Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T23: Quantum Hall Effect: Coherent Phenomena |
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Sponsoring Units: DCMP FIAP Chair: Vladimir Goldman, SUNY at Stony Brook Room: 325 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T23.00001: Low-field quantum Hall transport in an electron Fabry-Perot interferometer P.V. Lin, F.E. Camino, V.J. Goldman We report systematic experimental characterization of an interferometer device as a function of front-gate voltage at 10 mK. Application of front-gate voltage affects the constriction electron density, but the 2D bulk density remains unaffected. The low-field quantum Hall transport (filling $f > 4$) shows quantized plateaus in longitudinal resistance, while the Hall resistance is dominated by the low-density, low-filling constriction. This allows to determine independently both: the bulk and the constriction filling. At lower fields, when the quantum Hall plateaus fail to develop, we observe the bulk Shubnikov-de Haas oscillations in series corresponding to an integer number of the magnetoelectric subbands in the constriction. From a Fock-Darwin analysis, we obtain the constriction electron density as a function of the front-gate bias, and, extrapolating to the zero-field, the $B=0$ number of 1D electric subbands (conductance channels) resulting from the electron confinement in the constriction. [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T23.00002: Oscillatory transport in electron Fabry-Perot interferometers F.E. Camino, P.V. Lin, V.J. Goldman We report experiments on GaAs/AlGaAs heterostructure interferometers in the integer QH regime with filling $f=1-4$. Etch trenches define the device, which consists of an electron island connected to the 2DES bulk via two wide constrictions. Front gates deposited in the trenches permit to fine tune the device. When tunneling occurs in the constrictions, electrons perform closed orbits around the island, producing an Aharonov- Bohm oscillatory signal in the conductance. On QH plateau transition between $f+1$ and $f$, we observe $f$ oscillations per flux $h/e$. In contrast, for all fillings, we observe one oscillation per back-gate charging period $e$. We also report a linear dependence of magnetic field period on front-gate voltage for three devices, with the slope inversely proportional to $f$. We attribute this behavior to self-consistent electrostatics of the electron island, and discuss the models of edge channel structure. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T23.00003: Universal dephasing in chiral one-dimensional electron systems Florian Marquardt, Clemens Neuenhahn The Green's function of a chiral interacting one-dimensional fermion system obeys a power-law decay at high energies, at zero temperature. Surprisingly, we find that the exponent is universal, i.e. independent of the interaction strength, for (almost) arbitrarily shaped interaction potentials. This has direct implications for the interference contrast in ballistic interferometers, e.g. the Mach-Zehnder interferometer composed of edge channels in the integer quantum Hall effect. Our result is obtained using a straightforward and physically transparent ``semiclassical'' approach to dephasing by electron- electron interactions. This approach is shown to coincide with the exact bosonization results in the high-energy regime of interest. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T23.00004: Determination of the coherence length in the Integer Quantum Hall Regime. F. Portier, P. Roulleau, P. Roche, A. Cavanna, G. Faini, U. Gennser, D. Mailly 0ne of the basic length scales limiting quantum effects in electrical conductors is the phase coherence length L$_{\varphi }$, the typical length on which an excitation looses its phase coherence via coupling to other degrees of freedom. In quasi-1D diffusive wires, due to electron-electron interactions, L$_{\varphi }$ was shown to scale as T$^{-1/3}$, as predicted by Altshuler-Aronov-Khmelnitsky. Surprisingly, little is known about L$_{\varphi }$ in the Integer Quantum Hall Regime (IQHE), where transport occurs through 1D chiral wires, localized on the edges of the sample. The number of these 'edge states' is equal to the filling factor (the number of electron per flux quantum flux). Chirality should prevent momentum conserving energy exchange processes, leading to a very long coherence length.We present an experiment where we have determined L$_{\varphi }$ in the IQHE at filling factor 2, by measuring the visibility of quantum interferences in an electronic Mach-Zhender interferometer. L$_{\varphi }$ shows a T$^{-1}$ dependence, proved to result from the coupling between the two neighbouring edge states: the thermal charge noise in one edge state blur the phase on the other edge state, leading to a finite coherence length proportional to T$^{-1}$. . [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T23.00005: Edge-State Velocity and Coherence in a Quantum Hall Fabry-Perot Interferometer Douglas McClure, Yiming Zhang, Eli Levenson-Falk, Charles Marcus, Loren Pfeiffer, Ken West We present finite-bias measurements of electronic Fabry-Perot interferometers in the integer quantum Hall regime. In devices large enough that Coulomb blockade is absent, checkerboard-like patterns of oscillations as a function of magnetic field and dc bias appear. Comparing our data to predictions for electromagnetic Aharonov-Bohm interference, we extract edge-state velocities over a range of magnetic fields, finding dependence consistent with a crossover from skipping orbits at low fields to $E\times B$ drift at high fields. Suppression of visibility observed at high bias and high field is quantitatively accounted for by including an energy-dependent dephasing rate. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T23.00006: Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers Yiming Zhang, Douglas McClure, Eli Levenson-Falk, Charles Marcus, Loren Pfeiffer, Ken West Two different types of resistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes. Measuring these oscillations as a function of magnetic field, gate voltage, or both, we observe three signatures that distinguish the two types. The oscillations observed in a $2~\mathrm{\mu m^2}$ device are understood to arise from Coulomb blockade, and those observed in an $18~\mathrm{\mu m^2} $ device from Aharonov-Bohm interference. This work clarifies, provides ways to distinguish, and demonstrates control over, the physical origins of resistance oscillations seen in electronic Fabry-Perot interferometers. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T23.00007: Fabry-Perot interferometer in Fractional Quantum Hall regime Aveek Bid, N. Ofek, M. Heiblum, Ady Stern, V. Umansky, D. Mahalu We have measured Aharonov-Bohm/Coulomb blockade oscillations in an electronic Fabry-Perot interferometer in the Fractional Quantum Hall regime. At $\nu $ = 2/5, when the inner channel is partially reflected (with the outer channel (1/3) being fully transmitted); the total transmission of the device oscillates as a function of magnetic field or modulation gate voltage. This is true also for $\nu $ =2, 3, 4 when the interference is of a partially reflected lower lying channel (with the other channels being either fully transmitted or fully reflected). However, in the outermost channel of all filling factors ($\nu $ = 1/3, 2/5, 1, 4/3, 2, 3, 4, 5) we do not see any oscillations as a function of B. This we interpret to be due to interplay between the magnetic field (which tries to modify the area of the compressible island inside the interferometer) and Coulomb energy (which prevents the density of quasiparticles within the island from building up indefinitely). The period of oscillations in modulation gate voltage in the inner channel of $\nu $ = 2/5 (partially partitioned) is found to be one-third of that observed in the second channels of the integer filling fractions which probably is an indication that the oscillations are due to the tunneling of quasiparticles of fractional charge 1/3. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T23.00008: Nonequilibrium Dephasing in an Electronic Mach-Zehnder Interferometer Seok-Chan Youn, Hyun-Woo Lee, H.-S. Sim We study nonequilibrium dephasing in an electronic Mach-Zehnder interferometer. We demonstrate that the shot noise at the beam splitter of the interferometer generates an ensemble of nonequilibrium electron density configurations and that electron interactions induce configuration-specific phase shifts of an interfering electron. The resulting dephasing exhibits two characteristic features, a lobe pattern in the visibility and phase jumps of $\pi $, in good agreement with experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T23.00009: Generating Excitonic Supercurrent in Quantum Hall Bilayers Jung-Jung Su, Tami Pereg-Barnea, Allan H. MacDonald Among the many examples of Bose condensation considered in physics, exciton condensation has maintained special interest because of controversy about condensate properties. Although ideal condensates can support an exciton supercurrent, it has not been clear how such a current could be induced or detected. We discuss the circuit conditions required to induce a steady-state counterflow superfluid. In addition, we will discuss interpretations of tunnel, drag and counterflow experiments in quantum Hall exciton condensates. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T23.00010: Density imbalance effect on the Coulomb drag upturn in an undoped electron-hole bilayer Christian Morath, John Seamons, John Reno, Mike Lilly A low-temperature upturn of the Coulomb drag resistivity measured in an undoped electron-hole bilayer (uEHBL) device, possibly manifesting from exciton formation or condensation, was recently observed. The effects of density imbalance on this upturn are examined. Measurements of drag as a function of temperature in a uEHBL with a 20 nm wide Al$_{.90}$Ga$_{.10}$As barrier layer at various density imbalances n$\ne $p are presented. The results show drag increasing as the density of either two dimensional system was reduced, both within and above the upturn temperature regime and with a significantly stronger dependence than the (np)$^{-3/2}$ predicted by the weak-coupling theory. A comparison of the data with numerical calculations of drag in the presence of electron-hole pairing fluctuations, which qualitatively reproduce the drag upturn behavior and easily accommodates density imbalance effects, is also presented. The calculations predict a peak in drag at matched densities, which is not reflected in the measurements. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T23.00011: Anyons in a weakly interacting system Gilad Rosenberg, Babak Seradjeh, Conan Weeks, Marcel Franz I will present our recent theoretical proposal for the realization and manipulation of anyons in a weakly interacting system. This system consists of a two-dimensional electron gas in the integer quantum Hall regime, adjacent to a type-II superconducting film with an artificial array of pinning sites. The anyons are realized in response to defects in the pinned vortex lattice and carry charge $\pm$e/2 and have an exchange phase $\pi$/4. We establish this result using a 2D continuum model of electrons in the magnetic field caused by the vortex lattice of the superconducting film. The charge of the defects is evaluated numerically, using the Aharonov-Casher exact solution (for $g=2$) for the ground state of a 2D system in magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T23.00012: Transport Measurements Between Superconductors and Semiconducting Structures Stephanie Law, Michael Vissers, Kevin Inderhees, Timothy McArdle, Allison Dove, Paul Goldbart, Nadya Mason, James Eckstein We report IV characteristics and differential resistance measurements on two dimensional electron gases and degenerately doped semiconductors in high magnetic fields. The samples are fabricated into Hall bars for measurement. Differential resistance and IV characteristics are then measured in a novel three terminal setup which allows us to measure voltage changes upstream of the current as well as conventional two and four terminal setups. The measurements are made at low temperatures in magnetic fields up to 7T both at fields where chiral edge states exist and at those where they do not exist. We are thus able to investigate the effect of chirality on transport. Samples are made both with high critical field superconducting contacts and with normal metal contacts, allowing us to compare the effect of injecting Cooper pairs into the structure. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T23.00013: An edge index for topological insulators Emil Prodan Topological insulators display dissipationless currents flowing at the edges of the samples. These currents are associated to chiral edge modes, whose existence is intrinsically linked to the topology of the electronic states of the bulk. The edge modes can be easily investigated when the edges are smooth and have a periodicity, but as soon as the periodicity is absent, the problem becomes un-traceable by purely theoretical means. In my talk I will exemplify the use of non-commutative calculus to explore the properties, especially the stability of the edge modes. For example, using such techniques one can give a fairly elementary proof that the edge modes in Chern insulators survive even for a rough (random) edge. Similarly, for the Spin-Hall effect, one can define an observable and its associated current whose conductance remains quantized during various deformations of the Hamiltonian system. It turns out that in all cases, the edge conductance is given by the index of a Fredholm operator, which provides a new topological invariant linked directly to the edge rather than the bulk. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T23.00014: Topological phases and topological surface states of three-dimensional time-reversal invariant superconductors Andreas Schnyder, Shinsei Ryu, Akira Furusaki, Andreas Ludwig We study topological phases of time-reversal invariant singlet superconductors in three spatial dimensions. In these particle-hole symmetric systems the topological phases are characterized by a winding number [1], similar to the $Z_2$ invariant of the $Z_2$ topological insulators. At a two-dimensional surface the topological properties of this quantum state manifest themselves through gapless surface states, that are robust against localization from random impurities respecting the discrete symmetries of the system. We construct a tight-binding model on the diamond lattice that realizes the topologically nontrivial phase and perform numerical studies of the winding number and the surface states of this model. \\[3pt] [1] A. P. Schnyder, S. Ryu, A. Furusaki, and A. W. W. Ludwig, arXiv:0803.2786 (PRB in press). [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T23.00015: Helical Metal Inside a Topological Band Insulator Yi Zhang, Ying Ran, Ashvin Vishwanath Topological defects, such as domain walls and vortices, have long fascinated physicists. A novel twist is added in quantum systems like the B-phase of superfluid helium He$_3$, where vortices are associated with low energy excitations in the cores. Similarly, cosmic strings may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? Here we show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator', a novel band insulator believed to be realized in the bulk material Bi$_{0.9}$Sb$_{0.1}$. In contrast to fermionic excitations in a regular quantum wire, these modes are topologically protected like the helical edge states of the quantum spin-Hall insulator, and not scattered by disorder. Since dislocations are ubiquitous in real materials, these excitations could dominate spin and charge transport in topological insulators. Our results provide a novel route to creating a potentially ideal quantum wire in a bulk solid. [Preview Abstract] |
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