High-pressure studies of electronic states of impurity donors-D(X) centres-in high purity GaAs are reported. For the first time, the diamond anvil high-pressure cell technique has been applied to far-infrared (FIR) magneto-optical experiments. Absorption due to the 1s-2p⁺ transition for different shallow donor species is investigated in transmission experiments using far-infrared laser radiation at pressures up to 40 kbar. For pressures up to 20 kbar the photoconductivity technique with a standard Unipress optical cell is also used. A shallow-deep transition is observed for sulphur and silicon donors close to 25 and 30 kbar, respectively. The transition is similar to that for the Ge donor at 9 kbar reported previously. A deep state responsible for quenching of FIR absorption is further investigated in photoluminescence experiments. Large 'chemical shifts', anti-crossing effects and fast electron capture at low temperatures demonstrate the coexistence of strong localization, A₁ symmetry and the non-metastable (non-D(X) like) character of the observed deep donor states. The present investigation and earlier results with InSb demonstrate that three types of states-shallow and two types of deep states, D(X)-like and deep A₁-are generally expected to be formed by impurity donors which form D(X) centres.

The domain configuration in permalloy wires (30 nm thick, 10 μm wide, and 205 μm long) with a wide size range of a narrow central bridge (5 μm long and w μm wide; $\text{0.5⩽w⩽10\hspace{0.05em}μ}\mathrm{m})$ were investigated in both their demagnetized and remanent states using magnetic force microscopy and the results were confirmed by micromagnetic calculations. At the bridge region, domain walls were found to be shifted by a small external field. Scanning magneto-optical Kerr effect revealed that the coercivity in these structures are the same as that in a straight wire, suggesting that domain wall movement is the dominant process in the magnetization reversal of these structures.

$G{e}_{1-x}{\mathrm{Sn}}_x$ quantum wells can be incorporated into $\mathrm{Si}\text{−}\mathrm{Ge}$–based structures with low-carrier effective masses, high mobilities, and the possibility of direct band-gap devices with x ∼ 0.1. However, the electrical properties of p-type $G{e}_{1-x}{\mathrm{Sn}}_x$ devices are dominated by a thermally activated mobility and metallic behavior. At 30 mK the transport measurements indicate localization with a mobility of $380{\mathrm{cm}}^2$/Vs, which is thermally activated with a temperature-independent carrier density of 4 × ${10}^{11}{\mathrm{cm}}^{-2}$. This weakly disordered system with conductivity, σ ∼ $e^2$/h, where e is the fundamental charge and h is Planck’s constant, is a result of negatively charged “Sn-vacancy” complex states in the barrier layers that act as hole traps. A measured hole effective mass of 0.090 ± $0.005m_e$ from the Shubnikov-de Haas effect, where $m_e$ is the free electron mass shows that the valence band is heavy hole dominated and is similar to p-type $\mathrm{Ge}$ with the compressive strain playing the role of quenching the spin-orbit coupling and shifting the unoccupied light-hole states to higher hole energies. The $G{e}_{1-x}{\mathrm{Sn}}_x$ devices have a high quantum mobility of approximately 36 000 ${\mathrm{cm}}^2$/Vs that is not thermally activated. The ratio of transport-to-quantum mobility of approximately 0.01 in $G{e}_{1-x}{\mathrm{Sn}}_x$ devices is unusual and points to several competing scattering mechanisms in the different experimental regimes.

The vortex dynamics of an undoped uniaxially textured YBa₂Cu₃O_7−δ (YBCO) superconducting film grown onto a MgO (100) substrate was inspected by applying the vortex glass and collective-pinning models. The texture and structural characterization studied by X-ray diffraction revealed a uniaxially (00 l) YBCO layer, which coexists with minor Y₂BaCuO₅ and CuO phases. The temperature dependence of the magnetization in the superconducting state revealed a critical temperature T C = 88 K. By measuring the hysteresis loops ( M ( H ) ) at 10–70 K, the critical fields H C 1 , H C 2 , H P and H irr were estimated and a vortex matter diagram was sketched. By using the Bean model, the critical current density values J C ( T , H ) were obtained and the typical peak effect is observed. The vortex dynamics mechanism is discussed taking into account four vortex feature regimes in the double-logarithmical J C ( H ) curves. The vortex pinning mechanism is discussed by obtaining the pining force, F P , its normalization, f ( h ) , and magnetic relaxation M ( t ) measurements taken in field cooling mode at 10–60 K. The glassy exponent μ and the characteristic energy U ( J , T ) in the vortex glass model were estimated following the Maley method. The collective-pinning model is used to discuss the possible vortex regimes mechanism (individual flux lines, small bundles and large bundles of pinned flux). Eventually, the E ( J ) curves, expressed from the swept field and creep measurements, show a power-law behaviour, in agreement with the vortex matter.

In this Letter, we report a systematic study of a structure found in zero magnetic field at or near 0.2 ×(e2/h) in In0.75Ga0.25As heterostructures, where e is the fundamental unit of charge and h is Planck's constant. This structure has been observed in many samples and stays at near constant conductance despite a large range of external potential changes, the stability indicating a quantum state. We have also studied the structure in the presence of high in-plane magnetic fields and find an anisotropy which can be related to the Rashba spin–orbit interaction and agrees with a recent theory based on the formation of coherent back-scattering. A possible state with conductance at 0.25 ×(e2/h) has also been found. The quantum states described here will help with the fundamental understanding of low-dimensional electronic systems with strong spin–orbit coupling and may offer new perspectives for future applications in quantum information schemes.