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M. Shayegan

Publications and source records attributed to M. Shayegan.

At least 145 records · Page 8Linked to original sources
arXiv2006

High-mobility AlAs quantum wells with out-of-plane valley occupation

Employing state-of-the-art molecular beam epitaxy techniques to grow thin, modulation-doped AlAs quantum wells, we have achieved a low temperature mobility of 5.5 m$^2$/Vs with out-of-plane occupation, an order of magnitude improvement over previous studies. However, due to the narrow well width, mobilities are still limited by scattering due to interface roughness disorder. We demonstrate the successful implementation of a novel technique utilizing thermally-induced, biaxial, tensile strain that forces electrons to occupy the out-of-plane valley in thicker quantum wells, reducing interface roughness scattering and allowing us to achieve mobilities as high as 8.8 m$^2$/Vs.

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arXiv2006

Dependence of persistent gaps at Landau level crossings on relative spin

We report measurements of the quantum Hall state energy gap at avoided crossings between Landau levels originating from different conduction band valleys in AlAs quantum wells. These gaps exhibit an approximately linear dependence on magnetic field over a wide range of fields and filling factors. More remarkably, we observe an unexpected dependence of the gap size on the relative spin orientation of the crossing levels, with parallel-spin crossings exhibiting larger gaps than antiparallel-spin crossings.

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arXiv2006

Valley susceptibility of an interacting two-dimensional electron system

We report direct measurements of the valley susceptibility, the change of valley population in response to applied symmetry-breaking strain, in an AlAs two-dimensional electron system. As the two-dimensional density is reduced, the valley susceptibility dramatically increases relative to its band value, reflecting the system's strong electron-electron interaction. The increase has a remarkable resemblance to the enhancement of the spin susceptibility and establishes the analogy between the spin and valley degrees of freedom.

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arXiv2005

Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems

We report measurements and calculations of the spin-subband depopulation, induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole systems. The results reveal that the shape of the confining potential dramatically affects the values of in-plane magnetic field at which the upper spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the carrier-carrier interaction in 2D hole systems does not significantly enhance the spin susceptibility. We interpret our findings using a multipole expansion of the spin density matrix, and suggest that the suppression of the enhancement is related to the holes' band structure and effective spin j=3/2.

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arXiv2005

Observation of Quantum Hall Valley Skyrmions

We report measurements of the interaction-induced quantum Hall effect in a spin-polarized AlAs two-dimensional electron system where the electrons occupy two in-plane conduction band valleys. Via the application of in-plane strain, we tune the energies of these valleys and measure the energy gap of the quantum Hall state at filling factor $ν$ = 1. The gap has a finite value even at zero strain and, with strain, rises much faster than expected from a single-particle picture, suggesting that the lowest energy charged excitations at $ν=1$ are "valley Skyrmions".

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arXiv2005

Interaction and disorder in bilayer counterflow transport at filling factor one

We study high mobility, interacting GaAs bilayer hole systems exhibiting counterflow superfluid transport at total filling factor $ν=1$. As the density of the two layers is reduced, making the bilayer more interacting, the counterflow Hall resistivity ($ρ_{xy}$) decreases at a given temperature, while the counterflow longitudinal resistivity ($ρ_{xx}$), which is much larger than $ρ_{xy}$, hardly depends on density. On the other hand, a small imbalance in the layer densities can result in significant changes in $ρ_{xx}$ at $ν=1$, while $ρ_{xy}$ remains vanishingly small. Our data suggest that the finite $ρ_{xx}$ at $ν=1$ is a result of mobile vortices in the superfluid created by the ubiquitous disorder in this system.

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arXiv2005

Spin-dependent resistivity at transitions between integer quantum Hall states

The longitudinal resistivity at transitions between integer quantum Hall states in two-dimensional electrons confined to AlAs quantum wells is found to depend on the spin orientation of the partially-filled Landau level in which the Fermi energy resides. The resistivity can be enhanced by an order of magnitude as the spin orientation of this energy level is aligned with the majority spin. We discuss possible causes and suggest a new explanation for spike-like features observed at the edges of quantum Hall minima near Landau level crossings.

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arXiv2004

Thermopower of Interacting GaAs Bilayer Hole Systems in the Reentrant Insulating Phase near $ν=1$

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor $ν=1$. Our data show that as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.

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arXiv2004

Coulomb Drag near the metal-insulator transition in two-dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the $T^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single layer resistivity, suggesting they both arise from the same origin. In addition, layer spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single layer resistivity.

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arXiv2004

Low-temperature, in situ tunable, uniaxial stress measurements in semiconductors using a piezoelectric actuator

We demonstrate the use of a piezoelectric actuator to apply, at low temperatures, uniaxial stress in the plane of a two-dimensional electron system confined to a modulation-doped AlAs quantum well. Via the application of stress, which can be tuned in situ and continuously, we control the energies and occupations of the conduction-band minima and the electronic properties of the electron system. We also report measurements of the longitudinal and transverse strain versus bias for the actuator at 300, 77, and 4.2 K. A pronounced hysteresis is observed at 300 and 77 K, while at 4.2 K, strain is nearly linear and shows very little hysteresis with the applied bias.

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arXiv2004

Giant low-temperature piezoresistance effect in AlAs two-dimensional electrons

We present piezoresistance measurements in modulation doped AlAs quantum wells where the two-dimensional electron system occupies two conduction band valleys with elliptical Fermi contours. Our data demonstrate that, at low temperatures, the strain gauge factor (the fractional change in resistance divided by the sample's fractional length change) in this system exceeds 10,000. Moreover, in the presence of a moderate magnetic field perpendicular to the plane of the two-dimensional system, gauge factors up to 56,000 can be achieved. The piezoresistance data can be explained qualitatively by a simple model that takes into account intervalley charge transfer.

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arXiv2004

Negative differential Rashba effect in two-dimensional hole systems

We demonstrate experimentally and theoretically that two-dimensional (2D) heavy hole systems in single heterostructures exhibit a \emph{decrease} in spin-orbit interaction-induced spin splitting with an increase in perpendicular electric field. Using front and back gates, we measure the spin splitting as a function of applied electric field while keeping the density constant. Our results are in contrast to the more familiar case of 2D electrons where spin splitting increases with electric field.

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arXiv2004

Spin Splitting in GaAs (100) Two-Dimensional Holes

We measured Shubnikov-de Haas (SdH) oscillations in GaAs (100) two-dimensional holes to determine the inversion asymmetry-induced spin splitting. The Fourier spectrum of the SdH oscillations contains two peaks, at frequencies $f_-$ and $f_+$, that correspond to the hole densities of the two spin subbands and a peak, at frequency $f_\mathrm{tot}$, corresponding to the total hole density. In addition, the spectrum exhibits an anomalous peak at $f_\mathrm{tot}/2$. We also determined the effective masses of the two spin subbands by finding the inverse transform of the Fourier spectrum in the vicinity of $f_-$ and $f_+$, and then analyzing the temperature dependence of the SdH oscillations for each subband. We discuss our results in light of self-consistent calculations and previous experiments.

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arXiv2004

Counterflow measurements in strongly correlated GaAs hole bilayers: evidence for electron-hole pairing

We study interacting GaAs bilayer hole systems, with very small interlayer tunneling, in a counterflow geometry where equal currents are passed in opposite directions in the two, independently contacted layers. At low temperatures, both the longitudinal and Hall counterflow resistances tend to vanish in the quantum Hall state at total bilayer filling $ν=1$, demonstrating the pairing of oppositely charged carriers in opposite layers. The temperature dependence of the counterflow Hall resistance is anomalous compared to the other transport coefficients: even at relatively high temperatures ($\sim$600mK), it develops a very deep minimum, with a value that is about an order of magnitude smaller than the longitudinal counterflow resistivity.

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arXiv2004

Spin Polarization Dependence of the Coulomb Drag at Large $r_{s}$

We find that the temperature dependence of the drag resistivity ($ρ_{D}$) between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Near the apparent metal-insulator transition, the temperature dependence of the drag, given by $T^α$, weakens with the application of a parallel magnetic field ($B_{||}$), with $α$ saturating at half its zero field value for $B_{||} > B^{*}$, where $B^{*}$ is the polarization field. Furthermore, we find that $α$ is roughly 2 at the parallel field induced metal-insulator transition, and that the temperature dependence of $ρ_{D}/T^{2}$ at different $B_{||}$ looks strikingly similar to that found in the single layer resistivity. In contrast, at higher densities, far from the zero field transition, the temperature dependence of the drag is roughly independent of spin polarization, with $α$ remaining close to 2, as expected from a simple Fermi liquid picture.

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arXiv2004

Spin susceptibility of two-dimensional electrons in narrow AlAs quantum wells

We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45$Å$ wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band valley, rendering a system similar to two-dimensional electrons in Si-MOSFETs but with only one valley occupied. We observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased, following closely the prediction of quantum Monte Carlo calculations and continuing at finite values through the metal-insulator transition.

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arXiv2004

Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom

We report measurements of the spin susceptibility, $χ\propto g_v g^*m^*$, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one conduction-band valley to another ($g_v$ is the valley degeneracy, and $m^*$ and $g^*$ are the electron effective mass and g-factor). At a given density, when the two valleys are equally populated ($g_v=2$), the measured $g^*m^*$ is smaller than when only one valley is occupied ($g_v=1$). This observation counters the common assumption that a two-valley two-dimensional system is effectively more dilute than a single-valley system because of its smaller Fermi energy.

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