Shabani, J., Gokmen, T., Chiu, Y. T., Shayegan, M.
We report the observation of developing fractional quantum Hall states at Landau level filling factors $\nu = 1/2$ and 1/4 in electron systems confined to wide GaAs quantum wells with significantly...
Density dependence of valley polarization energy for composite fermions (2009)
Padmanabhan, Medini, Gokmen, T., Shayegan, M.
In two-dimensional electron systems confined to wide AlAs quantum wells, composite fermions around the filling factor $\nu$ = 3/2 are fully spin polarized but possess a valley degree of freedom. Here...
Gokmen, T., Padmanabhan, Medini, Vakili, K., Tutuc, E., Shayegan, M.
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...
Shabani, J., Gokmen, T., Shayegan, M.
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 ($\nu$) display a remarkable dependence...
Effective mass suppression in a ferromagnetic two-dimensional electron liquid (2009)
Asgari, Reza, Gokmen, T., Tanatar, B., Padmanabhan, Medini, Shayegan, M.
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...
Tsukazaki, A., Ohtomo, A., Kawasaki, M., Akasaka, S., Yuji, H., Tamura, K., ...
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,...
Parallel Magnetic Field Tuning of Valley Splitting in AlAs Two-Dimensional Electrons (2008)
Gokmen, T., Padmanabhan, Medini, Gunawan, O., Shkolnikov, Y. P., Vakili, K., De Poortere, E. P., ...
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...
Spin Orientation of Holes in Quantum Wells (2008)
Winkler, R., Culcer, Dimitrie, Papadakis, S. J., Habib, B., Shayegan, M.
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...
Enhancement of valley susceptibility upon complete spin-polarization (2008)
Padmanabhan, Medini, Gokmen, T., Shayegan, M.
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...
Dependence of Effective Mass on Spin and Valley Degrees of Freedom (2008)
Gokmen, T., Padmanabhan, Medini, Shayegan, M.
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...
High-quality quantum point contact in two-dimensional GaAs (311)A hole system (2008)
Shabani, J., Petta, J. R., Shayegan, M.
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...
Giant frictional drag in strongly interacting bilayers near filling factor one (2008)
Tutuc, E., Pillarisetty, R., Shayegan, M.
We study the frictional drag in high mobility, strongly interacting GaAs bilayer hole systems in the vicinity of the filling factor $\nu=1$ quantum Hall state (QHS), at the same fillings where the...
NONLINEAR TRANSPORT IN BALLISTIC SEMICONDUCTOR DIODES WITH NEGATIVE EFFECTIVE MASS CARRIERS (2008)
B. R. Perkins, Jun Liu, A. Zaslavsky, Z. S. Gribnikov, V. V. Mitin, ...
We are pursuing a new class of solid-state devices capable of generating highfrequency radiation: short ballistic semiconductor diodes in which the current flow is due to carriers injected into a...
Effective mass suppression in dilute, spin-polarized two-dimensional electron systems (2008)
Padmanabhan, Medini, Gokmen, T., Bishop, N. C., Shayegan, M.
We report effective mass (m*) measurements, via analyzing the temperature dependence of the Shubnikov-de Haas oscillations, for dilute, interacting, two-dimensional electron systems (2DESs) occupying...
Dynamics of Density Imbalanced Bilayer Holes in the Quantum Hall Regime (2008)
Misra, S., Bishop, N. C., Tutuc, E., Shayegan, M.
We report magnetotransport measurements on bilayer GaAs hole systems with unequal hole concentrations in the two layers. At magnetic fields where one layer is in the integer quantum Hall state and...
Strain-induced Fermi contour anisotropy of GaAs 2D holes (2008)
Shabani, J., Shayegan, M., Winkler, R.
We report measurements of magneto-resistance commensurability peaks, induced by a square array of anti-dots, in GaAs (311)A two-dimensional holes as a function of applied in-plane strain. The data...
Spin susceptibility of interacting two-dimensional electrons with anisotropic effective mass (2008)
Gokmen, T., Padmanabhan, Medini, Tutuc, E., Shayegan, M., De Palo, Stefania, Moroni, Saverio, ...
We report measurements of the spin susceptibility in dilute (rs up to 10) AlAs two-dimensional (2D) electrons occupying a single conduction-band valley with an anisotropic in-plane Fermi contour,...
Spin Susceptibility of Interacting Two-dimensional Electrons with Anisotropic Effective Mass (2007)
Gokmen, T., Padmanabhan, Medini, Tutuc, E., Shayegan, M., De Palo, S., Moroni, S., ...
We report measurements of the spin susceptibility in dilute (rs up to 10) AlAs two-dimensional (2D) electrons occupying a single conduction-band valley with an anisotropic in-plane Fermi contour,...
NMR Study of Large Skyrmions in Al_{0.13}Ga_{0.87}As Quantum Wells (2007)
Mitrović, V. F., Horvatić, M., Berthier, C., Lyon, S. A., Shayegan, M.
A nuclear magnetic resonance (NMR) study is reported of multiple (30) Al$_{0.13}$Ga$_{0.87}$As quantum well (QW) sample near the Landau level filling factor $\nu =1$. In these...
Anomalous giant piezoresistance in AlAs 2D electrons with anti-dot lattices (2007)
Gunawan, O., Gokmen, T., Shkolnikov, Y. P., De Poortere, E. P., Shayegan, M.
An AlAs two-dimensional electron system patterned with an anti-dot lattice exhibits a giant piezoresistance (GPR) effect, with a sign opposite to the piezoresistance observed in the unpatterned...
Valley polarization and susceptibility of composite fermions around nu=3/2 (2007)
Bishop, N. C., Padmanabhan, M., Vakili, K., Shkolnikov, Y. P., De Poortere, E. P., Shayegan, M.
We report magnetotransport measurements of fractional quantum Hall states in an AlAs quantum well around Landau level filling factor nu = 3/2, demonstrating that the quasiparticles are composite...
Tunneling between Dilute GaAs Hole Layers (2007)
Misra, S., Bishop, N. C., Tutuc, E., Shayegan, M.
We report interlayer tunneling measurements between very dilute two-dimensional GaAs hole layers. Surprisingly, the shape and temperature-dependence of the tunneling spectrum can be explained with a...
Anisotropic low-temperature piezoresistance in (311)A GaAs two-dimensional holes (2007)
Habib, B., Shabani, J., De Poortere, E. P., Shayegan, M., Winkler, R.
We report low-temperature resistance measurements in a modulation-doped, (311)A GaAs two-dimensional hole system as a function of applied in-plane strain. The data reveal a strong but anisotropic...
Strong Aharonov-Bohm oscillations in GaAs two-dimensional holes (2006)
Habib, B., Tutuc, E., Shayegan, M.
We measured Aharonov-Bohm resistance oscillations in a shallow two-dimensional GaAs hole ring structure, defined by local anodic surface oxidation. The amplitude of the oscillations is about 10% of...
Pinning modes and interlayer correlation in high magnetic field bilayer Wigner solids (2006)
Wang, Zhihai, Chen, Yong P., Engel, L. W., Tsui, D. C., Tutuc, E., Shayegan, M.
We report studies of pinning mode resonances in the low total Landau filling (\nu) Wigner solid of a series of bilayer hole samples with negligible interlayer tunneling, and with varying interlayer...
Tuning of the spin-orbit interaction in two-dimensional GaAs holes via strain (2006)
Habib, B., Shabani, J., De Poortere, E. P., Shayegan, M., Winkler, R.
We report direct measurements of the spin-orbit interaction induced spin-splitting in a modulation-doped GaAs two-dimensional hole system as a function of anisotropic, in-plane strain. The change in...
Dephasing Time of Two-Dimensional Holes in GaAs Open Quantum Dots (2006)
Faniel, S., Hackens, B., Vlad, A., Moldovan, L., Gustin, C., Habib, B., ...
We report magnetotransport measurements of two-dimensional holes in open quantum dots, patterned either as a single-dot or an array of dots, on a GaAs quantum well. For temperatures $T$ below 500 mK,...
Spin-valley phase diagram of the two-dimensional metal-insulator transition (2006)
Gunawan, O., Gokmen, T., Vakili, K., Padmanabhan, M., De Poortere, E. P., Shayegan, M.
Using symmetry breaking strain to tune the valley occupation of a two-dimensional (2D) electron system in an AlAs quantum well, together with an applied in-plane magnetic field to tune the spin...
AlAs 2D electrons in an antidot lattice: Electron pinball with elliptical Fermi contours (2006)
Gunawan, O., De Poortere, E. P., Shayegan, M.
We report ballistic transport measurements in a two-dimensional electron system confined to an AlAs quantum well and patterned with square antidot lattices of period $a = $0.6, 0.8, 1.0 and 1.5...
Quantized conductance in an AlAs 2D electron system quantum point contact (2006)
Gunawan, O., Habib, B., De Poortere, E. P., Shayegan, M.
We report experimental results on a quantum point contact (QPC) device formed in a wide AlAs quantum well where the two-dimensional electrons occupy two in-plane valleys with elliptical Fermi...
Two-dimensional electrons occupying multiple valleys in AlAs (2006)
Shayegan, M., De Poortere, E. P., Gunawan, O., Shkolnikov, Y. P., Tutuc, E., Vakili, K.
Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the X- points of the Brillouin zone. These valleys have large effective mass and g-factor compared to the...
Zeeman splitting of interacting two-dimensional electrons with two effective masses (2006)
Vakili, K., Tutuc, E., Shayegan, M.
We have realized an AlAs two-dimensional electron system in which electrons occupy conduction-band valleys with different Fermi contours and effective masses. In the quantum Hall regime, we observe...
High-mobility AlAs quantum wells with out-of-plane valley occupation (2006)
Vakili, K., Shkolnikov, Y. P., Tutuc, E., De Poortere, E. P., Padmanabhan, M., Shayegan, M.
Employing state-of-the-art molecular beam epitaxy techniques to grow thin, modulation-doped AlAs quantum wells, we have achieved a low temperature mobility of 5.5 m$^2$/Vs with out-of-plane...
Dependence of persistent gaps at Landau level crossings on relative spin (2006)
Vakili, K., Gokmen, T., Gunawan, O., Shkolnikov, Y. P., De Poortere, E. P., Shayegan, M.
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...
Valley susceptibility of an interacting two-dimensional electron system (2006)
Gunawan, O., Shkolnikov, Y. P., Vakili, K., Gokmen, T., De Poortere, E. P., Shayegan, M.
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...
Flatland Electrons in High Magnetic Fields (2005)
This paper provides a review of developments in the physics of two-dimensional electron systems in perpendicular magnetic fields.
Observation of Quantum Hall Valley Skyrmions (2005)
Shkolnikov, Y. P., Misra, S., Bishop, N. C., De Poortere, E. P., Shayegan, M.
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...
Interaction and disorder in bilayer counterflow transport at filling factor one (2005)
We study high mobility, interacting GaAs bilayer hole systems exhibiting counterflow superfluid transport at total filling factor $\nu=1$. As the density of the two layers is reduced, making the...
Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems (2005)
Winkler, R., Tutuc, E., Papadakis, S. J., Melinte, S., Shayegan, M., Wasserman, D., ...
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...
Spin-dependent resistivity at transitions between integer quantum Hall states (2005)
Vakili, K., Shkolnikov, Y. P., Tutuc, E., Bishop, N. C., De Poortere, E. P., Shayegan, M.
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...
Faniel, S., Tutuc, E., De Poortere, E. P., Gustin, C., Vlad, A., Melinte, S., ...
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...
Shayegan, M., Karrai, K., Shkolnikov, Y. P., Vakili, K., De Poortere, E. P., Manus, S.
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....
Giant low-temperature piezoresistance effect in AlAs two-dimensional electrons (2004)
Shkolnikov, Y. P., Vakili, K., De Poortere, E. P., Shayegan, M.
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...
Spin Splitting in GaAs (100) Two-Dimensional Holes (2004)
Habib, B., Tutuc, E., Melinte, S., Shayegan, M., Wasserman, D., Lyon, S. A., ...
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...
Negative differential Rashba effect in two-dimensional hole systems (2004)
Habib, B., Tutuc, E., Melinte, S., Shayegan, M., Wasserman, D., Lyon, S. A., ...
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...
Spin Polarization Dependence of the Coulomb Drag at Large $r_{s}$ (2004)
Pillarisetty, R., Noh, H., Tutuc, E., De Poortere, E. P., Tsui, D. C., Shayegan, M.
We find that the temperature dependence of the drag resistivity ($\rho_{D}$) between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Near the apparent...
Spin susceptibility of two-dimensional electrons in narrow AlAs quantum wells (2004)
Vakili, K., Shkolnikov, Y. P., Tutuc, E., De Poortere, E. P., Shayegan, M.
We report measurements of the spin susceptibility in dilute two-dimensional electrons confined to a 45$\AA$ wide AlAs quantum well. The electrons in this well occupy an out-of-plane conduction-band...
Shkolnikov, Y. P., Vakili, K., De Poortere, E. P., Shayegan, M.
We report measurements of the spin susceptibility, $\chi\propto g_v g^*m^*$, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one...
Coulomb Drag near the metal-insulator transition in two-dimensions (2004)
Pillarisetty, R., Noh, H., Tutuc, E., De Poortere, E. P., Lai, K., Tsui, D. C., ...
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...
Ballistic transport in AlAs two-dimensional electrons (2004)
Gunawan, O., Shkolnikov, Y. P., De Poortere, E. P., Tutuc, E., Shayegan, M.
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...
Tutuc, E., Shayegan, M., Huse, D. A.
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...
A Laterally Modulated 2D Electron System in the Extreme Quantum Limit (2003)
Melinte, Sorin, Berciu, Mona, Zhou, C., Tutuc, E., Papadakis, S. J., Harrison, C., ...
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...
A Hybrid Al(0.10)Ga(0.90)As/AlAs Bilayer Electron System (2003)
De Poortere, E. P., Shayegan, M.
We have fabricated a device composed of two closely coupled two-dimensional electron systems, one of which resides within an AlAs quantum well (QW) at the X point of the Brillouin zone (BZ), while...
Realization of an Interacting Two-Valley AlAs Bilayer System (2003)
Vakili, K., Shkolnikov, Y. P., Tutuc, E., De Poortere, E. P., Shayegan, M.
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,...
Layer charge instability in unbalanced bilayer systems in the quantum Hall regime (2003)
Tutuc, E., Pillarisetty, R., Melinte, S., De Poortere, E. P., Shayegan, M.
Measurements in GaAs hole bilayers with unequal layer densities reveal a pronounced magneto-resistance hysteresis at the magnetic field positions where either the majority or minority layer is at...
Critical Resistance of the Quantum Hall Ferromagnet in AlAs 2D Electrons (2003)
De Poortere, E. P., Tutuc, E., Shayegan, M.
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...
Field-Effect Persistent Photoconductivity (2003)
De Poortere, E. P., Shkolnikov, Y. P., Shayegan, M.
We report a persistent increase or decrease in the two-dimensional electron density of AlAs or GaAs quantum wells flanked by AlGaAs barriers, brought about by illuminating the samples at T ~ 4 K...
In-Plane Magnetodrag between Dilute Two-Dimensional Systems (2003)
Pillarisetty, R., Noh, Hwayong, Tutuc, E., De Poortere, E. P., Tsui, D. C., Shayegan, M.
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...
Tutuc, E., Melinte, S., De Poortere, E. P., Shayegan, M., Winkler, R.
We report measurements and calculations of the spin-polarization, induced by a parallel magnetic field, of interacting, dilute, two-dimensional electron systems confined to GaAs/AlGaAs...
Role of density imbalance in an interacting bilayer hole system (2002)
Tutuc, E., Melinte, S., De Poortere, E. P., Pillarisetty, R., Shayegan, M.
We study interacting GaAs hole bilayers in the limit of zero tunneling. When the layers have equal densities, we observe a phase coherent bilayer quantum Hall (QH) state at total filling factor...
Magnetic Field Induced Spin Polarization of AlAs Two-dimensional Electrons (2002)
De Poortere, E. P., Tutuc, E., Shkolnikov, Y. P., Vakili, K., Shayegan, M.
Two-dimensional (2D) electrons in an in-plane magnetic field become fully spin polarized above a field B_P, which we can determine from the in-plane magnetoresistance. We perform such measurements in...
Valley splitting of AlAs two-dimensional electrons in a perpendicular magnetic field (2002)
Shkolnikov, Y. P., De Poortere, E. P., Tutuc, E., Shayegan, M.
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...
How free are Composite Fermions at $\nu = 1/2$? A NMR investigation (2002)
Freytag, N., Horvatić, M., Berthier, C., Shayegan, M., Lévy, L. P.
NMR measurements of the electron spin polarization have been performed on a 2D electron system at and around half-filled lowest Landau level. Comparing the magnetic field and the temperature...
Spin polarization and transition from metallic to insulating behavior in 2D systems (2002)
Tutuc, E., De Poortere, E. P., Papadakis, S. J., Shayegan, M.
We have made quantitative measurements of the spin polarization of two-dimensional (2D) GaAs (100) electrons and GaAs (311)A holes, as a function of an in-plane magnetic field. The functional form of...
Aharonov-Bohm Oscillations with Spin: Evidence for Berry's Phase (2002)
Yau, Jeng-Bang, De Poortere, E. P., Shayegan, M.
We report a study of the Aharonov-Bohm effect, the oscillations of the resistance of a mesoscopic ring as a function of a perpendicular magnetic field, in a GaAs two-dimensional hole system with a...
Frictional Drag between Two Dilute Two-Dimensional Hole Layers (2002)
Pillarisetty, R., Noh, Hwayong, Tsui, D. C., De Poortere, E. P., Tutuc, E., Shayegan, M.
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...
Low-field magnetoresistance in GaAs 2D holes (2002)
Papadakis, S. J., De Poortere, E. P., Manoharan, H. C., Yau, J. B., Shayegan, M., Lyon, S. A.
We report low-field magnetotransport data in two-dimensional hole systems in GaAs/AlGaAs heterostructures and quantum wells, in a large density range, $2.5 \times 10^{10} \leq p \leq 4.0 \times...
NMR investigation of how free composite fermions are at ν=1/2 (2002)
Freytag, N., Horvatić, M., Berthier, C., Shayegan, M., Lévy, L. P.
NMR measurements of the electron spin polarization (P) have been performed on a 2D electron system at and around half- filled lowest Landau level. Comparing the magnetic field and the temperature...
Unpolarizable ground states in the fractional quantum Hall effect at ν=2/3 (2002)
Freytag, N., Tokunaga, Y., Horvatic, M., Berthier, C., Shayegan, M., Lévy, L. P.
Glicofridis, P. I., Finkelstein, G., Ashoori, R. C., Shayegan, M.
Using charge accumulation imaging, we measure the charge flow across an incompressible strip and follow its evolution with magnetic field. The strip runs parallel to the edge of a gate deposited on...
Spin polarization and g-factor of a dilute GaAs two-dimensional electron system (2001)
Tutuc, E., Melinte, S., Shayegan, M.
The effective g-factor ($g^{*}$) of a dilute interacting two-dimensional electron system is expected to increase with respect to its bare value as the density is lowered, and to eventually diverge as...
Anomalous Rashba spin splitting in two-dimensional hole systems (2001)
Winkler, R., Noh, H., Tutuc, E., Shayegan, M.
It has long been assumed that the inversion asymmetry-induced Rashba spin splitting in two-dimensional (2D) systems at zero magnetic field is proportional to the electric field that characterizes the...
Freytag, N., Tokunaga, Y., Horvatic, M., Berthier, C., Shayegan, M., Levy, L.
The average electron spin-polarization $\cal P$ of two-dimensional electron gas confined in $\rm GaAs/GaAlAs$ multiple quantum-wells was measured by nuclear magnetic resonance (NMR) near the...
Noh, H., Yoon, Jongsoo, Tsui, D. C., Shayegan, M.
We studied low temperature (T=50mK) in-plane magnetoresistance of a dilute two-dimensional hole system in GaAs/AlGaAs heterostructure that exhibits an apparent metal-insulator transition. We found an...
Unpolarizable ground states in the fractional quantum Hall effect at ν=2/3 (2001)
Freytag, N., Tokunaga, Y., Horvatić, M., Berthier, C., Shayegan, M., Lévy, L.
Magnetization and orbital properties of the two-dimensional electron gas in the quantum limit (2001)
Wiegers, S., Bibow, E., Lévy, L. P., Bayot, V., Simmons, M., Shayegan, M.
Resistance Spikes at Transitions between Quantum Hall Ferromagnets (2000)
De Poortere, E. P., Tutuc, E., Papadakis, S. J., Shayegan, M.
We report a new manifestation of first-order magnetic transitions in two-dimensional electron systems. This phenomenon occurs in aluminum arsenide quantum wells with sufficiently low carrier...
Tutuc, E., De Poortere, E. P., Papadakis, S. J., Shayegan, M.
Using a novel technique, we make quantitative measurements of the spin polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional holes as a function of an in-plane magnetic...
Confinement symmetry, mobility anisotropy, and metallic behavior in (311)A GaAs 2D holes (2000)
Papadakis, S. J., De Poortere, E. P., Shayegan, M.
We study two dimensional hole systems confined to GaAs quantum wells grown on the (311)A surface of GaAs substrates. Such samples exhibit an in-plane mobility anisotropy. At constant 2D hole density,...
Topographic Mapping of the Quantum Hall Liquid using a Few-Electron Bubble (2000)
Finkelstein, G., Glicofridis, P. I., Ashoori, R. C., Shayegan, M.
A scanning probe technique was used to obtain a high-resolution map of the random electrostatic potential inside the quantum Hall liquid. A sharp metal tip, scanned above a semiconductor surface,...
Spin-splitting in GaAs 2D holes (2000)
Papadakis, S. J., De Poortere, E. P., Shayegan, M., Winkler, R.
We present quantitative measurements and calculations of the spin-orbit induced zero-magnetic-field spin-splitting in two-dimensional (2D) hole systems in modulation-doped GaAs (311)A quantum wells....
Highly anisotropic g-factor of two-dimensional hole systems (2000)
Winkler, R., Papadakis, S. J., De Poortere, E. P., Shayegan, M.
Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations...
NMR determination of 2D electron spin polarization at ν=1/2 (2000)
Melinte, S., Freytag, N., Horvatić, M., Berthier, C., Lévy, L. P., Bayot, V., ...
Anisotropic magnetoresistance of GaAs two-dimensional holes (1999)
Papadakis, S. J., De Poortere, E. P., Shayegan, M., Winkler, R.
Experiments on high-quality GaAs (311)A two-dimensional holes at low temperatures reveal a remarkable dependence of the magnetoresistance, measured with an in-plane magnetic field ($B$), on the...
Spin-resolved commensurability oscillations (1999)
Lu, J., Shayegan, M., Wissinger, L., Rössler, Ulrich, Winkler, R.
We report the observation of commensurability oscillations, induced by a one-dimensional periodic potential, in a high-mobility GaAs two-dimensional hole system confined to a triangular quantum well....
Anomalous magneto-oscillations in two-dimensional systems (1999)
Winkler, R., Papadakis, S. J., De Poortere, E. P., Shayegan, M.
The frequencies of Shubnikov-de Haas oscillations have long been used to measure the unequal population of spin-split two-dimensional subbands in inversion asymmetric systems. We report...
Large Skyrmions in an Al(0.13)Ga(0.87)As Quantum Well (1999)
Shukla, S. P., Shayegan, M., Parihar, S. R., Lyon, S. A., Cooper, N. R., Kiselev, A. A.
We report tilted-field magnetotransport measurements of two-dimensional electron systems in a 200 Angstrom-wide Al(0.13)Ga(0.87)As quantum well. We extract the energy gap for the quantum Hall state...
NMR Determination of 2D Electron Spin Polarization at $\nu=1/2$ (1999)
Melinte, S., Freytag, N., Horvatic, M., Berthier, C., Levy, L. P., Bayot, V., ...
Using a `standard' NMR spin-echo technique we determined the spin polarization of two-dimensional electrons, confined to GaAs quantum wells, from the hyperfine shift of Ga nuclei in the wells....
Yoon, Jongsoo, Li, C. C., Shahar, D., Tsui, D. C., Shayegan, M.
A magnetic field applied parallel to the two-dimensional hole system in the GaAs/AlGaAs heterostructure, which is metallic in the absence of an external magnetic field, can drive the system into...
Spin resolved ballistic transport in GaAs two-dimensional holes (1999)
Lu, J., Shayegan, M., Wiesinger, L., Rössler, Ulrich, Winkler, R.
We report the observation of commensurability oscillations, induced by a one-dimensional periodic potential, in high-mobility GaAs two-dimensional hole system confined to a triangular quantum well....
Apparent Metallic Behavior at B = 0 of a two-dimensional electron system in AlAs (1999)
Papadakis, S. J., Shayegan, M.
We report the observation of metallic-like behavior at low temperatures and zero magnetic field in two dimensional (2D) electrons in an AlAs quantum well. At high densities the resistance of the...
The Effect of Spin Splitting on the Metallic Behavior of a Two-Dimensional System (1999)
Papadakis, S. J., De Poortere, E. P., Manoharan, H. C., Shayegan, M., Winkler, R.
Experiments on a constant-density two-dimensional hole system in a GaAs quantum well reveal that the metallic behavior observed in the zero-magnetic-field temperature dependence of the resistivity...
Anisotropic Transport of Two-Dimensional Holes in High Landau Levels (1999)
Shayegan, M., Manoharan, H. C., Papadakis, S. J., DePoortere, E. P.
Magnetoresistance data taken along $[\bar{2}33]$ and $[01\bar{1}]$ directions in a GaAs/AlGaAs two-dimensional hole sample with van der Pauw geometry exhibit significant anisotropy at half-integer...
Magnetic Properties of CoCl2-Intercalated Graphite, (1998)
Elahy,M., Shayegan,M., Szeto,K. Y., Dresselhaus,G.
The temperature and magnetic field dependence of the magnetic susceptibility and heat capacity of graphite-CoC12 are investigated to understand the role of dimensionality in the observed magnetic...
Electron Transport in Heterojunction Superlattices. (1998)
This is a final report on the research carried out under the ARO contract DAAL03-89-K-0036 (ARO proposal number 26015-PH) from April 15, 1989 to August 31, 1992.It describes briefly three...
Low-Dimension Electronic Precesses in High Mobility GaAs/Al(x) Ga(1-x)As Nanostructures (1998)
This final report briefly describes the construction and installation of an in situ MBE cleave-edge overgrowth apparatus and accomplishments in the following two research areas: (1) Novel...
Heat Capacity Evidence for the Suppression of Skyrmions at Large Zeeman Energy (1998)
Melinte, S., Grivei, E., Bayot, V., Shayegan, M.
Measurements on a multilayer two-dimensional electron system (2DES) near Landau level filling $\nu$=1 reveal the disappearance of the nuclear spin contribution to the heat capacity as the ratio...
Semicircle: An exact relation in the Integer and Fractional Quantum Hall Effect (1998)
Hilke, M., Shahar, D., Song, S. H., Tsui, D. C., Xie, Y. H., Shayegan, M.
We present experimental results on the quantized Hall insulator in two dimensions. This insulator, with vanishing conductivities, is characterized by the quantization (within experimental accuracy)...
Yoon, J., Engel, L. W., Shahar, D., Tsui, D. C., Shayegan, M.
Microwave frequency conductivity, Re($\sigma_{xx}$), of high quality two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B) is measured with the carrier density ($n_s$) of...
Anomalous spin-splitting of two-dimensional electrons in an AlAs Quantum Well (1998)
Papadakis, S. J., De Poortere, E. P., Shayegan, M.
We measure the effective Lande g-factor of high-mobility two-dimensional electrons in a modulation-doped AlAs quantum well by tilting the sample in a magnetic field and monitoring the evolution of...
Lu, Jian Ping, Yau, Jeng-Bang, Shukla, S., Shayegan, M., Wissinger, L., Rössler, Ulrich, ...
We report quantitative experimental and theoretical results revealing the tunability of spin splitting in high-mobility two-dimensional GaAs hole systems, confined to either a square or a triangular...
Yoon, Jongsoo, Li, C. C., Shahar, D., Tsui, D. C., Shayegan, M.
We report the transport properties of a low disorder two-dimensional hole system (2DHS) in the GaAs/AlGaAs heterostructure, which has an unprecedentedly high peak mobility of $7\times 10^5cm^2/Vs$,...
Universality at integer quantum Hall transitions (1998)
Yang, Kun, Shahar, D., Bhatt, R. N., Tsui, D. C., Shayegan, M.
We report in this paper results of experimental and theoretical studies of transitions between different integer quantum Hall phases, as well as transition between the insulating phase and quantum...
Chang, CL, Shukla, SP, Pan, W, Venkataraman, V, Sturm, JC, Shayegan, M
A two-dimensional hole gas in the Si1-x-yGexCy channel on a Si(100) substrate has been demonstrated at temperatures from 0.3 to 300 K. The hole mobility decreases as more C is added. The hole...
Magnetic Anisotropy in Quantum Hall Ferromagnets (1998)
Jungwirth, T., Shukla, S. P., Smrcka, L., Shayegan, M., MacDonald, A. H.
We show that the sign of magnetic anisotropy energy in quantum Hall ferromagnets is determined by a competition between electrostatic and exchange energies. Easy-axis ferromagnets tend to occur when...
Observation of Quantum Fluctuations of Charge on a Quantum Dot (1998)
Berman, D., Zhitenev, N. B., Ashoori, R. C., Shayegan, M.
We have incorporated an aluminum single electron transistor directly into the defining gate structure of a semiconductor quantum dot, permitting precise measurement of the charge in the dot. Voltage...
Spontaneous Interlayer Charge Transfer near the Magnetic Quantum Limit (1997)
Manoharan, H. C., Suen, Y. W., Lay, T. S., Santos, M. B., Shayegan, M.
Experiments reveal that a confined electron system with two equally-populated layers at zero magnetic field can spontaneously break this symmetry through an interlayer charge transfer near the...
Bayot, V., Grivei, E., Melinte, S., Shayegan, M.
Thermal measurements on a GaAs/AlGaAs heterostructure reveal that the state of the confined two-dimensional electrons dramatically affects the nuclear-spin diffusion near Landau level filling factor...
A different view of the quantum Hall plateau-to-plateau transitions (1996)
Shahar, D., Tsui, D. C., Shayegan, M., Shimshoni, E., Sondhi, S. L.
We demonstrate experimentally that the transitions between adjacent integer quantum Hall (QH) states are equivalent to a QH-to-insulator transition occurring in the top Landau level, in the presence...
The Nature of the Hall Insulator (1996)
Shahar, D., Tsui, D. C., Shayegan, M., Cunningham, J. E., Shimshoni, E., Sondhi, S. L.
We have conducted an experimental study of the linear transport properties of the magnetic-field induced insulating phase which terminates the quantum Hall (QH) series in two dimensional electron...
Phonon Emission from a 2D Electron Gas: Evidence of Transition to the Hydrodynamic Regime (1996)
Chow, Edmond, Wei, H. P., Girvin, S. M., Shayegan, M.
Using as a thermometer the temperature dependent magneto-transport of a two-dimensional electron gas, we find that effective temperature scales with current as $T_{\rm e} \sim I^a$, where $a=0.4 \pm...
Giant Low Temperature Heat Capacity of GaAs Quantum Wells near Landau Level Filling $\nu$=1 (1996)
Bayot, V., Grivei, E., Melinte, S., Santos, M. B., Shayegan, M.
We report low temperature ($T$) heat capacity ($C$) data on a multiple-quantum-well GaAs/AlGaAs sample in the quantum Hall regime. Relative to its low field magnitude, $C$ exhibits up to 10^5-fold...
Evidence for a Bilayer Quantum Wigner Solid (1996)
Manoharan, H. C., Suen, Y. W., Santos, M. B., Shayegan, M.
As the electronic charge distribution in a wide quantum well is tuned from a single-layer through an interacting bilayer configuration to weakly-coupled parallel layers, we observe an insulating...
Evidence for charge-flux duality near the quantum Hall liquid to insulator transition (1995)
Shahar, D., Tsui, D. C., Shayegan, M., Shimshoni, E., Sondhi, S. L.
We examine the longitudinal, non-linear, current-voltage characteristics near the quantum Hall liquid to insulator transition and show that a simple mapping exists between the characteristics on the...
Mao, D., Santos, M., Shayegan, M., Kahn, A., Lelay, G., Hwu, Y., ...
A study of the formation of In- and Au-GaAs(100) interfaces is reported. The metal overlayers are deposited in ultrahigh vacuum on room-temperature (RT) and low-temperature (LT) (100)GaAs grown by...