arXiv subjects

Ken W. West

Publications and source records attributed to Ken W. West.

At least 19 recordsLinked to original sources

arXiv2026

Discrete states and ballistic interference in quantum wires approaching macroscopic lengths

Increasing the size of a system showing quantum effects is a difficult task limited by decoherence, a diminishing quantum level spacing, and the effects of disorder spoiling the quantum behavior when growing in size. Systems in 1D offer very strong confinement in the transverse directions, thus generally enhancing quantum effects, but are notoriously sensitive to disorder. In this work, we present a system of 1D electrons exhibiting discrete quantum levels and fully ballistic coherent quantum interference with lengths of up to 18\,$μ$m. Tunneling spectroscopy between two parallel quantum wires with a central gated segment shows intricate interference patterns exhibiting several different periods in magnetic field and density. An analysis over three different wire lengths and a comparison with single particle numerical simulations without any free parameters remarkably explains the full pattern including the observed periods. Therefore, these wires are essentially ideal 1D systems with aspect ratios approaching 1'000. In addition, at low bias, we also observe not only the Coulomb charging energies but can clearly resolve the discrete orbital and spin states in up to 10\,$μ$m long wires when filling 100 electrons with the center gate. This is made visible by a state-of-the-art low temperature and low noise measurement system. The spin filling sequence is completely regular, strictly alternating spin up and down, avoiding high spin states, while the peak conductance is modulated in accordance with the previously discussed interference patterns. These striking results show that single particle Schrödinger quantum mechanics such as ballistic quantum interference and discrete quantum states may be observed, under the right conditions, in systems of up to 18\,$μ$m length, thus approaching macroscopic sizes.

cond-mat.mes-hall↗

arXiv2026

Effective-Hamiltonian reconstruction through Bloch-wave interferometry in bulk GaAs driven by strong THz fields

Reconstructing effective Hamiltonians of condensed matter systems directly from experimental data is challenging because of the intricate relationship between Hamiltonian parameters and observables. Here, we reconstruct an effective three-band electron-hole (e-h) Hamiltonian in bulk GaAs based on high-order sideband generation (HSG) induced by quasi-continuous NIR and THz lasers. We perform polarimetry of high-order sidebands while varying the wavelength and polarization of the NIR laser, as well as the strength of the THz field. An analytic model is derived to incorporate the effects of both dephasing and quantum fluctuations around the semiclassical e-h recollision pathways. Surprisingly, the contribution of quantum fluctuations to the decay of sideband intensity with increasing sideband order is comparable to the contribution of dephasing. We simultaneously and unambiguously determine through Bloch-wave interferometry the effective Hamiltonian parameter that determines the e-h reduced masses, the bandgap of GaAs, and two dephasing constants associated with two e-h species. We demonstrate that full Hamiltonian reconstruction can be achieved by combining HSG measurements with absorbance spectroscopy. Unexpectedly, we find that the extracted bandgap of GaAs is about 10 meV larger than the value inferred from previous absorbance measurements. Quantum-kinetic analysis suggests that, in the HSG experiments, the e-h energy may be renormalized through Fröhlich interaction that is modulated by the strong THz fields. We also show that the energy threshold for optical-phonon emission can be suppressed by applying a strong THz field, leading to nearly constant dephasing rates.

cond-mat.mtrl-sci↗

arXiv2025

Total Faraday rotation by the Hall effect in a 2D electron gas

We report the realization of near total Faraday rotation of $θ_F$=1.43 rad (82 degrees) on a single pass through a 2D electron gas (2DEG), approaching the ideal limit of $π/2$ rad (90 degrees). The corresponding Verdet constant V = $9.5\times10^{8}$ rad T$^{-1}$m$^{-1}$, exceeds by approximately one order of magnitude that reported in other material systems. Our measurements were conducted at microwave frequencies (f=9.2-11.2 GHz) in a 2DEG with a high dc mobility $μ$ = $7\times10^6$ cm$^2$V$^{-1}$s$^{-1}$, in a hollow waveguide at low-magnetic field (B < 200 mT). Near-total Faraday rotation is attributed to the Hall effect with weak radiative coupling to the 2DEG in the inertial, collisionless regime, $ωτ\gg 1$, where $τ$ is the charge transport scattering time. A conducting iris was used to realize weak radiative coupling. Under these conditions, Faraday rotation is strongly enhanced away from the dissipation peak at cyclotron resonance. Our work demonstrates that the classical Hall effect could be ideally suited for the implementation of ideal non-reciprocal devices.

cond-mat.mes-hall↗

arXiv2025

Edge State Selective Measurement of Quantum Hall Dispersions

Edge states reflect the key physical properties yet are difficult to probe individually, particularly when several states are present at an edge. We present momentum resolved tunneling spectroscopy between a quantum well and a quantum wire to extract the dispersions of the quantum Hall edge states. Momentum and energy selective tunneling allows to separately address the different states even if they are spatially overlapping. This delivers the edge state velocities over broad ranges of magnetic field and density, in excellent agreement with a hard-wall model. This technique provides a basis for future edge state selective spectroscopy on quantum materials.

cond-mat.mes-hall↗

arXiv2025

Collective neutral excitations as sensitive probe for the quality of 2D charge carrier systems in ultra-pure GaAs quantum wells

Ultra-clean low-dimensional interacting charge carrier systems are the basis to explore correlated states and phases. We report the observation of very narrow collective intersubband excitations (ISBE) of 2D electron systems (2DESs) with ultra-high mobilities in high quality GaAs quantum well structures. These findings from resonant inelastic light scattering (RILS) experiments are used as tools for exploration of links between transport mobility and collective electron behavior in 2DES of high perfection. We find that the linewidths of collective ISBE modes can be very narrow with values smaller than 80μeV. Comparison of ISBE measurements from several high-mobility samples exhibits a variation in linewidth of more than a factor of two. There is, however, a surprising lack of direct correlation between ISBE linewidth with mobility in the range 15x10^6 cm^2/Vs<μ<24x10^6 cm^2/Vs. ISBE by RILS are discussed as a sensitive probe to characterize the interacting electron systems for fractional quantum Hall effect (FQHE) studies.

cond-mat.mes-hall↗

arXiv2023

Breaking a Bloch-wave interferometer: quasiparticle species-specific temperature-dependent nonequilibrium dephasing

Recently, high-order sideband polarimetry has been established as an experimental method that links the polarization of sidebands to an interference of Bloch wavefunctions. However, the robustness of sideband polarizations to increasing dephasing remains to be explored. Here, we investigate the dependence of high-order sideband generation in bulk gallium arsenide on dephasing by tuning temperature. We find that the intensities of the sidebands, but not their polarizations, depend strongly on temperature. Using our polarimetry method, we are able to isolate the contributions of electron-heavy hole (HH) and electron-light hole (LH) pairs to sideband intensities, and separately extract the nonequilibrium dephasing coefficients associated with the longitudinal optical (LO) phonons and acoustic (A) phonons for each species of electron-hole pair. We find that $Γ_{\text{HH},\text{A}} = 6.1 \pm 1.6$ $μ$eV/K, $Γ_{\text{LH},\text{A}} < 1.5$ $μ$eV/K, $Γ_{\text{HH},\text{LO}} = 14 \pm 3$ meV, and $Γ_{\text{LH},\text{LO}} = 30 \pm 3$ meV.

cond-mat.mes-hall↗

arXiv2022

Enhanced thermalization of exciton-polaritons in optically generated potentials

Equilibrium Bose-Einstein condensation of exciton-polaritons, demonstrated with a long-lifetime microcavity [Phys. Rev. Lett. 118, 016602 (2017)], has proven that driven-dissipative systems can undergo thermodynamic phase transitions in the limit where the quasiparticle lifetime exceeds the thermalization time. Here, we identify the role of dimensionality and polariton interactions in determining the degree of thermalization in optically generated traps. To distinguish the effect of trapping from interactions and lifetimes, we measured the polariton distribution under four nonresonant Gaussian pumps in a square geometry and compared it with polariton distributions measured with each pump individually. We found that significant redistribution of polaritons arises by trapping and modification of the density of states. Surprisingly efficient polariton-polariton scattering below the condensation threshold is evidenced by the depletion of the inflection-point polaritons. Our work provides a deeper understanding of polariton distributions and their interactions under various geometries of optically generated potentials.

cond-mat.quant-gas↗

arXiv2022

Domain Textures in the Fractional Quantum Hall Effect

Impacts of domain textures on low-lying neutral excitations in the bulk of fractional quantum Hall effect (FQHE) systems are probed by resonant inelastic light scattering. We demonstrate that large domains of quantum fluids support long-wavelength neutral collective excitations with well-defined wave vector (momentum) dispersion that could be interpreted by theories for uniform phases. Access to dispersive low-lying neutral collective modes in large domains of FQHE fluids such as long wavelength magnetorotons at filling factor v=1/3 offer significant experimental access to strong electron correlation physics in the FQHE.

cond-mat.mes-hall↗

arXiv2021

Observation of flat bands in gated semiconductor artificial graphene

Flat bands near M points in the Brillouin zone are key features of honeycomb symmetry in artificial graphene (AG) where electrons may condense into novel correlated phases. Here we report the observation of van Hove singularity doublet of AG in GaAs quantum well transistors, which presents the evidence of flat bands in semiconductor AG. Two emerging peaks in photoluminescence spectra tuned by backgate voltages probe the singularity doublet of AG flat bands, and demonstrate their accessibility to the Fermi level. As the Fermi level crosses the doublet, the spectra display dramatic stability against electron density, indicating interplays between electron-electron interactions and honeycomb symmetry. Our results provide a new flexible platform to explore intriguing flat band physics.

cond-mat.mes-hall↗

arXiv2020

Incipient Formation of the Reentrant Insulating Phase in a Dilute 2D Hole System with Strong Interactions

A new reentrant insulating phase (RIP) in low magnetic fields has been reported in the literature in strongly interacting 2D carrier systems and was suggested to be related to the formation of a Wigner crystal [e.g. Qiu et al, PRL 108, 106404 (2012)]. We have studied the transformation between the metallic liquid phase and the low field RIP in a dilute 2D hole system with large interaction parameter $r_s$ (~20-30) in GaAs quantum wells. Instead of a sharp transition, increasing density (or lowering $r_s$) drives the RIP into a state where an incipient RIP coexists with the metallic 2D hole liquid. The non-trivial temperature dependent resistivity and the in-plane magnetic field induced enhancement of the RIP highlight the competition between two phases and the essential role of spin in this mixture phase, and are consistent with the Pomeranchuk effect in a mixture of Wigner crystal and Fermi liquid.

cond-mat.str-el↗

arXiv2019

Observation of new plasmons in the fractional quantum Hall effect: interplay of topological and nematic orders

Collective modes of exotic quantum fluids reveal underlying physical mechanisms responsible for emergent complex quantum ground states. We observe unexpected new collective modes in the fractional quantum Hall (FQH) regime: intra-Landau-level plasmons in the second Landau level measured by resonant inelastic light scattering. The plasmons herald rotational-symmetry-breaking phases in tilted magnetic fields and reveal long-range translational invariance in these phases. The fascinating dependence of plasmon features on filling factor provide new insights on interplays between topological quantum Hall order and nematic electronic liquid crystal phases. A marked intensity minimum in the plasmon spectrum at Landau level filling factor v = 5/2 strongly suggests that this paired state, which could support non-Abelian excitations, overwhelms competing nematic phases, unveiling the robustness of the 5/2 superfluid state for small tilt angles. At v = 7/3, a sharp and strong plasmon peak that links to emerging macroscopic coherence supports the proposed model of a FQH nematic state at this filling factor.

cond-mat.mes-hall↗

arXiv2018

Negative longitudinal magnetoresistance in GaAs quantum wells

Negative longitudinal magnetoresistances (NLMRs) have been recently observed in a variety of topological materials and often considered to be associated with Weyl fermions that have a defined chirality. Here we report NLMRs in non-Weyl GaAs quantum wells. In the absence of a magnetic field the quantum wells show a transition from semiconducting-like to metallic behaviour with decreasing temperature. We observed pronounced NLMRs up to 9 Tesla at temperatures above the transition and weak NLMRs in low magnetic fields at temperatures close to the transition and below 5 K. The observed NLMRs show various types of magnetic field behaviour resembling those reported in topological materials. We attribute them to microscopic disorder and use a phenomenological three-resistor model to account for their various features. Our results showcase a new contribution of microscopic disorder in the occurrence of novel phenomena. They may stimulate further work on tuning electronic properties via disorder/defect nano-engineering.

cond-mat.mes-hall↗

arXiv2018

Evolution of the quantum Hall bulk spectrum into chiral edge states

One of the most intriguing and fundamental properties of topological materials is the correspondence between the conducting edge states and the gapped bulk spectrum. So far, it has been impossible to access the full evolution of edge states with critical parameters such as magnetic field due to poor resolution, remnant bulk conductivity, or disorder. Here, we use a GaAs cleaved edge quantum wire to perform momentum-resolved tunneling spectroscopy. This allows us to probe the evolution of the chiral quantum Hall edge states and their positions from the sample edge with unprecedented precision from very low magnetic fields all the way to high fields where depopulation occurs. We present consistent analytical and numerical models, inferring the edge states from the well known bulk spectrum, finding excellent agreement with the experiment -- thus providing direct evidence for the bulk to edge correspondence. In addition, we observe various features beyond the single-particle picture, such as Fermi level pinning, exchange-enhanced spin splitting and signatures of edge-state reconstruction.

cond-mat.mes-hall↗

arXiv2016

A non-invasive method for nanoscale electrostatic gating of pristine materials

Electrostatic gating is essential for defining and control of semiconducting devices. However, nano-fabrication processes required for depositing gates inevitably degrade the pristine quality of the material of interest. Examples of materials that suffer from such degradation include ultra-high mobility GaAs/AlGaAs two-dimensional electron gases (2DEGs), graphene, topological insulators, and nanowires. To preserve the pristine material properties, we have developed a flip-chip setup where gates are separated from the material by a vacuum, which allows nanoscale electrostatic gating of the material without exposing it to invasive nano-processing. An additional benefit is the vacuum between gates and material, which, unlike gate dielectrics, is free from charge traps. We demonstrate the operation and feasibility of the flip-chip setup by achieving quantum interference at integer quantum Hall states in a Fabry-Pérot interferometer based on a GaAs/AlGaAs 2DEG. Our results pave the way for the study of exotic phenomena including fragile fractional quantum Hall states by preserving the high quality of the material.

cond-mat.mes-hall↗

arXiv2015

Nanoscale Mach-Zehnder interferometer with spin-resolved quantum Hall edge states

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.

cond-mat.mes-hall↗

arXiv2015

Induced superconductivity in high mobility two dimensional electron gas in GaAs heterostructures

Introduction of a Josephson field effect transistor (JoFET) concept sparked active research on proximity effects in semiconductors. Induced superconductivity and electrostatic control of critical current has been demonstrated in two-dimensional gases in InAs, graphene and topological insulators, and in one-dimensional systems including quantum spin Hall edges. Recently, interest in superconductor-semiconductor interfaces was renewed by the search for Majorana fermions, which were predicted to reside at the interface. More exotic non-Abelian excitations, such as parafermions (fractional Majorana fermions) or Fibonacci fermions may be formed when fractional quantum Hall edge states interface with superconductivity. In this paper we develop transparent superconducting contacts to high mobility two-dimensional electron gas (2DEG) in GaAs and demonstrate induced superconductivity across several microns. Supercurrent in a ballistic junction has been observed across 0.6 $μ$m of 2DEG, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields ($>16$ Tesla) in NbN contacts enables investigation of a long-sought regime of an interplay between superconductivity and strongly correlated states in a 2DEG at high magnetic fields.

cond-mat.mes-hall↗

arXiv2012

Evidence of inter-layer interaction in magneto-luminescence spectra of electron bilayers

Magneto-luminescence studies in electron bilayers reveal the hallmarks of the even-denominator and other quantum Hall states in the intensities and energies of the inter-band optical recombination lines. In the presence of a small tunneling gap between the layers the magneto-optical emission from the lowest anti-symmetric subband, not populated in a single-electron picture, displays maxima at filling factors 1 and 2/3. These findings uncover a loss of pseudospin polarization, where the pseudospin describes the layer index degree of freedom, that is linked to an anomalous population of the anti-symmetric level due to excitonic correlations. The results demonstrate a new realm to probe the impact of inter-layer Coulomb interaction in quantum Hall bilayers.

cond-mat.mes-hall↗

arXiv2012

Evidence for a fractional quantum Hall state with anisotropic longitudinal transport

At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau level (LL), a clean two-dimensional electron system (2DES) exhibits numerous incompressible liquid phases which display the fractional quantized Hall effect (FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break rotational symmetry, exhibiting resistivities which are isotropic in the plane. In contrast, at lower fields, when the Fermi level lies in the $N\ge2$ third and several higher LLs, the 2DES displays a distinctly different class of collective states. In particular, near half filling of these high LLs the 2DES exhibits a strongly anisotropic longitudinal resistance at low temperatures (Lilly et al., 1999; Du et al., 1999). These "stripe" phases, which do not exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing broken rotational symmetry and orientational order (Koulakov et al., 1996; Fogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999; Fradkin et al, 2010). Here we report a surprising new observation: An electronic configuration in the N=1 second LL whose resistivity tensor simultaneously displays a robust fractionally quantized Hall plateau and a strongly anisotropic longitudinal resistance resembling that of the stripe phases.

cond-mat.str-el↗