Strain-driven light polarization switching in deep ultraviolet nitride emitters

Residual strain plays a critical role in determining the crystalline quality of nitride epitaxial layers and in modifying their band structure; this often leads to several interesting physical phenomena. It is found, for example, that compressive strain in AlxGa1-xN layers grown on AlyGa1-yN (x<y) templates results in an anti-crossing of the valence bands at considerably much higher Al composition than expected. This happens even in the presence of large and negative crystal field splitting energy for AlxGa1-xN layers. A judicious magnitude of the compressive strain can support vertical light emission (out of the c-plane) from AlxGa1-xN quantum wells up to x\approx 0.80, which is desirable for the development of deep ultraviolet light-emitting diodes designed to operate below 250nm with transverse electric polarization characteristics

Magnetotransport of coupled electron-holes

The carriers in InAs-GaSb double quantum wells are hybrid ``electron-holes''. We study the magnetotransport properties of such particles using a two-component Keldysh technique, which results in a semi-analytic expression for the small-field current. We show that zero temperature current can be large even when the Fermi energy lies within the hybridization gap, a result which cannot be understood within a semiclassical (Boltzmann) approach. Magnetic field dependence of the conductance is also affected significantly by the hybridization of electrons and holes.Comment: 4 pages, 2 figure

Hopping magnetoresistance in ion irradiated monolayer graphene

Magnetoresistance (MR) of ion irradiated monolayer graphene samples with variable-range hopping (VRH) mechanism of conductivity was measured at temperatures down to $T = 1.8$ K in magnetic fields up to $B = 8$ T. It was observed that in perpendicular magnetic fields, hopping resistivity $R$ decreases, which corresponds to negative MR (NMR), while parallel magnetic field results in positive MR (PMR) at low temperatures. NMR is explained on the basis of the "orbital" model in which perpendicular magnetic field suppresses the destructive interference of many paths through the intermediate sites in the total probability of the long-distance tunneling in the VRH regime. At low fields, a quadratic dependence ($|\Delta R/R|\sim B^2$) of NMR is observed, while at $B > B^*$, the quadratic dependence is replaced by the linear one. It was found that all NMR curves for different samples and different temperatures could be merged into common dependence when plotted as a function of $B/B^*$. It is shown that $B^*\sim T^{1/2}$ in agreement with predictions of the "orbital" model. The obtained values of $B^*$ allowed also to estimate the localization radius $\xi$ of charge carriers for samples with different degree of disorder. PMR in parallel magnetic fields is explained by suppression of hopping transitions via double occupied states due to alignment of electron spins.Comment: 14 pages, 9 figures. As accepted for publication on Physica

Excitonic instability and electric-field-induced phase transition towards a two dimensional exciton condensate

We present an InAs-GaSb-based system in which the electric-field tunability of its 2D energy gap implies a transition towards a thermodynamically stable excitonic condensed phase. Detailed calculations show a 3 meV BCS-like gap appearing in a second-order phase transition with electric field. We find this transition to be very sharp, solely due to exchange interaction, and so, the exciton binding energy is greatly renormalized even at small condensate densities. This density gradually increases with external field, thus enabling the direct probe of the Bose-Einstein to BCS crossover.Comment: LaTex, 11 pages, 3 ps figures, To appear in PR

Nonlinear voltage dependence of the shot noise in mesoscopic degenerate conductors with strong electron-electron scattering

It is shown that measurements of zero-frequency shot-noise can provide information on electron-electron interaction, because the strong interaction results in the nonlinear voltage dependence of the shot noise in metallic wires. This is due to the fact that the Wiedemann-Franz law is no longer valid in the case of considerable electron-electron interaction. The deviations from this law increase the noise power and make it dependent strongly on the ratio of electron-electron and electron-impurity scattering rates.Comment: 4 pages, 2 figures, revised version according to referee's comment

Current noise in long diffusive SNS junctions in the incoherent MAR regime

Spectral density of current fluctuations at zero frequency is calculated for a long diffusive SNS junction with low-resistive interfaces. At low temperature, T << Delta, the subgap shot noise approaches linear voltage dependence, S=(2/ 3R)(eV + 2Delta), which is the sum of the shot noise of the normal conductor and voltage independent excess noise. This result can also be interpreted as the 1/3-suppressed Poisson noise for the effective charge q = e(1+2Delta/eV) transferred by incoherent multiple Andreev reflections (MAR). At higher temperatures, anomalies of the current noise develop at the gap subharmonics, eV = 2Delta/n. The crossover to the hot electron regime from the MAR regime is analyzed in the limit of small applied voltages.Comment: improved version, to be published in Phys. Rev.

Shot noise suppression in multimode ballistic Fermi conductors

We have derived a general formula describing current noise in multimode ballistic channels connecting source and drain electrodes with Fermi electron gas. In particular (at $eV\gg k_{B}T$), the expression describes the nonequilibrium ''shot'' noise, which may be suppressed by both Fermi correlations and space charge screening. The general formula has been applied to an approximate model of a 2D nanoscale, ballistic MOSFET. At large negative gate voltages, when the density of electrons in the channel is small, shot noise spectral density $S_{I}(0)$ approaches the Schottky value $2eI$, where $I$ is the average current. However, at positive gate voltages, when the maximum potential energy in the channel is below the Fermi level of the electron source, the noise can be at least an order of magnitude smaller than the Schottky value, mostly due to Fermi effects.Comment: 4 page

What you know can influence what you are going to know (especially for older adults)

Stimuli related to an individual's knowledge/experience are often more memorable than abstract stimuli, particularly for older adults. This has been found when material that is congruent with knowledge is contrasted with material that is incongruent with knowledge, but there is little research on a possible graded effect of congruency. The present study manipulated the degree of congruency of study material with participants’ knowledge. Young and older participants associated two famous names to nonfamous faces, where the similarity between the nonfamous faces and the real famous individuals varied. These associations were incrementally easier to remember as the name-face combinations became more congruent with prior knowledge, demonstrating a graded congruency effect, as opposed to an effect based simply on the presence or absence of associations to prior knowledge. Older adults tended to show greater susceptibility to the effect than young adults, with a significant age difference for extreme stimuli, in line with previous literature showing that schematic support in memory tasks particularly benefits older adults

Frequency-Dependent Shot Noise as a Probe of Electron-Electron Interaction in Mesoscopic Diffusive Contacts

The frequency-dependent shot noise in long and narrow mesoscopic diffusive contacts is numerically calculated. The case of arbitrarily strong electron-electron scattering and zero temperature of electrodes is considered. For all voltages, the noise increases with frequency and tends to finite values. These limiting values are larger than the Poissonian noise and increase nearly as voltage to power 4/3. This allows one to experimentally determine the parameters of electron-electron interaction.Comment: 3 pages, RevTeX, 3 eps figure

Effect of screening on shot noise in diffusive mesoscopic conductors

Shot noise in diffusive mesoscopic conductors, at finite observation frequencies $\omega$ (comparable to the reciprocal Thouless time $\tau_T^{-1}$), is analyzed with an account of screening. At low frequencies, the well-known result $S_I(\omega)=2eI/3$ is recovered. This result is valid at arbitrary $\omega \tau_T$ for wide conductors longer than the screening length. However, at least for two very different systems, namely, wide and short conductors, and thin conductors over a close ground plane, noise approaches a different fundamental level, $S_I(\omega) = eI$, at $\omega \tau _T\gg 1$.Comment: 5 pages, 3 figures. Published version. Also available in the journal's format at http://hana.physics.sunysb.edu/~yehuda/cv/papers/shotnoise.pd