Heavy atom quantum diffraction by scattering from surfaces

Typically one expects that when a heavy particle collides with a surface, the scattered angular distribution will follow classical mechanics. The heavy mass assures that the de Broglie wavelength of the incident particle in the direction of the propagation of the particle (the parallel direction) will be much shorter than the characteristic lattice length of the surface, thus leading to a classical description. Recent work on molecular interferometry has shown that by increasing the perpendicular coherence length, one may observe interference of very heavy species passing through a grating. Here we show, using quantum mechanical simulations, that the same effect will lead to quantum diffraction of heavy particles colliding with a surface. We find that the effect is robust with respect to the incident energy, the angle of incidence and the mass of the particle. It may also be used to verify the quantum nature of the surface and its fluctuations at very low temperatures.Comment: 9 pages, 3 figure

The glass transition and the Coulomb gap in electron glasses

We establish the connection between the presence of a glass phase and the appearance of a Coulomb gap in disordered materials with strongly interacting electrons. Treating multiparticle correlations in a systematic way, we show that in the case of strong disorder a continuous glass transition takes place whose Landau expansion is identical to that of the Sherrington-Kirkpatrick spin glass. We show that the marginal stability of the glass phase controls the physics of these systems: it results in slow dynamics and leads to the formation of a Coulomb gap

Nonexponential Relaxations in a Two-Dimensional Electron System in Silicon

The relaxations of conductivity have been studied in a strongly disordered two-dimensional (2D) electron system in Si after excitation far from equilibrium by a rapid change of carrier density n_s at low temperatures T. The dramatic and precise dependence of the relaxations on n_s and T strongly suggests (a) the transition to a glassy phase as T->0, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics.Comment: 5 pages, 5 figure

History-dependent relaxation and the energy scale of correlation in the Electron-Glass

We present an experimental study of the energy-relaxation in Anderson-insulating indium-oxide films excited far from equilibrium. In particular, we focus on the effects of history on the relaxation of the excess conductance dG. The natural relaxation law of dG is logarithmic, namely dG=-log(t). This may be observed over more than five decades following, for example, cool-quenching the sample from high temperatures. On the other hand, when the system is excited from a state S_{o} in which it has not fully reached equilibrium to a state S_{n}, the ensuing relaxation law is logarithmic only over time t shorter than the time t_{w} it spent in S_{o}. For times t>t_{w} dG(t) show systematic deviation from the logarithmic dependence. It was previously shown that when the energy imparted to the system in the excitation process is small, this leads to dG=P(t/t_{w}) (simple-aging). Here we test the conjecture that `simple-aging' is related to a symmetry in the relaxation dynamics in S_{o} and S_{n}. This is done by using a new experimental procedure that is more sensitive to deviations in the relaxation dynamics. It is shown that simple-aging may still be obeyed (albeit with a modified P(t/t_{w})) even when the symmetry of relaxation in S_{o} and S_{n} is perturbed by a certain degree. The implications of these findings to the question of aging, and the energy scale associated with correlations are discussed

Hopping models and ac universality

Some general relations for hopping models are established. We proceed to discuss the universality of the ac conductivity which arises in the extreme disorder limit of the random barrier model. It is shown that the relevant dimension entering into the diffusion cluster approximation (DCA) is the harmonic (fracton) dimension of the diffusion cluster. The temperature scaling of the dimensionless frequency entering into the DCA is discussed. Finally, some open questions about ac universality are mentioned.Comment: 6 page

Extreme(ly) mean(ingful): Sequential formation of a quality group

The present paper studies the limiting behavior of the average score of a sequentially selected group of items or individuals, the underlying distribution of which, $F$, belongs to the Gumbel domain of attraction of extreme value distributions. This class contains the Normal, Lognormal, Gamma, Weibull and many other distributions. The selection rules are the "better than average" ($\beta=1$) and the "$\beta$-better than average" rule, defined as follows. After the first item is selected, another item is admitted into the group if and only if its score is greater than $\beta$ times the average score of those already selected. Denote by $\bar{Y}_k$ the average of the $k$ first selected items, and by $T_k$ the time it takes to amass them. Some of the key results obtained are: under mild conditions, for the better than average rule, $\bar{Y}_k$ less a suitable chosen function of $\log k$ converges almost surely to a finite random variable. When $1-F(x)=e^{-[x^{\alpha}+h(x)]}$, $\alpha>0$ and $h(x)/x^{\alpha}\stackrel{x\rightarrow \infty}{\longrightarrow}0$, then $T_k$ is of approximate order $k^2$. When $\beta>1$, the asymptotic results for $\bar{Y}_k$ are of a completely different order of magnitude. Interestingly, for a class of distributions, $T_k$, suitably normalized, asymptotically approaches 1, almost surely for relatively small $\beta\ge1$, in probability for moderate sized $\beta$ and in distribution when $\beta$ is large.Comment: Published in at http://dx.doi.org/10.1214/10-AAP684 the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

Dielectric susceptibility of the Coulomb-glass

We derive a microscopic expression for the dielectric susceptibility $\chi$ of a Coulomb glass, which corresponds to the definition used in classical electrodynamics, the derivative of the polarization with respect to the electric field. The fluctuation-dissipation theorem tells us that $\chi$ is a function of the thermal fluctuations of the dipole moment of the system. We calculate $\chi$ numerically for three-dimensional Coulomb glasses as a function of temperature and frequency

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