On the passive electrode signal in X-ray detectors based on superconducting tunnel junctions

X-ray detectors based on superconducting tunnel junctions with the multilayer electrode structure described by the formula Ti/Nb/Al, AlO _x /Al/Nb/NbN were studied. The main signal arose during X-ray absorption in the top electrode and had an energy resolution of ∼90 eV at the 5.9-keV line. The bottom passive Ti/Nb electrode provided rapid absorption of excess quasiparticles. The residual signal of the passive electrode was from 7 to 17% of the main signal amplitude. The dependences of the amplitude of this signal on the voltage and the absorbed X-ray energy were measured for detectors with different thicknesses of the top and bottom electrodes. The rate of quasiparticle trapping by the energy trap in the Ti/Nb bilayer was estimated. The main mechanisms of the formation of the passive electrode signal formation were considered and methods for its suppression were proposed.     

A STJ X-ray detector with a β-tantalum sublayer

The first step in the research and development of detectors for ¹⁸¹Ta Mössbauer spectroscopy based on superconductive tunnel junctions (STJ) is taken. The idea of using β-tantalum as a trapping layer for the inactivation of the base electrode in a detector with the structure β-Ta/Nb/Al, AlO _x /Al/Nb/NbN is successfully executed.     

Nb/Al-AlOx/Nb junctions with controllable critical current density for qubit application

Nb/Al-AlOx/Nb tunnel junctions with controllable critical current density Jc are fabricated using the standard selective Nb etching process.Tunnel barriers are formed in different oxygen exposure conditions (oxygen pressure P and oxidation time t),giving rise to Jc ranging from 100 A/cm2 to above 2000 A/cm2.Jc shows a familiar linear dependence on P × t in logarithmic scales.We calculate the energy levels of the phaseand flux-type qubits using the achievable junction parameters and show that the fabricated Nb/Al-AlOx/Nb tunnel junctions can be used conveniently for quantum computation applications in the future.     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Energy efficiency of femtosecond laser ablation of refractory metals

We present the results of an experimental study of the ablation energy thresholds and ablated mass for a number of refractory metals (Ti, Zr, Nb, Mo) by femtosecond (τ _0.5 = 45–70 fs) exposed to laser pulses in the ultraviolet — near infrared range (λ = 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10^–2 Pa). We have analyzed the ablation efficiency (mass yield per unit energy of the acting coherent radiation) and ablation energy thresholds vs. the laser pulse duration and photon energy.     

Current transport along the [001] axis of YBCO in low-temperature superconductor-normal metal-high-temperature superconductor heterostructures

The electrophysical properties of heterojunctions several microns in size, obtained by successive deposition of the metal-oxide high-temperature superconductor YBa₂Cu₃O_x, a normal metal Au, and the low-temperature superconductor Nb, were studied experimentally. Current flows in the [001] direction of the epitaxial YBa₂Cu₃O_x film. It is shown, by comparing the experimental data with existing theoretical calculations, that for the experimentally realizable transmittances (D̄=10⁻⁵–10⁻⁶) of the YBa₂Cu₃O_x—normal metal boundary the critical current of the entire heterostructure is low (of the order of the fluctuation current) because of a sharp change in the amplitude of the potential of the superconducting carriers at this boundary. The current-voltage characteristics of the heterostructure studied correspond to tunnel junctions consisting of a superconductor with type symmetry of the superconducting wave function and a normal metal. Zh. éksp. Teor. Fiz. 116, 2140–2149 (December 1999)     

Developing topologies of thin-film SQUID sensors for measuring extremely subtle magnetic fields

A topology of thin-film SQUID sensors that are based on Nb/AlO_x/Nb tunnel junctions has been developed and optimized for nondestructive testing of materials and for other systems with a magnetic field sensitivity of <10 fT/Hz^1/2.     

Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 10¹³–10¹⁴ W/cm² with flat low-density (0.5–10 mg/cm³) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2ω ₀ and 3ω ₀/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material—a fast stage (0.1–0.3 ns) and a slow stage (1–3 ns), are in good agreement with the experimental data. Zh. éksp. Teor. Fiz. 115, 805–818 (March 1999)     

Josephson and single-particle currents between partially dielectricized superconductors with charge-density waves

The amplitudes of the nonstationary Josephson current I ¹, the interference current I ², and the quasiparticle current J through symmetric and asymmetric tunnel junctions, including superconductors with charge density waves, are calculated. In the symmetric (s) case the dependence of the Josephson current I _s ¹ on the voltage V on the junction has a logarithmic singularity at |eV|=2Δ, Δ+D, and 2D, where , Δ and Σ are the superconducting and dielectric order parameters, and e is the unit charge. At temperatures T≠0 jumps appear in the current-voltage characteristics I _s ¹ (V) at |eV|=D−Δ. Jumps and singularities are observed in the currents I _s ² and J _s at the same voltages at which singularities and jumps appear in I _s ¹ , respectively. In the nonsymmetric (ns) junctions which include an ordinary superconductor, singularities and jumps occur at |eV|=D+Δ_BCS, Δ+Δ_BCS, and (for T≠0) |D−Δ_BCS| and |Δ−Δ_BCS|, where Δ_BCS is the order parameter of an ordinary superconductor. The quasiparticle current J _ns is an asymmetric function of the voltage V and does not depend on the sign of Σ. The results are compared with experiment. Fiz. Tverd. Tela (St. Petersburg) 39, 991–999 (June 1997)     

230 and 492 GHz low noise sis waveguide receivers employing tuned Nb/AlOx/Nb tunnel junctions

We report results on two full height waveguide receivers that cover the 200–290 GHz and 380–510 GHz atmospheric windows. The receivers are part of the facility instrumentation at the Caltech Submillimeter Observatory on Mauna Kea in Hawaii. We have measured receiver noise temperatures in the range of 20K–35K DSB in the 200–290 GHz band, and 65–90K DSB in the 390–510 GHz atmospheric band. In both instances low mixer noise temperatures and very high quantum efficiency have been achieved. Conversion gain (3 dB) is possible with the 230 GHz receiver, however lowest noise and most stable operation is achieved with unity conversion gain.A 40% operating bandwidth is achieved by using a RF compensated junction mounted in a two-tuner full height waveguide mixer block. The tuned Nb/AlO _x /Nb tunnel junctions incorporate an end-loaded tuning stub with two quarter-wave transformer sections to tune out the large junction capacitance. Both 230 and 492 GHz SIS junctions are 0.49µm² in size and have current densities of 8 and 10 kA/cm² respectively.Fourier Transform Spectrometer (FTS) measurements of the 230 and 492 GHz tuned junctions show good agreement with the measured heterodyne waveguide response.     

On the oxidation and on the superconductivity of niobium

Most superconducting rf cavities consist on Nb and most Josephson tunnel junctions are Nb based. Due to the very small dissipation in these superconducting cavities and tunnel junctions quantitative and qualitative new surface effects emerge, which are related to the oxidation of Nb.The oxidation of Nb is dominated by the solution of oxygen in Nb and the growth of Nb₂O₅ microcrystallites. Both effect heavily strain the Nb-Nb₂O₅ interface creating O segregates and serrating the interface, growing with oxidation. These effects yield a serrated and eroded transition superconductor — insulator, which affects the properties of superconducting rf cavities and tunnel junctions significantly.Based upon a habilitation thesis, University of Karlsruhe (1985)     

0-π oscillations in nanostructured Nb/Fe/Nb Josephson junctions

The physics of the π phase shift in ferromagnetic Josephson junctions may enable a range of applications for spin-electronic devices and quantum computing. We investigate transitions from “0” to “π” states in Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to 5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of about 1.1 nm. By fitting the characteristic voltage oscillations with existing theoretical models we extrapolate an exchange energy of 256 meV, a Fermi velocity of 1.98 ×10⁵ m/s and an electron mean free path of 6.2 nm, in agreement with other reported values. From the temperature dependence of the I_CR_N product we show that its decay rate exhibits a nonmonotonic oscillatory behavior with the Fe barrier thickness.     

Josephson coupling and Fiske dynamics in ferromagnetic tunnel junctions

We report on the fabrication of Nb/AlO _x /Pd_0.82Ni_0.18/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to π-coupling is observed for a thickness d _F @\simeq 6 nm of the ferromagnetic Pd_0.82Ni_0.18 interlayer. The magnetic field dependence of the π-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd_0.82Ni_0.18 has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments.     

Effect of an applied magnetic field on the performance of a sis receiver near 300 GHz

We have successfully constructed and tested a superconductor-insulator-superconductor (SIS) receiver for operation at 265–280 GHz using 1 m² area Nb–AlO _x –Nb tunnel junctions fabricated at Stony Brook. The best performance to date is a double sideband (DSB) receiver noise temperature of 129 K at 278 GHz. We find that suppression of the Josephson pair currents with a magnetic field is essential for good performance and a stable DC bias point. Fields as high as 280 gauss have been used with no degradation of mixing performance. We illustrate the improvement in the intermediate frequency (IF) output stability with progressively increasing magnetic fields.     

Multidetector device for the detection of coincidences of charged particles and γ rays

A 4π position-sensitive, axisymmetrical assembly of Si-Au charged-particle detectors is proposed, implemented, and tested on a beam of heavy ions; the dimensions and structure of the device are conducive to the organization of coincidences of charged reaction products with discrete γ rays emitted by the daughter nucleus and registered by a system of ultrapure Ge detectors. First results are obtained from an investigation of the reaction ⁵⁸Ni(¹⁶O,α2pγ)⁶⁸Ge at E ₀=74.5 MeV. Zh. Tekh. Fiz. 68, 139–142 (April 1998)     

A low-noise SIS receiver measured from 480 GHz to 650 GHz using Nb junctions with integrated RF tuning circuits

A heterodyne receiver using an SIS waveguide mixer with two mechanical tuners has been characterized from 480 GHz to 650 GHz. The mixer uses either a single 0.5 × 0.5 µm² Nb/AlO_x/Nb SIS tunnel junction or a series array of two 1 µm² Nb tunnel junctions. These junctions have a high current density, in the range 8 – 13 kA/cm². Superconductive RF circuits are employed to tune the junction capacitance. DSB receiver noise temperatures as low as 200 ± 17 K at 540 GHz, 271 K ± 22 K at 572 GHz and 362 ± 33 K at 626 GHz have been obtained with the single SIS junctions. The series arrays gave DSB receiver noise temperatures as low as 328 ± 26 K at 490 GHz and 336 ± 25 K at 545 GHz. A comparison of the performances of series arrays and single junctions is presented. In addition, negative differential resistance has been observed in the DC I–V curve near 490, 545 and 570 GHz. Correlations between the frequencies for minimum noise temperature, negative differential resistance, and tuning circuit resonances are found. A detailed model to calculate the properties of the tuning circuits is discussed, and the junction capacitance as well as the London penetration depth of niobium are determined by fitting the model to the measured circuit resonances.     

Phase shift analysis and extraction of structure functions for Coulomb distortion on (e, e'p) reactions at high electron energies

New results for the double beta decay of ⁷⁶ Ge are presented. They are extracted from data obtained with the HEIDELBERG-MOSCOW experiment, which operates five enriched ⁷⁶ Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two-neutrino-accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half-life of T _1/2 ^2ν = [1.55±0.01(stat)^+0.19 _-0.15(syst)]×10²¹ y. The lower limit on the half-life of the 0νββ decay obtained with pulse shape analysis is T _1/2 ^0ν > 1.9×10²⁵(3.1×10²⁵) y with 90% C.L. (68% C.L.) (with 35.5 kg y). This results in an upper limit of the effective Majorana-neutrino mass of 0.35 eV (0.27 eV) using the matrix elements of A. Staudt et al.'s work (Europhys. Lett. 13, 31 (1990)). This is the most stringent limit at present from double beta decay. No evidence for a majoron-emitting decay mode is observed. Received: 22 August 2001 / Accepted: 18 October 2001     

Novel compact 60-kV Mott detector for spin-polarization electron spectroscopy

A compact 60-kV Mott polarimeter designed specially for the local analysis of surface and two-dimensional magnetism by spin-resolved electron spectroscopy is developed and tested. The use of a design which combines a spherical accelerating field and the absence of a retarding potential after scattering of the electron beam ensures high stability of the measured polarization even when the potential and diameter of the beam being investigated vary. As a result of optimization of the scattering angle (118°) and the use of surface-barrier detectors with a large collection angle (∼48°), the efficiency or figure of merit of the polarimeter, which determines the signal-to-noise ratio ɛ=(I/I ₀)·(S _eff)², equals 2.5×10⁻⁴. Specially developed electronic circuits and optimum positioning of the detectors provide a maximum electron counting rate as high as 5×10⁶ counts/s. Consequently, it is possible to calibrate the polarimeter (to find the effective Sherman function S _eff) by extrapolating the measured asymmetry to a high discrimination level. This instrument can also be used in other areas of solid-state physics, atomic physics, and high-energy physics. Zh. Tekh. Fiz. 68, 125–130 (August 1998)     

Metal point contacts and metal-oxide tunnel barriers fabricated with an AFM

We report the fabrication of atomic point contacts and lateral tunnel junctions by using anodic oxidation of thin metal films with an atomic force microscope.In situelectrical measurements were used as feedback to control the fabrication of metal nanowires that were subsequently anodized through their cross section to form point contacts and tunnel junctions. When the conductance of an Al device is reduced below 5×10⁻⁴S it starts to decrease in discrete steps of 2e²/h. In some devices we are able to stabilize the conductance at a value near 2e²/hwhich corresponds to a single, atomic-sized conducting channel. Similar experiments on Ti devices result in a continuous decrease of the conductance and the formation of stable tunnel junctions. This continuous behavior is a result of the large series resistance and the small oxide barrier height of the Ti/TiO_xsystem.     

Optical nutation at a Raman-active transition

Optical nutation at the Raman-active transition 6P _1/2−6P _3/2 of thallium atoms (ω _R /2πc=7793 cm ⁻¹) under resonant Raman excitation by a biharmonic picosecond pulsed field, giving rise to substantial motion of the population, is detected. Optical nutation appears as an oscillatory behavior of the energy of the anti-Stokes scattering of probe pulses, which follow with a fixed delay, as a function of the product of the energies of the excitation pulses. As a result of the dynamic Stark effect, which decreases the frequency of the transition under study, resonance excitation conditions are satisfied for negative initial detunings of the Raman excitation frequency from resonance. The Raman scattering cross section for the transition under study is estimated by comparing the experimental data with the calculations. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 1, 7–12 (10 July 1999)