Mesoscopic Junctions Research Articles

An electrometer based on a single mesoscopic junction

An electrometer based on a single mesoscopic junction is introduced. The junction is monitored using a microwave signal capacitively coupled to the junction via a tank circuit. An easy way to realize the tank circuit having a very low parasitic capacitance is to utilize the plasma oscillation of a Josephson junction array.

Phase-coherent conduction in mesoscopic normal-metal/superconductor hybrid junctions

In this paper we report about the transport properties of mesoscopic normal-metal/superconductor (NS) hybrid systems with two different junction layouts. One is a junction fabricated by overlaying S on N wire and the other with S sandwiched between two N wires, forming an NSN structure. At zero bias all the junctions exhibited a sharp decrease of d V/d I, which is believed to arise from the interference between the disorder-scattered and the Andreev-reflected quasiparticles. A small magnetic field corresponding to one flux quantum through the normal metal region, applied in parallel with the NS junction, easily suppressed the anomalous zero-bias conductance enhancement for the sandwich-junction sample. This high sensitivity to a small magnetic field observed in our systems of mesoscopic NS junctions in a diffusive-transport regime directly confirms the electron–hole-interference origin of the zero-bias anomaly.

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.

AC linear response of a resonant tunneling system

The Keldysh–Green function technique in the representation of scattering states is applied to study the alternating current (AC) linear current response of a mesoscopic tunneling junction. It is shown that, despite the generally accepted viewpoint [M. Büttiker, Phys. Rev., B45, 3807 (1992)], in the one-electron approximation the linear AC current response of the system cannot be expressed in terms of the one-electron scattering matrix. However, the response is completely determined by the full set of true electron wave functions in the region of the potential barrier. Our approach is used to consider the behavior of a resonant tunneling system under the influence of a weak coherent radiation field. In addition to high-frequency device applications, the study of the dynamical conductance has fundamental importance in that it provides information about the structure of the potential barrier. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1007–1010, 2000

A 64-channel whole-head SQUID system in a superconducting magnetic shield

A superconducting magnetic shield of high-Tc superconductor Bi(Pb)-Sr-Ca-Cu-Ox has been constructed whose diameter is 65 cm and length is 160 cm. We have successfully observed magnetic fields from somatosensory-evoked human brains in the superconducting magnetic shield by stimulating the median nerves of patients by current pulses. We made a 64-channel whole-head SQUID magnetometer of superconductor/normal metal/superconductor (SNS) junctions which do not show low-frequency telegraph noise. The sensitivities of the dc SQUID mesoscopic SNS junctions are around 5 fT Hz-1/2 even in rather unfavourable surroundings. The magnetic shield can reduce a magnetic field by around -80 dB or a factor of 10-4 even at as low a frequency as 0.05 Hz. Therefore SQUIDs of SNS junctions and a superconducting magnetic shield are a good combination.

Geometric phase in a mesoscopic Josephson junction with classical driving source

We examine a mesoscopic Josephson junction driven by the time periodic field and solve its time-dependent Schr\"odinger equation with the help of the time dependent invariant of the Hamiltonian operator. We show that the state of the system can evolve a generalized squeezed state if the junction is prepared initially in its $n\mathrm{th}$ excited state. It also turns out to be possible to produce coherent and ordinary squeezed states for the mesoscopic junction system. The explicit expression of the nonadiabatic geometric phase of the system is also obtained.

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.

COHERENT EVOLUTION AND TUNNELING CHARGE STEP IN A DRIVEN MESOSCOPIC JOSEPHSON JUNCTION

We consider a circular superconductor with a weak link mesoscopic Josephson junction in the presence of a dc voltage bias. Using the time-dependent perturbated method, we obtain the unitary evolution operator of the mesoscopic junction system and show that an initial vacuum state of the junction can evolve into a class of superposition of coherent states. Further, we show that the charge tunneling through the junction can exhibit step properties in the mesoscopic system.

Charge transfer in 1-D HTS mesoscopic tunneling junction arrays

The influence of superconducting energy gap on the performance of single electron transistor as electrometer has been studied and was found out to improve its charge sensitivity. This paper mainly deals with 1-D (dimensional) HTS mesoscopic tunneling junction array made up of HTS material Tl-1223 with capacitively coupling to a ground plan. The layered nature of Tl-1223 acts as ideal SIS junctions. The influence enforced by the superconducting energy gap on the static and dynamics of such an array is studied numerically based on the Orthodox Theory of single electronics. The charge transfer in the means of solitons introduced by the voltage source attached to the end of the array is also discussed.

Neuromagnetic SQUID measurement in a superconducting magnetic shield

A superconducting magnetic shield of high-Tc superconductor Bi(Pb)SrCaCuOx has been constructed whose diameter is 65 cm and length is 160 cm respectively. We have successfully observed somatosensory evoked magnetic fields coming out from human brains in the superconducting magnetic shield stimulating the median nerves of patients by current pulses. We made a 64-channel whole-head SQUID magnetometer of SNS (Superconductor/Normal metal/Superconductor) junctions. The sensitivities of the dc-SQUID of mesoscopic SNS junctions are around 5 fT//spl radic/(Hz) even in rather unfavorable surroundings. The magnetic shield can reduce a magnetic field to around -80 dB or 10/sup -4/ even at as low a frequency as 0.05 Hz.

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.

Nonequilibrium Josephson-like effects in wide mesoscopic SNS junctions

Mesoscopic superconducting–normal-metal–superconducting (SNS) junctions with a large separation between the superconducting electrodes (i.e. wide junctions) exhibit nonequilibrium supercurrents, even at temperatures for which the equilibrium Josephson effect is exponentially small. The second harmonic of the Josephson frequency dominates these currents, as observed in recent experiments. A simple description of these effects, in the spirit of the resistively shunted junction model, is suggested here. It is used to calculate dc I–V characteristics, and to examine the effects of various types of noise and of external microwave radiation (Shapiro steps). It is found that the nonequilibrium supercurrents are excited when the junction is driven by a dc bias or an ac bias, or even by external noise. In the case of junctions which are also long in the direction perpendicular to the current flow, thermodynamic phase fluctuations (thermal noise) alone can drive the quasiparticles out of local equilibrium. The magnetic flux is then predicted to be trapped in units of Φ0/2 =hc/4e .

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.

Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, current injection and electromagnetic radiation. In particular we will consider the ‘giant’ Josephson critical current in ‘long’ SIS tunnel junctions and the regular and anomalous nonequilibrium Josepson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).

Mesoscopic Josephson effect

In the classical Josephson effect the phase difference across the junction is well defined, and the supercurrent is reduced only weakly by phase diffusion. For mesoscopic junctions with small capacitance the phase undergoes large quantum fluctuations, and the current is also decreased by Coulomb blockade effects. We discuss the behavior of the current–voltage characteristics in a large range of parameters comprising the phase diffusion regime with coherent Josephson current as well as the supercurrent peak due to incoherent Cooper pair tunneling in the Coulomb blockade regime.

SPECTRAL DENSITY FOR THE TUNNELING CURRENT ZERO-FREQUENCY SHOT-NOISE IN A ONE-DIMENTIONAL MESOSCOPIC SYSTEM

We have derived the expression of the spectral density for the current shot-noise in terms of the nonequilibrium Green's functions(closed-path Green's functions) for a one-dimentional mesoscopic junction, and performed numerical calculations to discuss the dependence of the zero frequency spectral density for the currentshot-noise on other variables when the tunnling junction is pure or impure.

Quantum Chaotic Scattering and Resistance Fluctuations in Mesoscopic Junctions

Quantum Chaotic Scattering and Resistance Fluctuations in Mesoscopic Junctions

Matsubara-Green's function approach to tunnelling in mesoscopic junctions

An expression for the ensemble-averaged current in a mesoscopic tunnel junction that is connected to a voltage source, via an impedance, is presented. The quantum description of the system includes the quantum fluctuations due to the environment. Iterative calculation of the spectral density function associated with the Matsubara function of the external circuit yields the time-averaged characteristics.

Electron and photon noise suppression in mesoscopic systems—how to teach noisy photons to follow quiet electrons

Principles of squeezed state generation in semiconductor lasers and light emitting diodes (LEDs) are discussed. Shot noise suppression in electron transport in macroscopic conductors is ultimately due to Pauli exclusion principle for fermion particles, electrons. Shot noise suppression in electron injection in macroscopic pn junctions originates from collective Coulomb blockade effect for changed particles, electrons. Regulated single photon generation in mesoscopic pn junctions is also discussed.