Electroreflectance spectroscopy in self-assembled quantum dots: lens symmetry

Modulated electroreflectance spectroscopy $\Delta R/R$ of semiconductor self-assembled quantum dots is investigated. The structure is modeled as dots with lens shape geometry and circular cross section. A microscopic description of the electroreflectance spectrum and optical response in terms of an external electric field (${\bf F}$) and lens geometry have been considered. The field and lens symmetry dependence of all experimental parameters involved in the $\Delta R/R$ spectrum have been considered. Using the effective mass formalism the energies and the electronic states as a function of ${\bf F}$ and dot parameters are calculated. Also, in the framework of the strongly confined regime general expressions for the excitonic binding energies are reported. Optical selection rules are derived in the cases of the light wave vector perpendicular and parallel to $$. Detailed calculation of the Seraphin coefficients and electroreflectance spectrum are performed for the InAs and CdSe nanostructures. Calculations show good agreement with measurements recently performed on CdSe/ZnSe when statistical distribution on size is considered, explaining the main observed characteristic in the electroreflectance spectra

The Electron Glass in a Switchable Mirror: Relaxation, Aging and Universality

The rare earth hydride YH$_{3-\delta}$ can be tuned through the metal-insulator transition both by changing $\delta$ and by illumination with ultraviolet light. The transition is dominated by strong electron-electron interactions, with transport in the insulator sensitive to both a Coulomb gap and persistent quantum fluctuations. Via a systematic variation of UV illumination time, photon flux, Coulomb gap depth, and temperature, we demonstrate that polycrystalline YH$_{3-\delta}$ serves as a model system for studying the properties of the interacting electron glass. Prominent among its features are logarithmic relaxation, aging, and universal scaling of the conductivity

Monte-Carlo Simulations of the Dynamical Behavior of the Coulomb Glass

We study the dynamical behavior of disordered many-particle systems with long-range Coulomb interactions by means of damage-spreading simulations. In this type of Monte-Carlo simulations one investigates the time evolution of the damage, i.e. the difference of the occupation numbers of two systems, subjected to the same thermal noise. We analyze the dependence of the damage on temperature and disorder strength. For zero disorder the spreading transition coincides with the equilibrium phase transition, whereas for finite disorder, we find evidence for a dynamical phase transition well below the transition temperature of the pure system.Comment: 10 pages RevTeX, 8 Postscript figure

Attention bias and anxiety in young children exposed to family violence

Background—Attention bias towards threat is associated with anxiety in older youth and adults and has been linked with violence exposure. Attention bias may moderate the relationship between violence exposure and anxiety in young children. Capitalizing on measurement advances, the current study examines these relationships at a younger age than previously possible. Methods—Young children (mean age 4.7, ±0.8) from a cross-sectional sample oversampled for violence exposure (N = 218) completed the dot-probe task to assess their attention biases. Observed fear/anxiety was characterized with a novel observational paradigm, the Anxiety Diagnostic Observation Schedule. Mother-reported symptoms were assessed with the Preschool-Age Psychiatric Assessment and Trauma Symptom Checklist for Young Children. Violence exposure was characterized with dimensional scores reflecting probability of membership in two classes derived via latent class analysis from the Conflict Tactics Scales: Abuse and Harsh Parenting. Results—Family violence predicted greater child anxiety and trauma symptoms. Attention bias moderated the relationship between violence and anxiety

Temperature dependence of the electron spin g factor in GaAs

The temperature dependence of the electron spin $g$ factor in GaAs is investigated experimentally and theoretically. Experimentally, the $g$ factor was measured using time-resolved Faraday rotation due to Larmor precession of electron spins in the temperature range between 4.5 K and 190 K. The experiment shows an almost linear increase of the $g$ value with the temperature. This result is in good agreement with other measurements based on photoluminescence quantum beats and time-resolved Kerr rotation up to room temperature. The experimental data are described theoretically taking into account a diminishing fundamental energy gap in GaAs due to lattice thermal dilatation and nonparabolicity of the conduction band calculated using a five-level kp model. At higher temperatures electrons populate higher Landau levels and the average $g$ factor is obtained from a summation over many levels. A very good description of the experimental data is obtained indicating that the observed increase of the spin $g$ factor with the temperature is predominantly due to band's nonparabolicity.Comment: 6 pages 4 figure

On dispersive energy transport and relaxation in the hopping regime

A new method for investigating relaxation phenomena for charge carriers hopping between localized tail states has been developed. It allows us to consider both charge and energy {\it dispersive} transport. The method is based on the idea of quasi-elasticity: the typical energy loss during a hop is much less than all other characteristic energies. We have investigated two models with different density of states energy dependencies with our method. In general, we have found that the motion of a packet in energy space is affected by two competing tendencies. First, there is a packet broadening, i.e. the dispersive energy transport. Second, there is a narrowing of the packet, if the density of states is depleting with decreasing energy. It is the interplay of these two tendencies that determines the overall evolution. If the density of states is constant, only broadening exists. In this case a packet in energy space evolves into Gaussian one, moving with constant drift velocity and mean square deviation increasing linearly in time. If the density of states depletes exponentially with decreasing energy, the motion of the packet tremendously slows down with time. For large times the mean square deviation of the packet becomes constant, so that the motion of the packet is ``soliton-like''.Comment: 26 pages, RevTeX, 10 EPS figures, submitted to Phys. Rev.

Non-Markovian Configurational Diffusion and Reaction Coordinates for Protein Folding

The non-Markovian nature of polymer motions is accounted for in folding kinetics, using frequency-dependent friction. Folding, like many other problems in the physics of disordered systems, involves barrier crossing on a correlated energy landscape. A variational transition state theory (VTST) that reduces to the usual Bryngelson-Wolynes Kramers approach when the non-Markovian aspects are neglected is used to obtain the rate, without making any assumptions regarding the size of the barrier, or the memory time of the friction. The transformation to collective variables dependent on the dynamics of the system allows the theory to address the controversial issue of what are ``good'' reaction coordinates for folding.Comment: 9 pages RevTeX, 3 eps-figures included, submitted to PR

Electronic correlation effects and the Coulomb gap at finite temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type Germanium, using tunneling spectroscopy on mechanically controllable break junctions. The tunnel conductance was measured as a function of energy and temperature. At low temperatures, the spectra reveal a minimum at zero bias voltage due to the Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by thermal excitations. This behavior is reflected in the temperature dependence of the variable-range hopping resitivity measured on the same samples: Up to a few degrees Kelvin the Efros-Shkovskii ln$R \propto T^{-1/2}$ law is obeyed, whereas at higher temperatures deviations from this law are observed, indicating a cross-over to Mott's ln$R \propto T^{-1/4}$ law. The mechanism of this cross-over is different from that considered previously in the literature.Comment: 3 pages, 3 figure

Hopping Conduction in Uniaxially Stressed Si:B near the Insulator-Metal Transition

Using uniaxial stress to tune the critical density near that of the sample, we have studied in detail the low-temperature conductivity of p-type Si:B in the insulating phase very near the metal-insulator transition. For all values of temperature and stress, the conductivity collapses onto a single universal scaling curve. For large values of the argument, the scaling function is well fit by the exponentially activated form associated with variable range hopping when electron-electron interactions cause a soft Coulomb gap in the density of states at the Fermi energy. The temperature dependence of the prefactor, corresponding to the T-dependence of the critical curve, has been determined reliably for this system, and is proportional to the square-root of T. We show explicitly that nevlecting the prefactor leads to substantial errors in the determination of the scaling parameters and the critical exponents derived from them. The conductivity is not consistent with Mott variable-range hopping in the critical region nor does it obey this form for any range of the parameters. Instead, for smaller argument of the scaling function, the conductivity of Si:B is well fit by an exponential form with exponent 0.31 related to the critical exponents of the system at the metal- insulator transition.Comment: 13 pages, 6 figure

Fractional excretion of sodium after renal transplantation

Fractional excretion of sodium after renal transplantation. After renal transplantation low urinary sodium concentration (UNa) has been used to diagnose acute rejection (AR), for the early phase of AR is often associated with reduced renal perfusion. Early postoperative graft failure without low UNa favors the diagnosis of ischemic tubular damage (ATN). As fractional excretion of filtered sodium (FENa) better reflects glomerulotubular balance in renal sodium handling, FENa was analyzed during the first 2 weeks in 118 renal allografts. From data on 41 transplants with good early renal function (GEF), a temporal profile of FENa was obtained and used to evaluate the behavior of FFNa by means of standardized FENa (z score). Individual subjects followed their own profile with a small deviation (Δz < 1.4 for 2 days). In 31 instances, acute rejection was diagnosed. In 14 with AR, the z score deviated little; 2 responded to methylprednisolone given intravenously. In 17 with AR, the z score fell significantly (Δz > 1.5 for 2 days), an average of 2.6 days before the first rise in serum creatinine concentration; 15 responded to treatment. The difference between these two groups was significant (P < 0.001). This functional heterogeneity and different responses to treatment may indicate different immunologic mechanisms which damage different target cells in the graft in AR. In 46 patients with acute tubular necrosis after cadaver kidney transplantation FENa was significantly higher than it was in the GEF group as early as the first posttransplantation day and approached normal as the renal function recovered. This behavior of FENa was clearly different from that in AR.Excrétion fractionnelle du sodium après transplantation rénale. Après transplantation rénale une concentration urinaire de sodium (UNa) faible est considérée comme un signe de rejet aigu (AR), du fait que la phase précoce du rejet est souvent associée à une diminution du débit rénal. L'échec précoce d'une greffe sans abaissement de UNa est en faveur d'une tubulopathie ischémique (ATN). Puisque l'excrétion fractionnelle du sodium filtré (FENa) est le meilleur reflet de l'équilibre glomérulo-tubulaire concernant le sodium, FENa a été étudiée pendant les 2 premières semaines d'évolution de 118 allogreffes rénales. A partir de l'observation de 41 transplants ayant eu un bon fonctionnement précoce (GEF) un profil de FENa en fonction du temps a été obtenu et utilisé pour évaluer le comportement de FENa au moyen d'une FENa standardisée (test z). Les sujets ont suivi leur propre profil avec une déviation faible (Δz < 1,4 par 2 jours). Le rejet aigu a été diagnostiqué dans 31 cas. Quatorze d'entre eux avaient une déviation minime de z; deux ont répondu à la methylprednisolone i.v. Dix sept sujets avaient une déviation significative de z (Δz > 1,5 par 2 jours), 2,6 jours en moyenne avant la première augmentation de la créatinine; quinze ont répondu au traitement. La différence entre ces deux groupes est significative (P < 0,001). Cette hétérogénéité fonctionnelle et cette différence de réponse au traitement peuvent être la traduction de mécanismes immunologiques différents qui atteignent des cellules cibles de la greffe différentes au cours du rejet aigu. Chez 46 malades ayant des lésions ischémiques après transplantation de reins de cadavre, FENa était significativement plus élevée que dans le groupe GEF dès le premier jour après la transplantation et revenait vers la normale au fur et à mesure que la fonction rénale s'améliorait. Ce comportement de FENa est nettement différent de celui observé dans les rejets aigus