Mesoscopic SNS Junctions Research Articles

Impurity states in mesoscopic SNS junctions with a point defect

We study theoretically localized states in a clean three-dimensional SNS junction, containing one point defect (impurity) located in the normal layer. We find that the defect induces two quasibound states in a short junction, corresponding to different spin projections. The energies of these states coincide for a nonmagnetic defect and are generally different for a magnetic defect. The spatial structures of the impurity states resemble the structures of Shiba states in a bulk superconductor. We also consider the case when the impurity is located close to a fat edge of the junction. We find mesoscopic fluctuations of the energies of the quasibound states as a function of the distance between the impurity and the edge.

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.

Density of States below the Thouless Gap in a Mesoscopic SNS Junction

Quasiclassical theory predicts an existence of a sharp energy gap ${E}_{\mathrm{g}}\ensuremath{\sim}\ensuremath{\Elzxh}{D/L}^{2}$ in the excitation spectrum of a long diffusive superconductor--normal metal--superconductor (SNS) junction. We show that mesoscopic fluctuations remove the sharp edge of the spectrum, leading to a nonzero density of states (DOS) for all energies. Physically, this effect originates from the quasilocalized states in the normal metal. Technically, we use an extension of Efetov's supermatrix $\ensuremath{\sigma}$ model for mixed NS systems. A nonzero DOS at energies $E<{E}_{\mathrm{g}}$ is provided by the instanton solution with broken supersymmetry.

Nanometer SNS junctions and their application to SQUIDs

Mesoscopic SNS junctions have been studied both theoretically and experimentally. We have made a 64-channel whole-head SQUID magnetometer of SNS junctions for brain science. SNS junctions are free of low-frequency telegraph noise while insulator barriers of SIS tunnel junctions are electrically charged and discharged to generate telegraph noise. The combination of SQUIDs of SNS junctions and a superconducting magnetic shield for human bodies has yielded an excellent SQUID system for brain science. Experimental data of the SQUID system has demonstrated that mesoscopic SNS junctions are important not only theoretically but also practically.

Low-voltage negative-resistance mixers of nano-meter SNS junctions

The current carried by the bound quasi-particles in the N region of a mesoscopic SNS junction has dc and cosine components. The dc current component is carried by the pair charge (-2e) transferred by a couple of Andreev reflections at both of the NS interfaces. The pair-charge transfers decrease due to reduction of the allowed number, 2/spl Delta//V, of Andreev reflections when the voltage, V, increases. Therefore, the low-voltage negative-differential-resistance is observed on the I-V curves of mesoscopic SNS junction, when the junction is driven by the low impedance voltage-bias source. The supercurrent carried by the quasi-particle in the N-region is sensitive to the external high-frequency fields. In a mixer experiment using the nano-meter SNS junctions of NbN, prominent IF signal peaks are observed at low bias voltage. Each IF signal peak corresponds to the negative differential resistance region at low bias voltage.

Magnetic-field effect on the supercurrent of an SNS junction

In this paper we study the effect of a Zeeman field on the supercurrent of a mesoscopic SNS junction. It is shown that the supercurrent suppression is due to a redistribution of current-carrying states in energy space. A dramatic consequence is that (part of the) the suppressed supercurrent can be recovered with a suitable non-equilibrium distribution of quasiparticles.

Mesoscopic SNS junctions on the basis of superconducting PtSi films

Low-temperature transport measurements on superconducting film – normal metal wire – superconducting film (SNS) junctions fabricated on the basis of 6 nm thick superconducting polycrystalline PtSi films are reported. The structures with the normal metal wires of two different lengths L=1.5 and 6 μm and the same widths W=0.3 μm are studied. The zero bias resistance dip is observed for all junctions whereas the subharmonic energy gap structure (SGS) originating from multiple Andreev reflections occurs only in the SNS junctions with short wires.

Proximity effects and Andreev reflection in a mesoscopic SNS junction with perfect NS interfaces

Low temperature transport measurements on superconducting film - normal metal wire - superconducting film (SNS) junctions fabricated on the basis of 6 nm thick superconducting polycrystalline PtSi films are reported. The structures with the normal metal wires of two different lengths L=1.5 $\mu$m and L=6$\mu$m and the same widths W=0.3$\mu$m are studied. Zero bias resistance dip related to pair current proximity effect is observed for all junctions whereas the subharmonic energy gap structure originating from phase coherent multiple Andreev reflections have occurs only in the SNS junctions with short wires.

Multiple Andreev Reflections and Enhanced Shot Noise in Diffusive Superconducting-Normal-Superconductor Junctions

We study the dc conductance and current fluctuations in mesoscopic diffusive SNS junctions at subgap applied voltages $\mathrm{eV}<2\ensuremath{\Delta}$. We find that in spite of the multiple Andreev reflections, the current-voltage characteristic of a long SNINS structure obeys Ohm's law. At the same time, non-equilibrium heating of subgap electrons produces giant shot noise with pronounced subharmonic gap structure which corresponds to stepwise growth of the effective transferred charge. At $\mathrm{eV}\ensuremath{\rightarrow}0$, the shot noise approaches the magnitude of the Johnson-Nyquist noise with the effective temperature ${T}^{\ensuremath{\star}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Delta}/3$. We analyze the role of inelastic scattering and present a criterion of strong nonequilibrium.

Mesoscopic SNS junctions and their application to SQUIDs and millimeter-wave detectors

Mesoscopic SNS junctions have been studied both in the ballistic and diffusive regimes. SNS junctions in the ballistic regime behave as an ideal Fermion oscillator which is to be compared with the Boson oscillator or the Planck theory of blackbody radiation. The current of mesoscopic SNS junctions in the diffusive regime has the same phase dependence as that of dirty-limit short weak links derived by a transport equation. Recent theories of mesoscopic SNS junctions have successfully unified the theories of the tunnel Josephson junction, the clean-limit short weak link and the dirty-limit short weak link which look very different conceptionally. We can even observe transitions among the three types of junctions when we change the transmission coefficients of the barriers between the superconducting electrodes experimentally. We looked experimentally for the optimum transmission coefficient which gives the minimum low-frequency telegraph noise in order to make a low-noise SQUID magnetometor for brain science. We have observed signals of 5 fT from human brains with a good signal-to-noise ratio using the SQUID magnetometor of the SNS junctions. The 64-channel SQUID magnetometer of SNS junctions has confirmed that mesoscopic SNS junctions are important not only theoretically but also practically. These data could encourage people studying SNS junctions of high-Tc superconductors.

Coherent transport and nonlocality in mesoscopic SNS junctions: anomalous magnetic interference patterns

We show that in ballistic mesoscopic SNS junctions the period of critical current versus magnetic flux dependence (magnetic interference pattern), Ic(Φ), changes continuously and nonmonotonically from Φ0to 2 Φ0as the length-to-width ratio of the junction grows, or temperature drops. In diffusive mesoscopic junctions the change is even more drastic, with the first zero of Ic(Φ) appearing at 3 Φ0. The effect is a manifestation of nonlocal relation between the supercurrent density and superfluid velocity in the normal part of the system, with the characteristic scale ξT=ℏvF/2 πkBT(ballistic limit) orξT=ℏD/2 πkBT(diffusive limit), the normal metal coherence length, and arises due to restriction of the quasiparticle phase space near the lateral boundaries of the junction. It explains the 2 Φ0-periodicity recently observed by Heida et al. [Phys. Rev. B57, R5618 (1998)]. We obtained explicit analytical expressions for the magnetic interference pattern for a junction with an arbitrary length-to-width ratio. Experiments are proposed to directly observe the Φ0→ 2 Φ0- and Φ0→ 3 Φ0-transitions.

Multiple Andreev reflections in diffusive SNS structures

We report new measurements on subgap energy structures originating from multiple Andreev reflections in mesoscopic SNS junctions. The junctions were fabricated in a planar geometry with high-transparency superconducting contacts of Al deposited on highly diffusive and surface δ -dopedn++ -GaAs. For samples with a normal GaAs region of active length 0.3μ m, the Josephson effect with a maximal supercurrent Ic= 3μ A at T= 237 mK was observed. The subgap structure was observed as a series of local minima in the differential resistance at dc bias voltages V=±2Δ/(ne) with n= 1, 2, 4, i.e. only the even subgap positions. While at V=±2Δ/e(n= 1) only one dip is observed, then= 2 and the n= 4 subgap structures each consists of two separate dips in the differential resistance. The mutual spacing of these two dips is independent of temperature, and the mutual spacing of the n= 4 dips is half the spacing of the n= 2 dips. The voltage bias positions of the subgap differential resistance minima coincide with the maxima in the oscillation amplitude when a magnetic field is applied in an interferometer configuration, where one of the superconducting electrodes has been replaced by a flux-sensitive open loop.