Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J38: MetalInsulator Phase Transitions  Experiment II 
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Sponsoring Units: DCMP Chair: Genda Gu, Brookhaven National Lab Room: LACC 513 
Tuesday, March 22, 2005 11:15AM  11:27AM 
J38.00001: The Electronic Structure of (La, Ce)Te2 System: Interplay between different degrees of freedom Daniel Garcia, Shuyun Zhou, GeyHong Gweon, Jeff Graf, Chris Jozwiak, MyungHwa Jung, Alessandra Lanzara Rare earth tellurides (La,Ce)Te$_{2}$ are interesting layered materials showing Charge Density Wave (CDW) formation with transition temperature of the order of 1000K. By substituting La with Ce, coexistence between magnetism and CDW is observed. Angle resolved photoemission spectroscopy (ARPES) is an ideal tool to study the competition and coexistence of these two phases, since it directly probes quasiparticles and manybody interaction. Here we report a detailed ARPES study as a function of momentum, temperature and composition of the (La, Ce)Te$_{2}$ system. The Fermi surface geometry in the CDW phase, the formation of the CDW gap and its anisotropy, as well as the ARPES lineshapes will be presented. The competition and cooperation between different degrees of freedom will be discussed. [Preview Abstract] 
Tuesday, March 22, 2005 11:27AM  11:39AM 
J38.00002: Field Effect Devices for Controlling the Conductivity of Ultrathin Films K.H. Sarwa B. Tan, Kevin A. Parendo, Allen M. Goldman An electricfield effect device geometry that uses a mechanicallythinned strontium titanate dielectric substrate has been used to affect large charge transfer into ultrathin elemental films. A large, nonambipolar effect of the electric field on the conductance in a 10 {\AA} thick film of amorphous bismuth was found, as evidenced by shifting its superconducting transition temperature of 446 mK higher by as much as 56 mK with positive gate voltage, and shifting it lower by as much as10 mK with negative gate voltage. This work is supported in part by the National Science Foundation under grant NSF/DMR0138209. [Preview Abstract] 
Tuesday, March 22, 2005 11:39AM  11:51AM 
J38.00003: Temperature/frequency scaling of conductivity near the MI transition in doped semiconductors Erik Helgren, Li Zeng, Frances Hellman, Kenny Burch, Dimitri Basov Doped semiconductors undergo a metalinsulator transition (MIT) at T = 0, i.e. a quantum phase transition and can be probed by varying the dopant concentration for a set of samples through a critical concentration. In crystalline systems such as Si:P, this occurs at a dopant concentration near 10$^{18}$, but in amorphous systems such as aNbSi and aGdSi, it occurs closer to 1015 at.{\%}$^{1,2}$. Doping with Gd introduces the possibility of examining this transition with concentration as well as magnetic field tuning, as it has been shown that a magnetic field tunes a sample of aGdSi through the MIT$^{3}$. We present scaling results for families of DC conductivity and optical conductivity curves for both the concentration tuned as well as the magnetic field tuned MIT. Probing the same quantum critical point using two separate tuning parameters allows for a unique and separate determination of the success of dynamic scaling as a function of the tuning parameter. A comparison of the concentration tuned vs. the magnetic field tuned transition shows distinct differences. [1] Hertel et al., \textit{Phys. Rev. Lett. }\textbf{50}, 743 (1983) [2] Hellman et al., \textit{Phys. Rev. Lett.} \textbf{77}, 4652 (1996) [3] Teizer et al., \textit{Phys. Rev. B} \textbf{67}, 121102(R) (2003) [Preview Abstract] 
Tuesday, March 22, 2005 11:51AM  12:03PM 
J38.00004: Transport Measurement of Ultradilute 2DHoles in High Quality (100) GaAs HIGFETs Jian Huang, D. C. Tsui, L. N. Pfeiffer, K.W. West We have studied transport properties of high purity pchannel GaAs HIGFETs and observed a surprising temperature dependence of conductivity ($\sigma$) at ultralow densities from $p=2\times10^{10}cm^{2}$ to $p=8\times10^{8}cm^{2}$. For $p>4\times10^{9}cm^{2}$, as the temperature is decreased $\sigma$ first decreases linearly with T, then exhibits a clear upward bending which signifies a metallic behavior. The temperature at which the bending occurs shifts to a lower value as $p$ is decreased. However, the upturning of $\sigma$ weakens as the density goes lower until, at $4\times10^{9}cm^{2}$, it becomes almost independent of T between 80mK and 40mK. As $p$ is decreased from $4\times10^{9}cm^{2}$ to $2\times10^{9}cm^{2}$, $\sigma$ continues to show little temperature dependence and tend to saturate at some finite values below $80mK$. Meanwhile, the high temperature linear regions become almost parallel to each other. As $p$ is further decreased below $2\times10^{9}cm^{2}$, the results are even more striking: the linear Tdependence persists even though the slope of the linear region starts to decrease. Remarkably, $\sigma$ maintains the weak Tdependence below $80mK$. At $8\times10^{8}cm^{2}$, $\sigma$ retains a finite value of $0.045e^{2}/h$ at $40mK$. Consistent results were obtained through measuring three different samples. These observations contrast sharply with the metalinsulatortransition model which would predict a lowdensity insulating state, whereas we found no evidence of localization all the way down to the lowest density of $8\times10^{8}cm^{2}$. [Preview Abstract] 
Tuesday, March 22, 2005 12:03PM  12:15PM 
J38.00005: Temperature and Magnetic Field Enhanced Hall Slope of the Dilute 2D Holes in GaAs in the Ballistic Regime Xuan Gao, Greg Boebinger, Allen Mills, Art Ramirez, Loren Pfeiffer, Ken West We report the temperature($T$) and perpendicular magnetic field ($B$) dependence of the Hall resistivity $\rho_{xy}(B)$ of dilute metallic twodimensional(2D) holes in high mobility GaAs quantum wells over a broad range of temperature(0.021.25K). The low $B$ Hall coefficient, $R_H$, is found to be enhanced when $T$ decreases. Strong perpendicular magnetic fields further enhance the slope of $\rho_{xy}(B)$ at all temperatures studied. Coulomb interaction corrections of a Fermi liquid in the ballistic regime ($k_BT>\hbar/\tau$ with $\tau$ being the scattering time) can not explain the enhancement of $\rho_{xy}$ which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a Fermi liquid, these same interactions should reduce, {\it not enhance} the slope of $\rho_{xy}(B)$ as $T$ decreases and/or $B$ increases. Preprint available at condmat/0411391. [Preview Abstract] 
Tuesday, March 22, 2005 12:15PM  12:27PM 
J38.00006: Observation of abrupt firstorder metalinsulator transition in GaAsbased twoterminal devices HyunTak Kim, DooHyeb Youn, ByungGyu Chae, KwangYong Kang, YongSik Lim An abrupt firstorder metalinsulator transition (MIT) as a jump of the density of states is observed for Be doped GaAs, which is known as a semiconductor, by inducing very low holes of approximately $n_p\approx$5$\times$10$^{14}$~cm$^{3}$ into the valence band by the electric field; this is anomalous. In a higher hole doping concentration of $n_p\approx$6$\times$10$^{16}$~cm$^{3}$, the abrupt MIT is not observed at room temperature, but measured at low temperature. A negative differential resistance are also observed as further evidence of the MIT. The upper limit of the temperature allowing the MIT is deduced to be approximately 440K from experimental data. The abrupt MIT rather than the continuous MIT is intrinsic and can explain the ``breakdown'' phenomenon (unsolved problem) incurred by a high electric field in semiconductor devices.(ref:NJP 6 (2004)52:(www.njp.org)) [Preview Abstract] 
Tuesday, March 22, 2005 12:27PM  12:39PM 
J38.00007: 2D Metalinsulator transition behavior in a high mobility strained Si quantum well K. Lai, D.C. Tsui, S. Lyon, W. Pan, M. Muhlberger, F. Schaffler The apparent metalinsulator transition is observed in a high quality twodimensional electron system (2DES) in the strained Si quantum well of a Si/Si$_{1x}$Ge$_{x}$ heterostructure with mobility $\mu$=1.9$\times$10$^{5}$cm$^{2}$/Vs at $n$=1.45 $\times$10$^{11}$cm$^{2}$. The critical density $n_c$, where the thermal coefficient of low T resistivity changes sign, is 0.32$\times$10$^{11}$cm$^{2}$, much smaller than the $n_c$ of $\sim$0.8$\times10^{11}$cm$^{2}$ seen in clean SiMOSFET's (usually with a peak $\mu$$\sim$4$\times$10$^{4}$cm$^{2}$/Vs). This result is consistent with previous observations in the GaAs systems that $n_c$ decreases with increasing 2DES quality. Moreover, in low $n$ range, for 0.27$\times$10$^{11}$cm$^{2} $$<$$n$$<$0.35$\times$10$^{11}$cm$^{2}$, close to the transition region, the conductivity increases roughly linearly with $T$ around the Fermi temperature and, surprisingly, all the curves of different densities are parallel to each other for $T>1.2K$. In the higher density range where the 2DES shows metalliclike behavior, the inplane magnetoresistance $\rho$(B) first increases $\sim$B$^{2}$ and then saturates to a finite value $\rho$(B$_C$) for B$>$B$_C$. The full spinpolarization field B$_C$ decreases monotonically with $n$ but appears to saturate to a finite value as $n$$\rightarrow$0. We find $\rho (B_C)/\rho(0)$$\sim$1.8 for all the densities ranging from 0.35 to 1.45$\times10^{11}$cm$^{2}$ and, when plotted versus B/B$_C$, collapse onto a single curve. [Preview Abstract] 
Tuesday, March 22, 2005 12:39PM  12:51PM 
J38.00008: Shear strain profile of a driven CDW probed by Xray microbeam diffraction A.F. Isakovic, J. Kmetko, K. Cicak, R.E. Thorne, B. Lai, Z. Cai, P.G. Evans We have probed charge density wave (CDW) structure in stepped, whiskerlike NbSe$_{3}$ samples with lateral variations of pinning strength by Xray microbeam diffraction using 2IDD beamline at APSANL. The rotation of the CDW \textbf{\textit{q}}vector on the depinned side appears above the threshold field for CDW depinning, consistent with the picture of inhomogeneous pinning. The corresponding shear strain profile is determined with a resolution of 300 nm. The results demonstrate how the magnitude of shear strains changes with the DC bias applied along the direction of CDW motion. This profile is compared with finite element modeling. [Preview Abstract] 
Tuesday, March 22, 2005 12:51PM  1:03PM 
J38.00009: Forced localization in thin K films, investigated with the superconducting proximity effect Manjiang Zhang, Gerd Bergmann Thin films of alkali metals are forced into an insulating state by being covered with submonolayers of Pb. The superconducting proximity effect is used to investigate the electronic change in the alkali film. On the length scale of the film thickness the electronic properties of the alkali film do not change noticeably during the metalinsulator transition. [Preview Abstract] 
Tuesday, March 22, 2005 1:03PM  1:15PM 
J38.00010: Revealing the local structure of the chargedensitywave material, CeTe$_{3}$, using atomic pair distribution function analysis H.J. Kim, S.J.L. Billinge, D. Bilc, S.D. Mahanti, D. Wermeille, D. Robinson, C. Malliakas, M.G. Kanatzidis CeTe$_{3}$ is a layered charge density wave material whose Te square nets undergo a Peierls distortion. Formation of the CDW in this material has been validated through observing superlattice reflections by single crystal xray diffraction and Fermi surface nesting by ARPES. However, this electronically driven structural distortion is difficult to measure and has not been previously elucidated. We were recently able to solve this using single crystal xray diffraction for the first time. In addition, the $local$ structure of CeTe$_{3}$ has been studied using the atomic pair distribution function analysis of xray powder diffraction data. The study shows that the local structure of CeTe$_{3}$ is more distorted than the average structure. Interestingly, a bimodal TeTe bondlength distribution of Te nets is found from the local structural model whereas a Gaussian like distribution from the average structural model. We will discuss how the local and average structures can be reconciled by the existence of structural disorder in the Te network. [Preview Abstract] 
Tuesday, March 22, 2005 1:15PM  1:27PM 
J38.00011: Fabrication and characterization of end current injection contacts to the quasi1D conductor NbSe3 R. E. Thorne, K. Cicak, A. F. Isakovic We have successfully fabricated end current injection contacts to the CDW conductor NbSe$_{3}$ through a combination of electroplating and standard lithographic procedures. These contacts allow direct carrier injection along the direction of CDW motion and produce uniform transverse current and electric field profiles in this highly anisotropic material. The differential conductance of these endcontacted samples shows some quantitative differences from previous measurements [1,2] using side contacts, which are particularly evident in measurements of the condensate current versus phaseslip voltage. This may indicate differences in how phase dislocation loops convert normal carriers to condensate when shear components to the driving force are eliminated. [1] M. P. Maher \textit{et al.}, Phys. Rev. B \textbf{52}, 13850 (1995). [2] S. G. Lemay \textit{et al.}, Phys. Rev. B\textbf{ 57}, 12781 (1998). [Preview Abstract] 
Tuesday, March 22, 2005 1:27PM  1:39PM 
J38.00012: Electrodynamics of the 2D superconductorinsulator transition Peter Armitage Using microwave cavities and a novel cryogenic system we have probed the evolution of the low frequency electrodynamics of thin InO$_x$ films across the nominal 2D fieldtuned superconductor insulator quantum phase transition. Such a finite study allows us, at least in principle, to access the true phase coherent ($\hbar \omega > k_B T$) quantum critical behavior. A number of other interesting items are found including evidence for significant finite frequency superfluid density well into the ``insulating" regime of the phase diagram. Various scenarios for frequency dependent scaling are also investigated. [Preview Abstract] 
Tuesday, March 22, 2005 1:39PM  1:51PM 
J38.00013: Charge Density Wave Gap in $ZrTe_{3}$ Andrea Perucchi, Leonardo Degiorgi, Helmut Berger The transition metal trichalcogenides ($MX_{3}, M= Ta, Nb$) have been widely studied as prototype examples of linear chain conductors exhibiting novel charge density wave (CDW) phenomena. The title compound is semimetallic with linear chain structure but quasitwodimensional conductivity. At $T_{CDW}$=63 K it undergoes a phase transition, which most strongly affects its conductivity components perpendicular to the conducting chains, and it becomes superconducting at 2 K. We have measured the optical reflectivity from the farinfrared up to the ultraviolet spectral range as a function of temperature and with light polarized both along and perpendicular to the chains. Through KramersKronig transformation we have extracted the optical conductivity. We found optical evidence for the CDW gap, which opens over a tiny amount of the Fermi surface. The temperature dependence of the gap follows the BCS behaviour for an order parameter. The role played by fluctuation effects will be also discussed. [Preview Abstract] 

J38.00014: Conductivity of granular metals Yen Lee Loh, Vikram Tripathi, Misha Turlakov The conductivity of granular metals, which consist of isolated metallic regions embedded within an insulating matrix, is often experimentally seen to obey the `softactivation' law $\sigma\propto e^{\sqrt{T_0/T}}$ over a very large temperature range, as opposed to the Arrhenius `hardactivation' law $e^{E_c^*/T}$ that would have been expected from basic Coulomb blockade theory. Our extensive perturbative and pathintegral MonteCarlo analysis of the AmbegaokarEckernSch{\"o}n (AES) model for a regular array of grains gives an Arrhenius law and does not reveal a softactivation behaviour even at the lowest temperatures we considered. This result is in agreement with recent experiments on silver nanoparticle arrays with controllable disorder, and suggests that the softactivation law should be attributed to disorder. [Preview Abstract] 
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