Anomalous Tc-behavior of LaSn3 under pressure

The superconducting transition temperature T_c of LaSn₃ has been measured up to ～ 22 kbar and was observed to increase through a maximum under hydrostatic compression. The anomalous T_c-behavior is attributed to a pressure induced Fermi surface topology change.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

Normal state resistivity and Tç studies of superconducting Ti1−xSbx system

The superconducting transition temperature (T_c) and the temperature dependence of the normal state resistivity of the Ti_1?xSb_x system between T_c and 300 K have been studied. The T_c values are found to depend on the heat treatment of the samples. Below 40 K, all alloys show a T² dependence of the resistivity. However, the sample with x = 0.53 is not superconducting and shows a different behaviour of the resistivity.     

Critical current behavior in locally illuminated superconducting lead strips

We have used guided optical waves to locally illuminate superconducting thin lead strips and measured the critical current as a function of optical power. The measured results resemble the predictions of a simple heating model more closely than the Parker T¹ model in the temperature range investigated (5.5 K to T_c) and several possible reasons for this behavior are discussed.     

Transport properties in single phase superconductor Ba2YCu3O9-δ

The electrical resistivity and the thermopower are measured on the single phase superconductor Ba₂YCu₃O₉-δ (δ=2.1). The results indicate that the temperature dependences of the resistance and thermopower exhibit typical metallic behaviour, and the sample conducts via electrons at high temperatures. The behaviour of the thermopower can be described with Mott's semi-classical model. The specific heat of electrons in normal state has been estimated 780mJ/K·mole at 200K, i.e. γ=3.9mJ/K²·mole. Unusual phonon-drag effect is observed above the superconducting transition temperature T_c. Below T_c, the electrical resistivity and the thermopower all drop to zero corresponding to a superconducting ground state.     

Temperature dependent time delay of the voltage response to supercritical current pulses in superconducting aluminium

Current pulses larger than the critical current I_c are passed through narrow superconducting aluminium strips. The time delay between the start of the pulse and the onset of the voltage response is measured as a function of current amplitude for various temperatures (0.84<T/T_c<0.98). We find the delay time to be dependent on the temperature as (1 - T/T_c)^α with α changing from a value of - 0.5 for I/I_c close to 1 to a value of - 1.0 for I/I_c larger than 3.     

Pressure dependence of the superconducting transition temperature of a (Mo0.6Ru0.4)86B14 metallic glass

We have studied the pressure dependence of the superconducting transition temperature of amorphous (Mo_0.6Ru_0.4)₈₆B₁₄ for hydrostatic pressures up to P ～ 9 kbar. The transition temperature T_c decreases with pressure at a rate dT_c/dP=-(9±1) mK kbar^-1. We estimate the Grüneisen parameter and the volume dependence of the electron-phonon coupling constant.     

Superconductivity in polymeric sulfur nitride (SN)x at high pressure

Investigations of the pressure dependence of the superconducting transition temperature T_c up to 17 kbar, and of the normal conductivity up to 50 kbar are reported. It is observed that below 8 kbar, the value of T_c increases linearly with the pressure. In addition, there is a significant drop of T_c at about 9 kbar which may be due to a phase transition.     

Superconductivity in the Zr-D system under pressure

The superconducting transition temperature T _c of the ZrD_0.48 alloy is measured in the pressure range up to 41.5 GPa. The measurements are carried out in a high-pressure chamber with diamond anvils by the inductometric method. It is found that T _c(P) increases to 3.1 K at a pressure below 30 GPa, exhibits a sharp increase up to 8 K near 30 GPa, and then smoothly decreases to ～6.5 K at 41.5 GPa. A similar dependence T _c(P) is obtained for pure Zr. The similarity of the T _c(P) curves suggests that the dependence T _c(P) observed for ZrD_0.48 is due to the presence of ω-phase in this alloy at pressures P<30 GPa and the ω-β transition at P≈30 GPa, which leads to the establishment of new ratios between the phases in the Zr-D system. In the pressure range studied, no indications are observed for new superconducting phases similar to the phases of intermediate composition in the Ti-H(D) system, which are formed by the hydrogen transfer from tetrahedral to octahedral interstitials.     

Pressure dependence of the superconducting transition temperature of the stage-2 potassium mercurographitide intercalation compound

Measurements of the pressure (P) dependence of the superconducting transition temperature T_c of stage-two KHgC₈ are reported. T_c is found to decrease with applied pressure from a room pressure value of 1.85K at a rate dT_c/dP=-6.5 × 10^-5K/bar, similar to typical superconducting elements such as Sn. No superconductivity was detected for stage-one KHgC₄ or K_0.5Hg_0.5 amalgam to a limiting temperature T = 1.3K and a limiting pressure P = 22 kbar. These results are discussed in reference to the possible occurence of structural and charge density wave transitions in these materials and recent theoretical models of superconducting graphire intercalation compounds.     

Measurement of the pressure dependence of Tc for superconducting spinel compounds

The superconducting transition temperatures (T_c) of CuRh₂Se₄, CuRh₂S₄ and LiTi₂O₄ were all found to increase linearly under hydrostatic pressure up to 22 kbar, at a rate of 1–5 × 10^-5 Kbar^-1. These results are discussed in terms of the dependence of T_c on Debye temperature previously found for this set of compounds from heat capacity measurements at zero pressure.     

Suppression of superconductivity in YNi2B2C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature T _c , and the upper critical field H _c2(T) of a polycrystalline sample of YNi₂B₂C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval T _ann = 100–1000°C has been studied. It has been found that the irradiation of YNi₂B₂C with a fluence of 10¹⁹cm^?2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), T _c , and H _c2(T) values are almost completely recovered upon annealing at up to T _ann = 1000°C. The quadratic dependence ρ(T) = ρ₀ + a ₂ T ² is observed for the sample in the superconducting state (T _c = 5.5?14.5 K). The coefficient a ₂ (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of T _c on ρ₀ can be interpreted as the suppression of the two superconducting gaps, Δ₁ and Δ₂ (Δ₁ ～ 2Δ₂). The degradation rate of Δ₁ is about three times higher than that of Δ₂. The dependences dH _c2/dT on ρ₀ and T _c may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ₀ is on the order of the electron mean free path l _tr) under the assumption of a nearly constant electron density of states on the Fermi level N(E _F). The observed behavior of T _c obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.     

High pressure study of BaPb0.7Bi0.3O3 films

The resistance R, the superconducting transition temperature T_c and the energy gap Δ(T) have been measured on the BaPb_0.7Bi_0.3O₃ films up to 14 kbar. We have found that up to 14 kbar: (1) pressure suppresses T_c and Δ(T) while enhances R, (2) the value of 2Δ(0)/kT_c is 3.8±0.1, independent of pressure, and (3) the Δ(T)/Δ(0) varies with T/T_c in a BCS fashion but only for T/T_c<0.75 and independent of pressure. The results show that BaPb_1?xBi_xO₃ is a weak-coupling superconductor, but fail to provide information about the cause for the high T_c of the compound.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Comparison of amorphous melt-spun and vapor-quenched Zr-Rh alloys

Amorphous Zr-Rh alloys produced by melt spinning have a higher superconducting transition temperature T_c than materials generated by r.f. cosputtering techniques. However, the temperature dependence of the upper critical field H_c²(T is consistent with the GLAG theory for both materials. The T_c differences, as well as variations in the electronic density of states, can be partially understood in terms of slight differences in the average density, perhaps due to the inclusion of Ar gas during sputtering.     

Superconductivity in a simple model of the pseudogap state

An analysis is made of characteristics of the superconducting state (s-and d-pairing) using a simple, exactly solvable model of the pseudogap state produced by fluctuations of the short-range order (such as antiferromagnetic) based on a Fermi surface model with “hot” sections. It is shown that the superconducting gap averaged over these fluctuations is nonzero at temperatures higher than the mean-field superconducting transition temperature T _c over the entire sample. At temperatures T > T _c superconductivity evidently exists in isolated sections (“ drops”). Studies are made of the spectral density and the density of states in which superconducting characteristics exist in the range T > T _c however, in this sense the temperature T = T _c itself is no different in any way. These anomalies show qualitative agreement with various experiments using underdoped high-temperature superconducting cuprates.     

Effect of Ce impurities on superconductivity and nuclear spin-lattice relaxation in La1−cCecAl2

In the superconducting state of La_1?cCe_cAl₂ alloys ¹³⁹La spin-lattice relaxation times T_1s for relatively high c were found to agree with theory based on weak exchange coupling between conduction electrons and paramagnetic impurities. But for a dilute specimen the measured values of T_1s are significantly longer than expected. These results are tentatively attributed to the existence of a localized impurity band within the superconducting energy gap.     

Unusual behavior of nuclear relaxation in CeCu2Si2 “possible evidence for triplet superconductivity”

Nuclear relaxation of ⁶³Cu in the superconducting state of the Kondo-lattice system CeCu₂Si₂ has been studied with the use of the ⁶³Cu nuclear quadrupole resonance technique under zero field and down to 65mK. The nuclear spin-lattice relaxation rate (1/T₁) decreases drastically just below T_c=0.67 K down to 0.5T_c without the apparent enchanced behavior and then is found to be almost temperature independent below 0.3T_c. These results suggest that the superconductivity in CeCu₂Si₂ is not in the usual BCS regime. The analysis based upon the existing triplet pairing model with an anisotropic energy gap describes well the behavior from T_c down to 0.5T_c, while the temperature independence below 0.3T_c remains unexplained.