Two-dimensional Hole Gas System Research Articles

Scaling properties of the plateau transitions in the two-dimensional hole gas system

The behavior of phase coherence is studied in two-dimensional hole gas through the integer quantum Hall plateau-to-plateau transition. From the plateau transition as a function of temperature, scaling properties of multiple transitions are analyzed. Our results are in good agreement with the assumption of the zero-point fluctuations of the coherent holes, and support the intrinsic saturation of the coherence time at low temperature limit. The critical exponent $p$ can also be determined under the scheme of the zero-point fluctuations. The similarity and difference in experimental observations between quantum Griffiths singularity and plateau transition is discussed. The spin-orbit coupling effect's influence on the plateau transition is explored by comparing the results from different transitions.

Plasmons in spin-orbit coupled two-dimensional hole gas systems

We study the dynamical dielectric function of a two-dimensional hole gas, exemplified on [001]-GaAs and InAs quantum wells, within the Luttinger model extended to the two lowest subbands including bulk and structure inversion asymmetric terms. The plasmon dispersion shows a pronounced anisotropy for GaAs- and InAs-based systems. In GaAs this leads to a suppression of plasmons due to Landau damping in some orientations. Due to the large Rashba contribution in InAs, the lifetime of plasmons can be controlled by changing the electric field. This effect is potentially useful in plasmon field effect transistors as previously proposed for electron gases.

A first-principles study of dual acceptors behaviors in Mn [formula omitted]-doped digital ferromagnetic heterostructures

The effect of the dual acceptors (Mn and Be) on ferromagnetism in δ -doped GaAs/AlAs digital ferromagnetic heterostructures (DFH) has been studied by ab initio calculations. The dependence of the electronic and magnetic properties on both doping acceptor positions in the well and in the barrier are reported in the two-dimensional hole gas system, respectively. Our results also show that the δ -doping Mn layer in the well edge of the DFH is potential for high Curie temperature of a diluted magnetic semiconductor (DMS). The hole distributions and the magnetic moments are analyzed in terms of their orbital projected density of states. The ferromagnetic coupling is attributed to the overlap of the hole gases and the magnetic layer. However, the Be δ -doping layer in AlAs barrier strongly suppresses the ferromagnetic coupling in the DFH, the origin is not from the electrical doping impurities, but rather be the result of some structural modifications due to the presence of the Be.

Different acceptor behaviors in two-dimensional hole gas system formed in Mn-doped GaAs∕p-AlGaAs ferromagnetic heterostructures

The authors investigated the ferromagnetic property of dual acceptors (Be and Mn) doped GaAs∕AlGaAs heterostructures by means of numerical self-consistent field calculation. The dependence of the magnetic behavior on both acceptor doping concentrations and modes was studied in the two-dimensional hole gas system. The results show different dependences of Curie temperature on the concentration of each acceptor, which is attributed to different doping conditions and mechanisms. In addition, the ferromagnetic transition temperature can be increased by about 70K as a result of introducing Be δ-doping layer in AlGaAs barrier, as compared with that for Be bulk doping. The theoretical results indicate that double acceptor δ-doped ferromagnetic heterostructures have potential advantage in the realm of spintronic application.

Admittance spectroscopy ofSi∕Si1−xGex∕Siquantum well systems: Experiment and theory

We present a combined experimental and theoretical study on the admittance spectroscopy of $\mathrm{Si}∕{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}∕\mathrm{Si}$ single quantum well structures. Experimentally, the admittance spectra are measured for ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$-based two-dimensional hole gas systems. The dependence of the spectra on width of the quantum well and on content of Ge is investigated. Theoretically, in conjunction with the measurements we develop a simple and systematic approach to calculate the conductance and capacitance on the basis of a Boltzmann equation in which the emission rate induced by hole-phonon scattering is considered. We compare the theoretical results with those obtained experimentally and find that our model calculation can reproduce nicely the experimental findings.

Hall Effect in Organic Single-crystal Field-effect Transistors

ABSTRACTWe report Hall effect of charge carriers accumulated in organic field-effect transistors. Rubrene (C42H28) single crystals are shaped in to the Hall-bar congiguration in the devices so that the Hall signal is appropriately detected in external magnetic fields. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The observation of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system. The direct access to the density of mobile charge carriers provides a tool to understand nontrivial features of organic field-effect transistors such as gate electric field dependent mobility.

Numerical study of spin Hall transport in a two-dimensional hole-gas system

We present a numerical study of the spin Hall effect in a two-dimensional hole gas (2DHG) system in the presence of disorder. We find that the spin Hall conductance (SHC), extrapolated to the thermodynamic limit, remains finite in a wide range of disorder strengths for a closed system on torus. But there is no intrinsic spin Hall accumulation as induced by an external electric field once the disorder is turned on. The latter is examined by performing a Laughlin's Gedanken gauge experiment numerically with the adiabatical insertion of a flux quantum in a belt-shaped sample, in which the absence of level crossing is found under the disorder effect. Without disorder, on the other hand, energy levels do cross each other, which results in an oscillating spin-density-modulation at the sample boundary after the insertion of one flux quantum in the belt-shaped system. But the corresponding net spin transfer is only about one order of magnitude smaller than what is expected from the bulk SHC. These apparently contradictory results can be attributed to the violation of the spin conservation law in such a system. We also briefly address the dissipative Fermi surface contribution to spin polarization, which may be relevant to experimental measurements.

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).

Absence of floating delocalized states in a two-dimensional hole gas

By tracking the delocalized states of the two-dimensional hole gas in a $p$-type ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure as a function magnetic field, we mapped out a phase diagram in the density--magnetic-field plane. We found that the energy of the delocalized state from the lowest Landau level flattens out as the magnetic field tends toward zero. This finding is different from that for the two-dimensional electron system in an $n$-type ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure where delocalized states diverge in energy as $B$ goes to zero indicating the presence of only localized states below the Fermi energy. The possible connection of this finding to the recently observed metal-insulator transition at $B=0$ in the two-dimensional hole gas systems is discussed.

Many-body interactions, the quantum Hall effect, and insulating phases in bilayer two-dimensional hole-gas systems

We have studied the quantum Hall effect in a high mobility double-layer 2D hole gas in which we are able to control the carrier density in the two layers independently. We find that both the ν=1 and the ν= 3 2 bilayer quantum Hall (QH) states are destroyed with increasing carrier density. The collapse of ν=1 is extremely abrupt, occurring at a carrier density that corresponds to an inter-layer separation of 1.8 l B . However, although the ν= 3 2 QH state disappears without trace, the zero in ρ xx at ν=1 is replaced by a broad minimum similar to that observed at ν= 1 2 when only one layer is occupied. This suggests that we are observing a transition from a correlated incompressible QH state with total filling factor ν=1 to a compressible state consisting of two (possibly uncorrelated) layers of ‘composite fermions’. At very low densities and filling factors below ν=1, an insulating phase is observed, which is not present when only one layer is occupied. We demonstrate that this state is entirely due to inter-layer interactions and is strong evidence for the existence of a bilayer Wigner crystal.

Fractional quantum Hall effect in bilayer two-dimensional hole-gas systems.

We have studied the fractional and integer quantum Hall effect in high-mobility double-layer two-dimensional hole-gas systems. The large hole effective mass inhibits tunneling, allowing us to investigate the regime in which the interlayer and intralayer interactions are comparable without significant interlayer tunneling occurring. As the interlayer separation is reduced we observe the formation of bilayer-correlated quantum Hall states at total filling factor $\ensuremath{\nu}=\frac{3}{2}$ and $\ensuremath{\nu}=1$. We find that the bilayer $\ensuremath{\nu}=\frac{3}{2}$ state is rapidly destroyed by small carrier density imbalances between the layers, whereas the bilayer $\ensuremath{\nu}=1$ state evolves continuously into the single-layer $\ensuremath{\nu}=1$ state.

Effect of interface quality on the electrical properties of p-Si/SiGe two-dimensional hole gas systems

Electrical properties have been examined for single Si/Si0.8Ge0.2 p-type modulation-doped heterostructures which have been grown by molecular beam epitaxy. It is shown that the two-dimensional hole gas in a normal modulation-doped heterostructure (doped layer on the surface side) has a higher mobility than in an inverted structure (doped layer on the substrate side). Secondary-ion mass spectrometry analysis indicates that the lower mobility in the inverted structure is due to surface segregation of boron. Hole mobilities as high as 6000 cm2/V s at 2 K and 3800 cm2/V s at 6 K have been obtained which are the highest values reported so far for Si/SiGe heterostructures.