arXiv subjects

L. N. Pfeiffer

Publications and source records attributed to L. N. Pfeiffer.

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.

cond-mat.mes-hall↗

arXiv2026

Melting of quantum Hall Wigner and bubble crystals

A two-dimensional crystal melts via the proliferation and unbinding of topological defects, yet quantitatively predicting the melting temperature $T_m$ in real systems is challenging. Here we resolve this discrepancy in quantum Hall electron bubble phases by combining Corbino-geometry transport experiment in an ultraclean GaAs/AlGaAs quantum well for Landau levels 2 to 5 with Hartree--Fock elasticity and the full Kosterlitz--Thouless--Halperin--Nelson--Young melting criterion including the finite-temperature renormalization-group calculation. The theoretically obtained $T_m$ quantitatively captures the measured solid-liquid phase transition boundaries across all probed ranges, validating the bubble-crystal interpretation and establishing defect--mediated melting as a predictive framework for strongly interacting electronic solids. This agreement further supports using bulk transport to probe the energetics of topological defects and screening in quantum Hall physics, and the approach is readily extendable to other electronic crystals, including the generalized Wigner crystal in moiré Chern bands.

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arXiv2025

Resonant Magneto-phonon Emission by Supersonic Electrons in Ultra-high Mobility Two-dimensional System

We investigate resonant acoustic phonon scattering in the magneto-resistivity of an ultra-high mobility two-dimensional electron gas system subject to DC current in the temperature range 10 mK to 3.9 K. For a DC current density of $\sim$1.1 A/m, the induced carrier drift velocity $v_{drift}$ becomes equal to the speed of sound $s \sim$ 3 km/s. When $v_{drift} \gtrsim s$ very strong resonant features with only weak temperature dependence are observed and identified as phonon-induced resistance oscillations at and above the "sound barrier". Their behavior contrasts with that in the subsonic regime ($v_{drift} < s$) where resonant acoustic phonon scattering is strongly suppressed when the temperature is reduced unless amplified with quasi-elastic inter-Landau-level scattering. Our observations are compared to recent theoretical predictions from which we can extract a dimensionless electron-phonon coupling constant of $g^{2}$=0.0016 for the strong non-linear transport regime. We find evidence for a predicted oscillation phase change ' effect on traversing the "sound barrier". Crossing the "sound barrier" fundamentally alters the resulting phonon emission processes, and the applied magnetic field results in pronounced and sharp resonant phonon emission due to Landau level quantization.

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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

Density Dependence of the Phases of the $ν= 1$ Integer Quantum Hall Plateau in Low Disorder Electron Gases

Recent magnetotransport measurements in low-disorder electron systems confined to GaAs/AlGaAs samples revealed that the $ν= 1$ integer quantum Hall plateau is broken into three distinct regions. These three regions were associated with two phases with different types of bulk localization: the Anderson insulator is due to random quasiparticle localization, and the integer quantum Hall Wigner solid is due to pinning of a stiff quasiparticle lattice. We highlight universal properties of the $ν= 1$ plateau: the structure of the stability diagram, the non-monotonic dependence of the activation energy on the filling factor, and the alignment of features of the activation energy with features of the stability regions of the different phases are found to be similar in three samples spanning a wide range of electron densities. We also discuss quantitative differences between the samples, such as the dependence of the onset temperature and the activation energy of the integer quantum Hall Wigner solid on the electron density. Our findings provide insights into the localization behavior along the $ν= 1$ integer quantum Hall plateau in the low disorder regime.

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arXiv2025

Interaction-Driven Asymmetry in the Breakdown of the $ν$ = 1 Quantum Hall State

We report real-time detection of longitudinal and transverse transport responses across distinct frequency bands in a ferromagnetic filling factor $ν$ = 1 integer quantum Hall state. By tuning $ν$, we simultaneously access the evolution of the screening environment and bulk excitation structure. The resulting asymmetric breakdown, for $ν>1$ and $ν<1$, reveals that interaction effects, rather than a single-particle band picture, dominate the transport instability. Our findings highlight the indispensability of electron-electron interactions even in integer quantum Hall phases, suggesting that distinct many-body entanglement structures underlie both integer and fractional topological phases.

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arXiv2025

Pushing Photons with Electrons: Observation of the Polariton Drag Effect

We show the direct effect of free electrons colliding with polaritons, changing their momentum. The result of this interaction of the electrons with the polaritons is a change in the angle of emission of the photons from our cavity structure. Because the experiment is a photon-in, photon-out system, this is equivalent to optical beam steering of photons using a direct electrical current. The effect is asymmetric, significantly slowing down the polaritons when they move oppositely to the electrons, while the polariton momentum only slightly increases when electrons moving in the same direction. We present a theoretical model which describes this effect.

cond-mat.quant-gas↗

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.

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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.

cond-mat.mes-hall↗

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

Thermal Activation Signatures of the Anderson Insulator and the Wigner Solid forming near $ν=1$

When interactions overcome disorder, integer quantum Hall plateaus support topological phases with different bulk insulators. In the center of the $ν=1$ plateau the bulk is an Anderson-type insulator, while in the flanks of the plateau the bulk is the integer quantum Hall Wigner solid. We find that the activation energy along the $ν=1$ plateau exhibits a very dramatic non-monotonic dependence on the magnetic field, a dependence that is strongly correlated with the stability regions of the two phases. Furthermore, the activation energy has an unexpected minimum at the boundary between the Anderson insulator and the Wigner solid. Our findings constrain the theory of the integer quantum Hall Wigner solid, determine its thermodynamic properties, and reveal novel behavior at the boundary between the Anderson insulator and the Wigner solid.

cond-mat.str-el↗

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 $ν=1/6$.

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arXiv2024

Coherence of a non-equilibrium polariton condensate across the interaction-mediated phase transition

The emergence of spatial coherence in a confined two-dimensional Bose gas of exciton-polaritons with tuneable interactions offers a unique opportunity to explore the role of interactions in a phase transition in a driven-dissipative quantum system, where both the phase transition and thermalisation are mediated by interactions. We investigate, experimentally and numerically, the phase correlations and steady-state properties of the gas over a wide range of interaction strengths by varying the photonic/excitonic fraction of the polaritons and their density. We find that the first order spatial coherence function exhibits algebraic decay consistent with the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Surprisingly, the exponent of the algebraic decay is inversely proportional to the coherent density of polaritons, in analogy to equilibrium superfluids above the BKT transition, but with a different proportionality constant. Our work paves the way for future investigations of the phenomenon of phase transitions and superfluidity in a driven-dissipative setting

cond-mat.quant-gas↗

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.

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