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 F4: Condensed Matter Physics I |
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Chair: Hendrik Ohldag, SLAC National Accelerator Laboratory Room: Ball Room C |
Friday, October 28, 2016 2:00PM - 2:12PM |
F4.00001: Using Wavelets to Make an Adapted Basis Set Thomas Baker, Glen Evenbly, Anna Kesselman, Kieron Burke, Steven R. White A wavelet transformation is a special type of filter usually reserved for image processing and other applications. We show that wavelets can be used to coarse grain a low-level calculation, such as density functional theory or Hartree-Fock, for use as a basis set in a high-level method, such as density matrix renormalization group or quantum Monte Carlo, in one dimension. The goal is to adapt a basis set to a given quantum chemical system using 2-3 basis functions per electron. We compare a variety of orthogonal wavelets such as coiflets, symlets, and daubechies wavelets as well as a new type of orthogonal wavelet with dilation factor three. Extending the method to three dimensions is also considered. [Preview Abstract] |
Friday, October 28, 2016 2:12PM - 2:24PM |
F4.00002: Magnetic Order-Disorder Transitions on a 1/3 -- Depleted Square Lattice H.-M. Guo, T. Mendes, W. E. Pickett, R. T. Scalettar Quantum Monte Carlo simulations are used to study the magnetic and transport properties of the Hubbard Model, and its strong coupling Heisenberg limit, on a one-third depleted square lattice. This is the geometry occupied, after charge ordering, by the spin-{\$}$\backslash $frac\textbraceleft 1\textbraceright \textbraceleft 2\textbraceright {\$} Ni{\$}\textasciicircum \textbraceleft 1$+$\textbraceright {\$} atoms in a single layer of the nickelate materials La{\$}\textunderscore 3{\$}Ni{\$}\textunderscore 2{\$}O{\$}\textunderscore 6{\$} and La{\$}\textunderscore 4{\$}Ni{\$}\textunderscore 3{\$}O{\$}\textunderscore 8{\$}. Our model is also a description of strained graphene, where a honeycomb lattice has bond strengths which are inequivalent. For the Heisenberg case, we determine the location of the quantum critical point (QCP) where there is an onset of long range antiferromagnetic order (LRAFO), and the magnitude of the order parameter and compare with results of spin wave theory. An ordered phase also exists when electrons are itinerant. In this case, the growth in the antiferromagnetic structure factor roughly coincides with the transition from band insulator to metal in the absence of interactions. [Preview Abstract] |
Friday, October 28, 2016 2:24PM - 2:36PM |
F4.00003: Accurate Electronic and Transport Properties of Bulk wurtzite Beryllium Oxide (w-BeO) Cheick Bamba, Lashounda Franklin, Yuriy Malozovsky, Diola Bagayoko We present ab-initio, self -- consistent density functional theory (DFT) description of electronic, transport, and bulk properties of wurtzite Beryllium Oxide (w-BeO). We used a local density approximation potential (LDA) and the linear combination of atomic orbitals (LCOA) formalism. Our implementation of the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), ensures the full physical content of our local density approximation (LDA) calculations as per the derivation of DFT [ AIP advances,4,127104 (2014). We report the band gap, density of states, partial density of states, effective masses, and bulk modulus. Our calculated band gap of 10.29 eV, using an experimental lattice constant of 2.6979 at room temperature is in agreement with the experimental value of 10.6 eV. [Preview Abstract] |
Friday, October 28, 2016 2:36PM - 2:48PM |
F4.00004: Extreme Quantum Advantage when Simulating Strongly Coupled Classical Systems Cina Aghamohammadi, John R Mahoney, James P Crutchfield Classical stochastic processes can be generated by quantum simulators instead of the more standard classical ones, such as hidden Markov models. One reason for using quantum simulators has recently come to the fore: they generally require less memory than their classical counterparts. Here, we examine this quantum advantage for strongly coupled spin systemsâ€”the Dyson-like one- dimensional Ising spin chain with variable interaction length. We find that the advantage scales with both interaction range and temperature, growing without bound as interaction increases. In particular, it is impossible to classically simulate Dysonâ€™s original spin chain since it requires infinite memory, while quantum simulators can do so since they use only finite memory. Thus, quantum systems can very efficiently simulate strongly coupled classical systems. [Preview Abstract] |
Friday, October 28, 2016 2:48PM - 3:00PM |
F4.00005: Magnetic Phase Transitions of Local Moments Coupled to Multiple Conduction Bands Wenjian Hu, Richard Scalettar Interfaces between strongly and weakly correlated materials, and superlattices thereof, have been the topic of much work over the last decade. Quantum Monte Carlo (QMC) in this area has often described the strongly correlated system with a single band Hubbard model, which supports Mott insulating and magnetic ordering behaviors. Here we study, instead, using determinant quantum Monte Carlo, a bilayer system consisting of a Kondo insulator, represented by a symmetric periodic Anderson model, coupled to a metal. This introduces an additional richness to the problem by allowing consideration of the effect of the metallic band on the competition between antiferromagnetic (AF) order and singlet formation. To understand the magnetic phase transition qualitatively, we first carry out a self-consistent mean field theory (MFT). The basic conclusion is a stabilization of the AF phase to larger $fd$ hybridization $V$. We then employ a QMC treatment which, in combination with finite size scaling, allows us to evaluate the critical $V$ in an exact treatment of the interactions. This approach confirms the stabilization of AF order, which occurs through an enhancement of the Ruderman-Kittel-Kasuya-Yosida interaction by the coupling to the additional metallic band. [Preview Abstract] |
Friday, October 28, 2016 3:00PM - 3:12PM |
F4.00006: Systematic method for improving first principle calculations of materials under extreme conditions Justin Smith, Kieron Burke We develop a new exact method for calculating the density and free energy of electronic systems at moderate temperatures that is used in conjunction with existing computational approaches. This new method relies on an effective thermal potential (ETP) that can be explicitly defined. In practical calculations, a solver that is extremely accurate at very high temperatures is used in conjunction with an approximate ETP constructed from e.g. density functional theory to calculate the properties at a moderate temperature. In this work we lay out the formalism of the scheme and provide a proof-of-principle calculation using the asymmetric Hubbard dimer. We show in this case that our method improves the calculations of approximate densities and maintains the accuracy of the free energy. [Preview Abstract] |
Friday, October 28, 2016 3:12PM - 3:24PM |
F4.00007: Effects of dopant positions in a CdSe QD Jonathan Muliang, Ching-Yao Fong Quantum Dots (QDs) are isolated clusters of atoms with diameters ranging from the order of \textasciitilde 1nm-10nm and can be thought of as ``artificial atoms.''~ In addition to properties of QDs being size-dependent, doping provides another means of controlling the properties of the material, in particular the donor state, just as with bulk semiconductors.~ Using a CdSe QD model with In doping, we examine through ab initio methods the properties with the In dopant at different locations.~ Also, we investigate the effects of a dangling bond on the donor state in the CdSe QD. [Preview Abstract] |
Friday, October 28, 2016 3:24PM - 3:36PM |
F4.00008: Tensor Network Representations of Critical Quantum Systems via the Levin-Wen Construction Michael Flynn, Rajiv Singh, Mukund Rangamani, Andrew Essin We briefly introduce the Levin-Wen construction, a method for creating exactly solvable quantum mechanical lattice models which correspond to fixed-point continuum gauge theories and doubled Chern-Simons theories in $(2+1)D$. The Levin-Wen construction naturally leads to the introduction of tensor networks as a method to efficiently calculate partition functions and other data for finite-size systems. By specializing to the case of the $Z_{2}$ lattice gauge theory, we will show how to construct a tensor network representation of the theory's ground state, while gaining insight into the behavior of the time-evolution of the system. As expected, four topological sectors are observed whose ground states are equal-weight superpositions of the vortex-free states in each topological sector. We will then discuss generalizations of these techniques to other lattice models, and argue that similar schemes for constructing tensor networks should generate ground states for other Levin-Wen Hamiltonians. If time allows, a discussion of the application of tensor networks to the $U(1)\times U(1)$ Chern-Simons (double semion) theory will be included. [Preview Abstract] |
Friday, October 28, 2016 3:36PM - 3:48PM |
F4.00009: Numerical Challenges of a New Parametrized Description of Magnetic Josephson Junctions Brendan Chan, Andreas Bill We present a study of diffusive Josephson junctions made of two superconductors connected by a ferromagnetic film. When the link is sufficiently thin Cooper pairs can tunnel from one superconductor to the other generating a Josephson current. Inhomogeneous magnetic junctions are of particular interest because they display long-range triplet pair correlations. The generation of these correlations in the hybrid structure is studied by solving numerically the Usadel equations. In previous work we have shown that the rotation of the magnetization can be used to tune the relative weight of the singlet and triplet pair correlations, thereby tuning the current. In this talk we present a novel approach to the numerical treatment of the Usadel equations, using a parametrization that takes automatically into account a required constraint of the model. We show results for the weak magnetization case. We also discuss new challenges posed in the high magnetization regime, where the parameterization is accompanied by the appearance of `moving singularities'. Various methods are proposed in order to address this technical issue. The resolution of this problem is important to be able to include a position-dependent pair potential that will allow for the study of new phenomena. [Preview Abstract] |
Friday, October 28, 2016 3:48PM - 4:00PM |
F4.00010: Synthesis of L10 FeNi thin films via High Speed Rapid Thermal Annealing Julius De Rojas, Dustin Gilbert, June Lau, Kai Liu High magnetic anisotropy materials are critical for future ultrahigh density magnetic recording and permanent magnet technologies, particularly those that are rare-earth-free and precious-metal-free. Alloys of FeNi can form a metastable high anisotropy $L$1$_{\mathrm{0}}$ phase, but convenient methods of achieving the $L$1$_{\mathrm{0}}$ ordering have remained elusive. Here we report the synthesis of high-anisotropy $L$1$_{\mathrm{0}}$-Fe$_{\mathrm{50}}$Ni$_{\mathrm{50}}$ alloys via sputtering of alternating atomic layers of Fe and Ni, followed by a high speed rapid thermal annealing (RTA). Compared to as grown FeNi (4nm) films that are magnetically soft, RTA treated samples exhibit substantial increases of coercivity by 1-2 orders of magnitude. Magnetization reversal characteristics analyzed by the first-order reversal curve (FORC) technique show the emergence of a new high-anisotropy phase after RTA, and a phase fraction is extracted. Microstructure analysis by electron diffraction reveals a corresponding appearance of previously forbidden diffractions, consistent with the $L$1$_{\mathrm{0}}$ ordering. These results demonstrate a convenient approach to achieve high-anisotropy $L$1$_{\mathrm{0}}$-FeNi. [Preview Abstract] |
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