Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q21: Metal Insulator Transitions 
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Sponsoring Units: DCMP Chair: Dragana Popovic, National High Magnetic Field Laboratory Room: 201 

Q21.00001: ABSTRACT WITHDRAWN 
Wednesday, March 4, 2015 2:42PM  2:54PM 
Q21.00002: An apparent metal insulator transition in high mobility 2D InAs heterostructures Javad Shabani, Chris Palmstrom We report on the first experimental observation of an apparent metal insulator transition in a 2D electron gas confined in an InAs quantum well. At high densities we find that the carrier mobility is limited by background charged impurities and the temperature dependence of the resistivity shows a metallic behavior with resistivity increasing with increasing temperature. At low densities we find an insulating behavior below a critical density of $n_{c} = 5 \times 10^{10}$cm$^{2}$ with the resistivity decreasing with increasing temperature. We analyze this transition using a percolation model arising from the failure of screening in random background charged impurities [1]. We also examine the percolation transition experimentally by introducing remote ionized impurities at the surface. Using a bias during cooldown, we modify the screening charge at the surface which strongly affects the critical density. Our study shows that transition from a metallic to an insulating phase in our system is due to percolation transition. \\[4pt] [1] J. Shabani, S. Das Sarma, and C. J. Palmstr{\o}m, Phys. Rev. B 90, 161303(R) (2014). [Preview Abstract] 
Wednesday, March 4, 2015 2:54PM  3:06PM 
Q21.00003: Electrically induced MetalInsulator Transition in Nb/NbO$_{2}$/TiN Devices Yuhan Wang, Stuart.A Wolf, Jiwei Lu Niobium dioxide (NbO$_{2})$ exhibits a metal insulator transition (MIT) as well as a structural transition at 1081 K. It has also been observed that an MIT could be induced by applied electrical field, which makes it attractive as potential electric and optical switch applications. A reactive bias target ion beam deposition (RBTIBD) technique was employed to synthesize Nb/NbO$_{2}$ thin films on TiN/Si substrates, in which the Nb top layer was used as the top electrodes. Electrically induced MIT was observed in the Nb/NbO$_{2}$/TiN devices, showing threshold characteristics. The electrically induced MIT was unipolar with very minimal hysteresis behaviors and good reproducibility. Such transitions were observed up to 200 $^{\circ}$C without obvious phase changes of NbO$_{2}$, indicating good thermal stability. We will discuss the evolution of electrical fields, current densities and input power at switching with the number of switches, as well as their dependence on the size of contacts. To study the possible effects from the interfaces between NbO$_{2}$ films and contacts, annealing processing in forming gas was conducted and transition behaviors were compared before and after annealing. The possible mechanisms for this induced MIT will be discussed. [Preview Abstract] 
Wednesday, March 4, 2015 3:06PM  3:18PM 
Q21.00004: Electrical transport properties of CaB$_6$ Jolanta Stankiewicz, Javier Ses\'e, Geetha Balakrishnan, Zachary Fisk We report results from a systematic electrontransport study in a broad temperature range on twelve CaB$_6$ single crystals. None of the crystals were intentionally doped. The different carrier densities observed presumably arise from slight variations in the Ca:B stoichiometry. In these crystals, the variation of the electrical resistivity and of the Hall effect with temperature can be consistently explained by a variable charge state of intrinsic defects, most likely Bantisites (B atom replacing Ca atom). Our model is also consistent with the presence of a narrow, defect related, impurity band close to the Fermi level. Thus it may indicate the validity of defectdriven intrinsic ferromagnetism in alkalineearth hexaborides. The magnetotransport measurements reveal that most of the samples we have studied are close to a metalinsulator transition at low temperatures. The magnetoresistance changes smoothly from negativefor weakly metallic samplesto positive valuesfor samples in a localized regime. [Preview Abstract] 
Wednesday, March 4, 2015 3:18PM  3:30PM 
Q21.00005: Metallic Hydrogen Isaac Silvera, Mohamed Zaghoo, Ashkan Salamat Hydrogen is the simplest and most abundant element in the Universe. At high pressure it is predicted to transform to a metal with remarkable properties: room temperature superconductivity, a metastable metal at ambient conditions, and a revolutionary rocket propellant. Both theory and experiment have been challenged for almost 80 years to determine its condensed matter phase diagram, in particular the insulatormetal transition. Hydrogen is predicted to dissociate to a liquid atomic metal at multimegabar pressures and T$=$0 K, or at megabar pressures and very high temperatures. Thus, its predicted phase diagram has a broad field of liquid metallic hydrogen at high pressure, with temperatures ranging from thousands of degrees to zero Kelvin. In a bench top experiment using static compression in a diamond anvil cell and pulsed laser heating, we have conducted measurements on dense hydrogen in the region of 1.11.7 Mbar and up to 2200 K. We observe a firstorder phase transition in the liquid phase, as well as sharp changes in optical transmission and reflectivity when this phase is entered. The optical signature is that of a metal. The mapping of the phase line of this transition is in excellent agreement with recent theoretical predictions for the longsought plasma phase transition to metallic hydrogen. [Preview Abstract] 
Wednesday, March 4, 2015 3:30PM  3:42PM 
Q21.00006: Nanoscale Electrical Imaging of MetalInsulator Transition in IonGel Gated ZnO Field Effect Transistors Yuan Ren, Hongtao Yuan, Xiaoyu Wu, Yoshihiro Iwasa, Yi Cui, Harold Hwang, Keji Lai Electric doublelayer transistors (EDLTs) using ionic liquid as the gate dielectric have demonstrated a remarkably wide range of density modulation, a condition crucial for the study of novel electronic phases in complex quantum materials. Yet little is known microscopically when carriers are modulated in the EDLT structure because of the technical challenge to image the buried electrolytesemiconductor interface with nanoscale resolution. Using a cryogenic microwave impedance microscope, we demonstrate the realspace conductivity mapping in ZnO EDLTs with a spatial resolution of 100nm. A thin layer of ion gel, which solidifies below the glass transition temperature of 200K, was spincoated on the ZnO surface to induce the metalinsulator transition. The microwave images acquired at different channel conductance clearly showed the spatial evolution of local conductivity through the transition. In addition, by applying a large sourcedrain bias, electrical inhomogeneity was also observed across the source and drain electrodes. [Preview Abstract] 
Wednesday, March 4, 2015 3:42PM  3:54PM 
Q21.00007: Metalinsulator transition in 2D quantum walks Jonathan Edge, Janos Asboth We investigate the localisation properties due to disorder of several different twodimensional quantum walks. We find that, contrary to claims in the literature, the Hadamard quantum walk does not localise. In a different quantum walk system we find a way to induce localisation. By tuning the parameters of the system we further manage to drive the quantum walk through a metalinsulator transition and show that the transition is related to the plateau transition of the integer quantum Hall effect. [Preview Abstract] 
Wednesday, March 4, 2015 3:54PM  4:06PM 
Q21.00008: CompressionDriven Enhancement of Electronic Correlations in Simple Alkali Metals Gilberto Fabbris, Jinhyuk Lim, Larissa Veiga, Daniel Haskel, James Schilling Alkali metals are the best realization of the nearly free electron model. This scenario appears to change dramatically as the alkalis are subjected to extreme pressure, leading to unexpected properties such as the departure from metallic behavior in Li and Na, and the occurrence of remarkable lowsymmetry crystal structures in all alkalis. Although the mechanism behind these phase transitions is currently under debate, these are believed to be electronically driven. In this study the highpressure electronic and structural ground state of Rb and Cs was investigated through low temperature XANES and XRD measurements combined with \textit{ab initio} calculations. The results indicate that the pressureinduced localization of the conduction band triggers a Peierlslike mechanism, inducing the low symmetry phases. This localization process is evident by the pressuredriven increase in the number of $d$ electrons, which takes place through strong \textit{spd} hybridization. These experimental results indicate that compression turns the heavy alkali metals into strongly correlated electron systems. [Preview Abstract] 
Wednesday, March 4, 2015 4:06PM  4:18PM 
Q21.00009: From Coulomb Fluid to SelfGenerated Charge Glass due to LongRange Interactions and Geometric Frustration Samiyeh Mahmoudian, Louk Rademaker, Arnaud Ralko, Simone Fratini, Vladimir Dobrosavljevi\'c We show that introducing longrange Coulomb interactions immediately lifts the massive ground state degeneracy induced by geometric frustration for electrons on quarterfilled triangular lattices in the semiclassical regime. This produces not only a stripeordered (global) crystalline ground state, but also very many lowlying metastable states with amorphous ``stripeglass'' spatial structure. At intermediate temperatures, such a frustrated Coulomb liquid shows remarkably slow (viscous) dynamics, with very long relaxation times growing in Arrhenius fashion upon cooling, as typical of ``strong glass formers.'' On shorter time scales, the system falls out of equilibrium and displays the ``aging'' phenomena characteristic of supercooled liquids around the glass transition. Our results, which are obtained using mean field theory, classical Monte Carlo simulations and exact diagonalization, show remarkable similarity with the recent observations of chargeglass behavior in ultraclean triangular organic materials $\theta$RbZn and $\theta$CsZn.\footnote{F. Kagawa {\em et al.}, Nat. Phys. {\bf 9}, 419422, (2013)} [Preview Abstract] 
Wednesday, March 4, 2015 4:18PM  4:30PM 
Q21.00010: Strongdisorder renormalization group study of the Anderson localization transition in three and higher dimensions Hossein Javan Mard, Vladimir Dobrosavljevi\'c, Jos\'{e} A. Hoyos, Eduardo Miranda We implement an efficient strongdisorder renormalization group (SDRG) procedure\footnote{H. Javan Mard, Jos\'{e} A. Hoyos, E. Miranda, V. Dobrosavljevi\'c, \textit{Strongdisorder renormalizationgroup study of the onedimensional tightbinding model} , Phys. Rev. B 90, 125141(2014).} for disordered tightbinding models in dimension $D\ge 3$ , including the localization properties on Erd\H{o}sR\'{e}nyi random graphs, which represent an appropriate infinite dimensional limit. Our dramatically improved SDRG algorithm is based on a judicious elimination of most (irrelevant) new bonds generated under RG. It yields excellent agreement with exact numerical results for universal properties at the critical point, without significant increase of the required computer time, even as the spatial dimension is increased beyond $D=3$. This opens an efficient avenue to explore the critical properties of Anderson transition in the strongcoupling limit of high spatial dimensions. [Preview Abstract] 
Wednesday, March 4, 2015 4:30PM  4:42PM 
Q21.00011: Unstable DomainWall Solution in the MetalMott Insulator Coexisting Regime TsungHan Lee, Vladimir Dobrosavljevic, Jaksa Vucicevic, Darko Tanaskovic, Eduardo Miranda We employ Dynamical Mean Field Theory (DMFT) with multidimensional optimization (Conjugate Gradient and Broyden method) to investigate the transport properties of the unstable solution in the Mott metalinsulator coexisting regime. Physically, this solution is expected to describe the properties of the domain wall separating the metallic and the Mottinsulating regions in a spatially inhomogeneous case. We show that the multidimensional optimization can efficiently converge not only to the local minima of the free energy, describing the two coexisting phases, but also to the saddlepoint describing the unstable solution. This unstable solution represents a new phase of matter: its low temperature transport properties differ qualitatively from both the metal and the insulator, displaying incoherent metallic behavior down to lowest temperatures. [Preview Abstract] 

Q21.00012: ABSTRACT WITHDRAWN 
Wednesday, March 4, 2015 4:54PM  5:06PM 
Q21.00013: Large Disorder Renormalization Group Study of the Anderson Model of Localization Sonika Johri, R. N. Bhatt We describe a large disorder renormalization group (LDRG) scheme for the Anderson model of localization in one dimension which eliminates eigenstates based on the size of their wavefunctions rather than their energy[1] (as done in RG models to date). We show that our LDRG scheme flows to infinite disorder, and thus becomes asymptotically exact. We use it to obtain the disorderaveraged inverse participation ratio and density of states and compare these with results obtained by exact numerical diagonalization for the entire spectrum. A modified method is formulated for higher dimensions, which is found to be less efficient, but capable of improvement. The possibility of extending this scheme to manybody localized states will be discussed. 1. Sonika Johri and R. N. Bhatt, Phys. Rev. B \textbf{90}, 060205(R) (2014) [Preview Abstract] 
Wednesday, March 4, 2015 5:06PM  5:18PM 
Q21.00014: Quantum phase transitions of a boson Hubbard model in one and two dimensions: a modified TimeEvolving Block Decimation study JiWoo Lee, Sung Moon Kim, Hwan Bin Choi We study quantum phase transitions of a boson Hubbard model in one and two dimensions at zero temperature. The model has a repulsive energy term ($U$) between the bosons located at the same site and a hopping energy term ($t$) to nearest neighboring sites. We construct matrix product states for one dimension and projected entangled pair states for two dimensions to represent ground states by a modified TimeEvolving Block Decimation. By exploring the energies and correlation functions, we obtain a phase diagram for this model as a function of chemical potential ($\mu$) and hopping energy ($t$) as we fix $U=1$. Our results are compared with other methods, such as strongcoupling perturbation results and Monte Carlo results. Our method can be useful in calculating groundstate properties since we can control the accuracy of the ground state and the number of parameters for quantum entanglement. [Preview Abstract] 
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