arXiv subjects

D. Kamburov

Publications and source records attributed to D. Kamburov.

At least 19 recordsLinked to original sources

arXiv2021

Design rules for modulation-doped AlAs quantum wells

Thanks to their multi-valley, anisotropic, energy band structure, two-dimensional electron systems (2DESs) in modulation-doped AlAs quantum wells (QWs) provide a unique platform to investigate electron interaction physics and ballistic transport. Indeed, a plethora of phenomena unseen in other 2DESs have been observed over the past decade. However, a foundation for sample design is still lacking for AlAs 2DESs, limiting the means to achieve optimal quality samples. Here we present a systematic study on the fabrication of modulation-doped AlAs and GaAs QWs over a wide range of AlxGa1-xAs barrier alloy compositions. Our data indicate clear similarities in modulation doping mechanisms for AlAs and GaAs, and provide guidelines for the fabrication of very high quality AlAs 2DESs. We highlight the unprecedented quality of the fabricated AlAs samples by presenting the magnetotransport data for low density (~1X1011 cm2) AlAs 2DESs that exhibit high-order fractional quantum Hall signatures.

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arXiv2019

Composite fermions in a wide quantum well in the vicinity of the filling factor 1/2

Using acoustic method we study dependences of transverse AC conductance, $σ(ω)$, on magnetic field, temperature and the amplitude of AC electric field in a wide (75 nm) quantum well (QW) structure focusing on the vicinity of the filling factor $ν=1/2$. Measurements are performed in the frequency domain 30-307 MHz and in the temperature domain 20-500 mK. Usually, in wide QW structures closely to $ν=1/2$ the fractional quantum Hall effect (FQHE) regime is realized at some parameters of the sample. However, in our structure, at $ν=1/2$ it is a compressible state corresponding to gas of composite fermions which is observed. This is confirmed by apparent frequency independence and weakly decreasing temperature dependence of $\mathrm{Re}\, σ(ω)$. Comparing the dependences of this quantity on temperature and power of the acoustic wave we conclude that the observed nonlinear behavior of the conductance is compatible with heating of the composite fermions by the acoustic wave. For comparison, we also study the vicinity of $ν= 3/2$ where the FQHE regime is clearly observed.

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arXiv2019

Electronic band structure in $n$-type GaAs/AlGaAs wide quantum wells in tilted magnetic field

Oscillations of the real component of AC conductivity $σ_1$ in a magnetic field were measured in the n-AlGaAs/GaAs structure with a wide (75 nm) quantum well by contactless acoustic methods at $T$=(20-500)~mK. In a wide quantum well, the electronic band structure is associated with the two-subband electron spectrum, namely the symmetric (S) and antisymmetric (AS) subbands formed due to electrostatic repulsion of electrons. A change of the oscillations amplitude in tilted magnetic field observed in the experiments occurs due to crossings of Landau levels of different subbands (S and AS) at the Fermi level. The theory developed in this work shows that these crossings are caused by the difference in the cyclotron energies in the S and AS subbands induced by the in-plane magnetic field.

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arXiv2019

Geometric Resonance of Four-Flux Composite Fermions

Two-dimensional interacting electrons exposed to strong perpendicular magnetic fields generate emergent, exotic quasiparticles phenomenologically distinct from electrons. Specifically, electrons bind with an even number of flux quanta, and transform into composite fermions (CFs). Besides providing an intuitive explanation for the fractional quantum Hall states, CFs also possess Fermi-liquid-like properties, including a well-defined Fermi sea, at and near even-denominator Landau level filling factors such as $ν=1/2$ or $1/4$. Here, we directly probe the Fermi sea of the rarely studied four-flux CFs near $ν=1/4$ via geometric resonance experiments. The data reveal some unique characteristics. Unlike in the case of two-flux CFs, the magnetic field positions of the geometric resonance resistance minima for $ν<1/4$ and $ν>1/4$ are symmetric with respect to the position of $ν=1/4$. However, when an in-plane magnetic field is applied, the minima positions become asymmetric, implying a mysterious asymmetry in the CF Fermi sea anisotropy for $ν<1/4$ and $ν>1/4$. This asymmetry, which is in stark contrast to the two-flux CFs, suggests that the four-flux CFs on the two sides of $ν=1/4$ have very different effective masses, possibly because of the proximity of the Wigner crystal formation at small $ν$.

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arXiv2018

Intersubband scattering in n-GaAs/AlGaAs wide quantum wells

Slow magnetooscilations of the conductivity are observed in a 75 nm wide quantum well at heating of the two-dimensional electrons by a high-intensity surface acoustic wave. These magnetooscillations are caused by intersubband elastic scattering between the symmetric and asymmetric subbands formed due to an electrostatic barrier in the center of the quantum well. The tunneling splitting between these subbands as well as the intersubband scattering rate are determined.

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arXiv2017

Search for Composite Fermions at Filling Factor 5/2: Role of Landau Level and Subband Index

The pairing of composite fermions (CFs), electron-flux quasi-particles, is commonly proposed to explain the even-denominator fractional quantum Hall state observed at $ν=5/2$ in the first excited ($N=1$) Landau level (LL) of a two-dimensional electron system (2DES). While well-established to exist in the lowest ($N=0$) LL, much is unknown about CFs in the $N=1$ LL. Here we carry out geometric resonance measurements to detect CFs at $ν=5/2$ by subjecting the 2DES to a one-dimensional density modulation. Our data, taken at a temperature of 0.3 K, reveal no geometric resonances for CFs in the $N=1$ LL. In stark contrast, we observe clear signatures of such resonances when $ν=5/2$ is placed in the $N=0$ LL of the anti-symmetric subband by varying the 2DES width. This finding implies that the CFs' mean-free-path is significantly smaller in the $N=1$ LL compared to the $N=0$ LL. Our additional data as a function of in-plane magnetic field highlight the role of subband index and establish that CFs at $ν=5/2$ in the $N=0$ LL are more anisotropic in the symmetric subband than in the anti-symmetric subband.

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arXiv2017

Interplay between Quantum Well Width and Interface Roughness for Electron Transport Mobility in GaAs Quantum Wells

We report transport mobility measurements for clean, two-dimensional (2D) electron systems confined to GaAs quantum wells (QWs), grown via molecular beam epitaxy, in two families of structures, a standard, symmetrically-doped GaAs set of QWs with Al$_{0.32}$Ga$_{0.68}$As barriers, and one with additional AlAs cladding surrounding the QWs. Our results indicate that the mobility in narrow QWs with no cladding is consistent with existing theoretical calculations where interface roughness effects are softened by the penetration of the electron wave function into the adjacent low barriers. In contrast, data from AlAs-clad wells show a number of samples where the 2D electron mobility is severely limited by interface roughness. These measurements across three orders of magnitude in mobility provide a road map of reachable mobilities in the growth of GaAs structures of different electron densities, well widths, and barrier heights.

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arXiv2016

Geometric Resonance of Composite Fermions near Bilayer Quantum Hall States

Via the application of parallel magnetic field, we induce a single-layer to bilayer transition in two-dimensional electron systems confined to wide GaAs quantum wells, and study the geometric resonance of composite fermions (CFs) with a periodic density modulation in our samples. The measurements reveal that CFs exist close to bilayer quantum Hall states, formed at Landau level filling factors $ν=1$ and 1/2. Near $ν=1$, the geometric resonance features are consistent with half the total electron density in the bilayer system, implying that CFs prefer to stay in separate layers and exhibit a two-component behavior. In contrast, close to $ν=1/2$, CFs appear single-layer-like (single-component) as their resonance features correspond to the total density.

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arXiv2016

Anisotropic composite fermions and fractional quantum Hall effect

We study the role of anisotropy on the transport properties of composite fermions near Landau level filling factor $ν=1/2$ in two-dimensional holes confined to a GaAs quantum well. By applying a parallel magnetic field, we tune the composite fermion Fermi sea anisotropy and monitor the relative change of the transport scattering time at $ν=1/2$ along the principal directions. Interpreted in a simple Drude model, our results suggest that the scattering time is longer along the longitudinal direction of the composite fermion Fermi sea. Furthermore, the measured energy gap for the fractional quantum Hall state at $ν=2/3$ decreases when anisotropy becomes significant. The decrease, however, might partly stem from the charge distribution becoming bilayer-like at very large parallel magnetic fields.

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arXiv2015

Fermi Contour Disintegration of Quasi-2D Electrons in Parallel Magnetic Fields

In a quasi two-dimensional electron system with non-zero layer thickness, a parallel magnetic field (B||) can couple to the out-of-plane electron motion and lead to a severe distortion and eventual disintegration of the Fermi contour. Here we directly and quantitatively probe this evolution through commensurability and Shubnikov-de Haas measurements on electrons confined to a 40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour disintegration phenomenon, in good agreement with the results of semi-classical calculations. Experimentally we also observe intriguing features, suggesting magnetic-breakdown-type behavior when the Fermi contour disintegrates.

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arXiv2015

Composite Fermion Geometric Resonance Near ν = 1/2 Fractional Quantum Hall State

We observe geometric resonance features of composite fermions on the flanks of the even denominator ν = 1/2 fractional quantum Hall state in high-mobility two-dimensional electron and hole systems confined to wide GaAs quantum wells and subjected to a weak, strain-induced, unidirectional periodic potential modulation. The features provide a measure of how close to ν = 1/2 the system stays single-component and supports a composite fermion Fermi sea before transitioning into a ν = 1/2 fractional quantum Hall state, presumably the two-component Ψ331 state.

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arXiv2015

Composite Fermions with a Warped Fermi Contour

Via measurements of commensurability features near Landau filling factor $ν=1/2$, we probe the shape of the Fermi contour for hole-flux composite fermions confined to a wide GaAs quantum well. The data reveal that the composite fermions are strongly influenced by the characteristics of the Landau level in which they are formed. In particular, their Fermi contour is $\textit{warped}$ when their Landau level originates from a hole band with significant warping.

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arXiv2014

Determination of the Fermi Contour and Spin-polarization of $ν=3/2$ Composite Fermions via Ballistic Commensurability Measurements

We report ballistic transport commensurability minima in the magnetoresistance of $ν=3/2$ composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to $ν=3/2$, which we explain quantitatively by comparing three CF density models and concluding that the $ν=3/2$ CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of $\simeq 1.8 \times 10^{11}$ cm$^{-2}$, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin-polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field ($B_{||}$) leads to a significant distortion of the CF Fermi contour as $B_{||}$ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the $ν=1/2$ CF case at comparable $B_{||}$. Moreover, the applied $B_{||}$ further spin-polarizes the $ν=3/2$ CFs as deduced from the positions of the commensurability minima.

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arXiv2014

Fractional Quantum Hall Effect and Wigner Crystal of Two-Flux Composite Fermions

In two-dimensional electron systems confined to GaAs quantum wells, as a function of either tilting the sample in magnetic field or increasing density, we observe multiple transitions of the fractional quantum Hall states (FQHSs) near filling factors $ν=3/4$ and 5/4. The data reveal that these are spin-polarization transitions of interacting two-flux composite Fermions, which form their own FQHSs at these fillings. The fact that the reentrant integer quantum Hall effect near $ν=4/5$ always develops following the transition to full spin polarization of the $ν=4/5$ FQHS strongly links the reentrant phase to a pinned \emph{ferromagnetic} Wigner crystal of two-flux composite Fermions.

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arXiv2014

What Determines the Fermi Wave Vector of Composite Fermions?

Composite fermions (CFs), exotic particles formed by pairing an even number of flux quanta to each electron, provide a fascinating description of phenomena exhibited by interacting two-dimensional electrons at high magnetic fields. At and near Landau level filling $ν=1/2$, CFs occupy a Fermi sea and exhibit commensurability effects when subjected to a periodic potential modulation. We observe a pronounced asymmetry in the magnetic field positions of the commensurability resistance minima of CFs with respect to the field at $ν=1/2$. This unexpected asymmetry is quantitatively consistent with the CFs' Fermi wave vector being determined by the \textit{minority} carriers in the lowest Landau level. Our data indicate a breaking of the particle-hole symmetry for CFs near $ν=1/2$.

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arXiv2014

Even-denominator Fractional Quantum Hall Effect at a Landau Level Crossing

The fractional quantum Hall effect (FQHE), observed in two-dimensional (2D) charged particles at high magnetic fields, is one of the most fascinating, macroscopic manifestations of a many-body state stabilized by the strong Coulomb interaction. It occurs when the filling factor ($ν$) of the quantized Landau levels (LLs) is a fraction which, with very few exceptions, has an odd denominator. In 2D systems with additional degrees of freedom it is possible to cause a crossing of the LLs at the Fermi level. At and near these crossings, the FQHE states are often weakened or destroyed. Here we report the observation of an unusual crossing of the two \emph{lowest-energy} LLs in high-mobility GaAs 2D $hole$ systems which brings to life a new \emph{even-denominator} FQHE at $ν=1/2$.

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arXiv2013

Anisotropic Fermi Contour of (001) GaAs Electrons in Parallel Magnetic Fields

We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semi-quantitatively with the results of parameter-free calculations of the Fermi contours but there are significant discrepancies.

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arXiv2013

Fermi Contour Anisotropy of GaAs Electron-Flux Composite Fermions in Parallel Magnetic Fields

In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors $ν=$ 1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near $ν=1/2$, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field ($B_{||}$) applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing $B_{||}$ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high $B_{||}$. We present a simple, qualitative model to interpret our findings.

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