arXiv subjects

K. W. West

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

At least 109 records · Page 6Linked to original sources

arXiv2017

Low-Frequency Microwave Induced Quantum Oscillations in A Two-Dimensional Electron System

We study the magnetoresistance of an ultrahigh mobility GaAs/AlGaAs two-dimensional electron sample in a weak magnetic field under low-frequency (f < 20 GHz) microwave (MW) irradiation. We observe that with decreasing MW frequency, microwave induced resistance oscillations (MIRO) damp and multi-photon processes become dominant. At very low MW frequency (f < 4 GHz), MIRO disappears gradually and a new SdH-like oscillation develops. The analysis indicates that the new oscillation may originate from alternating Hall-field induced resistance oscillations (ac-HIRO), or can be viewed as a multi-photon process of MIRO in low MW frequency limit. Our findings bridge the non-equilibrium states of MIRO and HIRO, which can be brought into a frame of quantum tunneling junction model.

cond-mat.mes-hall↗

arXiv2017

Interaction-induced Interlayer Charge Transfer in the Extreme Quantum Limit

An interacting bilayer electron system provides an extended platform to study electron-electron interaction beyond single layers. We report here experiments demonstrating that the layer densities of an asymmetric bilayer electron system oscillate as a function of perpendicular magnetic field that quantizes the energy levels. At intermediate fields, this interlayer charge transfer can be well explained by the alignment of the Landau levels in the two layers. At the highest fields where both layers reach the extreme quantum limit, however, there is an anomalous, enhanced charge transfer to the majority layer. Surprisingly, when the minority layer becomes extremely dilute, this charge transfer slows down as the electrons in the minority layer condense into a Wigner crystal. Furthermore, by examining the quantum capacitance of the dilute layer at high fields, the screening induced by the composite fermions in an adjacent layer is unveiled. The results highlight the influence of strong interaction in interlayer charge transfer in the regime of very high fields and low Landau level filling factors.

cond-mat.mes-hall↗

arXiv2017

Observation of an anomalous density-dependent energy gap of the $ν=5/2$ fractional quantum Hall state in the low density regime

We have studied the $ν=5/2$ fractional quantum Hall state in a density-tunable sample at extremely low electron densities. For the densities accessed in our experiment, the Landau level mixing parameter $κ$ spans the $2.52<κ<2.82$ range. In the vicinity of $5.8 \times 10^{10}$~cm$^{-2}$ or $κ= 2.6$ an anomalously large change in the density dependence of the energy gap is observed. Possible origins of such an anomaly are discussed, including a topological phase transition in the $ν=5/2$ fractional quantum Hall state.

cond-mat.mes-hall↗

arXiv2017

Stability of the $N = 2$ bubble states in Corbino geometry under a tilted field

We measured the activation energy gap of the N = 2 bubble states using tilted-field technique in two GaAs/AlGaAs samples with Corbino geometry, and observed monotonically decreasing energy gaps with the increasing total magnetic field. The observed tilt angle dependence is discussed based on magneto-orbital coupling and finite-size effect. We also found a bubble state at an uncommon filling factor of 4.67, with an unanticipated energy gap of up to 5.35 K and opposite in-plane field dependence.

cond-mat.mes-hall↗

arXiv2017

Specific heat and entropy of fractional quantum Hall states in the second Landau level

Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information about the fractional quantum Hall effect as well. However, traditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron in the second Landau level by measuring the thermalization time between the electrons and phonons. We observe activated behaviour of the specific heat of the 5/2 and 7/3 fractional quantum Hall states, and extract the entropy by integrating over temperature. Our results are in excellent agreement with previous measurements of the entropy via longitudinal thermopower. Extending the technique to lower temperatures could lead to detection of the non-Abelian entropy predicted for bulk quasiparticles at 5/2 filling

cond-mat.mes-hall↗

arXiv2017

Evidence of a two-stage melting of Wigner solids in two dimensions

Two-dimensional (2D) solid-liquid transition (SLT)~\cite{Mermin1966Absence,Mermin1968Crystalline,Kosterlitz1972Long} concerns fundamental concepts of long-range correlations vital to magnetism, superconductivity, superfluidity, and topological matters. A long sought-after example is the melting of a Wigner Crystal (WC)~\cite{Wigner1934Interaction} of electrons. Detection efforts have targeted distinctive collective modes such as pinning by disorder, resonant-frequency absorption of Goldstone modes, and melting transition. However, only one-step second-order melting of softly-pinned modes was reported. Without the evidence of genuine pinning as exhibited in the charge density waves (CDWs)~\cite{PinningCDW}, these modes are likely intermediate phases which are only part of a complete SLT. To verify if there is a WC-intermediate phase transition will not only provide a solid proof of a WC, but will also unveil the nature of the SLT in relation to the two-stage Kosterlitz-Thouless (KT) model~\cite{Kosterlitz1972Long,Kosterlitz1973Ordering,Halperin1978Theory,Nelson1979Dislocation,hexatic_ceperley,hexatic_nelson, Young1979Melting}. %(or dislocation pairs) above a critical temperature $T_c$. Through transport studies of ultra-dilute high-purity 2D systems, this work presents evidence for not only a WC, but also for a two-stage WC-liquid SLT mediated by a first-order WC-intermediate phase transition.

cond-mat.str-el↗

arXiv2017

Spin-orbit coupling and transport of strongly correlated two-dimensional systems

Measuring the magnetoresistance (MR) of ultraclean {\it GaAs} two-dimensional holes in a large $r_s$ range of 20-50, two striking behaviors in relation to the spin-orbit coupling (SOC) emerge in response to strong electron-electron interaction. First, in exact correspondence to the zero-field metal-to-insulator transition (MIT), the sign of the MR switches from being positive in the metallic regime to being negative in the insulating regime when the carrier density crosses the critical density $p_c$ of MIT ($r_s\sim 39$). Second, as the SOC-driven correction $Δρ$ to the MR decreases with reducing carrier density (or the in-plane wave vector), it exhibits an upturn in the close proximity just above $p_c$ where $r_s$ is beyond 30, indicating a substantially enhanced SOC effect. This peculiar behavior echoes with a trend of delocalization long suspected for the SOC-interaction interplay. Meanwhile, for $p 40$, in contrast to the common belief that a magnet field enhances Wigner crystallization, the negative MR is likely linked to enhanced interaction.

cond-mat.str-el↗

arXiv2017

Tuning of Fermi Contour Anisotropy in GaAs (001) 2D Holes via Strain

We demonstrate tuning of the Fermi contour anisotropy of two-dimensional (2D) holes in a symmetric GaAs (001) quantum well via the application of in-plane strain. The ballistic transport of high-mobility hole carriers allows us to measure the Fermi wavevector of 2D holes via commensurability oscillations as a function of strain. Our results show that a small amount of in-plane strain, on the order of $10^{-4}$, can induce significant Fermi wavevector anisotropy as large as 3.3, equivalent to a mass anisotropy of 11 in a parabolic band. Our method to tune the anisotropy \textit{in situ} provides a platform to study the role of anisotropy on phenomena such as the fractional quantum Hall effect and composite fermions in interacting 2D systems.

cond-mat.mes-hall↗

arXiv2017

Search for Composite Fermions at Filling Factor 5/2: Role of Landau Level and Subband Index

The pairing of composite fermions (CFs), electron-flux quasi-particles, is commonly proposed to explain the even-denominator fractional quantum Hall state observed at $ν=5/2$ in the first excited ($N=1$) Landau level (LL) of a two-dimensional electron system (2DES). While well-established to exist in the lowest ($N=0$) LL, much is unknown about CFs in the $N=1$ LL. Here we carry out geometric resonance measurements to detect CFs at $ν=5/2$ by subjecting the 2DES to a one-dimensional density modulation. Our data, taken at a temperature of 0.3 K, reveal no geometric resonances for CFs in the $N=1$ LL. In stark contrast, we observe clear signatures of such resonances when $ν=5/2$ is placed in the $N=0$ LL of the anti-symmetric subband by varying the 2DES width. This finding implies that the CFs' mean-free-path is significantly smaller in the $N=1$ LL compared to the $N=0$ LL. Our additional data as a function of in-plane magnetic field highlight the role of subband index and establish that CFs at $ν=5/2$ in the $N=0$ LL are more anisotropic in the symmetric subband than in the anti-symmetric subband.

cond-mat.mes-hall↗

arXiv2017

Fine structure of high-power microwave-induced resistance oscillations

We report on observation of a fine structure of microwave-induced resistance oscillations in an ultraclean two-dimensional electron gas. This fine structure is manifested by multiple secondary sharp extrema, residing beside the primary ones, which emerge at high radiation power. Theoretical considerations reveal that this fine structure originates from multiphoton-assisted scattering off short-range impurities. Unique properties of the fine structure allow us to access all experimental parameters, including microwave power, and to separate different contributions to photoresistance. Furthermore, we show that the fine structure offers a convenient means to quantitatively assess the correlation properties of the disorder potential in high-quality systems, allowing separation of short- and long-range disorder contributions to the electron mobility.

cond-mat.mes-hall↗

arXiv2017

Transference of Fermi Contour Anisotropy to Composite Fermions

There has been a surge of recent interest in the role of anisotropy in interaction-induced phenomena in two-dimensional (2D) charged carrier systems. A fundamental question is how an anisotropy in the energy-band structure of the carriers at zero magnetic field affects the properties of the interacting particles at high fields, in particular of the composite fermions (CFs) and the fractional quantum Hall states (FQHSs). We demonstrate here tunable anisotropy for holes and hole-flux CFs confined to GaAs quantum wells, via applying \textit{in situ} in-plane strain and measuring their Fermi wavevector anisotropy through commensurability oscillations. For strains on the order of $10^{-4}$ we observe significant deformations of the shapes of the Fermi contours for both holes and CFs. The measured Fermi contour anisotropy for CFs at high magnetic field ($α_\mathrm{CF}$) is less than the anisotropy of their low-field hole (fermion) counterparts ($α_\mathrm{F}$), and closely follows the relation: $α_\mathrm{CF} = \sqrt{α_\mathrm{F}}$. The energy gap measured for the $ν= 2/3$ FQHS, on the other hand, is nearly unaffected by the Fermi contour anisotropy up to $α_\mathrm{F} \sim 3.3$, the highest anisotropy achieved in our experiments.

cond-mat.mes-hall↗

arXiv2017

Interplay between Quantum Well Width and Interface Roughness for Electron Transport Mobility in GaAs Quantum Wells

We report transport mobility measurements for clean, two-dimensional (2D) electron systems confined to GaAs quantum wells (QWs), grown via molecular beam epitaxy, in two families of structures, a standard, symmetrically-doped GaAs set of QWs with Al$_{0.32}$Ga$_{0.68}$As barriers, and one with additional AlAs cladding surrounding the QWs. Our results indicate that the mobility in narrow QWs with no cladding is consistent with existing theoretical calculations where interface roughness effects are softened by the penetration of the electron wave function into the adjacent low barriers. In contrast, data from AlAs-clad wells show a number of samples where the 2D electron mobility is severely limited by interface roughness. These measurements across three orders of magnitude in mobility provide a road map of reachable mobilities in the growth of GaAs structures of different electron densities, well widths, and barrier heights.

cond-mat.mes-hall↗

arXiv2017

Quantum Hall Spin Diode

Double layer two-dimensional electron systems at high perpendicular magnetic field are used to realize magnetic tunnel junctions in which the electrons at the Fermi level in the two layers have either parallel or anti-parallel spin magnetizations. In the anti-parallel case the tunnel junction, at low temperatures, behaves as a nearly ideal spin diode. At elevated temperatures the diode character degrades as long-wavelength spin waves are thermally excited. These tunnel junctions provide a demonstration that the spin polarization of the electrons in the $N=1$ Landau level at filling factors $ν= 5/2$ and 7/2 is essentially complete, and, with the aid of an in-plane magnetic field component, that Landau level mixing at these filling factors is weak in the samples studied.

cond-mat.mes-hall↗

arXiv2016

Multi-Photon Transitions in Coupled Plasmon-Cyclotron Resonance Measured by Millimeter-Wave Reflection

We construct a low-temperature microwave waveguide interferometer for measuring high-frequency properties of two-dimensional electron gases (2DEGs). Coupled plasmon-cyclotron resonance (PCR) spectra are used to extract effective mass, bulk plasmon frequency, and carrier relaxation times. In contrast to traditional transmission spectroscopy, this method does not require sample preparation and is nondestructive. PCR signals can be resolved with a microwave power source as low as 10 nW. We observe PCR in the multi-photon transition regime, which has been proposed to be relevant to the microwave-induced resistance oscillations.

cond-mat.mes-hall↗

arXiv2016

Photovoltage Detection of Edge Magnetoplasmon Oscillations and Giant Magnetoplasmon Resonances in A Two-Dimensional Hole System

In our high mobility p-type AlGaAs/GaAs two-dimensional hole samples, we originally observe the B-periodic oscillation induced by microwave (MW) in photovoltage (PV) measurements. In the frequency range of our measurements (5 - 40 GHz), the period (ΔB) is inversely proportional to the microwave frequency (f). The distinct oscillations come from the edge magnetoplasmon (EMP) in the high quality heavy hole system. In our hole sample with a very large effective mass, the observation of the EMP oscillations is in neither the low frequency limit nor the high frequency limit, and the damping of the EMP oscillations is very weak under high magnetic fields. Simultaneously, we observe the giant plasmon resonance signals in our measurements on the shallow two-dimensional hole system (2DHS).

cond-mat.mes-hall↗

arXiv2016

Sub-Cycle Optical Response Caused by Dressed State with Phase-Locked Wavefunctions

The coherent interaction of light with matter imprints the phase information of the light field on the wavefunction of the photon-dressed electronic state. Driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a sub-cycle timescale in a GaAs quantum well in the presence of a multi-cycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.

physics.optics↗

arXiv2016

Geometric Resonance of Composite Fermions near Bilayer Quantum Hall States

Via the application of parallel magnetic field, we induce a single-layer to bilayer transition in two-dimensional electron systems confined to wide GaAs quantum wells, and study the geometric resonance of composite fermions (CFs) with a periodic density modulation in our samples. The measurements reveal that CFs exist close to bilayer quantum Hall states, formed at Landau level filling factors $ν=1$ and 1/2. Near $ν=1$, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a two-component behavior. In contrast, close to $ν=1/2$, CFs appear single-layer-like (single-component) as their resonance features correspond to the total density.

cond-mat.mes-hall↗

arXiv2016

Charge metastability and hysteresis in the quantum Hall regime

We report simultaneous quasi-dc magnetotransport and high frequency surface acoustic wave measurements on bilayer two-dimensional electron systems in GaAs. Near strong integer quantized Hall states a strong magnetic field sweep hysteresis in the velocity of the acoustic waves is observed at low temperatures. This hysteresis indicates the presence of a metastable state with anomalously high conductivity in the interior of the sample. This non-equilibrium state is not revealed by conventional low frequency transport measurements which are dominated by dissipationless transport at the edge of the 2D system. We find that a field-cooling technique allows the equilibrium charge configuration within the interior of the sample to be established. A simple model for this behavior is discussed.

cond-mat.mes-hall↗