arXiv subjects

K. W. West

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

At least 325 records · Page 18Linked to original sources

arXiv2006

Reentrant anisotropic phases in a two-dimensional hole system

Anisotropic charge transport is observed in a two-dimensional (2D) hole system in a perpendicular magnetic field at filling factors nu=7/2 and nu=11/2 for temperatures below 150mK. In stark contrast, the transport at nu=9/2 is isotropic for all temperatures. Our results for a two-dimensional hole system differ substantially from 2D electron transport where no anisotropy has been observed at nu=7/2, the strongest anisotropy occurs at nu=9/2, and reentrant behavior is not evident. We attribute this difference to strong spin-orbit coupling in the hole system.

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arXiv2006

Resistively-Detected NMR in a Two-Dimensional Electron System near $ν= 1$: Clues to the Origin of the Dispersive Lineshape

Resistively-detected NMR measurements on 2D electron systems near the $ν= 1$ quantum Hall state are reported. In contrast to recent results of Gervais \emph{et al.} [Phys. Rev. Lett. $\bf 94$, 196803 (2005)], a dispersive lineshape is found at all RF powers studied and Korringa-like nuclear spin-lattice relaxation is observed. The shape of the unexplained dispersive lineshape is found to invert when the temperature derivative of the longitudinal resistance changes sign. This suggests that both Zeeman and thermal effects are important to resistively-detected NMR in this regime.

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arXiv2006

Interaction Effects and Pseudogap in Two-Dimensional Lateral Tunnel Junctions

Tunneling characteristics of a two-dimensional lateral tunnel junction (2DLTJ) are reported. A pseudogap on the order of Coulomb energy is detected in the tunneling density of states (TDOS) when two identical two-dimensional electron systems are laterally separated by a thin energy barrier. The Coulombic pseudogap remains robust well into the quantum Hall regime until it is overshadowed by the cyclotron gap in the TDOS. The pseudogap is modified by in-plane magnetic field, demonstrating a non-trivial effect of in-plane magnetic field on the electron-electron interaction.

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arXiv2006

Resistance scaling for Composite Fermions in the presence of a density gradient

The magnetoresistance, Rxx, at even-denominator fractional fillings, of an ultra high quality two-dimensional electron system at T ~ 35 mK is observed to be strictly linear in magnetic field, B. While at 35mK Rxx is dominated by the integer and fractional quantum Hall states, at T~1.2K an almost perfect linear relationship between Rxx vs B emerges over the whole magnetic field range except for spikes at the integer quantum Hall states. This linear Rxx cannot be understood within the Composite Fermion model, but can be explained through the existence of a density gradient in our sample.

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arXiv2005

Multiphoton processes in microwave photoresistance of 2D electron system

We extend our studies of microwave photoresistance of ultra-high mobility two-dimensional electron system (2DES) into the high-intensity, non-linear regime employing both monochromatic and bichromatic radiation. Under high-intensity monochromatic radiation $ω$ we observe new zero-resistance states (ZRS) which correspond to rational values of $ε=ω/ω_C$ ($ω_C$ is the cyclotron frequency) and can be associated with multiphoton processes. %Formation of these rational ZRS is accompanied by a dramatic reconstruction of the photoresistance spectrum which reveals diminishing, narrowing, and phase reduction of the resistance peaks, as well as overall suppression of resistance at $ε<1/2$. Under bichromatic radiation $ω_1,ω_2$ we discover new resistance minimum, possibly a precursor of bichromatic ZRS, which seems to originate from a frequency mixing process, $ω_1+ω_2$. These findings indicate that multiphoton processes play important roles in the physics of non-equilibrium transport of microwave-driven 2DES, and suggest new directions for theoretical and experimental studies.

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arXiv2005

Spin Susceptibility of a 2D Electron System in GaAs towards the Weak Interaction Region

We determine the spin susceptibility $χ$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a GaAs/AlGaAs heterostructure. The band structure effects, modifying mass and g-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $χ$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\times10^{11} cm^{-2}$. In the high density limit, $χ$ tends correctly towards $χ\to 1$ and compare well with recent theory.

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arXiv2005

Negative Differential Resistance and Astability of the Wigner Solid

We report an unusual breakdown of the magnetically induced Wigner solid in an exceptional two-dimensional electron gas. The current-voltage characteristic is found to be hysteretic in the voltage biased setup and has a region of negative differential resistance in the current biased setup. When the sample is current biased in the region of negative differential resistance, the voltage on and the current through the sample develop spontaneous narrow band oscillations.

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arXiv2005

Splitting of Long-Wavelength Modes of the Fractional Quantum Hall Liquid at $ν=1/3$

Resonant inelastic light scattering experiments at $ν=1/3$ reveal a novel splitting of the long wavelength modes in the low energy spectrum of quasiparticle excitations in the charge degree of freedom. We find a single peak at small wavevectors that splits into two distinct modes at larger wavevectors. The evidence of well-defined dispersive behavior at small wavevectors indicates a coherence of the quantum fluid in the micron length scale. We evaluate interpretations of long wavelength modes of the electron liquid.

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arXiv2005

The microwave induced resistance response of a high mobility 2DEG from the quasi-classical limit to the quantum Hall regime

Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from ~20 GHz up to ~4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency are presented and discussed.

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arXiv2005

Imaging Transport Resonances in the Quantum Hall Effect

We use a scanning capacitance probe to image transport in the quantum Hall system. Applying a DC bias voltage to the tip induces a ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves with the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island in the IS. These islands enable resonant tunneling across the IS, enhancing its conductance by more than four orders of magnitude. The images provide a quantitative measure of disorder and suggest resonant tunneling as the primary mechanism for transport across ISs.

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arXiv2005

Surface Acoustic Wave Propagation and Inhomogeneities in Low Density Two-Dimensional Electron Systems Near the Metal-Insulator Transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime ($<$ $10^{10}$ cm$^{-2}$) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES which become very important at low density. This has implications for the putative metal-insulator transition in two dimensions.

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arXiv2005

Spin-charge separation and localization in one-dimension

We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained from tunneling between two parallel wires in a GaAs/AlGaAs heterostructure while varying electron density. We observe two spin modes and one charge mode of the coupled wires, and map the dispersion velocities of the modes down to a critical density, at which spontaneous localization is observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that is not observed. The measured spin velocity is found to be smaller than theoretically predicted.

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arXiv2005

Localization Transition in a Ballistic Quantum Wire

The many-body wave-function of an interacting one-dimensional electron system is probed, focusing on the low-density, strong interaction regime. The properties of the wave-function are determined using tunneling between two long, clean, parallel quantum wires in a GaAs/AlGaAs heterostructure, allowing for gate-controlled electron density. As electron density is lowered to a critical value the many-body state abruptly changes from an extended state with a well-defined momentum to a localized state with a wide range of momentum components. The signature of the localized states appears as discrete tunneling features at resonant gate-voltages, corresponding to the depletion of single electrons and showing Coulomb-blockade behavior. Typically 5-10 such features appear, where the one-electron state has a single-lobed momentum distribution, and the few-electron states have double-lobed distributions with peaks at $\pm k_F$. A theoretical model suggests that for a small number of particles (N<6), the observed state is a mixture of ground and thermally excited spin states.

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arXiv2005

Quantization of the diagonal resistance: Density gradients and the empirical resistance rule in a 2D system

We have observed quantization of the diagonal resistance, R_xx, at the edges of several quantum Hall states. Each quantized R_xx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, R_xy. Peaks in R_xx occur at different positions in positive and negative magnetic fields. Practically all R_xx features can be explained quantitatively by a ~1%/cm electron density gradient. Therefore, R_xx is determined by R_xy and unrelated to the diagonal resistivity rho_xx. Our findings throw an unexpected light on the empirical resistivity rule for 2D systems.

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arXiv2005

Influence of a Parallel Magnetic Field on Microwave Photoconductivity in a High-Mobility 2D Electron System

We have studied experimentally the influence of a parallel magnetic field ($B_{//}$) on microwave-induced resistance oscillations (MIRO) and zero-resistance states (ZRS) previously discovered in a high-mobility 2D electron system. We have observed a strong suppression of MIRO/ZRS by a modest $B_{//}\sim 0.5$ T. In Hall bar samples, magnetoplasmon resonance (MPR) has also been observed concurrently with the MIRO/ZRS. In contrast to the suppression of MIRO/ZRS, the MPR peak is found to be enhanced by $B_{//}$. These findings have not been addressed by current models proposed to explain the microwave-induced effects.

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arXiv2005

Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer

We report on quantum interference between a pair of counterpropagating quantum Hall edge states that are separated by a high quality tunnel barrier. Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling between coupled Luttinger liquids that creates bound electronic states between pairs of tunnel centers that act like interference slits. We place a lower bound in the range of 20-40 $μ$m for the phase coherence length and directly confirm the extended phase coherence of quantum Hall edge states.

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arXiv2005

Strongly Enhanced Hole-Phonon Coupling in the Metallic State of the Dilute Two-Dimensional Hole Gas

We have studied the temperature dependent phonon emission rate $P$($T$) of a strongly interacting ($r_s\geq$22) dilute 2D GaAs hole system using a standard carrier heating technique. In the still poorly understood metallic state, we observe that $P$($T$) changes from $P$($T$)$\sim T^5$ to $P$($T$)$\sim T^7$ above 100mK, indicating a crossover from screened piezoelectric(PZ) coupling to screened deformation potential(DP) coupling for hole-phonon scattering. Quantitative comparison with theory shows that the long range PZ coupling between holes and phonons has the expected magnitude; however, in the metallic state, the short range DP coupling between holes and phonons is {\it almost twenty times stronger} than expected from theory. The density dependence of $P$($T$) shows that it is {\it easier} to cool low density 2D holes in GaAs than higher density 2D hole systems.

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arXiv2005

High mobility two-dimensional hole system in GaAs/AlGaAs quantum wells grown on (100) GaAs substrates

We report on the transport properties of a high mobility two-dimensional hole system (2DHS) confined in GaAs/AlGaAs quantum wells grown molecular-beam epitaxy on the (100) surface of GaAs. The quantum wells are modulation-doped with carbon utilizing a novel resistive filament source. At T=0.3K and carrier density p=1x10^11cm^-2, a mobility of 10^6cm^2/Vs is achieved. At fixed carrier density p=10^11cm^-2, the mobility is found to be a non-monotonic function of the quantum well width. The mobility peaks at 10^6cm^2/Vs for a 15nm well and is reduced for both smaller and larger well widths for these (100) samples. The mobility anisotropy is found to be small. Mobility along (0\bar11) is approximately 20% higher than along the (011) direction. In addition, the low temperature carrier density is found to have low sensitivity to light. The hole density increases by only ~10% after exposure to red light at T=4.2K. In structures designed for a lower carrier density of 3.6x10^10cm^-2, a mobility of 800,000cm^2/Vs is achieved at T=15mK.

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