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Journal of physics / Conference series 1643, 012181 (2020). doi:10.1088/1742-6596/1643/1/012181, Published by IOP Publ., Bristol

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.

An AC magnetic field can rotate a vortex-molecule composed of two fractional vortices in a multiband type of multicomponent superconductor, where two components couple with each other through the intercomponent Josephson interaction. It may directly relate to an abnormal AC loss peak, which we have found recently in the multilayer cuprate supercondutor CuxBa2Ca2Cu3Oy (Cu-1223).

We report the superconducting properties of (Cu, C)Ba2CuOy thin films deposited by RF magnetron sputtering from targets with a nominal composition of Ba2Cu1.5Oy. The thin films were grown in mixed Ar (5~40 mTorr), O2 (0~1.2 mTorr) and CO2 (0~1.8 mTorr) atmosphere. The epitaxial c-axis oriented thin films with c-axis length in the range between 7.96 A and 8.82 A were obtained on SrTiO3(100) substrates, but superconductivity was observed only for the films which have the c-axis length with 8.01 A and 8.32 A. By optimizing deposition conditions, the excellent (Cu, C)Ba2CuOy thin film with Tc = 62K and superconducting transition width ΔTc = 1.5 K was successfully obtained.

Ba-Cu-O films have been grown by pulsed laser deposition. Investigation of their transport properties and characterization of electronic structure by in-situ photoemission spectroscopy have been carried out. The films grown at 450 ~ 480 °C from BaCuO2 target had simple infinite layer structure with lattice parameter of a and c about 0.385 and 0.405 nm, respectively. Conductivity of these films were, however, poor. Usage of BaCu0.75Ox target and addition of CO2 into the growth atmosphere resulted in an expansion of c-axis length up to 0.810 ~ 0.835 nm and a significant rise of conductivity. This structure is represented as ''2c''. For the 2c phase, conductivity above of 103 S/cm was achieved by adjusting their growth temperature in the range of 500 ~ 530 °C. Most of the highly conductive 2c phase showed superconductivity. The maximum temperature of onset of the superconducting transition and zero resistance state obtained so far were 60 and 47 K, respectively. Finite spectral weights at Fermi level were successfully observed in in-situ photoemission spectra of the 2c phase. In valence band region, a prominent peak with u 3d character located at a binding energy around 1.6 eV. Though this low binding energy suggested a rather high hole concentration of mobile holes were around middle of 1021 cm−3 range. These results about electronic structure indicate a coexistence of conventional CuO2 planes and distorted ones. The latter structure might play charge-reserving blocks, though some portions of holes should localize in them. Correlations between the properties and electronic structure are to be discussed.

(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.

We have measured the AC susceptibility response in field-cooling conditions of (Cu,C)Ba2Ca2Cu3Oy with aligned crystallites. In a certain but large range of the applied DC magnetic field, the out-of-phase susceptibility has two dissipation peaks. Due to the characteristics of the sample, the shape of the second peak and its position's field-dependence, we ruled out the possibility that the anomalous second dissipation peak could be related to intergrain dissipation. Because this multilayered cuprate is known to be a two-gap superconductor, we believe that our results are an experimental indication of a two-component vortex matter in high-critical temperature 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 report a new sample synthesis technique for polycrystalline (Ca1−xNax)Fe2As2 (0

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.

Here we demonstrate the technique to stabilize the ErFeAsO-based superconductor with the smallest lattice constants in LnFeAsO1-y (Ln = lanthanide) series using hydrogen doping method. Polycrystalline samples were synthesized by heating pellets with nominal compositions of ErFeAsO1-y (1-y = 0.75 ~ 0.95) sandwiched between pellets of LaFeAsO0.8H0.8 compositions at 1100 {\deg}C under a pressure of 5.0 - 5.5 GP. The sample with lattice constants of a = 3.8219 {\AA} and c = 8.2807 {\AA} shows the highest superconducting critical temperatures (Tc) of 44.5 K and 41.0 K determined by onset transitions of resistivity and susceptibility, respectively. We discuss phase diagram of Ln dependence of Tc in LnFeAsO-based superconductors., Comment: 12 Pages, 5 Figures, Accepted for publication in EPL

We have synthesized an iodine-intercalated superconductor IxBa2Ca3Cu4O8(O,F)2. The intercalation was performed by heating a host compound of Ba2Ca3Cu4O8(O,F)2 with I2 in an evacuated quarts tube. The Tc significantly increased from 82 K to 121 K by the iodine intercalation. The Tc is the highest in four CuO2 layered cuprates excluding Hg- and Tl-system. The iodine will be intercalated between Ba-(O,F) layers. Composition analysis indicated that nearly stage-I iodine intercalation (about χ = 0.9) was realized. The iodine atoms expand the spacing between the double Ba-(O,F) layers by about 4.0 A which is comparable to the value of 3.5 A for the iodine intercalation to Bi-system.

Ploycrystalline samples of NaMn204 and LiMn2O4 with the calcium ferrite-type structure were synthesized using high pressure technique. For synthesis of LiMn2O4 samples, we applied for two different synthesis routes; one is directly obtained from the starting mixture of LiMn2O4 composition under high pressure of 4.5 GPa and the other is obtained by alkali ion exchange from high pressure synthesized NaMn2O4. From the powder X-ray diffraction measurements, these samples showed almost single phases. The crystal structures were determined from the results of neutron diffraction measurements. The data were refined with the space group orthorhombic Pnam. The valence states of Mn are determined by the Mn-0 distance using the bond valence sum analyses. Ordering of MnJ+/Mn4+ at different Mn sites caused for NaMn2O4 samples, whereas Mn3+ and Mn4+ ions are seem to distribute randomly for LiMn2O4 samples. The temperature dependence of static magnetic susceptibility of NaMn2O4 shows antiferromagnetic behaviour with Neel temperature(TN)=13K while the spin-glass-like behaviour is observed in both LiMn2O4 compounds in the low temperature region.

Transport properties of (Cu, C)-1201 thin films have been characterized by in-situ four probe method without breaking vacuum, subsequent to their growth by pulsed laser deposition, in order to clarify intrinsic transport properties. Owing to the in-situ measurements, degradation of contact resistance and normal state conductivity were successfully suppressed. Obtained results reveal the positive correlation between Tc and normal state conductivity δ at 290 K, and that between Tc and temperature coefficient of resistivity (TCR) at 290 K. The films exhibit Tc(ρ=0) > 20 K on the cases of δ[290 K] > 4 x 102 S/cm and TCR > 1.5 x 10-3 K-1. The high Tc with low conductivity of (Cu, C)-1201 films indicates the presence of extrinsic defects such as grain boundaries. The absence of saturation of Tc with an increase of TCR indicates doping level of the (Cu, C)-1201 films in this study should be in between under-doped to optimally-doped states. They suggest there would be some room for further increases of Tc.

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.

Multilayered cuprates having crystallographically inequivalent CuO2 planes are worthful materials to realize the mechanism of high-Tc superconductivity as well as a stage to ascertain new phenomena. The material synthesis of HgBa2Can-1CunOy multilayered cuprates by employing high-pressure synthesis technique is demonstrated. A starting mixture with a composition ratio of Hg:Ba:Ca:Cu:O = 1:2:n-1:n:2n+2+? was synthesized by mixing the precursor of Ba2Ca2Cu3Oy (y~7.5) with an appropriate amount of HgO, Ca2CuO3, CuO and Cu2O (or Cu). The oxygen content of the mixture was adjusted incisively by ascertaining a ratio of CuO, Cu2O (or Cu) to obtain samples. Almost single-phase samples up to n = 6 were synthesized. The X-ray diffraction analysis of the grain-aligned sample with nominal compositions of n ? 7 revealed that the sample simultaneously comprises from n = 6 up to 16 phases.

Hg-based multi- and super-multi-layered cuprates HgBa2Can-1CunOy (Hg:12(n-1)n) (n?4) are composed of the charge reservoir layer (CRL) HgBa2Ox, and of CuO2 planes: outer planes (OP) with pyramidal oxygen coordination and inner planes (IP) with square oxygen coordination, with different charge distribution. This fact gives rise to very interesting phenomena, like coexistence of superconductivity (SC) in OPs with antiferromagnetism (AF) in IPs in Hg:1245. The dependence of Tc on the number n of CuO2 planes is also interesting: for n increasing from 4 to 5 and to 6, Tc decrease from 125 to 108 and then to about 103 K; further increase in n keeps Tc constant at about 103 K. In Hg:1245 we show that the AF ordering at TN of 60 K has no influence on vortex dynamics, and there is a quite robust c-axis supercurrent. Vortex melting lines of various samples showed that Hg-based multi-layered cuprates (n = 4, 5) behaves in the usual way (increasing n resulted in decrease of Josephson coupling (JC) and a less sharp melting line) and are well described by the usual anisotropic GL-based theory. Samples with n ? 6 have all the same melting line, suggesting that, in this case, the short-range JC becomes much less important than the long-range magnetic coupling (MC). However, since the JC between two pancakes in OPs of adjacent unit cells (pancakes separated by the thin CRL) is large, we suggest that the vortex matter in Hg-based super-multi-layered cuprates is composed of magnetically-coupled pancake-vortex-molecules.

We observed two dissipation peaks in the field cooled AC susceptibility measurements on aligned multilayered cuprate Superconductor, (Cu,C)Ba2Ca2Cu3Oy[(Cu,C)-1223], and demonstrated that it is the direct experimental indication of the two-component vortex matter in the multilayered cuprates. We propose that, the second loss peak at lower temperature is due to the additional degree of freedom of the rotation of 'vortex molecule' composed of two fractional vortices, originated at two components, mediated by an i-soliton bond. To probe the dynamics of this vortex molecule, we measured frequency dependence of AC susceptibility response on aligned crystallites of (Cu,C)-1223 at different temperatures and DC fields. In second peak region, it gives resonance peak. The observed frequency dependence patterns of x(T), for 0.5 T HDC, are rescaled with the resonating frequencies and show tail at low frequency region indicating that the dissipation is due to rotation and twisting of vortex molecule. The temperature dependence of the average relaxation time shows that the vortex molecule rotation/twisting glass state follow critical slowing down process.

A long-running-time (T=0.45 K) germanium bolometer which has a compact charcoal adsorption pump with a novel 3He gas condenser has been constructed. The refrigerator provides continuous cooling of the bolometer element at 0.45 K for 24-hour measurements of spectra in the range 2 to 40 cm-1. Utilizing this bolometer system, transmission spectroscopy has been carried out successively, maintaining the temperature of the sample below 40 K and that of the bolometer element below 1.5 K without a thermal cycle. This experimental setup is essential for obtaining a reproducible spectrum of MEM(TCNQ)2. Thus, each resultant spectrum has good reproducibility even after one-week-long experiments.