arXiv subjects

W. Pan

Publications and source records attributed to W. Pan.

At least 37 records · Page 2Linked to original sources

arXiv2016

Quantum Oscillations at Integer and Fractional Landau Level Indices in ZrTe5

A three-dimensional (3D) Dirac semimetal (DS) is an analogue of graphene, but with linear energy dispersion in all (three) momentum directions.3D DSs have been a fertile playground in discovering novel quantum particles, for example Weyl fermions, in solid state systems.Many 3D DSs (e.g., ZrTe5) were theoretically predicted. We report here the results from the studies of aberration-corrected scanning transmission electron microscopy and low temperature magneto-transport measurements in exfoliated ZrTe5 thin flakes.Several unique results were observed. First, an anomalous-Hall-effect-like behavior was observed around zero magnetic field (B).Second, a non-trivial Berry's phase of π was obtained from the Landau level fan diagram of the Shubnikov-de Haas oscillations in the longitudinal resistivity. Third, the longitudinal resistivity shows linear B field dependence in the quantum limit. Most surprisingly, quantum oscillations were observed at fractional Landau level indices N = 2/3 and 2/5, demonstrating strong electron-electron interactions effects in ZrTe5.

cond-mat.mtrl-sci↗

arXiv2015

Impact of the modulation doping layer on the ν=5/2 anisotropy

We have carried out a systematic study of the tilted magnetic field induced anisotropy at the Landau level filling factor ν=5/2 in a series of high quality GaAs quantum wells, where the setback distance (d) between the modulation doping layer and the GaAs quantum well is varied from 33 to 164 nm. We have observed that in the sample of the smallest d electronic transport is anisotropic when the in-plane magnetic field (B_{ip}) is parallel to the [1-10] crystallographic direction, but remains more or less isotropic when B_{ip} // [110]. In contrast, in the sample of largest d, electronic transport is anisotropic in both crystallographic directions. Our results clearly show that the modulation doping layer plays an important role in the tilted field induced ν=5/2 anisotropy.

cond-mat.mes-hall↗

arXiv2015

Fractional Quantum Hall Effect at Landau Level Filling nu=4/11

We report low temperature electronic transport results on the fractional quantum Hall effect of composite fermions at Landau level filling nu = 4/11 in a very high mobility and low density sample. Measurements were carried out at temperatures down to 15mK, where an activated magnetoresistance Rxx and a quantized Hall resistance Rxy, within 1% of the expected value of h/(4/11)e^2, were observed. The temperature dependence of the Rxx minimum at 4/11 yields an activation energy gap of ~ 7 mK. Developing Hall plateaus were also observed at the neighboring states at nu = 3/8 and 5/13.

cond-mat.mes-hall↗

arXiv2014

Terahertz magneto-optical spectroscopy of two-dimensional hole and electron systems

We have used terahertz (THz) magneto-optical spectroscopy to investigate the cyclotron resonance in high mobility two-dimensional electron and hole systems. Our experiments reveal long-lived (~20 ps) coherent oscillations in the measured signal in the presence of a perpendicular magnetic field. The cyclotron frequency extracted from the oscillations varies linearly with magnetic field for a two-dimensional electron gas (2DEG), as expected. However, we find that the complex non-parabolic valence band structure in a two-dimensional hole gas (2DHG) causes the cyclotron frequency and effective mass to vary nonlinearly with the magnetic field, as verified by multiband Landau level calculations. This is the first time that THz magneto-optical spectroscopy has been used to study 2DHG, and we expect that these results will motivate further studies of these unique 2D nanosystems.

physics.optics↗

arXiv2014

Giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction

Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors [1-4]. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T-H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.

cond-mat.supr-con↗

arXiv2014

Superconducting properties in tantalum decorated three-dimensional graphene and carbon structures

We present here the results on superconducting properties in tantalum thin films (100nm thick) deposited on three-dimensional graphene (3DG) and carbon structures. A superconducting transition is observed in both composite thin films with a superconducting transition temperature of 1.2K and 1.0K, respectively. We have further measured the magnetoresistance at various temperatures and differential resistance dV/dI at different magnetic fields in these two composite thin films. In both samples, a much large critical magnetic field (~ 2 Tesla) is observed and this critical magnetic field shows linear temperature dependence. Finally, an anomalously large cooling effect was observed in the differential resistance measurements in our 3DG-tantalum device when the sample turns superconducting. Our results may have important implications in flexible superconducting electronic device applications.

cond-mat.supr-con↗

arXiv2014

Two-dimensional electron gas in monolayer InN quantum wells

We report in this letter experimental results that confirm the two-dimensional nature of the electron systems in monolayer InN quantum wells embedded in GaN barriers. The electron density and mobility of the two-dimensional electron system (2DES) in these InN quantum wells are 5x10^{15} cm^{-2} and 420 cm^2/Vs, respectively. Moreover, the diagonal resistance of the 2DES shows virtually no temperature dependence in a wide temperature range, indicating the topological nature of the 2DES.

cond-mat.mes-hall↗

arXiv2014

Competing quantum Hall phases in the second Landau level in low density limit

We present in this Letter the results from two high quality, low density GaAs quantum wells. In sample A of electron density n=5.0x10^10 cm^-2, anisotropic electronic transport behavior was observed at ν=7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1x10^10 cm^-2, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second landau level.

cond-mat.mes-hall↗

arXiv2013

Magnetoplasmons in quasi-neutral epitaxial graphene nanoribbons

We present infrared transmission spectroscopy study of the inter-Landau-level excitations in quasi-neutral epitaxial graphene nanoribbon arrays. We observed a substantial deviation in energy of the $L_{0(-1)}$$\to$$L_{1(0)}$ transition from the characteristic square root magnetic-field dependence of two-dimensional graphene. This deviation arises from the formation of upper-hybrid mode between the Landau level transition and the plasmon resonance. In the quantum regime the hybrid mode exhibits a distinct dispersion relation, markedly different from that expected for conventional two-dimensional systems and highly doped graphene.

cond-mat.mes-hall↗

arXiv2012

Chaotic Quantum Transport near the Charge Neutrality Point in Inverted Type-II InAs/GaSb Field-Effect Transistors

We present here our recent quantum transport results around the charge neutrality point (CNP) in a type-II InAs/GaSb field-effect transistor. At zero magnetic field, a conductance minimum close to 4e^2/h develops at the CNP and it follows semi-logarithmic temperature dependence. In quantized magnetic (B) fields and at low temperatures, well developed integer quantum Hall states are observed in the electron as well as hole regimes. Quantum transport shows noisy-like behavior around the CNP at extremely high B fields. Surprisingly, when the diagonal conductivity σ_{xx} is plotted against the Hall conductivity σ_{xy}, a circular conductivity law is discovered, suggesting a chaotic quantum transport behavior.

cond-mat.mes-hall↗

arXiv2012

Spin Transition in the ν=8/3 Fractional Quantum Hall Effect

We present here the results from a density dependent study of the activation energy gaps of the fractional quantum Hall effect states at Landau level fillings ν=8/3 and 7/3 in a series of high quality quantum wells. In the density range from 0.5 x 10^{11} to 3 x 10^{11} cm^{-2}, the 7/3 energy gap increases monotonically with increasing density, supporting its ground state being spin polarized. For the 8/3 state, however, its energy gap first decreases with increasing density, almost vanishes at n ~ 0.8 x 10^{11} cm^{-2}, and then turns around and increases with increasing density, clearly demonstrating a spin transition.

cond-mat.mes-hall↗

arXiv2012

Spin Polarization of the 12/5 Fractional Quantum Hall Effect

We have carried out tilt magnetic field (B) studies of the ν=12/5 fractional quantum Hall state in an ultra-high quality GaAs quantum well specimen. Its diagonal magneto-resistance Rxx shows a non-monotonic dependence on tilt angle (θ). It first increases sharply with increasing θ, reaches a maximal value of ~ 70 ohms at θ~ 14^o, and then decreases at higher tilt angles. Correlated with this dependence of Rxx on θ, the 12/5 activation energy (Δ_{12/5}) also shows a non-monotonic tilt dependence. Δ_{12/5} first decreases with increasing θ. Around θ= 14^{o}, Δ_{12/5} disappears as Rxx becomes non-activated. With further increasing tilt angles, Δ_{12/5} reemerges and increases with θ. This tilt B dependence at ν=12/5 is strikingly different from that of the well-documented 5/2 state and calls for more investigations on the nature of its ground state.

cond-mat.mes-hall↗

arXiv2011

Impact of disorder on the 5/2 fractional quantum Hall state

We compare the energy gap of the ν=5/2 fractional quantum Hall effect state obtained in conventional high mobility modulation doped quantum well samples with those obtained in high quality GaAs transistors (heterojunction insulated gate field-effect transistors). We are able to identify the different roles that long range and short range disorders play in the 5/2 state and observe that the long range potential fluctuations are more detrimental to the strength of the 5/2 state than short-range potential disorder.

cond-mat.mes-hall↗

arXiv2011

Terahertz Coherent Control of a Landau-Quantized Two-Dimensional Electron Gas

We demonstrate coherent control of cyclotron resonance (CR) in a two-dimensional electron gas (2DEG). We use a sequence of terahertz pulses to control the amplitude of CR oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the terahertz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using {\em single-particle} optical Bloch equations, showing no signatures of electron-electron interactions, which verifies the validity of Kohn's theorem for CR in the coherent regime.

cond-mat.mes-hall↗

arXiv2011

Termination of two-dimensional metallic conduction near metal-insulator transition in a Si/SiGe quantum well

We report in this Letter our recent low temperature transport results in a Si/SiGe quantum well with moderate peak mobility. An apparent metal-insulating transition is observed. Within a small range of densities near the transition, the conductivity $σ$ displays non-monotonic temperature dependence. After an initial decrease at high temperatures, $σ$ first increases with decreasing temperature T, showing a metallic behavior. As T continues decreasing, a downturn in $σ$ is observed. This downturn shifts to a lower T at higher densities. More interestingly, the downturn temperature shows a power law dependence on the mobility at the downturn position, suggesting that a similar downturn is also expected to occur deep in the apparent metallic regime at albeit experimentally inaccessible temperatures. This thus hints that the observed metallic phase in 2D systems might be a finite temperature effect.

cond-mat.mes-hall↗

arXiv2009

Scaling in Plateau-to-Plateau Transition: A Direct Connection of Quantum Hall Systems with Anderson Localization Model

The quantum Hall plateau transition was studied at temperatures down to 1 mK in a random alloy disordered high mobility two-dimensional electron gas. A perfect power-law scaling with κ=0.42 was observed from 1.2K down to 12mK. This perfect scaling terminates sharply at a saturation temperature of T_s~10mK. The saturation is identified as a finite-size effect when the quantum phase coherence length (L_ϕ ~ T^{-p/2}) reaches the sample size (W) of millimeter scale. From a size dependent study, T_s \propto W^{-1} was observed and p=2 was obtained. The exponent of the localization length, determined directly from the measured κand p, is ν=2.38, and the dynamic critical exponent z = 1.

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

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↗