Bulletin of the American Physical Society
2016 Annual Meeting of the Far West Section
Volume 61, Number 17
Friday–Saturday, October 28–29, 2016; Davis, California
Session S3: Condensed Matter Physics II |
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Chair: Andreas Bill, California State University, Long Beach Room: Ball Room B |
Saturday, October 29, 2016 2:00PM - 2:12PM |
S3.00001: Polarization induced Z2 and Chern topological phases in a periodically driving field Shu-Ting Pi, Sergey Savrasov Z2 and Chern topological phases such as newly discovered quantum spin Hall and original quantum Hall states hardly both co–exist in a single material due to their contradictory requirement on the time–reversal symmetry (TRS). We show that although the TRS is broken in systems with a periodically driving field, an effective TRS can still be defined provided the ac–field is linearly polarized or certain other conditions are satisfied. The controllable TRS provides us a route to manipulate contradictory phases by tuning the polarization. To demonstrate the idea, we consider a tight-binding model that is relevant to several monolayered materials as a benchmark system. Our calculation shows not only topological Z2 to Chern phase transition occurs but rich Chern phases are also observed. In addition, we also discussed the realization of our proposal in real materials, such as spin-orbit coupled graphene and crystal Bismuth. This opens the possibility of manipulating various topological phases in a single material and can be a promising approach to engineer new electronic states of matter. [Preview Abstract] |
Saturday, October 29, 2016 2:12PM - 2:24PM |
S3.00002: Calculating Nuclear Magnetic Relaxation (NMR) Rates with Numerical Linked Cluster Expansions Nicholas Sherman, Rajiv Singh Numerically calculating low frequency spectral weights and NMR relaxation rates for quantum spin models, beyond a regime where the quasi-particle picture is valid, remains a challenging task. We explore several ways of calculating these quantities using NLC. These include Moriya’s Gaussian approximation, as well as extrapolation from multiple frequency moments of the dynamical structure factor or of the spectral-weight function. We have employed these techniques on several spin models, including the antiferromagnetic Heisenberg model on the Ladder, Kagome, and Square Bilayer. We have found that Moriya’s Gaussian approximation provides a good first approximation for the NMR rates in all cases. However, we find that an extrapolation from multiple frequency moments can provide the most accurate calculations for the NMR rates. The challenge in the extrapolation method arises with the convergence of high frequency moments breaking down early in NLC, and so one must balance using a large number of moments and extrapolation with fewer parameters. We believe the method of extrapolation of multiple frequency moments can be a versatile computational tool for addressing NMR rates and other low frequency probes in quantum-spin and strongly-correlated electron systems. [Preview Abstract] |
Saturday, October 29, 2016 2:24PM - 2:36PM |
S3.00003: Topological insulator and Dirac semimetal in tetragonally strained alkaline earth - pnictide antiperovskites Wen Fong Goh, Warren Pickett Compounds with antiperovskite structure have been suggested to be potential topological insulators, due to their small band gap or gapless electronic characteristics. Using first principles calculations, we survey the entire class of $3\times5\times5$ cubic alkaline earth-pnictide antiperovskites, viz. $\mathrm{Ae_3Pn_APn_B}$, where $\mathrm{Ae=Ca,Sr,Ba}$ and $\mathrm{Pn_A,Pn_B=N,P,As,Sb,Bi}$, and classified these compounds into either trivial insulators or topological semimetals. For the trivial insulators, strain can invert the band ordering to produce topological insulators, while for the topological semimetals, where the band ordering has been inverted by spin-orbit coupling but leaving a gapless bulk state, strain can open up a gap while maintaining the inverted band ordering. Among the antiperovskites that show topological semimetals, $\mathrm{Ca_3BiP}$, a narrow gap semiconductor, is used as an example to illustrate the role played by the spin-orbit coupling and strain in the topological insulator to Dirac semimetal phase transition. Results show that it can be driven into a topological insulating phase under uniaxial compression, or a Dirac semimetallic state under uniaxial expansion. The band inversion diagram, topological surface states and Fermi arc will be presented. [Preview Abstract] |
Saturday, October 29, 2016 2:36PM - 2:48PM |
S3.00004: Landau Level Mixing Effects in the Graphene Fractional Quantum Hall Effect. Yonas Getachew, Michael Peterson A two-dimensional electron system exposed to a strong perpendicular magnetic field at low temperatures forms a new state of matter that exhibits the fractional quantum Hall effect (FQHE). This phenomenon has been observed in graphene, a naturally occurring two-dimensional electron system. Landau level mixing is intrinsic to graphene and must be taken into account in any realistic theoretical treatment [Phys. Rev. B 87, 245129 (2013)]. Recently, an effective model Hamiltonian including Landau level mixing has been formulated in terms of Haldane pseudopotentials: this model includes emergent three-body interactions in addition to renormalizing the two-body interactions. Furthermore, electrons in graphene have spin and valley degrees of freedom, complicating the physics and making exact diagonalization studies formidable. We discuss a real-space realistic Hamiltonian formalism that can be used in future variational Monte Carlo studies of the graphene FQHE. We benchmark this formalism by comparing the results of the Monte Carlo to exact diagonalization results that utilize the pseudopotential. [Preview Abstract] |
Saturday, October 29, 2016 2:48PM - 3:00PM |
S3.00005: Metropolis Monte Carlo and Wang-Landau Simulations of Tricriticality in Crossed Ising Chains Tyler Cary, Richard Scalettar, Rajiv Singh We explore the phase diagram of Ising spins on one-dimensional chains which criss-cross in two perpendicular directions and which are connected by interchain couplings. This system is of interest as a simpler, classical analog of a quantum Hamiltonian which has been proposed as a model of magnetic behavior in Nb$_{12}$O$_{29}$ and also, conceptually, as a geometry which is intermediate between one and two dimensions. Using mean field theory as well as Metropolis Monte Carlo and Wang-Landau simulations, we locate quantitatively the boundaries of four ordered phases. Each becomes an effective Ising model with unique effective couplings at large interchain coupling. Away from this limit we demonstrate non-trivial critical behavior, including tricritical points which separate first and second order phase transitions. Finally, we find that the crossover of the magnetization critical exponent from the Ising to tricritical Ising value shows an unusual non-monotonic behavior. [Preview Abstract] |
Saturday, October 29, 2016 3:00PM - 3:12PM |
S3.00006: Angle dependant NMR spectral studies of URu2Si2 Matthew Lawson, Blaine Bush, Kent Shirer, Nicholas Curro We have measured nuclear magnetic resonance spectra as a function of angle and temperature in isotopically enriched URu$_2$Si$_2$. The statistical moments of the spectra are reported as a function of temperature and angle. A strong enhancement of the third moment near the onset of hidden order is observed. [Preview Abstract] |
Saturday, October 29, 2016 3:12PM - 3:24PM |
S3.00007: Temperature Gradient Induced Movement of Liquid Alloy Droplets in Au on Ge(110) Bret Stenger, Alex Dorsett, James Miller, Erin Russell, Christopher Gabris, Shirley Chiang The growth of Au on Ge(110) was observed with Low Energy Electron Microscopy (LEEM). The objectives of this study were to control the growth of low-dimensional nanostructures and understand the temperature induced motion of islands. Ge(110) was dosed with 0.5-5 ML of Au and heated to 850C. During deposition, islands grew to 1-2 microns in width and 2-5 microns in length, all oriented along the (1,-1,0) direction. The larger islands began moving with speeds of 0.1-1.0 microns/s, absorbing smaller stationary islands upon collision and increasing in size up to 60 microns in width and 100 microns in length. This movement can be explained by a temperature gradient across the sample causing a Ge concentration gradient across the islands, inducing movement in the direction of increasing temperature. Optical microscopy confirmed that the large islands moved from the cooler edges of the sample toward the hotter center of the sample. As the temperature decreased, the island behavior was also studied and revealed rapid island contractions which left traces on the Ge(110) surface. Low Energy Electron Diffraction (LEED) showed a (4x1) reconstruction below 400C, a (4x4) reconstruction between 400C and 500C, and a (2x1) reconstruction above 500C. [Preview Abstract] |
Saturday, October 29, 2016 3:24PM - 3:36PM |
S3.00008: Thermal conductance of graded silicon germanium superlattices Shunda Chen, Ruochong Shen, Pablo Ferrando-Villalba, Javier Rodriguez-Viejo, Davide Donadio Minimizing the thermal conductivity of silicon germanium (SiGe) superlattices is of interest for thermoelectric applications of Si-based devices [1]. Recent experiments have shown unprecedented control in the growth of SiGe superlattices with graded concentration profiles, in which dramatic reduction of the thermal conductance was observed [2]. In this work we use atomistic lattice dynamics simulations to understand the microscopic mechanisms leading to such reduction of the thermal conductance in graded SiGe superlattices. An efficient implementation of the elastic scattering formalism [3] allows us to compute the transmission function and the thermal conductance of atomistic models with the same system size scale as the experimental systems, and to explore the relation between the thermal conductance and the structural parameters of graded superlattices. The database of results thus acquired is analyzed using machine learning methods [4], so to devise a predictive model for thermal transport in this class of systems. [1] C.J. Vineis, et al., Adv. Mater. 22, 3970 (2010) [2] P. Ferrando-Villalba, et al., Nano Research 8(9),2833 (2015) [3] I. Duchemin, D. Donadio, Phys. Rev. B 84, 115423 (2011) [4] T. Hastie, et al., The Elements of Statistical Learning (Springer, 2009) [Preview Abstract] |
Saturday, October 29, 2016 3:36PM - 3:48PM |
S3.00009: Complete Suppression of Magnetism in Gd/(La,Sr)CoO$_{3}$ Films via Redox Design of Oxygen Distributions Peyton Murray, Kai Liu, Aleksey Ionin, Rajesh Chopdekar, Yayoi Takamura, Dustin Gilbert, Alex Grutter, Brian Kirby, Brian Maranville, Julie Borchers, Alpha N'Diaye, Elke Arenholz We demonstrate full control of the magnetism of La0.7Sr0.3CoO3 (LSCO, nominally 40 nm) by interfacial redox reactions realized via a strongly reducing Gd capping layer (0-5nm). Polarized neutron reflectometry reveals full control of the oxygen depth-profile, and the corresponding magnetic profile, by tuning the Gd capping layer thickness. For large Gd thicknesses (5nm), oxygen is removed from deep within the film, fully suppressing the magnetism throughout the entire 40 nm LSCO. X-ray absorption and magnetic circular dichroism also show the suppression of the Co magnetization and an accompanying reduction in the Co oxidation state. X-ray diffraction and reciprocal space mapping show that leaching of oxygen by the Gd capping layer increases the c-axis lattice parameter in the LSCO. These results demonstrate a new level of control of magnetism through redox reactions, using interface reactions to tune and eventually fully suppress bulk magnetization. [Preview Abstract] |
Saturday, October 29, 2016 3:48PM - 4:00PM |
S3.00010: Utilizing Phase Front Propagation in Liquid Crystal Droplets to Control Quantum Dot Self-Assembly Charles Melton, Linda Hirst Topological defects in nematic liquid crystals are known to drive the assembly of included particles. For example, large colloids have been seen to interact and assemble at defect locations and into colloidal crystals via elastic forces. Similar effects have been observed with nano-sized particles, in particular gold particles and quantum dots. Anchoring conditions affect how the liquid crystal orders around the particle, so using specially designed surface ligands is important. Recently, our research group showed that mesogenic surface ligands allow for self-assembly of well defined structures at the isotropic -- nematic phase boundary. For liquid crystals in spherical geometries that possess a well known bipolar configuration we demonstrate that quantum dots co-assemble with the formation of defects upon cooling from the isotropic phase. Spatial and size controlled patterning of quantum dot clusters could be important in photonic applications such as developing a liquid crystal laser. [Preview Abstract] |
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