Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L11: Metal-Insulator Transition I |
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Sponsoring Units: DMP Chair: Sambandamurthy Ganapathy, State University of New York at Buffalo Room: Colorado Convention Center Korbel 1F |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L11.00001: Flow diagram of the metal-insulator transition in two dimensions Sergey Kravchenko, Svetlana Anissimova, Alexander Punnoose, Alexander Finkel'stein, Teun Klapwijk Recently, a two-parameter scaling theory comprehensively describing the metal-insulator transition in 2D was developed by two of us [1]. Here, we report experimental verification of the basis of this theory. We demonstrate, for the first time, that as a result of the interplay between the electron-electron interactions and disorder, both the resistance and the interactions become scale (temperature) dependent. We show that not only the resistance but also the interaction amplitude exhibits a fan-like spread as the MIT is crossed. We use these data to construct a resistance-interaction flow diagram of the MIT that clearly reveals a quantum critical point, as predicted in Ref.[1]. The metallic side of this diagram is accurately described by the theory without any fitting parameters. In particular, the temperature dependence of the resistance, which is non-monotonic, passes through a maximum when the interaction amplitude reaches a certain value $\gamma_2\approx0.45$ that is in remarkable agreement with the calculated one.\\ $[1]$ A. Punnoose and A.~M. Finkel'stein, \textit{Science} \textbf{310}, 289-291 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L11.00002: Quantum Coulomb glasses and electron assisted hopping Markus Muller, Lev Ioffe In Anderson insulators where the single particle localization length is much larger than the mean distance between electrons, Coulomb interactions drive the electrons into a strongly correlated quantum glass phase. In the limit of large localization length, the resulting quantum Coulomb glass can be studied analytically. The theory predicts many almost degenerate quantum states with a spectrum of gapless collective excitations in each of them. The latter can acts as a bath with which individual electrons can exchange energy. This is a crucial ingredient for activated transport, the collective modes of the quantum glass providing a natural mechanism for electron-assisted hopping conductance. In particular, for 2D systems we predict a weakly temperature dependent pre-exponential factor of order $e^2/h$ for variable range hopping, as has been reported in many recent experiments. [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L11.00003: Multifractality and Conformal Invariance at 2D Metal-Insulator Transition in the Spin-Orbit Symmetry Class Hideaki Obuse, Arvind Subramaniam, Akira Furusaki, Ilya Gruzberg, Andreas Ludwig We study the multifractality of critical wave functions at boundaries and corners at the Anderson metal-insulator transition for noninteracting electrons in the two-dimensional (2D) spin-orbit (symplectic) universality class. We find that the multifractal exponents near a boundary are different from those in the bulk. The exponents at a corner are found to be directly related to those at a straight boundary through a relation arising from conformal invariance. This provides direct numerical evidence for conformal invariance at the 2D spin-orbit metal-insulator transition. We also show that the presence of boundaries modifies the multifractality of the whole sample even in the thermodynamic limit. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L11.00004: Localization of interacting fermions at high temperature Vadim Oganesyan, David Huse We suggest that if a localized phase at nonzero temperature $T>0 $ exists for strongly disordered and weakly interacting electrons, as recently argued, it will also occur when both disorder and interactions are strong and $T$ is very high. We show that in this high-$T$ regime the localization transition may be studied numerically through exact diagonalization of small systems. We obtain spectra for one-dimensional lattice models of interacting spinless fermions in a random potential. As expected, the spectral statistics of finite-size samples cross over from those of orthogonal random matrices in the diffusive regime at weak random potential to Poisson statistics in the localized regime at strong randomness. However, these data show deviations from simple one-parameter finite-size scaling: the apparent mobility edge ``drifts'' as the system's size is increased. Based on spectral statistics alone, we have thus been unable to make a strong numerical case for the presence of a many-body localized phase at nonzero $T$. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L11.00005: Out-of-Equilibrium Dynamics of a Strongly Correlated Electron System in Two Dimensions Dragana Popovi\'c, Jan Jaroszy\'nski Slow, nonexponential relaxations of conductivity $\sigma (t)$ have been studied in a strongly disordered two-dimensional electron system (2DES) in Si MOSFETs in the vicinity of the metal-insulator transition (MIT). The 2DES is excited far from equilibrium by a rapid change of carrier density $n_s$ at low temperatures $T$. The dramatic and precise dependence of $\sigma(t)$ on $n_s$ and $T$ shows that (a) the equilibration time diverges exponentially as $T\rightarrow 0$, suggesting a glass transition at $T_g=0$, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics~[1]. The scaling of $\sigma(t,T)$ is also consistent with $T_g=0$. These results support conclusions based on earlier noise measurements~[2] that, in a 2DES in Si, the glass transition occurs in the metallic phase as a precursor to the MIT.\\ \noindent [1] J. Jaroszy\'nski and D. Popovi\'c, Phys. Rev. Lett. {\bf 96}, 037403 (2006).\\ \noindent [2] S. Bogdanovich and D. Popovi\'c, Phys. Rev. Lett. {\bf 88}, 236401 (2002); J. Jaroszy\'nski, D. Popovi\'c, and T. M. Klapwijk, Phys. Rev. Lett. {\bf 92}, 226403 (2004). [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L11.00006: Electric field gating near the metal-insulator transition using ionic liquid dielectrics Arthur Hebard, Rajiv Misra, Mitchell McCarthy Ionic liquids (ILs) are highly polar low-melting-temperature binary salts typically comprising nitrogen-containing organic cations and inorganic anions. Since there is no solvent, ILs are distinctly different from aqueous, organic, gel or polymer electrolytes. Using either coplanar or overlay gate configurations in which the IL is the gate dielectric, we demonstrate room temperature field-induced resistance changes on the order of a factor of 10$^{4}$ for thin conducting InOx films. There is a large asymmetry manifested by the significantly larger changes in impedance for negative gate voltage $V_{g}$ (electron depletion) compared to positive $V_{g}$ (electron enhancement). The pronounced frequency dependence over the range 10$^{-2}$--10$^{6}$ Hz, due to the low ionic mobilities in the dielectric fluid, is modeled well by a simple RC circuit from which an effective areal gate capacitance can be derived. The induced surface charge densities and field-effect mobilities noticeably exceed those that can be achieved on similar films using AlOx dielectrics. In addition, the charge state can be frozen in by reducing the temperature below the glass transition ($\sim $250K) of the IL, thus providing an opportunity for electric field tuning of metal-insulator transitions in a variety of novel thin-film systems. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L11.00007: Phase diagram of amorphous Ta thin films in B-T-disorder space Yize Li, Yongguang Qin, Carlos Vicente, Jongsoo Yoon We have studied the effect of temperature (T) and perpendicular magnetic fields (B) on the transport properties in amorphous Ta thin films. In the zero T limit, the films exhibit superconducting, metallic, and insulating phases with increasing B. Each phase can be identified by distinct nonlinear current-voltage (I-V) characteristics: the I-V curves in the superconducting phase are characterized by a hysteresis, in the metallic phase the differential resistance (dV/dI) increases with increasing I, while in the insulating phase dV/dI decreases with increasing I [1]. As demonstrated for the superconducting and metallic phase, these nonlinear transports arise from a non-thermal origin [2]. In order to understand the effect of B, T, and disorder on the electronic states and the nature of the resulting ground states, we construct a B-T-disorder space ``phase diagram''. Disorder is controlled by film thickness. The resulting phase diagram shows that the superconducting phase is completely surrounded by the metallic phase; in the zero temperature limit (B-disorder plane) a B-induced direct superconductor-insulator transition is not allowed, while a superconductor-metal-insulator or metal-insulator transition are possible depending on the degree of disorder in our 2D system. [1] Y. Qin et al., Phys. Rev. B 73, 100505(R) (2006). [2] Y. Seo et al., Phys. Rev. Lett. 97, 057005 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L11.00008: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L11.00009: First-order metal-insulator transition and structural phase transition: analysis of coherent phonons observed by femtosecond pulse laser in VO$_{2}$ Hyun-Tak Kim, Byung-Gyu Chae, Bong-Jun Kim, Yong Wook Lee, Sun Jin Yun, Kwang-Yong Kang, Kang-Jeon Han, Ki-Ju Lee, Yong-Sik Lim It has been well-known that VO$_{2}$ undergoes both a structural phase transition (SPT) (electron-phonon interaction) from monoclinic (insulator phase) to tetragonal (metal phase) and of a discontinuous first-order metal-insulator transition (MIT) (Jump) (electron-electron interaction) near 68$^{o}$C. Peierls transiton and Mott transition in VO$_{2}$ remain controversial. We have investigated a relation of the MIT and the SPT in VO$_{2}$ by observing coherent phonons using a laser with a femtosecond pulse width (10$\sim $20 ft). A coherent phonon indicating a metal phase is measured after MIT. This indicates that the SPT does not affect the MIT. This is confirmed by a micro-Raman scattering experiment and XRD. The speed of the first-order MIT is interpreted as about 100 femtosecond. This is different from a well-known analysis in which the SPT and the MIT simultaneously occur. (References on the MIT: New J. Phys. 6 (1994) 52 (http://www.njp.org), Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005) 76; cond-mat/0607577; cond-mat/0608085; cond-mat/0609033) [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L11.00010: Spin-Valley Phase Diagram of the 2D Metal-Insulator Transition O. Gunawan, T. Gokmen, K. Vakili, M. Padmanabhan, E.P. De Poortere, M. Shayegan It has been recognized that the spin degree of the freedom plays a crucial role in the controversial metal-insulator transition problem in 2D carrier systems. Here, we directly probe the role of another discrete electronic degree of freedom, namely the valley polarization. Using symmetry breaking strain to tune the valley occupation of a 2D electron system in an AlAs quantum well, together with an applied in-plane magnetic field to tune the spin polarization, we map out a spin-valley phase diagram for the 2D metal-insulator transition. The insulating phase occurs in the quadrant where the system is both spin- and valley- polarized. This observation establishes the equivalent roles of spin and valley degrees of freedom in the 2D metal-insulator transition. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L11.00011: Doping variation of orbitally-induced anisotropy in electronic structure of the perovskite-type vanadium oxides Jun Fujioka, Shigeki Miyasaka, Yoshinori Tokura Recently, the perovskite-type vanadium oxide LaVO$_{3}$ has been attracting much attention. In this system, the anisotropic charge dynamics due to the one-dimensional orbital exchange interaction is observed. In addttion, the filling control insulator-metal transition (FC-IMT) concomitant with the orbital ordering-disordering transition can be achieved in the hole doped system La$_{1-x}$Sr$_{x}$VO$_{3}$[1]. In this study, the variation of anisotropic charge dynamics in the course of FC-IMT in the perovskite-type vanadium oxide has been investigated by measurements of optical conductivity spectra with focus on the role of t$_{2g}$-orbital degree of freedom. The orbitally-induced anisotropic feature of the Mott-gap excitation as well as of the doping-induced mid-infrared excitation is suppressed with increasing the hole concentration, and instead the isotropic and incoherent dynamics of the doped hole dominates over the low-energy excitation near and above the IMT point. \newline \newline [1] S.Miyasaka et al., Phys.Rev.Lett. 85,5388(2000) [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L11.00012: Screening of disorder by the Hubbard interaction near a metal-insulator transition in two dimensions Prabuddha Chakraborty, Peter Denteneer, Richard Scalettar We present a determinant quantum Monte Carlo study of the metal-insulator transition in the Hubbard model on a square lattice with random site disorder. We show that beyond a critical value of the Hubbard interaction U, the Anderson insulator can undergo a phase transition to a two-dimensional metal. It is also shown that a further increase of the Hubbard interaction can lead to a decrease in conductivity, in direct analogy with the superfluid to Bose-glass transition in the bosonic Hubbard model. We point out that screening of disorder by the Hubbard interaction is not enough to explain the metal-insulator transition in the two-dimensional disordered Hubbard model. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L11.00013: The metal to insulator transition in manganites - evidence for changes in the kinetic energy up to 24 eV I. Mahns, A. Rusidy, G. Neuber, M. Bastjan, S. Mueller, P. Saichu, B. Schulz, M. Ruebhausen, R. Rauer, G. Stryganyuk, K. Doerr, G. A. Sawatzky The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.8}$Ca$_{0.2}$MnO$_3$ was investigated by a combination of dc conductivity, ellipsometry, and VUV-reflectance measurements covering an energy range from 0 to 24 eV. By performing a stabilized Kramer-Kronig transformation, we obtain the optical conductivity as a function of temperature around the metal to insulator transition. Our main findings are that changes in the kinetic energy exceed energies of more than 22 eV. In the spectral range between 0 and 24 eV the spectral weight is conserved within a fraction of 3/1000. The pronounced redistribution of the spectral weight between low and high energies has important ramifications for the construction and down-folding of effective low-energy Hamiltonians. We discuss the importance of local interactions to the electronic bandstructure such as the Coulomb onsite and Jahn-Teller effects. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L11.00014: Metal-Insulator Transition and Coulomb Gap: A Real-Space Dynamical Mean-Field Study of the Anderson-Hubbard Model Yun Song, W.A. Atkinson, R. Wortis The interplay between disorder and electron interactions in the two-dimensional paramagnetic Anderson-Hubbard model is studied by real-space dynamical mean-field theory (DMFT) with a Hubbard- I solver. At half-filling, the Mott gap evolves into a Coulomb- like gap with power law energy dependence $|E - E_F|$, suggesting a Mott insulator to Anderson insulator transition as a function of disorder. Away from half filling for strong interactions and disorder, we find a negative density of states (DOS) anomaly at the Fermi level that is distinct from the Mott gap. Far from half-filling, we obtain a positive DOS anomaly at the Fermi level. While this positive anomaly is consistent with paramagnetic mean-field calculations, the negative anomaly near half filling is a feature unique to strong correlation physics. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L11.00015: Jastrow theory of the Mott transition in bosonic Hubbard models Manuela Capello, Federico Becca, Michele Fabrizio, Sandro Sorella We show that the Mott transition occurring in bosonic Hubbard models can be successfully described by a simple variational wave function that contains all important long-wavelength correlations. Within this approach, a smooth metal-insulator transition is made possible by means of a long-range Jastrow correlation term that binds in real space density fluctuations. We find that the Mott transition has similar properties in two and three dimensions but differs in one dimension. We argue that our description of the Mott transition in terms of a binding-unbinding transition is of general validity and could also be applied to realistic electronic systems. [Preview Abstract] |
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