Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y7: Insulators: Transport, Spectroscopies, etc. |
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Sponsoring Units: DCMP Chair: Andrei Sushkov, University of Maryland Room: 006B |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y7.00001: Nanoscale Determination of the Mass Enhancement Factor in Lightly-Doped Bulk Insulator PbSe Kane Scipioni, Ilija Zeljkovic, Daniel Walkup, Yoshinori Okada, Wenwen Zhou, Raman Sankar, Guoqing Chang, Yung Jui Wang, Hsin Lan, Arun Bansil, Fangcheng Chou, Ziqiang Wang, Vidya Madhavan Phonons play a significant role in achieving the desired thermoelectric properties of many materials. Recent evidence suggests that electron-phonon coupling plays an important role in specifically the Lead and Bismuth Chalcogenides. Thus, quantifying the interaction between phonons and electrons is of immense importance for understanding of these systems. Nearly all information about electron-phonon coupling is contained in the Eliashberg function of the material, but its precise extraction has in part been limited due to the lack of local experimental probes. By utilizing Landau level spectroscopy, we construct a method to directly extract the Eliashberg function, and demonstrate its applicability to lightly-doped thermoelectric bulk insulator PbSe. In addition to high energy, and access to both occupied and unoccupied electronic states, this novel experimental method could be used to detect variations in the mass enhancement factor ($\lambda$) on microscopic length scales, which opens up a unique pathway for investigating the effects of chemical defects, surface doping and strain on $\lambda$. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y7.00002: Dimerization-Induced Fermi Surface Reconstruction in IrTe$_{2}$ ManJin Eom, Kyoo Kim, Youn Jung Jo, Junjie Yang, Eun Sang Choi, Byung Il Min, Jae-Hoon Park, Sang-Wook Cheong, Jun Sung Kim We report a de Haas-van Alphen (dHvA) oscillation study on IrTe$_{2}$ single crystals showing complex dimer formations. By comparing the angle dependence of dHvA oscillations with band structure calculations, we show distinct Fermi surface reconstruction induced by a 1/5-type and a 1/8-type dimerizations. This verifies that an intriguing quasi-two-dimensional conducting plane across the layers is induced by dimerization in both cases. A phase transition from the 1/5-type to the 1/8-type dimerizations reveals that local instabilities associated with intra- and inter-dimer couplings are the main driving force for complex dimer formations in IrTe$_{2}$. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y7.00003: Enhanced terahertz emission from a femtosecond-laser-ablated photoconductor Athanasios Margiolakis, Zhen-Yu Zhao, Peter Hale, Julien Madeo, Michael Man, Quan-Zhong Zhao, Wei Peng, Keshav Dani Terahertz (THz) emission properties from bow-tie antennas fabricated on a femtosecond-laser-ablated, semi-insulating gallium arsenide (SI-GaAs) photoconductor are investigated. The ablated material demonstrates increased photoabsorption resulting in increased photocurrent leading to a more efficient optical to THz efficiency. We use THz time-domain spectroscopy (THz-TDS) in order to compare the relative efficiency of the two fabricated devices. The influence of the excitation power and applied bias on the antennas electrodes for both ablated and non-ablated substrates is studied, highlighting the better performances of the ablated devices. A 60{\%} enhancement in THz emission amplitude is observed in the frequency range 0.5 - 4 THz of the ablated SI-GaAs antenna, compared to untreated SI-GaAs. Our experimental results are in agreement with Drude-Lorentz numerical simulations using previously reported absorption and photocurrent properties of femtosecond laser ablated SI-GaAs based photoconductors. This material treatment provides a new way to achieve THz-TDS systems based on SI-GaAs antennas with an improved signal-to-noise ratio. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y7.00004: Optical and Photo-stimulated EPR Studies on Intrinsic and Mn-doped Zinc Germinates Phosphors Xiaojun Wang, Zhiyi He, Li Ma Intrinsic zinc germinates (Zn2GeO4) and Mn-doped Zn2GeO4 phosphors have been prepared using solid state reaction and their photoluminescence and phosphorescence studied. Phosphorescence from both the Mn2$+$ ions and host defects in Zn2GeO4 has observed upon UV excitation, while the Mn2$+$ ions present a longer persistent time than the defects. The charging process has also been studied and different behaviors of Mn2$+$ and defects observed. Electron paramagnetic resonance (EPR) and photo (UV)-stimulated EPR spectra have been collected for both host defects and Mn dopants from 20 K to room temperature. UV-induced EPR signal and the decay processes have been analyzed and provided a better understanding of the trapping mechanism for the phosphorescence. EPR signal from Mn2$+$ has been found decreasing after the UV excitation, indicating that the population of Mn2$+$ ions decreases in the trapping state and the valence change from Mn2$+$ to Mn3$+$ when hole trapping occurred. [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y7.00005: Large thermal Hall effect in a frustrated pyrochlore magnet Max Hirschberger, Jason Krizan, Robert J. Cava, N. Phuan Ong In frustrated magnetism, the nature of the ground state and its elementary excitations are a matter of considerable debate. We present a detailed study of the full thermal conductivity tensor $\kappa_{ij}$, including the Righi-Leduc (or thermal Hall) effect, in single crystals of the frustrated quantum spin-ice pyrochlore Tb$_2$Ti$_2$O$_7$. The off-diagonal response $\kappa_{xy}/T$ is large in this insulating material, despite the absence of itinerant electrons experiencing the Lorentz force. Our experiments over the temperature range of $0.8 - 200\,$K and in fields up to $14\,$T reveal a remarkable phenomenology: A sizeable field-linear Hall effect $\kappa_{xy}/T$ is observed below $100\,$K, and its slope with respect to magnetic field increases strongly as we cool the sample. We observe significant curvature in the field dependence of $\kappa_{xy}/T$ below $15\,$K. At the lowest temperatures, both $\kappa_{xx}/T$ and the initial slope $\lim_{B\rightarrow 0}[\kappa_{xy}/TB]$ are constant in temperature, behavior reminiscent of fermionic heat conduction in dirty metals. Experimental methods and verification of the intrinsic nature of the effect will be discussed. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y7.00006: Time-dependent Elastic Deformation in Crystal: Insights from Metric Description and Berry Phase Effect Liang Dong, Qian Niu It is well known that elastic deformation in crystal can be described in the language of a metric. However, how the metric couples to the one-electron Hamiltonian in a deformed crystal is not very clear. By coordinate transformation from a Cartesian frame to lattice frame where all coordinates of ions are fixed, the metric emerges naturally both in the kinetic energy and potential energy of an electron. Besides, the velocity field of ions is also manifested in the Hamiltonian, which resembles the role of a vector potential. When the deformation slowly varies both in space and time, the wave-packet method can be used to study the Berry phase effect of deformation. This method applies to finite-strain cases and is accurate up to the first order of strain gradient. Different deformation effects are discussed, such as piezoelectricity, flexoelectricity and curving effect of a two-dimensional material [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y7.00007: Theoretical Studies of the Optical Properties of Eu doped Barium Mixed Halides: From X-ray Storage Phosphor to Bright Scintillator Slim Chourou, Gregory Bizarri, Andrew Canning The Eu doped Ba mixed halide family BaBrX (X=F,Cl,Br,I) changes from being a widely used X-ray Storage Phosphor (BaBrF:Eu) to one of the brightest know new gamma ray detector scintillators (BaBrI:Eu). To help understand these contrasting optical properties and guide in the design of new and improved scintillator detectors, in collaboration with experimental groups, we have performed first principles theoretical studies of these materials. In particular we have studied their electron and hole trapping mechanisms and how that can explain their very different optical properties. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y7.00008: Single-particle dispersion in tetragonal CuO Clemens Adolphs, Mona Berciu We study the single-hole dispersion in a layer of tetragonal CuO using a variational approach in which fluctuations between the copper spins are neglected. This approach has recently been applied to the single-particle dispersion in a CuO$_2$ layer, where it successfully reproduces the experimentally observed dispersion. Since the CuO lattice can be viewed as two interspersed CuO$_2$ lattices with weak intra-layer coupling, we expect this approach to be valid for CuO as well. The intra-layer coupling leads to an interesting spectral feature at $k = (\pi/2, \pi/2)$, where it turns the minimum found in the CuO$_2$ dispersion into a saddle point. This is a result of the intra-layer coupling lifting the degeneracy between quasiparticles occupying different sublattices. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y7.00009: Electric polarization and the photogalvanic effect in solids with a topological band structure Benjamin M. Fregoso It is known that solids without inversion symmetry can exhibit photogalvanic effects and intrinsic electric polarization, e.g., ferroelectrics. Understanding the relation between the strength of the induced current and the electric polarization has proven challenging. We report on model calculations with topologically non-trivial band structure aimed at quantifying these contributions. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y7.00010: Room Temperature Ferroelectricity in Ultrathin SnTe Films Kai Chang, Junwei Liu, Haicheng Lin, Kun Zhao, Yong Zhong, Shuai-Hua Ji, Ke He, Lili Wang, Xucun Ma, Liang Fu, Xi Chen, Qi-Kun Xue The ultrathin SnTe films with several unit cell thickness grown on graphitized SiC(0001) surface have been studied by the scanning tunneling microscopy and spectroscopy (STM/S). The domain structures, local lattice distortion and the electronic band bending at film edges induced by the in-plane spontaneous polarization along $\langle110\rangle$ have been revealed at atomic scale. The experiments at variant temperature show that the Curie temperature $T_c$ of the one unit cell thick (two atomic layers) SnTe film is as high as 280K, much higher than that of the bulk counterpart ($\sim$100K); and the 2-4 unit cell thick films even indicate robust ferroelectricity at room temperature. This $T_c$ enhancement is attributed to the stress-free interface, larger electronic band gap and greatly reduced Sn vacancy concentration in the ultrathin films. The lateral domain size varies from several tens to several hundreds of nanometers, and the spontaneous polarization direction could be modified by STM tip. Those properties of ultrathin SnTe films show the potential application on ferroelectric devices. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y7.00011: A comparison of homogeneous and inhomogeneous absorption broadening in Nd doped Gadolinium Gallium Garnet Christopher Ferri, Sayantani Ghosh We perform a temperature dependent optical absorption study of the $ ^{4}I_{9/2} \to ^{4}F_{3/2}$ ($Z_n \to R_n$)transition of the Neodymium dopant in Neodymium(III) doped Gadolinium Gallium Garnet. Absorption spectra are acquired for these transitions as a function of temperature from 12K to 300K. The absorption peaks are subsequently fit with Voigt profiles to measure the homogeneous and inhomogeneous broadening for each transition. We find that for the $Z_1 \to R_2$ transition the homogeneous and inhomogeneous broadening are of the same order. Below 80K, for the $Z_1 \to R_1$ transition, the homogeneous broadening is below the resolution of our spectrometer (1.85 cm$^{-1}$) resulting in a lower bound of $\sim 18$ps for the excited state lifetime which is likely much slower. Furthermore, because of the resolution limit, the ratio of inhomogeneous broadening to homogeneous broadening for this transition has a lower bound of $\sim 6$. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y7.00012: Thermal boundary conductance of hydrophilic and hydrophobic ionic liquids Takafumi Oyake, Masanori Sakata, Susumu Yada, Junichiro Shiomi A solid/liquid interface plays a critical role for understanding mechanisms of biological and physical science. Moreover, carrier density of the surface is dramatically enhanced by electric double layer with ionic liquid, salt in the liquid state. Here, we have measured the thermal boundary conductance (TBC) across an interface of gold thin film and ionic liquid by using time-domain thermoreflectance technique. Following the prior researches, we have identified the TBC of two interfaces. One is gold and hydrophilic ionic liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF4), which is a hydrophilic ionic liquid, and the other is N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide (DEME-TFSI), which is a hydrophobic ionic liquid. We found that the TBC between gold and DEME-TFIS (19 MWm$^{-2}$K$^{-1}$) is surprisingly lower than the interface between gold and DEME-BF4 (45 MWm$^{-2}$K$^{-1}$). With these data, the importance of the wetting angle and ion concentration for the thermal transport at the solid/ionic liquid interface is discussed. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y7.00013: Lattice dynamics of negative thermal expansion in ScF3 Jason Hancock, Sahan Handunkanda, Ayman Said, Bogdan Leu, Vladimir Voronov We present inelastic scattering spectroscopy of single-crystalline samples of negative thermal expansion material ScF3 to investigate the dispersion of lattice excitations in this unusual system. The spectra reveal important mechanistic information regarding the negative thermal expansion and show that a large class of fluorides can accommodate unconventional lattice dynamics. The connections to nonlinear and distinctly quantum phenomena will be discussed. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y7.00014: Low field, non-destructive investigation of the effect of moisture on the reliability of low-k dielectrics via dielectric relaxation Robert Laibowitz, Archana Raja, Eric Liniger, Thomas Shaw Assessing the reliability of low dielectric constant ($<$3.9) materials is an important problem for the scalability of integrated circuits to reduced dimensions. The decrease in capacitance ensures lower power consumption and shorter RC delays. The materials are typically nanoporous and are composed of Si, C, O and H. We present a non-destructive, low field investigation of the effect of moisture on Back End of Line integrated circuit capacitors as used in industry. The low-k dielectric layers are around 40nm thick with a k=2.5. Through the measurement of dielectric losses as a function of temperature and applied frequency, we assign two relaxation modes of water in the pores of the dielectric, having thermal activation energies of 0.30 and 0.56eV. Study of dielectric loss as a function of humidification yields a direct correlation to the time dependent dielectric breakdown, wherein increased water incursion into the low k dielectric reduces the lifetime of the device. [Preview Abstract] |
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