Quantum Hall Regime Research Articles

Coulomb oscillations of a quantum antidot formed by an airbridged pillar gate in the integer and fractional quantum Hall regime

Abstract Quantum antidots (QAD) are attractive for manipulating quasiparticles in quantum Hall (QH) systems. Here, we form a QAD in the integer and fractional QH regimes at nominal Landau-level filling factor ν = 2, 1, and 2/3 using a submicron pillar gate with an airbridge connection. After confirming the required conditions for a fully depleted QAD, we analyze the observed Coulomb oscillations in terms of the area of the QAD and the effective charge for the oscillation period in an identical gate voltage range. The area at ν= 2/3 is significantly smaller than that at ν = 2 and 1, in qualitative agreement with the previous report. By assuming a constant gate capacitance, the effective charge at ν= 2/3 is about 2/3 of that at ν = 2 and 1. The QAD device can be used to capture and emit charges in the unit of 2e/3.

Andreev Reflection in the Quantum Hall Regime at an Al/InAs Junction on a Cleaved Edge.

We have fabricated a superconductor/semiconductor junction composed of Al and InAs using cleaved edge overgrowth. By exploiting the unique geometry with a thin Al/Pt/Al trilayer formed on the side surface of an in situ cleaved InAs quantum well heterostructure wafer, we achieve a superconducting critical field of ∼5 T, allowing superconductivity and quantum Hall (QH) effects to coexist down to filling factor ν = 3. Andreev reflection at zero magnetic field shows a conductance enhancement limited solely by the Fermi velocity mismatch, demonstrating a virtually barrier-free junction. Bias spectroscopy in the QH regime reveals the opening of a superconducting gap, with the reduced downstream resistance indicating electron-hole Andreev conversion. Our results, obtained in a new experimental regime characterized by a clean edge-contacted junction with a superconducting electrode narrower than the coherence length, open new avenues for both theoretical and experimental studies of the interplay between superconductivity and QH effects.

Time-resolved counterpropagating edge transport in the topological insulating phase

We have investigated the time-resolved low-energy charge dynamics in counterpropagating edge channels formed in the quantum Hall regime of an electron-hole bilayer system in band-inverted InAs/InGaSb composite quantum wells. In the studied magnetic field range, the system remains in the topological phase where an equal number of counterpropagating electronlike and holelike edge channels exist in the bulk energy gap, analogous to the helical edge channels in a quantum spin Hall insulator. When the Fermi level was set in the conduction band, the injected charge pulses propagated unidirectionally as chiral edge magnetoplasmon (EMP) wave packets. In contrast, in the nonchiral regime with the Fermi level set in the band gap, the injected charge pulses propagated identically in both directions with significantly reduced velocity. In addition, as the channel length increased, we observed a sharp crossover from EMP transport to diffusive transport, manifested as pronounced waveform broadening and dissipation. Simulations using a distributed capacitance model including interchannel tunneling well reproduced the observed behavior in both chiral and nonchiral regimes, revealing that interchannel charge transfer between counterpropagating channels plays a crucial role. The slow EMP transport in the nonchiral regime and its crossover to diffusive transport are consequences of the strong interchannel interaction and interchannel tunneling, respectively. Our results will thus provide insights into charge dynamics and scattering mechanism in analogous one-dimensional systems, including a quantum spin Hall insulator with helical edge modes and its expected Tomonaga-Luttinger liquid behavior. Published by the American Physical Society 2024

Electron spin resonance impact on the longitudinal resistance in the quantum Hall regime

The electron spin resonance impact on the longitudinal resistance in the quantum Hall regime near the integer filling factors is considered. For the odd filling factors extra spin excitons are created in the process of the microwave absorption, and the resistance increase due to the effective temperature rise. For the even filling factors in the absorption process depending on the temperature inspired spin polarization leads to transition of the spin-flip excitations to the higher energy short-lived excitations, and as a result the resistance is decreased due to effective temperature decrease.

The Migdal jump under the quantum Hall regime

In two-dimensional electron systems at large values of the Wigner-Seits parameter rs and in the quantum Hall effect mode, the distribution function of particles over Landau levels was calculated. It turned out that at small filling factors, the tail of the distribution function and the magnitude of the Migdal jump are qualitatively different from the case of a Fermi liquid in a zero magnetic field. Due to the presence of the cyclotron energy gap, the Fermi-liquid distortion of the distribution function is significantly suppressed.

Universality of quantum phase transitions in the integer and fractional quantum Hall regimes

Fractional quantum Hall (FQH) phases emerge due to strong electronic interactions and are characterized by anyonic quasiparticles, each distinguished by unique topological parameters, fractional charge, and statistics. In contrast, the integer quantum Hall (IQH) effects can be understood from the band topology of non-interacting electrons. We report a surprising super-universality of the critical behavior across all FQH and IQH transitions. Contrary to the anticipated state-dependent critical exponents, our findings reveal the same critical scaling exponent κ = 0.41 ± 0.02 and localization length exponent γ = 2.4 ± 0.2 for fractional and integer quantum Hall transitions. From these, we extract the value of the dynamical exponent z ≈ 1. We have achieved this in ultra-high mobility trilayer graphene devices with a metallic screening layer close to the conduction channels. The observation of these global critical exponents across various quantum Hall phase transitions was masked in previous studies by significant sample-to-sample variation in the measured values of κ in conventional semiconductor heterostructures, where long-range correlated disorder dominates. We show that the robust scaling exponents are valid in the limit of short-range disorder correlations.

Gate tunable edge magnetoplasmon resonators

AbstractQuantum Hall systems are platforms of choice to study topological properties of condensed matter systems and anyonic exchange statistics. In this work we have developed a tunable radiofrequency edge magnetoplasmonic resonator controlled by both the magnetic field and a set of electrostatic gates, meant to serve as a versatile platform for future interferometric devices designed to evidence non-abelian anyons. In our device, gates allow us to change both the size of the resonant cavity and the electronic density of the two-dimensional electron gas. We show that we can continuously control the frequency response of our resonator, making it possible to develop an edge magnetoplasmon interferometer. As we reach smaller sizes of our resonator, finite size effects caused by the measurement probes manifest. In the future, such device will be a valuable tool to investigate the properties of non-abelian anyons in the fractional quantum Hall regime.

Static quantum dot on a large potential hilltop for generating and analyzing hot electrons in the quantum Hall regime

Static quantum dot on a large potential hilltop for generating and analyzing hot electrons in the quantum Hall regime

Conformity Experiment on Inelastic Scattering Exponent of Electrons in Two Dimensions.

The quantum Hall (QH) effect is one of the most widely studied physical phenomenon in two dimensions. The plateau-plateau transition within this effect can be comprehensively described by the scaling theory, which encompasses three pivotal exponents: the critical exponent κ, the inelastic scattering exponent p, and the universal exponent γ. Prior studies have focused on measuring κ and estimating γ, assuming a constant p value of 2 across magnetic fields. Here, our work marks a significant advancement by measuring all three exponents within a single graphene device and a conventional two-dimensional electron system. This study uniquely determines p at low magnetic fields (weak localization region and well outside the QH regime) and high magnetic fields (in the vicinity of the QH regime). Employing a comprehensive analytical approach that includes weak localization, plateau-plateau transitions, and variable range hopping, we have directly determined κ, p, and γ. Our findings reveal a distinct variation in p, shifting from 1 in the low magnetic field regime to 2 in the QH regime in graphene.

Optimization of submicron Ni/Au/Ge contacts to an AlGaAs/GaAs two-dimensional electron gas

We report on fabrication and performance of sub-micrometer Ni/Au/Ge contacts to a two-dimensional electron gas in an AlGaAs/GaAs heterostructure. Utilizing scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and low temperature electrical measurements, we investigate the relationship between contact performance and the mechanical and chemical properties of the annealed metal stack. Contact geometry and crystallographic orientation significantly impact performance. Our results indicate that the spatial distribution of germanium in the annealed contact plays a central role in the creation of high transmission contacts. We characterize the transmission of our contacts at high magnetic fields in the quantum Hall regime. Our work establishes that contacts with an area of 0.5 μm2 and resistance less than 400 Ω can be fabricated with high yield.

Quantum Hall resistance standards based on epitaxial graphene with p-type conductivity

In the last decade, quantum resistance metrology has benefited from the application of graphene as the base material for the fabrication of quantum Hall (QH) resistance standards since it allows for the realization of the resistance unit ohm in the revised International System of Units under relaxed experimental conditions. Here, we present a detailed magnetotransport investigation of p-type epitaxial graphene, which was doped by the molecular acceptor F4-TCNQ. High-accuracy measurements of the QH resistance show an excellent quantization and a reproduction of the nominal value, the half of the von Klitzing constant RK/2, within 2 nΩ/Ω. It underlines the universality of the QH effect and shows that p-type epitaxial graphene can also serve as the basis for future resistance standards for operation at relaxed experimental conditions. For the p-type devices, the onset of the QH plateau is observed at about 1 T higher magnetic fields, which can be attributed to an additional disorder or a non-symmetric charge transfer mechanism in the QH regime.

Signature of anyonic statistics in the integer quantum Hall regime

Anyons are exotic low-dimensional quasiparticles whose unconventional quantum statistics extend the binary particle division into fermions and bosons. The fractional quantum Hall regime provides a natural host, with the first convincing anyon signatures recently observed through interferometry and cross-correlations of colliding beams. However, the fractional regime is rife with experimental complications, such as an anomalous tunneling density of states, which impede the manipulation of anyons. Here we show experimentally that the canonical integer quantum Hall regime can provide a robust anyon platform. Exploiting the Coulomb interaction between two copropagating quantum Hall channels, an electron injected into one channel splits into two fractional charges behaving as abelian anyons. Their unconventional statistics is revealed by negative cross-correlations between dilute quasiparticle beams. Similarly to fractional quantum Hall observations, we show that the negative signal stems from a time-domain braiding process, here involving the incident fractional quasiparticles and spontaneously generated electron-hole pairs. Beyond the dilute limit, a theoretical understanding is achieved via the edge magnetoplasmon description of interacting integer quantum Hall channels. Our findings establish that, counter-intuitively, the integer quantum Hall regime provides a platform of choice for exploring and manipulating quasiparticles with fractional quantum statistics.

Testing the Renormalization of the von Klitzing Constant by Cavity Vacuum Fields

The value of fundamental physical constants is affected by the coupling of matter to the electromagnetic vacuum state, as predicted and explained by quantum electrodynamics. In this work, we present a millikelvin magnetotransport experiment in the quantum Hall regime that assesses the possibility of the von Klitzing constant being modified by strong cavity vacuum fields. By employing a Wheatstone bridge, we measure the difference between the quantized Hall resistance of a cavity-embedded Hall bar and the resistance standard, achieving an accuracy down to one part in 105 for the lowest Landau level. While our results do not suggest any deviation that could imply a modified Hall resistance, our work represents pioneering efforts in exploring the fundamental implications of vacuum fields in solid-state systems. Published by the American Physical Society 2024

Negative longitudinal resistance of monolayer graphene in the quantum Hall regime

In the quantum Hall regime, the charge current is carried by ideal one-dimensional edge channels where the backscattering is prohibited by topology. This results in the constant potential along the edge of the Hall bar leading to zero 4-terminal longitudinal resistance rxx. Finite scattering between the counter-propagating edge states, when the topological protection is broken, commonly results in rxx > 0. However, a local disorder, if allowing intersection of the edge states, can result in a counter-intuitive scenario when rxx < 0. In this work, we report the observation and a systematic study of such unconventional negative longitudinal resistance seen in an encapsulated monolayer graphene Hall bar device measured in the quantum Hall regime. We supplement our findings with the numerical calculations, which allow us to outline the conditions necessary for the appearance of negative rxx and to exclude the macroscopic disorder (contamination bubble) as the main origin of it.

Numerical simulation of Rashba spin–orbit coupling effect on quantum Hall resistivity within tight-binding approximation

A numerical simulation of the effect of Rashba spin–orbit coupling (RSO) on quantum transport properties of electrons confined to a two-dimensional electron gas (2DEG) in the integer quantum Hall regime is presented. The mechanism of quantization and the dependence of the Hall resistivity (HR) on the system built-in and external parameters are studied using the non-equilibrium Green’s function (NEGF) technique. Relevant Green’s functions are computed numerically using spectral decomposition method in combination with recursive techniques. The tight-binding-like representation of the system Hamiltonian including magnetic field is obtained within the finite difference method, using Dirichlet boundary conditions. Corresponding formula linking the Hall resistivity with the transmission function of the system has been derived analytically within the NEGF formalism in a general form, in the sense that the formula can also be used beyond the linear response limit and for nonzero temperatures. In particular, it was shown explicitly that in the presence of the RSO coupling the even- and odd plateaus emerging in the magnetic-field dependence of the quantum Hall resistivity are related to different physical mechanisms.

In-plane gate induced transition asymmetry of spin-resolved Landau levels in InAs-based quantum wells

The crossover from quasi-two- to quasi-one-dimensional electron transport subject to transverse electric fields and perpendicular magnetic fields is studied in the diffusive to quasi-ballistic and zero-field to quantum Hall regime. In-plane gates and Hall-bars have been fabricated from an InGaAs/InAlAs/InAs quantum well hosting a 2DEG with a carrier density of about 6.8 × 1011 cm−2, a mobility of 1.8 × 105 cm2/Vs, and an effective mass of 0.042me after illumination. Magnetotransport measurements at temperatures down to 50 mK and fields up to 12 T yield a high effective Landé factor of g*=16, enabling the resolution of spin-split subbands at magnetic fields of 2.5 T. In the quantum Hall regime, electrostatic control of an effective constriction width enables steering of the reflection and transmission of edge channels, allowing a separation of fully spin-polarized edge channels at filling factors ν = 1 und ν = 2. A change in the orientation of a transverse in-plane electric field in the constriction shifts the transition between Zeeman-split quantum Hall plateaus by ΔB ≈ 0.1 T and is consistent with an effective magnetic field of Beff ≈ 0.13 T by spin-dependent backscattering, indicating a change in the spin-split density of states.

One-dimensional proximity superconductivity in the quantum Hall regime.

Extensive efforts have been undertaken to combine superconductivity and the quantum Hall effect so that Cooper-pair transport between superconducting electrodes in Josephson junctions is mediated by one-dimensional edge states1-6. This interest has been motivated by prospects of finding new physics, including topologically protected quasiparticles7-9, but also extends into metrology and device applications10-13. So far it has proven challenging to achieve detectable supercurrents through quantum Hall conductors2,3,6. Here we show that domain walls in minimally twisted bilayer graphene14-18 support exceptionally robust proximity superconductivity in the quantum Hall regime, allowing Josephson junctions to operate in fields close to the upper critical field of superconducting electrodes. The critical current is found to be non-oscillatory and practically unchanging over the entire range of quantizing fields, with its value being limited by the quantum conductance of ballistic, strictly one-dimensional, electronic channels residing within the domain walls. The system described is unique in its ability to support Andreev bound states at quantizingfields and offers many interesting directions for further exploration.

Asymmetry of the Breakdown near Hot Spots in the Quantum Hall Regime in a Specially Shaped GaAs/AlGaAs Sample

Asymmetry of the Breakdown near Hot Spots in the Quantum Hall Regime in a Specially Shaped GaAs/AlGaAs Sample

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.

Electrical control of two-dimensional electron-hole fluids in the quantum Hall regime

Electrical control of two-dimensional electron-hole fluids in the quantum Hall regime