arXiv subjects

K. W. Baldwin

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

At least 19 recordsLinked to original sources

arXiv2026

Successive Phase Transitions from a Composite Fermion Liquid to a Fractional Quantum Hall State at ν=3/2 Driven by In-Plane Magnetic Field

We report an even-denominator fractional quantum Hall state at ν = 3/2 induced entirely by in-plane magnetic field B_|| in an ultra-high-mobility GaAs quantum well. As B_|| increases, the system undergoes two successive transitions: from a composite fermion liquid to a soft-gap FQH state (B_||~12.2 T), then via a topological phase transition to a hard-gap robust FQH state (B_||~14.7 T), accompanied by a daughter state at ν = 19/13. We present systematic data, and discuss a possible scenario in interpreting these findings. Our work demonstrates that topological order may be engineered through k-space Fermi contour splitting under an in-plane magnetic field.

cond-mat.str-el↗

arXiv2026

Radiowave-induced Resistance Oscillations

Microwave-induced resistance oscillations (MIROs) occur when a 2D electron gas is subjected to radiation of frequency $ω= 2 πf$ and varying magnetic field $B$. MIROs are periodic in $1/B$, with the period determined by the radiation frequency $ω$, and their amplitude scales with the radiation power. Stepping from single-photon transitions between Landau levels, MIROs are found on the low-field side of the cyclotron resonance, $ω_c \lesssim ω$, where $ω_c$ is the cyclotron frequency. Here, we report on another class of magneto resistance oscillations, which are induced by high-intensity radiation in the radio frequency range and occur at $ω_c \gg ω$. These oscillations are independent of frequency $ω$, can be either $1/B$ or $1/B^2$-periodic, and their period is controlled by the radiation electric field. We further show that using a displacement model in the limit of short-range (``sharp'') disorder we can extract the radiation field and the width of the cyclotron resonance.

cond-mat.mes-hall↗

arXiv2026

Engineering Quantum Criticality in the Integer Quantum Hall Regime through a Screening Layer

Disorder-induced localization of electrons and electron-electron interaction are among the most fundamental problems in condensed matter physics. In two-dimensional electron systems, extensive studies have led to the emergence of a scaling picture, characterized by a set of universal critical exponents that govern the transitions between the integer quantum Hall plateaus. From the temperature dependence of the plateau-to-plateau transitions, experiments primarily report k ~ 0.42, implying a dynamic exponent z = 1, consistent with a theoretical picture where electrons have a long-range (1/r) interaction. Theory also predicts that z = 2 for short-range electron interaction, but an experimental verification has remained elusive. Here, we directly probe the influence of Coulomb interaction on these transitions using a bilayer electron system confined to a GaAs double quantum well device. The two layers are in close proximity, with an interlayer distance approximately equal to the magnetic length at the relevant magnetic fields. By tuning the electron density in the top layer, we access both insulating and metallic phases of the electrons in this layer as a function of magnetic field, allowing in-situ control of the unscreened and screened interaction strengths in the bottom layer as it goes through its plateau-to-plateau transitions. In the unscreened case, we measure k ~ 0.42 consistent with the widely reported value. More importantly, when screening is introduced, k is reduced to ~ 0.22, implying z = 2. Our results provide direct experimental evidence for the role of electron-electron interaction in determining critical behavior in the quantum Hall regime, and demonstrate screening as a powerful tuning parameter for engineering quantum criticality.

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arXiv2026

Demonstration of a Field-Effect Three-Terminal Electronic Device with an Electron Mobility Exceeding 40 Million cm^2/(Vs)

We report the fabrication and operation of a source-drain-gate three-terminal field-effect electronic device with an electron mobility exceeding $40\times 10^6$ cm$^2$ / (Vs). Several devices were fabricated, with the highest achieved electron mobility obtained using a symmetrically-doped GaAs/AlGaAs quantum well forming a two-dimensional electron gas (2DEG) with a density of $1.47(1) \times 10^{11}$ cm$^{-2}$ and a pristine, pre-fabrication electron mobility of $44(2) \times 10^6$ cm$^2$/(\text{Vs}). To circumvent the well-known degradation of electron mobility during fabrication, devices were fabricated using a flip-chip technique where all lithographic processing steps were performed on a separate sapphire substrate. This method demonstrates the successful operation of various gate assembly designs on distinct 2DEGs without observable mobility degradation. This advance doubles the previous record for field-effect electronic device mobility and enables access to new regimes of quantum transport and applications that were previously unfathomable due to mobility limitations.

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arXiv2025

High-order two-component fractional quantum Hall states around filling factor $ν= 1$

Two-component fractional quantum Hall (2C-FQH) states in electron bilayers have been known for decades, yet their experimental realization remained limited to low-order fractions. Here we report on several families of high-order 2C-FQH states that emerge when an in-plane magnetic field drives a controlled monolayer-to-bilayer transition in an ultra-high-mobility GaAs quantum well. These families of states proliferate symmetrically toward the filling factor $ν= 1$, from both $ν= 2/3$ and $ν= 4/3$, thereby respecting particle-hole symmetry. Surprisingly, many unbalanced states (with unequal layer fillings) are more robust than their parent balanced states, defying the expected hierarchy of Jain sequences. Our findings substantially expand the known landscape of 2C-FQH states, highlighting the unexpected richness of the bilayer quantum Hall regime and opening new routes for probing the interplay of symmetry, topology, and interactions in quantum Hall systems.

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arXiv2025

Even-denominator fractional quantum Hall states with spontaneously broken rotational symmetry

The interplay between the fractional quantum Hall effect and nematicity is intriguing as it links emerging topological order and spontaneous symmetry breaking. Anisotropic fractional quantum Hall states (FQHSs) have indeed been reported in GaAs quantum wells but only in tilted magnetic fields, where the in-plane field explicitly breaks the rotational symmetry. Here we report the observation of FQHSs with highly anisotropic longitudinal resistances in purely perpendicular magnetic fields at even-denominator Landau level (LL) fillings ν = 5/2 and 7/2 in ultrahigh-quality GaAs two-dimensional hole systems. The coexistence of FQHSs and spontaneous symmetry breaking at half fillings signals the emergence of nematic FQHSs which also likely harbor non-Abelian quasiparticle excitations. By gate tuning the hole density, we observe a phase transition from an anisotropic, developing FQHS to an isotropic composite fermion Fermi sea at ν = 7/2. Our calculations suggest that the mixed orbital components in the partially occupied LL play a key role in the competition and interplay between topological and nematic orders.

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arXiv2025

Mott transition in excitonic Bose polarons

For a neutral system of positive and negative charges, such as atoms in a crystal, increasing the density causes the Mott transition from bound electrons to free electrons. The density of optically generated electron-hole systems can be controlled in situ by the power of optical excitation that enables the Mott transition from excitons, the bound pairs of electrons and holes, to free electrons and holes with increasing density. These Mott transitions occur in systems of pairs of the same kind, such as atoms or excitons. However, a different type of the Mott transition can occur for Bose polarons. A Bose polaron is a mobile particle of one kind in a Bose gas of particles of another kind. For the Mott transition in polarons, the polaron states vanish with increasing density of the surrounding gas. In this paper, we present the observation of this type of the Mott transition and the measurement of the Mott transition parameter $n_{\rm M}^{1/2} a_{\rm B}$ in 2D excitonic Bose polarons.

cond-mat.quant-gas↗

arXiv2025

Competing Many-Body Phases at Small Fillings in Ultrahigh-Quality GaAs 2D Hole Systems: Role of Landau Level Mixing

The fractional quantum Hall state (FQHS), an incompressible liquid state hosting anyonic excitations with fractional charge and statistics, represents a compelling many-body phase observed in clean two-dimensional (2D) carrier systems. The expected non-Abelian nature of the FQHSs at even-denominator Landau level (LL) fillings has particularly sparked considerable recent interest. At sufficiently small fillings, another exotic phase, namely a quantum Wigner crystal (WC) state, dominates. Here we report magneto-transport measurements in an ultrahigh-quality GaAs 2D \textit{hole} system where the large hole effective mass leads to a significant LL mixing (LLM) even at very high magnetic fields and affects the many-body states at very small fillings. We observe numerous developing FQHSs at both even- and odd-denominator fillings, deep in the insulating regime at $ν\lesssim$ 1/3 where WC states dominate. The FQHSs we observe at odd-denominator fillings on the flanks of $ν=$ 1/4 and 1/6 are consistent with the Abelian Jain sequence of four-flux and six-flux composite fermions, while the ones at even-denominator fillings $ν=$ 1/4 and 1/6 are likely non-Abelian states emerging from the pairing of these quasiparticles induced by severe LLM. Our results demonstrate that the competition between the FQHSs and WC phases is close at very small fillings even in the presence of severe LLM. We also measure activation energies of WC states near $ν=$ 1/6, and find that they are substantially larger than what has been reported for ultrahigh-quality GaAs 2D electrons. A moderate LLM is believed to lower the activation energy associated to the formation of WC intrinsic defects. The surprisingly large activation energy for our 2DHS with significant LLM is therefore puzzling, and may suggest a different type of intrinsic WC defect compared to that in 2D electrons.

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arXiv2025

Developing fractional quantum Hall states at even-denominator fillings 1/6 and 1/8

In the extreme quantum limit, when the Landau level filling factor $ν<1$, the dominant electron-electron interaction in low-disorder two-dimensional electron systems leads to exotic many-body phases. The ground states at even-denominator $ν=$ 1/2 and 1/4 are typically Fermi seas of composite fermions carrying two and four flux quanta, surrounded by the Jain fractional quantum Hall states (FQHSs) at odd-denominator fillings $ν=p/(2p\pm1)$ and $ν=p/(4p\pm1)$, where $p$ is an integer. For $ν<$ 1/5, an insulating behavior, which is generally believed to signal the formation of a pinned Wigner crystal, is seen. Our experiments on ultrahigh-quality, dilute, GaAs two-dimensional electron systems reveal developing FQHSs at $ν=p/(6p\pm1)$ and $ν=p/(8p\pm1)$, manifested by magnetoresistance minima superimposed on the insulating background. In stark contrast to $ν=$ 1/2 and 1/4, however, we observe a pronounced, sharp minimum in magnetoresistance at $ν=$ 1/6 and a somewhat weaker minimum at $ν=$ 1/8, suggesting developing FQHSs, likely stabilized by the pairing of composite fermions that carry six and eight flux quanta. Our results signal the unexpected entry, in ultrahigh-quality samples, of FQHSs at even-denominator fillings 1/6 and 1/8, which are likely to harbor non-Abelian anyon excitations.

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arXiv2024

Developing fractional quantum Hall states at $ν$ = $\dfrac{1}{7}$ and $\dfrac{2}{11}$ in the presence of significant Landau level mixing

Termination of the fractional quantum Hall states (FQHSs) and the emergence of Wigner crystal phases at very small Landau level filling factors ($ν$) have been of continued interest for decades. Recently, in ultra-high-quality, dilute GaAs 2D electron systems (2DESs), strong evidence was reported for FQHSs at $ν=1/7, 2/13$ and 2/11 which fall in the $ν= p/(6p\pm1)$ Jain series of FQHSs, interpreted as integer ($p = 1$, 2) QHSs of 6-flux composite fermions ($^6$CFs). These states are surrounded by strongly-insulating phases which are generally believed to be Wigner crystals. Here, we study an ultra-high-quality 2DES confined to an AlAs quantum well where the 2D electrons have a much larger effective mass ($m^*\simeq 0.45 m_e$) and a smaller dielectric constant ($ε\simeq10ε_0$) compared to GaAs 2D electrons ($m^*\simeq 0.067 m_e$ and $ε\simeq13ε_0$). This combination of $m^*$ and $ε$ renders the Landau level mixing parameter $κ$, defined as the ratio of the Coulomb and cyclotron energies, $\simeq 9$ times larger in AlAs 2DESs ($κ\propto m^*/ε$). Qualitatively similar to the GaAs 2DESs, we observe an insulating behavior reentrant around a strong $ν=1/5$ FQHS, and extending to $ν<1/5$. Additionally, we observe a clear minimum in magnetoresistance at $ν=2/11$, and an inflection point at $ν=1/7$ which is very reminiscent of the first report of an emerging FQHS at $ν=1/7$ in GaAs 2DESs. The data provide evidence for developing QHSs of $^6$CFs at very small fillings. This is very surprising because $κ$ near $ν\simeq 1/6$ in our sample is very large ($\simeq4$), and larger $κ$ has the tendency to favor Wigner crystal states over FQHSs at small fillings. Our data should inspire calculations that accurately incorporate $κ$ in competing many-body phases of $^6$CFs at extremely small fillings near $ν=1/6$.

cond-mat.str-el↗

arXiv2024

Excitonic Bose-polarons in electron-hole bilayers

Bose polarons are mobile impurities dressed by density fluctuations of a surrounding degenerate Bose gas. These many-body objects have been realized in ultracold atomic gasses and become a subject of intensive studies. In this work, we show that excitons in electron-hole bilayers offer new opportunities for exploring polarons in strongly interacting, highly tunable bosonic systems. We found that Bose polarons are formed by spatially direct excitons immersed in degenerate Bose gases of spatially indirect excitons (IXs). We detected both attractive and repulsive Bose polarons by measuring photoluminescence excitation spectra. We controlled the density of IX Bose gas by optical excitation and observed an enhancement of the energy splitting between attractive and repulsive Bose polarons with increasing IX density, in agreement with our theoretical calculations.

cond-mat.quant-gas↗

arXiv2024

Origin of pinning disorder in magnetic-field-induced Wigner solids

At low Landau level filling factors ($ν$), Wigner solid phases of two-dimensional electron systems in GaAs are pinned by disorder, and exhibit a pinning mode, whose frequency is a measure of the disorder that pins the Wigner solid. Despite numerous studies spanning the last three decades, the origin of the disorder that causes the pinning and determines the pinning mode frequency remains unknown. Here we present a study of the pinning mode resonance in the low-$ν$ Wigner solid phases of a series of ultralow-disorder GaAs quantum wells which are similar except for their varying well widths, $d$. The pinning mode frequencies,$f_p$, decrease strongly as $d$ increases, with the widest well exhibiting $f_p$ as low as $\simeq$35 MHz. The amount of reduction of \fp\ with increasing $d$ can be explained remarkably well by tails of the wave function impinging into the alloy-disordered Al$_x$Ga$_{1-x}$As barriers that contain the electrons. However, it is imperative that the model for the confinement and wave function includes the Coulomb repulsion in the growth direction between the electrons as they occupy the quantum well.

cond-mat.mes-hall↗

arXiv2024

Aharonov-Borm oscillation and Microwave-induced edge-magnetoplasmon modes enhanced by quantum point contact

AB oscillation in weak magnetic field (B<1.5kG) is observed in QPC due to interference between electrons propagating along different QPC channels. We also investigate photo-induced magnetoresistance oscillation in open-regime split-gate QPC under MW irradiation. It is attributed to EMPs interfering in the QPC region. The influence of MW power, frequency and split gate voltage is discussed thoroughly. We unify the result of photoconductance at B=0 with EMP theories.

cond-mat.mes-hall↗

arXiv2024

Signatures of correlated defects in an ultra-clean Wigner crystal in the extreme quantum limit

Low-disorder two-dimensional electron systems in the presence of a strong, perpendicular magnetic field terminate at very small Landau level filling factors in a Wigner crystal (WC), where the electrons form an ordered array to minimize the Coulomb repulsion. The nature of this exotic, many-body, quantum phase is yet to be fully understood and experimentally revealed. Here we probe one of WC's most fundamental parameters, namely the energy gap that determines its low-temperature conductivity, in record-mobility, ultra-high-purity, two-dimensional electrons confined to GaAs quantum wells. The WC domains in these samples contain $\simeq$ 1000 electrons. The measured gaps are a factor of three larger than previously reported for lower quality samples, and agree remarkably well with values predicted for the lowest-energy, intrinsic, hyper-corelated bubble defects in a WC made of flux-electron composite fermions, rather than bare electrons. The agreement is particularly noteworthy, given that the calculations are done for disorder-free composite fermion WCs, and there are no adjustable parameters. The results reflect the exceptionally high quality of the samples, and suggest that composite fermion WCs are indeed more stable compared to their electron counterparts.

cond-mat.mes-hall↗

arXiv2024

Superballistic flow of viscous electron fluid induced by microwave irradiation in quantum point contact

We measure the resistance oscillation of quantum point contact (QPC) under microwave (MW) radiation. What is different from the common resistance oscillation induced by edge magnetoplasmon (EMP) is that at lower magnetic field (ω>ω_c), photoconductance is positive (negative) with weak (strong) MW radiation. This transport phenomenon is proved to be related to superballistic flow of electrons through QPC. The distinction between the regions ω>ω_c and ω<ω_c is attributed to different absorption rate of MW radiation. Violent absorption occurs when cyclotron orbits or current domains are destroyed in QPC region.

cond-mat.mes-hall↗

arXiv2024

Evidence for Topological Protection Derived from Six-Flux Composite Fermions

The composite fermion theory opened a new chapter in understanding many-body correlations through the formation of emergent particles. The formation of two-flux and four-flux composite fermions is well established. While there are limited data linked to the formation of six-flux composite fermions, topological protection associated with them is conspicuously lacking. Here we report evidence for the formation of a quantized and gapped fractional quantum Hall state at the filling factor $ν=9/11$, which we associate with the formation of six-flux composite fermions. Our result provides evidence for the most intricate composite fermion with six fluxes and expands the already diverse family of highly correlated topological phases with a new member that cannot be characterized by correlations present in other known members. Our observations pave the way towards the study of higher order correlations in the fractional quantum Hall regime.

cond-mat.mes-hall↗

arXiv2024

Moving crystal phases of a quantum Wigner solid in an ultra-high-quality 2D electron system

In low-disorder, two-dimensional electron systems (2DESs), the fractional quantum Hall states at very small Landau level fillings ($ν$) terminate in a Wigner solid (WS) phase, where electrons arrange themselves in a periodic array. The WS is typically pinned by the residual disorder sites and manifests an insulating behavior, with non-linear current-voltage (\textit{I-V}) and noise characteristics. We report here, measurements on an ultra-low-disorder, dilute 2DES, confined to a GaAs quantum well. In the $ν< 1/5$ range, superimposed on a highly-insulating longitudinal resistance, the 2DES exhibits a developing fractional quantum Hall state at $ν=1/7$, attesting to its exceptional high quality, and dominance of electron-electron interaction in the low filling regime. In the nearby insulating phases, we observe remarkable non-linear \textit{I-V} and noise characteristics as a function of increasing current, with current thresholds delineating three distinct phases of the WS: a pinned phase (P1) with very small noise, a second phase (P2) in which $dV/dI$ fluctuates between positive and negative values and is accompanied by very high noise, and a third phase (P3) where $dV/dI$ is nearly constant and small, and noise is about an order of magnitude lower than in P2. In the depinned (P2 and P3) phases, the noise spectrum also reveals well-defined peaks at frequencies that vary linearly with the applied current, suggestive of washboard frequencies. We discuss the data in light of a recent theory that proposes different dynamic phases for a driven WS.

cond-mat.mes-hall↗

arXiv2023

Ultraclean two-dimensional hole systems with mobilities exceeding 10$^7$ cm$^2$/Vs

Owing to their large effective mass, strong and tunable spin-orbit coupling, and complex band-structure, two-dimensional hole systems (2DHSs) in GaAs quantum wells provide rich platforms to probe exotic many-body physics, while also offering potential applications in ballistic and spintronics devices, and fault-tolerant topological quantum computing. We present here a systematic study of molecular-beam-epitaxy grown, modulation-doped, GaAs (001) 2DHSs where we explore the limits of low-temperature 2DHS mobility by optimizing two parameters, the GaAs quantum well width and the alloy fraction ($x$) of the flanking Al$_x$Ga$_{1-x}$As barriers. We obtain a breakthrough in 2DHS mobility, with a peak value $\simeq 18 \times 10^6$ cm$^2$/Vs at a density of 3.8 $\times$ 10$^{10}$ /cm$^{2}$, implying a mean-free-path of $\simeq 57 μ$m. Using transport calculations tailored to our structures, we analyze the operating scattering mechanisms to explain the non-monotonic evolution of mobility with density. We find it imperative to include the dependence of effective mass on 2DHS density, well width, and $x$. We observe concomitant improvement in quality as evinced by the appearance of delicate fractional quantum Hall states at very low density.

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