Angular dependence of the superconducting transition temperature in ferromagnet-superconductor-ferromagnet trilayers

The superconducting transition temperature T(c) of a ferromagnet (F)-superconductor (S)-ferromagnet trilayer depends on the mutual orientation of the magnetic moments of the F layers. This effect has been previously observed in F/S/F systems as a T(c) difference between parallel and antiparallel configurations of the F layers. Here we report measurements of T(c) in CuNi/Nb/CuNi trilayers as a function of the angle between the magnetic moments of the CuNi ferromagnets. The observed angular dependence of T(c) is in qualitative agreement with a F/S proximity theory that accounts for the odd triplet component of the condensate predicted to arise for noncollinear orientation of the magnetic moments of the F layers.     

Calculated Nb superconducting transition temperature under hydrostatic pressure

Abstract

A full-potential linear muffin-tin orbital method (FP-LMTO) based linear-response approach is used to calculate the electron-phonon coupling in Nb under hydrostatic pressure. The superconducting transition temperature T_c is calculated using the Eliashberg equation. The calculated T_c agrees nicely with the experimental result at ambient pressure, but the agreement is only fair at high pressures. The T_c measured anomaly at 60–70GPa is understood in terms of the 2.5 order Lifshitz transition and its origin is traced back to the qualitative changes in the Fermi surface topology.     

Mechanism of the size dependence of the superconducting transition of nanostructured Nb

A direct measurement of the superconducting energy gap by point contact spectroscopy in nanostructured Nb films shows that the gap decreases with a reduction in the average particle size. The superconducting T(c), obtained from transport and magnetic measurements, also decreases with size and scales with the energy gap. The size dependence of the superconducting properties in this intermediate coupling type II superconductor is therefore governed by changes in the electronic density of states rather than by phonon softening. Consistent with the Anderson criterion, no T(c) was observed for sizes below 8 nm.     

Doping dependence of the superconducting transition temperature in high-Tc systems

Using a self consistent diagrammatic theory of the one band Hubbard model, a suppression of the superfluid density due to spin fluctuation induced scattering rates is found. This is most pronounced for underdoped systems and enforces fluctuations of the superconducting order parameter and a suppression of the superconducting mean field transition temperature for low doping. Consequently, an optimal doping concentration around χ_opt: = 0:14 occurs within the spin fluctuation mechanism.     

Measurement of improved pressure dependence of superconducting transition temperature

We describe a technique for making electrical transport measurements in a diamond anvil cell at liquid helium temperature having in situ pressure measurement option, permitting accurate pressure determination at any low temperature during the resistance measurement scan. In general, for four-probe resistivity measurements on a polycrystalline sample, four fine gold wires are kept in contact with the sample with the help of the compression from the soft solid (usually alkali halides such as NaCl, KCl, etc.) acting as a pressure-transmitting medium. The actual pressure on the sample is underestimated if not measured from a ruby sphere placed adjacent to the sample and at that very low temperature. Here, we demonstrate the technique with a quasi-four-probe resistance measurement on an Fe-based superconductor in the temperature range 1.2–300 K and pressures up to 8 GPa to find an improved pressure dependence of the superconducting transition temperature.     

Volume dependence of the superconducting transition temperature of quench-condensed be films

The volume dependence of the superconducting transition temperatureT _c of quench-condensed Be films is investigated by bending the substrate of the film. Tensile strain causes an increase, compressive strain a decrease ofT _c . The volume coefficientd lnT _c /d lnV is about 0.8, a value which is small compared with those of other weakcopling non-transition metals. In addition to pure films, Be films stabilized by codeposition of noble metals or Ge are investigated.Paper based in part on a Habilitationsschrift submitted to the Fakultät für Physik, Universität Karlsruhe (TH), FRG     

Pressure dependence of the superconducting transition temperature of Th4H15

Measurements of the superconducting transition temperature of Th₄H₁₅ under hydrostatic pressure up to 28 kbar are reported. The initial linear increase is found to be $\text{+42}\text{mK}\text{kbar}$.     

Enhancement of the superconducting transition temperature in Nb/permalloy bilayers by controlling the domain state of the ferromagnet

In (S/F) hybrids the suppression of superconductivity by the exchange field h(ex) of the ferromagnet can be partially lifted when different directions of h(ex) are sampled simultaneously by the Cooper pair. In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F layer yield a similar effect. We study the transport properties of Ni(0.80)Fe(0.20)/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples.     

On the pressure dependence of martensitic and superconducting transition temperature in intermetallic compounds

In order to explain the large pressure dependence of the cubic to tetragonal transition temperature T_M in LaAg an expression has been derived from ?T_M/?P for a two-fold degenerate electronic band interacting with the tetragonal strain mode. Analogy with the pressure dependence of the ferromagnetic transition temperature T_c in an itinerant system is pointed out. The nature of variation of the superconducting transition temperature with pressure is also discussed.     

Quasiparticles relaxation processes in Nb/CuNi bilayers

The dynamic instability of the moving vortex lattice at high driving currents has been studied in superconductor (S)/weak ferromagnet (F) bilayer, Nb/Cu_0.38Ni_0.62. Voltage-current, V(I), characteristics have been acquired as a function of both the temperature, T, and the magnetic field, H, and interpreted in the framework of the model proposed by Larkin and Ovchinnikov. From these analysis the values of the quasiparticle relaxation time, τ, have been estimated. The results confirm the high performance of S/F hybrids in terms of velocity in the energy relaxation process, compared to corresponding single superconducting thin films. Moreover the temperature dependence of τ _E is extremely smooth, also if compared with the data reported in literature for other weak ferromagnet S/F based systems. This last result has been tentatively ascribed to the disorder present in the CuNi alloy.     

Carbon concentration dependence of the superconducting transition temperature and structure of MgCxNi3

The crystal structure of the superconductor MgC_xNi₃ is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88<x<1.0. The superconducting transition temperature was found to decrease systematically with decreasing carbon concentration. The introduction of carbon vacancies has a significant effect on the positions of the Ni atoms. No evidence for long-range magnetic ordering was seen by neutron diffraction for carbon stoichiometries within the perovskite phase stability range.     

Interplay of magnetic and superconducting proximity effects in ferromagnet-superconductor-ferromagnet trilayers

We present theoretical results on the interplay of magnetic and superconducting orders in diffusive ferromagnet-superconductor-ferromagnet trilayers. The induced triplet superconducting correlations throughout the trilayer lead to an induced spin magnetization. We include self-consistency of the order parameter in the superconducting layer at arbitrary temperatures, arbitrary interface transparency, and any relative orientation of the exchange fields in the two ferromagnets. We propose to use the torque on the trilayer in an external magnetic field as a probe of the presence of triplet correlations in the superconducting phase.     

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.     

Anomalous behavior of superconducting transition temperature of amorphous and crystalline Nb3Ge under pressure

The superconducting transition temperature (T_c) of amorphous Nb₃Ge was studied under both hydrostatic and quasihydrostatic pressure to 3.5 and 13 GPa, respectively. Whereas hydrostatic pressure causes T_c to initially decrease, T_c is found to increase under higher quasihydrostatic pressures. T_c(p) was also studied on an A-15 crystalline Nb₃Ge sample obtained from the amorphous sample by annealing.     

Anomalous pressure dependence of the superconducting transition temperature of beta-pyrochlore AOs2O6 oxides

High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors AOs(2)O(6) (A = Cs,Rb,K) are studied by measuring resistivity under high pressures up to 10 GPa. The superconducting transition temperature T(c) first increases with increasing pressure in every compound and then exhibits a broad maximum at 7.6 K (6 GPa), 8.2 K (2 GPa), and 10 K (0.6 GPa) for A = Cs, Rb, and K, respectively. Finally, the superconductivity is suppressed completely at a critical pressure near 7 GPa and 6 GPa for A = Rb and K and probably above 10 GPa for A = Cs. Characteristic changes in the coefficient A of the T(2) term in resistivity and residual resistivity are observed, both of which are synchronized with the corresponding change in T(c).