Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides

A review of the oxidation of uranium dioxide at temperatures below 400°C

The formation of bound states at surfaces of materials with an energy gap in the bulk electron spectrum is a well known physical phenomenon. At superconductor surfaces, quasiparticles with energies inside the superconducting gap Δ may be trapped in bound states in quantum wells, formed by total reflection against the vacuum and total Andreev reflection against the superconductor. Since an electron reflects as a hole and sends a Cooper pair into the superconductor, the surface states give rise to resonant transport of quasiparticle and Cooper pair currents, and may be observed in tunnelling spectra. In superconducting junctions these surface states may hybridize and form bound Andreev states, trapped between the superconducting electrodes. In d-wave superconductors, the order parameter changes sign under 90° rotation and, as a consequence, Andreev reflection may lead to the formation of zero energy quasiparticle bound states, midgap states (MGS). The formation of MGS is a robust feature of d-wave superconductivity and provides a unified framework for many important effects which will be reviewed: large Josephson current, low-temperature anomaly of the critical Josephson current, π-junction behaviour, 0→π junction crossover with temperature, zero-bias conductance peaks, paramagnetic currents, time reversal symmetry breaking, spontaneous interface currents, and resonance features in subgap currents. Taken together these effects, when observed in experiments, provide proof for d-wave superconductivity in the cuprates.

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Andreev bound states in high-Tc superconducting junctions

Heat transport in the cuprate superconductors ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ and ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ was measured at low temperatures as a function of doping. A residual linear term ${\ensuremath{\kappa}}_{0}/T$ is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from ${\ensuremath{\kappa}}_{0}/T$ is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component $(\mathrm{ix})$ appearing at a putative quantum phase transition and argues against a nonsuperconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.

https://arxiv.org/pdf/cond-mat/0301105.pdf

Thermal conductivity across the phase diagram of cuprates: Low-energy quasiparticles and doping dependence of the superconducting gap

Columnar defects introduced by irradiation with very energetic heavy ions are the most effective pinning centres for flux lines in high-temperature superconductors. This correlated disorder generates large increases in the critical current densities and expansion of the irreversible regime in and the various Bi- and Tl-based compounds. In single crystals and thin films, the pinning enhancement is strongly angular-dependent, and maximizes when the applied magnetic field is parallel to the amorphous latent tracks. In contrast, in the much more anisotropic Bi- and Tl- based materials this unidirectional anisotropy is very small due to the quasi two-dimensional character of the vortices. Some of the extensive experimental studies on this topic are reviewed. Measurements of the temperature, field and defect density dependence of the persistent currents and their time relaxation in YBCO are presented. The analysis of these results based on recent theoretical models permits the identification of various pinning and creep regimes. Studies in other compounds, including technologically relevant Bi-based tapes, are summarized. The influence of the angular dispersion (splay) of the tracks on vortex pinning and creep is discussed.

https://iopscience.iop.org/article/10.1088/0953-2048/10/7A/003/pdf

Vortex pinning and creep in high-temperature superconductors with columnar defects

Single crystals of YBa2Cu3O7−δ were irradiated at 95 K by 5.6 GeV Xe ions and the evolutions of the superconducting resistive curves - measured parallel and perpendicular to the c axis - were determined in situ as a function of ion fluences. We find that ϱc and ϱab increase linearly with the fluence and even tend to saturate for the highest fluences. This behaviour strongly differs from that found for sintered samples irradiated under the same conditions for which a quasi-exponential increase of resistance was observed. Nevertheless our results confirm that energy deposition by electronic excitation plays a role in defect creation as previously reported in Xe irradiated sintered samples.

Irradiation of YBa2Cu3O7−δ single crystals by 5.6 GeV xenon ions

Defect production rates in normal and in superconducting states

Low temperature thermal conductivity, $\kappa$, of optimally-doped Bi2212 was studied before and after the introduction of point defects by electron irradiation. The amplitude of the linear component of $\kappa$ remains unchanged, confirming the universal nature of heat transport by zero-energy quasiparticles. The induced decrease in the absolute value of $\kappa$ at finite temperatures allows us to resolve a nonuniversal term in $\kappa$ due to conduction by finite-energy quasiparticles. The magnitude of this term provides an estimate of the quasiparticle lifetime at subkelvin temperatures.

Effect of controlled disorder on quasiparticle thermal transport in Bi 2 Sr 2 CaCu 2 O 8

Sintered samples and single crystals of YBa 2 Cu 3 O 7-δ have been irradiated at low temperature by 2.5 MeV electrons. Under irradiation the critical temperature T c decreases and the normal state resistance R increases. The ΔT c versus ΔR variations are nearly the same in single crystal as in sintered samples, indicating that degradations of T c and R result mostly from intragrain disorder. The evolution of the transport critical current j c of sintered YBa 2 Cu 3 O 7 measured in situ as a function of electron fluence show that weak links between grains are also damaged by irradiation defects. We also found that decrease rates of T c are higher in samples with lower initial values of T c . This behaviour is not restricted to YBa 2 Cu 3 O 7−δ but seems to be quite general in high- T c oxides.

F. Rullier-Albenque

Papers

Irradiation of YBa2Cu3O7−δ single crystals by 5.6 GeV xenon ions

Defect production rates in normal and in superconducting states

Effect of controlled disorder on quasiparticle thermal transport in Bi 2 Sr 2 CaCu 2 O 8

Effects of low temperature electron irradiation on single crystal and sintered YBa2Cu3O7−δ

Determination of displacement threshold energies in pure Ti and in γ-TiAl alloys by electron irradiation