SNS Junctions Research Articles

Tunneling spectroscopy in the Charge-Density Wave conductor NbSe3

The motion of Charge-Density Waves (CDWs) shows many similarities with transport in superconductors with the role of voltage and current reversed. Submicron superconducting devices are very important in both fundamental studies and applications of superconductivity. For CDWs, reliable fabrication methods for making similar devices are not as advanced and are still being developed. In search for new mesoscopic CDW physics, we have fabricated insulating longitudinal point contacts and weak links in the CDW conductor NbSe 3 . We have found conductance peaks Ht energies corresponding to the Peierls gap when the temperature is brought below the transitions. The peaks follow a mean-field theory temperature dependence, indicating that the transport through these devices are spectroscopy measurements. The insulating point contacts show tunnelling behaviour similar to SIS junctions, while the weak links show transport similar to SNS junctions. These devices allow us to study the CDW analogies of superconductors such as the Josephson relations, Andeev reflection, subgap features of phonon modes due to electron-phonon coupling and pair-breaking at high velocities.

Theory of anomalous magnetic interference pattern in mesoscopic superconducting/normal/superconducting Josephson junctions

The magnetic interference pattern in mesoscopic SNS Josephson junctions is sensitive to the scattering in the normal part of the system. In this paper we investigate it, generalizing Ishii's formula for current-phase dependence to the case of normal scattering at NS boundaries in an SNS junction of finite width. The resulting flattening of the first diffraction peak is consistent with experimental data for S-2DEG-S mesoscopic junctions.

Thermal transport in stacked superconductor–normal metal–superconductor Josephson junctions

Nb/MoSi 2 / Nb stacked superconductor–normal metal–superconductor (SNS) Josephson junctions has proven to be a good candidate for high-density series arrays for Josephson voltage-standard applications. As the junction density increases, self-heating becomes an issue because the high power density per junction (1 W/cm2) generates significant power dissipation under typical operating conditions. In this letter, we analyze the heating effect of these sandwich-type SNS junctions using a model to quantitatively estimate and predict thermal-transport properties of the stacked structures. We describe several strategies that reduce heating and demonstrate improved properties of stacked-junction arrays with enhanced cooling capacity.

Lattice-matched, large-grain HTS films for reproducible Josephson junctions

Several novel techniques are now available to dramatically increase the grain size of epitaxial Rare-earth-BCO films so they are effectively single crystals. Our motivation is to use single-crystal films for base electrodes of edge SNS junctions to improve their reproducibility. We employed the technique of tri-phase to grow films by liquid phase epitaxy in a pulsed laser deposition system and examined the surface morphology and crystal structure of the films. We tried several techniques for in-situ monitoring of a Ba-Cu liquid flux on the surface of the film during growth - including resistivity and emissivity measurements - but found that the only way to be certain that we had a liquid phase on the sample surface was from the dendritic crystal patterns that formed after cooling the samples. We observed series of small cracks in the resulting films, presumably due to stress from lattice-match and thermal expansion mismatch with the substrates so we developed a solid solution of YBa/sub 2/Cu/sub 3/O/sub 7/ and NdBa/sub 2/Cu/sub 3/O/sub 7/ to match the lattice of our substrates to better than 0.1%.

Isolated hybrid normal/superconducting ring in a magnetic flux: From persistent current to Josephson current

We investigate the ground state current of an isolated hybrid normal/superconducting ring (NS ring), threaded by an Aharonov-Bohm magnetic flux. We calculate the excitation spectrum of the ring for any values of the lengths of the normal metal and of the superconductor. We describe the nonlinear flux dependence of the energy levels above and below the gap edge. Using a harmonics expansion for the current, we isolate the contribution due to these nonlinearities and we show that it vanishes for large normal segment length. The remaining contribution is very easy to evaluate from the linearized low energy spectrum. This decomposition allows us to recover in a controlled way the current-flux relationships for SNS junctions and for NS rings. We also study the crossover from persistent current to Josephson current in a multichannel NS ring at finite temperature.

Magnetic properties of square Josephson arrays with SIS and SNS junctions

The field dependence of the magnetic moment of 100×100 SIS and SNS Josephson arrays was investigated by SQUID-magnetometer. It was found, that the magnetic flux penetrated into SIS array by means of randomly distributed avalanches of tens and hundreds of flux quanta. The distribution of jumps by their size is a power function, which gives an evidence of self-organised criticality in regular Josephson structures.On SNS array the similar jumps were not observed, but dynamics of magnetic field in array was considerably asymmetric. The periodic peaks at external field, corresponding to integer and semi-integer of flux quanta per cell, were well observed only while absolute field was increased, and was imperceptible if the field is decreased.

FABRICATION PROCESS OF Nb/Al/Nb SNS JOSEPHSON JUNCTIONS FOR SUPERCONDUCTING ELECTRONICS APPLICATIONS

Overdamped Josephson junctions are gaining interest in many fields of superconducting electronics. The characteristic voltage ICRN is the parameter limiting the highest possible speed and voltage resolution of the junctions. Nb/Al/Nb technology is a good candidate for these devices because of its wide range of characteristic voltages. In our laboratory we have so far fabricated Nb/Al/Nb SNS junctions having (a) high critical currents and smooth interfaces, (b) low critical currents and smooth interfaces, (c) low critical currents and rough interfaces. However, further improvements to this technology are still needed.

Shot noise in diffusive SNS and SIN junctions

We studied shot noise in metallic SNS and doped silicon-based SIN junctions. In SNS structures, the shot noise is very much enhanced due to incoherent multiple Andreev reflections (IMAR) which are truncated, at low voltages, by inelastic electron–electron interaction. These experimental results show good agreement with recent semiclassical theory. In SIN junctions, the zero-voltage conductance is increased by disorder-induced coherent MAR (reflectionless tunneling) and we found that the shot noise is double ( S I =4 eI) below the Thouless energy and equals the full shot noise ( S I =2 eI) above. We also present conductance measurements which show the same zero bias anomaly but in a double-barrier metallic SININ junction.

Density of prelocalized states in mesoscopic NS systems

The semiclassical theory of the proximity effect predicts the formation of a gap E g ~ " D / L 2 in the excitation spectrum of a diffusive contact between a normal metal and a superconductor (NS). Mesoscopic fluc- tuations lead to the emergence of states localized anomalously in the normal metal and weakly linked with the superconducting bank, creating a nonzero density of states for energies lower than E g . In this review, the behav- ior of the density of quasiparticle states below a quasi-classical gap is considered for various geometries of the NS system (special attention is paid to SNS junctions) and for the problem of a superconductor with a low con- centration of magnetic impurities, in which a similar effect is observed. Analysis is mainly carried out on the basis of a fully microscopic method of the supermatrix σ model; in this method, a nonzero density of states emerges due to instanton configurations with broken supersymmetry. In addition, the results of an alternative approach proceeding from the idea of universality of the spectra of random Hamiltonians with the given sym- metry are reviewed. In situations studied using both methods, the results are identical. They include the exact expression for the mean density of states of an NS system in the vicinity of E g . In the framework of 1D and 2D σ models, the subgap density of states is determined with an exponential accuracy. The contacts with a poor transparency of the NS interface are also considered. It is shown that the number of subgap states in the case of low transparency is much greater than unity. © 2003 MAIK "Nauka/Interperiodica".

MgB2 junctions and SQUIDs fabricated by focused ion beam

We have recently developed a technique for fabricating SNS junctions in MgB2 thin films using a focused ion beam. These junctions show a strong modulation of the critical current by applied magnetic field and microwaves. They also show large IcRN products in excess of 1 mV at 4.2 K making them attractive candidates for a range of superconducting electronics applications. We have made SQUIDs with directly coupled pick-up loops using this technique which show large voltage modulations (175 μV at 10 K) and noise comparable to HTS SQUIDs. Here we will present an overview of our devices along with our latest results.

A modified Ginzburg-Landau model for Josephson junctions in a ring

We consider the Ginzburg-Landau functional to analyze SNS junctions in a one-dimensional ring. We compare several canonical scalings. The linearized problem is solved to obtain the phase transition curves. We compute the Γ \Gamma -limit of the functional in the different scalings. The interaction of several junctions is analyzed. We study the zero set of the order parameter for distinguished values of the flux. Finally, we compute the currents in the weakly nonlinear regime.

Minigap in a long disordered SNS junction: Analytical results

We review and refine analytical results on the density of states in a long disordered superconductor--normal-metal--superconductor junction with transparent interfaces. Our analysis includes the behavior of the minigap near phase differences zero and $\ensuremath{\pi}$ across the junction, as well as the density of states at energies much larger than the minigap but much smaller than the superconducting gap.

Influence of dissipation on a low-voltage dc current in a long SNS junction

The dc current through a voltage-biased long transparent SNS junction in a dissipative regime is considered. The problem under certain conditions is mapped onto exactly solvable model of energy pumping into a quasiballistic 1D quantum ring driven by time-dependent magnetic flux. A rich peak-like structure of the subgap current at low voltages is predicted. The maxima in the current correspond to resonant energy absorption for fractional values of the normalized bias voltage.

Semiclassical Theory of Quasiparticles in the Superconducting State

We have developed a semiclassical approach to solving the Bogoliubov–de Gennes equations for superconductors. It is based on the study of classical orbits governed by an effective Hamiltonian corresponding to the quasiparticles in the superconducting state and includes an account of the Bohr–Sommerfeld quantisation rule, the Maslov index, torus quantisation, topological phases arising from lines of phase singularities (vortices), and semiclassical wave functions for multidimensional systems. The method is illustrated by studying the problem of an SNS junction and a single vortex.

Electrical and structural properties of Nb/Al/Nb overdamped Josephson junctions

Superconductor–normal metal–superconductor (SNS) Josephson junctions are gaining interest in many fields, related to superconducting electronics. Their main advantages are the high critical current density and non-hysteretic I– V characteristic. We have studied Nb/Al/Nb SNS junctions, where the combined effects of the thickness and deposition rate of the Al barrier play a relevant role in determining the electrical behaviour of the junction. The increase of the Al deposition rate from 0.5 to 5 nm/s increases the surface roughness of the junction by a factor of two, with a corresponding variation of the current density and of the I C R N product from 10 6 to 10 3 A/cm 2 and from 100 μV to 1 mV, respectively.

Direct current through a superconducting two-barrier system

We analyze the influence of the structure within a SNS junction on the multiple Andreev resonances in the subgap I-V characteristics. Coherent interference processes and incoherent propagation in the normal region are considered. The detailed geometry of the normal region where the voltage drops in superconducting contacts can lead to observable effects in the conductance at low voltages.

Density of states in a mesoscopic SNS junction

Semiclassical theory of proximity effect predicts a gap E_g \sim hD/L^2 in the excitation spectrum of a long diffusive SNS junction. Mesoscopic fluctuations lead to anomalously localized states in the normal part of the junction. As a result, a non-zero, yet exponentially small, density of states appears at energies below E_g. In the framework of the supermatrix nonlinear sigma-model these prelocalized states are due to instanton configurations with broken supersymmetry. The exact result for the DOS near the semiclassical threshold is found provided the dimensionless conductance of the normal part is large. The case of poorly transparent interfaces between the normal and superconductive regions is also considered. In this limit the total number of the subgap states may be large.

Engineering issues in high-frequency RSFQ circuits

This paper reports progress on several projects that contribute to advancing the state of the art of rapid single flux quantum (RSFQ) logic. The first project is aimed to demonstrate, with true digital testing, the performance of RSFQ circuits of significant size and importance at a frequency that challenges the best semiconductor circuits, with only a miniscule fraction of their power dissipation. The second is a demonstration of an internally shunted SNS junction that has a high I c R n product and is intended as a drop-in replacement for the now-common resistively shunted tunnel junction; the advantage of this device is reduction of size, minimization of parasitic inductances, as well as high I c R n product for higher frequency operation. In the third project, we are trying to break the memory bottleneck that has long plagued superconductor digital electronics by using a hybrid of Josephson and CMOS technologies.

The principle of a high-temperature superconductor three-dimensional nanoelectron system for a bistable or four-stable state

We propose the principle of a three-dimensional (3D) nanoelectron system for bistable or four-stable states based on metallic nanodot arrays and HTSC–N–HTSC (HTSC–SNS) junctions, where HTSC denotes the high-temperature superconductor and N is the normal conductor. We consider the possibility of implementing bistable or four-stable operations on the basis of a 3D nanoelectron system through charge interactions between metallic nanodots on the surface of HTSC–SNS junctions. We present a simple classical circuit model without considering single-electron effects between metallic nanodots, where HTSC–SNS junctions are described as a simplified model of Kümmel–Nicolsky theory. Finally, the calculations based on the Monte Carlo simulation for considering single-electron effects between metallic nanodots demonstrate that our classical model is comprehensive enough for describing this system in nature. We provide numerical results for this HTSC 3D nanoelectron system for different relevant device parameters. The results may be applied to nanologic circuits or nanomemories.

S-N-S Josephson Weak Links in Superluid 3He-B

The Josephson current is calculated for an SNS junction consisting of two reservoirs of superfluid 3He-B connected via thin cylindrical channels. The current-phase relations and maximal Josephson currents are calculated for channels of different diameter and length.