Normal Metal Electrodes Research Articles

Single cooper pair electronics

Abstract Single electron devices are based on small normal metal electrodes called ‘islands’ separated by tunnel junctions. In such circuits, electrons can be transfered one by one when the energy of thermal fluctuations is less than the electrostatic energy necessary to add an extra electron to an island. We present here experimental results in single electron devices for which all normal-metal electrodes are replaced by superconducting ones. In such devices coherent Cooper pair tunneling (i.e. the DC-Josephson effect) combines with Coulomb blockade. These effects can be tuned in such a way that signatures of the macroscopic quantum coherence between two well-defined charge states become observable.

Phase-coherent tunneling through a mesoscopic superconductor coupled to superconducting and normal-metal electrodes

Phase-coherent diffusive transport through mesoscopic hybrid superconductor/normal metal tunneling structures is investigated. For a N-s-S two-barrier tunneling system with bulk S and N electrodes coupled by a mesoscopic superconducting constriction s, zero-bias conductance and non-linear I-V curves are calculated under the assumption that the dwell times of quasiparticles in the s region is shorter than inelastic relaxation time. It is shown that the low voltage conductance of this system determined by the Andreev reflection processes may exceed the conductance in the normal state and its value is very sensitive to the weak pairing interaction of electrons in the s region. We show that even weak pairing electron interaction may result in the significant qualitative and quantitative change of the conductance temperature dependence with respect to the case of structures with the normal mesoscopic region. We calculate the I-V curves and show that they depend on the applied voltage in a non-monotonic way, therefore differential conductance becomes negative with increasing voltage. Such behavior is due to the voltage dependence of the order parameter in the constriction and the phase difference, phi, between the S and s superconductors. It is shown that if the tunneling processes determine the form of the quasiparticle distribution function in the s superconductor, the phase, phi, is stationary at arbitrary voltages. For quasiparticle tunneling interferometers in which the mesoscopic superconductor,s, couples the superconductor,S, and the normal metal,N, the zero bias conductance, as a function of the phase difference between the S electrodes is investigated. It is shown that the amplitude of the conductance oscillations may exceed the conductance of this structure in the normal state.

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.

HTS mixers based on the Josephson effect and on the hot-electron bolometric effect

We report on our experimental studies of high-T/sub c/ Josephson mixers and high-T/sub c/ hot-electron bolometric (HEB) effect mixers. Mixers based on high-T/sub c/ bicrystal Josephson junctions have been fabricated, and noise, conversion efficiency, and receiver bandwidth measurements have been performed in the frequency range between 90 and 550 GHz. The dependence of the mixer performance on the operation temperature has been studied. High-T/sub c/ HEB mixers have been fabricated on MgO and sapphire substrates. We successfully sized the dimensions of the effective device volume down and managed to fabricate 50-60 nm long devices. In such short structures phonon diffusion into the normal metal electrodes should significantly improve the mixer performance.

Andreev reflections and magnetoresistance in ferromagnet-superconductor mesoscopic structures

An analysis is made of the change in the resistance of a nanostructure consisting of a diffusive ferromagnetic (F) wire and normal metal electrodes, due to the onset of superconductivity (S) in the normal electrode and Andreev scattering processes. The superconducting transition results in an additional contact resistance arising from the necessity to match the spin-polarized current in the F-wire to the spinless current in the S reservoir, which is comparable to the resistance of a piece of F wire with length equal to the spin relaxation length. It is also shown that in the absence of spin relaxation the resistance of a two-domain structure is the same for a ferro-or antiferromagnetic configuration if one electrode is in the superconducting state.

Interface Properties Between SrTiO3 Thin Films and Electrodes

AbstractSrTiO3 (STO) thin films were grown by pulsed laser deposition on single crystal STO substrates with a SrRuO3 buffer layer, which also serves as a bottom electrode. Measurements of the low frequency dielectric properties were performed in a parallel plate capacitor configuration for a range of temperatures using different top electrode materials. The contribution to the interfacial potential from Schottky barriers was investigated. In comparison to STO single crystals, thin films have continued dielectric non-linearity above T ∼ 70 K. This complicates conventional Schottky barrier height measurements using C-V curves because both Schottky barriers and dielectric non-linearity result in a decrease in dielectric constant under applied electric fields. However, by using I-V data, difficulties related to field dependence of the dielectric constant may be removed. Barrier height measurements for both metal and oxide electrodes were performed for T > 70 K. Calculated barrier heights from a modified Schottky equation were very low for an oxide electrode, and an order of magnitude higher for a normal metal electrode.

Primary thermometry with nanoscale tunnel junctions

We have found current-voltage (I-V) and conductance (dI/dV) characteristics of arrays of nanoscale tunnel junctions between normal metal electrodes to exhibit suitable features for primary thermometry. The current through a uniform array depends on the ratio of the thermal energy kBT and the electrostatic charging energy E c of the islands between the junctions and is completely blocked by Coulomb repulsion at T = 0 and at small voltages eV/2 ≤ Ec. In the opposite limit, kBT ≫ Ec, the width of the conductance minimum scales linearly and universally with T and N, the number of tunnel junctions, and qualifies as a primary thermometer. The zero bias drop in the conductance is proportional to T−1 and can be used as a secondary thermometer. We will show with Monte Carlo simulations how background charge and nonuniformities of the array will affect the thermometer.

Electronic microrefrigerator based on a normal-insulator-superconductor tunnel junction

We present measurements on a novel electronic microrefrigerator that can cool conduction electrons significantly below the lattice temperature. A normal-insulator-superconductor tunnel junction is used to extract electrons from the normal metal electrode whose energy is higher than the Fermi energy. Electrons with an average energy equal to the Fermi energy are returned to the metal by a superconducting contact. Consequently, the high-energy thermal excitations are removed from the normal metal, thus cooling the electrons. For lattice temperatures higher than 100 mK the data can be explained by a simple theory incorporating the BCS density of states in the superconducting electrode and the coupling between electrons and phonons. At lower temperatures our measurement suggests that the electron energies in the normal electrode depart strongly from an equilibrium distribution.

Thermometry by arrays of tunnel junctions.

We show that arrays of tunnel junctions between normal metal electrodes exhibit features suitable for primary thermometry in an experimentally adjustable temperature range where thermal and charging effects compete. $I\ensuremath{-}V$ and $\frac{\mathrm{dI}}{\mathrm{dV}}$ vs $V$ have been calculated for two junctions including a universal analytic high temperature result. Experimentally the width of the conductance minimum in this regime scales with $T$ and $N$, the number of junctions, and its value (per junction) agrees with the calculated one to within 3% for large $N$. The height of this feature is inversely proportional to $T$.

Modification of the Blonder, Tinkham and Klapwijk theory of normal metal-superconductor point contact due to contact heterogeneity

A modification is proposed of the Blonder, Tinkham and Klapwijk (BTK) theory for heterogeneous normal metal-superconductor (N-S) point contact. Contact is heterogeneous when the normal state electronic dispersion relations in the left-hand and right-hand electrodes are different (in the original BTK theory dispersion relations in both electrodes are equal). For characterization of the dispersion relation difference the author uses the amplitude of reflection r(E) at the clear interface for electrons incoming from the normal metal electrode with energy EF+E when the 'superconductive' electrode is in the normal state. He calculates the amplitudes of ordinary and Andreev reflection at the N-S interface for electrons incoming from the normal metal electrode as functions of r(E). This enables him to obtain an expression for differential conductance of the point contact as a function of applied voltage and r(eV). For a suitable function r(eV) the calculated differential conductance decreases with increasing voltage in the low-voltage region (as is observed experimentally for some point contacts between normal metal-high-Tc superconductor) in contrast to the BTK theory which provides only non-decreasing dependences of differential conductance against voltage.

Electrochemically assessed corrosion reactivity of YBa 2Cu 3O 7 electrodes

This paper describes a series of electrochemical measurements designed to explore the surface reactivity of the oxide superconductor YBa 2Cu 3O 7. The electrochemical response of electrodes prepared from this material was found to be highly dependent on both the method of surface treatment and on the amount of water in the electrolytic fluid. In the absence of water, YBa 2Cu 3O 7 behaved as a normal metal electrode and reproducible voltammetry was obtained in the potential window of 1.3 V to −1.4 V vs. SCE. At these potentials, faradaic electron transfer reactions for solution dissolved redox couples were readily observed at YBa 2Cu 3O 7 and were similar in behavior to those obtained at noble metals such as Pt. At potentials outside this window, degradation of the electrode surface occurred. In addition, a very sensitive method has been developed for evaluating the quality of the surface of high temperature superconductors based on monitoring Δ E p values in cyclic voltammetry.

Coulomb blockade of single-electron tunneling, and coherent oscillations in small tunnel junctions

A microscopic approach to the theory of small, current-biased tunnel junctions is developed. This approach yields a natural account of the “secondary” quantization of both the single-electron (quasiparticle) and Cooper-pair (Josephson) current components. The theory shows that the current of the single electrons is blocked by their Coulomb interaction at low temperatures within a considerable range of the junction voltage. As a result of the blockade, coherent oscillations of the voltage can arise even in the absence of Josephson coupling, e.g., for single-electron tunneling (SET) between normal metal electrodes. The most significant features of these “SET” oscillations and their coexistence with Bloch oscillations in Josephson junctions are studied in detail. Prospects of experimental verification of the predicted effects and of their possible applications are discussed.

Electron currents through barriers between two metals

Abstract The current flow between two (normal) metal electrodes, separated by a vacuum gap or an insulating layer, is considered from a general point of view as a process of electron transits through and over a potential energy barrier. The main results of a unified theory of field and thermionic emission, which takes into account the band structure of the insulator, are discussed. Experiments confirming the theory both for the cases of electron emission in vacuum and in some insulators are also considered. Certain directions of future work on experimental tests of the independent-electron model for the emission process are suggested.

Collective Excitation Spectroscopy in Superconductor-Semiconductor Tunnel Junctions

The transfer-Hamiltonian description of the influence of electron-boson interactions on the electrical characteristics of metal-semiconductor tunnel junctions is extended by considering the combined effects of electrode self-energy phenomena and boson-assisted tunneling. The current associated with these processes is evaluated for superconducting metal electrodes described by the pairing model as well as for normal metal electrodes. Numerical evaluations of the conductance are made using models and parameters appropriate for the description of the influence of electron (hole) interactions with optical phonons ($\mathrm{energy}\ensuremath{\cong}\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$) on the tunneling characteristics of metal contacts on boron-doped silicon.

Phonon Emission and Self-Energy Effects in Normal-Metal Tunneling

We discuss the dependence of conductance on voltage for metal-insulator-metal junctions for the bias range from 0-1 V. The dependence is roughly parabolic but the minimum conductance need not occur at $V=0$. Small deviations from this parabolic conductance behavior are described. These are due to interactions of the tunneling electrons with impurities in the oxide, the oxide itself, or the surface layers of the metal electrodes. These emission processes are studied by calculating the even conductance and its derivative. This derivative, at low voltages, is a crude measure of the phonon density in the normal-metal electrode. We show that self-energy effects in the normal metal can be conveniently displayed by calculating the odd conductance. Such experimental effects are presented for normal Pb, and are in good agreement with the self-energy calculated from superconducting tunneling.