arXiv subjects

M. Shayegan

Publications and source records attributed to M. Shayegan.

At least 109 records · Page 6Linked to original sources

arXiv2011

Unequal Layer Densities in Bilayer Wigner Crystal at High Magnetic Field

We report studies of pinning mode resonances of magnetic field induced bilayer Wigner crystals of bilayer hole samples with negligible interlayer tunneling and different interlayer separations d, in states with varying layer densities, including unequal layer densities. With unequal layer densities, samples with large d relative to the in-plane carrier-carrier spacing a, two pinning resonances are present, one for each layer. For small d/a samples, a single resonance is observed even with significant density imbalance. These samples, at balance, were shown to exhibit an enhanced pinning mode frequency [Zhihai Wang et al., Phys. Rev. Lett. 136804 (2007)], which was ascribed to a one-component, pseudospin ferromagnetic Wigner solid. The evolution of the resonance frequency and line width indicates the quantum interlayer coherence survives at moderate density imbalance, but disappears when imbalance is sufficiently large.

cond-mat.mes-hall↗

arXiv2010

Fractional Quantum Hall Effect at High Fillings in a Two-subband Electron System

Magneto-transport measurements in a clean two-dimensional electron system confined to a wide GaAs quantum well reveal that, when the electrons occupy two electric subbands, the sequences of fractional quantum Hall states observed at high fillings ($ν> 2$) are distinctly different from those of a single-subband system. Notably, when the Fermi energy lies in the ground state Landau level of either of the subbands, no quantum Hall states are seen at the even-denominator $ν$ = 5/2 and 7/2 fillings; instead the observed states are at $ν= (i + p/(2p \pm 1))$ where $i$ = 2, 3, and $p$ = 1, 2, 3, and include several new states at $ν$ = 13/5, 17/5, 18/5, and 25/7.

cond-mat.mes-hall↗

arXiv2010

Transference of Transport Anisotropy to Composite Fermions

When interacting two-dimensional electrons are placed in a large perpendicular magnetic field, to minimize their energy, they capture an even number of flux quanta and create new particles called composite fermions (CFs). These complex electron-flux-bound states offer an elegant explanation for the fractional quantum Hall effect. Furthermore, thanks to the flux attachment, the effective field vanishes at a half-filled Landau level and CFs exhibit Fermi-liquid-like properties, similar to their zero-field electron counterparts. However, being solely influenced by interactions, CFs should possess no memory whatever of the electron parameters. Here we address a fundamental question: Does an anisotropy of the electron effective mass and Fermi surface (FS) survive composite fermionization? We measure the resistance of CFs in AlAs quantum wells where electrons occupy an elliptical FS with large eccentricity and anisotropic effective mass. Similar to their electron counterparts, CFs also exhibit anisotropic transport, suggesting an anisotropy of CF effective mass and FS.

cond-mat.mes-hall↗

arXiv2010

Temperature dependence of piezoresistance of composite Fermions with a valley degree of freedom

We report transport measurements of composite Fermions at filling factor $ν=3/2$ in AlAs quantum wells as a function of strain and temperature. In this system the composite Fermions possess a valley degree of freedom and show piezoresistance qualitatively very similar to electrons. The temperature dependence of the resistance (R) of composite Fermions shows a metallic behavior (dR/dT > 0) for small values of valley polarization but turns insulating (dR/dT < 0) as they are driven to full valley polarization. The results highlight the importance of discrete degrees of freedom in the transport properties of composite Fermions and the similarity between composite Fermions and electrons.

cond-mat.mes-hall↗

arXiv2010

Contrast between spin and valley degrees of freedom

We measure the renormalized effective mass (m*) of interacting two-dimensional electrons confined to an AlAs quantum well while we control their distribution between two spin and two valley subbands. We observe a marked contrast between the spin and valley degrees of freedom: When electrons occupy two spin subbands, m* strongly depends on the valley occupation, but not vice versa. Combining our m* data with the measured spin and valley susceptibilities, we find that the renormalized effective Lande g-factor strongly depends on valley occupation, but the renormalized conduction-band deformation potential is nearly independent of the spin occupation.

cond-mat.mes-hall↗

arXiv2010

Ferromagnetic Fractional Quantum Hall States in a Valley-Degenerate Two-Dimensional Electron System

We study a two-dimensional electron system where the electrons occupy two conduction band valleys with anisotropic Fermi contours and strain-tunable occupation. We observe persistent quantum Hall states at filling factors $ν= 1/3$ and 5/3 even at zero strain when the two valleys are degenerate. This is reminiscent of the quantum Hall ferromagnet formed at $ν= 1$ in the same system at zero strain. In the absence of a theory for a system with anisotropic valleys, we compare the energy gaps measured at $ν= 1/3$ and 5/3 to the available theory developed for single-valley, two-spin systems, and find that the gaps and their rates of rise with strain are much smaller than predicted.

cond-mat.str-el↗

arXiv2009

Observation of fractional quantum Hall effect at even-denominator 1/2 and 1/4 fillings in asymmetric wide quantum wells

We report the observation of developing fractional quantum Hall states at Landau level filling factors $ν= 1/2$ and 1/4 in electron systems confined to wide GaAs quantum wells with significantly $asymmetric$ charge distributions. The very large electric subband separation and the highly asymmetric charge distribution at which we observe these quantum Hall states, together with the fact that they disappear when the charge distribution is made symmetric, suggest that these are one-component states, possibly described by the Moore-Read Pfaffian wavefunction.

cond-mat.mes-hall↗

arXiv2009

Density dependence of valley polarization energy for composite fermions

In two-dimensional electron systems confined to wide AlAs quantum wells, composite fermions around the filling factor $ν$ = 3/2 are fully spin polarized but possess a valley degree of freedom. Here we measure the energy needed to completely valley polarize these composite fermions as a function of electron density. Comparing our results to the existing theory, we find overall good quantitative agreement, but there is an unexpected trend: The measured composite fermion valley polarization energy, normalized to the Coulomb energy, decreases with decreasing density.

cond-mat.str-el↗

arXiv2009

Effective mass suppression in a ferromagnetic two-dimensional electron liquid

We present numerical calculations of the electron effective mass in an interacting, ferromagnetic, two-dimensional electron system. We consider quantum interaction effects associated with the charge-density fluctuation induced many-body vertex corrections. Our theory, which is free of adjustable parameters, reveals that the effective mass is suppressed (relative to its band value) in the strong coupling limit, in good agreement with the results of recent experimental measurements.

cond-mat.mes-hall↗

arXiv2009

Effective mass suppression upon complete spin-polarization in an isotropic two-dimensional electron system

We measure the effective mass (m*) of interacting two-dimensional electrons confined to a 4.5 nm-wide AlAs quantum well. The electrons in this well occupy a single out-of-plane conduction band valley with an isotropic in-plane Fermi contour. When the electrons are partially spin polarized, m* is larger than its band value and increases as the density is reduced. However, as the system is driven to full spin-polarization via the application of a strong parallel magnetic field, m* is suppressed down to values near or even below the band mass. Our results are consistent with the previously reported measurements on wide AlAs quantum wells where the electrons occupy an in-plane valley with an anisotropic Fermi contour and effective mass, and suggest that the effective mass suppression upon complete spin polarization is a genuine property of interacting two-dimensional electrons.

cond-mat.mes-hall↗

arXiv2009

Correlated States of Electrons in Wide Quantum Wells at Low Fillings: The Role of Charge Distribution Symmetry

Magneto-transport measurements on electrons confined to a 57 nm-wide, GaAs quantum well reveal that the correlated electron states at low Landau level fillings ($ν$) display a remarkable dependence on the symmetry of the electron charge distribution. At a density of $1.93 \times 10^{11}$ cm$^{-2}$, a developing fractional quantum Hall state is observed at the even-denominator filling $ν= 1/4$ when the distribution is symmetric, but it quickly vanishes when the distribution is made asymmetric. At lower densities, as we make the charge distribution asymmetric, we observe a rapid strengthening of the insulating phases that surround the $ν= 1/5$ fractional quantum Hall state.

cond-mat.mes-hall↗

arXiv2008

Spin susceptibility and effective mass of two-dimensional electrons in MgxZn1-xO/ZnO heterostructures

We report measurements of the spin susceptibility and the electron effective mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through control of the barrier composition, the electron density was systematically varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in units of the effective Bohr radius. We used the coincidence technique, where crossings of the spin-split Landau levels occur at critical tilt angles of magnetic field, to evaluate the spin susceptibility. In addition, we determined the effective mass from the temperature dependence of the Shubnikov-de Haas oscillations measured at the coincidence conditions. The susceptibility and the effective mass both gradually increase with decreasing electron density, reflecting the role of electron-electron interaction.

cond-mat.mtrl-sci↗

arXiv2008

Parallel Magnetic Field Tuning of Valley Splitting in AlAs Two-Dimensional Electrons

We demonstrate that, in a quasi-two-dimensional electron system confined to an AlAs quantum well and occupying two conduction-band minima (valleys), a parallel magnetic field can couple to the electrons' orbital motion and tune the energies of the two valleys by different amounts. The measured density imbalance between the two valleys, which is a measure of the valley susceptibility with respect to parallel magnetic field, is enhanced compared to the predictions of non-interacting calculations, reflecting the role of electron-electron interaction.

cond-mat.mes-hall↗

arXiv2008

Spin Orientation of Holes in Quantum Wells

This paper reviews the spin orientation of spin-3/2 holes in quantum wells. We discuss the Zeeman and Rashba spin splitting in hole systems that are qualitatively different from their counterparts in electron systems. We show how a systematic understanding of the unusual spin-dependent phenomena in hole systems can be gained using a multipole expansion of the spin density matrix. As an example we discuss spin precession in hole systems that can give rise to an alternating spin polarization. Finally, we discuss the qualitatively different regimes of hole spin polarization decay in clean and dirty samples.

cond-mat.mes-hall↗

arXiv2008

Enhancement of valley susceptibility upon complete spin-polarization

Measurements on a two-dimensional electron system confined to an AlAs quantum well reveal that for a given electron density the valley susceptibility, defined as the change in valley population difference per unit strain, is enhanced as the system makes a transition from partial to full spin polarization. This observation is reminiscent of earlier studies in which the spin susceptibility of AlAs electrons was observed to be higher in a single-valley system than its two-valley counterpart.

cond-mat.mes-hall↗

arXiv2008

Dependence of Effective Mass on Spin and Valley Degrees of Freedom

We measure the effective mass (m*) of interacting two-dimensional electrons confined to an AlAs quantum well while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively. Compared to its band value, m* is enhanced unless the electrons are fully valley and spin polarized. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior.

cond-mat.mes-hall↗

arXiv2008

High-quality quantum point contact in two-dimensional GaAs (311)A hole system

We studied ballistic transport across a quantum point contact (QPC) defined in a high-quality, GaAs (311)A two-dimensional (2D) hole system using shallow etching and top-gating. The QPC conductance exhibits up to 11 quantized plateaus. The ballistic one-dimensional subbands are tuned by changing the lateral confinement and the Fermi energy of the holes in the QPC. We demonstrate that the positions of the plateaus (in gate-voltage), the source-drain data, and the negative magneto-resistance data can be understood in a simple model that takes into account the variation, with gate bias, of the hole density and the width of the QPC conducting channel.

cond-mat.mes-hall↗

arXiv2008

Giant frictional drag in strongly interacting bilayers near filling factor one

We study the frictional drag in high mobility, strongly interacting GaAs bilayer hole systems in the vicinity of the filling factor $ν=1$ quantum Hall state (QHS), at the same fillings where the bilayer resistivity displays a reentrant insulating phase. Our measurements reveal a very large longitudinal drag resistivity ($ρ^{D}_{xx}$) in this regime, exceeding 15 k$Ω/\Box$ at filling factor $ν=1.15$. $ρ^{D}_{xx}$ shows a weak temperature dependence and appears to saturate at a finite, large value at the lowest temperatures. Our observations are consistent with theoretical models positing a phase separation, e.g. puddles of $ν=1$ QHS embedded in a different state, when the system makes a transition from the coherent $ν=1$ QHS to the weakly coupled $ν=2$ QHS.

cond-mat.mes-hall↗