arXiv subjects

E. Tutuc

Publications and source records attributed to E. Tutuc.

At least 37 records · Page 2Linked to original sources

arXiv2006

Zeeman splitting of interacting two-dimensional electrons with two effective masses

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 both resistivity spikes and persistent gaps at crossings between the Landau levels originating from these two valleys. From the positions of the spikes in tilted magnetic field and measurements of the energy gaps away from the crossings, we find that, after occupation of the minority valley, the spin susceptibility drops rapidly, and the electrons possess a {\it single} interaction-enhanced g-factor, despite the dissimilarity of the two occupied valleys.

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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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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

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

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

Ballistic transport in AlAs two-dimensional electrons

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 $m_l/m_t=5.2\pm0.5$ for the ratio of the longitudinal and transverse electron effective masses.

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arXiv2003

A 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 $ν$, 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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arXiv2003

Critical Resistance of the Quantum Hall Ferromagnet in AlAs 2D Electrons

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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arXiv2003

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 magneto-transport measurements and the observation of a phase-coherent, bilayer $ν$=1 quantum Hall state flanked by a reentrant insulating phase.

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arXiv2003

Layer charge instability in unbalanced bilayer systems in the quantum Hall regime

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 Landau level filling factor one. At a fixed field in the hysteretic regions, the resistance exhibits an unusual time dependence, consisting of random, bidirectional jumps followed by slow relaxations. These anomalies are apparently caused by instabilities in the charge distribution of the two layers.

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arXiv2003

Role of density imbalance in an interacting bilayer hole system

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 $ν=1$, flanked by insulating phases at nearby fillings which suggest the formation of a pinned, bilayer Wigner crystal. As we transfer charge from one layer to another, the insulating phases disappear while, surprisingly, the $ν=1$ QH state becomes stronger. Concomitantly, a pronounced hysteresis develops in the longitudinal magnetoresistance at higher fillings, indicative of a first-order quantum phase transition.

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