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.     

UV to IR photon detection using superconducting tunnel junctions

Superconducting Tunnel Junctions have been investigated in the past as particle and X-ray detectors. We report here on results obtained illuminating Nb-Al-AlOx-Nb junctions with radiation covering the UV to IR (250 – 1100 nm) at 0.30 K. In the presence of continuous illumination, an increase of the subgap current is observed, while the change is found to be consistent with the amount of energy supplied to the junction. Several measurements have been performed in a pulsed mode, showing the detection of pulses containing about 100 photons each, at =850 nm. These results have allowed a determination of the mean energy required to create a single charge carrier in Nb, which agrees well with theoretical predictions. Such results open new possibilities for the fabrication of optical photon counting cameras based on Josephson junctions.     

Josephson-type superconductive tunnel junctions and applications

The physics and applications of the Josephson effect are reviewed in detail. A brief introduction to superconductivity and tunneling is provided. The configuration most fully discussed is the oxide junction. However, properties of bridges, point-contacts, and solder-blobs are also described. Among the applications reviewed are magnetometers, infrared detectors, microwave generators and mixers, voltmeters, computer logic and memory devices, relaxation oscillators, thermometers, and spectrometers.     

Superconducting tunnel junctions and quasiparticle trapping

The interaction of a nuclear particle or X-ray with a superconductor leads to the breaking of Cooper pairs and the creation of excess phonons and quasiparticles. The basic physics and the use of superconducting tunnel junctions as detectors of excess quasiparticles are reviewed. For superconducting absorbers of appreciable mass intrinsic limitations require the use of the phenomenon of quasiparticle trapping. The relaxation phonons released in the trapping process can also be detected and they can lead to amplification via further pair breaking. Superconducting tunnel junctions can also detect phonons produced by particle interactions in a substrate absorber. The use of series-connected arrays of junctions to achieve high sensitivity and good energy resolution is discussed.     

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.     

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.     

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.     

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.     

Seebeck effect in magnetic tunnel junctions

Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K?1 from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K?1. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.     

Electric-field-assisted switching in magnetic tunnel junctions

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.     

Y-Ba-Cu-O/Nb Josephson tunnel junctions

The authors fabricated Y-Ba-Cu-O/Au/AlO_x/Nb and Y-Ba-Cu-O/AlO_x/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Sintered Y-Ba-Cu-O was used as the base electrode. Superconducting Josephson current and hysteresis of the current-voltage characteristics, which are typical features of Josephson tunnel junctions, have been observed at 4.2 K. RF-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed, and RF-induced subharmonic steps have also appeared. The superconducting Josephson current was modulated by the magnetic field     

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.     

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.     

Graphene as a tunnel barrier: graphene-based magnetic tunnel junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic.     

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.     

On phonon structure in proximity tunnel junctions

We have iterated to convergence, for the first time, a set of four coupled real axis Eliashberg equations for the superconducting gap and renormalization functions on each side of a proximity sandwich. We find that the phenomenological procedures developed to extract the size of the normal side electron-phonon interaction from tunneling data are often reasonable but may in some cases need modifications. In all the cases considered the superconducting phonon structure reflected on the normal side, as well as other structures, shows considerable agreement with experiment as to size, shape, and variation with barrier transmission coefficient. Finally, we study the effects of depairing on these structures.     

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.     

磁性隧道结中自旋相关的隧穿输运

全面回顾和总结了磁性隧道结中自旋相关的隧穿这一研究领域的理论和实验方面的最新研究进展。讨论了影响磁性隧道结的自旋极化和隧穿磁电阻的各种因素及反映铁磁层和铁磁／绝缘层界面电子结构在隧穿中重要作用的理论模型和近期实验，同时也讨论了绝缘势垒和铁磁／绝缘层界面中的无序性在隧穿过程中对自旋极化与磁电阻效应的影响。     

Riedel singularity in tin-tin oxide-tin tunnel junctions

The Riedel peak in the Josephson supercurrent in Sn-Sn oxide-Sn tunnel junctions has been investigated by studying the amplitudes of microwave-induced steps. At low temperatures, the peak rises to a value approximately 2.8 times the zero-voltage supercurrent amplitude. This can be accounted for quantitatively in terms of the known bulk gap anisotropy of tin, reduced by a factor of 2–3 due to dirty-superconductor averaging and a further factor of about 17 due to preferred crystallographic orientation of the films comprising the junctions. At high temperatures, the truncation of the peak is in quantitative agreement with theoretical predictions of the effect of quasiparticle damping processes. Complementary measurements of the slope of the quasiparticle tunnel current at the gap voltage cannot be satisfactorily explained in a similar way, despite the common origin of the quasiparticle current jump at the gap voltage and the Riedel peak in the quasiparticle density-of-states peak at the gap edge. The quasiparticle current jump appears to be broadened by some mechanism which does not significantly affect the Riedel peak.Research supported by the Army Research Office (Durham) and the Advanced Research Projects Agency.     

Solid-state memories based on ferroelectric tunnel junctions

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1?×?10(6)?A?cm(-2)). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ～1?×?10(4)?A?cm(-2) at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism.