Session T1: Poster Session III (Thursday, 1:00 pm - 4:00 pm)

\textbf{Formation of graphene flakes from oxidation of SiC nanoparticle}

We have performed the largest (112 million atoms) reactive molecular dynamics simulation using reactive force fields to study the oxidation of a SiC nanoparticle on the full Blue Gene/P machine. The SiC nanoparticle of diameter 100 nm is surrounded by O$_{\mathrm{2}}$ gas and heated to 2,800 K. Subsequently, we let the SiC nanoparticle oxidize at 2,800K. We observe formation of graphene-like flakes despite the harsh oxidation condition. We find the fractal dimension of the flakes is 1.85. We will discuss the implication of reaction kinetics on the structure and distribution of graphene flakes.   

Bayesian Inference of Effective Classical Spin Hamiltonians from Hartree-Fock Calculation

A novel method is described for extracting effective classical spin Hamiltonians from mean-field type electronic structural calculations by means of Bayesian inference[1]. The method is applied to a NiS$_2$ triangular lattice in NiGa$_2$S$_4$ with a spin disordered ground state. Unrestricted Hartree-Fock calculations for the spin configurations of 16 Ni sites led to the estimation that not only the strongest superexchange interaction between the third nearest neighbor sites but also those between the nearest and the second nearest neighbor sites should be taken into account to extract effective classical spin Hamiltonians for NiGa$_2$S$_4$. Results obtained from the above calculations with the Boltzmann factor are also shown. It was estimated that the superexchange interaction between the nearest neighbor sites is ferromagnetic, which is consistent with magnetic experiment results. This supports the theory that the competition between the antiferromagnetic third neighbor interaction and the ferromagnetic nearest neighbor interaction may lead to the quantum spin liquid in NiGa$_2$S$_4$. [1]H. Takenaka, K. Nagata, T. Mizokawa, and M. Okada, J. Phys. Soc. Jpn. 83, 124706, (2014).   

Enhancing AFLOW Visualization using Jmol

The AFLOW library[1,2] is a database of theoretical solid-state structures and calculated properties created using high-throughput \textit{ab initio} calculations. Jmol[3] is a Java-based program capable of visualizing and analyzing complex molecular structures and energy landscapes. In collaboration with the AFLOW consortium, our goal is the enhancement of the AFLOWLIB database through the extension of Jmol's capabilities in the area of materials science. Modifications made to Jmol include the ability to read and visualize AFLOW binary alloy data files, the ability to extract from these files information using Jmol scripting macros that can be utilized in the creation of interactive web-based convex hull graphs, the capability to identify and classify local atomic environments by symmetry, and the ability to search one or more related crystal structures for atomic environments using a novel extension of inorganic polyhedron-based SMILES strings. [1] S. Curtarolo, \textit{et al.}, \textit{AFLOW: an automatic framework for high-throughput materials discovery}, ~Comp. Mat. Sci. \textbf{58}, 218-226 (2012). [\underline {\textbf{doi}}$=$\underline {\textbf{10.1016/j.commatsci.2012.02.005}}]; [2] S. Curtarolo, \textit{et al}., ~\underline {\textit{AFLOWLIB.ORG}}\textit{: a distributed materials properties repository from high-throughput ab initio calculations}, ~Comp. Mat. Sci. \textbf{58}, 227-235 (2012). [\underline {\textbf{doi}}$=$\underline {\textbf{10.1016/j.commatsci.2012.02.002}}]; [3] R. Hanson, \textit{Jmol -- A Paradigm Shift in Crystallographic Visualization, }~J. Appl. Cryst$.$ \textbf{2010} \textit{43}, 1250-1260.   

Composition-Dependent Phase Concentrations from First Principles: Simulating Combinatorial Libraries of Transition Metal Oxides

To identify material phases in experimental combinatorial libraries, we develop a theoretical model as a complementary approach to accelerate phase identification. In this approach, samples in a combinatorial library are simulated as mixtures in chemical equilibria. Each of these mixtures contains all the solid-state phases, which can possibly exist in the library. Using the total energies of these phases obtained in first-principle calculations, we calculate the Gibbs free energy changes in the corresponding chemical reactions, and subsequently evaluate the equilibrium concentrations of the phases in every sample according to the law of mass action. Furthermore, to test this approach, we simulate pseudobinary libraries MnxV1-xOy and CuxV1-xOy. Interestingly, we find that the composition-dependent phase concentrations calculated within our approach agree well with the experimental results measured with XRD spectroscopy.   

Bosonics: Phononics, Magnonics, Plasmonics in Nano-Scale Disorder(Nanonics), Metamaterials, Astro-Seismology (Meganonics): Brillouin-Siegel GENERIC: Generalized-Disorder Collective-Boson Mode-Softening Universality-Principle (G\textellipsis P) With PIPUB Many-Body Localization

Siegel and Matsubara[Statphys-13(`77); Intl.Conf.Lattice-Dyn.(`77);Scripta Met.13,913(`80)];JMMM:5, 1, 84 (`77);22,1:41,58(`80);Mag.Lett.(`80);Phys./Chem.Liquids:4,(4) (`75);5,(1)( 76)] generalization to GENERIC Siegel[J.Non-Xline-Sol.40,453(`80)] G\textellipsis P GENERIC Brillouin[Wave-Propagation in Periodic-Structures(`22)]-Landau[`41]-Feynman[`51]-de Boer[in Phonons/Phonon-Interactions(`64)]-Egelstaff[Intro.Liquid-State(`65)]-Hubbard-Beebe[J.Phys.C(`67)]-``Anderson''[1958]- Siegel [J.Non-Xl.-Sol. 40, 453(`80)] GENERIC many-body localization. GENERIC Hubbard-Beebe[J.Phys.C(`67)] static structure-factor S(k) modulated kinetic-energy $\omega $(k)$=\hslash $\textasciicircum (2)k\textasciicircum (2)/2mS(k) expressing G\textellipsis .P(``bass-ackwardly'') aka homogeneity and isotropy creates GENERIC G\textellipsis P with GENERIC pseudo-isotropic pseudo-Umklapp backscattering (PIPUB) for GENERIC many-body localization of and/or by mutually interacting collective-bosons: phonons(phononics) with magnons(magnonics) with plasmons(plasmonics) with fermions (electros, holes)\textellipsis etc. in nano-scale ``disorder'', metamaterials and on very-macro-scales (surprisingly) Bildsten et.al. astro-seismology(meganonics) of red-giant main-sequence stars(Mira, Betelguese)!   

Introduction to Electromagnetic Fields and Geodesics in a Tokamak

Photons mediate electromagnetic radiation such that electric and magnetic particles obey the principle of least action from the applied fields. Elastic and inelastic collisions arise after summation of Lagrangian geodesics. In the case of reacting tritium and deuterium, energy is released in the form of electromagnetic radiation, neutrons, and alpha particles. Within fusion tokamaks, alpha particle energies determine if a self sustaining reaction---or ignition---will proceed. If particle mean free path is confined by electric and magnetic fields, then fusion occurs at higher frequencies. If temperature is increased and particle velocity is increased, then collision frequency increases. Modeling the nucleons as polarizable quark dielectric liquid drops increases differentiation between scattering events and fusion. When the cross section of two reactant liquid drops is coincident, fusion occurs. If cross sections do not overlap sufficiently, Coulomb scattering occurs. One strives for understanding of geometric approaches to solving for reactants' cross sections and fusion collision frequency in order to determine power output per particle and critical density of reactants.   

Global and short-range entanglement properties in excited, many-body localized spin chains

Many-body localization is a manifestation of the violation of the eigenstate thermalization hypothesis. As one of many characteristic features, eigenstates in a many-body localized regime have been observed to obey an area law in the scaling of the entanglement entropy. Consequently, such states can be efficiently represented by matrix product states (MPS). Here, we use the SIMPS algorithm proposed by Yu, Pekker, and Clark to numerically access these excited states in spin chains with disorder, and study them from the perspective of their global and short range entanglement properties, as well as through other local observables. We compare the behavior across excited states as the strength of disorder varies.   

Atomistic Simulations of High-intensity XFEL Pulses on Diffractive Imaging of Nano-sized Systems

We have developed a large-scale atomistic computational method based on a combined Monte Carlo and Molecular Dynamics (MC/MD) method to simulate XFEL-induced radiation damage dynamics of complex materials. The MD algorithm is used to propagate the trajectories of electrons, ions and atoms forward in time and the quantum nature of interactions with an XFEL pulse is accounted for by a MC method to calculate probabilities of electronic transitions. Our code has good scalability with MPI/OpenMP parallelization, and it has been run on Mira, a petascale system at the Argonne Leardership Computing Facility, with particle number \textgreater 50 million. Using this code, we have examined the impact of high-intensity 8-keV XFEL pulses on the x-ray diffraction patterns of argon clusters. The obtained patterns show strong pulse parameter dependence, providing evidence of significant lattice rearrangement and diffuse scattering. Real-space electronic reconstruction was performed using phase retrieval methods. We found that the structure of the argon cluster can be recovered with atomic resolution even in the presence of considerable radiation damage.   

Canonical Quantization of Crystal Dislocation and Electron-Dislocation Scattering in an Isotropic Media

Crystal dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and decent quantum-mechanical theory of dislocation remains undiscovered for decades. Here we present an exact and manageable Hamiltonian theory for both edge and screw dislocation line in an isotropic media, where the effective Hamiltonian of a single dislocation line can be written in a harmonic-oscillator-like form, with closed-form quantized 1D phonon-like excitation. Moreover a closed-form, position dependent electron-dislocation coupling strength is obtained, from which we obtained good agreement of relaxation time when comparing with classical results. This Hamiltonian provides a platform to study the effect of dislocation to materials' non-mechanical properties from a fundamental Hamiltonian level.   

Electron binding energies using perturbative delta-SCF method

~~~~~ The knowledge of fundamental and optical gaps is of significant importance for organic photovoltaics. The electron binding~energies estimated from the Kohn-Sham eigenvalues are significantly underestimated. Here, we use our recently outlined perturbative delta-SCF approach to compute the electron binding energies of a number of aromatic organic molecules commonly used in organic photovoltaics. Further, the electron affinities are also computed for the C60, C70 and PCBM. The results show~that the perturbative delta-SCF provide adequate description of valence electron binding energies. We also applied the method to compute the core binding energies and the core-valence excited states.~ While the method can successfully~predict the core-valence excited states the results on the core-binding energies are mixed. The strategies for improvement of the core binding energies will be discussed. \newline   

Berry phase oscillations in a one-dimensional Dirac comb

In quantum mechanics, the Berry phase is a geometric phase acquired by a wave function over the course of a cycle, when subjected to adiabatic processes. In general, this phase is due to the geometry of the underlying parameter space and thus depends only on the path taken. In any system described by a periodic potential, the torus topology of the Brillouin zone itself can lead to such a phase. In this work, we numerically calculate the Berry phase for a one-dimensional Dirac comb described by $N$ distinct wells per unit cell. As expected, the resulting Berry phase exhibits a rich band-dependence. In the case where $N = 2$, we find that the Berry phase corresponding to the $n^\mathrm{th}$ energy band oscillates such that \begin{equation*} \gamma_n (x) = A_n \sin (\pi x) \cos [(2n -1) \pi x] , \end{equation*} where $A_n$ is a band-dependent constant and $0 < x < 1$ is the relative position of the two wells. This expression, obtained using perturbation theory, gives excellent agreement with exact numerical results, even at low energy levels. The Berry phase exhibits a similar behavior for cases where $N > 2$.   

Single Electron in Systems of Two and Three Quantum Dots

We consider the single electron confinement states in the system of two and three quantum dots (QDs). The InAs/GaAs QDs are modeled as laterally distributed dots, using single sub-band effective mass approach with effective potential simulating the strain effect. Electron localization in double quantum dots (DQDs) and in triple quantum dots (TQDs) is studied over the entire electron energy spectrum by varying the geometry parameters of these QDs arrays. It is shown that a small violation of the DQD shape symmetry drastically affects tunneling. This effect also appears as a numerical instability in calculations of spectral distribution of localized/delocalized electron states for small variations of the input parameters of numerical procedure. The effect of adding a third dot to a DQD is investigated. We show that the presence of a third dot increases the tunneling in the initial DQD. The spectral distribution of localized/delocalized states appears sensitive to the violation of the mirror symmetry of TQDs.   

State of the art for ab initio vs empirical potentials for HeH$^+$ (2e$^-$), BeH$^+$ (4e$^-$), BeH (5e$^-$), Li$_2$ (6e$^-$) and BH (6e$^-$)

For large internuclear distances, the potential energy between two atoms is known analytically, based on constants that are calculated from atomic \textit{ab initio} rather than molecular \textit{ab initio}. This analytic form can be built into models for molecular potentials that are fitted to spectroscopic data. Such empirical potentials constitute the most accurate molecular potentials known. For HeH$^+$, and BeH$^+$, the long-range form of the potential is based only on the polarizabilities for He and H respectively, for which we have included up to 4th order QED corrections. For BeH, the best \texit{ab initio} potential matches all but one observed vibrational spacing to < 1 cm${^-}$ accuracy, and for Li$_2$ the discrepancy in the spacings is < 0.08 cm$^{-1}$ for all vibrational levels. But experimental methods such as photoassociation require the absolute energies, not spacings, and these are still several in several cm$^{-1}$ disagreement. So empirical potentials are still the only reliable way to predict energies for few-electron systems. We also give predictions for various unobserved "halo nucleonic molecules" containing the "halo" isotopes: $^{6,8}$He, $^{11}$Li, $^{11,14}$Be and $^{8,17,19}$B.   

Efficient method of finite-size correction in quantum Monte Carlo calculations

We present a simple but efficient method of finite size correction for metallic crystals [1]. Our method is based on an accurate combination of twist averaging boundary condition and density functional theory. We compare our method with several previously introduced schemes. Our quantum Monte Carlo results for lithium and aluminium show the accuracy and practicality of our method. :1] Sam Azadi, and W. M. C. Foulkes, J. Chem. Phys. \textbf{143}, 102807 (2015).   

Microscopic Picture of Atomic Dynamics in the Double Perovskite, PrBaCo$_{2}$O$_{6}$

We have used a combination of lattice dynamics and \textit{ab initio} molecular dynamics to study atomic dynamics in PrBaCo$_{2}$O$_{6}$, a prototype material for a large class of layered compounds of both fundamental and technological interest. We find clear signatures of the layered structure of this compound on the overall atomic dynamics. In particular, we find that O atom dynamics in the PrO layer is predominantly in-plane (\textit{ab}-plane) in contrast to the predominantly out-of-plane dynamics in the CoO$_{2}$ layer. This suggests that the oxide ionic conductivity is dominated by the O atoms in the PrO layer, a finding of interest in solid-oxide fuel cells. Additionally, our results reveal sharp low-energy vibrational modes below $20$ meV for both Ba and Pr atoms leading to the intriguing possibility that this material may be engineered for thermoelectric applications.   

Obtaining model parameters for real materials from ab-initio calculations: Heisenberg exchange

An approach to compute exchange parameters of the Heisenberg model in plane-wave based methods is presented. This calculation scheme is based on the Green's function method and Wannier function projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, Li$_2$MnO$_3$, and KCuF$_3$. The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and KCuF$_3$. In the case of FeO our calculations explain the pressure dependence of the N\'{e}el temperature. Li$_2$MnO$_3$ turns out to be a Slater insulator with antiferromagnetic nearest neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling.   

Devil's staircase in a quantum dimer model on the hexagonal lattice

Quantum dimer models appear in different contexts when describing dynamics in constrained low-energy manifolds, such as for frustrated Ising models in weak transverse fields. In this talk, I address a particularly interesting case, where a quantum dimer model on the hexagonal lattice, in addition to the standard Rokhsar-Kivelson Hamiltonian, includes a competing potential term, counting dimer-free hexagons. It has a rich zero-temperature phase diagram that comprises a cascade of rapidly changing flux quantum numbers (tilt in the height language). This cascade is partially of fractal nature and the model provides, in particular, a microscopic realization of the ``devil's staircase'' scenario [E.\ Fradkin {\it et al.} Phys. Rev. B {\bf 69}, 224415 (2004)]. We have studied the system by means of quantum Monte-Carlo simulations and the results can be explained using perturbation theory, RG, and variational arguments.\\ References: arXiv:1507.04643, arXiv:1501.02242.   

Fermionic Quantum Monte Carlo simulations without fixed nodes

Recent restricted PIMC (RPIMC) simulations [PRL \textbf{110}, 146405 (2013)] of the uniform electron gas (UEG) at finite temperature have turned out to be surprisingly inaccurate [PRL \textbf{115}, 130402 (2015)]. Therefore, there exists a high need for alternative approaches which circumvent the fermion sign problem (FSP). In this work, we present two independent approaches which exhibit a complementary behavior. The configuration PIMC (CPIMC) method [Contrib. Plasma Phys. \textbf{51}, 687-697 (2011)], which operates in Fock space, excels at high density and allows for cutting edge results at strong degeneracy. In contrast, the permutation blocking PIMC (PB-PIMC) approach [New J. Phys. \textbf{17}, 073017 (2015)] is formulated in coordinate space and combines antisymmetric imaginary time propagators (determinants) with a higher order factorization of the density matrix. This leads to a significant reduction of the sign problem and extends the range of applicability of standard PIMC towards higher density and lower temperature [arXiv:1508.03221 (2015)]. Joining these two complementary methods allows us to present accurate thermodynamic results for the uniform electron gas over a broad parameter range and, therefore, to partly avoid the FSP.   

Modeling of Electromagnetic Phenomenon in Fractional Dimensional Space

Fractional dimensional space has emerged as an extremely useful concept in many areas of physics, including electromagnetic (EM) theory. The development made in the area of fractional calculus has made it possible to study the most important physical phenomenon in a generalized $D$-dimensional fractional space. It is worthwhile to mention that many natural objects, such as clouds, snowflakes, rough surfaces, cracks, turbulence in fluids, are aptly described by dimensions of fractional order. Therefore, EM wave propagation in such fractal media is best characterized by considering an effective space of non-integer (fractional) dimensions. Here we present the recent developments in the study of differential Maxwell equations in a $D$-dimensional fractional space, where $D$ is a non-integer value. Same examples will be used in order to show the transition to the traditional non-fractional conditions or settings.   

Optimized simulations of Olami-Feder-Christensen systems using parallel algorithms

The sequential nature of the Olami-Feder-Christensen (OFC) model for earthquake simulations limits the benefits of parallel computing approaches because of the frequent communication required between processors. We developed a parallel version of the OFC algorithm for multi-core processors. Our data, even for relatively small system sizes and low numbers of processors, indicates that increasing the number of processors provides significantly faster simulations; producing more efficient results than previous attempts that used network-based Beowulf clusters. Our algorithm optimizes performance by exploiting the multi-core processor architecture, minimizing communication time in contrast to the networked Beowulf-cluster approaches. Our multi-core algorithm is the basis for a new algorithm using GPUs that will drastically increase the number of processors available. Previous studies incorporating realistic structural features of faults into OFC models have revealed spatial and temporal patterns observed in real earthquake systems. The computational advances presented here will allow for studying interacting networks of faults, rather than individual faults, further enhancing our understanding of the relationship between the earth's structure and the triggering process.   

Ab initio molecular electrostatic potential of hexanuclear Cu, Ag, and Au clusters

DFT calculations of electrostatic potential (ESP) are carried out under the PBE/SDD theory level. Planar initial structures are given as input to perform DFT optimization with the aim of obtaining ground state structures. ESP is thus calculated and results show the existence of both, nucleophilic and electrophilic sites. In each case, the latter are located over the cluster planes while the former are observed in cluster vertices. Binding energy is provided, as well as structural parameters of ground state structures.   

Hyperpolarized 13C NMR lifetimes in the liquid-state: relating structures and T1 relaxation times

Among the various attempts to solve the insensitivity problem in nuclear magnetic resonance (NMR), the physics-based technique dissolution dynamic nuclear polarization (DNP) is probably the most successful method of hyperpolarization or amplifying NMR signals. Using this technique, liquid-state NMR signal enhancements of several thousand-fold are expected for low-gamma nuclei such as carbon-13. The lifetimes of these hyperpolarized 13C NMR signals are directly related to their 13C spin-lattice relaxation times T1. Depending upon the 13C isotopic location, the lifetimes of hyperpolarized 13C compounds can range from a few seconds to minutes. In this study, we have investigated the hyperpolarized 13C NMR lifetimes of several 13C compounds with various chemical structures from glucose, acetate, citric acid, naphthalene to tetramethylallene and their deuterated analogs at 9.4 T and 25 deg C. Our results show that the 13C T1s of these compounds can range from a few seconds to more than 60 s at this field. Correlations between the chemical structures and T1 relaxation times will be discussed and corresponding implications of these results on 13C DNP experiments will be revealed.   

Hyperpolarized $^{\mathrm{89}}$Y NMR spectroscopic detection of yttrium ion and DOTA macrocyclic ligand complexation: pH dependence and Y-DOTA intermediates

Dissolution dynamic nuclear polarization (DNP) is a rapidly emerging physics technique used to enhance the signal strength in nuclear magnetic resonance (NMR) and imaging (MRI) experiments for nuclear spins such as yttrium-89 by \textgreater 10,000-fold. One of the most common and stable MRI contrast agents used in the clinic is Gd-DOTA. In this work, we have investigated the binding of the yttrium and DOTA ligand as a model for complexation of Gd ion and DOTA ligand. The macrocyclic ligand DOTA is special because its complexation with lanthanide ions such as Gd$^{\mathrm{3+}}$ or Y$^{\mathrm{3+}}$ is highly pH dependent. Using this physics technology, we have tracked the complexation kinetics of hyperpolarized Y-triflate and DOTA ligand in real-time and detected the Y-DOTA intermediates. Different kinds of buffers were used (lactate, acetate, citrate, oxalate) and the pseudo-first order complexation kinetic calculations will be discussed.   

Temperature dependence of proton NMR relaxation times at earth's magnetic field

The theoretical description of relaxation processes for protons, well established and experimentally verified at conventional nuclear magnetic resonance (NMR) fields, has remained untested at low fields despite significant advances in low field NMR technology. In this study, proton spin-lattice relaxation (T$_{1})$ times in pure water and water doped with varying concentrations of the paramagnetic agent copper chloride have been measured from 6 to 92$^{o}$C at earth's magnetic field (1700 Hz). Results show a linear increase of T$_{1}$ with temperature for each of the samples studied. Increasing the concentration of the copper chloride greatly reduced T$_{1}$ and reduced dependence on temperature. The consistency of the results with theory is an important confirmation of past results, while the ability of an ultra-low field NMR system to do contrast-enhanced magnetic resonance imaging (MRI) is promising for future applicability to low-cost medical imaging and chemical identification.   

Impact of Gd$^{3+}$ doping and glassing solvent deuteration on $^{13}$C DNP at 5 Tesla

Dynamic nuclear polarization (DNP) is a technique which can be used to amplify signals in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) by several thousand-fold. The most commonly available DNP system typically operates at the W-band field or 3.35 T, at which it has been shown that $^{13}$C NMR signal can be enhanced by deuteration and Gd$^{3+}$ doping. In this work, we have investigated the applicability of these procedures at 5 T. Our results indicate that the deuteration of the glassing matrix still yields an enhancement of $^{13}$C DNP when 4-oxo-TEMPO free radical is used. The effect is attributed to the lower heat load of the deuterons compared to protons. An addition of a trace amount of Gd$^{3+}$ gives a modest enhancement of the signal when trityl OX063 is used, albeit with a less pronounced relative enhancement compared to the results obtained at 3.35 T. The results suggest that the enhancement obtained via Gd$^{3+}$ doping may become saturated at higher field. These results will be discussed using a thermodynamic model of DNP.   

STRUCTURAL AND ELEMENTAL ANALYSIS OF DEGRADED SINGLE JUNCTION AMORPHOUS SILICON SOLAR MODULE

Photovoltaic solar modules have different defects and degradation characteristic modes. These degradation modes normally heats up some regions in the PV module. Depending on the degree and size of the localised heat, the localized heat can raise above the temperature limit of the module and cause damage to the structural orientation. The presence of severe defect and degradation correlates with high temperature gradients that usually results in morphological damage especially under outdoor conditions. The present study investigates the effect of defect/degradation on the surface morphology of single junction amorphous silicon modules (a-Si:H) during outdoor deployment. The observed structural damage was analysed using scanning electron microscope (SEM) and energy dispersion X-ray (EDX) to ascertain the elemental composition. Results show huge discrepancies in the chemical composition constitute alone different regions. The presence of high concentration of carbon and oxygen was found in the affected region.   

Enhanced confinement in compositionally heterogeneous alloy quantum dots

While there is a growing need to increase solar cell efficiencies and reduce the cost per watt, reported efficiencies are still well below the thermodynamic limit of photovoltaic energy conversion. The major factor that affects the efficiency (by more than 40\%) is the lack of absorption or thermalization of electrons. To improve absorption, existing approaches, till date, are focused on combining multiple materials in the form of heterostructures. This talk will show the application of a physics-based mechanistic approach to engineer absorption by using alloy quantum dots and exploiting its heterogeneous compositional and deformation fields. Using a multiscale computational framework that combines density functional theory, k.p method and the finite element calculations, the work shows that heterogeneous distribution of composition and strain fields can lead to substantial confinement in alloy quantum dots. Subsequently alloy quantum dots that are much larger (on the order of 50 nm) in size -- compared to their single crystalline counterparts (which are on the order of 5 nm) -- can still provide significant confinement. The findings uncover new fundamental insights for engineering confinement that are unattainable under conventional homogenization approximations.   

Electric Transport Phenomena of Nanocomposite Organic Polymer Thin Films

We discuss herein the nanocomposite organic thin film diodes for the use of plasmonic solar cells. This experimental work follows the theoretical calculations done for plasmonic solar cells using the MNPBEM toolbox for MatLab. These calculations include dispersion curves and amount of light scattering cross sections for different metallic nanoparticles. This study gives us clear ideas on what to expect from different metals, allowing us to make the best choice on what to use to obtain the best results. One specific technique for light trapping in thin films solar cells utilizes metal nanoparticles on the surface of the semiconductor. The characteristics of the metal, semiconductor interface allows for light to be guided in between them causing it to be scattered, allowing for more chances of absorption. The samples were fabricated using organic thin films made from polymers and metallic nanoparticles, more specifically Poly(1-vinylpyrrolidone-co-2-dimethylaminoethyl methacrylate) copolymer and silver or gold nanoparticles. The two fabrication methods applied include spin coating and Langmuir-Blodgett technique. The transport properties are obtained by analyzing the I-V curves. We will also discuss the resistance, resistivity, conductance, density of charge carriers.   

Optimized growth of gold nanobars for energy responsive applications.

The aim of this research is to create a reliable protocol for the synthesis of plasmonic gold nano bars for energy responsive applications such as light harvesting. The mechanism of growth in these metallic structures is not fully understood. Symmetry breaking by twinning introduces anisotropy in the shape of the nanostructures. This also results in the formation of highly faceted tip geometries that support the propagation of surface plasmon polaritons. Gold nanobars have been synthesized through chemical reduction in the presence of surfactants: cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP). Synthesis is executed by varying the concentrations of CTAB and PVP, as well as adjusting the growth temperature. The influence of additives such as metal ions will be presented. Resulting plasmonic gold nanobars are viewed using darkfield microscopy and scanning electron microscopy to visualize the nanoparticle product mixture. Atomic force microscopy is employed to measure the length and width of the nanobelts. X-ray diffraction determines the degree of crystallinity in the synthesized gold nanobars.   

\textbf{Nanostructured SnO}$_{\mathrm{\mathbf{2}}}$\textbf{ current collectors for solar energy conversion devices: relating morphology and conductivity}

Large bandgap (3.8 eV), high bulk conductivity, and a low-lying valence band make nanostructured SnO2 a promising candidate material for extracting photoexcited electrons from absorbers in solar energy conversion devices. Efficient charge collection requires high surface to volume ratio of a nanostructured SnO2 network, which comes at a cost of reduced conductivity due to incorporation of defects and grain boundaries, and reduction of electrical connectivity. We use terahertz time-domain spectroscopy (THz-TDS) to measure conductivity in nanoporous SnO2 films and nanowire arrays with different average lengths and packing densities. THz-TDS allows a non-contact measurement of frequency-resolved conductivity over nanoscale distances. Modeling the THz-TDS data using the the Drude--Smith model, we extract intrinsic properties of SnO2 as well as the effects of morphology on nanoscale conductivity. We find that the intrinsic carrier mobility of SnO2 making up a nano-porous film is 100 cm$^{\mathrm{2}}$/Vs, while the nanoscale mobility is 25 cm$^{\mathrm{2}}$/Vs. Correlating THz conductivity of nanostructured SnO2 with morphology allows us to establish optimal morphology and growth conditions for achieving highest conductivity while maintaining high surface to volume ratio.   

Environmental Effects on the Terahertz Surface Plasmons in Epitaxial Graphene

We predict the existence of low-frequency nonlocal plasmons at the vacuum-surface interface of a superlattice of $N$ graphene layers interacting with conducting substrate. We derive a dispersion function that incorporates the polarization function of both the graphene monolayers and the semi-infinite electron liquid at whose surface the electrons scatter specularly. We find a surface plasmon-polariton that is not damped by particle-hole excitations or the bulk modes and which separates below the continuum mini-band of bulk plasmon modes. The surface plasmon frequency of the hybrid structure always lies below a limiting value of the surface plasmon frequency of the conducting substrate. The intensity of this mode depends on the distance of the graphene layers from the conductor's surface, the energy band gap between valence and conduction bands of graphene monolayer and, most importantly, on the number of two-dimensional layers. For a sufficiently large number of layers ($N$ \textgreater 7) the hybrid structure has no surface plasmon. The existence of two plasmons with different dispersion relations indicates that quasiparticles with different group velocity may coexist for various ranges of wavelengths determined by the number of layers in the superlattice.   

Modeling Wettability and Friction of Water on MoS$_2$ Surface

The molybdenum disulfide (MoS$_2$) nanosheet is a promising new two-dimensional (2D) material and has recently been used in biological sensing. While the electronic strudture of 2D MoS$_2$ sheet has been extensively studied, the role of its atomic structure and thus the interfacial interactions with bio-fluids are still elusive. Using Molecular dynamics simulations, we modeled the contact angle of water on the MoS$_2$ nanosheet and predicted the slip-length of water (that is not measurable in experiment yet). Simulation results suggest that the MoS$_2$ nanosheet is a hydrophobic and low-friction surface. We expect that our newly developed force fields for depicting surface atoms of MoS$_2$ will facilitate future research in understanding biomolecule-MoS$_2$ interactions in MoS$_2$-based biosensors.   

\textbf{Unraveling the microscopic pathway of homogeneous water crystallization at supercooled conditions from direct simulations}

By means of unbiased classical molecular dynamics simulations, we identify the microscopic pathways of spontaneous homogeneous crystallization in supercooled ST2 water. By introducing a new order parameter, we are able to monitor formation/disruption of locally ordered regions characterized by small ice clusters with intermediate range order. When two of these regions are close each other, they percolate and form a larger ordered region. The process is slow enough to allow for polymorphic selection in favor of cubic ice (Ic). The formation of an ice nucleus requires percolation of many small clusters so that the transformations at the interface of the nucleus do not involve its core, thus guaranteeing the stability of the nucleus. The growth of the crystalline nucleus is fast and involves direct transformation of interfacial liquid molecules as well as percolation of small Ic/Ih clusters. The growth is too fast to allow conversion of Ih into Ic sites, originating the formation of a stacking fault in the final crystal. We recognize Euclidean structures in the oxygen configuration of the second shell in Ic and Ih clusters. This new point of view allows us to explain the source of the ordered stacking fault geometry.   

Boson peak, Ioffe-Regel Crossover, and Liquid-Liquid phase transition in Supercooled Water

We have investigated the onset of Boson peak in a model of liquid water which exhibits a clear first-order phase transition between a low-density liquid phase and a high-density liquid phase of water at low temperature and high pressure. We find that the at low pressures, the onset of Boson peak coincides with the Widom-line of the system. At high pressures, the onset occurs at the transition temperature between the two liquids. Furthermore, we show that at both low and high pressure, the frequency of the Boson peak coincides with the Ioffe-Regel crossover of the transverse phonons, suggesting that the breakdown of Debye behavior is a general feature of Ioffe-Regel limit crossover in supercooled water. The frequency of the Boson peak is weakly pressure dependent and decreases with increasing pressure. Our work bridges gap between the experimental results on the Boson peak nanoconfined water and the behavior that one would expect from a bulk system.   

Testing and using the Lewin-Lieb bounds in density functional theory

Lewin and Lieb have recently proven several new bounds on the exchange-correlation energy that complement the Lieb-Oxford bound. We test these bounds for atoms, for slowly-varying gases, and for Hooke's atom, finding them usually less strict than the Lieb-Oxford bound. However, we also show that, if a generalized gradient approximation (GGA) is to guarantee satisfaction of the new bounds for all densities, new restrictions on the the exchange-correlation enhancement factor are implied.   

Ab-Initio Modeling of Embedded Subsystems that Exchange Energy and Charge with the Environment

We claim that a subsystem formulation of Density-Functional Theory simplifies both the theoretical framework and the computational effort for calculating the electronic structure of condensed phase systems. In addition, the naturally subsystem-like form of molecular aggregates makes subsystem DFT a better descriptor of the underlying physics than regular DFT of the supersystem. Our claims are substantiated by simulations of embedded ground and excited states (including charge transfer) of liquids, crystals, and layered systems. By suppressing the inter-subsystem self-interaction error inherent in the exchange--correlation functional, subsystem DFT yields substantially improved simulations compared to Kohn--Sham DFT at a fraction of the computational cost - achieving a dual saving: time to the researcher, and energy to our planet.   

Correcting for the Self-Interaction Error in Ab-Initio Molecular Dynamics Simulations

The Self-Interaction Error (SIE) in semilocal Kohn-Sham (KS) DFT is omnipresent and it can strongly affect the quality of the properties predicted by ab-initio molecular dynamics (AIMD). Liquid water offers two good examples of this behavior: (1) semilocal KS-DFT overestimates the hydrogen bond strength resulting in an overly structured liquid, similar to ice; (2) in the case of solvated radical species, such as OH, KS-DFT exhibits unphysical spin density leakage to neighboring water molecules. We identify the cause of such behavior to be the SIE in the interaction between different molecules (rather than within). Unfortunately, it is a challenge to only rid of the SIE in the intermolecular interactions without introducing spurious corrections for the intramolecular interactions. Semilocal formulations of subsystem DFT offer an elegant solution to this problem: they remove the intermolecular self interaction, and result in an optimal description of liquid water and solvated OH radical, as compared against the experiment. In addition, the subsystem DFT simulations involve a much reduced computational effort compared to KS-DFT. [1] A. Genova et al., JCP 2014, 141, 174101 [2] A. Genova et al., JPCM 2015, Accepted   

Time-dependent Liouville density functional theory for laser-induced ultrafast demagnetization in ferromagnets

Abstract: The traditional time-dependent density functional theory is very powerful to simulate the dynamic process, but is very time consuming. When it was first used to understand laser-induced ultrafast demagnetization in ferromagnets, the results were disappointing, with the laser amplitude at least three orders of magnitude larger than the experimental one to achieve the similar spin reduction. We develop a new theory within the density functional theory (DFT) for laser-induced ultrafast demagnetization in ferromagnets. We first solve the Liouville equation in the time domain and then feed the excited state density into the DFT code, so the dynamics proceeds on the excited and constraint potential surface. We test this for several magnetic systems and find a significantly larger demagnetization than the static approach, but is still smaller than the experimental finding. Both the local density approximation and the generalized gradient approximation fail. Our finding strongly suggests that a new functional must be developed. As a first test, we introduce a spin power scaling method. Some primitive results will be presented.   

Magnetic Exchange Couplings in Heterodinuclear Transition Metal Complexes based on Differential Local Spin Rotations

We assess the performance of a new method based on a generalized perturbative approach, which uses differential local spin rotations for the calculation of magnetic exchange couplings for the case of heterodinuclear transition metal complexes of Cu, Ni, and V. These types of complexes pose a challenge for estimating exchange couplings, mainly due to the asymmetrical spin on the metal centers and the different mapping schemes that can be applied to such systems. The reliability of calculated couplings has been examined by comparing with couplings obtained from the broken symmetry (BS) energy differences method with different exchange correlational functionals, and experimental values. Results show that our method to calculate magnetic exchange couplings can be reliably employed with heterodinuclear complexes, and gives results similar to BS energy differences, when a proper mapping is used.   

Large-Scale Hybrid Density Functional Theory Calculations in the Condensed-Phase: \textit{Ab Initio} Molecular Dynamics in the Isobaric-Isothermal Ensemble

Hybrid functionals are known to alleviate the self-interaction error in density functional theory (DFT) and provide a more accurate description of the electronic structure of molecules and materials. However, hybrid DFT in the condensed-phase has a prohibitively high associated computational cost which limits their applicability to large systems of interest. In this work, we present a general-purpose order(N) implementation of hybrid DFT in the condensed-phase using Maximally localized Wannier function; this implementation is optimized for massively parallel computing architectures. This algorithm is used to perform large-scale \emph{ab initio} molecular dynamics simulations of liquid water, ice, and aqueous ionic solutions. We have performed simulations in the isothermal-isobaric ensemble to quantify the effects of exact exchange on the equilibrium density properties of water at different thermodynamic conditions. We find that the anomalous density difference between ice I$h$ and liquid water at ambient conditions as well as the enthalpy differences between ice I$h$, II, and III phases at the experimental triple point (238 K and 20 Kbar) are significantly improved using hybrid DFT over previous estimates using the lower rungs of DFT. [1] X Wu, A Selloni, and R Car, PRB 79, 08510   

Calibration of a cavity ring down spectrometer and nephelometer using polystyrene spheres and Mie theory

The extinction and scattering cross section of 700 nm polystyrene spherical particles are measured in the 500-660 nm light wavelength range using CRD (Cavity Ring Down) Spectroscopy and an integrating nephelometer. The measurement using spherical particles can be compared with Mie theory predictions to evaluate sources of errors in the system in order to use the system for studying real aerosols. Measurement of optical properties of aerosols such as absorption and scattering cross sections and singe scattering albedo are important to quantify the radiative properties of aerosols for climate models.   

Investigation of Growth Patterns due to Environmental Factors on the Surface of Bivalve shells with LIBS and Raman Spectroscopy

Environmental disturbances (such as temperature or chemical disturbances) can cause bivalve mollusk shells to grow faster or slower and cause changes in color and surface ring pattern. We have selected a few shells from our local habitat in Pensacola Beach, FL to analyze without sample treatment to determine what factors may have come into play during growth and use this as a way to analyze our marine environment. Laser Induced Breakdown Spectroscopy (LIBS) uses high energy laser pulses (355 and 532nm) to ablate the sample and create a micro-plasma from which emission spectra can be recorded. Based upon the analysis of intensities, wavelengths, and band patterns of spectral emission bands, the spatial qualitative elemental composition of the shell samples can be determined. Raman spectra were also recorded and correlated to molecules in the sample. By analyzing these measurements using LIBS-Raman spectroscopic techniques, we will be able to see how the local environment is effecting growth, with the largest chemical disturbance in the area being the BP Oil Spill in the Gulf in 2010. Therefore if samples are selected from this period of time it may be possible to identify the effects on shell growth.   

Encaged molecules in external electric fields: a molecular `tug-of-war'

We investigate applying ab initio theoretical methods, the molecules Hydrogen peroxide, H$_{\mathrm{2}}$O$_{\mathrm{2}}$, and Methanol, CH$_{\mathrm{3}}$OH, encaged in hydrogen-bonded water ``buckyballs'' (H$_{\mathrm{2}}$O)$_{\mathrm{20}}$ , subjected to an externally applied electric field. While the water-cage (host) tends to confine the guest-molecule, the external electric field tends to stretch it along with its labile hydrogen-bonded host, resulting into a molecular `tug-of-war'. We appraise these two competing effects in terms of the extent of `screening' of the host by the cage and compare the response of the composite system in the form of the consequent structural mutations, redistributions in the electron density and the electrostatic potential leading to emergence and suppression of the covalent O-H characteristic frequency shifts in the infra-red vibrational spectrum. This study brings forth the cooperative effect of hydrogen-bonding up to a maximally sustainable threshold electric field, beyond which fragmentation of the water cage occurs.   

Insights into reactivity properties of the ground state structures of (CuS)x (x=1-7) using DFT

The extraordinary properties of nanoscale materials have generated an enormous interest in the study of nanomaterials, because of the difference of their properties as compared to the corresponding bulk materials. Polyatomic nanomaterials have become important in recent years, due to the possibility of synthesize new materials with similar or better physical and chemical properties, than those of the monoatomic materials, or with a lower cost, to be used in technological applications in medicine, biology, electronics, or catalysis. Among these materials, copper sulfide is one of the transition metal chalcogenides that exhibits different stoichiometric forms with crystal structure varying from orthogonal to hexagonal. In this work we obtained the ground state structures of cooper sulfide nanoparticles (CuS)x, x=1-7. The corresponding frontier orbitals (HOMO and LUMO) are analyzed, and different reactivity parameters are obtained. We also present the molecular electrostatic potential, which is used to determine the higher and lower electron density regions on the clusters' structure. All calculations were performed using the TZVP basis set for S and the Christianssen-Ermler pseudopotential for Cu, employing two different exchange-correlation functionals, PBE and PBE0.   

Theoretical Study of Carborane:Pyridine and Carborane:Pyrimidine Aggregates and Polymers.

The carboranes are cross-linked by the pyridines and pyrimidines to form aggregates and polymers. Their geometries and electronic structures are studied by the first-principle calculations. Our results show different connections influence the orientations of the aromatic rings of pyridines and pyrimidines, which would highly affect the electronic structures of carborane:pyridine and carborane:pyrimidine aggregates and polymers. This study might be helpful for the future design of new class of semiconducting boron carbides.   

INTERACTION OF BORON CLUSTERS WITH OXYGEN: A DFT STUDY

A controlled combustion involving aluminum nanoparticles has often been the focus of studies in the field of solid fuel propellants. However very little focus has been given to the study of boron nanoparticles in controlled combustion. In contrast to aluminum nanoclusters, boron nanoclusters (B$_{n})$ are known to exhibit a planar geometries even at the size of $n \quad =$ 19 -- 20, and thus offer a greater surface area for interaction with oxygen. Earlier experimental studies have shown that boron nanoclusters exhibit different reactivity with oxygen depending on their size and charge. In this poster, we present our recent density functional theory based results, focusing on the reactivity patterns of neutral and negatively charged B$_{\mathrm{5}}$ cluster with O$_{n}$, where $n \quad =$ 1 -- 5; and B$_{\mathrm{6}}$ cluster with O$_{n}$ ($n \quad =$ 1 -- 2). The effect of charge on the reactivity of boron cluster, variation in the stability of product clusters, i e., neutral and negatively charged B$_{\mathrm{5}}$O$_{n}$ ($n \quad =$ 1 -- 5) and B$_{\mathrm{6}}$O$_{n}$ ($n \quad =$ 1 -- 2) are also examined.   

High Temperature Raman Spectroscopy Study of the Conversion of Formate into Oxalate: Search for the Elusive CO$_{\mathrm{2}}^{\mathrm{2-}}$ Intermediate

Research on conversion of carbon dioxide into chemicals and fuels has the potential to address three problems of global relevance. (a) By removing carbon dioxide from the atmosphere, we are able to reduce the amount of greenhouse gases in the atmosphere, (b) by converting carbon dioxide into fuels, we are providing pathways for renewable energy sources, (c) by converting carbon dioxide into C2 and higher order compounds, and we are able to generate valuable precursors for organic synthesis. Formate salts are formed by the electrochemical reduction of carbon dioxide in aqueous media. However, in order to increase the utilization of carbon dioxide, methods need to be developed for the conversion of formate into compounds containing two carbon atoms such as oxalate or oxalic acid. Recently, we examined the thermal conversion of sodium formate into sodium oxalate utilizing a hydride ion catalyst. The proposed mechanism for this reaction involves the carbon dioxide dianion.   

Energy of the quasi-free electron in hydrogen, deuterium and oxygen: Probing intermolecular potentials within the local Wigner-Seitz model

We present for the first time the quasi-free electron energy $V_0(\rho)$ for H$_2$, D$_2$ and O$_2$ from gas to liquid densities, on noncritical isotherms and on a near critical isotherm in each fluid. These data illustrate the ability of field enhanced photoemission (FEP) to determine $V_0(\rho)$ accurately in strongly optically absorbing fluids (e.g., O$_2$) and fluids with extremely low critical temperatures (e.g., H$_2$ and D$_2$). We also show that the isotropic local Wigner-Seitz model for $V_0(\rho)$ -- when coupled with thermodynamic data for the fluid -- can yield optimized parameters for intermolecular potentials, as well as zero kinetic energy electron scattering lengths.   

Effect of dissolved ions on dipolar correlations in liquid water

Structural correlations in liquid water and the effect of dissolved ions on them have generally been characterized through short range density fluctuations. Recent simulation and experimental results have shown that there exists considerably longer ranged ($> 24$ Angstroms) orientational order in water that can be studied using dipolar correlations. Using extensive molecular dynamics simulations, we show that the spatially long-range nature of such structural correlations are suppressed by the presence of ions, through reduction in co-operativity in orientational fluctuations. At high ($\geq 2$M) concentrations, strongly solvated ions induce strong perturbations in the hydrogen bond network of water, leading to the formation of bulk like domains with defect sites on boundaries of such domains. Reorientational autocorrelation functions of dipole vectors of water molecules at such defect sites, which are beyond the first hydration shells of ions, also experience significant slowing of reorientation times. Our results show that the effect of ions on the properties of water can propagate well beyond the first solvation shells. Results are discussed in the context of hydrophobic effect and Hofmeister series.   

External dc bias field effects in the nonlinear ac stationary response of permanent dipoles in a uniaxial potential

External dc bias field effects on the nonlinear dielectric relaxation and dynamic Kerr effect of a system of permanent dipoles in a uniaxial mean field potential are studied via the rotational Brownian motion model. Postulated in terms of the infinite hierarchy of differential-recurrence equations for the statistical moments (the expectation value of the Legendre polynomials), the dielectric and Kerr effect ac stationary responses may be evaluated for arbitrary dc bias field strength via perturbation theory in the ac field. We have given two complementary approaches for treating the nonlinear effects. The first is based on perturbation theory allowing one to calculate the nonlinear ac stationary responses using powerful matrix methods. The second approach based on the accurate two-mode approximation [D.A. Garanin, Phys. Rev. E. \textbf{54}, 3250 (1996)] effectively generalizes the existing results for dipolar systems in superimposed ac and dc fields to a mean field potential. The results apply both to nonlinear dielectric relaxation and dynamic Kerr effect of nematics and to magnetic birefringence relaxation of ferrofluids. Furthermore, the given methods of the solution of infinite hierarchies of \textit{multi-term} recurrence relations are quite general and can be applied to analogous nonlinear response problems.   

The hydrogen bond network of water supports propagating optical phonon-like modes

The local structure of liquid water as a function of temperature is a source of intense research. This structure is intimately linked to the dynamics of water molecules, which can be measured using Raman and infrared spectroscopies. Vibrational modes in liquids are usually considered to be associated to the motions of single molecules or small clusters. Previously, the librational Raman peaks of water were assigned to the librational motions of single molecules. By comparing experimental Raman and IR spectra we show these assignments are problematic. Using molecular dynamics simulations we study the k-dependent dielectric susceptibility of water. We find dispersive optical phonon-like modes in water's librational and OH stretching bands. We argue that on subpicosecond time scales these modes propagate through water's hydrogen bond network over distances of up to two nanometers. In the long wavelength limit these optical modes exhibit longitudinal-transverse splitting, indicating the presence of coherent long range dipole-dipole interactions. Studying how LO-TO splitting evolves with temperature may yield insight into how local structure changes. Our results indicate the dynamics of liquid water have more similarities to ice than previously thought. Reference: arXiv:1507.06363   

Possible Existence of Two Amorphous Phases of D-Mannitol Related by a First-Order Transition

We report that the common polyalcohol D-mannitol may have two amorphous phases related by a first-order transition. Slightly above Tg (284 K), the supercooled liquid (SCL) of D-mannitol transforms to a low-energy, apparently amorphous phase (Phase X). The enthalpy of Phase X is roughly halfway between those of the known amorphous and crystalline phases. The amorphous nature of Phase X is suggested by its absence of birefringence, transparency, broad X-ray diffraction, and broad Raman and NIR spectra. Phase X has greater molecular spacing, higher molecular order, fewer intra- and more inter-molecular hydrogen bonds than the normal liquid. On fast heating, Phase X transforms back to SCL near 330 K. Upon temperature cycling, it shows a glass-transition-like change of heat capacity. The presence of D-sorbitol enables a first-order liquid-liquid transition (LLT) from SCL to Phase X. This is the first report of polyamorphism at 1 atm for a pharmaceutical relevant substance. As amorphous solids are explored for many applications, polyamorphism could offer a tool to engineer the properties of materials. (Ref: M. Zhu et al, J. Chem. Phys. 2015, 142, 244504)   

Microfaceting of Cu$_2$O and its implications in photochemistry

The high Miller-index microfacets e.g. \{211\}, \{311\}, and \{522\} have been proposed to play a key role in shape-controlled crystal engineering of Cu$_2$O polyhedrons for various clean energy applications. These Cu$_2$O microcrystals with high Miller-index microfacets are found to have a higher photocatalytic activity than those with octahedra and cube morphologies, and thus suggesting that the catalytically active sites are more abundant on the high Miller-index surfaces. Although much effort has been devoted to the actual synthesis and characterizations of these shaped Cu$_2$O nanocrystals with various morphologies, a firm theoretical understanding of these system are currently limited to low Miller-index facets of Cu$_2$O. Here, we perform first-principles density-functional theory (DFT) calculations to study the surface energetics and electronic structure of these high Miller-index Cu$_2$O surfaces, and evaluate their overpotential for water redox reactions on Cu$_2$O, in comparison with that for the low Miller-index surfaces.   

Water Adsorption and Dissociation on CeO2(111).

The complexity and flexibility of ceria surface hinders the fully understanding of its reactivity and real applications. Here, we use H$_{\mathrm{2}}$O/CeO$_{\mathrm{2}}$(111) as the model system to investigate the water effect on the electron localization and vacancy diffusion on CeO$_{\mathrm{2}}$(111) surface by the first-principle calculations. Our results indicate the water adsorption would high affect the electronic structures of CeO2(111) surface, which further induce the dissociation of H2O molecule. This molecular mechanism might provide more guidance to the future applications including the watergas shift reactions.   

Investigating Non-Equilibrium Fluctuations of Nanocolloids in a Magnetic Field Using Direct Imaging Methods

Nanoparticles of iron oxide have a high surface area and can be controlled by an external magnetic field. Since they have a fast response to the applied magnetic field, these systems have been used for numerous in vivo applications, such as MRI contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery, and cell separation. We performed three direct imaging experiments in order to investigate the concentration-driven fluctuations using magnetic nanoparticles in the absence and in the presence of magnetic field. Our direct imaging experimental setup involved a glass cell filled with magnetic nanocolloidal suspension and water with the concentration gradient oriented against the gravitational field and a superluminescent diode (SLD) as the light source. Nonequilibrium concentration-driven fluctuations were recorded using a direct imaging technique. We used a dynamic structure factor algorithm for image processing in order to compute the structure factor and to find the power law exponents. We saw evidence of large concentration fluctuations and permanent magnetism. Further research will use the correlation time to approximate the diffusion coefficient for the free diffusion experiment.   

Capillary Condensation in Polymer Blends: an Analysis of Phase Transitions

We explore herein the capillary condensation for various geometries. Capillary condensation is studied in the presence of van der Waals forces. We derive the grand free energy, and we analyze the phase transitions, the absorption isotherms and the triple point. Phase transitions between full, empty and two films are investigated and the shape of the liquid is calculated. We also analyze an important application of wetting phenomena and capillary condensation in binary polymer blends and investigate the type of wetting transitions presented and the phase diagram.   

Frustrated Total Internal Reflection applied to Quantum Tunneling

The objective of this project is to demonstrate an optical phenomenon, frustrated total internal reflection (FTIR), by numerically solving the time-dependent Schrodinger equation (TDSE) in quantum mechanics, and to illustrate the correlations between FTIR and the quantum tunneling in one-dimensional quantum structures. We will use a MATLAB program to numerically propagate a Gaussian wave packet to penetrate finite square barriers. The transmission coefficient is then calculated as a function of the distance between two rectangular barriers/wells. The results will be useful to elucidate the correlations between optical FTIR and quantum tunneling.   

Photodissociation Spectroscopy of Ruthenium Polypyridyl Complexes in Vacuo

Photoelectrochemical water oxidation is a direct way to produce solar fuels from renewable sources. Since this reaction has a high reaction barrier, a cost-effective catalyst is necessary. Ruthenium polypyridyl complexes are promising catalysts for water oxidation. However, the mechanism of catalytic action is not well understood. One major difficulty of a mechanistic understanding is the complexity of reactive solutions under turnover conditions. To~circumvent this problem, we applied electronic photodissociation spectroscopy in the UV and visible spectral range to a series of mass selected ruthenium polypyridyl complex ions~in vacuo. The ions in this work are of the form [Ru$^{II}$-L]$^{2+}$, where Ru$^{II}$ represents ruthenium(II)-bipyridine-terpyridine, a prototype catalyst belonging to the ruthenium-polypyridyl family. By varying the ligand L, we were able to study the ligand influence on the photophysical properties of the complex. The cases where L $=$ (H$_{2}$O)$_{1,2,3}$ are of particular interest because they are directly related to an intermediate in the catalytic cycle for water oxidation. Our experiment in vacuo is an essential complement to experiments in solution and provides unique information for understanding the photophysics and photochemistry of these complexes on a molecular level.   

Quantum and Classical Electrostatics Among Atoms

Quantum theory has been unquestionably successful at describing physics at the atomic scale. However, it becomes more difficult to apply as the system size grows. On the other hand, classical physics breaks down at sufficiently short length scales but is clearly correct at larger distances. The purpose of methods such as QM/MM is to gain the advantages of both quantum and classical regimes: quantum theory should provide accuracy at the shortest scales, and classical theory, with its somewhat more tractable computational demands, allows results to be computed for systems that would be inaccessible with a purely quantum approach. This strategy will be most effective when one knows with good accuracy the length scale at which quantum calculations are no longer necessary and classical calculations are sufficient. To this end, we have performed both classical and quantum calculations for systems comprising a small number of atoms for which experimental data is also available. The classical calculations are fully exact; the quantum calculations are at the MP4(SDTQ)/aug-cc-pV5Z and CCSD(T)/aug-cc-pV5Z levels. The precision of both sets of calculations along with the existence of experimental results allows us to draw conclusions about the range of utility of the respective calculations.   

Single micelle force microscopy reveals the coordination interaction between catechol and Fe3$+$.

Metal coordination bonds are widely found in natural adhesive, load-bearing, and protective materials, which are thought to be responsible for their high strength and toughness. However, it remains unknown how the metal-ligand complexes could give rise to such superb mechanical properties. Here, combining single molecule force spectroscopy and quantum calculation, we study the mechanical properties of individual catechol-Fe3$+$ complexes, the key elements accounting for the high toughness and extensibility of byssal threads of marine mussels. We find that catechol-Fe3$+$ complexes possess a unique combination of mechanical features, including high mechanical stability, fast reformation kinetics, and stoichiometry-dependent mechanics. Therefore, they can serve as sacrificial bonds to efficiently dissipate energy in the material, quickly recover the mechanical properties when load is released, and be responsive to environmental conditions. Our study provides the mechanistic understanding of the coordination bond-mediated mechanical properties of biogenetic materials, and could guide future rational design and regulation of the mechanical properties of synthetic materials.   

\textbf{Band-gap opening properties of graphene binding with low-concentration fluorine }

To better understand the effects of low-level fluorine (F) in graphene-based sensors, the structure and impact of low-concentration of fluorine defects on the electrical properties of single- and multi-layer graphene films were investigated by density functional theory with van der Waals dispersion interactions. When F bonds to a carbon atom of graphene, the carbon atom is pulled slightly above the graphene plane creating what is referred to as a C$_{\mathrm{F}}$ defect, and a valence band (B$_{\mathrm{F}})$ near the Fermi level is formed mainly from the $p$ orbitals of the F atoms with some small contribution from the $p$ orbitals of the bonded carbon atoms. Depending on the F binding sites, the B$_{\mathrm{F}}$ can serve as a valence band or a conduction band and only few configurations of the F-binding graphene can open a band gap. Such results indicate that the band gap opening for graphene with low F-adsorption level strongly depends on the F-binding configurations, which is different from the fully or highly partial fluorinated graphene. At low F-adsorption level, the interaction between neighboring pairs of F adatoms is negligible and the most important interaction is between the F and carbon atoms in the C$_{\mathrm{F}}$ defect. Such results are useful for sensor and nano-electronics developments.   

A simple model for electronic properties of surface adsorbed molecules.

We adapt a minimal approximation to one electron quantum theory of molecules referred as Fast Accurate Kinetic Energy method. This in principle handles large complex molecular structures with less computational effort to compute electronic properties of adsorbed molecules. Kinetic energy integrals are calculated accurately but multi-electron potential energy integrals are approximated. The neighboring atom interactions are included also. For layers of isopthalic acids formed on pyrolytic graphite the configuration changes as a function of length of hydrocarbon tails. We study properties of this system as a function of tail length.   

Spectroscopic Studies on Graphenes Dispersed Within Polymeric Matrices

Graphenes have been dispersed within various polymeric matrices (polyethylene, polyethylene oxide, polystyrene, and epoxy resins). Some have been used as purchased (pristine and functionalized graphene platelets from Cheap Tubes). Pristine and functionalized graphene oxides have been obtained in laboratory according to W. Hummers, R. Offeman, ("Preparation of Graphitic Oxide". J Am Chem Soc \textbf{80}, 6, 1339, 1958) and by original functionalization processes. All these samples were investigated by Raman spectroscopy using a Renishaw InVia spectrometer operating at 532 and 785 nm. Additional information has been obtained by Wide Angle X-Ray Scattering using a Bruker Discover 8 spectrometer. Raman spectra have been fitted by a convolution of modified Breit-Wigner-Fano line shapes and the main parameters (position, intensity, width, asymmetry factor) of each line are discussed. The research aims to a better identification of graphene related nanostructures isolated or dispersed within polymeric matrices by Raman spectroscopy.   

Study of near surface Nitrogen vacancy center (NV$^{-})$ neutralization in diamond

The performance of nitrogen vacancy based sensors strongly depends on the population of NV$^{-}$ near the diamond surface. The magnetic sensing capabilities of NV$^{-\, }$diamonds are diminished as the NV$^{-\, }$becomes neutralized and turns into NV$^{0}_{,\, }$where NV$^{0}$ represents the neutralized charge state of NV$^{-}$. A theoretical calculation is performed to obtain the electron transfer rate between the NV$^{-}$ and surface molecules using the Marcus theory of electron transfer where reorganization energy and electronic wave function coupling are considered. The electronic wave function coupling is determined using the density functional theory method. Band structure simulation is also performed to confirm the NV$^{-\, }$neutralization at the surface due to surface termination. The electron transfer rate is investigated for various surface terminations (hydrogen, oxygen). Moreover, an investigation of the stability of the NV$^{-}$ at different depths relative to the surface is conducted. This work provides the ratio of NV$^{-}$ to (NV$^{0}+$NV$^{-})$ at equilibrium, which demonstrates the effect of surface termination and contamination on NV$^{-\, }$neutralization and also depicts surface properties of NV$^{-\, }$diamonds.   

Electronic structure of novel charge transfer compounds: application of Fermi orbital self-interaction corrected density functional theory

We present our experimental and theoretical results on novel Picene/F4TCNQ and Manganese-Phthalocyanine/F4TCNQ donor / acceptor systems. We apply the recently developed Fermi-orbital based approach for self-interaction corrected density functional theory (FO-SIC DFT) to these materials and compare the results to standard DFT calculations and to experimental data obtained by photoemission spectroscopy. We focus our analysis on the description of the magnitude of the ground state charge transfer and on the details of the formed hybrid orbitals. Further, we show that for weakly bound donor / acceptor systems the FO-SIC approach delivers a more realistic description of the electronic structure compared to standard DFT calculations. [1] M. R. Pederson, A. Ruzsinszky, and J. P. Perdew, J. Chem. Phys. 140, 121103 (2014). [2] M. R. Pederson, J. Chem. Phys. 142, 064112 (2015).   

Au nanoparticles improve amorphous carbon to be gas sensors

In order to make the amorphous carbon possess the gas sensing capability transferring some sp3 orbits to sp2 is necessary. It is proposed that the metallic materials having a large charge exchange with sp3 carbon orbits are being catalysts to transfer the carbon orbits. We found embedding gold nanoparticles to the amorphous carbon will induce many compact sp2 orbits around the nanoparticles, which make the amorphous carbon be the candidate material for the gas sensors. The orbits of amorphous carbon near the interface of Au nanoparticles can be changed from sp3 to compact sp2 to reduce the surface energy of Au nanoparticles. Meanwhile, our molecular dynamics simulation has confirmed the fact, when an Au nanoparticle is embedded in the amorphous carbon system the ratio of sp2 orbits increases dramatically. Similar results also have been confirmed from the Raman spectrum measurements. We controlled the carrier transport by changing the hopping barriers formed by amorphous carbon matrix between the Au nanoparticles to modify the resistance. These nanocomposites exhibit a superior sensitivity to NH$_{\mathrm{3}}$ at room temperature as well as good reproducibility and short response/recovery times, which could have potential applications in gas sensors.   

Spin-charge separation of edge zero modes in one dimension

We propose a new type of edge zero modes that exhibit spin-charge separation, which can be realized at the boundaries in an exotic one dimensional topological matter that is both fermionic Haldane insulator and topological superconductor. We give a lattice model to illustrate the nature of edge zero modes, both from bosonization and mean-field analysis. Finally, We find that Haldane phase of spin-1 chain also owns spin-charge separation of edge zero modes when mapped into fermionic system by Jordan-Wigner transformation.   

Triggering Incipient Ferroelectricity in Calcium Copper Titanate (CaCu$_{3}$Ti$_{4}$O$_{12})$ ceramics through partial B-site substitution with Te$^{4+}$ ion.

Double perovskite structured dielectric ceramic CaCu$_{3}$Ti$_{4-}_{x}$Te$_{x}$O$_{12}$ (CCTTO) ($x \quad =$ 0, 0.05, 0.1, 0.15, 0.2) was fabricated from the powder obtained by conventional solid state synthetic route. The room temperature XRD patterns for the $x =$0, 0.05, 0.075 modified samples were confirmed to possess a single phase with cubic space group \textit{Im3 }by Rietveld refinement. But, the Rietveld refinement performed on XRD patterns recorded for the compositions corresponding to $x =$ 0.1, 0.15, 0.2 shows the coexistence of the cubic phase (space group \textit{Im3}; a $=$ 7.4065{\AA}) and tetragonal phase (space group \textit{I4/mcm}; a $=$ 7.369 {\AA} and c $=$ 6.967 {\AA}). The dielectric properties of these ceramics were studied over a wide frequency (40Hz--2MHz) and temperature range (30-400K). The Te$^{4+}$ doped samples (CCTTO) exhibited dielectric permittivity (?$_{r})$ value of \textasciitilde 23-33X10$^{3}$ which is more than twice that of undoped CCTO (\textasciitilde 11x10$^{3})$ at 1kHz. A decreasing trend in dielectric permittivity with increasing temperature, a signature of incipient ferroelectricity, was observed for all the samples. Barrett's formula was invoked to rationalize the dielectric permittivity variation as a function of temperature. The incipient ferroelectric behavior is correlated with soft phonon mode observed in temperature dependent Raman Spectroscopic studies. .   

Electric and Magnetic Characterization of patterned La$_{0.6}$Sr$_{0.4}$MnO$_{3}$/SrTiO$_{3}$/Si junctions using strained SrTiO$_{3}$ as a Ferroelectric Barrier.

Controlling a magnetic device via electrical means is a sought-after goal for technological devices and can be achieved through magnetoelectric coupling between ferroelectric and ferromagnetic materials. We investigate such as possibility through a by epitaxially growing a magnetic oxide, La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ (LSMO) as an active magnetic electrode on a ferroelectric oxide, strained SrTiO$_{3\, }$(STO) on Si. STO thin films grown on Si are compressively strained (1.7 {\%}) and can be ferroelectric at T$=$300 K when less than 5nm thick. LSMO is ferromagnetic up to 340 K (in bulk), has an in-plane crystal constant of $a=$0.3870 nm, and is closely lattice matched to STO ($a=$0.3905 nm) with a 0.9{\%} in-plane tensile strain. Since STO is compressively strained in Si, an even smaller lattice mismatch is expected between LSMO and STO/Si. We investigate the epitaxial growth of LSMO/STO/Si and electrical characteristics in a capacitor type structure fabricated using photolithography as a function of Temperature and Magnetic Field. Acknowledgements: Support by the NSF-Career grant, DMR-1255629, Hope College Frissel Research Fund, NSF-MRI Grant, CHE-1126462 is gratefully acknowledged.   

Observation of ferroelectricity at room temperature in $\sim$1 nm thick conducting BaTiO$_{3 - \delta}$

Efforts to search for new and multi-functionalities in thin-film systems have led important findings of unknown phenomena and functionality which do not appear in bulk systems. As film growth technique is advanced, one can decrease the film thickness even thinner down to $\sim$ nm, its unique physical properties are still appearing. For example, the superconducting metallic state of an LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructure was found where LAO is about 3-4 unit cells (uc). An SrRuO$_3$ film exhibited its ferromagnetic metallicity down to 4-6 uc; a few years later, its ferromagnetism was found to be disappeared at 2-3 uc. Meanwhile, theoretical methods have predicted existence of ferroelectrical properties mostly in prototype ferroelectric BaTiO$_3$ (BTO): 3-6 uc. However, experimental verification to find such predicted thickness was hindered by large leakage current. Here we observed that $\sim$1 nm-thick conducting BTO fillms show ferroelectric switching at room temperature (RT), and BTO films are fully-strained on LAO/STO heterostructures thicker than 5 nm thickness. Our experimental results will enlarge applicable functional oxide devices for future applications.   

Optical Properties and Electronic Transitions of YbFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$Thin Films

We present growth, structural, optical and electronic properties of Ytterbium-Iron-oxide, YbFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, thin films. YbFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$exhibits the unique physical properties due to the presence of Fe$^{\mathrm{2+}}$ and Fe$^{\mathrm{3+}}$ valance states within the triangular lattice structure. We prepared the compound by a solid state reaction starting with stoichiometric proportion of Yb$_{\mathrm{2}}$O$_{\mathrm{3}}$, Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$, and FeO. The material was then deposited on c-axis sapphire substrates using a reactive electron beam deposition technique to produce \textasciitilde 100 nm thick films. Absorption, reflectance, and transmittance of the YbFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ films were measured in the temperature range of 10 -- 450 K. The optical spectra contain Fe d to d on-site transitions as well as O 2p to Fe 3d, Yb 6$s$, and Yb 5$d$ charge-transfer transitions. In addition, the optical spectra exhibit strong temperature dependence, indicating evidence of a structural distortion of the crystal structure at \textasciitilde 180 \textpm 10 K as well as a magnetic transition at $\sim $250 K. The detail analysis of the optical data in comparison with theoretical studies will be presented.   

Structure and Magnetic Properties of Rare Earth Doped Transparent Alumina

Recent experimental studies of rare earth (RE) doped alumina suggest that the RE induced novel phase-dependent structural and magnetic properties [1]. Motivated by these efforts, the effects of RE doping of alpha and theta alumina on the local structure, magnetic properties, and phase stability have been examined in this first principles study. Although a direct correlation between the magnetic field dependent materials properties observed experimentally and calculated from first principles is not feasible because of the applied field and the scale, the internal magnetic properties and other properties of the doped materials are evaluated. The RE dopants are shown to increase the substitutional site volume as well as increasingly distort the site structure as a function of ionic radii. Doping both the alpha (stable) and theta (metastable) phases enhanced the relative stability of the theta phase. The energetic doping cost and internal magnetic moment were shown to be a function of the electronic configuration of the RE-dopant, with magnetic moment directly proportional to the number of unpaired electrons and doping cost being inversely related. [1] Pavlacka, Robert, et al. Ceramic Transactions 252: 3, 2015.   

Magnetic and Ferroelectric Anisotropy in Multiferroic FeVO$_{4}$

FeVO$_{4}$ has been studied as a model system for understanding the magnetoelectric interaction mechanisms in low symmetry multiferroics. Triclinic FeVO$_{4}$ is characterized by two antiferromagnetic phase transitions, occurring at $T_{N1}=$22 K and $T_{N2}=$15 K, with the latter transition signaling a break in the space inversion symmetry, accompanied by the development of a non-collinear magnetic order which induces ferroelectricity. Earlier measurements on polycrystalline FeVO$_{4}$ doped with magnetic (Cr and Mn) as well as non magnetic (Zn) dopants indicate the stability of the two antiferromagnetic transition temperatures. In this work, single crystals of both undoped and doped FeVO$_{4}$ were grown from flux. To track the changes in lattice parameters induced by changing the doping concentration (measured by EDAX), XRD and Raman spectra were obtained. By recording the magnetization along two different crystal orientations, we were able to confirm the easy magnetic axis in this structure. Finally, we obtain the crystal's ferroelectric polarization along two different directions in an attempt to further understand the mechanism responsible for the ferroelectric transition.   

Anomalous enhancement of Neel temperature and magnetic coupling for Bi$_{\mathrm{0.9}}$Ca$_{\mathrm{0.1}}$FeO$_{\mathrm{3-\delta }}$ and Bi$_{\mathrm{0.9}}$Pb$_{\mathrm{0.1}}$FeO$_{\mathrm{3-\delta }}$

Temperature dependent neutron diffraction patterns of the Ca-doped BiFeO$_{\mathrm{3\thinspace }}$and Pb-doped BiFeO$_{\mathrm{3}}$ show that their Neel temperatures (T$_{\mathrm{N}})$ increase to 710 K and 680 K, while pure BiFeO$_{\mathrm{3}}$ has a T$_{\mathrm{N}}$ \textasciitilde 643 K. X-ray absorption spectra clearly shows that there is no evidence of mixed valence states despite divalent cation doping in trivalent Bi-sites. X-ray photoemission spectroscopy study revealed that divalent doping has introduced oxygen vacancies in the system. Oxygen deficiency plays a significant role in contracting Fe-O bond length in Fe-O$_{\mathrm{6}}$ octahedra and hence increasing the Fe-O-Fe bond angle in Bi$_{\mathrm{0.9}}$Ca$_{\mathrm{0.1}}$FeO$_{\mathrm{3-\delta }}$ and Bi$_{\mathrm{0.9}}$Pb$_{\mathrm{0.1}}$FeO$_{\mathrm{3-\delta }}$. The decreased Fe-O bond length and increased Fe-O-Fe bond angle favors the Goodenough-Kanamori-Anderson (GKA) coupling. The GKA coupling increases the magnetic interaction between the spins and hence increases the T$_{\mathrm{N}}$. Additionally, doping of divalent cations (Ca$^{\mathrm{2+}}$ and Pb$^{\mathrm{2+}})$ results in the destruction of cycloidal spin structure and formation of a simple antiferromagnetic (AFM) structure. This structure can easily be canted near the heterogeneous interface with a ferromagnetic layer to induce the Dzyaloshinskii-Moriya (DM) interaction and enhance the magneto-electric (M-E) coupling.   

Phononic Structure Relationships in the Subgroup Phases of Ferroelectric Ca$_{3}$Mn$_{2}$O$_{7}$

The Ruddlesden-Popper (RP) compound, Ca$_{3}$Mn$_{2}$O$_{7}$, exhibits hybrid improper ferroelectric (FE) behavior in its \textit{A2$_{1}$am} phase. However, a new phase (space group \textit{Acaa}), co-existing with the FE phase (200-320 K) and exhibiting negative thermal expansion (NTE) was recently discovered. This discovery highlighted the complexity of the phase relationships in the subgroup structure of Ca$_{3}$Mn$_{2}$O$_{7}$. Successful exploitation of RP compounds for FE applications depends on a clear understanding of the phononic relationships among the different relevant subgroup phases. Accordingly, we have used density functional theory (DFT) to map out the total energy landscape for the principal subgroup phases relative to the tetragonal phase. In order to elucidate the interrelationships of the soft phonon modes among the different subgroup phases, we performed lattice dynamics and quasi-harmonic approximation calculations. In addition, the latter calculations enabled us to extract mode Gruneisen parameters leading to new insights into the NTE behavior of Ca$_{3}$Mn$_{2}$O$_{7}$. The implications of our findings are discussed in the context of the potential of RP compounds as FE materials.   

Signatures of Soft Phonons in Impedance Spectroscopy of Barium Titanate Colloidal Solutions

Barium titanate (BTO) is a widely used dielectric material in capacitor technologies due to a high bulk dielectric constant between 1500-2000 [1] at room temperature. Although bulk BTO has been extensively studied, it is still not entirely clear how varying BTO nanoparticle size affects the dielectric constant, particularly for non-sintered discrete nanoparticles. The most widely accepted and agreed upon behavior is that smaller BTO particles have lower dielectric constants due to lower tetragonality. However, Wada et al. reported that the BTO dielectric constant reached a high value of \textasciitilde 5000 near a small particle size of \textasciitilde 140 nm. This anomaly was attributed to the soft phonon, which reached a minimum frequency at the particle size of \textasciitilde 140 nm when observed in FIR reflection measurements [2]. The soft phonon explanation for the anomaly observed by Wada et al. implies that the measured value of the dielectric constant will depend on the frequency of the applied electric field when performing impedance measurements. Herein, we present an equivalent circuit model to fit BTO colloidal solution impedance spectra, which accounts for a distribution of capacitance values as a function of applied electric field frequency. This model fits reasonably well to experimental measurements obtained via impedance spectroscopy, which suggests that the soft phonon contribution to the dielectric constant is observed in the impedance spectra for BTO colloidal solutions.   

High-Mobility Sm-Doped Bi 2 Se 3 Ferromagnetic Topological Insulators and Robust Exchange Coupling

Here we prepare a new type of diluted magnetic semiconductor (Sm$_{x}$Bi$_{1-x})_{2}$Se$_{3}$Te$_{y}$. It reaches an anistropic ferromagnetic phase at x$=$0.05, which exhibits a Curie temperature of around 50K and a typical coeric field of 0.05T. Its magnetic strength and carrier density can be tuned by controling the dopant ratio of Sm and Te separately. Robust ferromagnetism is demonstrated by the fact that electronic control makes no significant influence on the Curie temperature. The density functional theory calculation supports the ferromagnetism origin from the Sm atoms. The carrier density is \textasciitilde 10$^{18}$/cm$^{3}$ and the mobility is over 18000cm$^{2}$/V s with the pronounced schubnikov de Haas oscillations. This suggests it is a high-mobility candidate of magnetic topological insulators. (\textit{Advan. Mater. 201501254(2015)})   

Gate-modulated magnetotransport and anomalous quantum oscillations in Dirac semimetal Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ nanowires

Magnetotransport studies of three-dimensional (3D) relativistic electrons in Dirac semimetals is critical for identifying exotic topological phenomena and quantum transport. Using gate-modulation method, we conducted systematic transport measurements over our fabricated Cd$_{\mathrm{3}}$As$_{\mathrm{2\thinspace }}$nanowire based devices under a variable magnetic field. We observe an obvious ambipolar-field-effect as tuning the gate-voltage bias without applied field and distinctive MR behaviors at different gate-voltage bias with applied field. Remarkably, anomalous quantum oscillations occurs at high fields, which may be in close relationship to the sought-after Fermi-arc surface state in Dirac semimetals. The presence of anomalous oscillations may suggest that Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ nanomaterials with a gate-tunable Fermi-surface may be a promising candidate as an excellent platform to explore the elusive surface state in topological semimetals.   

\textbf{Identification of topological surface states in (Bi}$_{\mathrm{\mathbf{1-x}}}$\textbf{Sb}$_{\mathrm{\mathbf{x}}}$\textbf{)}$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{3\thinspace }}}$\textbf{alloy films}

Topological insulators (TIs) have emerged as an exciting class of quantum materials, with an insulating bulk and spin-momentum-locked topologically-protected surface states, making them desirable for spintronics and other applications. Recently, tunable surface to bulk conduction has been demonstrated in ternary TI alloys (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$, providing an ideal candidate for TI spintronic devices. Although room-temperature topological surface transport is desirable for device applications, direct detection topological surface states at room temperature has yet to be demonstrated in (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ systems. Here, we use scanning tunneling microscopy and spectroscopy (STM/STS) to characterize the band structure of (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3\thinspace }}$alloy films and directly detect the presence of topological surface states at room temperature. We will discuss the thickness and composition dependence of the band structure, including the Fermi level energy, Dirac point, and carrier type, comparing STM/STS and macroscopic transport data.   

Microstructural and magneto-transport characterization of Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ topological insulator thin films grown by pulsed laser deposition method

Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ topological insulator thin films were grown on Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$(0001) substrate by pulsed laser deposition (PLD). XRD and other structural characterization measurements confirm the growth of the textured Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ thin films on Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$substrate. The magneto-transport properties of thick and thin Þlms were investigated to study the effect of thickness on the topological insulator properties of the Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x\thinspace }}$films. A pronounced semiconducting behavior with a highly insulating ground state was observed in the resistivity vs. temperature data. The presence of the weak anti-localization (WAL) effect with a sharp cusp in the magnetoresistance measurements confirms the 2-D surface transport originating from the TSS in Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ TI films. A high fraction of surface transport is observed in the Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ TI thin films which decreases in Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ TI thick films. The Cosine ($\theta )$ dependence of the WAL effect supports the observation of a high proportion of 2-D surface state contribution to overall transport properties of the Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ TI thin films. Our results show promise that high quality Bi$_{\mathrm{2}}$Se$_{\mathrm{x}}$Te$_{\mathrm{3-x}}$ TI thin films with significant surface transport can be grown by PLD method to exploit the exotic properties of the surface transport in future generation spintronic devices.   

Discovery of the first Weyl fermion semimetal and topological Fermi arcs in TaAs

Weyl semimetals have opened a new era in condensed matter physics and materials science. They host Weyl fermions as emergent quasiparticles and admit a topological classification that protects Fermi arc surface states on the boundary. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of the first Weyl semimetal, TaAs. We directly observe the Weyl fermions and the Fermi arcs in a TaAs single crystal and demonstrate its topological character. Our work opens the field for studying of Weyl fermions in table-top experiments.   

Condensation bottleneck as the driver of pseudogap physics in cuprates

We extend our previous GW calculations on cuprates [T. Das, R.S. Markiewicz, and A. Bansil, Advances in Physics 63, 151-266 (2014)] to include vertex corrections via self-consistent renormalization. We find that the antiferromagnetic transition is bottlenecked, as a continuous manifold of competing q-vectors attempts to soften at the same time. The resulting extended range of short-range order strongly resembles pseudogap physics. We discuss the strikingly different phenomena found for different cuprates, and try to determine their origins.   

Magnetism in Olivine-type LiCo$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$PO$_{\mathrm{4}}$ Cathode Materials: Bridging Theory and Experiment

We present a non-aqueous sol-gel synthesis of olivine type LiCo$_{\mathrm{1-}}_{x}$Fe$_{x}$PO$_{\mathrm{4}}$ compounds ($x \quad =$ 0.00, 0.25, 0.50, 0.75, 1.00). The magnetic properties of the olivines are measured experimentally and calculated using first-principles theory. Specifically, the electronic and magnetic properties are studied in detail with standard density functional theory (DFT), as well as by including spin-orbit coupling (SOC), which couples the spin to the crystal structure. We find that the Co$^{\mathrm{2+}}$ ions exhibit strong orbital moment in the pure LiCoPO$_{\mathrm{4}}$ system, which is partially quenched upon substitution of Co$^{\mathrm{2+}}$ by Fe$^{\mathrm{2+}}$. Interestingly, we also observe a non-negligible orbital moment on the Fe$^{\mathrm{2+}}$ ion. We underscore that the inclusion of SOC in the calculations is essential to obtain qualitative agreement with the observed effective magnetic moments. Additionally, Wannier functions were used to understand the experimentally observed rising trend in the N\'{e}el temperature, which is directly related to the magnetic exchange interaction paths in the materials. We suggest that out of layer M -- O -- P -- O -- M magnetic interactions (J$_{\mathrm{\bot }})$ are present in the studied materials.   

Electronic and magnetic properties of double-perovskite (La$_{1-x}$Sr$_{x})_{2}$CuIrO$_{6}$ compounds

Double perovskite oxides that combine 3d and 5d transition metal elements offer a model system to study novel electronic and magnetic states arising from the interplay of strong electron correlations and spin-orbit couplings (SOCs). In this work, we studied the electronic and magnetic properties of a double perovskite iridate La$_{2}$CuIrO$_{6}$ and its hole-doped compounds (La$_{1-x}$Sr$_{x})_{2}$CuIrO$_{6}$. Magnetic susceptibility measurements suggest that the Ir sublattice and the Cu sublattice both form antiferromagnetic order but at two different temperatures. Two-dimensional magnetism that was reported in many other Cu-based double-perovskites is not observed in our samples, indicating the existence of Cu-Ir interaction despite a weak orbital mixing. Sr-doping is shown to decrease the magnetic ordering temperatures and enhance the electrical conductivity. Density functional theory (GGA$+$SOC$+$U) calculations suggest that an isolated band is generated above the Fermi level as a result of strong SOC and U. The exchange coupling constants between transition metal ions are estimated by calculating the total energies for various magnetic ground states with expanded unit cells.   

Van der Waals Epitaxy of Ultrathin Halide Perovkistes

We present our understanding, with CH$_{3}$NH$_{3}$PbX$_{3}$ as a model system, on the 2D van der Waals growth and kinetics of 3D parent materials. We show the successful synthesis of ultrathin (sub-10 nm), large scale (a few tens of $\mu $m) single crystalline 2D perovskite thin films on layered mica substrate by van der Waals (VDW) epitaxy. Classical nucleation and growth model explaining conventional epitaxy has been modified to interpret the unique 2D results under VDW mechanism. The generalization of our model shows that a 3D crystal with low cohesive energy tends to favor the 2D growth while the one with strong cohesive energy has less kinetic window. With Monte Carlo simulations, we show that the fractal 2D morphology in perovskite precisely manifests the kinetic competition between VDW diffusivity and thermodynamic driving force, a unique phenomenon to VDW growth, suggesting a fundamental limit on the morphology stability of the 2D form of a 3D material. On the other hand, our single crystal thin film growth results and subsequent cryogenic study in the iodide perovskite provide a perfect resource for the exploration of its complex optical and electronic properties and unveiling the origins of its popularity in the energy conversion field.   

\textbf{Exciton spin dynamics in MAPbI}$_{\mathrm{\mathbf{3}}}$\textbf{ measured by Hanle effect}

The organic-inorganic hybrid perovskites have emerged as a highly promising class of semiconductors for photovoltaic applications. The properties responsible for the high photoconversion efficiency are under extensive investigation. There have; however, been fewer investigations of spin-dependent effects in this class of materials. We present energy dependent photoinduced Faraday rotation in polycrystalline thin film CH$_{\mathrm{3}}$NH$_{\mathrm{3}}$PbI$_{\mathrm{3}}$, which benefit from the band structure and optical selection rules. The Faraday rotation spectrum follows the exciton absorption band at low temperatures, indicating its excitonic origin. Through the Hanle effect, based on Faraday rotation, we found the coexistence of two spin components at 4 K, which was confirmed through time resolved measurements. Research supported by the NSF-MRSEC (DMR 1121252) at the University of Utah.   

Majorana transport in superconducting nanowire with Rashba and Dresselhaus spin-orbit couplings

The tunneling experiment is a key technique for detecting Majorana fermion (MF) in solid state systems. We use Keldysh non-equilibrium Green function method to study two-lead tunneling in superconducting nanowire with Rashba and Dresselhaus spin–orbit couplings. A zero-bias dc conductance peak appears in our setup which signifies the existence of MF and is in accordance with previous experimental results on InSb nanowire. Interestingly, due to the exotic property of MF, there exists a hole transmission channel which makes the currents asymmetric at the left and right leads. The ac current response mediated by MF is also studied here. To discuss the impacts of Coulomb interaction and disorder on the transport property of Majorana nanowire, we use the renormalization group method to study the phase diagram of the wire. It is found that there is a topological phase transition under the interplay of superconductivity and disorder. We find that the Majorana transport is preserved in the superconducting-dominated topological phase and destroyed in the disorder-dominated non-topological insulator phase.   

Chiral Topological Superconductor and Half-Integer Conductance Plateau from~Quantum Anomalous Hall Plateau Transition

We propose to realize a two-dimensional chiral topological superconducting (tsc) state from the quantum anomalous hall plateau transition in a magnetic topological insulator thin filmthrough the proximity effectto a conventional s-wave superconductor. The optimal condition for realizing such chiral tsc is to have inequivalent superconducting pairing amplitudes on top and bottom surfaces of the doped magnetic topological insulator. We further propose several transport experiments to detect the chiral tsc. The conductance will be quantized into a half-integer plateau at the coercive field in this hybrid system. In particular, with the point contact formed by a superconducting junction, the conductance oscillates between e\textasciicircum 2/h and e\textasciicircum /h with the frequency determined by the voltage across the junction.   

Creation of Dirac cones in two-dimensional HgTe honeycomb lattices produced by gate voltage

HgTe is the first 2D topological insulator that was confirmed experimentally. In this material, it is well known that HgTe quantum well manifests as a topological insulator only when the parameter M, which is determined by the energy difference between E1 and H1 bands, is negative (M \textless 0). In this study, we demonstrate that the topological feature can still be obtained in the HgTe quantum well with M \textgreater 0, if we construct a honeycomb mask on 2D HgTe and apply gate voltages. The newly developed topological state has very large and controllable band gaps and can be used for the realization of various topological properties such as fraction Chern insulator and a fractional quantum spin Hall effect. It should be stressed that the newly developed Dirac cones is not ascribed to the band inversion but is driven by the honeycomb mask. Obviously, this idea can be extended to other materials and devices.   

SdH oscillations and pressure effect of the Weyl semimetal NbAs

Via angular Shubnikov-de Hass (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs. The SdH oscillations consist of two frequencies: 20.8 T ($\alpha $-pocket) and 15.6 T ($\beta $-pocket). The analysis shows that the $\beta $-pocket has a Berry phase of $\pi $ and a small effective mass 0.033 m$_{\mathrm{0}}$, indicative of a nontrivial topology; whereas the $\alpha $-pocket has a trivial Berry phase of 0 and a heavier effective mass 0.066 m$_{\mathrm{0}}$. Subtle changes can be seen in the $\rho_{\mathrm{xx}}$(T) profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure, while the topological features of the two pockets remain unchanged. Specific heat measurements reveal a small Sommerfeld coefficient $\gamma _{\mathrm{0}}=$0.09(1) mJ/(mol\textbullet K$^{\mathrm{2}})$ and a large Debye temperature, $\Theta_{\mathrm{D}}=$450(9) K, confirming a ``hard'' crystalline lattice. The Kadowaki-Woods ratio and a suppressed transport scattering rate are also studied. \textbf{References:} [1] N. J. Ghimire \textit{et al}., JPCM \textbf{27}, 152201 (2015) [2] Y. Luo \textit{et al.}, arXiv: 1506.01751 (2015) [3] Y. Luo \textit{et al.}, arXiv: 1510.08538 (2015)   

Weak Antilocalization Effect in Metallic Bi$_{2}$Te$_{3}$ Topological Insulator

We have observed weak antilocalization effect in the metallic Bi$_{2}$Te$_{3}$ single crystals having different bulk carrier densities. The angle dependence of weak antilocalization with respect to the direction of the magnetic field showed the surface states dominating in the samples having lower carrier concentration. The surface states dominance in weak antilocalization does not depend on the nature of the bulk charge carriers (p or n-type). Using the Hikami-Larkin-Nagaoka (HLN) formula, we have found the number of conduction channels is smaller in the samples having lower carrier concentration.   

Ultrafast terahertz spectroscopy study of Kondo insulating thin film SmB$_{\mathrm{6}}$: evidence for an emergent surface state

We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB$_{\mathrm{6}}$, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at \textasciitilde T*$=$20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1$m_{e}$. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20K, indicative of another channel opening up in the low energy electrodynamics. Taken together, these results suggest the onset of a surface state well below the crossover temperature (100K) after long-range coherence of the f-electron Kondo lattice is established.   

Topological insulator with negative spin-orbit coupling

Based on the first-principles calculations, we reveal that TlN, a simple binary compound with Wurtzite structure, is a three-dimensional (3D) topological insulator (TI) with effectively negative spin-orbit coupling $\lambda_{eff} < 0$, which makes it distinguished from other TIs by showing opposite spin-momentum locking effect in its surface states. The sign of $\lambda_{eff}$ depends on the hybridization between N-$2p$ and Tl-$5d$ states, and can be tuned from negative to positive by lattice strain or chemical substitution, which drive the system into a Dirac semimetal with 3D Dirac cones in its bulk states. Such topological phase transition can be realized by electronic mechanism without breaking any crystal symmetry.   

Bosonic Dirac materials in two dimensions

We examine the low energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in honeycomb lattice structure. Two different types of collective phase oscillations are obtained, which are analogous to the massive Leggett and massless Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. It is shown that the spectra of these collective bosonic modes cross each other at the $K$ and $K'$ points in the Brillouin zone and form a Dirac node. Dirac node dispersion of bosonic excitations is representative of Bosonic Dirac Materials (BDM). We show that the Dirac node is preserved in presence of an inter-grain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with Fermionic Dirac materials.   

Black Arsenic-Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties

2D layered materials with diverse properties have attracted significant interest in the past decade. The layered materials discovered so far have covered a wide, yet discontinuous electromagnetic spectral range from semimetallic graphene, insulating boron nitride, to semiconductors with bandgaps from middle infrared to visible light. Here, we introduce new layered semiconductors, black arsenic-phosphorus (b-AsP), with highly tunable chemical compositions and electronic and optical properties. Transport and infrared absorption studies demonstrate the semiconducting nature of b-AsP with tunable bandgaps, ranging from 0.3 to 0.15 eV. These bandgaps fall into long-wavelength infrared (LWIR) regime and cannot be readily reached by other layered materials. Moreover, polarization-resolved infrared absorption and Raman studies reveal in-plane anisotropic properties of b-AsP. This family of layered b-AsP materials extend the electromagnetic spectra covered by 2D layered materials to the LWIR regime, and may find unique applications for future all 2D layered material based devices. Ref. Liu, B., et al., Black Arsenic-Phosphorus: Layered Anisotropic Infrared Semiconductors with Highly Tunable Compositions and Properties. Adv. Mater., 2015, 27, 4423-4429.   

High free carrier density in aluminum adsorbed graphene

Electronic properties of graphene are enriched by aluminum adsorption on surface. The Al adsorbate could be used in n-type transfer doping in graphene. From the first-principle density functional calculations, performed by Vienna ab initio simulation package, there are lots of free conduction electrons in the distorted Dirac-cone structure. Charges transferred from Al to C atoms are about 1.24 e, almost irrespective of the concentration and distribution of the adatoms. A high carrier density is estimated to be $\sim$6$\times$10$^{-14}$/cm$^{2}$ for a ratio of Al/C = 12.5\%. Such carriers mainly originate from the hybridization of Al 3$s$ and C $2p_z$ orbitals, as clearly indicated from the orbital-projected density states, charge distributions and atom-dominated energy bands. Aluminum adsorbed graphene is predicted to have the highest free carriers density except for Al/C $\geq\,$25\% compared with the other adatom-adsorbed systems.   

Zigzag phosphorene nanoribbons: one dimensional resonant tunnelling in two dimensional atomic crystals

We theoretically investigate phosphorene zigzag nanoribbons as a platform for constriction engineering. In the presence of a constriction at one of the edges, quantum confinement of edge protected states reveals breit-wigner-like resonant tunneling, if the edge is uncoupled to the other. If the constriction is narrow enough to promote coupling between edges, it gives rise to fano-like as well as anti-resonances in the transmission spectrum. These effects are shown to mimic an atomic chain like behavior in a two dimensional atomic crystal.   

Room-Temperature, Low-Barrier Boron Doping of Graphene

Doping graphene with boron has been difficult because of high reaction barriers. Here, we describe a low-energy reaction route derived from first-principles calculations and validated by experiments. We find that a boron atom on graphene on a ruthenium(0001) substrate can replace a carbon by pushing it through, with substrate attraction helping to reduce the barrier to only 0.1 eV, implying that the doping can take place at room temperature. High-quality graphene is grown on a Ru(0001) surface and exposed to B2H6. Scanning tunneling microscopy/spectroscopy and X-ray photoelectron spectroscopy confirmed that boron is indeed incorporated substitutionally without disturbing the graphene lattice. (L.D. Pan et al., Nano Lett. 2015, 15, 6464. In collaboration with Lida Pan, Yande Que, Hui Chen, Dongfei Wang, Jun Li, Chengmin Shen, Wende Xiao, Hongjun Gao in CAS, and S. Pantellides in Vanderbilt University.)   

Peculiarities of sliding friction in graphene, graphene fluoride, graphite: Comparison of experiment with atomistic simulations

Friction is the major source of energy dissipation at the nanoscale. We use atomic force microscopy (AFM) to study slide friction based on analysis of trace-minus-retrace (TMR) signals. To obtain the signals a directional dependence of the sliding friction using a rotational technique was used at the edges and interiors of the samples graphene (G), graphene fluoride (GF), and graphite. The friction coefficient experimental results were based on a methodology assuming orthotropic friction and found to be in the range of 10$^{\mathrm{-3}}$ to 10$^{\mathrm{-1}}$ over all samples. Supplementing experimental measurements, we also performed atomistic modeling and simulations to investigate tribological properties of G including the edges. Molecular dynamics simulations and geometry optimization calculations were carried and compared with experimental measurements. It is suggested that the atoms at the apex of the asperities and at the graphene edges have important effect on friction.   

\textbf{Recognition of DNA sequencing through binding of nucleobases to graphene}~

Graphene is one of the most promising materials in nanotechnology. Its large surface to volume ratio, high conductivity and electron mobility at room temperature are outstanding properties for use in DNA sensors. For this study, we used Density Functional Theory (DFT), ?with and without the inclusion of van der Waals (vdW) interactions, ?to investigate the adsorption of nucleobases (cytosine, guanine, adenine, thymine, and uracil) on pristine graphene and graphene with defects (Divacancy and Stone-Wales). We investigated the performance of two types of vdW-DF functional (optB86b-vdW and rPW86-vdW), as well as the PBE functional, and their description of the adsorption geometry and electronic structure of the nucleobase-graphene systems\textbf{. }The inclusion of defects results in an increase in binding energy, closer adsorption of the molecule to graphene and greater buckling in both the graphene structure and nucleobase.~   

Orientationally Misaligned Zipping of Lateral Graphene and Boron Nitride Nanoribbons with Minimized Strain Energy and Enhanced Half-Metallicity

Lateral heterostructures of two-dimensional materials may exhibit various intriguing emergent properties. Yet when specified to the orientationally aligned heterojunctions of zigzag graphene and hexagonal boron nitride (hBN) nanoribbons, realizations of the high expectations on their properties encounter two standing hurtles. First, the rapid accumulation of strain energy prevents largescale fabrication. Secondly, the pronounced half-metallicity predicted for freestanding graphene nanoribbons is severely suppressed. By properly tailoring orientational misalignment between zigzag graphene and chiral hBN nanoribbons, here we present a facile approach to overcome both obstacles. Our first-principles calculations show that the strain energy accumulation in such heterojunctions is significantly diminished for a range of misalignments. More strikingly, the half-metallicity is substantially enhanced from the orientationally aligned case, back to be comparable in magnitude with the freestanding case. The restored half-metallicity is largely attributed to the recovered superexchange interaction between the opposite heterojunction interfaces. The present findings may have important implications in eventual realization of graphene-based spintronics.   

Oscillatory behavior of the surface reduction process of multilayer graphene oxide at room temperature

The graphene oxide (GO) is one of 2D materials which continues to be studied intensively since it is thought can be used as a precursor of graphene. Recently, it was found that the chemical composition of multilayer GO is metastable on the time scale of one month even at room temperature. The observed changes in chemical composition were attributed to a reduction process controlled by the in-plane diffusion of functional groups which progresses through radical reactions. Here we report the observation of oscillatory oxidation-reduction (redox) reactions on the surface of multilayer GO films at room temperature. The redox reactions exhibited dampened oscillatory behavior with a period of about 5 days and found to be dependent on the time elapsed from GO deposition. The kinetic behavior of the processes and observed metastability of the surface functional groups are adequately described by two models involving reactions between functional groups of GO and reactant diffusion.   

Atomistic Simulation Studies on the Friction of 2D materials

Frictional properties of two-dimensional (2D) layerd materials including graphene, MoS$_{2}$, NbSe$_{2}$, and h-BN, have been revealed using atomic force microscopy (AFM) [1]. All the materials exhibit similar trends on friction: the thicker the sheet the lower the friction is. Puckering effect has been suggested as the primary mechanical reason for this thickness-dependent behaviors. Despite this novel findings, detailed atomic-scale processes during tip sliding against such atomically thin sheets are not fully understood yet. In this work, we provide a detailed study of the role of the buried interface between tip and surface on atomic friction using molecular dynamics (MD) simulation. We investigate the magnitude of puckering under various tip and surface conditions such as tip size and surface orientation, to unravel its effect on friction. Our systematic approach could provide a comprehensive understanding of friction phenomena at atomic level.\\[4pt] [1] C. Lee et al. \textit{Science} \textbf{328} (2010) 76   

Transferring graphene onto hydrated "soft" substrates using a modified H2 bubbling method

Graphene has many applications, most of which require its deposition onto a specific substrate. Several methods exist for transferring large areas of graphene. However, there is a lack of existing techniques for \newline proper transfer onto soft hydrated substrates. We demonstrate a method for transferring a large wrinkle-free area of graphene onto soft substrates using a modified bubbling technique. Widefield microscopy was used to characterize the results.   

Effect of the sample mounting geometry on the grain size of single-crystalline transition-metal dichalcogenide monolayers grown by chemical vapor deposition

In this presentation, it is reported that the local pressure near the surface of the substrate plays a significant role affecting the single crystal grain size of two-dimensional (2D) transition-metal dichalcogenide MX$_{\mathrm{2}}$ (M$=$Mo, W; X$=$S, Se) monolayers in addition to other regular growth parameters such as growth chamber pressure, temperature, and gas flow rates etc during the chemical vapor deposition growth. Different sample mounting geometries (such as substrate facing up, facing down, sandwiching) and vapor trapping techniques (such as vapor trapping tube) to introduce qualitatively various local pressure have been employed in the growth to systematically study this effect. The grain size, optical, and electrical properties of 2D MX$_{\mathrm{2}}$ monolayer samples grown at different local pressures are compared. It is observed that the enhanced local pressure facilitates larger single crystal grain size and higher quality of the 2D MX$_{\mathrm{2}}$ monolayers. The size of the single-crystalline MX$_{\mathrm{2}}$ monolayers achieved by this method were comparable to the literature reported largest size.   

Mechanisms of Bowtie- and Star-Shaped \textit{MX}$_{\mathrm{2}}$ Nanoisland Formation

A large number of experimental studies over the last few years observed the formation of unusual highly symmetric polycrystalline twinned nanoislands of transition metal dichalcogenides, resembling bowties or stars. Here we analyze their morphology in terms of equilibrium and growth shapes. We propose a mechanism for their formation via collision of concurrently growing islands and validate the theory with phase-field simulations. Finally, we use first-principles calculations to propose an explanation of the predominance of high-symmetry polycrystals with 60-degree lattice misorientation angles.   

Metal Contact Formation and Substrate Ferroelectric Poling: Effective Means of Determining MoS$_{2}$ Transport Properties

Monolayer transition metal dichalcogenides (TMDs) are of rising interest due to their direct band gap at the single-layer limit and pronounced spin splitting in the valence band. Metal contact formation to such materials is a persistent issue yet it holds tremendous opportunity for improving TMD transport properties: simply through their composition, metal contacts can increase the very low carrier numbers in single-layer films leading to significant shifts of the Fermi energy. X-ray photoelectron spectroscopy (XPS) measurements of the charge transfer during metal contact formation reveal the TMD valence band edge to approach the Fermi level underneath the contact, so that TMD devices resemble \textit{pnp}-junctions. We employ a combination of scanning photocurrent microscopy (SPCM) and surface acoustic spectroscopy on ferroelectric substrates to ascertain our findings. SPCM measurements allow us to probe the impact of electrical contacts on the photoconductivity of the materials. In contrast, surface acoustic spectroscopy allows access to the transport properties of the material even in the absence of contacts. The combination of these technique sheds new light on the band alignment in TMD materials between contacts and on ways to manipulate it.   

Substrate induced phase transformation of monolayer transition metal dichalcogenides

Using density functional theory calculations, we investigate the effects of a metal substrate on the structural and electronic properties of a monolayer of transition metal dichalcogenide (TMD). We find that a suitable choice of substrate can induce a transformation of the phase of the monolayer from 2H to 1T. We will discuss the impact of the results on some earlier studies of TMD/metal contacts as well as potential applications of our system in catalysis.   

HYDROGENATED GRAPHENE- METAL OXIDE NANOHYBRIDS: AN INVENTIVENESS PLINTH FOR SENSING DEVICES

Graphene- a two dimensional sheet of sp$^{\mathrm{2}}$ hybridized carbon atoms has been considered as promising materials in sensor design for detection of target molecule. Charge carriers in graphene obey linear dispersion relation and it behaves like mass less relativistic particles which act as base for enhanced electron transport. Thus the electrons move ballistically without scattering giving higher mobility even at room temperature. Further, the presence of oxygen containing functional group and crystal defects assisted via hydrogenation process take vital part in electrochemical adsorption of electro active species and catalyses the same. Though issues with selectivity, stability and sensitivity are limited for several nanostructured metal oxides sensing, the hybrid system started its effective role in design of sensing platform. Thus considering the potential important of hydrogenated graphene -metal oxide systems, a nanohybrid system is developed and its structural, morphological and optical properties were understood using respective characterization tool. Further, the prepared hybrid nanosystem used as a platform for bimolecule detection, where the sensor exhibits higher range of sensitivity and selectivity.   

Bias dependent transport property of defective phosphorene.

\textbf{Phosphorene is receiving great research interests because of its peculiar physical properties.} \textbf{Nonetheless, no systematic studies on the transport properties modified due to defects have been} \textbf{performed. Here, we present the electronic band structure, defect formation energy and bias} \textbf{dependent transport property of various defective systems. We found that the defect formation} \textbf{energy is much less than that in graphene. The defect configuration strongly affects the electronic} \textbf{structure. The band gap vanishes in single vacancy layers, but the band gap reappears in divacancy} \textbf{layers. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property.} \textbf{Unlike the common belief, we observe that the vacancy defect can contribute to greatly increasing} \textbf{the current. Along the zigzag direction, the current in the most stable single vacancy structure was} \textbf{significantly increased as compared with that found in the pristine layer. In addition, the current} \textbf{along the armchair direction was always greater than along the zigzag direction and we observed a} \textbf{strong anisotropic current ratio of armchair to zigzag direction.}   

Low-temperature optical spectroscopy of single-layer transition metal dichalcogenides

In recent years, layered materials beyond graphene have attracted immense interest in the scientific community. Among those, particularly the semiconducting transition metal dichalcogenides (TMDCs) in their monolayer form are in the focus of the current research due to their intriguing optical properties and their potential application in valleytronic-based devices. The optical properties are governed by excitonic features, even at room temperature. The excitons in monolayer TMDCs have unusually large binding energies due to the two-dimensional carrier confinement and weak dielectric screening. Here, we investigate the photoluminescence spectra of monolayer TMDCs at low temperatures. We present clear evidence for the existence of biexcitons in monolayer WS$_2$, exhibiting a superlinear behavior in excitation-power-dependent measurements. Applying a gate-voltage in a FET-configuration, we can identify charge-neutral and negatively charged excitons (trions) in the optical spectrum of different TMDCs. The trion binding energies range in the order of 30\,meV. The evolution of the excitonic peaks under the application of external magnetic fields give further insight into the internal structure of these materials.   

Valley polarization and coherence in atomically thin tungsten disulfide via optical spectroscopy

Atomically thin group-VI transition metal dichalcogenides (TMDC) has been emerging as a family of intrinsic 2-dimensional crystals with a sizeable bandgap, opening a potential avenue for ultimate electronics and optoelectronics. Besides, the characteristic structural inversion symmetry breaking in monolayers leads to non-zero but contrasting Berry curvatures and orbital magnetic moments at K/K' valleys. These features provide an opportunity to manipulate electrons' additional internal degrees of freedom, namely the valley degree of freedom, making monolayer TMDC a promising candidate for the conceptual valleytronics. Here, our experimental approach on valley dependent circular dichroism in monolayer and bilayer WS$_{\mathrm{2}}$ via optical spectroscopy are elaborated. Consequently, the polarization of photoluminescence inherits that of excitations, circularly and linearly polarized, confirming the valley dependent selectivity rule. However, the valley polarization and valley coherence in bilayer WS$_{\mathrm{2}}$ owing to the coupling of spin, valley and layer degrees of freedom, are anomalously robust compared with monolayer WS$_{\mathrm{2}}$. We propose potential mechanisms of the anomalous behavior in WS$_{\mathrm{2}}$ bilayers.   

Optical nano-imaging of waveguide exciton polaritons in transition-metal dichalcogenides

Exciton polaritons, which are collective oscillations of photons and excitons in semiconductors, trigger tremendous research interests in both fundamental physics and technological applications. Previous studies retain to spectroscopic studies of exciton polaritons confined in microcavities. Here, we report on optical nano-imaging study of waveguide exciton polaritons of thin flakes of transition-metal dichalcogenides (TMDCs) using the near-field scanning optical microscopy. The observed polaritons are formed by strong coupling between waveguide photons and A excitons in TMDCs. The wavelength of these exciton polaritons can reach as low as 300 nm. By tuning the laser frequency, we are able to map the entire polariton dispersion both above and below the A exciton energy. Further analysis indicates that polaritons in the lower-energy branch have a propagation length over many microns while the modes in the upper-energy branch are strongly damped due to the Landau damping.   

Anisotropic Electron transport and device applications of atomically thin ReS$_{\mathrm{2}}$

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behavior, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. In this talk, we will present atomically thin rhenium disulfide (ReS$_{\mathrm{2}})$ flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We first fabricated mono- and few-layer ReS$_{\mathrm{2}}$ field effect transistors, which exhibit competitive performance with large current on/off ratios (\textasciitilde 10$^{\mathrm{7}})$ and low subthreshold swings (100 mV dec$^{\mathrm{-1}})$. The observed anisotropic ratio along two principle axes reaches up to 3.1. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS$_{\mathrm{2}}$ anisotropic field effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Recent results on ultra-high responsivity (as high as 88,600 A W$^{\mathrm{-1}})$ phototransistors based on few-layer ReS$_{\mathrm{2}}$ will also be discussed. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic and optoelectronic applications.   

Nonlinear dynamics of the normal and superfluid dipolariton gas in transition-metal dichalcogenide-based heterostructures

Propagation of a dipolariton quantum gas in normal and superfluid states in a patterned mictrocavity in the presence of an external electric field is studied. The double layer transition-metal dichalcogenide structure is embedded into the microcavity. The dipolaritons are formed as a superposition of direct and indirect excitons in the layers and cavity photons. By numerically solving the Boltzmann equation for a gas of interacting dipolaritons in a normal state at room temperatures and the Gross-Pitaevskii equation for a dipolariton Bose-Einstein condensate in superfluid state at low temperatures we show that the dipolariton flow can be controlled by the electric field in the cavity. We also numerically studied the dipolariton propagation in channels of various geometries in the cavity and determine conditions when the dipolariton flow can be guided in the channels.   

Tunable valley polarization of quantum confined excitons in WSe$_{\mathrm{2}}$

The discovery of single photon emitters in two dimensional transition metal dichalcogenides opens up a new research direction in the field of two-dimensional layered materials. In order to understand the origin of the quantum confinement that is responsible for these localized excitonic states we perform polarization resolved optical measurements. The quantum dots are embedded in a diode-like device to control the quantum dot energy levels via the quantum confined Stark effect. In addition to applied electric field, an external magnetic field is also used to control the quantum dot exciton properties. In this work we present our findings that sheds light on the symmetry of the confinement potential. The observed extent of valley polarization indicates quantum confined exciton's valley degree of freedom protection from environmental disturbances.   

Phonons in Stabilized B or N doped graphene

Based on \textit{Ab-initio} density functional perturbation theory, we have investigated various doped B and N based graphene sheets by raising their concentrations upto 50{\%} of the host carbon. Although the doped structures seem to stabilize in 2-D configurations, but the resulting Phonon frequencies do not confirm the stability as the transverse modes above critical concentrations of B and N tend to result in negative eigenvalues. This essentially requires strained 2-D sheets when doped above such critical concentrations. We find the results of phonons and thermodynamics very interesting and attempt to report these in strained lattices. The motivation to do such a calculation results from our primary goal to address the issue of heat dissipation rate in the devices based on designable electronic and optical properties of such doped graphene reported already.   

Multi-Level Memory Effect of CVD Graphene Transferred on SiO$_{\mathrm{2}}$ by Controlled Hydron Adsorption at Interface

Memory effect of graphene based on the Fermi-level shift driven by external electric field has been studied in various ways. There have been several experimental reports exploring the fabrication of two-level memory devices relying on the hysteresis loop of channel current vs. gate voltage of Graphene/SiO$_{\mathrm{2}}$/Si field effect transistor (FET). This channel current hysteresis has been explained by the motion of the water molecules trapped between graphene and SiO$_{\mathrm{2}}$ insulator. In this study, we fabricated a CVD-grown graphene FET on a SiO$_{\mathrm{2}}$/Si substrate and found four different channel conductivity states tunable by varying the applied gate voltage pulse. It is noticed that the stabilization of reset state (lowest conductivity state) is one of the challenging issues in fabricating memory devices with graphene FET. We found that the stabilization of reset state can be achieved by positioning the Fermi-level in reset state as close to the charge neutrality point as possible during read-out. We propose one easy way to ensure the proper positioning of the reset state Fermi-level, which is to apply a constant gate voltage during read-out. Our study demonstrates the possibility of fabricating graphene-based multi-bit memory devices.   

Gate-tunable tunneling resistance in graphene/topological insulator vertical junctions

The emergence of graphene-based vertical heterostructures, especially stacked by various layered materials, opens up new promising possibilities for investigations and applications. The junction based on two famous Dirac materials, graphene and topological insulator, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, can considerably enlarge the family of van der Waals heterostructures, while the experimental approach to obtain controllable interface of these junctions is still a challenge. Here we show the experimental realization of the vertical heterojunction between Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and monolayer graphene. The tunneling-mediated quantum oscillations are identified to arise from several two-dimensional conducting layers. The electrostatic field induced by back gate voltage, as well as the magnetic field, is applied to tailor the available density of states near the Fermi surface. We observe exotic gate-tunable tunneling resistance in high magnetic field, which is attributed to semimetal-quantum Hall insulator transition in the underlying graphene.   

Theory of hot electron photoemission from graphene

Motivated by the development of Schottky-type photodetectors, some theories have been proposed to describe how the hot carriers generated by the incident photon are transported over the Schottky barrier through the internal photoelectric effect. One of them is Fowler's law proposed as early as 1931, which studied the temperature dependence of photoelectric curves of clean metals. This law is very successful in accounting for mechanism of detecting photons of energy lower than the band gap of semiconductor based on conventional metal/semiconductor Schottky diode. With the goal of achieving better performance, graphene/silicon contact-based- graphene/WSe2 heterostructure-based photodetectors have been fabricated to demonstrate superior photodetection efficiency. However, the theory of how hot electrons is photo-excited from graphene into semiconductor remains unknown. In the current work, we first examine the photoemission process from suspended graphene and it is found that traditional Einstein photoelectric effect may break down for suspended graphene due to the unique linear band structure. Furthermore, we find that the same conclusion applies for 3D graphene analog (e.g. 3D topological Dirac semi-metal). These findings are very useful to further improve the performance of graphene-based photodetector, hot-carrier solar cell and other kinds of sensor.   

Memristive Phenomena in Polycrystalline Single Layer MoS$_{2}$

Recently, a new class of layered two-dimensional semiconductors has shown promise for various electronic applications. In particular, ultrathin transition metal dichalcogenides (e.g. MoS$_{2})$ present a host of attractive features such as high carrier mobility and tunable band-gap. However, available growth methods produce polycrystalline films with grain-boundaries and point defects that can be detrimental in conventional electronic devices. In contrast, we have developed unconventional device structures that exploit these defects for useful electronic functions.[1] In particular, we observe grain-boundary mediated memristive phenomena in single layer MoS$_{2}$ transistors. Memristor current-voltage characteristics depend strongly on the topology of grain-boundaries in MoS$_{2}$. A grain boundary directly connecting metal electrodes produces thermally assisted switching with dynamic negative differential resistance, whereas a grain boundary bisecting the channel shows non-filamentary soft-switching. In addition, devices with intersecting grain boundaries in the channel show bipolar resistive switching with high on/off ratios up to \textasciitilde 10$^{3}$.[1] Furthermore, the gate electrode in the field-effect geometry can be used to control the absolute resistance of the on and off states. Correlated electrostatic force microscopy, photoluminescence, and Raman microscopy reveal the role of sulfur vacancies in the switching mechanism. \textit{Refs: 1. Sangwan et al., Nature Nanotech, 10 403 (2015) }   

Resonance frequency shifts due to quantized electronic states in atomically thin NEMS

The classic picture of the force exerted on a parallel plate capacitor assumes infinite density of states (DOS), which implies identical electrochemical and electrostatic potential. However, such assumption can breakdown in low-dimensional devices where the DOS is finite or quantized. Here we consider the mechanical resonance shift of a nanoelectromechanical (NEMS) resonator with small DOS, actuated and detected capacitively at fixed electrochemical potential. We found three leading correction terms to the classical picture: the first term leads to the modulation of static force due to the variation in chemical potential, and the second and third terms are related to the static and dynamic changes in spring constants, caused by quantum capacitance. The theory agrees well with recent experimental findings from graphene resonator in quantum Hall regimes, where the chemical potential and quantum capacitance are tuned by magnetic field, while the gate voltage is kept constant.   

A theoretical design of graphene-based spin field-effect transistors

The search for a feasible design of graphene-based materials for spintronics applications has been intensified in recent years. Encouraged by recent experimental achievements, here we propose a new scheme to realize graphene-based spin field-effect transistors. The new design is constituted of a half-hydrogenated graphene nanoroad embedded in a fully-hydrogenated graphene. Using first-principles density function theory calculations, we demonstrate that such a design can convert non-magnetic pristine graphene into a bipolar ferromagnetic semiconductor. More importantly, the magnetism of such a nanoroad is very robust: independent of its width and orientation. We also discuss the stability of such nanoroads, as well as a simple design of an all-electric controlled device for generation and detection of a fully spin-polarized electric current.   

Control of Rewriteable Doping Patterns in Graphene/Boron Nitride Heterostructures

Spatial control of charge doping in 2D materials is a prerequisite for designing future electronic devices and understanding novel physics. Electrostatic gating and chemical doping are two of the most common methods to achieve this. However, these approaches suffer from complicated fabrication processes that introduce impurities, change material properties irreversibly, and lack flexibility. Here we introduce a new method for patterning rewriteable doping profiles using an STM tip by way of local tip-voltage-induced ionization of defects in a BN substrate. We characterize these spatial doping patterns through local probe and transport techniques. This technique enables many novel device designs for 2D materials, including atomically thin p-n junctions and rewriteable memory devices.   

Effects of interface oxygen vacancies at the FeSe/SrTiO$_3$ interface

The effects of oxygen vacancies on the electronic bands at the interface of between monolayer and bilayer FeSe and SrTiO$_3$ are investigated by first-principles supercell calculations. Unfolded bands derived from the $k$-projection method reveal that the oxygen vacancy not only provides electron doping to the interface FeSe layer, but also significantly renormalizes the width of the Fe-3$d$ band near the Fermi level for the checkboard antiferromangetic (AFM) state. However, the effects of the oxygen vacancies on the electronic properties of the top layer of bilayer FeSe are limited. The $k$-projected bands for the checkboard AFM state are in good agreement with ARPES results.   

Even-dimensional topological semimetals under disorders

A topological theory of even-dimensional, chiral symmetry-preserving topological semimetal, which is known as the counterpart of Weyl semimetal, is developed in this work. We show that, in presence of disorder, an anisotropic topological $\theta$-term emerges in the action of effective non-linear sigma model, meanwhile, an anisotropic Chern character term in terms of $U(1)$ gauge response theory, which gives the electromagnetic response, whose stability against disorders is ensured by the former topological ?-term, has also been derived. Moreover, it is found that this topological semimetal can be included in the family of topological quantum matter preserving chiral symmetry. The relations of this topological semimetal to odd-dimensional topological insulator and even-dimensional Dirac fermion are revealed in both effective non-linear sigma model and gauge theory. And importantly, above results can be applied to graphene if we set dimension $d=2$ and thus reveal the topological character of this kind of $2$-dimensional topological semimetal with two Dirac cones with opposite chiralities.   

Supercurrent in the quantum Hall regime

Combining superconductivity and the quantum Hall (QH) effect is a promising route for creating new types of topological excitations. Despite this potential, signatures of superconductivity in the quantum Hall regime remain scarce, and a superconducting current through a QH weak link has so far eluded experimental observation. Here we demonstrate the existence of a novel type of Josephson coupling through a QH region at magnetic fields as high as 2 Tesla. The supercurrent is mediated by states encompassing QH edge channels, which are flowing on opposite sides of the sample. The edges are coupled together by the hybrid electron-hole modes at the interfaces between the QH region and the superconducting contacts. These chiral modes, which share some features with Majorana modes, are formed when electron and hole edge states are mixed by the superconductor.   

Cooper pairing protected by spin-valley locking in two-dimensional superconductivity on MoS$_{2}$

MoS$_{2}$ is an archetypal layered semiconductor; monolayer shows out-of-plane spin polarization at the K-points due to intrinsic Zeeman-type spin-orbit coupling (SOC) derived from its in-plane broken inversion symmetry. By ionic-liquid gating, almost all carriers are confined only to topmost layer, realizing two-dimensional superconductivity in this system [1]. We reported the first observation of a huge in-plane upper critical field of about 52 T and a clear saturating behaviour in the low temperatures using pulsed magnetic fields up to 55 T [2]. From first-principles-based tight binding supercell calculations followed by realistic numerical calculations of $H_{c2}$ based on the subband structure, we revealed that this unusual behavior is due to the moderately large Zeeman-type spin splitting of 13 meV at the Fermi level (vicinity of the K points) [3]. This forces Cooper pairs to be completely aligned to out-of-plane direction by spin-valley locking effect, thereby causing the dramatic enhancement of the Pauli limit Our calculation also indicates that even if the carrier density and then spin splitting (9--15 meV) at the Fermi level changes, the Pauli limit is predominantly controlled by both the Zeeman-type SOI and $T_{c}$, and the contribution of Rashba-type SOI is negligibly small. [3]. [1] Y. Saito et al. http://meetings.aps.org/link/BAPS.2014.MAR.T52.8 [2] Y. Saito, et al. http://meetings.aps.org/link/ BAPS.2015.MAR.G11.11 [3] Y. Saito et al. Nature Phys$.$ doi: 10.1038/nphys3580. (arXiv:1506.04146).   

Highly anisotropic Dirac fermions in square graphynes

Recently, there have been intense search of new 2D materials, and one especially appealing class of 2D materials is the all-carbon allotropes of Dirac materials. Here, we predict a new family of 2D carbon allotropes, square graphynes (S-graphynes) that exhibit highly anisotropic Dirac Fermions, using first-principle calculations within density functional theory. The equal-energy contour of their 3D band structure shows a crescent shape, and the Dirac crescent has varying Fermi velocities from 0.6 x 10$^{\mathrm{5}}$ to 7.2 x 10$^{\mathrm{5}}$ m/s along different k directions. Near the Fermi level, the Dirac crescent can be nicely expressed by an extended 2D Dirac model Hamiltonian. Furthermore, tight-binding band fitting reveals that the Dirac crescent originates from the next-nearest-neighbor interactions between C atoms. Our findings enrich the Dirac physics founded in other 2D Dirac systems, and offer a new design mechanism for creating Dirac band by tuning the interaction range. We envision that the highly anisotropic Dirac crescent may be exploited in all-carbon-based electronic devices for manipulating anisotropic electron propagation.   

Microwave Irradiation on Halloysite-Polypropylene Nanocomposites

Halloysite is an unique cyllindrical nanoclay characterized by poor electrical and thermal conductivity, which may become the filler of choice for the reinforcement of polymeric matrix, where electrical or thermal insulation are required. The main limits in the use of halloysite as replacement for carbon nanotube (CNT) are: 1. Smaller aspect ratio as halloysites are typically shorter than CNTs. 2. Smaller Young modulus of halloysites compared with CNTs. 3. Reduced thermal stability due to the loss of water upon heating. A research on halloysite dispersed within isotactic polypropylene is reported. To improve the interface between the halloysite and the polymeric matrix a microwave irradiation step has been considered. The local heating of the halloysite nanotubes is mediated by the absorbed/structural water content of the nanoclay. Nanocomposites loaded by various amounts of halloysite ranging from 0 {\%} to 20 {\%} wt. have been prepared by melt mixing by using a Haake RheoMixer. The as obtained nanocomposites have been subjected to microwave irradiation at 75 W in an Anton Paar Monowave 300 system and various irradiation times ranging from 5, 10, 15, 30, 45, and 60 minutes. The effect of microwave irradiation has been studied by Raman and FTIR spectroscopy   

On the Radial Breathing Mode in SWCNTs dispersed within PVC

The Radial Breathing Mode (RBM) is an unique set of Raman lines, characterized by shifts smaller than 500 cm$^{-1}$, assigned to vibrations that affect the diameter of carbon nanotubes. The position of the RBM lines is inversely proportional to the diameter of nanotubes. RBM was reported in Single Walled Carbon Nanotubes (SWCNTs) and Double Walled Carbon Nanotubes. This mode is very sensitive being frequently used to obtain information regarding the stress transfer from the polymeric matrix. Nanocomposites have been prepared by loading the polyvinylchloride (PVC) purchased from Sigma Aldrich with SWCNTs from Cheap Tubes Inc., by melt mixing, using Haake Rheomix equipped with two counter rotating screws. The concentration of SWCNTs dispersed within PVC ranged from 0 {\%} wt. up to 20 {\%} wt. The as recorded spectra have been deconvoluted into several individual lines characterized by an extended Breit-Wigner-Fano line shape. A full analysis of the Raman spectra of the polymeric matrix and of the matrix is reported with emphasize on the RBM features. The spectra have been recorded by using a Renishaw InVia spectrometer equipped with Eclipse filters that allow the recording of Raman lines starting from about 25 cm$^{-1}$.   

First Principles Study of Carbyne Structural Stability

Carbyne is composed of linear sp-hybridized carbon bonds and yields promising results to surpass graphene's mechanical and electrical properties. Carbyne has two semi-stable conformations: Polyyne (alternating triple and single bonds) and Polycumulene (repeating double bonds). This study investigated the stability of these forms at infinite chain lengths by using periodic boundary conditions. Geometric optimization was performed via DFT calculations using DMoL3 and PBE GGA functional group. Each configuration's chain was stretched or compressed until the most stable form -- lowest energy - was obtained. After comparing the energies, the most stable form alternated between Polyyne and Polycumulene as the number of carbon atoms within each boundary increased. Polyyne was the most stable form for odd number of carbons and Polycumulene was the most stable for even number of carbons. Finally, K-point sampling was increased in the direction of the chain axis to obtain a more accurate depiction of structural stability. As the number of k-points increased, the Polycumulene structure became more stable compared to Polyyne.   

Monoclonal Antibodies Attached to Carbon Nanotube Transistors for Paclitaxel Detection

Paclitaxel is a naturally-occurring pharmaceutical used in numerous cancer treatments, despite its toxic side effects. Partial inhibition of this toxicity has been demonstrated using weakly interacting monoclonal antibodies (3C6 and 8A10), but accurate monitoring of antibody and paclitaxel concentrations remains challenging. Here, single-molecule studies of the kinetics of antibody-paclitaxel interactions have been performed using single-walled carbon nanotube field-effect transistors. The devices were sensitized with single antibody attachments to record the single-molecule binding dynamics of paclitaxel. This label-free technique recorded a range of dynamic interactions between the antibody and paclitaxel, and it provided sensitive paclitaxel detection for pM to nM concentrations. Measurements with two different antibodies suggest ways of extending this working range and uncovering the mechanistic differences among different antibodies.   

Observation of dopant-induced impurity states in bottom-up graphene nanoribbons

Graphene nanoribbons (GNRs) provide a means for inducing energy gaps in graphene and are a promising candidate for many nanotechnological applications.~New bottom-up fabrication techniques allow the structure of GNRs to be tuned with atomic precision, thus providing new opportunities for modifying their electronic structure. Here we report the synthesis of bottom-up armchair GNRs (AGNRs) with isolated substitutional boron-dopant centers; thus creating localized impurity states in the GNR. These impurities are realized via dilute doping of pristine n$=$7 AGNRs with sparse boron-containing monomer units, resulting in uniform-width n$=$7 AGNR segments where only two carbon atoms have been substitutionally replaced by boron atoms. Scanning tunneling microscopy (STM) and spectroscopy (STS) were performed to study the electronic structure of these AGNR impurity systems, enabling us to observe localized mid-gap impurity states.   

\textit{In Situ} Characterization of Nanostructures Using Rayleigh Scattering

Controlling selective growth of nanotubes has posed a considerable challenge over the last two decades. A crucial step to overcoming such hurdle is to gain detailed knowledge of the early stage of nanomaterial syntheses for which $in\,situ$ measurements are required. Laser-based probes, such as Rayleigh scattering (RS), can potentially characterize the shape and size of nanoparticles $in\,situ$. The intensity of RS in a gas mixed with nanoparticles is proportional to the polarizabilities of the constituent particles, therefore, theoretical spectroscopy can complement such measurements. Here, we employed time-dependent density functional theory to compute the frequency-dependent polarizabilities of various nanostructures and predicted the corresponding RS intensity and depolarization. We found that with increasing length and asymmetry of the nanostructures the longitudinal polarizability exhibited characteristic resonances leading to measurable signatures in the RS intensity and depolarization. Also by considering gas-particle mixtures at estimated experimental conditions for nanoparticle synthesis on the periphery of an arch, we predict that $in\,situ$ characterization of a few nanometer long particles with concentration as low as one particle per million is feasible using RS.   

Giant Rashba spin-orbit splitting in $n-p$ codoped graphene

Enhancement of the spin-orbit coupling (SOC) in graphene may lead to various topological phenomena and also find applications in spintronics. However, increasing the SOC strength in graphene without drastically affecting the basic physical properties is proving extremely difficult. Here, we propose a new approach, based on compensated $n--p$ codoping, that can simultaneously address all the main shortcomings associated with single-element adsorption in graphene, effectively resulting in giant Rashba spin-orbit splitting. Our proposal is to deposit strong SOC adatoms with outer shell $p$ electrons, acting as $n$-type dopants, onto already $p$-doped (e.g., by B) graphene. We found that: (1) the electrostatic attraction between the $n$- and $p$-type dopants effectively enhances the adsorption of the metal adatoms and suppress their undesirable clustering, (2) considerable (\textasciitilde 130 meV) Rashba-type SO splitting can be achieved in the graphene $\pi $ bands, (3) the charge compensated nature and mutually screening each other of the $n--p$ codopants helps to preserve the Dirac nature of the charge carriers, and (4) the B doping effect together with intrinsic induced SOC by adatom also lead the codoped system open about 20 \textasciitilde 90 meV band gap.   

Flexible, Transparent and Conductive Carbon Nanotube Aerogels /PEDOT:PSS Electrodes created by Top-bottom Fabrication

The sheets of Carbon Nanotubes (CNT) have proven to be a good substitute for ITO. To improve their conductivity and increase optical transparency we have created composites which incorporate silver nanowires or other evaporated metals. Coating CNT/metals with PEDOT:PSS is important for creating hole transport/electron barrier layer functionality, but it is not easy to achieve using PEDOT:PSS solutions due to the hydrophobicity of CNT. We report a new top-to-bottom approach for the fabrication of highly flexible, transparent and conductive carbon nanotube-based electrodes using PDMS as a substrate. A uniform and smooth layer of approximately 50 nm of PEDOT:PSS was spin coated on top of a PDMS stamp followed by the deposition of vapor densified freestanding Multiwall Carbon Nanotube (MWNT) aerogels. An incorporation of silver nanowires, silver or Aluminum thin layer can be sprayed or evaporated on top of the freestanding MWNT aerogels in order to lower the sheet resistance even further. The PDMS substrate is drop cast on top of the configuration then the PDMS stamp is lifted-up. The PEDOT:PSS layer is selectively deposited on top of the MWNT only. The composite electrodes can be laminated on photovoltaic devices and on LEDs.   

Computational design of metal-organic frameworks with paddlewheel-type secondary building units

We employ the TOPOS package to study 697 coordination polymers containing paddlewheel-type secondary building units. The underlying nets are analyzed and 3 novel nets are chosen as potential topologies for paddlewheel-type metal organic frameworks (MOFs). Dicarboxylate linkers are used to build basic structures for novel isoreticular MOF series, aiming at relatively compact structures with a low number of atoms per unit cell. The structures are optimized using density functional theory. Afterwards the Grand Canonical Monte Carlo approach is employed to generate adsorption isotherms for CO$_2$, CO, and CH$_4$ molecules. We utilize the universal forcefield for simulating the interaction between the molecules and hosting MOF. The diffusion behavior of the molecules inside the MOFs is analyzed by molecular dynamics simulations.   

\textbf{Structural, Electronic and Magnetic Properties of Ti}$_{\mathbf{1+x}}$\textbf{FeSb and TiFe}$_{\mathbf{0.75}}$\textbf{M}$_{\mathbf{0.25}}$\textbf{Sb ( M}$=$\textbf{ Ni, Mn) Heusler Alloys}

Density functional theory calculations based on full potential linearized augmented plane-wave (FPLAPW) plus local orbital method in the framework of GGA-PBE, as embodied in the WIEN2k code, is used to investigate the structural, electronic and magnetic properties of intermetallic Ti$_{1+x}$FeS Heusler compounds, where ($x=$\textit{i/4, i}$=$\textit{-3,-2,-1,0,1,2,3,4}) and the TiFe$_{0.75}$M$_{0.25}$Sb (M $=$ Ni, Mn) quaternary semi-Heusler compounds. Moreover, the modified Becke-Johnson exchange potential, as a semi-local method, was employed to predict the band-gap more precisely. We examined the site preference of the parent compound TiFeSb and varying the electron concentration by doping or removing a Ti atom. It is found that they play a crucial role in physical properties of these material systems. The lattice parameters and spin magnetic moment calculated were consistent with the previous experimental and theoretical data available. Moreover, alloys with x\textless 0 are found to exhibit a ferrimagnetic phase, and the alloy with x$=$0.25 exhibit a non-magnetic properties, whereas the rest have shown ferromagnetic phase. The band-structure analysis of Ti$_{1.75}$FeSb, Ti$_{2}$FeSb and TiFe$_{0.75}$Ni$_{0.25}$Sb alloys suggested that they could be a ferromagnetic half-metallic members with band-gaps 0.67, 0.41 and 0.54 eV, respectively.   

Stability of Carbyne: First Principles Approach

Over the last decade, carbon based nanomaterials have gained attention due to the discovery of graphene and its extraordinary properties. This has inspired new research into other carbon allotropes to obtain their unique properties. Carbyne is one such allotrope composed of linear sp-hybridized carbon bonds that has promising results and characteristics to surpass graphene's mechanical strength and possess novel electrical properties. It has two semi-stable conformations: Polyyne (alternating triple and single bonds) and Polycumulene (repeating double bonds). We investigated the stability of these forms with infinite chain lengths by employing periodic boundary conditions. Geometric optimization was performed using DMoL3 with GGA PBE. After comparing the energies, the most stable form alternated between Polyyne and Polycumulene as the number of carbon atoms within each boundary increased; furthermore, every odd carbon atoms showed Polyyne as the most stable form, while every even number of carbon atoms showed Polycumulene as the most stable form. Considering k-point sampling resulted in the Polycumulene structure being the most stable as the number of k-points increased.   

Development and Validation of Polarized Models for Peptide-Graphene Interactions

Biosensor technologies require the understanding of interactions between organic and inorganic materials to tune electric response functions, such as peptide assembly on graphitic substrates. Laboratory characterization of specific interactions and molecular assembly of such biomolecules in atomic resolution remains challenging. These methods can be complemented by molecular simulations and quantum-mechanical analysis of band gaps and expected conductivity. We improved common dispersive~interatomic potentials for graphite and graphene to include pi~electron density at virtual sites. The new models~reproduce experimental X-ray structure, density, cleavage energy, hydration energy, and contact angles. As a result we have improved existing models which gave the~wrong sign of hydration energies and~deviations on the order of 30{\%} in other properties. The parameters are embedded in CHARMM, CVFF, TEAM-AMBER, and other common force fields as part of the INTERFACE force field. An analysis of binding residues, binding energies, conformations, and dynamic information of molecular mobility on the surfaces will be presented.   

Accurate prediction of the refractive index of polymers using first principles and data modeling

Organic polymers with a high refractive index (RI) have recently attracted considerable interest due to their potential application in optical and optoelectronic devices. The ability to tailor the molecular structure of polymers is the key to increasing the accessible RI values. Our work concerns the creation of predictive \textit{in silico} models for the optical properties of organic polymers, the screening of large-scale candidate libraries, and the mining of the resulting data to extract the underlying design principles that govern their performance. This work was set up to guide our experimentalist partners and allow them to target the most promising candidates. Our model is based on the Lorentz-Lorenz equation and thus includes the polarizability and number density values for each candidate. For the former, we performed a detailed benchmark study of different density functionals, basis sets, and the extrapolation scheme towards the polymer limit. For the number density we devised an exceedingly efficient machine learning approach to correlate the polymer structure and the packing fraction in the bulk material. We validated the proposed RI model against the experimentally known RI values of 112 polymers. We could show that the proposed combination of physical and data modeling is both successful and highly economical to characterize a wide range of organic polymers, which is a prerequisite for virtual high-throughput screening.   

Nanoscale Characterization of Organometal Trihalide Perovskite using Photothermal Induced Resonance (PTIR) Technique

Further improvement of the performance of organometal trihalide perovskites (OTP) solar cells can be aided by nanoscale characterization. Photothermal induced resonance (PTIR), is a novel scanning probe method that enable measuring vibrational and electronic absorption maps and spectra with a resolution as high as 20 nm. In this presentation, the chemical composition and bandgap of OTP thin films was characterized with PTIR: 1) to identify the origin of the switchable photovoltaic effect and 2) to quantify the local chloride content in mixed-halide perovskites. PTIR vibrational maps recorded in correspondence of methyl ammonium ions (MA$+)$ for a as prepared lateral structure solar cell were uniform but displayed stronger intensity in proximity of the cathode after electric poling. Those measurements provide the first direct proof of ion electron migration in OTP devices. Because chloride incorporation modifies the bandgap in MAPbI3-xClx perovskites, PTIR electronic maps and spectra were used to extract the local chloride content as a function of annealing. Results show that the as-prepared sample consist of a mixture of Cl-rich and Cl-poor phases that evolves into a homogenous Cl-poorer phase upon annealing. This measurement suggests that Cl- is progressively expelled from the film.   

High Pressure Raman Spectroscopic Studies on CuInTe$_{2}$ Quantum Dots

High pressure Raman spectroscopy studies were performed on CuInTe$_{2}$ Quantum Dots (QD) up to 7.7 GPa. At ambient conditions, the Raman modes of the QD loaded into a high-pressure diamond anvil cell (DAC) were observed at 125.1 cm$^{-1}$ (A$_{1}$ mode) and 142.8 cm$^{-1}$ (B$_{2}$ or E mode). As the pressure increases, the A$_{1}$ mode starts to split above 2 GPa and shifts to the left as indication of a structural change. A pressure-induced phase transition was observed around 2.9 GPa due to the collapse of the modes with the appearance of a new Raman peaks. The phase transition observed in our experiments compare well with the characteristics of bulk and larger nanoparticles. Further, it could be concluded that the phase transition pressure observed mainly depends on the particle size.   

Tunneling spectroscopy of multi-shell carbon fullerenes

Carbon allotropes such as fullerenes and nanotubes have generated considerable interest due possible exploitation of their mechanical and electrical properties for practical applications. Carbon onions are a type of fullerene consisting of multiple spherically concentric shells of curved graphitic sheets. Compared to single-shell fullerenes, few studies have been directed toward understanding the structural and electrical properties of carbon onions. Because carbon onions have proven difficult to fabricate in a controlled method, most of these studies have focused on synthesis methods. In this study, we investigate the electrical properties of carbon onions using a scanning tunneling microscope. Carbon onions were fabricated using ultrasonic agitation to break down isopropanol facilitated by a MoS$_{\mathrm{2}}$ catalyst. Particles suspended in the remaining solution were deposited onto atomically flat HOPG substrates. Scanning tunneling spectroscopy indicate that carbon onions can exhibit both metallic and semiconducting properties, similar to carbon nanotubes.   

Electro-optical properties of a bilayer $\beta $-graphyne

Graphynes (GYs) are graphene-like structures that can be constructed by replacing some bonds $=$ C $=$ C $=$ in graphene by acetylenic linkages, - C $\equiv $ C - [1]. According to first-principles calculations [2] the so-called $\beta $-graphyne, has a Dirac cone not located at the K and K' points of the Brillouin zone but on lines between the high symmetry $\Gamma $ and M points. In a previous work we show that a bilayer of $\beta $-graphyne can be metal or semiconductor, depending on the staking. An electric field applied perpendicular to the layers has remarkable effects on the electronic properties of this structure. We have found that the field can close the gap in the case of semiconductor bilayers [3]. In this work we perform a theoretical study of the electro-optical properties of a bilayer $\beta $-graphyne. Calculations are based on density functional theory (DFT) method. The indirect and direct band gap of the optimized lattice parameters is calculated by ABINIT. Finally, the dielectric function of the bilayer $\beta $-graphyne is calculated. Our results show that the optical properties of this type of graphyne are strongly anisotropic and that the optical band gap can be tuned by means of an external electric field. [1] Baughman RH et al., Chem. Phys., 87 (1987) 6687. [2] Malko D et al., Phys. Rev. Lett., 108 (2012) 086804 [3] Le\'{o}n A, Pacheco M, Chem. Phys. Lett., 620 (2015) 67   

Reactivity of Graphene Investigated by Density-Functional Theory

Using spin-polarized density-functional theory, we study the adsorption and reaction of hydrogen and fluorine with graphene. Graphene has a bipartite lattice with two different sublattices and hence, due to Lieb’s theorem, the inequality between two sublattices should lead to a net magnetic moment upon adsorption of hydrogen or fluorine. Our calculations using density-functional theory with the generalized gradient approximation predict a magnetic moment of 1 µB for a single hydrogen adsorbed on graphene but not for a single fluorine atom adsorbed on graphene. Switching to hybrid density-functional theory with the HSE functional [1], we obtain a magnetic moment of 1 µB for of a single fluorine atom adsorption on graphene. This is in line with work of Kim et al. [2], who also found in density-functional theory calculations with the HSE exchange-correlation functional spin-polarization for a fluorine adatom on graphene. Here, we present a systematic study of the reactivity and relevant adsorption mechanism for single-sided graphene, i.e., a graphene sheet which is accessible by an adsorbate from only one side with hydrogen and fluorine using hybrid density-functional theory. References 1)AV Krukau et al., J. Chem. Phys. 125,224106(2006) 2)H-J Kim et al., Phys. Rev B 87,174435(2013)   

Electronic structures of hybrid graphene/boron nitride nanoribbons with hydrogen adsorption

Electronic properties of hybrid graphene/boron nitride nanoribbons are investigated using density functional calculations. It is found that hydrogen adsorption on a graphene nanoribbon alters band structures drastically. Furthermore, H-vacancy chains and lines can effectively shape the conduction properties. Influences of edge atoms with nonzero magnetic moments and the interface between B and N are also prominent in the electronic structures.   

Role of Cooperative Interactions in the Intercalation of Heteroatoms between Graphene and a Metal Substrate

Graphene, a two-dimensional crystal of carbon atoms packed in a honeycomb structure, has many promising mechanical, electrical, and optical properties. The intercalation of heteroatoms between graphene and a metal substrate has been studied intensively over the past few years, due to its effect on the graphene properties, and as a method to create vertical heterostructures. Various intercalation processes have been reported with different combinations of heteroatoms and substrates. In this talk, I will present the investigation of the key processes governing the intercalation of heteroatoms between graphene and a substrate by combining atomic-scale characterization with density functional theory (DFT). Si intercalation between graphene and Ru(0001) is chosen as a test bed. We elucidate the role of cooperative interactions between heteroatoms, graphene, and substrate. By combining scanning tunneling microscopy with density functional theory, the intercalation process is confirmed to consist of four key steps, involving creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of an intercalated monolayer. Other combinations of heteroatoms (such as Ni, Pd and Pt) and substrates (such as Ir(111) and SiC(0001)) are also investigated to support the generality of our study. Both theory and experiments indicate that this mechanism applies also to other combinations of heteroatoms and substrates. (G. Li et al., J. Am. Chem. Soc. 137 (2015) 7099. In collaboration with G. Li, H.T. Zhou, L.D. Pan, Y. Zhang, L. Huang, W.Y. Xu, and H.J. Gao in CAS, Min Ouyang in MU, and A.C. Ferrari in U. Cambridge.)   

Optical properties of AAB-stacked trilayer graphene.

The band structures and optical properties of AAB-stacked trilayer graphenes (AAB-TLG) are calculated by the tight-binding model and gradient approximation. There are one pair of parabolic bands and two pairs of wavy bands at low energy, and three pairs of saddle points at the middle energy. At zero electric field, 3$^{2}$ excitation channels exist in both the low and middle frequencies, and cause the very rich joint density of states (JDOS). However, the structures in the JDOS do not appear in the absorption spectra completely. In the spectra, due to the velocity metric elements, the transitions between the same pair only make the slight contributions in the low frequency, except for the transition between the pair of the lowest bands. Furthermore, three transitions with the similar energies at the saddle points peaks lead to a strong peak in the middle frequency. The energy dispersions and the energy spacing exhibit obvious variations with the change of the electric field, and thus the absorption spectra..   

A new way of describing the Dirac bands in graphene

We develop a new way of describing the electronic structure of graphene, by treating the honeycomb lattice as a network of one-dimensional quantum wires. The electrons travel as free particles along these quantum wires and interfere at the three-way junctions formed by the carbon atoms. The model generates the linearly dispersive Dirac cone band structure as well as the chiral nature of the pseudo-spin sublattice wave functions. When vacancies are incorporated, we find that it also reproduces the well known zero mode states. This simple approach might have advantages over other methods for some applications, such as in analyzing electronic transport through graphene nanoribbons. In addition, this finding suggests new ways of constructing Dirac band materials in the laboratory by nano-patterning for investigating Dirac fermions.   

Electronic and geometrical properties of monoatomic and diatomic 2D honeycomb lattices. A DFT study

Since the discovery of graphene by Geim and Novoselov at 2004, several analogous systems have been theoretically and experimentally studied, due to their technological interest. Both monoatomic lattices, such as silicine and germanene, and diatomic lattices (h-GaAs and h-GaN) have been studied. Using Density Functional Theory we obtain and confirm the chemical stability of these hexagonal 2D systems through the total energy curves as a function of interatomic distance. Unlike graphene, silicine and germanene, gapless materials, h-GaAs and h-GaN exhibit electronic gaps, different from that of the bulk, which could be interesting for the industry. On the other hand, the ab initio band structure calculations for graphene, silicene and germanene show a non-circular cross section around K points, at variance with the prediction of usual Tight-binding models. In fact, we have found that Dirac cones display a dihedral group symmetry. This implies that Fermi speed can change up to 30$\%$ due to the orientation of the wave vector, for both electrons and holes. Traditional analytic studies use the Dirac equation for the electron dynamics at low energies. However, this equation assumes an isotropic, homogeneous and uniform space.   

Magneto-optical transitions in bilayer graphene nanoribbons

We utilize the tight-binding theory to study the magneto-optical transitions in bilayer graphene nanoribbons. The magneto-absorption spectra highly depend on the stacking, edge orientation, ribbon width, and strength of magnetic field. The competition between the magnetic quantization and lateral confinement results in the coexistence of edge-dependent selection rules and magneto-absorption selection rule. The magneto-electronic properties, including energy dispersions, density of states, and wave functions, are also discussed in detail.   

Semiconducting graphene nanoribbon fabricated by gate-controlled, edge-selective photo-oxidation of graphene.

Graphene is attracting much attention for its ultrahigh carrier mobility, and is expected as the next generation material which makes high speed communication possible. However, since graphene has no bandgap, its conductivity cannot be turned completely off. Thus, pristine graphene cannot be used as a transistor in logic applications, where high on/off current ratios are required. It is known that narrow graphene nanoribbons with nanometer-scale widths show semiconducting characteristics with sufficiently large bandgap [1]. In this presentation, we propose and demonstrate a new method to fabricate graphene nanoribbons at room temperature in air. We have found that ultraviolet(UV)-induced photochemical reactions can be controlled by a configuration of a field effect transistor, and graphene edges are selectively photo-oxidized by the UV irradiation under a negative gate voltage and a finite drain voltage [2]. By means of this gate-controlled, edge-selective photo-oxidation, we succeeded to improve the on/off ratio from 2.7 to 43 at room temperature. [1] Y. W. Son, M. L. Cohen, and S. G. Louie, Phys. Rev. Lett. \textbf{97}, 216803 (2006). [2] N. Mitoma and R. Nouchi, Appl. Phys. Lett. \textbf{103}, 201605 (2013).   

Controllably Inducing and Modeling Optical Response from Graphene Oxide

Graphene, a novel 2-dimensional sp$^{\mathrm{2}}$-hybridized allotrope of Carbon, has unique electrical and mechanical properties. While it is naturally a highly conductive zero band gap semiconductor, graphene does not exhibit optical emission. It has been shown that functionalization with oxygen-containing groups elicits an opening of band gap in graphene.~ In this work, we aim to induce an optical response in graphene via controlled oxidation, and then explore potential origins of its photoluminescence through mathematical modeling. We employ timed ozone treatment of initially non-fluorescent reduced graphene oxide (RGO) to produce graphene oxide (GO) with specific optical properties. Oxidized material exhibits substantial changes in the absorption spectra and a broad photoluminescence feature, centered at 532 nm, which suggests the appearance of a band gap. We then explore a number of possible mechanisms for the origin of GO photoluminescence via PM3 and ab initio calculations on a functionalized single sheet of graphene. By adjusting modeling parameters to fit experimentally obtained optical transition energies we estimate the size of the sp$^{\mathrm{2}}$ graphitic regions in GO and the arrangement of functional groups that could be responsible for the observed emission.~   

Plasmon-polaritonic bands in sequential doped graphene superlattices

Doped graphene has the extraordinary quality of supporting two types of surface excitations that involve electric charges (the transverse magnetic surface plasmons) or electric currents (the transverse electric modes). We have studied numerically the collective modes that result from the coupling of surface plasmons in doped graphene multilayers. By use of structured supercells with fixed dielectric background and inter layer separation, we found a series of plasmon-polaritonic bands of structure dependent on the doping sequence chosen for the graphene sheets. Periodic and quasiperiodic sequences for the graphene chemical potential have been studied. Our results show that transverse magnetic bands exist only in the low frequency regime but transverse electric bands arise within specific ranges of higher frequencies. Our calculations are valid for THz frequencies and graphene sheets with doping levels between 0.1 eV and 1.2 eV have been considered.   

Localization properties of graphene Landau levels: The role of edge states

The observation of the quantum Hall effect for graphene in 2005 represented an important landmark, proving the genuine two-dimensional nature of graphene. Here we use a tight-binding approach to investigate the localization properties of quantum Hall edge states of \underline {graphene} flakes with sharp edges. In order to identify which wave function is concentrated in the edges, or distributed in the bulk, we defined a quantity named ``Edge Fraction'',indicating the fraction of electronic probability densities over the atomic sites at distances from the edges limited to twice the magnetic length lB. We also calculate separately the fraction of the wave function amplitude over zigzag or armchair edges, observing an interesting and clear pattern for different energies between consecutive Landau levels. The edge states are manifested in the presence of states among the Landau levels. Here was explored the interplay of different square lattices sizes and disorder in the localization properties of the system. We observed that size variation do not affect the behavior of the Edge Fraction. Also it was found that exist a dependence between the behavior of the Edge Fraction for the armchair and zigzag contribution with respect to disorder.   

Optical Signatures of Weyl Points in TaAs

We present a systematic study of both the temperature and frequency dependence of the optical response in TaAs, a material that has recently been realized to host the Weyl semimetal state. Our study reveals that the optical conductivity of TaAs features a narrow Drude response alongside a conspicuous linear dependence on frequency. The width of the Drude peak decreases upon cooling, following a $T^{2}$ temperature dependence which is expected for Weyl semimetals. Two linear components with distinct slopes dominate the 5-K optical conductivity. A comparison between our experimental results and theoretical calculations suggests that the linear conductivity below $\sim$230~cm$^{-1}$ is a clear signature of the Weyl points lying in very close proximity to the Fermi energy.   

Theoretical analysis of high-resolution X-ray absorption spectra and 2p-3d resonant X-ray emission spectra of CeO$_{\mathrm{2}}$

The 3$d$ and 4$f$ systems show various attractive phenomena due to strong electron correlations. The X-ray core-level spectroscopy, such as X-ray absorption spectroscopy (XAS), is an efficient technique to investigate electronic states of the systems. Recent years, the experimental techniques have been rapidly developing, and, especially, the progress in the experimental resolution has enabled us to observe fine spectral features. For example, a pioneering work was made by K. Hämäläinen et al.[1] Recently, we performed high-resolution XAS experiments (partial fluorescence yield; PFY) and observed a peak corresponding to 2$p$-4$f $quadrupole transition around the pre-edge region of the $L_{\mathrm{3}}$-edge of CeO$_{\mathrm{2}}$. Conventionally, X-ray spectra have been analyzed using a phenomenological impurity Anderson model (IAM). However, such a simplified model does not seem to appropriate for analysis of high-resolution spectrum because of possible ambiguities from the choice of adjustable parameters included. Thus we constructed an IAM framework combined with a first principle band calculation. In this meeting, we report experimental results and theoretical analysis for the PFY spectrum and 2$p$-3$d$ resonant X-ray emission spectroscopy. [1] K. Hämäläinen et al., PRL 67 (1991) 2850.   

The 4f multipole ordering effect on core-level spectroscopies of Ce intermetallics

The 3$d$ transition metal compounds and 4$f$ rare earth compounds show attractive phenomena, such as superconductivity and Kondo effect, due to strong electron correlations among localized 3$d$ and 4$f$ electrons. Especially, multipole ordering of orbital and/or spin in 4$f$ and 5$f $compounds are attracting much attention these years. For example, CeB$_{6}$ is known to show antiferro-quadrupolar (AFQ) ordering below 3.2K. X-ray core-level spectroscopy is an efficient technique to investigate the electronic states of strongly correlated systems. Recent years, experimental techniques have been rapidly developing and, especially, the progress in experimental resolution has enabled us to observe fine spectral features, which were not formerly observed. These advantages will enable us to observe spectral fine features related with the multipole ordering. In this study, we discuss multipole ordering effects on X-ray spectra for CeB$_{6}$, especially paying attention on the polarization dependence. In order to simulate the electronic state of CeB$_{6}$ with the multipole ordering, we use an impurity Anderson model including realistic valence structure and a simplified RKKY interaction.   

Twisted Boundary Conditions for Lattice Monte Carlo Simulations

Numerical simulations for spatially correlated lattice models have made progress via Dynamical Mean Field Theory and Dynamical Cluster Approximation, but are still hindered by a computational cost that scales exponentially with lattice size. We present a method of addressing finite size errors in a computationally efficient manner by running simulations with twisted boundary conditions. Averaging over these boundary conditions allows for thermodynamic extrapolation of physical quantities of interest without the cost associated with large system simulations.   

\textbf{Structural and Magnetic Phase Coexistence in Oxygen Deficient Perovskites (Sr,Ca)FeO}$_{\mathrm{\mathbf{2.5+\delta }}}$

A variety of compounds crystallize into perovskite and similar structures, making them versatile laboratories for many phenomena and applications, including multiferroicity, superconductivity, and photovoltaics. Oxygen-deficient perovskites ABO$_{\mathrm{x}}$ have attracted interest for use in fuel cells and related applications due to high oxygen mobility and the possibility of charge disproportionation. Vast chemical flexibility is obtained through reductions in lattice symmetry and rotation/distortion of the BO$_{\mathrm{6}}$ octahedra, as well as ordering of oxygen vacancies. We have synthesized and studied the structural and magnetic properties of oxygen-deficient perovskites (Sr,Ca)FeO$_{\mathrm{2.5+\delta }}$ using x-ray diffraction and Mossbauer spectroscopy. While the ideal perovskite has $\delta =$0.5, this requires Fe$^{\mathrm{4+}}$, and hence strongly oxidizing environments. When grown in air, Fe$^{\mathrm{3+}}$ is favored, yielding $\delta \approx $0. SrFeO$_{\mathrm{2.5+\delta }}$ exhibits cubic symmetry and paramagnetism at 300K, but CaFeO$_{\mathrm{2.5+\delta }}$ crystallizes into the orthorhombic brownmillerite structure, and is magnetically ordered at 300K. In the doped intermediaries we find coexistence of cubic/paramagnetic and orthorhombic/magnetic phases over a wide range of Ca content.   

Introducing a New Capability at SSRL: Resonant Soft X-ray Scattering

Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC recently developed a setup for the resonant soft x-ray scattering (RSXS). In general, the RSXS technique uniquely probes not only structural information, but also chemical specific information. This is because this technique can explore the spatial periodicities of charge, orbital, spin, and lattice with spectroscopic aspect. Moreover, the soft x-ray range is particularly relevant for a study of soft materials as it covers the $K$-edge of C, N, F, and O, as well as the $L$-edges of transition metals and $M$-edges of rare-earth elements. Hence, the RSXS capability has been regarded as a very powerful technique for investigating the intrinsic properties of materials such as quantum- and energy-materials. The RSXS capability at the SSRL composes of in-vacuum 4-circle diffractometer. There are also the fully motorized sample-motion manipulations. Also, the sample can be cooled down to 25 K via the liquid helium. This capability has been installed at BL 13-3, where the photon source is from elliptically polarized undulator (EPU). Covering the photon energies is from 230 eV to 1400 eV. Furthermore, this EPU system offers more degree of freedoms for controlling x-ray polarizations (linear and circular). Using the advance of controlling x-ray polarization, we can also investigate a morphology effect of local domain/grain in materials. The detailed introduction of the RSXS end-station and several results will be touched in this poster presentation.   

Electric-field-driven resistive switching in dissipative Hubbard model

Understanding of solids driven out of equilibrium by external fields has been one of the central goals in condensed matter physics for the past century and is relevant to nanotechnology applications such as resistive transitions. We study how strongly correlated electrons on a dissipative lattice evolve from equilibrium when driven by a constant electric field, focusing on the extent of the linear regime and hysteretic non-linear effects at higher fields. We access the non-equilibrium steady states, non-perturbatively in both the field and the electronic interactions, by means of a non-equilibrium dynamical mean-field theory in the Coulomb gauge. The linear response regime is limited by Joule heating effects and breaks down at fields orders of magnitude smaller than the quasi-particle energy scale. For large electronic interactions, strong but experimentally accessible electric fields can induce a resistive switching by driving the strongly correlated metal into a Mott insulator. Hysteretic $I$-$V$ curves suggest that the non-equilibrium current is carried through a spatially inhomogeneous metal-insulator mixed state.\textsuperscript{1}\\\par [1]J. Li, C. Aron, G. Kotliar, J. E. Han, Phys. Rev. Lett. \textbf{114}, 226403 (2015)   

Unconventional but tunable phase transition above the percolation threshold by two-layer conduction in electroless-deposited Au nanofeatures on silicon substrate

Previous research has shown that disorder, dislocation, and carrier concentration are the main factors impacting transitions in the traditional metal--insulator transition (MIT) and metal--semiconductor transition (MST). In this study, it is demonstrated that a non-traditional MST governed by two-layer conduction is possible by tuning the conducting channel of one layer of the two-layer conduction system. By means of the electroless deposition method we produced Au nanofeatures (AuNFs) on p-type silicon (p-Si) as the two-layer conduction system, controlling AuNF coverage (Au{\%}) below and above the percolation threshold (pc). Even when the AuNF coverage percentage is larger than pc, the resistivities of the AuNFs on p-Si show MST as the temperature increases. We present a conduction model based upon two predominant parallel conductions by AuNFs and p-Si in the present paper. In the results, we show how the temperature of the MST is tuned from 145 to 232 K as Au{\%} is changed from 82.7 to 54.3{\%}.   

Fractal nature of metallic and insulating domain configurations in nearly grain-boundary-free VO$_{2}$/TiO$_{2}$ thin films

We investigated evolution of the surface work function ($W_{S})$ maps of epitaxial 15-nm-thick VO$_{2}$/TiO$_{2}$ thin films using Kelvin probe force microscopy (KPFM) measurements while the film undergoes the metal-insulator transition (MIT). The metallic and insulating domains coexist in the VO$_{2}$ thin films during the transition, since the MIT is the first-order phase transition. Nearly grain-boundary-free samples allowed observation of metallic and insulating domains with distinct $W_{S}$ values, throughout the transition. Each domain allowed us to obtain real space domain maps with nanoscopic spatial resolution. The two-dimensional percolation model well explained the relationship between the metallic domain fraction and the measured the resistivity. The domain maps also suggested that the percolation clusters formed a fractal surface.   

On metal-insulator transition in cubic fullerides

The interplay between degenerate orbital and electron correlation is a key to characterize the electronic phases in, for example, transition metal compounds [1,2] and alkali-doped fullerides [3]. Besides, the degenerate orbital couples to spin and lattice degrees of freedom ,giving rise to exotic phenomena. Here, we develop the self-consistent Gutzwiller approach for the simultaneous treatment of the Jahn-Teller effect and electron correlation, and apply the methodology to reveal the nature of the ground electronic state of fullerides [4]. For small Coulomb repulsion on site $U$, the fulleride is quasi degenerate correlated metal. With increase of $U$, we found the quantum phase transition from the metallic phase to JT split phase. In the latter, the Mott transition (MT) mainly develops in the half-filled subband, whereas the empty and the completely filled subbands are almost uninvolved. Therefore, we can qualify the metal-insulator transition in fullerides as an orbital selective MT [2] induced by JT effect. [1] Y. Tokura and N. Nagaosa, Science {\bf 288}, 462 (2000). [2] A. Koga, {\it et al.}, Phys. Rev. Lett. {\bf 92}, 216402 (2004). [3] O. Gunnarsson, Rev. Mod. Phys. {\bf 69}, 575 (1997). [4] N. Iwahara and L. F. Chibotaru, Phys. Rev. B {\bf 91}, 035109 (2015).   

Vanadium dioxide thin films prepared on silicon by low temperature MBE growth and ex-situ annealing

Vanadium dioxide (VO$_2$) is a material that shows an insulator to metal transition (IMT) near room temperature. This property can be exploited for applications in field effect devices, electro-optical switches and nonlinear circuit components. We have prepared VO$_2$ thin films on silicon wafers by combining a low temperature MBE growth with an ex-situ annealing at high temperature. We investigated the structural, electrical and optical characteristics of films with thicknesses ranging from $10$ to $100$ nm. We have also studied the influence of the substrate cleaning. The films grown with our method are polycrystalline with a preferred orientation in the ($011$) direction of the monoclinic phase. For the films produced on silicon with a native oxide, an IMT at around $75$ $^{\circ}$C is observed. The magnitude of the resistance change across the IMT decreases with thickness while the refractive index at room temperature corresponds with values reported in the literature for thin films. The successful growth of VO$_2$ films on silicon with good electrical and optical properties is an important step towards the integration of VO$_2$ in novel devices.   

Magnetic-order-driven topological transition in the Haldane-Hubbard model.

We study the Haldane model with on-site repulsive interactions at half-filling. We show that the mean-field Hamiltonian with magnetic order effectively modifies parameters in the Haldane Hamiltonian, such as sublattice energy difference and phase in next nearest hopping. As interaction increases, increasing of magnetic order corresponds to varying these parameters and consequently, drives topological transitions. At the mean-field level, one scenario is that the magnetic order continuously increases, and inevitably, the fermion gap closes at the topological transition point. Beyond the mean-field, fluctuation induced interaction can further open up the gap, rendering a first-order transition. Another scenario is a first-order transition at mean-field level across which a canted magnetic order develops discontinuously, avoiding the fermion gap closing. We find that both scenarios exist in the phase diagram of the Haldane-Hubbard model. Our predication is relevant to recent experimental realization of the Haldane model in cold atom system.   

Dilute magnetic topological semiconductors

Replacing semiconductors with topological insulators, we propose the problem of dilute magnetic topological semiconductors. Performing the renormalization group analysis for an effective field theory, where doped magnetic impurities give rise to a spatially modulated random axion term, we find a novel insulator-metal transition from either a topological or band insulating phase to an inhomogeneously distributed Weyl metallic state with such insulating islands, where extremely broad distributions of ferromagnetic clusters combined with strong spin-orbit interactions are responsible for the emergence of randomly distributed Weyl metallic islands. Since electromagnetic properties in a Weyl metal are described by axion electrodynamics, the role of random axion electrodynamics in transport phenomena casts an interesting problem beyond the physics of percolation in conventional disorder-driven metal-insulator transitions.   

Confinement transition of $Z_2$ gauge theory coupled to fermions. A sign problem free quantum Monte Carlo study.

In two space dimensions, the $Z_2$ lattice gauge theory is known to undergo a zero temperature confinement to de-confinment quantum phase transition . In this work, we study how this transition is modified in the presence of lattice fermions which are minimally coupled to the $Z_2$ gauge field. This may be viewed as an extreme version of the BEC-BCS transition where fermions are confined in the strong coupling phase. We investigate both a square lattice model with a large fermi surface and Dirac fermions realized on a $\pi$ flux and honeycomb lattices. The models are found to be free of the numerical sign problem for all fermion density. In addition, we introduce a numerical method to stochastically incorporate the Gauss law constraint in a quantum Monte Carlo (QMC) simulation. The phase diagram as a function of the model parameters, chemical potential and temperature is determined by means of a large scale determinant QMC.   

Angle dependence of Shubnikov-de Haas effect of filled skutterudite compounds CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$

Intriguingly the three filled skutterudite compounds CeOs$_4$Sb$_{12}$, PrOs$_4$Sb$_{12}$, and NdOs$_4$Sb$_{12}$ span the range from the Kondo insulator with a 1K antiferromagnetic (AFM) order, a 1.85K unconventional superconductor (SC), to a 1K mean-field type ferromagnet (FM), indicating that they reside near quantum critical points of AFM and FM with unconventional SC induced within this regime. Therefore, understanding the Fermi surfaces of NdOs$_4$Sb$_{12}$ and CeOs$_4$Sb$_{12}$ becomes crucial in elucidating the superconducting pairing mechanism in PrOs$_4$Sb$_{12}$. Penetration depths of single crystals of CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ were measured for temperatures down to 1.3 K and magnetic fields up to 60 tesla by using proximity detection oscillators in the Pulsed Field Facility at NHMFL/LANL. Angle dependence of Shubnikov-de Haas oscillations was detected for rotating the field with respect to the crystalline orientations [010] and [0-10]. The results indicate that LaOs$_4$Sb$_{12}$, PrOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ have similar Fermi surfaces. The Fermi surface of CeOs$_4$Sb$_{12}$ is rather isotropic and is much different from the other three compounds.   

Dipolar glass and strong magneto-electric coupling within a purely organic system

There is much interest in the search for novel materials that show ferroelectric as well as magneto-electric coupling, such as that observed in multiferroics. Within organic based materials the electronic polarisation can come from a charge distribution across a molecule or molecules and so one must search for systems that have a electronic (and magnetic) dipole that is intrinsic. One such material is tetraethylammonium bis-7,7,8,8-tetracyanoquinodimethane (TEA(TCNQ)$_2$) which is a charge transfer system where there is a single electron delocalised across a TCNQ dimer. We show that dielectric measurements yield anomalies at the Peierls structural distortion and on going through the spin-Peierls transition. In both cases the electric response is glassy and at low temperature the corresponding magnetic measurements evidence the strong magneto-electric coupling within the material showing analogies to spin glass systems.   

\textbf{Epitaxial Ni/VO}$_{\mathrm{\mathbf{2}}}$\textbf{ heterostructures on Si (001)}

VO$_{\mathrm{2}}$~is a strongly correlated oxide, undergoes a first order metal-insulator (MIT) well above the room temperature 340K. Previous works$^{\mathrm{\thinspace }}$have shown that the stress associated with structural changes across MIT, VO$_{\mathrm{2}}$~can produce significant changes in magnetic properties of over layer ferromagnetic films such as Ni. This control of the magnetic properties could be very important to many technological applications. However, the current use$^{\mathrm{\thinspace }}$of r-sapphire as substrate can be restrictive in the microelectronics industry. The previous works~focused their studies on polycrystalline Ni and VO$_{\mathrm{2}}$~films, which do not allow the precise controlling of the associated properties due to poor reproducibility of polycrystalline films. We have investigated the magnetic and electronic properties of Ni/VO$_{\mathrm{2}}$~films when epitaxially integrated on Si (001) by pulsed laser deposition using domain matching epitaxy paradigm. Ni was grown both in nanoscale islands and layered form. The XRD results showed that the Ni, VO$_{\mathrm{2~}}$and YSZ layers were grown epitaxially in single out of plane orientations. We found that the hysteresis in resistance vs. temperature curves in VO$_{\mathrm{2}}$~thin films was retained even when it is in close proximity with the Ni layer which helped confirm that VO$_{\mathrm{2}}$~layer preserves its characteristic features, revealed the fingerprint magnetic features of Ni layer. We will present and discuss our comprehensive experimental findings.   

Exact sign structure and variational wave function of t-J chain and ladder

The motion of a doped hole in the anti-ferromagnetic spin background generally induces a many-body phase shift, which is identified by an exact sign structure of the t-J model known as the phase string. We find that the characteristic momentum structure, the one dimensional (1D) Luttinger liquid behavior, the quantum phase interference of the hole under a periodic boundary condition, and the breakdown of Landau's quasiparticle description can all be attributed to it. Based on the exact sign structure, we introduce a variational wave function for the t-J model and calculate physical properties such as the momentum distribution, quasiparticle weight, and hole distribution of the single-hole 1D and ladder system in detail using Monte Carlo (MC) method. An excellent agreement is found between the MC and Density Matrix Renormalization Group results.   

Experimental Apparatus to Observe Dynamical Manifestations of Hamiltonian Monodromy

An experiment to observe a topological change in a classical system with nontrivial monodromy is presented. Monodromy is the study of the topological behavior of a system as it evolves along a closed path. If the system does not return to the initial topological state at the end of the circuit, that system exhibits nontrivial monodromy. Such a topological change has been predicted in certain mechanical systems, but has not yet been observed experimentally. One such system is a family of paths in a cylindrically symmetric champagne-bottle potential, with a classically forbidden region centered at the origin. We constructed this system with a long spherically symmetric pendulum and a permanent magnet attached at the end. Magnetic fields from coils are used to create the potential barrier and the external forces to drive the pendulum about a monodromy circuit. A loop of initial conditions, that is initially on one side of the forbidden region, is driven smoothly about this circuit such that it continuously evolves into a loop that surrounds the forbidden region. We will display this phenomena through numerical simulations and hopefully experimental measurement.   

Elastoconductivity measurements as a probe of broken mirror symmetries

We propose the possible detection of broken mirror symmetries in correlated two-dimensional materials by elastotransport measurements. Using linear response theory we calculate the shearconductivity $\Gamma_{xx,xy}$ , defined as the linear change of the longitudinal conductivity $\sigma_{xx}$ due to a shear strain $\epsilon_{xy}$ . This quantity can only be non-vanishing when in-plane mirror symmetries are broken and we discuss how candidate states in the cuprate pseudogap regime (e.g. various loop current or charge orders) may exhibit a finite shearconductivity. We also provide a realistic experimental protocol for detecting such a response, including the specific form of the elastoresistance for broken tetragonal symmetry.   

Casimir effect mechanism of pairing between fermions in the vicinity of a magnetic quantum critical point

We consider two spin $1/2$ fermions in a two-dimensional magnetic system that is close to the $ O(3)$ magnetic quantum critical point (QCP) which separates magnetically ordered and disordered phases. Focusing on the disordered phase in the vicinity of the QCP, we demonstrate that the criticality results in a strong long range attraction between the fermions, with potential $V(r) \propto -1/r^{\alpha}$, $\alpha \approx 0.75$, where $r$ is separation between the fermions. The mechanism of the enhanced attraction is similar to Casimir effect and corresponds to multi-magnon exchange processes between the fermions. While we consider a model system, the problem is originally motivated by recent experimental establishment of magnetic QCP in hole doped cuprates under the superconducting dome at doping of about 10\%. We suggest the mechanism of magnetic critical enhancement of pairing in cuprates.   

Application of the Debye formula to the computation of x-ray diffraction patterns of nanostructured diffusion couples

Time resolved optical pump/x-ray probe techniques have made it possible to acquire x-ray diffraction patterns corresponding to very early diffusion times in nanostructured diffusion couples. The analysis of these diffraction patterns, however, is complicated by significant line broadening and other finite size effects that appear in samples containing a relatively small number of scatterers. In order to better quantify these issues, x-ray diffraction patterns have been calculated by the direct application of the Debye formula to core/shell and thin film diffusion couples. In particular a series of diffraction patterns have been calculated as a function of the sample size and composition profile determined from the appropriate solutions to Fick's second law. The results of these calculations have been used to guide the interpretation of the measured diffraction patterns of Pt/Ni core/shell nanoparticles and Pt/Ni thin film multilayers.   

All-metal superconducting planar microwave resonator

There is common agreement that noise and resonance frequency jitter in superconducting microwave planar resonators are caused by presence of two-level systems, or fluctuators, in resonator materials- in dielectric substrate, in superconducting and dielectric layers and on the boundaries and interfaces. Scaling of noise with device dimensions indicate that fluctuators are likely concentrated around boundaries; physical nature of those fluctuators remains unclear. The presence of dielectrics is not necessary for the superconducting device functionality, and one can ask question about properties of all-metal device, where dielectric substrate and oxide films on metal are absent. Resonator made from of thin conducting layer with cuts in it is usually called slot line resonator. We report on the design, fabrication and initial testing of multiple split rings slot line resonator made out of thin molybdenum plate.   

Normal state above the upper critical feld in Fe1$+$yTe1-x(Se,S)x.

We have investigated characteristics of the normal state above the upper critical field (Hc2) in Fe1.14Te0.7Se0.3, Fe1.02Te0.61Se0.39, Fe1.05Te0.89Se0.11, and Fe1.06Te0.86S0.14. Superconductivity is suppressed in high magnetic fields above 60 Tesla, allowing for the insight into normal state below the superconducting transition temperature (Tc). We show that Fe1.14Te0.7Se0.3 and Fe1.02Te0.61Se0.39 resistivity above the Hc2 is metallic as T$\to $0, just like the normal state resistivity above Tc. On the other hand, Fe1.05Te0.89Se0.11 and Fe1.06Te0.86S0.14 normal state resistivity is nonmetallic as T$\to $0, reflecting the normal state resistivity above Tc. These results suggest that conductivity of normal states above Hc2 is connected with the details of crystal structure inhomogeneity.   

Observation of Superconducting Fluctuations above Tc in underdoped BaFe2-xNixAs2

Angular dependent torque measurements have been performed on the electron doped iron pnictide superconductors BaFe$_{2-x}$Ni$_x$As$_2$ with a series Ni doping (0.03 $\leq$ \emph{x} $\leq$ 0.3). In the superconducting state, an irreversibility, as the evidence for the pinning of vortex, is observed between the torque measured with increasing and decreasing angle. Our results in underdoped sample (\emph{x} = 0.065) show that the irreversible torque signal can survive up to a temperature $T_{irr}$ well above the superconducting transition temperature $T_c$, suggesting the existence of superconducting fluctuations (SCF) above $T_c$. The Ni doping dependent phase diagram both for $T_{irr}$ and $T_c$ with a strong SCF region in the underdoped samples, is summarized based on our results.   

Persistence of Dirac Node near Antiferromagnetic-to-Superconducting Phase Boundary in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Since the ground state of iron-pnictides changes from an antiferromagnetic (AF) phase to a superconducting (SC) phase, the evolution of electronic structure has attracted much attention. However, systematic investigation has been hindered by the intricate multiple bands arising from the orbital degree of freedom of iron $3d$ states. Here we performed a polarization-dependent ARPES study of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ across the AF-SC phase boundary. The doping-dependence of ARPES spectra has shown that the Dirac node reported in the AF phase of BaFe$_2$As$_2$ persists in $x=0.04$ near the AF-SC phase boundary, and that it disappears in the SC phase of $x=0.05$. We parametrized the cone-like dispersion in $x=0.04$. The polarization-dependence of our ARPES spectra is consistent with the view that the Dirac node is protected by Berry phase arising from orbital degree of freedom under the inversion symmetry.   

Unconventional superconductivity in CaFe$_{0.85}$Co$_{0.15}$AsF evidenced by torque measurements

Out-of-plane angular dependent torque measurements were performed on CaFe$_{0.85}$Co$_{0.15}$AsF single crystals. Abnormal superconducting fluctuation, featured by enhanced diamagnetism with magnetic field, is detected up to about 1.5 times superconducting transition temperature $T_c$. Compared to cuprate superconductors, the fluctuation effect in iron-based superconductor is less pronounced. Anisotropy parameter $\gamma$ is obtained from the mixed state torque data and it is found that $\gamma$ shows both magnetic field and temperature depenence, pointing to multiband superconductivity. The temperature dependence of penetration depth $\lambda(T)$ suggests unconventional superconductivity in CaFe$_{0.85}$Co$_{0.15}$AsF.   

High-$T$c Superconductivity and Raman Scattering Study of the phonon properties of electron doped (transition metal, rare-earth) - Oxygen-Free CaFeAsF and compared with RFeAsO system.

Quaternary CaFeAsF has ZrCuSiAs-type structure,(RO)$^{\mathrm{\delta +}}$ layer in RFeAsO replaced by (CaF)$^{\mathrm{\delta +}}$ layer,with tetragonal ($P$4/\textit{nmm})-orthorhombic (\textit{Cmma}) phase transition at 134K,while magnetic order,SDW sets in at 114K. Partial replacement of Fe with Co/Ni is direct electron doping to (FeAs)$^{\mathrm{\delta +}}$ layer.Tc \textasciitilde 15K in CaFe$_{\mathrm{0.9}}$Ni$_{\mathrm{0.1}}$AsF.Substitution of rare earth metal for alkaline earth metal suppresses anomaly in resistivity {\&} induces superconductivity.Tc \textasciitilde 52K in Ca$_{\mathrm{0.5}}$Pr$_{\mathrm{0.5}}$FeAsF.Characterized by resistivity, susceptibility,XRD {\&} EDX-SEM.Upper critical field estimated from magneto resistance.Bulk superconductivity proved by DC magnetization. Hall coefficient R$_{\mathrm{H}}$ revealed hole-like charge carriers in parent compound CaFeAsF, while electron-type (R$_{\mathrm{H}}$ in normal state is --Ve) for Ca$_{\mathrm{0.5}}$Pr$_{\mathrm{0.5}}$FeAsF.Evolution of Raman active phonons of Ca$_{\mathrm{1-x}}$Pr$_{\mathrm{x}}$FeAsF measured with polarized Raman spectroscopy at room temperature from \textit{ab }surfaces of impurity-free microcrystals.Spectra exhibit sharp phonon lines on very weak electronic scattering background.Frequency and symmetry of Raman phonons involving out-of-plane atomic vibrations are found at 162.5 cm$^{\mathrm{-1}}$ ($A$1$g$, Pr), 201 cm$^{\mathrm{-1}}$ ($A$1$g$, As), 215.5 cm$^{\mathrm{-1}}$ ($B$1$g$, Fe), 265 cm$^{\mathrm{-1}}$ (\textit{Eg}, Fe) and 334 cm$^{\mathrm{-1}}$ ($B$1$g$, F) for Ca$_{\mathrm{0.5}}$Pr$_{\mathrm{0.5}}$FeAsF.Observations are compared with RFeAsO unconventional superconductors also possibly related to magnetic fluctuations   

Functional renormalization group study of the pairing symmetry and pairing mechanism in iron-selenide superconductors

In iron-selenide superconductors only electron-like Fermi pockets survive, challenging the $s_{\pm }$ pairing based on the quasi-nesting between the electron and hole pockets(as in iron arsenides). By functional renormalization group study we show that an in-phase $s$-wave pairing on the electron pockets is realized. The pairing mechanism involve two competing driving forces: the strong C-type spin fluctuations cause attractive pair scattering between and within electron pockets via Cooperon excitations on the virtual hole-like pockets, while the G-type spin fluctuations cause repulsive pairing scattering. The latter effect is however weakened by the hybridization splitting of the electron-like pockets. The in phase $s$-wave pairing symmetry is consistent with the existing experiments.   

Electronic structure of the titanium-based oxypnictide superconductor Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O and direct observation of its charge density wave order

The unconventional superconducting ground state usually emerges in proximity to a spin or charge ordering state, such as that in cuprates, iron-based superconductors and layered chalcogenides. This unique character offers a platform for searching unconventional superconductivity in analogous layered compounds. Recently, superconductivity has been achieved in Ba$_{\mathrm{1x}}$Na$_{\mathrm{x}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O with maximum Tc at 5.5 K, which makes this material more interesting. Here we perform high resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy studies on the titanium-based oxypnictide superconductor Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O. The electronic structure shows both multi-orbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surface is well nested along the ($\pi $,$\pi )$ direction, which might probably be the driving force of the CDW transition. This is further proved by the scanning tunneling microscopy result, which directly observed a CDW wave vector at ($\pi $,$\pi )$ direction. However, due to the weak CDW coupling, we didn't observe the CDW gap here. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in Ba$_{\mathrm{0.95}}$Na$_{\mathrm{0.05}}$Ti$_{\mathrm{2}}$Sb$_{\mathrm{2}}$O   

Sustainable Materials for the Anthropocene

Civilization on our planet took a sharp turn about 250 years ago, at the beginning of the industrial revolution. Arguably, its impact on humankind is equivalent to that of the invention of fire. The enormous consequences of industrial activity, positive and negative, could not have been anticipated, but we live with them today: per capita global consumption of energy is higher than ever, and demand for materials (relative to the year 1900) has increased by factors of 3 to 6000, depending on the element. Total population, as well as those segments of the population doing the consuming, is also increasing. Now we speak (informally, thus far) of the Anthropocene, the first geological epoch in which human activity is deemed to have had an effect on the Earth's ecosystem. For how much longer can economic growth and demand for goods be sustained? Can the human ingenuity that started the industrial revolution mitigate its effects? In this talk I will address the meaning and definition of sustainable development, and its intersection with materials science. Every human endeavor should be informed by sustainable development, because none of our material resources are in?nite and only a few sources of energy are sustainable. The immediate and direct connections between sustainable development and materials science include ef?cient use of materials, materials life cycle assessment, replacement materials, and energy-related materials. Sustainable development is an enormous field, and we are learning there is nothing we do as humans to ensure our survival (food, water, materials, shelter, economy, health) that lies outside of its boundaries.   

Dynamic Stimulation of Superconductivity with Resonant Terahertz Phonons

Can superconductivity be induced at a temperature far above the equilibrium critical temperature Tc? While small enhancements were observed many years ago and associated with nonequilibrium electron distributions, it is proposed here that much larger enhancements (up to a factor of two in temperature) may be possible by generation of coherent phonons at appropriate resonant frequencies comparable to the gap frequency, in the terahertz range. These phonon standing waves may induce real-space electron localization that forms the basis for superconducting coherence within a novel model of superconductivity [1,2]. This concept may be generalized to dynamic stimulation using coherent spin waves for non-phonon mediated superconductors such as the cuprates. Several experiments to test this are proposed. [1] S.B. Kaplan and A.M. Kadin (2012), ``Superconductivity via Two-Phase Condensation of Localized Electrons'', \underline {http://absimage.aps.org/image/MAR12/MWS\textunderscore MAR12-2011-002491.pdf} [2] A.M. Kadin (2009), ``Superconductivity without Pairing?,'' \underline {http://arxiv.org/abs/0909.2901} .   

Break the electron- hole balance and pressure induced superconductivity in Tungsten Ditelluride.

Tungsten ditelluride has garnered immense interest due to the recent discovery of titanic unsaturated magnetoresistance up to 60 Tesla and its possible topological metal nature. The titanic unsaturated magnetoresistance is attributed to the perfect compensation between the opposite carriers in this material. Motivated by the small and sensitive Fermi surface of 5d electronic orbitals, we break the electron-hole balance by the application of high pressure. Superconductivity sharply appears at the pressure of 2.5 GPa, quickly reaching a maximum critical temperature of 7 K at around 16.8 GPa, and followed by a monotonic decrease in Tc with increasing pressure exhibiting the typical dome-shaped superconducting phase. What's more, linear magnetoresistance dominates the transport behavior under high pressure instead of semi-classical parabolic magnetoresistance, like in other topological metals. Refence: Nature Commun. 6, 7805 (2015), arXiv 1505, 07968.   

Photoemission spectra of charge density wave states in cuprates

Angle-resolved photoemission spectroscopy(ARPES) experiments have reported many exotic properties of cuprates, such as Fermi arc at normal state, two gaps at superconducting state and particle-hole asymmetry at the antinodal direction[1]. On the other hand, a number of inhomogeneous states or so-called charge density waves(CDW) states have also been discovered in cuprates by many experimental groups. The relation between these CDW states and ARPES spectra is unclear. With the help of Gutzwiller projected mean-field theory[2], we can reproduce the quasiparticle spectra in momentum space. The spectra show strong correspondence to the experimental data with afore-mentioned exotic features in it.$\backslash $pard1. I. Vishik et al, PNAS 109, 18332-18337(2012).. Wei-Lin Tu and Ting-Kuo Lee, arXiv: 1505.07728(2015).   

Quantum creep in a highly crystalline two-dimensional superconductor

Conventional studies on quantum phase transitions, especially on superconductor-insulator or superconductor-metal-insulator transitions have been performed in deposited metallic thin films such as Bismuth or MoGe. Although the techniques of thin films deposition have been considerably improved, unintentional disorder such as impurities and deficiencies, generating the pinning centers, seems to still exist in such systems. The mechanical exfoliated highly crystalline two-dimensional material can be a good candidate to realize a less-disordered 2D superconductor with extremely weak pinning, combined with transfer method or ionic-liquid gating. We report on the quantum metal, namely, magnetic-field-induced metallic state observed in an ion-gated two-dimensional superconductor based on an ultra-highly crystalline layered band insulator, ZrNCl [1]. We found that the superconducting state is extremely fragile against external magnetic fields; that is, zero resistance state immediately disappears, once an external magnetic field switches on. This is because the present system is relatively clean and the pinning potential is extremely weak, which cause quantum tunneling and flux flow of vortices, resulting in metallic ground state. [1] Y. Saito et al. Science 350, 409-413 (2015).   

Negative Oxygen Isotope Effect on the Static Spin Stripe Order in La$_{2-x}$Ba$_{x}$CuO$_{4}$ ($x$ = 1/8)

Cuprate high temperature superconductors (HTS's) are characterized by a complex interplay between lattice, charge, and spin degrees of freedom. One of the remarkable phases is a self-organized charge/spin structure, which is known as ''stripes'' and is observed in some cuprates near 1/8 doping. The microscopic origin of the stripe phase is still unclear at present. We report large negative oxygen-isotope ($^{16}$O/$^{18}$O) effects (OIE's) on the static spin-stripe ordering temperature $T_{\rm so}$ and the magnetic volume fraction $V_{\rm m}$ in La$_{2-x}$Ba$_{x}$CuO$_{4}$ ($x$ = 1/8) observed by means of muon spin rotation experiments [1]. The corresponding OIE exponents were found to be $\alpha_{T_{\rm so}}$ = -0.57(6) and $\alpha_{V_{\rm m}}$ = -0.71(9), which are sign reversed to $\alpha_{T_{\rm c}}$ = 0.46(6) measured for the superconducting transition temperature $T_{\rm c}$. This indicates that the electron-lattice interaction is involved in the stripe formation and plays an important role in the competition between bulk superconductivity and static stripe order in the cuprates. [1] Z. Guguchia et. al., Phys. Rev. Lett. 113, 057002 (2014).   

Calculation of the superconducting transition temperature of a graphene layer doped with titanium and palladium.

We explore the structural interactions between graphene and transition metals such as palladium (Pd) and titanium (Ti) and the possibility of inducing superconductivity in a graphene sheet in two cases, one by doping its surface with palladium atoms sit on the center of the hexagons of the graphene layer and other by covering the graphene layer with two layers of titanium metal atoms. The results here were obtained from first-principles density functional theory in the local density approximation. The Quantum-Espresso package was used with norm conserving pseudopotentials. All of the structures considered were relaxed to their minimum energy configuration. Phonon frequencies were calculated using the linear-response technique on several phonon wave-vector mesh. The electron-phonon coupling parameter was calculated with several electron momentum k-mesh. The superconducting critical temperature was estimated using the Allen-Dynes formula with $\mu $* $=$ 0.1 - 0.15. We note that palladium and titanium are good candidate materials to show a metal-to-superconductor transition. We thank Direcci\'{o}n General de Asuntos del Personal Acad\'{e}mico de la Universidad Nacional Aut\'{o}noma de M\'{e}xico, partial financial support by Grant IN-106514 and we also thank Miztli Super-Computing center the technical assistance.   

Formation of As-As bond and its effect on absence of superconductivity in the collapsed tetragonal phase of Ca0$.$86Pr0$.$14Fe2As2: An optical spectroscopy study

The temperature dependence of in-plane optical conductivity has been investigated for Ca$_{.}_{86}$Pr$_{0.14}$Fe$_{2}$As$_{2}$, which shows a structural transition from tetragonal (T) to collapsed tetragonal (cT) phase at $T_{cT} \sim $ 73 K. Upon entering the cT phase, drastic change characterized by the formation of a midinfrared peak near 3200 cm$^{-1\, }$(0.4 eV) in the optical conductivity is observed. Analysis of the spectral weight reveals reduced electron correlation after the cT phase transition. Based on the calculated band structure and simulated optical conductivity, we attribute the new feature around 0.4 eV to the formation of an interlayer As-As bond. The As-As bond strongly affects the Fe-As hybridizations and, in turn, drastically changes the Ca$_{.}_{86}$Pr$_{0.14}$Fe$_{2}$As$_{2}$ into a nonmagnetic Fermi liquid system without bulk superconductivity in the cT phase.$_{\mathbf{\, \, }}$   

Structural $\gamma$-$\varepsilon$ phase transition in Fe-Mn alloys: a CPA+DMFT study

We study the $\gamma$-$\varepsilon$ structural transition in paramagnetic Fe-Mn alloys for Mn content from 10 to 20 at.\% using CPA+DMFT method. This method employs the coherent potential approximation (CPA) combined with the dynamical mean-field theory (DMFT). The material-specific Hamiltonians in the Wannier function basis are obtained by density functional theory. The electronic correlations are found to play a crucial role in this transition. The calculated transition temperature decreases with increasing Mn content and is in a good agreement with experiment. We demonstrate that in contrast to the $\alpha$-$\gamma$ transition in pure iron, the $\gamma$-$\varepsilon$ transition in Fe-Mn alloys is driven by a combination of kinetic and Coulomb energies. The latter is found to be responsible for the decrease of the $\gamma$-$\varepsilon$ transition temperature with Mn content.   

An ab initio study of the structure and dynamics of bulk liquid Ag and its liquid-vapor interface.

Several static and dynamic properties of bulk liquid Ag at a thermodynamic state near its triple point have been calculated by means of \textsl{ab initio} molecular dynamics simulations. The calculated static structure shows a very good agreement with the available experimental data. The dynamical structure reveals collective density excitations with an associated dispersion relation which points to a small positive dispersion. Results are also reported at a slightly higher temperature in order to study the structure of the free liquid surface. The ionic density profile shows an oscillatory behaviour with two different wavelenghts, as the spacing between the outer and first inner layer is different from that between the other inner layers.   

Optical pump/x-ray probe studies of lattice parameter evolution in Pt nanoparticles due to sintering and grain growth

An \textit{in-situ }optical pump/x-ray probe technique has been developed to study the evolution in the lattice parameter of nanometer-sized Pt particles as a function of the particle size during sintering and grain growth. In particular, the lattice parameter of the as-prepared nanoparticles was observed to be about 0.4{\%} smaller than the corresponding bulk value in good agreement with the value expected for isolated spherical particles subject only to a simple surface stress. As the as-prepared nanoparticles sinter and grow as a result of the photo-thermal irradiation, however, the evolution in the lattice parameter reflects the effects of both an increasing grain size and the evolving particle-particle interface. As a result, the lattice parameter does not evolve monotonically with increasing grain size.   

The Magnetism of Mn$_{\mathrm{2-x}}$Fe$_{\mathrm{x}}$B Alloys : First-Principles Calculations

Magnetic, electronic and structural properties of Mn$_{\mathrm{2-x}}$Fe$_{\mathrm{x}}$B (0 $\le $ x $\le $ 2) are investigated by the First principles calculations with the virtual crystal approximation (VCA) based on the density-functional theory (DFT) with generalized gradient approximation (GGA). Although both of Mn$_{\mathrm{2}}$B and Fe$_{\mathrm{2}}$B have the same electronic structure of I4/MCM with different lattice constants, the former is anti-ferromagnetic and the latter ferromagnetic. All calculations, based on the four types of magnetic states of NM (non-magnetic), FM (ferromagnetic), AFM1 and AFM2 (anti-ferromagnetic), illustrate that there is a critical point of magnetic phase transition occurred at x as 0.5 from AFM2 to FM state under the condition of the lowest energy. Results are also in agreement with the Stoner model within the range of FM state.   

Magnetic Nanoparticles in Non-magnetic CNTs and Graphene

Magnetic nanoparticles were embedded in non-magnetic CNTs and graphene matrix to incorporate all the advantages and the unique properties of CNTs and graphene [1]. Composites of CNTs and graphene with magnetic nanoparticles may offer new opportunities for a wide variety of potential applications such as magnetic data storage, magnetic force microscopy tip, electromagnetic interference shields, thermally conductive films, reinforced polymer composites, transparent electrodes for displays, solar cells, gas sensors, magnetic nanofluids, and magnetically guided drug delivery systems. Magnetic nanoparticles coated CNTs can also be used as an electrode in lithium ion battery to replace graphite because of the higher theoretical capacity. Graphene nanocomposites, coated with magnetic sensitive nanoparticles, have demonstrated enhanced magnetic property. \textbf{1. D. Seifu, }S. Neupane, L. Giri, S. P. Karna, H. Hong,~and M. S. Seehra, ``Multilayered graphene acquires ferromagnetism in proximity with magnetite particles'', Appl. Phys. Lett., \textbf{106}, 212401, (2015).   

Ferromagnetic Fe$_{\mathrm{2}}$CrAl Nanowires

Heusler alloy Fe$_{\mathrm{2}}$CrAl (FCA) nanowires were grown on silicon substrates. Nanowires have diameters in the range 50 to 200 nm and lengths up to 100 \textmu m. They exhibit cubic L$_{\mathrm{21}}$ and A$_{\mathrm{2}}$ type structure with a space group, Pm m. Magnetic characterization reveals that they display ferromagnetic behavior and has a Curie temperature above 400 K. Magnetic behavior of FCA nanowires is different from the reported bulk behavior. Bulk FCA with L$_{\mathrm{21}}$ structure has a Curie temperature around 274 K.   

Magnetic Properties of 3D Printed Toroids

Transformers are ubiquitous in electronics today. Although toroidal geometries perform most efficiently, transformers are traditionally made with rectangular cross-sections due to the lower manufacturing costs. Additive manufacturing techniques (3D printing) can easily~achieve toroidal geometries by building up a part through a series of 2D layers. To get strong magnetic properties in a 3D printed transformer, a composite filament is used containing Fe dispersed in a polymer matrix. How the resulting 3D printed toroid responds to a magnetic field depends on two structural factors of the printed 2D layers: fill factor (planar density) and fill pattern. In this work, we investigate how the fill factor and fill pattern affect the magnetic properties of 3D printed toroids. The magnetic properties of the printed toroids are measured by a custom circuit that produces a hysteresis loop for each toroid. Toroids with various fill factors and fill patterns are compared to determine how these two factors can affect the magnetic field the toroid can produce. These 3D printed toroids can be used for numerous applications in order to increase the efficiency of transformers by making it possible for manufacturers to make a toroidal geometry.   

Electro-magneto-thermal characterization of ferromagnetic thin films.

In this work we report electro-magneto-thermal characterization of ferromagnetic multilayer thin films. These thin films include Co/Pd, Co/Pt and CoFeB/MgO multilayers. We carried out in-situ focused magneto optic Kerr effect based hysteresis measurement while the specimen was under DC bias to ascertain the electro-magnetic behavior. These experiments are then supplemented with in-situ transmission electron microscope studies to verify the microstructural changes. We also report thermal conductivity measurements using 3-omega method. Thermal conductivity measurements suggest thermo-magnetic resistance due to spin scattering at the interfaces in multilayer thin films.   

Brillouin Light Scattering study of the rotatable magnetic anisotropy in exchange biased bilayers of Ni81Fe19Ir20Mn80

It is known that when a ferromagnet (FM) is in atomic contact with an antiferromagnet (AF) the exchange coupling between the FM and AF spins at the interface induces a unidirectional anisotropy in the ferromagnetic film. This effect is known as exchange bias (EB). Despite the large amount of research on this topic there are still several aspects of the EB mechanism that are not well understood. One of this aspects is the origin of the rotatable anisotropy in polycrystalline AFs. By means of Brillouin Light Scattering (BLS) measurements, we investigated the dependence of the rotatable anisotropy field H$_{\mathrm{RA}}$ and exchange field H$_{\mathrm{E}}$ with the magnitude of the external magnetic field (H$_{\mathrm{o}})$ in FM/AM bilayers of Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$(10nm)/Ir$_{\mathrm{20}}$Mn$_{\mathrm{80}}$(t$_{\mathrm{AF}})$. We developed an algorithm to numerically fit the in-plane angular dependence of the magnon frequency, at a fixed value of H$_{\mathrm{o}}$ measured by BLS. From the fit parameters we were able to investigate H$_{\mathrm{RA}}$ and H$_{\mathrm{E}}$ dependency on H$_{\mathrm{o}}$. The results reveal that H$_{\mathrm{RA}}$ value depends on H$_{\mathrm{o}}$, so we argue that AF grain distribution at the interface is partially modified by the applied field strength. Contrary to this, the relation between H$_{\mathrm{E}}$ and H$_{\mathrm{o}}$ is not straightforward, remaining constant at high values of H$_{\mathrm{o}}$.   

\textbf{Atomic structure prediction of Zr-Co and Hf-Co nanoclusters using the evolutionary algorithm}

Nanostructures of Hf-Co and Zr-Co rare earth free magnetic material that exhibit a high room-temperature energy product. In our study, the evolutionary algorithm coupled with density functional (DFT) method is used to identify the global energy minimum atomic structure of Zr-Co and Hf-Co clusters. Using evolutionary crystal structure optimization algorithm, as implemented in USPEX, we studied the atomic structure, binding energies, magnetic properties, and anisotropy of Zr$_{\mathrm{x}}$Co$_{\mathrm{y}}$ and Hf$_{\mathrm{x}}$Co$_{\mathrm{y}}$ (x$=$1,2 and y$=$5,7,11) clusters. A set of metastable and global minimum atomic structures are identified. Several new lower energy configurations were identified for Zr$_{\mathrm{2}}$Co$_{\mathrm{11}}$, Zr$_{\mathrm{1}}$Co$_{\mathrm{5}}$, Zr$_{\mathrm{1}}$Co$_{\mathrm{7}}$, Hf$_{\mathrm{2}}$Co$_{\mathrm{11}}$, Hf$_{\mathrm{1}}$Co$_{\mathrm{5}}$ and Hf$_{\mathrm{1}}$Co$_{\mathrm{7\thinspace }}$clusters by our calculations. We discussed the magnetic interaction between the atoms of the clusters which is critical in finding the lowest energy structure. Our calculation show that Zr-Co and Hf-Co have ferromagnetic coupling and large magnetization. We will also discuss the magnetocrystalline anisotropy (MAE) variation in these clusters.   

Magneto-electric control of magnetization in a chain of circular nanomagnets as new paradigm for ultra low power binary information propagation.

Elliptical nanomagnets with bi stable magnetization states are traditionally employed for dipole coupled Bennett clocked nanomagnetic logic.Logic bits are propagated down a chain of nanomagnets by sequentially rotating their magnetizations with an electric field [1].In this talk,we present for the first time,the notion of replacing elliptical nanomagnets with circular nanomagnets that have no inherent shape anisotropy.The circular nanomagnets would develop bi stable magnetization orientations with the application of an electrical field to induce in-plane strain anisotropy.This new strategy provides two significant advantages for nanomagnetic logic applications:(i)re-orienting the magnetizations does not require overcoming a shape-anisotropy energy barrier and hence the electric field needed to reorient is reduced,leading to lower energy dissipation in the clocking process,and(ii)scalability to dimensions substantially smaller than what presently exists becomes possible.[1]J.Atulasimha and S.Bandyopadhyay, Appl. Phys. Lett., 97, 173105 (2010).This work was supported by NSF CAREER grant CCF-1253370 and by FAME,one of six centers of STARnet,Semiconductor Research Corporation program sponsored by MARCO and DARPA.   

\textbf{The impact of substrate stimulated functional interface on magnetic and magneto-transport signature of martensitic transformation in NiMnIn shape memory alloy}

We study the impact of the substrate on the martensite transformation of Ni-Mn-In thin films by Hall resistance measurements and discuss it using density functional theory calculations. Similarly to the bulk systems, thin films grown on MgO exhibit the martensitic transformation accompanied by large magnetoresistance and a sign reversal of the ordinary as well as anomalous Hall coefficient. Martensite transition temperature of films grown on (100) surface of MgO is near 170K, while the films grown on (111) surface of MgO show the change of Hall coefficient at 110K. The calculated total energy difference between FM austenite and FiM martensite states in Ni$_{\mathrm{2}}$Mn$_{\mathrm{1.5}}$In$_{\mathrm{0.5}}$~film on MgO (001) substrate (with Ni/MgO interface) is 0.20eV per NiMnIn f.u, compared to 0.24eV in the bulk at the same equilibrium lattice parameters, i.e. when film is ``unstrained''. When lattice parameters of~Ni$_{\mathrm{2}}$Mn$_{\mathrm{1.5}}$In$_{\mathrm{0.5}}$/MgO~are of those of MgO substrate, i.e. when the film experiences strong bi-axial tensile strain $\Delta a/a=$2.4{\%}, the energy difference is 0.08eV per NiMnIn f.u. These results clearly indicate strong interplay between lattice strain/stress and the relative stability martensite and austenite phase   

\textbf{Ferromagnetic properties of manganese doped iron silicide.}

We report the synthesis of high quality Iron silicide (FeSi) nanowires via Chemical Vapor Deposition (CVD). The materials exhibits excellent magnetic response at room temperature, especially when doped with manganese showing values of 2.0 X 10-04 emu for the FexMnySi nanowires. SEM and TEM characterization indicates that the synthesized nanowires have a diameter of approximately 80nm. MFM measurements present a clear description of the magnetic domains when the nanowires are doped with manganese. Electron Diffraction and XRD measurements confirms that the nanowires are single crystal forming a simple cubic structure with space group P213. First-principle calculations were performed on (111) FeSi surface using the Vienna ab initio simulation package (VASP). The exchange correlations were treated under the Ceperley-Alder (CA) local density approximation (LDA). The Brillouin Zone was sampled with 8x8x1 k-point grid. A total magnetic moment of about 10 $\mu $B was obtained for three different surface configuration in which the Iron atom nearest to the surface present the higher magnetization. To study the effect of Mn doping, Fe atom was replaced for a Mn. Stronger magnetization is presented when the Mn atom is close to the surface. The exchange coupling constant have been evaluated calculating the energy difference between the ferromagnetic and anti-ferromagnetic configurations.   

Effect of Milling Time on the Blocking Temperature of Nanoparticles of Magnetocaloric Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$.

Extensive research has been done on giant magnetocaloric material Gd$_{\mathrm{5}}$(Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}})_{\mathrm{4}}$ to improve adiabatic temperature/isothermal entropy change. However, there have been only a few reports on fabrication of nanostructure/nanoparticles that can be used to tune various properties by changing the length scale. Recently we have reported fabrication of room temperature ferromagnetic nanoparticles of Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$ using high energy ball milling. These nanoparticles have potential applications in biomedical engineering such as better T$_{\mathrm{2}}$ MRI contrast agents and in hypothermia. Here we report the effect of milling time on the blocking temperature, micro-structure, crystal structure, and magnetic properties of these nanoparticles. Magnetization vs. temperature at an applied field of 100 Oe is measured for all the ball milled samples. Bulk Gd$_{\mathrm{5}}$Si$_{\mathrm{4\thinspace }}$has a transition temperature of $\approx $340 K. There are two phase transitions observed in the nanoparticles, one near 300 K corresponding to the Gd$_{\mathrm{5}}$Si$_{\mathrm{4}}$ phase and another between 75-150 K corresponding to Gd$_{\mathrm{5}}$Si$_{\mathrm{3}}$. Zero Field Cooling (ZFC) and Field Cooling (FC) were measured. The blocking temperatures for the nanoparticles increase with decrease in milling time.   

Magnetic and optical properties of Co-doped and Mn-doped ZnO nanocrystalline particles.

We carried out a systematic study on the effect of Co doping and Mn doping on the structural, magnetic and optical properties of ZnO nanocrystalline particles, using x-ray diffraction, x-ray photoelectron spectroscopy (XPS), Quantum Design PPMS-9 magnetometry, and Ultra Violet-Visible spectroscopy. The Zn$_{1-}_{x}$Co$_{x}$O and Zn$_{1-}_{x}$Mn$_{x}$O nanoparticles with $0\le x\le 0.1$were successfully prepared by the formal solid-state reaction method. The XPS results and the XRD analysis with full structural Rietveld refinement reveal that both structures have hexagonal wurtzite structure. For all Co-doped ZnO nanoparticles under investigation, the field dependence of the magnetization curves exhibits ferromagnetic behavior with relatively small coercive fields at room temperature. In addition, we found a signature for antiferromagnetic ordering between the Co ions. For the Mn-doped ZnO nanoparticles, we observed ferromagnetic behavior only below 50 K. We also observed a strong correlation between the magnetic and optical behavior of the Co-doped ZnO nanoparticles. Optical diffuse reflectance and absorption spectra exhibit a red shift at room temperature in the absorption band edge with increasing Co-doping. The red shift is attributed to the \textit{sp-d} exchange interaction between free charge carriers in ZnO band and the localized magnetic moments.   

Investigation of the Magnetic Properties of Ni-implanted ITO Thin Films

Commercially available ITO thin films on fused silica substrates were implanted with 40 keV Ni$^+$ ions to fluences of (0.5,1.0{\&}1.5) \times 10$^{17}$ ions/cm$^2$ at room temperature. XRR measurements show that the thickness of the implanted films ($\sim$ 28.5 nm) does not change noticeably with the fluence, while the surface roughness increases essentially. SEM and EDX studies revealed a highly non-uniform distribution of Ni atoms. Room temperature ferromagnetism was observed in the samples with fluences of (1.0{\&}1.5)\times 10$^{17}$ ions/cm$^2$. VSM hysteresis curves and FMR signal point to the formation of a ferromagnetic near-surface layer in the implanted films due to agglomeration of closely-spaced metal Ni nanoparticles. The filling factor of the Ni ferromagnetic phase in the granular magnetic layer was estimated from the FMR results. Super- and para- magnetic phases were observed in the temperature dependence of magnetization by VSM. Superparamagnetic phase is attributed to the Ni nanoparticles located in deeper regions near the film/substrate interface, while paramagnetic phase is related to the impurity centers. For the samples with fluences of (1.0{\&}1.5) \times 10$^{17}$ ions/cm$^2$ average sizes of the superparamagnetic nanoparticles were calculated from the blocking temperatures $T_B$ observed in thermo-magnetic dependences.   

Magnetization reversal in the orthochromite Y0.5Gd0.5CrO3.

Complex oxide of transition metal with perovskite structure represent fascinating playground for basic solid state research: new electronics and exotic ground states emerge via the competing interplay like spin, orbital, charge as well as lattice degree of freedom. Accordingly, orthochromites are not exception to the rule. In these compounds have been found ferroelectric polarization, spin reorientation transition along with the characteristic behavior known as; magnetization reversal (MR) consisting that a characteristic temperature, T*, the system becomes diamagnetic. In this work, the magnetic behavior of the equimolar Y0.5Gd0.5CrO3 composition was studied. Negative magnetization was observed at T*\textasciitilde 70 K in FC mode, and applied field of 100 Oe. The characteristic hysteresis loop in the M-H graph of the pristine sample disappears for a wide range of temperature below of T$_{\mathrm{N}}$, and the characteristic spin reorientation is shifted from 14 K in GdCrO3 to 5 K for Y0.5Gd0.5CrO3. The negative magnetization is explained according the model that take into account the anisotropic and antisymmetric exchange interaction between Gd$+$3 -- Cr$+$3 sublattice.   

Investigation of Low Temperature Non-Linear Magnetization Behavior in Al and Ga$-$ Substituted La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$Mno$_{\mathrm{3}}$ Manganites.

Low temperature magnetization measurements have been carried out for the samples containing Al and Ga at B-site in La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$MnO$_{\mathrm{3}}$ manganites. The magnetization (M) vs. T(K) data shows strong ferromagnetic behavior with highest magnetization of 6.45 emu/g for La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$Mn$_{\mathrm{0.95}}$Al$_{\mathrm{0.05}}$O$_{\mathrm{3}}$ and 5.40 emu/g for La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$Mn$_{\mathrm{0.90}}$Al$_{\mathrm{0.1}}$O$_{\mathrm{3}}$ samples respectively for an applied magnetic field of H$=$100 Oe at T$=$20 K. Similarly at T$=$20 K for La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$Mn$_{\mathrm{0.95}}$Ga$_{\mathrm{0.05}}$O$_{\mathrm{3}}$ the highest magnetization (M$_{\mathrm{S}})$ was found to be 5.44 emu/g and for La$_{\mathrm{0.4}}$Bi$_{\mathrm{0.6}}$Mn$_{\mathrm{0.90}}$Ga$_{\mathrm{0.1}}$O$_{\mathrm{3}}$ the M$_{\mathrm{S}}$ is 5.05 emu/g. The decrease in magnetization with both Al and Ga substitution produces magnetic dilution with increasing concentrations. Both Al and Ga substituted samples exhibit non-linear behavior in their magnetization (M$_{\mathrm{NL}})$ curves around 40$-$120 K due to the frustrations arising from mismatch in their magnetic spin arrangements. The quantity non linear susceptibility, $\chi _{\mathrm{NL}}= \quad -$M$_{\mathrm{NL}}$/H, diverges as the temperature approaches the frustrated region T$_{\mathrm{f}}$ from above (i.e.T$_{\mathrm{C}})$. Further from d$\chi_{\mathrm{NL}}$/dT vs. T(K) plots and critical analysis with unusual critical exponent's $\gamma $ and $\beta $ gives an experimental evidence for the observed non linearity and magnetic frustration.   

Tunnel Magneto Resistance of Fe/Insulator/Fe }

Tri-layer thin films of Fe/Insulator/Fe were synthesized using magnetron DC/ RF sputtering with MgO insulator and Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ topological insulators as middle buffer layer. The multi-layered samples thus produced were studied using in-house built magneto-optic Kerr effect (MOKE) instrument, vibrating sample magnetometer (VSM), torque magnetometer (TMM), AFM, MFM, and magneto-resistance (MR). This system, that is Fe/Insulator/Fe on MgO(100) substrate, is a well-known tunnel magneto resistance (TMR) structure often used in magnetic tunnel junction (MTJ) devices. TMR effect is a method by which MTJs are used in developing magneto-resistive random access memory (MRAM), magnetic sensors, and novel logic devices. The main purpose behind this research is to measure the magnetic anisotropy of Fe/Insulator /Fe structure and correlate it to magneto-resistance [1]. In this presentation, we will present results from MOKE, VSM, TMM, AFM, MFM, and MR studies of Fe/Insulator/Fe on MgO(100). [1] A. Newman, S. Khatiwada,, S. Neupane, \textbf{D. Seifu}, "Nano Wires of Fe/MWCNTs and Nanometric Thin Films of Fe/MgO", J. of Appl. Phys., \textbf{117}, 144302 (2015).   

\textbf{Integration of Multifunctional Epitaxial Oxide Heterostructures with Si(001)}

Multifunctional heterostructures exhibit a wide range of functional properties, including colossal magneto-resistance, multiferroic behavior, and spin, charge, and orbital ordering. However, putting this functionality to work remains a challenge. To date, most of the previous works reported in the literature have dealt with heterostructures deposited on closely lattice matched (using lattice matching epitaxy-LME) insulating substrates such as DyScO$_{\mathrm{3}}$, NdGaO$_{\mathrm{3}}$, MgO, SrTiO$_{\mathrm{3}}$ and MBE-grown STO buffered Si(100). This presentation discusses the major advances in the integration of multifunctional oxide materials onto ubiquitous silicon semiconductor platform reported$^{\mathrm{1-6}}$ in the recent past by the presenting authors using a novel thin film growth approach, called `domain matching epitaxy'(DME), which minimizes the strain and nucleation of unwanted defects. The DME paradigm has been used across the large misfit scale (7-25{\%}).~Of particular interest, thin film heterostructures including two-phase multiferroics such as BiFeO$_{\mathrm{3}}$(BFO)/La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO), BaTiO$_{\mathrm{3}}$(BTO)/LSMO, and LSMO/SrRuO$_{\mathrm{3}}$(SRO). These significant materials advancements may herald a flurry of exciting new advances in CMOS-compatible multifunctional devices.$^{\mathrm{\mathbf{1}}}$\underline {\textbf{S. S. Rao}}\textbf{, }\textit{et al.}\textbf{, }Nano Letters \textbf{13}, 5814 (2013); J. Appl. Phys., \textbf{116}, 094103 (2014); J. Appl. Phys., \textbf{116}, 224104 (2014); J. Appl. Phys., \textbf{117}, 17D908 (2015); $^{\mathrm{5}}$J. Appl. Phys., \textbf{117}, 17B711 (2015); $^{\mathrm{\mathbf{6}}}$Current Opinion in Solid State and Materials Science. \textbf{19,} 301-304 (2015).   

Untangling the contributions of cerium- and iron- sublattices to the magnetism of Ce-doped yttrium iron garnet.

The remarkable magnetic properties of yttrium iron garnets (YIGs) underpin the use of these materials in a broad scope of spintronic and photonic applications. In particular, the addition of rare-earth metals in the structure enhances to a great extent the magneto-optical activity, which is beneficial for the development of nonreciprocal devices for communication along optical fibers. Yet, the physical mechanisms that lead to the observed enhanced gyrotropic response of doped YIG are not fully unveiled. Here we present a methodology based on magneto-optical spectroscopy that may be instrumental to better understand the optical response of these materials. In particular, we have exploited the wavelength selectivity of magneto-optics to identify a range of frequencies at which one can unravel the individual contributions to the magnetism and gyrotropic response arising from the individual cerium and iron sublattices. The approach outlined here paves the way to assess quantitatively the effect on the optical properties of rare-earth incorporation into YIG, providing an instrumental methodology towards tailoring the functional properties of YIG.   

Growth of EuO films on Si using Pulsed Laser Deposition

Epitaxial monolayers of europium monoxide (EuO) deposited on silicon (Si) wafers are suited for spintronic applications such as adding spin filter tunneling and spin current to Si technology, and for probing phenomena like Anomalous Hall effect and Topological Hall effect. However, the innate chemical reactivity of europium (Eu) and Si prevents a direct synthesis of EuO by pulsed laser deposition technique, without significant contamination of the EuO/Si interface and degradation of the EuO thin film. Silicon oxides (SiO$_{2-\delta}$) on the surface of Si substrates, partial pressure of oxygen (O$_2$) gas and water vapors in the vacuum chamber act as contaminants. Techniques like standard wet etching process, thermal annealing, and decomposition of SiO$_{2-\delta}$ by the bombardment of metal ions, and their effectiveness is studied using the X-Ray diffraction (XRD) system. Our goal is one-process in situ integration of spin-functional magnetic oxides seamless on Si wafers. Also the mechanism for the ferromagnetic order in oxygen-deficient europium monoxide (EuO$_{1-x}$) at temperatures higher than 69K (the Curie temperature of stoichiometric EuO) remains controversial. We have investigated the magnetization of EuO$_{1-x}$ thin films prepared via PLD as a function of (emu) vs (K)   

Room temperature ferromagnetism of Cr-doped In$_{\mathrm{2}}$O$_{\mathrm{3}}$ bi-layer consisted of a triangular crystal-amorphous interface

In$_{\mathrm{2}}$O$_{\mathrm{3}}$ film is a very conductive and can be modified to exhibit room temperature ferromagnetism upon doping of Cr. In this study, we developed a method, based on the RF power, to control the Cr-doped In$_{\mathrm{2}}$O$_{\mathrm{3}}$ (CIO) thin-films to form a crystalline phase, at a high power region, or an amorphous phase, at a low power region. When the RF power is set at a medium power, the CIO film self-assemble into a two layers system consisted of crystalline and amorphous layers with interface manifests zig-zag feature. The two layer system has a saturation magnetization M$_{\mathrm{s}}$, of \textasciitilde 0.27 to \textasciitilde 1.78 emu/c.c. with increase of Cr-doping content. In contrast, the M$_{\mathrm{s}}$ of the amorphous films are \textasciitilde 0.45 emu/c.c independent of Cr content. Electron energy loss spectroscopy (EELS) measurements suggested that Cr existed in mixed oxidation states in all films. The Cr with lower oxidation state prefers crystalline structure, while the higher oxidation state Cr prefers an amorphous structure. Due to this charge imbalance, a transport of charge across the interface originates the ferromagnetic interaction, and hence, we observe enhanced M$_{\mathrm{S}}$ in crystal-amorphous interface system.   

\textbf{Two phase multiferroics for voltage-induced entropy change with application in near-room-temperature refrigeration}

The demand for environmental friendly, cost-effective and energy efficient cooling drives the emerging technology of magnetic refrigeration at room temperature. We fabricate a two phase mutiferroic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-PbTiO$_{3}$(001) via pulsed laser deposition for application in advanced near room-temperature refrigeration and miniature cooling devices. The key innovation rests on utilizing the magnetocaloric effect in zero applied magnetic fields. The magnetocaloric effect of the composite is activated purely by electric field. We utilize strain originating from stress which is voltage-induced via the inverse piezoelectric effect of PMN-PT. The strain is carried over into the adjacent LSMO thin film thus changing its magnetic order. The voltage-induced variation in magnetization leads to change in isothermal entropy when the experiment is carried out in contact with a thermostat and gives correspondingly rise to an adiabatic temperature change when heat exchange is suppressed.   

Enhanced Magnetic Proximity Effect at Ferromagnetic Insulator / Magnetic Topological Insulator Interface

Magnetic proximity effect at magnetic insulator / topological insulator interface provides a promising approach to realize low-dissipation quantum devices. However, the commonly used magnetic insulators have in-plane anisotropy hence cannot magnetize topological insulator. Here we report an enhancement of proximity exchange coupling in ferromagnetic insulator / magnetic topological insulator EuS / Sb$_{\mathrm{2-x}}$V$_{\mathrm{x}}$Te$_{\mathrm{3}}$ hybrid heterostructure, where proximity effect is enhanced by a factor of 3 through the Vanadium doping. Moreover, an artificial antiferromagnetic-like structure is created between two strong ferromagnets, which may account for the proximity effect enhancement. The interplay between the proximity effect and doping in hybrid heterostructure provides insights into the engineering of magnetic ordering.   

Structural and magnetic properties of epitaxial FeMn$_{2}$O$_{4}$ film on MgO (100).

Fe$M_{2}X_{4}$ spinel structures, where $M$ is a transition metal and $X$ is oxygen or sulfur, are candidate materials for spin filters, one of the key devices in spintronics. On the other hand, the electronic and magnetic properties of these spinel structures could be modified via the control of cation distribution. Among the spinel oxides, iron manganese oxide is one of promising materials for applications. FeMn$_{2}$O$_{4}$ shows inverse spinel structure above 390 K and ferrimagnetic properties below the temperature. In this work, we report on the structural and magnetic properties of epitaxial FeMn$_{2}$O$_{4}$ thin film on MgO(100) substrate. The reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD) results indicated that films were epitaxially grown on MgO(100) without the impurity phases. The valance states of Fe and Mn in the FeMn$_{2}$O$_{4\, }$film were carried out using x-ray photoelectron spectrometer (XPS). The magnetic properties were measured by vibrating sample magnetometer (VSM), indicating that the samples are ferromagnetic at room temperature. The structural detail and origin of magnetic ordering in FeMn$_{2}$O$_{4}$ will be discussed.   

Study of Magnetic and Electric Properties in La2/3Sr1/3MnO3 Thin Film

La2/3Sr1/3MnO3 (LSMO) is the most interesting compound of the manganite perovskite family due to its Curie temperature above 300K that makes its remarkable properties desirable for practical applications. However, it is well known that ferromagnetic properties weaken when dimensions are reduced. We have grown LSMO thin films by sputtering DC in pure oxygen atmosphere on SrTiO3 (STO) and LaAlO3 (LAO) substrates at temperature of 830 \textdegree C. From x-ray diffraction analysis, we have found the Bragg peaks of LSMO thin films around (002) reflection, indicating a textured growth. We have characterized the morphology of the samples by atomic force microscopy. LSMO thin film was patterned using standard UV photolithography. Dependence of resistivity with temperature shows a behavior typical of ferromagnetic system with metal-insulator transition above 300 K. The electrical properties of the structured will be contrasted with thin film. We carried out isothermal resistance and magnetization versus applied magnetic field loops to characterize the samples. We study the dependence of magnetic transport properties with film thickness of 25nm and path size (5 micron) for potential applications like magnetic sensors.   

A Focker-Planck description of the spin Seebeck effect

Thermally driven spin-wave spin current in a ferromagnetic material FM and the resulting electric signal in a metal probe placed on the FM are theoretically investigated by considering a thermally fluctuating spin at the interface of a FM-metal junction. We develop an analytical formulation to establish a Focker Plank equation for the probability distribution as a function of magnetization components of the material, for calculating the spin Seebeck signal detected by the metal probe, which converts spin current to charge current by the inverse spin Hall effect. The spin current is induced in the metal probe via an exchange interaction when the metal senses the temperature gradient.   

Ferromagnetic thickness dependence of current-driven spin-orbit torques in different ferromagnetic and heavy metal bilayers

The spin-orbit torques in ferromagnetic (FM) and heavy metal (HM) bilayers have attracted extensive research interests recently because of the rich physical phenomena and potential applications. We measured the effective fields of field-like torques in Ni/Pt, NiFe/Pt and CoFeB/Pt bilayer systems by the second-order planar Hall effect. When the FM layers are less than 2nm, the effective fields increase rapidly with decreasing the FM layer thickness for all three different FM layers. Among the three FMs, the effective field in Ni is largest, followed by NiFe, then CoFeB. Above 2nm, the effective fields decrease much slower with increasing the FM layer thickness and level off to the Orested field due to the current in the Pt layer. Through FM layer thickness dependence of the field-like torque study, we found that the spin dephasing length in the FM layer, which is related to the scattering in FM layer, plays an important role in determining the magnitude of field-like spin-orbit torque in FM/HM bilayers.   

spin pumping occurred under nonlinear spin precession

Spin pumping occurs when a pure-spin current is injected into a normal metal thin layer by an adjacent ferromagnetic metal layer undergoing ferromagnetic resonance, which can be understood as the inverse effect of spin torque, and gives access to the physics of magnetization dynamics and damping. An interesting question is that whether spin pumping occurring under nonlinear spin dynamics would differ from linear case. It is known that nonlinear spin dynamics differ distinctly from linear response, a variety of amplitude dependent nonlinear effect would present. It has been found that for spin precession angle above a few degrees, nonlinear damping term would present and dominated the dynamic energy/spin-moment dissipation. Since spin pumping are closely related to the damping process, it is interesting to ask whether the nonlinear damping term could be involved in spin pumping process. We studied the spin pumping effect occurring under nonlinear spin precession. A device which is a Pt/YIG microstrip coupled with coplanar waveguide was used. High power excitation resulted in spin precession entering in a nonlinear regime. Foldover resonance lineshape and nonlinear damping have been observed. Based on those nonlinear effects, we determined the values of the precession cone angles, and the maximum cone angle can reach a values as high as 21.5 degrees. We found that even in nonlinear regime, spin pumping is still linear, which means the nonlinear damping and foldover would not affect spin pumping process.   

Spin Hall and spin Nernst effects: temperature dependence

We have considered temperature dependence of spin Hall and spin Nernst effect in two-dimensional electron gas with spin-orbit interaction of Rashba type [arXiv:1510.03080]. In our considerations we have employed the approach based on the Matsubara Green functions. The formalism used in the case of electric field as a driving force was subsequently adopted to the situation of a spin current driven by a temperature gradient. To achieve this, we have used the concept of an auxiliary vector field. Such a description gives the possibility to consider all mechanisms leading to the spin Hall and spin Nernst effect on equal footing and also their behavior at finite temperatures. Both spin Hall and spin Nernst conductivities were calculated in the approximation including the vertex correction. The total spin Hall conductivity, including vertex correction, has been shown to vanish exactly in the whole temperature range. Thus, our results extend the earlier ones to an arbitrary temperatures. In turn, the total spin Nernst conductivity remains finite when the vertex corrections are included. Using the Ioffe-Regel localization criterion, we have also estimated the range of parameters where the calculated results for the spin Hall and spin Nernst conductivities are applicable.   

Non-equilibrium spin polarization and spin-orbit torque induced by electric field and temperature gradient in a magnetized two-dimensional electron gas with Rashba spin-orbit interaction

We have considered theoretically temperature dependence of non-equilibrium spin polarization of electrons that appears in a magnetized two-dimensional electron gas with Rashba spin-orbit interaction due to external electric field and/or temperature gradient. To do this we have employed the approach based on the Matsubara Green function formalism. We analyzed in detail variation of the induced spin polarization with position of the Fermi level, temperature, and Rashba coupling constant. Moreover, we analyzed the temperature dependence of the electrically and thermally induced spin polarization in the temperature regime, where the spin relaxation time can be assumed constant (independent of temperature). In contrast to the case of nonmagnetic Rashba gas, all three components of the induced spin polarization are now nonzero. The induced spin-polarization is exchange-coupled to the local equilibrium magnetization and therefore exerts a torque on the magnetization vector. We have considered in detail the temperature behavior of spin-orbit torque induced by electric field and by temperature gradient for specific relative orientation of the magnetization and electric field or temperature gradient, respectively.   

Cross-tunneling and phonon bottleneck effects in the relaxation pheomena of the XY pyrochlore antiferromagnet Er$_{\mathbf{2}}$\textbf{Ti}$_{\mathbf{2}}$\textbf{O}$_{\mathbf{7}}$\textbf{.}

Multiple time-scale relaxation dynamics are revealed by alternating-current (ac) susceptibility measurements of a single crystal of Er$_{2}$Ti$_{2}$O$_{7}$ studied at high temperatures ($k_{B}T$ \textgreater \textgreater $J/k_{B})$ and in a wide-range of static magnetic fields. The analysis of the frequency dependence of the ac susceptibility revealed the existence of two relaxation mechanisms identified as an Orbach process with a pronounced effect of phonon bottleneck and cross-tunneling. The origin of the phonon bottleneck is attributed to a resonant phonon trapping. The relevance of the obtained results for relaxation phenomena found in other rare--earth pyrochlores, studied under similar conditions, is discussed.   

Characterization of defect structures on triangular antiferromagnet PdCrO$_{2}$ using scanning tunneling microscopy and spectroscopy

Frustrated magnetic systems have received significant attentions due to the possibility as source of ferroelectricity. We studied atomic structures and electronic structures of defects in two dimensional triangular-lattice antiferromagnet PdCrO$_{2}$ using scanning tunneling microscopy and spectroscopy (STM / STS). In CrO$_{2}$ layers and Pd layers, atomic resolution STM images showed distinguishable defects and differential conductance images showed unusual standing wave patterns. We identified the origin of defect structures using STS measurements, atomic ball model analysis and Density Functional Theory (DFT) calculations. Atomic structure analysis of defects gives the opportunity to understand a ground state of frustrated magnetic materials.   

Imaging Magnetic Order and Frustration on Distinct Sublattices in Artificial Quasicrystals

Scanning electron microscopy with polarization analysis (SEMPA) was used to acquire direct images of as-grown magnetization textures for Permalloy thin films patterned into Penrose P2 tilings (P2T). Simulations yield a low-energy manifold of textures composed of two distinct, perfectly ordered sublattices and two sublattices that remain frustrated. As-grown P2T samples exhibited large domains of the two ordered sublattices in the room-temperature SEMPA images. Higher resolution Monte Carlo simulations based on long-range dipolar interactions predict the two frustrated sublattices will order. These results indicate 3$^{\mathrm{rd}}$ generation P2T will offer the first example of magnetic order in a quasicrystalline material.   

Itinerant Ferromagnetism in Rashba Spin-Orbit Coupled Semiconductors

We theoretically studied the itinerant ferromagnetism in the Rashba spin-orbit coupled 2D electron system by oppositely shifting the two spin-split Fermi surfaces, on condition that electric current vanishes. We found that the system is stable for the infinitesimal displacements only if rs is smaller than around 10. But when rs becomes much larger than 10, the system becomes unstable at the original state, and finally we obtained another stable state with nonzero spin polarisation.   

Magnetic torque measurements in a chiral magnet CrNb$_{\mathrm{3}}$S$_{\mathrm{6}}$

Chiral magnetic orders emerge in a particular class of magnetic materials with a chiral crystal structure. As a consequence of the competition between Heisenberg exchange and Dyzaloshinskii-Moriya (DM) interactions in the presence of external magnetic field, chiral helimagnetic order (CHM) formed at zero magnetic field transforms into a nonlinear magnetic superlattice called chiral soliton lattice (CSL) under magnetic fields perpendicular to the chiral axis. The CSL consists of forced ferromagnetic (FM) regions periodically partitioned by chiral soliton kinks of spins. The period of the CSL increases gradually with increasing magnetic field. The CSL is the ground state and exhibits a phase transition into forced FM state above the critical field. To understand the nature of the phase transition, it is important to examine thermodynamic quantities such as magnetization. Furthermore, it is interesting to explore the possibility of the discretization of such physical quantities in a finite CSL system. In this talk, we will present the development of magnetic torque measurement method using micro cantilever in order to precisely measure the magnetization of a micro-sized sample and a set of experimental data obtained by magnetic torque measurements performed in chiral magnet CrNb$_{\mathrm{3}}$S$_{\mathrm{6}}$. Hysteresis and stepped behavior of magnetization observed are discussed.   

Realization of Ground State Artificial Skyrmion Lattices at Room Temperature

Artificial skyrmion lattices stable at ambient conditions offer a convenient and powerful platform to explore skyrmion physics and topological phenomena and motivates their inclusion in next-generation data and logic devices. In this work we present direct experimental evidence of artificial skyrmion lattices with a stable ground state at room temperature [1]. Our approach is to pattern vortex-state Co nanodots (560 nm diameter) in hexagonal arrays on top of a Co/Pd multilayer with perpendicular magnetic anisotropy; the skyrmion state is prepared using a specific magnetic field sequence. Ion irradiation has been employed to suppress PMA in the underlayer and allow imprinting of the vortex structure from the nanodots to form skyrmion lattices, as revealed by polarized neutron reflectometry. Circularity control is realized through Co dot shape asymmetry, and confirmed by microscopy and FORC magnetometry. The vortex polarity is set during the field sequence and confirmed by magnetometry. Spin-transport studies further demonstrate a sensitivity to the skyrmion spin texture.Work supported by NSF (DMR-1008791, ECCS-1232275 and DMR-1543582). [1]. D. A. Gilbert, et al, Nat. Commun. 6, 8462 (2015)].   

Unexpected observation of splitting of skyrmion phase in Zn doped Cu$_{2}$OSeO$_{3}$

Polycrystalline (Cu$_{1-x}$Zn$_{x})_{2}$OSeO$_{3}$ (0 $\le $ x $\le $ 0.2) samples were characterized by X-ray diffraction. The effect of Zn doping upon saturation magnetization (M$_{S})$ indicates that the Zn favors to occupying Cu(II) square pyramid crystallographic site. The Zn doping concentration is found to greatly affect the M-$T$ and $\chi \prime _{ac}$-$T$. The skyrmion phase has been inferred from the $\chi \prime _{ac}$-H data, and then indicated within the H-$T$ phase diagrams for various Zn doping concentrations. The striking and unexpected observation is that the skyrmion phase region becomes split upon Zn doping concentration. Interestingly, second conical boundary accompanied by second skyrmion phase was also observed from d$\chi \prime_{ac}$/dH vs. H curves. Atomic site disorder created by the chemical doping modulates the delicate magnetic interactions via changes in the Dzyaloshinskii-Moriya (DM) vector of distorted Cu(II) square pyramid, thereby splitting of skyrmion phase might occurred. These findings illustrate the potential of using chemical and atomic modification for tuning the temperature and field dependence of skyrmion phase of Cu$_{2}$OSeO$_{3}$.   

\textbf{Vanadyl Phthalocyanine (C}$_{\mathrm{\mathbf{32}}}$\textbf{H}$_{\mathrm{\mathbf{16}}}$\textbf{N}$_{\mathrm{\mathbf{8}}}$\textbf{VO): a near-perfect molecular paramagnet}

Transition-metal-doped phthalocyanines (TMPc, TM $=$ Mn, Fe, Co, Ni, and Cu) are semiconductors with interesting photoconductive properties and so have potential applications in optoelectronic devices [1]. TMPc are planar molecules with the TM atom at the center bound to four N atoms and forming a linear chain along the monoclinic b-axis. Recent magnetic studies reported in CuPc, CoPc, and MnPc show that the exchange coupling between the TM ions are either ferromagnetic as in MnPc [2] or antiferromagnetic as in CuPc [3] and CoPc [4]. In contrast to TMPc, VOPc has a five- coordinate square pyramidal structure with a single electron associated with VO$^{\mathrm{2+}}$ ion [5]. Here we report results from detailed investigations of the magnetic properties of powder sample of VOPc X-ray diffraction of which shows it to be triclinic. Temperature dependence of magnetization M from 2 K to 300 K in H $=$ 1 kOe fits the Curie-Weiss (CW) law with $\theta \quad =$ 0 K, $\mu \quad =$ 1.665$\mu_{\mathrm{B}}$ and g $=$ 1.922 for spin S $=$ 1/2 which indicates VOPc is paramagnetic without any exchange coupling between VO$^{\mathrm{2+}}$ ions, quite different from CuPc, CoPc and MnPc. Also, M vs. H data (up to 90 kOe) at 2 K, 5 K, 10 K, 25 K, 50 K, 100 K, and 300 K fit well with the Brillouin function variation for S $=$ \textonehalf , again confirming perfect paramagnetism in VOPc. [1]S.Heutz et al. Adv. Mater.19, 3618 (2007). [2]J.E.Brumboiu et al, J. Phys. Chem. A. 118, 927(2014). [3]Z.Wang et al, IEEE Trans. Magn. 51, 2700104(2015). [4]M.Serri et al, Nature Commun. 5, 3079(2014). [5]H.Adler et al, J. Phys. Chem. C. 119, 8755 (2015).   

Quantum mechanical forces in the presence of spin and rotational states of nanomagnets

We study nanomagnets that are free to rotate about their anisotropy and display quantum mechanical forces originated from quantum tunneling between classically degenerate magnetic states. Employing superpositions of spin and rotational states, we show that such forces can exist in the presence of a microwave field and a static magnetic field with a gradient. The optimal conditions for the observation of the oscillating force with quantum beats are presented.   

How to probe transverse magnetic anisotropy of a single-molecule magnet by electronic transport?

We propose an approach for \emph{in-situ} determination of the transverse magnetic anisotropy (TMA) of an individual molecule by electronic transport measurements, see Phys. Rev. B \textbf{91}, 035442 (2015). We study a Fe4 single-molecule magnet (SMM) captured in a gateable junction, a unique tool for addressing the spin in different redox states of a molecule. We show that, due to mixing of the spin eigenstates of the SMM, the TMA significantly manifests itself in transport. We predict and experimentally observe the pronounced intensity modulation of the Coulomb peak amplitude with the magnetic field in the linear-response transport regime, from which the TMA parameter $E$ can be estimated. Importantly, the method proposed here does not rely on the small induced tunnelling effects and, hence, works well at temperatures and electron tunnel broadenings by far exceeding the tunnel splittings and even $E$ itself. We deduce that the TMA for a single Fe4 molecule captured in a junction is substantially larger than the bulk value.   

Micromagnetic Simulation for Exploring Spin-wave Filtering Effects

Spin wave propagation in periodical magnetic structures has been studied in experiments and simulations over last few years and offers potential applications in spin wave filters. We conduct simulation studies on the band gap structure of three types of structures for potential spin filter applications: first, antidot lattice based on yttrium iron garnet (YIG); second, magnonic crystal with iron dots embedded in YIG film; third, grating coupler formed by iron dots above YIG film. We found that the width and frequency position of band gap vary among these different structures. In addition, we investigate spin filter properties depend on the geometry parameters of the periodic pattern, e.g. lattice period, dot diameter. Such studies can be helpful for the realization of the spin filter device with optimized periodical structure and geometry parameters using nanotechnology.   

Propagating spin waves in YIG micro-channel on Silicon

Recently the utilization of spin waves in the field of information processing has been widely developed because it is free of Joule heat dissipation and beneficial to miniaturization of the magnon based devices. Here we study spin waves in yttrium iron garnet (YIG) with a low damping property. The YIG film is fabricated on silicon substrate using pulsed laser deposition and the measured FMR linewidth is only a few Gauss. Using ebeam lithography, we are able to pattern the YIG film into a micro-channel and integrate sub-meter waveguides to generate and detect spin waves of wavelength down to 1$\mu$m or below. We show results of propagating spin waves in the YIG micro-channel measured by the $S_{12}$ parameter of the vector network analyzer.   

User-friendly software for modeling collective spin wave excitations

There exists a great need for user-friendly, integrated software that assists in the scientific analysis of collective spin wave excitations measured with inelastic neutron scattering. SpinWaveGenie is a C$++$ software library that simplifies the modeling of collective spin wave excitations, allowing scientists to analyze neutron scattering data with sophisticated models fast and efficiently. Furthermore, one can calculate the four-dimensional scattering function S(Q,E) to directly compare and fit calculations to experimental measurements. Its generality has been both enhanced and verified through successful modeling of a wide array of magnetic materials. Recently, we have spent considerable effort transforming SpinWaveGenie from an early prototype to a high quality free open source software package for the scientific community.   

Field-driven domain wall motion in ferromagnetic nanowires with Dzyaloshinskii-Moriya interaction

Field-driven domain-wall (DW) motion in ferromagnetic nanowires with easy- and hard-axis anisotropies was studied theoretically and numerically in the presence of the Dzyaloshinskii-Moriya interaction (DMI) based on the Landau-Lifshitz-Gilbert equation. We proposed a new trial function and found the exact solution for the DW motion along a uniaxial nanowire driven by an external magnetic field. A new strategy was suggested to speed up the DW motion in a uniaxial magnetic nanowire with large DMI parameters. In the presence of the hard-axis anisotropy, we found that the breakdown field and velocity of the DW motion was strongly affected by the strength and sign of the DMI parameter under external fields. The work may be useful for future magnetic information storage devices based on the DW motion.   

Ultralow field magnetization reversal of two-body Stoner particles system

Magnetic mechanism of nanoparticles has attracted explosive attention in the development of modern information industry. On the base of Landau-Lifshitz-Gilbert equation, we studied the magnetization reversal in a system of two Stoner particles with uniaxial anisotropies and static magnetic interaction. Using micromagnetic simulation, two typical geometrical configurations of perpendicular(PERP) and parallel(PARA) configuration where the diameter of each particle is 20nm are considered. We found that when the separation between two particles has 23nm in PERP configuration ultralow switching field strength, 17mT can be realized, which satisfies the zero-field condition in our previous works[J. Appl. Phys. 109, 104303(2011)] according to the chosen parameters of cobalt material. For other separation values the switching field are multiple of lowest field. However, in PARA configuration the switching field changes with the separation faintly. This two-body system considered in our work might be implement as a composite information bit and our results offer further possibilities for its applications in information storage and/or fast magnetic response.   

Static Magnetic Properties of Films Measured by Means of Angular Perturbative Magnetoresistance

In this work we introduced a new technique to measure magnetic anisotropies and magnetoelectrical properties, such as Anisotropic Magnetoresistance (AMR) and Giant Magnetoresistance (GMR) amplitudes. The Perturbative Magnetoresistance (PMR) consist of a regular collinear four probe magnetoresistance set up with an AC magnetic field (h$_{ac}$) applied perpendicular to the DC (H$_{dc}$) one. h$_{ac}$ amplitude is about 1.0 Oe and oscillate at 270 Hz. We successfully interpreted the signal response from the voltage measured by lock-in amplifier and proposed a model based on energy minimization to extract magnetic anisotropies, AMR and GMR amplitudes. Measuring the in-plane angular dependency of PMR signal we were able to identify the usual magnetic anisotropy, such as uniaxial, unidirectional and cubic. Taking into account the perturbative nature of this technique (small h$_{ac}$ amplitude and low frequency), we argue that angular PMR can be used to investigate some dynamic magnetic effects where static technique can not provide such information. A distinct feature of angular PMR is the capability to be used in saturated and non-saturated regime, so revealing magnetic properties dependency on applied field strength. We addressed the Rotatable Anisotropy as an example in this work.   

New mechanism of kinetic exchange interaction induced by strong magnetic anisotropy

It is well known that the kinetic exchange interaction between single-occupied magnetic orbitals (s-s) is always antiferromagnetic, while between single- and double-occupied orbitals (s-d) is always ferromagnetic and much weaker. Here we show that the exchange interaction between strongly anisotropic doublets of lanthanides, actinides and transition metal ions with unquenched orbital momentum contains a new s-d kinetic contribution equal in strength with the s-s one [1]. In noncollinear magnetic systems, this s-d kinetic mechanism can cause an overall ferromagnetic exchange interaction which can become very strong for transition metal ions. The importance of the s-d kinetic interaction and the possibility of the ferromagnetic interaction are confirmed in some existing complexes on the basis of the density functional theory calculations. \noindent [1] N. Iwahara and L. F. Chibotaru, arXiv:1502.04180.   

Itinerant ferromagnetism in fermionic systems with $SP(2N)$ symmetry

The Ginzburg-Landau free energy of systems with $SP(2N)$ symmetry describes a second order phase transition on the mean field level, since the Casimir invariants of the $SP(2N)$ group can be only of even order combinations of the generators of the $SP(2N)$ group. This is in contrast with systems having the $SU(N)$ symmetry, where the allowance of cubic term generally makes the phase transition into first order. In this work, we consider the Hertz-Millis type itinerant ferromagnetism in an interacting fermionic system with $SP(2N)$ symmetry, where the ferromagnetic orders are enriched by the multi-component nature of the system. The quantum criticality is discussed near the second order phase transition point.   

Partially Screened Edgemagnetoplasmons

We present a study of edgemagnetoplasmons in a partially-screened system of electrons on a helium surface. We compare experiment results with Fetter's theory fits the mode frequency versus field and screening parameter for small magnetic fields. Fetter's theory fits the mode frequency versus field and screening parameter for small magnetic fields. Deviations at larger fields occur near the point where the penetration length becomes shorter than the width of the density profile at the sample perimeter. At larger fields, the mode frequencies are in reasonable agreement with the theoretical predictions of Volkov and Mikhailov. The linewidths are in fair qualitative agreement with their theory.   

First-principles investigation of transient spin transfer torque in magnetic layers

By employing the nonequilibrium Green's function (NEGF) method, the transient current-induced spin transfer torque (STT) of the magnetic layered system is investigated based on the density functional theory (DFT). The computational cost of the transient STT is huge due to the dense mesh of $k$-sampling for the layered system. In order to speed up the calculation, the Hamiltonian of leads is replaced by the complex absorbing potential (CAP) so that the Green's function can be cast into the wide-band form. After employing the Pad\'e spectrum decomposition, the energy integrals in the formalism of transient electric current and STT, including that of the Fermi distribution function, can be analytically calculated by the theorem of residue, which dramatically reduces the computational complexity of the transient STT. As an application, the NEGF-DFT-CAP formalism with the Pad\'e approximation is implemented to study the transient electric current and current-induced STT of Co/Cu/Co trilayers under an upward pulse of bias with different rotating angles of magnetization direction between two leads. The oscillation behavior is obtained for the transient STT when it approaches the steady state.   

Generalized non-Local Resistance Expression and its Application in F/N/F Spintronic Structure with Graphene Channel

We report our work on the spin transport properties in the F/N/F(ferromagnets/normal metal/ferromagnets) spintronic structure from a new theoretical perspective. A significant problem in the field is to explain the inferior measured order of magnitude for spin lifetime. Based on the known non-local resistance formula and the mechanism analysis of spin-flipping within the interfaces between F and N, we analytically derive a broadly applicable new non-local resistance expression and a generalized Hanle curve formula. After employing them in the F/N/F structure under different limits, especially in the case of graphene channel, we find that the fitting from experimental data would yield a longer spin lifetime, which approaches its theoretical predicted value in graphene.   

Theoretical Study on Twofold and Fourfold Symmetric Anisotropic Magnetoresistance Effect

We theoretically study the twofold and fourfold symmetric anisotropic magnetoresistance (AMR) effect $[1]$. We first extend our previous model $[2]$ to a model including the crystal field effect $[1]$. Using the model, we next obtain an analytical expression of the AMR ratio, i.e., ${\rm AMR}(\phi)$=$C_0 + C_2 \cos (2 \phi) + C_4 \cos (4 \phi)$, with $C_0$=$C_2 - C_4$ $[1]$. Here, $\phi$ is the relative angle between the magnetization direction and the electric current direction and $C_2$ ($C_4$) is a coefficient of the twofold (fourfold) symmetric term. The coefficients $C_2$ and $C_4$ are expressed by a spin-orbit coupling constant, an exchange field, a crystal field, and s-s and s-d scattering resistivities. Using this expression, we analyze the experimental results for Fe$_4$N $[3]$, in which $|C_2|$ and $|C_4|$ increase with decreasing temperature. The experimental results can be reproduced by assuming that the tetragonal distortion increases with decreasing temperature. \\ $[1]$ S. Kokado {\it et al}., J. Phys. Soc. Jpn. {\bf 84} (2015) 094710. \\ $[2]$ S. Kokado {\it et al}., J. Phys. Soc. Jpn. {\bf 81} (2012) 024705. \\ $[3]$ M. Tsunoda {\it et al}., Appl. Phys. Express {\bf 3} (2010) 113003.   

Dynamical correlation functions of the transverse Ising model with next-nearest-neighbor interactions

We investigate the effects of next-nearest-neighbor (NNN) interactions on the dynamics of the one-dimensional spin-1$/$2 transverse Ising model in the high temperature limit. Using exact diagonalization of finite chains, we obtain the time-dependent transverse correlation function and the corresponding spectral density for a tagged spin. Our results for chains of 13 spins with periodic boundary conditions produce results which are valid in the infinite-size limit. In general we find that the NNN coupling produces slower dynamics accompanied by an enhancement of the central mode behavior. Even in the case of a strong transverse field, if the NNN coupling is sufficiently large there is a crossover from collective mode to central mode behavior. We also obtain several recurrants for the continued fraction representation of the relaxation function.   

Precise quantum control on solid-state spins

Precise quantum control is of great importance for quantum information processing, high resolution spectroscopy, and quantum metrology. One of the key obstacles to realizing precise quantum control on solid-state spins is the noises arising from both environment and control field. Here, we design a composite pulse to realize precise quantum control on a single electron spin in diamond by suppressing the effect of both noises simultaneously. The control is experimentally demonstrated to be with a low error rate of 4.8E-5. We improve quantum optimal control method to realize precise two-qubit quantum control on a system comprised by a single electron spin and $^{14}$N nuclear spin. With the improved quantum optimal control method, we design a pulse sequence for CNOT gate to suppress the noises simultaneously. The error rate of CNOT gate is measured to be 8E-3. To the best of our knowledge, the control we have realized stands for the state of art in precise quantum control on solid-state spins.   

Mircomagnetic Simulation of Amorphous TbFeCo Thin Films with Self Exchange Bias.

Amorphous ferrimagnetic TbFeCo thin films are found to exhibit self exchange bias effect near compensation temperature by magnetic hysteresis loop measurement. Atom probe tomography, scanning transmission electron microscopy, and energy dispersive spectroscopy mapping have revealed two nanoscale amorphous phases with different Tb concentrations distributed within the amorphous films. The observed exchange anisotropy originates from the exchange interaction between the two nanoscale amorphous phases. Here, we present a computational model of TbFeCo with two nanoscale amorphous phases using micromagnetic simulation. To obtain a structure similar to the two nanoscale amorphous phases, two kinds of cells with different Tb concentration are distributed within the simulated space. Each cell contains separated Tb and FeCo components, forming two antiferromagnetically coupled sublattices. Using this model, we show exchange bias effect near compensation temperature, in agreement with experimental results. The effect can be tuned by controlling the microstructure and composition.   

Spin-to-charge-current conversion in yttrium iron garnet-graphene hybrid structure

The use of graphene in spintronic devices depends, among other things, on its ability to convert a spin excitation into an electric charge signal, a phenomenon that requires a spin-orbit coupling (SOC). In this work we report the observation of two effects that show the existence of SOC in large-area CVD grown single-layer graphene (SLG) deposited on a single crystal film of the ferrimagnetic insulator yttrium iron garnet (YIG). The first is a magnetoresistance of graphene induced by the magnetic proximity effect with YIG. The second is the detection of a DC voltage along the graphene layer resulting from the conversion of the spin current generated by spin pumping from microwave driven FMR into charge current. We interpret the spin-to-charge conversion as arising from the inverse Rashba-Edelstein effect (IREE) made possible by the extrinsic spin-orbit coupling in graphene. These observations show that spin orbit coupling can be extrinsically enhanced in graphene by the proximity effect with a ferromagnetic layer. This result opens new possibilities for the use of graphene in spintronic devices with unique functionalities.   

Magnetic Anisotropy and Crystalline Electric Field in Quaternary Intermetallic Compounds.

All isostructural compounds RNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C (R$=$Er, Ho, Dy) show some magnetic transitions in magnetization isotherms at certain applied magnetic fields and temperatures above and below Neel and superconducting temperatures (T$_{\mathrm{N}}$, T$_{\mathrm{C}})$ where T$_{\mathrm{N}}$/T$_{\mathrm{C\thinspace }}$varies from 0.57 to 1.66 for ErNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C and DyNi$_{\mathrm{2}}$B$_{\mathrm{2}}$C. By using theoretical group analysis of D$_{\mathrm{4h}}$ (I4/mmm) to the energy level scheme of crystalline electric field of magnetization isotherms anisotropy at various temperatures, we have obtained some possible ground state energy levels such as singlet $\Gamma_{\mathrm{4}}$ and first excited doublet state $\Gamma _{\mathrm{5}}$ in addition to another excited singlet $\Gamma _{\mathrm{1.}}$ Our crystalline electric field energy scheme analysis shows some qualitative agreement between theoretical calculation and experiments at high magnetic fields regime only, which means the interplay between antiferromagnetsm and superconductivity should be included.   

Tailoring of SmCo$_{\mathrm{5\thinspace }}$for optimal structure, magnetic anisotropy, and reduced criticality

SmCo$_{\mathrm{5\thinspace }}$forms hexagonal CaCu$_{\mathrm{5}}$-type structure with three non-equivalent sites: Sm (1a), Co (2c), and Co (3g). Sm lies in the middle of the Co (2c) hexagonal layers. Advanced density functional theory calculations employing Hubbard model show crystal field split localized Sm 4f states, which are responsible for the large part of the magnetic anisotropy exhibited by this system. In addition, the hexagonal Co (2c) layers help enhancing the anisotropy. Due to the partially quenched Sm 4f orbital moment, there is a net Sm 4f moment, which also helps enhancing magnetic moment. The substitution of some of the Sm sites by Nd adds Nd 4f multiplet thereby enhancing crystal field split 4f states and overall magnetic moment. The substitution of Co (2c) by Fe is preferred over Co (3g) but the compound becomes chemically unstable. The criticality issues could be addressed by substituting abundant Ce.   

Rare-earth element based permanent magnets: a theoretical investigation

Permanent magnetic materials with large magnetization and high magnetocrystalline anisotropy are important for technical applications. In this context rare-earth (R) element based materials are good candidates because of their localized 4$f$ electrons. The 4$f$ crystal field splitting provides large part of magnetic anisotropy depending upon the crystal environment. The $d$ spin orbit coupling of alloyed transition metal component provides additional anisotropy. RCo$_{\mathrm{5}}$ and its derivative R$_{\mathrm{2}}$Co$_{\mathrm{17}}$ are known compounds for large magnetic anisotropy. Here we have performed electronic structure calculations to predict new materials in this class by employing site substitutions. In these investigations, we have performed density functional theory including on-site electron correlation (DFT$+$U) and L-S coupling calculations. The results show that the abundant Ce substitution in R sites and Ti/Zr substitutions in some of the Co sites help reduce criticality without substantially affecting the magnetic moment and magnetic anisotropy in these materials.   

Magnetostrictive stress reconfigurable thin film resonators in vacuum

The magnetic response of microdevices is significantly enhanced at structural resonance allowing for improved sensitivity and signal-to-noise ratio. The magnetic field resolution of these devices can be further improved when operating in vacuum due to an increase in mechanical quality factor. In this work, free-standing thin film CoFe bridge resonators have been fabricated and investigated. A strong magnetic field dependence of the fundamental resonance frequency is a function of magnetic field orientation due to a large unidirectional anisotropy. Under vacuum, a quality factor of up to 25 times greater than at atmosphere was revealed as well as an increased magnetic field sensitivity. Such stress reconfigurable sensors offer the possibility of broadband sensing with high resolution, and may therefore represent a new approach to fully integrated resonant magnetic field sensing technology.   

Heisenberg antiferromagnetic chain with multiple spin 1/2 particles of different flavors per site

Motivated by the discoveries of quasi-1D magnetic systems, we studied a quantum mechanical spin lattice system consisting of a one-dimensional antiferromagnetic Heisenberg chain. In this system we considered M spin 1/2 particles of different flavors per site, and the low-lying states, ground state included, of the Hamiltonian was solved numerically using the exact diagonalization method for finite cluster sizes. We have also obtained the corresponding solutions for systems of the same chain length but with one spin M/2 particle per site. The low energy spectra of both systems are then compared. For M$=$ 2 and M$=$3, our result shows that the two spin chain systems (one spin M/2 per site vs. M spin 1/2 of different flavors per site) have the same excitation spectra at low energy and the number of overlapped states increases as the size of the cluster increases. The observed overlap also indicates that low energy excitations of the M flavored spin 1/2 chain system selects the high spin states, effectively satisfying the Hund's Rule even though the system does not possess the orbital angular momentum.   

Comparison of Magnetic Behavior in Nanostructured and Bulk-Crystalline Mn$_{x}$TaS$_{\mathrm{2}}$

The magnetic behavior of nanostructured Mn$_{x}$TaS$_{\mathrm{2}}$ for several different Mn concentrations $x$ have been studied and a magnetic phase diagram obtained. For $x$ values between 0.15 (the lowest measured) and 0.19, the nanostructured system shows cluster-glass behavior as evidenced by spin relaxation well described by the Vogel-Fulcher-Tammann law as well as aging effects in the dc magnetization and ac susceptibility. For $x$ values between 0.19 and 0.24, nanostructured Mn$_{x}$TaS$_{\mathrm{2}}$ displays re-entrant cluster-glass characteristics, with transitions from paramagnetism to ferromagnetism at higher temperature and ferromagnetism to cluster glass at lower temperature. The tricritical point where all three phases converge seems to be close to $x =$ 0.19. Results for single-crystal Mn$_{x}$TaS$_{\mathrm{2}}$ from the literature show single transitions: paramagnetism to ferromagnetism for $x =$ 0.25 and paramagnetism to cluster glass for $x$ values in the range 0.02--0.1. For comparable $x$ values, Curie temperatures seem to be slightly higher and cluster-glass transition temperatures slightly lower in the nanostructures. Bulk crystalline samples with Mn concentrations in the range 0.1--0.25 are currently \quad being studied to more comprehensively compare the magnetic phase diagrams.   

Effects of disorder in the Sr$_{\mathrm{2}}$FeMoO$_{\mathrm{6\thinspace }}$double perovskite via first principles calculations.

First principles calculations were done in the double perovskite Sr$_{\mathrm{2}}$FeMoO$_{\mathrm{6}}$ regarding the effects of cationic disorder and electronic correlation in the ground-state properties such as spin polarization and magnetic moment. We used the Generalized Gradient Approximation (GGA) method including a U Hubbard term. Disorder is introduced via atomic substitution with a ratio of 25{\%} and 12.5{\%}. It is found a magnetic saturation of 2.22\textmu $_{\mathrm{B}}$ and 2.99\textmu $_{\mathrm{B}}$ for 25{\%} and 12.5{\%} of disorder respectively, in agreement with neutron magnetic scattering experiments. The half-metallic behavior of the above double perovskite remains only for a 12.5{\%} of disorder.   

Magnetic Properties of Single-Crystal and Polycrystalline YIG Films Using a Custom Broad-Band FMR System

We present a comparison of ferromagnetic resonance data acquired from single-crystal and polycrystalline yttrium iron garnet thick films deposited by liquid phase epitaxy and aerosol deposition, respectively. Data were taken using a custom broad-band measurement system consisting of a 1.2 T dc magnet and a 40 GHz vector network analyzer, which is used to track the ferromagnetic resonance signal up to 40 GHz. Ferromagnetic resonance data of the films were also taken using a high-sensitivity cavity system operating at 9.5 GHz. We include details on the experimental configuration and include an empirical conversion scheme relating frequency-swept to field-swept linewidths obtained from analysis of {\em S}-parameter data. Our results show that using these complementary measurement techniques can provide insight into dynamic magnetization characteristics of ferromagnetic materials.   

Alternatives to Rare Earth Permanent Magnets for Energy Harvesting Applications

Direct-drive permanent magnet generators (DDPMGs) offer increased reliability and efficiency over the more commonly used geared doubly-fed induction generator, yet are only employed in less than 1 percent of utility scale wind turbines in the U.S. One major barrier to increased deployment of DDPMGs in the U.S. wind industry is NdFeB permanent magnets (PMs), which contain critical rare earth elements Nd and Dy. To allow for the use of rare earth free PMs, the magnetic loading, defined as the average magnetic flux density over the rotor surface, must be maintained. Halbach cylinders are employed in 3.5kW Halbach PMGs (HPMGs) of varying slot-to-pole ratio to concentrate the magnetic flux output by a lower energy density PM over the rotor surface. We found that for high pole and slot number, the increase in magnetic loading is sufficient to allow for the use of strontium iron oxide hard ferrite PMs and achieved rated performance. Joule losses in the stator windings were found to increase for the hard ferrite PMs due to increased inductance in the stator windings. However, for scaling of the HPMG designs to 3MW, rated performance and high efficiency were achieved, demonstrating the potential for elimination for rare earth PMs in commercial scale wind turbines.   

Triple Halo Coil: Development and Comparison with Other TMS Coils

Transcranial Magnetic Stimulation (TMS) is a non-invasive stimulation technique that can be used for the treatment of various neurological disorders such as Parkinson's Disease, PTSD, TBI and anxiety by regulating synaptic activity. TMS is FDA approved for the treatment of major depressive disorder. There is a critical need to develop deep TMS coils that can stimulate deeper regions of the brain without excessively stimulating the cortex in order to provide an alternative to surgical methods.~We have developed a novel multi-coil configuration called ``Triple Halo Coil'' (THC) that can stimulate deep brain regions. Investigation of induced electric and magnetic field in these regions have been achieved by computer modelling. Comparison of the results due to THC configuration have been conducted with other TMS coils such as ``Halo Coil'', circular coil and ``Figure of Eight'' coil. There was an improvement of more than 15 times in the strength of magnetic field, induced by THC configuration at 10 cm below the vertex of the head when compared with the ``Figure of Eight'' coil alone.   

Anomalous Hall effect sensors based on magnetic element doped topological insulator thin films

Anomalous Hall effect (AHE) is recently discovered in magnetic element doped topological insulators (TIs), which promises low power consumption highly efficient spintronics and electronics. This discovery broaden the family of Hall effect (HE) sensors. In this work, both HE and AHE sensor based on Mn and Cr doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI thin films will be systematically studied. The influence of Mn concentration on sensitivity of Mn$_{\mathrm{x}}$Bi$_{\mathrm{2-x}}$Te$_{\mathrm{3}}$ HE sensors will be discussed. The Hall sensitivity increase 8 times caused by quantum AHE will be reported. AHE senor based on Cr-doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI thin films will also be studied and compared with Mn doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ AHE sensor. The influence of thickness on sensitivity of Cr$_{\mathrm{x}}$Bi$_{\mathrm{2-x}}$Te$_{\mathrm{3}}$ AHE sensors will be discussed. Ultrahigh Hall sensitivity is obtained in Cr doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$. The largest Hall sensitivity can reach 2620 $\Omega $/T in sensor which is almost twice higher than that of the normal semiconductor HE sensor. Our work indicates that magnetic element doped topological insulator with AHE are good candidates for ultra-sensitive Hall effect sensors.   

Enhanced Response of Magneto-active Elastomers by Anisotropy

We have investigate the magnetostriction of magneto-active elastomers which have random or aligned packing and magnetic filler particles of varying aspect ratios. We have systematically also varied the volume fraction. In general, we find that the response is a complex function of both particle aspect ratio as well as volume fraction. While for any given aspect ratio, there is a maximum in the magnetostriction as a function of volume fraction, for a given volume fraction, there is a local minimum for spherical particles. That is, stubby rods and thick disks show maximal response. We discuss these results in terms of competition between the magnetic dipole interactions, magnetic torque, and elastic response.   

Surface magnetic properties and magnetoimpedance of Co-rich amorphous and nanocrystalline (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ ribbons with oxide layer formed by long-term exposure to air

The surface magnetic properties and magnetoimpedance (MI) of amorphous and nanocrystalline (Co$_{1-x}$Fe$_{x})_{89}$Zr$_{7}$B$_{4}$ melt-spun ribbons with x$=$ 0, 0.025, 0.05 {\&} 0.1 was investigated. A 540\textdegree C heat treatment for 1 hour under a 2 T transverse field formed a large volume fraction of nanocrystalline phases in the ribbons, in addition to a well-defined transverse anisotropy indicated by x-ray diffraction and magneto-optical Kerr effect microscopy. After the heat treatment, the ribbon samples were exposed to open air for an extended period of time producing a visible oxide layer on the surfaces. High frequency magnetoimpedance measurements in the driving frequency range of 1-1000 MHz were made to characterize the potential impact of the surface oxide layer on the ac magnetization process. Unique field-dependent behavior of the real and imaginary components of the MI was found in nanocrystalline ribbons with higher Co content (x $\ge $ 0.05), showing multiple peaks above 50 MHz driving current.   

Interface and surface effects on Magnetic Properties in FeRh

FexRh1-x alloy systems has different magnetic properties depending of composition and temperature. When x is around 0.5, it is antiferromagnetic at room temperature and has phase transition around 370K from antiferromagnetic to ferromagnetic ordering. Due to this property, the FeRh alloy has a big potential for technological applications. In this study, effects of growing parameters, using buffer and cap layers on magnetic ordering and on phase transition have been studied. All films were grown on MgO(100) surface by sputter technique at different substrate temperatures. Rh and Pt were deposited on substrate as buffer and cap layer. Structural properties of the film were investigated by X-ray diffraction. Magnetization measurements were performed as a function of temperature by PPMS. And FMR spectra were registered and the results were analyzed for the ferromagnetic phases. Magnetization measurements showed that growing temperature has noteworthy effect on magnetic properties and structure of FeRh thin films. Depending of growth temperature, ferromagnetic and antiferromagnetic ordered samples were observed at room temperature. Phase transition also was controlled successfully by using buffer and cap layers.   

Analysis of ringing due to magnetic core materials used in pulsed nuclear magnetic resonance applications

Oil-field well logging instruments employ pulsed nuclear magnetic resonance (NMR) techniques and use inductive sensors to detect and evaluate the presence of particular fluids in geological formations. Acting as both signal transmitters and receivers most inductive sensors employ magnetic cores to enhance the quality and amplitude of signals recorded during field measurements. It is observed that the magnetic core also responds to the applied input signal thereby generating a signal (`ringing') that interferes with the measurement of the signals from the target formations. This causes significant noise and receiver dead time and it is beneficial to eliminate/suppress the signals received from the magnetic core. In this work a detailed analysis of the magnetic core response and in particular loading of the sensor due to the presence of the magnetic core is presented. Pulsed NMR measurements over a frequency band of 100 kHz to 1MHz are used to determine the amplitude and linewidth of the signals acquired from different magnetic core materials. A lower signal amplitude and a higher linewidth are vital since these would correspond to minimal contributions from the magnetic core to the inductive sensor response and thus leading to minimized receiver dead time.   

Ground State of the One Dimensional Heisenberg Model with {\it NNN} Interactions

A great number of insights into a variety of complex physical many-body systems have been gleaned from the study of the of the one-dimensional Heisenberg model. There exists a number of quasi one-dimensional inorganic compounds such as $T T F -C U S_{4} C_{4} (C F_{3})_{4}$, $S R C U_{2} O_{3}$, $V O_{2} P_{2} O_{7}$ and $C u G e O_{3}$ for which this Hamiltonian system is relevant. For this work we shall study the one-dimensional Heisenberg Model with nearest, next nearest and next -next nearest interactions. The Hamiltonian is given by: \[ H =J_{1} \sum _{k}{\mathbf{S}}_{k} \cdot {\mathbf{S}}_{k +1}+ J_2\sum_k {\mathbf{S}}_k \cdot {\mathbf{S}}_{k+2}+ J_3 \sum_k {\mathbf{S}}_k \cdot {\mathbf{S}}_{k+3} \] where ${\mathbf{S}}_{k}$ represents the spin $1/2$ operator along a chain of $N$ sites and periodic boundary conditions is assumed for the closed chain. We note that it is further possible to describe the Coulomb interaction subject to the Pauli exclusion principle for two quantum dots an $X Y$ model. Here we shall study the ground-state energy as well as the energy gap of this system using both a Lanczos (tridiagonal) scheme as well as a generalized Moments approach.   

Ground-State of the Bose-Hubbard Model

The Bose-Hubbard Model represents a s simple theoretical model to describe the physics of interacting Boson systems. In particular it has proved to be an effective description of a number of physical systems such as arrays of Josephson arrays as well as dilute alkali gases in optical lattices. Here we wish to study the ground-state of this system using two disparate but related moments calculational schemes: the Lanczos (tridiagonal) method as well as a Generalized moments approach. The Hamiltonian to be studied is given by (in second-quantized notation): \[H = -t\sum _{ }b_{i}^{\dag }b_{j} +\frac{U}{2}\sum _{i}n_{i}\left (n_{i} -1\right ) -\mu \sum _{i}n_{i} . \] Here $i$ is summed over all lattice sites, and~$ $ denotes summation over all neighbhoring sites $i$ and $j$, while $b_{i}^{\dag }$ and $b_{i}$ are bosonic creation and annihilation operators. $n_{i} =$$b_{i}^{\dag }b_{i}$ gives the number of particles on site $i$. Parameter $t$ is the hopping amplitude, describing mobility of bosons in the lattice. Parameter $U$ describes the on-site interaction, repulsive, if $U >0$, and attractive for $U <0$. $\mu $ is the chemical potential. Both the ground-state energy and energy gap are evaluated as a function of $t$, $U$ and $\mu$.   

Atomic structure prediction of Zr-Co and Hf-Co nanoclusters using the evolutionary algorithm.

Nanostructures of Hf-Co and Zr-Co rare earth free magnetic materials exhibit a high room-temperature energy product. In our study, the evolutionary algorithm coupled with density functional theory (DFT) is used to identify the global energy minimum atomic structures of Zr-Co and Hf-Co clusters. Using evolutionary crystal structure optimization algorithm, as implemented in USPEX, we studied the atomic structure, binding energies, magnetic properties, and anisotropy of Zr$_{\mathrm{x}}$Co$_{\mathrm{y}}$ and Hf$_{\mathrm{x}}$Co$_{\mathrm{y}}$(x$=$1,2 and y$=$5,7,11) clusters. A set of metastable and global minimum atomic structures are identified. Several new lower energy configurations were identified for Zr$_{\mathrm{2}}$Co$_{\mathrm{11}}$, Zr$_{\mathrm{1}}$Co$_{\mathrm{5}}$, Zr$_{\mathrm{1}}$Co$_{\mathrm{7}}$, Hf$_{\mathrm{2}}$Co$_{\mathrm{11}}$, Hf$_{\mathrm{1}}$Co$_{\mathrm{5}}$ and Hf$_{\mathrm{1}}$Co$_{\mathrm{7\thinspace }}$clusters by our calculations. We discussed the magnetic interaction between the atoms of the clusters which is critical in finding the lowest energy structure. Our calculations show that Zr-Co and Hf-Co clusters have ferromagnetic coupling and large magnetization. Magnetocrystalline anisotropy energies (MAE) of these clusters were also found to be large.   

Analytical Phase Equilibrium Function for Mixtures Obeying Raoult's and Henry's Laws

When a mixture of two substances exists in both the liquid and gas phase at equilibrium, Raoults and Henry's laws (ideal solution and ideal dilute solution approximations) can be used to estimate the gas and liquid mole fractions at the extremes of either very little solute or solvent. By assuming that a cubic polynomial can reasonably approximate the intermediate values to these extremes as a function of mole fraction, the cubic polynomial is solved and presented. A closed form equation approximating the pressure dependence on mole fraction of the constituents is thereby obtained. As a first approximation, this is a very simple and potentially useful means to estimate gas and liquid mole fractions of equilibrium mixtures. Mixtures with an azeotrope require additional attention if this type of approach is to be utilized.   

Oxide double quantum dot - an answer to the qubit problem?

We propose that oxide-based double quantum dots with only one electron (tunnelling between the dots) can be regarded as a qubit with little decoherence; these dots can possibly meet future challenges of miniaturization. The tunnelling of the $e_g$ electron between the dots and the attraction between the electron and the hole on adjacent dots can be modelled as an anisotropic Heisenberg interaction between two spins with the total z-component of the spins being zero. We study two anisotropically interacting spins coupled to optical phonons; we restrict our analysis to the regime of strong coupling to the environment, to the antiadiabatic region, and to the subspace with zero value for $S_{zT}$ (the z-component of the total spin). In the case where each spin is coupled to a different phonon bath, we assume that the system and the environment are initially uncorrelated (and form a simply separable state) in the polaronic frame of reference. By analyzing the polaron dynamics through a non-Markovian quantum master equation, we find that the system manifests a small amount of decoherence that decreases both with increasing nonadiabaticity and with enhancing strength of coupling g.   

Chiral projected entangled-pair state with topological order

We show that projected entangled-pair states (PEPS) can describe chiral topologically ordered phases. For that, we construct a simple PEPS for spin-1/2 particles in a two-dimensional lattice. We reveal a symmetry in the local projector of the PEPS that gives rise to the global topological character. We also extract characteristic quantities of the edge conformal field theory using the bulk-boundary correspondence.   

OIST - A New Model of Graduate Education.

I would present the Okinawan Institute of Science and Technology Graduate University. The Graduate University offers outstanding opportunities for students looking to study for a Ph.D. I would outline what makes OIST different from other Graduate Schools and explain the opportunities for students but also the posts open for Faculty. The Okinawa Institute of Science and Technology is an interdisciplinary graduate school offering a 5-year PhD program in Science. Over half of the faculty and students are recruited from outside Japan, and all education and research is conducted entirely in English.   

Controlling Surface Chemistry of Gallium Liquid Metal Alloys to Enhance their Fluidic Properties

Gallium liquid metal alloys (GaLMAs) are one of the key components of emerging technologies in reconfigurable electronics, such as tunable radio frequency antennas and electronic switches. Reversible flow of GaLMA in microchannels of these types of devices is hindered by the instantaneous formation of its oxide skin in ambient environment. The oxide film sticks to most surfaces leaving unwanted metallic residues that can cause undesired electronic properties. In this report, residue-free reversible flow of a binary alloy of gallium (eutectic gallium indium) is demonstrated via two types of surface modifications where the oxide film is either protected by an organic thin film or chemically removed. An interface modification layer (alkyl phosphonic acids) was introduced into the microfluidic system to modify the liquid metal surface and protect its oxide layer. Alternatively, an ion exchange membrane was utilized as a ‘sponge-like’ channel material to store and slowly release small amounts of HCl to react with the surface oxide of the liquid metal. Characterization of these interfaces at molecular level by surface spectroscopy and microscopy provided with mechanistic details for the interfacial interactions between the liquid metal surface and the channel materials.   

Hellmann-Feynman Forces within the DFT+U in Wannier functions basis

The most general way to describe localized atomic-like electronic states in strongly correlated materials is to use Wannier functions. In the present paper we continue development of widely-used DFT+U method with the Wannier function basis set and propose a technique to calculate Hubbard contribution to atomic forces. The technique was implemented as a part of plane-waves pseudopotential code Quantum-ESPRESSO and tested on two compounds: charge transfer insulator NiO with cubic crystal structure and correlated metal SrVO$_3$ with perovskite structure.   

Generation of nondegenerate narrow-band photon pairs for hybrid quantum network

In a hybrid quantum network, the linking two types of quantum nodes through photonic channels requires excellent matching of the central frequency and bandwidth between both nodes and their interfacing photons. However, pre-existing photon sources cannot fulfill this requirement. Using a novel conjoined double-cavity strategy, we report the generation of nondegenerate narrow-band photon pairs by cavity-enhanced spontaneous parametric down-conversion. The central frequencies and bandwidths of the signal and idler photons are independently set to match with trapped ions and solid-state quantum memories. With this source we achieve the bandwidths and central frequencies of 4 MHz at 935 nm and 5 MHz at 880 nm for the signal and idler photons, respectively, with a normalized spectral brightness of 4.9/s/MHz/mW. Due to its ability to be independently locked to two different wavelengths, the conjoined double-cavity is universally suitable for a hybrid quantum network consisting of various quantum nodes.   

Optimization of the profile of a pulsed slow positron beam extracted from a buffer-gas positron trap for the production of a variable energy positronium beam

In this poster we will present some details of steps taken to optimize the beam profile of a pulsed slow positron beam extracted from a buffer-gas positron trap. The beam will be employed for the production of a novel positronium beam by the acceleration and photodetachment of positronium negative ions [1, 2]. The TUS group is planning on using this beam to study positronium diffraction from solid surfaces, providing a unique neutral-particle spectroscopic method with several advantages over conventional neutral-particle spectroscopy, such as a reduced particle mass and, hence, the reduction of damage to the sample surface. [1] K. Michishio, et al. Phys. Rev. Lett.~\textbf{106}, 153401 (2011) [2] K. Michishio, et al. Appl. Phys. Lett.~\textbf{100}, 254102~(2012)   

Synthesis, Characterization and Electrochemical Analysis of Composite Cathode Material 0.5Li$_{\mathrm{2}}$MnO$_{\mathrm{3}}$-0.25LiMn$_{\mathrm{2}}$O$_{\mathrm{4}}$-0.25LiNi$_{\mathrm{0.5}}$Mn0.5O$_{\mathrm{2}}$ for LIB applications.

Structural stability, environment friendliness, low cost as well as good electrochemical performances are the major requirements for cathode materials. Li$_{\mathrm{2}}$MnO$_{\mathrm{3}}$ based composite cathode materials are one of the widely investigated positive cathode materials due to their ability to provide high discharge capacity and good rate capability. We have synthesized layered- spinel composite cathode material 0.5Li$_{\mathrm{2}}$MnO$_{\mathrm{3}}$-0.25LiMn$_{\mathrm{2}}$O$_{\mathrm{4}}$-0.25LiNi$_{\mathrm{0.5}}$Mn0.5O$_{\mathrm{2}}$ by sol-gel synthesis technique and surface characterized using XRD, Raman, SEM and EDX. Peaks corresponding to layered and spinel structures are identified by XRD and Raman studies. SEM images depict the nano-sized particles and EDX data confirms the presence of constituent transition metals and oxygen. Electrochemical studies were performed on coin cells, which were assembled in the Ar- filled glove box using Li as anode and spread material as cathode. LiPF$_{\mathrm{6}}$ with EC:DMC::1:2 ratio was used as the electrolyte. CV, EIS and charge discharge studies shows that the developed cathode material is a promising electrode for next generation Li ion batteries.   

Propulsion and Levitation with a Large Electrodynamic Wheel

We constructed an electrodynamic wheel using a motorized bicycle wheel with a radius of 12 inches and 36 one-inch cube magnets attached to the rim of the wheel. The radial magnetic field on the outside of the wheel was maximized by arranging the magnets into a series of Halbach arrays which amplify the field on one side of the array and reduce it on the other side. Rotating the wheel produces a rapidly oscillating magnetic field. When a conductive metal ``track'' is placed in this area of strong magnetic flux, eddy currents are produced in the track. These eddy currents create magnetic fields that interact with the magnetic fields from the electrodynamic wheel. The interaction of the magnetic fields produces lift and drag forces on the track which were measured with force gauges. Measurements were taken at a variety of wheel speeds, and the results were compared to the theoretical prediction that there should be a linear relationship between the lift and drag forces with increasing wheel speed. Partial levitation was achieved with the current electrodynamic wheel. In the future, the wheel will be upgraded to include 72 magnets rather than 36 magnets. This will double the frequency at which the magnetic field oscillates, increasing the magnetic flux. Electrodynamic wheels have applications to the transportation industry, since multiple electrodynamic wheels could be used on a vehicle to produce a lift and propulsion force over a conductive track.   

The Microwave Hall Effect Measured Using a Waveguide Tee

We describe a simple microwave apparatus to measure the Hall effect in semiconductor wafers. This technique does not require contacts on the sample or the use of a resonant cavity. Our method consists of placing a semiconductor wafer into a slot in an X-band (8 - 12 GHz) waveguide series tee, injecting microwave power into the two opposite arms of the tee, and measuring the microwave output at the third arm. A magnetic field is applied perpendicular to the wafer and produces a microwave Hall signal that is linear in the magnetic field and which reverses phase when the magnetic field is reversed. The microwave Hall signal is proportional to the semiconductor mobility, which we compare for calibration purposes with d. c. mobility measurements obtained using the van der Pauw method. We obtain the resistivity by measuring the microwave reflection coefficient of the sample. We determine a calibration constant as a function of the ratio of thickness to skin depth for two and three inch silicon and germanium samples doped with boron or phosphorus. The measured mobilities ranged from 270 to 3000 cm$^{\mathrm{\mathbf{2}}} \quad /($Vsec)   

Adsorption of Water Molecule on Silicene-FE System

After graphene synthesis[1], there have been numerous studies on similar systems in two dimensions so, we have the borophene [2], germanene [3], silicene [4,5], phosphorene [6], etc. Following this line, I do a study that takes the silicene system at its starting point, system to which it add Fe atom. At first, the stability of SILICENE-Fe system is studied, which is stable. Then a water molecule is added to the SILICENE-Fe system, which is captured, as is bounded to the Fe atom. To make this study I used Functional Density Theory, Born-Openheimer Approximation, Atomic Pseudopotentials and Molecular Dynamics. \newline \noindent [1] Novoselov, K.S.;Geim, A.K. et al, Science, {\bf 306 }, 666 (2004).\newline [2] Yang, X., et al, PRB,{\bf 77}, 041402(R) (2008). \newline [3] Bianco, E., et al, ACS Nano, {\bf 7}, 4414 (2013). \newline [4] Kamal, C, et al, J.Phys.: Condens. Matter, {\bf 25}, 085508 (2013). \newline [5] Drummond, N.P., et al, PRB,{\bf 85}, 075423 (2012). \newline [6] Liang, L., et al, Nanolett., {\bf 14}, 6400 (2014).   

Effect of immersion in simulated body environment on mechanical properties of twist-oriented poly(lactic acid) screws.

Poly(lactic acid) (PLA) has been applied to bone fixation devices, since it has high biocompatibility. In order to apply PLA device to a higher loaded part, mechanical properties of PLA have been improved by uniaxial drawing. However, mechanical properties along the other loading direction than the drawing direction such as torsion were not improved. Therefore, surgeon should be carefully conducted not to brake the reinforced PLA screw when tightening. In this study, screw is focused on as a bone fixation device. In order to improve torsional strength of a PLA screw, twist-orientation method was developed. PLA screw is prepared through a series of routes including casting, extrusion drawing, twist-orientation and forging. This screw was immersed in the phosphate buffered solution for 0, 8, 16 and 24 weeks, then shear strength, orientation function, crystallinity and molecular weight were measured. As a result, twist-orientation improves the initial torsional strength of PLA screw without the decrease in initial shear strength. In addition, the shear strength on twist-oriented screw is equivalent that of non-twist oriented screw during immersion until 24 weeks. This result shown that the twist-orientation does not decrease shear strength after immersion.   

Lift to Drag Ratio Analysis in Magnetic Levitation with an Electrodynamic Wheel

Our experiments explored inductive magnetic levitation (MagLev) using simple permanent magnets and conductive tracks. Our investigations used a circular Halbach array with a 1 Tesla variable magnetic field on the outer rim of the ring. Such a system is usually called an Electrodynamic Wheel (EDW). Rotating this wheel around a horizontal axis above or below a flat conducting surface should induce eddy currents in said surface through the variable magnetic flux. The eddy currents produce, in turn, their own magnetic fields, which interact with the magnets of the EDW. We constructed a four-inch diameter Electrodynamic Wheel using twelve Neodymium permanent magnets and demonstrated that the magnetic interactions produce both lift and drag forces on the EDW. These forces can be used for levitation and propulsion of the EDW to produce magnetic levitation without coils and complex control circuitry. We achieved full levitation of the non-magnetic aluminum and copper plates. Our results confirm the expected behavior of lift to drag ratio as proportional to (L/R)$\omega $, with L and R being the inductance and resistance of the track plate, and $\omega $ being the angular velocity of the magnetic flux.   

Electromigration in focused ion beam deposited tungsten single nanowires

As the focused ion beam induced deposited (FIBID) nanowires (NWs) of W, Pt are being used in nanoelectronic technology to connect individual nanodevices, repairing damaged interconnects in integrated circuit (IC), electromigration study in FIBID-NWs has become essential. Briefly, when a thin conductor, like metallic Al, Cu interconnects in an IC chip carry quite high current density \textasciitilde 10$^{12}$ A/m$^{2}$, ions or atoms start migrating. Such migration causes void and hillock formation leading to interconnect discontinuity, short circuit and ultimately IC failure. Our electromigration study in single FIBID-NWs of W reveals that failure in NWs of width and thickness $\approx $100 nm occurs typically at 10$^{11}$ A/m$^{2}$. Most notably, void and hillock always form in opposite polarity compared to typical metallic NWs. Such distinctly new outcome is explained via electromigration driven by direct force (ionic charge*electric field) opposed to wind force driven migration observed in metallic NWs. As FIBID-NWs are composite in nature, different species (e.g., Ga, W and C) migrate with different degree and direction depending on their oxidation state, leading to redistribution of species across NW length and formation of a Ga rich hillock.   

Robust surface states in epitaxial Bi(111) thin films

Bulk Bi a prototype semimetal with trivial electronic band topology. Unanticipatedly, we show the Altshuler-Aronov-Spivak and Aharonov-Bohm effects in epitaxial Bi(111) thin films. Meanwhile, we clearly identify the interaction of the top and bottom surface states via quantum tunneling by the electrical conductance and weak anti-localization measurements. These results have significantly enriched our understanding about the electronic structure of Bi, which might be helpful for clearing up some of its longstanding subtle issues.   

Ternary Synaptic Plasticity Arising from Memdiode Behavior of TiOx Single Nanowire

Electric field-induced resistive switching (RS) effect has been widely explored as a novel nonvolatile memory over the past few years. Recently, the RS behavior with continuous transition has received considerable attention for its promising prospect in neuromorphic simulation. Here, the switching characteristics of a planar-structured TiOx single nanowire device were systematically investigated. It exhibited a strong electrical history-dependent rectifying behavior that was defined as a "memdiode". We further demonstrated that a ternary synaptic plasticity could be realized in such a TiOx nanowire device, characterized by the resistance and photocurrent responses. For a given state of the memdiode, a conjugated memristive characteristic and a distinct photocurrent can be simulaneously obtained, resulting in a synchronous implementation of various Hebbian plasticities with the same temporal order of spikes. These intriguing properties of TiOx memdiode provide a feasible way toward the designing of multifunctional electronic synapses as well as programmable artificial neural network   

Formation of silicene-germanene heterostructures by Ge deposition on epitaxial silicene

Silicene and germanene are two dimensional honeycomb sheets composed of Si and Ge atoms. Epitaxial silicene and germanene form spontaneously on ZrB2(0001) thin films grown on Si(111) [1] and Ge(111) [2] substrates and can be identified by (2X2)- and ($\surd $3X$\surd $3)-reconstruction of ZrB2(0001) respectively. In the present work, we demonstrate that silicene-germanene heterostructures can be formed by deposition of Ge on epitaxial silicene and by subsequent annealing. LEED and STM analysis revealed the growth of the following Si-Ge structures depending on preparation conditions. (1): After annealing at 830 K, (2X2)- and ($\surd $3X$\surd $3)-reconstructed areas existed side by side, which suggests that a two-dimensional silicene-germanene heterostructures is formed. (2): After annealing at 1070 K, the surface is (2X2)-reconstructed, with a heterogenous atomic contrast different from silicene which suggests the incorporation of Ge atoms in the silicene lattice. (3): After annealing this mixed Si-Ge layer at 830 K, a (2$\surd $3X2$\surd $3)-reconstruction is observed, in agreement with the overlapping of ($\surd $3X$\surd $3)- and (2X2)-reconstructed layers. The structure is presumably a silicene-germanene heterostack structure. [1] A.Fleurence, et al., Phys. Rev. Lett. 108 245501(2012). [2] A.Fleurence, et al., APS March Meeting 2016.   

Analysis of magnetic anisotropy on BaTiO3/La2/3Sr1/3MnO3 bilayers

We have deposited BTO/LSMO bilayers as a possible route to design systems with artificial magnetoelectric coupling. We maintain a fixed ferroelectric layer thickness (tBTO$=$ 100 nm) and ferromagnetic layer (tLSMO $=$ 25 nm). We analyze the influence of direction substrate on electrical and magnetic properties of manganite. From XRD we found that the BTO layer for STO(001) growth textured with almost two different distribution of domains (c domains in plane and out-plane) with cBTO$=$4.108 {\AA} and LSMO layer growth textured with cLSMO$=$3.855 {\AA}. Interestingly, for STO(110) and STO(111) the BTO layer and LSMO layer the growth is textured with cBTO$=$4.037 {\AA} and cBTO$=$4.018 {\AA} while LSMO growth is also textured with cLSMO$=$3.867 {\AA} and cLSMO$=$3.858 {\AA}, respectively. Magnetization with temperature curves shows a ferromagnetic transition for all bilayers at room temperature with a magnetization between 280-320 emu/cm3. Measures of anisotropy at 300 K show a change in magnetic anisotropy for bilayer growth on STO(001) from biaxial magnetic ordering (LSMO/STO) to uniaxial magnetic ordering (bilayer), probably due to BTO layer influence on magnetic properties on LSMO layer. This work has been supported by ``Colciencias-CENM Research Project CI7917-CC 10510 contract 0002-2013 and CI 7978.   

The anomalous Hall effect of Fe(001) in ultrathin film regime

The anomalous Hall effect (AHE) in ultrathin film regime is investigated in Fe(001)(1 nm-3 nm) epitaxial on MgO(001). We find the intrinsic anomalous Hall conductivity (AHC) is reducing when the thickness decreasing. The reduction of the intrinsic AHC is interpreted as modification of electronic band structure of iron through boundary confinement. We also observe localization correction to longitudinal resistivity, while the anomalous Hall resistivity of different temperature can be figured out by a set of variable skew scattering coefficients and a constant side-jump contribution. The analysis indicates that localization correction has a significant impact on skew scattering but little on side-jump.   

Unusual magnetic properties of superconducting Bi/Ni bilayers

Superconductivity and ferromagnetism are two incompatible phenomena. However, the interaction between them attracts numerous physicists' interests for both theoretical and experimental purposes. Recently, increasing experimental discoveries reveal unconventional effects in superconductor and ferromagnet hybrids, which stimulates a new field called superconducting spintronics. In present work, we report various intriguing magnetic properties of an unexpected superconducting bilayer consisting of non-superconducting Bi and ferromagnetic Ni. A large spontaneous magnetization is induced when the temperature is decreased below the superconductivity transition temperature, which indicates a complex interaction between superconductivity and ferromagnetism in this bilayer. The zero field cooling results show normal Meissner effect while the field cooling results show paramagnetic Meissner effect. Besides, magnetic hysteresis loops in low temperatures show flux pinning and flux jumping effects. Our findings pave the way for exploring unconventional superconductivity coupled to ferromagnetism and potential applications in superconducting spintronics.   

Long-lived non-equilibrium states in a quantum-Hall Tomonaga-Luttinger liquid

The existence of long-lived non-equilibrium states without showing thermalization, which has previously been demonstrated in time evolution of ultracold atoms (quantum quench), suggests the possibility of their spatial analogue in transport behavior of interacting electrons in solid-state systems. Here we report long-lived non-equilibrium states in one-dimensional edge channels in the integer quantum Hall regime. For this purpose, non-trivial binary spectrum composed of hot and cold carriers is prepared by an indirect heating scheme using weakly coupled counterpropagating edge channels in an AlGaAs/GaAs heterostructure. Quantum dot spectroscopy clearly reveals that the carriers with the non-trivial binary spectrum propagate over a long distance (5 - 10 um), much longer than the length required for electronic relaxation (about 0.1 um), without thermalization into a trivial Fermi distribution. This observation is consistent with the integrable model of Tomonaga-Luttinger liquid. The long-lived spectrum implies that the system is well described by non-interacting plasmons, which are attractive for carrying information for a long distance.   

A unified understanding of the thickness-dependent bandgap transition in hexagonal layered semiconductors

Over the past few years, it has been recognized that a single layer of hexagonal two-dimensional semiconductors---such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs)---has a direct bandgap, while it becomes an indirect semiconductor as the number of layers increases to two or more. Understanding and control of the direct-to-indirect bandgap transition (DIBT) of hexagonal layered semiconductors is of great scientific and technological importance, because the DIBT converts multilayer hBN or TMD into optically less active materials. Here, taking hBN and MoS$_{\mathrm{2}}$ as examples, we provide a microscopic understanding of the DIBT of hexagonal layered semiconductors based on our symmetry analysis and direct first-principles calculations. Starting from a simple phenomenological explanation of the DIBT within the first-order perturbation theory of multilayer phases, we show how the bandgap transition arises from the selective orbital couplings in hexagonal layered semiconductors.   

Making Distinct Dynamical Systems Look Identical Spectrally

We use tracking control techniques to match the optical responses of distinct dynamical systems interacting with laser pulses in both quantum and classical regimes. As a result we provide illustrations where a variety of open and closed systems display the same optical response, demonstrating that the optical spectrum alone is not enough to uniquely characterize general dynamical systems. These findings have important implications in quantum inversion techniques encompassing both linear and nonlinear optics. Moreover, the presented results can be applied to designing materials with desired optical responses.   

Searching for Correlations with the HCO+ 4-3 Molecular Spectra of Protostars

The assignment is based on HCO+ J=4-3 spectral line molecular observations of protostars from the James Clerk Maxwell Telescope, which has the 15 m diameter dish and located in Mauna Kea, Hawaii, USA. Data of 20 protostars are taken from the public LOMASS database and analyzed. We looked for correlations between a few observational quantities.   

Dependence of the Crossing Time on the Sequence Length in a Diploid Discrete-Time Mutation-Selection Model for a Finite Population

This study examined the crossing time in the diploid discrete-time mutation-selection model in a finite population for a range of dominance parameters and selective advantages by switching on a diploid, asymmetric, bridged landscape, from an initial state, a steady state in a diploid, bridged landscape. The dependence of the crossing time on the sequence length was examined for a fixed extension parameter, which was defined as the mean Hamming distance from the optimal allele of the initial steady state divided by the sequence length. The boundary between the deterministic and stochastic regions in the diploid discrete-time mutation-selection model was characterized using the same formula as that in the haploid discrete-time mutation-selection model. The crossing time in a finite population with various population sizes, dominance parameters and selective advantages began to deviate from the crossing time for an infinite population at the critical sequence length. The crossing time for a finite population in the stochastic region was found to be an exponentially increasing function of the sequence length, whose rate was unchanged, regardless of changes in the population size, dominance parameter and selective advantage with a fixed extension parameter.   

Molecular Origins of Higher Harmonics in Large-Amplitude Oscillatory Shear Flow: Shear Stress Response

Recent work has focused on understanding the molecular origins of higher harmonics that arise in the shear stress response of polymeric liquids in large-amplitude oscillatory shear flow. These higher harmonics have been explained using only the orientation distribution of a dilute suspension of rigid dumbbells in a Newtonian fluid, which neglects molecular interactions and is the simplest relevant molecular model of polymer viscoelasticity [R.B. Bird et al., \textit{J Chem Phys}, \textbf{140}, 074904 (2014)]. We explore these molecular interactions by examining the Curtiss-Bird model, a kinetic molecular theory that accounts for restricted polymer motions arising when chains are concentrated [Fan and Bird, \textit{JNNFM}, \textbf{15}, 341 (1984)]. For concentrated systems, the chain motion transverse to the chain axis is more restricted than along the axis. This anisotropy is described by the link tension coefficient, $\epsilon $, for which several special cases arise: $\epsilon =0_{\mathrm{\thinspace }}$corresponds to reptation, $\epsilon >1 \mathord{\left/ {\vphantom {1 8}} \right. \kern-\nulldelimiterspace} 8_{\mathrm{\thinspace }}$to rod-climbing, $1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2\ge \epsilon \ge 3 \mathord{\left/ {\vphantom {3 4}} \right. \kern-\nulldelimiterspace} 4$ to reasonable shear-thinning predictions in steady simple shear flow, and $\epsilon =1_{\mathrm{\thinspace }}$to a dilute solution of chains. We examine the shapes of the shear stress versus shear rate loops for the special cases, $\epsilon =\left( {{0,1} \mathord{\left/ {\vphantom {{0,1} {8,3 \mathord{\left/ {\vphantom {3 8}} \right. \kern-\nulldelimiterspace} 8}}} \right. \kern-\nulldelimiterspace} {8,3 \mathord{\left/ {\vphantom {3 8}} \right. \kern-\nulldelimiterspace} 8},1} \right)$, of the Curtiss-Bird model, and we compare these with those of rigid dumbbell and reptation model predictions.   

Micro-architected Composite Lattices with Tunable Negative Thermal Expansions

Solid materials with minimum or negative thermal expansion (NTE) have broad applications, from dental fillings to thermal-sensitive precision instruments. Previous studies on NTE structures were mostly focused on theoretically design and 2D experimental demonstrations. Here, aided with multimaterial projection micro-stereolithography, we experimentally fabricate multi-material composite lattices that exhibit significant negative thermal expansion in three directions and over a large range of temperature variations. The negative thermal expansion is induced by the structural interaction of material components with distinct thermal expansion coefficients. The NTE performance can be tuned over a large range by varying the thermal expansion coefficient difference between constituent beams and geometrical arrangement. Our experimental results match qualitatively with a simple scaling law and quantitatively consistently with computational models.   

Surface Tension of Methanol as a Function of cut-off Radius and Temperature Controllers.

Molecular dynamics is used to calculate the surface tension of van Leeuwen methanol. The van Leeuwen model of methanol is chosen over other models of methanol, since this model is widely used to study nucleation at low temperature. Usually, scientists use the cut-off radius to be three order of magnitude of segment diameter. In this study, we varied the cut-off radius to estimate the best cut-off at which the surface tension reaches its plateau. After deciding the best cut-off radius for van der Waals and Coulomb interactions (CUT-OFF and PME were used for Coulomb interaction), we varied the temperature controller (van-Housen, Berendsen, and v-rescale) to decide the best temperature controller to be used to study methanol. In all simulations, Gromacs is used at T$=$200-300K with periodic boundary conditions in all dimensions.   

Geometrical design of self-phoretic colloids

Within a unified formalism we study the generic properties of self-phoretic particles for source-or-sink (such as self-diffusiophoresis and self-thermophoresis) and sink-and-source (such as self-electrophoresis) flux distribution across a continuous range of geometries from disk-like to sphere to rod-like shapes. We obtain new insights into the performance of self-phoretic particles as a function of the distribution of surface flux and their shape. Surprisingly, upon varying the geometry between the sphere and rod-like shape, the velocity is not simply an interpolation, but has a nonmonotonic dependent on particle geometry.   

Effect of Graphene, Carbon Black, and Multi-wall Carbon Nano Tube as fillers on Dielectric Response Function of Polystyrene Matrix.

A set of thin samples of three different fillers (graphene, carbon black, and multi-wall carbon nano-tubes) has been prepared in the laboratory. The thin composite films have different concentration of the filler (weight concentration). The dielectric response function of the prepared samples shows strong relaxation at high frequency (\textgreater 500 KHz), the position of this peak is found to be a function of weight percent of the composite.   

SANG-a kernel density estimator incorporating information about the measurement error.

Analyzing nominally large data sets having a measurement error unique to each entry is evaluated with a novel technique. This work begins with a review of modern analytical methodologies such as histograming data, ANOVA, regression (weighted and unweighted) along with various error propagation and estimation techniques. It is shown that by assuming the errors obey a functional distribution (such as normal or Poisson), a superposition of the assumed forms then provides the most comprehensive and informative graphical depiction of the data set's statistical information. The resultant approach is evaluated only for normally distributed errors so that the method is effectively a Superposition Analysis of Normalized Gaussians (SANG). SANG is shown to be easily calculated and highly informative in a single graph from what would otherwise require multiple analysis and figures to accomplish the same result. The work is demonstrated using historical radiochemistry measurements from a transuranic waste geological repository's environmental monitoring program.   

Interface enhanced superconductivity in single unit-cell FeSe films on SrTiO$_{\mathrm{3}}$(110)

The advent of enhanced superconductivity in FeSe/STO(001) has instigated great interests in other interfacial systems both experimentally and theoretically. To figure out the key role of substrate, STO(110) substrate is of great interest because it resembles STO(001) in high density subsurface oxygen vacancies but distinguishes itself by anisotropic in-plane lattice constants and dielectric constant. Here, we investigated molecular beam epitaxy growth of 1-UC FeSe films on STO(110) substrates and studied the superconducting properties by combined \textit{in-situ} scanning tunneling spectroscopy (STS) and \textit{ex-situ} transport measurement. By STS we observed a superconducting gap as large as 17 meV. Transport measurements on 1-UC FeSe/STO(110) capped with FeTe layers reveal superconductivity with an onset transition temperature ($T_{C})$ of 31.6 K and an upper critical magnetic field of 30.2 T. We also find that $T_{C}$ can be further increased by an external electric field, but the effect is weaker than that on STO(001) substrate. Our study highlights the important roles of interface related charge transfer and electron-phonon coupling in the high temperature superconductivity of FeSe/STO. References: [1] Q. Y. Wang, \textit{et al.}, Chin. Phys. Lett., \textbf{29}, 037402 (2012). [2] J. J. Lee\textit{ et al.}, Nature \textbf{515}, 245 (2014).   

Micro-Structure of Iron in \textbf{\textit{Petroselinum crispum}} and its dependence with the chemical nature of the soil.

The micro structure of the iron Iron in \textbf{\textit{Petroselinum crispum }} is investigated using synchrotron X-ray Absorption and Mossbauer spectroscopy. Plants were grown under controlled soil conditions with different pH, and iron concentrations. The correlation between the micro structure of the iron in \textbf{\textit{Petroselinum crispum}} plants and the soil conditions were studied. Most of the iron present in the plants has the form Fe$^{\mathrm{3+}}$ or electron density at the site of the iron nucleus similar to that of Fe$^{\mathrm{3+}}$. But the amount of iron absorbed by the plants depends on the soil conditions. These findings will help establish soil conditions necessary to increase Fe$^{\mathrm{2+}}$ intake by plants similar to the form of iron present in most supplements.   

Numerical study of the butterfly effect on the solutions of the logistic difference equation using arbitrary significant digits

The solutions of the logistic difference equation when they are under the influence of the chaotic regime are very sensitive to initial conditions due to the butterfly effect. In this study we used arbitrary significant digits to generate solutions of the logistic difference equation under the influence of chaos, and a follow of its effects along each digit of the solutions was made. A large amount of significant digits to generate the solutions is necessary since it is the only way of naturally appreciating the implications of chaos on these solutions. We compared digit by digit the numerical solutions that were generated by several different initial conditions that contain modifications in a very far significant digit, with respect to the solution of another initial condition that was selected for a control solution. The results shown that it is possible to track the butterfly effect and easily predict the moment on which its effects will be noticeable.   

Experimental determination of fluxoid quantum's contribution to magnetic moment for force metrology

Utilizing a cantilever torque-magnetometry equipped with fiber-scanning capability, we have executed precision measurements of the magnetic moment of a micron-sized Nb ring in superconducting state at $T$ = 4 K, which is a key element for sub-piconewton force standard previously suggested by some of the authors. The magnetic moments due to diamagnetic response and individual magnetic fluxoid have been independently determined with a resolution of sub-femto-Am$^2$. Within the accuracy of the spring constant determined from a thermal noise method, the results are very consistent with the estimation by Brandt and Clem’s model that considers finite-penetration-depth effect.   

Analytical Phase Equilibrium Function for Mixtures Obeying Raoult's and Henry's Laws.

When a mixture of two substances exists in both the liquid and gas phase at equilibrium, Raoults and Henry's laws (ideal solution and ideal dilute solution approximations) can be used to estimate the gas and liquid mole fractions at the extremes of either very little solute or solvent. By assuming that a cubic polynomial can reasonably approximate the intermediate values to these extremes as a function of mole fraction, the cubic polynomial is solved and presented. A closed form equation approximating the pressure dependence on mole fraction of the constituents is thereby obtained. As a first approximation, this is a very simple and potentially useful means to estimate gas and liquid mole fractions of equilibrium mixtures. Mixtures with an azeotrope require additional attention if this type of approach is to be utilized.   

Possible Unconventional superconductivity in YCo$_{0.7}$C$_{2}$

Non-centrosymmetric superconductors as CePt$_{3}$Si [1,2] and sequicarbides (La,Y)$_{2}$C$_{3}$ [3] are remarkable examples of unusual properties displayed associated to unconventional pairing due to an antisymmetric spin-orbit coupling. Another interesting case is the family of compounds belonging to the CeNiC$_{2}$ type structure, in which more than thirty stable compounds have found to crystallize in this structure. Here we report magnetization, resistivity, and heat capacity measurements on poly-crystalline samples of non-centrosymmetric YCo$_{0.7}$C$_{2}$, showing clear evidence of bulk superconductivity with a critical temperature of T$_{c}=4$ K. Interestingly the specific heat of the superconducting state deviates from conventional exponential temperature dependence, which is suggestive of possible unconventional superconducting behavior in YCo$_{0.7}$C$_{2}$, similar to that seen in the isostructural and isoelectronic superconductor LaNiC$_{2}$ [4]. Besides, these results strongly suggest that this material is a strong candidate of multiband superconductivity. \\ References: [1] E. Bauer, G. Hilscher, H. Michor, C. Paul and P. Rogl, Phys. Rev. Lett. 92 (2004) 027003.   

Structural and electronic properties of atomically thin germanium selenide polymorphs

Using comprehensive density functional theory calculations, we systematically investigate the structure, stability, and electronic properties of five polymorphs of GeSe monolayer, and highlight the differences in their structural and electronic properties. Our calculations show that the five free-standing polymorphs of GeSe are stable semiconductors. $\beta $-GeSe, $\gamma $-GeSe, $\delta $-GeSe, and $\varepsilon $-GeSe are indirect gap semiconductors, whereas $\alpha $-GeSe is a direct gap semiconductor. We calculated Raman spectra and scanning tunneling microscopy images for the five polymorphs. Our results show that the $\beta $-GeSe monolayer is a candidate for water splitting.   

Structure and Self-Assembly of Oligocarbonate-Fluorene End Functionalized Poly (ethylene glycol) ABA Triblock Polymer

Hierarchical structures of oligocarbonate-fluorene end-functionalized poly(ethylene glycol) triblock copolymer (P(F-TMC)m-PEG444-P(F-TMC)m) were characterized by light scattering, atomic force microscopy, and Ultraviolet-visible spectroscopy in dilute regime in water, a poor solvent of F-TMC block. The evidence for pai-pai stacked of F-TMC block in self-assembled structure was provided. The self-assembly behavior is highly dependent on concentration and F-TMC block length, m. The presence of clusters dominates the population of scatterers once m is larger than 2, where there is no clear evidence of a separation of micelles and clusters. The molecular aggregation driven by F-TMC groups appears too strong to permit labile micelle-cluster dynamics as observed with m $=$ 2 and 1.2. The non-mean field scaling of the aggregation number, when compared to models for triblock copolymers, highlights the need for a molecular-based model to predict the self-assembly at low end-group numbers. In our case, the end-groups are oligomers, so the comparison to Flory scaling may not be justified.   

Negative magnetization and exchange bias effect in Ni$_{\mathrm{1.4}}$Mn$_{\mathrm{2}}$Ga$_{\mathrm{0.6}}$

Ni-Mn-X$_{\mathrm{\thinspace }}$based Heusler alloys have attracted significant interest in recent years due to their multifunctional properties. Exchange bias (EB) is one such property that results from competing magnetic interactions in these alloys. The EB effect is typically observed in materials where ferromagnetic (FM) and antiferromagnetic (AFM) interactions co-exist. Since the discovery of EB effect in CoO (AFM) coated Co (FM) nanoparticles by Meikle John and Bean in 1956, a significant amount of research efforts have been made on this subject. Here, we have performed an experimental study on the magnetic and exchange bias properties of polycrystalline Ni$_{\mathrm{1.4}}$Mn$_{\mathrm{2}}$Ga$_{\mathrm{0.6}}_{\mathrm{\mathbf{\thinspace }}}$alloy by X-ray diffraction, dc magnetization, and ac susceptibility measurements. The material exhibits a ferromagnetic Curie temperature of $\sim $300 K. The magnetization versus field data obtained at 5 K under zero field condition exhibits a double shifted hysteresis loop that disappears at higher temperatures. When the sample is cooled from room temperature to 5 K in applied magnetic fields, exchange bias is observed, whose magnitude is strongly dependent on the cooling field. A maximum exchange bias field of 730 Oe is observed under field cooling condition at 5 K. A negative magnetization is observed in the magnetization versus temperature data obtained at magnetic fields smaller than 75 Oe. The experimental results are explained in terms of the competing ferromagnetic and antiferromagnetic exchange interaction that exist in the materials due to the Mn atoms occupying multiple crystalline sites resulting in a spin glass-type frustrated ground state.   

Anomalous transport properties of Ni$_{\mathrm{2}}$Mn$_{\mathrm{1-x}}$Cr$_{\mathrm{x}}$Ga Heusler alloys at the martensite-austenite phase transition

The Heusler alloy Ni$_{\mathrm{2}}$MnGa exhibits a first order martensitic phase transition at T$_{\mathrm{M\thinspace }}\approx $ 202 K. During this transition, the high temperature cubic L2$_{\mathrm{1}}$ phase (austenite) of the alloy transforms to a low temperature phase (martensite) with a lower symmetry. In both stoichiometric and off-stoichiometric Ni$_{\mathrm{2}}$MnGa based materials, jump-like anomalies are observed in the resistivity versus temperature data in the vicinity of T$_{\mathrm{M}}$. The magnitude of the jump has been reported to vary from less than 1{\%} to a few percent. This variation in the magnitude of resistivity change has been attributed to the difference in scattering on the vibrational motion of the lattice between the austenitic and martensitic phases and the reconstruction of the electronic structure. Although, several reports can be found in existing literature that discuss the change of resistivity of Heusler alloys at MPT, detailed study of a complete system that shows a systematic change of resistivity at MPT is missing. Here we report an experimental study on a series of Ni$_{\mathrm{2}}$Mn$_{\mathrm{1-}}_{x}$Cr$_{x}$Ga Heusler alloys. A detailed study has been performed on this previously unexplored system by magnetization and transport measurements. Sharp step-like anomalies are observed in the resistivity data of the alloys, in the vicinity of the T$_{\mathrm{M}}$, that changes dramatically with increasing Cr concentration. The magnitude of the jump in resistivity changes dramatically from less than 1 {\%} to nearly 18 {\%}. The results provide a further understanding of the mechanisms that may cause the change in resistivity in the vicinity of MPT.   

Information jet: Handling noisy big data from weakly disconnected network

Sudden aggregation (information jet) of large amount of data is ubiquitous around connected social networks, driven by sudden interacting and non-interacting events, network security threat attacks, online sales channel etc. Clustering of information jet based on time series analysis and graph theory is not new but little work is done to connect them with particle jet statistics. We show pre-clustering based on context can element soft network or network of information which is critical to minimize time to calculate results from noisy big data. We show difference between, stochastic gradient boosting and time series-graph clustering. For disconnected higher dimensional information jet, we use Kallenberg representation theorem (Kallenberg, 2005, arXiv: 1401.1137) to identify and eliminate jet similarities from dense or sparse graph.   

Scale Invariant Fluctuations of Proteins Native States

Long-range correlations in biological systems often hints for the presence of universal mechanism at work. Here we study protein native dynamics by analyzing a large set of structure ensembles determined by solution NMR. For proteins of diverse sizes, the average distance-dependent cross-correlation functions $\phi(r)$ and its correlation length $\xi_\phi$ are analyzed. The analysis uncovered the presence of nontrivial scaling in the proteins' equilibrium dynamics around native states. We show that the correlation length is proportional to the gyration radius of the molecule, implying that the motion of any residue could influence all the others, up to the entire molecule. In addition, it is found that certain shapes are favored, such that for any given protein size the folding process ``chooses" the shape with the maximum susceptibility. These results suggest that the proteins native state is critical in the same sense with other slowly built self-organized critical systems, which once posed near the minimum of the energy landscape, preserve their dynamic flexibility.   

Verwey transition of nano-sized magnetite crystals investigated by $^{\mathrm{57}}$Fe NMR

It is well known that magnetite crystals undergo a metal-insulator transition at the Verwey transition temperature, T$_{\mathrm{V}} \quad =$ 123 K. In this work, we studied the Verwey transition of nano-sized crystals with $^{\mathrm{57}}$Fe NMR. In the metallic state above Tv, the NMR spectrum shows a single sharp peak, which broadens below T$_{\mathrm{V}}$ indicating the Verwey transition. We measured the spectra of the nano-crystals with radii of 16 nm, 25 nm, and 40 nm and compared with that of a bulk. The transition temperature obtained from the NMR spectra depends on both the crystal size and crystallinity. When the crystal size decreases from bulk to 16 nm, the transition temperature drops from 123 K to 100 K. The transition temperature of the samples kept dry air decrease due to aging.   

Cr$^{3+}$ NMR for Multiferroic Chromium spinel ZnCr$_{2}$Se$_{4}$

Multiferroic systems including ZnCr$_{2}$Se$_{4}$, the chromium spinel with helical spin structure, have been in huge interest for decades due to its physical variety and applicability. In the temperature range between 21K and 80K, this material shows negative thermal expansion. Due to the bond frustration, the spins of the chromium ions order helically below the transition temperature, 21K, though the exchange constant tends to make a ferro-order. The anomalous 1$^{st~}$order-like magnetic transition is yet clarified and still an interesting topic. To probe microscopic origin of these features, we measured zero-field NMR of Cr$^{3+}$~ ions having nuclear spin 3/2. Six peaks were observed revealing Nuclear Quadrupole Resonance(NQR) and anisotropic hyperfine field at chromium sites. The NQR spectrum reveals that the structure is highly distorted below the magnetic transition temperature where the normal Jahn-Teller distortion is absent. Temperature dependence of the spectrum is also measured to obtain the magnetization as a function of temperature.   

Topological Analysis, Modeling, and Imaging of Gelatin-Based Hydrogels

Gelatin is a component of~natural biocompatible scaffolds used in tissue engineering constructs. However, due its supra-molecular structure, the mesh size is drastically larger compared to synthetic polymers having the same moduli, and therefore the Rubber Elastic Theory cannot be used to describe properties of gelatin. Gelatin forms distinct fibrils, bundles of triple helix chains, which form rigid areas. We experimented with two different gel moduli, made possible by varying the concentration of microbial transglutaminase (mTG). mTG forms permanent cross links and affects the morphology of the gelatin by changing the number of fibrils formed. Thus, the mesh size calculated from the Rubber Elastic Theory was much smaller than the actual size of the mesh, as measured from cryoscanning electron microscopy images and~fluorescent bead particle migration. We also observed the en-mass migration behavior of dermal fibroblast cells as a function of the substrate rheological response. Our results will present the ability of the cells to sense the structure of the underlying substrate, as well as the absolute value of the modulus. Furthermore, the data will be interpreted in terms of a modified theoretical model, which takes into account the structure and mesh size of the gel.   

Charge transport in the organic doped spin-liquid candidate, $\kappa$-(ET)$_{4}$Hg$_{2.89}$Br$_{8}$, under Pressure

The family of layered organic conductors $\kappa$-(ET)$_{2}$X plays an important role in the study of Mott physics, which is a major subject in the condensed matter physics. While most $\kappa$-(ET)$_{2}$X compounds have half-filled bands and antiferromagnetic nature, the title compound $\kappa$-(ET)$_{4}$Hg$_{2.89}$Br$_{8}$ ($\kappa$-HgBr) is an exceptional doped system which is supposed to be the only doped spin-liquid candidate up to the present. The transport study under controlled pressure, which enables us to investigate this intriguing system with tuning the correlation strengths, revealed that $\kappa$-HgBr shows a transition or crossover from a non-Fermi liquid to a Fermi-liquid as pressure increases.\footnote{H. Taniguchi \textit{et al}., J. Phys. Soc. Jpn. \textbf{76}, 113709 (2007).}$^{,}$\footnote{H. Oike \textit{et al}., Phys. Rev. Lett. \textbf{114}, 067002 (2015).} In the present work, we have carried out the detailed transport measurement under pressure for $\kappa$-HgBr with static magnetic fields applied normal to the conducting layers. I will discuss the in-plane and out-of-plane charge transport in normal and superconducting states in this doped spin-liquid candidate with variable electron correlation.   

Analytical Lower and Upper Bounds for the Threshold Surfaces of Quantum Error Correcting Codes

If all the physical gates in a fault tolerant code construction have a failure probability below a certain value, the failure probability of the construction approaches zero after many concatenations. This value is called the threshold value of the code and lower bounds for it for various codes have been reported in the literature. However, these approaches do not take into account that the failure probability of each species of logical gate depends on that of many different species of physical gates, and that the distribution of logical failure probability depends on that of many different physical gates. How can we reconcile the interdependency of the failure probabilities of all the various species of gates? Direct simulation would be one of the possible ways to attack this question, but it would be difficult to be done at high concatenation levels because of the exponential growth of simulation time. Here, we deal with this question by instead considering a multidimensional space of the failure probabilities of the physical gates and study the set of points that approach zero error after a large number of concatenations. We present a way to obtain lower and upper bounds for the boundary of this set, what we call the threshold surface, given a particular code and constructions of logical gates. Our method uses only the logical failure probabilities after one concatenation, and moreover the running time of the algorithm scales linearly with respect to concatenation levels. We hope this will establish a reasonable goal for experiments to work towards a scalable quantum computer.   

Measurement of Diffraction Properties of Colloidal Crystals

Close-packed, self-assembled arrays of micrometer polystyrene or silica spheres are high quality artificial crystals that generate well-defined diffraction patterns in the visible range. Such crystals are explored as possible substrates for deposition of nanomaterials such as graphene. Quasi-monochromatic visible light diffraction microscopy is used to characterize effective refractive index and crystal structure, specifically grain size, orientation, and lattice parameters. These parameters can be used to monitor deformations of the colloidal crystal lattice during transfer of nanomaterials.   

Nonparametric estimation of quantum states, processes and measurements

Quantum state, process, and measurement estimation methods traditionally use parametric models, in which the number and role of relevant parameters is assumed to be known. When such an assumption cannot be justified, a common approach in many disciplines is to fit the experimental data to multiple models with different sets of parameters and utilize an information criterion to select the best fitting model. However, it is not always possible to assume a model with a finite (countable) number of parameters. This typically happens when there are unobserved variables that stem from hidden correlations that can only be unveiled after collecting experimental data. How does one perform quantum characterization in this situation? We present a novel nonparametric method of experimental quantum system characterization based on the Dirichlet Process (DP) that addresses this problem. Using DP as a prior in conjunction with Bayesian estimation methods allows us to increase model complexity (number of parameters) adaptively as the number of experimental observations grows. We illustrate our approach for the one-qubit case and show how a probability density function for an unknown quantum process can be estimated.   

Adsorption and Transport of Methane Molecules through One-Dimensional Channels in Dipeptide-Based Materials

The development of new materials for use in energy and environmental applications is of great interest, in particular in the areas of gas separation and carbon capture, where molecular transport plays a significant role. The dipeptides are organic molecules that offer an attractive possibility in such areas, because they form open hexagonal crystalline structures (space group P61) with quasi one-dimensional channels of tunable pore diameters in the range 3-6 {\AA}. These molecular crystals exhibit selective adsorption, as well as, water and gas transport properties: these are believed to result from collective vibrations of the crystal structure that are coupled to the motions of the guest molecules within the channels. Current studies focus on characterizing the system methane and L-Isoleucyl-L-Valine (IV): this was initially done with high-resolution adsorption isotherms; then, high-resolution Inelastic Neutron Scattering measurements at the Spallation Neutron Source (BASIS spectrometer) revealed clear rotational tunneling peaks, offering details to unravel the potential energy surface of the system, as well as, evidences that channels flexibility and dynamical motion of the molecules have influence on the dipeptides adsorption properties.   

Development of Cellulose/PVDF-HFP Composite Membranes for Advanced Battery Separators

Improvements in battery technology are necessary as Li-ion batteries transition from consumer electronic to vehicular and industrial uses. An important bottle-neck in battery efficiency and safety is the quality of the separators, which prevent electric short-circuits between cathode and anode, while allowing an easy flow of ions between them. In this study, cellulose acetate was dissolved in a mixed solvent with poly(vinylpyrrolidone) (PVP), and the mixture was forcespun in a peudo paper making process to yield nanofibrillated nonwoven mats. The mats were soaked in NaOH/Ethanol to strip PVP and regenerate cellulose from its acetate precursor. The cellulose mats were then dipped in poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) to yield the cellulose/PVDF-HFP composte membranes. These membranes were characterized chemically through FTIR spectroscopy and solvent-stability tests, thermally through DSC, physically by stress/strain measurements along with weight-based electrolyte uptake, and electrically by AC-impedance spectroscopy combined with capacitative cycling.   

An Automated, High-Throughput System for GISAXS and GIWAXS measurements of thin films

Grazing incidence small-angle X-ray scattering (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) are important techniques for characterizing thin films. In order to meet rapidly increasing demand, the SAXSWAXS beamline at the Advanced Light Source (beamline 7.3.3) has implemented a fully automated, high-throughput system to conduct SAXS, GISAXS and GIWAXS measurements. An automated robot arm transfers samples from a holding tray to a measurement stage. Intelligent software aligns each sample in turn, and measures each according to user-defined specifications. Users mail in trays of samples on individually barcoded pucks, and can download and view their data remotely. Data will be pipelined to the NERSC supercomputing facility, and will be available to users via a web portal that facilitates highly parallelized analysis.   

Energy storage by droplet/bubble capillary force

In present research, a capillary energy storage device is designed by a channel-expansion chamber structure. In the proposed model, the energy is stored in the form of compressed droplet/ bubble in a smaller channel with the release of energy in the form of capillary driven flow. The power output curve for this device is provided by numerical studies. Trials are also engaged to design a continuous output supply by considering the power output and the viscous loss. This device can potentially be used in both micro- and nano- scale energy storage.   

Imaging ballistic carrier trajectories in graphene using scanning gate microscopy

Graphene layers encapsulated by hexagon boron-nitride enable charge carriers to travel ballistically over several microns and provide an opportunity to realise electron optics with Dirac fermions. Scanning gate microscopy is a valuable tool for directly imaging such effects and has recently been applied to investigate coherent scattering in graphene \textit{pnp} junctions [1]. In this work we use SGM to image magnetic focusing of ballistic carriers in a graphene device [2]. By locally varying the carrier concentration and electrostatic potential with the tip we are able to image electrons bouncing from the graphene edges. Moreover, by refocusing misaligned electrons back to collector, our results show how scanning probe tips can be used as mobile lenses for manipulating Dirac fermions in novel device concepts. [1] E.D. Herbschleb, et al., Phys. Rev. B 92, 125414 (2015) [2] S. Morikawa, et al., Appl. Phys. Lett. 107, 243102 (2015); S. Bhandari, et al., arXiv:1510.05197 (2015).   

\textbf{NEXT GENERATION GAMMA RAY DIAGNOSTICS FOR THE NATIONAL IGNITION FACILITY}

Fusion reaction history and ablator areal density measurements based on gamma ray detection are an essential part of Inertial Confinement Fusion (ICF) experiments on the National Ignition Facility (NIF). Capability improvements are being implemented in sensitivity, temporal and spectral response relative to the existing Gamma Reaction History diagnostic (GRH-6m). The ``Super'' Gas Cherenkov Detector (GCD) [1] will provide 200x more sensitivity, reduce the effective temporal resolution from 100 to 10 ps, and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH-6m. The Gamma-to-Electron Magnetic Spectrometer (GEMS) [2] - a Compton spectrometer intended to provide true gamma energy resolution ($\le $5{\%}) for isolation of specific lines such as t(d,$\gamma )$, D(n,$\gamma )$, $^{\mathrm{12}}$C(n,n'$\gamma )$ and energetic charged particle nuclear reactions indicative of ablator/fuel mix. [1] H.W. Herrmann, et al., Rev. Sci. Instrum. 85, 11E124 (2014) [2] Y. Kim, et al., Rev. Sci. Instrum. 85, 11E122 (2014)   

Thermodynamic, Modeling and Neutron Investigations of Cycloalkanes Adsorbed on MgO (100) and Graphite Basal Plane

Understanding the adsorption of molecules on solid surfaces is central to many scientific and technological challenges. Solid surfaces such as metal oxides, carbonaceous archetypes, porous silica, and metal organic frameworks currently represent significant components of nanomaterial research because of their widespread use in optoelectronics, separation chemistry, and catalysis. Understanding the interaction between adsorbed molecules and surfaces is a necessity for developing synthetic methods to produce materials with specific functional properties. An investigation of the effects of molecular and adsorbate symmetry is proposed in this study. The principal aim of this work is to identify the role of surface and molecular symmetry on the physicochemical properties of 2D layers of cyclic molecules adsorbed on metal oxide and semiconductor substrates. Initially our characterization will focus on the thermodynamic and microscopic structure and dynamics of cyclopentane and cyclohexane on the MgO (100) surface and graphite basal plane. In order to realize this goal adsorption isotherms, inelastic neutron scattering (INS) and molecular dynamics (MD) simulation studies will be performed to investigate the structure, dynamics and wetting properties.   

Resistance-Strain Relation On Vanadium Dioxide Thin Films

Vanadium dioxide is a strongly correlated material with a sharp metal to insulator transition at \textasciitilde 341 K. It is well known that the strain along c-axis can change the transition temperature, but the other effects of the strain have not been drawing much attention. In this work we have studied the effects of the strain on resistance changes in the polycrystalline and epitaxial films. Polycrystalline films of VO$_{\mathrm{2}}$ are deposited on the Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) using a SiO$_{\mathrm{2}}$ buffer layer. The strain on film is tuned by applying a bias electric field through the piezoelectric substrate, and the resistance is measured using four-probe method. The epitaxial films of VO$_{\mathrm{2}}$ are grown on TiO$_{\mathrm{2}}$ (001) and have been glued to PMN-PT substrate to transfer strain. The change in the resistance of the epitaxial films is measured to be only about 30{\%} more than polycrystalline films for the same amount of strain. We have studied the strain-induced resistance changes as a function of temperature. we have shown that the resistance is more sensitive to strain in the metallic phase.   

Nonlinear Dynamic Model Explains The Solar Dynamic

Nonlinear mathematical model in torus representation describes the solar dynamic. Its graphic presentation shows that without perturbing force the orbits of the planets would be circles; only perturbing force could elongate the circular orbits into ellipses. Since the Hubble telescope found that the planetary orbits of other stars in the Milky Way are also ellipses, powerful perturbing force must be present in our galaxy. Such perturbing force is the Sagittarius Dwarf Galaxy with its heavy Black Hole and leftover stars, which we see orbiting around the center of our galaxy. Since observations of NASA’s SDO found that magnetic fields rule the solar activity, we can expect when the planets align and their magnetic moments sum up, the already perturbed stars to reverse their magnetic parity (represented graphically as periodic looping through the hole of the torus). We predict that planets aligned on both sides of the Sun, when their magnetic moments sum-up, would induce more flares in the turbulent equatorial zone, which would bulge. When planets align only on one side of the Sun, the strong magnetic gradient of their asymmetric pull would flip the magnetic poles of the Sun. The Sun would elongate pole-to-pole, emit some energy through the poles, and the solar activity would cease. Similar reshaping and emission was observed in stars called magnetars and experimentally observed in super-liquid fast-spinning Helium nanodroplets. We are certain that NASA’s SDO will confirm our predictions.   

Reconstructing quantum states from local data

Quantum spin chains are systems of extreme complexity, in the sense that the number of parameters that fully characterize the state of a quantum spin chain grows exponentially with the number of spins. Yet, physically relevant subsets of all quantum states can be well-approximated by a small number of parameters using well-known methods such as Matrix Product States (MPS). The structure of such states can guarantee reconstruction of the state from the measurement of a small number of simple observables, merely growing linearly with the number of spins. \\ We compare two classes of quantum states which admit efficient reconstruction from incomplete, local information: States which have vanishing conditional mutual information, and the recently introduced class of states with non-decreasing operator Schmidt rank under partial traces which includes generic Matrix Product Operators (MPO). It is well-known that R\'{e}nyi entropies can be used to characterize the bond dimension of a pure MPS, i.e. the number of parameters required to describe the state. For mixed MPOs, no similar relation is known. Our comparison provides a first relation between the mutual information and the bond dimension of an MPO representation of a mixed state.   

Validation of Capillarity Theory at the Nanometer Scale by Atomistic Computer Simulations of Water Droplets and Bridges in Contact with Hydrophobic and Hydrophilic Surfaces

Capillarity is the study of interfaces between two immiscible liquids or between a liquid and a vapor. Capillarity theory (CT) was created in the early 1800s and it is applicable to macroscopic (\textgreater 1 $\mu $m) systems. In general, macroscopic theories are expected to fail at \textless 10 nm scales where molecular details may become relevant. We show that, surprisingly, CT provides satisfactory predictions at 210 nm scales. Specifically, we perform atomistic molecular dynamics (MD) simulations of water droplets and capillary bridges of different symmetry in contact with various surfaces. The surfaces correspond to hydroxilated silica, modified to cover a wide range of hydrophobicity/hydrophilicity. In agreement with CT, it is found that (i) water contact angle is independent of the droplet/bridge geometry and depends only on the surface employed; (ii) CT provides the correct droplet/bridge profile for all hydrophobic/hydrophilic surfaces considered; and, remarkably, (iii) CT works even for the very small droplets/bridges studied, for which the smallest dimension is $\approx $2 nm. We confirm the self-consistency of CT at 210 nm scales by calculating the \textit{capillary forces} between different surfaces induced by capillary bridges; the agreement between MD simulations and CT theory is remarkable   

A novel cell penetrating peptide carrier for the delivery of nematocidal proteins drug

Nematodes have recently become a primary source of harmful diseases to the environment that inflict harsh damages to pine trees and marine species. However, nematodes cannot be killed by normal pesticides or chemicals due to their thick outer protective layer mainly composed of collagen and cuticles. Thus, a novel approach to trigger intracellular delivery of chemicals through the layers of nematodes is required. In this study, the selection of the novel CPP was carefully progressed through protein database and serial digested fragmentation, internalization of each amino sequence was analyzed through flow cytometry and confocal microscope. As one of the most effective CPP material, JH 1.6 was compared with other major CPPs and its cellular toxicity was investigated. Furthermore, JH 1.6 was attached to various RNA, DNA, and proteins and internalization efficiency was evaluated for mammalian cells. To examine its effects on nematodes in vivo, JH 1.6 was conjugated with nematocidal protein - botulinum neurotoxin (BnT) and treated in C.elegans as a model animal. The results showed that JH 1.6 had high relative internalization rate and low cellular toxicity compared to other major CPP such as TAT and GV1001 peptides.   

The Role of Ligand in the Mechanical Properties of Self-Assembled Nanoparticle Films

Self-assembled films of nanoparticles (NP) capped with ligands at the air/water interface exhibit rich mechanical responses to compression including hashing, wrinkling, and folding, which are the combined result of particle- and ligand-based interactions. Previous studies have shown that a high concentration of ligands inhibits wrinkling and folding, but the mechanism remains elusive. By using inductively coupled plasma optical emission spectrometry (ICP-OES) to measure the ligand concentration of our NP solutions and then back-adding excess ligands at controlled amounts, we precisely control ligand-based interactions, enabling an investigation of how these interactions guide self-assembly and correspondingly on mechanical properties. Our experiments reveal that increasing the ligand concentration of the films causes the formation of free-ligand islands in addition to an increase in the interparticle separation. These effects are correlated with the previously observed inhibition of wrinkling and folding, as well a decrease in the dilatational and shear moduli. This work was supported by the University of Chicago Materials Research Science and Engineering Center, NSF-DMR-1420709.   

Interleukin 18 secretion and its effect in improving Chimeric Antigen Receptors efficiency

Clinical trials have shown that chimeric antigen receptor T cells modified to target cancer cells expressing a surface antigen found on immature B-cells. The purpose of this experiment is to take a pro-inflammatory cytokine, and analyze its effect in improving the efficiency of the T cells. IL-18 has been previously shown to recruit T cells to the tumor site and improve their secretion of cytotoxic cytokines. A human model of the proposed armored T cell has been created and has shown success in combating cancer cells in vitro. The next step is to design and produce a murine model to test in vivo in immunocompetent mice. This research project aimed to create two models: one utilizing 2A peptides and another utilizing IRES elements as a multicistronic vector. Both models would require the insertion of the desired genes into SFG backbones. IRES, a DNA element which acts as a binding site for the transcriptional machinery to recognize which part of the DNA to transcribe, commonly found in bicistronic vectors, is large with 500-600 base pairs, and has a lower transgene expression rate. P2A is smaller, only consisting of about 20 amino acids, and typically has a higher transgene expression rate, which may or may not result in higher effectiveness of the model.   

Towards Producing Black Nobel Laureates Affiliated with "African Universities"

While Africa has produced a handful Nobel laureate in literature and peace, it has continued to shy away from producing any in the other categories. The reason is not farfetched; our university system is not up to standard. It is saddening that in this century, African countries place emphasis on certificates and not on knowledge. This has made the continent produce students that lack the intellectual capability, experimental ability, fundamental training, creativity, and motivation to excel except they get a foreign training. It is this backdrop that precipitated the research into the methods of teaching and research in universities across Africa. The study is designed to identify the problems and proffer solution to them. Two important questions immediately come to mind. (1) What factors account for the difficulty in producing Nobel laureates affiliated with African universities? (2) What strategies could be adopted to improve teaching and research in African universities? Several factors were investigated which revolve around funding, the competence of the lecturers, quality of students admitted, attitude of the students, parents and government. Nigerian universities were investigated and important deductions were made. During the study an inquiry was made on the method of instruction at various universities, from result obtained, the study therefore concluded that adequate funding, the presence of erudite scholars and brilliant minds will produce future Nobel laureate affiliated with the continent. The study therefore recommended admission and employment of only students and lecturers who have got a thing for academics into the universities and adequate funding of universities and research centres.   

Investigation of Local Structure and Cation Ordering in Dielectric Oxide Microwave Ceramics with stoichiometry A(Li$_{\mathrm{x}}$(Nb,Ta)$_{\mathrm{y}})$O$_{\mathrm{3\thinspace }}$Using $^{\mathrm{7}}$Li and $^{\mathrm{93}}$Nb solid-state NMR spectroscopy.

The local structure and cation ordering in dielectric oxide microwave ceramics with stoichiometry A(Li$_{\mathrm{x}}$(Nb,Ta)$_{\mathrm{y}})$O$_{\mathrm{3\thinspace }}$are investigated using $^{\mathrm{7}}$Li and $^{\mathrm{93}}$Nb solid-state NMR spectroscopy. For all samples, $^{\mathrm{7}}$Li MAS NMR spectra show one strong and sharp resonance peak indicating one unique environment which corresponds to local lithium environment of nearest B-site neighbor (nBn) surrounded by 4 LiO$_{\mathrm{6\thinspace }}$octahedra and 2 NbO$_{\mathrm{6}}$ octahedra (TaO$_{\mathrm{6}}$ in some samples). In addition to this, $^{\mathrm{7}}$Li MAS NMR spectrum of (Ca$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ show one additional weak and broad resonance peak which can be assigned to nBn of 3 LiO$_{\mathrm{6\thinspace }}$octahedra and 3 NbO$_{\mathrm{6}}$ octahedra. $^{\mathrm{93}}$Nb MAS NMR spectra of samples with niobium content, show a resonance peak with tail toward the low frequency limit, an evidence to the existence of chemical shifts and quadrupole couplings distributions. Both (Sr$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ and Ca(Li$_{\mathrm{1/4}}$Nb$_{\mathrm{3/4}})$O$_{\mathrm{3}}$ spectra show one broad resonance peak, which can be interpreted as one NbO$_{\mathrm{6}}$ octahedron nBn with many slight variations through out the sample. While (Ca$_{\mathrm{2/3}}$La$_{\mathrm{1/3}})$(Li$_{\mathrm{1/3}}$Nb$_{\mathrm{2/3}})$O$_{\mathrm{3}}$ spectra show four peaks correspond to four distinct NbO$_{\mathrm{6}}$ octahedra local nBn environments with the nBn configuration as: (i) 3 LiO$_{\mathrm{6}}$ and 3 NbO$_{\mathrm{6}}$; (ii) 2 LiO$_{\mathrm{6}}$ and 4 NbO$_{\mathrm{6}}$; (iii) 1 LiO$_{\mathrm{6}}$ and 5 NbO$_{\mathrm{6}}$; (iv) all 6 NbO$_{\mathrm{6}}$   

3D cancer cell migration in a confined matrix

Cancer cell migration is widely studied in 2D motion, which does not mimic the invasion processes in vivo. More recently, 3D cell migration studies have been performed. The ability of cancer cells to migrate within the extracellular matrix depends on the physical and biochemical features of the extracellular matrix. We present a model of cell motility in confined matrix geometry. The aim of the study is to study cancer migration in collagen matrix, as a soft tissue, to investigate their motility within the confined and surrounding collagen environment. Different collagen concentrations have been used to show the ability of these cancer cells to move through such a complex structure by measuring Cancer cell migration velocity as well as the displacement.   

Blackhole formula and example relativity

Black hole formula 1) Second dimension (x,y) f(x)=y Energy E=m*c^2 2) Third dimension (x,y,z) really x=y=z Black hole formula Root(c^2)=c=Root(E/m) As mass go the velocity of light, mass become black hole so there are energy as multiply by mass. Example relativity When E=m*c^2 1) Root(c^2)=c=Root(E/m) 2) 3*c*Root(c^2)=3*c*Root(E/m)=3*c^2 From 1) to 2) as an example, As velocity is faster, mass increased. It means when velocity is increased, sec(time) is slower, and m(distance) is increased.   

Epitaxial growth of Cu films on Ag(111) characterized on monitoring the evolution of their surface states

The growth of Cu on Ag(111) attracts interest because of its unusual growth behavior. Previous STM work indicated that, when Cu was deposited on Ag(111) at room temperature, Cu islands formed with a Cu coverage as small as 0.02 ML. However, a (9×9) reconstruction was observed on the surface of these Cu islands, suggesting that the surface of the Cu islands on Ag(111) might be covered with one atomic layer of Ag. The suggestion was not verified because the STM lacked the capability to discern various chemical species. To address the issue, we characterized the growth of Cu films on Ag(111) with a novel approach based on ARPES. On monitoring the evolution of the surface states, we showed that the surface of the Cu islands on Ag(111) with the (9×9) reconstruction was indeed covered with Ag. Our results also showed that the mobility of Ag on Cu(111) greatly depends on temperature. Ag does not migrate on the surface of the Cu islands at a low temperature, but does at 300 K and significantly at 380 K. In addition, the migration of Ag on Cu is associated with the existence of the Ag(111) surface; Ag atoms migrate to the Cu(111) surface not through the Cu film but along the walls of the holes in the Cu films that penetrate deeply into the Ag substrate.   

Studying the Enhanced Ductility of Bimodal Nanocrystalline Copper Using a Coarse-Grained Model

Viewing a bimodal configuration of nanocrystalline copper as composed of soft grains containing stiff cores, we proposed a coarse-grained model with systematically tunable stiffness of grains to study the enhanced ductility of bimodal nanocrystalline copper [Y. Wang, M. Chen, F. Zhou, and E. Ma, Nature 419 (2002) 912]. Using molecular dynamics simulations, we shear our model quasistatically. Our results not only qualitatively confirms that a bimodal configuration could behave more ductile than a monomodal one but also predicts there exists a range of ratio of soft/stiff domains that best minimizes shear localization. Moreover, our model indicates that a bimodal configuration could sometimes exacerbate shear localization and therefore jeopardize ductility if the ratio of soft/stiff domains is not properly chosen. This may explain why some experimental results are hard to be reproduced.   

Biocompatible Ferromagnetic Cr-Trihalide Monolayers

Cr with an electronic configuration of 3d$^{\mathrm{5}}$4s$^{\mathrm{1}}$ possesses the largest atomic magnetic moment (6\textmu $_{\mathrm{B}})_{\mathrm{\thinspace }}$of$_{\mathrm{\thinspace }}$all elements in the 3d transition metal series. Furthermore, the trivalent chromium (Cr$^{\mathrm{3+}})$ is biocompatible and is widely found in food and supplements. Here using first principles calculations combined with Monte Carlo simulations based on Ising model, we systematically study a class of 2D ferromagnetic monolayers CrX$_{\mathrm{3}}$ (X$=$ Cl, Br, I). The feasibility of exfoliation from their layered bulk phase is confirmed by the small cleavage energy and high in-plane stiffness. Spin-polarized calculations, combined with self consistently determined Hubbard U that accounts for strong correlation energy, demonstrate that CrX$_{\mathrm{3}}$ (X$=$Cl, Br, I) monolayers are ferromagnetic and Cr is trivalent and carries a magnetic moment of 3\textmu $_{\mathrm{B}}$, the resulting Cr$^{\mathrm{3+}}$ ions are biocompatible. The corresponding Curie temperatures for CrCl$_{\mathrm{3}}$ CrBr$_{\mathrm{3}}$ CrI$_{\mathrm{3\thinspace }}$are are found to 66, 86, and 107 K, respectively, which can be increased to 323, 314, 293 K by hole doping. The biocompatibility and ferromagnetism render these Cr-containing trichalcogenide monolayers unique for applications.   

Spectral Gauss quadrature method with subspace interpolation for Kohn-Sham Density functional theory

Algorithms with linear-scaling ($\mathcal{O}(N)$) computational complexity for Kohn-Sham density functional theory (K-S DFT) is crucial for studying molecular systems beyond thousands of atoms. Of the $\mathcal{O}(N)$ methods that use a polynomial-based approximation of the density matrix, the linear-scaling spectral Gauss quadrature (LSSGQ) method (Suryanarayana \emph{et al.}, JMPS, 2013) has been shown to exhibit the fastest convergence. The LSSGQ method requires a Lanczos procedure at every node in a real-space mesh, leading to a large computational pre-factor. We propose a new interpolation scheme specific to the LSSGQ method that lift the need to perform a Lanczos procedure at every node in the real-mesh. This interpolation will be referred to as subspace interpolation. The key idea behind subspace interpolation is that there is a large overlap in the Krylov-subspaces produced by the Lanczos procedures of nodes that are close in real-space. The subspace interpolation scheme takes advantage of the block-Lanczos procedure to group the Krylov-subspaces from a few representative nodes to approximate the density matrix over a large collection of nodes. Subspace interpolation outperforms cubic-spline interpolation by several orders of magnitude.   

Quantum Monte Carlo with known sign structures

We investigate the merits of different Hubbard-Stratonovich transformations (including fermionic ones) for the description of interacting fermion systems, focusing on the single band Hubbard model as a model system. In particular we revisit an old proposal of Batrouni and Forcrand (PRB 48, 589 1993) for determinant quantum Monte Carlo simulations, in which the signs of all configurations is known beforehand. We will discuss different ways that this knowledge can be used to make more accurate predictions and simulations.   

High-temperature transport in the Hubbard Model

We examine the general behavior of the frequency and momentum dependent single-particle scattering rate and the transport coefficients, of many-body systems in the high-temperature limit. We find that the single-particle scattering rate always saturates in temperature, while the transport coefficients always decay like $\frac{1}{T}$, where $T$ is the temperature. A consequence of this is a resistivity which is ubiquitously linear in $T$ at high temperatures. For the Hubbard model, by using the high-temperature series, we are able to calculate the first few moments of the single particle scattering rate $\Sigma(\vec{k},\omega)$ and the conductivity $\sigma(\vec{k},\omega)$ in the high-temperature regime in $d$ spatial dimensions. Further in the case of $d\to \infty$, we are able to calculate a large number of moments using symbolic computation. We make a direct comparison between these moments and those obtained through Dynamical Mean Field Theory (DMFT). Finally, we use the moments to reconstruct the $\omega$-dependent optical conductivity $\sigma(\omega) = \lim_{k\to0} \sigma(\vec{k},\omega)$ in the high-temperature regime.   

Anharmonicity alters the stability of Fe$_7$C$_3$ at the conditions of the Earth's core

Recently, a new orthorhombic phase of iron carbide (Fe$_7$C$_3$) with an unusually high Poisson's ratio was discovered experimentally, raising the possibility that it may be important at the Earth's core. However, calculations of the Gibbs free energy in the quasiharmonic approximation suggested that it would be metastable with respect to the well known hexagonal phase at the pres sure and temperature of the Earth's core. We present new anharmonic calculations of the Gibbs free energy using the temperature-dependent effective potential (TDEP) method, which suggest that the orthorhombic phase is more stable at the conditions of the Earth's core. Anharmonicity is shown to be important at relatively low temperatures, and has a decisive effect on the phase diagram. Moreover, we show that Fe$_7$C$_3$ decomposes to form Fe$_2$C and Fe$_3$C over a narrow region of the phase diagram between the orthorhombic and hexagonal phases.   

Role of Nanostructure Coating Quality in Delay of Surface Flooding during Jumping Droplet Condensation

Vapor condensation is commonly observed in everyday life and routinely used in industry as an effective means of transferring heat. In industrial systems, condensed vapor typically forms a thin liquid film which is not desired due to the large thermal resistance to heat transfer; however, if the condensing surface is functionalized with a hydrophobic coating, the condensate forms discrete liquid droplets which shed at sizes approaching the capillary length and refresh the surface for re-nucleation, resulting in a 5--7x heat transfer improvement. Furthermore, when a micro- or nanostructured surface is functionalized, a superhydrophobic surface can be created on which small ($\approx $10-100 \textmu m) droplets coalesce and can spontaneously jump away from the surface due to release of excess surface energy; this jumping droplet mode of condensation has been shown to increase heat transfer by an additional 30 -- 40{\%}. However, at elevated supersaturations, nanostructured superhydrophobic surfaces can become flooded with condensate and form pinned droplets which cannot jump, thereby eliminating the desired heat transfer improvement. In this work, we experimentally demonstrated a delay in the supersaturation at which surface flooding occurs by reducing the hydrophobic coating defect density. This resulted in a lower proportion of structure unit cells occupied by condensate, which allowed higher droplet mobility and jumping at elevated supersaturation.   

Analysis of Adsorbed Natural Gas Tank Technology

With gasoline being an ever decreasing finite resource and with the desire to reduce humanity's carbon footprint, there has been an increasing focus on innovation of alternative fuel sources. Natural gas burns cleaner, is more abundant, and conforms to modern engines. However, storing compressed natural gas (CNG) requires large, heavy gas cylinders, which limits space and fuel efficiency. Adsorbed natural gas (ANG) technology allows for much greater fuel storage capacity and the ability to store the gas at a much lower pressure. Thus, ANG tanks are much more flexible in terms of their size, shape, and weight. Our ANG tank employs monolithic nanoporous activated carbon as its adsorbent material. Several different configurations of this Flat Panel Tank Assembly (FPTA) along with a Fuel Extraction System (FES) were examined to compare with the mass flow rate demands of an engine.   

Development of facile property calculation model for adsorption chillers based on equilibrium adsorption cycle.

Facile property calculation model for adsorption chillers was developed based on equilibrium adsorption cycles. Adsorption chillers are one of promising systems that can use heat energy efficiently because adsorption chillers can generate cooling energy using relatively low temperature heat energy. Properties of adsorption chillers are determined by heat source temperatures, adsorption/desorption properties of adsorbent, and kinetics such as heat transfer rate and adsorption/desorption rate etc. In our model, dependence of adsorption chiller properties on heat source temperatures was represented using approximated equilibrium adsorption cycles instead of solving conventional time-dependent differential equations for temperature changes. In addition to equilibrium cycle calculations, we calculated time constants for temperature changes as functions of heat source temperatures, which represent differences between equilibrium cycles and real cycles that stemmed from kinetic adsorption processes. We found that the present approximated equilibrium model could calculate properties of adsorption chillers (driving energies, cooling energies, and COP etc.) under various driving conditions quickly and accurately within average errors of 6{\%} compared to experimental data.   

A learner's multiple views of the connection between mathematics and quantum mechanics

Students' physical intuitions and prior knowledge are critical to making sense of and solving problems in classical mechanics. In quantum mechanics (qm), coordinating concepts connected to such everyday thinking becomes more difficult. How then can students develop coherence in their knowledge of qm? Consider how experts do it: they build meaning in, around, and through the mathematics of the theory. This view on the role of mathematics, which is one of among many possible to take, seems most productive for qm. In our work to characterize student views of knowledge that emerge in the context of qm coursework, we came to analyze one student who mostly adopted such a view until a shift in context moved him to express an alternative. We present his case and discuss important implications for instruction.