insulator Bi$_2$Se$_3$ films on a $\textit{d}$-wave superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi$_2$Se$_3$ films with a large pairing gap up to 15 meV. Interestingly, distinct from the $\textit{d}$-wave pairing of Bi2212, the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant $\textit{s}$-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.   

Symmetry-protected topological invariant and Majorana impurity states in time-reversal invariant superconductors

We address the question of whether individual nonmagnetic impurities can induce zero-energy states in time reversal invariant topological superconductors, and define a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength [1]. These symmetries allow the definition of a position space topological $Z_2$ invariant, which is related to the standard bulk topological $Z_2$ invariant. Our general results are applied to the time reversal invariant $p$-wave phase of the doped Kitaev-Heisenberg model [2], where we demonstrate how a lattice of impurities can drive a topologically trivial system into the non-trivial phase. Finally, signatures of impurity states in the spin-susceptibility are described.\\[4pt] [1] L.~Kimme, T.~Hyart, and B.~Rosenow, arXiv:1308.2496, (2013).\\[0pt] [2] T.~Hyart, A.R.~Wright, G.~Khaliullin, and B.~Rosenow, Phys. Rev. B 85, 140510(R) (2012).   

Topological Blount's theorem of odd-parity superconductors

Nontrivial nodal structures are one of the most salient features of gap functions of the unconventional superconductors. In a system with spin-orbit coupling and crystal field, the group theory plays a key role to determine the node of the gap function [1]. From the group theoretical ground, Blount proved that the line node is ``vanishingly improbable'' in spin-triplet superconductors [2]. Namely, it is impossible to create a stable line node in odd-parity superconductors. Our motivation is to compare the group theoretical result with topological stability of nodes by K-theory [3] As a result, we found that K-theory not only rebuilds the original Blount's argument but also exhibits counterexamples with the stable line node. In this talk, we will show the physical interpretation of them. [1] M. Sigrist and K. Ueda, Rev. Mod. Phys. \textbf{63}, 239 (1991). [2] E. I. Blount, Phys. Rev. B, \textbf{32}, 2935 (1985). [3] P. Horava, Phys. Rev. Lett. \textbf{95}, 016405 (2005); Y. X. Zhao and Z. D. Wang, Phys. Rev. Lett. \textbf{110}, 240404 (2013).   

Quantum oscillations of $Cu_{x}Bi_{2}Se_{3}$ in intense magnetic field

Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. Recently there has been much interest in resolving the Fermi Surface of $Cu_{x}Bi_{2}Se_{3}$ to shed light on the nature of its superconducting state - in particular to determine if it is a topological superconductor, an exotic class of material. Using torque magnetometry, quantum oscillations in magnetization (the de Haas--van Alphen effect) were observed [1] in $Cu_{x}Bi_{2}Se_{3}$ up to 90 degrees in polar angle with respect to the sample surface. The doping of Cu in $Bi_{2}Se_{3}$ increases the carrier density and its ellipsoidal Fermi Surface becomes increasingly elongated. The detailed study of the temperature dependence at different tilt angles reveals strong effective mass anisotropy. The comparison of oscillation data in magnetization with that in magnetoresistance [2] helps elucidate the electronic structure of this interesting material. \\[4pt] [1] B.J. Lawson, Y.S. Hor, Lu Li, Phys. Rev. Lett. {\bf 109}, 226406 (2012).\\[0pt] [2] E. Lahoud, {\it et al.}, Phys. Rev. B {\bf 88}, 195107 (2013).   

Double Berry monopoles and topological surface states in the superconducting B-phase of UPt$_3$

The recent phase sensitive measurements in the superconducting B-phase of UPt$_3$ provide strong evidence for the triplet, chiral $k_z(k_x \pm ik_y)^2$ pairing symmetries, which endow the Cooper pairs with orbital angular momentum projections $L _z= \pm 2$ along the c-axis. Such pairing can support both line and point nodes, and both types of nodal quasiparticles possess nontrivial topology in the momentum space. We show that the point nodes located at the intersections of the closed Fermi surfaces with the c-axis, act as the double monopoles and the antimonopoles of the Berry curvature, and generalize the notion of Weyl quasiparticles. Consequently, the B phase should support an anomalous thermal Hall effect, various magnetoelectric effects such as the polar Kerr effect, in addition to the protected Fermi arcs on the (1,0,0) and the (0,1,0) surfaces. The line node at the Fermi surface equator acts as a vortex loop in the momentum space and gives rise to the zero energy, dispersionless Andreev bound states on the (0,0,1) surface. At the transition from the B-phase to the A-phase, the time reversal symmetry is restored, and only the nodal ring survives inside the A-phase.   

Absence of zero-energy surface bound states in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$ via a study of Andreev reflection spectroscopy

Cu$_{x}$Bi$_{2}$Se$_{3}$ has been proposed as a potential topological superconductor characterized by an odd-parity full bulk superconducting gap and zero-energy surface Andreev bound states (Majorana fermions). A predicted consequence of such Majorana fermions is a peak in the zero-energy density of states which should lead to a persistent zero-bias-conductance-peak (ZBCP) in Andreev reflection (AR) or tunneling experiments. Here we employ a newly developed nanoscale AR spectroscopy method to study normal metal/superconductor (N-S) devices featuring Cu$_{x}$Bi$_{2}$Se$_{3}$. The results show that a ZBCP can be tuned in or out from Cu$_{x}$Bi$_{2}$Se$_{3}$ samples depending on the N-S barrier strength. While the appearance of ZBCP may be traced to different origins, its absence under finite barrier strength represents the absence of zero-energy Majorana fermions. The present observations thus call for a reexamination of the intriguing superconductivity in Cu$_{x}$Bi$_{2}$Se$_{3}$.   

Point Contact Spectroscopy in Half-Heusler Compounds

The half-Heusler family of compounds have been predicted to exhibit topologically non-trivial behavior. Some members, including YPtBi and LuPtBi, exhibit superconductivity, suggesting the possibility of a topological superconductor. We have performed soft point contact spectroscopy measurements on the superconducting half-Heusler compound YPtBi to investigate gap structure and density of states as a function of temperature and magnetic field. We report properties of the conductivity spectra and discuss implications of the superconducting gap features for interpretation of the nature of superconductivity in these compounds.   

Fully gapped topological surface states in Bi$_2$Se$_3$ films induced by a $d$-wave high-temperature superconductor

Topological insulators are a new class of materials which are insulating in bulk but exhibit robust conducting surface states protected by time-reversal symmetry. The coupling between such symmetry-protected surface states and symmetry-broken states (for example, superconductivity) may lead to novel quantum phenomena, such as Majorana zero modes which are crucial for fault-tolerated quantum computation. Using molecular beam epitaxy, we have successfully grown high quality topological insulator Bi$_2$Se$_3$ films on high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. In this talk, I will present our recent work on superconducting proximity effect in Bi$_2$Se$_3$ films induced by high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. Using angle-resolved photoemission spectroscopy, we observe a proximity-induced gap up to 15 meV on the topological surface states of Bi$_2$Se$_3$ [1]. Bi$_2$Se$_3$/Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ heterostructure not only provides new opportunities for investigating the intriguing coupling between a topological insulator thin film and a d-wave superconductor, but also may be a new system for realizing Majorana zero modes. \\[4pt] [1] Eryin Wang et.al, {\it Nature Physics.}{\bf 9}, 621 (2013).   

Normal state electrodynamics of superconducting Cu$_x$Bi$_2$Se$_3$

Using infrared spectroscopy, we have studied the bulk electronic structure of superconducting Cu$_x$Bi$_2$Se$_3$ (x = 0.15, 0.2, 0.3, 0.4), a candidate topological superconductor. The screened plasma frequency is observed to red-shift monotonically with doping, ranging from 198 meV for x = 0.15 to 156 meV for x = 0.4. We have also investigated the effects of electron-boson coupling in this compound. An extended Drude analysis of the free carrier charge dynamics suggests a mass enhancement $m^*/m_b$ of roughly 1.2 for x = 0.2.   

Point-contact spectroscopy study of the pairing symmetry of candidate topological superconductors

The recently proposed topological superconducting materials are predicted to have odd parity paring and host Majorana fermions on the surface. Here we investigate the pairing symmetry of candidate topological superconductors, including CuxBi2Se3, Sn1-xInxTe, etc., via point-contact spectroscopy. The measurements are performed using both normal-metal gold tips and s-wave superconducting niobium tips. For samples with s-wave pairing, one would expect standard Andreev reflection in gold tip case and supercurrent-like behavior in niobium tip case. For CuxBi2Se3, however, we observe robust zero-bias conductance peak (ZBCP) in the differential conductance spectra with gold point contact, while with niobium point contact we find the height of the peak exhibiting an unusual non-monotonic temperature dependence. We argue that both observations cannot be explained by Andreev reflection within the standard BTK model, but signifying unconventional superconductivity in this material. For Sn1-xTnxTe samples, we observe ZBCP in the differential conductance spectra with gold point contact, while with niobium point contact, the temperature dependence of ZBCP is monotonic as expected from conventional theory, leaving the nature of the superconductivity of Sn1-xTnxTe still an open question.   

Proximity effect in the 3D topological insulator Bi2Te3

Topological insulators (TI) are electronic materials with a bulk band gap that is supplemented by protected conducting states on their edges or surfaces in the 2- and 3- dimensional cases, respectively. This study reports the magnetotransport response observed in the 3D topological insulator Bi$_{2}$Te$_{3}$ with indium superconducting electrodes, and demonstrates two critical transitions in the magnetoresistive response with decreasing temperatures below T$=$ 3.4K. Here, the first transition is attributed to superconductivity in the In electrodes, as the second transition is attributed to the proximity effect in this hybrid TI/SC structure.   

Classification of Two Dimensional Topological Crystalline Superconductors and Majorana Bound States at Disclinations

We classify discrete-rotation symmetric topological crystalline superconductors (TCS) in two dimensions and provide the criteria for a zero energy Majorana bound state (MBS) to be present at composite defects made from magnetic flux, dislocations, and disclinations. In addition to the Chern number that encodes chirality, discrete rotation symmetry further divides TCS into distinct stable topological classes according to the rotation eigenspectrum of Bogoliubov-de Gennes quasi-particles. Conical crystalline defects are shown to be able to accommodate robust MBS when a certain combination of these bulk topological invariants is non-trivial as dictated by the index theorems proved within. The number parity of MBS is counted by a $Z_2$-valued index that solely depends on the disclination and the topological class of the TCS. We also discuss the implications for corner-bound Majorana modes on the boundary of topological crystalline superconductors   

Study of the Topological Crystalline Insulator, SnTe and Sn-In-Te systems in the form of nanomaterials

We have studied the Topological Crystalline Insulator SnTe and the superconducting variant which arises due to the partial substitution of Sn atoms with indium, Sn$_{\mathrm{1-x}}$In$_{\mathrm{x}}$Te, in this rock salt structure. The observable topological features are thought to be enhanced by increasing the surface area to volume ratio of the materials and therefore suppressing contributions from the bulk. We report the first evidence for the growth of SnTe and SnInTe nanowires starting from bulk crystals. The nanowires produced are typically 20 $\mu$m in length and 20 nm wide. The stoichiometries of these structures have been determined to compare with that of the source material. Various morphologies of nanomaterials are observed and the optimal conditions and processes involved to obtain these are discussed.   

Session M42: Spectroscopy of Topological Insulators

Appearance and suppression of electron interference patterns on the surface of Ho$_{0.05}$Bi$_{1.95}$Se$_{3}$

We present study on the surface property of a magnetically doped topological insulator, Ho$_{0.05}$Bi$_{1.95}$Se$_{3}$. By using scanning tunneling microscopy (STM) and spectroscopy (STS), we obtained topographic images with several distinct types of defects, some of which originate from Holmium dopant atoms and conductance maps at various bias voltages. It is found that conductance maps in the upper and lower branches of helical Dirac cone show interference patterns around the defects. Interestingly, the interference patterns were substantially suppressed in the range of 40 meV close to the Dirac point. Our analysis shows that the surface states of Ho$_{0.05}$Bi$_{1.95}$Se$_{3}$ behave differently than other magnetically doped topological insulators.   

Quasiparticle interference on the surfaces of the of the layered topological superlattice Bi$_{4}$Se$_{3}$

Three-dimensional topological insulators (TIs) host robust surface states with massless Dirac-like dispersion and helical spin texture. The possibility of layering TIs with other materials in a superlattice is especially intriguing, as exotic phenomena are predicted to occur at their boundary. Here, we present scanning tunneling microscopy and spectroscopy (STM/STS) results on one of the simplest such superlattices, Bi$_{4}$Se$_{3}$, which consists of alternating layers of a three-dimensional TI, Bi$_{2}$Se$_{3}$, and a two-dimensional TI, Bi$_{2}$. STM topographs reveal two distinct, alternating surfaces, each of which harbors dispersing surface states. By using Fourier-transform STS, we characterize the dispersion of these states, which is considerably more complex than that of the single Dirac cone found in prototypical three-dimensional TIs. In addition, we show that the surface states of Bi$_{4}$Se$_{3}$ are strongly influenced by proximity to atomic defects.   

Evolution of Topological Surface States in Tunable Topological Insulators

Bi$_{\mathrm{1-x}}$Sb$_{x}$ (0.07\textless $x$\textless 0.22) is the first generation of 3D topological insulators (TIs), possessing a bandgap and topological surface states (SSs) generated by spin-orbit coupling In fact, within the whole range of 0\textless $x$\textless 1 (i.e. from pure Bi to pure Sb), a topological phase transition has to occur as the system is twisted from topologically trivial to nontrivial phases, even though it becomes a semimetal hosting a negative indirect gap Therefore, taking advantage of Fourier-transform scanning tunneling spectroscopy (FT-STS) and \textit{ab initio} calculations, we investigate the progressive evolution of topological SSs in the tunable Bi$_{\mathrm{1-x}}$Sb$_{x}$ (0\textless $x$\textless 1) materials grown by means of molecular beam epitaxy In alloys with several representative compositions, quasiparticle interference (QPI) patterns of SSs exhibit dramatic dependence on $x$ values, indicating that intra-surface scatterings are ultimately determined by band structures and the associated spin textures. Additionally, the corresponding simulated QPI patterns are also revealed based on \textit{ab initio} calculations. Such systematic studies of the Bi$_{\mathrm{1-x}}$Sb$_{x}$ alloy family can be further explored to tailor surface energetic and transport properties for potential applications in quantum information, spintronics and many other topological quantum phenomena.   

Scanning tunneling spectroscopic (STS) studies of the bulk magnetic doping effects on the surface state of Bi$_{2}$Se$_{3}^{\ast }$

We report STS studies of MBE-grown undoped and Cr-doped Bi$_{2}$Se$_{3}$ bi-layers on InP (111) and as a function of the updoped layer thickness and the Cr-doping level ($x)$. Our studies reveal \textit{gapless} Dirac spectra at all temperatures ($T)$ for samples with an undoped top layer larger than 5 QLs, implying that the interlayer magnetic correlation length $\xi_{\bot }$ is \textless $\sim$ 5-QL. For samples with an undoped top layer smaller than 5 QLs, STS reveals \textit{gapped} spectra at $T$ \textless $T_{c}=$ (260 $\pm$ 20) K. The gap is spatially inhomogeneous and increases with decreasing $T$, reaching an $x$-independent maximum $\Delta =$ (0.8 $\pm$ 0.2) eV at $T$ \textless \textless $T_{c}$. Further, the gap inhomogeneity increases with decreasing $x$, showing magnetic clusters separated by gapless regions and an in-plane magnetic correlation length $\xi_{\vert \vert }$ $\sim$ 8-QL. We also find spatially localized double and single resonance peaks in the gapless regions, and their areal densities peak near $T_{c}$. We attribute the resonance sites to isolated Cr impurities, which couple with the spins of surrounding Dirac electrons and form localized topological spin textures of a long lifetime. With increasing interlayer magnetic field, the resonance sites diminish and the gap distribution becomes more homogeneous.   

Anisotropic Dirac Cones in SrMnBi2 and CaMnBi2 Revealed by Angle-Resolved Photoemission

By carrying out high resolution angle-resolved photoemission measurements on SrMnBi2 and CaMnBi2, we have directly revealed the existence of anisotropic Dirac cone in these two compounds. In particular, we find that SrMnBi2 and CaMnBi2 exhibit markedly different anisotropy in their Dirac cone. In SrMnBi2, four Dirac cones are well separated in the first Brillouin zone forming four crescent moon-like Fermi surface sheets and the ratio of Fermi velocities along and perpendicular to Gamma-M direction is $\sim$18. In CaMnBi2, however, the Dirac cones are connected in the first Brillouin zone forming a ridge-like Fermi surface topology indicating CaMnBi2 as a new Dirac material with a single Dirac cone and giant Dirac anisotropy. The dichotomy of the Dirac cone anisotropy between SrMnBi2 and CaMnBi2 originates from a different local Sr (Ca) arrangement surrounding the Bi square net. It demonstrates the feasibility in engineering the Dirac cone anisotropy by manipulating the environment of the Bi square net.   

Probing the pressure-induced topological phase transition in BiTeI

The recent intensive development of topological insulators was boosted by the discovery of real material systems. To expand the topological insulator family, pressure has been proposed to induce topological phase transitions in a variety of topologically trivial materials, but experimental realization is hindered by the lack of appropriate surface-sensitive probes applicable under high pressure. Here we discuss signatures of pressure-induced band gap closing and reopening in the bulk of the narrow-gap semiconductor BiTeI from infrared spectroscopy and x-ray powder diffraction. Combined with theoretical calculations, our experimental results strongly support a pressure-induced topological phase transition in BiTeI. These experimental methods can be valuable in the study of certain topological phase transitions when surface-sensitive probes are not applicable.   

Infrared Magnetospectroscopy of Dirac Plasmons in Topological Insulator Ribbons

We present the infrared spectroscopy study of magnetoplasmons in patterned topological insulator (Bi$_{2}$Se$_{3})$ ribbon arrays. The measurement is performed in Faraday configuration with incident infrared light polarized parallel or perpendicular to the ribbon direction and in a high magnetic field up to 18 T. We demonstrate that the collective oscillations of Dirac fermions (i.e., plasmons) in topological insulators can be coupled with the cyclotron resonance, forming the so-called upper-hybrid-mode. This mode exhibits a characteristic magnetic field dependence, with an effective mass consistent with that obtained from the unpatterned two-dimensional sample. Due to the high quality of the MBE grown topological insulator thin films, a higher order plasmon mode is also evidenced in our measurements. This work is supported by the DOE (DE-FG02-07ER46451), the NSF (DMR-0845464), and the ONR (N000141210456).   

Optical properties of the doped Bi$_{2}$Te$_{3}$ single crystals: electron-phonon coupling and bulk-boundary correspondence

We studied the optical properties of Bi$_{2}$Te$_{3}$ single crystals doped with La (8{\%}), Ce (8{\%}), Fe(8{\%}, 15{\%}) via a combination of terahertz time-domain spectroscopy and spectroscopic ellipsometry. We observed the Drude peak and the in-plane phonon near 60 cm$^{-1}$ in the optical conductivity in the terahertz regime as well as the absorptions corresponding to the bulk interband transitions in the far-infrared and the visible-ultraviolet regions. We confirmed that the terahertz in-plane phonon asymmetry can be employed as spectroscopic evidence in order to investigate the electron-phonon interaction arising from external dopants while the bulk band structure can be determined from ellipsometry.   

Raman Scattering studies of Topological Insulators Bi$_2$Te$_3$, Bi$_2$Se$_3$, and Sb$_2$Te$_3$

Raman scattering spectroscopy has been used to probe the bismuth and antimony chalcogenides Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$. Room temperature spectra of these topological insulators reveal new phonon modes in spectral regions not previously investigated. Polarization studies have been done to characterize these modes. New results on the temperature dependence of the Raman active phonon modes in these materials will also be presented.   

Optics of midgap impurity states on a surface of a topological insulator

The time reversal symmetry breaking on a surface of a topological insulator leads to a pronounced anomalous quantum Hall effect that can manifest itself in the quantized Kerr and Faraday effects [1,2]. The symmetry can be broken by magnetic impurities introduced to the TI bulk which tend to order and to gap the surface spectrum. We investigate the role of midgap impurity states in the surface spectrum in optics, particularly in the Faraday and Kerr effects. 1. J. Maciejko, X.L. Qi, H.D. Drew, S.C. Zhang, Phys. Rev. Lett., 105, 166803 (2010). 2. W.-K. Tse and A. H. MacDonald, Phys. Rev. B 82, 161104 (2010).   

Spectroscopic Evidence for the Emergence of a Half-Metallic Surface State on the Bulk Insulator Sodium Cobaltate

In recent years Na$_{x}$CoO$_{2}$ has attracted much attention for its unconventional superconductivity and antiferromagnetic phases. More recently, the stoichiometric compound NaCoO$_{2}$ has been proposed as a platform for achieving topological superconductivity through its predicted half-metallic surface state. We characterize this surface state and its relationship to local sodium concentration using low-temperature scanning tunneling spectroscopy (STS) and tuning fork-based atomic force microscopy. We also examine the magnetic moment of the surface state through temperature-dependent STS and Kerr rotation spectroscopy. These results are compared with density functional theory-calculated band structure and local density of states.   

Session M43: Electronic Structures of Insulators and Superconductors

Stress induced tunable dielectric LiAsSe2

Tunable dielectric materials have many applications for the new electric devices. Traditionally, electric field is applied in order to tune the dielectric. We proposed a new material - LiAsSe2 (space group $Cc$) as the potential tunable dielectric material. First-principle calculations show that with applying stress along [100] and [010] direction, dielectric constant can be enlarged by around factor of 3 depending on the stress magnitude. With calculated electronic structure, optical dielectric and absorption spectrum also show very large magnitude difference before and after applying stress. This large difference comes from the structure change. By shrinking in these two directions, the original alternating bonds length between neighboring As and Se atomic chain become even bonds. The top of valence band and bottom of conduction bands tend to exclusively contain Se $p_z$ orbital and As $p_z$ orbital. As a result, inter-band transition between there bands gives much larger transition dipoles. DFT+$U$ method is used in order to correct self-interaction error, especially to As atoms.   

Valence band study of Sm$_{0.1}$Ca$_{0.9-x}$Sr$_{\mathrm{x}}$MnO$_{3}$ using high resolution photoemission spectroscopy

We have studied the valence band electronic structure of Sm$_{0.1}$Ca$_{\mathrm{0.9-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ (x $=$ 0, 0.1, 0.3 and 0.6) at various temperatures using high resolution photoemission spectroscopy (HRPES). The data were taken using a Scienta R4000 energy analyser and the resolution was set at 5 meV. The doping dependent studies of Sm$_{0.1}$Ca$_{\mathrm{0.9-x}}$Sr$_{\mathrm{x}}$MnO$_{3}$ at 50 K, 100 K and 295 K are quite interesting. The density of e$_{\mathrm{g}}$ states near the Fermi level decreases with Sr substitution at the Ca site at 50 K. Also the similar trend has been observed at 100 K. At 295 K the changes in the e$_{\mathrm{g}}$ states is quite different than the earlier temperatures where the intensity remains the same for x $=$ 0, 0.1 and 0.3 and then decreases for x $=$ 0.6. These changes in the density of states near the Fermi level will be explained by taking into account the structural, electrical and magnetic properties associated with this system.   

Investigation of the Band Alignment at h-BN/SiX Dielectric Interfaces utilizing X-ray Photoemission

Due to a wide band gap ($\sim $ 6 eV), close lattice matching (\textless 2{\%}) and atomic planarity, hexagonal boron nitride (h-BN) is of interest as a potential substrate and gate dielectric in graphene channel transistor devices. A key property for the success of h-BN as a gate dielectric in such devices is its interfacial band alignment with graphene, the gate contact metallization and the surrounding insulating dielectric materials. In this regard, we have utilized x-ray photoelectron spectroscopy (XPS) to determine the Schottky barrier and valence band offsets present at the interfaces between plasma enhanced chemically vapor deposited amorphous h-BN:H and a variety of materials including graphene, Cu, SiO$_{\mathrm{2}}$, a-SiN$_{\mathrm{x}}$:H, a-SiC:H, and Si. We show that in many instances the valence and conduction band offsets are significant ($\ge $ 2 eV) and favorable for a variety of possible h-BN/graphene transistor devices.   

Ab initio calculation of d-metal L-edge RIXS spectra using many-body quantum chemistry methods

We designed a fully ab initio quantum chemistry scheme for the computation of both d-d excitation energies and intensities as measured by resonant inelastic x-ray scattering (RIXS) in d-electron systems. RIXS has recently emerged as a powerful tool to reliably probe the charge, spin, and orbital degrees of freedom of correlated electrons in solids [1,2]. As a first application we picked up Li$_2$CuO$_2$, a quasi-1D Cu 3$d^9$ oxide with a simple valence configuration in the intermediate state. We use embedded-cluster MCSCF and MRCI techniques [3], including scalar relativistic effects, spin-orbit coupling, and the valence orbital relaxation in the presence of the core hole. The transition matrix elements of the dipole operator are obtained by non-orthogonal configuration interaction. A careful analysis of the RIXS spectra is important for understanding the interplay between local distortions and longer-range lattice anisotropy and its effect on the d-level electronic structure [3,4] and magnetic interactions [4]. [1] L. Ament {\it et al.} Rev. Mod. Phys. {\bf 83}, 705 (2011); [2] J. Schlappa{\it et al.} Nature {\bf 485}, 82 (2012); [3] H.-Y. Huang {\it et al.} Phys. Rev. B {\bf 84}, 235125 (2011); [4] N.~A. Bogdanov {\it et al.} Phys. Rev. Lett. {\bf 110}, 127206 (2013).   

The electronic structure of thorium halides predicted by HSE and GW

Recently, there has been a significant experimental push to measuring the VUV nuclear excitation of $^{229}$Th using optical spectroscopy. Large band gap Thorium halides such as ThF$_{4}$ and Na$_{2}$ThF$_{6}$ have been suggested as candidate materials for studying this nuclear transition, as they are transparent to the relevant optical frequencies. In this work, we compare the many body GW approach, hybrid density functional theory, and local density approximation calculations of the electronic structure of these materials, as well as the rest of the binary thorium halides (ThX$_{4}$, X=Cl,Br,I).   

The low temperature Fermi surface of IrTe2 probed by quantum oscillations.

The transition metal dichalcogenide IrTe2 undergoes a structural transition at 280K [1]; doping on the Ir site suppresses this transition and induces superconductivity with $T$c of about 3K [2]. The nature of the structural transition is possibly driven by charge disproportionation and the effect this has on the electronic structure of the superconducting state is not fully understood. We report a low temperature investigation of the Fermi surface of IrTe2 from quantum oscillations, using torque measurements performed in magnetic fields up to 33T and temperatures down to 0.3K. The observed extremal areas of the Fermi surface likely correspond to frequencies of a reconstructed Fermi surface, with light effective masses below 0.8me. The angular dependence of these frequencies across multiple crystals of IrTe2 suggests these materials are prone to domain formation upon cooling. We compare our measured Fermi surface with those predicted by electronic structure calculations, based upon the existing structural models, for both above and below the structural transition. This work was supported by EPSRC (UK) and partly by EuroMagnet (EU contract number 228043). [1] Matsumoto et al., J. Low Temp. Phys. 117, 1129 (1999) [2] Fang et al., Sci. Rep. 3, 1153 (2013)   

Structural phase transition induced by van Hove singularity in 5$d$ transition metal compound IrTe$_{2}$

Comprehensive studies of the electronic states of Ir 5$d$ and Te 5$p$ have been performed to elucidate the origin of the structural phase transition in IrTe$_{2}$ by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5$d$ electronic states across the transition, indicating that the Ir 5$d$ orbitals are not involved in the transition, we reveal a van Hove singularity at the Fermi level ($E_{\mathrm{F}})$ related to the Te $p_{\mathrm{x}}+p_{\mathrm{y}}$ orbitals, which is removed from $E_{\mathrm{F}}$ at low temperatures. The wavevector connecting the adjacent saddle points is consistent with the in-plane projection of the superstructure modulation wavevector. These results can be qualitatively understood with the Rice-Scott ``saddle-point'' mechanism, while effects of the lattice distortions need to be additionally involved.   

Investigation of strain effects in the charge density waves of $2H$-NbSe$_2$

The transition metal dichalcogenide $2H$-NbSe$_2$ possesses coexisting superconducting and charge density wave (CDW) ordered states, which possibly compete below the critical transition temperature, $\sim$7.2K. Previous studies of this system have seen that the CDW is very susceptible to alterations due to local strain. This suggests proximity to a quantum critical point. In this study, we use scanning tunneling microscopy and spectroscopy to investigate the intimate relationship between mechanical strain and the electronic properties of $2H$-NbSe$_2$, and observe the evolution of electronic states on atomic length scales.   

Enhanced Electron Interactions on Hydrogen Adsorbed ZnO Surface

Zinc oxide (ZnO) is a wide band- gap semiconductor (3.37 eV), and is widely used as a catalyst, a chemical sensor, and a variety of electronic and photonic devices. Recent studies have revealed that a two-dimensional electron gas (2DEG) is formed on a hydrogen adsorbed ZnO(10$\bar{1}$0) surface. However, a precise structure of the 2DEG on a ZnO surface is still uncertain. We have investigated the electronic states using angle-resolved photoemission spectroscopy (ARPES), and found the clear incoherent states associated with the coherent metallic peaks near the Fermi-level, giving direct evidence of many-body interactions inherent to 2D metallic states. The incoherent states are enhanced by the hydrogen adsorptions. Thus, we suggest that the incoherent peaks are originated from electron-phonon and electron-electrons interactions enhanced by the electron doping on the surface.   

Angle-Resolved Synchrotron Photoemission Spectroscopy and Density Functional Theory Studies on Iridium Modified Si(111) Surface

The physical and electronic properties of Ir modified Si(111) surface has been investigated with the help of Angle Resolved Photoemission Spectroscopy (ARPES) and Density Functional Theory (DFT). The surface consists of Ir-ring clusters that form $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstruction. The band structure around the Fermi level is dominated by the projected bulk states and the states originating from the `1x1' domains of the underlying Si substrate. The dispersion of these states is heavily modified due to umklapp scattering from the surface Brillouion zone. The morphology of the surface and the origins of the observed electronic states are explored and confirmed by DFT calculations.   

3-dimensional electronic structures of CaC6

There is still remaining issues on origin of superconductivity in graphite intercalation compounds, especially CaC6 because of its relatively high transition temperature than other GICs. There are two competing theories on where the superconductivity occurs in this material; intercalant metal or charge doped graphene layer. To elucidate this issue, it is necessary to confirm existence of intercalant driven band. Therefore, we performed 3 dimensional electronic structure studies with ARPES to find out 3d dispersive intercalant band. However, we could not observe it, instead observed 3d dispersive carbon band. This support the aspect of charge doped graphene superconductivity more than intercalant driving aspect.   

Doping dependence of dispersion renormalizations in strongly correlated materials with electron-phonon coupling

The renormalization of band dispersions by interactions provides insight into the underlying physics in a strongly correlated material. Coupling between electrons and bosons leads to dispersion kinks at the boson energy, while electronic interactions alone can lead to strong correlation kinks, which generally occur at a higher energy scale. Both of these types of kinks have been observed by ARPES in a variety of strongly correlated materials. Since dispersion kinks yield information on interaction strength, in a system with multiple strong interactions, it is important to disentangle the effects of these interactions on dispersion renormalizations. In order to gain insight into this issue, we simulate the single-band Hubbard-Holstein model using determinant quantum Monte Carlo, a numerically exact technique which allows the electron-electron and electron-phonon interactions to be treated on an equal footing. By analyzing the single particle spectral function for a variety of electron-electron and electron-phonon interaction strengths, we characterize how the interplay of these interactions influences the kink structure, and in particular, its evolution with hole and electron doping.   

Real-time studies of the atomic layer deposition of metal oxides using Ambient pressure x-ray photoelectron spectroscopy

Performing atomic layer deposition (ALD) of metal oxides at pressures around 0.01 mbar slows the half reactions of the process to allow \textit{in situ} real-time probing of changes in the surface electronic structure using Ambient pressure x-ray photoelectron spectroscopy (APXPS). By monitoring the ALD process as it occurs, new details on the mechanisms of interface formation and thin film growth can be obtained. The deposition of HfO$_{2}$ on InAs and the deposition of TiO$_{2}$ on rutile titania from transition metal complexes and water were studied with APXPS. Predictable, cyclic chemical shifts of ligand and substrate ionizations are seen in the growth of the films, but the kinetics of the film growth differs for the two systems. Upon exposure to the titania surface, the titanium precursor reacts straightaway and gradually proceeds to completion. In contrast, the hafnium precursor does not interact with the surface immediately. Once an activation barrier is surpassed, the reaction occurs instantaneously. By understanding the reactivity of different precursors, the ALD process can be more easily optimized in applications that require thin films of metal oxides such as metal-oxide-semiconductor devices and catalytic surfaces.   

The relative influence of many body and molecular motion effects for near-threshold N K x-ray emission spectra in ionic nitrogen insulating compounds

Our previous studies of the factors affecting the N K x-ray emission spectra of ammonium chloride and ammonium nitrate have revealed the importance of many body effects and molecular motion at an excitation energy well above the K edge. Quasiparticle lifetimes of the valence bands and zero point motion of the molecular groups have proven to be unusually significant. Large changes are observed experimentally in the x-ray emission spectra of these compounds as the excitation energy is progressively lowered towards threshold. We compare experimental results with initial calculations of the spectra including excitonic effects, self-energy contributions, and molecular motion.   

Effects of phonon broadening on x-ray near-edge spectra in molecular crystals

Calculations of near-edge x-ray spectra are often carried out using the average atomic coordinates from x-ray or neutron scattering experiments or from density functional theory (DFT) energy minimization. This neglects disorder from thermal and zero-point vibrations. Here we look at the nitrogen K-edge of ammonium chloride and ammonium nitrate, comparing Bethe-Salpeter calculations of absorption and fluorescence to experiment. We find that intra-molecular vibrational effects lead to significant, non-uniform broadening of the spectra, and that for some features zero-point motion is the primary source of the observed shape.   

Session M44: Focus Session: Defects in Semiconductors: Oxides

OH centers and the conductivity of hydrogen doped In$_{2}$O$_{3}$ single crystals

Mechanisms for the n-type conductivity of In$_2$O$_3$ have been controversial. Recent experiments suggest that O vacancies are the cause of conductivity.\footnote{S. Lee and D.C. Paine, Appl. Phys. Lett. \textbf{102}, 052101 (2013).} However, other recent experiments find that the H-doping of thin films gives rise to shallow donors.\footnote{T. Koida \textit{et al.}, Jpn. J. Appl. Phys. \textbf{46}, L685 (2007).} Theory also finds that interstitial H and H at an O vacancy are shallow donors in In$_{2}$O$_{3}$.\footnote{S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009).} We have performed a series of IR absorption experiments to determine the properties of OH and OD centers in In$_{2}$O$_{3}$ single crystals. Annealing In$_{2}$O$_{3}$ samples in H$_{2}$ or D$_{2}$ at temperatures near 450$^{\circ}$C (30 min) produces an n-type layer $\approx $0.05 mm thick with an n-type doping of 2x10$^{9}$ cm$^{-3}$. The resulting free-carrier absorption is correlated with an OH center with a vibrational frequency of 3306 cm$^{-3}$ that we associate with interstitial H.\footnote{M. Stavola, J. Appl. Phys., to be published.}   

Hydrogen dynamics in indium oxide

It has been recognized that hydrogen is a shallow donor in several transparent oxides, including In$_{2}$O$_{3}$ [1]. Both interstitial H and H at an O vacancy have been suggested as candidates [2]. We have used the CRYSTAL06 code[3] with a hybridized DFT Hamiltonian to determine equilibrium positions and vibrational frequencies for both of these cases as well as for H at an In vacancy. While the bixbyite structure [4] of In$_{2}$O$_{3}$ has overall cubic symmetry, its remarkable internal asymmetries lead to a number of candidate locations for the H, each of which has different vibrational frequencies. This enables potential assignments of the experimental IR results obtained by Yin \textit{et al} [5]. Furthermore, the unique topology of In$_{2}$O$_{3}$ leads to constraints on possible H diffusion paths, which we have also investigated. \\[4pt] [1] P. D. C. King \textit{et al.}, Phys. Rev. B \textbf{80}, 081201(R) (2009).\\[0pt] [2] S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009).\\[0pt] [3] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual }(University of Torino, Torino, 2006). \\[0pt] [4] M. Marezio, Acta Crystallogr. \textbf{20}, 723 (1966).\\[0pt] [5] W. Yin \textit{et al.}, this meeting; M. Stavola \textit{et al.}, J. Appl. Phys. (to be published).   

Structural Properties of Amorphous Indium-Based Oxides

Amorphous transparent conducting and semiconducting oxides exhibit similar or even superior properties to those observed in their crystalline counterparts. To understand how the structural properties change upon amorphization and how chemical composition affects the local and long-range structure of the amorphous oxides, we employ first-principles molecular dynamics to generate amorphous In-X-O with X$=$Zn, Ga, Sn, Ge, Y, or Sc, and compare their local structure features to those obtained for amorphous and crystalline indium oxide. The results reveal that the short-range structure of the Metal-O polyhedra is generally preserved in the amorphous oxides; however, different metals (In and X) show quantitatively or qualitatively different behavior. Some of the metals retain their natural distances and/or coordination; while others allow for a highly distorted environment and thus favor ``defect'' formation under variable oxygen conditions. At the same time, we find that the presence of X increases both the average In-O coordination and the number of the 6-coordinated In atoms as compared to those in IO. The improved In coordination may be responsible for the observed reduction in the carrier concentration as the substitution level in In-X-O increases.   

Bandgap narrowing of TiO$_{2}$ via codoping for enhanced photocatalytic reactions

We investigated the growth of Cr$-$N codoped single-crystal anatase TiO$_{2}$(001) (A-TiO$_{2})$ thin films using pulsed laser deposition with a target of Cr$_{2}$O$_{3}$ and TiN mixture. N concentrations were finely tuned under different growth temperatures and oxygen pressures, and high quality films with atomically flat terraces were obtained. UV$-$Vis absorption measurements showed that the band-gap of the codoped A-TiO$_{2}$ film is significantly narrowed in comparison with the undoped and monoelement doped films. We further systematically investigated the structures and the activity of the oxidized and reduced (1 $\times$ 4) reconstructed surfaces of A-TiO$_{2}$ epitaxially grown on SrTiO$_{3}$ using scanning tunneling microscopy/spectroscopy, X-ray/ultraviolet photoemission spectroscopy and first-principles calculations. Quite unexpectedly, it is found that the perfect (1 $\times$ 4) surface of A-TiO$_{2}$ is not even active for H$_{2}$O and O$_{2}$ adsorption at room temperature. Two types of intrinsic point defects are identified, among which only the Ti$^{3+}$ defect site on the reduced surface demonstrates considerable activity for H$_{2}$O and O$_{2}$ adsorption. The perfect surface itself should be fully oxidized, but shows no obvious activity. We thus propose an oxidized ridge model for the reconstructed (1 $\times$ 4) surface, where the Ti atoms at the normal ridge sites are sixfold coordinated. The Ti-rich point defects on reduced surface are fourfold-coordinated. This model provides consistent explanations for our experimental observations We have compared the results with those from rutile TiO$_{2}$(001)-(1 $\times$ 1) surface in our investigations. Our findings suggest that the activity of the A-TiO$_{2}$ surface should depend on its reduction status, similar to that of rutile TiO$_{2}$ surfaces.   

Hole polaron-polaron interaction in transition metal oxides and its limit to p-type doping

Traditionally the origin of the poor p-type conductivity in some transition metal oxides (TMOs) was attributed to the limited hole concentration: the charge-compensating donor defects, such as oxygen vacancies and cation interstitials, can form spontaneously as the Fermi energy shifts down to near the valence band maximum. Besides the thermodynamic limit to the hole concentration, the limit to the hole mobility can be another possible reason, e.g., the hole carrier can form self-trapped polarons with very low carrier mobility. Although isolated hole polarons had been found in some TMOs, the polaron-polaron interaction is not well-studied. Here we show that in TMOs such as TiO$_2$ and V$_2$O$_5$, the hole polarons prefer to bind with each other to form bipolarons, which are more stable than free hole carriers or separated polarons. This pushes the hole states upward into the conduction band and traps the holes. The rise of the Fermi energy suppresses the spontaneous formation of the charge-compensating donor defects, so the conventional mechanism becomes ineffective. Since it can happen in the impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs.   

Electronic and Magnetic Structure of C and N doped rutile-TiO$_{2}$: an \textit{ab-initio} DFT study

We study the electronic and magnetic structure of carbon and nitrogen impurities and interstitials in rutile TiO${_2}$. To this end we perform \textit{ab-initio} calculations of a 48-atom supercell employing the VASP code. In order to obtain a realistic description of the size of the band gap, exchange and correlation are treated with functionals beyond ordinary LSDA. Both, atomic positions and cell dimensions are fully relaxed. Substitutional carbon and nitrogen are found to have a magnetic moment of 2 and 1$\mu{_B}$, respectively, with a tendency for anti-ferromagnetic long range order. For C/N on interstitial sites we find that carbon is non-magnetic while nitrogen always possesses a magnetic moment of 1$\mu{_B}$. We find that these interstitial positions are on a saddle point of the total energy. The stable configuration is reached when both carbon and nitrogen form a CO and NO dimer with a bond length close to the double bond for CO and NO. This result is in agreement with earlier experimental investigations detecting such NO entities from XPS measurements. For all configurations investigated both C and N states are found inside the TiO${_2}$ gap. These new electronic states are discussed with respect to tuning doped TiO${_2}$ for the application in photocatalysis.   

A Coupling Mechanism between Co-doped Acceptors and its Control on Optical Absorption Edge of TiO2

A hole-strain-mediated coupling between dopants in anatase TiO$_2$ is revealed by first-principles calculations. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor-acceptor codoping. The calculated absorption spectra show that acceptor-acceptor codopings could shift the absorption edge to the visible light region.   

Synthesis conditions and electronic structures of heavily N-doped TiO$_{2}$

TiO$_{2}$ has drawn a lot of attention for its notable photocatalytic properties. Unfortunately, however, only a small portion of solar spectrum is utilized for photocatalytic activities of TiO$_{2}$ because of its wide band gap. To harvest solar energy more efficiently, TiO$_{2}$ must be sensitized under the irradiation of visible light which accounts for nearly 50 {\%} of solar light reaching ground surface. Although N-doped TiO$_{2}$ is a well-known visible-light driven photocatalyst, its photoabsorption cross section is still limited. In order to enhance visible-light absorption, high-concentration doping of N should be a promising solution. Here, we propose the synthesis conditions of heavily N-doped TiO$_{2}$ both for rutile and anatase structures based on the density-functional theory. We use supercell models with several different N concentrations to clarify the concentration dependence of the synthesis conditions. To discuss the synthesis conditions, we enforce a constraint to avoid the precipitation of other compounds, e.g. TiN, TiO, Ti$_{2}$O$_{3}$, TiO$_{2}$, during the synthesis of heavily N-doped TiO$_{2}$, which is described as a set of inequalities with respect to chemical potentials of N and O ($\mu _{\mathrm{N}}$ and $\mu_{\mathrm{O}})$ The results show that $\mu _{\mathrm{N}}$ must be larger than zero, which should be the upper limit for chemical potentials, in order for heavily N-doped TiO$_{2}$ to deposit stably. This means that high-concentration N doping is energetically difficult to be realized. Also, we will discuss the local arrangement of N atoms in connection with O vacancies and the electronic structures of examined models.   

Bandgap Modulation of CeO2 Nanoparticles by Codoping of Y and Co Impurities

Interplay of trivalent and divalent dopants in (Y, Co) codoped CeO2 nanoparticles with dominantly tetravalent host cations has been investigated using a variety of structural and optical techniques. The nanoparticle samples were prepared by a Polyol method. As revealed by the x-ray diffraction (XRD) data, all nanocrystal samples under investigation have similar average particle size. The concentration of O vacancies in the samples was found to increase with Y doping level as indicated by the Raman spectroscopy, extended x-ray absorption fine structure (EXAFS), and x-ray absorption near edge structure (XANES) data. As determined from diffuse reflectance spectra, the bandgap of the sample appears to decrease with increasing Y concentration. However, a series of Co-free samples measured for comparison show no dependence of bandgap on Y concentration. We have proposed a theoretical model and performed numerical simulation using the Vienna ab initio simulation package (VASP) to explain such bandgap modulation effect.   

Metastable defects are the origin of high conductivity in gallium doped zinc oxide

Doping in wide-bandgap materials is often counteracted by formation of intrinsic compensating defects of opposite charge. One prototypical exception to this general rule is gallium doped zinc oxide (ZnO:Ga) used as transparent conductor in numerous applications. High conductivities (1,000-10,000 S/cm) in ZnO:Ga are typically achieved during the growth at 250 - 350C around 10\textasciicircum -8 atm. The corresponding electron concentration exceed by a factor of 100,000 the values expected in equilibrium from first principles calculations. In contrast at high temperature and high oxygen partial pressure conductivity of ZnO:Ga measured in-situ shows good agreement with the ab initio theoretical thermodynamic model. The results of this study indicate that degenerate levels of doping in ZnO:Ga transparent conductive oxide used in practical applications are enabled by non-equilibrium concentration of both extrinsic donors and compensating acceptors. [1] A. Zakutayev et al, Appl. Phys. Lett.   

Optical Properties of Aluminum Doped Zinc Oxide Thin Film Grown by Remote PEALD

Transparent conducting oxides (TCO) are applied in optoelectronic devices and solar cells due to their high transmittance and low resistivity. Al-doped zinc oxide (AZO) film have been considered as a promising alternative to ITO. In this research, we investigated the optical properties of ZnO and AZO thin films deposited by remote plasma-enhanced atomic layer deposition (PEALD) using dimethylaluminum (DMAI) and dimethylzinc (DMZ) precursors. The substrates were double side polished amorphous quartz cleaned with acetone, methanol and dried with UHP nitrogen. Remote PEALD was then employed to deposit $\sim$ 100 cycles of ZnO thin films or AZO thin films (with Al: Zn cycle ratios from 1:6 to 1:2). In situ XPS indicated Al:Zn atomic ratios that increased from 3.2{\%} to 15.9{\%}. Transmittances higher than 90{\%} were measured from $\sim$ 500nm to 800nm for a film thickness of $\sim$ 20nm. XRD showed the films were crystalline on the amorphous substrates. A blue shift in the transmittance cut off was observed, and the optical band gap increased from 3.1 eV to 3.6 eV with increasing aluminum concentration. The results indicate that PEALD-grown AZO thin films are potential candidates for applications in high transparency TCO-based devices.   

Short-range order and its effects on electrons in (GaN)$_{(1-x)}$(ZnO)$_x$ alloys

Prior work by Li et al. gives ``cluster expansion" parameters for (GaN)$_{(1-x)}$(ZnO)$_x$ alloys. From these, by Monte-Carlo calculations, large representative unit cells can be generated at any chosen temperature. We choose mainly T=1200K, typical of the temperature at which experimental samples fall out of equilibrium. The atoms are distributed on the wurtzite anion and cation sublattices with significant short-range order. A periodic supercell with 432 atoms is chosen as a compromise between accurate self-averaging and fully self-consistent and relaxed density-functional (DFT) computation. Composition- and temperature-dependent short-range order (SRO) parameters of the alloys are discussed. Entropy is related to the SRO parameters. DFT relaxation finds significant bond-length alterations. Typical Zn-O distances are larger by 10$\%$ than Ga-N distances in the alloy, even though in pure ZnO and GaN, bond lengths are nearly equal. Electronic properties of the alloys, and in particular, the influences of short-range order and bond-length fluctuations, will be discussed.   

First-principles studies of n-type and p-type doping in hematite $\alpha$-Fe$_{2}$O$_{3}$

Spin-polarized density functional theory calculations are conducted to study the substitutional anionic doping in hematite $\alpha$-Fe$_{2}$O$_{3}$ crystal. Selective Group V and VII impurities, as p-type and n-type dopants, respectively, are investigated. The formation energies are lower under Fe-rich environment in general. The impurity levels are discussed both by the transition levels of the formation energies and the single-particle levels. And the local magnetic structure around an impurity is also analyzed.   

Session M45: Semiconductor Electronic Structure: Theory & Spectra II

Ab-initio Calculations of Accurate Electronic Properties of ZnS

We present the results from \textit{ab-initio}, self consistent, local density approximation (LDA) calculations of the electronic and related properties of zinc-blende zinc sulphide (zb-ZnS). We employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism in our non-relativistic computations. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method includes a methodical search for the optimal basis set that yields the minima of the occupied energies. This search entails increasing the size of the basis set and related modifications of angular symmetry and of radial orbitals. Our calculated, direct gap of 3.725 eV, at the $\Gamma $ point, is in excellent agreement with experiment. We have also calculated the total (DOS) and partial (pDOS) densities of states, electron and hole effective masses and total energies that agree very well with available, corresponding experimental results. Acknowledgement: This research is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE-NA0001861), LaSPACE, and LONI-SUBR.   

Inverted band structure and excitons in halide perovskites

The halide perovskites CsSnX$_3$, X=I, Br, Cl have recently received attention for their application in photovoltaics. The high hole-mobility and optimal band gap (1.3 eV) of $\gamma$-CsSnI$_3$ make it attractive as both absorber and hole transport material in all solid-state Gr\"atzel cells. We present self-consistent $GW$-calculations of the electronic band structures of CsSn$I_3$, CsSnBr$_3$, CsSnCl$_3$ in different phases. The most important finding is that these materials have an ``inverted'' band structure in the sense that the CBM consists of $p$-like Sn states while the VBM consists of Sn-$s$ antibonding with halide $p$-states. The reasons for the location in {\bf k}-space of the direct gap and the nature of the band edge states are explained. The intra SnI$_6$ cluster bonding is the origin of various anomalies, such as the low hole mass, the anomalous temperature dependence of the gap with lattice expansion, the relative insensitivity to the anion, and the strong dipole allowed optical transitions. We show that when the phonon contribution to the screening is included, the exciton binding energy is two orders of magnitude smaller than with electronic screening only. The observed luminescence with high binding energy is argued to be due to a defect bound exciton.   

Origin of the failed ensemble averaged rule for the band gaps of the disordered nonisovalent semiconductor alloys

Recent calculations show that the nonisovalent random alloy, such as Zn0.5Sn0.5P, has a band gap much smaller than their ordered phases; i.e., the band gap of random alloy is not the ensemble averaged value of the ordered structures, as observed in most isovalent semiconductor alloys and predicted by cluster expansion theory. We show that this abnormal behavior in nonisovalent alloys is caused by strong wavefunction localization of the band edge states. Moreover, we show that although the disordered phase of isovalent alloys are similar to the random phase, for nonisovalent alloys the disordered phase deviates significantly from the random phase, and the completely random phase is not achievable for nonisovalent alloys under equilibrium growth conditions.   

An $O(h^{2})$ Coulomb singularity correction for the Bethe-Salpeter equation

We present an improved numerical correction, at no extra computational cost, for the Coulomb singularity in the Bethe Salpeter equation for bound excitonic states. This method leads to modifications of the off-diagonal matrix elements of the Bethe-Salpeter matrix with quadratic scaling of the asymptotic error. This method is particularly well suited for systems where hybrid Brillouin sampling schemes are ineffective, e.g., systems with an indirect fundamental band gap or large supercells containing defects. Numerical results are presented for the binding energy of the ground state excitons in the two-band model as well as the scintillator material CsI.   

The effect of semicore electrons on the polarizability and band gaps in \textit{ab initio }planewave-pseudopotential (PW-PP) GW calculations

Understanding the effect of semicore electrons on \textit{ab initiio }PW-PP GW calculations is currently of great interest due to the increasing importance of complex materials with active semicore electrons, e.g.,the transition metal dichalcogenides. While past research has found a significant effect due to the inclusion of semicore electrons, it did not fully explore the nature of the various deviations of traditional valence-only PW-PP GW calculations from calculations that include the semicore electrons. We study this issue in the simple system of the Si atom, where the effect is more easily isolated, and then extend our results to bulk Si, and other bulk systems. We present results showing the effect of semicore electrons on various contributions to the GW self energy, and discuss the nature of differences with the traditional PW-PP approach. We present methods to efficiently include the effect of semicore electrons in a hierarchy of computational cost and accuracy.   

Electrical Analogues of Optical {\&} EELS Spectra: Silicon

We have explored an analogy between optical and electrical-circuit resonances that yields insight into single-particle and collective excitations. The analogy rests on the similarity of the differential equations for the Drude-Lorentz model of optics and the impedance of ac circuits. A parallel combination of capacitive (C) and inductive-capacitive (L-C) branches is a suitable circuit model. The L-C branches correspond to single-particle excitations. The C branch accounts for the electric-field term in the displacement, or equivalently the free-space susceptibility. Collective excitations represent combination resonances of the L-C and C branches. These excitations involve only \textit{internal} mesh currents that can flow in the absence of an \textit{external} (input) current. In this case, the admittance of the circuit is zero corresponding to the vanishing of the dielectric function at the plasmon resonance in optics (absent resistive losses). Circuit impedance corresponds to charged-particle energy loss. In contrast, circuit admittance (inverse impedance) corresponds to optical measurements. The interference of mesh currents in the circuit model plays the role of Coulomb screening in energy-loss spectra.   

Zero-point motion effect on the bandgap of diamond: validation of codes

Verification and validation of codes, as well as new theoretical methods, are of utmost importance if one wants to provide reliable results. In this work we present a rigorous and careful study of all the quantities that enters into the calculation of the zero point motion renormalization of the direct band gap of diamond due to electron-phonon coupling. This study has been done within the Allen-Heine-Cardona (AHC) formalism as implemented into Abinit and Yambo on top of Quantum Espresso. We aim at quantifying the agreement between the codes for the different quantities of interest. This study shows that one can get less than $10^{-5}Ha/at$ differences on the total energy, 0.07 cm$^{-1}$ on the phonon frequencies, 0.5\% on the electron-phonon matrix elements and less than 4 meV on the zero-point motion renormalization. At the LDA level, the converged direct bandgap renormalization in diamond due to electron-phonon coupling in the AHC formalism is -409 meV (reduction of the band gap) [1]. \\[4pt] [1] S. Ponc\'e \textit{et al.}, arXiv:1309.0729 [cond-mat.mtrl-sci] and submitted for publication in Comput. Mat. Science (2013).   

Carrier separation at intra-grain partial dislocation pairs in CdTe

Using aberration corrected scanning transmission electron microscopy (STEM), we have determined the atomic configuration of CdTe intra-grain Shockley partial dislocation pairs. Counter-intuitively, density-functional theory calculations indicate that these partial dislocation pairs do not create deep states in the band gap, instead, they lead to local band bending that separates electrons and holes, therefore reducing undesirable carrier recombination. STEM images also show that the intra-grain partial dislocation pairs are seen to annihilate and regenerate under the influence of the electron beam. A systematical examination about the annihilation of the dislocation is done. Band calculations about the dislocation pairs with different core-core distance suggest that the band bending caused by the charge transfer between the cores, which helps to separate carriers and further avoid their recombination, becomes significant when the distance increases, but does not change when the distance is larger than a critical value, dc.   

Electronic Structure of Perovskite Solid Solutions (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$

Band gap engineering of oxide perovskite materials is of great interest for electronics and photocatalysis. In this study we demonstrate that the band gap of SrTiO$_{3}$ is narrowed by mixing it with the oxinitride LaTiO$_{2}$N. Using hybrid density functional calculations, we study the electronic structure of LaTiO$_{2}$N and (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ solid solutions. We show that the valence-band maximum (VBM) of (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ is raised as the LaTiO$_{2}$N concentration increases, while the conduction-band minimum (CBM) remains almost unchanged. This is explained by the atomic orbitals that composed the VBM and CBM in the two parent compounds: in LaTiO$_{2}$N the VBM is derived from N 2p states, which are higher in energy than the O 2p that composed the VBM in SrTiO$_{3}$. The band gap of (SrTiO$_{3})_{\mathrm{1-x}}$(LaTiO$_{2}$N)$_{\mathrm{x}}$ is quantified and discussed in terms of the valence- and conduction-band offsets of SrTiO$_{3}$/LaTiO$_{2}$N.   

Electronic structure of p-type transparent conducting oxide CuAlO$_2$

CuAlO$_2$ is a prototypical p-type transparent conducting oxide. Despite its importance for potential applications and number of studies on its band structure and gap characteristics, experimental study on the momentum-resolved electronic structure has been lacking. We present angle-resolved photoemission data on single crystalline CuAlO$_2$ using synchrotron light source to reveal complete band structure. Complemented by the x-ray absorption and emission spectra, we also study band gap characteristics and compare them with theory.   

Electronic and optical properties of tantalum pentoxide polymorphs from first principles calculations

Tantalum oxide has been extensively studied due to its attractive properties as dielectric films, anti-reflection coatings, and resistive switching memory. Although various crystalline structures of tantalum pentoxide (Ta$_{2}$O$_{5}$) have been reported, the structural and electronic/optical properties still remain a controversial issue. We investigate the electronic and optical properties of crystalline and amorphous Ta$_{2}$O$_{5}$ structures using first-principles calculations in the GW approximation. The calculated band gaps of the crystalline structures are too small to explain the experimental measurements. The amorphous structure exhibits a strong exciton binding energy and an optical band gap ($\sim$ 4eV) similar to experiment. We determine the atomic orbitals that form the conduction band of each polymorph and analyze the dependence of the band gap on the atomic geometry. Our results establish the connection between the underlying structure and the electronic and optical properties of Ta$_{2}$O$_{5}$.   

Applicability check of ZnO crystals for device applications

There has always been vital interest in wide-band gap semiconductors for their applicability in short-wavelength photonic devices and in electronic devices operating in high frequency regime. Historically, ZnO was never favored as a potential material for the above applications primarily because of difficulty in growing it. This situation, however, has improved drastically in the past decade thereby renewing the attention on this material system. Hence, ZnO is being proposed for potential light emitting devices in the blue and UV regions of electromagnetic spectrum. ZnO single crystals are also being considered for high power transistors. In this work, we present investigations of optical properties of pure (99.99{\%}) ZnO performing transmittance, reflectance, Raman, and photoluminescence measurements. The ZnO single crystals employed in this work, were obtained commercially. We present detailed analysis of the measured data through theoretical calculations. Our results identify the state-of-the-art application potential of commercially available ZnO, revealing its advantages and limitations when compared to similar materials such as GaN.   

Real-time Approach for Core-hole Dynamics in X-ray Spectra

We present a real time method to calculate dynamical core hole effects in x-ray spectra using single-determinant wavefunctions to evaluate the time-correlation series. Starting with the system in the ground state, the x-ray field suddenly creates a particle-hole pair which is propagated in real-time in the presence of the dynamically screened core-hole potential. The approach is implemented in our software package RTXS\footnote{A. J. Lee, F. D. Vila, and J. J. Rehr, Phys. Rev. B \textbf{86}, 115107 (2012)}, a local time-correlation based program for the calculation of x-ray absorption and emission spectra. RTXS uses GPAW, a grid based electronic structure code, to calculate the time-evolution operator with a Crank-Nicolson algorithm and projector augmented wave transition matrix elements. This implementation builds in full-potential electronic structure and dynamical core-hole screening, resulting in a practical and generally applicable code. Recent improvements include electron-electron interactions which had been previously neglected. By introducing the single-determinant wavefunctions, we now model intrinsic and extrinsic interactions. The method is illustrated in a number of cases, including diamond and C60.   

The First Picosecond after Sunlight Absorption in Si, GaAs, and CdTe from First-Principles Calculations

Sunlight absorption in semiconducting materials generates out-of-equilibrium electron populations $-$ also known as hot carriers $-$ relaxing towards equilibrium through a host of scattering processes at the subpicosecond time scale. While such dissipation processes typically result in the loss of more than half of the energy associated with the absorbed sunlight, a microscopic understanding of this ultrafast regime is still missing. In this talk, we provide a detailed picture of the first picosecond after sunlight absorption in semiconductors of wide use in photovoltaics (PV) such as Si, GaAs, and CdTe. Our results are based on ab initio calculations combining density functional theory and the GW plus Bethe-Salpeter Equation (GW-BSE) approach together with electron-phonon interactions. We computed the lifetimes and k-space dependence of electron-electron and electron-phonon scattering events responsible for ultrafast solar energy dissipation. Using this information, we simulated the ultrafast dynamics of hot carriers using an empirical-parameter-free formulation of the Boltzmann equation. A clear understanding of hot carrier dynamics emerges for several materials of interest in PV, and novel engineering paradigms are suggested.   

Session M46: Hidden Order, URu2Si2 and Other U-Based Heavy-Fermion Systems

Magnetic Nematicity in the Hidden Ordered Compound URu2Si2

The Hund's rule exchange interaction promotes a second-order phase transition to a coupled spin and orbital density wave state in the underscreened Anderson Lattice Model. The spin-flip part of the Hund's rule coupling stabilizes a spontaneous spin-dependent mixing of 5f quasiparticle bands which, in the normal state, have pure orbital characters. The transition breaks the spin-rotational invariance and leads to an asymmetric pseudo-gap forming in the density of states. When a magnetic field is applied, the electronic dispersion relations become dependent on the relative orientation of the field and the spontaneously chosen quantization axis. We show that this results in the magnetic susceptibility becoming anisotropic below the critical temperature, but without the development of a static magnetization.   

Absence of a static in-plane magnetic moment in the `hidden-order' phase of URu$_2$Si$_2$

We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URu$_2$Si$_2$ to probe the possible existence of a small magnetic moment parallel to the tetragonal basal plane in the `hidden-order' phase. This small in-plane component of the magnetic moment $S_\parallel$ on the uranium sites has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component $S_\perp \approx 0.01-0.04 ~\mu_B${/}U. $S_\parallel$ and $S_\perp $ components were separated by using the fact that only the components of the magnetic structure that are perpendicular to the scattering vector $Q$ contribute to the magnetic neutron scattering. We find no evidence for an in-plane magnetic moment $S_\parallel$. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is $S_{\parallel}^{max} \approx 1\times10^{-3} ~\mu_B${/}U. Analysis of diffuse neutron scattering shows no presence of short-range magnetic correlations within the measured limit.   

The Hidden Order Gap and In-Gap Excitation Mode in URu$_2$Si$_2$ Revealed by Electronic Raman Scattering

The heavy fermion compound URu$_2$Si$_2$ displays a phase transition into the so called ``hidden order'' state at $T_{HO}=17.5\,$K. Using polarized electronic Raman scattering, we show that the Raman response in the $A_{2g}$ symmetry channel ($D_{4h}$): (1)~at high temperatures can be described by a Drude-like continuum with the scattering rate decreasing from 46\,cm$^{-1}$ at 300\,K to 16\,cm$^{-1}$ at 70\,K; (2)~develops a low energy peak due to spectral weight transfer through Fano interference in the temperature range of 70-20\,K; (3)~below $T_{HO}$ develops a gap of about 55\,cm$^{-1}$ in the continuum, and a sharp in-gap mode centered at 14\,cm$^{-1}$. In addition, we show that the real part of the static Raman susceptibility in the $A_{2g}$ symmetry is proportional to the \textit{c}-axis static magnetic susceptibility above $T_{HO}$. The implication of these observations will be discussed in the talk.   

Microwave Conductivity Measurements of URu$_2$Si$_2$

Narrow, free-carrier spectra occur in the heavy fermion metals, in which the high quasiparticle effective mass renormalizes the width of the Drude conductivity peak from infrared frequencies, where it is found in more typical metals, down to the microwave region. This narrow peak has been observed [1] in a thin film of UPd$_2$Al$_3$, occurring with a Lorentzian lineshape due to the dominant defect-scattering in the film. We have taken microwave conductivity measurements on URu$_2$Si$_2$ in a temperature range in which the sample has entered into its hidden order state but above the superconducting transition. These measurements exhibit a narrow Lorentzian lineshape, indicating high sample purity and considerably reduced impurity scattering. If the inelastic scattering is sufficiently low, there is the possibility for an observation of electron-electron scattering. Results to date will be presented. \\[4pt] [1] Scheffler, M. $\textit{et al.}$, $\textit{Nature}$ $\textbf{438}$, 1135 (2005).   

Optical study of the hidden order state in URu$_2$Si$_2$

We discuss recent optical experiments [1,2] both in the normal state above the hidden order transition at 17.5 K in URu$_2$Si$_2$ and in the hidden order state itself. In the normal state the focus is on the development of coherence as shown by the Drude peak and the pseudo-hybridization gap that develops at 12 meV. In the hidden order state a gap opens up with $2\Delta=6$ meV but its temperature evolution is not that of a mean field transition. To gain insight into the nature of the transitions we discuss the transfer of spectral weight between the various features in the optical conductivity spectrum as the temperature is changed. \\[4pt] [1] U. Nagel {\it et al.} PNAS {\bf 109} 19161 (2012).\\[0pt] [2] J. Hall {\it t al.} Phys. Rev. B {\bf 86}, 035132 (2012).   

Anomalous Nernst effect of the heavy-fermion superconductor URu$_{2}$Si$_{2}$

The heavy-fermion material URu$_{2}$Si$_{2}$ exhibits the ``hidden order'' and superconducting phase transitions at T$_{\rm{HO}}$ = 17.5 K and T$_{\rm{SC}}$ = 1.4 K, respectively. Below T$_{\rm{HO}}$ a significant decrease of carrier density has been observed, and the remaining carriers condense into the superconducting state below T$_{\rm{SC}}$. The superconducting symmetry is suggested to be chiral d-wave with time reversal symmetry breaking. We have recently measured the Nernst coefficient ${\nu}$(T) in an ultraclean single crystal of URu$_{2}$Si$_{2}$ with RRR $\sim$ 700, which is much larger than the previous report [1]. We observed an increase of in $\nu$(T) below T$_{\rm{HO}}$ which shows an additional steep increase below $\sim$ 3T$_{\rm{SC}}$. The magnitude of $\nu$(T) is much larger than the previous report and reaches $\sim$ 200 $\mu$V/KT at 1 T. We show that such a giant Nernst effect in an ultraclean sample cannot be explained by conventional Gaussian superconducting fluctuations. Possible origins including fluctuations of exotic chiral superconductivity will be discussed.\\[4pt] [1] R. Bel $\it{et}$ $\it{al}$., Phys. Rev. B $\bf{70}$, 220501 (2004).   

$^{29}$Si NMR study of the paramagnetic state of URu$_2$Si$_2$ under pressure

We report $^{29}$Si nuclear magnetic resonance measurements in a single crystal of URu$_2$Si$_2$ in the hidden order, antiferromagnetic, and paramagnetic phases under pressure. We find evidence for partial suppression of the density of states below 30 K at ambient pressure. We study how this behavior varies under pressure as hidden order gives way to antiferromagnetism. We analyze the data in light of various recent models.   

Abrupt electronic structure changes in URu$_{2}$Si$_{2}$ at the Hidden Order transition

In recent years, high-resolution angle-resolved photoemission (ARPES) measurements of URu$_{2}$Si$_{2}$ [1] have attempted to characterized the temperature dependent behavior of $f$-states close to the Fermi level in the range of photon energies of 7-31 eV and in a narrow $k$-space range around the surface zone center revealing varying ways in which energy shifts, backfolding of states and/or spectral weight sharpening correlate to the hidden order transition at 17.5K. In this study we expand the temperature dependent ARPES measurements to a broader range of photon energy and emission angles in order to probe the full 3-dimensional electronic structure. We find particular $k$-space regions close to incommensurate wave vector separation that exhibit dramatic electronic structure changes that are abrupt at the hidden order transition and whose evolution to higher temperatures correlates to a gradual Kondo coherence transition below $\sim$50K. A critical assessment of these electronic structure changes in relation to LDA band structure predictions is discussed. \\[4pt] [1] A.F. Santander-Syro, Nat. Phys. 2009; R. Yoshida, PRB 2010; G.L. Dakovski, PRB 2011; F.L. Boariu, PRL 2013; S. Chatterjee, PRL 2013; J.Q. Meng, PRL 2013.   

Hybridization in Kondo lattice heavy fermions via quasiparticle scattering spectroscopy (QPS)

Band renormalization in a Kondo lattice via hybridization of the conduction band with localized states has been a hot topic over the last several years. In part, this has to do with recently reignited interest in the hidden order problem in URu$_{2}$Si$_{2}$. Despite recent developments regarding the electronic structure in this compound, it remains to be resolved whether the hidden order phase transition is related to the opening of a hybridization gap. Our quasiparticle scattering spectroscopy (QPS) has shown they are not related directly [1]. This can be understood naturally since in principle band renormalization does not involve symmetry breaking. To deepen our understanding, we extend to other Kondo lattice compounds. For instance, when applied to YbAl$_{3}$, a vegetable heavy-fermion system, QPS reveals conductance signatures for hybridization in a Kondo lattice such as asymmetric Fano background along with characteristic energy scales. Presenting new results on these materials, we will discuss a broader picture.\\[4pt] [1] W. K. Park \textit{et al}., PRL \textbf{108}, 246403 (2012).   

Temperature Dependence of the London Penetration Depth and Nodal Gap Structure of UPt$_3$ from Small Angle Neutron Scattering

Despite the fact that the heavy-fermion superconductor UPt$_3$ has attracted substantial experimental and theoretical attention for nearly thirty years, the nature of the unconventional superconducting order parameter is not settled. There are many theories that attempt to explain the superconducting state, all of which differ in the nodal structure of the superconducting gap. Our recent measurements of the temperature dependence of the small angle neutron scattering from the superconducting vortex lattice provides a bulk measurement of the anisotropic temperature dependence of the London penetration depth and thus a direction specific probe of quasi particle excitations sensitive to gap nodes. Our measurements and their theoretical analysis favor an odd parity, time reversal symmetry breaking order parameter with $E_{2u}$ symmetry.   

History Dependence of the Vortex Lattice Rotation in the B-phase of UPt$_3$

The unconventional superconductor UPt$_3$ is widely believe to be a triplet superconductor, where the low temperature superconducting B-phase is a chiral state. We have performed small angle neutron scattering from the vortex lattice (VL) of UPt$_3$ in the B-phase with magnetic fields parallel to the hexagonal $c$-axis. Our field dependent measurements show scattering from multiple VL domains, with a subtle magnetic field history dependence of the domain orientation; VL's prepared with the magnetic field parallel or antiparallel with respect to the angular momentum from the circulating screening currents show different field history dependence. These results indicate a coupling of a chiral superconducting order parameter with the applied magnetic field, in agreement with other recent results.   

NMR, Thermodynamics and Magnetic Disorder in Kondo Intermetallics: UCu4Pd and UCu4Ni

We compare and contrast the static magnetic character of UCu4Pd and UCu4Ni as probed by local Cu-NMR and bulk thermodynamic measurements. For the Pd case, evidence for magnetic disorder has been well established by most bulk (magnetization, specific heat, etc.) and local measurements, particularly Cu-NMR. For the Ni case, on the contrary, the local Cu-NMR data appear to suggest that thermodynamic divergences are not directly controlled by static magnetic inhomogeneity, even though the Ni material possesses a higher degree of structural disorder than UCu4Pd. We discuss to what extent Cu-NMR can be used to elucidate differences between the two systems and shed light on the notion of disorder-driven quantum criticality.   

Low Temperature Specific heat of U$_{2}$PtC$_{2}$

We present specific heat data of the moderately heavy superconductor U$_{2}$PtC$_{2}$ with T$_{\mathrm{c}} =$ 1.34 K, and normal state Sommerfeld coefficient $\gamma =$ C/T $\approx $ 150 mJ/mol K$^{2}$, at temperatures down to below 100 mK and in fields up to 9 T, exceeding the superconducting critical field. Zero-field data show systematic deviation from the weak-coupling BCS fit, with excess contribution at low temperature. The field evolution of the residual $\gamma _{0}$(T$=$0) shows $\surd $H dependence for H \textless 1 T. Together, these results suggest an unconventional nature of superconductivity in this compound.   

Superconductivity and magnetism in the doping series U$_{2}$Rh$_{\mathrm{x}}$Pt$_{\mathrm{(1-x)}}$C$_{2}$

U$_{2}$PtC$_{2}$ has been known for many years to exhibit nearly-heavy-fermion behavior, as well as superconductivity, T$_{\mathrm{c}}$ $\sim$ 1.5 K. Little is known about the nature of the superconductivity, but many other uranium based heavy fermion superconductors, such as UPt$_{3}$ and UBe$_{13}$, have been shown to be unconventional. U$_{2}$RhC$_{2}$ also shows nearly-heavy-fermion behavior, but it is non-superconducting and reported to be antiferromagnetic, T$_{\mathrm{N}}$ $\sim$ 18 K. These observations have motivated our study of the doping series of U$_{2}$Rh$_{\mathrm{x}}$Pt$_{\mathrm{(1-x)}}$C$_{2}$ in order to investigate the evolution from the antiferromagnetic to the superconducting groundstate, as well as the role of the antiferromagnetism in the superconductivity. Through measurement of the resistivity, magnetic susceptibility and heat capacity of polycrystalline samples, we show the suppression of antiferromagnetism, the presence of competing ferromagnetism, and emergence of superconductivity with doping. Furthermore, we present evidence that the emergence of superconductivity, which deviates from single-gap BCS theory, is not directly related to the suppression of antiferromagnetism.   

The Fermi volume as a probe of hidden order

Quantum oscillations are a highly sensitive probe of electronic structure, and provide valuable information about the Fermi surface and quasiparticle properties. We demonstrate that the volume of the Fermi surface, measured very precisely using de Haas-van Alphen (dHvA) oscillations, can be used to probe changes in the nature and occupancy of localized electronic states. In systems with unconventional ordered states, this allows an underlying electronic order parameter to be followed to very low temperatures. PrOs$_4$Sb$_{12}$ has an unusual antiferroquadrupolar (AFQ) ordered phase that forms at low temperature and high magnetic field. We find that the phase of dHvA oscillations is sensitively coupled, through the Fermi volume, to the configuration of Pr f-electron states that are responsible for AFQ order. Specifically, a given sheet of the Fermi surface expands or shrinks as the occupancy of competing localized Pr crystal field states changes. In addition, the low temperature sensitivity of the dHvA effect reveals a strong and previously unrecognized influence of hyperfine coupling on the order parameter below 300mK within the AFQ phase. Our approach to quantum oscillations in PrOs$_4$Sb$_{12}$ might be more widely applicable and provide new insights in hidden order systems.   

Session M47: Metal-Insulator and Other Electronic Phase Transitions: Experiment III

Size effect on voltage-induced metal-insulator transition in VO$_{2}$ film grown by direct thermal oxidation method

Metal-insulator transition (MIT) in vanadium dioxide (VO$_{2}$) can be induced by diverse stimuli such as temperature, voltage, light and pressure. Voltage induced MIT is of special interest for both scientific understanding and future applications. However, it is still under the debate whether the origin of voltage induced MIT comes from electrical breakdown or Joule heating effect. To figure out this origin, the electrically triggered MIT from the strip-line VO$_{2}$ film devices with dimensions of fixed width (W) of 100 $\mu $m and varied length (L) of 10, 20, 40, and 80 $\mu $m were investigated by temperature and external bias voltage dependent electrical transport, and optical microscopy. It was found that the magnitude of critical electric filed at MIT and its temperature dependence were dependent on the length of the device. In this talk, we will present the size effect on the voltage-induced metal-insulator transition in VO$_{2}$ film grown by direct thermal oxidation method and discuss the origins of voltage driven MIT and its implications.   

Effect of long-range correlations on the Metal-Insulator Transitions in Vanadium Oxides

The role of long-range electronic correlations in the metal-insulator (MIT) and structural phase (SPT) transitions in V$_{2}$O$_{3}$ and VO$_{2}$ are still under debate. In order to investigate the effect of disorder on the long-range correlations we irradiated V$_{2}$O$_{3}$ and VO$_{2}$ thin films with O$^{+}$ ion at different doses. We studied the effects on the transport and crystallographic properties as a function of the temperature across the phase transition. Both materials are sensitive to the irradiation, but effects on the transport and crystallographic properties across the phase transitions are different. We find changes in the transition temperature, lattice constant and magnitude of the MIT in both oxides. We interpreted this result as a change of the long-range order in vanadium oxides by ion irradiation. The response of VO$_{2}$ and V$_{2}$O$_{3}$ to the irradiation shed light on the SPT and MIT mechanisms.   

Electric field-induced carrier accumulation at the vanadium dioxide-dielectric interface

Classical rigid band semiconductors respond to an electric field at a dielectric interface by accumulating or depleting carriers at the interface. We investigate electrostatic field-effects in thin film devices formed from the prototypical, strongly-correlated insulator, vanadium dioxide VO$_{2}$. This material exhibits a temperature driven insulator to metal transition near room temperature. Therefore, non-trivial electric field driven electronic effects can be anticipated. We find that excess carriers can be introduced in our devices with concentrations of up to $\sim$ 5x10$^{13}$cm$^{-2}$: these field induced carriers exhibit an activated low mobility at low temperatures that is characteristic of electron localization. Field-effect conductance modulation and depletion are highly inhibited with excess carriers confined near the interface. Signatures of defect-dominated scenarios are absent. The field-effect behavior that is exhibited by our VO$_{2}$ based devices is fundamentally different from that of a classical semiconductor.   

Thermochromic characteristics of Ti-doped VO2 thin film

Utilizing metal-to-insulator transition (MIT) properties of V-oxide film, stable VO2 phase is necessary. In sputtering deposition of VO2, simple target preparation and high deposition rate are recommendable. For this, VO2 film was deposited on quartz substrate by RF magnetron sputter system under low working pressure using V2O5 target. Due to the lower sputtering yield of oxygen compared to vanadium, oxygen ion contents is usually deficient from that of target. So, the reduction of V ions was a result of charge compensation with the oxygen ions. Under lower working pressure, deposition rate become higher so that this deficiency is getting larger to cause further reduction to destabilize VO2. Preventing this, titanium oxide co-deposition was suggested to enrich oxygen source. When TiO2 was used, Ti ion has stable +4 charge state so that extra oxygen sputtered prevents V ion reduction below +4 state. But, in case of TiO, Ti ions were oxidized from +2 to +3 and +4 state and V ions with less oxidation potential should be reduced to +3 or so. Pure VO2 film had MIT at 66$^{\circ}$C and large resistivity ratio of 4 orders of magnitude from 30$^{\circ}$C to 90$^{\circ}$C. Under low working pressure, (V2O5 + TiO2) system yield fairly good films, while films with poor or absence of MIT were produced with TiO case.   

Pressure Evolution of X-ray Raman Spectra in a Novel Monoclinic V2O3 Metal

V2O3 is a prototypical metal-to-insulator transition system, where the transition always coincides with a corundum-to-monoclinic structural transition in temperature-dependent studies. However, recent pressure-dependent study demonstrates that the two transitions can be decoupled, showing a novel monoclinic metallic phase above a critical pressure Pc around 33 GPa. Here we study the corresponding pressure evolution of electronic structure with X-ray Raman scattering. The spectra do not exhibit any appreciable difference at low pressures, but broaden substantially across Pc. Multiplet calculations with additional screening channels from coherent quasiparticles indicate a weakened screening effect at high pressures. This could result from a decreased coherent quasiparticle strength due to enhanced electronic correlation, suggesting that V2O3 in the high-pressure monoclinic phase is a critical correlated metal on the verge of Mott- insulating behavior.   

Ab Initio calculations of phonons in metallic rutile and insulating monoclinic M1 and M2 VO$_2$

Vanadium dioxide (VO$_2$) undergoes a first-order metal-insulator transition (MIT) at 340\,K from a metallic, high-temperature rutile phase (R) to an insulating, low-temperature monoclinic phase (M1). Under tensile strain, two other insulating phases, a second monoclinic phase (M2) and a low symmetry triclinic phase (T), are also known to exist. Recently, Park {\em et al.}\footnote{J. H. Park, Nature {\bf 500}, 431 (2013).} observed a solid-state triple point of these phases in strained VO$_2$ nanobeams. More recently, phonon frequencies for strain-stabilized M2 have been observed.\footnote{M. M. Qazilbash, private communication} Understanding the vibrational properties of these phases may help resolve questions surrounding the long-debated issue of the respective roles of electronic correlation and Peierls mechanisms in driving the MIT. We will present {\em ab initio} DFT and DFT+U calculations of phonon frequencies for the M2 phase and compare these to measured results and to previous calculations and measurements for the R and M1 phases.\footnote{T. J. Huffman et al., PRB {\bf 87},115121 (2013).}   

Anharmonicity in complex oxides: case of VO$_2$

Harmonic and quasi-harmonic models of lattice dynamics are widely successful in explaining thermodynamic properties of materials, including in complex oxides. However, in some cases, strong anharmonicity can critically affect physical properties, and a (quasi) harmonic model is not sufficient to capture these important features. In this talk, we present the results of ab initio molecular dynamics studies of anharmonicity in VO$_2$. Our simulations provide good agreement with measurements of phonon dispersions and diffuse scattering. Other implications of strong anharmonicity will also be discussed.   

Hole-lattice Coupling and Photo-induced Insulator-metal Transition in VO2

In this talk, we will present a theory [PRB \textbf{88}, 035119 (2013)] that is able to explain the photo-induced insulator-metal transition in VO2 and the related transient and multi-time-scale structural dynamics upon photo-excitation. Holes created by photo-excitation weaken the V-V bonds and eventually break V-V dimers in the M1 phase when the laser fluence reaches a critical value. The breaking of the V-V bonds in turn leads to an immediate electronic phase transition from an insulating to a metallic state while the crystal lattice remains monoclinic in shape.   

Dynamically Tracking the Strain Across the Metal-Insulator Transition in VO$_2$ Measured Using Electromechanical Resonators

We study the strain state of doubly clamped VO$_2$ nanobeam devices by dynamically probing resonant frequency of the nanoscale electromechanical device across the metal$-$insulator transition. Simultaneous resistance and resonance measurements indicate M1-M2 phase transition in the insulating state with a drop in resonant frequency concomitant with an increase in resistance. The resonant frequency increases by 7~MHz with the growth of metallic domain (M2-R transition) due to the development of tensile strain in the nanobeam. Our approach to dynamically track strain coupled with simultaneous resistance and resonance measurements using electromechanical resonators enables the study of lattice-involved interactions more precisely than static strain measurements.   

Two-dimensional metal-insulator transition in RuO$_{2}$ films

The complex chemical and structural nature of oxide materials make them highly susceptible to disorder. This disorder strongly influences the transport properties of these systems. By systematically varying the disorder and/or carrier concentration, many oxides can be driven through the metal--insulator transition (MIT). We have performed temperature dependant magneto-transport measurements (1.75K\textless T\textless 300K and 0\textless B\textless 8T) on 10-30 nm thick films of RuO$_{2}$ as they were driven through the MIT through calcination. The results reveal an unexpected 2-d insulator to metal transition as a function of decreasing disorder. The presentation will include an introduction to the concepts of localization in disordered materials, an overview of the thin-film sample preparation and characterization, a comparison with a 3-d oxide system (In$_{2}$O$_{3}$), and a discussion of the results in the context of a localization model.   

Insulator to Metal Transition in WO$_{3}$ Induced by Electrolyte Gating

We have modified the transport properties of thin WO$_{3}$ films by the electric field effect using ionic liquids and solid electrolytes. Atomically flat films were prepared on different substrates by RF sputtering. The huge electric field that is generated in the double-layer induces an extraordinarily large change of the mobile charge carrier density in the sample. The sheet resistance of the gated film drops by more than 10 orders of magnitude at the lowest temperature, and a clear insulator-to-metal transition is observed. The thickness dependence has been studied and the mechanism of doping by electrolyte gating will be discussed.   

Pressure Induced Insulator to Metal Transition in FBBO

We have measured resistivity vs. temperature and pressure on the fluoro-substituted oxobenzene-bridged bisdithiazolyl radical, FBBO. This is a layered, single component organic compound that is a Mott insulator at ambient pressure, due to the singly occupied molecular orbitals and an intrinsically high inter-molecular charge transfer energy barrier. Previous room temperature infrared absorption and conductivity measurements suggest that the charge gap of 0.1eV closes and the sample may become metallic at pressures above 3GPa[1]. We report direct transport measurements under various pressures on powder samples of FBBO down to low temperature, measured in an anvil pressure cell. [1] A. Mailman, \textit{et al}., J. Am. Chem. Soc. \textbf{134}, 9886 (2012).   

Metal-insulator transition of (CeO)MnAs by carrier doping

(LaO)MnPn ; (Pn $=$ P, As, Sb) are antiferromagnetic semiconductors with high N\'eel temperature by the strong Mn -- Mn magnetic interaction and they seem to be a robust system against carrier doping. (CeO)MnPn are suitable materials to study the electron correlation because the Ce 4$f$ electrons in the Ce$^{3+}$ state constitute a Mott insulator which is expected to control by carrier doping due to weaker magnetic interaction than that of the Mn case. In this study, we have investigated the carrier doping effects on the physical properties of (CeO)MnAs. The parent material (CeO)MnAs is also a magnetic semiconductor as same as the analogous case of (NdO)MnAs [1]. In this material, there are two magnetic components, one is the antiferromagnetic ordered Mn 3$d$ component, the other is the Ce paramagnetism. The CeO deficiencies provide enough carriers to change the electrical resistivity from insulating to metallic. The deficient samples show Fermi liquid like behaviors at low temperature. These drastic changes are thought to be controlled by Mott transitions.   

Magnetotransport in Iron Cobalt Silicide Nanowires

Iron silicide is a small gap insulator that can be made metallic and magnetic when doped with cobalt. With the incorporation of cobalt, Fe$_{\mathrm{1-x}}$Co$_{x}$Si undergoes an insulator-to-metal transition becoming a half metal for a wide range of $x$. The magnetic ground state is helimagnetic with distinct itinerant character. It has been demonstrated by others that an exotic intermediate magnetic vortex or skyrmion state exists between the helimagnetic and ferromagnetic phases in small applied fields. Electron transport in bulk Fe$_{\mathrm{1-x}}$ Co$_{x}$Si has been found to be dominated by electron-electron interaction effects similar to what has been found in prototypical semiconductors such as Si:P. Here we probe low temperature magnetotransport in CVD-grown Fe$_{\mathrm{1-x}}$ Co$_{x}$Si nanowires with x$=$0.05. The reduced size presents the opportunity to characterize the quantum contributions to the conductivity where the electrons are effectively confined to one dimension. The dimensionality is determined by the wire diameter which can be smaller than the electron's inelastic scattering length at low temperatures. Results of these measurements will be presented.   

Session M48: Invited Session: Advances in Correlated Electron Systems

Evidence of f-electron localization at a heavy-fermion quantum critical point

The prototypical heavy-fermion compound YbRh$_{2}$Si$_{2}$ exhibits a magnetic-field ($B)$ induced antiferromagnetic quantum critical point (QCP) at $B_{c}$ ($\bot $c) $\approx $ 60 mT. As inferred from transport and thermodynamic measurements a quantum-critical energy scale, k$_{B}T$*($B)$, indicating a crossover of the Fermi surface, has been established for this system [1]. Upon extrapolating finite-temperature ($T)$ data to $T =$ 0, one concludes (i) a vanishing of $T$*($B)$ [2] and (ii) an abrupt drop in the (normal) Hall coefficient $R_{H}(B)$ [2, 3] at $B = B_{c}$, verifying the proposal of a Kondo destroying QCP [4,5]. The dynamical processes underlying this apparent break-up of the Kondo singlets have been explored [6-8] by studying the Lorenz ratio $L$/L$_{0}$ as a function of $T $and $B$. Here, $L = \rho /w$ is the ratio of the electrical ($\rho)$ and thermal ($w =$ L$_{0}T$/$\kappa )$ resistivities, with $\kappa $ being the thermal conductivity and L$_{0} =$ ($\pi $k$_{B})^{2}$/3e$^{2}$ Sommerfeld's constant. By properly taking care of bosonic (magnon/paramagnon) contributions to the heat current which exist at finite temperature only, extrapolation of the measured data to $T =$ 0 yields a purely electronic Lorenz ratio $L$/L$_{0} =$ 1 at $B \ne B_{c}$. At $B =$ $B_{c}$, we extrapolate $L$/L$_{0} \approx $ 0.9. Therefore, the Wiedemann Franz (WF) law holds at any value of the control parameter $B$, except for the field-induced QCP [6], as is also illustrated by a pronounced heating of the sample when measuring the low -- $T$ electrical resistivity in the vicinity of the critical magnetic field [8]. This violation of the WF law is ascribed to scatterings of the electronic heat carriers from \textit{fermionic} quantum-critical fluctuations, namely those of the Fermi surface. Work done in collaboration with H. Pfau, S. Lausberg, P. Sun, U. Stockert, M. Brando, S. Friedemann, C. Krellner, C. Geibel, S. Wirth, S. Kirchner, E. Abrahams and Q. Si. \\[4pt] [1] P. Gegenwart et al., Science \underline {315}, 969 (2007).\\[0pt] [2] S. Friedemann et al., Proc. Natl. Acad. Sci. USA \underline {107}, 14547 (2010).\\[0pt] [3] S. Paschen et al., Nature \underline {432}, 881 (2004).\\[0pt] [4] Q. Si et al., Nature \underline {413}, 804 (2001).\\[0pt] [5] P. Coleman et al., J. Phys.: Condens. Matter \underline {13}, R 723 (2001).\\[0pt] [6] H. Pfau et al., Nature \underline {484}, 493 (2012).\\[0pt] [7] H. Pfau et al., Phys. Rev. Lett. \underline {110}, 256403 (2013).\\[0pt] [8] F. Steglich et al., arXiv: 1309.7260.   

Time reversal symmetry breaking in heavy fermion superconductors

Heavy fermion materials have been of interest for decades because of the numerous ordered phases they exhibit at low temperatures, often resulting in novel bulk properties including various forms of magnetic ordering and unconventional superconductivity. A full understanding of these phases and their associated order parameters requires knowledge of their corresponding symmetries. In this talk we discuss specifically the role of time reversal symmetry (TRS) breaking, as probed by polar Kerr effect (PKE) measurements, in the canonical heavy fermion superconductors UPt$_3$ and URu$_2$Si$_2$. In UPt$_3$, we observe the onset of PKE below a temperature $T_{\mathrm{Kerr}}$ that coincides with the low temperature ``B phase'' superconducting transition temperature $T_{c-}\sim480$mK. In contrast, no change in Kerr effect is observed through either the high temperature ``A phase'' superconducting transition at $T_{c+}\sim 550$mK or the small-moment antiferromagnetic (AF) transition at $T_N\sim 5$K. These results indicate that TRS is broken only in the B phase, independently of the higher temperature AF order, thus placing strong restrictions on the theory of superconductivity in this system. The case of URu$_2$Si$_2$ is more complex. At relatively high temperatures, there is a Kerr effect associated with the so-called ``hidden order'' (HO) transition at $T_{HO}\sim 17.5$K whose magnitude appears to depend on impurity concentration. At lower temperatures, an additional Kerr signal appears below the superconducting transition $T_c\sim1.5$K, which is independent of impurity concentration and which can be trained independently of the HO signal in an external magnetic field. Finally, we consistently observe an anomaly in the Kerr data at $\sim 0.8-1$K whose origins remain a puzzle, suggesting that there is more to be learned about URu$_2$Si$_2$ within the superconducting state.   

Mott criticality and multiferroicity in organic $\kappa$ -(BEDT-TTF)$_{2}$X salts

Layered organic charge-transfer (CT) salts of the $\kappa$-(BEDT-TTF)$_{\mathrm{2}}$X family show a wealth of electronic phases resulting from the interplay of strong electron-electron correlations, reduced dimensions and magnetic frustration. Of particular interest has been the bandwidth-controlled Mott transition, separating an antiferromagnetic (afm) insulating state from a correlated metallic and superconducting state. Whereas the hydrogenated X $=$ Cu[N(CN)$_{\mathrm{2}}$]Br salt is located on the metallic side, the deuterated variant, denoted $\kappa $-D8, is situated in splitting distance to the Mott transition, enabling the s-shaped transition line $T_{\mathrm{MI}}$ to be crossed via temperature sweeps. The talk will address the following aspects: 1) Thermal expansion measurements on single crystalline $\kappa $-D8 reveal discontinuous changes of the lattice parameters on crossing the Mott transition line and a huge anomaly close to the second-order critical end point of $T_{\mathrm{MI}}$ [1]. By elaborating on a scaling theory [2], we found that (i) the latter effect is a consequence of an almost divergence of the Gr\"{u}neisen parameter $\Gamma $ at the finite-$T$ critical end point, and (ii) that the expansivity data of [1] are in excellent agreement with the Mott criticality lying within the 2D Ising universality class [2], at variance with results from conductivity measurements [3]. Thermal expansion measurements under Helium-gas pressure are underway for providing thermodynamic information at variable pressure. 2) Surprisingly, for the isostructural X $=$ Cu[N(CN)$_{\mathrm{2}}$]Cl salt, located close to the Mott transition on the insulating side, we found that besides the well-established afm order at $T_{\mathrm{N}}$ $\sim$ 27 K, the system also reveals a ferroelectric transition at $T_{\mathrm{FE}}$, making this material the first multiferroic CT salt [4]. Most remarkably, the measurements reveal $T_{\mathrm{FE}} \approx T_{\mathrm{N}}$, suggesting a close interrelation between both types of ferroic order.\\[4pt] The work was performed in collaboration with M. de Souza, L. Bartosch, P. Lunkenheimer, J. M\"{u}ller, S. Krohns, A. Loidl, B. Hartmann, J. A. Schlueter \\[4pt] [1] M. De Souza \textit{et al.}, Phys. Rev. Lett. \textbf{99}, 037003 (2007) \\[0pt] [2] L. Bartosch, M. de Souza, M. Lang, Phys. Rev. Lett. \textbf{104}, 245701 (2010) \\[0pt] [3] F. Kagawa, K. Miyagawa, K. Kanoda, Nature \textbf{436}, 543 (2005) \\[0pt] [4] P. Lunkenheimer \textit{et al.}, Nature Mater. \textbf{11}, 755 (2012)   

Session M49: Focus Session: Oxide Thin Films: Growth and Properties

Atomic layer-by-layer growth of oxide thin films by laser MBE

We have studied an atomic layer-by-layer thin film growth technique for complex oxide thin films and heterostructures. By using a laser-MBE system and monitoring the reflection high-energy electron diffraction (RHEED) intensity to control the flux for each atomic layer in-situ, we actively control the structure and stoichiometry down to the atomic layer level. In the growth of SrTiO$_{3}$ from the separate SrO and TiO$_{2}$ targets, or from metal Sr and oxide TiO$_{2}$ target, we studied the phases of the specular and diffraction spot intensities as well as that of the Kikuchi lines. UV Raman spectroscopy was used to probe the symmetry breaking due to the cation off-stoichiometry. Similar stoichiometry control as shown by reactive MBE has been demonstrated. We also studied the target preparation of various oxides, including the highly reactive La$_{2}$O$_{3}$ and BaO. We have successfully applied this atomic layer-by-layer growth method to the deposition of LaAlO$_{3}$ and LaNiO$_{3}$ thin films and superlattices.   

Optimization of atomically smooth and metallic surface of SrTiO$_{3}$ for the growth of ultra-thin manganite films

Atomically smooth, TiO$_{2}$ terminated SrTiO$_{3}$ substrates can be prepared using a combination of chemical and thermal annealing treatments. Such substrates have been widely used to grow sharp oxide interfaces between SrTiO$_{3}$ and materials such as LaAlO$_{3}$. Insulating SrTiO$_{3}$ can also be made metallic by inducing oxygen vacancies or by doping with metals such as niobium. However, such treatments usually generate a rough surface. Thus, further growth of epitaxial thin films or study of the surface itself has been limited. Here, we report the optimal conditions to fabricate atomically smooth and metallic SrTiO$_{3}$ surfaces which show steps of one unit cell height. We directly confirmed the metallic characteristic of SrTiO$_{3}$ using sheet resistance vs. temperature ($R(T))$ measurements. The $R(T)$ data provides information on the physical origin of metallic behavior in SrTiO$_{3}$, which might also be relevant to the current research interest in 2DEG SrTiO$_{3}$ and oxide interfaces. We will also discuss the thin film growth of strain-induced insulating manganites on top of atomically smooth and metallic SrTiO$_{3}$ using pulsed laser deposition.   

Spectroscopic ellipsometry study on metal-insulator transition for ultrathin La-doped SrTiO$_{3}$ films

We investigated the metal-insulator transition for ultrathin La-doped SrTiO$_{3}$ (LSTO) films by using spectroscopic ellipsometric technique. As the film thickness decreased below 10 unit cells, phase transition from metal to insulator occurred through interplay of charge, spin, orbital, and lattice degrees of freedom. The optical spectra below the charge transfer gap near 3 eV changed significantly through the insulator-metal transition, exhibiting the coherent-incoherent crossover behavior. We detail our spectroscopic results on the LSTO ultrathin films, and compare them with the transport and structural properties of the films.   

Bimodal island size distribution in heteroepitaxial growth

A bimodal size distribution of two dimensional islands is inferred during interface formation in heteroepitaxial growth of Bismuth Ferrite on (001) oriented SrTiO$_3$ by sputter deposition. Features observed by in-situ x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on SrTiO$_3$. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones.   

Epitaxial strain induced phase transitions in La-doped BiFeO$_{3}$ thin films on Si substrates

Epitaxial strain is a powerful pathway to trigger phase transitions with emergent phenomena in oxide thin films, e.g., strain induced ferroelectric to ferroelectric (PE-PE) phase transition from tetragonal-like to rhombohedral-like phase in Pb(Zr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}})$O$_{3}$ and BiFeO$_{3}$ films. In this study, we report a strain driven antiferroelectric to ferroelectric (AFE-FE) phase transition from orthorhombic (O) to rhombohedral (R) phase in La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ (LBFO) thin film on Si substrates. The ground state of La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ bulk is antiferroelectric PbZrO$_{3}$ type orthorhombic phase. We show that epitaxial strain from Si substrates can stabilize a rhombohedral structure of LBFO in 20 nm films and intermediate strains position LBFO into a nanoscale mixture of rhombohedral and orthorhombic phases in 30-100 nm films and then strain relaxation in 125nm films leads to the orthorhombic phase. Transmission electron microscopy (TEM) shows atomically sharp O/R morphotropic phase boundary (MPB) with O phase domains larger than 10 nm in width. In summary, our findings open a new path to drive AFE-FE phase transition in LBFO and provide a route to study O/R MPB.   

Bimodal island size distribution in heteroepitaxial film growth: BiFeO$_{3}$ on SrTiO$_{3}$

Growth and control of complex oxide thin films on an atomic level is highly critical in understanding the behavior of both interfaces and complex oxide system. Real time X-ray scattering measurements during heteroepitaxial film deposition provide details of initial nucleation and growth giving insight into atomic scale processes and growth mechanisms. In this work we present experimental data for growth of multiferroic epitaxial BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ substrates using \textit{in-situ} diffuse x-ray scattering. A bimodal size distribution of two dimensional islands where monodispersed set of large clusters and a broad distribution of smaller islands are observed during coalescence evident from two different components of diffuse scattering. Features observed by \textit{in-situ} x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on SrTiO$_{3}$. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones. \textit{Ex-situ} atomic force microscopy (AFM) was used to measure the final surface morphology of the films at each stage.   

Epitaxial growth of polar KTaO$_{3}$ thin-films on polar perovskite substrates

The atomic polarity plays an important role in a wide range of physical phenomena at heterointerfaces. For example, the polar/non-polar nature of a LaAlO$_{3}$/SrTiO$_{3}$ system induces partial conducting electrons at the heterointerfaces to avoid diverging electrostatic potential, the so-called ``polar catastrophe,'' which results in intriguing two-dimensional transport and magnetic properties. In this presentation, we discuss another system in which the role of the polar interface is important: the KTaO$_{3}$/GdScO$_{3}$ (KTO/GSO) polar/polar system. At the KTO/GSO interface, there is a ``polar conflict'' heterointerface along the [001] direction, where the AO and BO$_{2}$ layers have reversed net charges so that there is a conflict between the chemical bonding and the electrostatic charges, i.e. K$^{1+}$O$^{2-}$(1-)/Sc$^{3+}$O$_{2}^{4-}$(1-) or Ta$^{5+}$O$_{2}^{4-}$(1$+)$/Gd$^{3+}$O$^{2-}$(1$+)$, which is unstable in the electrostatic point of view. We ask a fundamental question: ``How is the polar conflict resolved in the atomically flat heterointerfaces of such polar/polar systems?'' We have synthesized epitaxial KTO thin films on GSO substrates using pulsed laser deposition. From X-ray diffraction and high-resolution transmission electron microscopy, we have observed that the polar conflict is quite effectively avoided by forming only two non-polar mono-layers at the heterointerface, resulting in high-quality epitaxial thin films on top of the layers. Our result suggests a new way to create two-dimensional confined layers using the polar conflict of the heterointerfaces of two polar materials.   

Epitaxy of polar semiconductor $Co_{3}O_{4}$ (110): growth, structure, and characterization

The (110) plane of catalytic $Co_{3}O_{4}$ exhibits significantly higher rates of carbon monoxide conversion due to the presence of active Co$^{3+}$ species at the surface. However, experimental studies of $Co_{3}O_{4}$ (110) surfaces and interfaces have been limited due to the difficulties in growing high-quality films. In this paper, we present thin (1- 25nm) $Co_{3}O_{4}$ films grown by molecular beam epitaxy in the polar (110) direction on $MgAl _{2}O_{4}$ substrates. Reflection high-energy electron diffraction, atomic force microscopy, x-ray diffraction, and transmission electron microscopy measurements attest to the high quality of the as-grown films. We note that the film surface roughens at intermediate thickness, but slowly smoothens as growth continues, returning to an RMS surface roughness less than 1 {\AA}. Furthermore, we investigate the electronic structure and optical properties of this material by core level and valence band x-ray photoelectron spectroscopy, first-principles density functional theory calculations, and ellipsometry. A valence band offset of 3.5 eV is measured for the $Co_{3}O_{4}/MgAl _{2}O_{4}$ heterostructure. Magnetic measurements show the signature of antiferromagnetic ordering at 46 K.   

Effect of Particle Size Distribution on the Magnetostrictive Properties of Cobalt Ferrite

Magnetostrictive materials are technologically useful for developing stress sensors and actuators. Oxide based magnetostrictive materials such cobalt ferrite are more appropriate especially in situations where it is desirable to avoid losses due to eddy current. A very important factor that strongly affects the magnetostrictive properties of this class of materials is the microstructure. This study investigates the relationship between the pre-sintering particle size distribution and the magnetostrictive properties of cobalt ferrite. This is important because final microstructure, hence the magnetostrictive properties of ferrites prepared via the solid-state reaction technique will depend strongly on the pre-sintering particle size distribution. Samples derived by combining powders with the smallest and largest particle size distributions gave the highest magnetostriction amplitude and strain sensitivity for measurement in the parallel direction. Samples from the largest particle size distribution gave the least in the parallel direction but highest in the perpendicular direction.   

Room-Temperature Ferroelectricity in Hexagonal TbMnO$_{3}$ Thin Films

Magnetoelectric multiferroics exhibit coupling between the ferroelectric and magnetic order parameters, allowing control of electric polarization by a magnetic field or magnetization by an electric field. This property is appealing for novel device applications but they require room-temperature functionality. Among a limited group of single-phase multiferroic materials, rare-earth manganites, such as TbMnO$_{3}$, are promising due to their strong magnetoelectric coupling. However, the ferroelectric transition temperature of TbMnO$_{3}$ in the bulk orthorhombic phase is very low. Here, we report room-temperature ferroelectricity of epitaxially-stabilized hexagonal TbMnO$_{3}$ thin films which is accompanied by significant polarization-dependent resistive switching. The first principle calculation and group theoretical analysis reveals that the ferroelectric polarization of hexagonal TbMnO$_{3}$ is associated with the lattice instability of prototypical paraelectric phase at the zone boundary and is also an improper ferroelectric similar to other manganites such as YMnO$_{3}$. Our results demonstrate a possibility to engineer new single-phase multiferroics by epitaxial growth, which broadens the range of functional materials desirable for novel electronic devices.   

The Control of Anisotropic Transport in Manganites by Stripy Domains

Epitaxial thin film acts as a significant tool to investigate novel phenomena of complex oxide systems. Extrinsic constraint1 of uniform or certain designed buffer layer strain could be easily implanted to these materials. However, the strain distribution might be quite complicated by involving micro- or nano-lattice distortions which could partially relax the strain and determine the complex phase diagrams of thin film, meanwhile introducing structural and physical inhomogeneities. In this work , we report 71$^{\circ}$ striped ferroelectric domains created in BFO can also epitaxially lock the perovskite manganites leading to the emerge of ordered structural domain. LSMO/BFO hetero-epitaxial samples are deposited by PLD. The 71$^{\circ}$ periodic striped domains and coherent growth are demonstrated by PFM and X-ray analysis. Plan-view TEM and X-ray RSM have been used to confirm the epitaxial relationships of the functional layers and IP lattice constant. Both the simulation and structural analysis demonstrate we can create a periodic ordered stripe structural domain in LSMO. And this will leave an anisotropic distribution of structural domain walls which makes it possible to capture the anisotropic tunneling for strong electron--lattice coupling in manganites. Temperature-dependent resistivity measurements reveal a substantial anisotropic resistivities and a remarkable shift of the MI transition between the perpendicular and parallel to the stripe domain directions.   

Structural Characterization of Strain Relaxed (100)-Oriented Nd$_{0.5}$Sr$_{0.5}$MnO$_3$ Thin Films

Half-doped manganites exhibit intriguing charge ordering (CO) properties. Pseudocubic (110)-oriented thin films can preserve bulk properties and show a charge ordering-ferromagnetic (CO-FM) transition. However, for (100) oriented films grown on traditional perovskite substrates, no CO-FM transition has been reported so far. Here we successfully realized the CO-FM transition in (100)-oriented $\mathrm{Nd_{0.5}Sr_{0.5}MnO_3}$ (NSMO) thin films grown on $\mathrm{SrTiO_3}$ substrates by inserting a perovskite-like flexible buffer layer $\mathrm{Sr_3Al_2O_6}$. From temperature-dependent X-ray elastic scattering, we observed changes in the NSMO lattice constants exactly at the CO-FM transition temperature determined from transport and magnetization measurements. Moreover, we observed CO peaks suggesting a different ordering pattern compared to the bulk or (110)-oriented thin films. These results provide new opportunities to create and study novel electronic ground states unexplored in films grown on the rigid substrates used up to now.   

Atomic manipulation with Scanning Tunneling Microscopy on the surface of a manganite thin film

Manganites have attracted significant attention in the past two decades, due to an extraordinarily rich spectrum of phenomena stemming from inherent complexity linking spin, charge, lattice and orbital degrees of freedom that result in properties including half-metallicity and giant magnetoresistance. Here, we report atomic manipulation with STM on the surfaces of 25 unit-cell thick La5/8Ca3/8MnO3 (LCMO) SrTiO3 (STO) substrates. We demonstrate that by applying triangular first-order reversal curve (FORC) waveforms of increasing amplitude to STM tips in-situ, it is possible from both A and B terminations to individually extract single units, form vacancies, remove units from layers below, rearrange atoms in the surrounding lattice, and therefore cause reactions to occur at the atomic level. These experiments point to the possibility of STM to manipulate atoms on the surfaces of manganites, opening up further avenues of research into fundamental physical properties defined at atomic scales. This research was sponsored by the Division of Materials Sciences and Engineering (RKV, AT, SVK) and by the Scientific User Facilities Division (APB) of BES, DOE. Research was conducted at the CNMS, which is sponsored at ORNL by the Scientific User Facilities Division, BES, DOE.   

Critical thickness for ferromagnetism in insulating LaMnO$_{3}$ films

The interplay between exchange interactions, interfacial charges, and confinement effects controls the electronic, magnetic, and transport properties of complex oxide thin films. Here we report the emergence of ferromagnetism in insulating LaMnO$_{3}$ thin films grown on SrTiO$_{3}$ substrates beyond a critical thickness. LaMnO$_{3}$ (001) films are deposited by a pulsed laser deposition technique with thicknesses varying from 1 unit cell to 24 unit cells. The position dependent local magnetization is then mapped with micrometer resolution using scanning superconducting quantum interference device microscopy. We find that the magnetic ground state switches from non-ferromagnetic to ferromagnetic within a change of one unit cell above the critical thickness of 5 unit cells with characteristic domain size of about 20 $\mu $m. Further increase of film thickness up to 24 unit cells leads to reduction of the domain size to about 10 $\mu $m. The critical thickness is qualitatively explained in terms of the charge transfer in polar LaMnO$_{3}$ (001) thin films based on results of additional experimental data, density-functional calculations, and the electrostatic modeling.   

Atomically-Resolved In-Situ Studies of Surface Structure Evolution of PLD-Grown La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ Thin Films

Here, we report atomically resolved in-situ Scanning Tunneling Microscopy (STM) studies of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ (LCMO) thin films grown by RHEED-assisted PLD. Films were grown on TiO$_{2}$-terminated (001) SrTiO$_{3}$ substrates at a substrate temperature of 750 $^{\circ}$C and O$_{2}$ pressure of 50 mTorr. \textit{In-situ} UHV STM was performed at room temperature. LCMO is known to grow in layer-by layer (LBL) mode. We find that the initial growth does not follow the best physically possible LBL growth (with only three u.c. layers exposed). RHEED oscillations decay during deposition of the first 10-15$^{\rm th}$ unit cells. Subsequently, the RHEED intensity oscillations grow and remain persistent. STM images of 16 u.c.-thick films revealed surfaces with up to five u.c. layers being exposed in a stepped island-like morphology with 1/2 u.c. step heights. Such morphology allowed studies of atomic surface structure of both terminations. 25 u.c.-thick samples were found to be almost single-terminated. The minor termination is ordered and exhibits (1x1) reconstructions; RHEED suggests that this termination is the La/Ca-O termination. A 250 u.c.-thick film was found to be single-terminated with only three u.c. layers exposed.   

Session M50: Nanostructures and Metamaterials

Single-molecule Raman mapping with sub-nm resolution

Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, bio-imaging, molecular nanotechnology, and material science. Molecular vibrations provide a valuable ``fingerprint'' for this identification. The spectroscopy based on tip-enhanced Raman scattering (TERS) has opened a path to obtain enhanced vibrational signals thanks to the strong localized plasmonic field originated at the tip apex. However, the best spatial resolution of the TERS imaging reported to date is still limited to a few nm, obviously not adequate for resolving a single molecule chemically. Here we demonstrate unprecedented sub-molecular Raman spectroscopic mapping with spatial resolution below 1 nm, resolving even the inner structure of a single molecule and its configuration on the surface [1]. This is achieved by creating a double-resonance nonlinear process via spectral matching, particularly by matching the resonance of the nanocavity plasmon to the downward molecular vibronic transitions [2]. Such exquisite tuning capability is provided by a combination of low-temperature ultrahigh-vacuum scanning tunneling microscopy with ultrasensitive optical detection. Our nonlinear TERS technique features the use of only a continuous wave laser rather than two pulse lasers. Our finding of Raman spectromicroscopy going intra-molecular and sub-nanometer may open up a new avenue to probe surface chemical identification, optical processes and photochemistry at the single-molecule scale. \\[4pt] [1] R. Zhang, et al., Nature 498, 82 (2013).\\[0pt] [2] Z.C. Dong, et al., Nature Photonics 4, 50 (2010).   

Effect of dielectric spacer layers on SERS with Au nanoparticle arrays on silicon substrates

The optical response of a plasmonic nanostructure is often highly dependent on the nature of the substrate supporting it. To study the effect of the substrate on surface enhanced Raman scattering (SERS), we have fabricated a series of SERS substrates consisting of a hexagonal array of Au nanoparticles self assembled on block copolymer films, a silicon oxide (dielectric) layer and a silicon substrate. The inter-particle distance and the dielectric layer thickness were controlled. The SERS Enhancement Factors (EF) were calculated by comparing the Raman spectra of 4-aminothiophenol adsorbed on the surface of the Au nanoparticles and in a standard solution. The SERS EF were found to be strongly affected by the inter-particle distance and silicon oxide thickness. Changing the inter-particle spacing induced a 10$^{\mathrm{2}}$ variation in the EF while changing the oxide thickness increased the range of EF values by an additional factor of 10. Maximal enhancement factors were found with oxide layer thicknesses between 50 nm and 100 nm beneath the 30 nm polymer film. This geometry both improved the resonance condition with the probe laser and reduced the absorption by the substrate. This work illustrates that optimization of plasmonic-based sensors should consider both the metallic and the surrounding structures.   

Mechanisms of surface-enhanced Raman scattering on metal oxide nanowires

For several decades, surface-enhanced Raman scattering (SERS) from various analytes adsorbed on metal has been studied and utilized for optoelectronic and biochemical devices. There are two well accepted mechanisms of SERS on metal: One is the electromagnetic enhancement and the other is the charge transfer enhancement. On the other hand, another mechanism depending on the geometry, for example diameter, aspect ratio, etc., of the sample has been considered recently to explain SERS from molecules adsorbed on dielectric nanostructures where far less free charges exist. In this study, we would like to explain the mechanism of SERS on metal oxide nanostructures. To study enhancement effects, we measured Raman scattering signal from molecules adsorbed on metal oxide nanowires and nanocones excited by lasers with three different wavelengths. We observed that the Raman signal was enhanced regardless of excitation wavelengths, even though the enhancement factor showed slight wavelength dependence. Importantly, we observed that the enhancement was always larger when the analytes were adsorbed on nanocones. From our understanding, we can suggest a way to systematically create, or control ``hot spots'' for enhancement of light field using one dimensional metal oxide nanostructures.   

Electrostatic gating and single-molecule Raman spectroscopy

Simultaneous electronic and optical measurement on molecular junctions provide rich microscopic information about electronic and vibrational energy distributions at the atomic and molecular scale. We fabricate nanoscale gold bowtie structures as surface enhanced Raman (SERS) substrates. Following electromigration, these nanostructure with nanoscale interelectrode gaps support highly localized surface plasmon resonances, resulting in single-molecule sensitivity due to the high electromagnetic enhancement. In prior electronic transport studies, these structures have proven to be suitable tools to examine electronic and vibrational properties of single molecules, in which the underlying substrate is used as a gate electrode to capacitively shift the molecular level relative to the Fermi levels of the source and drain, enabling the studies in the nonresonant, resonant and Coulomb blockade regime. We will present preliminary results on the effect of gate modulation on the SERS and electrical properties of molecules in such junctions.   

Raman spectroscopic investigation of lithium niobate nanoparticles

Recently there has been a large interest in synthesizing nanoscale structures from ferroelectric materials. Due to the tendency of the nanoscale structures to form aggregates, characterizing the properties of isolated nanostructures can be challenging. Through combining Raman spectroscopy with an optical trap, we investigated the properties of lithium niobate nanoparticles synthesized by the sol-gel method. Analysis of the Raman spectrum shows that the stoichiometry of the nanoparticles is dependent on the starting stoichiometric ratio of lithium to niobium in the synthesis step. We also demonstrate the power of this technique to determine the orientation of ferroelectric nanoparticles in an external applied field.   

Crystal Structure Anisotropy Explains Anomalous Elastic Properties of Metal Nanorods

It is demonstrated that the frequency of the extensional vibrational mode of a nanorod made of an elastically anisotropic crystalline material deviates widely from the predictions of the theories based on the analysis of the long-wavelength limit. The dispersion relation for the fundamental extensional mode of a gold rod grown in the $[100]$ direction is calculated and found to be in an excellent agreement with experimental data obtained from the transient optical absorption measurements on gold nanorods.\footnote{H. Petrova, J. Perez-Juste, Zh. Zhang, J. Zhang, T. Kosel, and G.V. Hartland, J. Mater. Chem. {\bf 16}, 3957 (2006)} This explains an anomaly in the elastic properties of nanorods which was previously attributed to a 26\% decrease in Young's modulus for nanorods compared to its bulk value.   

A New Model with Internal and External Traps explaining Fluorescence Intermittency in a Quantum Dot Studied with Scanning Tunneling Spectroscopy

Recent studies on fluorescence intermittency, or called blinking, in quantum dots (QDs) show that complete control of this phenomenon is near at hand. Although a number of models deal with the transitions between on/off states in the intermittency, they do not consider the spatial and energy distribution of traps in a single QD. In this study, we measured the spatial and energy distribution of traps using scanning tunneling microscopy and spectroscopy. The trap states of CdSe/ZnS QD exhibit two distinct energy states and intensities in the tunneling spectra according to their residing positions (inside or surface of QD). We were able to simulate trapping dynamics of the fluorescence intermittency from the measured energy and spatial distribution. We used Monte Carlo method to render transitions between the trap states in this model. We can successfully explain the power-law distribution of on/off time, which is a characteristic feature of the blinking. The dependence is a consequence of a two-step trapping process through inner and surface traps. The simulation also predicts the suppression of the long tail in the power-law distribution by reducing the surface traps. This result is in good agreement with a recent fluorescence lifetime-intensity distribution measurement.   

Exploring the charge/energy transfer process at the graphene/giant nanocrystal quantum dots interfaces

Due to its transparency in wide spectral range and high charge mobilities, graphene has been considered to utilize as transparent electrode for nanocrystal based photo-voltaic and light emitting diodes.~ A detail understanding on charge/energy transfer (CT/ET) processes between zero dimensional quantum dots and 2D graphene layer hold the key in optimizing the performance of these devices.~ To attain this understanding, we conduct a systematic study on CT and ET processes between a graphene layer and~ CdSe/CdS giant nanocrystal quantum dots (g-NQD) as the function of CdS shell thickness.~ In addition to analyzing PL quenching and change of PL decay dynamic, we also perform 2$^{nd}$ order photon correlation spectroscopy studies to investigate the effect of graphene layer on dynamic and emission efficiency of g-NQDs' multi-exciton states.~ In case of g-NQDs over coated with a thick 16 ML CdS shell, we observed a surprising increase of multi-exciton emission efficiency.   

Session M51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties VI

Edge contact to BN encapsulated graphene -- towards contamination-free 2D system

Since the first discovery in 2004, graphene has been electrically contacted by metal layers deposited on its surface. In the process, this single atomic layer sheet experiences polymers and solvents contaminations and thermal annealing. Our recent progress shows that we can metalize only the one-dimensional (1D) edge of a graphene layer in a BN/G/BN structure. In addition to outperforming conventional surface contacts, the edge contact geometry allows a complete separation of the layer assembly and contact metallization processes. For the first time, the graphene surface has never contacted any polymer or solvent contaminations, and thermal annealing is found to be unnecessary. Over 1000 um$^{\mathrm{2}}$ bubble free BN/G/BN stack is achieved and carrier mean free path on these devices are found to be over 20 um. 1 000 000 cm$^{\mathrm{2}}$/Vs carrier mobility is for the first time observed at carrier density as high as 3 X 10$^{\mathrm{12}}$/cm$^{\mathrm{2}}$.   

Thermal Interface Conductance across a Graphene-BN Heterojunction

We deposit graphene on h-BN flakes and measure the thermal interface conductance of a graphene/h-BN interface by passing current through the graphene sheet to create Joule heating while monitoring the temperatures of the graphene and h-BN using Raman spectroscopy. During the electrical heating, the Raman G band, 2D band, and h-BN frequency downshift with the increasing of the applied power, indicating the heating in graphene and h-BN. The Raman temperature coefficient of the G band, 2D band, and h-BN frequency are calibrated by heating the device from 300K to 400K in a temperature-controlled stage, yielding values of 0.0102 and 0.0215 cm$^{-1}$/K for graphene, and 0.0246 cm$^{-1}$/K for h-BN. These electrical heating and the temperature calibration results suggest a maximum temperature gradient of 60K at the graphene/h-BN interface during the electrical heating. Multiple electrical heating experiments are conducted, showing consistent results, validating the reliability of the acquired data. From the power dependence of the temperature difference between the graphene and h-BN, we are able to establish the interface thermal conductance across the graphene/h-BN interface to be 7.41 $\pm$ 0.43 MW/m2K.   

Transport Properties of Crystallographically Aligned Heterostructures of Graphene and Hexagonal Boron Nitride

Graphene and hexagonal boron nitride (hBN) heterostructures have been heavily studied due to graphene's high electronic mobility in this system. Hexagonal BN also shows possibilities to alter graphene's electronic properties. Recently several research groups have demonstrated accurate placement of graphene on hBN with crystallographic alignment[1][2][3]. Due to the resulting superlattice formed in the graphene/hBN heterostructures, an energy gap, secondary Dirac Points, and Hofstadter quantization in a magnetic field have been observed. However, many aspects of the electronic properties of graphene/hBN heterostructures remain unexplored. Using aligned layer transfer we are able to produce graphene/hBN heterostructures with ~1 degree alignment accuracy, and measure the transport properties of the resulting systems. We will discuss our latest transport data, which contribute towards a greater understanding the electron motion in the graphene/hBN interface. [1] P. A. Ponomarenko et al., Nature 497, 594-597 (2013). [2] C. R. Dean, et al., Nature 497, 598-602(2013). [3] B. Hunt, et al., Science 340, 1427(2013).   

Wafer-scale integration of graphene heterojunction transistors for low-power electronics

Recently, vertical graphene field effect transistors (VGFETs) in which graphene is combined with semiconductors including layered two-dimensional materials have attracted much attention for application in digital electronics. Work-function tunability of graphene was employed in VGFETs to modulate energy barrier between graphene and semiconductors, and therefore Ion/Ioff was dramatically increased. Here, we demonstrate wafer-scale VGFETs based on graphene-thin semiconductor-metal asymmetric junctions on a transparent 150 $\times$ 150 mm2 glass. In this system, a triangular energy barrier between the graphene and metal is designed by selecting a metal with a proper work function. We obtain a maximum Ion/Ioff up to 1,000,000 with an average of 3,010 over 2,000 devices at ambient conditions. Furthermore, an inverter that combines complementary n-type and p-type devices was demonstrated to operate at a bias of only 0.5 V.   

Edge Contact to 2D materials

Electrically interfacing 3D metal electrodes to atomically thin 2D materials presents a fundamental technical challenge.~ Recently we demonstrated that metalizing along the 1D edge of graphene enables a new device topology with remarkably low contact resistance.~ Here we expand the capability of this edge-contact technique by integrating leads with complex properties, such as ferromagnetic and superconducting metals.~ Implications for realizing novel electronic behavior in 2D layered materials will be discussed. ~ ~[1] L. Wang, \textit{et al.,} One-Dimensional Electrical Contact to a Two-Dimensional Material, \textit{Science}, \textbf{342} (6158), 614-617. (2013)   

First-principles Simulations of a Graphene-Based Field Effect Transistor

We propose a first-principles method to simulate the effect of a gate voltage on a two-dimensional channel. Because of the aperiodic structure of field effect devices, we adopt the effective screening medium (ESM) method, which enables us to solve the Poisson equation with free boundary conditions, to simulate nanoscale field effect devices. With this approach, we investigated a graphene-based vertical field effect tunneling transistor with a graphene$|h$-BN$|$graphene multilayer structure. The calculated carrier density on the graphene away from the gate electrode is sublinear with respect to the gate voltage, and decreases as the thickness of the $h$-BN barrier increases. The band structure of graphene layers near the Fermi energy exhibit a $\sim 0.05 \,\mathrm{eV}$ gap opening due to interactions with $h$-BN, as well as the chemical potential difference between the two graphene monolayers. This work paves the road to first-principles simulations of nanoscale field effect transistors and opens a new avenue towards computational guided device design.   

Investigation of Charge Transfer in Graphene-Based Heterostructure by Raman Spectroscopy

Van der Waals heterostructures are an emerging research area in novel electronics and optoelectronics. They can be assembled layer by layer with any stacking order via precise micromechanical manipulation. Moreover, their material properties can be theoretically tailored on demand. Two interesting properties, the interfacial interaction between dissimilar layer materials and the accordingly modified bandstructure, are important; however, they are rarely studied. Herein, we take graphene-MoS$_{2}$ as an example, and we utilize the well-studied Raman fingerprint of graphene to inspect charge transfer between graphene and an adjacent material. The layer dependence of charge transfer is observed as well as the edge effect. Further fundamental understanding of this heterostructure will be presented. Our work will serve as a platform to study the band alignment of graphene-based heterostructures.   

Synthesis of white graphene (h-BN) from polymeric precursors and its physical properties

Large area and high quality white graphene (h-BN, hexagonal boron nitride) has been effectively synthesized from borazine oligomer on Ni catalysts. Synthetic methods for white graphene only included spin coating and subsequent annealing steps, and the thickness of white graphene was controlled with variation of spin coating speed. Characterization methods such as TEM, SAED, XPS, Raman, and EELS revealed highly crystalline boron nitride structures with stoichiometric B/N ratio close to 1. Catalytic activity of Ni catalyst for the phase conversion reaction through crosslinking and BN crystallization was clearly demonstrated and proper mechanism was suggested. Considering thermal conductivity and capacitance measured from this white graphene, it has potential applications such as gate dielectric layers for graphene, and thermal management coatings.   

Nanoscale Capacitors Composed of Graphene and Boron Nitride Layers: Size Effects at Small Separation

A nanoscale dielectric capacitor model consisting of two-dimensional, hexagonal h-BN layers placed between two commensurate and metallic graphene layers is investigated using self-consistent field density functional theory. The separation of equal amounts of electric charge of different sign in different graphene layers is achieved by applying electric field perpendicular to the layers. The stored charge, energy, and the electric potential difference generated between the metallic layers are calculated from the first-principles for the relaxed structures. Predicted high-capacitance values exhibit the characteristics of supercapacitors. The capacitive behavior of the present nanoscale model is compared with that of the classical Helmholtz model, which reveals crucial quantum size effects at small separations, which in turn recede as the separation between metallic planes increases.   

SPM Study and Growth Mechanism of Graphene Directly CVD-Grown on h-BN Film

We present our Scanning Tunneling Microscopy (STM)/Spectroscopy (STS) and Kelvin Probe Force Microscope (KPFM) study for graphene directly CVD-grown on h-BN film. High resolution STM image shows perfect honeycomb lattice structure of graphene on top surface and Moir\'{e} pattern indicating the structural interference patter with the underlying h-BN crystal. Non-disturbed electronic structure of graphene on h-BN film is also confirmed by spatially-resolved STS measurements, which show very sharp and symmetric V shape with a Dirac point at Fermi level. To confirm the graphene growth mechanism on h-BN film/Cu foil, careful Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) measurements were performed on different thickness of h-BN film on a SiO$_{2}$ substrate to unveil the catalytic origin of graphene growth on h-BN/Cu. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (Grant Numbers: 2009-0083540, 2011-0030046, 2012R1A1A2020089 and 2012R1A1A1041416).   

Synthesis of Large Scale MoS2-Graphene Heterostructures

A rapidly progressing field involves the stacking of multiple two dimensional materials to form heterostructures. These heterosctructures have exhibited unique and interesting properties. For the most part, heterostructure devices are produced via mechanical exfoliation followed by careful aligning and stacking of the various components, limiting dimensions to micron-scale devices. Chemical vapor deposition (CVD) has proven to be a useful tool in the production of graphene and has very recently been investigated as a means for the growth of other 2D materials such as MoS2, hexagonal boron nitride and WS2. Using a two-step CVD process we are able to synthesize MoS2 on CVD grown graphene. AFM and Raman microscopy of the MoS2-graphene heterostructure show a uniform and continuous film on the cm scale.   

A stable path to ferromagnetic hydrogenated-graphene growth

Based upon first principle calculations, we present results that indicate the presence of a preferential site on one sublattice for hydrogen adsorption due to the screening effect of hexagonal boron nitride (h-BN). Our results show the effect of h-BN increases the hydrogen migration barrier on top of graphene. We propose a functional heterostructure as a TMR device, which is exploiting the screening effect caused by h-BN and the insulating properties of this exotic 2-D material. The density of states (DOS) calculations, with 1, 2 and 3 h-BN layers sandwiched in between two layers of graphone, show a half metallic state for these new heterostructures.   

Toward disorder-free graphene

Integration of monolayer graphene with BN dielectrics has enabled substantial reduction in disorder with recent graphene devices exhibiting ballistic transport over tens of microns. However, low density response and magneto transport in the quantum Hall effect regime indicate a remnant disorder temperature above a few Kelvin. In my talk I will show how low field Shubnikov-de Haas oscillations measured from encapsulated BN/G/BN devices are consistent with a remote-impurity scattering model, suggesting that the residual source of disorder may lie outside the heterostack. Using our recently developed fabrication techniques together with the edge contact geometry, we will present new strategies to eliminate this residual scattering.   

Atomic-scale study of lateral graphene/h-BN hybrid structure

Recently, atomically sharp 1D interfaces have been successfully implemented in lateral graphene/hexagonal boron nitride (h-BN) hybrid structures. Graphene/h-BN interfaces are of particular interest, because their bandgap and magnetic properties can be engineered by controlling the arrangement of nonmagnetic B, C and N atoms. Despite the enormous interest in graphene/h-BN, there has been very limited experimental success in determining the local atomic structure of the graphene/h-BN interface. Here, using state-of-the-art scanning tunneling microscopy, we report the direct and precise observation of a graphene/h-BN interface bonding structure at the atomic scale. Based on the detailed atomic structure, first-principles density-functional calculations show that graphene zigzag edge states and the h-BN polarity are strongly coupled to each other near the interface and induce spatial modulation of physical properties along the lateral direction. In addition, we investigate how the d-orbitals of metal surfaces (Cu (111), Cu (001)) and the pi-orbital of graphene/h-BN hybridize and predict resulting modification of the electronic properties of graphene/h-BN.   

Session M52: ARPES in Copper-oxide Superconductors

From insulator to d-wave superconductor: a persisting nodal gap and its interaction with superconductivity

Gapless quasiparticle excitations along (pi,pi) direction in cuprate superconductors are enforced by sign-changing d-wave pairing symmetry. However, increasing number of evidences has been uncovered to support a finite gap along nodal direction in various families of underdoped cuprate superconductors. In this work, we will demonstrate with very comprehensive doping and temperature dependent nodal gap evolution in single layer LSCO system. The continuous doping evolution of the gap extending from insulating region of phase diagram all the way into superconducting dome indicates an origin other than d-wave superconductivity that interacts fundamentally with the superconducting order parameter. By lifting the symmetry-protected node, this gap coexists with superconductivity, meanwhile shows vivid interaction with superconductivity in low temperature underdoped region.   

Angle resolved photoemission spectroscopy study of HgBa$_{2}$CuO$_{4+\delta}$

HgBa$_{2}$CuO$_{4+\delta}$(Hg1201) is a model cuprate for scattering, optical, and transport experiments, but angle-resolved photoemission spectroscopy (ARPES) data are still lacking owing to the absence of a charge-neutral cleavage plane. We report on progress in achieving the optimal experimental conditions where quasiparticles can be observed in the near-nodal region. The superconducting gap, Fermi surface, and nodal kink were measured and quantified by ARPES for the first time in Hg1201, providing a crucial momentum space complement to other experimental probes.   

Ultrafast Momentum-resolved Gap and Quasiparticle Dynamics in Bi2212

Time- and angle-resolved photoemission spectroscopy (trARPES) is a direct and multifaceted probe of electron dynamics in crystalline solids. In cuprate high-temperature superconductors, it holds promise for understanding the interactions potentially linking superconductivity, charge density waves, and antiferromagnetism, which occur in close proximity in the cuprate phase diagram. We have used trARPES to measure the nonequilibrium gap and quasiparticle relaxation dynamics in Bi2212 while varying a range of dopings, temperatures, pump fluences, binding energies, and crystal momenta. Measurements reveal distinct signatures corresponding to superconducting and non-superconducting ordering tendencies, with potential indications of phase coexistence and/or competition.   

Investigating the Effects of Doping Inhomogeneity on the ARPES Spectrum of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$

Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ (Bi2212) is a dirty material. This is readily seen with STM, which shows nanoscale variations in the gap parameter and local doping level. Through numerical simulation we investigate the effects of this doping inhomogeneity on the ARPES spectrum of Bi2212. We find that the main effect of the inhomogeneity is to broaden the spectrum in momentum, with an increasing magnitude towards the antinode. This has implications for the scattering rates extracted from MDC or EDC analysis. We show that after removing the doping inhomogeneity the scattering rate measured with ARPES qualitatively agrees with that from STM and Optical Reflectivity.   

Zhang-Rice singlet hopping on bipartite tetragonal CuO

In the superconducting cuprates, corner sharing CuO$_4$ plaquettes host the formation and propagation of the Zhang-Rice singlet. Adding a further Cu atom to the center of such plaquettes results in a rare edge sharing geometry. The cupric oxide CuO indeed crystallizes in a lower-symmetry monoclinic form. At beamline 7.0.1 of the Advanced Light Source, we have grown tetragonal CuO thin films by pulsed laser deposition. By in situ angle-resolved photoemission (ARPES), we show that the first ionization state is a singlet propagating on two nearly independent corner sharing sublattices, and we resolve an inter-plaquette coupling of the order of 100 meV.   

New Insights about the Strip Order in LBCO-1/8 by X-ray Natural Circular Dichroism

Recent studies on LBCO-1/8 showed that the Kerr rotation of the infrared light polarization becomes non-zero below its charge stripe ordering temperature, and the sign of the Kerr rotation stays the same on opposite surfaces of the sample. This result has been interpreted as an evidence for a broken chiral symmetry of the stripe order. Such an interpretation is highly relevant to the nature of the broken symmetries that characterize the normal (pseudogap) state of cuprates as similar Kerr onsets have also been observed in several other cuprate families in the pseudogap state. Given its importance, insights from complementary experimental probes are required to shed new light on this issue. X-ray natural circular dichroism (XNCD), which measures circular dichroism in x-ray absorption, has been known as a useful probe for chirality of materials. Different from the Kerr effect, it is element-specific and predominantly measures a different part of the optical activity tensor that manifests a unique orientational dependence. We will present our first XNCD study of LBCO-1/8 as functions of the x-ray incident angle and temperature. A circular dichroic signal was observed within the stripe order state and shows a clear onset behavior. We will compare this result with the Kerr effect and neutron/x-ray diffraction results previously obtained on the same piece of sample.   

Spin-charge interplay in antiferromagnetic LSCO studied by the muons, neutrons, and ARPES techniques

We performed temperature dependent angle resolved photo emission spectroscopy (ARPES) measurements on an antiferromagnetic (AFM) LSCO sample with $x=1.92$\%. We find: quasiparticle peaks, Fermi surface, antinodal gap, and below $45$ K a nodal gap. Muon spin rotation measurements ensures that the sample is AFM and that the doping is close, but below, the spin glass phase boundary. In addition, we performed elastic neutron scattering measurements on the same sample, and determined the thermal evolution of the commensurate and incommensurate magnetic order. Our major finding is that nodal gap opens at a temperature well below the commensurate ordering at $140 $ K, and close to the incommensurate ordering temperature of $30$~K.   

Accurate variational solution to describe a hole doped in a cuprate layer

Using a semi-analytical variational scheme, we study the spectral properties of a single hole doped in a CuO$_2$ plane, based on a model that includes the O 2p orbitals explicitly. We verify our choice of variational space by showing that its increase has little effect on the hole's dispersion. Our method predicts a hole dispersion that agrees very well with exact diagonalization results performed for 32-site clusters of the same model. This is quite remarkable given the much less computational effort required in our approach, which deals with an infinite lattice. We find that the low-energy states have considerable overlap with the Zhang-Rice singlet, however the overlap decreases for higher energy states, signalling the breakdown of the one-band effective treatment of this problem. Finally, we show that our results also explain the unusual drop in ARPES spectral weight observed experimentally to occur outside the magnetic Brillouin zone.   

Quasiparticle lifetime in Optimally Doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ by Angle Resolved Photoemission Spectroscopy

Quasiparticle lifetime of optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ has been systematically investigated by state-of-the-art angle resolved photoemission spectroscopy with energy, momentum and temperature resolution. In the normal state, Momentum Distribution Curve (MDC) width shows linear temperature dependence in all momentum and energy range, consistent with Marginal Fermi Liquid picture. In the superconducting state, the temperature dependent MDC width is strongly energy and momentum dependent. It shows a rapid drop below T$_C$. The drop is observed on the whole Fermi surface but more obvious in antinodal region. A sharp peak structure is revealed in energy dependent MDC width in antinodal region, which is a result of strong electron - boson mode coupling. The peak intensity has similar temperature dependence with spin resonance mode measured by neutron scattering. Detailed comparison between our ARPES results and theory calculation will be discussed.   

Probing the temperature scales in cuprate high-temperature superconductor by ultrafast angle-resolved photoemission

We used time- and angle-resolved photoemission (trARPES) to measure the non-equilibrium electronic states of Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. Detailed temperature and pump fluence dependent measurements were taken on these samples to study the temperature scales in cuprate, giving a phase diagram from the view of the non-equilibrium study. These results indicate that trARPES is a powerful tool in probing the quantum phase transitions in materials.   

ARPES study on annealing induced doping effects in electron doped cuprates

In contrast to the hole doped cases, electron doped cuprates Ln$_{2-x}$Ce$_x$CuO$_{4-\delta}$, (Ln: rare earths) become superconducting after proper annealing which affects the oxygen content. Their normal state spin and charge dynamics also significantly depend on the heat treatment. While it is expected that oxygen deficiency and extra oxygen naturally induces electron and hole doping respectively, quantitative investigation has not been carried out so far. In this study, we prepared single crystalline samples of Pr$_{1-x}$LaCe$_x$CuO$_{4-\delta}$ (x=0.1, 0.18) and heat treated them under various conditions. We then performed ARPES and magnetic susceptibility measurements. It is found that the Fermi surface volume of the oxygen reduced x=0.1 system (T$_c$=25K) is comparable to that of the re-oxidized x=0.18 sample (T$_c$=19K), indicating that the electron carriers introduced by the Ce substitution is indeed compensated by the post-annealing (oxidation) process. Based on the results, we discuss the relationship between x, $\delta$ and T$_c$ of the electron doped cuprate superconductors in detail.   

A Revision of the Phase Diagram of Overdoped BSCCO

A re-examination of the temperature dependence of the anti-nodal gap observed in the overdoped region of the phase diagram BSCCO by angle-resolved photoemission spectroscopy (ARPES) suggests that the transition temperature, at which the gap, closes was under-estimated previously. In this study both oxygen and calcium doping were used as a means of achieving overdoping with both methods yielding similar results. Higher experimental resolution and new ways of data analysis improve the accuracy of extracting superconducting gap value. The present studies result in a modification to the accepted phase diagram.   

A spectroscopic fingerprint of electron correlation in high temperature superconductors

The so-called ``strange metal phase'' of high temperature (high Tc) superconductors remains at the heart of the high Tc mystery. Better experimental data and insightful theoretical work would improve our understanding of this enigmatic phase. In particular, the recent advance in angle resolved photoelectron spectroscopy (ARPES), incorporating low photon energies ($\approx$ 7 eV), has given a much more refined view of the many body interaction in these materials. Here, we report a new ARPES feature of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ that we demonstrate to have the key ability to distinguish between different classes of theories of the normal state. This feature--the anomaly in the nodal many body density of states (nMBDOS)--is clearly observed in the low energy ARPES data, but also observed in more conventional high energy ARPES data, when a sufficient temperature range is covered. We show that key characteristics of this anomaly are explained by a strong electron correlation model; the electron-hole asymmetry and the momentum dependent self energy emerge as key required ingredients. In particular, we find that, among many theories available for comparison, the phenomenological extremely correlated Fermi liquid (ECFL) model scores the best in terms of explaining the new anomaly feature.   

Session M53: Surfaces, Interfaces and Thin Films: Electronic Structures and Size Effects

Electronic structure of stacking faults in hexagonal graphite

We present results of self-consistent, full-potential electronic structure calculations for slabs of hexagonal graphite with stacking faults. There are two types of stacking faults, which differ qualitatively in their chemical bonding picture. We find, that both types induce localized interface bands near the symmetry line K-M in the Brillouin zone and a related peak in the local density of states (LDOS) very close to the Fermi energy, which should give rise to a dominating contribution of the interface bands to the local conductivity at the stacking faults. In contrast, a clean surface does not host any surface bands in the energy range of the $\pi$ and $\sigma$ bands, and the LDOS near the surface is even depleted. On the other hand, displacement of even one single surface layer induces a surface band near K-M. A special role play unsaturated monomer $p_{z}$-orbitals in the vicinity the stacking faults. The formation energy of both types of stacking faults and the surface energy are discussed.   

Electromagnetic fluctuations near thin metallic films

We compute the electromagnetic fluctuations due to evanescent-wave Johnson noise in the vicinity of a thin conducting film, such as a metallic gate or a 2-dimensional electron gas. This noise can decohere a nearby qubit and it is also responsible for Casimir forces. We have improved on previous calculations by including the nonlocal dielectric response of the film, which is an important correction at short distances. Remarkably, the fluctuations responsible for decoherence of charge qubits from a thin film are greatly enhanced over the case of a conducting half space. The decoherence times can be reduced by over an order of magnitude by decreasing the film thickness. This appears to be due to the leakage into the vacuum of modes that are well localized in the perpendicular direction. There is no corresponding effect for spin qubits (magnetic field fluctuations). We also show that a nonlocal dielectric function naturally removes the divergence in the Casimir force at vanishing separation between two metallic sheets or halfspaces. We include a treatment of both a Drude conductor and a superconducting material.   

Temperature Dependence of Lateral Charge Transport in Silicon Nanomembranes

Thin sheets of single-crystal silicon (nanomembranes), electrically isolated from a bulk substrate by a dielectric layer, are an exceptional tool for studying the electronic transport properties of surfaces in the absence of an extended bulk. Under UHV, we measure the conductivity, and a back gate allows us to look into the depletion region, where we can determine the minimum conductance. For hydrogen-terminated Si(001) NMs, for which the surface has no conductivity, the minimum conductance decreases with decreasing NM thickness (220-42nm), demonstrating the reduction in carriers for thinner NMs. For the clean Si(2$\times$1)surface, mobile charge exists in the $\pi^{\star}$ surface band [1]. For thicknesses below 200nm surface conduction dominates, rendering the thickness independence of the minimum. We determine a surface charge mobility of $\sim$50cm$^{2}$V$^{-1}$s$^{-1}$[2]. We have measured the temperature dependence of the conductance of a 42nm thick HF treated SiNM. The results show that the Fermi level is pinned 0.21 $\pm0.01$ eV below the conduction band minimum, in agreement with XPS results [3].\\[4pt] [1] P. P. Zhang, et al., Nature 439, 703-706 (2006);\\[0pt] [2] W. Peng, et al., Nature Commun. 4, 1339 (2013);\\[0pt] [3] R. Schlaf, et al., J. Vac. Sci. Technol. A 17, 164 (1999).   

Lateral extension of quantum well states: scanning tunneling spectroscopy study

Quantum well states(QWS) in thin metal films have been extensively studied mostly by laterally averaging techniqes such as photoemission or inverse photoemission.A complementary approach is opened by scanning tunneling spectroscopy(STS) and microscopy(STM), which extends the range of this extremely surface sensitive device into the interior of the sample, and make it possible to image features of a buried interface with lateral resolution on the atomic scale. We present low temperature STS results of ocuupied sp-QWS localized in Ag(111) films. For thin film with local varying thickness,we recall the fundamental question-how the transition of QWS takes place,and at what length scale? We demonstrate that the QWS of thin Ag(111) films are highly perturbed within the proximity of a step edge.Atomic resolved scanning tunneling microscopy/spectroscopy indicates that the energy of these states has a strong distance dependence within the proximity of the step edge with large energetic shift equaling up to $\sim$ 200meV. For an Ag layer of 30ML thick, we obtain a lateral extension of the QWS in the order of $\sim$ 10\AA. This spatial extension of QWS can be understood within the context of electron scattering within the proximity of the buried interface.   

Nanoscale Schottky Barrier Height Mapping at Metal/Semiconductor Interfaces

Metal/semiconductor interfaces form a rectifying contact known as a Schottky diode characterized by a barrier height that is governed by the charge transfer and localized bonding at the interface. Conventional current voltage spectroscopy measures a spatially averaged barrier height. Ballistic electron emission microscopy (BEEM) is a scanning tunneling microscopy (STM) technique that can measure barrier heights with nanoscale resolution due to the nano-positioning of the STM tip. In this presentation, the Schottky barrier height is mapped with nanoscale resolution at several metal/silicon interfaces. These maps provide insight into the distribution and spatial homogeneity of the barrier height. In addition, they have the potential to identify and differentiate between different metal species at the interface as well as identify oxides and defects as well.   

Band alignment and interface charge decomposition for abrupt and polar-compensated Si/ZnS interfaces

Using the DFT+$U$ method, we study abrupt and polar-compensated Si/ZnS interfaces in the (100), (110), and (111) directions, examining computational and physical aspects of asymmetric supercells. Distinct interfaces derived from the (100) and (111) directions have valence band offsets (VBO) near either -0.9 eV or -2.1 eV. This bimodal interface dipole distribution is surprisingly obeyed by single-substitution polar-compensated interfaces as well as abrupt polar interfaces. The VBO in the non-polar direction (110) is near the mean value of the distribution (-1.5 eV). Examining one stoichiometric (111) superlattice, we find that the interface free charge, estimated directly from partial occupation of nominally unoccupied surface states, is in good agreement with prediction from the calculated electric fields and dielectric properties. Specifically, the sum of the free charge determined by state occupation, the induced bound charge, and the compositional charge (interface theorem bound charge) is equal to the total interface charge.   

Metal-insulator transition and nanoscale phase separation in a hole-doped surface reconstruction

Doping, the deliberate introduction of impurities to alter electronic or magnetic properties, has been a tremendously successful method to study and understand systems with multiple competing interactions, as reflected in both the widespread use of doped semiconductors and in the large number of emergent electronic phases in doping-dependent phase diagrams of e.g. complex oxides. In low dimensional systems, however, the perturbation to the crystal lattice by the dopant atoms can overwhelm a delicate balance of interactions in e.g. a ground state with coexisting phases. Here we introduce a modulation doping technique used to dope holes in a surface reconstruction of Sn on Si(111). Using variable and low temperature scanning tunneling microscopy and spectroscopy, we observe a doping-induced metal-insulator phase transition that is of a displacive nature, contrasting with the order-disorder nature of other surface phase transitions. Moreover, the transition leads to an intrinsic nanoscale phase coexistence at 5 K never before observed on semiconductor surfaces. Clearly, modulation doping allows us to study the delicate balance of interactions in the phase diagram of low-dimensional electronic surface systems that is otherwise experimentally inaccessible.   

Measurement of work function difference between Pb/Si(111) and Pb/Ge/Si(111) by high-order Gundlach oscillation

Ge films can be grown between the Pb overlayer and Si(111) substrate by the surfactant-mediated epitaxy. We detect the high-order Gundlach oscillation revealed in scanning tunneling microscopy (STM) to measure the work function difference between Pb/Si(111) and Pb/Ge/Si(111). Owing to different dielectric responses of Si and Ge, the tunneling current on Pb/Si has to be larger than that on Pb/Ge/Si by a factor of 2-3 to establish the same electric field in STM gap on both regions. This condition leads us to obtain a work function difference of 200 meV from observing Gundlach oscillation. It is believed that the method developed in this work can be extended to measure the work function difference of bulk materials as well.   

Effect of surface and interface states on the piezoresistivity of 2D electrons in III/V heterojunctions

Uniaxial strains applied along the interface of n-AlGaAs/GaAs heterojunctions induce piezoelectric fields normal to the interface and change both the density $N_{\mathrm{s}}$ of 2D electrons and the resistance $R$ of the channel. We have measured this piezoresistance in a group of samples grown on (111)B, where the inversion symmetry is absent. Resistance changes$\Delta R/R$ of typically 1\% were observed for the external strain of 5 $\times$ 10$^{-4}$, indicating that the electron density $N_{\mathrm{s}}$ changed by about 10$^{9}$/cm$^{2}$. It should be noted that the change of $N_{\mathrm{s}}$ is affected not only by changes in the polarization charges at the surfaces and interfaces, resulting from the piezoelectric field, but also by changes in localized charges on the surface and interface states. Indeed, our measurements have shown that the magnitude of piezoresistance depends sensitively on whether the sample surface is kept bare or clad by SiO$_{2}$ and/or metal film. By analyzing these data, we have shown that the density of surface states can be quantitatively evaluated. This method is extended also to study n-AlGaAs/GaAs samples formed on (100) surface, in which external strains break the inversion symmetry.   

Size effects in thin gold films: Discrimination between electron-surface and electron-grain boundary scattering by measuring the Hall effect at 4 K

We report the Hall effect measured in gold films evaporated onto mica substrates, the samples having an average grain diameter D that ranges between 12 and 174 nm, and a thickness t of approximately 50 nm and 100 nm. The Hall mobility was determined at low temperatures T (4K $\le $ T $\le $ 50K). By tuning the grain size during sample preparation, we discriminate whether the dominant collision mechanism controlling the resistivity of the samples at 4 K is electron-surface or electron-grain boundary scattering, based upon whether the Hall mobility depends linearly on film thickness t or on grain diameter D. \\[4pt] References:\\[0pt] R. Henriquez et al, Appl. Phys. Lett. \textbf{102} (2013) 051608.   

Electrolyte gating with ionic liquids -- structural and electronic characterization of the interface

Oxide dielectrics used for traditional gating suffer from breakdown, which limits the electric field that can be applied and thus the carrier density. Because of their wide electrochemical window, ionic liquids have recently been used to replace oxide dielectrics in hope of achieving higher carrier densities. We show that the specific capacitance of the interfacial layer for several common ionic liquids is 5 - 10 $\mu$F/cm$^2$, that the breakdown voltage is 3 - 6 volts, and that the characteristic time for the double layer to form is several milliseconds. We also show that the DC behavior of the ionic liquid interface at large potentials is not purely electrostatic.   

Anomalous Hall effect in Pt thin films induced by ionic gating

Pt is an exchange-enhanced paramagnetic material, in which the Stoner criterion for ferromagnetism is nearly satisfied and thus external stimuli may induce unconventional magnetic characteristics. For example, nano-structure formation such as particles\footnote{Y. Yamamoto {\it et al.}, Physica B {\bf 329-333}, 1183 (2003).} or wires\footnote{X. Teng {\it et al.}, Angew. Chem. Int. Ed. {\bf 47}, 2055 (2008).} provides Pt with ferromagnetic-like properties even at room temperature. In this presentation, we report that a nonmagnetic perturbation in the form of a gate voltage applied through an ionic liquid induces a nonlinear Hall effect in Pt thin films,\footnote{S. Shimizu {\it et al}., Phys. Rev. Lett, in press.} which resembles the anomalous Hall effect induced by the contact to yttrium iron garnet.\footnote{S. Y. Huang {\it et al.}, Phys. Rev. Lett. {\bf 109}, 107204 (2012).} Analysis of detailed temperature and magnetic field experiments indicates that the evolution of the nonlinear Hall effect can be explained in terms of large local moments. The applied electric field triggers an electrochemical reaction at the solid/liquid interface and induces magnetic moments as large as $\sim$10 $\mu_{\rm B}$ that follow the Langevin function.   

Novel Collective Excitation in Quantum Confined Pb(111) Films

Atomically smooth Pb(111) films were deposited on a Ge(111)-2x8 substrate and studied by high-resolution electron energy loss spectroscopy. A plasmonic feature with a positive momentum dispersion is observed at low energy, ranging in energy location from 0.3 eV in eight monolayer (ML) thin films to 1.7eV in 40 ML thick films. This excitation is no longer observable below 5 ML. Time dependent density functional theory (TDDFT) calculations of the dielectric function strongly suggest that in the bulk limit, the 1.7 eV feature can be visualized as being due to a modulation of the dielectric function by interband transitions in an otherwise simple-metal-like material. Ab initio calculations of the electronic structure within DFT have been performed to identify the specific nature of these interband transitions and their energy dependence as a function of the film thickness, from the ultrathin quantum size regime to the bulk limit.   

Quantum confinement induced oscillatory electric field on stepped Pb(111) films and its influence on surface reactivity

Using first-principles calculations, we showed that such quantum size effects (QSE) can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even or odd layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment.   

Ohmic contact formation assisted by metallic states at the interface of Cu2Te/CdTe

Ohmic contact to CdTe is a formidable task, but is indispensable for achieving high-efficiency in CdTe thin film solar cells. Using first-principles calculations within density functional theory, we study the structures and Schottky barrier heights (SBHs) of the widely used Cu$_{2}$Te/CdTe contact interfaces. We obtain two main structural configurations of the Cu$_{2}$Te layers on CdTe(111), with physically reasonable formation energies: epitaxial and bulk-like Cu$_{2}$Te layers. The epitaxial Cu$_{2}$Te layers possess Cu-Te bonds with the CdTe(111) surface, maintaining the hexagonal, unreconstructed CdTe(111) structure. In contrast, for the bulk-like Cu$_{2}$Te layered, both unreconstructed and reconstructed CdTe(111) surfaces are possible due to weak interactions between the Cu$_{2}$Te and CdTe interfaces. Detailed calculations of the SBHs at the Cu$_{2}$Te/CdTe interfaces show that the interfaces with an unreconstructed CdTe(111) surface have a low SBH of \textless 0.24 eV, consistent with our experimental observation of \textless 0.3 eV. These findings may serve as an important guidance in future efforts for improving contact quality of semiconductor devices.