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...
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...
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...
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...
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 = $0.6, 0.8, 1.0 and 1.5...
Quantized conductance in an AlAs 2D electron system quantum point contact (2006)
Gunawan, O., Habib, B., De Poortere, E. P., Shayegan, M.
We report experimental results on a quantum point contact (QPC) device formed in a wide AlAs quantum well where the two-dimensional electrons occupy two in-plane valleys with elliptical Fermi...
Two-dimensional electrons occupying multiple valleys in AlAs (2006)
Shayegan, M., De Poortere, E. P., Gunawan, O., Shkolnikov, Y. P., Tutuc, E., Vakili, K.
Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the X- points of the Brillouin zone. These valleys have large effective mass and g-factor compared to the...
High-mobility AlAs quantum wells with out-of-plane valley occupation (2006)
Vakili, K., Shkolnikov, Y. P., Tutuc, E., De Poortere, E. P., Padmanabhan, M., Shayegan, M.
Employing state-of-the-art molecular beam epitaxy techniques to grow thin, modulation-doped AlAs quantum wells, we have achieved a low temperature mobility of 5.5 m$^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...
Single-Walled Carbon Nanotubes as Shadow Masks for Nanogap Fabrication (2006)
De Poortere, E. P., Huang, L. M., Huang, M., Wind, S. J., O'Brien, S., Hone, J., ...
We describe a technique for fabricating nanometer-scale gaps in Pt wires on insulating substrates, using individual single-walled carbon nanotubes as shadow masks during metal deposition. More than...
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...
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 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...
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...
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, $2.5 \times 10^{10} \leq p \leq 4.0 \times...
Resistance Spikes at Transitions between Quantum Hall Ferromagnets (2000)
De Poortere, E. P., Tutuc, E., Papadakis, S. J., Shayegan, M.
We report a new manifestation of first-order magnetic transitions in two-dimensional electron systems. This phenomenon occurs in aluminum arsenide quantum wells with sufficiently low carrier...
Tutuc, E., De Poortere, E. P., Papadakis, S. J., Shayegan, M.
Using a novel technique, we make quantitative measurements of the spin polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional holes as a function of an in-plane magnetic...
Confinement symmetry, mobility anisotropy, and metallic behavior in (311)A GaAs 2D holes (2000)
Papadakis, S. J., De Poortere, E. P., Shayegan, M.
We study two dimensional hole systems confined to GaAs quantum wells grown on the (311)A surface of GaAs substrates. Such samples exhibit an in-plane mobility anisotropy. At constant 2D hole density,...
Spin-splitting in GaAs 2D holes (2000)
Papadakis, S. J., De Poortere, E. P., Shayegan, M., Winkler, R.
We present quantitative measurements and calculations of the spin-orbit induced zero-magnetic-field spin-splitting in two-dimensional (2D) hole systems in modulation-doped GaAs (311)A quantum wells....
Highly anisotropic g-factor of two-dimensional hole systems (2000)
Winkler, R., Papadakis, S. J., De Poortere, E. P., Shayegan, M.
Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations...
Anisotropic magnetoresistance of GaAs two-dimensional holes (1999)
Papadakis, S. J., De Poortere, E. P., Shayegan, M., Winkler, R.
Experiments on high-quality GaAs (311)A two-dimensional holes at low temperatures reveal a remarkable dependence of the magnetoresistance, measured with an in-plane magnetic field ($B$), on the...
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...
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...
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...