Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Z1: Invited Session: Time- and Angle- Resolved Photoemission Spectroscopy of Complex Materials
Sponsoring Units: DCMP GIMSChair: Thomas Devereaux, SLAC National Acceleratory Laboratory
Room: Ballroom I
Friday, March 22, 2013 11:15AM - 11:51AM |
Z1.00001: Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi$_2$Se$_3$ Invited Speaker: Jonathan Sobota Bi$_2$Se$_3$ is a material which has gained great attention since it was recognized to be a topological insulator. Due to their topologically-protected spin-textured Dirac surface states, topological insulators have been recognized for their potential in device applications, particularly for spintronics. Thus, much of the experimental focus has been on ways to electronically or optically couple to the surface spin-texture. Using time- and angle- resolved photoemission spectroscopy (TR-ARPES), we optically excite p-type Bi$_2$Se$_3$ and study the dynamical response of its electronic structure on a femtosecond timescale. The strength of this technique is that its energy- and angle- resolution allows us to study these dynamics directly within the electronic band structure, so that surface and bulk contributions can be separately resolved. We find that optical excitation produces a metastable population of bulk carriers due to the presence of the bandgap. We discuss the coupling of these carriers to the Dirac surface state, which results in a long-lived nonequilibrium surface carrier distribution. This spin-textured population may present a channel in which to drive transient spin-polarized currents. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z1.00002: Ultrafast momentum-dependent quasiparticle dynamics in high-$T_{c}$ superconductors Invited Speaker: Uwe Bovensiepen Femtosecond time- and angle-resolved photoelectron spectroscopy trARPES facilitates insight into electronic relaxation and electronic structure of non-equilibrium states of matter [1]. Hot electrons and holes relax in metals on ultrafast time scales due to the screened Coulomb interaction [2]. In superconductors the relaxation rates of quasiparticles at energies close to the superconducting gap edge are reduced because of the loss of quasiparticle states near $E_{F}$. Since in the superconducting state the relaxation of optically excited carriers proceeds partly by Cooper pair reformation, the study of the quasiparticle dynamics bears the potential to analyze the interaction responsible for Cooper pair formation. Results of trARPES will be discussed for optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in the superconducting state [2] and on EuFe$_{2}$As$_{2}$ in the antiferromagnetic state [3]. In the cuprate system we find a predominant excitation of quasiparticles at momenta near the antinode. We show furthermore, that at excitation densities of several 10 $\mu $J/cm$^{2}$ quasiparticle relaxation is dominated by Cooper pair reformation, which again proceeds near the antinode. In the Fe-pnictide material we monitor a difference in the relaxation rate for electrons and holes near the Fermi momentum, which disappears above the Neel temperature. We conclude that this anisotropic relaxation of electrons and holes is a consequence of the optical modification of the antiferromagnetic order. Analysis of energy transfer from electrons to phonons allows to determine the momentum averaged electron-phonon coupling constant $\lambda $. We find values below 0.25 for Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ [5] and below 0.15 for EuFe$_{2}$As$_{2}$ [4]. \\[4pt] [1] Bovensiepen and Kirchmann, Laser Photonics Rev. 6, 589 (2012).\\[0pt] [2] Kirchmann et al., Nature Physics 6, 782 (2010).\\[0pt] [3] Cortes et al., Phys. Rev. Lett. 107, 097002 (2011).\\[0pt] [4] Rettig et al., Phys. Rev. Lett. 108, 097002 (2012).\\[0pt] [5] Perfetti et al., Phys. Rev. Lett. 99, 197001 (2007). [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z1.00003: Measurement of intrinsic Dirac fermion cooling of a topological insulator with time- and angle- resolved photoemission spectroscopy Invited Speaker: Yihua Wang Three-dimensional topological insulator (TI) is a new phase of matter with exotic surface electronic properties.~Even though the bulk states have a bandgap, the surface electrons possess a linear energy-momentum dispersion that is protected by the nontrivial topology of TI to cross the Fermi level. These properties provide a promising platform for new physics and applications in future electronics and computers including high-speed quantum information processing, whose performance depends critically on the dynamics of hot carriers. Unlike the case in graphene, helical Dirac fermions in a TI interact not only with phonons but also with an underlying bulk reservoir of electrons. In this talk, we will present our recent results of time- and angle-resolved photoemission spectroscopy (TrARPES) study of a prototypical TI Bi2Se3. We show that TrARPES is a powerful tool to distinguish the coupled dynamics between these different degrees of freedom. With the combined sub-picosecond time resolution and energy-momentum resolution, we have directly visualized the coupling between surface and bulk electrons through phonons. At low temperature, such coupling is suppressed and the unique cooling of surface Dirac fermions by acoustic phonons is revealed through the power law cooling rate dependence on doping level. The effect on the TrARPES spectra from varying excitation photon energy will also be discussed. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z1.00004: Time-resolved ARPES and f-electron coherence Invited Speaker: Tomasz Durakiewicz The coherence temperature, T*, sets an important energy scale in correlated f-electron systems. In this scale the hybridization gap opens at or in the vicinity of the Fermi level and the gap magnitude scales with effective quasiparticle mass. The new quasiparticle bands are heavy, as demonstrated by their small dispersion, and the quasiparticle lifetime is long, as seen by the narrow width of the peaks. Unless magnetic ordering suppresses the gap or mass enhancement is observed due to, e.g., magnetic excitations, the gap scales with effective mass in a universal manner across the heavy fermion systems. Possible deviations from this pattern, e.g. a small finite gap persisting at high temperatures above T* require models beyond a mean-field approach, and may be understood within e.g. the model of periodic array of Anderson impurities with correlations described by coupling to specific boson modes. \\ Self-energy approach is commonly used in ARPES of correlated systems. The coherent part of the self-energy corresponding to the gap formation is reduced at high temperatures, and the incoherent part corresponds to quasiparticle scattering. The coherent term in the self-energy expresses the mixing of f and d bands and is directly responsible for repulsion, producing the hybridization gap. This theoretical framework provides a direction towards understanding quasiparticle dynamics in correlated electron systems through ultrafast self-energy measurements and modeling. Here we show examples of time-resolved ARPES measurements of f-electron systems, providing valuable information about the evolution of coherence and the dynamics of the related quasiparticle states. \\ References \\ 1) Phys. Rev. B 84, 161103 (Rapid Comm.) (2011). \\ 2) Phys. Rev. B 84, 161101 (Rapid Comm) (2011).\\ 3) Phys. Rev. Lett. 106, 207402 (2011).\\ 4) J.Phys.C. 23, 094211 (2011).\\ 5) Rev. Sci. Instr. 81, 073108 (2010). \\ 6) Europhys. Lett. 84, 37003 (2008). \\ 7) Phys. Rev. Lett. 101, 016403 (2008). [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z1.00005: Ultrafast quasiparticle dynamics and pair recombination in cuprate high-temperature superconductors Invited Speaker: Chris Jozwiak Understanding how superconductivity emerges from other competing phases and how this balance evolves through the phase diagram is one of the biggest challenges in the field of high-$T_\textrm{c}$ superconductors. By using high resolution time- and angle-resolved photoemission spectroscopy (tr-ARPES) we are able to directly probe the effects of optical excitation on the electronic structure of cuprate superconductors, and study the resulting quasiparticle, superconducting gap, and Cooper pair formation dynamics near their natural time scales. In particular, we observed a pump-induced meltdown of quasiparticles, which occurs only within the energy scale defined by a particular boson mode. This meltdown was observed only below $T_\textrm{c}$, suggesting a link between superconductivity and quasiparticles in momentum space where the superconducting gap is zero. We observed that the excited quasiparticle decay dynamics were strongly pump-fluence dependent and consistent with the picture that the observed dynamics reflect actual Cooper pair formation. Further, these quasiparticle recombination dynamics were strongly momentum dependent, increasing away from the superconducting nodes. Direct measurements of momentum dependent superconducting gap dynamics and the evolution of other non-equilibrium spectral phenomena through the phase diagram further illustrate the power of this unique time- and momentum-resolved spectroscopy. These results reveal new windows into the nature of the pairing interaction in high-$T_\textrm{c}$ superconductors. [Preview Abstract] |
Session Z2: Invited Session: Jamming and Rheology of Disordered Systems
Sponsoring Units: DCMP GSNPChair: Bulbul Chakraborty, Brandeis University
Room: Ballroom II
Friday, March 22, 2013 11:15AM - 11:51AM |
Z2.00001: Impact-activated solidification of dense suspensions Invited Speaker: Scott Waitukaitis Shear-thickening, non-Newtonian fluids have typically been investigated under steady-state conditions. This approach has produced two pictures for suspension response to imposed forcing. In the weak shear-thickening picture, the response is typically attributed to the hydrodynamic interactions giving rise to hydroclusters, small groups of particles interacting through lubrication forces. At the other end of the spectrum, in the discontinuous shear-thickening regime, the response can be seen as a system-wide jamming that is ultimately limited in strength by the system boundaries. While these steady-state pictures have proven extremely useful, some of the most interesting phenomena associated with dense suspensions is transient and local in character. A prototypical example is the extraordinarily large impact resistance of dense suspensions such as cornstarch and water. When poked lightly these materials respond like a fluid, but when punched or kicked they seem to temporarily ``solidify'' and provide enormous resistance to the motion of the impacting object. Using an array of experimental techniques, including high-speed video, embedded force and acceleration sensing, and x-ray imaging, we are able to investigate the dynamic details this process as it unfolds. We find that an impacting object drives the rapid growth of a jammed, solid-like region directly below the impact site. Being coupled to the surrounding fluid by grain-mediated lubrication forces, this creates substantial peripheral flow and ultimately leads to the sudden extraction of the impactor's momentum. With a simple jamming picture to describe the solidification and an added mass model to explain the force on the rod, we are able to predict the forces on the impactor quantitatively. These findings highlight the importance of the non-equilibrium character of dense suspensions near jamming and might serve as a bridge between the weak and discontinuous shear-thickening pictures. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z2.00002: Dilatancy and shear thickening of particle suspensions Invited Speaker: Daniel Bonn Shear thickening is a fascinating subject, as 99.9{\%} of complex fluids are thinning; thickening systems thus are the ``exception to the rule'' that needs to be understood. Moreover, such tunable systems show very promising applications, e.g. to block large underground pores in oil recovery to maintain a constant oil flow by plugging water filled pores (an approach used in oil recovery by e.g. Shell), or to manufacture bulletproof vests that are comfortable to wear, but stop bullets nonetheless. We study the rheology of non-Brownian particle suspensions (notably, cornstarch) that exhibit shear thickening. Using magnetic resonance imaging (MRI), the local properties of the flow are obtained by the determination of local velocity profiles and concentrations in a Couette cell. We also perform macroscopic rheology experiments in different geometries. The results suggest that the shear thickening is a consequence of dilatancy: the system under flow attempts to dilate but instead undergoes a jamming transition, because it is confined. This proposition is confirmed by an independent measurement of the dilation of the suspension as a function of the shear rate. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z2.00003: Simulations of shear-induced jamming in athermal particulate systems Invited Speaker: Corey O'Hern We perform simulations of athermal particulate systems that are prepared in unjammed states with zero static shear modulus and then subjected to successive pure or simple quasistatic shear strains at either fixed packing fraction or fixed pressure. In response to applied shear, these systems jam, forming anisotropic networks of interparticle contacts. We determine the onset of shear-induced jamming as a function of the amplitude of the shear strain, packing fraction, pressure, and system size. We find that the parameter space for shear-induced jamming expands for particles with frictional interactions and nonspherical shapes. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z2.00004: Dilatancy and Diffusion in Sheared Granular Materials Invited Speaker: Joshua Dijksman Disordered materials such as sand, foams and emulsions display a wide variety of different forms of mechanical behavior. Currently the origin of this rich dynamics is the subject of intense study. Experimentally it has proved difficult to probe the microscopic dynamics in these systems. We present an overview of experimental investigations that have been successful in giving more insight into the microstructural dynamics of disordered systems. We focus on shear induced dilatancy and diffusion in quasi statically deformed granular materials and suspensions and contrast the behavior of low and high friction particulate materials. We shall discuss the consequences of our observations in the context of shear banding and jamming phenomena. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z2.00005: Rigidity of Dry Granular Solids Invited Speaker: Dapeng Bi Solids are distinguished from fluids by their ability to resist shear. In traditional solids, the resistance to shear come as an energy cost of straining, which works to distort density modulations that exists in both crystalline or amorphous structures. In our recent work,we focus on the emergence of shear-rigidity in a special class of solids: dry (non-cohesive) granular materials which have no energetically preferred density modulations. In contrast to traditional solids, the emergence of mechanical rigidity in these marginal granular solids is a collective process, which is controlled solely by boundary forces, the constraints of force and torque balance, and the positivity of the contact forces. We develop a theoretical framework based on these constraints, and show that these solids can be characterized by topological invariants and that, in two dimensions, they have internal patterns that are most naturally represented in the space of gauge field of the stress. Broken translational invariance in this gauge space is a necessary condition for rigidity in granular solids. We apply our theory to experimentally shear-jammed states as well as numerically generated jammed force networks to show that the statistics of stress fluctuations, and the ability of jammed configurations to resist deformations can be understood within this theoretical framework. [Preview Abstract] |
Session Z3: Invited Session: Integration of Research and Teaching Excellence: Cottrell Scholars
Sponsoring Units: FEdChair: Richard Wiener, Research Corporation
Room: Ballroom III
Friday, March 22, 2013 11:15AM - 11:51AM |
Z3.00001: Cottrell Scholars Collaborative -- Integrating Research and Teaching Invited Speaker: Jairo Sinova Higher education reform needs to move towards a more interactive and integrated model, in which there is greater curricular emphasis in skill development, multi-discipline integration, and innovative connectivity, rather than traditional content driven curricula. This is even more crucial in STEM education, given our current slow down relative to other countries and the need to remain competitive in a global environment. Successful reforms require a seamless integration of research and teaching where education excellence and research excellence are not viewed by faculty as a zero sum game but as mutually benefitting ingredients of academic success. Cottrell scholars are selected among top ranking young academics with an equal passionate commitment to research excellence and education. Recently, these national group of academics sponsored by the Research Corporation for Science Advancement have created the Cottrell Scholars Collaborative (CSC) which aims at creating a self-supporting group that promotes integration of research and teaching at a national level with different initiatives. I will describe in these talk the aim of this group and the different sponsored projects that CSC is undertaking and the types of collective and individual efforts that are making a difference in sustainable education reform. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z3.00002: Optics for Biophysics: An Interdisciplinary course in Optics for Physicists and Life Science Students Invited Speaker: Jennifer Ross Optics is an applied sub-field of physics that life science researchers utilize daily. Indeed, one cannot open a biological science research journal without seeing five beautiful images of cells. To bridge the gap and educate more life science students in the field of physics, I have developed a new course called ``Optics for Biophysics,'' an interdisciplinary course engaging students from physics, chemistry, life science, and engineering. The course is a team-based learning or studio physics approach combined with a semester-long project. Mini-lectures of 20 minutes are given before students do hands-on group work to understand the concepts. In the project, the students design and build a modern transmitted light microscope. The final aspect of the project is to build a unique module onto the microscope to address a specific biological question. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z3.00003: Stimulating Creativity by Integrating Research and Teaching Across the Academic Disciplines Invited Speaker: Richard Taylor Creativity is a human adventure fueled by the process of exploration. But how do we explore our intellectual interests? In this talk, I'll propose that we seek out our creative opportunities using an inherent natural process. This process might, therefore, exploit search strategies found across diverse natural systems -- ranging from the way animals forage for food to the way the human eye locates information embedded within complex patterns. The symbolic significance of this hypothesis lies in its call for educational institutes to provide environments that encourage our natural explorations rather those that stamp restrictive, artificial `order' on the process. To make my case, I'll review some of my own research trajectories followed during my RCSA Cottrell Scholarship at the University of Oregon (UO). My first conclusion will be that it is fundamentally unnatural to declare divides across disciplines. In particular, the infamous `art-science divide' is not a consequence of our natural creative searches but instead arises from our practical inability to accommodate the rapid drive toward academic specialization. Secondly, divides between research and teaching activities are equally unnatural -- both endeavors are driven by the same creative strategy and are intertwined within the same natural process. This applies equally to the experiences of professors and students. I will end with specific success stories at the UO. These include a NSF IGERT project (focused on accelerating students' transitions from classroom to research experiences) and a collaboration between architects and professors to design a building (the recently opened Lewis Integrative Science Building) that encourages daily encounters between students and professors across research disciplines. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z3.00004: Integrated Concentration in Science (iCons): Undergraduate Education Through Interdisciplinary, Team-Based, Real-World Problem Solving Invited Speaker: Mark Tuominen Attitude, Skills, Knowledge (ASK) -- In this order, these are fundamental characteristics of scientific innovators. Through first-hand practice in using science to unpack and solve complex real-world problems, students can become self-motivated scientific leaders. This presentation describes the pedagogy of a recently developed interdisciplinary undergraduate science education program at the University of Massachusetts Amherst focused on addressing global challenges with scientific solutions. Integrated Concentration in Science (iCons) is an overarching concentration program that supplements the curricula provided within each student's chosen major. iCons is a platform for students to perform student-led research in interdisciplinary collaborative teams. With a schedule of one course per year over four years, the cohort of students move through case studies, analysis of real-world problems, development of potential solutions, integrative communication, laboratory practice, and capstone research projects. In this presentation, a track emphasizing renewable energy science is used to illustrate the iCons pedagogical methods. This includes discussion of a third-year laboratory course in renewable energy that is educationally scaffolded: beginning with a boot camp in laboratory techniques and culminating with student-designed research projects. Among other objectives, this course emphasizes the practice of using reflection and redesign, as a means of generating better solutions and embedding learning for the long term. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z3.00005: Living the good life: pursuing excellence as a scientist and as a teacher Invited Speaker: Erica Carlson Do research and teaching represent competing demands on our time and energy, or have we bought into a false dichotomy? As a scientist, my job is to find truth. As a teacher, my job is to teach the next generation how to find truth. In this talk, I discuss the ways in which research and teaching have been synergistic in my experience, as well as the tension commonly felt among professors (myself included) as to how to ``split our time'' between the two. I will share a brief synopsis of my teaching philosophy, and I hope to give some insight into what (in my opinion) makes or breaks you as a teacher. I will also share some of my experience in this great adventure we call scientific progress. [Preview Abstract] |
Session Z5: Focus Session: Graphene: Transport and Optical Phenomena: Hot Electrons and Photocurrents
Sponsoring Units: DCMPChair: Alex Holleitner, Technische Universitaet Muenchen
Room: 301
Friday, March 22, 2013 11:15AM - 11:27AM |
Z5.00001: Hot carriers, phonons and electron-phonon decoupling in graphite Tung-Wu Hsieh, Chih-Wei Lai Visible and near-IR radiation and hot phonons are observed in HOPG graphite following the excitation of picosecond laser pulses at 1.58 eV of fluences exceeding 1000 J/m$^2$. The optically generated electron-hole carriers lead to non-thermal radiation ranging from 1.2 to 2.8 eV, including black-body-like emissions above the excitation and a broad spectral peak near 1.4 eV. We determine an effective electronic temperature (Te) by fitting the high energy radiation to a Plank spectrum; Tg from G-mode Stokes/anti-Stokes Raman peaks; Tl from spectral line shifts of G-mode. With increasing incident fluence from 10$^3$ to 10$^4$ J/m$^2$, Te, Tg and Tl are decoupled and increase from 1000 to 5000, 1000 to 2500, and 300 to 500K, respectively. At a fluence below 10$^3$ J/m$^2$, Te approaches Tg near 2000K, which is $\sim$ 1000K above Tl. This is indicative of quasi-equilibrium, but decoupled, distributions of carriers and phonons. The transient radiation decays within 2ps, limited to instrument response. Similar effects are observed for excitations at 1.53 and 1.49 eV. Experiments are conducted in vacuum at ambient T$=$300K. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z5.00002: Doping dependence of the ultrafast relaxation dynamics of hot electrons in graphene Liang Zhao, Jie Shan, Kin Fai Mak, Tony Heinz The ultrafast relaxation dynamics of highly excited electrons in graphene has attracted much attention due to both its fundamental interest and its practical importance in relation for optoelectronic devices. Several mechanisms including electron-optical phonon scattering and disorder assisted electron-phonon scattering have been proposed to be responsible for electron cooling on the picosecond time scale. In this work, we apply the technique of two-color femtosecond pump-probe spectroscopy to investigate the electron relaxation dynamics as a function of doping. The photo-induced absorption in graphene is seen to vary rapidly in the first a few 100's fs, followed by a slower decay of $\sim$ ps. The dynamics depend sensitively on the doping level. We will present our analysis of the results in terms of the transient electron chemical potential and temperature and discuss the role of different doping mechanisms, in particular, in the regime where the Fermi energy approaches half of the probe photon energy. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z5.00003: Hot Carrier Transport at the Graphene-Metal Interface Induced by Strong Lateral Photo-Dember Effect Chang-Hua Liu, You-Chia Chang, Nanditha Dissanayake, Yaozhong Zhang, Zhaohui Zhong Ultrafast photo-excitation in a semiconductor can lead to transient spatial charge gradient if electrons and holes have different drift velocities. The charge gradient builds up the transient electric field and causes the subsequent terahertz pulse emission. This phenomenon, known as the photo-Dember effect, was typically considered insignificant in graphene due to its similar electron and hole mobilities. Here, we observe hot carrier transport at the graphene-metal interface driven by the photo-Dember electric field under femtosecond pulse laser excitation. The polarity of hot carrier transport is determined by the asymmetry of electron and hole mobilities of the graphene device and cannot be flipped sign by tuning graphene doping level. This indicates the formation of strong photo-Dember field, dominating over the graphene/metal built-in electric field or thermal electric field. We further analyze the spatial distribution and temporal evolution of the transient electric field near the contact edge by using the drift-diffusion model. The modeling results suggest that strong photo-Dember effect is caused by the low electronic specific heat of graphene and a huge charge gradient near the graphene-metal interface under pulse laser excitation. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z5.00004: Giant Nonlocal Photocurrent at the Charge Neutrality Point in Graphene Qiong Ma, Nathan Gabor, Nityan Nair, Wenjing Fang, Jing Kong, Pablo Jarillo-Herrero Graphene based photosensitive devices have attracted considerable attention due to monolayer graphene's broadband optical absorption and gate tunable capacities. As the quality of graphene increases, emergent phenomena are being observed in both transport and optical measurements. Here we report measurements of giant nonlocal photocurrent that emerges at the charge neutrality point in graphene transistor devices. Scanning photocurrent imaging of uniformly undoped monolayer graphene transistors reveals highly ordered spatial patterns with alternating photocurrent signs as a function of laser position. The charge density dependence of the photoresponse, combined with in-situ improvement of device mobility, reveals a strong correlation between the nonlocal photocurrent and the derivative of the thermopower as a function of charge density. Photocurrent enhancement is pronounced in high-mobility devices and at intermediate temperatures. Such behaviors are suggestive of phonon drag effects that emerge at the charge neutrality point under photoexcitation. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z5.00005: Hot carrier response in gapped bilayer graphene Grant Aivazian, Jason Ross, K. Watanabe, T. Taniguchi, K. Kitamura, David Cobden, Xiaodong Xu Recently bilayer graphene has been shown to develop a bandgap upon breaking of inversion symmetry by a perpendicular electric field that is \textit{in situ }tunable between zero and several hundred meV (corresponding to wavelengths in the mid-IR). Such unique tunability offers bilayer graphene a niche in mid-IR optoelectronic devices where a lack of high performance photodetectors exists. In this work we have performed spatially and temporally resolved photocurrent measurements in a dual-gated bilayer graphene FET under continuous-wave and pulsed laser excitation. We find that photocurrent generation in native bilayer graphene is dominated by hot carriers, as is the case in monolayer graphene, but it behaves very differently from monolayer graphene once a bandgap has been opened. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z5.00006: Supercollision cooling in undoped graphene Sung Ho Jhang, Andreas Betz, Emiliano Pallecchi, Robson Ferreira, Gwendal Feve, Jean-Marc Berroir, Bernard Placais We have investigated the electron-lattice cooling rate in graphene by means of GHz Johnson noise thermometry. For phonon temperatures ($T_{ph}$) larger than Bloch-Gr\"{u}neisen temperature ($T_{\mathrm{BG}}$), we find the energy relaxation rate $J$ obeys a cubic law as a function of electron temperature $T_e$. In this regime, the small Fermi surface of graphene drastically restricts the allowed phonon energy in ordinary electron-phonon scattering, and disorder-assisted supercollisions dominate over the conventional electron-phonon collisions. In the low-temperature regime, for $T_{ph} < T_{\mathrm{BG}}$, we regain $J \propto T_e^{4}$ dependence, which is the signature of standard electron-phonon interaction in the 2D graphene. Beside its implication for electron-phonon physics, our observations are of direct relevance for the performance of graphene bolometers and photo-detectors. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z5.00007: Focused Laser Induced Spatially Controllable p-n junction in Graphene Field-Effect Transistor Young Duck Kim, Myung-Ho Bae, Jung-tak Shu, Young Seung Kim, Joung Real Ahn, Seung-Hyun Chun, Yun Daniel Park Tunable local doping on graphene is an important issue for future graphene-based electronics. Here we investigate a local doping effect by a focused laser irradiation and demonstrate a spatially controllable p-n junction in graphene field-effect transistor. Scanning photocurrent microscopy with varying back-gate voltages reveals the local charge trap in gate oxide near the laser-irradiated region. This is manifested by itself as double peaks in resistance as a function of gate voltage in graphene device, where the region between the double peaks corresponds to the p-n junction. Irradiation of a focused laser on graphene device suggests a new pave to spatially control the doping level, position and size of doped segment on graphene channel in a nondestructive way without high electrical bias, local gate electrode and chemical process. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z5.00008: Multiple regimes of carrier cooling in photoexcited graphene probed by time-resolved terahertz spectroscopy A.J. Frenzel, N.M. Gabor, P.K. Herring, W. Fang, J. Kong, P. Jarillo-Herrero, N. Gedik Energy relaxation and cooling of photoexcited charge carriers in graphene has recently attracted significant attention due to possible hot carrier effects, large quantum efficiencies, and photovoltaic applications. However, the details of these processes remain poorly understood, with many conflicting interpretations reported. Here we use time-resolved terahertz spectroscopy to explore multiple relaxation and cooling regimes in graphene in order to elucidate the fundamental physical processes which occur upon photoexcitation of charge carriers. We observe a novel negative terahertz photoconductivity that results from the unique linear dispersion and allows us to measure the electron temperature with ultrafast time resolution. Additionally, we present measurements of the relaxation dynamics over a wide range of excitation fluence. By varying the pump photon energy, we demonstrate that cooling dynamics of photoexcited carriers depend on the amount of energy deposited in the graphene system by the pump pulse, not the number of absorbed photons. The data suggest that fundamentally different regimes are encountered for different excitation fluences. These results may provide a unifying framework for reconciling various measurements of energy relaxation and cooling in graphene. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z5.00009: Terahertz generation and picosecond photo-thermoelectric currents in graphene Alexander Holleitner We demonstrate that THz radiation is generated in optically pumped bilayer graphene. The electro-magnetic radiation is detected via a time-domain THz spectroscopy utilizing coplanar metal stripline circuits in combination with an on-chip pump/probe scheme [1]. The striplines act as highly sensitive near-field antennae with a bandwidth of up to 1 THz. Our ultrafast experiments further clarify the optoelectronic mechanisms contributing to the photocurrent generation at graphene-metal interfaces. We verify that both built-in electric fields, similar to those in semiconductor-metal interfaces, and a photo-thermoelectric effect give rise to the photocurrent at graphene-metal interfaces at different time scales. We particularly discuss how the picosecond photocurrents in monolayer graphene depend on the geometry and the thermal coupling of the devices to the environment [2]. We acknowledge the very fruitful cooperation with L. Prechtel, S. Manus, D. Schuh, W. Wegscheider, L. Song, and P. Ajayan.\\[4pt] [1] L. Prechtel, L. Song, P. Ajayan, D. Schuh, W. Wegscheider, A.W. Holleitner, Nature Communications 3, 646 (2012).\\[0pt] [2] A. Brenneis et al. (2013). [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z5.00010: Optical pure spin current injection in graphene Julien Rioux, Guido Burkard Pure spin current injection by optical methods is investigated in single-layer and bilayer graphene within the tight-binding model, including bias and interlayer coupling effects. Interlayer coupling in bilayer graphene has a distinct qualitative effect on the polarization dependence of the spin current injection. In combination with interlayer coupling, which induces trigonal warping of the electronic bands, the bias voltage allows to control the warping at the Fermi surface. The resulting implications for the spin current injection are presented. Unlike the previously presented charge current injection [J. Rioux et al., PRB 83, 195406 (2011)], the effect presented here relies on a single monochromatic beam. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z5.00011: Probing the Optoelectronic Response of a Monolayer MoS$_{2}$ Field-Effect Transistor Kathryn L. McGill, Kin Fai Mak, Joshua W. Kevek, Jiwoong Park, Paul L. McEuen Two-dimensional materials contain a wealth of interesting optoelectronic properties. Single-layer molybdenum disulfide (MoS$_{2})$, with its broken inversion symmetry, is of particular interest. This broken symmetry results in the formation of direct band gaps at the K and K' valleys in its band structure, allowing long-lived optical excitations. Furthermore, monolayer MoS$_{2}$ has valley-dependent electronic properties allowing confinement of charge carriers to a single valley by optical pumping [1]. We have fabricated two- and four-terminal devices based on single layers of MoS$_{2}$. We observe an efficient photocurrent response at the two-dimensional semiconductor-metal interface displaying Shottky diode behavior, in which an interfacial field splits excitons at the contacts to produce current. We also find that the photocurrent drastically increases under reverse biasing of the diode. Additionally we are exploring the dependence of this photocurrent response on the polarization state of incident light.\\[4pt] [1] Xiao, D., \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{108}, 196802 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z5.00012: Photoresponse of Quasi-One Dimensional Graphene Nanostructures Tu Hong, Zeynab Jarrahi, Yunhao Cao, Alex Huffstutter, Yaqiong Xu Here, we perform simultaneous photocurrent and photoluminescence measurements of free-standing graphene nanostructures. Their photocurrent intensities show a linear relationship with the incident laser power, whereas their photoluminescence intensities increase non-linearly when the incident power rises. The photoluminescence may result from the thermal radiation generated during hot carrier relaxation. The power dependences of their photoluminescence reveal that these graphene nanostructures are quasi-one dimensional materials. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z5.00013: Hybrid graphene-organic molecule transistors with large photoresponse Shao-Yu Chen, Yi-Ying Lu, Fu-Yu Shih, Po-Hsun Ho, Chun-Wei Chen, Yang-Fang Chen, Yit-Tsong Chen, Wei-Hua Wang We present large photoresponse in hybrid graphene-organic molecule transistors, which exhibit high gain and large responsitivity. High-quality graphene phototransistors are achieved via resist-free fabrication and noncovalent bonding of the organic molecules. The photocurrent of the devices is tunable with back gate which enables high controllability by electrical means. The strong photoresponse can be attributed to charge transfer and photogating effect in the layer of organic molecules. High photo-sensitivity in the hybrid graphene-organic molecule transistors is promising for the future development of graphene-based optoelectronic applications. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z5.00014: Photocurrent Response of Graphene Heterostructures Joaquin Rodriguez-Nieva, Mildred S. Dresselhaus One of the obstacles to the use of graphene as an alternative to silicon electronics has been the absence of a band gap. One solution to some of the limitations that this obstacle introduces is to integrate graphene into a heterostructure such as a field-effect tunneling transistor that uses an atomically thin dielectric [1]. We explore theoretically some of the interesting properties of optically excited graphene heterostructures, where novel behaviors can appear due to the tunability of the Fermi level and thus, of the charge carrier densities and intrinsic electronic cooling mechanisms. We also discuss possible applications of such types of optically activated heterostructures in different areas of science and engineering. References: [1] L. Britnell, R. V. Gorbachev, R. Jalil, et al., Science, 335, 947-950 (2012) [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z5.00015: Near-field spectroscopy of graphene during ultrafast photoexcitation Martin Wagner, Zhe Fei, Alexander McLeod, Aleksandr Rodin, Wenzhong Bao, Lingfeng Zhang, Zeng Zhao, Eric Iwinski, Mark Thiemens, Michael Fogler, Antonio Castro-Neto, Chunning Lau, Fritz Keilmann, Dimitri Basov Recently, impressive progress in nanoplasmonics of graphene using near-field spectroscopy and imaging has been reported [Z. Fei et al., Nano Lett. 11, 4701 (2011); Z. Fei et al., Nature 487, 82 (2012)]. However, these studies of the interaction of the graphene plasmon with the SiO2 substrate surface phonon were time-independent. Here we combine imaging and material characterization on the nano scale with ultrafast sub-picosecond time resolution and present optical pump broadband mid-infrared probe spectroscopy of graphene. We discuss the optical pump induced changes of the coupled plasmon-phonon modes with respect to carrier density and time-dependence. The difference between ultrafast photoexcitation and conventional electrostatic doping via the field effect is analyzed and compared with modeling. [Preview Abstract] |
Session Z6: Nanotubes and Nanowires (non-carbon): Other Phenomena
Sponsoring Units: DCMPChair: Yanjie Zhou, Purdue University
Room: 302
Friday, March 22, 2013 11:15AM - 11:27AM |
Z6.00001: Thin-Film Nanowire Networks for Transparent Conductor Applications: Simulations of Sheet Resistance and Percolation Thresholds Karen I. Winey, Rose M. Mutiso, Michelle C. Sherrott, Aaron R. Rathmell, Benjamin J. Wiley Thin-film metal nanowire networks are being pursued as a viable alternative to the expensive and brittle indium tin oxide (ITO) for transparent conductors. For high performance applications, nanowire networks must exhibit high transmittance at low sheet resistance. Previously, we have used complimentary experimental, simulation and theoretical techniques to explore the effects of filler aspect ratio (L/D), orientation, and size-dispersity on the electrical conductivity of three-dimensional rod-networks in bulk polymer nanocomposites. We adapted our 3D simulation approach and analytical percolation model to study the electrical properties of thin-film rod-networks. By fitting our simulation results to experimental results, we determined the average effective contact resistance between silver nanowires. This contact resistance was then used to quantify how the sheet resistance depends on the aspect ratio of the rods and to show that networks made of nanowires with L/D greater than 100 yield sheet resistances lower than the required 100 Ohm/sq. We also report the critical area fraction of rods required to form a percolated network in thin-film networks and provide an analytical expression for the critical area fraction as a function of L/D. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z6.00002: A simple method to make an electrical connection between ZnO microwire and substrate through nanoscale metal evaporation Hakseong Kim, Jinkyung Lee, Hoyeol Yun, Sang Wook Lee We developed a simple method to make an electrical connection with nanoscale electrodes on microscale wire using suspended Poly(methyl methacrylate) (PMMA) strings. Less than 90 nm height of Ti/Au made a complete electrical connection on the ZnO microwires of which diameter is around 2 $\mu $m. A cross linked PMMA string was bridged between ZnO microwire and substrate for making good electrical connection. The contact resistance of ZnO microwire fabricated by this method was much lower than that of device fabricated by standard E-beam lithography and evaporation. This fabrication method is readily extendible to prepare nano scale electrodes on various micro sized materials and serves as a pathway for studying their mesoscopic transport phenomena. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z6.00003: High-performance Flexible Photodetectors based on Aligned Cadmium Sulfide Nanowire Networks Dong-Guk Cho, Kwang Heo, Hyungwoo Lee, Yongju Park, Jinho Park, Hyun-Jin Lim, Duhee Yoon, Changhee Lee, Miyoung Kim, Hyeonsik Cheong, Jonghyurk Park, Jikang Jian, Seunghun Hong We developed a method to mass-produce aligned cadmium sulfide (CdS) nanowire (NW) network channels for highly flexible and high-performance photodetectors. In this method, CdS NWs were aligned along the molecular patterns on flexible substrates by a directed assembly strategy. The aligned CdS NW patterns were utilized as the channel of flexible photodetectors. The photodetectors based on \textit{aligned} CdS NWs showed $\sim$ 10 times higher photosensitivity and $\sim$ 100 times faster photoresponse than those based on \textit{randomly-oriented} CdS NW networks. Additionally, the stable photoconductive characteristics of our flexible photodetectors were maintained even when the photodetectors were bent down to a 0.2 mm radius of curvature. This simple but efficient strategy should pave the way for the large-scale fabrication of low-cost and high-performance flexible photodetectors based on the aligned CdS NW networks. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z6.00004: Growth of aligned Mo$_6$S$_6$ nanowires on a Cu(111) Maral Aminpour, Duy Le, Dezheng Sun, Wenhao Lu, Chen Wang, Quan Ma, Ludwig Bartels, Talat S. Rahman We report the possibility of using the Cu(111) surface for growing molybdenum sulfide nanowire (Mo$_{6}$S$_{6})$ based on density functional theory and scanning tunneling microscopy investigations [1]. A small lattice mismatch between the nanowires and strong substrate interactions lead to epitaxial growth of the nanowires at alignment with the substrate crystallographic axes and at a preferred inter-wire separation.\\[4pt] [1] Duy Le, Dezheng Sun, Wenhao Lu, Maral Aminpour, Chen Wang, Quan Ma, Talat S. Rahman, Ludwig Bartels [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z6.00005: Structural Characterization and Transport Properties of GaN nanowires in non-serrated and newly discovered serrated morphologies Zheng Ma, Adam Friedman, Latika Menon We present results on the synthesis, structural characterization and transport properties of single crystal GaN nanowires in two different morphologies (non-serrated and serrated nanowires). The synthesis of these two types of nanowires is carried out in chemical vapor deposition with Au catalysts. Different from the regularly non-serrated GaN nanowires, the GaN nanowires in ``serrated'' morphology have been newly discovered by our group. By controlling the growth conditions, it has been demonstrated that GaN nanowires with regular periodic serrations along the surface of the nanowire can be produced under specific conditions as for large-sized Au catalysts and excess concentration of gallium oxide. Detailed structural and morphological characterization studies reveal interesting features for these two growth modes. In an attempt to understand how these structural and morphological variations impact the electrical properties, transport studies on single GaN nanowires (both serrated and non-serrated) are currently underway. The transport properties, namely current versus voltage will be obtained for such nanowires which in turn will reveal important information on the potential applications of such wires in optoelectronic devices. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z6.00006: On/off-current Ratio and Ambipolar Behavior of Narrow Bandgap III-V Nanowire FETs Yanjie Zhao, Drew Candebat, Collin Delker, Yunlong Zi, David Janes, Joerg Appenzeller, Chen Yang III-V nanowires (NW) are promising candidates for future device applications due to the high bulk mobility. Yet the small bandgap may result in undesirable high off-current. Here we establish a simple but reliable model that quantitatively explains how channel bandgap and Schottky barriers at metal contacts affect the ambipolar characteristics and the achievable on/off-current ratios of NW-FETs. Thus one can gain insights of the expected transfer characteristics of a given channel material with certain device structure, and the optimal choice of materials for different device applications in ultimately scaled cases. The physics of electron transport in both ideal case (no Schottky barrier) and practical case (with Schottky barrier) is studied. The impact of Schottky barriers is evaluated by numerical calculation of the tunneling current, and is found to play a critical role for the different characteristics observed. A universal plot of on/off ratio vs. bandgap is presented. The excellent agreement between our simulation predictions and experiment results from InAs, InSb, Ge NWs and CNTs highlights the potential of our approach for understanding narrow bandgap NW-FETs, bridging material development and device applications, and guiding future transistor design. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z6.00007: Resistive Switching in Metal-Nanowire/Polymer Nano-Gap Devices Rose M. Mutiso, James K. Kikkawa, Karen I. Winey We recently presented the first examples of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions near the percolation threshold, Ag nanowire-polystyrene composites exhibit reversible resistive switching upon increase voltage at room temperature. We proposed that switching in these materials is the result of the field-induced formation of Ag filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample's bulk resistivity. To further understand the switching mechanism and explore possible applications, we have designed and fabricated single-gap nanowire devices comprised of lithographically-defined metal lines separated by polymer-filled nano-gaps. We have successfully demonstrated reversible resistive switching in our nano-gap Ag/PS devices when the gap size is 20 - 100nm, observing highly reversible switching behaviors in some samples with high on/off ratios for over 50 cycles. Preliminary ex-situ high resolution imaging of the devices shows significant gap remodeling after a switching event, implying that the switching mechanism is linked to some form of electromigration of Ag electrodes. Additional ex- and in-situ characterization studies to elucidate observed trends are in progress. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z6.00008: Nonlocal Response of Plasmonic Nanowire Metamaterials in the ENZ Regime Brian Wells, Anatoly Zayats, Viktor Podolskiy Nanowire metamaterials are a class of materials formed by an array of aligned plasmonic nanowires embedded in a dielectric host which exhibit strongly anisotropic behavior. For a wide range of excitation frequencies, the optical properties of these systems are dominated by two waves with different polarizations. In contrast to this behavior, in the epsilon-near-zero (ENZ) frequency range, excitation of additional wave mode has been observed. In this frequency range the contribution of spatial dispersion becomes increasingly important and a modified dispersion relationship for the anisotropic metamaterials must be used. The properties of the additional wave need to be taken into consideration during design and analysis of the properties of nanowire-based systems$.$ Here we present analytical and computational studies of the nonlocal optical response of plasmonic nanowire metamaterials. Dispersion of photonic modes of plasmonic metamaterials have been studied as a function of wavelength, geometry, and material parameters. A new analytical description of the optical properties of nonlocal nanowire systems has been developed. It is shown that the optical response of the system results from the coupling of conventional effective-medium-dominated oscillations with plasmon-polariton-type oscillations. The presented model is in agreement with numerical solutions of Maxwell's equations. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z6.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z6.00010: First-principles study of bio-conjugated ultra-thin silicon nanowires: Interaction with a PNA-RNA double helix Xiaoliang Zhong, William Slough, Ravindra Pandey, Craig Friedrich We present the results of a first-principles study based on density functional theory of peptide nucleic acid (PNA) - ribonucleic acid (RNA) double helix conjugated silicon nanowires (SiNWs). The effects of a hexane linker functionalization, probe PNA strand immobilization, and target RNA strand hybridization on the electronic states of the ultra-thin SiNWs in a dry condition are investigated. All of these effects appear to marginally modify the core silicon states of the nanowires, manifested by a low level of p-doping in SiNWs. The intrinsic energy gap of the SiNWs is essentially unchanged, though there exist mid-gap states contributed by the PNA/RNA molecules which tend to localize near the Fermi energy. Overall, the bio-conjugation considered does not appear to significantly affect the intrinsic electronic and transmission states of the ultra-thin SiNWs. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z6.00011: Wide bandwidth nanowire electromechanics on insulating substrates at room temperature Abhilash Sebastian, John Mathew, Shamashis Sengupta, Maheshwar Gokhale, Arnab Bhattacharya, Mandar Deshmukh We present a simple fabrication scheme for nano-scale devices on insulating substrates. Doubly clamped InAs nanowire resonators with local gate configuration are fabricated on sapphire substrates. Parasitic capacitance is reduced on insulating substrates thus enabling measurements at all temperatures and particularly above room temperature, an essential requirement for NEMS sensors. Mechanical motion of the nanowire is capacitively actuated and detected using a network analyser. This technique provides wide bandwidth radio frequency transduction and allows the nanowire oscillations to be probed at a much faster rate compared to mixing techniques. Both in-plane and out-of-plane vibrational modes of the nanowire are observed and the non-linear response of the resonators is studied. Quality factor of the resonator increases at low temperatures. We also study the relation between mechanical motion and thermal strains in the nanowire. This opens up a new approach in studying thermal properties of nanostructures. Our method of fabrication can be extended to NEMS devices on flexible substrates and other nanostructures. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z6.00012: Enhanced Performance in Flexible Binder-free SWCNT Membrane EDLC Danhao Ma, Pralav Shetty, Kofi Adu, Ramakrishnan Rajagopalan We present results on an aqueous symmetric double layer electrochemical capacitor (EDLC) constructed with flexible binder-free single wall carbon (SWCNTs) membrane as electrodes. The capacitors were cycled from 0 to 1V @ 10 A/g for 10,000 cycles with 99.9{\%} coulombic efficiency and 94{\%} energy efficiency, and 100{\%} depth of discharge. The power performance of the aqueous symmetric SWCNTs membrane capacitor is almost 100 --1000 times better than commercial non-aqueous EDLC capacitors. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z6.00013: Synthesis and HRTEM Electron Diffraction Characterization of Monocrystalline V$_2$O$_5$ Luisa Tafoya, Luis Rendon, Patricia Santiago, Elizabeth Chavira, Ernesto E. Marinero, Vicente Garibay, Leonardo Gonzalez We have synthesized V$_{2}$O$_{5}$ nanorods via solvothermal synthesis. By controlling the synthesis conditions, unidirectional crystalline growth is achieved. HRTEM and XRD studies reveal that the resulting nanorods are monocrystalline and are on average 80 nm in width and readily grow to a few microns in length. Utilizing electron diffraction we investigate the growth of these nanostructures along preferential crystalline planes. XRD confirms also that the crystalline phase of the nanorods is orthorhombic. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z6.00014: Narrow peaks in the current power spectra of nanomechanical resonators. Dong Liu, Adrian Bachtold, Joel Moser, Alex Levchenko, Mark I. Dykman We show that the power spectrum of current through a nanomechanical resonator has narrow peaks at the frequencies of mechanical modes. These peaks can be selectively downshifted to low frequencies by applying almost resonant ac source-drain or gate voltage. Our analysis refers to the Coulomb blockade regime, where the current is limited by tunneling through the contacts. Where the tunneling rate largely exceeds the vibration frequency, the analysis can be done in terms of the conductance that adiabatically depends on the displacement of the nanoresonator. In a more general case the current power spectrum near the narrow vibration-induced peaks is related to the vertex correction for the corresponding Green function. The spectral peak at low frequency can result also from the vibration nonlinearity in the absence of inversion symmetry. We note that measuring the power spectra of the current noise provides an alternative to the often complicated direct measurements of the absorption spectrum of coupled electron-vibrational systems. [Preview Abstract] |
Session Z7: Focus Session: Carbon Nanotubes: Synthesis
Sponsoring Units: DMPChair: Eric Stach, Brookhaven National Laboratory
Room: 303
Friday, March 22, 2013 11:15AM - 11:51AM |
Z7.00001: Developing Single-Wall Carbon Nanotubes into an Industrial Material through the Super-Growth CVD Method Invited Speaker: Don Futaba Since the discovery of the carbon nanotube (CNT) 20 years ago, extensive effort has been made to utilize their exceptional intrinsic properties toward industrial applications. However, availability has significantly thwarted these endeavors. In one section of my presentation, I will describe our efforts toward the economical mass-production of single-walled carbon nanotubes (SWCNT) based on the water-assisted chemical vapor deposition technique, from which highly efficient synthesis of vertically aligned SWCNTs grow from substrates (SWCNT forests). Further, I will discuss our work to promote the industrial use of SWCNTs as a member of the Technology Research Association for Single-Walled Carbon Nanotubes (TASC) (A consortium of five companies and AIST founded for the specific purpose of developing SWCNT industrial technology.) Specifically, I will present our progress on developing the technology for the synthetic control of SWCNTs and the development of standardized evaluation techniques for the purpose of understanding the relationship between the SWCNT forest structure, e.g. length, density, crystallinity, etc and the targeted property, e.g. conductivity, mechanical reinforcement, etc. Finally, I will present several examples of applications from composites to CNT-based devices. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z7.00002: Oxygen-Assisted Synthesis of Single-Walled Carbon Nanotubes O. Tolga Gul, Arith J. Rajapakse, Philip G. Collins Water-assisted chemical vapor deposition (CVD) has become a standard synthesis method for high quality single-walled carbon nanotubes (SWCNTs). Some drawbacks of the water-assisted method, however, include good control of water concentrations in the feedstock and poor control of SWCNT diameters below 2.0 nm. Here, we describe a variation of water-assisted CVD that uses dry feedstocks with a small, controlled quantity of molecular oxygen. Reactions of oxygen with hydrogen in the reaction zone provide all the benefits of water-assisted growth at the substrate while maintaining dry valves and flowmeters. In addition, the oxygen-based technique allows water concentrations in the system to be varied precisely and with short time constants. Perhaps because of the improved control, we find that the SWCNT diameter can be easily tuned by changing the oxygen concentration during the growth phase. Changing the oxygen concentration over the range of 0.5{\%} to 1{\%} varied the resulting SWCNT diameters from 1.5 to 0.5 nm, with typical diameter distributions less than $+$/- 30{\%}. Control of SWCNT growth within this diameter range is ideal for probing opto-electronic properties of individual SWCNTs and SWCNT devices. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z7.00003: Chirality-controlled synthesis of single-wall carbon nanotubes using vapour phase epitaxy Jia Liu, Chuan Wang, Xiaomin Tu, Bilu Liu, Liang Chen, Ming Zheng, Chongwu Zhou Due to the superior electrical properties such as high intrinsic carrier mobility and current-carrying capacity, single wall carbon nanotubes (SWCNT) hold great promise for electronic application. Since the electronic property of a SWCNT strongly depends on its chirality, the lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of SWCNTs Here we demonstrate a general strategy for producing carbon nanotubes with predefined chiralities by using purified single-chirality nanotubes as seeds for subsequent metal-catalyst-free growth, resembling vapour phase epitaxy commonly used for semiconductor films. In particular, we have successfully synthesized (7, 6), (6, 5), and (7, 7) nanotubes, and used Raman spectroscopy to show unambiguously that the original chiralities of the nanotube seeds are preserved. Furthermore, we have performed electrical measurements on synthesized individual (7, 6) and (6, 5) nanotubes, confirming their semiconducting nature. The vapour phase epitaxy approach is found to be highly robust and should enable a wide range of fundamental studies and technological developments. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z7.00004: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z7.00005: Towards Single-Chirality Armchair Carbon Nanotube Ensembles using Combined Size Exclusion Chromatography and Density Gradient Ultracentrifugation Erik Haroz, Junichiro Kono, Robert Hauge, Stephen Doorn, Constantine Khripin, Ming Zheng Recently, density gradient ultracentrifugation (DGU) has been shown to produce aqueous ensembles enriched in armchair carbon nanotubes (CNTs), introducing new experimental insight into the photophysics of one-dimensional metals. However, despite these successes, DGU-produced armchair CNT ensembles contain multiple armchair species, which is not ideal for extracting chirality-specific optical quantities. Sample heterogeneity is partly due to tube-to-tube variability in other CNT properties such as end-capping, CNT diameter and length, resulting in differences in the observed CNT mass density. For example, CNT sedimentation velocity increases with decreasing tube length, resulting in a given CNT species appearing in multiple separated fractions after DGU. Here, using surfactant-based, size exclusion chromatography, high-concentration, uniform length CNT fractions were produced. These fractions were subsequently used for armchair enrichment DGU with the expectation that greater uniformity of the starting CNT material will lead to more monodispersed fractions, enhancing separation towards the goal of single-chirality armchair ensembles. The resulting separated fractions were analyzed using optical absorption and resonant Raman spectroscopy to assess improvement in separation. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z7.00006: Removal of surfactants and adducts from solution-processed single-walled carbon nanotubes Alexander Kane The use of single-walled carbon nanotubes (SWCNTs) in scalable electronics and optoelectronics requires purification of the material to remove contaminants from the growth, and enrichment of the semiconducting fraction of the material through sorting. Centrifugation of aqueous suspensions of SWCNTs allows for both purification and sorting in successive steps with the aid of surfactants, but the suspension process causes oxidative damage to the SWCNTs and the surfactants are difficult to remove from the SWCNT sidewall after deposition on the substrate. These residual surfactants and adductive defects negatively impact device performance. We present a two-step approach towards reducing this disorder post-deposition using mild oxidation to remove the surfactant followed by vacuum annealing to heal the SWCNT sidewall. Thermal gravimetric analysis and temperature programmed desorption show the optimal conditions and fundamental mechanisms. Characterization of the results using Raman spectroscopy, atomic force microscopy, and electronic transport measurements show that the quality of the material is maintained. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z7.00007: Directed assembly of one-dimensional functional nanostructures Erika Penzo, Matteo Palma, Risheng Wang, Shalom J. Wind One-dimensional (1D) nanostructures have unique electronic, optical and mechanical properties that have attracted intense interest over the past two decades. Single wall carbon nanotubes (SWNTs) and semiconducting nanorods have long been recognized as potential candidates for future nanoelectronic applications. The small size and the fact that these nanostructures are synthesized either at high temperatures or in solution make it difficult to organize them in complex architectures, a key requirement for their exploitation. As a step toward this goal, we are developing approaches leading to the controlled and ordered arrangement of nanoobjects on lithographically patterned, chemically (or biochemically) functionalized surfaces. One approach consists in patterning metallic nanodots that serve as anchors by selective functionalization with single stranded DNA (ssDNA) or with other chemical moieties. End functionalized nanostructures are attached to the dots through DNA hybridization or through a covalent bond. A second approach consists in patterning hydrophilic regions on hydrophobic substrates. Ion-complexed nanostuctures selectively bind to the hydrophilic pattern. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z7.00008: A novel fabrication process of hundreds of field effect transistors around one single carbon nanotube for molecular assembly Xian Zhang, Daniel Chenet, Bumjung Kim, Jaeeun Yu, Colin Nuckolls, James Hone Carbon nanotube field effect transistors (CNTFETs) can be used both as stand-alone electronic devices and as basis for other devices, but high-throughput fabrication remains an important challenge. In one specifically demanding application, CNTFETs are lithographically `cut' and rejoined with single molecules in the gap, to yield circuits for studying transport properties of single molecules. Because of the extreme precision required, such devices have a fabrication yield of only a few percent, which severely limits the speed of implementing CNT-molecule devices. In addition, the diversity of CNT structures provides an additional source of heterogeneity that makes collection of meaningful statistics difficult. Here we report a novel fabrication method to produce a chip with over 600 CNTFETs fabricated on one CNT. We use long (1cm) flow-aligned CNTs grown by chemical vapor deposition. Two photolithography steps are then used to pattern contacts and define a mask to burn away extra CNTs by oxygen plasma. We present the statistics of the transport properties of these devices including threshold voltage and on-state resistance. The devices are then lithographically cut and reconnected with DNA to provide consistent measurements of DNA conductance. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z7.00009: Can diamond nanowires grow inside carbon nano\-tubes? Zhen Zhu, David Tom\'anek, Yanquan Feng We investigate the possibility of templated growth of diamond nanowires from functionalized diamondoid molecules enclosed in a carbon nanotube (CNT). Our {\em ab initio} density functional theory studies identify suitable candidate molecules and conditions, under which such molecules may fuse to narrow diamond nanowires with C$_8$H$_8$ or C$_7$H$_8$ unit cells inside a CNT. We find that the unique environment inside a narrow carbon nanotube, which can be suitably represented by a cylindrical potential, subjects enclosed molecules to a high pressure, caused by ``capillary'' forces, and orients them in a suitable way favoring fusion and constraining the resulting structure. Based on total energy calculations, we find that fusion of C$_{10}$H$_{16}$ adamantane molecules requires additional energy, whereas fusion of C$_{14}$H$_{18}$(COOH)$_2$ diamantane di-acid molecules in hydrogen atmosphere occurs as an exothermic reaction. Our canonical molecular dynamics calculations at elevated temperatures indicate likely intermediate products occurring during this reaction. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z7.00010: Characteristics of thin graphene sheets prepared by a laser ablation method Maki Nakamura, Takazumi Kawai, Michiko Irie, Ryota Yuge, Sumio Iijima, Shunji Bandow, Masako Yudasaka Graphenes are innovative carbon materials having a sheet-like structure; these materials are thought to have many applications in the fields of electrochemistry, biomedicine, and so on. In this study, we showed that thin graphene sheets (TGSs) prepared by a laser ablation method had a distinctive structure: even-number layered graphenes (2-, 4-, 6- and 8-layers) were preferentially grown (ca. 90{\%}), and their population decreased as the layer number increased. These phenomena have not been observed in graphenes prepared with other methods. Our results suggest a new growth mechanism in which single-layer graphene is unstable and bends to form bi-layers, and the bi-layers then go on to stack and form thicker TGSs. The inter-layer distances estimated by transmission electron microscope images were about 15{\%} larger than that of bulk graphite in the bi-layer TGSs, and they approached the bulk value as the layer number increased. Furthermore, we showed surface-selective functionalization of TGSs by mild oxidation with H$_{2}$O$_{2}$ at room temperature, indicating the possibility of multi-modal functionalization, which will make the graphene more attractive in various applications. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z7.00011: Plasmon Heat Transport Between Vertical Carbon Nanotube Forest and Different Substrates Andrei Nemilentsau, Slava Rotkin Near-field radiative heat transfer between vertical forest of carbon nanotubes and different metallic and dielectric substrates has been studied using the formalism of the fluctuational electrodynamics. Proper matching between surface plasmons in nanotubes and surface polaritons in the substrate was demonstrated to be crucial for the efficient thermal coupling across the interface. Particularly, thermal Kapitza conductance between nanotubes and such polar dielectrics as quartz, sapphire and GaAs (with surface phonon-polariton energies $\sim$ 30-50 meV) is substantially higher than that between nanotubes and BN and SiC (with polartion energies $>$ 100 meV), or metals (with plasmon-polaritons in the visible range). Further optimization of heat transport can be achieved by tweaking nanotube length. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z7.00012: Unusual thermal conduction characteristics of phase change composites with single-walled carbon nanotube inclusion Sivasankaran Harish, Kei Ishikawa, Shohei Chiashi, Junichiro Shiomi, Shigeo Maruyama Thermal energy storage using phase transition materials is often employed in many engineering applications. However, the low thermal conductivity of such materials inhibits its use for large scale applications. Recently, Zheng et al. [Nature Comm. 2011] demonstrated an efficient technique using graphite suspensions to tune the thermal and electrical conductivity using temperature regulation. In this work, we report large contrasts in thermal conductivity enhancement of nano composites with single walled carbon nanotube (SWCNT) inclusions using first order phase transition process. SWCNTs synthesized by alcohol CVD were dispersed in n-octadecane by tip-sonication with sodium deoxycholate as the surfactant. Thermal conductivity measurements were carried out with transient hot-wire system [Mater. Express 2012]. Thermal conductivity enhancement in the liquid state was found to be nominal and is consistent with the predictions of Maxwell- Garnett type effective medium theory. However, in the frozen state nearly a 2.5 fold increase in thermal conductivity was observed. Similar temperature dependent thermal conductivity contrast was observed when exfoliated graphite nanoplatelets were used as the inclusions. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z7.00013: Explosive Characteristics of Carbonaceous Nanoparticles Leonid Turkevich, Joseph Fernback, Ashok Dastidar Explosion testing has been performed on 20 codes of carbonaceous particles. These include SWCNTs (single-walled carbon nanotubes), MWCNTs (multi-walled carbon nanotubes), CNFs (carbon nanofibers), graphene, diamond, fullerene, carbon blacks and graphites. Explosion screening was performed in a 20 L explosion chamber (ASTM E1226-10 protocol), at a (dilute) concentration of 500 g/m$^{3}$, using a 5 kJ ignition source. Time traces of overpressure were recorded. Samples exhibited overpressures of 5-7 bar, and deflagration index K$_{\mathrm{St}} =$ V$^{1/3}$ (dp/pt)$_{\mathrm{max}}$ $\sim$ 10 - 80 bar-m/s, which places these materials in European Dust Explosion Class St-1 (similar to cotton and wood dust). There was minimal variation between these different materials. The explosive characteristics of these carbonaceous powders are uncorrelated with particle size (BET specific surface area). Additional tests were performed on selected materials to identify minimum explosive concentration [MEC]. These materials exhibit MEC $\sim$ 10$^{1}$ -10$^{2}$ g/m$^{\mathrm{3}}$ (lower than the MEC for coals). The concentration scans confirm that the earlier screening was performed under fuel-rich conditions (i.e. the maximum over-pressure and deflagration index exceed the screening values); e.g. the true fullerene K$_{\mathrm{St}}$ $\sim$ 200 bar-m/s, placing it borderline St-1/St-2. [Preview Abstract] |
Session Z8: Topological Insulators: Transport and interfaces
Sponsoring Units: DCMPChair: Rolando Valdes Aguilar, Los Alamos National Laboratory
Room: 307
Friday, March 22, 2013 11:15AM - 11:27AM |
Z8.00001: Time-reversal anomaly and Josephson effect in time-reversal invariant topological superconductors Suk Bum Chung, Joshua Horowitz, Xiao-Liang Qi Topological superconductors are gapped superconductors with protected Majorana surface/edge states on the boundary. In this paper, we study the Josephson coupling between time-reversal invariant topological superconductors and $s$-wave superconductors. The Majorana edge/surface states of time-reversal invariant topological superconductors in all physical dimensions 1, 2, 3 have a generic topological property which we named as time-reversal anomaly. Due to the time-reversal anomaly, the Josephson coupling prefers a nonzero phase difference between topological and trivial superconductors. The nontrivial Josephson coupling leads to a current-flux relation with a half period in a SQUID geometry, and also a half period Fraunhofer effect in dimension higher than one. We also show that an in-plane magnetic field restores the ordinary Josephson coupling, as a sharp signature that the proposed effect is a consequence of the unique time-reversal property of the topological edge/surface states. Our proposal provides a general approach to experimentally verify whether a superconductor is topological or not. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z8.00002: Symmetry Protected Josephson Supercurrents in Three-Dimensional Topological Insulators Sungjae Cho, Brian Dellabetta, Alina Yang, John Schneeloch, Zhijun Xu, Tonica Valla, Genda Gu, Matthew Gilbert, Nadya Mason Coupling the surface state of a topological insulator (TI) to an s-wave superconductor is predicted to produce the long-sought Majorana quasiparticle excitations. However, superconductivity has not been measured in surface states when the bulk charge carriers are fully depleted, i.e., in the true topological regime relevant for investigating Majorana modes. Here, we report measurements of DC Josephson effects in TI-superconductor junctions as the chemical potential is moved through the true topological regime characterized by the presence of only surface currents. We compare our results to 3D quantum transport simulations, and determine the effects of bulk/surface mixing, disorder, and magnetic field; in particular, we show that the supercurrent is largely carried by surface states, due to the inherent topology of the bands, and that it is robust against disorder. Our results thus clarify key open issues regarding the nature of supercurrents in TIs. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z8.00003: Majorana fermions in a superconductor quantum wire connected to normal leads Edson Vernek, Ant\^onio C.F. Serid\^onio, Jos\'e C. Egues We study the appearance of Majorana fermions in a quantum wire connected to a normal lead. We employ a Kitaev model for the wire with induced superconductivity with a full coupling with a normal wire. In comparison with previous study of this problem, our approach has the advantage of allowing us to fine tune the Kitaev Hamiltonian model all the way from its normal to its superconducting topological phase. By developing an exact Green's function calculation scheme, we are to explore the full parameter space of the model via analysis of the electron and the Majorana density of states. Our results show clearly that the main effect of a particle-hole symmetric lead is the broadening of the Majorana density of states at the end of the wire, while particle-hole asymmetric leads are detrimental to the Majorana bound states. We also study the transmission through a quantum dot connected to two normal leads and to a superconducting wire. We show that by driving the wire from its normal to its topological phase, a great change in the transmission function through the dot is observed, clearly indicating the emergence of a Majorana mode in the wire. Although such a signature has already been predicted in recent works, our model leads to substantially different results. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z8.00004: Majorana Fermions Under Stress Ming Gong, Li Mao, Sumanta Tewari, Chuanwei Zhang Spin-orbit coupled semiconductor nanowires with Zeeman splitting in proximity contact with bulk s-wave superconductivity have recently been proposed as a promising platform for realizing Majorana fermions. However, in this setup the chemical potential of the nanowire is generally pinned by the Fermi surface of the superconductor. This makes the tuning of the chemical potential by external electrical gates, a crucial requirement for unambiguous detection of Majorana fermions, very challenging in experiments. Here we show that tunable topological superconducting regime supporting Majorana fermions can be realized in semiconductor nanowires using uniaxial stress. For n-type nanowires the uniaxial stress tunes the effective chemical potential, while for p-type systems the effective pairing may also be modified by stress, thus significantly enhancing the topological minigap. The required stress is within current experimental reach using conventional piezo crystals. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z8.00005: The soft superconducting gap in semiconductor Majorana nanowires So Takei, Benjamin M. Fregoso, Hoi-Yin Hui, Alejandro M. Lobos, Sankar Das Sarma We theoretically consider the mysterious topic of the soft gap in the tunneling conductance of the proximity-induced superconductivity in a semiconductor-superconductor hybrid structure, where the observation of a zero-bias conductance peak has created considerable excitement because of its possible connection with the elusive zero-energy Majorana mode. The observed experimental superconducting tunneling gap in the semiconductor nanowire looks v-shaped with considerable subgap conductance even at very low temperatures in sharp contrast to the theoretically expected hard BCS gap with exponentially suppressed subgap conductance. We systematically study, by solving the appropriate BdG equations both numerically and analytically, a number of physical mechanisms (e.g. magnetic and non-magnetic disorder, finite temperature, dissipative Cooper pair breaking, interface fluctuations), which could, in principle, lead to a soft gap, finding that only the interface fluctuation effect is a quantitatively and qualitatively viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface would go a long way in enhancing the gap in the hybrid structures being used for studying the Majorana mode. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z8.00006: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z8.00007: Majorana edge modes of topological exciton condensate with superconductors Babak Seradjeh I study the edge states of the topological exciton condensate formed by a Coulomb interaction between two parallel surfaces of a strong topological insulator. When the condensate is contacted by superconductors with a $\pi$ phase shift across the two surfaces, a pair of counterpropagating Majorana modes close the gap at the boundary. I propose a nanostructured system of topological insulators and superconductors that hosts unpaired Majorana fermions when and only when the exciton condensate forms. Therefore, measuring the Majorana signal in this structure provides a way of detecting the topological exciton condensate that is uniquely related to its topological nature. The relevant experimental signatures as well as implications for related systems are discussed. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z8.00008: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z8.00009: Entanglement in a Cooper-pair Splitter based on a Topological Insulator Koji Sato We theoretically study a solid state device producing entangled electron pairs that are spatially separated by coupling a superconductor to the helical edge states of a two-dimensional topological insulator. The interacting regions of the edge states are taken to be finite length around the tunneling region to capture the effect of non-interacting leads, and the ends of a given edge are further connected to a beam splitter. By controlling the scattering through such beam splitters, we show that Bell test can be performed via measurement of the current-current correlations. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z8.00010: Current-phase relationship of planar Josephson junctions mediated by the surface states of a topological insulator C. Kurter, A.D.K. Finck, C.D. English, Y.S. Hor, D.J. Van Harlingen It is predicted that the presence of Majorana fermions manifests itself with a $4\pi$ periodic current-phase relation (CPR) in planar Josephson junctions formed with topological weak links. To test this proposal, we have fabricated planar junctions by depositing Nb leads on exfoliated Bi$_{2}$Se$_{3}$ single crystals. The temperature and magnetic field dependence of the proximity-induced supercurrent have been examined in various doping regimes accessed via top gating. The critical current modulation with magnetic field deviates from the usual Fraunhofer diffraction pattern, suggesting modifications to a sinusoidal CPR consistent with a $sin(2\phi)$ component. We are corroborating those results with direct measurements of the CPR using a phase-sensitive SQUID interferometry technique. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z8.00011: Signatures of Majorana Fermions in Topological Insulator Josephson Junction Devices Benjamin Wieder, Fan Zhang, Charles Kane We study theoretically the electrical current and low-frequency noise for a linear Josephson junction structure on a topological insulator, in which the superconductor forms a closed ring, and currents are injected from normal regions inside and outside the ring. We find that this geometry offers a unique signature for the presence of gapless 1D Majorana fermion modes that are predicted to exist in the channel when the phase difference $\phi$, controlled by the magnetic flux through the ring, is $\pi$. We show that for low temperature, the linear conductance jumps by $2e^2/h$ when $\phi$ passes through $\pi$, accompanied by non-local correlations between the currents from the leads inside and outside of the ring. We compute the dependence of these features on temperature, voltage, and linear dimensions, and discuss the implications for experiments. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z8.00012: Spinful Majorana fermions and magnetoelectricity in junctions of semiconductor / superconductor heterostructures Panagiotis Kotetes, Alexander Shnirman, Gerd Schoen Recently, the interest in topological quantum computing has grown due to the appearance of promising platforms for realizing Majorana fermions. The most prominent proposal involves a 1D semiconducting quantum wire in proximity to a bulk s-wave superconductor, where in addition a Zeeman field is applied. Here we investigate the Josephson effect in TNT and NTN junctions, consisting of topological (T) and non-topological (N) phases of semiconductor-superconductor 1D heterostructures in the presence of a Zeeman field [1]. A key feature of our setup is that, in addition to the variation of the phase of the superconducting order parameter, we allow the orientation of the magnetic field to change along the junction. We find a novel magnetic contribution to the Majorana Josephson coupling that permits the Josephson current to be tuned by changing the orientation of the magnetic field along the junction. We also predict that a spin current can be generated and additionally controlled by a finite superconducting phase difference. This new type of coupling not only constitutes a unique fingerprint of Majorana fermions but also provides an alternative pathway for manipulating and braiding topological qubits.\\[4pt] [1] P. Kotetes, A. Shnirman, G. Sch\"{o}n, arXiv:1207.2691. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z8.00013: Proximity-effect-induced superconductivity in Bi$_2$Se$_3$ and Bi$_2$Te$_3$ Li Lu, Jie Shen, Yue Ding, Fanming Qu, Fan Yang, Jun Chen, Zhongqing Ji, Guangtong Liu, Jie Fan, Xiunian Jing, Changli Yang In this talk I will present our experimental investigations on the proximity effect between conventional superconductors such as Sn, Pb and the strong spin-orbit coupling materials Bi$_2$Se$_3$ or Bi$_2$Te$_3$ [1-3]. Several types of hybrid devices were fabricated, and their electron transport properties were measured down to $\sim$10 milli-Kelvin temperatures. The results show that a superconducting phase can be easily induced in Bi$_2$Se$_3$ and Bi$_2$Te$_3$ single crystals by superconducting Pb electrodes that are deposited on the surface of the former. The induced superconducting phase can be regarded as a true superconducting phase, i.e., it has an energy gap of the order 0.1 meV, and carries a Josephson supercurrent over a distance as far as several microns. The conductance spectrum of the interface between the induced superconducting phase and the normal phase of Bi$_2$Se$_3$ or Bi$_2$Te$_3$ exhibits a zero-bias peak. Based on the induced superconducting phase, single Josephson junction devices and superconducting quantum interference devices (SQUIDs) were constructied, and their critical supercurrent were investigated as a function of applied magnetic flux. We will discuss the implication of the results in terms of the pairing symmetry of the induced superconducting phase.\\[4pt] [1] F. Yang, et al., Phys. Rev. B 85, 104508 (2012).\\[0pt] [2] F. M. Qu, et al., Scientific Reports 2, 339 (2012). \\[0pt] [3] F. Yang, et al., Phys. Rev. B 86, 134504 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z8.00014: Spin-polarized tunneling current through a thin film of a topological insulator in a parallel magnetic field Victor Yakovenko, Sergey Pershoguba We calculate the tunneling conductance between the surface states on the opposite sides of an ultra-thin film of a topological insulator in a parallel magnetic field $B$. The parallel magnetic field produces a relative shift of the in-plane momenta of the two surfaces states. An overlap between the shifted Fermi circles and spinor wave functions result in unusual non-monotonic dependence of the tunneling conductance $\sigma(B)$ on the magnetic field $B$. The conductance $\sigma(B)$ grows with the magnetic field $B$, which corresponds to a negative magnetoresistance observed in an experiment [2], until it drops down abruptly to zero at the critical magnetic field $B_{\rm cr}$. Because spin orientation of the electronic surface states in topological insulators is locked to momentum, spin polarization of the tunneling current can be controlled by the magnetic field.\\[4pt] [1] Sergey S. Pershoguba and Victor M. Yakovenko, Phys. Rev. B {\bf 86}, 165404 (2012).\\[0pt] [2] H. B. Zhang et al., Adv. Mater. {\bf 24}, 132 (2012). [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z8.00015: Polarization selective micro-Raman spectroscopy of gated 3D topological insulators Jeff Secor, Milan Begliarbekov, Lukas Zhao, Haiming Deng, Lia Krusin-Elbaum One of the majors challenges to understanding the behavior of the quantum states in 3D topological insulators (TIs) is a significant carrier conduction in the bulk. Understanding phonons and electron-phonon interactions can shed light on the link between surfaces and the bulk and are critical in potential applications based on TIs. Raman scattering is a fast nondestructive technique used to analyze electron lattice interactions. In this work we study micro-Raman scattering of few quintuple layer thin $2^{nd}$ generation excellent crystalline quality 3D TIs, such as Sb$_2$Te$_3$, Be$_2$Te$_3$, and Bi$_2$Se$_3$ in the 15-300~K temperature range in order to probe the interaction of circularly polarized light between the lattice phonon modes and helical surface states of TI's. Circularly and linearly polarized light combined with an applied gate bias and the temperature dependence is used to examine the helicity dependence of Raman scatter to analyze the strength of electron-phonon coupling in these systems. [Preview Abstract] |
Session Z10: Invited Session: Elastic Instabilities and Pattern Formation in Structureless Solids
Sponsoring Units: DFD DPOLY GSNPChair: Benny Davidovitch, University of Massachusetts Amherst
Room: 309
Friday, March 22, 2013 11:15AM - 11:51AM |
Z10.00001: Coarsening of patterns from scale free instabilities in soft solids Invited Speaker: Evan Hohlfeld Soft materials such as rubbery solids have hidden, scale-free instabilities that are undetectable by linearized analysis, yet which have no energy barrier for onset. Examples include the nucleation of sharply creased surface folds resembling the sulci on the brain and the nucleation and growth of cavities. These instabilities can be understood as quasi-phase transitions: they have well defined binodal points, form via a nucleation and growth process, and have finite energies of transformation; however, there is no clear phase boundary dividing the ``nucleated phase'' from the surrounding elastomer. First anticipated by Weierstrass more than 100 years ago, our understanding of these instabilities---so called ``Weierstrass needles''---is now rapidly developing as an increasing number of physical examples are being identified. Recent experimental and theoretical work has continued to deepen the analogy between a Weierstrass needle and a more traditional phase transition. Along this line, I will present new results showing how the coarsening of a crease pattern can be understood as a form of Ostwald ripening. I will also discuss classes of systems which might support other examples of Weierstrass needles. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z10.00002: Instabilities in axisymmetrically constrained sheets Invited Speaker: Jos\'e Bico We propose to describe three different situations where a circular sheet is submitted to axisymmetric loads resulting from capillary forces or constrained boundary conditions. In a first case, a thin annulus floating on water is radially compressed by a surface pressure induced by the addition of surfactant molecules outside the annulus. As a consequence the annulus is compressed in the orthoradial direction and wrinkles are observed beyond a critical load. In a second situation, a planar disk is deposited on an adhesive sphere. Can the sheet accommodate the change in gaussian curvature? Wrinkles actually appear at the edge of the disk if the diameter exceeds a critical value. A third experiment finally involves a planar disk squeezed in a spherical mold. While low confinement induces the formation of localized folds, these folds eventually evolve into a cascade of orthoradial wrinkles. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z10.00003: The generation of stress-focusing features in confined elastic sheets Invited Speaker: Robert Schroll Crumpling is the canonical example of stress focusing in a confined elastic sheet. Subject to a large biaxial confinement, the sheet must bend in multiple directions, which induces Gaussian curvature and therefore strain. This strain is best accommodated by focusing the stress into small regions. In a crumpled sheet, multiple stress-focusing features appear apparently randomly. Here, I present two systems in which stress-focusing features are created in a controlled manner. In the first, a thin sheet is floated on a droplet of water. As the curvature of the droplet is increased, first wrinkles and then a focused features appear on the edge of the sheet. In the second, a focused feature appears at the transition between wrinkle patters of two different wavelengths. The degree of the focusing can be controlled by the confinement, the thickness, and the tension applied transverse to the confinement. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z10.00004: Compression-triggered instabilities of multi-layer systems: From thin elastic membranes to lipid bilayers on flexible substrates Invited Speaker: Howard A. Stone Instabilities are triggered when elastic materials are subjected to compression. We explore new features of two distinct systems of this type. First, we describe a two-layer polymeric system under biaxial compressive stress, which exhibits a repetitive wrinkle-to-fold transition that subsequently generates a hierarchical network of folds during reorganization of the stress field. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the final network morphology. Some analogies to the venation pattern of leaves are indicated. Second, motivated by the confined configurations common to cells, which are wrapped in lipid bilayer membranes, we study a lipid bilayer, coupled to an elastic sheet, and demonstrate that, upon straining, the confined lipid membrane is able to passively regulate its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. The two distinct systems illustrate the influence of the substrate on finite amplitude shape changes, for which we describe the time-dependent shape evolution as the stress relaxes. This talk describes joint research with Manouk Abkarian, Marino Arroyo, Pilnam Kim, Mohammad Rahimi and Margarita Staykova. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z10.00005: Electromechanical instability in soft materials: Theory, experiments and applications Invited Speaker: Zhigang Suo Subject to a voltage, a membrane of a dielectric elastomer reduces thickness and expands area, possibly straining over 100\%. The phenomenon is being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. The behavior of dielectric elastomers is closely tied to electromechanical instability. This instability may limit the performance of devices, and may also be used to achieve giant actuation strains. This talk reviews the theory of dielectric elastomers, coupling large deformation and electric potential. The theory is developed within the framework of continuum mechanics and thermodynamics. The theory attempts to answer commonly asked questions. How do mechanics and electrostatics work together to generate large deformation? How efficiently can a material convert energy from one form to another? How do molecular processes affect macroscopic behavior? The theory is used to describe electromechanical instability, and is related to recent experiments. [Preview Abstract] |
Session Z11: Invited Session: Nonlinear Mechanics of Glassy Polymers
Sponsoring Units: DPOLYChair: Robert Hoy, University of South Florida
Room: 310
Friday, March 22, 2013 11:15AM - 11:51AM |
Z11.00001: Rate- and Temperature-Dependent Softening in Polymer Glasses Invited Speaker: Leon Govaert It is well established that physical aging in polymer glasses leads to an increase in density, elastic modulus, yield stress and also strain softening. The latter, sometimes referred to as ``mechanical rejuvenation,'' is the phenomenon where the post-yield stress level initially decreases with further deformation until strain hardening sets in. In all constitutive models for glasses proposed until now, the rate and temperature-dependence of the yield stress is regarded to remain unchanged during strain softening. In the present study, it is demonstrated that a large number of polymer glasses (PMMA, PLLA, PS, PVC) display a pronounced change in kinetics (strain-rate dependence) after yield. The phenomenon finds its origin in the fact that, in specific ranges of temperature and strain rate, two different molecular mechanisms may contribute to the yield stress. Due to strain softening the post-yield response is only controlled by one of the two, resulting in a strain-rate and temperature dependence of the yield drop. The universality of the phenomenon is discussed in connection to the alleged influence of secondary transitions on the impact response of polymer glasses. A modification to the traditional models is proposed that enables an accurate description of the mechanical response of solid polymers in the transition range. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z11.00002: How deformation enhances mobility in a polymer glass Invited Speaker: Daniel Lacks Recent experiments show that deformation of a polymer glass can lead to orders-of-magnitude enhancement in the atomic level dynamics. To determine why this change in dynamics occurs, we carry out molecular dynamics simulations and energy landscape analyses. The simulations address the coarse-grained polystyrene model of Kremer and co-workers, and the dynamics, as quantified by the van Hove function, are examined as the glass undergoes shear deformation. In agreement with experiment, the simulations find that deformation enhances the atomic mobility. The enhanced mobility is shown to arise from two mechanisms: First, active deformation continually reduces barriers for hopping events, and the importance of this mechanism is modulated by the rate of thermally activated transitions between adjacent energy minima. Second, deformation moves the system to higher-energy regions of the energy landscape, characterized by lower barriers. Both mechanisms enhance the dynamics during deformation, and the second mechanism is also relevant after deformation has ceased. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z11.00003: Can intrachain contributions dominate the stress response of polymer glasses under large deformation? Invited Speaker: Shi-Qing Wang Polymer glasses are a structural hybrid in their mechanical responses to large deformation. The primary structure due to the short-range inter-segmental van der Waals bonds yields at small strains. In presence of chain connectivity, brittle failure may be avoided if the chain networking is adequately dense. We show in this presentation how the interplay between the primary structure and chain network dictates deformation, yielding, strain softening, strain localization and ``strain hardening'' during continuous uniaxial extension at room temperature of a variety of polymer glasses from the brittle (e.g., PS, PMMA) to the ductile (e.g., PC). In particular, our results identify straining of the chain network as the dominant contribution to the mechanical stress in the post-yield regimes. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z11.00004: A Simple Model for Yielding and Strain Hardening in Glassy Polymers Invited Speaker: Ron Larson Strain hardening has long been an observed feature of polymer glasses in extension; explanations to date have often been phenomenological. Ediger and coworkers (Lee et al. \textit{Science} 323, 231, 2009) have shown in experiments on PMMA glasses that, in addition to strain hardening, polymeric glasses show a remarkable non-monotonicity in the segmental relaxation time both in loading and unloading of stress. Here, we develop a simple constitutive equation that combines recent theories for yielding in simple glasses (Brader et al. PNAS, 106, 15186, 2009) to represent local segmental modes in the polymer, with a dumbbell model for the slow polymer relaxation modes. For a polymer glass under uniaxial loading, the model predicts that the liquefaction of the segmental modes permits strain hardening of the polymer modes to emerge, and once this emerges, it slows the deformation of the material under constant load enough to partially re-vitrify the segmental modes even though the sample remains under stress. In this way, the observed non-monotonicity in the segmental relaxation modes is produced. We show the extension of the work to simple shearing flows, and make (as yet) untested predictions about segmental relaxation rates in shear flows. We also show how to extend the model to include Rouse chain dynamics in place of the over-simplified dumbbell. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z11.00005: Impact-Induced Glass Transition in Elastomeric Coatings Invited Speaker: C.M. Roland When an elastomer layer is applied to the front surface of steel, the resistance to penetration by hard projectiles increases significantly. It is not obvious why a soft polymer should affect this property of metals, and most rubbers do not. However, we have found that a few are very effective; the requirement is that the polymer undergo a viscoelastic phase transition upon impact. This means that the frequency of its segmental dynamics correspond to the impact frequency. The latter is estimated as the ratio of the projectile velocity to the coating thickness, and is on the order of 10$^{5}$ s$^{-1}$ for the experiments herein. Our data and a non-linear dynamics finite-element analysis offer support for this resonance condition as a primary mechanism underlying the penetration-resistance of elastomer-coated metal substrates. The impact-induced phase transition causes large energy absorption, decreasing the kinetic energy of the impacting projectile. However, this energy absorption only accounts for about half the enhanced stopping power of the elastomer/steel bilayer. An additional mechanism is lateral spreading of the impact force, resulting from the transient hardening of the elastomeric during its transition to the glassy state -- the modulus of the rubber increases 1000-fold over a time period of microseconds. The penetration-resistance is a very nonlinear function of the coating thickness. Moreover, tests on various metals show that hardness is the principal substrate parameter controlling the contribution of the coating. [Preview Abstract] |
Session Z12: Focus Session: Thermoelectric Magnetothermoelectric Magnetocaloric
Sponsoring Units: DMP GERA FIAPChair: David Singh, Oak Ridge National Laboratory
Room: 314
Friday, March 22, 2013 11:15AM - 11:27AM |
Z12.00001: Enhanced thermoelectric properties via oxygen non-stoichiometry in La$_2$NiO$_4$ and SrTiO$_3$ Victor Pardo, Antia S. Botana, Paul M. Bach, Victor Leboran, Francisco Rivadulla, Daniel Baldomir We present the results of transport properties calculations and experiments on various oxides. A large enhancement of the thermoelectric properties is predicted\footnote{PRB 86, 165114 (2012).} via ab initio calculations for La$_2$NiO$_{4+\delta}$, with electronic-only thermoelectric figure of merit ($zT$) values exceeding unity for oxygen excess $\delta$ $\leq$ 0.10. The effects of lattice strain (caused, e.g. by growth of thin films on different substrates) enhance even further the thermoelectric response. A similar result is obtained at very low electron-doping in bulk SrTiO$_3$ via oxygen removal. This is analyzed experimentally via thermal annealing that depletes oxygen ($\sim$ 1 oxygen vacancy per 10$^6$ unit cells). In both these systems, the increase in conductivity reached in the metallic limit retains a large thermopower, with the corresponding enhancement of $zT$. In the case of SrTiO$_3$, experiments indicate\footnote{arxiv:1211.1615.} that such a small oxygen vacancy level reduces drastically the thermal conductivity by introducing random scattering centers. In the talk, we will discuss the electronic structure origin of the enhancement of the thermoelectric response and how this can be tuned. Results are general and applicable to other non-stoichiometric oxides. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z12.00002: Thermometry and power sensing with SNS proximity structures Russell Lake, Joonas Govenius, Ville Pietil\"{a}, Kuan Yen Tan, Mikko M\"{o}tt\"{o}nen We present our experimental progress on thermometry employing the superconductor proximity effect in a normal-metal (N) mesoscopic wire between two superconducting (S) electrodes. We have fabricated Al/Au/Al SNS structures with junction lengths in the diffusive transport regime and performed electrical measurements between 300 K and 8 mK. Temperature dependence of the differential resistance shows sensitivity at the millikelvin level at a bath temperature of 8 mK. Specifically, the shape of the proximity effect induced dip in the differential resistance at zero current-bias serves as a direct probe of the N wire temperature. We show that the energy scale of proximity superconductivity in the N wire can be controlled by changing the wire length or by applying a perpendicular magnetic field to tune the temperature detection range. Results are discussed in terms of the temperature and resolving power noise for a thermometer and a power meter, respectively. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z12.00003: Chemical Pressure Effect and Dimer Formation in (Ba,Sr)Ni$_2$As$_2$ Solid Solutions Tyler Drye, Shanta Saha, Johnpierre Paglione Although both BaNi$_2$As$_2$ and SrNi$_2$As$_2$ form in ThCr$_2$Si$_2$ structure, these materials display very different behaviors, owing in part to an important structural difference: while the Sr compound exhibits As-As bonds between layers, the Ba compound lacks these interlayer bonds. Thus, substitution of Sr into BaNi$_2$As$_2$ produces a positive chemical pressure effect on the system that pulls the NiAs layers closer together and towards As-As dimer formation. We will present the resulting phase diagram as determined by x-ray, chemical composition, electrical resistivity and magnetization measurements. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z12.00004: Quantifying the Local Seebeck Coefficient using Scanning Thermoelectric Microscopy (SThEM) Jenna Walrath, Yen-Hsiang Lin, Kevin Pipe, Rachel Goldman Thermoelectric (TE) devices allow reliable solid-state conversion of heat to electricity. The efficiency of a TE device is determined by the figure of merit, ZT, which is sensitive to the Seebeck coefficient, S. A promising alternative to traditional macroscale measurements of S is scanning thermoelectric microscopy (SThEM), which can profile S with nm resolution [1]. In SThEM, an unheated scanning tunneling microscopy tip acts as a high-resolution voltmeter probe to measure the thermally-induced voltage, V, in a heated sample. However, the temperature (T) gradient is not localized to the sample, and the measured V is a convolution of voltages within the region of non-zero temperature gradient. Therefore we have developed a 1D Fourier heat conduction model to predict the T gradient in the tip and to deconvolute the measured V within the sample. This approach enables direct conversion between the measured V and the local S. [1] H.K. Lyeo et al., Science \textbf{303}, 816 (2004). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z12.00005: Profiling the Local Seebeck Coefficient with Nanometer Resolution Using Scanning Thermoelectric Microscopy (SThEM) Yen-Hsiang Lin, Jenna Walrath, Rachel Goldman Thermoelectric (TE) devices offer a method of recovering waste heat through solid state conversion of heat to electricity. Nanostructured thermoelectric materials may provide the key to increased efficiencies, which are sensitive to the Seebeck coefficients (S) However, traditional bulk measurement techniques can only provide a spatially averaged measurement of S over the whole sample, which can hardly investigate the effects of nanostructures on S on the nanoscale. A novel technique known as scanning thermoelectric microscopy (SThEM) has recently been developed to measure induced thermal voltages with nanometer resolution In SThEM, an unheated scanning tunneling microscopy tip acts as a high-resolution voltmeter probe to measure the thermally-induced voltage, V, in a heated sample. Here we present a local S measurement using SThEM across an InGaAs P-N junction. The thermovoltage shows an abrupt change of sign within 10 nanometers, which reveals nanometer spatial resolution. We will discuss local S measurements of AlAs/GaAs superlattices (SLs) with various SL periods and compare the local S with scanning tunneling spectroscopy measurements, which will reveal how local electronic states influence thermoelectric properties. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z12.00006: Scaling of electrical and thermal conductivities in an almost integrable chain Joel Moore, Christoph Karrasch, Roni Ilan Many low-dimensional materials are well described by integrable one-dimensional models such as the Hubbard model of electrons or the Heisenberg model of spins. However, the small perturbations to these models required to describe real materials are expected to have singular effects on transport quantities: integrable models often support dissipationless transport, while weak non-integrable terms lead to finite conductivities. We use translation-invariant matrix-product-state methods to obtain quantitative values of electrical and thermal conductivities in an almost integrable chain (an XXZ spin chain with staggered fields, or equivalently a spinless fermion chain with staggered on-site potentials). [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z12.00007: Flexible thermoelectric films using the spin Seebeck effect Akihiro Kirihara, Masahiko Ishida, Hiroko Someya, Koichi Kondo, Naoharu Yamamoto, Ken-ichi Uchida, Eiji Saitoh, Shigeru Kohmoto, Tomoo Murakami Thermoelectric (TE) technologies have been of great interest, since they can directly generate electricity from thermal energy that is available in various places. For making full use of such omnipresent heat, TE devices using the spin Seebeck effect (SSE) potentially open opportunities for large-area TE applications, because of their favorable features such as a simple film structure and convenient scaling capability [1]. In this work, we show a SSE-based flexible TE device, which consists of metallic and magnetic-insulator films on a 25-um-thick polyimide substrate. Novel fabrication processes enabled us to form the magnetic insulator, having a good spin-current-conduction property for the SSE, on the highly flexible organic film. Such flexible TE sheets are readily implementable on various curved or uneven surfaces, leading to versatile energy-harvesting and heat-sensing applications. [1] A. Kirihara, et al., Nature Mat. 11, 686 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z12.00008: Nonlocal thermoelectric effects and nonlocal Onsager relations in a three-terminal superconducter/ferromagnet proximity system Matthias Eschrig, Peter Machon, Wolfgang Belzig Heterostructures of ferromagnets and superconductors are presently subject of intense study since they show interesting phenomena based on the singlet-triplet conversion of pairing amplitudes at the interfaces, and the resulting spin-dependent proximity effect. Spectacular examples are long-range triplet Josephson currents due to inhomogeneous magnetic order, or due to the spin-dependence of the interface reflection and transmission amplitudes, which were confirmed in a set of pivotal experiments in 2010. Here, we study thermal and charge transport in a three-terminal setup consisting of a superconducting and two ferromagnetic contacts. We predict that the simultaneous presence of spin-filtering and of spin-dependent scattering phase shifts at each of the two interfaces will lead to giant non-local thermoelectric effects both in clean and in disordered systems. The symmetries of thermal and electric transport coefficients are related to fundamental thermodynamic principles by the Onsager reciprocity. Our results show that a non-local version of the Onsager relations for thermoelectric currents holds in a three terminal quantum coherent ferromagnet-superconductor heterostructure including spin-dependent crossed Andreev reflection and coherent electron transfer processes. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z12.00009: Magneto Themoelectric Generator with Carbon Nanotube Thermal Interfaces Patrick T. McCarthy, Timothy S. Fisher, Ernesto E. Marinero We report the thermal behavior of Gd foils used in a magneto thermoelectric generator cells. The device exploits the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ``shuttle'' whose mechanical energy is converted to electrical form and which enhances heat transfer through both conduction and convection. Efficient heat transfer at mechanical interfaces is critical to increase shuttle speed and the commensurate rate of heat transfer. The synthesis and characterization of carbon nanotube thermal interfaces for the Gd foils are described. The samples generated in this study were consistently measured with total thermal interface resistances in the range of 65--105 mm$^{2}$ K/W, a reduction of 55--70{\%} compared to bare Gd (R$_{\mathrm{int}}$ $\sim$ 230 mm$^{2}$ K/W). The addition of carbon nanotube arrays did not alter the magnetic properties of the gadolinium foils and only a slight decrease in the magnetic moment of the gadolinium samples (8--13{\%}) was measured after growth. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z12.00010: Thermomagnetic effects in elemental rare-earth single crystals Audrey M. Chamoire, Joseph P. Heremans Thermomagnetic properties and magnetothermal conductivity of elemental rare-earth (R-E) metals are for the first time systematically presented from 80 to 400 K. Measurements are given with heat flux applied along the [100] and the [111] directions since R-E present mainly a hexagonal symmetry at room temperature. This work is motivated by the complex Fermi surfaces of the R-E's and by their magnon contributions to the thermal conductivity. Elemental rare-earths are multicarrier systems involving electron ($e)$ and hole ($h)$ pockets and have a very small thermopowers ($S)$, which can result in large Nernst coefficients. This would be suitable for transverse Nernst cooler since they could be used as a single material with a particular design, then resolving the problems of contact resistances of actual Peltier coolers where materials need to be cascaded. Magnetic field dependent thermal conductivity is used to extract magnon heat conduction. Magnons are bosons, but unlike acoustic phonons they can have energy gaps. Taken together, these two properties should theoretically lead to a non-linear thermal conductivity in the presence of a magnetic field gradient. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z12.00011: Magnetic refrigeration capabilities of magnetocaloric Ni2Mn:75Cu:25Ga S.K. Mishra, C.A. Jenkins, I. Dubenko, T. Samanta, N. Ali, S. Roy Doping-driven competition between energetically similar ground states leads to many exciting materials phenomena such as the emergence of high-\emph{T$_{c}$} superconductivity, diluted magnetic semiconductors, and colossal magnetoresistance. Doped Ni$_{2}$MnGa Heusler alloy, which is a multifunctional ferromagnetic alloy with various exotic physical properties demonstrates this notion of rich phenomenology \emph{via} modified ground spin states. Adopting this generic concept, here we will present a novel doped Ni$_{2}$Mn$_{.75}$Cu$_{.25}$Ga alloy that offers unprecedented co-existence of the magnetocaloric effect and fully controlled ferromagnetism at room temperature. Application of site engineering enables us to manipulate the ground spin state that leads to the decrease in magnetic transition temperature and also increases the delocalization of the Mn magnetism. SQUID magnetometery suggests that Cu doping enhances the saturation magnetization, coercive field and clarity of magnetic hysteresis loops. By exploiting x-ray absorption techniques and measuring element specific magnetic hysteresis loops, here we will describe the microscopic origin of enhnaced magnetocaloric properties and \emph{d-d} interaction driven charge transfer effects in Ni$_{2}$Mn$_{.75}$Cu$_{.25}$Ga [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z12.00012: First successful growth of magnetic thin films of meta-stable monoclinic Gd$_{5}$(Si$_{x}$Ge$_{1-x)4}$ David C. Jiles, Ravi L. Hadimani, Ikenna C. Nlebedim, Yevgen Melikhov We report on the first successful growth of magnetic thin films of the giant magnetocaloric material Gd$_{5}$(Si$_{x}$Ge$_{1-x})_{4}$. This material has been widely studied for its unusual properties including the coupled magnetic-structural phase transition. We report on the successful growth of films of Gd$_{5}$Si$_{2.09}$Ge$_{1.91}$ that can be used in micro-cooling applications. The film was grown by Pulsed Laser Deposition (PLD) on a (001) silicon wafer deposited at 200$^{\circ}$C from a polycrystalline target. PLD was achieved using a femtosecond laser with a repetition rate of 1kHz, pulse energy of up to 3.5mJ. The deposited film thickness was $\sim$ 400nm measured using Scanning Electron Microscopy and the composition of the film was analyzed using Energy Dispersive Spectroscopy and found to be close to the target composition. Magnetic measurements were carried out in a SQUID magnetometer. Magnetic moment vs. magnetic field measurement confirmed that the film was ferromagnetic at 200K. The transition temperature of the film was measured from magnetic moment vs. temperature measurements using inflection point. Transition temperature was measured at 280K which was close to the 1$^{st}$ order phase transition temperature of bulk material. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z12.00013: Strain induced ferromagnetism and magnetocaloric effect in LaFe2Si2 thin film Guixin Cao, German Samolyuk, Siwei Tang, Liang Qiao, Wenbin Wang, Jieyu Yi, Thomas Zac Ward, Michael Biegalski, Wolter Siemons, David Mandrus, Malcolm Stocks, Zheng Gai Great interest in magnetic refrigeration techniques based on the magnetocaloric effect (MCE) has grown recently due to its high efficiency and environmental friendliness. Although the thin film form of the materials is very important in both application and fundamental research, as the properties of films can be tailored by parameters like epitaxial strain, studies on MCE in single crystal films are limited by the difficulty of the growth. In this work, LaFe2Si2 thin films are successfully tuned from Pauli paramagnetic to ferromagnetic, and MCEs are observed around 50K. The ferromagnetic transition is a first order transition, and the magnetic entropy $\Delta $S $\approx $-8.5 J/Kg K is obtained under a magnetic field of 7T. The magnetocaloric effect is characterized by a 14 K hysteresis in the field cooling and field warming process. Our temperature dependent X-ray measurements exclude the correlation between the striking MCE of the thin film and structural transition. Density functional theory (DFT) calculations indicate that the strain induced distance variations of Si-Fe bonds control the magnitude of the magnetic moment and MCE. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z12.00014: Entropy changes and the caloric effects in R$_{5}$Si$_{2}$Ge$_{2}$ (R$=$Gd and Tb) Nilson de Oliveira It has been experimentally shown that at ambient pressure, the compound Gd$_{5}$Si$_{2}$Ge$_{2}$ undergoes a first order transition with giant magnetocaloric effect around this room temperature. Experimental data also show that an applied pressure increases the critical temperature of this compound and keeps the first order phase transition. On the other hand, experimental data show that the compound Tb$_{5}$Si$_{2}$Ge$_{2}$ undergoes a second order phase transition with a normal magnetocaloric effect around 100 K. It has also been shown that an applied pressure increases its critical temperature without changing the order of the phase transition. In this work, we calculate the magnetocaloric and barocaloric effects in and Gd$_{5}$Si$_{2}$Ge$_{2}$ and Tb$_{5}$Si$_{2}$Ge$_{2}$. For this purpose, we use a model of localized magnetic moments including the magnetoelastic interaction. In the model, the order of the phase transition is controlled by the ratio between the exchange interaction and the magnetoelastic coupling parameter. Our calculations show that these compounds exhibit large values of the entropy changes upon pressure variation in good agreement with the available experimental data. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z12.00015: Energy-harvesting at the Nanoscale Andrew Jordan, Bj\"orn Sothmann, Rafael S\'anchez, Markus B\"uttiker Energy harvesting is the process by which energy is taken from the environment and transformed to provide power for electronics. Specifically, the conversion of thermal energy into electrical power, or thermoelectrics, can play a crucial role in future developments of alternative sources of energy. Unfortunately, present thermoelectrics have low efficiency. Therefore, an important task in condensed matter physics is to find new ways to harvest ambient thermal energy, particularly at the smallest length scales where electronics operate. To achieve this goal, there is on one hand the miniaturizing of electrical devices, and on the other, the maximization of either efficiency or power the devices produce. We will present the theory of nano heat engines able to efficiently convert heat into electrical power. We propose a resonant tunneling quantum dot engine that can be operated either in the Carnot efficient mode, or maximal power mode. The ability to scale the power by putting many such engines in a ``Swiss cheese sandwich'' geometry gives a paradigmatic system for harvesting thermal energy at the nanoscale. [Preview Abstract] |
Session Z14: Permanent Magnet Materials
Sponsoring Units: GMAGChair: George Hadjipanayis, University of Delaware
Room: 316
Friday, March 22, 2013 11:15AM - 11:27AM |
Z14.00001: Magnetic Hardening of Ce$_{2}$Fe$_{\mathrm{14-x}}$Co$_{\mathrm{x}}$B J.F. Herbst, E.J. Skoug, M.S. Meyer, F.E. Pinkerton Permanent magnets based on R$_{2}$Fe$_{14}$B (R $=$ rare earth element) are essential to a wide variety of applications, among them automotive traction motors. Current state-of-the-art materials rely on R $=$ Nd and Dy, both of which are currently subject to supply and cost instability. A possible alternative is R $=$ Ce, the most abundant rare earth, but Ce$_{2}$Fe$_{14}$B has several disadvantages, including a low Curie temperature (T$_{\mathrm{c}})$ that restricts the maximum operating point to well below that required for some applications. Given that substitution of Co for Fe is known to enhance T$_{\mathrm{c}}$ significantly in other R$_{2}$Fe$_{14}$B compounds, we systematically investigate magnetic hardening of Ce$_{2}$Fe$_{\mathrm{14-x}}$Co$_{\mathrm{x}}$B by melt spinning alloys having compositions guided by our previous work on the Ce-Fe-B system. We find the range of Co solubility in Ce$_{2}$Fe$_{14}$B to be markedly lower than for other R$_{2}$Fe$_{14}$B materials, a consequence of the fact that Ce$_{2}$Co$_{14}$B apparently does not form. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z14.00002: Mechanochemical synthesis of (Sm,Pr)$_2$(Co,Fe)$_{17}$ powders for nanocomposite permanent magnets George Hadjipanayis, Alexander Gabay, Wanfeng Li Bottom-up fabrication of nanocomposite permanent magnets with enhanced maximum energy product requires large quantities of high-coercivity powder with crystallographically anisotropic particles tens of nanometers in size. In this work, we report a systematic study aimed to employ combination of intensive mechanical milling and calciothermic reduction for preparation of polydispersed (Sm,Pr)$_2$(Co,Fe)$_{17}$ powders with a predominant-to-significant part of the particles smaller than 100 nm. In addition to the effects of Pr and Fe on the hard magnetic properties of the particles, the study analyzes the influence of excess reducing agent Ca and that of the heat treatment on the particle size distribution, their chemical/structural homogeneity and crystallographic anisotropy. Emphasized is the likely role of the excess Ca facilitating the diffusion-enabled particle growth. Remanent magnetization up to 106 emu/g and intrinsic coercivity up to 14 kOe were obtained. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z14.00003: High Coercivity Anisotropic Nd2Fe14B Nanoparticles Produced by Planetary Ball Milling Ozlem Koylu-Alkan, George C. Hadjipanayis, Dimitris Niarchos The bottom-up fabrication of anisotropic exchange-coupled nanocomposites brings out the necessity of fabrication of magnetically hard nanoparticles with high coercivity. In this study, we have fabricated Nd$_{2}$Fe$_{14}$B nanoparticles from die-upset Nd-Fe-B (MQ3) precursor materials using planetary milling. The MQ3 alloy consists of platelets which are approximately 80 nm in thickness and 500 nm in diameter. Using planetary ball milling we were able to produce Nd$_{2}$Fe$_{14}$B nanoparticles with a size down to 20 nm. However, the size distribution of the milled particles is very broad ranging between 20 nm and 20 $\mu m$. A sedimentation experiment was used to separate the different size particles. By allowing bigger particles to sediment in a viscous liquid, we were able to separate different size nanoparticles with a size smaller than 200 nm. The coercivity of particles is found to decrease with particle size. After 60 min sedimentation the collected particles had an average size 100 nm with a coercivity value of 5.4 kOe. The objective of this study is to obtain nanoparticles with a size below 100 nm and a coercivity greater than 10 kOe for the fabrication of anisotropic exchange-coupled nanocomposites. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z14.00004: Synthesis of CeFe$_{10.5}$Mo$_{1.5}$ with ThMn$_{12}$-Type Structure by Melt Spinning Chen Zhou, Misle Tessema, Martin Meyer, Frederick Pinkerton Rare earth compounds RFe$_{12-x}$M$_{x}$ with tetragonal ThMn$_{12}$-type structure are of great interest for potential applications as permanent magnets. These materials serve as precursors for nitriding and hydriding, processes which can dramatically increase the Curie temperature, spontaneous magnetization, and affect the magnetic anisotropy. We report the phase study of CeFe$_{10.5}$Mo$_{1.5}$ samples melt spun at various surface wheel speeds v$_{s}$ between 5 and 60 m/s. The results from quantitative Rietveld analysis indicate that the as-spun ribbons are a mixture of primary CeFe$_{10.5}$Mo$_{1.5\, }$phase with impurity phases such as Ce$_{2}$Fe$_{17}$, Fe-Mo compound and CeFe$_{2}$. At wheel speeds v$_{s}$ below 25 m/s, CeFe$_{10.5}$Mo$_{1.5\, }$phase accounts for greater than 85 wt{\%}, while the Fe-Mo compound is the only detectable impurity phase. Above v$_{s\, }=$ 25 m/s, as the wheel speed increases, CeFe$_{10.5}$Mo$_{1.5\, }$phase decreases monotonically to about 60 wt{\%} at v$_{s\, }=$ 60 m/s while the amounts of impurity phases increase. Thermogravimetric measurement indicates that the Curie temperature T$_{c}$ of the CeFe$_{10.5}$Mo$_{1.5\, }$phase is 340 K. As a result, the best performing sample melt spun at v$_{s}=$15m/s only exhibits an energy product BH$_{max}=$0.121 MGOe at room temperature. Although such a number is modest for a permanent magnet, nitriding is expected to greatly enhance the Curie temperature, and hence the magnetic performance. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z14.00005: Giant magnetic anisotropy in Li$_{3-x}$Fe$_x$N permanent magnets Anton Jesche, Srinivasa Thimmaiah, Sergey Bud'ko, Paul Canfield Single crystals of Li$_2$(Li$_{1-x}$Fe$_x$)N were successfully grown out of Li-flux. Fe-concentrations and lattice parameters were determined by means of single crystal and powder diffraction which also confirmed the substitution of Fe on only one of the Li sites resulting in Li$_{1-x}$Fe$_x$ layers separated by Li$_2$N layers. Magnetization measurements revealed a ferromagnetically ordered ground state with Curie temperatures of $\sim 60$\,K for Fe concentrations of $x \approx 20$\%. Large saturation moments of up to 5\,$\mu_B$ per Fe atom were found along the hexagonal crystallographic $c$-axis. These values exceed the spin-only contribution of Fe and are also reflected in correspondingly large effective moments at room temperature. The anisotropy field at $T = 2$\,K, defined as intersection of the magnetization for $M \parallel c$ and $M \perp c$, can be estimated to lie well beyond 100 Tesla. Electrical resistivity measurements show insulating behavior and raise questions about the nature of the underlying magnetic exchange mechanism. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z14.00006: Combinatorial search of rare-earth free permanent magnets Tieren Gao, Ichiro Takeuchi, Sean Fackler, Lei Fang, Ying Zhang, Matthew Krammer, Iver Anderson, Bill McCallum Permanent magnets play important roles in modern technologies such as in generators, motors, speakers, and relays. Today's high performance permanent magnets contain at least one rare earth element such as Nd, Sm, Pr and Dy. However, rare earth elements are increasingly rare and expensive, and alternative permanent magnet materials which do not contain them are needed by the industry. We are using the thin film composition spread technique to explore novel compositions of permanent magnets without rare-earth. Ternary co-sputtering is used to generate composition spreads. We have thus far looked at Mo doped Fe-Co as one of the initial systems to search for possible compounds with enhanced coercive fields. The films were deposited on Si (100) substrates and annealed at different temperatures. The structural properties of films are mapped by synchrotron diffraction. We find that there is a structural transition from a crystalline to an amorphous state at about 20{\%} atomic Mo. With increasing annealing temperature, the Mo onset concentration of the structural transition increases from 25{\%} for 600$^{\circ}$C to 35{\%} for 700$^{\circ}$C. We find that some of compounds display enhanced coercive field. With increasing Mo concentration, the magnetization of Fe-Co-Mo begins to switch from in-plane to out-of-plane direction. This work is funded by the BREM (Beyond Rare-earth Magnet) project (DOE EERE). [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z14.00007: Electronic structure and equation of state of Sm$_2$Co$_{17}$ from first-principles DFT+$U$ Patrick Huang, Nicholas P. Butch, Jason R. Jeffries, Scott K. McCall Rare-earth intermetallics have important applications as permanent magnet materials, and the rational optimization of their properties would benefit greatly from guidance from ab initio modeling. However, these systems are particularly challenging for current electronic structure methods. Here, we present an ab initio study of the prototype material Sm$_2$Co$_{17}$ and related compounds, using density functional theory with a Hubbard correction for the Sm 4$f$-electrons (DFT+$U$ method) and ultrasoft pseudopotentials. The Hubbard $U$ parameter is derived from first principles [Cococcioni and de Gironcoli, PRB 71, 035105 (2005)], not fit to experiment. Our calculations are in good agreement with recent photoemission measurements at ambient pressure and the equation of state up to 40 GPa, thus supporting the validity of our DFT+$U$ model. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z14.00008: Effects of pressure on the sturctural and magnetic properties on Sm based permanent magnets Scott McCall, Nicholas Butch, Jason Jeffries, Patrick Huang The magnetic properties of the rare earth-transition metal permanent magnets are sensitive to interatomic spacing and can be tuned by adjusting these parameters. We report the effects of high pressure on the crystal structure and magnetic properties of Sm$_{2}$Co$_{17}$ and Sm$_{2}$Fe$_{17}$ measured in diamond anvil cells. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z14.00009: Site-preference and valency for rare-earth sites in (R-Ce)2Fe14B [R$=$La,Nd] magnets Aftab Alam, Mahmud Khan, R.W. McCallum, D.D. Johnson Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two symmetry distinct R-sites (Wyckoff 4f and 4g) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R{1-x}Ce$_{x}$)Fe14B [R=La,Nd] using density functional theory (DFT) methods. The Fe moments compare well with neutron scattering data -- remain weakly affected by Hubbard U, but improved with spin-orbit coupling. In (La,Ce)2Fe14B, Ce alloys for 0 $<$ x $<$ 1 with a preference for smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas in (Nd,Ce)2Fe14B, Ce is predicted to have limited alloying (x $<$ 0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and segregation. Curie temperatures versus $x$ were predicted for a typical sample processing and verified experimentally. We shall also present some initial results on the critical mixed valency of Ce in related compounds. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z14.00010: Nucleation-Mode Localization in Hard-Soft Nanocomposites Ralph Skomski, Balamurugan Balasubramanian, Bhaskar Das, D. J. Sellmyer Aligned hard-soft nanocomposites continue to be an active research area in permanent magnetism, challenged by demanding processing requirements but also encouraged by experimental proofs of principle. The approach was initially outlined by Kneller and Hawig (1991), who advocated hard-soft multilayers. Skomski and Coey (1993) considered three-dimensional nanostructures, such as soft spheres in a hard matrix, and predicted an upper energy-product limit of about 1000 kJ/m$^{\mathrm{3}}$. It is well-established that the dimensions of the soft regions cannot be larger than twice the domain-wall width of the hard phase, but otherwise it was believed that geometry has a rather secondary effect. However, our recent research reveals substantial differences. Soft-in-hard geometries are better than hard-in-soft geometries and embedded soft spheres are better than multilayers. This is in close analogy to the dimensionality-dependent quantum-mechanical delocalization of electrons in an inhomogeneous potential and to the behavior of impurity states in the band gaps of solids. Transparent analytical nucleation-field solutions are found for some geometries and in the limit of very small soft inclusion as a function of the hard-phase coercivity and hysteresis-loop shape. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z14.00011: Frequency dependence of Verdet constant of Bismuth-Doped Rare-Earth Iron Garnets for Magneto-Optic Sensor Applications Mannix Shinn, Rongjia Tao, Dong Ho Wu, Anthony Garzarella There is growing interest in applying magneto-optic materials toward sensor applications. One of these applications is to exploit the Faraday Effect to measure magnetic fields. Bismuth-doped rare-earth iron garnets have proven to be highly sensitive Faraday rotators, but their frequency response and dynamic range to magnetic fields require further study. The Faraday Effect was studied in two samples of bismuth-doped rare-earth iron garnets grown in different conditions, and experiments were performed in a static field as well as in a RF field. Static magnetic fields up to 3 kG were used, and we found that the Faraday rotation became saturated at high fields, indicating that the field dependence follows the hyperbolic tangent function. We extracted each sample's Verdet constant from the Faraday rotation at low magnetic fields of \textless\ 0.1 kG. These experiments were repeated using different laser probe beam wavelengths, ranging from 405 nm to 2000 nm. We measured the transmission coefficient and the Verdet constant for each sample for different probe beam wavelengths and for an external magnetic field at various frequencies. We will discuss the implication of our experimental results. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z14.00012: Magnetism in Mo-doped Yttrium Iron Garnet S. Khanra, Y. Kolekar, M. Langhoff, P. Kahol, K. Ghosh Yttrium iron garnet (YIG) is a synthetic garnet and ferrimagnetic, with chemical formula Y$_{3}$Fe$_{5}$O$_{12}$. In YIG, five iron (III) ions occupy two octahedral and three tetrahedral sites, with the yttrium (III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behavior. Bulk YIG has been synthesized systematically by solid state reaction method. The formation of pure YIG have been investigated through X-ray diffraction (XRD) beginning from weighing in molar proportions of Y$_{2}$O$_{3}$ and Fe$_{2}$O$_{3}$, mixing and grinding, pre-sintering and final sintering at 1300 $^{\circ}$C. XRD study shows that YIG exhibits cubic structure with lattice constant of about 12 {\AA}. Magnetization with varying field and temperature has been measured using a SQUID magnetometer. Magnetic measurement of Mo YIG has shown that magnetic moment increase initially and then decreases with Mo doping. Detailed results will be discussed in this presentation. This work is supported by National Science Foundation (Award Number DMR-0907037). [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z14.00013: Local imaging of the phase transition in single crystal Nd$_2$Fe$_{14}$B Magdalena Huefner, Adam Pivonka, Cun Ye, Martin Blood-Forsythe, Ruslan Prozorov, Paul Canfield, Jennifer Hoffman The magnetic microstructure of hard magnets is of interest for immediate industrial applications and for fundamental understanding of the relationship between microscopic and macroscopic magnetic properties in materials. Of particular interest is Nd$_2$Fe$_{14}$B, which shows strong anisotropy with an easy axis along the c-axis at room temperature, but undergoes a phase transition around T$\sim$135K to an easy cone magnetization where the magnetic moments are canted away from the c-axis. Here we present magneto-optical Kerr effect (MOKE) and magnetic force microscope (MFM) measurements to investigate the spin-reorientation phase transition in single crystal Nd$_2$Fe$_{14}$B. The MFM measurements resolve a continuous change in the domain structure from rounded flower-shaped domains of a lateral extent $\sim$200 nm-$\sim$7$\mu$m to larger rectangular features of typical width $\sim$1$\mu$m and length $\sim$10$\mu$m-$\sim$30$\mu$m. By imaging the same surface area in small temperature steps across the phase transition we track the evolution of single features. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z14.00014: Magnetization and scanning tunneling spectroscopy studies of Mn$_{1.5}$Ga films on GaAs(001) Jaesuk Kwon, Xin Zhang, John Hatch, Archana Kumari, Hui Xing, Payam Taherirostami, Hao Zeng, Jeffrey Cogswell, Joseph A. Gardella, Hong Luo Hard magnetic materials have applications in permanent magnets and data storage media. Free of rare earth elements, L1$_{0}$ structured Mn$_{1.5}$Ga with high magnetic anisotropy is a potential candidate for such applications. Epitaxial films of Mn$_{1.5}$Ga with different thicknesses (35 nm -- 200 nm) were grown on GaAs(001) by molecular beam epitaxy. Films with thicknesses of 35 nm and 50 nm present uniform surface morphology which consists of overlapping rectangular features with widths and lengths on the order of a few tens to a few hundred nanometers. Measurements of X-ray diffraction reveal the presence of an interfacial layer of Mn$_{2}$As between the substrate and L1$_{0}$ Mn$_{1.5}$Ga. The 200 nm thick film presents a mixture of two different surface structures: domains which consist of faceted tent-like structures and domains with flat terraces (with lateral dimensions of about 500 nm). The magnetic properties of all samples are studied by vibrating sample magnetometer and their correlation with their surface morphology and stoichiometry will be presented. Scanning tunneling spectroscopy measurements of the 200 nm thick Mn$_{1.5}$Ga film reveal electrical inhomogeneity correlated to the two morphologies. This work was supported by NSF DMR1006286. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z14.00015: Magnetic anisotropy of strained MnGa alloys Renat Sabirianov, Nabil Al-Aqtash MnGa is a promising candidate for Rare Earth free permanent magnet applications because it has a large magnetocrystalline anisotropy. We examine the variation of the magnetocrystalline anisotropy of these alloys as function of bi-axial in-plane strain using ab-initio electronic structure calculations. We employed force theorem to calculate the MAE$=$E(\textbar \textbar )-E($\bot )$ as difference of energies of the system with magnetization along and perpendicular to the easy axis. Using projector augmented wave method implemented in VASP we have calculated MAE in MnGa, Mn$_{3}$Ga and Mn$_{\mathrm{1+x}}$Ga$_{\mathrm{1-x}}$ alloys. We find that the MAE is 2.5MJ/m$^{3}$ (0.42meV/u.c.) and 0.12MJ/m$^{3}$ (0.07meV/u.c.) in unstrained MnGa and Mn$_{3}$Ga, respectively. MAE decreases if bi-axial strain is applied in MnGa. Thus, the anisotropy of this system can be affected by the strain. We also discuss the effect of Mn disorder on MAE in Mn$_{\mathrm{1+x}}$Ga$_{\mathrm{1-x}}$ alloys. [Preview Abstract] |
Session Z15: Focus Session: Frustrated Systems: Artificial and Disordered
Sponsoring Units: GMAG DMPChair: John Cumings, University of Maryland
Room: 317
Friday, March 22, 2013 11:15AM - 11:27AM |
Z15.00001: Two-Dimensional Magnetic Correlations and Partial Long-Range Order in Geometrically Frustrated Sr$_2$YRuO$_6$ Eduardo Granado, Jeffrey W. Lynn, Renato F. Jardim, Milton S. Torikachvili Geometrically frustrated magnets are fascinating materials displaying a rich variety of physical states. The simplest three-dimensional structure leading to frustrated magnetism and the first one to be investigated is the face-centered cubic (FCC) lattice with antiferromagnetic nearest-neighbor interactions. Sr$_2$YRuO$_6$ is a particular example of this, crystallizing in the ordered double perovskite structure with the Ru$^{5+}$ ions defining an FCC magnetic network. Neutron diffraction experiments were performed on this material, revealing planar magnetic correlations that condense into a partial long-range ordered state with coupled alternate antiferromagnetic (AFM) YRuO$_{4}$ square layers coexisting with the short-range correlations below $T_{N1}=32$ K. A second transition to a fully ordered AFM state below $T_{N2}=24$ K is observed. The reduced dimensionality of the spin correlations in an FCC lattice is arguably due to a cancellation of the magnetic coupling between consecutive AFM square layers. The interesting magnetic phenomena observed here in Sr$_2$YRuO$_6$ are entirely driven by its lattice geometry, and may also occur in other FCC antiferromagnets. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z15.00002: Neutron Scattering Study of the Double Perovskite FCC Antiferromagnet Ba$_2$YRuO$_6$ J.P. Carlo, J.P. Clancy, K. Fritsch, C.A. Marjerrison, G.E. Granroth, H.A. Dabkowska, B.D. Gaulin Magnetic cations in the rock-salt ordered double perovskite structure comprise a geometrically frustrated FCC network of edge-sharing tetrahedra. Previous measurements of the $4d^3$ Ru$^{5+}$ system Ba$_2$YRuO$_6$ [1] indicated the existence of long-range commensurate antiferromagnetic order below $T_N$ = 36K, a factor f $\sim$ 15 lower than the Curie-Weiss temperature $\Theta_W$ = -522K. We report time-of-flight neutron spectroscopy of Ba$_2$YRuO$_6$ confirming the existence of the long-range ordered state below $T_N$. The magnetic inelastic scattering extends over a bandwidth of $\sim$ 15 meV, and develops a $\sim$ 5 meV gap at the [100] magnetic ordering wavevector at and below $T_N$. Strong spin-orbit coupling in this $4d^3$ system is expected to result in a $j_{eff}$=3/2 magnetic moment. This is distinct from the $4d^1$, $j_{eff}$=3/2 moment which arises in its sister antiferromagnetic FCC compound, Ba$_2$YMoO$_6$, which displays an apparent singlet ground state and a $\sim$ 28 meV singlet-tripet gap at low temperatures [2], matching its |$\Theta_W$| $\sim$ 300K. [1] T. Aharen et al. Phys. Rev. B 80, 134423 (2009). [2] J. P. Carlo et al. Phys. Rev. B 84, 100404(R) (2011). [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z15.00003: Phase diagram of split 2D dipolar spin ice Tommaso Roscilde, Louis-Paul Henry Long-ranged dipolar interactions, which are very natural in artificial square-lattice spin ice, can mask some of the most relevant aspects of spin-ice physics, as they remove the extensive degeneracy of the ground state manifold to give a unique ground state, and they bind monopole pairs into localized spin flips. Following an earlier idea of G. M\"oller and R. Moessner [Phys. Rev. Lett. 96, 237202 (2006)] we investigate how adding a third direction to square ice allows to recover fundamental traits of spin-ice physics even in the presence of dipolar interactions. Using Monte Carlo simulations based on a generalized loop algorithm, we explore the phase diagram of square dipolar spin ice in which horizontal and vertical dipoles are spatially separated in a third direction (split 2D spin ice). As a function of the splitting we recover a two-fold degenerate staggered state for coplanar dipoles, and a four-fold degenerate ``Manhattan'' state for strongly split dipoles, separated by a first order transition. The competition between the two states at intermediate splitting leads to a strong suppression of the ordering transition temperatures, and makes space for the observation of a hallmark of spin-ice physics in the paramagnetic phase: pinch points in the static structure factor. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z15.00004: Phase diagram of quantum square ice Louis-Paul Henry, Peter Holdsworth, Frederic Mila, Tommaso Roscilde We have investigated the ground-state and finite-temperature phase diagram of quantum square ice - realized by the transverse-field Ising model on a checkerboard lattice - using both linear spin-wave (LSW) theory and quantum Monte Carlo (QMC). We generalize the model with different couplings between nearest ($J_1$) and next-to-nearest ($J_2$) neighbors on the checkerboard lattice. Our QMC approach generalizes the loop algorithm - very efficient in the study of constrained classical systems - to a ``brane algorithm'' for quantum systems. At the LSW level the vast degeneracy of the ground-state for $J_1=J_2$ and $J_2>J_1$ remains intact; moreover LSW theory breaks down in extended regions of the phase diagram, pointing at non-classical states [1]. Our QMC study goes beyond perturbative schemes and addresses directly the nature of the low-temperature phases. We have critically examined the possibility of a resonating-plaquette state for $J_1=J_2$, suggested by degenerate perturbation theory on the ice-rule manifold for weak fields. Our QMC results for finite fields confirm the absence of N\'eel or collinear order, but they do not confirm the presence of resonating-plaquette order, pointing at a possibly more complex non-classical state.\\[4pt] [1] L.-P. Henry et al., PRB 85, 134427 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z15.00005: Resonant dynamics of Dirac monopoles and strings in an artificial spin-ice lattice Olle Heinonen, Sebastian Gliga, Attila Kakay, Riccardo Hertel Spin ices can occur in atomic structures, in which the magnetic interaction with neighboring vertices cannot be simultaneously minimized at each atomic vertex, leading to frustration. Artificial spin-ice lattices are arrays of patterned elements geometrically arranged to mimic the frustration in such atomic lattices, and have the advantage that their properties are directly observable using microscopy techniques. Artificial spin-ices can support topological defects, such as Dirac monopoles and Dirac strings connecting the monopoles. We present micromagnetic simulations of the resonant dynamics of a square spin-ice lattice. The simulations predict that topological defects give rise to specific signatures in the excitation spectrum of the lattice and that, moreover, the amplitude of a defect-specific resonant peak increases linearly with the number of defects or length of the Dirac line. A measured spectrum therefore allows to both identify the defects in an array as well as to determine their number. In addition, we observe that the main bulk-like FMR spectral peak is split in the presence of defects in the lattice, compared to a defect-free reference state. This splitting is caused by Dirac strings, in which the FMR frequency is increased due to the different magnetostatic coupling of the elements within a string as compared to the rest of the lattice. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z15.00006: Magnon lifetime in the easy-plane antiferromagnets Alexander Chernyshev, Michael Zhitomirsky Considering a prototypical 2D easy-plane antiferromagnet on a square lattice in which gapped excitations coexist with acoustic spin waves, we find that random disorder induces a relaxation of the gapped magnon with the rate that greatly exceed the effect of conventional magnon-magnon scattering. Anomalous disorder-induced $T$-dependence of the energy gap of the optical magnon and of the scattering rate of the acoustic magnon are also discussed. These can be readily probed by the high-resolution neutron-resonance spin-echo technique. Implications for other systems are discussed and comparison with the available experimental data is presented. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z15.00007: Magnetic enhancement and cluster-glass behavior in (Sc$_{1-x}$Er$_x$)$_{3.1}$In Eteri Svanidze, Emilia Morosan Sc$_3$In is a weak itinerant ferromagnet with no magnetic constituents. In this talk, we will present DC and AC magnetization data on Sc$_{3.1}$In doped with Er$^{3+}$ local moment ions. As x increases in (Sc$_{1-x}$Er$_x$)$_{3.1}$In, the Weiss temperature nearly triples up to $x \leq 0.1$. The effective moment per formula unit is larger than the simple sum of the itinerant moment in pure Sc$_{3.1}$In and the Er$^{3+}$ local moment. Moreover, Er doping of as little as $x = 0.02$ induces a cluster-glass state. The glassy behavior persists up to $x = 0.1$, and a structural transition likely occurs for higher doping levels. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z15.00008: Spin Glass Phase in the Disordered Spin Systems Nvsen Ma, Dao-Xin Yao, Anders Sandvik We use quantum Monte Carlo simulations to study a glassy ground state of S = 1/2 quantum spins by using a dimerized J1-J2-J3 Heisenberg model on the square lattice. J1 corresponds to weak bonds, and J2 and J3 are stronger bonds which are randomly distributed on columnar rungs forming coupled 2-leg ladders. By tuning the average value of J2 and J3, the system undergoes Neel glass paramagnetic quantum phase transition. The size of the glass region is affected by the value of the disorder strength. In the glass phase, we find that the uniform susceptibility decreases with T according to $exp(1/T^a)$ with $a<1$; thus the state is incompressible at $T = 0$ and classified as a Mott glass (MG). At the Neel-MG transition, the susceptibility behaves as $T^{2/z-1}$, where z is the dynamical exponent and it is close to 1. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z15.00009: Microscopic Coexistence of Antiferromagnetic and Spin-Glass States Shravani Chillal, Matthias Thede, Fred Jochen Litterst, Severian Gvasaliya, Tatiana Shaplygina, Sergey Lushnikov, Andrey Zheludev The disordered antiferromagnet PbFe1/2Nb1/2O3 (PFN) is investigated in a wide temperature range by combining Mossbauer spectroscopy and neutron diffraction experiments. It is demonstrated that the magnetic ground state is a microscopic coexistence of antiferromagnetic and spin-glass orders. This speromagnet-like phase features frozen-in short-range fluctuations of the Fe3$+$ magnetic moments that are transverse to the long-range ordered antiferromagnetic spin component. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z15.00010: Randomness effects on the distorted triangular-lattice antiferromagnets Toru Sakai, Hiroki Nakano The quantum spin liquid-like behaviors were observed on some distorted tirangular-lattice antiferromagnet organic compounds[1]. If the lattice vibration is much slower than the spin dynamics, the lattice distortion possibly plays a role of the bond randomness in the spin system. Thus inorder to explain the observed spin liquid behavior of the organic compound, we consider the antiferromagnetic Heisenberg model on some distorted triangular lattices with the bond randomness. Using the numerical exact diagonalization we calculated the standard spin glass order parameter, as well as the ordinary Neel order parameter. The present study suggested that the spin glass order can survive in some region where the Neel order vanishes. We will discuss a scenario of the observed spin liquid behavior of the distorted triangular-lattice antiferromagnets, based on the result. [1] Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato and G. Saito, Phys. Rev. Lett. 91 (2003) 107001. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z15.00011: Frustrated antiferromagnetism in bulk Ti-doped BiFeO$_3$ ceramics Miguel Angel Garcia, Mara Bernardo, Teresa Jardiel, Marco Peiteado, Federico Mompean, Mar Garcia-Hernandez, Marina Villegas, Amador Caballero We present here a magnetic characterization of Ti-doped BiFeO$_3$ prepared by a ceramic route. A detailed analysis of the microstructure revealed that Ti$^{+4}$ is not homogeneously distributed but partially segregated towards Ti enriched grain boundary regions that define Ti-deficient domains with a size distribution of the order of tens of nanometers. Since the size of these domains is smaller than the spin cycloid wavelength (64 nm) they hold a net magnetic moment. Consequently the material exhibits frustrated antiferromagnetism with hysteresis, coercivity and remanance. This ferromagnetic-like behavior vanishes at the Neel temperature of the BiFeO$_3$. The small magnetic moment per domain (M$_{\mathrm{S}}$ $\sim$0.2 emu/g) yields very large coercive fields of 27 KOe at 5 K and 17 KOe at room temperature. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z15.00012: Modeling geometric frustration with magnetic colloids Gabi Steinbach, Sibylle Gemming, Artur Erbe, Dennis Nissen, Manfred Albrecht The implementation of artificial frustrated spin systems can give insight into the mechanisms which lead to the different equilibrium configurations in geometrically frustrated magnetic materials. Prominent artificial systems are the patterns of superconducting rings or nanometer-sized ferromagnetic islands. These are Ising systems allowing two possible in-plane orientations for each macroscopic moment. Here we present an alternative method using magnetically interacting colloids. The spherical shape of the particles allows the modeling of spin systems with continuous symmetry. Micrometer-sized silica particles are half capped with a Co/Pd multilayer thin film. Such thin films on curved surfaces have a predefined net magnetic moment called macrospin. The interaction of such particles in the self-arranged close-packed 2D structure is frustrated. Using video microscopy, the direction of the individual macrospins can be visualized by the spatial orientation of the magnetic caps. This setup allows us to investigate geometric frustration in static systems and in dynamic processes, e.g. during the particle-wise cluster growth. Further, we evaluate the impact of intrinsic defects and control defect formation extrinsically e.g. by varying the growth conditions. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z15.00013: Effect of proton irradiation on the magnetic and magneto-transport properties of TbFeCo metglass Nattawut Anuniwat, Manli Ding, Joseph Poon, Jiwei Lu, Brad Weaver The ferrimagnetism in amorphous rare-earth transition metal alloys is well known, and has recently been investigated for applications in perpendicular magnetic random access memory (p-MRAM), which is considered to be a universal memory technology due to the low power dissipation and the non-volatility. The amorphous TbFeCo thin films were deposited by rf magneton sputtering. The as-deposited film exhibited a low saturation magnetization ($M_{S}\sim $100 emu/cc) and a high perpendicular anisotropy ($K_{U}\sim $10$^{6}$ erg/cc). Hall-bar devices were fabricated for characterizing the magneto-transport behaviors. Both thin film samples and Hall bar devices were exposed to 2 MeV-energy protons with incremental fluences. Magneto-transport and standard magnetic measurements are employed to investigate defects/displacement damages. The magneto-transport suggests that compensation temperature of the film decreases after irradiations. The changes in saturation moments and coercive fields will be discussed as a function of total fluence, which may be related to structural damages. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z15.00014: Heat Capacity and Magnetic Properties of NiMnIn, NiCoMnIn and NiCoMnSn Metamagnetic Materials Jing-Han Chen, Joseph H. Ross, Jr., N. Bruno, J. Monroe, I. Karaman, Jianguo Li NiMnIn and NiMnSn Heusler materials feature coupled glassy magnetic and martensitic structural transformations. Co substitution can induce a large magnetocaloric effect near room temperature with little hysteresis, leading to interest for solid-state refrigeration as well as energy recovery. Recent work has also identified NiMnIn compositions with extremely sharp coupled magnetic-structural transformations. We report thermodynamic measurements for a number of these systems. NiMnCoSn melt-spun tapes can be processed to exhibit more well-defined martensitic transformations, however we find the magnetic contributions to the heat capacity to be similar to that of bulk materials. In both cases the magnetic entropy agrees with what is expected for $J$ = 2 Mn ions according to the alloy composition. A significant difference in electronic $\gamma$ points to electronic differences despite the same $e/a$ ratio. NiMnIn alloys include compositions exhibiting sharp $\lambda$-like transitions, while in bulk NiCoMnIn compositions the magnetic contribution is increased over the Mn-only value. The kinetic arrest behavior reduces the total magnetic entropy in these materials, and we also examine low-temperature anomalies as further indications of the glassy properties of these materials. [Preview Abstract] |
Session Z17: Focus Session: Orbital Order
Sponsoring Units: DMP GMAGChair: Despina Louca, University of Virginia
Room: 319
Friday, March 22, 2013 11:15AM - 11:27AM |
Z17.00001: Local structure and orbital ordering in YTiO$_3$ Bing Li, Keeseong Park, Shinichiro Yano, Despina Louca, Biao Hu, Jianshi Zhou, John Goodenough YTiO$_3$, with the strongest GdFeO$_3$-typed distortion in RTiO$_3$, is a ferromagnet below $T_C$ of 30 K and many theoretical and experimental studies suggest it is of antiferro-orbital ordering due to magnetic superexchange. Here, the local atomic structure of YTiO$_3$ has been investigated by using elastic (inelastic) neutron scattering and the (dynamic) pair density function analysis from 5 to 350 K. Deviations are observed of the local from the average crystal symmetry and these are attributed to distortions involving the Y and O atoms. In the case of Y, the in-plane $x-y$ displacements result in an antiferrodistortive motion exerting influence on Y-O1 (apical sites of octahedral) bonds seen in the temperature dependence. At the same time, the O ion site in the basal plane of the octahedron is split to two (O2 and O3), giving rise to two unequivalent Ti-O bonds, which results consequently in different tilting of basal plane of octahedra (0.5$^\circ$ larger in O3), A-O covalency and about 2$^\circ$ larger Ti-O-Ti bond angle in O3 sites. These facts may be regarded as the structural evidences on antiferro-orbital ordering in YTiO$_3$ and suggest electron-lattice interaction may play an important role in the orbital ordering, in addition to magnetic superexchange interation. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z17.00002: Quasiparticle mass enhancement and temperature evolution of the electronic structure in ferromagnetic SrRuO$_{3}$ Daniel Shai, Carolina Adamo, Dawei Shen, Charles Brooks, John Harter, Eric Monkman, Bulat Burganov, Darrell Schlom, Kyle Shen We report high-resolution angle-resolved photoemission spectroscopy (ARPES) studies of epitaxial thin films of the correlated $4d$ transition metal oxide ferromagnet SrRuO$_{3}$. The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles, exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence, but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO$_{3}$. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z17.00003: Temperature-dependent photoluminescence studies of GdTiO$_{3}$ thin films Amit Verma, Santosh Raghavan, Vladimir Protasenko, Susanne Stemmer, Debdeep Jena GdTiO$_{3}$ (GTO), a Mott-insulator, has acquired increased prominence in last few years since the discovery of a 2-dimensional electron gas (2DEG) at its heterojunction with the band-insulator SrTiO$_{3}$. These 2DEGs have very large electron densities ($\sim $3x10$^{14}$ cm$^{-2})$ amounting to half electron per unit cell. To realize many possible applications of this large 2DEG, an understanding of the GTO bandstructure is needed. With this goal in mind, in this work we present photoluminescence (PL) studies of GTO thin films (10nm and 20nm) grown by molecular beam epitaxy (MBE) on (001) LSAT substrates. When pumped with a 325 nm He-Cd laser, we observe a red PL ($\sim $683 nm at 300K) from both GTO thin films at RT. Upon lowering the temperature from 300K to 80K, the PL peak blue shifts by $\sim $0.14eV. Interestingly, the reported activation energy of the resistivity of MBE-grown GTO thin films is also $\sim $0.14eV (Moetakef et al., J. Crystal Growth 355, 166 (2012)). We connect the observed temperature-dependent PL data with the expected electronic bandstructure and electrical resistivity, and explain the sharp transition in the peak that occurs between 120K-200K from $\sim $636nm to $\sim $683nm. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z17.00004: Spin Switching and Magnetization Reversal in Single-Crystal NdFeO$_3$ Shujuan Yuan, Fang Hong, La Chen, Yabin Wang, Wei Ren, Jincang Zhang, Shixun Cao, Gang Cao We report results of our recent study of single-crystal NdFeO$_3$ that features a strong interaction between 3d and 4f electrons, which generates \textit{two distinct magnetic states} ordered at 17 K and 170 K. This study reveals novel magnetic behavior that is highly sensitive to the orientation and history of magnetic field and is characterized by the following: \textbf{(1)} sharply contrasting magnetization, M(T), along the \textbf{\textit{a}} and \textbf{\textit{c}}-axis; \textbf{(2)} an abrupt spin-switching along the \textbf{\textit{a}}-axis via a first-order transition below 17 K when the system is \textit{zero-field-cooled}; and \textbf{(3)} a progressive magnetization reversal when the system is \textit{field-cooled}. Such behavior suggests an exotic ground state driven by an extraordinary coupling between spin, orbit and lattice degrees of freedom. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z17.00005: Itinerant ferromagnetism in the oxygen-deficient EuTiO$_3$: A first-principles investigation Hai-Xia Cao, Hai-Shuang Lu, Tian-Yi Cai, Sheng Ju Effects of oxygen vacancy on the electronic structure and magnetism in the quantum paraelectric EuTiO$_{3}$ were investigated from first-principles. In contrast to antiferromagnetism in the pristine EuTiO$_{3}$, itinerant ferromagnetism was revealed in the oxygen-deficient EuTiO$_{3}$. The origin lies in the spin-polarized Ti 3$d$ states, which mediate a ferromagnetic exchange interaction between almost localized Eu 4$f$ spins. In addition, this ferromagnetic exchange coupling was strengthened via the partial occupation of Eu $5d$ states. These findings not only explain the observation of ferromagnetism in the unstrained EuTiO$_{3}$ thin films, but also demonstrate the potential application of EuTiO$_{3}$ in magnetoelectronics. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z17.00006: Spin and orbital order separation in colossal magnetoresistive transition M.A. Hossain, M.H. Burkhardt, E. Weschke, E. Schierle, M.S. Golden, Y. Tomioka, Y. Tokura, J. St\"{o}hr, H.A. D\"{u}rr Understanding the Colossal magnetoresistive (CMR) process in manganites is one of the grand challenges of modern physics. While the metallic ferromagnetic phase is relatively well understood, the triggering mechanism of the metal-insulator transition is not clear and it is believed that lattice strain in term of polarons play an important role in the mysterious insulating phase. Lattice strain occurs in the charge-orbitally ordered insulating phase via the Jahn-Teller type distortion and therefore, to understand the CMR it is critical to understand the interplay of ferromagnetism and orbital order during the CMR transition itself. In this letter, with high magnetic field dependent Resonant Soft X-ray Scattering measurements, we show that during the CMR process, an insulating antiferromagnetic phase, which is extremely susceptible to magnetic field and temperature, directly competes with metallic ferromagnetism while the robust CE type spin and orbitally ordered regions act as a catalyst to seed these antiferromagnetic regions. This allows us to construct a picture of the competing forces at the heart of CMR. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z17.00007: Charge-ordering transitions without charge differentiation Yundi Quan, Victor Pardo, Warren Pickett The distorted perovskite nickelate system RNiO$_3$ (R=rare earth except La) undergoes a metal-insulator transition (MIT) at a temperature that varies smoothly with the R ionic radius. This MIT is accompanied by structural transition which leads to two inequivalent Ni sites in the cell, and has been explained by charge ordering (CO): charge is transferred between the Ni1 and Ni2 sites in a long-range ordered fashion. Experimental data on core binding energies, ionic radii, and Mossbauer shifts are interpreted in terms of Ni cation charges of 3$\pm \delta$ with, for example, $\delta \approx$ 0.3 for YNiO$_3$. Making use of first principles DFT results and a new approach not invoking integration of the charge density, we find\footnote{Y. Quan, V. Pardo, and W. E. Pickett, Phys. Rev. Lett. (2012, in press)} that the Ni $3d$ occupation is identical (to high accuracy) for the two Ni sites. We also present results for other compounds (La$_2$VCuO$_6$, YNiO$_3$, CaFeO$_3$, AgNiO$_2$, V$_4$O$_7$), all of which have distinct ``charge states'' that have identical $3d$ occupation. This quantitative procedure will be discussed and some implications will be outlined. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z17.00008: Real Space Imaging of Phase Separation in Mangnites Kai Du, Lifeng Yin, Jian Shen Electronic phase separation (EPS) in manganites is generally considered to be responsible for their unusual colossal magneto resistance (CMR) [1-2]. However, the dynamic behavior of EPS and the formation mechanism are still not very clear. Magnetic force microscopy (MFM) is one of the most powerful techniques which enables us to study the magnetic domains and direct image the EPS in real space without damaging the samples. In this work, we use a PPMS (Quantum Design) compatible MFM to study the magnetic domains of La0.7Sr0.3MnO3 (LSMO) thin films on NdGaO3(110) and La0.325Pr0.3Ca0.375MnO3 (LPCMO) thin films on SrTiO3 (100) grown by pulsed laser deposition technique (PLD). The LSMO system shows clear stripe domain pattern [3], while the LPCMO system exhibits large scale domains corresponding to charge-ordered insulating phase and ferromagnetic metallic phase [1]. Their transport properties were studied under a variety of temperatures and magnetic fields. The phase separation in submicron scale and their percolative transport have been confirmed by MFM images and the transport measurement during the imaging.\\[4pt] [1] M. Uehara, Nature (London) 399, 560 (1999)\\[0pt] [2] E. Dagotto, Phys. Rep. 344, 1(2001); and references therein\\[0pt] [3] Y. Jiang, Solid State Communications 150, 2028(2010) [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z17.00009: Effective magnetoelectric tensor of a composite material David Stroud, Mehul Dixit We calculate the effective magnetoelectric coefficient tensor of a composite of two single-phase magnetoelectrics in which effect of strain is unimportant. We obtain exact relations for elements of the effective magnetoelectric coefficient tensor entirely in terms of the elements of the individual components, and the composite geometry. The problem is solved by a decoupling transformation that reduces the problem to finding the effective coefficients in a composite of the {\emph same geometry} but with two {\emph independent, curl-free fields}. The decoupling transformation is found to be identical to that used in the problem of composite thermoelectrics \footnote{D. J. Bergman and O. Levy. Thermoelectric properties of a composite medium. J. Appl. Phys., 70:6821 - 6833, 1991}. Details of the calculation will be presented. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z17.00010: Growth and characterization of high crystalline quality Co$_{2}$FeAl$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ Heusler alloy films on MgAl$_2$O$_4$(001) substrates Brian Peters, Christian Blum, Patrick Woodward, Sabine Wurmehl, Fengyuan Yang A number of Heusler alloys have been predicted to be half-metallic and are thus ideal candidates for use in spintronics. Co$_{2}$FeAl$_{\mathrm{x}}$Si$_{\mathrm{1-x}}$ has been predicted and shown to have some of the highest Tc, saturation magnetization and lowest magnetic damping constant among Heusler half-metals. Here we outline the growth and characterization of the highest crystalline quality epitaxial Heusler films using a novel off-axis UHV sputtering technique. We grow these films onto a closely lattice matched MgAl$_{2}$O$_{4}$(001) substrate, without the need for a Cr-buffer layer or post annealing, as has been done previously. This eliminates the diffusion of Cr across the interface, thus improving the purity and crystallinity of the films at the interface. X-ray diffraction results demonstrate epitaxial films with distinct Laue oscillations and rocking curves of FWHM as low as 0.0035$^{\circ}$, which demonstrates the highest crystalline quality for Heusler films reported to date. Magnetic measurements show highly square hysteresis loops with a remanence of 95-98{\%}, near ideal saturation magnetization, very small coercivities - between 3-8 Oe, pronounced magnetocrystalline anisotropy. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z17.00011: Small bound polarons for ultrafast holography in dielectric LiNbO$_3$ Mirco Imlau, Hauke Bruening, Holger Badorreck, Andreas Buescher Small bound polarons allow for hologram recording with single laser pulses and exceptional photosensitivity in nominally undoped, thermally reduced LiNbO$_3$ [1]. This new type of recording mechanism is of particular interest for the field of nonlinear and ultrafast photonics because of small bound polaron generation on the fs-scale. In this contribution we present our latest results on the emerging field of small bound polarons for fs-holography in the visible spectral range as well as the successful application of the effect for holographic imaging. The impact of two-photon absorption and nonlinear index of refraction is highlighted by studying the temporal dynamics of the diffraction efficiency as a function of time-delay between hologram recording and probing (-6 fs $< \delta t <$ 6 ns). The analysis of the temporal dynamics supports our recent approach (see Ref. [2]) to explain the build-up of the space-charge field on the sub-ps-time scale in the model of optical absorption of small polarons: optically induced polaron hopping to next-neighboring lattice sites is responsible for a fast and efficient charge transport on the nanoscale.\\[4pt] [1] M.~Imlau et al., Opt. Express 19, 15322 (2011)\\[0pt] [2] H.~Bruening et al. Opt. Express 20, 13326 (2012) [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z17.00012: Atomically resolved data for oxide surface analyzed using a local crystallography analysis method Zheng Gai, Wenzhi Lin, K. Fuchigami, T. Ward, P. Snijders, J. Shen, Stephen Jesse, Sergei Kalinin, Arthur Baddorf The emergent physical phenomena of oxides have attracted increasing scientific attention. Here, we report an approach for studying local surface chemistry and order parameter fields based on a local crystallographic analysis of scanning probe microscopy data of oxide surfaces. We obtained initial estimated atom locations by finding the centroid of the remaining isolated regions of pixels, after applying thresholds to the topographic images. Then we determined the refined positions by automatically fitting each atom individually using a shape function. With the refined locations, we can further derive and quantify properties that are not readily clear in the topographic images. This approach was applied to analyze scanning tunneling microscopy data for the surface of La5/8Ca3/8MnO3 (001) and demonstrated distortion domains with different distortion orientations. These studies provide a new pathway to extract and quantify local properties for scanning probe microscopy images. Research was supported (W.L., S.V.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted at and supported by (Z.G., S.J., A.P.B.) the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z17.00013: Electronic structure and strain-induced Lifshitz transition in epitaxial Ba$_{2}$RuO$_{4}$ thin films as studied by ARPES Bulat Burganov, Carolina Adamo, Daniel Shai, Andrew Mulder, Masaki Uchida, John Harter, Craig Fennie, Darrell Schlom, Kyle Shen We employ oxide molecular beam epitaxy and \emph{in situ} ARPES to synthesize epitaxial thin films of Ba$_{2}$RuO$_{4}$, which is isostructural and isoelectronic to the unconventional superconductor Sr$_{2}$RuO$_{4}$, and characterize its Fermi surface topology and multiorbital quasiparticle dynamics. Although Ba$_{2}$RuO$_{4}$ cannot be synthesized as bulk single crystals, we epitaxially stabilize thin films on TbScO$_{3}$ or SrTiO$_{3}$ substrates. We report a full parametrization of the band structure and compare our results to first-principles calculations as well as our data on Sr$_{2}$RuO$_{4}$. Unlike in Sr$_{2}$RuO$_{4}$ we do not observe a surface reconstruction in line with our expectations for the larger Ba cations. We use ARPES to demonstrate that the combination of a larger cation radius, together with epitaxial strain, can be employed to drive a Lifshitz transition in the d$_{xy}$-like $\gamma$ band from electron-like in Sr$_{2}$RuO$_{4}$ to hole-like in Ba$_{2}$RuO$_{4}$. The ability to control the Fermi surface topology by epitaxial strain is a promising tool for investigating the role of the near-E$_{F}$ van Hove singularity in superconductivity and magnetism in ruthenates, as well as a general tool for controlling and studying correlated electronic materials. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z17.00014: Conducting states caused by a surface electric dipole in CrN(001) very thin films Antia S. Botana, Victor Pardo, Daniel Baldomir, Peter Blaha The changes in the electronic structure of oxides and other correlated compounds caused by electronic reconstructions at their surface and interfaces has attracted much attention recently. CrN shows a magnetostructural phase transition as a function of temperature and controversial electronic properties. It has been argued recently that, with the onset of antiferromagnetic order, CrN as a bulk is always semiconducting, but very close to a metal-insulator transition [1]. In order to check if a small perturbation in the system could drastically change its conduction properties, we have performed electronic structure calculations for CrN in a thin film geometry within the LDA+U method. For thin films with increasing thickness (4-10 layers) starting with a critical thickness of 10 (cubic symmetry) or 6 layers (orthorhombic) the gap closes and conducting states appear. The appearence of metallic states is connected with a structural relaxation at the surface, where Cr (N) atoms buckle inside (outside) forming an effective surface dipole moment. Being CrN a low-gap system, these electric dipoles at the surface are able to shift the bands around the Fermi level significantly enough to drive those thin films metallic. \\[4pt] [1] A. S. Botana et al. Phys. Rev. B 85, 235118 (2012) [Preview Abstract] |
Session Z18: Focus Session: Spin-torque and Related Magnetic Oscillations
Sponsoring Units: DMP FIAP GMAGChair: Robert McMichael, National Institute of Standards and Technology
Room: 320
Friday, March 22, 2013 11:15AM - 11:51AM |
Z18.00001: Decoherence and mode-hopping in spin-torque oscillators Invited Speaker: Pranaba Kishor Muduli A Spin Torque Oscillator (STO) is a nano-sized magneto-resistive device that can produce microwave signals in the GHz range as a result of spin transfer torque [1, 2]- a phenomena which is receiving increasing importance in contemporary spintronics research both for fundamental spin physics as well as a number of possible microwave applications e.g., oscillator, detectors and modulators. A very important question, both for fundamental physics as well for applications, is what limits the coherence time of the STO. This is a subject of significant interest recently. Until now theoretical studies have investigated decoherence through thermal noise assuming that only a single mode is excited [3]. On the other hand, experiments clearly show both the existence of multiple modes and persistent mode-hopping between several modes. The impact on coherence time of such mode-hopping has been largely unexplored and a theoretical study of its origin is entirely lacking. In this work, we will present first ever systematic experimental investigations of mode hopping, and its impact on the coherence time in a magnetic tunnel junction based spin torque oscillator [4]. We will discuss micromagnetic simulations and a theoretical treatment to show that the non-conservative fields due to finite damping-either positive or negative (spin torque) -couple individual modes and, in the presence of thermal noise, govern the experimentally observed mode-hopping. Using quantitative analysis of both coherence and dwell times, we will show that mode-hopping could be a limiting factor for STO coherence. Finally we show how our theoretical treatment can be extended to the case of a metallic nanocontact based STO, where anomalous temperature dependence of linewidth is found as result of the mode coupling [5].\\[4pt] References:\\[0pt] [1] L. Berger, Phys. Rev. B 54, 9353 (1996).\\[0pt] [2] J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [3] A. Slavin and V. Tiberkevich, IEEE Trans. Magn. 45, 1875 (2009).\\[0pt] [4] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. Lett. 108,207203 (2012).\\[0pt] [5] P. K. Muduli, O. G. Heinonen, and J. {\AA}kerman, Phys. Rev. B 86, 174408 (2012). [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z18.00002: Influence of Interlayer Exchange Coupling and Exchange Bias on the Ferromagnetic Resonance Spectra Dirk Backes, Bartek Kardasz, Juergen Langer, Andrew D. Kent We present a study of the influence of exchange bias and exchange coupling on the shape and width of ferromagnetic resonance (FMR) spectra. Such interactions are employed in pinned synthetic antiferromagnets (SAF), layer stacks in which two ferromagnetic layers are antiferromagnetically coupled due to interlayer exchange coupling (IEC). One of the ferromagnetic layers shares an interface with an antiferromagnet, thus pinning its magnetization due to the exchange bias (EB) effect. It has been shown that quantitative values for the IEC and EB interactions can be determined from FMR dispersion relations [1]. In this work we study how these interactions manifest themselves in the peak intensities and line widths of FMR spectra. For this we adjust the strength of exchange bias and IEC by varying the thickness of PtMn and Ru in PtMn/ 2 CoFe/ Ru/ 2.3 CoFeB layer stacks (thicknesses in nm). We investigate various cases: i) presence or absence of an exchange bias field, combined with different kinds of IEC: ii) strong and weak antiferromagnetic, weak parallel, and no coupling. [1] D. Backes et al., JAP 111, 07C721 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z18.00003: Spin torque nanooscillators: new applications in information processing Ferran Macia, Andrew D. Kent, Frank C. Hoppensteadt Nanonometer scale electrical contacts to ferromagnetic thin films (STNOs) can provide sufficient current densities to excite magnetic-moment dynamics resulting in emission of short wave-length spin waves. We discuss several applications of spin-wave patterns created from STNOs and their interaction with background oscillations. We review how to encode information in STNOs signals $-$modulating their amplitude, frequency or phase$-$ and stability against noise. We first model arrays of STNOs in extended ferromagnetic thin films and define conditions to control spin-waves emission directions. We also study arrays of oscillators in patterned ferromagnetic thin films and we put forward a method to build an STNO lookup tables or an STNO based network analyzer. Using spin waves complements digital semiconductor technologies and offers new possibilities for increased memory capacity and computation performance.\\[4pt] [1] F. Maci\`a \emph{et al.} Nanotechnology 22 095301 (2011)\\[0pt] [2] F. Maci\`a \emph{et al.} Journal of Applied Physics 109, 07C733 (2011). [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z18.00004: Microwave emission by a spin Hall nano-oscillator Ronghua Liu, Weng L. Lim, Sergei Urazhdin In a recently developed class of spintronic devices, the spin Hall effect (SHE) produces a pure spin current controlling the magnetization of ferromagnets. While SHE opens possibilities for new material combinations and device geometries, it also requires innovative approaches to device engineering. We demonstrate coherent microwave generation by a spintronic nanodevice that utilizes local injection of spin current generated by SHE into an extended magnetic film to generate magnetization oscillations, and anisotropic magnetoresistance of the magnetic layer to convert the oscillations into a microwave signal. We will describe our measurements of the dependence of spectral characteristics of the oscillations on current, temperature, and magnetic field. The dependence on current was remarkably similar to the spin-valve nano-oscillators. However, the dependence on temperature was different from the traditional magnetic nano-oscillators, indicating a significant temperature dependence of both the magnetization dynamics and the SHE. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z18.00005: Spintronics r.f. oscillator driven by magnetic field feedback Ashwin Tulapurkar, Dinesh Dixit, Katsunori Konishi, C.V. Tomy, Yoshishige Suzuki Magnetic tunnel junctions (MTJ) can be used as nano-scale rf oscillators using spin-transfer torque effect. Here we present an alternative novel mechanism of ``magnetic field feedback'' for driving MTJs into precessional states. To realize this effect, MTJ needs to be fabricated on top of a co-planar wave-guide. A dc current is passed through MTJ to produce a fluctuating voltage across it as a combination of thermal fluctuations of free layer and magneto-resistance effect. This voltage is applied across co-planar wave-guide to create a fluctuating magnetic field which acts on the free layer to enhance its fluctuations. If the dc current exceeds a critical value, precessional states of free layer are excited. We have derived expression for the critical current using lineralized LLG equation, modified to include the ``feedback'' magnetic field. We have verified the feedback effect by numerical simulation of stochastic LLG equation including random magnetic field: we find that the damping of the free layer can be increased/decreased by applying --ve/$+$ve dc current. Simulations show that by applying dc current more than critical current, large amplitude oscillations with high quality factors are possible. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z18.00006: Parallel pumping instabilities of spin wave modes in a nanodisk Robert McMichael, Feng Guo, Lyuba Belova The confined spin wave modes in a magnetic nanostructure are fundamental to the magnetization dynamics, and the majority of studies of these spin waves have used uniform transverse pumping fields to excite the modes. However, our recent ferromagnetic resonance force microscopy measurements have shown that parallel pumping reveals a richer set of resonances than the more conventional transverse pumping. This talk presents measurements and micromagnetic modeling of the parallel pumping process in a 500 nm diameter 25 nm thick Permalloy disk with fields applied in plane. In the experiments, the linear, transversely pumped spectrum at 5.2 GHz is simple, with a main resonance at 38 mT applied field and a weaker mode at 54 mT. At a doubled excitation frequency of 10.4 GHz and high pumping power, five resonances are excited by parallel pumping. Two of these resonances occur at the same fields as the modes observed under transverse pumping, but the most intense mode is one that does not appear in the transverse spectrum. The modeling results show similar behavior, and also provide images of the excited modes. The lowest thresholds for parallel pumping belong not to the nearly uniform ``main'' precession mode, but to standing waves that propagate along the field direction. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z18.00007: Evidence for non-local damping in individual nano structures with a single magnetic layer Hans Nembach, Justin Shaw, Carl Boone, Tom Silva The spin excitation damping $\alpha$ in nanomagnets is a critical parameter for switching in STT-MRAM because the required power depends on $\alpha^2$. We experimentally demonstrate that intralayer spin-pumping is a significant source of damping. Ferromagnetic resonance spectra were measured by heterodyne magneto-optical microwave microscopy for individual Ni80Fe20 nanomagnets down to 100 nm. Micromagnetic simulations show that one spin-wave mode, i.e. the ``center-mode,'' is distributed throughout the nanomagnet, whereas the two ``end-modes'' are localized at the ends. $\alpha$ is found to increase for the ``center-mode'' with decreasing nanomagnet size but shows the opposite trend for the ``end-modes.'' It was proposed that dissipative transverse spin-currents can increase $\alpha$. Calculations of this additional damping are in agreement with the experimental data. We also used micromagnetics to test the hypothesis that an area of increased damping close to the edges of the nanomagnets forms during patterning. Such simulations predict that $\alpha$ for both spin-wave modes increases with decreasing size of the nanomagnets, contrary to our experimental observations. Thus, we conclude that non-local contributions to $\alpha$ are the dominant mechanism for size-dependence of $\alpha$. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z18.00008: Ferromagnetic resonance (FMR) spin-pumping in FM/I/NM heterostructures Yong Pu, C. Du, H. Wang, R. Adur, A. Berger, J. Beardsley, A. Hauser, P. Odenthal, A. Swartz, R. Kawakami, J. Pelz, E. Johnston-Halperin, F. Yang, P.C. Hammel The recent demonstration of the injection of a pure spin current via ferromagnetic resonance (FMR) in the FM electrode, spin-pumping, with no need for an accompanying charge current, promises low-power high-efficiency spin injection in a wide variety of materials. Here we report the demonstration of FMR spin-pumping in Ferromagnet/Insulator/Non-magnetic materials heterostructures via different spin detection techniques, and characterizations of the dynamically injected spin. Our investigation proves the possibility that one can both utilize the advantages of FMR spin-pumping, and simultaneously overcome the well-known resistance mismatch problem, which usually happens for spin injection through a FM/NM direct contact and drastically suppresses the efficiency of spin injection into NM. Furthermore, by individually and systematically varying the magnetic, electrical and mechanical properties of each element of the FM/I/NM heterostructures, we are able to study the fundamental mechanisms for FMR spin-pumping, e.g. coupling range and strength, and role of and interplay between spin, charge, lattice, magnon and phonon degree of freedoms. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z18.00009: Numerical study of spin-dependent transition rates within pairs of dipolar and exchange coupled spins with (s=1/2) during magnetic resonant excitation Mark Limes, Jinqi Wang, William Baker, Sang-Yun Lee, Brian Saam, Christoph Boehme The effect of dipolar and exchange interactions within pairs of paramagnetic electronic states on Pauli-blockade-controlled spin-dependent transport and recombination rates during magnetic resonant spin excitation is studied numerically using the superoperator Liouville-space formalism. The simulations reveal that spin-Rabi nutation induced by magnetic resonance can control transition rates which can be observed experimentally by pulsed electrically (pEDMR) and pulsed optically (pODMR) detected magnetic resonance spectroscopies. When the dipolar coupling exceeds the difference of the pair partners' Zeeman energies, several nutation frequency components can be observed, the most pronounced at $\sqrt{2}\gamma B_1$ ($\gamma$ is the gyromagnetic ratio, $B_1$ is the excitation field). Exchange coupling does not significantly affect this nutation component; however, it does strongly influence a low-frequency component $< \gamma B_1$. Thus, pEDMR/pODMR allow the simultaneous identification of exchange and dipolar interaction strengths. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z18.00010: Configurational Dependence of the Magnetization Dynamics in Spin Valve Systems Ruslan Salikhov, Radu Abrudan, Frank Bruessing, Kurt Westerholt, Hartmut Zabel, Florin Radu, Ilgiz A. Garifullin Spin current related phenomena in F1/N/F2 spin valve heterostructures, where F is a ferromagnetic layer and N is a nonmagnetic metal layer, are important in modern magnetism. Spin valve theory predicts a spin pumping effect with a precessional relaxation rate that depends on the configuration of F1 and F2 [1]. Using time-resolved x-ray resonant magnetic scattering we report on the precessional dynamics of spin valve systems with parallel (P) and antiparallel (AP) orientation. We observe in Co/Cu/Py spin valve systems an increase of the magnetic damping parameter in Py with changing magnetization direction of Py and Co layers from P to AP orientation [2]. Furthermore we studied the temperature dependence of the spin pumping effect and possible other causes for the configurational dependence of the damping parameter, such as domain wall induced coupling or magnetic dipole coupling [3]. The main focus is on Co/Cu/Py and on Co$_2$MnGe/V/Py trilayers with spin valve properties.\\[4pt] [1] J.-V. Kim, C. Chappert, JMMM \textbf{286}, 56 (2005)\\[0pt] [2] R. Salikhov \textit{et al.}, APL \textbf{99}, 092509 (2011)\\[0pt] [3] R. Salikhov \textit{et al.}, PRB \textbf{86}, 144422 (2012) [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z18.00011: Domain-wall-controlled transverse spin injection Rembert Duine We propose an effect whereby a charge current accross a domain wall in a magnetic wire injects a transverse pure spin current in an adjacent normal metal. We compute how this effect may be measured via inverse spin Hall effect detection, and consider its effect on enhancement of spin transfer. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z18.00012: Magnetic Bloch Oscillations in 1D ferromagnets Olav Sylju{\aa}sen, Sergey Shinkevich Domain-walls in certain 1D ferromagnets can oscillate when exposed to a static magnetic field. Such magnetic Bloch oscillations have however not been observed experimentally to date. We have calculated neutron scattering signatures of magnetic Bloch oscillations for the material ${\rm{CoCl}_2\cdot 2\rm{H}_2\rm{O}}$, and investigated numerically the possibility of using a laser to generate such oscillations at low temperatures. Our results are positive, and may be used to assist the experimental search for magnetic Bloch oscillations. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z18.00013: Spin-Wave Generation by DW Collision Seonghoon Woo, Tristan Delaney, Geoffrey Beach Spin waves (SWs) in nanoscale metallic ferromagnets have generated much recent interest. Micromagnetic simulations have shown that SWs can couple to and propel DWs by exciting internal resonances, and this effect could be used as a means of low-power DW manipulation. However, generating and detecting large-amplitude exchange-mode SWs is challenging due to their very short wavelengths, which cannot be directly excited by. Here we show, through micromagnetic (OOMMF) simulations, that DWs can be used both to efficiently generate and detect exchange-mode SWs. We first examine SW emission resulting from field-driven DW collisions in Permalloy nanowires. DW annhilation generates intense SW bursts that almost uniformly populate the available SW spectrum across a broad frequency range. The SW power spectrum was characterized as a function of nanowire width, DW topology, and driving field used to induce DW collision. SW bursts were detected through their influence on a third DW pinned at a notch a fixed distance from the DW collision point. SWs induced DW depinning in the presence of background field significantly below the DW depinning field in the absence of SW excitations. The reduction in depinning field dropped with distance between the collision point and the pinned DW, consistent with the decay length due to Gilbert damping. These results show DWs can act as efficient sources of large-amplitude SWs, which can be detected by their influence on a nearby DW. The design of experiments to test these predictions will be discussed. [Preview Abstract] |
Session Z19: f-Electron System Properties - Theory & Experiment
Sponsoring Units: DCMPChair: Ryan Baumbach, Los Alamos National Laboratory
Room: 321
Friday, March 22, 2013 11:15AM - 11:27AM |
Z19.00001: Tuning thermoelectric power factor by crystal-field and spin-orbit couplings in Kondo lattice materials Seungmin Hong, Pouyan Ghaemi, Joel Moore, Philip Phillips We study thermoelectric transport at low temperatures in correlated electron materials, motivated by the recent observation of a high thermoelectric figure of merit(ZT) in $FeSb_2$ at $T \sim 10K$. Even at room temperature, correlations have the potential to lead to high ZT, as in $YbAl_3$, one of the most widely used thermoelectric metals. At low temperature correlation effects are especially worthy of study because fixed band structures are unlikely to give rise to the very small energy gaps $E_g \sim 5 K$ necessary for a weakly correlated material to function efficiently at low temperature. We explore the possibility of improving the thermoelectric properties of correlated Kondo insulators through tuning of crystal field and spin-orbit coupling and present a framework to design more efficient low-temperature thermoelectrics based on our results. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z19.00002: Quantum Oscillations of Nitrogen Atoms in Uranium Nitride S.E. Nagler, A.A. Aczel, G.E. Granroth, D.L. Abernathy, W.J.L. Buyers, G.J. MacDougall, G.D. Samolyuk, G.M. Stocks The quantum harmonic oscillator is among the very few soluble fundamental models in quantum mechanics and the foundation for understanding phonons in crystalline solids. Inelastic neutron scattering typically reveals acoustic and optic one phonon modes at low energies, and as energy increases a complex continuum of many-phonon excitations. In contrast, measurements using chopper spectrometers at the SNS have shown that for the binary crystal uranium nitride, where the nitrogen atoms are very light compared to the uranium atoms, the response above the optic phonon modes exhibits a remarkable spectrum of well-defined local levels that are equally spaced by 50 meV intervals and that extend to the tenth order ~500 meV. The levels are attributed to nearly isotropic, quantum harmonic oscillator behavior of the nitrogen atoms vibrating within a largely static uranium cage. See Nature Communications 3, 1124 (2012). [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z19.00003: First-principles study of the Kondo physics of a Pu impurity in a Th host Jian-Xin Zhu, K. Haule, R.C. Albers, J.M. Wills From the viewpoint of condensed matter physics properties, crystal structure, and metallurgy, plutonium is the most complicated element in the Periodic Table, including a phase diagram with six allotropic phases. Its anomalous properties are related to the special position of Pu in the Periodic Table, which is at the boundary of the light actinides that have itinerant 5$f$ electrons and the heavy actinides with localized 5$f$ electrons, indicative of a very strongly correlated state. To reveal the role of electronic correlations in Pu, we investigate the electronic structure of a Pu atom embedded in a Th host by combining density functional theory within the local density approximation with the continuous-time quantum Monte Carlo simulation of a Pu impurity. As a hallmark of electronic correlations, the Kondo resonance peak around the Fermi energy is obtained in the local density of states on the Pu impurity. Furthermore, we show that the resonance peak width is narrower for Pu atoms that are at the surface of Th than when compared to those in the bulk, due to a weakened Pu 5$f$-ligand hybridization in the former geometry. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z19.00004: Electron Spin Resonance in Antiferro-Quadrupolar Ordered CeB$_6$ Pedro Schlottmann CeB$_6$ is a {\it cubic} heavy fermion compound with a $\Gamma_8$ ground-quartet with antiferro-quadrupolar (AFQ) order below 4 K. An ESR signal was observed [1] in the AFQ phase. Single ions with a $\Gamma_8$ ground-multiplet should display four transitions, but only one resonance was observed. Several fundamental questions arise: (1) why is only one transition seen, (2) why was this transition observed if the Kondo temperature is larger than the linewidth of the resonance, and (3) can the resonance be explained with localized moments or is an itinerant picture of heavy electron spins necessary? The interplay of AFQ and ferromagnetic correlations on the phase diagram, the magnetization and the ESR linewidth are discussed [2]. In contrast to other Yb and Ce heavy fermion systems displaying an ESR signal, CeB$_6$ does not have strong magnetic anisotropy with ferromagnetic correlations, rendering an observable narrow resonance [3,4]. The AFQ state is necessary for an ESR signal in the present case [2].\\[4pt] [1] S.V. Demishev {\it et al}, Phys. Rev. B {\bf 80}, 245106 (2009);\\[0pt] [2] P. Schlottmann, Phys. Rev. B {\bf 86}, 075135 (2012);\\[0pt] [3] E. Abrahams and P. W\"olfle, Phys. Rev. {\bf 78}, 104423 (2008); {\bf 80}, 235112 (2009);\\[0pt] [4] P. Schlottmann, Phys. Rev. B {\bf 79}, 045104 (2009). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z19.00005: Heavy antiferromagnetic phases in Kondo lattices Ilya Vekhter, Leonid Isaev We propose a microscopic physical mechanism that stabilizes coexistence of the Kondo effect and antiferromagnetism in heavy-fermion systems. We consider a two-dimensional quantum Kondo-Heisenberg lattice model and show that long-range electron hopping leads to a robust antiferromagnetic Kondo state. By using a modified slave-boson mean-field approach we analyze the stability of the heavy antiferromagnetic phase across a range or parameters, and discuss transitions between different phases. We also address connection to experiments on heavy fermion compounds. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z19.00006: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z19.00007: Possible Itinerant Moment Contributions to the Magnetic Excitations in Gd, Studied by Neutron Spectroscopy G.E. Granroth, A.A. Aczel, J.A. Fernandez-Baca, S.E. Nagler Many experimental features in magnetic superconductors are also present when these complex materials are in the normal state. Therefore studies of simpler itinerant magnets may help provide understanding of these phenomena. We chose to study Gd as it is has an $\sim 0.6\mu_B$ itinerant moment in addition to a $\sim 7.0 \mu_B$ localized moment. The SEQUOIA spectrometer, at the Spallation Neutron Source at Oak Ridge National Laboratory, was used in fine resolution mode with $E_i$=50 meV neutrons, to measure the magnetic excitations in a 12 gm $^{160}$Gd single crystal. The crystal was mounted with the $h0l$ plane horizontal and rotated around the vertical axis to map out the excitations. The measured magnetic structure factor for the acoustic modes in the $hh0$ direction has an intensity step at $h \sim 0.3$. Electronic band structure calculations (W. M. Temmerman and P. A. Sterne, J. Phys: Condes. Matter,{\bf 2}, 5529 (1990) ) show this $Q$ position to be near several band crossings of the Fermi surface. A detailed analysis, including instrumental resolution, is presented to clarify any relationship between the magnetic structure factor and the electronic band structure. [Preview Abstract] |
Session Z21: Solid State Lighting and Other Semiconductors
Sponsoring Units: FIAPChair: Angelo Mascarenhas, National Renewable Energy Laboratory
Room: 323
Friday, March 22, 2013 11:15AM - 11:27AM |
Z21.00001: Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes Emmanouil Kioupakis, Qimin Yan, Chris G. Van de Walle The wider adoption of solid-state lighting is hampered by the significant efficiency reduction of nitride light-emitting diodes (LEDs) at high power. Although Auger recombination has been shown to contribute to this efficiency loss, many of the supporting studies focused on bulk materials. In contrast, LEDs consist of quantum wells that exhibit polarization fields, which strongly influence the recombination rates. We use Schrodinger-Poisson calculations in order to investigate the effect of polarization fields in nitride quantum wells on the carrier recombination rates and the efficiency of nitride LEDs. Our results demonstrate that both the efficiency-droop and green-gap problems can be attributed to the combined effect of Auger recombination and the spatial separation of electrons and holes induced by the polarization fields. Our results show that the suppression of polarization fields is a promising solution to improve the high-power efficiency of nitride LEDs. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z21.00002: CuPt atomic ordering and band gap reduction in AlInP for green LED applications Daniel Beaton, Kunal Mukherjee, Kirstin Alberi, Theresa Christian, Angelo Mascarenhas, Eugene Fitzgerald Efficient light emission in the wavelength range of 575-595nm (green/amber) is necessary for high colour rendering index (CRI) colour-mixed white LED light sources. The present lack of efficient light emitters in this range is known as the 'green gap'. However, it is possible to achieve efficient green/amber light emission with III-V semiconductor alloys, specifically by using direct band gap AlInP alloys, where carrier confinement for device application can result from the band off-set between ordered and disordered material of the same composition. The greater size discrepancy between Al and In results in higher degrees of CuPt atomic ordering and larger band gap reductions than typically reported for other order materials, such as GaInP. Samples are grown lattice matched to InGaAs graded buffer layers grown on GaAs substrates and atomic ordering is observed by TEM. Photoluminescence and modulated reflectance characterization are used to quantify the band gap shift as a function of order parameter. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z21.00003: Demonstration of amber-green light emitting diodes with lattice-mismatched AlInP active region Theresa Christian, Daniel Beaton, Kunal Mukherjee, Kirstin Alberi, Angelo Mascarenhas, Eugene Fitzgerald Future solid-state lamps based on all-LED white light emission will require four emitter colors (red, amber, green, and blue) to achieve good color rendering while maintaining high efficiency. Traditional LED material systems are well-suited to the red and blue ends of the spectrum but there is not yet a clear front-runner material for efficient light emission in the amber-green wavelength range (570 -- 595 nm). The compound semiconductor alloy Al$_x$In$_{1-x}$P has the potential to achieve this target due to its high direct bandgap. This talk will present results from our recent fabrication of amber-green LEDs featuring an AlInP double-heterostructure device structure. Cladding layers for carrier confinement are achieved through control of atomic ordering in the AlInP material. To fully exploit the high direct bandgap that occurs at lattice constants below that of GaAs, these devices are grown on InGaAs/GaAs virtual substrates. Devices are characterized in terms of current-voltage behavior, electroluminescence emission spectra and drive current dependence. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z21.00004: Theoretical Analysis of the Band Offsets and Band Bending in (0001) In$_{x}$Ga$_{\mathrm{1-x}}$N/GaN Heterostructures and Quantum Wells Liang Dong, S. Pamir Alpay Valence band offsets ($\Delta E_{V})$ and built-in electric fields of (0001) In$_{x}$Ga$_{\mathrm{1-x}}$N/GaN heterostructures and quantum wells are studied as a function of In composition $x$ using first principles calculations based on density functional theory (DFT). These properties determine the degree of quantum confinements and wave function overlapping of electrons and holes, and thus the overall efficiencies of electronic/optoelectronic devices based on these structures. We show that with increasing $x$, $\Delta E_{V}$ of (0001) In$_{x}$Ga$_{\mathrm{1-x}}$N/GaN displays a parabolic bowing in both strain-free (fully relaxed) heterostructures and pseudomorphic quantum wells on $c$-plane GaN substrates. $\Delta E_{V}$ of (0001) InN/GaN in these two cases (0.98 eV and 0.64 eV, respectively) can be used to explain the deviations in experimental results that vary from 1.1 eV to 0.58 eV. We also show that the DFT calculated built-in electric fields in these constructs agree with continuum-level electrostatic analysis based on Maxwell and Poisson's relations, taking into account the first and second order of piezoelectric couplings. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z21.00005: Effects of quantum-well shape and polarization on simulations of InGaN/GaN multi-quantum-well light-emitting diodes Patrick McBride, Qimin Yan, Chris Van de Walle We investigate the effects of different InGaN quantum well (QW) profiles in $c$-plane InGaN/GaN 3-QW blue light-emitting diodes (LEDs) by employing a semi-empirical drift-diffusion model. Our results show that changing the typically assumed square indium profile to one with a smoother interfacial transition leads to a significant modification of the band diagram, carrier overlap, and current-voltage characteristics. In previous works, an \emph{ad hoc} reduction of the polarization field has often been used to generate simulated results that match experiment while the realistic indium profile is not taken into account. However, our results indicate that the indium profile plays an important role in determining the current vs. voltage characteristics of InGaN/GaN heterostructure LEDs. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z21.00006: Plasma assisted molecular beam epitaxy of strain-balanced a-plane InGaN/AlGaN periodic structures Ryan Enck, Nathaniel Woodward, C.S. Gallinat, G.D. Metcalfe, Hongen Shen, Michael Wraback A-plane nitride semiconductors have a tunable anisotropic absorption edge that can be exploited to create a compact, broad spectrum THz radiation detector which leverages fiber lasers operating at telecommunication wavelengths. Incident THz radiation is detected by observing the anisotropic change in the anisotropic absorption in the semiconductor of a femtosecond probe pulse by monitoring the polarization rotation of the probe. The sensitivity of this detection method requires a high quality a-plane wurtzite semiconductor with sufficient thickness to provide a large enough polarization rotation as required by the detection scheme. We report on the growth and characterization of strain balanced InGaN/AlGaN periodic structures on various substrates and buffer layers to obtain thick epilayers while maintaining a large absorption anisotropy. We use x-ray diffraction to determine the strain, composition, degree of relaxation, and thickness of our samples and polarization dependent transmission spectroscopy to measure the anisotropic absorption and polarization rotation in these materials [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z21.00007: Blue and Green light InGaN/GaN Multiquantum-Well grown by plasma-assisted molecular beam epitaxy Chia-Hsuan Hu, Ikai Lo, Cheng-Hung Shih, Wen-Yuan Pang, Cheng-Da Tsai, Yu-Chiao Lin High-efficiency red, green and blue light-emitting diodes (LEDs) can be used in the construction of full color display. We have grown green and blue light InGaN/GaN multiquantum-well (MQW) thin film on sapphire substrate with GaN buffer by using plasma-assisted molecular beam epitaxy. The optical properties of the samples were analyzed by photoluminescence measurement in room temperature. Under constant nitrogen flux condition, we obtained the blue and green emitting bands from different samples by modified the Indium and Gallium flux ratio in MQW. In high nitrogen flux condition, the wavelength shifts to 560nm, which provides an effective way to reach high Indium incorporation LED. In order to improve the quality, we can control the growth temperature and InGaN/GaN thickness. There are more than five order satellite peaks in Double Crystal X-ray Diffraction data. Smooth surface morphology has been verified in our samples by scanning electron microscope. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z21.00008: The epitaxial relationship between M-Plane and c-plane GaN grown on gamma-LiAlO$_{2}$ Ying-Chieh Wang, Ikai Lo, Cheng-Hong Shih, Chia-Hsueng Hu, Cheng-Da Tsai, Shou-Ting You The M-plane and c-plane oriented GaN have been found co-existed in the $\gamma $-LiAlO$_{2}$ substrate grown by molecular beam epitaxy (MBE). Two-step growth with different N/Ga ratios has been used in the experiment at growth temperature 670$^{\circ}$C. By the SEM images, the smooth M-plane surface was observed and the c-plane GaN 3-D structures homogenously spread on it. From the scanning of $\omega $-2$\theta $ X-ray diffraction pattern, the peaks at 32.295$^{\circ}$ and 34.505$^{\circ}$ were attributed to the M-plane and c-plane GaN. The microstructure of the samples was investigated by transmission electron microscopic (TEM) study. It was found that the c-plane and M-plane GaN both were oriented from the substrate with the same growth direction and arranged vertically to the substrate with a periodical pattern. The interfaces between the c-plane and M-plane GaN can be confirmed clearly by one or two monolayer in high resolution TEM images. The stacking faults were found at the edge of c-plane side at the interface that released the dislocation between the M- and c-plane GaN. In summary, we found that the M-plane and c-plane GaN can be assembled on $\gamma $-LiAlO$_{2}$ substrate with a clear phase-transition interface. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z21.00009: Gallium-monochalcogenides mechanically exfoliated at temperatures above room temperature Jose Fonseca Vega, Hui Fang, Ali Javey, Oscar Dubon In recent years, there has been an increased interest toward layered 2D materials beyond graphene. Among these III-VI metal-chalcogenide layered semiconductors are interesting materials for 2D applications as the digitally controlled crystal thickness (by the number of layers) opens a new degree of freedom to tailor electronic properties. In this work, thin layers of GaSe and GaTe were obtained via micromechanical exfoliation and transferred onto SiO2/Si substrates at temperatures ranging from room temperature to 75 C. Exfoliation above room temperature showed a dramatic increase in yield and mean surface area for the exfoliated single-crystalline flakes, 75 C and 50 C being the optimum conditions for GaSe and GaTe, respectively. Few-layer flakes were observed through optical microscopy. It was found that GaTe offered an additional challenge for exfoliation; this was attributed to its monoclinic crystal structure, contrasting GaSe's hexagonal structure. Atomic force microscopy thickness measurements determined the amount of layers in the exfoliated flakes. Micro-Raman and photoluminescence spectroscopy reveal an evolution in properties in these materials as a function of thickness. Results from measurements of field-effect transistors will be presented. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z21.00010: Structural and Electronic Properties in multilayer (BiSe)$_n$(TiSe$_2$)$_m$ Misfit compounds Benjamin Trump, Maxime Siegler, Ken Livi, Tyrel McQueen The nature of the charge density wave (CDW) transition in 1T-TiSe$_2$ has been hotly debated, and variously described as a simple CDW to the formation of an excitonic insulator. Here we report the synthesis and basic physical properties of the incommensurate layered chalcogenides (BiSe)$_n$(TiSe$_2$)$_m$. Their structure consists of a rock-salt type BiSe layer separated by one or more edge-sharing TiSe$_2$ octahedral layer. These octahedral layers are isomorphic to the layers found in 1T-TiSe$_2$, and thus provide a mechanism to study the electronically driven structural transition in TiSe$_2$ as a function of the number of layers. Structural determinations from electron and x-ray diffraction, using 4/5-D superspace approach, will be presented, and the implications of our results on the understanding of CDW formation in TiSe$_2$ will be discussed. The effects of doping with Cu to observe possible superconducting behavior is also explored. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z21.00011: Optically lossless semiconductors reached by means of bichromatic irradiation Adil-Gerai Kussow, Alkim Akyurtlu Non-omhic semiconductors are theoretically studied in the fields of two parametrically coupled electromagnetic waves. A second-order non-linearity due to the non-omhicity couples waves and causes exchange energy between the modes. Based on Maxwell's equations and coherence requirements, it is demonstrated that the optical losses in the probe mode are compensated due to the flow of energy from the support mode. Estimates are made to show that the total loss suppression can be realized in semiconductors with low optical dispersion, e.g. zinc telluride (ZnTe), within the mid-IR to Long Wavelength --IR regime, and the appropriate design for the experimental validation is suggested. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z21.00012: Growth morphology of boron doped single crystal diamond Sunil Karna, Yogesh Vohra The growing demand of wide band semiconductors entice researcher to investigate electronic properties in diamond. The chemical vapor deposition (CVD) method has shown that various level of doping can be possible in diamond films. The purpose of this study was to investigate the growth morphology and quality of boron doped diamond film with deposition parameters. Various level of boron doped diamond films were synthesized epitaxially on synthetic (100) ib type diamond substrate using microwave plasma assisted CVD. The structural, optical and electrical characterizations were made to study effect of deposition parameters and pretreatment of substrates on surface morphology and growth quality. Raman spectra showed shape modification of the zone center optical phonon line and its downshift with the increasing boron content in the film. Additional bands were also observed in lower wavelength region below optical phonon line. Surface modification of films with increasing boron content has been observed in atomic force microscopy. High growth rate and high quality films were obtained with the addition of a few ppm of nitrogen in feed gas during deposition with little compromise on conductivity. Electrical measurement showed carriers have been transported via two different conduction mechanisms. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z21.00013: Growth and Contrast of Hexagonal Boron Nitride: From Submonolayer Islands to Multilayer Films Justin Koepke, Joshua Wood, Eric Pop, Joseph Lyding Strong interest in hexagon boron nitride (h-BN) as a substrate for graphene devices [1] or as a template for growth of other layered compounds [2] has motivated recent attempts to synthesize large scale h-BN by chemical vapor deposition (CVD). We synthesize h-BN by low pressure CVD on polycrystalline Cu foil in a hot wall tube furnace with a heated ammonia borane precursor carried downstream by Ar and H$_{\mathrm{2}}$ gas. Transmission electron microscopy (TEM) diffraction patterns show that the resulting growths are highly crystalline, with several layers obtained for longer growth times. Short growth times show that the h-BN nucleates in triangular islands at a higher precursor temperature than previously reported in [3] and a lower temperature than reported in [4]. In-air calcination of the Cu foils after partial h-BN growth allows optical contrast of the previously transparent h-BN islands on the Cu foil. This observed resistance to oxidation suggests that grown h-BN films can serve as an insulating anti-corrosion layer. \\[4pt] [1] J. Xue, et al., Nat. Mater. 10, 282 (2011).\\[0pt] [2] P. Gehring, et al., Nano Lett. 12, 5137 (2012).\\[0pt] [3] K.K. Kim, et al., \textit{Nano Lett.} 12, 161 (2012).\\[0pt] [4] N. Guo, et al., Nanotechnology. 23, 415605 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z21.00014: MOCVD grown hexagonal BN epilayers for DUV photonics Sashikanth Majety, Jing Li, Jingyu Lin, Hongxing Jiang Hexagonal boron nitride (hBN) has attracted a lot of interest recently owing to its excellent physical properties and its potential use as a template in graphene electronics. We report on the successful growth of hBN epilayers using metal organic chemical vapor deposition (MOCVD) on sapphire and n-AlGaN substrates. P-type conductivity control was also achieved by in-situ Mg doping. This provides us with an opportunity to solve the problem of low quantum efficiency of DUV devices using Al-rich AlGaN alloys due to their extremely low p-type conductivity. Mg doped hBN epilayers grown on insulating templates were p-type with an in-plane resistivity of 2.3 $\Omega $ cm. Diode behavior in the p-n structures of p-hBN/n-Al$_{0.62}$Ga$_{0.38}$N has been demonstrated. Our results indicate that hBN epilayers have potential for DUV optoelectronic devices and also demonstrate the feasibility of using highly conductive p-type hBN as electron blocking and p-contact layers for AlGaN based deep UV emitters. This work is supported by DOE. [Preview Abstract] |
Session Z22: He3 - He4 Quantum Fluids
Sponsoring Units: DCMPChair: Benjamin Hunt, Massachusetts Institute of Technology
Room: 324
Friday, March 22, 2013 11:15AM - 11:27AM |
Z22.00001: Chiral Phases of Superfluid $^3$He in an Anisotropic Medium James Sauls I report theoretical results for the phases of superfluid $^3$He infused into homogeneous uniaxial aerogel. Ginzburg-Landau (GL) theory for a class of equal-spin-pairing (ESP) states in a medium with uniaxial anisotropy is developed and used to analyze recent experiments on uniaxially strained aerogels. For $^3$He in an axially ``stretched'' aerogel GL theory predicts a transition from normal liquid into a \emph{chiral} ABM phase in which the chirality axis is aligned along the strain axis. This state is protected from random fluctuations in the anisotropy direction, has a positive NMR shift, a sharp NMR resonance line and is in quantitative agreement with NMR in the high-temperature ESP-1 phase of superfluid $^3$He in axially stretched aerogel. A second transition into a bi-axial phase is predicted to onset at a slightly lower temperature. This phase is an ESP state, breaks time-reversal symmetry, and is defined by an order parameter that spontaneously breaks axial rotation symmetry. The bi-axial phase has a continuous degeneracy associated with broken axial symmetry. Theoretical predictions for the NMR frequency shifts provide an identification of the ESP-2 phase as the bi-axial state, partially \emph{disordered} by random anisotropy (Larkin-Imry-Ma effect). [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z22.00002: Topological current at an interface between superfluid $^3$He A- and B-phases Yasumasa Tsutsumi At a surface of the superfluid $^3$He, the surface Andreev bound state accompanied with edge current emerges due to a topological phase transition. The topological phase transition at the surface is occurred because the superfluid gap of the superfluid $^3$He among topological superfluids is closed at the interface of a topologically trivial vacuum. Since the paring symmetries are different between the superfluid $^3$He A- and B-phases, topological features are quite different between the A- and B-phases. The A-phase is a chiral superfluid with the spontaneous edge mass current while B-phase is a helical superfluid with the spontaneous edge spin current. At an interface between the A- and B-phase, a topological phase transition is also occurred because they belong in a different topological classification . Then, based on the quasiclassical Eilenberger theory, we discuss topological mass and spin current carried by the bound state at an interface between the A- and B-phases. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z22.00003: Nanofabricated cells for confined $^3$He Nikolay Zhelev, Robert Bennett, Rob Ilic, Jeevak Parpia, Lev Levitin, Andrew Casey, John Saunders We describe methods for fabrication of Silicon-Glass and all-silicon cells with a height specified to be between 100nm and 1100nm, and with areas on the order of cm x cm. These cells need to meet different requirements, including pressure capability to 30 bar with minimal distortion, and surface roughness which can be characterized and modified as needed to alter the transport characteristics of the confined $^3$He. The cells are suitable for NMR and Torsion Oscillator measurements on the superfluid phases of $^3$He. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z22.00004: Study of Liquid $^{3}$He Films with MEMS Devices Pan Zheng, Miguel Gonzalez, Yoonseok Lee, Ho Bun Chan Liquid $^{3}$He films with thicknesses of 0.75 and 1.25 $\mu$m were established and probed by micro-electro-mechanical (MEMS) resonators each of which consists of a pair of parallel plates with a well defined separation. The mechanical resonances of the devices immersed in liquid $^{3}$He were studied in a wide range of temperatures from 10 to 800 mK and at sample pressures of 3, 21, and 29 bar. A crossover from Fermi liquid to classical fluid was observed on warming. In the Fermi liquid regime, the damping coefficient associated with the film exhibits an unexpected temperature dependence below 100 mK. This work demonstrates the capacity of MEMS devices as sensitive probes suitable for the study of quantum fluids in a micrometer scale. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z22.00005: Spin and mass currents on the surface of the topological superfluid, $^3$He-B Hao Wu, James Sauls The surface excitation spectrum of superfluid $^3$He-B is discussed for a translationally invariant interface (specular surface). We report calculations of surface spectral spin-current and mass current densities originating from the Andreev bound state and the continuum response. Two branches of gapless Fermions, bound to the surface, disperse linearly with momentum $\vec{p}_\parallel$ along the surface. These states are spin polarized transverse to their direction of propagation, $\vec{p}_\parallel$. The spectral functions reveal the subtle role of the spin-polarized surface states in relation to the ground-state spin current. By contrast, these states do not contribute to the ground-state mass current density. However, the surface states do give rise to a power law suppression of the superfluid mass current for $0 \ll T \ll T_c$, providing a direct signature of the Majorana branches of surface excitations in the fully gapped 3D topological superfluid, $^3$He-B. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z22.00006: Observation of the Larkin-Imry-Ma Effect in Superfluid $^3$He-A in Aerogel J.I.A. Li, J. Pollanen, A.M. Zimmerman, C.A. Collett, W.J. Gannon, W.P. Halperin It was proposed by Volovik that $^3$He-A in aerogel will be a superfluid glass owing to the Larkin-Imry-Ma (LIM) effect where arbitrarily small amounts of disorder can disrupt long range directional coherence of a vector order parameter in a condensed system. Several reports of NMR experiments in $^3$He-A have been interpreted as evidence for this phenomenon. However it is not trivial to distinguish the LIM effect induced from disorder on a microscopic scale from macroscopic non-uniformity or anisotropy in the aerogel sample. Order parameter disorder from these two possible mechanisms have very different distributions of order parameter orientations directly observable in the width of the NMR spectrum. If a complete LIM effect is operative there should be no contribution to the line width, contrary to previous reports. On warming from the low temperature isotropic state, we find NMR spectrum shifts characteristic of the dipole-locked axial state, i.e. no sign of a LIM superfluid glass. However, on cooling from the normal state this same phase is fully disordered in a LIM state. We will discuss the origin of the different order parameter structures in superfluid $^3$He-A that result when prepared from the normal state, as compared with warming from the B-phase. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z22.00007: Phase transitions and critical currents in superfluid $^3$He films Anton Vorontsov, James Sauls Using the quasiclassical theory of superfluidity we investigate thermodynamic and transport properties of superfluid $^3$He in confined geometries. Classic flow experiments, as well as more recent NMR and flow experiments on superfluidity in slab and film geometries, exhibit inconsistencies between experimental results and existing theoretical models of confinement effects. In order to explain the origin of some of these inconsistencies we describe a theoretical model for confinement effects based on scattering of quasiparticles from rough surfaces that is more general than the `specular' and `diffusive' scattering models. Using the this more general boundary scattering model we report theoretical results for (a) the suppression of the superfluid critical temperature $T_c^{film}$, (b) the confinement-driven transition between A and B phases, $T_{AB}$, and (c) effects of the surface roughness on the critical current. The new scattering model should provide a more complete framework for analysis of the properties of confined superfluid $^3$He. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z22.00008: Unusual Behavior of a MEMS Resonator in Superfluid $^{4}$He Miguel Gonzalez, Pan Zheng, Byoung Hee Moon, Erik Garcell, Yoonseok Lee, Ho Bun Chan Mechanical resonators based on micro-electro-mechanical systems (MEMS) technology were developed for the study of superfluid $^{4}$He [1]. The MEMS device is composed of a movable plate (200 $\times$ 200 $\mu$m$^{2})$ suspended above the substrate by four serpentine springs. The suspended plate moves parallel to the substrate while maintaining a uniform gap between them. A specific device with a 1.25 $\mu$m gap was tested in the superfluid phase of $^{4}$He down to 100 mK. The device exhibits an extreme sensitivity to the excitation level below 400 mK, displaying a nonlinear and hysteretic behavior accompanied by switching. This phenomenon might be related to quantum turbulence generated by a rather simple oscillating plate.\\[4pt] [1] M. Gonzalez, B. Moon, P. Zheng, E. Garcell, H. B. Chan, and Y. Lee. \textit{Journal of Low Temperature Physics, Online First}$^{TM}$\textit{, 22 August 2012}, DOI: 10.1007/s10909-012-0682-8. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z22.00009: Superfluid helium-4 in one dimensional channel Duk Y. Kim, Samhita Banavar, Moses H. W. Chan, John Hayes, Pier Sazio Superfluidity, as superconductivity, cannot exist in a strict one-dimensional system. However, the experiments employing porous media showed that superfluid helium can flow through the pores of nanometer size. Here we report a study of the flow of liquid helium through a single hollow glass fiber of 4 cm in length with an open id of 150 nm between 1.6 and 2.3 K. We found the superfluid transition temperature was suppressed in the hollow cylinder and that there is no flow above the transition. Critical velocity at temperature below the transition temperature was determined. Our results bear some similarity to that found by Savard {\it et. al.} [1] studying the flow of helium through a nanohole in a silicon nitrite membrane.\newline [1] M. Savard, G. Dauphinais, and G. Gervais, Phys. Rev. Lett. {\bf107}, 254501 (2011) [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z22.00010: Effect of Helium on Vycor Glass: Anomalous Thermal Conductivity Reduction Zhigang Cheng, Samhita Banavar, Moses H. W. Chan There is a long history of studying helium adsorbed in Vycor. In this talk we present the results showing that helium can have a profound effect on the thermal conductivity property of Vycor glass. Although the thermal conductivity of liquid $^4$He is four orders of magnitude higher than that of Vycor, the filling of liquid $^4$He inside the Vycor pores brings about a three-fold reduction of the thermal conductivity as compared with empty Vycor between 0.06 and 0.5 K. By comparing these results with that of superfluid films, liquid $^3$He and solid helium in the Vycor pores, we found that heat is conducted primarily through the silica network even when the pores are filled with solid or liquid helium. The dramatic reduction is brought about by the presence of slow sound mode in liquid $^4$He that greatly facilitates the quantum tunneling of the two level systems (TLS) in the silica which enhances the scattering of the thermal phonons. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z22.00011: Observation of a New Casimir Effect in Saturated Superfluid $^4$He Films John Abraham, Gary Williams, Konstantin Penanen We report the results of experiments on saturated superfluid $^4$He films in the vicinity of the bulk superfluid transition temperature $T_\lambda$, measuring the film thickness with a capacitance technique and the superfluid density with third sound. For moderately slow temperature sweep rates (0.5 mK/hr) we measure the critical Casimir film-thinning effect with good resolution, and find that the Kosterlitz-Thouless superfluid onset in the film occurs just at the start of the dip in film thickness. When warming through $T_\lambda$ at extremely slow rates (a few $\mu$K/hr), however, we have observed a sudden large increase in the film thickness (nearly 25 {\AA} in a film initially 480 {\AA} thick) within microkelvins of $T_\lambda$. We propose that this is a new type of Casimir effect arising from the viscous suppression of second sound modes in the film, leading to a large free energy difference in the superfluid state that disappears abruptly when second sound ceases to propagate in the bulk helium at $T_\lambda$. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z22.00012: Cubic interactions in superfluid $^4$He Bjorn F{\aa}k, Thomas Keller, Michael Zhitomirsky, Alexander Chernyshev High-resolution neutron resonance spin-echo measurements of superfluid $^4$He show that the roton energy does not have the same temperature dependence as the inverse lifetime, in contrast to the Landau-Khalatnikov theory. We present a diagrammatic analysis that attributes this effect to the interaction of rotons with thermally excited phonons via both four- and three-particle processes, the latter being allowed by the broken gauge symmetry of the Bose condensate. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z22.00013: Photon-Roton Modes in Liquid $^4$He coexist with Bose-Einstein Condensation Henry R. Glyde, Jacques Bossy, Jacques Ollivier, Helmut Schober We present neutron scattering measurements of the phonon-roton (P-R) and layer modes of liquid $^4$He confined in MCM-41 under pressure up to 38 bar. The data shows unambiguously that the P-R mode exists at low temperature only. As temperature is increased there is a gradual transfer of intensity from the P-R mode to the normal liquid response, which lies at a lower energy at higher pressure. The transfer takes place with no observable mode broadening. The loss of P-R modes is identified with the loss of Bose-Einstein condensation (BEC). The mode giving rise to the specific heat, $c_V$, of liquid $^4$He in porous media (e.g. gelsil) at higher temperature is the layer mode since the energy of the mode extracted from $c_V$ and the layer mode energy are the same. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z22.00014: Helium-4 crossover from a 3d superfluid to a 1d Luttinger liquid in a nanopore Bohdan Kulchytskyy, Adrian Del Maestro, Guillaume Gervais Quantum Monte Carlo studies of helium-4 below the bulk superfluid transition temperature show that when it is confined to flow in narrow cylindrical pores with nanometer radii, it tends to form concentric shells around a possible inner core. The latter potentially represents an experimental playground for exploring the implications of Luttinger liquid theory for one dimensional quantum fluids. We have performed large scale numerical simulations investigating the crossover from a bulk three dimensional superfluid to a one dimensional Luttinger liquid as the nanopore radius is reduced at low temperature. Measurements of the superfluid density employing both stiffness and angular momentum estimators provide new insights into confinement induced fluctuation effects in strongly interacting quantum fluids. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z22.00015: Metastable pin sites for a superfluid vortex Rena Zieve, Ingrid Neumann Circulation trapped around a straight, fine wire can be detected through its effect on the wire's vibration. Here we use such a wire in a cylindrical cell to examine pinning of a superfluid helium vortex line at a macroscopic bump. Hydrodynamic considerations imply that, as long as the fluid velocity is fixed and not too large, a vortex can pin at a unique place on the bump. However, for two separate geometries we find that the vortex has metastable locations both at the apex of the bump and near its edge. In one case, the vortex is trapped around the wire, which terminates in the center of a bump on the cylinder endcap. We find that the vortex can follow the entire length of the wire to the bump apex, or it can leave the wire and make its way through the fluid to the edge of the bump. The former situation is more stable, but the latter can also persist for long times. The second geometry involves a free vortex that extends from the wire to a bump on the cylindrical wall of the container. Again our measurements show pinning at multiple sites on the bump. Interaction of the vortex with the surface curvature may produce the unexpected additional pin sites. [Preview Abstract] |
Friday, March 22, 2013 2:15PM - 2:27PM |
Z22.00016: Recent Progress in Low-Temperature Research from the Davis Lab at the University of Alberta John P. Davis, Xavier Rojas, Yikai Yang, Andrej Duh, Greg Popowich In this talk I will briefly describe our recent progress towards new low-temperature experiments at the University of Alberta in the Davis Lab. We are currently setting up two nuclear demagnetization fridges - one new cryostat that has two independent 9 T magnets (the second magnet being useful for a double demag stage or combined high field and low temperature experiments). The other fridge is an older unit that is extensively refurbished, with all new pumping systems. We are planning numerous experiments at the intersection of low-temperature physics and nanoscience, including quantum properties of nanomechancial resonators and quantum fluids in confined geometries. Concerning the latter, we have fabricated high quality microfluidic devices suitable for low-temperature research. We will discuss our progress towards quantum fluids measurements using these devices. [Preview Abstract] |
Session Z23: Semiconductors: Thermodynamic & Optical Properties II
Sponsoring Units: FIAPRoom: 325
Friday, March 22, 2013 11:15AM - 11:27AM |
Z23.00001: Excitons in Ultrathin PbI$_{2}$ Crystals Alexis Toulouse, Benjamin Isaacoff, Guangsha Shi, Emmanouil Kioupakis, Marie Matuchov\'{a}, Roberto Merlin Due to their weak inter-layer van der Waals bonding, layered materials offer the unique possibility to produce natural quantum wells in the form of single and few atomic layer samples. A technique known as micromechanical cleavage, involving repeated cleaving, is used to isolate samples of all thicknesses [1]. Here, we present a combined experimental and theoretical study of band-edge excitons in the layered compound PbI$_{2}$ and, in particular, on their behavior as a function of sample thickness. Results of photoluminescence and reflection experiments are reported on samples with thicknesses ranging from a few micrometers down to a few monolayers, as determined by atomic force microscopy measurements. The data display striking and well reproducible changes in the transition from three to two-dimensions, which will be compared with results of first-principles calculations of the electronic band structure based on density functional and many-body perturbation theory. Computational resources were provided by the DOE NERSC facility. [1] A. K. Geim et. al. Nature Materials 6, 183 (2007) [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z23.00002: Second harmonic generation from few-layer MoS2 and BN Yilei Li, Yi Rao, Kin Fai Mak, Yumeng You, Shuyuan Wang, Tony Heinz We have measured optical second harmonic generation (SHG) from few-layer MoS2 and hexagon BN samples. In both materials, we observe SHG for odd numbers of layers. However, no appreciable SHG signal is observed for samples with even numbers of layers. This general behavior is compatible with the fact that individual layers of each material are non-centrosymmetric, thus allow SHG in the dipole approximation. For even layer thickness, on the other hand, the overall structures become centrosymmetric, with adjacent layers producing canceling contributions. In the case of odd layer thickness, we observe strong in-plane variation of the SHG signal with polarization. From this dependence, we can determine crystallographic axes in the material by a purely optical measurement. We also discuss the evolution of the signal strength in the two material systems with thickness (for odd layer numbers), considering both propagation effects and the evolution of the electronic structure of the material with thickness [1].\\[4pt] [1] Kin Fai Mak, Changgu Lee, James Hone, Jie Shan and Tony F. Heinz, Phys. Rev. Lett, \textbf{105}, 136805, 2010 [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z23.00003: Low Temperature Optical Spectroscopy of Excitons and Trions in Monolayer MoS2 Changjian Zhang, Haining Wang, Farhan Rana Monolayer MoS2 is a two-dimensional (2D) semiconductor with optical properties different from conventional inorganic semiconductors. We will present our results on low temperature absorption and photoluminescence (PL) spectroscopy of monolayer MoS2 crystals. As a result of the large carrier effective masses and low dielectric screening in the 2D geometry, the excitons in MoS2 are tightly bound with large binding energies. We find that the prominent peak at $\sim$1.9 eV in both PL and absorption spectra at low temperatures is split in two: an exciton peak and a trion peak. The binding energies of trions, measured relative to the excitons, are extremely large and in the 30-35 meV range. We find that the trion peak acquires more spectral weight than the exciton peak as the electron density increases, and also broadens due to increased scattering with electrons. The temperature dependence of the exciton and trion PL intensities enables us to determine the radiative recombination efficiencies as a function of the temperature. We also observe Stokes shifts of $\sim$5 meV of both exciton and trion peaks, indicative of lattice distortions accompanying the quasiparticles (i.e. polarons) in this highly polar crystal. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z23.00004: Ultrafast Optical Pump-Probe Studies of Photoexcited Carrier Dynamics in Mono-Layer MoS2 Haining Wang, Changjian Zhang, Farhan Rana Mono-layer MoS2 is a 2D semiconductor with a direct bandgap. We present, for the first time, ultrafast optical pump-probe measurements results on the relaxation dynamics of photoexcited carriers in monolayer MoS2. Pulses at energies 2.74 eV and 1.37 eV with width ~100 fs are used in our experiments. The pump photon energy is larger than the bandgap, and the probe is below the optical absorption edge. Our results show that the differential transmission of the probe is negative with three distinct features: i) an initial probe absorption due to two-photon absorption involving also the pump pulse ii) a fast relaxation transient lasting to about ~1ps in which the differential transmission recovers by almost ~90\%. iii) a very slow recovery of the transient that lasts about 200ps. We explain the observed transients in terms of the relaxation of the carriers to the conduction band bottom, followed by the formation of excitons and trions, and the response of the excitons and trions. The extremely large exciton and trion binding energies make these states much more preferable than the free carrier states in the bands. The contributions of interband and intraband processes to the observed transients will be explained and the extracted relaxation and recombination rates will be discussed. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z23.00005: Optical behavior of native defects in ZnO John L. Lyons, Daniel Steiauf, Audrius Alkauskas, Anderson Janotti, Chris G. Van de Walle The behavior of native defects in ZnO has been fiercely debated for years, yet questions still remain regarding their fundamental properties. Once blamed for causing unintentional $n$-type conductivity, it is now well-established that native donors are highly unlikely to act as shallow donors in as-grown material. Still, both native donors and acceptors may be present in some samples, acting as either compensating acceptors or deep donors that may inhibit attempts to obtain high-conductivity $n$-type ZnO. In this work, we re-examine the properties of native donors and acceptors in ZnO using hybrid density functional calculations, which allow for the quantitative prediction of defect transition levels and formation energies. We focus on the optical and electrical properties of these defects, and calculate both their optical and thermodynamic transition levels. Most of the defects give rise to deep, broad luminescence signals that can serve as a means of experimentally verifying the nature of the center. We also examine how interactions with hydrogen interstitials affect the properties of these defects. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z23.00006: Strain-gradient dominated emission energy shift of pure-bending ZnO wire Dapeng Yu, Xuewen Fu, Xiaobing Han, Qiang Fu, Wanlin Guo, Zhuhua Zhang High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain gradient from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fine excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. Strain-gradient is found to dominate the overall red-shift of the emission energy at a pure bending configuration. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z23.00007: Electronic structure and optical properties of CuYO$_2$ nanocrystals Muhammad Huda, Yanfa Yan, John A. Turner, Mowafak M. Al-Jassim A unique class of highly stabile, self-saturated and self-charge-compensated delafossite nanocrystals has been identified. The density functional theory (DFT) study of structural and electronic properties of these nano-crystalline CuYO$_{2}$ will be presented. To have a better estimate of the electronic excitation energies, and consequently the optical gap, time dependent DFT has been employed as well. The goal is to show, first of all, that these unique set of nanocrystals exists, and to study whether the nano-phase can modify the electronic properties for enhanced optical absorption. It has potential application as photocatalysts for H$_{2}$ production by water splitting. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z23.00008: Contactless Electroreflectance Characterization of a Triple Asymmetric Coupled Quantum Well Active Region of a ZnCdMgSe-Based Quantum Cascade Laser Joel De Jesus, Thor Garcia, Siddharth Dhomkar, Arvind Ravikumar, Claire Gmachl, Aidong Chen, Maria Tamargo Quantum cascade lasers (QCL) with emission at wavelengths below 4$\mu $m are difficult to achieve from conventional GaAs and InP based systems due to the small conduction band offset (CBO) of those materials. The II-VI materials ZnCdSe/ZnCdMgSe, with as much as 1.1 eV CBO and no inter-valley scattering, are excellent candidates to achieve this goal. We grew by MBE a QCL structure made of ZnCdSe and ZnCdMgSe lattice matched to InP. Triple asymmetric coupled quantum well (3ACQW) structures were also grown which contain only the active region of the QCL separated by quaternary barrier layers. The 3ACQW structure was characterized by contactless electroreflectance (CER). A model based on the transfer matrix method (TMM) was used to identify the CER transitions and to predict the Fourier transform infra-red (FT-IR) absorption spectrum of the full QCL structure. Excellent agreement between the predicted and the experimental FT-IR absorption peaks was observed. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z23.00009: Design of N-doped anatase TiO$_{2}$ photocatalyst with visible-light-response based on Ti-O bond weakening L.-C. Yin, G. Liu, H.-M. Cheng Nitrogen bulk doping is an effective strategy to change the electronic structures of anatase TiO$_{2}$ photocatalyst for visible light response improvement. Unfortunately, it is hard to achieve nitrogen bulk doping in practice, due to both limited thermodynamic solubility of substitutional nitrogen and N-induced recombination centers. It remains challenging yet highly desirable to develop new doping approach to increase nitrogen solubility in bulk. This challenge is originally stemmed from both strong Ti-O bond and charge difference (O$^{2-}$ versus N$^{3-})$ between lattice oxygen and nitrogen dopant. In this work, we propose a new doping approach to promote the bulk substitution of lattice oxygen with nitrogen in bulk anatase TiO$_{2}$, based on the Ti-O bond weakening by pre-implanted interstitial boron.$^{1}$ By using the first-principles calculations, we study the interstitial boron induced Ti-O bonding weakening and the thermodynamics/kinetics changes for nitrogen bulk doping.$^{2}$ In experiment, we realize to synthesize a bulk gradient B-N co-doping red anatase TiO$_{2}$ microsphere which has an extended absorption edge up to ca. 700 nm covering the full visible light spectrum and has a bandgap varying from 1.94 eV on its surface to 3.22 eV in its core by gradually elevating VBM. This approach could be extended to modify other electronic materials that demand bulk substitutional doping. 1. G. Liu, J. Pan, L. C. Yin et al., Adv. Funct. Mater., 2012, 22, 3233. 2. G. Liu, L. C. Yin, J. Q. Wang et al., Energy Environ. Sci. 2012, 5, 9603. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z23.00010: Ultrafast Time- and Phase-Resolved Second Harmonic Generation James McIver, Changmin Lee, Darius Torchinsky, Nuh Gedik Ultrafast pump-probe experiments typically measure the changes in the amplitude of the probe light after it interacts with a sample as a function of pump time delay. However, measured amplitude change is typically a result of multiple processes happening in the sample. It is usually not possible to isolate these different processes from the measured amplitude response. Here we show using GaAs and Bi2Se3 as test samples that by probing interferometrically, phase information about the pump-probe signal can also be acquired. We find that different components of the signal in general have different optical phases associated with them, which can be isolated by changing the path length difference within the interferometer. We show that phase information about second harmonic light generated by the sample can also be obtained and we report progress toward realizing simultaneous~phase- and time-resolved second harmonic pump-probe measurements. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z23.00011: First-principles study of $\gamma$-ray detector materials in perovskite halides Jino Im, Hosub Jin, Constantinos C. Stoumpos, Duck Young Chung, Zhifu Liu, John A. Peters, Bruce W. Wessels, Mercouri G. Kanatzidis, Arthur J. Freeman In an effort to search for good $\gamma$-ray detector materials, perovskite halide compounds containing heavy elements were investigated. Despite the three-dimensional network of the corner shared octahedra and the extended nature of the outermost shell, its strong ionic character leads to a large band gap, which is one of the essential criteria for $\gamma$-ray detector materials. Thus, considering high density and high atomic number, these pervoskite halides are possible candidate for $\gamma$-ray detector materials. We performed first-principles calculations to investigate electronic structures and thermodynamic properties of intrinsic defects in the selected perovskite halide, CsPbBr$_3$. The screened-exchange local density approximation scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the calculated band gap of CsPbBr$_3$ is found to be suitable for $\gamma$-ray detection. Furthermore, defect formation energy calculations allow us to predict thermodynamic and electronic properties of possible intrinsic defects, which affect detector efficiency and energy resolution. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z23.00012: Temporal long-range order in exciton-polariton condensates Alex Hayat, Christoph Lange, Lee Rozema, Rockson Chang, Shreyas Potnis, Henry van Driel, Aephraim Steinberg, Mark Steger, David Snoke, Loren Pfeiffer, Kenneth West We demonstrate temporal long-range off-diagonal order in microcavity exciton-polariton dynamic condensation, by interference between two separate condensates, generated at different times and with different momenta. In our pulsed resonantly-injected condensates, stimulated polariton-polariton scattering results in spectral narrowing of the dynamic condensates and thus in longer coherence times. We study the temporal decay of the long-range order by monitoring the interference visibility between the condensates. We show that it strongly depends on the excitonic fraction of the polaritons and the corresponding polariton-polariton interaction strength, as well as on the temperature and pump intensity. Moreover, polariton interaction yields a blue shift of the condensate energy, which appears as a time-dependent shift in the interference pattern. These results show a direct evidence of temporal long-range order in dynamic condensates as well as demonstrate a new method for probing their ultrafast dynamics, opening new directions in the fundamental study of coherence in matter. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z23.00013: Statistics of Data Fitting: Flaws and Fixes of Polynomial Analysis of Channeled Spectra William Karstens, David Y. Smith Starting from general statistical principles, we have critically examined Baumeister's procedure* for determining the refractive index of thin films from channeled spectra. Briefly, the method assumes that the index and interference fringe order may be approximated by polynomials quadratic and cubic in photon energy, respectively. The coefficients of the polynomials are related by differentiation, which is equivalent to comparing energy differences between fringes. However, we find that when the fringe order is calculated from the published IR index for silicon* and then analyzed with Baumeister's procedure, the results do not reproduce the original index. This problem has been traced to 1. Use of unphysical powers in the polynomials (e.g., time-reversal invariance requires that the index is an even function of photon energy), and 2. Use of insufficient terms of the correct parity. Exclusion of unphysical terms and addition of quartic and quintic terms to the index and order polynomials yields significantly better fits with fewer parameters. This represents a specific example of using statistics to determine if the assumed fitting model adequately captures the physics contained in experimental data. The use of analysis of variance (ANOVA) and the Durbin-Watson statistic to test criteria for the validity of least-squares fitting will be discussed. *D.F. Edwards and E. Ochoa, Appl. Opt. 19, 4130 (1980). [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z23.00014: Resolving sub-phonon wavelength superlattices using photoacoustic spectroscopy Jeremy Curtis, Andrew Steigerwald, John Reno, David Hilton, Norman Tolk We have investigated the coherent acoustic phonon spectroscopic response of a mutilayer GaAs/Al$_x$Ga$_{1-x}$As/AlAs heterostructure with varying layer thickness and Al concentration. The optical response shows a low frequency effective Brillouin oscillatory response and an additional time-dependent change in reflectivity arising from the multilayer features. We can also resolve structural features less than the spatial width of the acoustic strain pulse. We model the optical response from each feature and develop general criteria of the layer thickness and the acoustic strain width that determine the total response. This allows us to determine whether the optical response of a given layer will be superimposed on the overall Brillouin response or will provide an individual Brillouin response from within the layer. Our results help provide a basis for a nondestructive method of determining material properties in stratified media. [Preview Abstract] |
Session Z24: Quantum Many-Body Systems and Methods II
Sponsoring Units: DCOMPChair: Jia-An Yan, Towson University
Room: 326
Friday, March 22, 2013 11:15AM - 11:27AM |
Z24.00001: Universal properties of the Higgs mode near quantum critical points Snir Gazit, Daniel Podolsky, Assa Auerbach Spontaneous symmetry breaking of relativistic models with $O(N)$ symmetry results in the emergence of two elementary excitations: the Goldstone modes and the Higgs mode. The massive Higss mode can decay into pairs of Goldstone modes, broadening the spectral line and hence questioning its visibility. Recently a set of \emph{scalar} response functions was introduced, in which the Higgs mode appears as a well defined peak [1]. We investigate the universal properties of the scalar susceptibility near the quantum critical point in 2+1 dimensions for $N=2$ and $N=3$ using Monte Carlo simulation. We demonstrate that the scalar spectral function contains a peak associated with the Higgs mode, which remains well-defined even upon approach to the critical point. We extract properties that characterize the Higgs peak, including the fidelity of the peak and the ratio $\omega_H/\Delta$ between the Higgs energy on the ordered side and the single particle gap on the disordered side. The universal nature of these results make them relevant to a broad range of experiments in condensed matter and atomic systems.\\[4pt] [1] D. Podolsky, A. Auerbach, and D. P. Arovas, Phys. Rev. B 84, 174522 (2011) [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z24.00002: Deconfined quantum criticality in bipartite SU($N$) antiferromagnets in two dimensions Matthew S. Block, Ribhu K. Kaul The theory of deconfined quantum criticality shatters the celebrated paradigm of the Landau-Ginzburg-Wilson description of phase transitions by allowing for direct, continuous, quantum phase transitions between conventional, ordered phases that spontaneously break fundamentally different symmetries of the system. In this talk, I will present new results of a quantum Monte Carlo study of a local, SU($N$) symmetric, antiferromagnetic spin model on the honeycomb and anisotropic rectangular lattices. In particular, I will show evidence for the existence of a continuous phase transition separating conventional N\'{e}el and valence bond solid ordered phases, as well as comparisons of the extracted critical exponents for sufficiently large values of $N$ to those calculated analytically via a $1/N$ expansion solution of the CP$^{N-1}$ gauge field theory that is believed to accurately describe the behavior at the critical point. In combination with previous results of a similar study on the square lattice, this allows for a robust understanding of how the existence of deconfined quantum criticality depends on the lattice symmetries as a function of $N$, and therefore gives a complete picture of the phenomenon in bipartite SU($N$) systems in two dimensions. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z24.00003: Valence bond solid order and phase transitions of honeycomb lattice models Kenji Harada, Haruhiko Matsuo, Takafumi Suzuki, Synge Todo, Naoki Kawashima We investigate the ground states of generalized SU($N$) Heisenberg models on honeycomb lattices. From large-scale quantum Monte Carlo simulations, we confirm the columnar valence bond solid (c-VBS) orders for $N \ge 5$ at low temperatures, which corresponds to Kekul\'e distortion. It is consistent with Read and Sachdev's prediction[N.~Read and S.~Sachdev, Phys Rev B {\bf 42}, 4568 (1990)]. If we introduce the designed six-body interactions on hexagonal plaquettes, the c-VBS order occurs even in the SU(2) case. While the c-VBS state on a square lattice breaks $Z_4$ rotational symmetry, $Z_3$ rotational symmetry breaks on a honeycomb lattice. The difference may changes the nature of c-VBS phase. In particular, we will report phase transitions from a c-VBS phase to a paramagnetic or N\'eel phase in details. These results give us insight for deconfinement critical phenomena. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z24.00004: Exotic quantum criticality in triangular lattice anti-ferromagnets Ribhu Kaul We introduce and study a generalized sign-problem free quantum anti-ferromagnet on the triangular lattice. Our Hamiltonian is shown to be a natural generalization of the popular bipartite SU($N$) anti-ferromagnet to non-bipartite lattices. At $N = 2$ our model is unitarily equivalent to a model of an XY superfluid (SF). Consistent with a large-N mapping to a certain quantum dimer model, we find evidence for valence bond solid (VBS) order with a large $\sqrt{12} \times\sqrt{12}$ unit cell. We show that there is a direct transition between these two phases that takes place between $N = 11$ and $N = 12$. For $N = 10, 11$ we use a four spin coupling parameter to tune through a new exotic ``deconfined'' continuous transition between SF and VBS. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z24.00005: Efficient computation of GW energy level corrections for molecules described in a plane wave basis Bruno Rousseau, Jonathan Laflamme Janssen, Michel C\^ot\'e An efficient computational approach is presented to compute the ionisation energy and quasiparticle band gap at the level of the GW approximation when the Hilbert space is described in terms of plane waves. The method relies on ab initio calculations as a starting point. Then, the use of the Sternheimer equation eliminates slowly convergent sums on conduction states. Further, the Lanczos method is used to efficiently extract the most important eigenstates of the dielectric operator. This approach avoids the explicit computation of matrix elements of the dielectric operator in the plane wave basis, a crippling bottleneck of the brute force approach. The method is initially applied to organic molecules of current interest in the field of organic photovoltaics. Given the completeness of the plane wave basis, systematic convergence studies can be conducted. Furthermore, the method can readily be extended to describe polymers, which are also of interest for photovoltaic applications, but remain a significant computational challenge for methods based on localized basis sets. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z24.00006: Thermodynamics of the 2D t-J Model William Putikka Very accurate calculations for the temperature dependence of the energy of the 2D Heisenberg AF on a square lattice have been done recently. By combining the results of these calculations with the known low temperature behavior of the Heisenberg entropy and results from high temperature series expansions at higher temperatures the Heisenberg entropy can be accurately calculated for all temperatures. This allows the Heisenberg entropy to be used as a known quantity in the calculation of the doped t-J model entropy. The high temperature series for the entropies of the t-J, Heisenberg and spinless fermion models can be combined as $S_{tJ}-S_{AF}(J^*)- S_{SF}(n^*)$ to produce a small difference which can then be extrapolated to low temperatures. Here $S_{AF}(J^*)$ is the Heisenberg entropy evaluated at a shifted value of $J$ and $S_{SF}(n^*)$ is the spinless fermion entropy evaluated at a modified density. By choosing $J^*$ and $n^*$ appropriately very good convergence for the series of the entropy differences can be obtained. The final t-J entropy is then found by readding the known functions $S_{AF}(J^*)$ and $S_{SF}(n^*)$. The integrated entropy is then fit to the high temperature free energy to find the ground state energy and the full temperature dependent free energy. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z24.00007: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z24.00008: Description of renormalization effect of multiband systems and its application within CMRA theory Jun Liu, Yongxin Yao, Chen Liu, Wencai Lu, Cai-Zhuang Wang, Kai-Ming Ho Many interesting physical phenomena, especially those observed in strongly correlated systems, incur a multiband description. A relatively accurate description of these systems is very important to clarify the origin of the observed physics. The recently proposed correlated matrix renormalization approximation (CMRA) introduces a new route to address this problem. As a variational approach, it makes use of possible renormalizations on the density matrix to correctly absorb effects resulting from strong electron-electron interactions. It performs quite well on different H systems. However, the generalization to multiband cases can be nontrivial. In this talk, I will discuss about how renormalization effects can be incorporated into the density matrix in the multiband case, and show the performance of the resulting CMRA on different dimer systems. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z24.00009: Continuum limits of 12 flavor QCD Yannick Meurice |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z24.00010: Condensation of Anyons in Frustrated Quantum Magnets Rolando Somma, Cristian Batista One dimensional quantum magnets can realize exotic states of matter such as Luttinger liquids, valence bond solids, and spin supersolids. A unique feature of 1D systems is that transmutations of particle statistics preserve the range and local nature of interactions. This is the main reason behind the success of spin-fermion transformations, such as the Jordan-Wigner mapping, for solving 1D quantum magnets. A simple generalization of such transformations allows for a mapping between spins and anyons, unusual particles that generalize the concepts of bosons and fermions. By exploiting this generalization, in this talk we will present the exact ground states of S$=$1/2 frustrated XXZ ladders, and introduce an efficient method for computing the relevant correlation functions. The novel states we find are \textit{anyon condensates} that spontaneously break the Hamiltonian symmetry associated with the particle-number conservation. In contrast to the familiar Bose-Einstein condensates, the condensed particles satisfy anyonic statistics. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z24.00011: Fulde-Ferrell-Larkin-Ovchinnikov and topological superconducting phase in one dimensional optical lattice Ruilin Chu, An Zhao, Ming Gong, Shunqing Shen, Chuanwei Zhang The recent experimental realization of spin-orbit coupling in ultracold atom systems provides new arena for us to explore new quantum states. In this work, we explore the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase and topological superconducting phase of spin-orbital coupled Fermions in one dimensional optical lattice using the Density matrix renormalization group (DMRG) method. We demonstrate that the FFLO phase is energetically favored for in-plane Zeeman field while the topological superconducting phase is favored for out-of-plane Zeeman field. The entanglement entropy for these two phases are also examined. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z24.00012: Probing Lee-Yang Zeros and Time-domain Phase Transitions Bo-Bo Wei, Ren-Bao Liu As a foundation of statistical physics, Lee and Yang in 1952 proved that the partition functions of thermal systems can be zero at certain points (called Lee-Yang zeros) on the complex plane of magnetic field. In the thermodynamic limit, the Lee-Yang zeros approach to real numbers at the critical temperature. However, the imaginary Lee-Yang zeros have not been regarded as experimentally observable since they occur at imaginary field or temperature, which are unphysical. Here we show that the coherence of a probe spin coupled to a many-body system presents zeros as a function of time that are one-to-one mapped to the Lee-Yang zeros of the many-body system. In the thermodynamic limit, of which the Lee-Yang zeros form a continuum, the probe spin coherence presents a sudden death and a sudden birth at critical times corresponding to the edge singularities of the Lee-Yang zeros. By measuring the probe spin coherence, one can directly reconstruct the partition function of a many-body system. These discoveries establish the concept of critical times for phase transition in analogue to critical temperature, and also provide a universal approach to studying interacting many-body systems through measuring coherence of only one probe spin (or one qubit in quantum computing). [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z24.00013: Strength of the effective Coulomb interaction at metal and insulator surfaces Ersoy Sasioglu, Christoph Friedrich, Stefan Bl\"{u}gel The effective on-site Coulomb interaction (Hubbard $U$) between localized electrons at surfaces of solids is expected to be enhanced due to the reduced coordination number and the subsequent reduced screening. By means of first-principles calculations in conjunction with the constrained random-phase approximation [1] within the FLAPW method, we show that this is indeed the case for simple metals and insulators but not necessarily for transition metals and insulators that exhibit pronounced surface states [2]. In the latter case, the screening contribution from surface states as well as the influence of the band narrowing can increase the electron polarization to such an extent that the expected decrease is overcompensated. In some cases the $U$ parameter is substantially reduced, e.g. by around 30\% for the Cr(100) surface, contrary to conventional wisdom. It also depends on the properties of the surface states for different surface orientations, e.g. 10\% [2\%] reduction [enhancement] of $U$ for MgO (110) [MgO (100)]. We show a systematic study for prototype materials including transition-metal surfaces.\\[4pt] [1] E. \c{S}a\c{s}{\i}o\u{g}lu et al., Phys. Rev. B 83, 121101(R) (2011). \newline [2] E. \c{S}a\c{s}{\i}o\u{g}lu et al., Phys. Rev. Lett. 109, 146401 (2012). [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z24.00014: A Symmetrized Basis for Transitions in the Heisenberg Model Roger Haydock, C.M.M. Nex The spin-S Heisenberg model has 2S+1 states on each site, for which there are (2S+1)$^{2}$ possible transitions between these states. For N sites there are (2S+1)$^{N}$ states and (2S+1)$^{2N}$ transitions between states. This rapid increase in the number of transitions with sites appears to limit calculations to just a few sites. However for transitions induced by spin-spin interactions, we construct a symmetrized basis which only grows as 2$^{N-3}$, making possible computations for much larger systems. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z24.00015: Failure of the Holstein model to describe strong electron-phonon coupling Clemens P.J. Adolphs, Mona Berciu We point out an inconsistency in the most widely used theoretical models that describe systems with strong electron-phonon coupling. Both the Holstein and the Fr\"ohlich models assume that lattice distortions are sufficiently small to justify treating them to linear order. At strong coupling, however, it is well established that these models predict the formation of a small polaron, with potentially considerable local lattice distortions, invalidating the original assumption. Here we use the momentum average approximation to study the effect of higher-order coupling terms in the Holstein model. We show that they have drastic consequences on the properties of the polaron when compared to the linear model, and that these effects cannot be captured by a linear model with renormalized parameters. [Preview Abstract] |
Session Z28: Focus Session: Wrinkling
Sponsoring Units: GSNPChair: Douglas Holmes, Virginia Polytechnic Institute and State University
Room: 336
Friday, March 22, 2013 11:15AM - 11:27AM |
Z28.00001: Wrinkling patterns of thin sheets glued to a negative curvature surface Rastko Sknepnek, Mark Bowick, Xu Ma, Zhenwei Yao Gauss's Theorema Egregium provides an intimate connection between the metric and the Gaussian curvature of a surface. If a thin sheet is adhered to a substrate with a negative Gaussian curvature it will experience stress due to the curvature-driven change of its metric. In the inextensible limit any changes of metric are not possible and the sheet will relieve the stress by locally deforming via wrinkles or folds. Using geometric arguments and numerical simulations of a non-linear elastic model we analyse the wrinkling pattern as a function of the shape of the adhering substrate. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z28.00002: Drag Control through Wrinkling on Curved Surfaces Denis Terwagne, Pedro Reis We present the results of an experimental investigation on the wrinkling of positively curved surfaces and explore their use towards drag reduction applications. In our precision model experiments we make use of rapid prototyping techniques to cast samples with custom geometry and material properties out of silicone-based rubbers. Our structures consist of a thin stiff shell that is chemically bonded to a thicker soft substrate. The substrate contains a spherical cavity that can be depressurized, under controlled volume conditions, to compress the ensemble structure. Under this compressive loading, the initially smooth outer-shell develops complex wrinkling patterns. We systematically characterize and quantify the morphology of the various patterns and study the phase diagram of the system. We consider both geometric and material quantities in the parameter space. Moreover, since the wrinkling patterns can be actuated dynamically using a pressure signal, we systematically characterize the aerodynamic behavior of our structures in the context of fluid drag reduction. An added advantage of our novel mechanism is that it allows for both dynamic switching and tuning of the surface morphology, thereby opening paths for drag control. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z28.00003: Folds and crease from wrinkles Mazen Diab, Teng Zhang, Ruike Zhao, Huajian Gao, Kyung-Suk Kim We present stability and post bifurcation analyses of free-surface deformation from wrinkles to folds and creases, caused by lateral compression of a neo-Hookean material with varying elastic modulus with depth from the free surface. The post-bifurcation behavior of the wrinkle mode is investigated by high order perturbation as well as finite element analyses. We show that there is a critical strain beyond which the initial wrinkle mode is unstable. Using the finite element software ABAQUS, we reveal other deformation mode that may emerge due to the nonlinear bifurcation of the material surface. Bifurcation chart is constructed and shows that localized modes such as crease and fold may emerge depending on the geometric and material properties. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z28.00004: Mechanics of Graded Wrinkles Shabnam Raayai-Ardakani, Mary Boyce Shark skin is known for its anti-fouling and self-cleaning properties. In attempts to mimic this pattern for getting similar properties, different surface patterns such as Sharklet and wrinkles have been previously introduced. Wrinkled patterns have gained importance in applications such as microfluidics, wetting and adhesion. Through buckling of a thin film of stiff material on a substrate of softer material, and maintaining symmetric geometries, ordered wrinkled patterns can be created. However, it can be shown that using the same principle, by changing the geometry of the surface, the dimensions of the wrinkles can be altered. This alteration turns ordered wrinkles into graded wrinkles which have more resemblance to shark skin than the ordered wrinkles, maintaining the same wave length while each wave having different amplitude. Here using finite element models, experiments and analytical solutions, the relations between different geometries and the resulting patterns were investigated. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z28.00005: Wrinkling in Cellular Structured Composites Narges Kaynia, Yaning Li, Mary C. Boyce Many structured composites found in nature possess undulating and wrinkled interfacial layers that regulate mechanical, chemical, acoustic, adhesive, thermal, electrical and optical functions of the material. This research focused on the formation of wrinkling patterns in cellular structured composites and the effect of the wrinkling pattern on the overall structural response. The cellular composites consisted of stiffer interfacial layers constructing a network submerged in a soft matrix. Analytical and finite element models were developed to capture various aspects of the wrinkling mechanism. The characteristics of the undulation patterns and the instability modes were investigated as functions of model geometry and material composition. Mechanical experiments were designed to further explore the modeling results. The cellular composite samples were fabricated by using different types of elastomers and by varying the geometry and the material properties. The experimental and numerical results were consistent with the analytical predictions. The results in this research improve understanding of the mechanisms governing the undulation pattern formation in cellular composites and can be used to enable on-demand tunability of different functions to provide, among others, active control of wave propagation, mechanical stiffness and deformation, and material swelling and growth. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z28.00006: Numerical Simulation of the Combined Bending, Stretching, and Wrinkling of Thin Sheets Michael Taylor, David Steigmann, Katia Bertoldi A two-dimensional theory of plates and shells derived from three-dimensional finite elasticity is presented. The approach is based on a systematic small thickness expansion of the exact three-dimensional strain energy density of the plate or shell. The theory involves the small thickness explicitly and accounts for both bending and stretching in a unified framework. Thus, wrinkling instabilities in thin sheets are accommodated as a natural outgrowth of the model. The plate model is demonstrated numerically via a specially designed finite difference code utilizing the method of dynamic relaxation. The code is used to simulate several equilibrium deformations of thin sheets and plates undergoing finite deformation with wrinkling. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z28.00007: Understanding and Controlling Morphological Transitions of Wrinkles Invited Speaker: Alfred Crosby The ability to generate micron and sub-micron structures across extensive lengths on soft materials surfaces is critical for numerous technologies, yet current fabrication methods do not provide cost-effective solutions for these diverging demands. In Nature, elastic instabilities often are used to produce materials structures on small scales from simple building blocks to achieve necessary performance on larger, macroscopic size scales. We present an overview of our efforts to understand and use elastic instabilities, such as wrinkling and folding, to define surface structures with advantageous properties. In particular, we address two questions related to morphological transitions: the roles of overstress and curvature on selecting the specific wrinkle morphology created under equibiaxial stress conditions; and non-linear transitions, including wrinkle-to-fold, and the suppression of such transitions to achieve high-aspect ratio wrinkle structures. The lessons described provide new insight into the physics of these complex material deformations while also introducing scalable methods that are expected to help transfer elastic instabilities into current technologies. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z28.00008: Transition from wrinkles to crumples in an elastic sheet Hunter King, Narayanan Menon A circular sheet confined to a surface of increasing curvature initially breaks azimuthal symmetry creating a finite pattern of radial wrinkles along its perimeter. At larger curvature, sharp crumpled features emerge and dominate the shape. Using optical profilometry, we study the transition from wrinkling to crumpling of a polystyrene sheet floating on a drop of glycerol by measuring the spatial distribution of curvatures of the sheet as a function of drop curvature. We observe that collisions of neighboring wrinkles at their tips generate cusps. These cusps subsequently sharpen and merge to produce large crumpled features, around which gaussian curvature focuses. Surprisingly, the stress field in the central, unwrinkled portion is not sensitive to the appearance of crumpled features. The transition shows little hysteresis and is smooth with respect to measured quantities. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z28.00009: Wrapping a sphere: stress relaxation by wrinkling Evan Hohlfeld, Benny Davidovitch The low energy deformations of thin elastic sheets are isometries because these incur no stretching energy while the cost of bending is small. Since there is no isometric map of a flat sheet, i.e. a developable surface, onto the surface of a sphere, it is natural to suspect that any such map must cost finite stretching energy. However, I will show that there are an enormous number of almost isometric mappings which approximate a sphere with arbitrary accuracy and with arbitrarily small stretching energy. I will construct an example using multiscale analysis of a radial wrinkle pattern in a thin elastic sheet bent over a sphere. These techniques could be applied to other wrinkling problems and to problems connected to developable surfaces, e.g. textures in smectic liquid crystals. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z28.00010: Wrinkling of Inhomogeneously Strained Thin Polymer Films Yu-Cheng Chen, Alfred J. Crosby Wrinkles occur due to a mechanical instability when sufficient strain is applied to an incompressible thin film attached to a deformable substrate. For wrinkles made with a polymer film supported on a soft elastomer, the amplitude is directly proportional to the wavelength and the square root of the applied strain. This dependence has been confirmed with ideal substrates where the global strain is homogeneously distributed, but the influence of strain inhomogeneity has not been considered previously. We use the contact line wrinkling technique to prepare polystyrene thin films with periodic regions of different wrinkle amplitudes, hence strains, on soft substrates. The surfaces with inhomogeneous wrinkle amplitudes and directions approach a homogeneous structure upon the application of sufficiently large strains. The surface becomes homogeneous at a relatively small strain due to the growth rate difference between pre-wrinkles and new wrinkles. Moreover, we find the pre-wrinkled region starts strain localizing prior to the initially flat region. We derive relationships to describe these processes, providing fundamental knowledge of the wrinkling mechanism. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z28.00011: Hydrostatic and Flow Measurements on Wrinkled Membrane Walls Ozgur Ozsun, Kamil L. Ekinci In this study, we investigate structural properties of wrinkled silicon nitride (SiN) membranes, under both hydrostatic perturbations and flow conditions, through surface profile measurements. Rectangular SiN membranes with linear dimensions of $15$~mm $\times~ 1.5$~mm $\times~ 1~\mu$m are fabricated on a 500$-\mu$m-thick silicon substrate using standard lithography techniques. These thin, initially flat, tension-dominated membranes are wrinkled by bending the silicon substrate. The wrinkled membranes are subsequently incorporated as walls into rectangular micro-channels, which allow both hydrostatic and flow measurements. The structural response of the wrinkles to hydrostatic pressure provides a measure of the various energy scales in the problem. Flow experiments show that the elastic properties and the structural undulations on a compliant membrane completely dominate the flow, possibly providing drag reduction. These measurements pave the way for building and using compliant walls for drag reduction in micro-channels. [Preview Abstract] |
Session Z29: Complex Networks and Their Applications II
Sponsoring Units: GSNPChair: Erin Rericha, Vanderbilt University
Room: 337
Friday, March 22, 2013 11:15AM - 11:27AM |
Z29.00001: Consensus and transitions in coupled Sznajd networks Matthew Ludden In this work we investigate two coupled square lattice networks undergoing Sznajd model dynamics. The coupling between the networks is quantified by a coupling strength $p$. Monte Carlo simulations indicate that the exit probability of each network (to reach either all spins up or all down) depends on $p$ and the initial density of up spins $d$ in the other network. For fixed initial densities, we find a critical coupling $p_c$, above which no further changes in the exit probability are observed. We also find $p_c$ to decrease linearly with increasing $d$. The consensus time scales with system size as $L^{\alpha}$, where $\alpha$ = $\alpha$($p$,$d$). The conditions that must be met for the two networks to reach consensus are also considered. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z29.00002: Topological Influence On Network Of Coupled Chemical Oscillators Jie Zhao, Erin Rericha Networks of interacting nodes are ubiquitous in biological and communication systems. Recently the manner of the network connections, be it through of activator or inhibitor signals, and the topology of the network has received theoretical attention with the goal of finding networks with optimal synchronization and information transmission properties. In preparation for building an experimental system to examine these predictions, we numerically explore networks of Belousov-Zhabotinsky oscillatory nodes connected through unidirectional links of activator species. We measure the time required for the nodes to synchronize as a function of the network topology. While we observe a trend of smaller synchronization times with increasing first non-zero eigen values, we find that the most important factor in determining synchronization time is the initial phase difference between the oscillators. We find that the synchronization times for a given network topology, as determined from a uniform distribution of initial phase differences, is best described with a skewed Gaussian. To better understand the factors underlying this distribution, we look at the synchronization times in a three-node network as a function of both initial conditions and model parameters. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z29.00003: Extreme Fluctuations in Stochastic Network Synchronization with Time Delays D. Hunt, B.K. Szymanski, G. Korniss We study the effects of nonzero time delay on the extreme fluctuations about the mean in complex networks with local relaxation dynamics in the presence of noise. This extends our previous results for average fluctuations \footnote{D. Hunt, G. Korniss, B.K. Szymanski, PRL \textbf{105}, 068701 (2010)}$^,$\footnote{D. Hunt, B.K. Szymanski, G. Korniss, http://arxiv.org/abs/1209.4240} by considering the typical behavior of the worst-case node as the system evolves in the steady state. Within our previously established framework of the synchronizability of such systems, we consider the changes in the distribution of extremes for various delays in particular networks and the scaling behavior of the average extremal values vs. system size across ensembles of similar networks. For networks with sufficient randomness in their structure, the distribution of the global extreme is in the same universality class as that of an ensemble of independent variables, similarly to the case of zero time delay. Specifically, it asymptotically approaches the Fisher-Tippet-Gumbel extreme-value limit distribution. The local trends for individual nodes (esp. those of high degree) within the network, as well as the scaling behavior of the global extreme, however, can be adversely affected by large time delays. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z29.00004: The Joint Effect of Network Topology and Update Functions on the Stability of Boolean Networks Shane Squires, Andrew Pomerance, Edward Ott, Michelle Girvan Boolean networks are dynamical systems commonly used to model biological systems such as gene regulatory networks and neural networks. In a Boolean network, the state of each node can take one of two values, which is updated at discrete time steps using an update function that depends only on the states of its inputs on the previous time step. We study the stability of attractors in a Boolean network with respect to small perturbations. While recent past work has addressed the separate effects on stability of nontrivial network topology and update functions, only very crude information exists on how these effects interact. We present a general solution for finding the stability of Boolean networks, considering the joint effects of network topology and update functions. In particular, we show that the predictions of our approach agree with simulations of Boolean networks with threshold update functions. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z29.00005: Asymptotically inspired moment-closure approximation for adaptive networks Maxim Shkarayev Dynamics of adaptive social networks, in which nodes and network structure co-evolve, are often described using a mean-field system of equations for the density of node and link types. These equations constitute an open system due to dependence on higher order topological structures. We propose a systematic approach to moment closure approximation based on the analytical description of the system in an asymptotic regime. We apply the proposed approach to two examples of adaptive networks: recruitment to a cause model and adaptive epidemic model. We show a good agreement between the mean-field prediction and simulations of the full network system. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z29.00006: Optimizing Nutrient Uptake in Biological Transport Networks Henrik Ronellenfitsch, Eleni Katifori Many biological systems employ complex networks of vascular tubes to facilitate transport of solute nutrients, examples include the vascular system of plants (phloem), some fungi, and the slime-mold \emph{Physarum}. It is believed that such networks are optimized through evolution for carrying out their designated task. We propose a set of hydrodynamic governing equations for solute transport in a complex network, and obtain the optimal network architecture for various classes of optimizing functionals. We finally discuss the topological properties and statistical mechanics of the resulting complex networks, and examine correspondence of the obtained networks to those found in actual biological systems. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z29.00007: Paradoxical Behavior of Granger Causality Annette Witt, Demian Battaglia, Alexander Gail Granger causality is a standard tool for the description of directed interaction of network components and is popular in many scientific fields including econometrics, neuroscience and climate science. For time series that can be modeled as bivariate auto-regressive processes we analytically derive an expression for spectrally decomposed Granger Causality (SDGC) and show that this quantity depends only on two out of four groups of model parameters. Then we present examples of such processes whose SDGC expose paradoxical behavior in the sense that causality is high for frequency ranges with low spectral power. For avoiding misinterpretations of Granger causality analysis we propose to complement it by partial spectral analysis. Our findings are illustrated by an example from brain electrophysiology. Finally, we draw implications for the conventional definition of Granger causality. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z29.00008: Direct and indirect effects in causal networks Andreas Kr\"amer Literature-derived networks of biomolecular interactions representing cause-effect relationships generally contain many indirect relationships where the actually observed causal effect results from a sequence of events represented in the same network. A statistical method is developed, based on an Ising-like spin model operating on the edges of the network, to distinguish between direct and indirect effects using only the network structure itself. This allows to identify paths representing likely causation mechanisms. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z29.00009: Extraction of hidden information by efficient community detection in networks Jooyoung Lee, Juyong Lee, Steven Gross Currently, we are overwhelmed by a deluge of experimental data, and network physics has the potential to become an invaluable method to increase our understanding of large interacting datasets. However, this potential is often unrealized for two reasons: uncovering the hidden community structure of a network, known as community detection, is difficult, and further, even if one has an idea of this community structure, it is not a priori obvious how to efficiently use this information. Here, to address both of these issues, we, first, identify optimal community structure of given networks in terms of modularity by utilizing a recently introduced community detection method. Second, we develop an approach to use this community information to extract hidden information from a network. When applied to a protein-protein interaction network, the proposed method outperforms current state-of-the-art methods that use only the local information of a network. The method is generally applicable to networks from many areas. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z29.00010: Scaling of Minimum Dominating Sets in Various Scale-Free Network Ensembles F. Molnar, S. Sreenivasan, B.K. Szymanski, G. Korniss We study the scaling behavior of the size of minimum dominating sets (MDS) in scale-free networks, with respect to network size $N$ and power-law exponent $\gamma $ [Nacher et al., NJP 073005 (2012)]. Network samples are constructed by either the configuration model (CM) via multigraphs, or exact degree sequence sampling methods. The MDS is found by a sequential greedy algorithm. We control the average degree by setting an appropriate lower degree cutoff $k_{\min } $. Two subtypes of networks are studied according to the maximum degree cutoff $k_{\max } $. Our results show that when $k_{\max } =\sqrt N $ all networks have similar scaling. The size of MDS is linear with respect to $N$, and for a given $N$, it increases for low $\gamma $ values. When $k_{\max } =N-1$, we find a structural difference between CM networks, and networks constructed by exact sampling methods. For the latter, we find a scaling transition of the MDS size from O(N) to O(1) at approximately $\gamma \approx 1.9$, due to the appearance of star subgraphs with O(N) central degree. For a given $N$, the size of MDS increases for higher $\gamma $ values. However, in CM networks the MDS scales linearly with $N$, and for a given $N$, it is non-monotonic with respect to $\gamma $. Finally, we find that a partial MDS, which dominates only a certain fraction of the network, has the same scaling as full domination, even for as low as $30\% $ dominated fraction. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z29.00011: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z29.00012: Graphicality of random scale-free networks with general degree cutoffs Yongjoo Baek, Daniel Kim, Meesoon Ha, Hawoong Jeong We study graphicality of random scale-free networks with arbitrary degree cutoffs in the thermodynamic limit, which refers to realizability of degree sequences randomly generated with the degree exponent $\gamma$ and the upper degree cutoff $k_c$ as the number of nodes $N$ goes to infinity. While a recent study\footnote{C. I. Del Genio, T. Gross, and K. E. Bassler, Phys. Rev. Lett. {\bf 107}, 178701 (2011).} found that only degree sequences with $\gamma > 2$ or $\gamma < 0$ are graphical if $k_c = N-1$ using the graphicality criterion proved by Erd\H{o}s and Gallai,\footnote{P. Erd\H{o}s and T. Gallai, Matematikai lapok {\bf 11}, 264 (1960).} we generalize the study to different upper cutoffs. To ensure graphicality of degree sequences, it is found that the upper cutoff must be lower than $k_c \sim N^{1/\gamma}$ for $\gamma < 2$, whereas any upper cutoff is allowed for $\gamma > 2$. This is also numerically verified, using both random and deterministic sampling of degree sequences. Our result can be interpreted as giving a fundamental constraint on the structure of random scale-free networks. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z29.00013: Phase transition of biconnected components in scale-free networks Purin Kim, Deok-Sun Lee, Byungnam Kahng In information-transport and biological systems, there can be more than one pathway between two nodes, so that there is a backup in case one pathway is inactive. The size of such biconnected nodes can be an important measure of the robustness of a system. The giant biconnected components of diverse real-world networks suggest the importance of scale-free topology in the biconnectivity. Thus, here, we consider a critical behavior of the largest biconnected component as links are added and form a random scale-free network. The critical exponents $\beta_{\rm (BC)}$ and $\beta_{\rm (SC)}$ associated with the order parameter of the percolation transition of biconnected and single-connected components, respectively, are compared. We obtain that $\beta_{\rm (BC)}/\beta_{\rm (SC)}=\lambda-1$ for $2 < \lambda < 3$ and 2 for $\lambda > 3$, where $\lambda$ is the exponent of the degree distribution in scale-free networks. We also obtain the finite-size scaling behavior of the order parameter analytically and numerically. [Preview Abstract] |
Session Z31: New Computational Methods in Polymer & Soft Matter Physics
Sponsoring Units: DPOLYChair: Gary Leuty, The University of Akron
Room: 339
Friday, March 22, 2013 11:15AM - 11:27AM |
Z31.00001: Simulations of Coarse Grain Entangled Polymeric Systems: From Thermodynamics to Rheology Abelardo Ramirez-Hernandez, Juan De Pablo Coarse-grained models have been proposed for description of soft materials over length and time scales unattainable by using atomistic models. Polymeric materials present particular challenges, because characteristic length and time scales generally span several orders of magnitude. Most coarse-grained models resort to soft effective interaction potentials, with the result that important effects are lost, including those created by the non-crossability of long polymer chains. In this work we generalize a particle-based coarse-grained approach, which has been successfully used in the past to describe the structure and thermodynamics of homopolymers and block polymers, to the study of linear and non-linear rheology in polymer melts well above the entanglement molecular weight. Entanglements are represented by slip-springs introduced at the two-chain level, as fluctuating interactions between neighboring pairs of polymeric molecules. The model is shown to exhibit scaling laws for the mean square displacement and shear viscosity that are consistent with those observed in tube theories and in experiments. Comparison between simulation and experimental results shows that the model is capable of describing quantitatively the linear and non-linear rheology of homopolymer melts and blends [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z31.00002: Model for the shear viscosity of suspensions of star polymers and other soft particles Carlos Mendoza We propose a model to describe the concentration dependence of the viscosity of soft particles. We incorporate in a very simple way the softness of the particles into expressions originally developed for rigid spheres. This is done by introducing a concentration-dependent critical packing, which is the packing at which the suspension looses fluidity. The resultant expression reproduces with high accuracy the experimental results for suspensions of star polymers in good solvents. The model allows to explain a weak increase of the viscosity observed in the case of diblock copolymer stars suggesting that the reason for this peculiar behavior is mainly a consequence of the softness of the particles. In the semi-dilute regime, suspensions of star polymers are modeled using the Daoud-Cotton picture to complete the description in the whole concentration regime. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z31.00003: Systematic and Simulation-Free Coarse-Graining of Polymer Melts using Soft Potentials Delian Yang, Qiang Wang Full atomistic simulations of many-chain systems such as polymer melts are not feasible at present due to their formidable computational requirements. Coarse-grained models have to be used instead, where the segments interact with soft potentials that allow complete overlapping. This enables systematic coarse-graining with different $N$ (number of segments on each chain) at constant invariant degree of polymerization controlling the system fluctuations. In this work we use integral-equation theories and a relative entropy framework for coarse-graining to investigate how the soft potential varies with $N$ and how well the coarse-grained models can reproduce both structural and thermodynamic properties of the original system. This will provide us with a quantitative basis for choosing small $N$-values that can still capture the chain conformational entropy, a characteristics of polymers. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z31.00004: Solvent Entropy in and Coarse-Graining of Polymer Lattice Models Qiang Wang, Pengfei Zhang In conventional lattice models for polymeric systems, each lattice site is occupied by at most one polymer segment, and an unoccupied lattice site is often treated as a solvent molecule. This self- and mutual-avoiding walk requires that all lattice sites, polymer segments, and solvent molecules have the same volume. Since a polymer segment here is the coarse-grained representation of a group of real monomers, this incorrectly accounts for the solvent entropy (i.e., size ratio between polymer segments and solvent molecules). It also limits the coarse-graining capability of such models, where the invariant degree of polymerization controlling the system fluctuations is too small (thus exaggerating the fluctuations) compared to that in most experiments. Here we show how to properly account for the solvent entropy in new lattice models with multiple occupancy of lattice sites [Q. Wang, \textbf{Soft Matter 5}, 4564 (2009); \textbf{6}, 6206 (2010)], and present a quantitative coarse-graining strategy that ensures both the solvent entropy and fluctuations in experimental systems are properly accounted for using the new lattice models. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z31.00005: Recent developments in the VOTCA package for coarse-graining Christoph Junghans Coarse-graining is a systematic way of reducing the number of degrees of freedom used to represent a system of interest. The Versatile Object-oriented Toolkit for Coarse-graining Applications (VOTCA) provides a uniform interface to commonly used coarse-graining techniques such as iterative Boltzmann inversion, force-matching, and inverse Monte Carlo. Further, it provides a flexible modular platform for the further development of new coarse-graining techniques. Recently two new methods for coarse-graining have been added to the package and got tested on SPC/E water and methanol-water mixtures. We will discuss these results in comparison to earlier structure-based studies, but also talk about the development of non-structure-based model. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z31.00006: Coarse-graining of Polystyrene in Various Environments by Iterative Boltzmann Inversion Roland Faller, Beste Bayramoglu We have developed mesoscale models for polystyrene (PS) oligomers in various environments following the Iterative Boltzmann Inversion Technique for polymer coarse--graining with and without confinement. Bond, bending angle, torsion angle distributions and radial distribution functions between PS monomers show that local structures were reproduced very well, while a small discrepancy remained in the reproduction of global structures (radii of gyration and end--to--end distances), which is probably due to end effects. Speed--up in polymer dynamics with each model was monitored by scaling factors calculated based on characteristic relaxation times of the end monomers as well as diffusivities of the chains. Results show that coarse--graining is most successful for the highest concentration system (melt) and least for the lowest concentration (dilute solution) due to the stronger slowdown of diffusive and rotational dynamics in atomistic simulations with concentration. The speed--up in the confined solution system was found to be greater than in the unconfined solution system due to the same reason except that confinement slows down the dynamics in that situation. We also characterize the limits to which extent the same models can be used for different degrees of confinement. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z31.00007: Complex Langevin Simulation of the Coherent States Formulation of Polymer Field Theory Xingkun Man, Kris Delaney, Henri Orland, Glenn Fredrickson In 1969, Edwards and Freed adapted the ``coherent state'' methods employed in the second quantization formalism of quantum many-body theory to study polymer networks. Since its introduction into polymer science, this formalism has been largely neglected and to our knowledge, has never been applied as a basis for numerical simulations, even for linear polymers. However, in contrast to the Edwards auxiliary-field framework, this alternative polymer field theory has several attractive features, including an action or effective Hamiltonian with an explicit, finite-order, and semi-local polynomial character. We thus revisited the CS formalism and show that these characteristics have advantages both for analytical and numerical studies of linear polymers at equilibrium. For this purpose, we developed a new Complex Langevin sampling scheme that allows for simulations within the CS formalism with stable and efficient numerical characteristics. We anticipate that this methodology will facilitate efficient simulations of a wide range of systems, including complicated branched and networked polymers and liquid crystalline polymers. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z31.00008: Using adaptive-mesh refinement in SCFT simulations of surfactant adsorption Scott Sides, Rajeev Kumar, Ben Jamroz, Robert Crockett, Alex Pletzer Adsorption of surfactants at interfaces is relevant to many applications such as detergents, adhesives, emulsions and ferrofluids. Atomistic simulations of interface adsorption are challenging due to the difficulty of modeling the wide range of length scales in these problems: the thin interface region in equilibrium with a large bulk region that serves as a reservoir for the adsorbed species. Self-consistent field theory (SCFT) has been extremely useful for studying the morphologies of dense block copolymer melts. Field-theoretic simulations such as these are able to access large length and time scales that are difficult or impossible for particle-based simulations such as molecular dynamics. However, even SCFT methods can be difficult to apply to systems in which small spatial regions might require finer resolution than most of the simulation grid (eg. interface adsorption and confinement). We will present results on interface adsorption simulations using PolySwift++, an object-oriented, polymer SCFT simulation code aided by the Tech-X Chompst library that enables via block-structured AMR calculations with PETSc. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z31.00009: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z31.00010: Embedding methods: application and development Jin Cheng, Florian Libisch, Emily Carter Correlated-wavefunction/density functional theory (CW/DFT) embedding methods aim to combine the formally exact correlation treatment in CW methods with the high efficiency of DFT. By partitioning a system into a cluster and its environment, each part can be treated independently. Different embedding schemes have been proposed. The density-based scheme searches for a global embedding potential mediating the interaction on the DFT level. The potential can then be used in CW calculations, e.g., to investigate hot-electron assisted H$_{\mathrm{2}}$ dissociation on Al and Au surfaces. Experimentally, optical excitations of plasmons efficiently create the required hot electrons. The embedded CW calculations validates that the hot electrons play a key role. However, this method neglects the back-action of the cluster on the environment. To solve this problem, a potential-based scheme has been proposed [\textit{J. Chem. Phys.},~135, 194104 (2011)] that allows for a self-consistent combination of different ab-initio methods. Such an embedding potential thus goes beyond the DFT level. The heterogeneity involved poses various numerical challenges. We report on efforts to construct appropriate basis sets and pseudopotentials as well as to optimize the numerical procedure. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z31.00011: Ion distributions near dielectric interfaces from Car-Parrinello molecular dynamics Vikram Jadhao, Francisco Solis, Monica Olvera de la Cruz Free charges in media characterized by different dielectric constants and separated by thin boundaries are basic models for studying phenomena in both biological and synthetic materials. Knowing the distributions of ions near the dielectric interfaces between these media is crucial towards understanding the structural and physical properties of these systems. We present a new Car-Parrinello molecular dynamics method for simulating charges in heterogeneous media and computing such distributions. This method is founded on a true energy functional of induced charge density which enables the replacement of the expensive solution of the Poisson equation at each simulation step with an on-the-fly computation of polarization effects. Our simulations track the exact induced density at all times and demonstrate excellent energy conservation. The method is applied to study models of a charged colloid in polar solvent, ions near a liquid-liquid emulsion droplet, and charged biological macromolecule in aqueous solution. Results for ionic density profiles for different dielectric contrasts, ion concentrations, ion valencies, and different interfacial shapes are presented. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z31.00012: Monte Carlo approaches for a particle at a diffusivity interface and the ``Ito-Stratonovich dilemma'' Mykyta V. Chubynsky, Hendrick W. de Haan, Gary W. Slater Diffusion of a particle in a fluid is often described by the overdamped Langevin equation (OLE). However, when the fluid is inhomogeneous, the stochastic term in the OLE is ambiguous (the ``Ito-Stratonovich dilemma''). Different interpretations of this term correspond to different stochastic calculi that may be appropriate in different physical situations. Concentrating on the case when two fluids with different viscosities are separated by a sharp interface, we develop two lattice Monte Carlo algorithms, both giving the choice between calculi (including Ito, Stratonovich, and ``isothermal''). We validate the algorithms considering a 1D system with the interface in the middle between two walls and particles starting at the interface and comparing the simulation results to both theory and molecular dynamics simulations, with Langevin Dynamics corresponding to isothermal and Brownian Dynamics to Ito calculi. This simple system turns out to have surprisingly rich behavior. The algorithms have also been applied to a model of polymer translocation. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z31.00013: Application of atomic-orbital projections to the study of the electronic properties of metal-organic frameworks Luis Agapito, Arrigo Calzolari, Andrea Ferretti, Marco Nardelli Metal-organic frameworks (MOF) are a new class of artificial crystalline materials. Because of their flexibility for synthesis and instrinsic ultrahigh surface area and porosity, MOFs show superior performance in gas storage, catalysis, and sensing applications. We use an efficient projection of plane-wave wavefunctions onto atomic orbitals for studying the electronic properties of these intriguing materials. The present scheme harnesses the robust periodic algorithms and systematic convergence of the plane-wave method for an atomistic electronic (Landauer conductance) and chemical (charge transfer, bond and atomic charge) analysis that provides guidelines for the design of MOF electronic materials. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z31.00014: MOVED TO F32.013 |
Session Z32: Micro/Nanofluidics II
Sponsoring Units: DFDChair: German Drazer, Rutgers University
Room: 340
Friday, March 22, 2013 11:15AM - 11:27AM |
Z32.00001: Electro-coflow as a means to study whipping instabilities in electrified liquid jets Josefa Guerrero Millan, Venkat Gundabala, Alberto Fernandez-Nieves Whipping is a non-axisymmetric instability that appears in electrified jets. In air, it usually manifests in a chaotic fashion preventing its detailed experimental characterization. We use electro-coflow to generate a steady-state whipping structure and quantify its wave-like properties, which we understand from simple force balances. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z32.00002: Do electroviscous effects impact the hydraulic conductance of xylem? A theoretical inquiry Michael Santiago, Vinay Pagay, Abraham Stroock Experiments show that the hydraulic conductance of plant xylem (K) varies with the ionic-strength (I) and pH of the sap, a behavior usually attributed to the swelling of hydrogels that cover bordered pits---conduits that interconnect individual xylem vessels. These gels are believed to swell at low I or large pH, and thus decrease the flow cross-section and K. But experiments have shown behaviors that contradict this hypothesis, where a decrease in I serves to increase K. Here, we investigate whether these observations could be explained by electroviscous effects in the pores of bordered pits, since the literature suggests that pits are covered by materials that develop electric charge in aqueous solution, e.g. lignin and pectin. We use experimental measurements from the literature, combined with standard electrokinetic theory, to estimate the electroviscous effect of I and pH on K. We find that K varies non-monotonically with I and can drop to a minimum of 0.8 of its maximum value, and that our predictions fit the available experimental data for physiologically relevant conditions in I and pH. We conclude that electrokinetics could explain, at least partially, the observed changes in K, and propose experiments to test this hypothesis. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z32.00003: Microfluidic route to generation of celloidosomes Venkata Gundabala, Sergio Martinez-Escobar, Samantha Marquez, Manuel Marquez, Alberto Fernandez-Nieves Here we present a microfluidic method to generate alginate particles with a liquid core and a shell with yeast cells encapsulated in it. This particular class of celloidosomes with cells embedded into the thin shell region at the surface, allows for easy access of oxygen to the cells improving their viability. The liquid core opens the possibility of encapsulating multiple types of cells into the core and the shell. The microfluidic method involving double emulsion technology employed here ensures robust control over the size of the particles and density of the encapsulated cells. The study has shown that the stability of the inner core is very much dependent on the viscosity of the oil used for collecting the emulsion. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z32.00004: Dynamics assembly of magnetic microparticles suspended in moving droplets under the influence of magnetic fields Helmut Strey, Eric Brouzes, Travis Kruse Droplet microfluidics has experienced tremendous growth, particularly since it is well suited for single-cell manipulation and analysis. As mature methods for high throughput droplet manipulation have been developed a technological bottleneck of current droplet microfluidics is that because droplets are separated, sequential chemical reactions are more difficult to achieve. For example, it is very difficult to concentrate target molecules, especially since every reaction step adds volume to the droplets. Our solution to this problem is to employ functionalized magnetic beads inside droplets. The basic idea is that an external magnetic field could be used to concentrate the magnetic beads in one part of the droplet and those could then be extracted by splitting the droplet. Here we present an experimental study of the self-assembly of superparamagnetic microparticles that are suspended in moving droplets and experience a combination of forces due to the internal fluid flow fields and external magnetic fields. We observed that this interplay of flow fields coupled to the formation of particle assemblies leads to the formations of stable patterns depending on the flow speed and magnetic field strength. An understanding of this dynamic assembly is critical in employing external forces for applications in separation and sorting. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z32.00005: Droplet pairing and coalescence control for generation of combinatorial signals Eujin Um, Matthew Rogers, Howard Stone A co-flowing aqueous phase with an immiscible oil phase in a microchannel generates uniformly spaced, monodisperse droplets, which retain their shape by not touching each other or by being stabilized with surfactants at the oil-water interface. However, droplet coalescence is required in many advanced applications, which can be achieved by a complex channel geometry or size differences in the droplets, and as well as by procedures to reduce the effect of a surfactant. These approaches, again, hinder the stability of droplets further downstream. We designed a microchannel which consistently inserts gas-bubble between droplets so that pairing and coalescence of droplets occurs even in the presence of surfactant, and yet prevents unwanted merging with other droplets. Aqueous droplets placed between the bubbles alter their relative speeds and spacing, and consequently we study the change in the number of droplet pairings in relation to the characteristics of the bubbles and the volume of aqueous droplets. By integrating this approach with droplets of different materials, we can program the output sequence of droplet compositions, and such complex combinatorial signals generated are aimed for concentration gradient generation and dynamic stimulation of biological cells with chemicals. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z32.00006: Microfluidic Printing and Ablation of Metallic Films by Modulated Capillary and Maxwell Stresses Gerry Della Rocca, Sandra Troian Liquid dosing strategies for micro/nanofluidic applications normally rely on interior flow driven by external pressure gradients. To maintain a constant flow rate, the effective pressure drop over a given length conduit must scale inversely as the fourth power in the conduit radius, as prescribed by the Hagen-Poiseuille relation. For micron or nanoscale capillaries, this constraint requires enormous pressure gradients and external control mechanisms. This burden, coupled with the likelihood of occlusions due to gas bubbles, contaminants or carrier particles, limits the usefulness of internal flow strategies for applications involving emission of charged droplets or ions. In this talk, we focus on capillary flow in slender V grooves as a more robust and self-regulating fluidic delivery system. When coupled with spatiotemporal modulation of Maxwell stresses induced by an external electric field, beams of droplets or ions can be metered reliably and effectively. Here we explore the steady state, transient and oscillatory flow characteristics of microscale metallic films in V-grooves subject to capillary and Maxwell stresses. The geometry investigated will focus on printing and ion ablation of thin films for electronic circuits and photovoltaic displays. [Preview Abstract] |
Session Z33: Focus Session: Organic Electronics and Photonics - Morphology and Structure II
Sponsoring Units: DPOLYChair: Brian Collins, National Institute of Standards and Technology
Room: 341
Friday, March 22, 2013 11:15AM - 11:27AM |
Z33.00001: Correlating polymer solution conformation and thin film nanostructure: Implications for BHJ processing Rajeev Dattani, Alisyn Nedoma, Natalie Stingelin, Jenny Nelson, Joao Cabral We study the solution properties of polymer-fullerene mixtures by a combination of dynamic light scattering, viscometry, small angle neutron scattering and microscopy. Specifically, the kinetics of polymer conformation (Rg and Rh) and interaction changes are mapped as function of polymer-particle concentration, overall concentration in solution and age. A model system of polystyrene and C60 fullerene was selected for this study, in addition to the P3HT/PCBM pair, which is currently explored in photovoltaic applications. The solution properties show a clear correlation to the resulting thin film nanostructured composite morphology. Our future work will further link it to bulk heterojunction solar cell performance. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z33.00002: Chemical Effects in Solution on the Formation of Film Morphology in Bulk Heterojunction Organic Solar Cells Jong Kuk Koh, Won Tae Choi, Kookheon Char A novel method to control the active layer morphology of bulk heterojunction(BHJ) organic solar cells will be presented in this study. The effect of solvent quality, chemical effect in solution, on the morphology of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) active layer has been investigated. The solubility of solvent can be controlled by mixing other types of additional solvents (additives) to the P3HT:PCBM blend solution, which could adjust the association and/or solvation characteristics for both P3HT and PCBM solutes in mixed solvents. As a result, the control over the solubility has a definitive effect on the film morphology. We report a new additive, 2-chlorophenol, which could drive P3HT to have more association character and, PCBM to have more solvation character in the mixed solvent. Higher P3HT crystallinity was achieved due to more association character in the presence of 2-chlorophenol. Also, the higher solvation character of PCBM leads to the reduced size of PCBM agglomerates, as confirmed by SANS measurements. Based on these results, P3HT:PCBM BHJ solar cell devices were fabricated, with maximum power conversion efficiency of 3.24{\%}, which is 43{\%} enhancement when compared with the reference. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z33.00003: Effect of solvent annealing on phase separation of donor/acceptor species in organic mixtures Miriam Cezza, Qian Shao, Shy-Hauh Guo, Raymond J. Phaneuf Studies on phase separation of mixtures of tetranitro zinc- phthalocyanine (tn-ZnPc) and [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) were performed in which we controlled the evaporation rate of the solvent (chloroform). Phase-contrast AFM analysis reveals that slowing down the evaporation rate of the solvent facilitates the nucleation of the donor component, and the two components phase-separate. The size of the molecular agglomerates and single small particles decreases for slow solvent evaporation and the density of small particles per unit area increases by an order of magnitude over the range studied. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z33.00004: Precise Structural Development and its Correlation to Function in Conjugated Polymer: Fullerene Thin Films by Controlled Solvent Annealing Huipeng Chen, Sheng Hu, Huidong Zang, Bin Hu, Mark Dadmun The structural evolution and function of solvent processed poly(3-hexylthiophene):[6,6]-phenyl-C$_{\mathrm{61}}$-butyric acid methyl ester (P3HT:PCBM) bilayers with controlled exposure to ortho-dichlorobenzene solvent vapor is examined. Different from thermal annealing, where the structure develops (P3HT crystallization and PCBM phase separation) in seconds, solvent vapor annealing provides more precise morphological control and a more detailed picture of the competing processes that drive the structural development. This work shows that P3HT crystallization and PCBM phase separation occur in different stages with solvent annealing. The interdiffusion of PCBM and P3HT and crystallization of P3HT occurs in the first stage, while in the second stage, the phase separation of PCBM from P3HT and agglomeration of PCBM occurs. Therefore, the sequential nature of these processes clearly documents that the phase separation of PCBM from P3HT \textit{is not} driven by P3HT crystallinity, but by the thermodynamic driving force of mixing (the miscibility limit of PCBM in P3HT) Correlation of the morphology to device performance indicates that both sufficient P3HT crystallization and PCBM phase separation are crucial in the optimization of the morphology of the active layer. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z33.00005: Controlling donor/acceptor interface structure by processing solvents in organic solar cells Wei Ma, Long Ye, Gann Eliot, Jianhui Hou, Harald Ade The nature of the interface structure between donor and acceptor are known to be critical for fullerene-based solar cells, yet have not been widely studied due to limitations of common characterization techniques. We show that processing solvents are an effective way to control the interface structure (sharp, fractal, or diffuse) of the active layer and thus impact device performance. Six different solvents or solvent mixtures are used as processing solvents in PDPP3T with PC$_{71}$BM blends to investigate the impact of solvents on interface properties. Interface roughness is revealed by analysing the scaling of high-q data of resonant soft x-ray scattering profiles. We find that with the presence of DIO, rough interfaces are always observed. While rough interfaces provide shorter average distances for excitons to reach donor/acceptor interfaces, they also enhance recombination and are thus not ideal. When CF is used as one component, a sharp or slightly diffuse interface is induced. However, over-pure domains (especially mixed CF with DIO) are also created that seem to negatively impact performance. Overall, the mixture of ternary yields the highest PCE of 6.7{\%}. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z33.00006: A comparative study of the morphology of flow and spin coated P3HT:PCBM films Jose Chapa, Alamgir Karim Polymer solar cells are attractive due to the possibility of using cheaper materials and processing techniques for mass production of solar panels. Previous methods of fabricating polymer solar cells are suitable in laboratory conditions but are not scalable for industrial production. In this study, thin films of the photoactive blend of poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were prepared by flow coating, which is suitable for industrial manufacturing of solar cells. P3HT:PCBM blends were cast from different solvents, and the morphology of flow coated and spin coated films was compared. The surface morphology and optical properties of P3HT:PCBM films were characterized with optical microscopy, AFM, and UV-vis absorption spectroscopy. The degree of P3HT order was higher in flow coated films, as compared to spin coated films. Films flow coated using chloroform solutions had a higher thermal stability and an enhanced degree of phase separation as compared to spin coated films. Flow coated films from chlorobenzene solutions had a lower thermal stability and a smaller length scale of phase separation. This study demonstrates that flow coating is a suitable alternative technique for fabricating polymer solar cells. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z33.00007: Microstructure of self-assembled all-conjugated donor-acceptor block copolymers for organic solar cells Michael Brady, Sung-Yu Ku, Justin Cochran, Craig Hawker, Edward Kramer, Michael Chabinyc All-conjugated diblock copolymers (CBCPs), with donor and acceptor blocks, form intriguing alternatives to polymer/fullerene bulk heterojunction (BHJ) blends as low-cost photovoltaics. BHJs comprise a phase-separated thin film microstructure, in which chemically distinct domains of donor and acceptor enable exciton dissociation at their interface and transport of free charges through continuous n- and p-type paths to the electrodes. GIWAXS, AFM, soft X-ray spectroscopy (NEXAFS), and resonant scattering (RSoXS) are used to probe the structure of films of CBCPs that have an electron-donating P3HT block and an electron-accepting poly-(diketopyrrolopyrrole-terthiophene) (DPP) block. Thermal annealing after casting causes these CBCP films to form ordered domains on the scale of the exciton diffusion length, with ca. 50 nm in-plane lamellar spacings, with crystallites of each block present. GIWAXS diffraction peaks from the (100), (200), and (300) alkyl chain stacking planes for crystals of each block show (h00) orientation toward the out-of-plane direction, with the (010) pi-stacking vectors in the film plane. CBCP processing-structure studies have enabled the control of chain ordering and orientation at both length scales, and thus the formation of optimal BHJ morphologies. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z33.00008: Controlled Domain Swelling for Block Copolymer-Based Solar Cells Alisyn Nedoma, Rajeev Dattani, James Bannock, Paul Westacott, Joao Cabral Block copolymers seem ideally suited materials for solar cells because they self-assemble to form highly-ordered domains on the same length scale as the diffusion length of an exciton. Success has thus far been limited by the tendency of block copolymers to disorder at low loadings of fullerene; a consequence of Timmerman's Rule whereby preferential interactions between the fullerene and one block of the copolymer tend to destabilize the microstructure. We present a method for balancing the volumetric swelling of one block by swelling the other block with a commensurate amount of the homopolymer. This technique is demonstrated for a model polymer system and extended to a conjugated rod-coil block copolymer. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z33.00009: Observation the Nanoscale Blending Morphology of P3HT:PCBM Bulk-Heterojunction by Energy-Filtered TEM and Contrast Transfer Function. Nopporn Rujisamphan, Ismat Shah The efficiency of bulk-heterojucntion organic solar cells is strongly related to the blending morphology of donor and acceptor materials. By understanding the intermixed morphology would improve device performance. Herein, we present the ways to improve contrast images in the transmission electron microscopy of P3HT:PCBM. In general, TEM images took at the focus point gives one low contrast. We take advantage of the contrast transfer function (CTF) to improve contrast images in bright field TEM. By changing the defocus values, the fibril structure of the P3HT is obviously observed and distinguished. In order to observe the nanoscopic blending morphology, fibril size, and distribution of those fibrils, we carry out the energy filtered TEM (EFTEM). The energy window centered at 19 eV with the slit width energy of 7 ev is selected for looking only P3HT domain. In contrast, the energy window is centered at 25, and 30 eV for observing PCBM domains with the same slit width energy for a comparison. When used the window at 19 ev, we are able to clearly observe the P3HT fibril structure with the diameter and the length of $15\pm 1$ nm and $51\pm 20$nm, respectively. The diameter size of those fibrils did not change even in the annealed samples implying that the PCBM diffused only into an amorphous region of P3HT. The distribution of those fibrils seemed to be homogeneous without any preferred direction. Together with XRD results, we found that in only one P3HT fibrils, there are 40 pi-pi stacking layers with 9 layers parallel to the fibril length. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z33.00010: Cross-sectional nanoscale morphology and interfacial band alignment of phase-separated polymer/fullerene by scanning tunneling microscopy and spectroscopy M.C. Shih, Y.P. Chiu, B.C. Huang, C.C. Lin, S.S. Li, C.S. Chang, C.W. Chen The efficiency of organic films based on poly(3-hexylthiophene) (P3HT) and methanofullerene derivative (PCBM) was shown to be strongly dependent on the crystalline order inside. Through the suitable annealing process, the well-crystallized organic P3HT:PCBM films can be fabricated to enhance their charge transport. To further improve the efficiency of photo-induced charge separation and transport as well as the corresponding photocurrent, more detailed electronic information at both interfaces of the donors/accepters and photoactive-layer/electrode will be essential. In this work, cross-sectional scanning tunneling microscopy and spectroscopy were employed to investigate the interfacial properties of P3HT:PCBM films. The vertical phase distribution and local electronic structures across the interfaces of substrate/organic film and P3HT/PCBM are obtained at the atomic resolution. These electronic structures also provide direct observations of the interfacial band alignments, suggesting the possible carrier transport mechanism of P3HT:PCBM organic films. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z33.00011: Enhanced Photocurrent in a Photovoltaic Cell involving a Nonconjugated Conductive Polymer, Poly($\beta $-pinene) M. Sangal, G. Telang, M. Thakur Photovoltaic cells have been fabricated using titanium dioxide/doped poly($\beta $-pinene)/carbon on ITO glass-substrates. Photocurrents and photo-voltages for different intensities of light (from a white illuminant light bulb, emission at 300-700 nm) have been measured. Use of iodine-doped nonconjugated conductive polymer film has led to significant enhancement of photocurrent compared to previous reports which included a different cell structure with undoped polymer-C$_{60}$ composites. A maximum photocurrent of about 0.3 mA was observed for a light intensity of about 5mW/cm$^{2}$. The maximum photo-voltage as observed was about 0.6 V for the same light intensity. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z33.00012: Molecular Imaging of Ultrathin Pentacene Films: Evidence for Homoepitaxy Yanfei Wu, Greg Haugstad, C. Daniel Frisbie Ultrathin polycrystalline films of organic semiconductors have received intensive investigations due to the critical role they play in governing the performance of organic thin film transistors. In this work, a variety of scanning probe microscopy (SPM) techniques have been employed to investigate ultrathin polycrystalline films (1-3 nm) of the benchmark organic semiconductor pentacene. By using spatially resolved Friction Force Microscopy (FFM), Kelvin Probe Force Microscopy (KFM) and Electrostatic Force Microscopy (EFM), an interesting multi-domain structure is revealed within the second layer of the films, characterized as two distinct friction and surface potential domains correlating with each other. The existence of multiple homoepitaxial modes within the films is thus proposed and examined. By employing lattice-revolved imaging using contact mode SPM, direct molecular evidence for the unusual homoepitaxy is obtained. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z33.00013: Understanding the growth of nanoscale organic semiconductors: the role of substrates Mina Yoon, Kai Xiao, Kendal W. Clark, An-Ping Li, David Geohegan, Bobby Sumpter, Sean Smith Our recent studies have demonstrated how substrates can be used to control the synthesis of nanoscale organic semicorductors. In particular, we study the growth mechanism of oriented crystalline organic nanowires consisting of M-TCNQF4 (M=Cu or Ag) from vapor-solid chemical reaction (VSCR). Our experimental and theoretical study combining time-resolved in situ X-ray diffraction and first-principles atomistic calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal substrates. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting. Another example is the growth of another promising organic semiconductor, CuPc assemblies on graphene(s) and Si substrates, where we investigate the role of the substrates in controlling the orientational arrangement of the molecules and their growth modes. Our theoretical study supports the various experimental observations from STM, TEM, and GIXS. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z33.00014: Temperature Dependent Anisotropic Step-Flow Growth of Metal Phthalocyanine on Silicon Studied by Scanning Probe Microscopy Sean Wagner, Richard Lunt, Pengpeng Zhang Control of highly ordered organic molecular thin films is currently of intense interest for integration into modern electronics due to the tunable nature of organic molecules. Here, we study the initial growth of archetypal zinc phthalocyanine (ZnPc) and copper phthalocyanine (CuPc) on the deactivated Si(111) surface. Using scanning probe microscopy (SPM), we demonstrate access to a new quasi-epitaxial anisotropic step-flow growth for both ZnPc and CuPc with a \textit{single} dominant long-range ordered relationship between the organic crystalline film and the substrate, uniquely distinct from inorganic epitaxial step-flow growth. This growth mode is largely attributed to the molecular diffusion and preferential nucleation at step edges enabled by the deactivated Si surface. We demonstrate the transition of growth modes by varying substrate temperature during deposition, altering the balance between diffusion and step- and island- nucleation rates. Access to the anisotropic step-flow growth offers new potential for the integration of highly-ordered organic thin films in silicon-based electronics. [Preview Abstract] |
Session Z34: Polymeric Glasses
Sponsoring Units: DPOLYChair: Yunlong Guo, Princeton University
Room: 342
Friday, March 22, 2013 11:15AM - 11:27AM |
Z34.00001: Nanostructured glassy polymer films deposited via matrix assisted pulsed laser evaporation Kimberly Shepard, Rodney Priestley It has recently been illustrated that nanostructured glassy polymer films can be formed via Matrix Assisted Pulsed Laser Evaporation (MAPLE). During the MAPLE process, a pulsed laser beam strikes a target, which is made of a frozen dilute polymer solution held under high vacuum. The interaction between laser light and target causes phase explosion and subsequent formation of a plume, containing clusters of polymer and solvent. The solvent is pumped off as the plume travels away from the target. The plume is collected on a temperature-controlled substrate, where a polymer film forms at a controlled, slow growth rate. The glassy films formed by MAPLE can exhibit an unusual combination of material properties. For instance, a significant reduction in density may be accompanied with a simultaneous increase in thermal/kinetic stability. These interesting material properties are a result of the films' nanostructured morphology, i.e., they exhibit a nanoglobular morphology. Here, we present further evidence connecting the global film properties to those of the nanoscale building blocks, i.e., the nanoglobules. In addition, we explore the impact of concentration (a key processing parameter) on the morphology of the films. Finally, we demonstrate the generality of nanostructured film formation via MAPLE for a series of poly(n-methacrylate)s. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z34.00002: Acid Diffusion in a Reacting Polymer Glass Abhijit Patil, Ginusha Perera, Yogendra Pandey, Manolis Doxastakis, Gila Stein The acid-catalyzed deprotection of glassy polymer films is an important process in photolithography. It is well-established that acid diffusion controls the deprotection kinetics, but simple Fickian transport models cannot capture experimental data. We examined the acid-catalyzed deprotection of a glassy poly(4-hydroxystyrene-co-tertbutylacrylate) resin using infrared absorbance spectroscopy and stochastic simulations. Experimental data were interpreted with a model that explicitly accounts for acid transport, where heterogeneities at local length scales are introduced through a non-exponential distribution of waiting times between successive hopping events. Subdiffusive behavior predicts key attributes of the observed deprotection rates, such as fast reaction at short times, slow reaction at long times, and a non-linear dependence on acid loading. These studies suggest that macroscopic deprotection rates are controlled by a strongly non-Fickian acid transport in the glassy polymer resin. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z34.00003: Effect of Hydrogenation on the Glass Transition Temperatures of Novel Ring-Opened Polynorbornenes Adam Burns, Sheng Li, Richard Register Ring-opening metathesis polymerization (ROMP) of norbornene-type monomers has been demonstrated as a facile way to produce block copolymers incorporating semicrystalline, glassy, and rubbery blocks. Of particular interest are block copolymers, made by ROMP, with thermoplastic elastomeric properties. For this application we seek blocks with glass transition temperatures (T$_{\mathrm{g}})$ in excess of 100 $^{\mathrm{o}}$C. To this end, novel substituted norbornene-type monomers with large, rigid substituents have been investigated. A key consequence of the ROMP mechanism is that unsaturation in the monomer is preserved in the polymer. Unsaturation in the polymer backbone is susceptible to degradation; therefore, hydrogenation is required to enhance the long-term stability of these polymers. Hydrogenation can also have a significant impact on the thermal behavior. To investigate this, we have synthesized ROMP polymers of 5-phenyl-2-norbornene and 5-cyclohexyl-2-norbornene. Hydrogenation yielded derivatives with saturated backbones. This series of polymers provides a systematic study on the influence of hydrogenation on the T$_{\mathrm{g}}$ of glassy ROMP polymers. We find that saturation of the side group increases the T$_{\mathrm{g}}$ by 14 $^{\mathrm{o}}$C, irrespective of backbone saturation. Conversely, saturation of the backbone reduces T$_{\mathrm{g}}$ by 17 $^{\mathrm{o}}$C for both aromatic and cycloaliphatic side groups. When compared to analogous studies on other ROMP polymers, it becomes clear that these trends are difficult to predict, highlighting the importance of experimental measurements. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z34.00004: Observing density-dependent formation of a fragile glass in surface-bound molecular chains L.I. Clarke, M.P. Roman, D.R. Stevens, M.C. Scott, J.R. Bochinski Dynamics within a monolayer collection of surface-bound substituted-alkyl chains are studied with narrow-band dielectric spectroscopy. A transition from independent (intra-molecular) motion to complex, glassy (inter-molecular) motion is observed as the surface density increases. At high density, both the glassy mode [1,2] and the sub-T$_{g}$ relaxation [3] have a direct analogy to the equivalent relaxations in polyethylene. Thus, this experimental approach enables observation of the formation of a fragile glass as an explicit function of density. Addition of a strong terminal dipole shows the transition occurring at lower density, dipole-mediated interacting dynamics in the low density regime, and increased dominance of the sub-T$_{g}$ local mode. We will discuss results from monolayers and an analogous siloxane-based substrate where alkyl chain-chain distance can be similarly controlled. [1] M. C. Scott et al. \textit{ACS Nano} \textbf{2}, 2392 (2008). [2] M. Beiner and H. Huth, \textit{Nat. Mater.} \textbf{2}, 595 (2003). [3] Q. Zhang et al., \textit{J. Phys. Chem. B} \textbf{110}, 4924 (2006). [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z34.00005: States of Water in Non-Equilibrium Glassy Polymers Eric Davis, Yossef Elabd For many applications (e.g., packaging, medical devices) a deeper fundamental understanding of the molecular nature of water in glassy polymer coatings is of significant interest. In this study, the sorption and diffusion of water in two glassy polymers, poly(methyl methacrylate) (PMMA) and poly(styrene) (PS), were measured with both quartz crystal microbalance (QSM) and time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Non-Fickian diffusion was observed in both PMMA and PS using both experimental techniques due to the non-equilibrium state of the polymers. The specific states of water were observed with FTIR-ATR spectroscopy, where dimers exist in PMMA below a critical concentration and larger clusters were observed above this concentration. Contrastingly, water only exists in PS as larger clusters over the entire sorption isotherm. A correlation between the states of water and the diffusive activation energy of water was observed. Additionally, the pseudo-equilibrium water sorption isotherms in PMMA and PS were accurately predicted with the non-equilibrium statistical associating fluid theory (NE-SAFT). We predict that the combination of time-resolved FTIR-ATR spectroscopy and NE-SAFT can be used on other water-glassy polymer systems to provide a molecular understanding of non-equilibrium sorption and diffusion. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z34.00006: Role of quantum effects in the glass transition Vladimir Novikov, Alexei Sokolov It is shown that quantum effects lead to a significant decrease of the glass transition temperature $T_{g}$ with respect to the melting temperature $T_{m}$, so that the ratio $T_{g}$/$T_{m}$ can be much smaller than the typical value of 2/3 in materials where $T_{g}$ is near or below $\sim$ 60 K. Furthermore, it is demonstrated that the viscosity or structural relaxation time in such low temperature glass-formers should exhibit highly unusual temperature dependence, namely a decrease of the apparent activation energy upon approaching $T_{g}$ (instead of traditional increase). [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z34.00007: Potential energy landscape contribution to the dynamic heat capacity John McCoy, Jonathan Brown The dynamic heat capacity of a simple polymeric, model glass former was computed using molecular dynamics simulations by sinusoidally driving the temperature and recording the resultant energy. The underlying potential energy landscape of the system was probed by taking a time series of particle positions and quenching them. The resulting dynamic heat capacity demonstrates that the long time relaxation is the direct result of dynamics resulting from the potential energy landscape. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z34.00008: Coarse grained dynamics in the glass phase Anton Smessaert, J\"org Rottler Atomic scale dynamics in glasses is dominated by extended periods of localized vibration, where the crowded surroundings of a particle act as a cage. Collective motion is necessary to escape the cage, and the succession of particle jumps or hops leads to diffusion. Each jump is an elementary relaxation event since the local structure is stable until a jump occurs. The link between local dynamics and structural properties has become of increasing interest in recent years. Aging of the mechanical response has been tied to a power-law distribution of persistence times in the cages, and concentration of hops into dynamical heterogeneities (DH) was observed in granular media and simulations of supercooled liquids in 2D. These studies were limited to small systems or hop detection in subsets, because of the post processing requirements. We present results based on a new algorithm that allows us to detect the hops of all particles during a molecular dynamics simulation. This complete coarse-grained ``map'' of the dynamics allows us to directly investigate temporal and spatial correlations between relaxation events. Furthermore, we can readily identify DH using a cluster algorithm and we explore the impact of aging and deformation on the size and shape of DH. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z34.00009: Dynamic Deformation of Thermosetting Polymers---All Atomistic Simulations Mesfin Tsige, Natalia Shenogina, Sharmila Mukhopadhyay, Soumya Patnaik We are using all-atom molecular dynamics simulations to investigate the interconnection between structural and mechanical properties of highly cross-linked polymer networks. In this study we focused on the widely used resin-hardener system composed of DGEBA epoxy oligomers and aromatic amine hardener DETDA. Accurate cross-linked models were developed using the effective cross-linking procedure that enables to generate thermoset structures with realistic structural characteristics. These models were used to examine the elastic properties of thermosetting networks with various degrees of curing and length of resin strands both in glassy and rubbery states. In our recent study we employed static deformation approach to estimate potential energy contribution to the mechanical response. In the present work we are using dynamic deformation approach which takes into account both potential energy and thermal motions in the structure. Uniaxial, volumetric and shear dynamic deformation modes were used to obtain Young's, bulk, shear moduli and Poisson's ratio directly. We also calculated elastic constants using formulae of linear elasticity and analyzed the results obtained by direct deformation and interconversion methods. The elastic properties determined from these two approaches are in good agreement with each other and also with experimental data. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z34.00010: How melt stretching affect the brittle-ductile transition temperature of polymer glasses Shiwang Cheng, Shi-Qing Wang Upon increasing temperature a brittle polymer glass can turn ductile. PMMA is a good example. For a while this brittle-ductile transition (BDT) was thought to be determined by the emergence of a secondary relaxation....$^{\mathrm{1-3}}$ On the other hand, it has been known for a long time...$^{\mathrm{4-6}}$ that predeformation in the melt state (e.g., melt stretching) can also make brittle glasses behave in a ductile manner. This transformation has recently received a satisfactory explanation based on a picture of structural hybrid for polymer glasses....$^{\mathrm{7}}$ It appears that BDT is dictated by the relative mechanical characteristics of the primary structure (due to the van der Waals bonds) and the chain network. The present work, based on conventional Instron tensile extension tests and DMA tests, shows that melt stretching does not alter the secondary relaxation behavior of PMMA and PC yet can turn them the brittle PMMA ductile and the ductile PC brittle. Moreover, sufficient melt stretching makes the brittle PS ductile although it does not produce any secondary relaxation process. .1. Monnerie, L.; Laupretre, F.; Halary, J. L. \textit{Adv. Polym. Sci }\textbf{2005,} 187, 35-213. 2. Monnerie, L.; Halary, J. L.; Kausch, H. \textit{Adv. Polym. Sci }\textbf{2005,} 187, 215-364. 3. Wu, S. \textit{J. Appl. Polym. Sci. }\textbf{1992,} 46, (4), 619-624. 4. Vincent, P. I. \textit{Polymer }\textbf{1960,} 1, (0), 425-444. 5. Harris, J. S.; Ward, I. M. \textit{J. Mater. Sci. }\textbf{1970,} 5, (7), 573-579. 6. Ender, D. H.; Andrews, R. D. \textit{J. Appl. Phys. }\textbf{1965,} 36, (10), 3057-3062. 7. Zartman, G. D.; Cheng, S.; Li, X.; Lin, F.; Becker, M. L.; Wang, S.-Q. \textit{Macromolecules }\textbf{2012,} 45, (16), 6719-6732. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z34.00011: Influence of entanglements on glass transition temperature of polystyrene Toshiaki Ougizawa, Yoshinori Kinugasa Chain entanglement is essential behavior of polymeric molecules and it seems to affect many physical properties such as not only viscosity of melt state but also glass transition temperature (Tg). But we have not attained the quantitative estimation because the entanglement density is considered as an intrinsic value of the polymer at melt state depending on the chemical structure. Freeze-drying method is known as one of the few ways to make different entanglement density sample from dilute solution. In this study, the influence of entanglements on Tg of polystyrene obtained by the freeze-dried method was estimated quantitatively. The freeze-dried samples showed Tg depression with decreasing the concentration of precursor solution due to the lower entanglement density and their depressed Tg would be saturated when the almost no intermolecular entanglement was formed. The molecular weight dependence of the maximum value of Tg depression was discussed. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z34.00012: Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids Philip Griffin, Alexander Agapov, Alexei Sokolov It is generally accepted that room temperature ionic liquids (RTILs) have many characteristics in common with prototypical molecular glass formers. In order to understand the glassy dynamics of RTILs, we have measured the temperature dependence of structural relaxation time and self diffusion in three imidazolium based RTILs. We demonstrate that self diffusion decouples from structural relaxation in these systems as the temperature is decreased toward Tg, but the degree of decoupling is shown to be exceptionally small. In addition to the weak decoupling, we demonstrate that the temperature dependence of structural relaxation time in all three liquids can be well described by a single Vogel-Fulcher-Tammann (VFT) function over 13 decades in time. Furthermore, the stretching of the structural relaxation is shown to be temperature independent over the same range of timescales, i.e. time-temperature superposition is valid for these ionic liquids in the entire temperature range. These properties are at odds with the usual behavior of most ``fragile'' glass forming liquids. We suggest that these differences may result from strong and directional intermolecular interactions characteristic to RTILs. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z34.00013: Statistical Properties of Fluctuating Local Phases and Fluctuating Local Relaxation Rates in Glass-forming Liquids Gcina Mavimbela, Horacio E. Castillo, Azita Parsaeian Using our recently developed method [1], we determine fluctuating local phases, and their time derivatives, the ``local relaxation rates'', in simulation data of glass forming systems. We determine probability distribution functions (PDFs)and power spectra of the time derivatives at different temperatures. Some of the temperatures are such that the systems are aging for the duration of the simulations and for some of the temperatures, the systems reach equilibrium during the duration of the simulations. We study how the power spectra change with temperature. For the aging systems, we study how the PDFs vary with time.\\[4pt] [1] G.~A.~Mavimbela, H.~E. Castillo and A.~Parsaeian, arxiv:1210.1249. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z34.00014: The Defect Diffusion Model of Glass-Forming Liquids John Fontanella, John Bendler, Mary Wintersgill, Michael Shlesinger The defect diffusion model (DDM) provides an explanation of many properties of glass-forming liquids. For example, it has been used to interpret dielectric relaxation (alpha and beta relaxations and the boson peak), viscosity, ionic conductivity, (including the effects of temperature and pressure) positron annihilation lifetime spectroscopy data, the physical basis of fragility, scaling, the ratio of the apparent isochoric activation energy to the isobaric activation enthalpy and its relationship to monomer volume, and correlation lengths. In the model, the glass transition, Tg, occurs because of rigidity percolation. In addition the transition at T$_{\mathrm{B}}$ (or T$_{\mathrm{LL}})$ is associated with mobility percolation. In the simplest form of the DDM, a supercooled liquid contains mobile single defects (MSDs) and immobile, clustered single defects (ICSDs). Consequently, dynamic heterogeneity is a natural feature of the model. If the glass transition did not intervene, all MSDs would disappear at a critical temperature Tc. In the present talk, the model will be used to comment on the change of heat capacity, thermal expansion coefficient and compressibility at Tg. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z34.00015: Phase behaviour of a 2D system exhibiting inverse melting Ahmad Almudallal, Sergey Buldyrev, Ivan Saika-Voivod We calculate the phase diagram for a square-shoulder square-well potential in two dimensions using Monte Carlo simulation techniques. This potential has been previously used as a model for understanding the connection between the anomalous properties of liquid water and a hypothesized metastable liquid-liquid critical point. In our phase diagram, we find that melting lines appear to be first order, and that one of them exhibits a maximum temperature as well as a maximum pressure, indicating inverse melting (crystallization upon heating) over a small range in pressure. We apply Hamiltonian Gibbs-Duhem integration to find potential parameters that maximize the pressure range over which inverse melting occurs. [Preview Abstract] |
Session Z42: Focus Session: Single Molecule Studies of Protein Nanomachines
Sponsoring Units: DBIOChair: Jing Xu, University of California, Merced
Room: Hilton Baltimore Holiday Ballroom 3
Friday, March 22, 2013 11:15AM - 11:51AM |
Z42.00001: Casein Kinase 2 Reverses Tail-Independent Inactivation of Kinesin-1 Invited Speaker: Jing Xu Kinesin-1 is a plus-end microtubule-based motor, and defects in kinesin-based transport are linked to diseases including neurodegeneration. Kinesin can auto-inhibit via a head-tail interaction, but is believed to be active otherwise. Here we report a tail-independent inactivation of kinesin, reversible by the disease-relevant signalling protein, casein kinase 2 (CK2). The majority of initially active kinesin (native or tail-less) loses its ability to interact with microtubules in vitro, and CK2 reverses this inactivation (approximately fourfold) without altering kinesin's single motor properties. This activation pathway does not require motor phosphorylation, and is independent of head--tail auto-inhibition. In cultured mammalian cells, reducing CK2 expression, but not its kinase activity, decreases the force required to stall lipid droplet transport, consistent with a decreased number of active kinesin motors. Our results (Nat. Commun., 3:754, 2012) provide the first direct evidence of a protein kinase upregulating kinesin-based transport, and suggest a novel pathway for regulating the activity of cargo-bound kinesin. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z42.00002: Investigation of the kinesin stepping mechanism via simulated annealing B.D. Jacobson, S.J. Koch, S.R. Atlas As kinesin processes along the microtubule, the cycle of different chemical states and physical conformations that the protein assumes can be represented by a kinetic model. Such models are preferred for numerical calculations since information about the kinesin stepping mechanism at all levels, from the atomic to the microscopic scale, is fully contained in the particular states of the cycle, in how states transition, and in the rate constants associated to each transition. This greatly simplifies the model of the mechanism while providing a reliable physical picture. We have developed a methodology that optimizes a kinetic model for kinesin built with a minimum of a priori assumptions about the mechanism. We combine Markov chain calculations and simulated annealing optimization to find the rate constants that effectively fit experimental data on kinesin speed and processivity. This optimization scheme leads us to choose the cycle that is most likely to realize the kinesin step. We report details of our kinetic model simulations which best fit experimental data for both single-molecule and gliding motility assays at varying ATP concentrations. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z42.00003: The Role of Secondary Structure on the Mechanical Properties of Titin Ravi Kappiyoor, Daniel Dudek, Ishwar Puri Elastomeric proteins are characterized by high resilience and low stiffness. Recent work suggests that charge interactions between the proteins and water have a large role in these mechanical properties. However, some elastomeric proteins are nonpolar, and, as such, do not have high charge interactions with the surrounding water. This indicates that there are also other factors at work. We consider the role of secondary structure (i.e. alpha helices and beta sheets) on the mechanical properties of one such elastomeric protein, titin. Molecular dynamics simulations are performed on four different configurations: (i) the PEVK domain of titin (little secondary structure in its natural state), (ii) an immunoglobulin-like domain of titin (high secondary structure in its natural state), (iii) the same immunoglobulin-like domain with all of its secondary structure artificially removed, and finally (iv) the PEVK domain linked to the immunoglobulin-like domain in its natural state. These simulations will provide key insight on the role of secondary structure on mechanical properties, which can be used to more efficiently design smart materials. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z42.00004: Revealing Transient Interactions between Phosphatidylinositol-specific Phospholipase C and Phosphatidylcholine--Rich Lipid Vesicles Boqian Yang, Tao He, C\'edric Grauffel, Nathalie Reuter, Mary Roberts, Anne Gershenson Phosphatidylinositol-specific phospholipase C (PI-PLC) enzymes transiently interact with target membranes. Previous fluorescence correlation spectroscopy (FCS) experiments showed that \textit{Bacillus thuringiensis} PI-PLC specifically binds to phosphatidylcholine (PC)--rich membranes and preferentially interacts with unilamellar vesicles that show larger curvature. Mutagenesis studies combined with FCS measurements of binding affinity highlighted the importance of interfacial PI-PLC tyrosines in the PC specificity. All-atom molecular dynamics simulations of PI-PLC performed in the presence of a PC membrane indicate these tyrosines are involved in specific cation-pi interactions with choline headgroups. To further understand those transient interactions between PI-PLC and PC-rich vesicles, we monitor single fluorescently labeled PI-PLC proteins as they cycle on and off surface-tethered small unilamellar vesicles using total internal reflection fluorescent microscopy. The residence times on vesicles along with vesicle size information, based on vesicle fluorescence intensity, reveal the time scales of PI-PLC membrane interactions as well as the curvature dependence. The PC specificity and the vesicle curvature dependence of this PI-PLC/membrane interaction provide insight into how the interface modulates protein-membrane interactions. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z42.00005: Single-Molecule Discrimination within Dendritic Spines of Discrete Perisynaptic Sites of Actin Filament Assembly Driving Postsynaptic Reorganization Invited Speaker: Thomas A. Blanpied In the brain, the strength of synaptic transmission between neurons is principally set by the organization of proteins within the receptive, postsynaptic cell. Synaptic strength at an individual site of contact can remain remarkably stable for months or years. However, it also can undergo diverse forms of plasticity which change the strength at that contact independent of changes to neighboring synapses. Such activity-triggered neural plasticity underlies memory storage and cognitive development, and is disrupted in pathological physiology such as addiction and schizophrenia. Much of the short-term regulation of synaptic plasticity occurs within the postsynaptic cell, in small subcompartments surrounding the synaptic contact. Biochemical subcompartmentalization necessary for synapse-specific plasticity is achieved in part by segregation of synapses to micron-sized protrusions from the cell called dendritic spines. Dendritic spines are heavily enriched in the actin cytoskeleton, and regulation of actin polymerization within dendritic spines controls both basal synaptic strength and many forms of synaptic plasticity. However, understanding the mechanism of this control has been difficult because the submicron dimensions of spines limit examination of actin dynamics in the spine interior by conventional confocal microscopy. To overcome this, we developed single-molecule tracking photoactivated localization microscopy (smtPALM) to measure the movement of individual actin molecules within living spines. This revealed inward actin flow from broad areas of the spine plasma membrane, as well as a dense central core of heterogeneous filament orientation. The velocity of single actin molecules along filaments was elevated in discrete regions within the spine, notably near the postsynaptic density but surprisingly not at the endocytic zone which is involved in some forms of plasticity. We conclude that actin polymerization is initiated at many well-separated foci within spines, an organization that may be necessary for the finely tuned adjustment of synaptic molecular content that underlies functional plasticity. Indeed, further single-molecule mapping studies confirm that actin polymerization drives reorganization of molecular organization at the synapse itself. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z42.00006: Solvated dissipative electro-elastic network model of hydrated proteins Daniel Martin Elastic network models coarse grain proteins into a network of residue beads connected by springs. We add dissipative dynamics to this mechanical system by applying overdamped Langevin equations of motion to normal-mode vibrations of the network. In addition, the network is made heterogeneous and softened at the protein surface by accounting for hydration of the ionized residues. Solvation changes the network Hessian in two ways. Diagonal solvation terms soften the spring constants and off-diagonal dipole-dipole terms correlate displacements of the ionized residues. The model is used to formulate the response functions of the electrostatic potential and electric field appearing in theories of redox reactions and spectroscopy. We also formulate the dielectric response of the protein and find that solvation of the surface ionized residues leads to a slow relaxation peak in the dielectric loss spectrum, about two orders of magnitude slower than the main peak of protein relaxation. Finally, the solvated network is used to formulate the allosteric response of the protein to ion binding. The global thermodynamics of ion binding is not strongly affected by the network solvation, but it dramatically enhances conformational changes in response to placing a charge at the a the active site. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z42.00007: Single Molecule Electron Paramagnetic Resonance Richelle M. Teeling-Smith, Ezekiel Johnston-Halperin, Michael G. Poirier, P. Chris Hammel Electron paramagnetic resonance (EPR) is a powerful spectroscopic tool for studying the dynamics of biomolecular systems. EPR measurements on bulk samples using a commercial X-band spectrometer provide insight into atomic-scale structure and dynamics of ensembles of biomolecules. Separately, single molecule measurements of biomolecular systems allow researchers to capture heterogeneous behaviors that have revealed the molecular mechanisms behind many biological processes. We are merging these two powerful techniques to perform single molecule EPR$.$ In this experiment, we selectively label double-stranded DNA molecules with nitrogen-vacancy (NV) center nanodiamonds and optically detect the magnetic resonance of the NV probe. Shifts and broadening of our EPR peaks indicate the changing position of the attached DNA relative to the applied magnetic field. Using this new technique, we have successfully measured the first EPR spectrum of a single biomolecule. By controlling the geometry of the diamond and the applied magnetic field, we will quantitatively determine the rotational and translational dynamics of single biomolecules. This research provides the foundation for an advanced single molecule magnetic resonance approach to studies of complex biomolecular systems. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z42.00008: Turning a Single Molecule into an Electric Motor Charles Sykes Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically-driven motors have only just been demonstrated [1,2] after many theoretical proposals. Studying the rotation of molecules bound to surfaces offers the advantage that a single layer can be assembled, monitored and manipulated using the tools of surface science. Thioether molecules constitute a simple, robust system with which to study molecular rotation as a function of temperature, electron energy, applied fields, and proximity of neighboring molecules. A butyl methyl sulphide (BuSMe) molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunneling microscope are used to drive directional motion of the BuSMe molecule in a two terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular-scale in real time. The direction and rate of the rotation are related to the chiralities of the molecule and the tip of the microscope (which serves as the electrode), which illustrates the importance of the symmetry of the metal contacts in atomic-scale electrical devices. [1] Experimental Demonstration of a Single-Molecule Electric Motor H. L. Tierney, C. J. Murphy, A. D. Jewell, A. E. Baber, E. V. Iski, H. Y. Khodaverdian, A. F. McGuire, Nikolai Klebanov and E. C. H. Sykes - Nature Nanotechnology 2011, 6, 625-629 [2] Electrically driven directional motion of a four-wheeled molecule on a metal surface Kudernac, T., Ruangsupapichat, N., Parschau, M., Macia, B., Katsonis, N., Harutyunyan, S. R., Ernst, K.-H., Feringa, B. L. - Nature 2011, 479, 208--211 [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z42.00009: Characterization of cellular traction forces at the single-molecule level Alexander Dunn The ability of cells to generate and respond to mechanical cues is an essential aspect of stem cell differentiation, embryonic development, and our senses of touch and hearing. However, our understanding of the roles of mechanical force in cell biology remains in its infancy, due largely to a lack of tools that measure the forces generated by living cells at the molecular scale. Here we describe a new technique termed Molecular Force Microscopy (MFM) that visualizes the forces exerted by single cellular adhesion molecules with nm, pN, and sub-second resolutions. MFM uses novel FRET-based molecular tension sensors that bind to a glass coverslip and present a binding site for integrins, a ubiquitous class of cell adhesion proteins. Cell-generated forces stretch the MFM sensor molecules, resulting in decreased FRET with increasing load that can be imaged at the single-molecule level. Human foreskin fibroblasts adhere to surfaces functionalized with the MFM probes and develop robust focal adhesions. FRET values measured using MFM indicate forces of between 1 and 4 pN per integrin, thus providing the first direct measurement of the tension per integrin molecule necessary to form stable adhesions. The relatively narrow force distribution suggests that mechanical tension is subject to exquisite feedback and control at the molecular level. [Preview Abstract] |
Session Z43: Catalysis and Chemical Reaction Dynamics
Sponsoring Units: DCPChair: James Skinner, University of Wisconsin
Room: Hilton Baltimore Holiday Ballroom 2
Friday, March 22, 2013 11:15AM - 11:27AM |
Z43.00001: Room-temperature self-cleaning molecular sensing by catalytic reactions Keith H. Warnick, Bin Wang, David E. Cliffel, David W. Wright, Richard F. Haglund, Sokrates T. Pantelides New sensing techniques using self-cleaning nanosensors for molecular detection are in demand. Here we describe a room-temperature process in which a nanostructured substrate catalyzes the reaction of a target molecule with atmospheric oxygen and the reaction energy is absorbed by the substrate, where it can in principle be detected. Specifically, we report first-principles calculations describing a reaction catalyzed by Fe-porphyrin at room temperature that breaks O$_{2}$, incorporates an oxygen into the methyl group of 2,4-dinitrotoluene (DNT) and releases 1.9 eV per reaction. The atomic oxygen left on the Fe site can be removed by reacting with another DNT molecule, making the whole process self-cleaning. The reaction energy absorbed by the substrate can in principle be detected optically, as for example, by detecting the metal-insulator phase transition in VO$_{2}$. We further explore issues of sensitivity and selectivity in exploiting this reaction for solid-state molecular sensing. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z43.00002: A density functional theory study of structure-property relationships for Pt-Ni alloy catalysts Liang Cao, Tim Mueller The ORR (Oxygen Reduction Reaction) is an important reaction in devices such as metal-air batteries and PEMFCs (Polymer Electrolyte Membrane fuel cells). Pure Pt is one of the most successful electrode catalysts for this key reaction. However, due to its expense, numerous efforts have been made to find a new catalysis system based on Pt bimetallic alloys, in which Pt is partially replaced by less expensive metals, such as Ni, Co and Fe. Experimental and theoretical works have shown that Pt3Ni alloys have a higher ORR activity than pure Pt. In order to investigate the enhanced catalytic activity, cluster expansions corresponding to a simplified 9-layer Pt-Ni slab model are built to accurately and quickly predict the energies of surfaces as a function of atomic order. With the help of this model, we can study systematically the atomic structure and the surface geometry of Pt3Ni surface system at a variety of temperature and chemical environments, and we can calculate the adsorption binding energies of O, OH and H on both equilibrium and non-equilibrium Pt-Ni(111) surfaces. Also, we can investigate the effects of off-stoichiometry on surface by searching for stable ground states under different concentrations. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z43.00003: First-principles design of a dynamically tunable catalyst for CO$_{2}$ capture and conversion Babatunde Alawode, Alexie Kolpak Due to its role in climate change, there is great interest in finding ways to take advantage of the vast amount of waste CO$_{2}$ we produce by its conversion to useful substances. This approach is currently impractical due to the high temperatures and pressures generally required for the synthesis of compounds using CO$_{2}$ as a precursor. To make direct CO$_{2}$ capture and conversion economically viable, new materials able to catalyze the conversion reactions at significantly milder conditions will be essential. In this work, we use DFT computations to design a dynamically tunable ferroelectric oxide-supported thin film catalyst that can capture CO$_{2}$ directly from the emission stream and convert it into methanol. One promising candidate for a dynamically tunable catalyst of this type is Zn$_{\mathrm{x}}$O$_{\mathrm{y}}$/PbTiO$_{3}$. We demonstrate that switching the polarization of the ferroelectric substrate substantially changes the surface atomic and electronic properties of the heterostructure, thereby alternately encouraging strong CO$_{2}$ adsorption and desorbing the products. Our approach may lead not only to new technologies for reducing emissions, but also to novel catalysts that could decrease energy consumption for industrial-scale synthetic processes. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z43.00004: Preferential condensation of $\beta$ RDX on In metal surfaces Terrence Jach, Ilana G. Goldberg, Fernando D. Vila The energetic compound cyclotrimethylene-trinitramine (RDX) normally crystallizes out of solution at standard temperature and pressure in the $\alpha$ form. This consists of two nitro groups in pseudoaxial positions in relation to the C-N ring, and one nitro group in a pseudoequatorial position in an orthorhombic lattice. A metastable phase, labeled the $\beta$ phase, is difficult to create and rarely observed. It consists of all three nitro groups in pseudoaxial positions, occupying a trigonal lattice. We have observed by means of Raman spectroscopy that RDX crystallized from solution on In metal foil preferentially adopts the $\beta$ phase. We discuss a possible mechanism for this behavior in the context of recently published DFT calculations for RDX on a metal cluster. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z43.00005: Catalytic properties of Pt nanoclusters on defective graphene Ioanna Fampiou, Ashwin Ramasubramaniam Metal nanoparticles on carbon supports hold promise as electrocatalysts in direct methanol fuel cells, proton-exchange membrane fuel cells, and hydrogen fuel cells. Pt nanoclusters on carbon supports have been shown to possess superior catalytic activity and increased selectivity in a variety of electrochemical reactions as compared to bulk Pt electrodes; however, the underlying mechanisms remain poorly understood. We examine the interaction of Pt nanoclusters with point defects in graphene using first-principles density functional theory. The presence of defects in graphene supports enhances the Pt-carbon bonding, which suppresses cluster sintering thus allowing for sustained catalytic performance. Furthermore, stronger binding of clusters at defects is found to increase the tolerance of bound Pt nanoparticles towards CO poisoning. Finally, we examine the role of defective graphene supports on the activity of the cluster for the CO oxidation reaction and obtain estimates for CO-oxidation kinetics. Our results suggest possible avenues for controlling the dispersion and catalytic activity of Pt nanoclusters on carbon supports via defect engineering. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z43.00006: Catalytic Role of Au Nanowires Edison da Silva, Ana Paula F. Nascimento, Miguel A. San-Miguel The oxidation of CO in linear atomic chains (LACs) of Au nanowires (NW) is studied by means of density functional theory calculations using quasi-static (T=0) and finite temperature {\it ab initio} molecular dynamics simulations. The adsorption of O$_2$ and CO molecules on the LAC lead to the formation of an intermediate O$_2$CO complex. Upon thermal activation at room temperature, the complex is able to proceed to oxidation forming a CO$_2$ molecule and leaving an atomic O impurity into the Au LAC. We report the conditions under which this oxidation pathway takes place. This process also explains the appearance of unusual large Au-Au bond distances in the LAC and attributed to the presence of atomic impurities. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z43.00007: Rationale for the high reactivity of the interfacial sites in methanol reaction on Au/TiO2(110) Sampyo Hong, Talat Rahman We have performed density functional theory calculations of methanol decomposition on gold nanoparticle supported on a partially reduced TiO2(110) surface. Our calculations show that the adsorption geometry of 13 atom gold nanoparticle strongly depends on the reduction level of the TiO2(110) surface such that a 30{\%} reduced TiO2(110) surface prefers a hemispherical shape while a 10{\%} reduced TiO2(110) surface prefers a flat shape. This hemispherical geometry of gold nanoparticle has a highest density of interfacial sites among the investigated geometries (flat, spherical, hemispherical ones), which may be a reason for the known high reactivity of interfacial sites towards various reactions on supported gold nanoparticles. We have found that methanol decomposition reaction occurring in the interfacial sites is much facile than that occurring in the non-interfacial sites of TiO2(110) surface in agreement with experiment [1]. We have found that the high activity of the interfacial sites is in fact, a result of charge transfer induced Coulomb interaction among the gold, reactant, and reducible TiO$_{\mathrm{2\thinspace }}$atoms through the formation of ionic O-Au bond between gold and methoxy in the active sites, which turns the participating perimeter gold atom cationic. A direct result of such charge transfer induced repulsion is tilting of the methoxy axis, which leads to facile reaction of methoxy through C-H scission with the bridge oxygen atoms that are readily available from the reducible support. Work supported by DOE Grant No. DE-FG02-07ER15842. [1] S. A. Tenney, B. Cagg, M. Levine, W. He, K. Manandhar, and D. A. Chen, \textit{Surf. Sci.} 606, 1233 (2012). [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z43.00008: Dispersion-corrected first-principles calculation of terahertz vibration, and evidence for weak hydrogen bond formation Masae Takahashi, Yoichi Ishikawa, Hiromasa Ito A weak hydrogen bond (WHB) such as CH--O is very important for the structure, function, and dynamics in a chemical and biological system WHB stretching vibration is in a terahertz (THz) frequency region Very recently, the reasonable performance of dispersion-corrected first-principles to WHB has been proven. In this lecture, we report dispersion-corrected first-principles calculation of the vibrational absorption of some organic crystals, and low-temperature THz spectral measurement, in order to clarify WHB stretching vibration. The THz frequency calculation of a WHB crystal has extremely improved by dispersion correction. Moreover, the discrepancy in frequency between an experiment and calculation and is 10 1/cm or less. Dispersion correction is especially effective for intermolecular mode. The very sharp peak appearing at 4 K is assigned to the intermolecular translational mode that corresponds to WHB stretching vibration. It is difficult to detect and control the WHB formation in a crystal because the binding energy is very small. With the help of the latest intense development of experimental and theoretical technique and its careful use, we reveal solid-state WHB stretching vibration as evidence for the WHB formation that differs in respective WHB networks [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z43.00009: Superior Long range Electric Transport of Organometallic Wire via Stepping Stone mechanism and First Principles Study of Length dependence of Thermoelectric Effects Hisao Nakamura, Takao Ishida, Yoshihiro Asai We revealed the role of metal centers for superior long-range electric transport in organometallic-complex wires via stepping stone mechanism, which is recently proposed in Ref. 1]. We also found that the transport properties of organometallic molecular wire have some advantages to create thermoelectric devices, such as phonon mismatching effect, superior long range transport, and quantum interferences of conducting orbitals. We analyzed the length dependence and metal species dependence of the figure of merit (ZT) with including phonon thermal conductivity based on the first principles calculations. [1] K. Terada, H. Nakamura, K. Kanaizuka, M. Haga, Y. Asai, and T. Ishida, ACS Nano, 6, 1988-1999 (2011). [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z43.00010: Oxygen Molecule Adsorption and Dissociation on Boron-doped Fullerene BC$_{59}$ Shizhong Yang, Lei Zhao, Feng Gao, Guang-Lin Zhao, Ebrahim Khosravi, Diola Bagayoko We studied the oxygen molecule adsorption and dissociation on boron-doped fullerene (B-C$_{59})$ from first principles spin polarized density functional theory method simulation. The results show that O$_{2}$ molecule can be adsorbed and partially reduced on the Pauling sites of B-C$_{59}$. The results are compared with those of nitrogen-doped fullerene (N-C$_{59})$. From the comprehensive simulation results, some implications in catalyst application are given. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z43.00011: Reaction Energies of Oxides using Random Phase Approximation Jun Yan, Jens Hummelshoej, Jens N{\O}rskov Oxides are widely used in industrial heterogeneous catalysis, photo catalysis, electrochemistry and in making batteries and fuel cells. To facilitate the computational engineer and design of novel materials in these fields, it is vital important to quantitatively predict the formation and reactions energies of the oxides. LDA/GGA, the success of which has largely relied on the mysterious error cancellation in the exchange-correlation term, generally failed for these oxides, showing systematic and non-canceling errors. Recently, the use of exact exchange (EXX), plus correlation energy from Random Phase Approximation (RPA) emerges as a promising approach to obtain non-empirical exchange-correlation terms. Exact exchange energy is free of self-interaction error, while RPA correlation energy takes into account dynamic electronic screening and is fully non-local. EXX$+$RPA has shown to systematically improve lattice constants, atomization energies, adsorption energies, reaction barriers for a wide range of systems that have ironic, covalent and van der Waals interactions. In this talk I will present our results comparing RPA and GGA functional for the formation and reaction energies of oxides. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z43.00012: Oxygen vacancy formation in doped ceria: Effects of electron localization and ion local distortion Zhenpeng Hu Density functional theory with plus U approximation has been used to study property of doped ceria, especially oxygen vacancy formation energy on doped CeO$_{\mathrm{2}}$ surface. Surfaces with substitutional dopants having lower valence than Ce(IV) have been studied in detail. Based on our results, there are two factors affecting the formation energy of oxygen vacancy: electron localization to form polaron, and local distortion around dopant while vacancy generating. We discuss related application for these rules in catalysis process. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z43.00013: Two-State Reactivity in Hydrocarbon Oxidation by FeO$+$: New Insight through Temperature Dependent Kinetics Shaun Ard, Josh Melko, Nick Shuman, Albert Viggiano Oxidative activation of C-H and C-C bonds is the rate limiting step in many catalytic applications. Transition metals and their oxides are the active component in numerous catalysts as they have proven to be efficient in the activation of these bonds. We report the temperature dependence of reaction kinetics from 120-700K for reactions of FeO$^{\mathrm{+}}$ with CH$_{\mathrm{4}}$, C$_{\mathrm{2}}$H$_{\mathrm{2}}$, C$_{\mathrm{2}}$H$_{\mathrm{4}}$, and C$_{\mathrm{2}}$H$_{\mathrm{6}}$ for the first time, in an effort to improve the mechanistic understanding, and from that the efficiency of these important reactions. The rate constants were found to decrease smoothly with temperature for each hydrocarbon, except for that with methane which displayed an abrupt change in temperature dependence. The branching fractions for the alcohol producing channels were also found to decrease with temperature for each hydrocarbon, with the exception of ethane where it remained constant. Implications of these results towards catalytic applications and theoretical modeling of these systems will be discussed. Specifically, the role of spin orbit coupling in determining the probability of spin inversion, and thus the importance of the ``two-state reactivity'' model applied to many transition metal oxide and hydrocarbon reactions will be addressed. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z43.00014: Charge relaxation dynamics of an electrolytic nanocapacitor Vaibhav Thakore, James Hickman Understanding charge relaxation dynamics in confined nanospaces with overlapping electric double layers (EDLs) is critical for the development of efficient electrochemical energy storage, energy conversion and bioelectrochemical sensing devices. Using Lattice Boltzmann (LB) method, results from simulations of an electrolytic nanocapacitor subjected to a step potential at t $=$ 0 are presented here for various degrees of EDL overlap, solvent viscosities, ratios of cation to anion diffusivity and electrode separations. A continuously varying molecular speed dependent relaxation time is proposed for use with the LB equation that, unlike the single relaxation time Bhatnagar-Gross-Krook approximation, recovers the correct microscopic description of molecular collision phenomena and holds promise for enhancing the stability of the LB algorithm. Simulations for large EDL overlap showed oscillatory behavior for ionic current densities as opposed to monotonic relaxation to equilibrium for low EDL overlap. Further, at low solvent viscosities and large EDL overlap, an anomalous plasma-like collective behavior of oscillating ions at a frequency much lower than the plasma frequency of the electrolyte was observed and as such it appeared to be purely an effect of nanoscale confinement. [Preview Abstract] |
Session Z44: Focus Session: Cell Mechanics III
Sponsoring Units: DBIOChair: Eric Dufresne, Yale University
Room: Hilton Baltimore Holiday Ballroom 1
Friday, March 22, 2013 11:15AM - 11:27AM |
Z44.00001: Energy barriers for cellular rearrangements in tissues Dapeng Bi, J.H. Lopez, J.M. Schwarz, M. Lisa Manning The behavior of cellular aggregates strongly influences morphogenesis, cancer growth and wound healing. While single cell mechanics has been extensively studied, the collective dynamics of cells inside a tissue is not well understood. Recent experiments have shown cells in tissues behave like fluids on long timescales and solids on shorter timescales, and exhibit caging behavior at intermediate timescales as they are more tightly packed. These observations are reminiscent of dynamic slowing down and dynamical heterogeneities due to mutual confinement and crowding of particles glassy systems. A common and crucial feature of glassy systems is the existence of a Potential Energy Landscape (PEL) for local rearrangements. For thermal glassy materials, when these barriers are large compared to thermal fluctuations, its rheology is dependent on the PEL and external mechanical driving. In contrast, cells in a tissue are non-thermal and overcome energy barriers in the PEL mainly through local active processes, i.e. making new adhesions and cell shape changes. We numerically map the PEL of a confluent tissue as functions of different transition pathways and single cell properties. Analytical calculations are also performed to find the minimal energy shapes for 2-D confluent cell packings. [Preview Abstract] |
Friday, March 22, 2013 11:27AM - 11:39AM |
Z44.00002: Motion of individual and coupled amoebae during collective migration Chenlu Wang, Meghan Driscoll, Sagar Chowdhury, Satyandra K. Gupta, Carole Parent, Wolfgang Losert Collective migration is a ubiquitous natural phenomenon. We analyzed the migration of Dictyostelium Discoideum amoebae, which migrate both individually and collectively. We previously found that individually and collectively migrating cells have similar speed and straightness. We analyzed the effects of cell-cell contact and cell-surface contact on cell characteristics, such as adhesion, speed, and shape. We found that in the absence of cell-surface contact, cells form irregular clumps, yet are still able to migrate collectively in response to an external signal. Individually migrating cells exhibit waves of high boundary curvature that travel from the fronts to the backs of cells. By comparing the shape dynamics of individual cells and groups of cells, we found that these boundary curvature waves can be transmitted from one cell to another. [Preview Abstract] |
Friday, March 22, 2013 11:39AM - 11:51AM |
Z44.00003: Cellular Particle Dynamics simulation of biomechanical relaxation processes of multi-cellular systems Matthew McCune, Ioan Kosztin Cellular Particle Dynamics (CPD) is a theoretical-computational-experimental framework for describing and predicting the time evolution of biomechanical relaxation processes of multi-cellular systems, such as fusion, sorting and compression. In CPD, cells are modeled as an ensemble of cellular particles (CPs) that interact via short range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through numerical integration of their equations of motion. Here we present CPD simulation results for the fusion of both spherical and cylindrical multi-cellular aggregates. First, we calibrate the relevant CPD model parameters for a given cell type by comparing the CPD simulation results for the fusion of two spherical aggregates to the corresponding experimental results. Next, CPD simulations are used to predict the time evolution of the fusion of cylindrical aggregates. The latter is relevant for the formation of tubular multi-cellular structures (i.e., primitive blood vessels) created by the novel bioprinting technology. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z44.00004: Effects of TNF-alpha on Endothelial Cell Collective Migration Desu Chen, Di Wu, Jose Helim Aranda-Espinoza, Wolfgang Losert Tumor necrosis factor (TNF-alpha) is a small cell-signaling protein usually released by monocytes and macrophages during an inflammatory response. Previous work had shown the effects of TNF-alpha on single cell morphology, migration, and biomechanical properties. However, the effect on collective migrations remains unexplored. In this work, we have created scratches on monolayers of human umbilical endothelial cells (HUVECs) treated with 25ng/mL TNF-alpha on glass substrates. The wound healing like processes were imaged with phase contrast microscopy. Quantitative analysis of the collective migration of cells treated with TNF-alpha indicates that these cells maintain their persistent motion and alignment better than untreated cells. In addition, the collective migration was characterized by measuring the amount of non-affine deformations of the wound healing monolayer. We found a lower mean non-affinity and narrower distribution of non-affinities upon TNF-alpha stimulation. These results suggest that TNF-alpha introduces a higher degree of organized cell collective migration. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z44.00005: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z44.00006: Mitotic wavefronts mediated by mechanical signaling in early Drosophila embryos Louis Kang, Timon Idema, Andrea Liu, Tom Lubensky Mitosis in the early Drosophila embryo demonstrates spatial and temporal correlations in the form of wavefronts that travel across the embryo in each cell cycle. This coordinated phenomenon requires a signaling mechanism, which we suggest is mechanical in origin. We have constructed a theoretical model that supports nonlinear wavefront propagation in a mechanically-excitable medium. Previously, we have shown that this model captures quantitatively the wavefront speed as it varies with cell cycle number, for reasonable values of the elastic moduli and damping coefficient of the medium. Now we show that our model also captures the displacements of cell nuclei in the embryo in response to the traveling wavefront. This new result further supports that mechanical signaling may play an important role in mediating mitotic wavefronts. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z44.00007: Cells as Drops and Drops as Cells Invited Speaker: Eric R. Dufresne How do the mechanical properties of tissues emerge from the interactions of individual cells? To shed some light on this fundamental biological question, we consider a model system of clusters of cohesive cells adherent to soft substrates. We quantify traction forces over a wide range of cluster sizes. The scaling of traction stresses with cluster size suggests the emergence of an apparent surface tension for large colonies. To explore the possible impact of cellular surface tension on physiology, we consider the behavior of liquid droplets on soft substrates. In this case, we find that the competition of surface tension and substrate elasticity can lead to rich phenomenology, mimicking certain aspects of the physiology of cells and tissues. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z44.00008: Control Parameter Description of Eukaryotic Chemotaxis Eberhard Bodenschatz, Gabriel Amselem, Albert Bae, Mathias Theves, Carsten Beta The chemotaxis of eukaryotic cells depends both on the average concentration of the chemoattractant and on the steepness of its gradient. For the social amoeba Dictyostelium discoideum, we test quantitatively the prediction by Ueda and Shibata [ Biophys. J. 93 11 (2007)] that the efficacy of chemotaxis depends on a single control parameter only, namely, the signal-to-noise ratio (SNR), determined by the stochastic fluctuations of (i)~the binding of the chemoattractant molecule to the transmembrane receptor and (ii)~the intracellular activation of the effector of the signaling cascade. For SNR 1, the theory captures the experimental findings well, while for larger SNR noise sources further downstream in the signaling pathway need to be taken into account. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z44.00009: Mathematical Modeling of Bacterial Growth Samina Masood We develop a mathematical model for the study of bacterial growth. This model reproduces the growth curve from one equation as well as we can fit it to the experimental data. All the parameters of the model are discussed and compared with the already existing models. Experimental data for the bacterial growth is shown to fit this model. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z44.00010: Measuring the correlation between cell mechanics and myofibroblastic differentiation during maturation of 3D microtissues Ruogang Zhao, Weigang Wang, Thomas Boudou, Christopher Chen, Daniel Reich Tissue stiffness and cellular contractility are two of the most important biomechanical factors regulating pathological transitions of encapsulated cells, such as the differentiation of fibroblasts into myofibroblasts - a key event contributing to tissue fibrosis. However, a quantitative correlation between tissue stiffness and cellular contraction and myofibroblast differentiation has not yet been established in 3D environments, mainly due to the lack of suitable 3D tissue culture models that allow both tissue remodeling and simultaneous measurement of the cell/tissue mechanics. To address this, we have developed a magnetic microtissue tester system that allows the remodeling of arrays of cell-laden 3D collagen microtissues and the measurement of cell and tissue mechanics using magnetically actuated elastomeric microcantilevers. By measuring the development of cell/tissue mechanical properties and the expression level of $\alpha $-smooth muscle actin ($\alpha $-SMA, a marker for myofibroblast differentiation) during a 6 day culture period, we found microtissue stiffness increased by 45{\%} and $\alpha $-SMA expression increased by 38{\%}, but tissue contraction forces only increased by 10{\%}, indicating that tissue stiffness may be the predominant mechanical factor for regulation of myofibroblast differentiation. This study provides new quantitative insight into the regulatory effect of cell and tissue mechanics on cellular function. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z44.00011: Effects of Polymer Surfaces on Proliferation and Differentiation of Embryonic Stem Cells and Bone Marrow Stem Cells Sisi Qin, Wenbin Liao, Yupo Ma, Marcia Simon, Miriam Rafailovich Currently, proliferation and differentiation of stem cell is usually accomplished either \textit{in vivo}, or on chemical coated tissue culture petri dish with the presence of feeder cells. Here we investigated whether they can be directly cultured on polymeric substrates, in the absence of additional factors. We found that mouse embryonic stem cells did not require gelatin and could remain in the undifferentiated state without feeder cells at least for four passages on partially sulfonated polystyrene. The modulii of cells was measured and found to be higher for cells plated directly on the polymer surface than for those on the same surface covered with gelatin and feeder cells. When plated with feeder cells, the modulii was not sensitive to gelatin. Whereas the differentiation properties of human bone marrow stem cells, which are not adherent, are less dependent on either chemical or mechanical properties of the substrate. However, they behave differently on different toughness hydrogels as oppose to on polymer coated thin films. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z44.00012: An Elastic Model of Blebbing in Nuclear Lamin Meshworks Chloe Funkhouser, Rastko Sknepnek, Takeshi Shimi, Anne Goldman, Robert Goldman, Monica Olvera de la Cruz A two-component continuum elastic model is introduced to analyze a nuclear lamin meshwork, a structural element of the lamina of the nuclear envelope. The main component of the lamina is a meshwork of lamin protein filaments providing mechanical support to the nucleus and also playing a role in gene expression. Abnormalities in nuclear shape are associated with a variety of pathologies, including some forms of cancer and Hutchinson-Gilford progeria syndrome, and are often characterized by protruding structures termed nuclear blebs. Nuclear blebs are rich in A-type lamins and may be related to pathological gene expression. We apply the two-dimensional elastic shell model to determine which characteristics of the meshwork could be responsible for blebbing, including heterogeneities in the meshwork thickness and mesh size. We find that if one component of the lamin meshwork, rich in A-type lamins, has a tendency to form a larger mesh size than that rich in B-type lamins, this is sufficient to cause segregation of the lamin components and also to form blebs rich in A-type lamins. The model produces structures with comparable morphologies and mesh size distributions as the lamin meshworks of real, pathological nuclei. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z44.00013: The actin cytoskeleton of chemotactic amoebae operates close to the onset of oscillations Christian Westendorf, Jose Negrete Jr., Albert Bae, Rabea Sandmann, Eberhard Bodenschatz, Carsten Beta We report evidence that the actin machinery of chemotactic Dictyostelium cells operates close to an oscillatory instability. The averaged F-actin response of many cells to a short-time pulse of cAMP is reminiscent of a damped oscillation. At the single-cell level, however, the response dynamics ranged from short, strongly damped responses to slowly decaying, weakly damped oscillations. Furthermore, in a small subpopulation, we observed self-sustained oscillations in the cortical F-actin concentration. We systematically exposed a large number of cells to periodic pulse trains. The results indicate a resonance peak at periodic inputs of around 20 s. We propose a delayed feedback model that explains our experimental findings based on a time-delay in the actin regulatory network. To quantitatively test the model, we performed stimulation experiments with cells that express GFP-tagged fusion proteins of Coronin and Aip1. These served as markers of the F-actin disassembly process and thus allow us to estimate the delay time. Based on this independent estimate, our model predicts an intrinsic period of 20 s, which agrees with the resonance observed experimentally. [Preview Abstract] |
Session Z45: Focus Session: From Molecules to Cells
Sponsoring Units: DBIOChair: Herbert Levine, Rice University
Room: Hilton Baltimore Holiday Ballroom 4
Friday, March 22, 2013 11:15AM - 11:51AM |
Z45.00001: Molecular Circuits that control Bacillus sporulation Invited Speaker: Gurol Suel |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z45.00002: Signal processing in eukaryotic chemotaxis Igor Segota, Archana Rachakonda, Carl Franck Unlike inanimate condensed matter, living cells depend upon the detection of chemical signals for their existence. First, we experimentally determined the chemotaxis response of eukaryotic \emph{Dictyostelium} cells to static folic acid gradients and show that they can respond to gradients as shallow as 0.2\% across the cell body. Second, using Shannon's information theory, we showed that the information cells receive about the gradient exceeds the theoretically predicted information at the receptor-ligand binding step, resulting in the violation of the data processing inequality. Finally, we analyzed how eukaryotic cells can affect the gradient signals by secreting enzymes that degrade the signal. We analyzed this effect with a focus on a well described \emph{Dictyostelium} cAMP chemotaxis system where cAMP signals are affected by an extracellular cAMP phosphodiesterase (PDE) and its inhibitor (PDI). Using a reaction-diffusion model of this set of interactions in the extracellular space, we show that cells can effectively sense much steeper chemical gradients than naively expected (up to a factor of 12). We also found that the rough estimates of experimental PDE and PDI secretion rates are close to the optimal values for gradient sensing as predicted by our model. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z45.00003: Bridging from Replication to Translation with a Thermal, Autonomous Replicator Made from Transfer RNA Dieter Braun, Friederike M. M\"oller, Hubert Krammer Central to the understanding of living systems is the interplay between DNA/RNA and proteins. Known as Eigen paradox, proteins require genetic information while proteins are needed for the replication of genes. RNA world scenarios focus on a base by base replication disconnected from translation. Here we used strategies from DNA machines to demonstrate a tight connection between a basic replication mechanism and translation [1]. A pool of hairpin molecules replicate a two-letter code. The replication is thermally driven: the energy and negative entropy to drive replication is initially stored in metastable hairpins by kinetic cooling. Both are released by a highly specific and exponential replication reaction that is solely implemented by base hybridization. The duplication time is 30s. The reaction is monitored by fluorescence and described by a detailed kinetic model. The RNA hairpins usetransfer RNA sequences and the replication is driven by the simple disequilibrium setting of a thermal gradient [2] The experiments propose a physical rather than a chemical scenario for the autonomous replication of protein encoding information.\\[4pt] [1] Physical Review Letters 108, 238104 (2012).\\[0pt] [2] Physical Review Letters 104, 188102 (2010) [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z45.00004: Symmetrical charge-charge interactions in ionic solutions and implications for cell division Eshel Faraggi As is well known in electrolyte theory, electrostatic fields are attenuated by the presence of mobile charges in the solution. This seems to limit the possibility of an electrostatic repulsion model of biological interactions such as cell division. However, for a system of two charges in an ionic solution it is found that in the context of the symmetries of the system, the electrostatic repulsion between the two parts of a dividing cell are considerably increased as compared to the electrostatic repulsion between two bare charges in a dielectric. This increase in repulsion, directly resulting from interactions between the symmetrical parts of the solute system, was found to be dependent on the magnitude of the charges and the separation between them. It was also found that this increases reaches a steady state for separation greater than a solvent determined length scale related to the Debye length. These findings strongly suggest that electrostatic interactions can play a crucial part in the physical forces that are involved in biological interactions. Most fundamentally this work presents a general physical force by which one can mechanically understand cell division. Such understanding will lead to unforetold new ways in medicine, biology, chemistry, and physics. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z45.00005: Motorized Glasses and Crystals: Microscopic Models of Active Matter and the Cytoskeleton Invited Speaker: Peter Wolynes The interior of cells is constantly forming and reconfiguring via molecular processes that dissipate chemical energy. I will discuss simulations and analytical theories of the quasi-equilibrium phase diagram of simple models of motorized crystals and motorized network glasses. The nonequilibrium nature of molecular motors leads also to dynamical transitions to states with collective sustained flows. Analogies of these dynamical transitions seem to occur in natural and artificially reconstituted cytoskeletons. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z45.00006: Tension-dependent dynamic microtubule model for metaphase and anaphase phenomena Edward Banigan, Michael Lampson, Andrea Liu During cell division, chromosome pairs align at the center of a bipolar microtubule (MT) spindle and oscillate as MTs attaching them to the cell poles polymerize and depolymerize. Pairs later separate as shrinking MTs pull each chromosome toward its respective cell pole. We present a minimal model for these processes. We use the measured tension-dependence of single MT kinetics [1] and extrapolate for compressed MTs. We apply these to a stochastic many MT model, which we solve numerically and with master equations. We find that tension dependence enhances the speed of chromosome pulling by retracting MTs. The force-velocity curve for the single chromosome system is bistable and hysteretic. Above some threshold load, tension fluctuations induce MTs to spontaneously switch from a pulling state into a growing, pushing state. To recover pulling from the pushing state, the load must be reduced far below the threshold. This leads to oscillations in the two-chromosome system. Unlike other models, our model also captures breathing oscillations. We also explore how various components control chromosome dynamics through MT rate constants alone. [1] Akiyoshi et al. (2010) Nature 468, 576. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z45.00007: Cytoskeleton dynamics studied by dispersion-relation fluorescence spectroscopy Ru Wang, Lei Lei, Yingxiao Wang, Alex Levine, Gabriel Popescu Fluorescence is the most widely used microscopy technique for studying the dynamics and function in both medical and biological sciences due to its sensitivity and specificity. Inspired by the spirit of spatial fluorescence correlation spectroscopy, we propose a new method to study the transport dynamics over a broad range of spatial and temporal scales. The molecules of interest are labeled with a fluorophore whose motion gives rise to spontaneous fluorescence intensity fluctuations that can be further analyzed to quantify the governing molecular mass transport dynamics. We analyze these data by the dispersion relation in the form of a power law,$\Gamma \left( q \right)\sim q^{\alpha }$, which describe the relaxation rate of fluorescence intensity fluctuations, $\Gamma $, vs. the wavenumber, q. We used this approach to study the interplay of various cytoskeletal components in intracellular transport under the influence of protein-motor inhibitors. We found that after actin is depolymerized, the transport becomes completely random for a few minutes and then it starts to organize deterministically again. We conclude that the disrupted cytoskeletal components first diffuse in the cytoplasm, but then become attached to microtubules and get transported deterministically. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z45.00008: Stress Generation by Actin-Myosin Networks and Bundles Anders Carlsson, Nilushi Dasanayake Forces and stresses generated by the action of myosin minifilaments are calculated in idealized computer-generated actin networks and bundles. The networks are generated as random collections of actin filaments in two dimensions, and bundles are obtained by constraining the filament orientations. The actin filaments are crosslinked and attached to two fixed walls. Myosin minifilaments are placed on actin filament pairs and allowed to move and deform the network so that it exerts forces on the walls. The vast majority of simulation runs end with contractile minifilament stress, because minifilaments rotate into energetically stable contractile configurations. This process is aided by the bending of actin filaments, which accomodates minifilament rotation. Stresses for bundles are greater than those for isotropic networks, and antiparallel filaments generate more tension than parallel filaments. The forces transmitted by the actin network to the walls of the simulation cell often exceed the tension in the minifilament itself. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z45.00009: Cytoplasmic streaming emerges naturally from hydrodynamic self-organisation of a microfilament suspension Francis Woodhouse, Raymond Goldstein Cytoplasmic streaming is the ubiquitous phenomenon of deliberate, active circulation of the entire liquid contents of a plant or animal cell by the walking of motor proteins on polymer filament tracks. Its manifestation in the plant kingdom is particularly striking, where many cells exhibit highly organised patterns of flow. How these regimented flow templates develop is biologically unclear, but there is growing experimental evidence to support hydrodynamically-mediated self-organisation of the underlying microfilament tracks. Using the spirally-streaming giant internodal cells of the characean algae Chara and Nitella as our prototype, we model the developing sub-cortical streaming cytoplasm as a continuum microfilament suspension subject to hydrodynamic and geometric forcing. We show that our model successfully reproduces emergent streaming behaviour by evolving from a totally disordered initial state into a steady characean ``conveyor belt'' configuration as a consequence of the cell geometry, and discuss applicability to other classes of steadily streaming plant cells. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z45.00010: Coordinated Switching of Bacterial Flagellar Motors in a Single E. Coli Bo Hu, Yuhai Tu The swimming of Escherichia coli is propelled by its multiple flagellar motors. Each motor spins either clockwise or counterclockwise, under the control of an intracellular regulator, CheY-P. A long standing question is whether these motors work independently or not. There can be two mechanisms (extrinsic and intrinsic) to coordinate the switching of bacterial motors. The extrinsic one arises from the fact that different motors in the same cell sense a common biochemical signal (CheY-P) which fluctuates near the motors' response threshold. An alternative, intrinsic mechanism is direct motor-motor coupling which makes synchronized switching energetically favorable. Here, we develop simple models for both mechanisms and uncover their different hallmarks. A quantitative comparison to the recent experiments suggest that the direct coupling mechanism may be accountable for the observed sharp correlation between motors in a single E. coli. Possible origins of this coupling are discussed. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z45.00011: High-Content Movement Analysis as a Diagnostic Tool in \textit{C. elegans} Peter Winter, Andrea Lancichinetti, Leah Krevitt, Luis Amaral, Rick Morimoto Many neurodegenerative diseases manifest themselves through a loss of motor control and give us information about the underlying disease. This loss of coordination is observed in humans and in the model organisms used to study neurodegeneration. In \textit{Caenorhabditis elegans}, there is an extensive genetic library of strains that lack functional neuronal signaling pathways and expressing proteins associated with neurodegenerative diseases. While most of these strains have decrease motility or cause paralysis, relatively few have been screened to look for more subtle changes in motor control such as stiffness, twitching, or other changes in behavior. we use high-resolution position and posture data to automatically analyze the movement of worms from different genetic backgrounds and characterize 14 movement characteristics. By creating a quantitative mapping between the movement characterization and an online database of gene annotation, gene expression, and anatomy, we aim to predict a likely set of cellular and molecular disruptions. This work provides a proof of concept for the use of detailed movement analysis to uncover novel disruptions in certain motor control processes. Knowledge of the molecular origin of these disruptions provided by our understanding of \textit{C. elegans} genetics and physiology could lead to new diagnostic and therapeutic targets for neurodegenerative disease. [Preview Abstract] |
Session Z46: Focus Session: Physics of Proteins IV
Sponsoring Units: DBIOChair: Andrea Markelz, State University of New York at Buffalo
Room: Hilton Baltimore Holiday Ballroom 5
Friday, March 22, 2013 11:15AM - 11:51AM |
Z46.00001: Is tertiary structure really required for specific function of a protein? Invited Speaker: Mikio Kataoka A protein is folded into the unique tertiary structure spontaneously based on the information encoded in the amino acid sequence. It has been believed that the unique tertiary structure is required for the expression of its specific function. However, the discovery of intrinsically disordered proteins (IDP) raised a question whether the structure is really required to function. Some IDP's are folded by the recognition and binding of their targets called coupled folding and binding. We have created many mutants of staphylococcal nuclease (SNase) which have interesting properties. One category of mutants cannot take native structures but show enzymatic activity. Another type of mutants takes stable native structures without activity, despite that the active site residues are completely conserved. The former can be regarded as a model system of IDP. They show ligand-induced folding which is similar to the coupled folding and binding. The mechanism of induced folding has been studied intensively by stopped-flow CD. The reason why activity is lost in the latter mutants will be discussed based on the crystal structure. Consequently, I would like to discuss about the relationship among structure, function and dynamics. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:03PM |
Z46.00002: Structure--Function Studies on Receptor Activation of Photoactive Yellow Protein Sandip Kaledhonkar, Shuo Dai, Rachana Rathod, Wouter Hoff, Aihua Xie Biological signaling in cells starts with detection of stimuli from ever changing environment, results in relay of signal, and finishes with particular cellular response. Photoactive yellow protein (PYP) from a salt loving \textit{Halorhodospira halophila} bacterium is a blue light photoreceptor protein for negative phototaxis and a structural prototype of PAS domain superfamily of signaling and regulatory proteins. Upon absorption of a blue photon by its negatively charged $p$-coumaric acid ($p$CA) chromophore, the receptor state (off-state) undergoes photocyclic process, leading to large amplitude protein quake that results in PYP receptor activation. To understand the structural basis of receptor activation we employ time-resolved FTIR spectroscopic techniques combined with site-specific mutation to search for a key residue involved in protein quake. We will discuss the strategies and experimental results in light of hydrogen bonding network, active site structure and protein quake in PYP. The signaling mechanism leaned from PYP may have implication to understand signal transduction in other proteins. [Preview Abstract] |
Friday, March 22, 2013 12:03PM - 12:15PM |
Z46.00003: Reconciling the concurrent fast and slow cycling of proteins on gene promoter Wei Wang, Yaolai Wang, Feng Liu Proteins appeared to cycle on and off the gene promoters with both long and short periods. We proposed a model to explore the dynamics of promoter-protein interactions, which enable gene transcription to proceed orderly and cyclically. We analytically proved that the intervals between two successive productive interactions are less than tens of seconds. Fitting of the model to the experimental data suggests that proteins rapidly cycle on and off the promoter, with the binding time less than several minutes. Different proteins kick in at different phases of the transcriptional cycle, and the percentage of promoters bound by specific proteins in a cell population oscillates with a period of 40min. We thus reconcile the fast and slow cycling of proteins and reveal the essential mechanism of transcription dynamics. [Preview Abstract] |
Friday, March 22, 2013 12:15PM - 12:27PM |
Z46.00004: Dynamics and pathway of electron tunneling in repair of damaged DNA by photolyase Zheyun Liu, Xunmin Guo, Chuang Tan, Jiang Li, Ya-Ting Kao, Lijuan Wang, Aziz Sancar, Dongping Zhong Through electron tunneling, photolyase, a photoenzyme, restores damaged DNA into normal bases. Here, we report our systematic characterization and analyses of three electron transfer processes in thymine dimer restoration by following the entire dynamical evolution during enzymatic repair with femtosecond resolution. Using (deoxy)uracil and thymine as dimer substrates, we unambiguously determined the electron tunneling pathways for the forward electron transfer to initiate repairing and for the final electron return to restore the active cofactor and complete the repair photocycle. Significantly, we found that the adenine moiety of the unusual bent cofactor is essential to mediating all electron-transfer dynamics through a super-exchange mechanism, leading to a delicate balance of time scales. The active-site structural integrity, unique electron tunneling pathways and the critical role of adenine assure these elementary dynamics in synergy in this complex photorepair machinery to achieve the maximum repair efficiency close to unity. [Preview Abstract] |
Friday, March 22, 2013 12:27PM - 12:39PM |
Z46.00005: Electronic measurements of single-molecule processing by DNA polymerase I Yongki Choi, Tivoli Olsen, Tolga Gul, Brad Corso, Chengjun Dong, William Brown, Gregory Weiss, Philip Collins A single-molecule nanocircuit technique is applied to continuously monitor DNA replication activity by the enzyme DNA polymerase I (Pol I). Using single copies of Pol I bound to a single-walled carbon nanotube device, an electrical signal was generated to reveal enzymatic function and dynamic variability. Continuous, single-molecule-resolution recordings were obtained for Pol I processing homopolymeric DNA templates over 10 minutes and through \textgreater 10,000 DNA replication events. Processivity of up to 40 nucleotide bases was directly observed, and statistical analysis of the recordings determined key kinetic parameters for the enzyme's open and closed conformations. We observe that the closed complex forms a phosphodiester bond in a highly efficient process \textgreater 99.8{\%} of the time, with a mean duration of only 0.3 ms for all four dNTPs. The rate-limiting step for replication occurs during the enzyme's open state, but with a duration that is nearly twice as long for dATP or dTTP incorporation than for dCTP or dGTP. Taken together, the results provide a wealth of new information complementing prior work on the mechanism and dynamics of DNA polymerase. [Preview Abstract] |
Friday, March 22, 2013 12:39PM - 12:51PM |
Z46.00006: Coupling between Switching Regulation and Torque Generation in Bacterial Flagellar Motor Jianhua Xing, Fan Bai, Tohru Minamino, Zhanghan Wu, Keiichi Namba The bacterial flagellar motor plays a crucial role in both bacterial locomotion and chemotaxis. Recent experiments reveal that the switching dynamics of the motor depend on the rotation speed of the motor, and thus the motor torque, nonmonotonically. Here we present a unified mathematical model that treats motor torque generation based on experimental torque-speed curves and the torque-dependent switching based on the Ising type conformational spread model. The model successfully reproduces the observed switching rate as a function of the rotation speed, and provides a generic physical explanation independent of most details. A stator affects the switching dynamics through two mechanisms: accelerating the conformational flipping rate of individual rotor-switching units, which contributes most when the stator works at a high torque and thus a low speed; and influencing a larger number of rotor-switching units within unit time, whose contribution is the greatest when the motor rotates at a high speed. Consequently, the switching rate shows a maximum at intermediate speed, where the above two mechanisms find an optimal output. The load-switching relation may serve as a mechanism for sensing the physical environment, similar to the chemotaxis mechanism for sensing the chemical environment. [Preview Abstract] |
Friday, March 22, 2013 12:51PM - 1:03PM |
Z46.00007: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 1:03PM - 1:15PM |
Z46.00008: Investigating a simple model of protein folding for evidence of self-organized criticality Joelle Murray, Andrew Cleland, Addison Wisthoff Protein folding is a complex, multi-faceted process with many drivers. Systems of this type are ubiquitous in nature and many behave as self-organizing critical (SOC) systems. Does protein folding exhibit self-organizing critical behavior? To answer this question, we developed a simple model of the folding process and searched for evidence of self-organized critical behavior. Furthermore, we investigated whether or not the parameters defining self-organization can shed light on the protein folding process. [Preview Abstract] |
Friday, March 22, 2013 1:15PM - 1:27PM |
Z46.00009: Investigating the nature of folded protein structure with the aid of crystalline and amorphous models Deniz Turgut, Osman Okan, Angel Garcia, Rahmi Ozisik Three-dimensional structure of a protein is closely tied with its function. Understanding the folded shape of a protein provides crucial information both in identifying the function and engineering custom proteins that will perform desired functions. In the current work, based on the symmetries present in the local neighborhood of residues in the folded protein structure, we investigated the possibility of creating protein-like structures from crystalline and amorphous models. Parameters like Radial Distribution Function and Bond Orientational Order Parameter [Steinhardt PJ, Nelson DR, Ronchetti M, Phys Rev B 1983, 28, 784] were used to identify the similarities between the created model structures and over 400 folded protein structures. The results show both similarities and differences between folded protein structures and those obtained from crystalline or amorphous models. [Preview Abstract] |
Friday, March 22, 2013 1:27PM - 1:39PM |
Z46.00010: The Dynamical Transition and DNA hybridization Deepu George, Katherine Niessen, Andrea Markelz Terahertz spectroscopy has contributed to the understanding of the so-called biomolecular dynamical transition [1,2], which has been related to the anharmonic motions necessary to biomolecular function. It has been established that the 220 K transition is associated with solvent dynamics. Recently there has been some evidence that correlated motions of proteins also contribute to the THz response, and possibly a lower temperature dynamical transition arises from internal molecular motions. Here we examine how the temperature dependent THz response changes upon binding of single stranded DNA polynucleotide chains. THz time-domain spectroscopy (THz TDS) transmission measurements are performed on solution phase single stranded DNA (5 bases in length) for a specific sequence GCGCG, its complement CGCGC, and the hybridized pair. Our preliminary results show that while we have consistent decrease in the net dielectric response with binding, the dynamical transition does not change.\\[4pt] [1] Y. He et al ``Protein Dynamical Transition Does Not Require Protein Structure,'' Phys. Rev. Lett., vol. 101, p. 178103, 2008.\\[0pt] [2] F. Lipps et al ``Hydration and temperature interdependence of protein picosecond dynamics,'' Phys. Chem. Chem. Phys. vol. 14, pp. 6375-6381, 2012. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 1:51PM |
Z46.00011: The power of hard-sphere models for proteins: Understanding side-chain conformations and predicting thermodynamic stability Alice Qinhua Zhou, Corey O'Hern, Lynne Regan We seek to dramatically improve computational protein design using minimal models that include only the dominant physical interactions. By modeling proteins with hard-sphere interactions and stereochemical constraints, we are able to explain the side-chain dihedral angle distributions for Leu, Ile, and other hydrophobic residues that are observed in protein crystal structures. We also consider inter-residue interactions on the distribution of side-chain dihedral angles for residues in the hydrophobic core of T4 lysozyme. We calculate the energetic and entropic contributions to the free energy differences between wildtype T4 lysozyme and several mutants involving Leu to Ala substitutions. We find a strong correlation between the entropy difference and the decrease in the melting temperature of the mutatants. These results emphasize that considering both entropy and enthalpy is crucial for obtaining a quantitative understanding of protein stability. [Preview Abstract] |
Friday, March 22, 2013 1:51PM - 2:03PM |
Z46.00012: Role of the different factors contributing to long lived quantum coherence in the FMO complex Nayeli Zuniga-Hansen, Russell Ceballos, Mark S. Byrd The Fenna-Matthews-Olson (FMO) complex is one of the most widely studied photosynthetic complexes. It occurs as a trimer with three identical subunits that contain eight bacteriochlorphylls embedded in a protein environment. The observation of long lived quantum coherence and the remarkably high efficiency with which energy transfer takes place in the FMO complex has brought much attention to try to understand the mechanism behind it. We study the different factors that contribute to the long lived coherence in this complex by looking at the interplay of different parameters within the intermediate regime, where the strength of the coupling to the environment is comparable to the strength of the coupling between the sites of the system. We attempt to verify if the environmental modes due to the protein backbone have an effect on the energy transfer or if it is inherently robust due to its structure. [Preview Abstract] |
Friday, March 22, 2013 2:03PM - 2:15PM |
Z46.00013: Dielectric response of hydrated proteins Dmitry Matyushov We study dipolar susceptibility of hydrated proteins, representing the average dipole moment induced at the hydrated protein by a uniform external field. This parameter shows remarkable variation among proteins. We find a negative value of the dipolar susceptibility for some proteins, which implies a dia-electric dipolar response and negative dielectrophoresis. Such proteins, even though carrying significant permanent dipole moments, repel from the electric field. This outcome is the result of a negative cross-correlation between the protein and water dipoles, compensating for the positive variance of the intrinsic protein dipole in the overall dipolar susceptibility. We therefore suggest that the dipolar response of proteins in solution is strongly affected by the coupling of the protein surface charge to the hydration water. The protein-water dipolar cross-correlations are long-ranged, extending approximately 2 nm from the protein surface into the bulk. A similar correlation length of about 1 nm is found for the electrostatic potential. The model is applied to the analysis of light absorption by protein solutions in the THz window of radiation. Here we also find significant deviations of the absorption coefficient from the predictions of traditional theories. [Preview Abstract] |
Session Z47: Invited Session: Active, Non-Equilibrium Dynamics in Complex Cellular Networks
Sponsoring Units: DBIO GSNPChair: Margaret Gardel, University of Chicago
Room: Hilton Baltimore Holiday Ballroom 6
Friday, March 22, 2013 11:15AM - 11:51AM |
Z47.00001: Probing mechanics and activity of cytoskeletal networks using carbon nanotubes Invited Speaker: Nikta Fakhri We use single-walled carbon nanotubes (SWNTs) as multi-scale micro-probes to monitor transport and fluctuations in cytoskeletal networks. SWNTs are nanometer-diameter hollow carbon filaments with micrometer lengths and a tunable bending stiffness. Their persistence length varies between 20-100 microns. We study the motion of individual SWNTs in reconstituted actin networks by near-infrared fluorescence microscopy. At long times, SWNTs reptate through the networks. At short times, SWNTs sample the spectrum of thermal fluctuations in the networks. We can calculate complex shear moduli from recorded fluctuations and observe power-law scaling in equilibrium actin networks. In the non-equilibrium cytoskeleton of cells we have targeted SWNTs to kinesin motors and thereby to their microtubule tracks. We observe both transport along the tracks as well as active fluctuations of the tracks themselves. [Preview Abstract] |
Friday, March 22, 2013 11:51AM - 12:27PM |
Z47.00002: F-actin Buckling Coordinates Contractility and Severing in a Biomimetic Actomyosin Cortex Invited Speaker: Michael Murrell |
Friday, March 22, 2013 12:27PM - 1:03PM |
Z47.00003: Criticalities in crosslinked actin networks due to myosin activity Invited Speaker: Michael Sheinman Many essential processes in cells and tissues, like motility and morphogenesis, are orchestrated by molecular motors applying internal, active stresses on crosslinked networks of actin filaments. Using scaling analysis, mean-field calculation, numerical modelling and \textit{in vitro} experiments of such active networks we predict and observe different mechanical regimes exhibiting interesting critical behaviours with non-trivial power-law dependencies. Firstly, we find that the presence of active stresses can dramatically increase the stiffness of a floppy network, as was observed in reconstituted intracellular F-actin networks with myosin motors and extracellular gels with contractile cells. Uniform internal stress results in an anomalous, critical mechanical regime only in the vicinity of the rigidity percolation points of the network. However, taking into account heterogeneity of motors, we demonstrate that the motors, stiffening any floppy network, induce large non-affine fluctuations, giving rise to a critical mechanical regime. Secondly, upon increasing motor concentration, the resulting large internal stress is able to significantly enhance unbinding of the network's crosslinks and, therefore, disconnect the initially well-connected network to isolated clusters. However, during this process, when the network approaches marginal connectivity the internal stresses are expected to drop drastically such that the connectivity stabilizes. This general argument and detailed numerical simulations show that motors should drive a well connected network to a close vicinity of a critical point of marginal connectivity. Experiments clearly confirm this conclusion and demonstrate robust critical connectivity of initially well-connected networks, ruptured by the motor activity for a wide range of parameters. M. Sheinman, C.P. Broedersz and F.C. MacKintosh, Phys. Rev. Lett, in press. J. Alvarado, M. Sheinman, A. Sharma, F.C. MacKintosh and G. Koenderink, in preparation. [Preview Abstract] |
Friday, March 22, 2013 1:03PM - 1:39PM |
Z47.00004: Nonequilibrium stabilization of an RNA/protein droplet emulsion by nuclear actin Invited Speaker: Clifford Brangwynne Actin plays a structural role in the cytoplasm. However, actin takes on new functions and structures in the nucleus that are poorly understood. The nuclei of the large oocytes of the frog \textit{X. laevis }specifically accumulate actin to reach high concentrations; however, it remains unclear if this actin polymerizes into a network, and what, if any, structural role such an actin network might play. Here, we use microrheological and confocal imaging techniques to probe the local architecture and mechanics of the nucleus. Our data show that actin forms a weak network that spatially organizes the nucleus by kinetically stabilizing embedded liquid-like RNA/protein bodies which are important for cell growth. In actin-disrupted nuclei this RNA/protein droplet emulsion is destabilized leading to homotypic coalescence into single large droplets. Our data provide intriguing new insights into why large cell nuclei require an actin-based structural scaffold. [Preview Abstract] |
Friday, March 22, 2013 1:39PM - 2:15PM |
Z47.00005: Nonequilibrium motion of chromosomal loci in living cells Invited Speaker: Julie Theriot |
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