arXiv subjects

K. W. West

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

At least 271 records · Page 15Linked to original sources

arXiv2009

Temperature Dependence of Microwave Photoresistance in 2D Electron Systems

We report on the temperature dependence of microwave-induced resistance oscillations in high-mobility two-dimensional electron systems. We find that the oscillation amplitude decays exponentially with increasing temperature, as $\exp(-αT^2)$, where $α$ scales with the inverse magnetic field. This observation indicates that the temperature dependence originates primarily from the modification of the single particle lifetime, which we attribute to electron-electron interaction effects.

cond-mat.mes-hall↗

arXiv2008

Distinct Signatures For Coulomb Blockade and Aharonov-Bohm Interference in Electronic Fabry-Perot Interferometers

Two distinct types of magnetoresistance oscillations are observed in two electronic Fabry-Perot interferometers of different sizes in the integer quantum Hall regime. Measuring these oscillations as a function of magnetic field and gate voltages, we observe three signatures that distinguish the two types. The oscillations observed in a 2.0 square micron device are understood to arise from the Coulomb blockade mechanism, and those observed in an 18 square micron device from the Aharonov-Bohm mechanism. This work clarifies, provides ways to distinguish, and demonstrates control over, these distinct physical origins of resistance oscillations seen in electronic Fabry-Perot interferometers.

cond-mat.mes-hall↗

arXiv2008

Charge Imbalance and Bilayer 2D Electron Systems at $ν_T = 1$

We use interlayer tunneling to study bilayer 2D electron systems at $ν_T = 1$ over a wide range of charge density imbalance, $Δν=ν_1-ν_2$, between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic $ν_T = 1$ phase at small layer separation can survive at least up to an imbalance of $Δν$ = 0.5, i.e $(ν_1, ν_2)$ = (3/4, 1/4). Phase transitions between the excitonic $ν_T = 1$ state and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing $d/\ell$, the temperature $T$, or the charge imbalance, $Δν$. We observe that close to the phase boundary the coherent $ν_T = 1$ phase can be absent at $Δν$ = 0, present at intermediate $Δν$, but then absent again at large $Δν$, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical $d/\ell$ shifts linearly with temperature, while at $Δν$ = 1/3 the critical $d/\ell$ is only weakly dependent on $T$. At $Δν$ = 1/3 we report the first observation of a direct phase transition between the coherent excitonic $ν_T = 1$ bilayer integer quantum Hall phase and the pair of single layer fractional quantized Hall states at $ν_1$ = 2/3 and $ν_2=1/3$.

cond-mat.mes-hall↗

arXiv2008

Local magnetization fluctuations in superconducting glasses resolved by Hall sensors

We report on magnetization measurements performed on a series of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses with $0\leq x\leq0.5$ using the Hall effect of a nearby 2-dimensional electron gas (2DEG) in a GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructure as a local probe. The great sensitivity of the Hall effect of the 2DEG in such heterostructure is exploited to determine the magnetization of the superconductor due to the Meissner effect and flux trapping. The data is used to determine the lower critical field B$_{c1}$ of the superconductors as a function of temperature. Surprisingly large fluctuations in the magnetization are also observed and attributed to the presence of large flux clusters in the superconductor.

cond-mat.supr-con↗

arXiv2008

Pinning mode resonances of 2D electron stripe phases: Effect of in-plane magnetic field

We study the anisotropic pinning-mode resonances in the rf conductivity spectra of the stripe phase of 2D electron systems (2DES) around Landau level filling 9/2, in the presence of an in-plane magnetic field, B_ip. The polarization along which the resonance is observed switches as B_ip is applied, consistent with the reorientation of the stripes. The resonance frequency, a measure of the pinning interaction between the 2DES and disorder, increases with B_ip. The magnitude of this increase indicates that disorder interaction is playing an important role in determining the stripe orientation.

cond-mat.mes-hall↗

arXiv2008

Microwave Photoresistance in dc-driven 2D Systems at Cyclotron Resonance Subharmonics

We study microwave photoresistivity oscillations in a high mobility two-dimensional electron system subject to strong dc electric fields. We find that near the second subharmonic of the cyclotron resonance the frequency of the resistivity oscillations with dc electric field is twice the frequency of the oscillations at the cyclotron resonance, its harmonics, or in the absence of microwave radiation. This observation is discussed in terms of the microwave-induced sidebands in the density of states and the interplay between different scattering processes in the separated Landau level regime.

cond-mat.mes-hall↗

arXiv2008

Observation of a Fractional Quantum Hall State at $ν=1/4$ in a Wide GaAs Quantum Well

We report the observation of an even-denominator fractional quantum Hall (FQH) state at $ν=1/4$ in a high quality, wide GaAs quantum well. The sample has a quantum well width of 50 nm and an electron density of $n_e=2.55\times10^{11}$ cm$^{-2}$. We have performed transport measurements at $T\sim35$ mK in magnetic fields up to 45 T. When the sample is perpendicular to the applied magnetic field, the diagonal resistance displays a kink at $ν=1/4$. Upon tilting the sample to an angle of $θ=20.3^o$ a clear FQH state at emerges at $ν=1/4$ with a plateau in the Hall resistance and a strong minimum in the diagonal resistance.

cond-mat.mes-hall↗

arXiv2008

Contrasting Behavior of the 5/2 and 7/3 Fractional Quantum Hall Effect in a Tilted Field

Using a tilted field geometry, the effect of an in-plane magnetic field on the even denominator nu = 5/2 fractional quantum Hall state is studied. The energy gap of the nu = 5/2 state is found to collapse linearly with the in-plane magnetic field above ~0.5 T. In contrast, a strong enhancement of the gap is observed for the nu = 7/3 state. The radically distinct tilted-field behaviour between the two states is discussed in terms of Zeeman and magneto-orbital coupling within the context of the proposed Moore-Read pfaffian wavefunction for the 5/2 fractional quantum Hall effect.

cond-mat.mes-hall↗

arXiv2008

Photo-current and photo-voltage oscillations in the two-dimensional electron system: screening and "anti-screening" of a potential profile

We observe in state-of-the-art GaAs based 2D electron systems microwave induced photo-current and photo-voltage oscillations around zero as a function of the applied magnetic field. The photo-signals pass zero whenever the microwave frequency is close to a multiple of the cyclotron resonance frequency. They originate from built-in electric fields due to for instance band bending at contacts. The oscillations correspond to a suppression (screening) or an enhancement ("anti-screening") of these fields by the photo-excited electrons.

cond-mat.mes-hall↗

arXiv2008

Non-linear magnetotransport in microwave-illuminated two-dimensional electron systems

We study magnetoresistivity oscillations in a high-mobility two-dimensional electron system subject to both microwave and dc electric fields. First, we observe that the oscillation amplitude is a periodic function of the inverse magnetic field and is strongly suppressed at microwave frequencies near half-integers of the cyclotron frequency. Second, we obtain a complete set of conditions for the differential resistivity extrema and saddle points. These findings indicate the importance of scattering without microwave absorption and a special role played by microwave-induced scattering events antiparallel to the electric field.

cond-mat.mes-hall↗

arXiv2008

Area dependence of interlayer tunneling in strongly correlated bilayer 2D electron systems at $ν_T=1$

The area and perimeter dependence of the Josephson-like interlayer tunneling signature of the coherent $ν_T=1$ quantum Hall phase in bilayer two-dimensional electron systems is examined. Electrostatic top gates of various sizes and shapes are used to locally define distinct $ν_T=1$ regions in the same sample. Near the phase boundary with the incoherent $ν_T=1$ state at large layer separation, our results demonstrate that the tunneling conductance in the coherent phase is closely proportional to the total area of the tunneling region. This implies that tunneling at $ν_T=1$ is a bulk phenomenon in this regime.

cond-mat.mes-hall↗

arXiv2008

Quasiparticle Tunneling in the Fractional Quantum Hall State at ν= 5/2

Theory predicts that quasiparticle tunneling between the counter-propagating edges in a fractional quantum Hall state can be used to measure the effective quasiparticle charge e* and dimensionless interaction parameter g, and thereby characterize the many-body wavefunction describing the state. We report measurements of quasiparticle tunneling in a high mobility GaAs two dimensional electron system in the fractional quantum Hall state at nu=5/2 using a gate-defined constriction to bring the edges close together. We find the dc-bias peaks in the tunneling conductance at different temperatures collapse onto a single curve when scaled, in agreement with weak tunneling theory. Various models for the ν=5/2 state predict different values for g. Among these models, the non-abelian states with e*=1/4 and g=1/2 are most consistent with the data.

cond-mat.mes-hall↗

arXiv2008

Observation of pinning mode of stripe phases of 2D systems in high Landau levels

We study the radio-frequency diagonal conductivities of the anisotropic stripe phases of higher Landau levels near half integer fillings. In the hard direction, in which larger dc resistivity occurs, the spectrum exhibits a striking resonance, while in the orthogonal, easy direction, no resonance is discernable. The resonance is interpreted as a pinning mode of the stripe phase.

cond-mat.mes-hall↗

arXiv2008

Anisotropic modification of the effective hole $g$-factor by electrostatic confinement

We investigate effects of lateral confinement on spin splitting of energy levels in 2D hole gases grown on [311] GaAs. We found that lateral confinement enhances anisotropy of spin splitting relative to the 2D gas for both confining directions. Unexpectedly, the effective $g$-factor does not depend on the 1D energy level number $N$ for $B\|[0\bar{1}1]$ while it has strong $N$-dependence for $B\|[\bar{2}33]$. Apart from quantitative difference in the spin splitting of energy levels for the two orthogonal confinement directions we also report qualitative differences in the appearance of spin-split plateaus, with non-quantized plateaus observed only for the confinement in $[0\bar{1}1]$ direction. In our samples we can clearly associate the difference with anisotropy of spin-orbit interactions.

cond-mat.mes-hall↗

arXiv2008

Soft Spin Wave Near nu=1: Evidence for a Magnetic Instability in Skyrmion Systems

The ground state of the two dimensional electron gas near $ν$=1 is investigated by inelastic light scattering measurements carried down to very low temperatures. Away from $ν$=1, the ferromagnetic spin wave collapses and a new low-energy spin wave emerges below the Zeeman gap. The emergent spin wave shows soft behavior as its energy increases with temperature and reaches the Zeeman energy for temperatures above 2 K. The observed softening indicates an instability of the two dimensional electron gas towards a magnetic order that breaks spin rotational symmetry. We discuss our findings in light of the possible existence of a Skyrme crystal.

cond-mat.mes-hall↗

arXiv2008

Intrinsic Gap of the nu=5/2 Fractional Quantum Hall State

The fractional quantum Hall effect is observed at low field, in a regime where the cyclotron energy is smaller than the Coulomb interaction. The nu=5/2 excitation gap is measured to be 262+/-15 mK at ~2.6 T, in good agreement with previous measurements performed on samples with similar mobility, but with electronic density larger by a factor of two. The role of disorder on the nu=5/2 gap is examined. Comparison between experiment and theory indicates that a large discrepancy remains for the intrinsic gap extrapolated from the infinite mobility (zero disorder) limit. In contrast, no such large discrepancy is found for the nu=1/3 Laughlin state. The observation of the nu=5/2 state in the low-field regime implies that inclusion of non-perturbative Landau level mixing may be necessary to better understand the energetics of half-filled fractional quantum hall liquids.

cond-mat.mes-hall↗

arXiv2008

Evidence for a finite temperature phase transition in a bilayer quantum Hall system

We study the Josephson-like interlayer tunneling signature of the strongly correlated $ν_T = 1$ quantum Hall phase in bilayer two-dimensional electron systems as a function of the layer separation, temperature and interlayer charge imbalance. Our results offer strong evidence that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The transition temperature is dependent on both the layer spacing and charge imbalance between the layers.

cond-mat.mes-hall↗

arXiv2008

Experimental studies of the fractional quantum Hall effect in the first excited Landau level

We present a spectrum of experimental data on the fractional quantum Hall effect (FQHE) states in the first excited Landau level, obtained in an ultrahigh mobility two-dimensional electron system (2DES) and at very low temperatures and report the following results: For the even-denominator FQHE states, the sample dependence of the nu=5/2 state clearly shows that disorder plays an important role in determining the energy gap at nu=5/2. For the developing nu=19/8 FQHE state the temperature dependence of the Rxx minimum implies an energy gap of ~5mK.The energy gaps of the odd-denominator FQHE states at nu=7/3 and 8/3 also increase with decreasing disorder, similar to the gap at 5/2 state. Unexpectedly and contrary to earlier data on lower mobility samples, in this ultra-high quality specimen, the nu=13/5 state is missing, while its particle-hole conjugate state, the nu=12/5 state, is a fully developed FQHE state. We speculate that this disappearance might indicate a spin polarization of the nu=13/5 state. Finally, the temperature dependence is studied for the two-reentrant integer quantum Hall states around nu=5/2 and is found to show a very narrow temperature range for the transition from quantized to classical value.

cond-mat.mes-hall↗