Temperature-dependent resonant tunneling in disordered quasi-one-dimensional <Emphasis Type="Italic">N</Emphasis>–<Emphasis Type="Italic">I</Emphasis>–<Emphasis Type="Italic">N</Emphasis> junctions

Formulas are obtained for the current–voltage characteristics and conductance of a quasi-one-dimensional N–I–N junction (where N is an ordinary metal and I is an insulator) with quantum resonance percolation trajectories (QRPTs) in a disordered I-layer at temperatures T > 0 K and the energy of local single-impurity electron level being equal to the Fermi energy ε₀ = ε_F. Under these conditions, the impact QRPTs have on the current through the junctions weakens as the temperature grows, and the conductance drops; this is in contrast to the rise in conductance of an empty junction (with no impurities in the I-layer).     

Scattering of Josephson plasmons on random quantum jumpers in a disordered <Emphasis Type="Italic">I</Emphasis>-layer of an <Emphasis Type="Italic">S</Emphasis>–<Emphasis Type="Italic">I</Emphasis>–<Emphasis Type="Italic">S</Emphasis> junction

A formula for the relaxation time of Josephson plasmons on random quantum jumpers, i.e., quantum resonant-percolation trajectories (QRPT) in a disordered I-layer of a tunnel S–I–S junction is derived. Domain Ω_r (μ ? E₀, c), in which the strongest plasmon damping takes place, is plotted in the plane of parameters (μ ? E₀, c).     

Low-field peak effect in type II superconductors

The combined effect of a surface (edge) barrier and volume pinning on the dependence of critical current I _c on the magnetic field (I⊥H ₀) in bulk type II superconductors is investigated. In low magnetic fields, there is a portion of the curve I _c(H ₀) where I _c grows with H ₀, causing a nontrivial peak effect in this field range. Such behavior is explained by the combined effect of a surface (edge) barrier and volume pinning, the latter being rather sensitive to the transport current density distribution in a superconductor.     

Josephson effect in S<Subscript>F</Subscript>XS<Subscript>F</Subscript> junctions

We investigate the Josephson effect in S_FXS_F junctions, where S_F is a superconducting material with a ferromagnetic exchange field, and X is a weak link. The critical current I_c increases with the (antiparallel) exchange fields, if the distribution of transmission eigenvalues of the X layer has its maximum weight at small values. This exchange-field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an S_FIS_F junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference in the exchange fields h₁-h₂ in an S_FIS_F junction corresponds to the potential difference V₁-V₂ in an SIS junction; i.e., the singularity in I_c [in an S_FIS_F junction] at |h₁-h₂|=Δ₁+Δ₂ is the analogue of the Riedel peak. We also discuss the AC Josephson effect in S_FIS_F junctions.     

Critical current in superconducting quantum point contacts

The quantization of the critical current I _c in a quantum point contact is studied on varying the number of free channels (varying the constriction width d ₀ in 2DEG). It is shown that the shape of the quantum I _c is not universal and depends on the parameters of the contact, in particular, on the properties of the 2DEG-S boundaries. Because of the effect of normal reflection from the S-2DEG boundaries, the quantum critical current nonmonotonically depends on d ₀ and may have a resonance structure. If E _F coincides with a quasi-stationary level of the square potential well formed by the contact walls, the critical current has a local maximum (resonance) equal (in the first channel) to the ratio of the electron charge to its time of travel through the contact. The critical current at the maximum is determined by the lowest positive Andreev level (for the phase difference π).     

Influence of experimental conditions on the electric field effect in the ceramic (BiPb)2Sr2Ca2Cu3Ox

An experimental study is reported of the influence of temperature (T), electric field polarity (±E), as well as of changes in the electrode/insulator/superconductor (E/I/S) measuring system on the field effect in the ceramic (BiPb)₂Sr₂Ca₂Cu₃O_x. It has been established that at 77 K and for E⩾60 MV/m the critical current I _c and conductivity of the sample increase for I>I _c, irrespective of the field polarity. For lower fields and a negative electrode potential the conductivity in an electric field may decrease. The field effect decreases with increasing temperature, to practically vanish near T _c where the sample is still in superconducting state. Experiments carried out with more complex systems E/I/M/I/S and E/I/M/S (M stands for a metallic foil) support the conclusion that it is the external electric field that is responsible for the observed effects. Fiz. Tverd. Tela (St. Petersburg) 39, 1967–1970 (December 1997)     

Metal-insulator transition in the spinel-type Cu(Ir1−xCrx)2S4 system

A normal thiospinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition around 230 K with structural transformation, showing hysteresis on heating and cooling. On the other hand, CuCr₂S₄ has the same normal spinel structure without the structural transformation. CuCr₂S₄ has been found to be metallic and ferromagnetic with the Curie temperature T_c~377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir_1−xCr_x)₂S₄. The M-I transition temperature shifts to lower temperature with increasing Cr-concentration x and this transition is not detected above x~0.05. The ferromagnetic transition temperature decreases as x is decreased and the transition does not occur below x~0.20.     

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I _c?(Φ/Φ₀) = I _c+(Φ/Φ₀ + Δ?) of the critical current measured in opposite directions. This shift means that the position of I _c+(Φ/Φ₀) and I _c?(Φ/Φ₀) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I _{c, an}(Φ/Φ₀) = I _c+(Φ/Φ₀) ? I _c?(Φ/Φ₀) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.     

Current-induced resistive state of superconducting tin whiskers with indium impurities

We have investigated the current-induced step-like structure in theV?T andV?I characteristics of tin whiskers with a mean free pathl for the electrons which is reduced by indium impurities (up to 3.6 at %). The extrapolation of theV?I characteristics beyond the first voltage step shows a zero-voltage interceptI ₀≈0.5I _c (I _c=critical current). From the differential resistance beyond the first voltage step a lengthL _An1 can be obtained which is proportional tol ^1/2. These results can be understood by assuming phase-slip centers in the whisker which carry a time-averaged supercurrent of approximately 0.5I _c above the critical current, and at which a voltage is developed by the “healing” of nonequilibrium quasiparticles.     

Effect of an electric field on the I–V curves of DyBa2Cu3−x Oy/1 wt % Pt HTSC ceramics

This paper reports on an experimental study of the effect of a magnetic field, B≤70 G, and an electric field, E=120 MV/m, on the critical current I _c and I–V curves of DyBa₂Cu_3?x O_y HTSC ceramics (x=0 and 0.2), both undoped and doped with 1 wt % Pt. It has been established that, in stoichiometric ceramics (x=0) at 77 K, I _c drops sharply (by more than an order of magnitude) already at very low B<1 G. In copper-deficient ceramics (x=0.2), I _c decreases with increasing B slowly, with Pt-doped samples exhibiting [on the dropping I _c (B) dependence] a peak effect, i.e., an increase rather than decrease of I _c at B≈10 G. As for the effect of an electric field on I _c and the I–V curves (the E effect), it is not observed in ceramics of a stoichiometric composition. DyBa₂Cu_2.8O _y samples acted upon by an electric field reveal a substantial increase in I _c and a decrease in the resistance R for I>I _c . In the case of DyBa₂Cu_2.8O_y/Pt, the electric field practically does not affect I _c but R decreases for I>I _c . In a sample placed in a magnetic field, the magnitude of the E effect is observed to correlate with the I _c (B) dependence. In particular, in Pt-doped samples, the E effect decreases with increasing magnetic field B not gradually but with a maximum appearing at B ≈10 G, i.e., in the region of the peak effect in the I _c (B) dependence. The data obtained suggest the conclusion that the electric-field effect in ceramics exhibiting weak links of the superconductor-insulator-superconductor (SIS) type correlates with magnetic vortex pinning.     

Influence of quantum resonance-percolation trajectories in a disordered I layer on the critical current of a Josephson S-I-S junction

Formulas for a critical current and the magnitude of its mesoscopic structural fluctuations are obtained in the form of sums over quantum resonance-percolation trajectories [1] randomly formed in a disordered I layer and connecting opposite S banks of the junction at T = 0 in the energy region of tunnel S-I-S resonances (S denotes the superconductor, I, insulator) for a tunnel junction with weak structural disorders (low impurity concentrations) in the I layer.     

Equation of state for rotating nuclei at finite temperature

The equation of state ω(T, I) is derived for temperatures in the range0 ? T ? 0.5 MeV and spins in the range 0 ? I ? 24. It is demonstrated that if the yrast line backbends, then the equation of state contains a critical point at temperature T_c, and that backbending disappears for T > T_c.     

Disordered ferromagnetic alloys — Effective medium

The transition temperature and critical concentration of randomly disordered A_χB_1?χ alloys, where A is the magnetic constituent of the alloy, have been investigated. The effects of second neighbor A-A and of nearest neighbor A-B interactions, both ferromagnetic and antiferromagnetic, have been included. Molecular field, assumed to hold within small enough regions, together with percolation theory are used to formulate an effective magnetic medium.     

Correlation between the electric-field effect and weak-link type in YBa2Cu3−x Oy and YBa2Cu3−x Oy /Agx high-T c superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current I _c and I-V characteristics of yttrium-based high-T _c superconducting ceramics. The materials studied were copper-deficient ceramics, YBa₂Cu_3−x O_y (D samples), and YBa₂Cu_3−x O_y/Ag_x ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases I _c and reduces substantially R for I>I _c, whereas in S samples no field effect is observed. Measurements of the I _c(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type I _c =const(1−T/T _c )^α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries. Fiz. Tverd. Tela (St. Petersburg) 40, 1195–1198 (July 1998)     

Enhancing of the critical temperature of an in-plane FFLO state in heterostructures by the orbital effect of the magnetic field

It is well-known that the orbital effect of the magnetic field suppresses superconducting T _c . We show that for a system, which is in the Larkin-Ovchinnikov-Fulde-Ferrell (FFLO) state at zero external magnetic field, the orbital effect of an applied magnetic field can lead to the enhancement of the critical temperature higher than T _c at zero field. We concentrate on two systems, where the in-plane FFLO state was predicted recently. These are equilibrium S/F bilayers and S/N bilayers under nonequilibrium quasiparticle distribution. However, it is suggested that such an effect can take place for any plane superconducting system, which is in the in-plane FFLO state (or is close enough to it) at zero applied field.     

A Mössbauer-effect study of the isomer shifts of iron in nickel aluminides

The room-temperature isomer shifts of iron in nickel aluminides have been measured. A single Mössbauer-resonance line is observed for alloys of composition (NiAl)_100-cFe_c, 0 ? c ? 5 at. %, and Al₅₀Ni^50-cFec, 2 ? c ? 50 at. %. The isomer shift for dilute Fe concentration in stoichiometric NiAl is compared with that for FeAl at stoichiometric composition.     

The correlation function of the Ising model in the MFA and their sum rule

It is shown that in the correctly performed molecular field approximation the correlation function 〈S(q) S(-q)〉 fulfills the sum rule N^-1Σ_q〈S(q) S(-q)〉 = 1. This can be proved for ferro- and antiferromagnets and the disordered phases of o-hydrogen.     

Magnetic field effect on the critical EPR-dynamics of the cubic ferromagnets CdCr2Se4 and CdCr2S4

We present measurements of the critical behaviour of the EPR linewidths at frequencies 9.21 and 35.5 GHz. In the exchange critical region above T_c (4πx ? 1) the strong field dependence of the linewidths is observed, even when the field variation of susceptibility x is small. This phenomenon is explained by the spin diffusion effect on the linewidth. The spin diffusion coefficients D for CdCr₂Se₄ and CdCr₂S₄ are determined from the linewidths data. The temperature dependence D is found to be consistent with the predictions of the dynamical scaling theory.     

Characterization of conductance under finite bias for a self-assembled monolayer coated Au quantized point contact

We have demonstrated that an experimental cross-wire junction set-up can be used to measure the I-V characteristics of a self-assembled monolayer (SAM) stabilized metal quantized point contact. The increased stability due to the presence of the SAM allows the measurement of the I-V characteristics. However, the SAM also provides additional conductance paths in addition to the pure metal point contact. The presence of the SAM may contribute to the non-integral quantum conductance transition and the non-linear I-V characteristics of the quantum contact. Nonetheless, a straight I-V curve is obtained for the Au quantized point contact from 0 to 1 V with a conductance of approximately 1G₀, in contrast to previous work reported in the literature.