Normal Metal Insulator Superconductor Research Articles

Electronic refrigerators: optimization studies

Electric current flowing through a normal metal-insulator-superconductor (NIS) junction accompanied by the heat transfer from normal metal to superconductor is able to cool electrons in the normal metal electrode below the lattice temperature under certain conditions. The tunneling electric current dependence on biasing voltage at different normal metal electrode electron temperatures is calculated. Refrigerators with one and with two NIS junctions are investigated. Further, the cooling power of SNIS and SINIS refrigerators, its dependence on biasing voltage and on the temperature of normal metal electrode electrons are investigated. With reduced insulator layer thickness the resistivity of a NIS junction decreases. Thus, the cooling power of a refrigerator increases. But simultaneously the probability for Andreev reflections which diminish the efficiency of the refrigerator increases. Therefore, the optimal thickness of the insulator layer needs to be found. A method for calculating an optimal insulator layer thickness based on the introduction of a new function that needs to be minimized is described. Geometrical limits on the influence of Andreev reflection are determined.

Point contact tunnelling apparatus with temperature and magnetic field control

The design and testing of a new device for point contact tunnelling measurements in superconductors are described. The insert is designed for use with a continuous flow cryostat which allows for a large range of sample temperatures from 1.5 K to room temperature. The use of nonmagnetic parts allows tunnelling measurements to be performed in high magnetic fields. Testing was carried out on a conventional superconductor, Nb, in fields up to 6 T using Nb and Au tips to obtain superconductor–insulator–superconductor and superconductor–insulator–normal metal junctions. In addition, a moderate- T c oxide superconductor, Ba 1−xK xBiO 3, was used to obtain temperature and magnetic field dependencies of the energy gap.

Quantum levels and quasi-local states of SINIS structures

Quantum states of a superconductor–insulator–normal metal–insulator–superconductor sandwich (the SINIS structure) are investigated on the basis of the Bogoliubov–de Gennes equations. The dispersion equation is obtained for the quasiparticle spectrum for energies E<Δ (Δ is the energy gap in the superconductor) taking into account the Andreev scattering as well as conventional electron reflection at the interfaces of the SINIS structure. The spectrum makes it possible to calculate the Josephson current in the system. The transparency coefficient of the system for electrons with a continuous energy spectrum is calculated, and quasi-local states (“resonance levels” of transparency) are determined for the structure under investigation.

Fabrication and properties of all-refractory Nb/Al–AlOx–Ti junctions for microbolometers and microrefrigerators

Fabrication of all-refractory Nb/Al–AlOx–Ti superconducting tunnel junctions using selective titanium etching process (STEP) is described. Results including anodization properties of Ti, and junction’s I–V characteristics and subgap currents measured in the temperature range of 0.4–9.2 K are presented. The junctions show fairly high quality with respect to their stability and reproducibility. Possible utilization of these junctions as superconductor–insulator–normal metal type devices operating around 100 mK and above for ultrasensitive microbolometer and electronic microrefrigerator applications is discussed.

Thermal characteristics of silicon nitride membranes at sub-Kelvin temperatures

We have performed calorimetric measurements on 200 nm thin silicon nitride membranes at temperatures from 0.07 to 1 K. Besides full windows, membranes cut into a thermally isolating suspended bridge geometry were investigated. Based on dc and ac measurements employing normal-metal/insulator/superconductor (NIS) tunnel junctions both as a thermometer and a heater, we report on heat transport and thermal relaxation in silicon nitride films. The bridge structure improves thermal isolation and, consequently, energy sensitivity by two orders of magnitude over those of the full membrane with the same size, and makes such a structure very attractive for bolometric and microrefrigeration applications.

Evidence ofdx2−y2symmetry in the tunneling conductance density of states ofTl2Ba2CuO6

Tunnel junctions have been made on single crystals of ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CuO}}_{6}$ with a ${T}_{c}$\ensuremath{\sim}91 K using a mechanical point-contact method. Both superconductor--insulator--normal-metal (SIN) and superconductor-insulator-superconductor (SIS) junctions were obtained using Au and Nb tips, respectively. The tunneling conductances obtained are reproducible, with sharp, well-defined energy gap structures and are characterized by a cusplike feature at zero bias. These conductances can be fit to a simple, momentum-averaged density of states (DOS) obtained from a superconducting order parameter with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry. The SIS data display a weak Nb gap feature which is consistent with the subgap DOS observed in the SIN data.

Anomalies in Tunneling through Localized States

We study the I–V characteristics of normal-metal/insulator/superconductor (NIS) and superconductor/insulator/superconductor (SIS) junctions with InOx as the insulator. The characteristics of both types of devices exhibit structure at sub-gap as well as above-gap biases. The voltage position of the above-gap structure seems to scale with the gap energy. SIS junctions exhibit only part of the above-gap structure. These effects are associated with the presence of localized states in the barrier region.

The current–voltage characteristic in an ultrasmall junction

The current–voltage characteristics of ultrasmall superconductor–insulator–normal metal (S–I–N), and a superconductor–insulator–superconductor (S–I–S) junctions are computed in the presence of a dissipative transmission line. The amplitude of the discontinuous jump at the energy gap of a single-particle current is greatly influenced by the size of the capacitance and the impedance of the external transmission line in the small junction. The results agree with Ambegaokar–Baratoff in the limit of vanishing impedance of a transmission line or large junction capacitance.

Microrefrigeration by normal-metal/insulator/superconductor tunnel junctions

A normal-metal/insulator/superconductor (NIS) tunnel junction can be applied to cool electrons by biasing the junction suitably with external voltage. Because of the symmetry with bias voltage, two NIS junctions in series can form an efficient microrefrigerator. So far our SINIS microrefrigerator has been capable of reaching electronic temperatures of about 100 mK starting from 300 mK. To achieve appreciable refrigeration of the underlying lattice, microrefrigerator must be thermally decoupled from the bulk substrate. We have demonstrated experimentally the reduction of lattice temperature of a few mK at 200 mK by extending the normal electrode on a thin dielectric membrane. Methods to improve the efficiency of lattice refrigeration are discussed.

Microrefrigeration by NIS tunnel junctions

By using a normal metal-insulator-superconductor (NIS) tunnel junction one can manipulate the Fermi-Dirac distribution of the electrons in the normal electrode. If the junction is biased close to the superconducting gap, Δ, only “hot electrons” above Fermi level can tunnel from the normal electrode to the superconductor. Thus, due to the decoupling of the conduction electrons from the lattice at low temperatures, there exists a possibility to decrease the electronic temperature by this method. Because of the symmetry with bias voltage, two NIS tunnel junctions in series can form an efficient microrefrigerator. Temperature can be measured with two additional junctions by considering the variations of their IV curves. So far our SINIS microrefrigerator has been capable of reaching temperatures of about 100 mK starting from 300 mK.

Efficient Peltier refrigeration by a pair of normal metal/insulator/superconductor junctions

We suggest and demonstrate in experiment that two normal metal/insulator/superconductor (NIS) tunnel junctions combined in series to form a symmetric SINIS structure can operate as an efficient Peltier refrigerator. Specifically, it is shown that the SINIS structure with normal-state junction resistances 1.0 and 1.1 kΩ is capable of reaching a temperature of about 100 mK starting from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total junction area of 0.8 μm2 at T=300 mK.

Coherence effects in a normal-metal-insulator-superconductor junction.

Coherence effects arising due to Andreev reflections in a superconductor--insulator--normal-metal junction are considered. It is shown that in the absence of electron-electron interaction in the metal at low temperatures, the electrical potential drop on the insulator ${\mathit{I}}_{0}$${\mathit{R}}_{\mathit{I}}$, caused by the current density ${\mathit{I}}_{0}$ through the junction, vanishes, although the resistance of the device ${\mathit{R}}_{\mathrm{\ensuremath{\infty}}}$ measured by the two-probe method can be large. The electron-electron interaction determines the zero-temperature value of ${\mathit{R}}_{\mathit{I}}$. The implications of these effects for the theory of the superconductor--normal-metal--superconductor junction are discussed.

Enhanced conductance near zero voltage bias in mesoscopic superconductor-semiconductor junctions.

We have studied the conductance enhancement near zero voltage bias of double-barrier Nb-${\mathit{p}}^{++}$Si-E junctions, where we chose for the counterelectrode E either Nb, Al, or W. The experiments show a large correction, \ensuremath{\Delta}G\ensuremath{\approxeq}0.1${\mathit{G}}_{\mathit{N}}$, on the classical superconductor--insulator--normal-metal (SIN) conductance. We present measurements of the temperature, magnetic-field, and voltage dependence, and we interpret the observed results within the available theoretical models for coherent Andreev reflection, as provided by several authors.

Superconducting Ba1−xKxBiO3 thin films and junctions

Superconducting Ba1−xKxBiO3 (BKBO) thin films and heteroepitaxial multilayers were deposited by in situ off-axis sputtering on various substrate at 400 °C. A typical superconducting BKBO thin film on the SrTiO3 and MgO substrates had a very sharp zero resistance transition at a temperature of 22–24 K as measured by transport four point and mutual inductance probes. The normal state resistivity exhibited metallic to semiconductive characteristics depending on film composition with the lowest room temperature resistivity of about 0.25 mΩ cm. The film orientation was highly epitaxial to both SrTiO3 and MgO substrates. Normal metal-insulator-superconductor (NIS) type tunnel functions with native insulator layer and deposited MgO layer were reproducibly fabricated. The superconducting energy gap of about 3.6 meV was obtained from those NIS junctions. Heteroepitaxial superconductor- insulator-superconductor (SIS) trilayer junctions were also fabricated using MgO insulator layers. For a 5 nm thick insulator layer, quasiparticle SIS tunnel junctions with nonhysteric I-V characteristics were obtained. By reducing MgO insulator layer thickness down to 3 nm, SIS tunnel junctions with Josephson supercurrent were obtained with hysteric I-V characteristics.

Measurement of the energy gap in oxygen-annealedBi2Sr2CaCu2O8+δhigh-Tcsuperconductors by tunneling spectroscopy

The superconducting energy gap has been determined in homogeneous, oxygen-annealed ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (BSCCO) single crystals by tunneling spectroscopy. Normal-metal--insulator-- superconductor (NIS) point-contact tunneling measurements made on cleaved surfaces of these BSCCO crystals exhibit a well-defined gap with a low conductance, G(V), within the gap region. The conductance and change of G(V) within the gap region differs from the behavior expected for either a gapless or BCS-like superconductor. Analysis of data from both NIS and superconductor-insulator-superconductor junctions provides a consistent scale for superconductivity with 2\ensuremath{\Delta}\ensuremath{\approxeq}6.8${\mathit{kT}}_{\mathit{c}}$. In addition, since this gap structure is reproducible at different surface positions of these oxygen-annealed crystals we conclude that spatial variations in 2\ensuremath{\Delta} and a high density of states within the gap are not intrinsic to the BSCCO system.

Fabrication and properties of superconductor–insulator–normal metal tunnel junctions

This article first describes the preparation and properties of small area (∼1 μm2)Pb/Bi/In-oxide–Ag junctions made by evaporation through metal masks. These junctions have successfully been used in a 230 GHz receiver. We then describe the preparation and properties of two slightly different types of Nb/Al-based superconductor–insulator–normal conductor (SIN) junctions applying a ‘‘trilayer technique’’ similar to that used in the fabrication of Nb–Al-oxide–Nb superconductor–insulator–superconductor junctions. For one type of the Nb/Al-based SIN junctions the base layer consists of Nb, for the other type, the base layer is made of Al. In practical applications these devices will operate at temperatures where the Al is in the normal state. Junctions with areas down to about 0.5 μm2 have been made.

Electron-electron scattering and phase fluctuations in superconducting films.

The effects of weak disorder on superconducting aluminum films are studied over temperatures 0.4${T}_{c}$\ensuremath{\le}T\ensuremath{\le}0.98${T}_{c}$ and resistances per square of 1.7\ensuremath{\le}${R}_{\ensuremath{\square}}$\ensuremath{\le}22 \ensuremath{\Omega}. The primary effects of disorder are found to be disorder-enhanced electron-electron scattering and thermal phase, or supercurrent fluctuations, both of which cause pair-breaking effects. This conclusion is drawn from measurement of the low-voltage resistance of low-resistance superconductor--insulator--normal-metal (SIN) tunnel junctions with aluminum as the S electrode. The effective pair-breaking rates from both processes, evaluated at ${T}_{c}$, increases linearly with ${R}_{\ensuremath{\square}}$, in semiquantitative agreement with model calculations. The electron-electron scattering rate decreases very rapidly as T decreases below ${T}_{c}$, in agreement with theory. The pair-breaking rate from phase fluctuations decreases roughly linearly with decreasing temperature. Although distinct effects, phase fluctuations, and electron-electron scattering share a common origin in electron-density fluctuations, which are enhanced by disorder. A microscopic theory of disordered superconductors incorporating both electron-electron scattering and thermal phase fluctuations is needed.

Tunnel junctions using oxide superconducting thin films epitaxially grown on SrTiO3

Normal-metal/insulator/superconductor (NIS) junctions were fabricated using thin films of YBa2Cu3O7−x (YBCO) or ErBa2Cu3O7−x. These were epitaxially grown on single-crystal SrTiO3 by the activated reactive evaporation method. For some NIS junctions prepared on SrTiO3 (110) substrates, we observed multipeaks in the differential conductance versus voltage curve. NIS junctions using single-crystal YBCO films on SrTiO3 (100) showed a set of peaks, from which we obtained a gap parameter of 11.5±1.5 meV at 4.4 K and a coupling constant of 3.2±0.4.

Theory of the effect of supercurrents on the low-voltage resistance of superconductor-insulator-normal-metal tunnel junctions.

The theory of charge imbalance in tunnel junctions is adapted to describe the low-voltage resistance R/sub j/(T) of low-resistance superconductor--insulator--normal-metal (SIN) tunnel junctions. R/sub j/ is shown to be very sensitive to the pair-breaking effect of supercurrents. Numerical calculations for hypothetical SIN junctions illustrate how one can obtain information about the magnitude and temperature dependence of electron scattering rates, coherence factors, and the density of superconducting electrons from measurements of R/sub j/ in the presence of supercurrents either applied directly with a current supply or induced with a magnetic field applied parallel to the superconducting film. Such measurements should yield information similar to that obtained from other nonequilibrium techniques like NMR and ultrasonic attenuation.

Phonon structure and proximity-effect tunnelling in strongly gapless superconductors

Abstract The tunnelling characteristics of normal-metal-insulator-superconductor (NIS) and NINS junctions in the presence of a parallel magnetic field larger than the bulk critical field have been measured. The state that results from this surface superconductivity is gapless. By comparing data taken with no applied field and above HC. we are able to study the strong coupling effects in a superconductor and the interference effects in a proximity sandwich as the order parameter is varied. In the case of the proximity effect, we are also able to obtain gaplessness by the addition of magnetic impurities to the normal metal N. For tunnelling into a single superconducting film, the amplitude of the observed phonon structure scales with the order parameter squared. Although surface superconductivity is usually reduced by proximity with a normal metal, the surface sheath remains superconducting for a layer of silver up to ∼ 800 A thick for Ag-Pb sandwiches. For thin silver layers (∼200A), the discrepancy betwee...