We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.Comment: 4 pages, 3 figures, submitted to Phys. Rev.

InAs-AlSb quantum wells are investigated by transport experiments in magnetic fields tilted with respect to the sample normal. Using the coincidence method we find for magnetic fields up to 28 T that the spin splitting can be as large as 5 times the Landau splitting. We find a value of the g-factor of |g| ≈ 13. For small even-integer filling factors the corresponding minima in the Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt angles. This indicates the anti-crossing of neighboring Landau and spin levels. Furthermore we find for particular tilt angles a crossover from even-integer dominated Shubnikov-de Haas minima to odd-integer minima as a function of magnetic field.73. 50.-h, 72.20.-i, 72.90.+y

We have measured the intersubband resonances of an InAs͞AlSb quantum well with two occupied subbands from cryogenic temperatures to well above room temperature. The higher energy mode is very robust with increasing temperature; the lower energy mode, however, broadens above 200 K. We explain the results in terms of Landau damping and argue generally that the collective nature of the intersubband resonance is crucial for an understanding of the scattering mechanisms that determine the intersubband resonance linewidth. [S0031-9007(98)05471-4] PACS numbers: 73.20.Dx, 73.20.Mf, 78.66.Fd Intersubband resonance (ISR) is a fundamental excitation of a low-dimensional semiconductor system. In a single-particle picture, the resonance corresponds to the transition between two quantized states. However, ISR is not a single-particle process [1,2]. Instead, ISR is a collective phenomenon better described as a plasmon, or charge-density excitation. The most obvious consequence of the collective effects is a shift of the ISR away from the energy separating the single-particle states. This shift tends to be only a small proportion of the resonance energy as the direct electron-electron interaction (depolarization field) and the exchange-correlation interaction (exciton effects) cause blueshifts and redshifts, respectively, and tend to cancel [3,4].Recently, the collective effects have been dramatically revealed by studying systems with a broad single-particle density of states. Nevertheless, for large densities the ISR is a single, narrow line. For instance, in a system with a highly nonparabolic energy dispersion, the single-particle transition energy is a strong function of wave vector k, being smaller at the Fermi wave vector k f than at k 0. The single-particle spectrum is then broad, yet the ISR is a sharp peak [5][6][7]. The collective effects condense all the available oscillator strength into a single mode. A similar effect also occurs in weakly coupled quantum wells which have a broad and complicated single-particle spectrum yet a narrow ISR can be observed [8,9]. In the nonlinear regime where ISR is excited with a very intense source, optical pumping of carriers induces a redshift as the collective effects weaken [10].These experiments show very convincingly that ISR is indeed a collective phenomenon. It has been argued that in the best samples the resonance is homogeneously broadened [11,12], in which case one can pose the question: What are the scattering mechanisms which destroy the coherence of the plasma oscillation? Surprisingly perhaps, a microscopic theory to answer this question does not seem to exist. In fact, the ISR linewidth is usually described with the single-particle scattering rates [11,12], ignoring the collective nature of the resonance. Calculations of the ISR width at low temperature in Si͞SiO 2 from charged ion scattering [13] suggest that this approach is likely to be misleading. Generally, it appears to be unclear how the prevalent single-particle picture of electron scattering, particularly w...

The growth of modulation-doped InAs/(Al,Ga)Sb quantum wells on GaAs substrates employing molecular beam epitaxy requires care in the nucleation and the use of buffer layers to achieve high quality material. Despite a 7% lattice mismatch between the substrate and the active layers, fully relaxed epitaxial growth can be accomplished, and quantum wells with electron sheet concentrations of 7×1012 cm−2 having low-temperature mobilities as high as 300 000 cm2/V s have been routinely fabricated recently in our laboratory. In the present work the combination of atomic force microscopy and van der Pauw measurements is used to investigate and explain the strong influence of the buffer layers on the morphology in the quantum well that is shown to be responsible for the great differences in the observed low-temperature mobilities.

We have studied the current-voltage characteristics of superconducting weak links in which the coupling medium is the 2-D electron gas in InAs-based semiconductor quantum wells, with relatively large (typically 0.5µm) separations between niobium electrodes. The devices exhibit Josephson-like current-voltage characteristics; however, the falloff of the differential resistance with decreasing temperature is thermally activated, and is orders of magnitude slower than for more conventional weak links. Most unexpectedly, the thermal activation energies are found to be proportional to the width of the device, taken perpendicular to the current flow. This behavior falls outside the range of established theories; we propose that it is a fluctuation effect caused by giant shot noise associated with multiple Andreev reflections. The possibility of non-equilibrium effects is discussed.