Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V19: Correlated Phases in 2-D Systems and Shot Noise |
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Sponsoring Units: DCMP Chair: Alan Dorsey, Univ. of Florida Room: LACC 406B |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V19.00001: Anisotropic Wigner crystal phases of two-dimensional electrons in high magnetic fields F.D. Klironomos, A.M. Ettouhami, A.T. Dorsey We study the collective states formed by a two-dimensional electron gas in the presence of a perpendicular magnetic field. We numerically solve the static (time-independent) Hartree-Fock (HF) equations projected onto a given Landau level of index $n\geq 2$. We find that a triangular isotropic Wigner crystal (WC) is favored at small values of the partial filling factor $\nu^*$, but that at higher values of $\nu^*$ an anisotropic triangular WC becomes energetically more favorable. This anisotropic WC has a channel-like structure and can be viewed as a periodic arrangement of one-dimensional stacks of electron guiding centers that is reminiscent of the stripe state. Our results indicate that this ``striped" state is the ground state of the system over a wider region of partial filling factors than predicted within previous HF treatments. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V19.00002: Elastic properties of electronic solids: the smectic phase A.M. Ettouhami, F.D. Klironomos, A.T. Dorsey We calculate the elastic moduli of the anisotropic triangular Wigner crystal (WC) of two-dimensional electrons, which we predict will appear in Landau levels of index $n\geq 2$ at intermediate and high values of the partial filling factor $\nu^*$. We find that these anisotropic WC's, which can also be viewed as a periodic arrangement of one-dimensional stacks (or channels) of electron guiding centers, have a vanishingly small shear modulus for uniform shear deformations along the direction of the channels. This suggests that the channels can slide freely past each other, and that the anisotropic WC is in fact a smectic. We discuss the experimental implications of our results in light of recent microwave conductivity measurements. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V19.00003: Effective elasticity theory for quantum Hall liquid crystals Cintia Lapilli, Carlos Wexler, Giovanni Vignale A variety of recent experiments probing the low-temperature transport properties of quantum Hall systems have been successfully interpreted in terms of liquid crystalline mesophases dubbed {\em quantum Hall liquid crystals}. Making use of the single mode approximation in combination with a variational calculation of the static structure factor we have recently found that density fluctuations in these systems exhibit a striking spectrum of collective modes. In the limit of zero wavevector $\vec q$ the dispersion of these modes is singular, with a gap that is dependent on the direction along which $\vec q=0$ is approached, for both {\em nematic} and {\em tetratic} liquid crystalline states, but remains regular in the {\em hexatic} state. Here we provide a physical interpretation of these results in terms of an effective elasticity theory for anisotropic systems projected onto the lowest Landau level, consistent with the symmetry groups of the different putative states. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V19.00004: Topological quantum crystals in the lowest Landau level Chia-Chen Chang, Gun Sang Jeon, Jainendra Jain There has been much interest in the crystal phase produced when electrons in two dimensions are subjected to a strong magnetic field. By comparing to exact solutions for finite systems, we demonstrate that the composite-fermion crystal, wherein quantized vortices are bound to electrons, provides an excellent representation of the actual crystal. The optimal number of vortices tied to an electron in the crystal increases as the filling factor is reduced; numerical evidence is seen for as many as 38 vortices bound to each electron. The quantum mechanical and topological nature of the crystal may have observable consequences, which will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V19.00005: Measurement of the local potential in the stripes and bubble phase Jens Martin, Basile Verdene, Jurgen Smet, Loren Pfeiffer, Ken West, Amir Yacoby A single electron transistor is used to probe the local chemical potential of a two- dimensional electron gas in the quantum Hall regime. Our measurements focus on the stripe and bubble phases that occur between filling factors 4 and 5. In the absence of any external transport current we see clear electrostatic signature of the bubble phases at filling 4.25 and 4.75. Once an additional transport current is applied the changes in potential at these filling factors become more pronounced. In contrast, at filling factor 4.5 we cannot detect any measurable change in potential regardless of whether the current is flowing along the hard or easy direction. Increasing the transport current leads eventually to the destruction of the bubble phases. This is observed as a jump and hysteretic behavior in the local potential as well as in the 2-terminal resistance. Surprisingly, the change in potential is an order of magnitude larger than the change in Hall voltage across the sample. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V19.00006: Tilt Induced Melting of CDW Phases of the Second Landau Level C.L. Vicente, J.S. Xia, G.A. Csathy, E.D. Adams, N.S. Sullivan, H.L. Stormer, D.C. Tsui, L.N. Pfeiffer, K.W. West We have explored the behavior with tilted magnetic fields of the various electronic phases forming in the second Landau level. We have found that the Hall resistance evolves from reentrant integer quantization towards the classical Hall values. We regard this behavior as another experimental signature for the recently discovered collective charge density wave (CDW) phases forming at reentrant quantization and interpret the transport data as melting of the CDW phases into the classical Hall gas. Our observations are inconsistent with a fully spin-polarized CDW and are therefore suggestive of antiferromagnetic ordering of the CDW. We think that such an ordering is due to the considerable exchange processes that have a dominant antiferromagnetic term. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V19.00007: Microwave Conductivity Spectra in High Landau Levels under Tilted Magnetic Fields G. Sambandamurthy$^{1,2}$, R. M. Lewis$^{1,2}$, Z. Wang$^{3,1}$, Y. Chen$^{2,1}$, L. W. Engel$^1$, D. C. Tsui$^2$, L. N. Pfeiffer$^4$, K. W. West$^4$ We present the results of a systematic study of the microwave conductivity in high Landau levels (LLs) of very clean two- dimensional electron systems (2DES), in the 20 MHz to 6 GHz range. When magnetic field ($B$) is applied perpendicular to the 2DES, a resonance that appears between LL fillings ($\nu$) of 4.2 and 4.4 is interpreted as due to the pinning mode of bubble phase of electrons [1]. We observe that the frequency of this resonance increases as a function of {\it in-plane} $B$ ($B_{|| }$). A resonance associated with a Wigner crystal composed of top LL electrons or holes [2] in the ranges 1.85$<\nu<$1.95 and 2.05$<\nu<$2.15 similarly increases with $B_{||}$. However, the peak frequency increases asymmetrically with $B_{||}$ on either side of $\nu$=2.\\ \noindent [1] R.M. Lewis {{\it et al.}}, PRL {\bf 89}, 136804 (2002)\\ \noindent [2] Y. Chen {\it {et al.}}, PRL {\bf 91}, 016801 (2003) [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V19.00008: Melting Temperatures of 2D Electron Solids in the Lowest Landau Level from Microwave Spectroscopy Y.P. Chen, S. Ganapathy, R.M. Lewis, L.W. Engel, D.C. Tsui, Z.H. Wang, P.D. Ye, L.N. Pfeiffer, K.W. West We studied the temperature($T$) dependence of the microwave conductivity spectra of two dimensional electron systems in the high magnetic field ($B$) insulating phase (HBIP) for Landau filling factor $\nu$$<\sim$1/5. Such an insulating phase, believed to be a pinned electron solid, supports a characteristic pinning resonance in the conductivity spectrum. Two samples were studied. Sample 1 is a heterojunction with density $n$$\sim$7$\times$10$^{10} $cm$^ {-2}$ and mobility $\mu$$\sim$5$\times$10$^{6}$cm$^2$/Vs and has a single resonance in the HBIP. Sample 2 is a 65nm-wide QW with $n$$\sim$6$\times$10 $^{10}$cm$^{-2}$ and $\mu$$\sim$10$\times$10$^{6}$cm$^2$/V and was recently found to have two resonances in the HBIP, interpreted as corresponding to two different solid phases, with one crossing over to the other as $\nu$ is reduced [1]. We studied the higher-$T$ behavior of the resonances at many different combinations of $n$ (through backgating) and $B$, and measured the characteristic temperatures $T_c$ at which the resonances disappear. We found$T_c$ is a non-increasing function of $\nu$ for either sample, although the function differs significantly for both samples. We interpret $T_c$ as the melting temperature of the electron solid(s) to a quantum liquid, for which $\nu$ captures the importance of inter-electron quantum correlation. [1] Y.P. Chen et al., Phys.Rev.Lett. 93, 206805 (2004) [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V19.00009: ac conductivity of a Coulomb glass from computer simulations Sergey Basylko, Victor Onischouk, Anders Rosengren A method for calculating the photon-induced hopping ac conductivity of a Coulomb glass by computer simulation is proposed. Results obtained by using an effective relaxation algorithm for two three-dimensional models of a Coulomb glass are reported. The ac conductance data clearly demonstrate the transition from super-linear to a sub-quadratic power law. We argue that the same qualitative behavior should be expected for compensated semiconductors. It is shown that the transition is driven by the Coulomb energy of sites forming resonant pairs and not by the width of the Coulomb gap. \begin{thebibliography}{99} \bibitem{} S. A. Basylko, V. A. Onischouk, and A. Rosengren, Phys. Rev. B {\bf 70}, 024201 (2004). \end{thebibliography} [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V19.00010: Berry Phase Correction to Electron Density of States in Solids Di Xiao, Junren Shi, Qian Niu We show that Berry phase effect on the semiclassical dynamics of Bloch electrons leads to a modification of the density of states, which can be controlled, for instance, by an external magnetic field. In metals, the predicted effect leads to the break down of the Luttinger’s theorem on the Fermi sea volume. In insulators, it results in a novel magneto-resistance. The Berry phase correction to the density of states also has a non-trivial contribution to the orbital magnetization. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V19.00011: Antidot lattice in AlAs 2D electron system: Electron pinball with elliptical Fermi contours Oki Gunawan, Yakov Shkolnikov, Emanuel Tutuc, Kamran Vakili, Mansour Shayegan We report ballistic transport experiments in AlAs quantum well antidot lattices with isotropic periods ranging from 0.6 $\mu$m to 1.5 $\mu$m. In this system two conduction-band valleys with in-plane elliptical Fermi contours are occupied. The magnetoresistance exhibits two clear peaks associated with classical commensurability of the electron orbits of these two valleys with the antidot lattice. By analyzing the density dependence of these peaks, we extract an anisotropy ratio for the longitudinal and transverse effective mass of 5.3$\pm $0.6. Our experiment demonstrates ballistic transport in an antidot lattice for a 2D electron system with elliptical Fermi contours, and allows for a direct determination of the mass anisotropy. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V19.00012: Shot Noise in Tunnel Barriers Yuanzhen Chen, Richard Webb We report a comprehensive study of the shot noise in tunnel barriers fabricated in GaAs/AlGaAs heterostructures. For most barriers tested a full shot noise 2$eI$ is observed, in good agreement with theory. However, deviations from full shot noise, both suppression and enhancement, have also been observed. Shot noise in these barriers shows a complicated dependence on bias conditions and barrier settings. In some cases the observed deviation can be related to conduction processes involving impurities. Resonant tunneling through impurities is identified to cause shot noise suppression, while interacting impurity states is believed to be responsible for super enhanced shot noise. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V19.00013: Drastic Reduction of Shot Noise in Semiconductor Superlattices W. Song, A.K.M. Newaz, J.K. Son, E.E. Medez We have found experimentally that when the states in adjacent wells of GaAs-GaAlAs superlattices are coupled, the shot noise of the current \emph{I} flowing through a superlattice photodiode (a \emph{p-i-n} photodiode with an embedded superlattice in the insulating region) is reduced in comparison with the Poissonian noise value \emph{2eI}. For very strongly coupled wells the shot noise is about 1/3 of the Poissonian value, while for wells with weaker coupling the reduction is less pronounced. On the other hand, when the coupling is destroyed by a strong electric field along the superlattice direction the current noise becomes Poissonian. Although our results are qualitatively consistent with existing theories, these theories cannot account for either the dependence of noise reduction on superlattice coupling or the abrupt field-induced transition from sub-Poissonian to Poissonian noise that we have observed. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V19.00014: Shot Noise Charateristics of InGaAs-InAlAs Triple Barrier Resonant Tunneling Diodes A.K.M. Newaz, W. Song, E.E. Mendez, Y. Lin, J. Nitta We have found that the shot noise of Triple-Barrier Resonant- Tunneling Diodes (TBRTD) shows distinct differences with that of Double-Barrier Resonant-Tunneling (DBRTD) diodes. Our measurements were done at 4.2 K in In$_{0.53}$Ga$_{0.47}$As-In$_ {0.52}$Al$_{0.48}$As heterostructures grown by MOCVD on InP substrates. Each InAlAs barrier at the two ends of a structure was 100 \AA \, thick, while the central barrier was either 52 or 100 \AA \, thick, depending on the sample; the InGaAs wells were 82 and 52 \AA \, wide. We observed that, as in DBRTDs, in the quasi-linear region of the current-voltage characteristics of our TBRTDs the noise was smaller than the corresponding Poissonian value of $2eI$, while in their negative-conductance region the noise was enhanced significantly relative to $2eI$. There were important differences, though, between this behavior and that found in DBRTDs. First, in TBRTDs the noise reduction was more pronounced than predicted by a sequential-tunneling theory. And second, the enhancement found for one of the two bias polarities did not follow the accepted rule that the larger (in absolute value) the negative-differential conductance, the larger the noise enhancement. Our results suggest that the current understanding of shot noise in multibarrier systems is incomplete. [Preview Abstract] |
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