Quantum Transport in Nanowire-based Hybrid Devices

We have studied the low-temperature transport properties of nanowires contacted by a normal metal as well as by superconducting electrodes. As a consequence of quantum coherence, we have demonstrated the electron interference effect in different aspects. The mesoscopic phase coherent transport properties were studied by contacting the semiconductor InAs and InSb nanowires with normal metal electrodes. Moreover, we explored the interaction of the microscopic quantum coherence of the nanowires with the macroscopic quantum coherence of the superconductors. In superconducting Nb contacted InAs nanowire junctions, we have investigated the effect of temperature, magnetic field and electric field on the supercurrent. Owing to relatively high critical temperature of superconducting Nb ($T_{c}sim9,$K), we have observed the supercurrent up to 4,K for highly doped nanowire-based junctions, while for low doped nanowire-based junctions a full control of the supercurrent was achieved. Due to low transversal dimension of the nanowires, we have found a monotonous decay of the critical current in magnetic field dependent measurements. The experimental results were analyzed within narrow junction model which has been developed recently. At high bias voltages, we have observed subharmonic energy gap structures as a consequence of multiple Andreev reflection. Some of the nanowires were etched, such that the superconducting Nb electrodes are connected to both ends of the nanowire rather than covering the surface of the nanowire. As a result of well defined nanowire-superconductor interfaces, we have examined quasiparticle interference effect in magnetotransport measurements. Furthermore, we have developed a new junction geometry, such that one of the superconducting Nb electrodes is replaced by a superconducting Al. Owing to the smaller critical magnetic field of superconducting Al ($B_{c}sim15-50,$mT), compared to superconducting Nb ($B_{c}sim3,$T), we were able to studied the Andreev reflection of quasiparticles at single interface, by suppressing the superconductivity of Al with small magnetic fields, as well as at double interface for zero magnetic field. The junction geometry was further changed by replacing the InAs nanowire with the InAs tube. In this case the GaAs/InAs core/shell tubular nanowires were contacted by two superconducting Nb electrodes. For this junction geometry we have demonstrated the interference of phase conjugated electron-hole pairs in the presence of coaxial magnetic. The effect of temperature, constant dc bias current and gate voltage on the magnetoresistance oscillations were examined. In the last part of this thesis, we have fabricated and characterized the single crystal Au nanowire-based proximity superconducting quantum interference device (SQUID)

Quantum Transport in Nanowire-based Hybrid Devices

We have studied the low-temperature transport properties of nanowires contacted by a normal metal as well as by superconducting electrodes. As a consequence of quantum coherence, we have demonstrated the electron interference effect in different aspects. The mesoscopic phase coherent transport properties were studied by contacting the semiconductor InAs and InSb nanowires with normal metal electrodes. Moreover, we explored the interaction of the microscopic quantum coherence of the nanowires with the macroscopic quantum coherence of the superconductors. In superconducting Nb contacted InAs nanowire junctions, we have investigated the effect of temperature, magnetic field and electric field on the supercurrent. Owing to relatively high critical temperature of superconducting Nb ($T_{c}\sim9\,$K), we have observed the supercurrent up to 4\,K for highly doped nanowire-based junctions, while for low doped nanowire-based junctions a full control of the supercurrent was achieved. Due to low transversal dimension of the nanowires, we have found a monotonous decay of the critical current in magnetic field dependent measurements. The experimental results were analyzed within narrow junction model which has been developed recently. At high bias voltages, we have observed subharmonic energy gap structures as a consequence of multiple Andreev reflection. Some of the nanowires were etched, such that the superconducting Nb electrodes are connected to both ends of the nanowire rather than covering the surface of the nanowire. As a result of well defined nanowire-superconductor interfaces, we have examined quasiparticle interference effect in magnetotransport measurements. Furthermore, we have developed a new junction geometry, such that one of the superconducting Nb electrodes is replaced by a superconducting Al. Owing to the smaller critical magnetic field of superconducting Al ($B_{c}\sim15-50\,$mT), compared to superconducting Nb ($B_{c}\sim3\,$T), we were able to studied the Andreev reflection of quasiparticles at single interface, by suppressing the superconductivity of Al with small magnetic fields, as well as at double interface for zero magnetic field. The junction geometry was further changed by replacing the InAs nanowire with the InAs tube. In this case the GaAs/InAs core/shell tubular nanowires were contacted by two superconducting Nb electrodes. For this junction geometry we have demonstrated the interference of phase conjugated electron-hole pairs in the presence of coaxial magnetic. The effect of temperature, constant dc bias current and gate voltage on the magnetoresistance oscillations were examined. In the last part of this thesis, we have fabricated and characterized the single crystal Au nanowire-based proximity superconducting quantum interference device (SQUID)

Comparison of real-time polymerase chain reaction assay methods for detection of RHD gene in amniotic fluid

Hemolytic disease of the newborn is the clinical condition in which Rh blood group antigens in couples are incompatible with each other and mother is negative for the antigen, whereas father is positive. Although RHD antigen encoded by RHD gene that is localized on chromosome 1 determines person's Rh genotyping, this incompatibility can lead to delivery as anemia, jaundiced, or dead in mother's uterus. In recent years, improvements have occurred in the prenatal diagnosis of Rh incompatibility. Quantitative real-time polymerase chain reaction (Real-time PCR) has been improved and determining rapidly, reliably, and sensitively has been possible. In this study, the determination of RHD genotyping was investigated using fetal DNA obtained from amniotic fluid and SYBR Green I and TaqMan probe methods were compared, and reliability in prenatal diagnosis of these methods was determined. We studied 35 pregnant women in the second trimester of pregnancy. “SYBR Green I” and “TaqMan” probes results for RHD gene of genomic DNA extracted from total 35 different amniotic fluid samples acquired from 10 RHD (-) and 25 pregnant women randomly were analyzed. DNA extracted from amniotic fluid was analyzed for RHD gene with real-time PCR and the results were then compared with the RHD fetal genotype determined on RHD phenotype of the red blood cells of the infants at birth. The results of RHD TaqMan probes PCR analysis of amniotic fluid DNA were completely concordant with the fetal blood group analysis after birth. Real-time PCR using the TaqMan probes has proven to be more sensitive, accurate, and specific for RHD gene than SYBR Green I method

Scanning tunneling microscopy with InAs nanowire tips

Indium arsenide nanowires grown by selective-area vapor phase epitaxy are used as tips for scanning tunneling microscopy (STM). The STM tips are realized by positioning the wires manually on the corner of a double cleaved gallium arsenide wafer with sub-μm precision and contacting them lithographically, which is fully compatible with further integrated circuitry on the GaAs wafer. STM images show a z noise of 2 pm and a lateral stability of, at least, 0.5 nm on a Au(111) surface. I(z) spectroscopy reveals an exponential decay indicating tunneling through vacuum. Subsequent electron microscopy images of the tip demonstrate that the wires are barely modified during the STM imaging

Supercurrent and Magnetoresistance Oscillations inNb/InAs-Nanowire/Nb Josephson junctions

One of the common goals in semiconductor/superconductor hybrid de-vices is to fabricate Schottky barrier free contacts at the interface of the two materials.[1] The natural formation of an electron accumu-lation layer on InAs surfaces prohibits the formation of a Schottky barrier. Therefore this material became the most preferred one for semiconducting weak links in Josephson junctions. This unique prop-erty of InAs in combination with the bottom-up growth approach of nanowires, led to many interesting experiments, e.g. tunable super-currents or Cooper pair beam splitters.[3] In these experiments aluminum (Al) was used as a superconducting material, which has a low critical temperature (Tc) and a low critical magnetic eld (Bc). As an alternative, we have used superconduct-ing Niobium (Nb) with a high Tc and Bc that oers the advantage to study Josephson properties in dierent regimes. In this report, we have used InAs nanowires with two dierent bulk carrier concentra-tions, i.e. 10 18 cm 3 (low doped) and 10 19 cm 3 (highly doped). The contacting process of Nb electrodes has been realized by standard electron beam lithography. We systematically investigated the basic Josephson properties, i.e. the eect of temperature, magnetic eld and electric eld on the super-current through InAs nanowires. By taking advantage of the high Tc ( 9:3K) of the superconducting Nb, we were able to measure a super-current up to 4.0K. The highest critical current Ic 100nA has been measured at 0.4K for a junctions with a highly doped InAs nanowire. For low doped nanowire Josephson junctions, a full control of the su-percurrent has been achieved by applying a gate bias. We have found a monotonous dependence of the measured critical current in the pres-ence of a perpendicular magnetic eld rather than a Fraunhofer-like diraction pattern. The experimental results have been compared to a recent theoretical model of Ref.[4] In addition, we studied the supercur-rent and conductance uctuations as a function of gate voltage. Here, a remarkable enhancement of the conductance uctuation amplitude has been observed. In the last part, we have studied the magnetore-sistance oscillations in the voltage state of Josephson junctions. [1] Th. Schapers, Superconductor/Semiconductor Junctions, 174 (Springer Tracts on Modern Physics, 2001) [2] Y.-J. Doh, J. A. van Dam, A. L. Roest, E. P. A. M. Bakkers, L. P. Kouwenhoven, and S. D. Franceschi, Science 309, 272 (2005) [3] L. Hofstetter, S. Csonka, J. Nygard, and C. Schonenberger, Na-ture 461, 960 (2009) [4] J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007