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Prototypes for hot-carrier solar cells based on type-II InAs/AlAsSb multiple quantum wells are examined for AC photoconductivity as a function of lattice temperature and photoexcitation energy to determine the photoexcited charge carrier transport. These samples previously exhibit an excitation energy onset of a metastable regime in their short time charge carrier dynamics that potentially improves their applicability for hot-carrier photovoltaic applications. The transport results illustrate that the AC photoconductivity is larger in the dynamic regime corresponding to the metastability as a result of higher excitation photocarrier densities. In this excitation regime, the AC photoconductivity is accompanied by slightly lower carrier mobility, arising from the plasma-like nature of carriers scattered by Auger recombination. Outside of this regime, higher mobility is observed as a result of a lower excitation density that is more readily achievable by solar concentration. Additionally, at ambient temperatures, more scattering events are accompanied by slightly lower mobility, but the excitation dependence indicates that this is accompanied by an ambipolar diffusion length that is greater than half a micron. These transport properties are consistent with good quality inorganic elemental and III-V semiconductor solar cells and far exceed those of novel materials. The transport results complement the dynamics observed in type-II InAs/AlAsSb and can guide the design of hot-carrier solar cells based on these and related materials., Comment: 7 pages, 5 figures, journal preprint

The nuclear spin-lattice relaxation time T1 of the nu = 2 quantum Hall ferromagnet (QHF) formed in a gate-controlled InSb two-dimensional electron gas has been characterized using a pump-probe technique. In contrast to a long T1 of quantum Hall states around nu = 1 that possesses a Korringa-type temperature dependence, the temperature-independent short T1 of the nu = 2 QHF suggests the presence of low energy collective spin excitations in a domain wall. Furthermore, T1 of this ferromagnetic state is also found to be filling- and current-independent. The interpretation of these results as compared to the T1 properties of other QHFs is discussed in terms of the domain wall skyrmion, which will lead to a better understanding of the QHF., Comment: 19 pages, 5 figures, including Supplementary Material

In-plane magnetoconductance of InSb quantum wells (QW) containing a two dimensional electron gas (2DEG) is presented. Using a vector magnet, we created a magnetoconductance map which shows the suppression of weak antilocalization (WAL) as a function of applied field. By fitting the in-plane field response of the 2DEG, we estimate material disorder and g-factor as a function of crystal direction. The in-plane WAL suppression is found to be dominated by the Zeeman effect and to show a small crystal-orientation-dependent anistropy in disorder and g-factor. These measurements show the utility of multi-directional measurement of magnetoconductance in analyzing material properties.

Resistively detected NMR (RDNMR) based on dynamic nuclear polarization (DNP) in a quantum Hall ferromagnet (QHF) is a highly-sensitive method for the discovery of fascinating quantum Hall phases; however, the mechanism of this DNP and in particular the role of quantum Hall edge states in it are unclear. Here we demonstrate the important but previously unrecognized effect of chiral edge modes on the nuclear spin polarization. A side-by-side comparison of the RDNMR signals from Hall bar and Corbino disk configurations allows us to distinguish the contributions of bulk and edge states to DNP in QHF. The unidirectional current flow along chiral edge states makes the polarization robust to thermal fluctuations at high temperatures and makes it possible to observe a reciprocity principle of the RDNMR response. These findings help us better understand complex NMR responses in QHF, which has important implications for the development of RDNMR techniques., Comment: 34 pages,10 figures

InAs/AlAs$_{x}$Sb$_{1-x}$ quantum wells are investigated for their potential as hot carrier solar cells. Continuous wave power and temperature dependent photoluminescence indicate a transition in the dominant hot carrier relaxation process from conventional phonon-mediated carrier relaxation below 90 K to a regime where inhibited radiative recombination dominates the hot carrier relaxation at elevated temperatures. At temperatures below 90 K photoluminescence measurements are consistent with type-I quantum wells that exhibit hole localization associated with alloy/interface fluctuations. At elevated temperatures hole delocalization reveals the true type-II band alignment; where it is observed that inhibited radiative recombination due to the spatial separation of the charge carriers dominates hot carrier relaxation. This decoupling of phonon-mediated relaxation results in robust hot carriers at higher temperatures even at lower excitation powers. These results indicate type-II quantum wells offer potential as practical hot carrier systems., Comment: 7 pages, 6 figures, submitted to "Progress in Photovoltaics: Research and Applications"

We investigated the gate control of a two-dimensional electron gas (2DEG) confined to InSb quantum wells with an Al2O3 gate dielectric formed by atomic layer deposition on a surface layer of Al0.1In0.9Sb or InSb. The wider bandgap of Al0.1In0.9Sb compared to InSb resulted in a linear, sharp, and non-hysteretic response of the 2DEG density to gate bias in the structure with an Al0.1In0.9Sb surface layer. In contrast, a nonlinear, slow, and hysteretic (nonvolatile-memory-like) response was observed in the structure with an InSb surface layer. The 2DEG with the Al0.1In0.9Sb surface layer was completely depleted by application of a small gate voltage (-0.9 V)., Comment: 12 pages, 3 figures

We report magnetotransport measurements of a gated InSb quantum well (QW) with high quality Al2O3 dielectrics (40 nm thick) grown by atomic layer deposition. The magnetoresistance data demonstrate a parallel conduction channel in the sample at zero gate voltage (Vg). A good interface between Al2O3 and the top InSb layer ensures that the parallel channel is depleted at negative Vg and the density of two-dimensional electrons in the QW is tuned by Vg with a large ratio of 6.5x1014 m-2V-1 but saturates at large negative Vg. These findings are closely related to layer structures of the QW as suggested by self-consistent Schrodinger-Poisson simulation and two-carrier model., Comment: 11 pages,3 figures

We report on the demonstration of the resistively detected nuclear magnetic resonance (RDNMR) of a single InSb two-dimensional electron gas (2DEG) at elevated temperatures up to 4 K. The RDNMR signal of 115In in the simplest pseudospin quantum Hall ferromagnet triggered by a large direct current shows a peak-dip line shape, where the nuclear relaxation time T1 at the peak and the dip is different but almost temperature independent. The large Zeeman, cyclotron, and exchange energy scales of the InSb 2DEG contribute to the persistence of the RDNMR signal at high temperatures., Comment: 11pages,3figures

The spin polarization (P) of high-density InSb two-dimensional electron systems (2DESs) has been measured using both parallel and tilted magnetic fields. P is found to exhibit a superlinear increase with the total field B. This P-B nonlinearity results in a difference in spin susceptibility between its real value Xs and Xgm ~ m*g*(m* and g* are the effective mass and g factor, respectively) as routinely used in experiments. We demonstrate that such a P-B nonlinearity originates from the linearly P-dependent g* due to the exchange coupling of electrons rather than from the electron correlation as predicted for the low-density 2DES., Comment: 14 pages, 4 figures

We present dynamic nuclear polarization (DNP) in the simplest pseudospin quantum Hall ferromagnet (QHF) of an InSb two-dimensional electron gas with a large g factor using tilted magnetic fields. The DNP-induced amplitude change of a resistance spike of the QHF at large current enables observation of the resistively detected nuclear magnetic resonance of the high nuclear spin isotope 115In with nine quadrupole splittings. Our results demonstrate the importance of domain structures in the DNP process. The nuclear spin relaxation time T1 in this QHF was relatively short (~ 120 s), and almost temperature independent., Comment: 16 pages, 4 figures, PRB rapid communications, selected as an Editors' suggestion

We report time resolved measurements of spin relaxation in doped and undoped InSb quantum wells using degenerate and two-color magneto-optical Kerr effect techniques. We observed that the photo-excited spin dynamics are strongly influenced by laser excitation fluence and the doping profile of the samples. In the low fluence regime, an oscillatory pattern was observed at low temperatures ($\leq$ 77 K) in the samples with an asymmetric doping profile which might be attributed to the quasi-collision-free spin relaxation regime. Our measurements also suggest the influence of the barrier materials (Al$_{x}$In$_{1-x}$Sb) on the spin relaxation in these material systems.

We first give the solution for the local approximation of a four parameter family of generalized one-dimensional point interactions within the framework of non-relativistic model with three neighboring $\delta$ functions. We also discuss the problem within relativistic (Dirac) framework and give the solution for a three parameter family. It gives a physical interpretation for so-called $\epsilon$ potential. It will be also shown that the scattering properties at high energy substantially differ between non-relativistic and relativistic cases., Comment: LaTeX 4 Pages, Talk to be presented at The 1999 International Symposium on Nonlinear Theory and Its Applications (NOLTA99) Hilton Waikoloa Village, Hawaii, USA, Nov 28 - Dec 2, 1999

We investigate whether the generalized second law is valid, using two dimensional black hole spacetime, irrespective of models. A time derivative form of the generalized second law is formulated and it is shown that the law might become invalid. The way to resolve this difficulty is also presented and discussed., Comment: 12 pages, 3 figures, revtex

We propose a new method to construct a four parameter family of quantum-mechanical point interactions in one dimension, which is known as all possible self-adjoint extensions of the symmetric operator $T=-\Delta \lceil C^{\infty}_{0}({\bf R} \backslash\{0\})$. It is achieved in the small distance limit of equally spaced three neighboring Dirac's $\delta$ potentials. The strength for each $\delta$ is appropriately renormalized according to the distance and it diverges, in general, in the small distance limit. The validity of our method is ensured by numerical calculations. In general cases except for usual $\delta$, the wave function discontinuity appears around the interaction and one can observe such a tendency even at a finite distance level., Comment: LaTeX 6pgs, IEICE doubl column, three figures (incl.), See also the website http://www.mech.kochi-tech.ac.jp/cheon/

In order to give some insight into a role of small impurities on the electron motion in microscopic devices, we examine from a general viewpoint, the effect of small obstacles on a particle motion at low energy inside microscopic bounded regions. It will be shown that the obstacles disturb the electron motion only if they are weakly attractive., Comment: LaTeX 5 Pages, Talk presented at The 1998 International technical Conference on Circuits/Systems, Computers and Communications (ITC-CSCC'98), Sokcho, Korea, July 13-15, 1998

We discuss spectral properties of the Laplacian with multiple ($N$) point interactions in two-dimensional bounded regions. A mathematically sound formulation for the problem is given within the framework of the self-adjoint extension of a symmetric (Hermitian) operator in functional analysis. The eigenvalues of this system are obtained as the poles of a transition matrix which has size $N$. Closely examining a generic behavior of the eigenvalues of the transition matrix as a function of the energy, we deduce the general condition under which point interactions have a substantial effect on statistical properties of the spectrum., Comment: Manuscript for Proceedings of The 8th International Colloquium on Differential Equations Plovdiv, Bulgaria, 18-23 August, 1997

We clarify from a general perspective, the condition for the appearance of chaotic energy spectrum in quantum pseudointegrable billiards with a point scatterer inside., Comment: Manuscript for Proceedings of The 1997 International Symposium on Nonlinear Theory and its Applications (NOLTA'97), Hilton Hawaiian Village, Hawaii, Nov.29-Dec.3, 1997