Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U19: Metal-Insulator Transitions II |
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Sponsoring Units: DCMP Chair: Rongwei Hu, University of Maryland Room: 321 |
Thursday, March 21, 2013 11:15AM - 11:27AM |
U19.00001: MBE synthesis and characterization of charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_3$ thin films Rebecca Sichel-Tissot, Robert Devlin, Philip Ryan, Jong-Woo Kim, Alex Dagg, Steven May La$_{1/3}$Sr$_{2/3}$FeO$_3$ (LSFO) is a transition metal oxide which exhibits strongly correlated electronic behavior. When cooled below 180-190K, an electronic phase transition occurs during which the resistivity abruptly increases. LSFO was deposited on (001) SrTiO$_3$ substrates using molecular beam epitaxy (MBE). The transition temperature T* = 183 K was measured from a sharp increase in the resistivity and confirmed by the appearance of x-ray reflections with wavevectors of q = n/3[111]. Oxygen loss from the film over a period of 8 months was observed to have significant effects on the structural and electronic properties, but was shown to be reversible by annealing in oxygen. This work is supported by the Office of Naval Research under grant number N00014-11-1-0664. Work at the Advanced Photon Source is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences under contract DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U19.00002: Strain dependence of the electronic phase transition in epitaxial La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ films Robert Devlin, Rebecca Sichel-Tissot, Phillip Ryan, Jong-Woo Kim, Steve May The electronic transport properties of La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ thin films were experimentally investigated as a function of epitaxial strain. In bulk, this compound exhibits a first-order electronic phase transition at 198 K accompanied by an abrupt change in resistivity. In order to investigate how different epitaxial strain states affect the abruptness and temperature of the transition, thin La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ films were grown using molecular beam epitaxy on SrTiO$_{3}$ DyScO$_{3}$ and (La,Sr)(Al,Ta)O$_{3}$ imparting $+$0.9{\%} $+$1.8{\%} and -0.05{\%} strain, respectively. The transition temperatures were determined through resistivity measurements as well as synchrotron x-ray diffraction of (4/3 4/3 4/3) peaks, which are a direct signature of an additional ordering below the transition temperature. We find that the transition temperature measured through resistivity and the integrated intensity of the (4/3 4/3 4/3) peaks are in excellent agreement. The variation in transition temperature and the abruptness of the transition will be presented for the films grown on the various substrates. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U19.00003: Electrical Transport in Iron Cobalt Silicide Nanowires Drew Rebar, John DeGrave, Song Jin, John DiTusa Iron silicide is a small gap insulator with fascinating physical properties that can be made metallic and magnetic when doped with cobalt. With the substitution of cobalt for iron, Fe1-xCoxSi, the material undergoes an insulator-to-metal transition becoming a half metal for a wide range of x. The ground state is helimagnetic with distinct itinerant character. It has been demonstrated by others that an exotic intermediate magnetic vortex or skyrmion state exists between the helimagnetic and ferromagnetic phases in small applied fields. Electron transport in bulk Fe1-xCoxSi has been found to be dominated by electron-electron interaction effects similar to what has been found in prototypical semiconductors such as Si:P. Here we probe low temperature electron transport in CVD-grown Fe1-xCoxSi nanowires with x$=$0.05. The reduced dimensionality presents the opportunity to characterize the conductivity where only the phase-coherent contribution may be constrained to one dimension. Results of low temperature transport measurements of these wires will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U19.00004: Exploring Fe$_{1-y}$Co$_x$Si near the insulator-to-metal transition Yan Wu, Brad Fulfer, Julia Chan, David Young, John DiTusa FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. Doping FeSi with Mn or Co introduces hole or electron a charge carriers as well as additional magnetic moments. Our previous investigations show that for Mn doping near the insulator-metal-transition(IMT) an intriguing field sensitive non-Fermi-Liquid behavior results from the underscreening of the $S=1$ impurity moments. Here we explore the case of electron doping via Co substitution for concentrations very near the IMT. Our magnetic susceptibility measurements indicate an underlying competition between screening of the magnetic moments at low y and ferromagnetic ordering at higher Co-concentrations. Our carrier transport measurements indicate that the IMT occurs near $y=0.01$ and that above 2 K electron-electron interaction effects dominate the magnetoresistance. However, for $T<1$ K, high magnetic fields induce an enhance charge carrier mobility for samples with $y\sim 0.01$. We will present data comparing the magnetotransport of the Co and Mn doped samples in order to compare electron and hole doping in proximity to the IMT. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U19.00005: Doping induced metallization of a narrow gap insulator FeGa$_{3}$ Monika Gamza, Akshat Puri, Jan Tomczak, Jim Quinn, Meigan Aronson Narrow gap semiconductors attract great interest owing to an unusual metallization process which remains poorly understood despite decades of extensive research [1]. Here, we report on the effects of hole doping on properties of a nonmagnetic semiconductor FeGa$_{3}$ with a band gap of 0.4 eV [2]. By means of electrical resistivity, magnetization and specific heat measurements performed on single crystals grown from gallium flux we have found that a substitution of Mn for Fe in Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$Ga3 (0.005\textless\ x \textless\ 0.03) yields an insulating state at high temperatures with residual magnetic moments. With lowering temperature, resistivity deviates from an activation-type behavior and nearly saturates at T\textless 100 K. Finally, it drops by as much as two orders of magnitude at temperature of 6 K, indicating a metal-insulator transition. Magnetization measurements did not show magnetic order associated with the transition. When an external magnetic field is applied, the metal-insulator transition moves to lower temperatures and eventually the resistivity returns to the insulating-type behavior in fields higher then of 5 Tesla. \\[4pt] [1] M. Imada et al, Rev. Mod. Phys., 70, 1039 (1998)\\[0pt] [2] M. Arita et al., Phys. Rev. B 83, 245116 (2011); Y. Hadano et al., J. Phys. Soc. Jpn. 78, 013702 (2009) [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U19.00006: Role of long range Coulomb interaction near the disorder driven metal-insulator transition in Ga$_{1-x}$Mn$_x$As S. Mahmoudian, E. Miranda, V. Dobrosavljevic Surprising signatures of interaction effects on disorder-driven localization have recently been observed by scanning tunneling microscopy of Ga$_{1-x}$Mn$_x$As, where visualizing the electronic wave function near the metal-insulator transition revealed\footnote{A. Richardella {\em et al.}, Science {\bf 327}, 665 (2010).} a pronounced suppression of the local tunneling density of states (LDOS) and enhanced localization only near the Fermi energy. These features highlight the limitation of the non-interacting picture, and point to the crucial importance of the long-range Coulomb interaction. Here, we implement a theoretical approach based on the recently developed Typical-Medium Theory,\footnote{V. Dobrosavljevi\'c, Int. J. Mod. Phys. B {\bf 24}, 1680 (2010).} the conceptually simplest approach to interaction-localization. We show that the presence of long-range Coulomb interaction leads to the simultaneous opening of a soft pseudogap in both the typical (geometrically averaged) and the average (algebraically averaged) LDOS, as the transition is approached. This result is consistent with the experimentally observed features of the STM spectra, suggesting new experiments that should be performed to fully characterize the quantum critical behavior at the metal-insulator transition [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U19.00007: Fitting of Diverging Thermoelectric Power in a Strongly Interacting 2D Electron System of Si-MOSFETs Hyun-Tak Kim The diverging-effective mass (DEM) in a metallic system is evidence of strong correlation between fermions in strongly correlated systems. The identification of the DEM still remains to be revealed The effective mass, m*$=$m$_{\mathrm{o}}$/(1-$\rho^{4})$ [1] where $\rho $ is band filling helps clarify the diverging thermoelectric power, S, measured in inhomogeneous Si-MOSFET systems [2]. As a carrier density n$_{\mathrm{s}}$ decreases, S increases rapidly This is regarded as the metal-insulator transition (MIT) near n$_{\mathrm{c}}\approx $79x10$^{-1}$cm$^{-2}$, where n$_{\mathrm{c}}$ is about 0.02{\%} to n$_{\mathrm{Si}}\approx $3.4x10$^{-14}$cm$^{-2}$ in Si. This can be solved in assuming that $\rho =$n$_{\mathrm{c}}$/n$_{\mathrm{s}}$ increases as n$_{\mathrm{s}}$ decreases. n$_{\mathrm{c}}$ is an excited(doped) carrier density in the semiconductor induced by gate and can be also regarded as a metallic carrier density, that is, n$_{\mathrm{c}}\equiv $n$_{\mathrm{seminon}}=$n$_{\mathrm{metal}}$. n$_{\mathrm{s}}$ is given as n$_{\mathrm{tot}}\equiv $n$_{\mathrm{s}}=$n$_{\mathrm{c}}+$n$_{\mathrm{seminon}}$ where n$_{\mathrm{seminon}}$ is a carrier density in a nonmetallic phase. The carrier density measured by Hall effect is the sum of carriers both induced by gate field and generated by MIT. Moreover, a larger metallic phase is not made due to a conducting path in the field-effect structure after a metallic phase is formed. Thus, increasing n$_{\mathrm{s}}$ indicates increasing n$_{\mathrm{non}}$; this corresponds to an over-doping to increase inhomogeneity. It's fitting is given from S$=(\alpha \pi ^{3}$k$^{2}_{\mathrm{B}}$T/3e)(1/E$_{\mathrm{F}})$ $=(\alpha $8$\pi ^{3}$k$^{2}_{\mathrm{B}}$T/3h$^{2})$(m*/e*n$_{\mathrm{c}})$ $=$S$_{\mathrm{o}}$(1/$\rho )$(1/(1-$\rho^{\mathrm{4}}))$, where e*$=\rho $e [1], $\rho =$n$_{\mathrm{c}}$/n$_{\mathrm{s}}$, T$=$0.8K, m*$=$m$_{\mathrm{o}}$/(1-$\rho^{4})$ [1], $\alpha =$0.6, and S$_{\mathrm{o}}=(\alpha $8$\pi ^{3}$k$^{2}_{\mathrm{B}}$T/3h$^{2})$(m$_{\mathrm{o}}$/en$_{\mathrm{c}})$ $\approx $12.36 are used. The data S [2] are closely fitted by m* [1] Physica C 341-348(2000)259. [2] Phys. Rev. Lett. 109 (2012) 096405. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U19.00008: Metal-insulator and glass transitions in a 2D electron system in Si MOSFETs with a screened Coulomb interaction Ping V. Lin, Dragana Popovi\'c We present a study of conductivity $\sigma$ of a 2D electron system (2DES) in Si MOSFETs with the oxide thickness $d_{ox}=7$~nm. In the low density regime of interest, the average electron-electron ($e$-$e$) separation is larger than $d_{ox}$, so that the $e$-$e$ interaction is screened by the metallic gate. The carrier density $n_s$ was changed at a high temperature $T\approx 20$~K, the 2DES was then cooled to a desired $T$ with a fixed $n_s$, and $\sigma$ was measured as a function of time $t$. At the lowest $n_s$, in the insulating regime, transport occurs via variable-range hopping. Near the critical density $n_c$ on the metallic side of the metal-insulator transition (MIT), the time-averaged $\langle\sigma(T)\rangle$ follows a power-law behavior, giving a reliable extrapolation of $\langle\sigma(n_s, T=0)\rangle$. The critical exponents are discussed and compared to the case of the MIT with long-range Coulomb interactions. The statistical analysis of the fluctuations in $\sigma(t)$ provides evidence for the glassy freezing of electrons for $n_s |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U19.00009: Valence Band Character of NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ using 3p-3d Resonant ARPES Garam Han, Yeongkwan Kim, Yoonyoung Koh, Beomyoung Kim, Dongjoon Song, Jungjin Seo, Wonshik Kyung, Kyungdong Lee, Changyoung Kim Understanding the strong correlated system is one of the most challenging tasks in condensed matter physics. Especially, the metal insulator transition (MIT) has been one of the major topics recent few decades. NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is known as one of famous material which has MIT. The cubic pyrite NiS$_{2}$ is a charge-transfer (CT) insulator. NiS$_{2}$ attracts particular interest as it easily forms a solid solution with NiSe$_{2}$ (NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}})$ which, while being isoelectronic and isostructural to NiS$_{2}$, is nevertheless a good metal. MIT, induced by Se alloying, is observed at low temperature (T) for x$=$0.45. Perucchi and his collaborators revealed closed relation between MIT and band width through comparison of infrared spectroscopy result and LDA calculation. However, it was only an indirect observation, and is inconsistent with recent proposal that NiS$_{2}$ is not a CT insulator but an insulator due to the bonding-antibonding splitting in the S -- S (Se -- Se) dimers. To reveal the true mechanism in the MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$, resonant photoemission experiment is essential. According to competing theories (CT insulator and insulator due to bonding-antibonding splitting), it is expected that the character of the main band that is responsible for the MIT should be different. Therefore, we performed 3p-\textgreater 3d resonant ARPES for various Se dopings (x$=$0.43; insulator, x$=$0.5, 0.7, 2.0; metal) and observed a significant change between on- and off-resonances near the MIT. Our experimental result supports that the origin of MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is the CT theory rather than the dimer theory. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U19.00010: Low temperature conductance spectra of STO at the nanoscale Alireza Mottaghizadeh, Qian Yu, Alexandre Zimmers, Herve Aubin The electronic properties of transition metal oxide materials depend on the electronic carrier density, which can be tuned with the oxygen stoichiometry. In binary MOx or ternary perovskite ABOx, it has been shown that upon applying a strong electric field, oxygen vacancies can be created or displaced in the material. This effect is responsible for the memristive behavior recently discovered in TiO2 materials by HP laboratory and launched a worldwide renew interest into ionics. We present a study of oxygen ions vacancies displacement in SrTiO3, the archetype perovskite oxide. For this work, metallic electrodes, separated by distances about 100 -- 300 nm, are deposited on the surface of a STO crystal and ions migration procedures and current-voltage characteristics measurements are done at low temperature, T $\sim$ 260 mK. Upon applying large voltage up to 30 V, oxygen vacancies migration is identified as the apparition of resistance switching events in current-voltage characteristics. Detailed measurements of the junction show that the switching event led to the formation of a nanosized region of highly doped STO, located within the electrodes where the current-voltage characteristics show the presence of the doped in-gap states. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U19.00011: Crystalline and Magnetic Anisotropy of the 3$d$ Transistion-Metal Oxides Andreas Schr\"on, Claudia R\"odl, Friedhelm Bechstedt The 3$d$ transition-metal oxides (TMOs) are subject of debate since many decades due to their extraordinary properties, such as the formation of an antiferromagnetic ordering AFM2 below their N\'eel temperature. Many studies, both experimental and theoretical, focus only on MnO and NiO, where the crystalline anisotropy is solely driven by exchange striction along the unique symmetry axis in the [111] direction and where the magnetic anisotropy is explained in terms of magnetic dipole interactions. In the other TMOs, FeO and CoO, however, orbital magnetization and spin-orbit interaction play an additional, yet crucial role for both crystalline and magnetic anisotropy. We present density-functional theory (DFT) studies including an on-site interaction $U$ of the crystalline and magnetic anisotropy of the electronic systems with non-collinear spins. The influence of the (semi-)local description of exchange and correlation (XC) by means of the local density approximation (LDA) and generalized gradient approximation (GGA) on the orbital moments in FeO and CoO and the implications on the aforementioned properties is investigated. We discuss the quenching of the orbital magnetization due to the gradient corrections. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U19.00012: Spatially resolved dynamic susceptibilities of disordered two dimensional Hubbard model Nandini Trivedi, Oinam Nganba Meetei We predict the existence of an emergent metallic phase in the disordered two dimensional Hubbard model [1] that has recently been confirmed by experiments on 1T-TaS$_2$ intercalated with Cu. The metallic state has a finite dc conductivity but unusual dynamical properties. We present here a comprehensive analysis of the spatially resolved spin susceptibility, screened charge density, and optical conductivity of the disordered Hubbard model. We develop a new method in which the exact eigenstates from inhomogeneous mean-field theory are used to calculate dynamical susceptibilities within the random phase approximation. By combining the non-perturbative effects of self-consistent mean-field theory with analytical perturbative methods, this approach gives insights about fluctuations near the quantum phase transitions. We make several predictions which can be directly tested in spatially resolved experiments. \\[4pt] [1] D. Heidarian and N. Trivedi, Phys. Rev. Lett. {\bf 93}, 126401 (2004) [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U19.00013: Dual fermion approach for disordered interacting fermion systems Shuxiang Yang, Patrick Haase, Hanna Terletska, Ziyang Meng, Juana Moreno, Mark Jarrell, Thomas Pruschke Understanding the combined effect of electron-electron interaction and disorder is one of the crucial questions in condensed matter physics. There is an obvious need of theoretical tools which allow to treat both these effects on equal footing. To study the intricate interplay of these effects, we generalize our recently proposed dual fermion approach to include both electron-electron interaction and disorder. Since the constraint imposed on the dual-space Feynman diagrams in the disordered case does not apply to those generated due to interactions, it is essential to treat elastic scattering processes due to the disorder separately from the inelastic scattering processes due to the pure interaction and mixed contributions. I will discuss the resulting diagrammatic formalism and an algorithm for its implementation. The possible applications for the Anderson Falicov-Kimball and the Anderson-Hubbard models are also discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U19.00014: Dual fermion method for disordered electronic systems Hanna Terletska, Shuxiang Yang, Zi Yang Meng, Juana Moreno, Mark Jarrell While the coherent potential approximation (CPA) is the most commonly used theoretical method to study disordered systems, it by construction misses non-local correlations and Anderson localization. We have recently extended the dual fermion approach [1] to disordered non-interacting systems using the replica method, which allows one to included such non-local physics. Our method utilizes an exact transform to the dual variables, and includes inter-site scattering via diagrammatic perturbation theory in dual fermion space, with the CPA being a zeroth-order approximation. Analyzing one-particle quantities we demonstrate good agreement between our results and those from the dynamical cluster extension of the CPA. Moreover, by calculating the dc conductivity we show that our approach successfully captures weak localization missing in the CPA. This method as a natural extension of CPA, and presents a powerful alternative to existing cluster extensions of CPA. It can be used in various applications, including systems with disorder and interactions. \\[4pt] [1] A.N. Rubtsov, et. al., Phys. Rev. B 77, 033101 (2008). [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U19.00015: Metal-Insulator Transitions in Crystalline Phase Change Materials Wei Zhang, Alexander Thiess, Peter Zalden, Rudolf Zeller, Peter Dederichs, Jean-Yves Raty, Matthias Wuttig, Stefan Bl\"ugel, Riccardo Mazzarello Phase-change materials are capable of undergoing fast and reversible transitions between amorphous and crystalline phase upon heating and have been exploited in data storage applications based on the strong optical/electrical contrast between the two phases. Recently, compelling evidence for a metal-insulator transition (MIT) solely due to disorder has been observed in the crystalline PCM Ge$_{1}$Sb$_{2}$Te$_{4}$ (GST) and similar compounds: upon annealing at temperatures T below 548K, the system exhibits insulating behavior due to Anderson localization; at higher T, it shows metallic behavior. In contrast to the MITs observed in other systems such as P-doped Si, in GST correlation effects do not play a role and the MIT occurs at fixed stoichiometry. In this work, we present a Density Functional Theory study of this effect. We consider a set of very large models of GST containing one to several thousand atoms and different degree of disorder. We identify the microscopic mechanism that localizes the electron wavefunctions near the Fermi energy in the insulating phase: these states are localized inside regions having large vacancy consequent dissolution of these vacancy clusters. These results could help to develop new device based on multiple resistance states. [Preview Abstract] |
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