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We study a superconducting quantum point contact made of a narrow In$_{0.75}% $Ga$_{0.25}$As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor--superconductor hybrid Josephson junction involving only a single ballistic channel., 6 pages, 4 figures

A gated inverted ${\mathrm{In}}_{0.52}{\mathrm{Al}}_{0.48}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.52}{\mathrm{Al}}_{0.48}\mathrm{As}$ quantum well is studied via magnetotransport. By analyzing the gate-voltage-dependent beating pattern observed in the Shubnikov--de Haas oscillation, we determine the gate voltage (or electron concentration) dependence of the spin-orbit coupling parameter \ensuremath{\alpha}. Our experimental data and its analysis show that the band nonparabolicity effect cannot be neglected. For electron concentrations above $2\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2},$ it causes a reduction of \ensuremath{\alpha} up to 25%. We report the \ensuremath{\alpha} value for the second subband.

We report on measurement of the electron-hole effective $g$-factors (${g}_{\mathrm{eff}}^{*}$) depending on electron filling factors $\ensuremath{\nu}$ from magnetophotoluminescence spectroscopy at low magnetic fields $Bl1$ T in low electron density regime in a GaAs/Al${}_{0.33}$Ga${}_{0.67}$As-gated quantum well. Enhancement of ${g}_{\mathrm{eff}}^{*}$ at odd $\ensuremath{\nu}$ is observed. The oscillatory behavior of ${g}_{\mathrm{eff}}^{*}$ is compared with results of a theory that takes into account the lowest-order exchange interaction of the screened Coulomb interaction. Good agreement of the observed ${g}_{\mathrm{eff}}^{*}$ with theoretical results is obtained except $\ensuremath{\nu}$ at around 3. The enhancement of ${g}_{\mathrm{eff}}^{*}$ at even $\ensuremath{\nu}$ with decrease in electron density has not been observed.

The differential resistance in an InAs-inserted-channel ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As heterostructure-coupled superconducting junction is enhanced within the Nb superconducting gap energy by applying a magnetic field. However, the enhancement saturates under a magnetic field of several mT. This behavior suggests the reduction of Andreev-reflection (AR) probabilities by the magnetic field. The AR probabilities are calculated as a function of the pair-potential penetration length into the InAs channel. The enhanced AR probabilities decrease with decreasing pair-potential penetration length. The magnetic field reduces the penetration of the pair potential in the channel and therefore reduces the AR probability. This reduction of AR probability causes an enhancement of dV/dI.

We report on carrier transport across a superconductor/ferromagnetic semiconductor junction with a highly transparent metallic contact, Nb/$p$-In${}_{0.96}$Mn${}_{0.04}$As. Below $~$10 K, $p$-In${}_{0.96}$Mn${}_{0.04}$As becomes ferromagnetic, as evidenced by the hysteretic transverse resistance caused by the anomalous Hall effect. Below the superconducting critical temperature $T$${}_{c}$ of the Nb electrodes (8.2 K), a conductance reduction occurs within the bias voltage that is comparable to the Nb superconducting energy gap. A rather moderate slope in the differential conductance curves within the gap region indicates the partial suppression of the Andreev reflection caused by spin-polarized carriers in $p$-In${}_{0.96}$Mn${}_{0.04}$As. Spin polarization $P$ in $p$-In${}_{0.96}$Mn${}_{0.04}$As has been extracted by fitting the measured differential conductance curves with a newly modified Blonder-Tinkham-Klapwijk model with both spin polarization and the inverse proximity effect (mod2-BTK model). The extracted $P$ value is $P$ $=$ 0.725 at 0.5 K, and it decreases gradually with increasing temperature.

Supercurrent is observed in a superconductor-semiconductor-superconductor (S-Sm-S) junction using a two-dimensional-electron-gas system in an InAs-inserted-channel ${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As/${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As heterostructure. The temperature dependence of the supercurrent cannot be explained by a simple exponential expression, but is here explained in terms of the clean-limit theory, taking account of the higher-order effect of the coherence length. The differential resistance of the junction shows clear subharmonic energy-gap structures due to multiple Andreev reflection since the phase coherence is maintained during the Andreev-reflection process. The Cooper pair transport explained by the clean-limit theory is attributed to the fact that the mean free path is longer than the coherence length and the length between the superconducting electrodes.

Both the electron and the optically created hole effective masses are found to be density dependent in a two-dimensional electron system of a $\mathrm{GaAs}∕{\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ back-gated quantum well by magnetophotoluminescence spectroscopy. We show that the density-dependent electron effective mass increases with a decrease in the electron density $({n}_{s})$ to ${n}_{s}l1\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. It is found that the electron effective masses determined from the lowest and the second Landau levels are larger than those from the higher Landau levels. The hole effective mass is found to increase with a decrease in ${n}_{s}$ and the hole is found to localize at ${n}_{s}l3\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. We observe an upward convex curve of the photoluminescence peak energy at $2l\ensuremath{\nu}l3$ depending on the electron-hole distance divided by the magnetic length. These results clearly show the important roles of both electron-electron and electron-hole interactions in the recombination of a valence hole with a high-quality two-dimensional electron system.

We have measured the conductance and shot-noise of superconductor-normal metal (S-N) junctions between a Niobium (Nb) film and a 2-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a submicrometer beam-splitter. The current shot-noise measured through one arm of the beam-splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching approx. 60-70 %. The present device can be seen as a quasi-ballistic S-N beam-splitter junction., full paper including figures can be retrieved from: http://www.unibas.ch/phys-meso/Research/Papers/2004/Shot-Noise-Yjunction.pdf

We study the influence of Cooper pair injection into niobium ~Nb!/quasidiffusive multiwalled carbon nanotube ~MWNT’s!/aluminum ~Al! junctions possessing weak localization. Evaporation of the Nb electrode on the open top end of MWNT’s standing in nanopores of alumina membranes makes end-bonded structures possible, leading to highly transparent Nb/MWNT interfaces. We found a proximity-induced conductance increase ~PIC!, which was enhanced by the extremely large diffusion constant of the Cooper pair in the MWNT’s, with onset temperatures as high as T56 ‐ 9 K. In contrast, we clarified that this PIC was very sensitive to the transparency of the MWNT/Al interface at low temperatures. High transparency led to reentrant conductance due to diffusion out of the Cooper pairs, while we successfully found superconductivity at T50.6 K, implying enhanced critical magnetic fields, in some of the low-transparency samples.

We investigate polarization-resolved fine structure in the photoluminescence (PL) in the fractional quantum Hall effect regime at $B=4$--6 T, where small Zeeman energy allows spin-depolarized ground states. We observe up to five distinct peaks with characteristic polarization and temperature dependence in the vicinity of $\ensuremath{\nu}=1/3$ and quenching of the PL from triplet charged quasiexcitons at around $\ensuremath{\nu}=1/4$. Those findings appear to be consistent with results of exact diagonalization on a Haldane sphere including all spin configurations and are understood to be PL from fractionally charged quasiexcitons.