Bulletin of the American Physical Society
2013 Annual Meeting of the California-Nevada Section of the APS
Volume 58, Number 14
Friday–Saturday, November 1–2, 2013; Rohnert Park, California
Session D2: Condensed Matter I |
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Chair: Hendrik Ohldag, SLAC National Accelerator Laboratory Room: Darwin 29 |
Friday, November 1, 2013 4:12PM - 4:24PM |
D2.00001: Magnetic Flux Creep Studies in High Temperature Superconductors Scott Sawyer, Dominique Davenport, Rose Zhang In order to understand the mixed state properties of type II superconductors, high-Tc ceramic superconductors with varying numbers of Cu-O layers were prepared, including La2CuO4$+$d, YBa2Cu3O7-d, Bi1.7Pb0.3Sr2CaCu2O8-d, and TlBa2Ca3Cu4O11-d. In addition to studying the transition temperatures and Meissner fractions, the magnetic time relaxations were measured, analyzed, and compared. Maintaining superconductivity in the mixed state of type II superconductors relies on the pinning of flux lines within the superconductor. Flux creep occurs when the flux lines overcome the pinning force while the magnetic current density is still less than the critical current density. Unlike conventional superconductors where flux creep is a small effect, the new high Tc superconductors (HTS) have measurable magnetic time relaxations. This is due to the higher thermal energy obtained by the higher temperatures of HTS, and because the energy barrier to flux creep is relatively smaller in HTS. The magnetization of the HTS can be modeled by a logarithmic function of time. This relationship determines the barrier energy to flux creep. Measuring the magnetic time relaxations for various HTS shows how properties such as crystal structure, anisotropy, and inter planar spacing effect the flux pinning of HTS. [Preview Abstract] |
Friday, November 1, 2013 4:24PM - 4:36PM |
D2.00002: Anisotropic behavior in CaFe2As2 under uniaxial pressure Miles Frampton For many years, magnetism was believed to destroy superconductivity. However, the recently discovered Iron-based superconductors seem to require magnetism in order to turn superconducting. We study CaFe2As2, an Iron-based superconductor. With optimal doping, superconducting transition temperatures (Tc) have been observed in this compound up to 38K. Studying CaFe2As2 will hopefully shed light on why superconductivity exists in all the Iron-based superconductors. CaFe2As2 has a complicated phase diagram, and we study a specific structural/magnetic transition in CaFe2As2 around 170K under uniaxial pressure in multiple directions. It is believed this transition is strongly tied to superconductivity. Uniaxial pressure is applying pressure in a single direction while leaving the other directions free. Applying uniaxial pressure is useful in this compound because it has anisotropic behavior in magnetism and resistivity. CaFe2As2 grows naturally in flat platelets. Applying pressure perpendicular to the platelet face is easy. Applying pressure parallel to the platelet face is challenging. We would like to present resistivity data of the 170K transition under uniaxial pressure both parallel and perpendicular to the platelet faces. [Preview Abstract] |
Friday, November 1, 2013 4:36PM - 4:48PM |
D2.00003: Long Range Effects in Tunable Superconducting-Magnetic Proximity Systems Thomas E. Baker, Adam Richie-Halford, Ovidiu E. Icreverzi, Andreas Bill We analyze Josephson junctions that contain an exchange spring magnet as interlayer between the two superconductors. Widening the interlayer and varying its magnetic properties have dramatic effects on the current. Here, we accurately model the exchange spring magnet and show that placed as interlayer in the Josephson setup produces not only a long range part to the order parameter but also a new type of 0$-\pi$ current transition resulting from the triplet component. The domain wall in the exchange spring is tunable and thus all effects in the junction are tunable as well. We also investigate the transition of the triplet pairs back into singlets due to the variable magnetization and a resulting re-emergent singlet current through very wide Josephson junctions. [Preview Abstract] |
Friday, November 1, 2013 4:48PM - 5:00PM |
D2.00004: Topological Entanglement Entropy of Fractional Quantum Hall States Clare Abreu, Raul Herrera, Edward Rezayi Quantum information theoretic concepts have been widely used to study topological phases of condensed matter.In particular, quantum entanglement has proven to be a useful tool to probe the topological order of fractional quantum Hall states. We present studies of topological entanglement entropy for some model fractional Hall states in spherical and toroidal geometries. We implement bipartitioning of the system with both orbital and real space cuts for small size systems. We extend these studies to generic Hamiltonians and discuss the prospect of obtaining the topological entanglement entropy from finite size calculations in these systems. [Preview Abstract] |
Friday, November 1, 2013 5:00PM - 5:12PM |
D2.00005: Realistic effects on the electron Wigner crystal energy in the quantum Hall regime Ryan Hashi, Michael Peterson Systems in the quantum Hall regime undergo a quantum phase transition from a quantum liquid to presumably an electron Wigner crystal as the filling factor is lowered below approximately 1/5. Theoretically, one studies this transition by comparing the ground state energies of the quantum liquid and the quantum Wigner crystal. Previous calculations do not include realistic effects such as the finite thickness of the experimental quantum well, and it is unknown how these effects affect the crystal energy. We expand on the classic work by Maki and Zotos [Phys. Rev. B 28, 4349 (1983)] to include various realistic effects in the calculation of the Wigner crystal energies. [Preview Abstract] |
Friday, November 1, 2013 5:12PM - 5:24PM |
D2.00006: Quantum Shock Waves in Collisions of Ultracold Atomic Clouds Sebastiano Peotta, Massimiliano Di Ventra Ultracold atomic gases represent an ideal toolbox to study quantum effects that are difficult to probe using other systems. Here, we use a time-dependent density matrix renormalization group approach to show that the collision of two interacting bosonic clouds in one dimension gives rise to shock waves with a concomitant local energy distribution typical of population inversion, i.e., an effective negative temperature. A classical hydrodynamic description compares well with the exact quantum dynamics only up to the gradient catastrophe time. Such a highly nonequilibrium local distribution, however, does not prevent the system from recovering its initial state after an oscillation period which is renormalized by the interaction. All these results can be tested experimentally. [Preview Abstract] |
Friday, November 1, 2013 5:24PM - 5:36PM |
D2.00007: Calculation of Multipolar Exchange Interactions in Spin-Orbital Coupled Systems Shu-Ting Pi, Ravindra Nanguneri, Sergey Savrasov A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in LDA+U electronic structure calculation. Within mean-field approximation, exchange constants can be mapped into a series of total energy calculations by pair-flip technique. Application to Uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but ferromagnetic in quadrupoles which is very different from past studies. Further calculation of spin-lattice interaction indicates it is of the same order with superexchange and characterizes the overall behavior of quadrupolar part as a competition between them. [Preview Abstract] |
Friday, November 1, 2013 5:36PM - 5:48PM |
D2.00008: Effects of $^{18}$O/$^{16}$O Isotopic substitution on the specific heat of Single Crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ Carlos Sanchez, Victor Aguilar, Oscar Bernal, GuoMeng Zhao Substantial studies of magnetic susceptibility and specific heat on Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ have demonstrated the existence of ferromagnetic (FM), antiferromagnetic (AFM) and charge ordering transitions (CO). In this work, effects of oxygen isotopic substitution on the specific heat of single crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ were studied on two samples, each containing $^{16}$O or high concentration of $^{18}$O. Specific heat was measured for each sample from 3K to 350 K in zero and 50 kOe applied magnetic fields using a Quantum Design Physical Property Measurement System (PPMS) with Specific Heat option. Below 30 K, a Schottky-like anomaly was found for both samples, which seemed to be unaffected by isotope substitution. The FM, AFM and CO transitions were present in both samples, and the CO temperature (T$_{\mathrm{co}}$) seems to depend strongly on the oxygen isotope mass in both zero and 50 kOe fields, which seems to agree with previous magnetization measurement in powder samples. [Preview Abstract] |
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