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Register a new userJosephson radiation and shot noise of a semiconductor nanowire junction
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We measured the Josephson radiation emitted by an InSb semiconductor nanowire junction utilizing photon assisted quasiparticle tunneling in an AC-coupled superconducting tunnel junction. We quantify the action of the local microwave environment by evaluating the frequency dependence of the inelastic Cooper-pair tunneling of the nanowire junction and find the zero frequency impedance $Z(0)=492,Omega$ with a cutoff frequency of $f_0=33.1,$GHz. We extract a circuit coupling efficiency of $etaapprox 0.1$ and a detector quantum efficiency approaching unity in the high frequency limit. In addition to the Josephson radiation, we identify a shot-noise contribution with a Fano factor $Fapprox1$, consistently with the presence of single electron states in the nanowire channel.
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We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. Current-Voltage characteristic curve shows clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The sub-gap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.
We demonstrate simultaneous measurements of DC transport properties and flux noise of a hybrid superconducting magnetometer based on the proximity effect (superconducting quantum interference proximity transistor, SQUIPT). The noise is probed by a cryogenic amplifier operating in the frequency range of a few MHz. In our non-optimized device, we achieve minimum flux noise $sim 4;muPhi_0/Hz^{1/2}$, set by the shot noise of the probe tunnel junction. The flux noise performance can be improved by further optimization of the SQUIPT parameters, primarily minimization of the proximity junction length and cross section. Furthermore, the experiment demonstrates that the setup can be used to investigate shot noise in other nonlinear devices with high impedance. This technique opens the opportunity to measure sensitive magnetometers including SQUIPT devices with very low dissipation.
We have studied mesoscopic Josephson junctions formed by highly $n$-doped InAs nanowires and superconducting Ti/Pb source and drain leads. The current-voltage properties of the system are investigated by varying temperature and external out-of-plane magnetic field. Superconductivity in the Pb electrodes persists up to $ sim 7$ K and with magnetic field values up to 0.4 T. Josephson coupling at zero backgate voltage is observed up to 4.5 K and the critical current is measured to be as high as 615 nA. The supercurrent suppression as a function of the magnetic field reveals a diffraction pattern that is explained by a strong magnetic flux focusing provided by the superconducting electrodes forming the junction.
Odd frequency (odd-$omega$) electron pair correlations naturally appear at the interface between BCS superconductors and other materials. The detection of odd-$omega$ pairs, which are necessarily non-local in time, is still an open problem. The main reason is that they do not contribute to static measurements described by time-local correlation functions. Therefore, dynamical measurements, which depend on non-local time correlations, are suitable for detecting these pairs. In this work, we study the signatures of odd-$omega$ pairs in the supercurrent noise through a weak link between two superconductors at different superconducting phases. We show that the finite frequency current noise can be decomposed into three different contributions coming from even frequency (even-$omega$), odd-$omega$ pair amplitudes, and electron-hole correlation functions. Odd-$omega$ pairing, which is inter-lead (between electrons at different sides of the junction), provides a positive contribution to the noise, becoming maximal at a superconducting phase difference of $pi$. In contrast, intra-lead even-$omega$ pair amplitude tends to reduce the noise, except for a region close to $pi$, controlled by the transmission of the junction.
We have found experimentally that the noise of ballistic electron transport in a superconductor/semiconductor/superconductor junction is enhanced relative to the value given by the general relation, S_V=2eIR^2coth(eV/2kT), for two voltage regions in which this expression reduces to its thermal and shot noise limits. The noise enhancement is explained by the presence of large charge quanta, with effective charge q*=(1+2Delta/eV)e, that generate a noise spectrum S_V=2q*IR^2, as predicted in Phys. Rev. Lett. 76, 3814 (1996). These charge quanta result from multiple Andreev reflections at each junction interface, which are also responsible for the subharmonic gap structure observed in the voltage dependence of the junctions conductance.
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