arXiv subjects

K. W. West

Publications and source records attributed to K. W. West.

At least 235 records · Page 13Linked to original sources

arXiv2011

Giant microwave photoresistivity in a high-mobility quantum Hall system

We report the observation of a remarkably strong microwave photoresistivity effect in a high-mobility two-dimensional electron system subject to a weak magnetic field and low temperature. The effect manifests itself as a giant microwave-induced resistivity peak which, in contrast to microwave-induced resistance oscillations, appears only near the second harmonic of the cyclotron resonance and only at sufficiently high microwave frequencies. Appearing in the regime linear in microwave intensity, the peak can be more than an order of magnitude stronger than the microwave-induced resistance oscillations and cannot be explained by existing theories.

cond-mat.mes-hall↗

arXiv2011

Non-linear response of a high mobility two-dimensional electron system near the second harmonic of the cyclotron resonance

Recent experiments on microwave-irradiated high-mobility two-dimensional electron systems revealed a novel photoresistivity peak in the vicinity of the second cyclotron resonance harmonic. Here we report on the nonlinear transport measurements and demonstrate that the peak can be induced by modest dc fields and that its position is not affected even by strong dc fields, in contrast to microwave-induced resistance oscillations that shift to higher magnetic fields. These findings reinforce the notion that the peak cannot be described by existing models and provides important constraints for further theoretical considerations.

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arXiv2011

Zero-Bias Anomalies in Narrow Tunnel Junctions in the Quantum Hall Regime

We report on the study of cleaved-edge-overgrown line junctions with a serendipitously created narrow opening in an otherwise thin, precise line barrier. Two sets of zero-bias anomalies are observed with an enhanced conductance for filling factors $ν> 1$ and a strongly suppressed conductance for $ν< 1$. A transition between the two behaviors is found near $ν\approx 1$. The zero-bias anomaly (ZBA) line shapes find explanation in Luttinger liquid models of tunneling between quantum Hall edge states. The ZBA for $ν< 1$ occurs from strong backscattering induced by suppression of quasiparticle tunneling between the edge channels for the $n = 0$ Landau levels. The ZBA for $ν> 1$ arises from weak tunneling of quasiparticles between the $n = 1$ edge channels.

cond-mat.mes-hall↗

arXiv2011

Hall field-induced resistance oscillations in tilted magnetic fields

We have studied the effect of an in-plane magnetic field on Hall field-induced resistance oscillations in high mobility two-dimensional electron systems. We have found that the oscillation frequency depends only on the perpendicular component of the magnetic field but the oscillation amplitude decays exponentially with an in-plane component. While these findings cannot be accounted for by existing theories of nonlinear transport, our analysis suggests that the decay can be explained by an in-plane magnetic field-induced modification of the quantum scattering rate.

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arXiv2010

Nonconventional odd denominator fractional quantum Hall states in the second Landau level

We report the observation of a new fractional quantum Hall state in the second Landau level of a two-dimensional electron gas at the Landau level filling factor $ν=2+6/13$. We find that the model of noninteracting composite fermions can explain the magnitude of gaps of the prominent 2+1/3 and 2+2/3 states. The same model fails, however, to account for the gaps of the 2+2/5 and the newly observed 2+6/13 states suggesting that these two states are of exotic origin.omposite fermion model. However, the weaker 2+2/5 and 2+6/13 states deviate significantly suggesting that these states are of exotic origin.

cond-mat.mes-hall↗

arXiv2010

Observation of exchange Coulomb interactions in the quantum Hall state at nu=3

Coulomb exchange interactions of electrons in the nu=3 quantum Hall state are determined from two inter-Landau level spin-flip excitations measured by resonant inelastic light scattering. The two coupled collective excitations are linked to inter-Landau level spin-flip transitions arising from the N=0 and N=1 Landau levels. The strong repulsion between the two spin-flip modes in the long-wave limit is clearly manifested in spectra displaying Coulomb exchange contributions that are comparable to the exchange energy for the quantum Hall state at nu=1. Theoretical calculations within the Hartree-Fock approximation are in a good agreement with measured energies of spin-flip collective excitations.

cond-mat.mes-hall↗

arXiv2010

Spatially probed electron-electron scattering in a two-dimensional electron gas

Using scanning gate microscopy (SGM), we probe the scattering between a beam of electrons and a two-dimensional electron gas (2DEG) as a function of the beam's injection energy, and distance from the injection point. At low injection energies, we find electrons in the beam scatter by small-angles, as has been previously observed. At high injection energies, we find a surprising result: placing the SGM tip where it back-scatters electrons increases the differential conductance through the system. This effect is explained by a non-equilibrium distribution of electrons in a localized region of 2DEG near the injection point. Our data indicate that the spatial extent of this highly non-equilibrium distribution is within ~1 micrometer of the injection point. We approximate the non-equilibrium region as having an effective temperature that depends linearly upon injection energy.

cond-mat.mes-hall↗

arXiv2010

Observation of a cyclotron harmonic spike in microwave-induced resistances in ultraclean GaAs/AlGaAs quantum wells

We report the observation of a colossal, narrow resistance peak that arises in ultraclean (mobility 3X10^7cm^2/Vs) GaAs/AlGaAs quantum wells (QWs) under millimeterwave irradiation and a weak magnetic field. Such a spike is superposed on the 2nd harmonic microwave-induced resistance oscillations (MIRO) but having an amplitude > 300% of the MIRO, and a typical FWHM ~50 mK, comparable with the Landau level width. Systematic studies show a correlation between the spike and a pronounced negative magnetoresistance in these QWs, suggesting a mechanism based on the interplay of strong scatterers and smooth disorder. Alternatively, the spike may be interpreted as a manifestation of quantum interference between the quadrupole resonance and the higher-order cyclotron transition in well-separated Landau levels.

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arXiv2010

Tilt-Induced Anisotropic to Isotropic Phase Transition at $ν= 5/2$

A modest in-plane magnetic field \Bpar\ is sufficient to destroy the fractional quantized Hall states at $ν= 5/2$ and 7/2 and replace them with anisotropic compressible phases. Remarkably, we find that at larger \Bpar\ these anisotropic phases can themselves be replaced by isotropic compressible phases reminiscent of the composite fermion fluid at $ν= 1/2$. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands. We also report surprising dependences of the energy gaps at $ν= 5/2$ and 7/3 on the width of the confinement potential.

cond-mat.str-el↗

arXiv2010

Unexpected features of branched flow through high-mobility two-dimensional electron gases

GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states, and serve as the basis for fast transistors, research on electrons in nanostructures, and prototypes of quantum-computing schemes. All these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from microns to hundreds of microns. Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes. Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.

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arXiv2010

Electron interferometer formed with a scanning probe tip and quantum point contact

We show an electron interferometer between a quantum point contact (QPC) and a scanning gate microscope (SGM) tip in a two-dimensional electron gas. The QPC and SGM tip act as reflective barriers of a lossy cavity; the conductance through the system thus varies as a function of the distance between the QPC and SGM tip. We characterize how temperature, electron wavelength, cavity length, and reflectivity of the QPC barrier affect the interferometer. We report checkerboard interference patterns near the QPC and, when injecting electrons above or below the Fermi energy, effects of dephasing.

cond-mat.mes-hall↗

arXiv2010

Observations of Plasmarons in a System of Massive Electrons

Calculations of the single particle density of states (SPDOS) of electron liquids have long predicted that there exist two distinct charged excitations: the usual quasiparticle consisting of an electron or hole screened by a correlation hole, and a "plasmaron" consisting of a hole resonantly bound to real plasmons in the Fermi sea(1,2). Using tunneling spectroscopy to measure the SPDOS of a two-dimensional electronic system, we demonstrate the first detection of the plasmaron in a system in which electrons have mass. We monitor the evolution of the plasmaron with applied magnetic field and discover unpredicted "magnetoplasmarons" which appear in spectra as negative index Landau levels. These sharp features corresponding to long-lived quasiparticles appear at high energies where SPDOS structure is ordinarily broadened by electron-electron interactions.

cond-mat.mes-hall↗

arXiv2010

Observation of pinning mode in Wigner solid of 1/3 fractional quantum Hall excitations

We report the observation of a resonance in the microwave spectra of the real diagonal conductivities of a two-dimensional electron system within a range of ~ +- .0.015 $ from filling factor $ν=1/3$. The resonance is remarkably similar to resonances previously observed near integer $ν$, and is interpreted as the collective pinning mode of a disorder-pinned Wigner solid phase of $e/3$-charged carriers .

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arXiv2010

Low Temperature Electrostatic Force Microscopy of a Deep Two Dimensional Electron Gas using a Quartz Tuning Fork

Using an ultra-low temperature, high magnetic field scanning probe microscope, we have measured electric potentials of a deeply buried two dimensional electron gas (2DEG). Relying on the capacitive coupling between the 2DEG and a resonant tip/cantilever structure, we can extract electrostatic potential information of the 2DEG from the dynamics of the oscillator. We present measurements using a quartz tuning fork oscillator and a 2DEG with a cleaved edge overgrowth structure. The sensitivity of the quartz tuning fork as force sensor is demonstrated by observation of Shubnikov de Haas oscillations at a large tip-2DEG separation distance of more than 500 nm.

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arXiv2010

Delocalized-localized transition in a semiconductor two-dimensional honeycomb lattice

We report the magneto-transport properties of a two-dimensional electron gas in a modulation-doped AlGaAs/GaAs heterostructure subjected to a lateral potential with honeycomb geometry. Periodic oscillations of the magneto-resistance and a delocalized-localized transition are shown by applying a gate voltage. We argue that electrons in such artificial-graphene lattices offer a promising approach for the simulation of quantum phases dictated by Coulomb interactions.

cond-mat.mes-hall↗

arXiv2010

Zero differential resistance in two-dimensional electron systems at large filling factors

We report on a state characterized by a zero differential resistance observed in very high Landau levels of a high-mobility two-dimensional electron system. Emerging from a minimum of Hall field-induced resistance oscillations at low temperatures, this state exists over a continuous range of magnetic fields extending well below the onset of the Shubnikov-de Haas effect. The minimum current required to support this state is largely independent on the magnetic field, while the maximum current increases with the magnetic field tracing the onset of inter-Landau level scattering.

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arXiv2010

Resonant Subband Landau Level Coupling in Symmetric Quantum Well

Subband structure and depolarization shifts in an ultra-high mobility GaAs/Al_{0.24}Ga_{0.76}As quantum well are studied using magneto-infrared spectroscopy via resonant subband Landau level coupling. Resonant couplings between the 1st and up to the 4th subbands are identified by well-separated anti-level-crossing split resonance, while the hy-lying subbands were identified by the cyclotron resonance linewidth broadening in the literature. In addition, a forbidden intersubband transition (1st to 3rd) has been observed. With the precise determination of the subband structure, we find that the depolarization shift can be well described by the semiclassical slab plasma model, and the possible origins for the forbidden transition are discussed.

cond-mat.other↗

arXiv2010

Alternation and interchange of e/4 and e/2 period interference oscillations as evidence for filling factor 5/2 non-Abelian quasiparticles

It is a theoretical conjecture that 5/2 fractional quantum Hall state charge e/4 excitations may obey exotic non-Abelian statistics. In edge state interference these purported non-Abelian quasiparticles should display period e/4 Aharonov-Bohm oscillations if the interfering quasiparticle encircles an even number of localized e/4 charges, but suppression of oscillations if an odd number is encircled. To test this hypothesis, here we perform swept area interference measurements at 5/2. We observe an alternating pattern of e/4 and e/2 period oscillations in resistance. This aperiodic alternation is consistent with proposed non-Abelian properties: the e/4 oscillations occur for encircling an even number of localized quasiparticles, e/2 oscillations are expressed when encircling an odd number. Aperiodic alternation corresponds to the expected area sweep sampling the localized quasiparticles. Importantly, adding localized quasiparticles to the encircled area by changing magnetic field induces interchange of the e/4 and e/2 oscillation periods, specifically consistent with non-Abelian e/4 quasiparticles.

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