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 k_BT 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 E_c. In the opposite limit, k_BT E_c, 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.     

Josephson tunnel junctions with refractory electrodes

Fabrication of high quality Josephson tunnel junctions with refractory electrodes is made difficult by the electrode's short coherence lengths and the sensitivity of the their superconducting properties to tunnel barrier processing. Several innovative procedures have been developed to address this problem. In this review, we describe these tunnel barrier processes and the junction characteristics that were obtained using them. In particular, attention is focused on the junction's subgap conductance and the reproducibility of the maximum Josephson current because of their importance as device design parameters for integrated circuits.     

InAs nanowire superconducting tunnel junctions: Quasiparticle spectroscopy,thermometry, and nanorefrigeration

We demonstrate an original method based on controlled oxidation for creating high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires (NWs).We show clean tunnel characteristics with a current suppression by ＞4 orders of magnitude for a junction bias well below the Al gap of A0 ≈ 200 μeV.The experimental data agree well with the BardeenCooper-Schrieffer theoretical expectations for a superconducting tunnel junction.The studied devices employ small-scale tunnel contacts functioning as thermometers as well as larger electrodes that provide proof-of-principle active cooling of the electron distribution in the NWs.A peak refrigeration of approximately δT =10 mK is achieved at a bath temperature of Tbath ≈ 250-350 mK for our prototype devices.This method introduces important perspectives for the investigation of the thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.     

Superconducting Nb-Ta-Al-AlOx-Al tunnel junctions for x-ray detection

We report progress on the microlithographic fabrication of Nb-Ta-Al-AlOx-Al structures designed for x-ray detection. These structures use bandgap engineering both for quasiparticle trapping to increase the collection efficiency and to prevent quasiparticle diffusion out through the leads. Non-standard tunnel junction geometries are used to reduce the magnetic field needed to suppress the Josephson current for stable biasing. The performance of these devices as alpha particle detectors is presented.     

Fluctuation conductivity of tunnel junctions

Tunneling of electrons is studied at joints between two metals separated by a thin dielectric layer. The conductivity of electrons in the case of the same metals at temperatures higher (close) to the temperature of superconducting transitions has a strong temperature dependence. Tunneling of electrons between a superconductor and a normal metal separated by a thin dielectric layer at temperatures higher than that of the superconducting transition of the normal metal is also considered. The fluctuation correction to the current increases strongly when the temperature approaches the transition temperature and the voltage applied to the barrier becomes of the order of magnitude of that of the energy gap of the superconductor.     

Magnetoresistance in magnetic tunnel junctions with amorphous electrodes

Magnetic tunnel junctions (MTJs) with amorphous CoFeB and Co/sub 2/MnSi electrodes were fabricated and examined. In the case of [Co/sub 90/Fe/sub 10/]/sub 100-x/B/sub x/, the x=32% boron addition reduces the magnetization by 30% compared to Co/sub 90/Fe/sub 10/, yet the reduction of the tunnel magnetoresistance (TMR) is over 95%. On the contrary, in the case of Co/sub (100-x-y)/Mn/sub x/Si/sub y/, although net magnetization is very small at room temperature, the TMR can be as large as 7%. The character of each metalloid (boron and silicon) could be responsible for the peculiar behavior to each system.     

Quantum interference filters based on oxide superconductor junctions for microwave applications

We have studied superconducting quantum interference filters (SQIFs) based on bicrystal neodymium gallate substrates, which can be used in the microwave frequency range. The characteristics of a serial SQIF have been compared for the first time with those of a single superconducting quantum interference device (SQUID) and a chain of serially connected SQUIDs with equal areas of superconducting loops. The regime of SQIF operation with a voltage-flux (V-Φ) characteristic determined by the magnetic-field dependence of the critical current in the Josephson junction has been analyzed. It is shown that the output noise of a SQIF measured with a cooled amplifier in the 1–2 GHz range is determined by the slope of the V-Φ characteristic. The influence of a spread in the parameters of Josephson junctions in the SQIF on the integral V-Φ characteristic of the whole structure is considered.     

Ni-NiO-Ni tunnel junctions for terahertz and infrared detection

We present complete experimental determinations of the tunnel barrier parameters (two barrier heights, junction area, dielectric constant, and extrinsic series resistance) as a function of temperature for submicrometer Ni-NiO-Ni thin-film tunnel junctions, showing that when the temperature-invariant parameters are forced to be consistent, good-quality fits are obtained between I-V curves and the Simmons equation for this very-low-barrier system (measured phi approximately 0.20 eV). A splitting of approximately 10 meV in the barrier heights due to the different processing histories of the upper and lower electrodes is clearly shown, with the upper interface having a lower barrier, consistent with the increased effect of the image potential at a sharper material interface. It is believed that this is the first barrier height measurement with sufficient resolution for this effect to be seen. A fabrication technique that produces high yields and consistent junction behavior is presented as well as the preliminary results of inelastic tunneling spectroscopy at 4 K that show a prominent peak at -59 meV, shifted slightly with respect to the expected transverse optic phonon excitation in bulk NiO but consistent with other surface-sensitive experiments. We discuss the implications of these results for the design of efficient detectors for terahertz and IR radiation.     

Analysis of nonequilibrium double tunnel junctions

The double tunnel junction M₁-S₂-M₃ consists of generator and detector tunnel junctions with a common electrode S₂. The quantity R ₂(E), the time rate of change of the quasiparticle distribution function (produced by the generator current), is calculated for each branch of the excitation spectrum in S₂. The rates of change of the quasiparticle number density \.n ₂, the particle branch imbalance \.Q ₂, and the distribution function branch imbalance \.Q ₂ ^*are also calculated. The detector excess current-voltage characteristic I _dvs. V _d is calculated for arbitrary excess distributions f _< (E) and f _>(E) in S₂. Computed characteristics are obtained assuming square distributions of appropriate amplitudes for f _<and f _>; these characteristics show a zero-voltage current I _d(0) and asymmetric structure at eV_d = ±(₂ + ₂) and eV_d = ±(eV_g – ₁ ± ₃). The square distributions are only approximate; the actual distributions would produce additional structure at eV_d = ±(eV_g + ₁ ± ₃). The results are discussed for M₁ and M₃ either normal or superconducting.This work was supported in part by the National Science Foundation through grant DMR 74-23661.     

High-frequency impedance of superconducting tunnel junctions

Comparison is made between the high-frequency impedance of a Josephson tunnel junction predicted by the RSJ model and that predicted by the Werthamer theory. Results obtained analytically, numerically, and by electronic simulation illustrate several fundamental differences between the two models. In particular, the small-signal rf resistance of a tunnel junction described by the Werthamer theory exhibits strong variations with temperature and frequency. In addition, the Werthamer theory predicts that a tunnel junction will have a plasma-like resonance in the absence of any electrode capacitance, and that the junction can have a nonzero reactance when current-biased outside of a constant-voltage step.     

Magnetic tunnel junctions for magnetic random access memory applications

Magnetic Tunnel Junctions (MTJ) are in the center of a large number of studies in spin polarized transport, because of their potential use in Magnetic Random Access Memory (MRAM) applications. The most commonly used tunnel junction is composed of an aluminium-oxide as a tunnel barrier and presents very interesting tunnel magnetoresistance (TMR) of about 40%. Nevertheless, this barrier needs an oxidation step and has a large resistance, which is not suitable for electronic devices. Thus a MTJ with an alternative ZnS barrier grown by sputtering on Si(111) substrate at room temperature is presented here with the following structure: Fe_{6 nm}Cu_{30 nm}(CoFe)_{1.8 nm}Ru_{0.8 nm}(CoFe)_{3 nm}ZnS_x(CoFe)_{1 nm}Fe_{4 nm}Cu_{10 nm}Ru_{3 nm}. The hard magnetic bottom electrode consists of the artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. This alternative barrier is most suitable for electronic devices and we will discuss its possible application in MRAM.     

Optimization of YBCO surfaces of tunnel junctions

It is established that in YBa₂Cu₃O₇ films prepared by annealing amorphous oxide deposits, Ba segregation in the amorphous phase and YBCO decomposition after recrystallization are the major causes of surface degradation. The authors have grown films by entirely in-situ processing in which these effects are minimized. The films were epitaxially grown on (100) SrTiO ₃ substrates with the a-axis normal to the film plane. Both structural and chemical analyses indicated that they are homogeneous and have proper stoichiometry up to their surfaces. At 4.2 K, contact resistivities below 4×10^-10 ohm-cm² were obtained with gold overlayers. Junctions have been formed by depositing thin Au proximity layers over the YBCO films followed by MgO barriers and Nb counterelectrodes. In some of the junctions weak-link shorts were observed, providing unambiguous evidence that the growth procedures used can produce films that are superconducting up to their surfaces     

Vapor technique for the formation of organic monolayer tunnel junctions

A vapor technique for the formation of organic monolayers within a vacuum chamber is described. The organic dimer molecules are vaporized and then broken down into monomer molecules which chemisorb onto a fresh metal film. Subsequent layers only physically absorb and are driven off by heat, leaving a monolayer which can be used as a tunneling barrier. Details of the fabrication process and the electrical characteristics of experimental Pb-organic-Al tunnel junctions are presented.     

Ab initio simulation of magnetic tunnel junctions

In this paper, we present the mathematical and implementation details of an ab initio method for calculating spin-polarized quantum transport properties of atomic scale spintronic devices under external bias potential. The method is based on carrying out density functional theory (DFT) within the Keldysh non-equilibrium Green's function (NEGF) formalism to calculate the self-consistent spin densities. We apply this method to investigate nonlinear and non-equilibrium spin-polarized transport in a Fe/MgO/Fe trilayer structure as a function of external bias voltage.     

Surface morphology of epitaxial magnetic tunnel junctions

The surface morphology of epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions, which show the giant tunneling magnetoresistance effect, was investigated by in situ scanning tunneling microscopy. It was observed that an epitaxial MgO barrier layer forms flat surface structures. The surface was flatter with distinct steps and terraces after annealing, which would lead to an increase of the tunneling magnetoresistance ratio. Examination of the local electronic structures of 1.05-nm-thick MgO barrier layers by scanning tunneling spectroscopy revealed no pinholes in the layers, so they would be perfect barriers in magnetic tunnel junctions.     

Properties of niobium nitride-based Josephson tunnel junctions

High T_c (16.8 K) niobium nitride thin films have been prepared by rf diode sputtering in N₂/Ar atmosphere and subsequent high temperature annealing. Josephson tunnel junctions have been made by thermal oxidation of the films. The geometry is defined by high resolution photolithography. The Josephson junctions have been characterized by magnetic field diffraction measurements and other techniques and are shown to be particularly suitable for applications especially in superconducting microwave integrated devices.     

Iron electrodiffusion of bulk YBaCuO superconductor under different magnetic fields

In this study, diffusion mechanism of iron impurities in bulk YBa₂Cu₃O₇ (YBaCuO) superconductor prepared by standart solid state reaction method and its effect on lattice structure under different magnetic field have been examined. Correlation between characteristic X-ray intensities and surface dopant density were investigated. The effects of different magnetic field intensities on crystal structure of iron diffused samples have been investigated by quantitative Energy Dispersive X-Ray Fluorescence (EDXRF) and X-ray Diffraction (XRD) techniques. It was found that Fe diffusion increases with increasing magnetic field.     

Plasma frequency of the tunnel Josephson junctions with anharmonic current-phase relationship

The effect of alternating current on the plasma frequency of a tunnel Josephson junction with anharmonic current-phase relationship under the conditions of simultaneous passage of direct (dc) and alternating (ac) current has been studied. It is established that the presence of the ac component decreases, whereas the anharmonicity increases the plasma frequency of the tunnel Josephson junction.     

Theoretical investigation of Josephson tunnel junctions with spatially inhomogeneous superconducting electrodes

The microscopic theory of the Josephson effect in tunnel structures with electrodes having spatially inhomogeneous superconducting properties is formulated. Two mechanisms of inhomogeneity are considered. The first is associated with the presence of a thin transition normal layer located near the tunnel barrier, which is relevant for junctions based on refractory superconductors. The second case is the trapping of Abrikosov vortices by junction electrodes. The tunnel current components are calculated numerically in the whole temperature range 0<T<T _c and magnetic field range 0<H<H _c2. It is shown that the tunnel current is extremely sensitive to the type of smearing of the singularities of the classical tunnel theory ateV=2. The results allow experimental determination of the characteristics of real tunnel junctions.     

Upper critical field for a superconductor with energy-dependent density of states: Possible application to heavy-fermion systems

The Eilenberger equations are applied to calculate the upper critical field for a superconductor with an energy-dependent density of states. The influence of the electron-paramagnon scattering is also considered. These two effects may be important in the explanation of the temperature dependence of the upper critical field of a heavy-fermion superconductor.