Magnetization Fluctuation Analysis and Superconducting Parameters of La1.5−x Ba1.5+x−y Ca y Cu3O z Superconductor

In this work we report analysis of experimental data of the magnetization of La_1.5?x Ba_1.5+x?y Ca _y Cu₃O _z superconducting system. The data are analyzed in terms of thermal fluctuations on the magnetization excess M(T) for different values of temperature in each of the samples. We describe a procedure for extracting the penetration depth in the ab plane (1537–1650 Å) and the coherence length in the ab plane (21–23 Å) parameters from the magnetization, as a function of the applied magnetic field. This procedure was performed for polycrystalline samples using the theory of Bulaevskii, Ledvij and Kogan, which analyzes the vortex fluctuation in superconducting materials within the Lawrence-Doniach framework. These data allowed one to determine the characteristic temperature value T ^? (53–73 K) in the magnetization curves for several magnetic fields. We calculated the data of magnetization excess from the curves of the magnetization as a function of the logarithm of the applied field. We notice that the values for these superconducting parameters are in agreement with reports for high-temperature superconductors. The value obtained of the superconducting volumetric fraction is compared with the value obtained through the measure of the Meissner effect.     

Microwave Surface Impedance of Nd-rich Nd 1+x Ba 2−x Cu 3 O 7−δ Thin Films

Surface impedance measurements on highly c-axis epitaxial Nd _1+x Ba _2–x Cu ₃ O ₇ (x=0, 0.09 and 0.12) films grown by d.c. magnetron sputtering on LaAlO ₃ substrates are presented. It is found that the zero temperature London penetration depth correlates well with the critical temperature of the films and with the corresponding number of carriers. The low temperature penetration depth follows a linear T law for optimally doped Nd123 sample and a T ² law in Nd-rich samples. In the case of the heavily underdoped samples (T _c < 60K) the T ² law extends to temperatures higher than T _c/2. The possible role of the Nd/Ba ions substitution on the penetration depth and surface resistance is discussed.     

A method for the accurate measurement of the complex conductivity of high-T c superconductive thin films

In this paper, we introduce a method which makes it possible to rapidly and accurately determine the penetration depth as a function of temperature for superconductive thin film samples. A key feature of the approach described here is that it is derived only from electrodynamic definitions and makes no use of a selected model of superconductivity. Another advantage relative to some of the alternative methods presently in use is that it can be expected to give useful results for films with thicknesses that are as much as three times the zerotemperature effective penetration depth. When combined with an accurate evaluation ofR _S as a function of temperature for the same samples, these penetration depth data enable the computation of the complex conductivity for a wide variety of samples. One shortcoming of the method is the fact that it performs well only at temperatures below about 0.95T _c.     

Upper limit ofT c for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c. A new parameter, “intrinsic”T _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

The penetration depth of HgBa2CuO4+δ with 0.07 ≤ δ ≤ 0.35

The magnetic penetration depth λ(T) of three HgBa₂CuO_4+δ samples with 0.16 < δ ≤ 0.27 has been determined from the reversible magnetization. The obtained λ follows a BCS-like correlation of 1/λ² ∝ 1?(T/T_c)² over whole measured temperature range in an underdoped sample with T_c ～ 90 K, but deviates significantly from similar fits in an overdoped sample with the same T_c and an optimum doped sample, whose 1/λ² 's depends on T nearly linearly below T_c/2. This asymmetry between the underdoped and overdoped samples suggests that the T-dependence of 1/λ² is affected by doping in a complicated way.     

Systematic investigation of electronic structure in BEDT-TTF based organic superconductors withT c above 10 K; κ-(BEDT-TTF)2 X (X=Cu(NCS)2, Cu[N(CN)2]Br,and Cu(CN)[N(CN)2])

The electronic structure of the title superconductors has been investigated by electrical resistivity, complex susceptibility, and electron paramagnetic resonance (EPR) measurements. The superconducting properties (pressure dependence ofT _c , magnetic penetration depth, upper critical field, and so on) of these three salts are similar to each other, while transport properties in the normal state have shown a large variety in the temperature dependence. In order to clarify the electronic structure in the normal state, the EPR parameters, the spin susceptibility (χ _spin), and the linewidth (ΔH _pp), are compared. An anomalous temperature dependence of theg-value has been observed below 150 K in the Cu(NCS)₂ and Cu(CN)[N(CN)₂] salts.     

Pair and Quasiparticle States of YBa2Cu3O7−x Films Deduced from the Surface Impedance and a Comparison with Nb3Sn

Surface impedance data at 19 and 87 GHz of high-quality epitaxial YBCO films on different substrates are compared with data for Nb₃Sn films on sapphire in terms of pair and quasiparticle (qp) transport. Surface resistance R _s and penetration depth λ of YBCO are strongly affected by temperature dependent qp scattering, which is depressed in films with enhanced lattice strain. All films showed a comparable residual resistance R _res(19 GHz)～90 μΩ constituting a qp reservoir which is likely to be caused by the electronic configuration and by impurities. Subtracting R _res from R _s (T) revealed activated behavior with a reduced energy gap Δ₀/k _B T _c～0.9 for a film on sapphire, but power-law behavior for the other films. The penetration depth did not reveal power-law dependences at T≤0.5 ·T _c, but was consistent with a reduced energy gap of 0.45 for a film on MgO. The increase of λ(T) at T≥0.5 · T _c was related to qp scattering, which also caused an extremal conductivity σ₁(T). A shoulder in λ(T) at T=(0.6–0.7) · T _c confirmed evidence for the existence of two superconducting bands. The magnetic-field induced recovery of λ(B) of various YBCO films hinted for an important role of magnetic scattering. The results are in contradiction to a d-wave symmetry of the order parameter, at least for the chain band.     

Parallel-plate resonator of variable spacer thickness for accurate measurements of surface impedance of high-T c superconductive films

Measurements of microwave surface impedance of high-T _c films at gigahertz frequencies and nitrogen temperature are performed. A simple technique employing a parallel-plate resonator with liquid nitrogen as a dielectric spaces is suggested. The use of a precise mechanical device provides smooth changing of distance between films from 200μm down to zero. Coupling to the resonator is accomplished by means of two small antennas-half-wave vibrators for frequency 10 GHz. The method for determining resistivity and magnetic field penetration depth was based on the analysis of spacer thickness dependences of the resonator quality factor and frequency. YBa₂Cu₃O₇ films produced by a laser deposition technique on CaNdAlO₄ substrates withT _c =91 K andj _c =10⁷ A/cm² and on NdGaO₃ substrates withT _c =91 K andj _c =10⁶ A/cm² are examined, and the valuesR _s =0.6 mΩ,λ=348 nm atf=8.97 GHz andR _s =0.5 mΩ,λ=250 nm atf=10.12 GHz, respectively, are obtained at 77 K.     

Optical Properties of the Cu–O Plane in the Bi2Sr2−x La x CuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi₂Sr_2?x La _x CuO₆, has been measured for hole concentrations per Cu site 0.03≤p≤0.18, and for 6 K≤T≤300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E _u modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel–Glover–Tinkham sum rule requires an integration up to Ω?6Δ and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron–phonon interaction.     

Low-Field microwave measurements of YBa2Cu3O7−x near the superconducting transition temperature

Novel microwave absorption and dispersion measurements have been performed on well-characterized single-crystal platelets of the high-T _c superconductor YBa₂Cu₃O_7?x . The results are explained in terms of the rapid variation of the penetration depth near and belowT _c . Since EPR measurements are very sensitive to small changes in absorption and dispersion, this technique should be very useful in the understanding of the transition temperature region in both “new” and “old” superconducting materials.     

Anisotropic Magnetocaloric Effect in Single-crystalline Pr0.52Sr0.48MnO3

We report the magnetocaloric effect (MCE) in a Pr_0.52Sr_0.48MnO₃ single crystal estimated from the isothermal magnetization curve using the Maxwell relation. Isothermal magnetization curves are measured over the range 20 K to 320 K where the field was applied parallel (??) and perpendicular (??) to the [110] direction of the perovskite structure with Pbnm space group. A peak in the temperature (T) dependence of magnetic entropy change (??S _M) with a fairly large negative value (???3.3 J/kg?K) is observed at 275 K close to the Curie temperature (T _C) for a change in field of ??H=40 kOe. The ?? and ?? components of ??S _M deviate from each other below ??260 K and an inverse MCE is observed below ??150 K. We note that the Landau theory of phase transitions satisfactorily explains the ??S _M vs T plot around the second-order transition at T _C.     

Thermally stimulated currents in a-Se99.5Bi0.5 thin films

The present paper reports on thermally stimulated current measurements in a-Se_99.5Bi_0.5 thin films at different heating rates (β₁ = 2.17 K/min, β₂ = 2.85 K/min, β₃ = 5.00 K/min). A well-defined TSC peak at a particular temperature has been observed which shifts toward higher temperatures (T₁ = 362 K, T₂ = 372 K, T₃ = 392 K) on increasing the heating rate (β). Using three different approaches the trap depth has been calculated giving good agreement with each other. The average value of trap depth is 0.29 eV. Trap density has also been calculated to be 6.62 × 10¹⁹ cm⁻³.     

Thermodynamic Temperatures of the Triple Points of Mercury and Gallium and in the Interval 217 K to 303 K

We measured the acoustic resonance frequencies of an argon-filled spherical cavity and the microwave resonance frequencies of the same cavity when evacuated. The microwave data were used to deduce the thermal expansion of the cavity and the acoustic data were fitted to a temperature-pressure surface to deduce zero-pressure speed-of-sound ratios. The ratios determine (T–T₉₀), the difference between the Kelvin thermodynamic temperature T and the temperature on the International Temperature Scale of 1990 (ITS-90). The acoustic data fall on six isotherms: 217.0950 K, 234.3156 K, 253.1500 K, 273.1600 K, 293.1300 K, and 302.9166 K and the standard uncertainties of (T−T₉₀) average 0.6 mK, depending mostly upon the model fitted to the acoustic data. Without reference to ITS-90, the data redetermine the triple point of gallium T_g and the mercury point T_m with the results: T_g/T_w = (1.108 951 6 ± 0.000 002 6) and T_m/T_w= (0.857 785 5 ± 0.000 002 0), where T_w = 273.16 K exactly. (All uncertainties are expressed as standard uncertainties.) The resonator was the same one that had been used to redetermine both the universal gas constant R, and T_g. However, the present value of T_g is (4.3 ± 0.8) mK larger than that reported earlier. We suggest that the earlier redetermination of T_g was erroneous because a virtual leak within the resonator contaminated the argon used at T_g in that work. This suggestion is supported by new acoustic data taken when the resonator was filled with xenon. Fortunately, the virtual leak did not affect the redetermination of R. The present work results in many suggestions for improving primary acoustic thermometry to achieve sub-millikelvin uncertainties over a wide temperature range.     

Temperature dependence of ac-losses in multifilamentary Bi-2223 tapes between 4.2 K and Tc

The ac-losses of twisted multifilamentary Bi-2223/Ag tapes were measured in the temperature range between 4.2 K and the critical temperature T_c∼110 K. Stacks of tapes in perpendicular and in parallel field were investigated. The loss measurement is performed by the standard magnetisation technique, the absolute loss value is obtained by a calorimetric calibration measurement. For a fixed ac-field amplitude the energy loss per cycle Q is determined, at a given temperature, as a function of frequency f. The extrapolation to f→0 gives the hysteresis loss. Coupling losses are obtained from the slope of the loss curve Q(f) in the low frequency range ωτ?1. The measured total losses as a function of temperature show, at small field amplitudes, a minimum around 50 K. This is explained by the different temperature dependence of hysteresis and coupling losses. While coupling losses decrease with increasing temperature, hysteresis losses increase for field amplitudes below the penetration field. Coupling losses show a much weaker temperature dependence than the silver conductivity, which is explained by the existence of an interfacial resistance between filaments and silver matrix. Measured hysteresis losses were compared with available theoretical models and good agreement was found in parallel field.     

Determining the contact temperature in the zone of friction of a polymer coating on steel

Temporal variation of the contact temperature in the zone of friction of a coating based on a mixture of polyamides 6.6 (PA6.6) and 6 (PA6) on steel 45 has been studied using a thermocouple junction situated at a distance of ≈1.5 mm from the friction zone. On the background of a monotonic temperature growth, there are two troughs in which the temperature (T ₁ and T ₂) stabilizes for a certain period of time and then increases again. It is established that the values of T ₁ and T ₂ are close to the temperatures of glass transitions in PA6.6 and PA6, respectively. The appearance of troughs is related to a sharp increase in the energy of friction that is spent for the segmental mobility during the glass transitions in the polymer composition. A comparison of the T ₁ and T ₂ values to the exact glass transition temperatures shows that the contact temperature in the friction zone is 10–20 K higher than the value measured by the thermocouple.     

Evidence for s+d Wave Pairing in Copper Oxide Superconductors from an Analysis of NMR and NQR Data

Knight shift and spin-lattice relaxation rate data of high temperature copper oxide superconductors are analyzed within a two-band model for superconductivity with coupled s+d wave superconducting gaps. The two-gap approach leads to substantial modifications of the coherence factors, which reflects itself in the Knight shift and the relaxation rate 1/T ₁ T. From the analysis it is concluded that the data are consistent with 40% s-wave and 60% d-wave gap admixtures, in agreement with earlier penetration depth data.     

Preparation of Pb-Bi film by alloy evaporation I: Film composition control

Pb-Bi films in h.c.p. phase (ε-PbBi) suitable for lead alloy Josephson junction counterelectrodes are typically prepared by evaporating a Pb-Bi alloy from a single r.f.-induction-heated crucible. The composition of such films is generally expected to be different from that of the source. In order to obtain a single-phase ε-PbBi film which covers only a small range of bismuth content (about 27–32 wt.% Bi), good control of film composition is required. The relationship between film and source composition can be theoretically estimated, using the Hertz-Knudsen-Langmuir (HKL) equation and assuming comparable sticking coefficients among the alloy constituents. Taking into account the contribution of bismuth monomers and dimers to the total partial pressure of bismuth in Pb-Bi vapor, we predicted that there exists an optimum source temperature T_o, or equivalently an optimum evaporation rate r_o, for alloy evaporation to obtain Pb-Bi films of the same composition as the source. It was also predicted that T_o should be a function of source composition because of the contribution of bismuth dimers present in bismuth vapor. Experiments were carried out to evaluate these predictions. Several alloy sources of known composition were evaporated to depletion, each at a different fixed rate. Films of 1–2 μm thickness were collected onto silicon substrates at a series of predetermined evaporation time intervals. The film composition was determined by controlled-potential coulometry which is accurate to ±0.2 wt.% Bi for film weights of 2 mg or more. Experimental results confirmed the existence of an optimum evaporation rate, corresponding to T_o, for a given source composition as predicted from the HKL equation. The optimum evaporation rate was also found to increase as expected with decreasing bismuth content. The optimum evaporation temperatures for 27% Bi and 29% Bi sources were measured to be 1080 ± 10 K and 1022 ± 10 K respectively, which are about 100 K higher than the predicted values. The discrepancy is believed to be mainly due to the use of an open crucible in this work causing the vapor pressure over the melt, at a given temperature, to be lower than the equilibrium value and the T_o value to increase. The difference in T_o values for two sources of different bismuth contents used in this experiment is in good agreement with the predicted value.     

Temperature and stress dependences of creep in single crystals of lead between 20 and 300 K

The influence of temperature and applied stress on creep in single crystals of lead has been investigated. Between 20 and 180 K a thermally activated “logarithmic” creep is observed that is easily interpreted in terms of theory based on a work hardening concept. In case recovery processes are absent the weakening of the “logarithmic” creep is determined by hardening of the crystal during the creep process and the level of effective stresses. The creep vanishes when the effective stresses become zero. The disappearance of creep at the point T ₀ has been shown experimentally. The temperature T ₀ is that at which critical shear stresses reach the temperature-independent branch of the yield-point curve Τ _T (T). At temperatures higher than T ₀ a steady-state creep is observed. At a fixed stress the creep rate is determined by the rate of recovery processes. The results obtained are described in terms of thermally activated dislocation motion.     

Equilibrium vacancy parameters and limit temperature of nonequilibrium melting in osmium

For osmium, based on experimental data on enthalpy and the averaged heat capacity obtained by the method of mixing within the temperature range of 1150?C2960 K, for the first time the parameters of the equilibrium vacancies for this metal have been determined: the vacancy formation energy E = 1.8 eV, the vacancy concentration at melting c = 3.3%, and the vacancy formation entropy S = 25.6 J/(mol K). The limit temperature of the onset of nonequilibrium melting of osmium T _m = 4256 K and its relative value T _lim/T _m = 1.30 has been calculated.