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S. J. Papadakis

Publications and source records attributed to S. J. Papadakis.

17 recordsLinked to original sources
arXiv2008

Spin Orientation of Holes in Quantum Wells

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 electron systems. We show how a systematic understanding of the unusual spin-dependent phenomena in hole systems can be gained using a multipole expansion of the spin density matrix. As an example we discuss spin precession in hole systems that can give rise to an alternating spin polarization. Finally, we discuss the qualitatively different regimes of hole spin polarization decay in clean and dirty samples.

cond-mat.mes-hall↗
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.

cond-mat.mes-hall↗
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.

cond-mat.mes-hall↗
arXiv2002

Torsional response and stiffening of individual multi-walled carbon nanotubes

We report on the characterization of torsional oscillators which use multi-walled carbon nanotubes as the spring elements. Through atomic-force-microscope force-distance measurements we are able to apply torsional strains to the nanotubes and measure their torsional spring constants and effective shear moduli. We find that the effective shear moduli cover a broad range, with the largest values near the theoretically predicted value. The data also suggest that the nanotubes are stiffened by repeated flexing.

cond-mat.mtrl-sci↗
arXiv2002

Spin polarization and transition from metallic to insulating behavior in 2D systems

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 the in-plane magnetoresistance is shown to be intimately related to the spin polarization. Moreover, for three different 2D systems, namely GaAs (100) electrons, GaAs (311)A holes, and AlAs (411)B electrons, the temperature dependence of the in-plane magnetoresistance reveals that their behavior turns from metallic to insulating before they are fully spin polarized.

cond-mat.mes-hall↗
arXiv2002

Low-field magnetoresistance in GaAs 2D holes

We report low-field magnetotransport data in two-dimensional hole systems in GaAs/AlGaAs heterostructures and quantum wells, in a large density range, $2.5 \times 10^{10} \leq p \leq 4.0 \times 10^{11}$ cm$^{-2}$, with primary focus on samples grown on (311)A GaAs substrates. At high densities, $p \gtrsim 1 \times 10^{11}$ cm$^{-2}$, we observe a remarkably strong positive magnetoresistance. It appears in samples with an anisotropic in-plane mobility and predominantly along the low-mobility direction, and is strongly dependent on the perpendicular electric field and the resulting spin-orbit interaction induced spin-subband population difference. A careful examination of the data reveals that the magnetoresistance must result from a combination of factors including the presence of two spin-subbands, a corrugated quantum well interface which leads to the mobility anisotropy, and possibly weak anti-localization. None of these factors can alone account for the observed positive magnetoresistance. We also present the evolution of the data with density: the magnitude of the positive magnetoresistance decreases with decreasing density until, at the lowest density studied ($p = 2.5 \times 10^{10}$ cm$^{-2}$), it vanishes and is replaced by a weak negative magnetoresistance.

cond-mat.mes-hall↗
arXiv2000

Resistance Spikes at Transitions between Quantum Hall Ferromagnets

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 densities and appears as a set of hysteretic spikes in the resistance of a sample placed in crossed parallel and perpendicular magnetic fields, each spike occurring at the transition between states with different partial magnetizations. Our experiments thus indicate that the presence of magnetic domains at the transition starkly increases dissipation, an effect also suspected in other ferromagnetic materials. Analysis of the positions of the transition spikes allows us to deduce the change in exchange-correlation energy across the magnetic transition, which in turn will help improve our understanding of metallic ferromagnetism.

cond-mat.mes-hall↗
arXiv2000

In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems

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 field. As the field is increased the system gradually becomes spin polarized, with the degree of spin polarization depending on the orientation of the field relative to the crystal axes. Moreover, the behavior of the system turns from metallic to insulating \textit{before} it is fully spin polarized. The minority-spin population at the transition is ~8*10^{9} cm^{-2}, close to the density below which the system makes a transition to an insulating state in the absence of a magnetic field.

cond-mat.mes-hall↗
arXiv2000

Confinement symmetry, mobility anisotropy, and metallic behavior in (311)A GaAs 2D holes

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, we vary the symmetry of the quantum well potential and measure the temperature dependence of the resistivity along two different current directions, in a regime where the samples exhibit metallic behavior. The symmetry has a significant and non-trivial effect on the metallic behavior. Moreover, differences between the temperature-dependence of the resistivity along the two mobility directions point to specific scattering mechanisms being important in the expression of the metallic behavior.

cond-mat.mes-hall↗
arXiv2000

Spin-splitting in GaAs 2D holes

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. The results show that the splitting is large and tunable. In particular, via a combination of back- and front-gate biases, we can tune the splitting while keeping the 2D hole density constant. The data also reveal a surprising result regarding the magnetoresistance (Shubnikov-de Haas) oscillations in a 2D system with spin-split energy bands: the frequencies of the oscillations are {\it not} simply related to the population of the spin-subbands. Next we concentrate on the metallic-like behavior observed in these 2D holes and its relation to spin-splitting. The data indicate that the metallic behavior is more pronounced when two spin-subbands with unequal populations are occupied. Our measurements of the magnetoresistance of these 2D hole systems with an in-plane magnetic field corroborate this conclusion: while the system is metallic at zero magnetic field, it turns insulating when one of the spin-subbands is depopulated at high magnetic field.

cond-mat.mes-hall↗
arXiv2000

Highly anisotropic g-factor of two-dimensional hole systems

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 of an in-plane magnetic field B relative to the crystal axes. This mechanism has no analogue in the bulk band structure. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (113) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B.

cond-mat.mes-hall↗
arXiv2000

Anisotropic magnetoresistance of GaAs two-dimensional holes

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 direction of $B$ relative to both the crystal axes and the current direction. The magnetoresistance features, and in particular the value of $B$ above which the resistivity exhibits an insulating behavior, depend on the orientation of $B$. To explain the data, the anisotropic band structure of the holes and a re-population of the spin-subbands in the presence of $B$, as well as the coupling of the orbital motion to $B$, need to be taken into account.

cond-mat.mes-hall↗
arXiv1999

Anomalous magneto-oscillations in two-dimensional systems

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 self-consistent numerical calculations and experimental results which indicate that these oscillations are not simply related to the zero-magnetic-field spin-subband densities.

cond-mat.mes-hall↗
arXiv1999

Anisotropic Transport of Two-Dimensional Holes in High Landau Levels

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 filling factors. The anisotropy appears to depend on both the density and symmetry of the hole charge distribution.

cond-mat.mes-hall↗
arXiv1999

Apparent Metallic Behavior at B = 0 of a two-dimensional electron system in AlAs

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 sample decreases with decreasing temperature, but as the density is reduced the behavior changes to insulating, with the resistance increasing as the temperature is decreased. The effect is similar to that observed in 2D electrons in Si-MOSFETs, and in 2D holes in SiGe and GaAs, and points to the generality of this phenomenon.

cond-mat.mes-hall↗
arXiv1998

Anomalous spin-splitting of two-dimensional electrons in an AlAs Quantum Well

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 the magnetoresistance oscillations. The data reveal that |g| = 9.0, which is much enhanced with respect to the reported bulk value of 1.9. Surprisingly, in a large range of magnetic field and Landau level fillings, the value of the enhanced g-factor appears to be constant.

cond-mat.mes-hall↗