The dependence of the critical current density j c on the ferromagnetic interlayer thickness d F was determined for Nb/ Al 2 O 3 / Cu/ Ni/ Nb Josephson tunnel junctions with ferromagnetic Ni interlayer thicknesses from very thin films ͑ϳ1 nm͒ upward and classified into F-layer thickness regimes showing a dead magnetic layer, exchange, exchange+ anisotropy and total suppression of j c . The Josephson coupling changes from 0 to as function of d F , and-very close to the crossover thickness-as function of temperature. The strong suppression of the supercurrent in comparison to nonmagnetic Nb/ Al 2 O 3 / Cu/ Nb junctions indicated that the insertion of a F layer leads to additional interface scattering. The transport inside the dead magnetic layer was in dirty limit. For the magnetically active regime fitting with both the clean and the dirty limit theories was carried out, indicating dirty limit condition, too. The results were discussed in the framework of literature.

Ternary (Bi 1−x Sb x ) 2 Te 3 films with an Sb content between 0 and 100% were deposited on a Si(111) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by X-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi 1−x Sb x ) 2 Te 3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n-to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n-to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states.

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 4 K. 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.

We report the detection of electromagnetic radiation at about 500GHz from current-biased intrinsic Bi2Sr2CaCu2O8 single crystal Josephson junctions. We used two silicon lenses to quasioptically couple radiation from our samples to an integrated superconducting heterodyne receiver. The estimated maximum Josephson radiation power which reached the receiver antenna was about 1pW. We attribute the observed radiation to individual Josephson junctions of the stack and discuss a possibility of the phase locking of a larger number of junctions.

We experimentally studied the Josephson supercurrent in Nb/InN-nanowire/Nb junctions. Large critical currents up to 5.7 µA have been achieved, which proves the good coupling of the nanowire to the superconductor. The effect of a magnetic field perpendicular to the plane of the Josephson junction on the critical current has been studied. The observed monotonous decrease of the critical current with magnetic field is explained by the magnetic pair-breaking effect in planar Josephson junctions of ultra-narrow width [J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007)] Superconductor/normal-conductor/superconductor (SNS) junctions with a semiconductor employed as the N-weak link material offer the great advantage that here the Josephson supercurrent can be controlled by means of the field effect. 1,2 Gate-controlled superconductor/semiconductor hybrid devices such as superconducting field effect transistors 3 or split-gate structures 4 have been fabricated which find no counterpart in conventional SNS structures. In addition, the high carrier mobility attainable in semiconductors in combination with the phase-coherent Andreev reflection leads to novel unique phenomena in the magnetotransport. 5-7 Usually for these devices the semiconductor is patterned by conventional lithography. As an elegant alternative one can also directly create semiconductor nanostructures, i.e. nanowires, by epitaxial growth. 8 By using InAs nanowires connected to superconducting electrodes tunable Josephson supercurrents, supercurrent reversal, and Kondo-enhanced Andreev tunneling have been realized. [9][10][11] Among the various materials used for semiconductor nanowires InN is of particular interest for semiconductor/superconductor hybrid structures, since the surface accumulation layer in InN can provide a sufficiently low resistive contact to superconducting electrodes. 12-14 Due to almost ideal crystalline properties of InN nanowires electronic transport along the wires, contacted by normal metal electrodes, shows quantization phenomena, i.e. flux periodic magnetoconductance oscillations. 15 Furthermore, the carrier concentration in the surface electron gas is of the order of 10 13 cm −2 and thus about a factor of ten larger than in InAs. Consequently when combined with superconducting electrodes one can expect low resistive SNS junctions.Here, we report on transport studies of Nb/InNnanowire/Nb junctions. We succeeded in observing a pronounced Josephson supercurrent and a relatively large I c R N product of up to 0.44 mV. The latter factor, the critical current times the normal resistance, is an important figure of merit for Josephson junctions. We devoted special attention to the dependence of the critical current I c on an external magnetic field B, where a monotonous decrease of I c with B was found. This experimental finding is interpreted in the framework of a recent theoretical model for the proximity effect in narrow-width junctions with dimensions comparable or smaller than the magnetic length ξ B = Φ 0 /B,...

We have investigated the differential resistance of hybrid planar Al-(Cu/Fe)-Al submicron bridges at low temperatures and in weak magnetic fields. The structure consists of a Cu/Fe bilayer forming a bridge between two superconducting Al electrodes. In the superconducting state of Al electrodes, we have observed a double-peak peculiarity in differential resistance of the S-(N/F)-S structures at a bias voltage corresponding to the minigap. We claim that this effect (the splitting of the minigap) is due to an electron spin polarization in the normal metal which is induced by the ferromagnet. We have demonstrated that the double-peak peculiarity is converted to a single peak at a coercive applied field corresponding to zero magnetization of the Fe layer.

We report on the fabrication and characterization of symmetric nanowire-based Josephson junctions, that is, Al- and Nb-based junctions, and asymmetric junctions employing superconducting Al and Nb. In the symmetric junctions, a clear and pronounced Josephson supercurrent is observed. These samples also show clear signatures of subharmonic gap structures. At zero magnetic field, a Josephson coupling is found for the asymmetric Al/InAs-nanowire/Nb junctions as well. By applying a magnetic field above the critical field of Al or by raising the temperature above the critical temperature of Al the junction can be switched to an effective single-interface superconductor/nanowire structure. In this regime, a pronounced zero-bias conductance peak due to reflectionless tunneling has been observed.

We report on the Shubnikov-de Haas oscillations in the longitudinal resistance of thin films of three-dimensional topological insulator Sb 2 Te 3 grown by means of molecular beam epitaxy. The oscillations persist up to the temperatures of 30 K, and the measurements at various tilt angles reveal that they originate from a two-dimensional system. Using a top gate, we further study the change of oscillation amplitude and frequency, which in combination with the standard Hall measurements suggest the origin of oscillations to be at the interface between the film and the Si substrate.