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Change of electronic structure of superconducting (Cu, C)-1201 thin films grown by pulsed laser deposition with superconducting critical temperature Tc has been characterized by means of in situ photoemission spectroscopy. The finite spectral weight at the Fermi level in the valence band of (Cu, C)-1201 superconducting thin films and a rigid-band-like approach of the main peak of the valence band toward the Fermi level with a rise of Tc has been observed, which reveals hole-doping-induced superconductivity of this system. The obtained Ba 4d and Cu 2p signals with an increase of Tc suggest excess oxygen should be introduced around Ba ions in the early stage of the emergence of superconductivity. Then, they should be preferentially introduced around Cu ions. The changes of core signals with Tc and comparisons of the obtained data with infinite-layer and other cuprate superconductor compounds suggest that the (Cu, C)–O charge reservoir in this system is in the heavily hole-doped state, similar to that of the Cu–O chain in YBa2Cu3O7−δ.

Superconducting (Cu, C)?Ba?O thin films have been epitaxially grown on (100) SrTiO3 at a low growth temperature of 500?600??C by pulsed laser deposition. The dependences of their crystallinity and transport properties on preparation conditions have been investigated in order to clarify the dominant parameters for carbon incorporation and the emergence of superconductivity. It has been revealed that the CO3 content in the films increases with increasing both the parameters of partial pressure of CO2 during film growth and those of growth rate and enhancement of superconducting properties. The present study has also revealed that the structural and superconducting properties of the (Cu, C)?Ba?O films are seriously deteriorated by the irradiation of energetic particles during deposition. Suppression of the radiation damage is another key for a high and uniform superconducting transition. By these optimizations, a superconducting onset temperature above 50?K and a zero-resistance temperature above 40?K have been realized.

(Cu,C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for yLiF = 0–0.1 if the amount of z mol AgO oxidizer is increased linearly from zAgO = 0.45 to 0.73 and for yLiF = 0.1–0.2 if zAgO = 0.73 = constant. Transport measurements (ρ(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases Tc. The critical point at yLiF = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for yLiF>0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu,C-12 (n−1)n with . LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

Single crystals of multi-layered superconductors of Ba2Can−1CunO2n(O,F)2 (F-02(n − 1)n, n = 3 and 4) have been grown under a pressure of 4.5 GPa. Crystals were obtained by cooling samples with nominal compositions of Ba2Can−1CunO2nF2 (n = 2, 2.5, 3 and 4) across the melting point. The crystals are thin plate-like shapes with typical sizes of 500 µm in plane length and 30 µm in thickness. The crystals are found to show cleavage, which will be convenient for surface sensitive measurements. The F-0223 and F-0234 crystals showed Tc of 90 K and 70 K, respectively after oxygen annealing. The temperature dependence of resistivity showed pronounced underdoped behaviour. Composition analysis showed that a ratio of Ba:Ca:Cu is close to the stoichiometric composition one. We think that the F-system is a rare multi-layered cuprate that can largely grow under high pressure and its crystals are quite valuable to study both high-Tc superconductivity and the multi-layered system.

A slice of the (Cu,C)Ba2Ca3Cu4O12−y bulk superconductor grown by high-pressure synthesis was polished down to a thickness of 50 μm, and then irradiated with high-energy (240 MeV) Au15+ ions (1011 cm−2). Since the length of the ion tracks was estimated to be 12.2 μm, our sample consists of two different types of grain, with and without columnar defects, having very different critical current densities. Using the third-harmonic response of the sample, we were able to separate the small dissipation in the irradiated grains from the much larger dissipation in the non-irradiated grains, in a certain experimental range of temperature, field and current density. The critical current density was estimated from critical state models. For temperatures in the range of 85–105 K, we observed a peak effect at high dc magnetic fields, suggesting the possibility of a record irreversibility field at 77.3 K.

A TlBa2Ca2Cu3Oy (Tl-1223) superconductor with an ideal composition has a much higher Tc than the established value (~120 K). The intrinsic Tc arises by suppression of Tl substitution for Ba and Ca sites, which inherently occurs in samples prepared by conventional methods. The substitution of Tl was suppressed by preserving the samples in a heavily over-doped state (Tc~100 K) which was realized by a high-pressure (solid and oxygen) synthesis method. In addition, we found several synthesis conditions for which we could obtain high-Tc samples with good reproducibility. Residual carbon in the precursor was found to be harmful for superconductivity, so that it had to be reduced as much as possible before the high-pressure synthesis. The synthesis temperature and starting concentration of Tl, which also affect Tc, had to be controlled properly to enhance Tc. Samples show that Tc≥130 K by post-annealing in a reducing atmosphere at 650-700 °C for 0.5 h to 12 h. The sample prepared with the starting composition of Tl0.75Ba2Ca2Cu3.25Oy had an almost ideal grain composition and showed a Tc of 133.5 K. This Tc is comparable with that of HgBa2Ca2Cu3Oy (Hg-1223) which is known to have the highest Tc in superconductors.

Multicomponent superconductivity is realized in multiband superconductors when an interband pairing interaction is considerably weaker than the intraband interactions. There is a new quantum phase that originates from the interband phase difference in this superconducting condensate. Firstly, we discuss the applicability of this physical viewpoint for known multiband superconductors. Secondly, topics related to the interband phase difference are treated. Finally, we mention that the Bardeen–Cooper–Schrieffer formalism and the Ginzburg–Landau formalism may not be fully guaranteed when we introduce a fluctuation in the interband phase difference mode. We also address plausible new superconducting electronics using the interband phase difference.

We have investigated superconductivity and structure in a gold–antimony–tellurium ternary alloy system (AuxSb1−x−yTey) synthesized under high pressure. We found that Au0.125Sb0.75Te0.125 and AuSb6Te when crystallized in simple cubic structures (α-Po type), which are reported to be semiconductors above 20 K, are superconductors with a transition temperature (Tc) of 6.7 K. The structure and Tc are mapped on an Au–Sb–Te triangular diagram. A β-Po-type rhombohedral structure appears before the crystal structure changes from As type (Sb) to α-Po type. Superconductivity is observed in the β-Po-type structure as well as in the α-Po-type structure. Tc increases with decreasing Te concentration toward the Te-free Au–Sb end of the diagram. A Tc of 8.1 K was achieved for the Au–Sb alloy with a typical composition of Au0.15Sb0.85. This Tc value is the highest among materials with the α-Po-type structure under ambient pressure. Our Au0.125Sb0.75Te0.125 sample exhibits a weak metallic behavior in resistivity. The discrepancy in the normal state resistivity behaviors between the previous study and ours must originate from a difference in the number of lattice defects in the samples.

We have measured the vortex melting lines of high-pressure-synthesized Ba2Ca3Cu4O8(O1−yFy)2 (nominal composition, 2y = 1.3, 1.6 and 2.0) F(2y)-0234 multilayered high-Tc superconductor using fundamental and third harmonic susceptibility responses carried out on preferentially oriented crystallites with very low ac field amplitude (5 µT) and in applied dc fields up to 6 T. The vortex melting lines of all three F-substituted samples show interesting doping dependence and are very well described by the commonly accepted theory of melting lines. Interestingly, we discovered that the models describing the temperature dependence of the in-plane London penetration depth also depend on the doping level: the 3D XY model for the nearly-optimum-doped sample [F(1.3)], the mean-field theory for the under-doped sample [F(1.6)] and the two-fluid model for the heavily-under-doped sample [F(2.0)]. We found that the vortex melting lines in F-0234 are determined by the interplay in the coupling of pancake vortices through the charge reservoir layer (CRL), and through the block of inner CuO2 planes (IPs), respectively. The anisotropy values (47 for the near-optimally-doped sample) derived from experimental vortex melting lines are consistent with the values obtained by first-principles electronic-band-structure calculations.

The Abrikosov lattice in the multilayer cuprate superconductor CuBa2Ca3Cu3Oy (Cu-1223) has been experimentally and theoretically demonstrated to be composed of vortex molecules. Cu-1223 is considered to be a typical multicomponent superconductor. We show that in such a system the rotational freedom around the axis of the vortex molecular tube generates orientational disorder and the orientational glass (or crystal) phase, which is never present in conventional vortex lattices consisting of axisymmetric vortices. The emergence of the orientational glass phase and orientational order phase with orthorhombic distortion is a general property of vortex molecule lattices of the multiband type of multicomponent superconductors.