Origin of hysteresis in a proximity josephson junction

We investigate hysteresis in the transport properties of superconductor-normal-metal-superconductor (S-N-S) junctions at low temperatures by measuring directly the electron temperature in the normal metal. Our results demonstrate unambiguously that the hysteresis results from an increase of the normal-metal electron temperature once the junction switches to the resistive state. In our geometry, the electron temperature increase is governed by the thermal resistance of the superconducting electrodes of the junction.     

Electrical detection of spin pumping due to the precessing magnetization of a single ferromagnet

We report direct electrical detection of spin pumping, using a lateral normal-metal/ferromagnet/normal-metal device, where a single ferromagnet in ferromagnetic resonance pumps spin-polarized electrons into the normal metal, resulting in spin accumulation. The resulting backflow of spin current into the ferromagnet generates a dc voltage due to the spin-dependent conductivities of the ferromagnet. By comparing different contact materials (Al and/or Pt), we find, in agreement with theory, that the spin-related properties of the normal metal dictate the magnitude of the dc voltage.     

Dispersionless modes and the superconductivity of ultrathin films

A self-consistent analytical solution of the problem of the superconductivity of ultrathin metal films is found within the tight-binding model for normal-metal electrons with a simple example of a film of three atomic layers. Superconductivity is not destroyed in atomically thin films if the energies of the electron subsystem lie near the Fermi surface at least for some values of the quasimomentum component along the film. A substantial increase in the critical temperature of an ultrathin metal film as compared to its bulk value is possible if the electron excitation spectrum contains low-energy modes with an anomalously weak dispersion.     

Insensitivity of sub-Kelvin electron-phonon coupling to substrate properties

We have examined the role of the substrate on electron-phonon coupling in normal-metal films of Mn-doped Al at temperatures below 1 K. Normal metal-insulator-superconductor junctions were used to measure the electron temperature in the films as a function of Joule heating power and phonon temperature. Theory suggests that the distribution of phonons available for interaction with electrons in metal films may depend on the acoustic properties of the substrate, namely, that the electron-phonon coupling constant Σ would be larger on the substrate with smaller sound speed. In contrast, our results indicate that within experimental error (typically ±10%), Σ is unchanged among the two acoustically distinct substrates used in our investigation.     

Investigation of the Speed of a SINIS Bolometer at a Frequency of 350 GHz

We have measured the time of the optical response of a 350-GHz radiation detector based on a superconductor–insulator–normal metal–insulator–superconductor tunnel structure with a suspended normal-metal bridge integrated into a planar log-periodic antenna. The transient characteristics of the detector were recorded when irradiated by a fast cryogenic blackbody source with a rise time of the order of microseconds. For this purpose, a short intense heating pulse was fed to an emitting NiCr film radiation source with a low heat capacity. The measured response time was 1.8 ± 0.5 μs at a bolometer electron temperature of 0.17 K, with a detection sensitivity of 10^–17–10^–18 W Hz^–1/2 being potentially achievable.     

Electron cooling in a normal-metal hot-electron bolometer

Normal-metal hot-electron bolometers, each of which contains two superconductor-insulator-normal metal (SIN) junctions for electron cooling and two SIN junctions for temperature measurements, were fabricated and experimentally studied. The electron cooling by SIN junctions is an analog of the Peltier effect and allows one to reduce the effective electron temperature of a bolometer. The electron temperature was determined from the ratio of the differential resistance to normal one for several values of a constant bias. At a phonon temperature of 250 mK, the resistance ratio at zero bias reached 1000, which was close to the theoretical value for an ideal SIN junction. A decrease in the electron temperature from 250 to 90 mK was obtained.     

Thermal conductance of Andreev interferometers

We calculate the thermal conductance G(T) of diffusive Andreev interferometers, which are hybrid loops with one superconducting arm and one normal-metal arm. The presence of the superconductor suppresses G(T); however, unlike a conventional superconductor, G(T)/G(T)(N) does not vanish as the temperature T-->0, but saturates at a finite value that depends on the resistance of the normal-superconducting interfaces, and their distance from the path of the temperature gradient. The reduction of G(T) is determined primarily by the suppression of the density of states in the proximity-coupled normal metal along the path of the temperature gradient. G(T) is also a strongly nonlinear function of the thermal current, as found in recent experiments.     

Microscopic description of superconductor-normal-metal superlattices

We study a model for superconductor-normal-metal superlattices in which adjacent layers are coupled via single-particle hopping. Examples include the high-T _c superconductor Bi₂Sr₂CaCu₂O_8+δ, where the BiO sheets seem to have normal metallic character. Using a BCS treatment, we investigate the influence of the interlayer hopping between the superconducting and the normal-metal slabs on the superconducting density of states, the tunneling characteristics for tunneling into both superconducting and normal-metal slabs as well as the temperature dependence of the London penetration depth.     

Long-range triplet Josephson current driven by the bias voltage

We study the long-range triplet Josephson current in a clean junction composed of two s-wave superconductors and a normal-metal/ferromagnet/normal-metal trilayer. Through applying the bias voltages on the metal regions by two antiparallel half-metal electrodes, we show that the amplitude and direction of this long-range current can be controlled flexibly. Such current arises from the fact that the applied voltage can produce a nonequilibrium spin-dependent quasiparticle distribution in the metal regions so that the Cooper pairs entering these regions acquire extra momenta, which will lead to a spin-transition process in the metal regions. This process can produce the parallel spin-triplet pairs in the central ferromagnet layer. In particular, if the voltage is applied only to one metal region, we further find that the recently discovered long-range superharmonic Josephson current will appear because of the transport of an even number of parallel spin-triplet pairs.     

Circuit theory for crossed Andreev reflection and nonlocal conductance

Nonlocal currents, in devices where two normal-metal terminals are contacted to a superconductor, are determined using the circuit theory of mesoscopic superconductivity. We calculate the conductance associated with crossed Andreev reflection and electron transfer between the two normal-metal terminals, in addition to the conductance from direct Andreev reflection and quasiparticle tunneling. Dephasing and proximity effect are taken into account. PACS 74.45.+c, 74.25.Fy, 73.23.-b     

Quasiparticle interferometer controlled by quantum-correlated andreev reflection

We propose a quasiparticle interferometer for experimentally confirming the predicted phase interaction between a quasiparticle and a superconducting state at the superconductor- normal-metal (S-N) interface. The phase interaction is caused by Andreev reflection. The proposed interferometer consists of a Josephson junction and a Y-junction composed of normal electron waveguides. In the setup, the phase interaction due to Andreev reflection affects the resistance across the end of a waveguide and an electrode of the Josephson junction. Thus the amount of supercurrent flowing through the Josephson junction can control the resistance.     

Spin-polarized transport in all-metal mesoscopic spin-valve devices

In spintronic devices the spin of the electron as well as its charge is utilized. We have fabricated a spin-valve device consisting of two permalloy electrodes which are connected by a normal-metal strip made of aluminum. By changing the relative alignment of the magnetizations of the electrodes the resistance of the device can be controlled. The domain configurations of the electrodes are imaged by magnetic-force microscopy in external fields at room temperature. In transport measurements at temperatures between 1.8 and 150 K we identify the observed magnetoresistance as a spin-valve effect.     

Unconventional surface impedance of a normal-metal film covering a spin-triplet superconductor due to odd-frequency cooper pairs

We discuss the dynamic response of odd-frequency Cooper pairs to an electromagnetic field. By using the quasiclassical Green function method, we calculate the impedance (Z=R-iX) of a normal-metal thin film which covers a superconductor. In contrast with the standard relation (i.e., R?X), the impedance in spin-triplet proximity structures shows anomalous behavior (i.e., R>X) in the low frequency limit. This unusual relation is a result of the penetration of odd-frequency pairs into the normal metal and reflects the negative Cooper pair density.     

Thermoelectric effects in normal metal/superconductor interface structures

The thermopower of Andreev interferometers, which are doubly connected loops in which one arm is a superconductor and one arm is a normal metal, oscillates as a function of magnetic field with a fundamental period corresponding to a flux quantum h / 2 e through the area of the loop. While the magnetoresistance of an Andreev interferometer is symmetric with respect to the magnetic field, the thermopower can be either symmetric or antisymmetric, depending on the topology of the sample. The temperature dependence of the thermopower oscillations is nonmonotonic. This nonmonotonic behavior does not appear to be related to the reentrance observed by many groups in the conductance of normal-metal/superconductor (NS) structures.     

Limitations in cooling electrons using normal-metal-superconductor tunnel junctions

We demonstrate both theoretically and experimentally two limiting factors in cooling electrons using biased tunnel junctions to extract heat from a normal metal into a superconductor. First, when the injection rate of electrons exceeds the internal relaxation rate in the metal to be cooled, the electrons do not obey the Fermi-Dirac distribution, and the concept of temperature cannot be applied as such. Second, at low bath temperatures, states within the gap induce anomalous heating and yield a theoretical limit of the achievable minimum temperature.     

Full counting statistics of a superconducting beam splitter

We study the statistics of charge transport in a mesoscopic three-terminal device with one superconducting terminal and two normal-metal terminals. We calculate the full distribution of transmitted charges into the two symmetrically biased normal terminals. In a wide parameter range, we find large positive cross correlations between the currents in the two normal arms. We also determine the third cumulant that provides additional information on the statistics not contained in the current noise.     

Thermoelectric effects in superconducting proximity structures

Attaching a superconductor in good contact with a normal metal gives rise to a proximity effect where the superconducting correlations leak into the normal metal. An additional contact close to the first one makes it possible to carry a supercurrent through the metal. Forcing this supercurrent flow along with an additional quasiparticle current from one or many normal-metal reservoirs leads many interesting effects. The supercurrent can be used to tune the local energy distribution function of the electrons. This mechanism also leads to finite thermoelectric effects even in the presence of electron–hole symmetry. Here we review these effects and discuss to which extent the existing observations of thermoelectric effects in metallic samples can be explained through the use of the dirty limit quasiclassical theory. PACS  74.25.Fy; 73.23.-b; 74.45.+c; 74.40.+k     

Coherent quantum transport in normal-metal/d-wave superconductor/normal-metal double tunnel junctions

Taking into account the effects of quantum interference and interface scattering, combining the electron current with hole current contribution to tunnel current,we study the coherent quantum transport in normal-metal/d-wave superconductor/normal-metal (NM/d-wave SC/NM) double tunnel junctions by using extended Blonder-Tinkham-Klapwijk (BTK) approach. It is shown that all quasiparticle transport coefficients and conductance spectrum exhibit oscillating behavior with the energy, in which periodic vanishing of Andreev reflection (AR) above superconducting gap is found.In tunnel limit for the interface scattering strength taken very large, there are a series of bound states of quasiparticles formed in SC.     

Contribution of Andreev reflection to the increase in the resistance of the normal metal in a bimetallic N-S structure

We study the resistive properties of 3D normal-metal-superconductor systems in the pure mean-free-path limit l _N,S ≫ξ(T) (l _N,S are the mean free paths in the metals, and ξ is the coherence length) at liquid helium temperatures. In contrast to the situation where l≪ξ, which is common in experiments involving either sandwiches or mesoscopic samples, here the N-S system exhibits unusual temperature behavior that cannot be described by existing theories of boundary resistance. What is most remarkable is a rise in normal resistance in regions that do not incorporate the N-S boundary as the temperature decreases, with asymptotic behavior resembling that of the temperature curve of the gap of a superconductor in contact with a normal metal. We show that this effect, not observed earlier in 3D systems, is due to the nonequivalence of the cross sections of scattering by normal-metal impurities of electron and hole excitations in conditions of Andreev reflection. We also show that in standard measurements of the contribution of the N-S boundary lying between the test contacts, this effect is masked by accompanying effects, the proximity effect and the boundary resistance, whose estimate requires taking into account the presence on the N-S boundary of an electrostatic barrier of the Schottky type, a barrier that redistributes the probabilities of ordinary and Andreev reflections of quasiparticles in the nonequilibrium conditions due to current flow. Zh. éksp. Teor. Fiz. 113, 1064–1070 (March 1998)