This article 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.

We report experimental results on a quantum point contact ͑QPC͒ device formed in a wide AlAs quantum well where the two-dimensional electrons occupy two in-plane valleys with elliptical Fermi contours. To probe the closely-spaced, one-dimensional electric subbands, we fabricated a point contact device defined by shallow etching and a top gate that covers the entire device. The conductance versus top gate bias trace shows a series of weak plateaus at integer multiples of 2e 2 / h, indicating a broken valley degeneracy in the QPC and implying the potential use of QPC as a simple "valley filter" device. A model is presented to describe the quantized energy levels and the role of the in-plane valleys in the transport. We also observe a well-developed conductance plateau near 0.7ϫ 2e 2 / h which may reflect the strong electron-electron interaction in the system.

We measured Aharonov-Bohm resistance oscillations in a shallow two-dimensional GaAs hole ring structure, defined by local anodic surface oxidation. The amplitude of the oscillations is about 10% of the ring resistance, the strongest seen in a hole system. In addition we observe resistance oscillations as a function of front gate bias at zero magnetic field. We discuss the results in light of spin interference in the ring and possible applications to spintronics.The Aharonov-Bohm (AB) effect and related phenomenon in mesoscopic semiconductor structures have attracted much attention over the last years. The AB effect reflects the fact that when magnetic flux pierces through a ring structure, a phase difference develops between the electron wavefunctions traveling in the ring's two arms [1]. This phase difference is 2π(Φ/Φ 0 ), where Φ = πr 2 B is the magnetic flux through the ring, r is the ring radius, B is the applied magnetic field in the perpendicular direction, and Φ 0 = h/e is the flux quantum. Hence the resistance of the ring exhibits oscillations periodic in B with a period equal to πr 2 /(h/e).In semiconductors, the AB effect has been seen in rings made from two-dimensional (2D) electron systems in various materials [2] but has been difficult to observe in 2D hole systems (2DHSs). Recently, AB oscillations with an amplitude of about 0.1% (of the total ring resistance) were observed by Yau et al.[3] in a 500 nm radius ring structure patterned via electron-beam lithography in GaAs 2D holes. In the present work we aimed to increase the amplitude of the AB oscillations in this system by fabricating smaller rings [4]. For this purpose we employed the local anodic oxidation (LAO) technique using an atomic force microscope (AFM). Here we present our measurements of AB oscillations in a GaAs 2D hole sample with r ≃ 160 nm. The observed AB oscillation amplitude is about 100 times larger than in Ref. [3]. A further motivation for our experiments is the fact that the 2DHS in GaAs exhibits strong spin-orbit interaction which is tunable with gate bias [5]. Nitta et al. [6] highlighted that the resistance in a ring structure in such a system can be modulated by changing the gate bias even at B = 0 and proposed it as a spin interference device. In our ring structure, we do indeed observe oscillations as a function of gate bias at B = 0; however, the origin of these oscillations is unclear.Our sample was grown on a GaAs (311)A substrate by molecular beam epitaxy and contains a modulationdoped 2DHS confined to a GaAs/Al 0.3 Ga 0.7 As interface. The interface is separated from an 11 nm-thick Si-doped Al 0.3 Ga 0.7 As layer (Si concentration of 1.64 × 10 19 cm −3 ) by a 15 nm Al 0.3 Ga 0.7 As spacer layer. A 5 nm GaAs layer caps the structure resulting in the 2D hole layer residing ≃ 31 nm below the surface. We fabricated Hall bar samples via optical lithography and used alloyed In/Zn for the ohmic contacts. Metal gates were deposited on the sample's front and back to control the 2D hole density (p). Before patterning the s...

We demonstrate experimentally and theoretically that two-dimensional (2D) heavy hole systems in single heterostructures exhibit a 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.In a solid that lacks inversion symmetry, the spin-orbit interaction leads to a lifting of the spin degeneracy of the energy bands, even in the absence of an applied magnetic field, B. In such a solid, the energy bands at finite wave vectors are split into two spin subbands with different energy surfaces, populations, and effective masses. The problem of inversion asymmetry-induced spin splitting in two-dimensional (2D) carrier systems in semiconductor heterojunctions and quantum wells [1,2,3,4] has become of renewed interest recently [5] because of their possible use in realizing spintronic devices such as a spin field-effect transistor [6,7], and for studying fundamental phenomena such as the spin Berry phase [8,9].In 2D carrier systems confined to GaAs/AlGaAs heterostructures, the bulk inversion asymmetry (BIA) of the zinc blende structure and the structure inversion asymmetry (SIA) of the confining potential contribute to the B = 0 spin splitting [4,5]. While BIA is fixed, the so called Rashba spin splitting [10] due to SIA can be tuned by means of external gates that change the perpendicular electric field (E ⊥ ) in the sample. For many years it has been assumed that the Rashba spin splitting in 2D carrier systems is proportional to E ⊥ that characterizes the inversion asymmetry of the confining potential [4]. 2D holes contained in a GaAs square quantum well provide an example [11]. On the contrary, in the present work we show both experimentally and theoretically that for heavy holes confined to a triangular well at the GaAs/AlGaAs interface, spin splitting decreases with an increase in E ⊥ . We demonstrate this negative differential Rashba effect by analyzing the Shubnikov-de Haas oscillations in this system at a constant density. We note that hole systems have recently gained great attention for spintronics applications [12] because ferromagnetic (III,Mn)V compounds are intrinsically p type. A detailed understanding of the B = 0 spin splitting in hole systems is thus of great importance.The sample used in our study was grown on a GaAs (001) substrate by molecular beam epitaxy and contains a modulation-doped 2D hole system confined to a GaAs/AlGaAs heterostructure [ Fig. 1(a)]. The Al 0.3 Ga 0.7 As/GaAs interface is separated from a 16 nm thick Be-doped Al 0.3 Ga 0.7 As layer (Be concentration of 3.5 × 10 18 cm −3 ) by a 25 nm Al 0.3 Ga 0.7 As spacer layer. We fabricated Hall bar samples via lithography and used In/Zn alloyed at 440 • C for the ohmic contacts. Metal gates were deposited on the sample's front and back to c...

We present quantitative measurements and calculations of the spin-orbit-induced zero magnetic field spin-splitting in GaAs two-dimensional hole systems. The magneto-resistance oscillations in this system show a clear beating pattern which results from the unequal densities of the two spin subbands. We present a Fourier analysis technique that de-convolves the magneto-oscillations of the two subbands. The temperature dependence of the de-convolved oscillations allows us to deduce the effective masses of the subbands without assuming parabolic dispersions for the two subbands. Next, we demonstrate that heavy hole systems in heterostructures exhibit a decrease in spin-splitting with an increase in perpendicular electric field. Our results are in contrast to the more familiar case of electrons where spin-splitting increases with electric field. Another external parameter that effects spin-splitting is strain. We report direct measurements of the spin-orbit interaction-induced spin-splitting in a hole system as a function of anisotropic, in-plane strain. In addition, our piezoresistance measurements reveal a strong dependence on density and the direction along which the resistance is measured. In the end, we present Aharonov-Bohm resistance oscillations in a shallow two-dimensional GaAs hole ring structure, defined by local anodic surface oxidation. The amplitude of the oscillations is about 10% of the ring resistance, the strongest seen in a hole system.