arXiv subjects

D. Gruetzmacher

Publications and source records attributed to D. Gruetzmacher.

6 recordsLinked to original sources

arXiv2012

Supercurrent in Nb/InAs-Nanowire/Nb Josephson junctions

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. In junctions with high doping concentrations in the nanowire Josephson supercurrent values up to 100\,nA are found. Owing to the use of Nb as superconductor the Josephson coupling persists at temperatures up to 4K. In all junctions the critical current monotonously decreased with the magnetic field, which can be explained by a recently developed theoretical model for the proximity effect in ultra-small Josephson junctions. For the low-doped Josephson junctions a control of the critical current by varying the gate voltage has been demonstrated. We have studied conductance fluctuations in nanowires coupled to superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.

cond-mat.mes-hall↗

arXiv2011

Robust surface electronic properties of topological insulators: Bi2Te3 films grown by molecular beam epitaxy

The surface electronic properties of the important topological insulator Bi2Te3 are shown to be robust under an extended surface preparation procedure which includes exposure to atmosphere and subsequent cleaning and recrystallization by an optimized in-situ sputter-anneal procedure under ultra high vacuum conditions. Clear Dirac-cone features are displayed in high-resolution angle-resolved photoemission spectra from the resulting samples, indicating remarkable insensitivity of the topological surface state to cleaning-induced surface roughness.

cond-mat.mtrl-sci↗

arXiv2005

Intra-valence-band mixing in strain-compensated SiGe quantum wells

We explore the midinfrared absorption of strain-compensated p-Si0.2Ge0.8/Si quantum wells for various well thicknesses and temperatures. Owing to the large band offset due to the large bi-axial strain contrast between the wells and barriers, the intersubband transitions energies from the ground state to the excited heavy hole (hh), light hole (lh) and split-off hole (so) states are resolved to ~0.5 eV. When hh2 is within ~30 meV of lh1 or so1 a partial transfer of the hh1-hh2 oscillator strength to the hh1-lh1 or hh1-so1 transitions is observed, which is otherwise forbidden for light polarized perpendicular to the plane of the wells. This is a clear sign of mixing between the hh and lh or so-states. A large temperature induced broadening of hh2 peak is observed for narrow wells indicating a non-parabolic dispersion of the hh2 states due to the mixing with the lh/so continuum. We found that the 6-band k.p theory gives a quantitative account of the observations. A possible role of many-body effects in the temperatureinduced negative peak shift is discussed.

cond-mat.other↗

arXiv2000

Single hole transistor in a p-Si/SiGe quantum well

A single hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.

cond-mat.mes-hall↗

arXiv1999

Metal-Insulator Transition in a 2-Dimensional System with an Easy Spin Axis

The low-temperature resistivity of a SiGe 2-dimensional hole gas has been studied using the gate controlled carrier density as a parameter. A metal-insulator transition is seen both in the temperature and in the electric field behaviour. Values of 1 for the dynamical exponent and 2.85 for the correlation length exponent are obtained from scaling plots. No quenching of the metallic phase in a parallel magnetic field is observed. Since in our system there is an easy axis for magnetization, this result supports the hypothesis that the transition is related to spin interactions.

cond-mat.mes-hall↗

arXiv1999

Coexistence of Weak Localization and a Metallic Phase in Si/SiGe Quantum Wells

Magnetoresistivity measurements on p-type Si/SiGe quantum wells reveal the coexistence of a metallic behavior and weak localization. Deep in the metallic regime, pronounced weak localization reduces the metallic behavior around zero magnetic field without destroying it. In the insulating phase, a positive magnetoresistivity emerges close to B=0, possibly related to spin-orbit interactions.

cond-mat.mes-hall↗