Normal-metal Junctions Research Articles

Transport through normal-metal–graphene contacts

Conductance of zigzag interfaces between graphene sheet and normal metal is investigated in the tight-binding approximation. Boundary conditions, valid for a variety of scattering problems, are constructed and applied to the normal metal -- graphene -- normal metal (NGN) junctions. At the Dirac point, the conductance is determined solely by the evanescent modes and is inversely proportional to the length of the junction. It is also independent on the interface resistance. Away from the Dirac point, the propagating modes' contribution dominates. We also observe that even in the junctions with high interface resistance, for certain modes, ideal transmission is possible via Fabry-Perot like resonances.

Influence of Charge and Energy Imbalances on the Tunneling Current through a Superconductor–Normal Metal Junction

We consider quasiparticle charge and energy imbalances in a thin superconductor weakly coupled with two normal-metal electrodes via tunnel junctions at low temperatures. Charge and energy imbalances, which can be created by injecting quasiparticles at one junction, induce excess tunneling current $I_{\rm ex}$ at the other junction. We numerically obtain $I_{\rm ex}$ as a function of the bias voltage $V_{\rm det}$ across the detection junction. We show that $I_{\rm ex}$ at the zero bias voltage is purely determined by the charge imbalance, while the energy imbalance causes a nontrivial $V_{\rm det}$-dependence of $I_{\rm ex}$. The obtained voltage-current characteristics qualitatively agree with the experimental result by R. Yagi [Phys. Rev. B {\bf 73} (2006) 134507].

Subgap conductivity in SIN-junctions of high barrier transparency

We investigate the current–voltage characteristics of high-transparency superconductor–insulator–normal metal (SIN) junctions with the specific tunnel resistance ρ≲30Ωμm2. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in rapid-single-flux-quantum (RSFQ) circuitry for controlling Josephson qubits.

Dynamics of Josephson junctions and single-flux-quantum networks with superconductor–insulator–normal-metal junction shunts

Within the framework of the microscopic model of tunneling, we modelled the behavior of the Josephson junction shunted by the Superconductor-Insulator-Normal metal (SIN) tunnel junction. We found that the electromagnetic impedance of the SIN junction yields both the frequency-dependent damping and dynamic reactance which leads to an increase in the effective capacitance of the circuit. We calculated the dc I-V curves and transient characteristics of these circuits and explained their quantitative differences to the curves obtained within the resistively shunted junction model. The correct operation of the basic single-flux-quanta circuits with such SIN-shunted junctions, i.e. the Josephson transmission line and the toggle flip-flop, have also been modelled.

Minigap in a superconductor–normal metal junction with paramagnetic impurities

We study the effect of spin-flip scattering on the density of states in a long diffusive S-N junction with transparent interface. We calculate the critical value of the spin-flip scattering rate at which the minigap closes and give the dependence of the minigap on the spin-flip rate. For the system we consider, the minigap and the critical spin-flip rate have the scale of the Thouless energy, and not of the superconducting gap.

Location of the minimum of the differential tunneling resistance $ \mathsf {R(V)}$ in a superconductor-degenerate semiconductor Schottky contact

Measurements of differential resistance in a superconductor-degenerate semiconductor junction Nb - n + + GaAs at T = 1.6 K show close similarity to those for a conventional superconductor-insulator- normal metal junction, except for the position of the minimum which is located at 3.6 meV. Using a simple model for the charge screening at the Schottky barrier, we give an argument why this minimum is by far displaced with respect to the superconducting gap energy ( Δ g = 1.5 meV for bulk Nb). We argue that a rebuilding of the density of states takes place at the barrier, due to the imperfect metal screening in the degenerate semiconductor. Energy states close to the degenerate semiconductor Fermi energy are depleted at the barrier and are not available for tunneling, up to an energy Eg which adds to the superconducting gap Δ g .

Time evolution of LaxMnO3/normal metal point contact resistance

Resistance vs. time characteristics of La x MnO 3 /normal metal junctions was studied in order to investigate the surface properties of epitaxial La x MnO 3 thin films. The change of the resistance of La x MnO 3 /normal metal point contact was observed. The results are explained in the frame of oxygen out-diffusion from La x MnO 3 surface layer and creation of an insulating barrier from La x MnO 3 and/or metal.

Charge transport across a mesoscopic superconductor–normal metal junction: coherence and decoherence effects

We present a simple scattering approach to the charge transport across a realistic superconductor–normal injector interface of a finite transmittance that is modeled by a double-barrier mesoscopic junction. For a d-wave pairing symmetry, our calculations combine a fully quantum-mechanical scattering formalism with a self-consistent estimation of Andreev reflection coefficients within the quasi-classical Eilenberger equation scheme for a free specular superconducting surface. Numerical simulations confirm experimental criteria of Cucolo for the unconventional superconducting origin of conductance anomalies in high-temperature oxides. A discussion of dephasing effects caused by inelastic scattering processes in the interlayer and their impact on the conductance spectra is given.

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa2Cu3O x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO3(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.

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.

Effects of Tunnel Resistance and Lead Resistance in Ultrasmall Single Tunnel Junctions

We report measurements on a series of ultrasmall Al single tunnel junctions. The leads were made of either multiple junctions connected in various ways or high-resistance NiCr films. The effects of tunnel resistance and lead resistance were studied extensively for normal-metal junctions and Josephson junctions. The dependence of the Coulomb blockade in normal-metal junctions on those resistances was compared quantitatively with theories. The observed tunnel-resistance dependence in Josephson junctions was explained as a manifestation of the dissipative phase transition predicted by the theory. The effect of lead resistance in Josephson junctions was also studied.

Magnetic-field dependence of conductance in microjunctions employing nonhomogeneous superconducting electrodes.

Zero bias anomaly phenomena in conductance spectra relative to superconductor/normal metal--constriction--normal metal (${\mathit{S}}_{1}$/${\mathit{N}}_{1}$-c-${\mathit{N}}_{2}$) junctions are interpreted in terms of the proximity effect. Attention is focused in particular on the influence of the magnetic field on the conductance in point contact junctions and microjunctions with nonhomogeneous base electrodes (${\mathit{S}}_{1}$/${\mathit{N}}_{1}$). We find that the highly transmissive nature of the ${\mathit{N}}_{1}$-c-${\mathit{N}}_{2}$ interface is fundamental in determining the weak dependence of zero bias conductance on the applied magnetic field. The predictions of our model are compared with experimental data on Nb based junctions by Xiong et al. [Phys. Rev. Lett. 71, 1907 (1993)], supplying helpful information on the actual morphology of the structures. \textcopyright{} 1996 The American Physical Society.

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.

Asymmetry in the normal-metal to high-Tc superconductor tunnel junction.

We show that the observed asymmetry in the $I\ensuremath{-}V$ characteristics of high-${T}_{c}$ material to normal-metal junctions can be explained within the resonating-valence-bond model. For a bias current with electrons moving from the superconductor to the normal metal, the current is quadratic in the bias voltage, and in the opposite case, with electrons moving from the normal metal to the superconductor, the current is linear in $V$.

Anomalous Tunneling in Normal Metal Junctions using Ferromagnetic Films

Measurements of tunnel current flow through normal metal junctions made with the ferromagnetic metals Fe, Ni, and Co have been carried out in the range 1°–300°K. The insulating layer has been an oxide of the above metals or aluminum oxide and the second metal either aluminum or silver. In the case of Fe-FeOx-Al a giant zero-bias anomaly corresponding to a resistance maximum has been observed at helium temperatures and this persists up to room temperature. Co-CoOx-Al also shows a similar but smaller anomaly while Ni-NiOx-Al shows an anomalous behavior of reverse sign. Data on the dynamic resistance as a function of voltage and temperature are presented in this paper. The effects of magnetic fields up to 50 kOe have also been studied.