Dilute Hole Research Articles

Linear collapse of the depolarization shift in very dilute two-dimensional hole gases

Very dilute two-dimensional hole systems were probed by resonant inelastic light scattering. This extremely high mobility system occurs in AlxGa1−xAs∕GaAs quantum wells designed specifically for their light insensitivity. We find that the depolarization shift in intersubband transitions decreases linearly for hole densities below p=6.6×1010cm−2, dropping to zero below p=1×1010cm−2.

Lightly doped dimerized spin chain in the one-dimensionalt−J−J′model

We study the one-dimensional $t\ensuremath{-}J\ensuremath{-}{J}^{\ensuremath{'}}$-model in the limit of small hole doping x and small $J/t,$ ${J}^{\ensuremath{'}}/t.$ Special emphasis is put on the regime ${J}^{\ensuremath{'}}/J\ensuremath{\approx}0.5,$ where a spin gap is present at small doping and the undoped spin chain is strongly dimerized. Using a perturbative approach and Luttinger liquid arguments, we demonstrate for this nonintegrable class of models that the charge degrees of freedom behave as noninteracting spinless solitons in the dilute hole limit. Our approach is also used to evaluate the energy and mass renormalization of a single hole. Interestingly, the corrections of these quantities are in powers of $\sqrt{J/t}.$ At ${J}^{\ensuremath{'}}/J=0.5$ we construct a variational spin-polaron wave function for the hole and find good agreement with our perturbative results.

Frictional drag between dilute two-dimensional hole systems

The frictional drag between dilute two-dimensional hole systems, exhibiting metallic-like behavior, has been studied. Our main findings are that the drag exhibits a two to three order of magnitude enhancement and deviations to the quadratic temperature dependence when compared to that expected from Boltzmann theory. In addition, we have found that both the drag and the single layer resistivity exhibit the same qualitative dependence on in-plane magnetic field, with the magnetodrag exhibiting sensitivity to matched density, unlike the drag at zero field.

Linear temperature dependence of conductivity in the apparent insulating regime of dilute two-dimensional holes in GaAs

The conductivity of extremely high mobility dilute two-dimensional holes in GaAs changes linearly with temperature in the insulating side of the metal-insulator transition. Hopping conduction, characterized by an exponentially decreasing conductivity with decreasing temperature, is not observed when the conductivity is smaller than ${e}^{2}/h.$ We suggest that strong interactions in a regime close to the Wigner crystallization must be playing a role in the unusual transport.

Interaction corrections to two-dimensional hole transport in thelarge−rslimit

The metallic conductivity of dilute two-dimensional holes in a GaAs heterojunction insulated-gate field-effect transistor with extremely high mobility and large ${r}_{s}$ is found to have a linear dependence on temperature, consistent with the theory of interaction corrections in the ballistic regime. Phonon scattering contributions are negligible in the temperature range of our interest, allowing comparison between our measured data and theory without any phonon subtraction. The magnitude of the Fermi liquid interaction parameter ${F}_{0}^{\ensuremath{\sigma}}$ determined from the experiment, however, decreases with increasing ${r}_{s}$ for ${r}_{s}\ensuremath{\gtrsim}22,$ a behavior unexpected from theoretical calculations valid for small ${r}_{s}.$

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.

Frictional Drag in Dilute Bilayer 2D Hole Systems

We develop a theory for frictional drag between two 2D hole layers in a dilute bilayer GaAs hole system, including effects of hole-hole and hole-phonon interactions. Our calculations suggest significant enhancement of hole drag transresistivity over the corresponding electron drag results. This enhancement originates from the exchange induced renormalization of the single-layer compressibility and the strong dependence of single-layer conductivity on density. We also address the effect of hole-phonon interaction on the drag temperature dependence. Our calculated results are in reasonable quantitative agreement with recent experimental observations.

Resistance noise scaling in a dilute two-dimensional hole system in GaAs.

We have measured the resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well, around the 2D metal-insulator transition (MIT) at zero magnetic field. The normalized noise power S(R)/R(2) increases strongly when the hole density p(s) is decreased, increases slightly with temperature (T) at the largest densities, and decreases strongly with T at low p(s). The noise scales with the resistance, S(R)/R(2) approximately R2.4, as for a second order phase transition such as a percolation transition. The p(s) dependence of the conductivity is consistent with a critical behavior for such a transition, near a density p(*) which is lower than the observed MIT critical density p(c).

Topological doping of a three-dimensional Peierls system: Predicted structure of dopedBaBiO3

At hole concentrations below x=0.4, Ba_(1-x)K_xBiO_3 is non-metallic. At x=0, pure BaBiO3 is a Peierls insulator. Very dilute holes create bipolaronic point defects in the Peierls order parameter. Here we find that the Rice-Sneddon version of Peierls theory predicts that more concentrated holes should form stacking faults (two-dimensional topological defects, called slices) in the Peierls order parameter. However, the long-range Coulomb interaction, left out of the Rice-Sneddon model, destabilizes slices in favor of point bipolarons at low concentrations, leaving a window near 30% doping where the sliced state is marginally stable.

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.

Weak-localization-like temperature-dependent conductivity of a dilute two-dimensional hole gas in a parallel magnetic field.

We have studied the magnetotransport properties of a high mobility two-dimensional hole gas (2DHG) in a 10 nm GaAs quantum well with densities in the range of (0.7-1.6) x 10(10) cm(-2) on the metallic side of the zero-field "metal-insulator transition." In a parallel field well above B(c) that suppresses the metallic conductivity, the 2DHG exhibits a conductivity Delta(g)(T) approximately (1/pi) (e(2)/h)lnT reminiscent of weak localization for Fermi liquids. The experiments are consistent with the coexistence of two phases in our system: a metallic phase and a weakly insulating Fermi liquid phase.

Two-dimensional metal in a parallel magnetic field.

We have investigated the effect of an in-plane parallel magnetic field (B(axially) on two high mobility metallic-like dilute two-dimensional hole gas systems in GaAs quantum wells. The experiments reveal that, while suppressing the magnitude of the low temperature resistance drop, B(axially) does not affect E(a), the characteristic energy scale of the metallic resistance drop. The field B(c) at which the metallic-like resistance drop vanishes is dependent on both the width of the quantum well and the orientation of B(axially). It is unexpected that E(a) is unaffected by B(axially) up to B(c) despite the fact that the Zeeman energy at B(c) is roughly equal to E(a).

Optical investigation of high-mobility dilute two-dimensional hole gases in GaAs (3 1 1)A quantum structures

We have investigated the photoluminescence (PL) and inelastic light-scattering spectra of dilute two-dimensional (2D) hole gases of ultra-high mobility. The samples were GaAs–Al x Ga 1− x As (3 1 1) A quantum structures, modulation-doped with Si. We find these samples not to deplete fully on illumination. The PL displays both excitonic and 2D hole gas transitions. We observe inelastic light-scattering, resonating on these intersubband transitions. Our measured intersubband excitation energies are in agreement with current calculations. These resonance enhancements are consistent with an electron-like dispersion of the first-excited hole subband at zero magnetic field.

Metallic behavior in dilute two-dimensional hole systems.

We have studied the metallic behavior in low-density two-dimensional p-GaAs systems, close to the apparent metal-insulator transition. Two observations are made concerning the origins of the metallic-like behavior. Within a given sample the strength of the metallic behavior is almost independent of the asymmetry of the confining potential, and is predominantly determined by the low-temperature resistivity (i.e., by k(F)l). In all our samples we find that at low densities, close to the transition from insulating to metallic behavior, the fractional decrease in conductivity with increasing temperature scales as T/T(F).

Nonmonotonic magnetic field and density dependence of in-plane magnetoresistance in dilute two-dimensional holes inGaAs/AlxGa1−xAs

We studied low temperature $(T=50$ mK) in-plane magnetoresistance of a dilute two-dimensional hole system in GaAs/AlGaAs heterostructure that exhibits an apparent metal-insulator transition. We found an anisotropic magnetoresistance, which changes dramatically at high in-plane fields ${(B}_{\ensuremath{\Vert}}\ensuremath{\gtrsim}5$ T) as the hole density is varied. At high densities where the system behaves metallic at ${B}_{\ensuremath{\Vert}}=0,$ the transverse magnetoresistance is larger than the longitudinal magnetoresistance. With decreasing the hole density the difference becomes progressively smaller, and at densities near the ``critical'' density and lower, the longitudinal magnetoresistance becomes larger than the transverse magnetoresistance.

Fermi-liquid behavior of the low-density 2D hole gas in a GaAs/AlGaAs heterostructure at large values of r(s).

We examine the validity of the Fermi-liquid description of the dilute 2D hole gas in the crossover from "metallic"-to-"insulating" behavior of rho(T). It has been established that, at r(s) as large as 29, negative magnetoresistance does exist and is well described by weak localization theory. The dephasing time, extracted from the magnetoresistance, is dominated by the T2 term due to hole-hole scattering in the clean limit. The effect of hole-hole interactions, however, is suppressed when compared with the theory derived for small r(s).

Metallic behavior and related phenomena in two dimensions

For about twenty years, it has been the prevailing view that there can be no metallic state or metal-insulator transition in two dimensions in zero magnetic field. In the last several years, however, unusual behavior suggestive of such a transition has been reported in a variety of dilute two-dimensional electron and hole systems. The physics behind these observations is presently not understood. We review and discuss the main experimental findings and suggested theoretical models.

Localisation and the metal–insulator transition in two dimensions

We investigate the insulating and metallic behaviour found in strongly interacting, dilute 2D GaAs hole systems. At the lowest carrier densities insulating behaviour is observed with the resistivity increasing with decreasing temperature. As the carrier density is increased a transition to metallic behaviour occurs. Despite exhibiting all the properties of the metallic behaviour observed in other material systems, localising corrections due both to weak localisation and hole–hole interactions are still present in the “metallic” regime. These results suggest that the metallic behaviour may only be a finite temperature effect, and we investigate various semi-classical mechanisms that might be responsible for this metallic-like behaviour.

Slow holes in the triangular Ising antiferromagnet

We consider the problem of the doped Ising antiferromagnet on the frustrated triangular lattice in the limit where the hole kinetic energy is much smaller than the Ising exchange. For a single hole we prove a ``frustrated Nagaoka theorem'' showing that the ground state is magnetized and breaks translational symmetry, in contrast to the parent insulating state that is unmagnetized and spatially homogeneous. The extension of this physics to finite dopings depends on the strength of a density-density coupling that is inevitably present---we find either phase separation of the holes, or a superconducting state that is also magnetized and breaks translational symmetry in a feat of spatial self-organization. Finally, we derive an effective interaction between dilute holes at temperatures in excess of the hopping and find an oscillatory, long-ranged form reflective of the correlations in the underlying classical magnet which presages the breaking of translational symmetry at zero temperature.

Parallel magnetic field induced transition in transport in the dilute two-dimensional hole system in GaAs

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 insulating at a finite field through a well-defined transition. The value of resistivity at the transition is found to depend strongly on density.