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The electronic structure of the LaS surface and CdS/LaS interface is calculated by means of a first-principles electronic-structure method for slab and bulk geometries. The calculations were based on the local-density approximation to density-functional theory and made use of linear muffin-tin orbitals as basis functions for solving the Kohn-Sham equations. The observed low work function of LaS is reproduced by our theory and the calculated surface relaxation of LaS is found to be very small. It is further found that NaCl structured layers of LaS should grow in a well-behaved, epitaxial way on a CdS substrate with a ZnS structure. The interlayer spacing between the first atomic layer of LaS and the topmost layer of the CdS substrate is found to be close to the interlayer distances in both LaS and CdS. The LaS/CdS geometry should thus be possible to grow, and may be a good candidate for cold electron emitter devices.

Starting with the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian, we develop a scattering-matrix approach to study coherent hole transport through the valence-band energy profile across the emitter-base junction of typical abrupt and graded $\mathrm{Pnp}$ heterojunction bipolar transistors. The tunneling and reflection coefficients of heavy and light holes are calculated for the upper and lower $2\ifmmode\times\else\texttimes\fi{}2$ Hamiltonians obtained through a unitary transform of the $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger-Kohn Hamiltonian. The probability for a transition from heavy (light) to light (heavy) hole while tunneling across the emitter-base junction is calculated as a function of the value of the wave vector parallel to the emitter-base heterointerface for both abrupt and graded heterojunctions. For holes injected from the emitter into the base, the probability of heavy- to light-hole conversion is shown to be quite different when calculated with the upper and lower Hamiltonians. On the other hand, the probability of light- to heavy-hole conversion is nearly the same for the upper and lower Hamiltonians. The energy dependence of the heavy- and light-hole tunneling coefficients is shown to be quite different from those calculated using a parabolic-band model, in which the effects of mixing and anisotropy in the valence band are neglected.

We identify a physical mechanism responsible for the onset of chaos in an asymmetric superconducting Wheatstone bridge of overdamped Josephson junctions while focusing on the dynamics of the transverse junction. The dynamics of the transverse junction are shown to be affected by an effective-noise-current term whose presence eventually leads to the onset of chaos in the bridge. This effective-noise-current term results from the competition of circulating currents in the upper and lower loops of the bridge. For some bridges, the effective-noise-current term has a profound influence on the value of the dc biasing current at which a nonzero average voltage appears across the transverse junction of the bridge.

We report a measurement of the ensemble-averaged transverse spin relaxation time T 2 in bulk and few molecules of the organic semiconductor tris-8-hydroxyquinolinolato aluminum or Alq3. This system exhibits two characteristic T 2 times: the longer of which is temperature independent and the shorter is temperature dependent, indicating that the latter is most likely limited by spin-phonon interaction. Based on the measured data, we infer that the single-particle T2 time is probably long enough to meet Knill’s criterion for fault-tolerant quantum computing even at room temperature. Alq3 is also an optically active organic, and we propose a simple optical scheme for spin qubit readout. Moreover, we found that the temperature-dependent T 2 time is considerably shorter in bulk Alq3 powder than in few molecules confined in 1–2-nm-sized cavities. Because carriers in organic molecules are localized over individual molecules or atoms but the phonons are delocalized, we believe that this feature is caused by phonon bottleneck effect.

A quantum wire with strong Rashba spin-orbit interaction is known to exhibit spatial modulation of spin density along its width owing to coupling between subbands caused by the Rashba interaction. This is known as spin texturing. Here, we show that a transverse external magnetic field introduces additional complex features in spin texturing, some of which reflect the intricate details of the underlying energy dispersion relations of the spin-split subbands. One particularly intriguing feature is a 90\ifmmode^\circ\else\textdegree\fi{} phase shift between the spatial modulations of two orthogonal components of the spin density, which is observed at moderate field strengths and when only the lowest spin-split level is occupied by electrons. Its origin lies in the fact that the Rashba interaction acts as an effective magnetic field whose strength is proportional to the electron's velocity.

We numerically calculate the energy dispersion relations of the spin-split subbands in a quantum wire subjected to a transverse magnetic field in the presence of Rashba and Dresselhaus spin-orbit interactions. The spin splitting energy at zero wave vector is found to be neither equal to the bare Zeeman splitting nor linear in the magnetic field in any subband. This happens because the expectation value of the spin angular momentum operator varies along the width of the wire, causing a spatial modulation of the spin density. We also show that spin splitting energy is subband dependent and has a complex dependence on the external magnetic field. In some subbands, it can vanish entirely at nonzero values of the external magnetic field. The effective spin polarization of carriers in any subband can be changed in both magnitude and sign with an external electrostatic potential, applied, for example, via a gate terminal. This has practical applications in quantum computing and other areas.

We have observed both peaks and troughs in the magnetoresistance of organic nanowires consisting of three layers---cobalt, 8-hydroxy-quinolinolato aluminum $(\mathrm{Al}{\mathrm{q}}_{3})$, and nickel. They always occur between the coercive fields of the ferromagnetic layers, and we attribute them to the normal and inverse spin-valve effect. The latter is caused by resonant tunneling through localized impurity states in the organic material. Peaks are always found to be accompanied by a positive monotonic background magnetoresistance, while troughs are accompanied by a negative monotonic background magnetoresistance. This curious correlation suggests that the background magnetoresistance, whose origin has hitherto remained unexplained, is probably caused by the recently proposed phenomenon of magnetic-field-induced enhancement of spin-flip scattering in the presence of spin-orbit interaction [Cahay and Bandyopadhyay, Phys. Rev. B 69, 045303 (2004)].

We study the conductance modulation of gate controlled electron spin interferometers (also known as spin field effect transistors) based on the Rashba spin--orbit coupling effect. It is found that the modulation is dominated by Ramsauer (or Fabry-Perot) type transmission resonances rather than the Rashba effect in typical structures. These transmission resonances are due to reflections at the interferometer's contacts caused by large interface potential barriers and effective mass mismatch between the contact material and the semiconductor. They are particularly strong in quasi-one-dimensional structures which, in fact, are preferred for spin interferometers because of the energy independence of the spin precession angle. Thus, unless particular care is taken to eliminate Ramsauer resonances by proper contact engineering, any observed conductance modulation of spin interferometers may not have its origin in the Rashba effect.

We study high-field spin transport in a quantum wire using a semiclassical approach. Spin dephasing (or spin depolarization) in the wire is caused by D'yakonov-Perel' relaxation associated with bulk inversion asymmetry (Dresselhaus spin-orbit coupling) and structural inversion asymmetry (Rashba spin-orbit coupling). The depolarization rate is found to depend strongly on the initial polarization of the spin. If the initial polarization is along the axis of the wire, the spin depolarizes \ensuremath{\sim}100 times slower compared to the case when the initial polarization is transverse to the wire axis. We also find that in the range 4.2\char21{}50 K, temperature has a weak influence and the driving electric field has a strong influence on the depolarization rate. The steady state distribution of the spin components parallel and transverse to the wire axis also depend on the initial polarization. If the initial polarization is along the wire axis, then the steady state distribution of both components is a flat-topped uniform distribution, whereas if the initial polarization is transverse to the wire axis, then the distribution of the longitudinal component resembles a Gaussian, and the distribution of the transverse component is U shaped.

Rare-earth monosulfides are shown to offer attractive alternatives to the commonly used cesiated surfaces to reach negative electron affinity at various III-V semiconductor surfaces. This is illustrated by theoretical calculations of metallic overlayers of LaS on a semiconducting InP substrate using first-principles techniques. The combination of these materials is demonstrated to result in a very low work function ~0.8 eV!. The interface between the LaS overlayer/InP substrate is shown to be quite stable, and the lattice relaxation of the surface atoms and the atoms at the interface is found to be small.

In the past, the conductance of disordered systems has been extensively studied with use of the Anderson tight-binding Hamiltonian. In this paper we use a different model, which views the semiconductor as regions of free propagation with occasional elastic scattering by a random array of scatterers. Each impurity is characterized by a scattering matrix which can, in principle, be derived for any arbitrary scattering potential. The randomness is introduced through the impurity location. The overall scattering matrix of the device is calculated by combining (using the appropriate law of composition) the scattering matrices of successive sections. The conductance is then evaluated with use of the multichannel Landauer formula. One advantage of this approach is that the quantum conductance can be compared with the semiclassical conductance, which is determined by combining the probability scattering matrices obtained by replacing each element of the (amplitude) scattering matrices by its squared magnitude. This comparison allows us to see clearly the effects of quantum interference. Numerical examples illustrating the onset of weak and strong localization, as well as conductance fluctuations, are presented. Even for samples shorter than the electron elastic mean free path, the size of the conductance fluctuations is close to the universal value if the two-probe conductance formula is used, though it is much larger when the four-probe formula is used.