Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J23: Optical Excitations, Defects and Synthesis of Dielectrics |
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Sponsoring Units: DCMP Chair: Javier Junquera, Universidad de Cantabria, Spain Room: 325 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J23.00001: Unconventional Transport of Spin Bipolarons on an Antiferromagnetic buckled hexagonal lattice of half-filled $d$-band Mn$^{2+}$ ions Verner Thorsmolle, Alexander Ignatov, Maria Pezzoli, Kristjan Haule, David Kolchmeyer, Alexander Lee, Jack Simonson, Meigan Aronson, Girsh Blumberg CaMn$_2$Sb$_2$ presents a magnetic system with a buckled hexagonal lattice of half-filled $d$-band Mn$^{2+}$ ions. AC resistivity and susceptibility exhibit non-monotonic temperature dependence at 85-210~K. Below 85~K it has an antiferromagnetic (AF) phase with an activation energy of 28~meV, and above 210~K a paramagnetic phase. Using Raman spectroscopy we find a mode at 32~meV which develops below the AF transition. We attribute this excitation to the activation energy associated with the motion of spin bipolarons. Here, hybridization between Sb and Mn results in extra electrons for the Mn 3$d$-shells. It is energetically favorable for these extra carriers to form spin-singlets. These spin-bipolarons cover two Mn sites with a binding energy of $\sim$80~meV and conduction proceed via photo-assisted hopping with an activation energy of $\sim$32~meV. This spin bipolaron model explains the spectroscopic features providing a self-consistent picture of this conductivity mechanism that also clarifies reported unusual temperature-dependent magnetic and transport data. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J23.00002: Infrared evidence for multiple structural transitions in single crystal Cu$_3$(SeO$_3)_2$Cl Kevin H. Miller, Helmuth Berger, David B. Tanner Infrared reflection and transmission over a broad temperature range (10-300 K) have been measured on the anisotropic single-crystal Cu$_3$(SeO$_3)_2$Cl. Two distinct space groups have previously been reported for Cu$_3$(SeO$_3)_2$Cl at 300 K (monoclinic C2/m and triclinic P1bar). Comparing the number of observed infrared active phonons with group theoretical predictions points towards the existence of the triclinic structure at 300 K; however, an impurity-rich monoclinic structure cannot be ruled out. New phonon modes are observed upon cooling below 90 K, and again upon cooling below 40 K. The latter temperature range corresponds to the onset of long range magnetic order in the material. The structural and magnetic properties of Cu$_3$(SeO$_3)_2$Cl will be discussed in terms of our infrared spectra, group theoretical predictions, and comparisons to related compounds. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J23.00003: Optical spectroscopy and Fermi Surface studies of the Rashba spin-splitting compound BiTeI Catalin Martin, K.H. Miller, S. Buvaev, A.F. Hebard, E.D. Mun, V. Zapf, H. Berger, D.B. Tanner We measured the temperature dependent optical reflectivity $R(\omega$) and Shubnikov-de Haas oscillations in samples of BiTeI with different carrier concentrations. The electronic excitation spectrum, although consistent with Rashba spin-splitting of the bulk electronic bands, reveals additional features: a low energy excitation band and a larger number of phonons than expected from crystal structure. Some of the vibrational modes have strongly asymmetric line-shape. The period of quantum oscillations scales remarkably well with the component of magnetic field along the crystallographic $c$-axis and is rapidly suppressed when the field is tilted from this axis. We discuss our results in connection with possible charge accumulation at the surface of BiTeI. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J23.00004: Enhancement of charge and spin orders in a photoexcited one-dimensional strongly correlated system Hantao Lu, Shigetoshi Sota, Hiroaki Matsueda, Janez Bonca, Takami Tohyama By using the time-dependent Lanczos method, the nonequilibrium pro- cess of the half-filled one-dimensional extended Hubbard model, driven by a transient laser pulse, is investigated. In the case of large on-site Coulomb interactions, there are two phases separated by a first order quantum phase transition, i.e., spin-density-wave (SDW) and charge-density-wave (CDW) phases, which are characterized by algebraic decay of spin cor- relations and a long-range (staggered) charge order, respectively. When the system is subjected to the irradiation of a laser pulse, from the SDW side near the phase boundary, with proper laser frequency and strength, a sustainable charge order enhancement can be realized while local spin correlations remain. Analogously, from the CDW side, the suppression of long-range charge order is accompanied with a local spin correlation enhancement. We analyze the conditions and investigate possible mecha- nisms of the emerging order enhancements. In off-resonance region, more extended recovery of spin correlations which may come from nonlinear effect is also observed. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J23.00005: Computational study of novel half metallic compounds Zhijian Wu, Jing Wang Half-metallic (HM) materials are metallic for one spin direction while at the same time semiconducting for the other spin direction [1]. The unique feature of HM material is that it has an integer spin magnetic moment. For a carefully selected material, the integer can be zero (compensated). Besides ferromagnetic (FM) parallel spin arrangements, ferrimagnetic or even antiferromagnetic (AFM) alignments are also possible. In particular, half-metallic antiferromagnet (HM-AFM) possesses no macroscopic magnetization, yet their carriers are fully spin-polarized. In this work, half metallic compounds have been predicted by using the first principles, such as NiMoO$_3$ [2].\\[4pt] [1] de Groot, R. A.; Mueller, F. M.; van Engen, P. G.; Buschow, K. H. J. Phys. Rev. Lett., \textbf{\textit{1983}}, \textbf{\textit{50}}\textit{, 2024}\\[0pt] [2] Wang, J., Wu, Z. J. Appl. Phys. Lett, \textbf{\textit{2012}}, \textbf{\textit{101}}\textit{, 042414} [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J23.00006: A non-perturbative general expression for the conductance through a leaky chiral edge mode Kun Woo Kim, Alexandra Junck, Israel Klich, Gil Refael Chiral edge modes of topological insulators and Hall states exhibit non-trivial behavior of conductance in the presence of impurities or additional channels. We will present a simple formula for the conductance through a chiral edge mode coupled to a disordered bulk. For a given coupling matrix between the chiral mode and bulk modes, and a Green function matrix of bulk modes in real space, the renormalized Green function of the chiral mode is expressed in a closed form ratios of determinants. We will conclude with examples of how the formula could be applied to describe the behavior of a chiral mode coupled to different types of bulk systems. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J23.00007: Generalization of the Peierls phase for gauge-invariant Green functions Sylvia D. Swiecicki, J.E. Sipe Solids in time-varying fields can be characterized with the non-equilibrium Green function formalism. If the interaction is described through potentials, the identification of sum rules is necessary to remove unphysical divergences that can appear in low frequency response calculations. For isolated atoms divergences are avoided by moving to a gauge-invariant Hamiltonian with the Power-Zienau-Woolley transformation.\footnote{W. Healy, Non-relativistic quantum electrodynamics (1982)} For solids, a gauge-invariant Green function formalism was proposed by Levanda and Fleurov\footnote{M. Levanda, V. Fleurov, J. Phys: Cond. Matt. \textbf{6} (1994) 7889}; in the generalization of the Peierls phase they introduced they consider only straight lines in spacetime. We extend this work to arbitrary paths in spacetime and show that the results for isolated atoms can be derived as a special case. More general applications are considered. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J23.00008: Evolution of the Coherent State and the Electronic Structure in the Kondo Insulator SmB$_6$ Xiaohang Zhang, N.P. Butch, P. Syers, S. Ziemak, R.L. Greene, J. Paglione As an exemplary Kondo insulator, SmB$_6$ has been studied for several decades; however, direct evidence for the development of the Kondo coherent state and the evolution of the electronic structure in the material has not been obtained due to the rather complicated electronic and thermal transport behavior. Recently, these open questions attracted increasing attention as the emergence of a time-reversal invariant topological surface state in the Kondo insulator has been suggested [1]. Here, we use the quasiparticle tunneling spectroscopy technique to directly investigate the temperature dependence of the electronic states in SmB$_6$. As a signature of the Kondo screening effect in the material, a Fano-like resonance line shape is observed in the tunneling spectroscopy at temperatures below $\sim$ 100 K. We further demonstrate that inter-ion correlation has to be taken into account [2] in order to precisely describe the observed asymmetric tunneling conductance at low temperatures. Our quasiparticle tunneling spectroscopy results also provide important implications for the predicted nontrivial topology in the Kondo insulator.\\[4pt] [1] Dzero et al., PRL 104, 106408 (2010);\\[0pt] [2] Maltseva et al., PRL 103, 206402 (2009) [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J23.00009: Intervalley scattering and localization behaviors of group-VI transition metal dichalcoginides Haizhou Lu, Wang Yao, Di Xiao, Shun-Qing Shen We study the quantum diffusive transport of multi-valley massive Dirac cones coupled by intervalley spin-orbit scattering. We show that the intervalley spin-orbit scattering and intravalley spin-conserved scattering can be distinguished from the quantum conductivity that corrects the semiclassical Drude conductivity, due to their distinct symmetries and localization trends. In immediate practice, it allows transport measurements to estimate the intervalley scattering rate in hole-doped monolayers of group-VI transition metal dichalcogenides (e.g., molybdenum dichalcogenides and tungsten dichalcogenides), an ideal class of materials for valleytronics applications. The results can be generalized to a large class of multi-valley massive Dirac systems where time-reversal symmetry demands opposite spins in opposite valleys. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J23.00010: The structural origin of energy band gap in ultraviolet borates. Zheshuai Lin, Ran He Borate crystals have been intensively studied for their broad significant application in nonlinear optics materials, fluorescent materials, and laser crystals, especially in the ultraviolet (UV) spectral region (photon energy larger than 6.2 eV). However, due to the structural complexity the mechanism determining the energy band gap in the UV borates still hides in clouds. In this work, the structural origins of the energy band gaps in UV borates are systematically studied by ab initio methods and modeling considerations. Through analyzing the electronic band structures, we find that the top of valence bands in UV borates are dominant from the orbitals on oxygen. These orbitals construct the non-bonding states which determine the energy band gaps and their magnitudes depend on the local environments around oxygen atoms. Accordingly, the UV borates are categorized into three structural types, and in each type the ideal energy band gaps by removing the non-bonding states are almost the same. Moreover, a modified Bond Valence Sum method is adopted to parameterize the local environment around oxygen atoms, and the good agreement between the calculated and experimental energy band gaps within the accuracy of 0.3 eV can be achieved in UV borates. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J23.00011: Electron Transport in Edge Metal-Insulator-Metal Tunnel Junctions Modulated by Underlying Ferroelectric Polarization Switching Kibog Park, Youngeun Jeon, Sungchul Jung, Han Byul Jin, Jae-Hyeon Go, Soon-Yong Kwon, Nam Kim The electron energy band profile in an Edge Metal-Insulator-Metal tunnel junction (EMIM) on a Insulator/Ferroelectric thin film was calculated by performing finite-element electrostatic modeling. It is found that the energy band profile in the EMIM junction alters significantly near the underlying Insulator/Ferroelectric layer depending on the polarization direction of ferroelectric layer. The energy band profile shows pinch-off when the interface bound charge at Insulator/Ferroelectric interface is negative while it shows a valley-like shape when the interface bound charge is positive. The change of the energy band profile depending on ferroelectric polarization was confirmed to result in a significant change of electron tunneling current by using WKB method. It is believed that this switching of electron tunneling resistance in the EMIM junction opens up a way to develop non-volatile ferroelectric memory devices using non-destructive read-out. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J23.00012: Material Designs and Combinational Growth Techniques to Enable Novel Multiferroic Devices Melanie Cole, Eric Ngo, Mathew Ivill, S. Gary Hirsch, Cliff Hubbard, Ryan Toonen, Wendy Sarney Voltage control of magnetism in magnetic/ferroelectric bilayers has been most recently demonstrated in ultrathin metallic magnetic films through an electric field induced spin polarized charge screening effect. Voltage-controlled magnetism in magnetic/ferroelectric multilayers would provide a unique opportunity for integrating voltage-tunable RF/microwave magnetic devices on integrated circuits. It has been theoretically predicted that the voltage-control of magnetism in ferromagnetic/ferroelectric heterostructures can be significantly enhanced by utilizing high-K dielectrics. The critical challenge is how to enhance the permittivity of the ferroelectric film while maintaining low loss and low leakage characteristics and accomplishing this in an affordable manner by employing industry standard processing methods and large area low cost substrates. In this work we demonstrate the achievement of high-k, low loss and low leakage BST films utilizing optimized sputtered SrTiO3 buffer layers combined with a MOSD grown Mg-doped Ba0.60Sr0.40TiO3 overgrowth film on affordable large area substrates. Results of this research serves to promote enhanced EM coupling to enable a new class of charge mediated integratable voltage control multiferroic devices exploiting the converse ME effect. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J23.00013: Synchroton Soft X-ray Absorption Studies of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0.0 $\le$ x $\le$ 1.0) Perovskites P. Olalde-Velasco, W.L. Yang, C. Hernandez, E. Chavira, I. Rosales, A. Tejada, L. Huerta, J. Jimenez-Mier, E.E. Marinero This work aims to correlate the interplay between structure-bonding (O2p-TM3d) and magnetic properties (TM 3d) in YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0.0 $\le$ x $\le$ 1.0) perovskites which are synthesized by the solid state reaction method. We have investigated by XAS the O2p and the magnetic TM3d unoccupied states of YbFe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$O$_{3}$ (0$\le $x$\le $1). We find that increasing Mn doping promotes the creation of new states at the O2p band, it also induces a shifts towards lower energies of the O K pre-edge (with reference to the O2p-TM3d hybridization) and changes the spectral distribution in the region of TM 4s, p -- O 2p- Yb 5d hybridization. These changes are most marked for x \textgreater 0.2. A correlated effect with Mn doping is observed in the Fe L$_{\mathrm{2,3}}$ spectra where again new electronic states and systematic changes are observed x \textgreater\ 0.2. This is in contrast with Mn L$_{2,3}$ spectra, where all the spectra are very similar except for x$=$0.2. Thus, we provide a comprehensive picture of the electronic structure evolution in the conduction band in these materials as a function of the Mn content. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J23.00014: Thermodynamic stability of radiogenic Ba in CsAlSi$_2$O$_6$ pollucite John Jaffe, Ren\'ee Van Ginhoven, Weilin Jiang Pollucite, a zeolite-like nanoporous aluminosilicate structure with nominal composition CsAlSi$_2$O$_6$, has been suggested as a nuclear waste storage form for fission-product radioactive isotopes of cesium, especially $^{137}$Cs. One factor affecting the long-term stability of this waste form is the valence change associated with the beta decay that converts Cs into barium. We have used first-principles density functional total energy calculations to evaluate the thermodynamic stability of pollucite with Ba replacing Cs at regular lattice sites with respect to the precipitation of Ba, Cs or their oxides. We included small clusters of substitutional Ba$_{\mathrm{Cs}}$ as well as localized complexes of Ba$_{\mathrm{Cs}}$ with compensating electron donor defects, specifically Cs vacancies and interstitial oxygen. We conclude that Cs-Ba pollucite is thermodynamically stable against precipitation of Cs or its oxide, but that partial precipitation of Ba or BaO may be thermodynamically favored under some conditions. Even this change may be kinetically limited, however. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J23.00015: Metal-induced gap states in ferroelectric capacitors and its relationship with complex band structures Javier Junquera, Pablo Aguado-Puente At metal-isulator interfaces, the metallic wave functions with an energy eigenvalue within the band gap decay exponentially inside the dielectric (metal-induced gap states, MIGS). These MIGS can be actually regarded as Bloch functions with an associated complex wave vector. Usually only real values of the wave vectors are discussed in text books, since infinite periodicity is assumed and, in that situation, wave functions growing exponentially in any direction would not be physically valid. However, localized wave functions with an exponential decay are indeed perfectly valid solution of the Schrodinger equation in the presence of defects, surfaces or interfaces. For this reason, properties of MIGS have been typically discussed in terms of the complex band structure of bulk materials. The probable dependence on the interface particulars has been rarely taken into account explicitly due to the difficulties to include them into the model or simulations. We aim to characterize from first-principles simulations the MIGS in realistic ferroelectric capacitors and their connection with the complex band structure of the ferroelectric material. We emphasize the influence of the real interface beyond the complex band structure of bulk materials. [Preview Abstract] |
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