Biomedical subjects

M Shayegan

Publications and source records attributed to M Shayegan.

At least 19 recordsLinked to original sources

PubMed2006

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 an 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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PubMed2006

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 the 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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PubMed2005

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 nu = 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 nu = 1 are "valley Skyrmions."

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PubMed2005

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 the spikelike features observed at the edges of quantum Hall minima near Landau level crossings.

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PubMed2005

Thermopower of interacting GaAs bilayer hole systems in the reentrant insulating phase near nu=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 nu=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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PubMed2005

Spin polarization dependence of the coulomb drag at large r(s).

We find that the temperature dependence of the drag resistivity between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Our main observation is that near the apparent metal-insulator transition, the temperature dependence of the drag, given by Talpha, weakens significantly with the application of a parallel magnetic field (B(||)), with alpha saturating at half its zero field value for B(||)>B(*), where B(*) is the polarization field.

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PubMed2004

Ballistic electron transport in AlAs quantum wells.

We report the observation of commensurability oscillations in an AlAs two-dimensional electron system where two conduction-band valleys with elliptical in-plane Fermi contours are occupied. The Fourier power spectrum of the oscillations shows two frequency components consistent with those expected for the Fermi contours of the two valleys. From an analysis of the spectra we deduce m(l)/m(t)=5.2+/-0.5 for the ratio of the longitudinal and transverse electron effective masses, a fundamental parameter that cannot be directly measured from other transport experiments.

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PubMed2004

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 nu=1, demonstrating the pairing of oppositely charged carriers in opposite layers. The counterflow Hall resistance decreases much more strongly than the longitudinal resistances as the temperature is reduced.

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PubMed2004

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

We report measurements of the spin susceptibility, chi proportional, variant g(v)g*m*, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one ellipsoidal 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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PubMed2004

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 A 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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PubMed2004

Realization of an interacting two-valley AlAs bilayer system.

By using different widths for two AlAs quantum wells comprising a bilayer system, we force the X-point conduction-band electrons in the two layers to occupy valleys with different Fermi contours, electron effective masses, and g factors. Since the occupied valleys are at different X points of the Brillouin zone, the interlayer tunneling is negligibly small despite the close electron layer spacing. We demonstrate the realization of this system via magnetotransport measurements and the observation of a phase-coherent, bilayer nu=1 quantum Hall state flanked by a reentrant insulating phase.

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PubMed2004

Laterally modulated 2D electron system in the extreme quantum limit.

We report on magnetotransport of a two-dimensional electron system (2DES), located 32 nm below the surface, with a surface superlattice gate structure of periodicity 39 nm imposing a periodic modulation of its potential. For low Landau level fillings nu, the diagonal resistivity displays a rich pattern of fluctuations, even though the disorder dominates over the periodic modulation. Theoretical arguments based on the combined effects of the long-wavelength, strong disorder and the short-wavelength, weak periodic modulation present in the 2DES qualitatively explain the data.

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PubMed2003

Critical resistance in the AlAs quantum Hall ferromagnet.

Magnetic transitions in AlAs two-dimensional electrons give rise to sharp resistance spikes within the quantum Hall effect. Such spikes are likely caused by carrier scattering at magnetic domain walls below the Curie temperature. We report a critical behavior in the temperature dependence of the spike width and amplitude, from which we deduce the Curie temperature of the quantum Hall ferromagnet. Our data also reveal that the Curie temperature increases monotonically with carrier density.

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PubMed2003

Role of density imbalance in an interacting bilayer hole system.

We study interacting GaAs hole bilayers in the limit of zero interlayer tunneling. When the layers have equal density, we observe a phase-coherent bilayer quantum Hall state (QHS) at a total filling factor nu=1, flanked by a reentrant insulating phase at nearby fillings which suggests the formation of a pinned, bilayer Wigner crystal. As we transfer charge from one layer to another, the phase-coherent QHS becomes stronger, evincing its robustness against charge imbalance, but the insulating phase disappears, suggesting that its stability requires the commensurability of the two layers.

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PubMed2003

In-plane magnetodrag between dilute two-dimensional systems.

We performed in-plane magnetodrag measurements on dilute double layer two-dimensional hole systems, at in-plane magnetic fields that suppress the apparent metallic behavior, and to fields well above those required to fully spin polarize the system. When compared to the single layer magnetoresistance, the magnetodrag exhibits exactly the same qualitative behavior. In addition, we have found that the enhancement to the drag from the in-plane field exhibits a strong maximum when both layer densities are matched.

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PubMed2002

NMR investigation of how free composite fermions are at nu=1 / 2.

NMR measurements of the electron spin polarization (P) have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature dependence of P to models of free and interacting composite fermions (CF), the imbalance of spin-up and spin-down CF Fermi seas is mapped as a function of Zeeman energy. Independent measurements of the CF effective mass, g factor, and Fermi energy are obtained from the thermal activation of P in tilted fields. The filling factor dependence of the P for 2 / 5<nu<2 / 3 reveals a broken particle-hole symmetry for the partially polarized CF Fermi sea.

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PubMed2002

Valley splitting of AlAs two-dimensional electrons in a perpendicular magnetic field.

By measuring the angles at which the Landau levels overlap in tilted magnetic fields (the coincidence method), we determine the splitting of the conduction-band valleys in high-mobility two-dimensional electrons confined to AlAs quantum wells. The data reveal that, while the valleys are nearly degenerate in the absence of magnetic field, they split as a function of perpendicular magnetic field. The splitting appears to depend primarily on the magnitude of the perpendicular component of the magnetic field, suggesting electron-electron interaction as its origin.

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PubMed2002

Frictional drag between two dilute two-dimensional hole layers.

We report drag measurements on dilute double layer two-dimensional hole systems in the regime of r(s) = 19-39. We observed a strong enhancement of the drag over the simple Boltzmann calculations of Coulomb interaction, and deviations from the T2 dependence which cannot be explained by phonon-mediated, plasmon-enhanced, or disorder-related processes. We suggest that this deviation results from interaction effects in the dilute regime.

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