Low Temperature Properties and Superconductivity of LuB12

We have investigated the low temperature properties of LuB₁₂ by measuring its magnetic susceptibility, heat capacity, and electrical resistivity, as well as by point-contact spectroscopy using both the spear-anvil type technique and mechanically controllable break junctions. Our specific heat measurements and point - contact spectroscopy results indicate that LuB₁₂ is a simple weak-coupling BCS-type superconductor with T_C ≈ 0.4 K, a superconducting energy gap of 2 Δ ≈ 0.12 meV, and a very small critical field B_C ≈ 1 mT. From the dU/dI(U) characteristics in the superconducting state, the energy gap 2Δ, the critical current I_C and the Andreev-reflection excess current I_ex as a function of normal-state point contact resistance R_N have been determined. At low R_N all three parameters are strongly suppressed, possibly due to the self-magnetic field. At contacts with large R_N the model of resistively and capacitively shunted Josephson junctions (RCSJ) allowed us to estimate the superconducting current plasma frequency and the Josephson coupling energy. Moreover, from the d²U/d I²(U) spectra in the normal state the (point-contact) electron-phonon interaction function and the characteristic phonon energies of LuB₁₂ have been determined.     

Rib waveguides based on Zn-substituted LiNbO3 films grown by liquid phase epitaxy

The fabrication of epitaxially grown Zn-substituted LiNbO₃ (Zn:LiNbO₃) waveguide films and rib waveguides is reported and detailed investigations about microstructure, morphology and optical waveguide properties are provided. Zn:LiNbO₃ films were grown on congruent X-cut LiNbO₃ substrates by a modified liquid phase epitaxy in solid–liquid coexisting solutions. The homogeneously Zn-substituted films exhibit high crystalline perfection and extremely flat surfaces with averaged surface roughness of rms = 0.2–0.3 nm. At the film/substrate interface a Zn-containing transient layer has been observed, which allows the growth of elastically strained Zn:LiNbO₃ film lattices. X-ray diffraction reciprocal-space measurements prove the pseudomorphic film growth. The refractive index difference between substrate and film depends on the zinc substitution content, which increase with rising growth temperatures. For films with 5.3 mol% Zn (Δn_o ≈ +5 × 10⁻³) only ordinary ray propagation was observed, while for films with 7.5 mol% Zn (Δn_o ≈ +8 × 10⁻³, Δn_e ≈ +5 × 10⁻³) both modes, TM and TE propagate. Stress-induced refractive index changes are in the order of Δn ≈ 10⁻⁴. In rib waveguide microstructures singlemode propagation with nearly symmetrical field distribution has been observed. To demonstrate the potential of the proton exchange-assisted dry-etching technique interferometer microstructures were fabricated.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7?x

The effects of addition to YBa₂Cu₃O_7–x of lithium halides (YBa₂Cu₃O_7–x (LiF) _y , and YBa₂Cu₃O_7–x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7–x (LiF) _y and 91.30 K in YBa₂Cu₃O_7–x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _cB ^–, with a lithium-concentration-dependent . The voltage–current characteristics follow a law typical for granular superconductors, V(I–I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Flux trapping and its effect on AC susceptibility of granular superconductors

Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highT _c superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highT _c superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.     

Development of a Nb3Sn multifilamentary composite conductor for a.c. use

A Nb₃Sn composite conductor with ≈ 10 000 submicron diameter filaments has been manufactured using the external diffusion process. A.c. losses were greatly reduced by the use of a fine filament size (0.53 μm, design value), a tight twist pitch (0.87 mm) and a small wire diameter (0.153 mm) with a bronze matrix. In an a.c. field with a frequency of 50 Hz and amplitude of 2.0 T, the hysteresis loss and the coupling current loss were observed to be 465 kW m⁻³ and 26 kW m⁻³, respectively. A triplex conductor was constructed by cabling three strands at a twisting pitch of 3 mm, and a small coil was wound from this cable (i.d. 11 mm, o.d. 33 mm, axial length 19 mm). With d.c. the coil generated a field of 1.3 T at the critical current, l_c of 37.4 A. When the coil was operated at 50 Hz, with an exciting current of I_c, the observed loss averaged over the windings was 240 kW m⁻³. The quenching current for 50 Hz operation was 53 A at a maximum field of 1.8 T. This was considerably higher than the critical values under d.c. conditions. Preliminary studies have shown that, if this conductor is used in superconducting armature windings of rotating machines, economical benefits are obtained compared with the use of conventional armatures.     

‘Electromaglev’-magnetic levitation of a superconducting disc with a DC field generated by electromagnets: Part 1. Theoretical and experimental results on operating modes, lift-to-weight ratio, and suspension stiffness

We present results of a comprehensive study, both theoretical and experimental, of an ‘electromaglev’ system, in which a high-temperature superconducting bulk sample, e.g. YBa₂Cu₃O_7−δ (YBCO), is levitated stably in a DC magnetic field generated by electromagnets placed underneath the floating object. Results of the zeroth-order theory agree quite well with experimental results on lift-to-weight ratio and suspension stiffness for three bulk samples: (1) a solid YBCO disc; (2) a YBCO annulus; and (3) a YBCO annulus with a neodymium-iron-boron (Nd-Fe-B) permanent magnet disc (PMD) filling the centre. The experiment has also verified the need to satisfy two requirements to achieve stable levitation with a DC magnetic field only: (1) the spatial flow of the supercurrent in the sample must have at least two degrees of freedom; and (2) the electromagnets must generate a magnetic field profile that satisfies spatial requirements for lateral and pitch stability. A permanent magnet disc has only one degree of freedom for its Amperian current, thus it cannot be levitated stably in this system; the experiment has also demonstrated that an HTS solenoid (wound with silver-sheathed BSCCO-2223 tape) cannot be levitated stably, because the solenoid supercurrent flow is also restricted to the azimuthal direction only. The zeroth-order theory together with the Bean model shows that the supercurrent induced in a YBCO sample is independent of the critical current density, J_c, of the material but is directly proportional to the axial component of the field and that the lift of the sample is directly proportional to the product of the axial and radial components of the magnetic field generated by the electromagnets.     

Possible explanation for trapped field enhancement on REBaCuO bulk by modified multi-pulse technique with stepwise cooling (MMPSC)

The time evolutions of the local fields B_L(t) have been measured on the surface of the superconducting bulk disk magnetized by a two-stage pulse-field magnetizing technique, called a modified multi-pulse technique combined with stepwise cooling (MMPSC), and the magnetic flux movement and the flux trapping have been investigated. The optimum concaved (“M-shaped”) trapped field profile, which is a necessary condition at the first stage to enhance the final trapped field B_T, makes a larger magnetic gradient (dB/dx) at the bulk periphery in the ascending stage of the applied magnetic pulse at the second stage due to the large viscous force F_v. The magnetic fluxes, which stay at the bulk periphery, start to flow to the center of the bulk, after the applied pulse field reaches a maximum, at which the flux velocity v is nearly zero and then F_v decrease. As a result, a large number of the magnetic fluxes are trapped at the bulk center. The effect of the “M-shaped” profile at the first stage in MMPSC on the enhancement of B_T is discussed.     

Hysteresis of critical currents of superconducting bridges in low perpendicular magnetic fields

Hysteresis of critcal currentsI _c of superconducting bridges with In, Nb, and NbN has been studied in low perpendicular magnetic fields. Influences of bridge geometry, small field sweep, trapped flux, and bombardment of argon ions on the hysteresis were made clear. The experimental results suggest that the edge pinning and trapped flux in the bank of bridges are associated with the hysteresis. The peak value ofI _c of NbN bridges, as well as granular Al and In bridges reported before, in decreasing fields agrees with the calculated pair-breaking current. The origin of the hysteresis is discussed.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

Effect of magnetic field and temperature on the voltage-current characteristics of YBCO superconductor

Voltage-current characteristics of YBCO superconductor was studied under magnetic field up to 0.4 T at different temperatures below T_c. The critical temperature decreases and the transition width broadens under magnetic field. V-I data below T_c were fitted to a power law expression V I^(T,B) in which (T,B) is found to decrease with increase of magnetic field and temperature, gradually approaching unity as T approaches T_c, being independent of magnetic field. Similarly, (T,B) approaches unity as magnetic field increases being independent of temperature.     

Emissions properties of Ho: I7 → I8 transition sensitized by Er and Yb in fluorophosphate glasses

Emission properties of Ho³⁺ at 2.0 μm and the energy transfer mechanism between Yb³⁺, Er³⁺ and Ho³⁺ ions in fluorophosphate glasses are investigated. The measured emission spectra show that the ⁵I₇ → ⁵I₈ transition of Ho³⁺ upon 980 nm laser diode excitation is strong. Judd–Ofelt intensity parameters (Ω_λ, λ = 2, 4, 6), spontaneous transition probability (A_rad), radiative lifetime (τ_r), absorption cross section (σ_a), stimulated emission cross section (σ_e) and FWHM ×  for the transition of Ho³⁺: ⁵I₇ → ⁵I₈ are calculated and discussed. The obtained results show that the present Yb³⁺/Er³⁺/Ho³⁺ triply-doped fluorophosphate glass can be identified to be a promising material at 2.0 μm emission.     

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1?xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B₁), x = 0.06 (B₂), and x = 0.10 (B₃) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B₁, B₂, and B₃ are superconducting with T _c ^R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T _c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T _c values of three samples.     

Current controlled high Tc superconducting switch

When a current is applied above the critical current l_c of a superconductor, the material is in its normal state and has a finite resistance. Below l_c the material becomes a superconductor with zero resistance. Switching between these two states can be achieved by modulating a current through the sample. Various high T_c superconducting (HTS) line structures have been made. In the normal state these structures are ordinary resistors with resistances ranging from 10 μ to 100 kμ. The critical currents are in the range 10 μA–100 mA. Switching behaviour has been observed in a simple divider circuit using the HTS lines at 77 K. Applications of the current controlled HTS switch to digital and logic circuits are discussed.     

High Magnetic Fields in Superconducting Permanent Magnets

We report on the temperature dependence of the maximum trapped field B _o(T) in the high-T _c superconductor YBa₂Cu₃O_7–x (YBCO). Trapped fields of bulk melt-textured YBCO samples are limited by their pinning behaviour and mechanical properties. The mechanical properties of bulk YBCO samples were improved by the addition of silver. Furthermore, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process. High trapped fields up to 13.3 T were measured at 33 K on the surface of a single YBCO disk containing Ag additions and reinforced with steel tubes. The flux pinning properties of this YBCO disk were improved by neutron irradiation and Zn doping resulting in a significant shift of the trapped field curve B _o(T) to higher temperatures. In YBCO mini-magnets, consisting of two YBCO disks, maximum trapped fields up to 16.0 T were achieved at 24 K by a combination of Zn-doping and Ag addition. The improved reinforcement, which was used in this case, was found to withstand the maximum trapped field of 16 T without cracking.     

Anisotropic Superconductivity of Quasi-Two-Dimensional Tight-Binding Electrons in a Strong Magnetic Field

Based on the mean field theory, we have investigated the transition temperature T _c (H) of anisotropic superconductivity in quasi-two-dimensional (Q2D) tight-binding electrons in a strong magnetic field, where we assume the nearest-site attractive interaction. By taking account of the quantum effect of electronic motion in a strong magnetic field parallel to the 2D conducting plane, T _c (H) of the Q2D superconductor has been shown to increase in an oscillatory manner as the magnetic field becomes large and to reach T _c (0) in a strong magnetic field limit for the spin-triplet superconductor. We get the different magnetic field dependencies from that of on-site case.     

Heat transfer from metallic filaments (whiskers) into helium under overcritical pressure for temperatures between 3.7 and 7.2 K

The heat transfer coefficient α, near the critical temperature, T_cO, was determined for several whiskers from the In-Pb alloy system. For this purpose the hysteresis of the voltage-temperature (V-T_B) transition curves at fixed currents, I, and of the V-I characteristics at fixed helium bath temperature, T_B, was determined. The advantage of using measurements made with whiskers is that there is no heat transfer to a substrate and negligible heat transfer to the contracts. The only heat transfer is that to the surrounding helium.     

On one selected problem in mathematical modeling of superconducting cylindrical coils in case of using the Ic(B) characteristic linearized in parts

The task to design the dimensions of cylindrical superconducting coil, which generates the maximum central field within a constant overall conductor length, represents mathematically the problem to find for a solution of the constrained extremum of a function. This problem can be transformed into the task of solving a non-linear equation because the value of gradient in the position of extremum (maximum) must be zero. The optimization procedure requires inter alia to evaluate the derivative of the I_c(B) characteristic of the conductor concerned. We show that a certain problem may arise if the I_c(B) characteristic is expressed by the function linear in parts. In this particular case, the derivative of the I_c(B) is a discontinuous function consisting of constants. As a consequence, the results of optimization needn’t be correct. The aim of this study is to analyze the nature of the above problem and to propose a way to minimize it. We show that the computational error in determining the dimensions of optimum coil reduces with decreasing a distance between the I_c(B) nodal points. Moreover, the error is completely eliminated if the I_c(B) is expressed in analytical form. This effect is typical for superconducting conductors with a strong non-linearity in I_c(B), such as e.g. MgB₂. On the other hand, the effect is not applied in case of e.g. NbTi superconducting conductors, the I_c(B) of which is linear in a broad field range. In this work we only study the case of isotropic I_c(B) characteristic.     

Vortex-induced rectification in type II superconductors

We report a large rectification effect in superconducting films in a parallel magnetic field. This rectification effect is manifested in two features in current-voltage characteristics: The critical current, I_c,is found to differ by as much as 40% for negative and positive currents, and beyond I_c,the magnitude of the voltage is different for positive and negative currents, ¦V(+I)¦ ¦V(–I)¦.Furthermore, the critical current difference ¦I_c+¦ – ¦I_c–¦shows complicated behavior, changing sign as temperature and magnetic field are varied. We discuss a model based on the Bean-Livingston surface barrier and inhomogeneous bulk pinning that accounts for all observed behavior.     

The effect of LiOH addition to the superconducting properties of YBa2Cu3O7-x

The effect of LiOH addition to YBa₂Cu₃O_7-x is presented. The crystal structure remains orthorhombic, bat the orthorhombic splitting decreases from 0.0171 at 2 at.% Li to 0.0158 at 50 at. / Li. The critical temperature exhibits a decrease from 91.18 to 83.79 K in the same range of lithium concentrations. From the magnetization curves were obtained the lower critical fields and the intragranular critical current density. The latter exhibits a negative power law,j _c∞ B^-α with α≈ 1/2.V-I characteristics are typical for grained superconducting materials:Vα(I-I_c)_n(B,t). The transport critical current dependence on temperature and magnetic field is also presented.     

Three-dimensional measurement of ice crystals in frozen dilute solution

A Micro-Slicer Image Processing System (MSIPS) has been applied to observe the ice crystal structures formed in frozen dilute solutions. Several characteristic parameters were also proposed to investigate the three-dimensional (3-D) morphology and distribution of ice crystals, based on their reconstructed images obtained by multi-slicing a frozen sample with the thickness of 5 μm. The values of characteristic parameters were determined for the sample images with the dimension of 530×700×1000 μm. The 3-D morphology of ice crystals was found to be a bundle of continuous or dendrite columns at any freezing condition. The equivalent diameter of ice crystals were in the range of 73–169 μm, and decreased exponentially with increasing freezing rate at the copper cooling plate temperature of −20 to −80 °C. At the T_cp −40 °C, the volumes of ice crystals were in the range of 4.6×10⁴ μm³ to 3.3×10⁷ μm³, and 36 ice columns were counted in the 3-D image.