Peak effect in surface resistance at microwave frequencies in Dy-123 thin films

A pronounced peak in the microwave (at frequency 9.55 GHz) surface resistance, R _s vs. T plot (where T is the temperature) has been observed in epitaxial DyBa₂Cu₃O_7−y superconducting thin films in magnetic fields (parallel to c-axis) in the range 2 to 8 kOe, and temperatures close to the superconducting transition temperature T _c(H). Our data suggest that the nature of peaks observed in the two films is different, thereby indicating different defect structures in the films.     

Surface resistance of an YBaCuO film in the megahertz range determined from the Q factor of a planar inductance coil: Effect of extrinsic factors

High-quality YBaCuO films are used to sequentially prepare a 10-GHz disk resonator and a planar inductance coil. The Q factor of the planar inductance coil at its resonance frequency (64 MHz) is much higher than the values reported for analogous structures. The measurement results are used to estimate the surface resistance of the films at frequencies of 10 GHz and 64 MHz. The surface resistance measured at 64 MHz is more than fourfold of that calculated from the surface resistance measured at 10 GHz by the dependence R _sur ∼ ω². Our analysis demonstrates that extrinsic factors cannot substantially affect the measurement results; therefore, the deviation from the R _sur ∼ ω² dependence in the megahertz range is determined by the intrinsic properties of the superconducting strip.     

Comparison of superconducting and electronic properties of σ-and A-15 phase Nb−Al

For both, the σ-phase and the A-15 phase structure of Nb−Al binary alloys the superconducting properties were investigated in connection with measurements of normal state electronic properties like resistance and NMR-frequency and line width. The A-15 phase shows a transition temperature ofT _c ≈17.5 K. The σ-phase shows only weak transitions at the same temperature which can be explained by A-15 inclusions in a non-superconducting σ-matrix. The Al²⁷-Knight shift at room temperature is reduced from 0.162% for pure Al to (0.024±0.002)% and (0.026±0.0025)% for the A-15 and the σ-phase, respectively. The corresponding temperature coefficients differ in sign: positive for the A-15 and negative for the σ-phase; this may have a relation to the different superconducting properties. The NMR line width and the resistance show no different behaviour for the two phases. In general, the electronic properties seems to be less important for the occurance of superconductivity in the Nb−Al system than the lattice structures.     

Observation of sharp resistance anomalies near Tc in granular superconductors

Granular films prepared from well-defined In clusters embedded in Kr (? 10-40 at % Kr) show sharp resistance anomalies very close to the superconducting transition temperature. The observed resistance peak(s), which go(es) above the normal state resistance R_N and which can be as high as ? 1.5 R_N, can be explained by a “mesoscopic” effect: Superconducting percolation aggregates of size ξ_ρ, larger than the superconducting coherence length ξ_S, but smaller than the quasiparticle relaxation length λ_Q*, have a superconducting gap, but are not yet able to carry a supercurrent.     

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     

快冷Fe-Al合金中的原子缺陷弛豫

在多功能内耗仪上用自由衰减和强迫振动方法研究了不同Al含量淬火Fe-Al合金中的两个弛豫型内耗峰.结果显示：P1(180℃)和P2(340℃)两个内耗峰只出现在淬火样品的加热过程中,而在随后的冷却过程中不出现.P1峰是由在α和β(或γ)点阵上的空位组成的最近邻双空位偶极子在应力诱导下的重新取向产生的,其弛豫强度随Al含量非单调地变化,在大约25% Al(原子百分比,以下同)处出现最大值.Al含量较低的Fe-Al合金无 关键词： 空位 弛豫 Al含量     

Resistive transitions in Nb/Cu0.41Ni0.59/Nb trilayers

The superconducting phase transition in Nb/Cu_0.41Ni_0.59/Nb trilayers, with superconducting (S) Nb and ferromagnetic (F) Cu_0.41Ni_0.59, has been experimentally studied as a function of the F-layer thickness by measuring the temperature dependence of the electrical resistance R(T). It is shown that the shape and the width of the R(T) curves depends on the Cu_0.41Ni_0.59 thickness, in particular in the regime where π is the coupling between the S layers, which can be expected. To explain the data, we developed a qualitative model which makes the interconnection between the superconducting phase transition and the 0 to π transition in SFS structures are more evident. The text was submitted by the authors in English.     

Zeeman effects on d-wave superconductor and tunneling spectrum in normal-metal/d-wave superconductor tunnel junction

We study the Zeeman effect on the d-wave superconductor and tunneling spectrum in normal-metal(N)/d-wave superconductor(S) junction by applying a Zeeman magnetic field to the S. It is shown that: (1) the Zeeman magnetic field can lead to the S gap decreasing, and with the increase in Zeeman energy, the superconducting state is changed to the normal state, exhibiting a first-order phase transition; (2) the Zeeman magnetic field may make the zero-bias conductance peak split into two peaks, and the energy difference between the two splitting peaks in the conductance spectrum is equal to 2h ₀ (h ₀ is the Zeeman energy); (3) both the barrier strength of interface scattering and the temperature can lower the magnitudes of splitting peaks, of which the barrier strength can lead to the splitting peaks becoming sharp and the temperature can smear out the peaks, however, neither of them can influence the Zeeman effect.     

Controlling the charging energy of arrays of tunnel junctions

We describe a new technique to control in situ charging energy of systems of coupled metallic or superconducting islands. To illustrate the technique, we have fabricated two-dimensional arrays of Al islands on GaAs/AlAs heterostructures. Each island is coupled to its nearest-neighbor by a submicron Al/AlO_x/Al tunnel junction and to the three-dimensional electron gas (3DEG) located below the surface of the heterostructure by a capacitance C_g. We vary C_g, which dominates the charging energy of the array, by depleting the electrons in the 3DEG by means of a negative voltage applied to the array. With the array driven normal by a magnetic field, a decrease in C_g increases in both the offset voltage and the period of the Coulomb blockade oscillations.     

Transverse voltage in superconductors at zero applied magnetic field

A systematic study of the transverse voltage at zero magnetic field in the superconducting state is reported. The effects of warming rate, temperature, applied magnetic field, and electrical current on the transversal resistance (R_XY) of polycrystalline superconducting sample are taken into account. At zero magnetic field two peaks are observed in R_XY(T) curves which are related to the double superconducting transition in the R_XX(T) component. In the superconducting (R_XX = zero) and normal states no transverse voltage has been detected at zero magnetic field as expected. The results are discussed within the framework of the motion of Abrikosov and Josephson vortices and anti-vortices. A new scaling relation between transverse and longitudinal components given by R_XY ～ dR_XX/dT has been confirmed.     

磁性半导体/磁性d波超导结中的自旋极化输运

通过求解磁性d波超导中的能隙与磁交换能的自恰方程,利用推广的Blonder-Tinkham-Klapwijk 理论研究磁性半导体/磁性d波超导结中自旋极化准粒子输运系数与微分电导. 计算表明： 1) 磁性d波超导结中的磁交换能h₀可导致零偏压电导峰与能隙电导峰劈裂,劈裂的宽度为2h₀;2) 磁性半导体中的磁交换能h_FS可使零偏压电导峰劈裂的峰值变低. 而由能隙电导峰劈裂的两个子峰,当两种磁性材料的磁 关键词： 磁性半导体 磁性d波超导体 自旋极化输运     

Bi2Sr2CaCu2O8超导单晶中的电阻反常行为

测量了Bi₂Sr₂CaCu₂O₈单晶的ab面和c轴方 向电阻,在其超导转变温度附近发现了反常的电阻峰出现.其随外磁场(>100Gs)和电流的增 加而逐渐消失.文章认为这个反常的电阻峰是由于单晶中超导相的不均匀分布而导致的准再 进入行为. 关键词： 2Sr₂CaCu₂O₈单晶')" href="#">Bi₂Sr₂CaCu₂O₈单晶 反常电阻峰 准再入行为     

Thermal effect in phase-periodic electron transport in disordered mesoscopic normal metal/superconductor structures

We report measurements of the temperature dependence of the amplitude of phase-periodic conductance oscillations in disordered normal metal (Ag) structures, attached to a superconducting (Al) wire at two points. The amplitude of oscillations reaches its maximum at temperatureT ^*, when the Thouless energy is of the order ofk _B T. The results are in agreement with recent calculations by Nazarov and Stoof [Phys. Rev. Lett. 76 (1996) 823].     

Dynamic and static phases of vortices under an applied drive in a superconducting stripe with an array of weak links

Static and dynamic properties of superconducting vortices in a superconducting stripe with a periodic array of weakly-superconducting (or normal metal) regions are studied in the presence of external magnetic and electric fields. The time-dependent Ginzburg-Landau theory is used to describe the electronic transport, where the anisotropy is included through the spatially-dependent critical temperature T _c . Superconducting vortices penetrating into the weak-superconducting region with smaller T _c are more mobile than the ones in the strong superconducting regions. We observe periodic entrance and exit of vortices which reside in the weak link for some short interval. The mobility of the weakly-pinned vortices can be reduced by increasing the uniform applied magnetic field leading to distinct features in the voltage vs. magnetic field response of the system.     

Structure and properties of solid solutions in the Mg-Al-B system

Compounds with the general formula Mg_1−x Al _x B₂ were obtained by two-step ceramic synthesis. All compounds were characterized by X-ray diffraction, NMR spectroscopy, and by four point probe resistivity measurements in various magnetic fields method. The diborides unit cell parameters were determined as a function of the Al mole fraction. With the vaues of x up to 0.40 (where x is the composition of the stock prepared for sintering), the unit cell parameters of Mg_1−x Al _x B₂ are similar to those of pure MgB₂ and the superconducting transition temperature was lowered. For stock compositions of 0:25 ≤ x ≤ 0:60, the products contain a superstructure, also superconducting phase, which becomes the only product at x = 0:50, and at x > 0:60 this phase is replaced by AlB₂-based solid solutions.     

Vortex patterns and nucleation of superconductivity in mesoscopic rectangles and in hybrid superconductor/ferromagnet structures

Nucleation of superconductivity and vortex patterns have been studied in mesoscopic samples providing the crossover between square and rectangular geometries. The measured nucleation line, T _c (H), has been analyzed in the framework of the linearized Ginzburg-Landau theory. A very good agreement has been found between the theoretical T _c (H) boundary and the experimental data for rectangles with different aspect ratios. The superconductor/ferromagnet hybrids, such as magnetic Co/Pd dot in a superconducting loop and the dot on top of a superconducting disk have also been investigated. Pronounced effects of the dot on the T _c (H) boundary have been found, including strong asymmetry with respect to the field polarity.Received: 26 February 2004, Published online: 3 August 2004PACS: 74.25.Dw Superconductivity phase diagrams - 74.78.Na Mesoscopic and nanoscale systems     

Preparation and characterization of Y1−xPrxBa2Cu3O7−δ single crystals

We present a reliable method for growing single crystals of Y_1−xPr_xBa₂Cu₃O_7−δ high-T_c superconductors in ZrO₂ crucibles. This method results in crystals with greatly improved superconducting properties compared to crystals grown with the previously reported methods which use Al₂O₃ crucibles. We describe techniques for crystal growth in both Al₂O₃ and ZrO₂ crucibles using an excess of BaCo₃ and CuO as the flux. The crystals were characterized by means of DC magnetic-susceptibility measurements, electrical-resistivity measurements, and electron microprobe analysis. The effects of Al contamination on the conditions for crystal growth and on the superconducting properties of the crystals are found to be quite significant.     

1-THz low-noise SIS mixer with a double-dipole antenna

A quasi-optical mixer containing two Nb/Al/AlO_x/Nb superconducting tunnel junctions integrated into a NbTiN/SiO₂/Al microstrip line is studied experimentally in the 800–1000 GHz frequency range. The mixer is developed as an optional front end of the heterodyne receiver operating in frequency band 3 or 4 and incorporated into the HIFI module of the Herschel space-borne telescope. The double-dipole antenna of the mixer is made of NbTiN and Al films; the quarter-wavelength reflector, of a Nb film. The mixer is optimized for the IF band of 4–8 GHz. The double-sideband noise temperature T _RX measured at 935 GHz is 250 K at a mixer temperature of 2 K and an IF of 1.5 GHz. Within 850–1000 GHz, T _RX remains below 350 K. The antenna pattern is symmetrical with a sidelobe level below −16 dB.     

A.C. loss and related effects in type II superconductors

This review gathers together information important to the understanding of superconducting materials under a.c. conditions, and points out some features which are at present not too well understood. The idea is to try to build a consistent picture of the performance of superconductors rather than to present a historical account of the subject.

The basic behaviour of fluxons in the bulk of type II superconductors can be explained by the critical state model, but some of the details of flux pinning and the method of flux motion are in doubt. Flux instabilities under d.c. conditions have received a lot of attention and so have the methods of stabilization, but instabilities under a.c. conditions have not been studied in so much detail, and the problem of stabilization is more difficult than for f.c.

Generally surface effects are less well understood than bulk effects, partly because there are a number of possible phenomena to contend with. A.C. loss in the Meissner state can be explained by field enhancement and flux penetration at peaks in the surface. For fields between H _c3 and H _c2 present solutions of the Ginzburg-Landau equations for the currents in the surface sheath do not give sufficient agreement with experiment and some alternative explanation, such as flux pinning, may be necessary. There are a number of effects between H _c2 and H _c1, but the most important are Meissner currents and flux pinning.     

Occurrence of A-15 Phases

Three binary A-15 compounds (Zr₄Sn, Ta₃Sb, Ta₄Au) were reinvestigated and all three found to be superconducting below 1 ?K. Ternary compounds V₃(Al, X), (X=As, Ge, Au) were also investigated in the composition range where the A-15 structure is stable. The extrapolated superconducting transition temperature of the compound V₃Al, if it existed in the A-15 form, is ～ 17 ?K.