arXiv subjects

G. A. Csáthy

Publications and source records attributed to G. A. Csáthy.

At least 19 recordsLinked to original sources

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.

cond-mat.mes-hall↗

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

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↗

arXiv2023

A Highly Correlated Topological Bubble Phase of Composite Fermions

Strong interactions and topology drive a wide variety of correlated ground states. Some of the most interesting of these ground states, such as fractional quantum Hall states and fractional Chern insulators, have fractionally charged quasiparticles. Correlations in these phases are captured by the binding of electrons and vortices into emergent particles called composite fermions. Composite fermion quasiparticles are randomly localized at high levels of disorder and may exhibit charge order when there is not too much disorder in the system. However, more complex correlations were predicted when composite fermion quasiparticles cluster into a bubble, then these bubbles order on a lattice. Such a highly correlated ground state was termed the bubble phase of composite fermions. Here we report the observation of this bubble phase of composite fermions, evidenced by the reentrance of the fractional quantum Hall effect. We associate this reentrance with a bubble phase with two composite fermion quasiparticles per bubble. Our results demonstrate the existence of a new class of strongly correlated topological phases driven by clustering and charge ordering of emergent quasiparticles.

cond-mat.str-el↗

arXiv2021

A ruthenium oxide thermometer for dilution refrigerators operating down to 5 mK

At the lowest temperatures achieved in dilution refrigerators, ruthenium oxide resistance thermometers often saturate and therefore lose their sensitivity. In an effort to extend the range of such temperature sensors, we built a thermometer which maintains sensitivity to 5~mK. A key component of this thermometer is an in situ radio frequency filter which is based on a modern rf absorption material. We show that the use of such a filter is only effective when it is encased in the same rf-tight enclosure as the ruthenium oxide sensor. Our design delivers an attenuation level that is necessary to mitigate the effects of parasitic heating of a fraction of pW present in our circuit. Furthermore, we show that the likely origin of this parasitic heating is the black body radiation present within the experimental space of the refrigerator. We found that the equilibration time of the thermometer increases rapidly as the temperature is lowered; below 5~mK this thermometer becomes impractical because of the prohibitively long equilibration times.

physics.app-ph↗

arXiv2021

Magnetotransport patterns of collective localization near $ν=1$ in a high-mobility two-dimensional electron gas

We report complex magnetotransport patterns of the $ν=1$ integer quantum Hall state in a GaAs/AlGaAs sample from the newest generation with a record high electron mobility. The reentrant integer quantum Hall effect in the flanks of the $ν=1$ plateau indicates the formation of the integer quantum Hall Wigner solid, a collective insulator. Moreover, at a fixed filling factor, the longitudinal resistance versus temperature in the region of the integer quantum Hall Wigner solid exhibits a sharp peak. Such sharp peaks in the longitudinal resistance versus temperature so far were only detected for bubble phases forming in high Landau levels but were absent in the region of the Anderson insulator. We suggest that in samples of sufficiently low disorder sharp peaks in the longitudinal resistance versus temperature traces are universal transport signatures of all isotropic electron solids that form in the flanks of integer quantum Hall plateaus. We discuss possible origins of these sharp resistance peaks and we draw a stability diagram for the insulating phases in the $ν$-$T$ phase space.

cond-mat.str-el↗

arXiv2021

Particle-Hole Symmetry and the Reentrant Integer Quantum Hall Wigner Solid

The interplay of strong Coulomb interactions and of topology is currently under intense scrutiny in various condensed matter and atomic systems. One example of this interplay is the phase competition of fractional quantum Hall states and the Wigner solid in the two-dimensional electron gas. Here we report a Wigner solid at $ν=1.79$ and its melting due to fractional correlations occurring at $ν=9/5$. This Wigner solid, that we call the reentrant integer quantum Hall Wigner solid, develops in a range of Landau level filling factors that is related by particle-hole symmetry to the so called reentrant Wigner solid. We thus find that the Wigner solid in the GaAs/AlGaAs system straddles the partial filling factor $1/5$ not only at the lowest filling factors, but also near $ν=9/5$. Our results highlight the particle-hole symmetry as a fundamental symmetry of the extended family of Wigner solids and paint a complex picture of the competition of the Wigner solid with fractional quantum Hall states.

cond-mat.str-el↗

arXiv2021

Stability of multielectron bubbles in high Landau levels

We study multielectron bubble phases in the $N=2$ and $N=3$ Landau levels in a high mobility GaAs/AlGaAs sample. We found that the longitudinal magnetoresistance versus temperature curves in the multielectron bubble region exhibit sharp peaks, irrespective of the Landau level index. We associate these peaks with an enhanced scattering caused by thermally fluctuating domains of a bubble phase and a uniform uncorrelated electron liquid at the onset of the bubble phases. Within the $N=3$ Landau level, onset temperatures of three-electron and two-electron bubbles exhibit linear trends with respect to the filling factor; the onset temperatures of three-electron bubbles are systematically higher than those of two-electron bubbles. Furthermore, onset temperatures of the two-electron bubble phases across $N=2$ and $N=3$ Landau levels are similar, but exhibit an offset. This offset and the dominant nature of the three-electron bubbles in the $N=3$ Landau level reveals the role of the short-range part of the electron-electron interaction in the formation of the bubbles.

cond-mat.str-el↗

arXiv2020

Disorder broadening of even denominator fractional quantum Hall states in the presence of a short-range alloy potential

We study energy gaps of the $ν=7/2$ and $ν=5/2$ fractional quantum Hall states in a series of two-dimensional electron gases containing alloy disorder. We found that gaps at these two filling factors have the same suppression rate with alloy disorder. The dimensionless intrinsic gaps in our alloy samples obtained from the model proposed by Morf and d'Ambrumenil are consistent with numerical results, but are larger than those obtained from experiments on pristine samples published in the literature. The disorder broadening parameter has large uncertainties. However, a modified analysis relying on shared intrinsic gaps yields consistent results for both the $ν=5/2$ and $7/2$ fractional quantum Hall states and establishes a linear relationship between the disorder broadening parameter and alloy concentration. Furthermore, we find that we can separate contributions to the disorder broadening of the long-range and short-range scattering.

cond-mat.str-el↗

arXiv2019

Electron Bubbles and the Structure of the Orbital Wavefunction

Stripe-like and bubble-like patterns spontaneously form in numerous physical, chemical, and biological systems when competing long-range and short-range interactions banish uniformity. Stripe-like and the related nematic morphology are also under intense scrutiny in various strongly correlated electron systems. In contrast, the electronic bubble morphology is rare. Some of the most intriguing electron bubbles develop in the two-dimensional electron gas subjected to a perpendicular magnetic field. However, in contrast to bubbles forming in classical systems such as the Turing activator-inhibitor reaction or Langmuir films, bubbles in electron gases owe their existence to elementary quantum mechanics: they are stabilized as wavefunctions of individual electrons overlap. Here we report a rich pattern of multi-electron bubble phases in a high Landau level and we conclude that this richness is due to the nodal structure of the orbital component of the electronic wavefunction.

cond-mat.str-el↗

arXiv2018

Electron-Electron Interactions and the Paired-to-Nematic Quantum Phase Transition in the Second Landau Level

In spite of its ubiquity in strongly correlated systems, the competition of paired and nematic ground states remains poorly understood. Recently such a competition was reported in the two-dimensional electron gas at filling factor $ν=5/2$. At this filling factor a pressure-induced quantum phase transition was observed from the paired fractional quantum Hall state to the quantum Hall nematic. Here we show that the pressure induced paired-to-nematic transition also develops at $ν=7/2$, demonstrating therefore this transition in both spin branches of the second orbital Landau level. However, we find that pressure is not the only parameter controlling this transition. Indeed, ground states consistent with those observed under pressure also develop in a sample measured at ambient pressure, but in which the electron-electron interaction was tuned close to its value at the quantum critical point. Our experiments suggest that electron-electron interactions play a critical role in driving the paired-to-nematic transition.

cond-mat.str-el↗

arXiv2017

Observation of an anomalous density-dependent energy gap of the $ν=5/2$ fractional quantum Hall state in the low density regime

We have studied the $ν=5/2$ fractional quantum Hall state in a density-tunable sample at extremely low electron densities. For the densities accessed in our experiment, the Landau level mixing parameter $κ$ spans the $2.52<κ<2.82$ range. In the vicinity of $5.8 \times 10^{10}$~cm$^{-2}$ or $κ= 2.6$ an anomalously large change in the density dependence of the energy gap is observed. Possible origins of such an anomaly are discussed, including a topological phase transition in the $ν=5/2$ fractional quantum Hall state.

cond-mat.mes-hall↗

arXiv2017

Onset of Quantum Criticality in the Topological-to-Nematic Transition in a Two-dimensional Electron Gas at Filling Factor $ν=5/2$

Under hydrostatic pressure, the ground state of a two-dimensional electron gas at $ν=5/2$ changes from a fractional quantum Hall state to the stripe phase. By measuring the energy gap of the fractional quantum Hall state and of the onset temperature of the stripe phase we mapped out a phase diagram of these competing phases in the pressure-temperature plane. Our data highlight the dichotomy of two descriptions of the half-filled Landau level near the quantum critical point: one based on electrons and another on composite fermions.

cond-mat.str-el↗

arXiv2017

Theory of topological excitations and metal-insulator transition in reentrant integer quantum Hall effect

The reentrant integer quantum Hall effects (RIQHE) are due to formation of electronic crystals. We show analytically and numerically that topological textures in the charge density distribution in these crystals in the vicinity of charged defects strongly reduce energy required for current-carrying excitations. The theory quantitatively explains sharp insulator-metal transitions experimentally observed in RIQHE states. The insulator to metal transition in RIQHE emerges as a thermodynamic unbinding transition of topological charged defects.

cond-mat.mes-hall↗

arXiv2015

Observation of a transition from a topologically ordered to a spontaneously broken symmetry phase

Until the late 1980s, phases of matter were understood in terms of Landau's symmetry breaking theory. Following the discovery of the quantum Hall effect the introduction of a second class of phases, those with topological order, was necessary. Phase transitions within the first class of phases involve a change in symmetry, whereas those between topological phases require a change in topological order. However, in rare cases transitions may occur between the two classes in which the vanishing of the topological order is accompanied by the emergence of a broken symmetry. Here, we report the existence of such a transition in a two-dimensional electron gas hosted by a GaAs/AlGaAs crystal. When tuned by hydrostatic pressure, the $ν=5/2$ fractional quantum Hall state, believed to be a prototype non-Abelian topological phase, gives way to a quantum Hall nematic phase. Remarkably, this nematic phase develops spontaneously, in the absence of any externally applied symmetry breaking fields.

cond-mat.str-el↗

arXiv2015

Impact of Heterostructure Design on Transport Properties in the Second Landau Level of in-situ Back-Gated Two-Dimensional Electron Gases

We report on transport in the second Landau level in \emph{in-situ} back-gated two-dimensional electron gases in GaAs/Al$_x$Ga$_{1-x}$As quantum wells. Minimization of gate leakage is the primary heterostructure design consideration. Leakage currents resulting in dissipation as small as $\sim$ 10 pW can cause noticeable heating of the electrons at 10 mK, limiting the formation of novel correlated states. We show that when the heterostructure design is properly optimized, gate voltages as large as 4V can be applied with negligible gate leakage, allowing the density to be tuned over a large range from depletion to over 4 $\times$ 10$^{11}$ cm$^{-2}$. As a result, the strength of the $ν= 5/2$ state can be continuously tuned from onset at n $\sim 1.2 \times 10^{11}$ cm$^{-2}$ to a maximum $Δ_{5/2} = 625$ mK at n = $3.35 \times 10^{11}$ cm$^{-2}$. An unusual evolution of the reentrant integer quantum Hall states as a function of density is also reported. These devices can be expected to be useful in experiments aimed at proving the existence of non-Abelian phases useful for topological quantum computation.

cond-mat.mes-hall↗

arXiv2014

Observation of Incompressibility at $ν=4/11$ and $ν=5/13$

The region of filling factors $1/3<ν<2/5$ is predicted to support new types of fractional quantum Hall states with topological order different from that of the Laughlin-Jain or the Moore-Read states. Incompressibility is a necessary condition for the formation of such novel topological states. We find that at 6.9~mK incompressibility develops only at $ν=4/11$ and $5/13$, while the states at $ν=6/17$ and $3/8$ remain compressible. Our observations at $ν=4/11$ and $5/13$ are first steps towards understanding emergent topological order in these fractional quantum Hall states.

cond-mat.str-el↗

arXiv2014

Anomalous Gap Reversal of the $3+1/3$ and $3+1/5$ Fractional Quantum Hall States

In this work we report the opening of an energy gap at the filling factor $ν=3+1/3$, firmly establishing the ground state as a fractional quantum Hall state. This and other odd-denominator states unexpectedly break particle-hole symmetry. Specifically, we find that the relative magnitudes of the energy gaps of the $ν=3+1/3$ and $3+1/5$ states from the upper spin branch are reversed when compared to the $ν=2+1/3$ and $2+1/5$ counterpart states in the lower spin branch. Our findings raise the possibility that the former states have a non-conventional origin.

cond-mat.str-el↗