ABSTRACT
We experimentally demonstrate Cooper pairs' drastic enhancement of the band-to-band radiative recombination rate in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode. The recombination of a Cooper pair with p-type carriers dramatically increases the photon generation probability of a light-emitting diode in the optical-fiber communication band. The measured radiative decay time rapidly decreases with decreasing temperature below the superconducting transition temperature of the niobium electrodes. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.
ABSTRACT
We report on the mapping of quantum-Hall edge states by quasiresonant photovoltage measurements using a near-field scanning optical microscope. We have observed fine structures near sample edges that shift inward with an increase in magnetic field in accordance with the shift of the positions of the quantum-Hall edge states. We have found a transition from the weak disorder regime where compressible-incompressble strips are visible to the strong disorder regime where fluctuations smear out incompressible strips.
ABSTRACT
We study the Kondo effect in a semiconductor quantum dot in contact with a spin-accumulated lead. The spin accmulation in a nonmagnetic semiconductor is realized by spin injection from a spin-polarized quantum point contact in combination with magnetic focusing, thus creating spin-unbalanced chemical potentials. We demonstrate that the spin splitting of the Kondo densities of states (DOS) for spin-up and spin-down electrons can be controlled by selectively shifting only the spin-up DOS using spin accumulation. We also show the possibility to recover the Kondo effect in a high magnetic field, by compensating for Zeeman splitting by spin accumulation.
ABSTRACT
We investigate low-temperature transport characteristics of a side-coupled double quantum dot where only one of the dots is directly connected to the leads. We observe Fano resonances, which arise from interference between discrete levels in one dot and the Kondo effect, or cotunneling in general, in the other dot, playing the role of a continuum. The Kondo resonance is partially suppressed by destructive Fano interference, reflecting novel Fano-Kondo competition. We also present a theoretical calculation based on the tight-binding model with the slave boson mean field approximation, which qualitatively reproduces the experimental findings.
ABSTRACT
We clearly identify single-electron-localization (SEL), nonlinear screening (NLS), and linear screening (LS) regimes of gate induced electrons in a GaAs quantum well from photoluminescence spectra and intergate capacitance. Neutral and charged excitons observed in the SEL regime rapidly lose their oscillator strength when electron puddles are formed, which mark the onset of NLS. A further increase in the density of the electrons induces the transition from the NLS to LS, where the emission of a charged exciton changes to the recombination of two-dimensional electron gas and a hole.
ABSTRACT
We study switching current statistics in moderately damped Nb-InAs-Nb and intrinsic Bi2Sr2CaCu2O8+delta) Josephson junctions. A paradoxical collapse of thermal activation with increasing temperature is reported and explained by the interplay of two conflicting consequences of thermal fluctuations, which can both assist in premature escape and help in retrapping back into the stationary state. We analyze the influence of dissipation on the thermal escape by tuning damping with a gate voltage, magnetic field, temperature, and an in situ capacitor.
