arXiv subjects

K. W. West

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

At least 343 records · Page 19Linked to original sources

arXiv2005

Evidence for Skyrmion crystallization from NMR relaxation experiments

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate $(1/T_{1})$ was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of $^{75}$As is found to relax much more efficiently with $T\to 0$ and when a well developed quantum Hall state with $R_{xx}\simeq 0$ occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e. a Skyrmion crystal.

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arXiv2004

Competition Between Fractional Quantum Hall Liquid, Bubble and Wigner Crystal Phases in the Third Landau Level

Magnetotransport measurements were performed in a ultra-high mobility GaAs/AlGaAs quantum well of density $\sim 3.0 \times 10^{11}$ $cm^{-2}$. The temperature dependence of the magnetoresistance $R_{xx}$ was studied in detail in the vicinity of $ν={9/2}$. In particular, we discovered new minima in $R_{xx}$ at filling factor $ν\simeq 4{1/5}$ and $4{4/5}$, but only at intermediate temperatures $80\lesssim T\lesssim 120$ mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0.

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arXiv2004

Tilt Induced Localization and Delocalization in the Second Landau Level

We have investigated the behavior of electronic phases of the second Landau level under tilted magnetic fields. The fractional quantum Hall liquids at $ν=$2+1/5 and 2+4/5 and the solid phases at $ν=$2.30, 2.44, 2.57, and 2.70 are quickly destroyed with tilt. This behavior can be interpreted as a tilt driven localization of the 2+1/5 and 2+4/5 fractional quantum Hall liquids and a delocalization through melting of solid phases in the top Landau level, respectively. The evolution towards the classical Hall gas of the solid phases is suggestive of antiferromagnetic ordering.

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arXiv2004

Measurements of the density-dependent many-body electron mass in 2D GaAs/AlGaAs Heterostructures

We determine the density-dependent electron mass, m*, in two-dimensional (2D) electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov deHaas measurements. Using very high quality transistors with tunable electron densities we measure m* in single, high mobility specimens over a wide range of r_s (6 to 0.8). Toward low-densities we observe a rapid increase of m* by as much as 40%. For 2>r_s>0.8 the mass values fall ~10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail.

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arXiv2004

Microwave resonance of the reentrant insulating quantum Hall phases in the 1st excited Landau Level

We present measurements of the real diagonal microwave conductivity of the reentrant insulating quantum Hall phases in the first excited Landau level at temperatures below 50 mK. A resonance is detected around filling factor $ν=2.58$ and weaker frequency dependence is seen at $ν=2.42$ and 2.28. These measurements are consistent with the formation of a bubble phase crystal centered around these $ν$ at very low temperatures.

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arXiv2004

Temperature and Magnetic Field Enhanced Hall Slope of a Dilute 2D Hole System in the Ballistic Regime

We report the temperature($T$) and perpendicular magnetic field($B$) dependence of the Hall resistivity $ρ_{xy}(B)$ of dilute metallic two-dimensional(2D) holes in GaAs over a broad range of temperature(0.02-1.25K). The low $B$ Hall coefficient, $R_H$, is found to be enhanced when $T$ decreases. Strong magnetic fields further enhance the slope of $ρ_{xy}(B)$ at all temperatures studied. Coulomb interaction corrections of a Fermi liquid(FL) in the ballistic regime can not explain the enhancement of $ρ_{xy}$ which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, {\it not enhance} the slope of $ρ_{xy}(B)$ as $T$ decreases and/or $B$ increases.

cond-mat.str-el↗

arXiv2004

Microwave induced resistance oscillations on a high-mobility 2DEG: absorption/reflection and temperature damping experiments

In this work we address experimentally a number of unresolved issues related to microwave induced resistance oscillations (MIRO) and the zero-resistance states observed recently on very high-mobility 2D electron gases in GaAs/AlGaAs heterostructures. In particular, we examine electrodynamic effects via reflection/absorption experiments and study the exact waveform of MIRO and their damping due to temperature. It is shown that electrodynamic effects due to metallic-like reflection and plasmons are important producing a wide cyclotron resonance line and a number of oscillations which do not coincide with the MIRO. To describe the MIRO waveform a simple model was employed involving radiation-induced scattering with displacement. A very good correlation was found between the temperature dependencies of the quantum lifetime from MIRO and the transport scattering time from the electron mobility. The results are compared with measurements of Shubnikov-de Haas oscillations down to 30 mK on the same sample.

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arXiv2004

Oscillatory and Vanishing Resistance States in Microwave Irradiated 2D Electron Systems

Giant-amplitude oscillations in dc magnetoresistance of a high-mobility two-dimensional electron system can be induced by millimeterwave irradiations, leading to zero-resistance states at the oscillation minima. Following a brief overview of the now well-known phenomenon, this paper reports on aspects of more recent experiments on the subject. These are: new zero-resistance states associated with multi-photon processes; suppression of Shubnikov-de Haas oscillations by high-frequency microwaves; and microwave photoconductivity of a high-mobility two-dimensional hole system.

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arXiv2004

Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems

Using a combination of heat pulse and nuclear magnetic resonance techniques we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at $ν_T = 1$ in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.

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arXiv2004

Spin separation in cyclotron motion

Charged carriers with different spin states are spatially separated in a two-dimensional hole gas. Due to strong spin-orbit interaction holes at the Fermi energy have different momenta for two possible spin states travelling in the same direction and, correspondingly, different cyclotron orbits in a weak magnetic field. Two point contacts, acting as a monochromatic source of ballistic holes and a narrow detector in the magnetic focusing geometry are demonstrated to work as a tunable spin filter.

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arXiv2004

Microwave absorption/reflection and magneto-transport experiments on high-mobility electron gas

We have performed simultaneous measurements of microwave absorption/reflection and magneto-transport characteristics of a high mobility two-dimensional electrons in GaAs/AlGaAs heterostructure in regime of Microwave-Induced Resistance Oscillations (MIROs). It is shown that the electrodynamic aspect of the problem is important in these experiments. In the absorption experiments a broad cyclotron resonance line was observed due to a large reflection from the highly conductive electron gas. There were no additional features observed related to absorption at harmonics of the cyclotron resonance. In near-field reflection experiments a very different oscillation pattern was revealed when compared to MIROs. The oscillation pattern observed in the reflection experiments is probably due to plasma effects occurring in a finite-size sample. The whole microscopic picture of MIROs is more complicated than simply a resonant absorption at harmonics of the cyclotron resonance. Nevertheless, the experimental observations are in good agreement with the model by Ryzhii et al. involving the photo-assisted scattering in the presence of a crossed magnetic field and dc bias. The observed damping factor of MIROs may be attributed to a change in the electron mobility as a function of temperature.

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arXiv2004

Transition from a fractional quantum Hall liquid to an electron solid at Landau level filling nu = 1/3 in tilted magnetic fields

We have observed in a low density two-dimensional hole system (2DHS) of extremely high quality (with hole density p=1.6x10^{10} cm^{-2} and mobility μ=0.8x10^6 cm^2/Vs) that, as the 2DHS is continuously tilted with respect to the direction of the magnetic field, the ν=1/3 fractional quantum Hall effect (FQHE) state is weakened and its magnetoresistivity rises from ~ 0.4 kohm/square in the normal orientation to ~ 180 kohm/square at tilt angle θ\~ 80 degrees. We attribute this phenomenon to the transition of the 2DHS from the FQHE liquid state to the pinned Wigner solid state, and argue that its origin is the strong coupling of subband Landau levels under the tilted magnetic fields.

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arXiv2004

Magnetic Field Induced Insulating Phases at Large $r_s$

Exploring a backgated low density two-dimensional hole sample in the large $r_s$ regime we found a surprisingly rich phase diagram. At the highest densities, beside the $ν=1/3$, 2/3, and 2/5 fractional quantum Hall states, we observe both of the previously reported high field insulating and reentrant insulating phases. As the density is lowered, the reentrant insulating phase initially strengthens, then it unexpectedly starts weakening until it completely dissapears. At the lowest densities the terminal quantum Hall state moves from $ν=1/3$ to $ν=1$. The intricate behavior of the insulating phases can be explained by a non-monotonic melting line in the $ν$-$r_s$ phase space.

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arXiv2004

AC Magnetotransport in Reentrant Insulating Phases of Two-dimensional Electrons near 1/5 and 1/3 Landau fillings

We have measured high frequency magnetotransport of a high quality two-dimensional electron system (2DES) near the reentrant insulating phase (RIP) at Landau fillings ($ν$) between 1/5 and 2/9. The magneto\textit{conductivity} in the RIP has resonant behavior around 150 MHz, showing a \textit{peak} at $ν$$\sim$0.21. Our data support the interpretation of the RIP as due to some pinned electron solid. We have also investigated a narrowly confined 2DES recently found to have a RIP at 1/3$<$$ν$$<$1/2 and we have revealed features, not seen in DC transport, that suggest some intriguing interplay between the 1/3 FQHE and RIP.

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arXiv2004

Evidence for Two Different Solid Phases of Two Dimensional Electrons in High Magnetic Fields

We have performed RF spectroscopy on very high quality two dimensional electron systems in the high magnetic field insulating phase, usually associated with a Wigner solid (WS) pinned by disorder. We have found two different resonances in the frequency dependent real diagonal conductivity spectrum and we interpret them as coming from \textit{two} different pinned solid phases (labeled as "WS-A" and "WS-B"). The resonance of WS-A is observable for Landau level filling $ν$$<$2/9 (but absent around the $ν$=1/5 fractional quantum Hall effect (FQHE)); it then \textit{crosses over} for $ν$$<$0.18 to the different WS-B resonance which dominates the spectrum at $ν$$<$0.125. Moreover, WS-A resonance is found to show dispersion with respect to the size of transmission line, indicating that WS-A has a large correlation length (exceeding $\sim$100 $μ$m); in contrast no such behavior is found for WS-B. We suggest that quantum correlations such as those responsible for FQHE may play an important role in giving rise to such different solids.

cond-mat.str-el↗

arXiv2004

Electron correlation in the second Landau level; a competition between many, nearly degenerate quantum phases

At a very low temperature of 9mK, electrons in the 2nd Landau level of an extremely high mobility two-dimensional electron system exhibit a very complex electronic behavior. With varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed $ν=2+2/5$ state separated from the even-denominator $ν=2+1/2$ state by an insulating phase and a $ν=2+2/7$ and $ν=2+1/5$ state surrounded by such phases. A developing plateau at $ν=2+3/8$ points to the existence of other even-denominator states.

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arXiv2004

2D Metal-Insulator transition as a percolation transition

By carefully analyzing the low temperature density dependence of 2D conductivity in undoped high mobility n-GaAs heterostructures, we conclude that the 2D metal-insulator transition in this system is a density inhomogeneity driven percolation transition due to the breakdown of screening in the random charged impurity disorder background. In particular, our measured conductivity exponent of $\sim 1.4$ approaches the 2D percolation exponent value of 4/3 at low temperatures and our experimental data are inconsistent with there being a zero-temperature quantum critical point in our system.

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arXiv2004

Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance

Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer phase coherence in bilayer two-dimensional electron systems at total Landau level filling factor $ν_T = 1$. We find that the phase boundary between the interlayer phase coherent state and the weakly-coupled compressible phase moves to larger layer separations as the electron density distribution in the bilayer is imbalanced. The critical layer separation increases quadratically with layer density difference.

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