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Chiral fluctuation in frustrated superconductivity unlocks the interband phase difference at the superconducting transition temperature calculated by the standard Bardeen–Cooper–Schrieffer (BCS) theory when an interband interaction is much weaker than an intraband interaction. At low temperature, a first-order phase transition locks the interband phase difference. This transition is demonstrated in degenerate chiral superconductivity, which adds a pair entropy term to the BCS theory.

Maintaining pair entropy assists in the evolution of a gap in frustrated multiband superconductors. A finite and persistent gap is always open at finite temperatures due to the fluctuation of the interband phase difference when the interband interaction is much smaller than the intraband interaction. We demonstrate this gap evolution by combining the Bardeen–Cooper–Schrieffer (BCS) free energy with an entropy contribution originating from the internal freedom of the pair.

Competition between an interband Josephson interaction and a biquadratic interband interaction can break time-reversal symmetry when the interband interactions are very weak compared with the intraband interaction. We demonstrate this using the phenomenological Ginzburg–Landau free energy, taking into account terms essential for this phenomenon. This time-reversal symmetry-breaking state peels away a sheet of an interband phase difference soliton wall. This sheet is the domain wall between two different chiral states. The peeling reduces the formation energy of the domain wall. The domain wall can simultaneously erase the Meissner effect and the specific heat jump when the entropy invokes many domain walls, and its life time is prolonged by a pinning owing to an inhomogeneity such as surface roughness.

(Cu, C)–Ba–O thin films have been deposited at low growth temperature of 450–570 °C by pulsed laser deposition method. A control of CO 2 gas pressure and the growth temperature, usage of BaCu y O x pellet target resulted in an expansion of twice c -axis length of BaCuO 2 structure (2c phase) and a significant rise of conductivity. Measurements of in-situ XPS suggest that the 2c phase should be (Cu, C)Ba 2 CuO x [(Cu, C)-1201]. The maximum temperature of onset of the superconducting transition and zero resistance state obtained so far were 60 and 47 K, respectively.

Zero- and longitudinal-field muon-spin rotation (ZF-μSR and LF-μSR) measurements were carried out on a multi-layered HgBa 2 Ca 4 Cu 5 O y (Hg-1245) superconductor with T c of 108 K. With decreasing temperature, the observed ZF-μSR function changed from a Gaussian-type to an exponential-type below 60 K and zero field muon precession was also observed below 45 K. These μSR signals are attributed to the development of the antiferromagnetic ordering in inner CuO 2 planes with a low carrier concentration. These results seem to indicate clear evidence that antiferromagnetism microscopically coexists with superconductivity in the Hg-1245 superconductor below 60 K.

The electronic structure of HgBa 2 Ca n −1 Cu n O 2 n +2 ( n =3,4, Hg12( n −1) n ) is investigated by using the scalar-relativistic full-potential linearized augmented-plane-wave method. We calculated Fermi velocities v F and Fermi surface volume, and found that the anisotropy defined by the ratio v F x / v F z is similar to that in TlBa 2 Ca 2 Cu 3 O 9 . This anisotropy is also in good agreement with the experimental value. Each of the large Fermi surfaces contains 0.93–1.11 holes in Hg1223 and 0.90–1.14 holes in Hg1234. These values are similar to that in CuBa 2 Ca 3 Cu 4 O 9 F, which are designed to have an optimum set of the Fermi surface for superconductivity.

We controlled the dynamics of the superconducting order parameter keeping the same T c on Cu x Ba 2 Ca 3 Cu 4 O y . There are two pair-breaking bands corresponding to two superconducting gaps in Raman spectrum of the as-synthesized sample. One of them smeared out by a simple annealing.

To further enhance the critical current density ( J c ) and irreversibility field ( H irr ) of (Cu 1− x C x )Ba 2 Ca n −1 Cu n O 2 n +4− δ ( n =4; hereafter described as (Cu,C)-1234), pinning centers were introduced by heavy-ion irradiation. The polycrystalline samples were irradiated with Au 15+ ions (240 MeV energy) at various fluence of 3.5 × 10 10 , 6.5 × 10 10 , 1 × 10 11 , 2.5 × 10 11 and 5 × 10 11 ions/cm 2 . The intragrain J c was determined from M – H curves using Bean’s critical state model. With increase of the fluence, J c shows a rapid increase and reaches a maximum value, 4.1 × 10 6 A/cm 2 (77 K and 1 T) for the fluence of 1 × 10 11 ions/cm 2 and above which it decreases slowly. The maximum value of H irr (77 K), determined by extrapolating J c curves to a 10 3 A/cm 2 criterion, is about 14.5 T for the 2.5 × 10 11 ions/cm 2 . The α value in equation H irr ( T )= H irr (0)(1− T / T c ) α decreases from 2.97 for un-irradiated sample to 1.87 for the fluence of 1 × 10 11 ions/cm 2 . These results indicate the possibility of J c (77 K and 1 T) enhancement and reaching a high H irr (77 K) at the fluence of the 1.5 × 10 11 ions/cm 2 .

Polycrystalline samples of Ba 2 Ca 3 Cu 4 O 8 (O 1-y F y ) 2 (0234F) with a variety of T c (55-105 K) were synthesized under high pressure. The dynamic range of the T c 50 K in an underdoped region, has been found to be remarkably wide. We also found that plate-like single crystals of 0234F (∼1 mm 2 in plane area and 30 μm in thickness) grow when samples with a nominal composition of y ∼ 1.0 are heated at high temperature (∼1150 °C). Cleavage was observed for the crystals. The 0234F will be a convenient material for investigations of physics of the multi-layered superconductors and/ or the underdoped state that has a key for understanding high-T c superconductivity.

A 50 μm thick slice of (Cu,C)Ba 2 Ca 3 Cu 4 O y (Cu:1234) bulk superconductor grown by high-pressure synthesis technique was irradiated with high-energy (240 MeV) Au 15+ ions (10 11 cm −2 ). Complimentary DC magnetization measurements up to 5 T showed an increase in critical current density by two orders of magnitude. AC susceptibility studies were performed in DC magnetic fields up to 14 T and at various temperatures, using various AC field amplitudes (up to 15 Oe) and frequencies (up to 10 kHz). Critical current densities at various temperatures and fields were estimated from critical state model. Using the third harmonic response of the sample we were able to separate different contributions to the dissipation: inter-grain regions, grains without columnar defects (heavy-ions penetration depth is about one quarter of sample’s thickness), and grains with columnar defects. In irradiated grains, we could observe a peak-effect in the field dependence of critical current density at high DC fields and temperatures between 85 and 105 K, suggesting the possibility of the highest irreversibility field at liquid nitrogen temperature.

The effects of residual carbon in precursors on phase formation of single Tl–O layer superconductors of TlBa 2 Ca n −1 Cu n O y ( n =3 and 4) have been investigated. The samples were prepared from Tl 2 O 3 and precursors of Ba 2 Ca n −1 Cu n O 2 n +1+ δ (C x ) ( n =3 and 4) with various residual carbon contents ( x =0.02–0.19). Even a small amount of carbon promotes the formation of double Tl–O layer phases of Tl 2 Ba 2 Ca n −1 Cu n O y ( n =3 and 4). Using the carbon reduced precursor ( x =0.02), a TlBa 2 Ca 2 Cu 3 O y phase is synthesized directly in a silver-sheathed tape without such elements as Pb, Bi and Sr.

Superconducting compounds of Ba2Can−1CunO2n(O,F)2 (n=2–5) whose apical oxygen is partially substituted by fluorine have been synthesized under high pressure. The nearly single-phase samples of n=3 and 4 were prepared from almost stoichiometric compositions (δ is small) of Ba2Ca2Cu3O6(O1−y±δFy)2 (y=0.6–0.8) and Ba2Ca3Cu4O8(O1−y±δFy)2 (y=0.5–0.7), respectively. The samples showed sharp superconducting transitions in both resistivity and susceptibility measurements. The samples showed typical under-doped behavior (S-shape) in the temperature dependence of resistivity. The Tc for the n=3 phase monotonically increased with decreasing a nominal fluorine content y, which shows the electronic state shifts from under-doped toward optimally one with increasing hole density by substitution of apical O2− for F−. The highest Tc measured so far for n=2, 3, 4 and 5 phases were 90, 120, 105 and 90 K, respectively. This compound shows a maximum Tc at n=3 like in many other homologous series. In spite of a short spacing between superconducting blocks (≈7.36 A), the critical current density (Jc) and irreversibility field (Hirr) of the n=3 phase decreased rapidly with increasing temperature provably due to the small carrier (holes) density.

TlBa 2 Ca n −1 Cu n O y ( n =3: Tl-1223 and n =4: Tl-1234) have been found to have comparable T c with Hg-system which is known to show the highest T c in superconductors. Samples were synthesized under a pressure of 3.5 GPa using precursors with small residual carbon concentrations. We found that starting composition of Tl and synthesis temperature also affect the T c . Polycrystalline samples showed T c of 133.5 (Tl-1223) and 127 K (Tl-1234) after the post-annealing in nitrogen flow, respectively. Composition analysis suggested that suppression of the substitution of Tl for Ba- and Ca-site enhances the T c .

The electronic structure of TlCa 2 Ba 2 Cu 3 O 9 (Tl1223) and its change by oxygen reduction or anion substitution is investigated by using the scalar-relativistic full-potential linearized augmented-plane-wave method. Electronic structure of Tl1223 is almost similar to the homologous system, Tl1212. However, reduction of oxygen, or substitution of flourine by oxygen induce a great change for Tl6s band, and this band crosses the Fermi level and additional carriers are introduced onto this new band. This behavior is confirmed by the X-ray photoemission spectroscopy.

The Sr x Ca 1− x CuO 2 thin films and [CaCuO 2 ] m /[SrCuO 2 ] n superlattices (SLs) have been prepared by multi-target off-axis rf magnetron sputtering on treated SrTiO 3 (1 0 0) with periodic step arrays of atomic control on substrate surfaces. X-ray diffraction analyses have confirmed the formation of the tetragonal, infinite layer crystal structure with c -axis orientation. The obtained SLs and the Sr x Ca 1− x CuO 2 thin films showed semiconducting behavior. The Sr x Ca 1− x CuO 2 thin films with x =0.7 had remarkable resistive anomalies around 90 K and clear Meissner signals at temperature below 45 K. For the CaCuO 2 /SrCuO 2 artificial SLs, SL chemical modulation was observed for structures with individual layers as thin as two unit cells. The peak attributed to the SL periodicity was also observed, indicating good control of the film growth.

We measured for the first time the fundamental and higher harmonic components of AC susceptibility of CuBa 2 Ca 3 Cu 4 O y superconductor produced by the high-pressure technique, in various experimental conditions (temperature, DC magnetic field, AC field amplitude and frequency). The results are discussed qualitatively in the frame of the critical state and other models in the literature. By using a very simple frequency-dependent critical state model, we estimated the parameters of practical interest, critical current density and pinning potential at various DC fields and temperatures and the irreversibility field. We also extrapolated our estimations to both practical temperature of 77 K and longer time scale as of DC magnetization and transport measurements. The results appeared to be quite self-consistent.

On a superconducting bi-layer with thickness much smaller than the penetration depth, λ, a vortex molecule might form. A vortex molecule is composed of two fractional vortices and a soliton wall. The soliton wall can be regarded as a Josephson vortex missing magnetic flux (degenerate Josephson vortex) due to an incomplete shielding. The magnetic energy carried by fractional vortices is less than in the conventional vortex. This energy gain can pay a cost to form a degenerate Josephson vortex. The phase diagram of the vortex molecule is rich because of its rotational freedom.

A superconducting gap resonating with a phonon makes a structure in the phonon spectral function even in the d-wave pairing. This is demonstrated by a theoretical calculation. The conditions employed in our calculations are q→0 and the clean limit where q is the momentum of the phonon. The difference in the line shape of the phonon between the s-wave model and d-wave mode is noticeable. The resonance structure would be observed in the Raman spectrum when the phonon propagates in ab-plane, i.e., the directions of the incident light and scattered light are perpendicular to c-axis. We propose a new method to investigate the symmetry of superconducting gap function in the cuprate superconductor through this resonance structure.

The c -axis-oriented films of Bi 2 (La 1− x Sr x ) 2 CuO y (BLSCO) with a composition region 0.5≤ x ≤1.0 were grown on a single crystal MgO (100) substrate using the spin-coating pyrolysis method. Transmittance and reflectivity spectra were taken in the photon energy region from 2.5 to 13 meV and from 0.12 to 2.5 eV at room temperature. The absorption coefficient α was calculated from the film thickness, transmittance and reflectivity. The absorption spectra exhibited a constant edge of 1.2 eV which did not vary with strontium content x . This phenomenon demonstrates that the CT gap of BLSCO remains unchanged with the substitution of Sr for La. The absorption spectra in the infrared to visible light region were insensitive to the excess oxygen content, while those in the mid- and far-infrared regions showed strong dependence on the excess oxygen content. The absorption coefficient in the far-infrared region from 2.5 to 13 meV was very close to that calculated from the dc conductivity. The effective hole number calculated using the sum rule agreed with that calculated, assuming that one excess oxygen atom creates two holes in the composition region 0.5≤ x ≤0.8. The temperature dependence of conductivity showed the existence of an energy gap between the valence band and the acceptor level originated from the excess oxygen. An electronic model where the excess oxygen acts as the acceptor was proposed to interpret the observed results.

Single crystals of CuBa 2 Ca 3 Cu 4 O y (Cu-1234) with dimensions up to 200×100×20 μ m 3 have been synthesized under high pressure using a belt type anvil apparatus. The impurity of Au, emanating from Au capsule, was found in the crystals by SEM–EDS analysis. These crystals showed superconducting transition temperature ( T c ) ranging from 105 K to 68 K and are lower than that of Au-free polycrystals ( T c =118 K). The T c values are found to decrease linearly with Au content at a substantially high rate of 7.0±0.4 K%, indicating the substitution of Au ions for Cu-sites. Superconducting anisotropy γ as estimated from the resistivity data of Cu-1234 single crystal just above T c (=102 K) has been found to be higher ( γ =5.8) than that for the Au-free polycrystals.

We demonstrate that infrared spectroscopy is a useful technique to analyze the CO 3 group in the Cu based superconductors. Using this technique it was found that the quality of the precursor affects the presence of the CO 3 group in this compound.

From DC magnetization studies in fields up to 14 T and temperatures between 20 and 100 K, we estimated the intra- and inter-grain critical current density J c of (Cu,C):1234 high- T c superconductors, in the frame of critical state models. The inter-grain J c was determined by comparing the magnetization loops of as-grown sample and of the ground sample. Finally, short comments on J c resulted from AC susceptibility measurements and of the impressive increase of intra-grain J c due to heavy-ion and neutron irradiation are presented.

Tl 2 Ba 2 CaCu 2 O y (Tl-2212) and Tl(Ba,Sr) 2 Ca 2 Cu 3 O y (Tl-1223) superconductor thin films have been prepared on CeO 2 buffered sapphire substrate by an ex situ process namely, amorphous phase epitaxy (APE) method. In the case of Tl-1223, an amorphous layer of TlSr 2 CaCu 2 O y (TlSr-1212) was deposited on top of CeO 2 layer to avoid chemical reaction between Tl-1223, containing Ba, and CeO 2 layers. All films are epitaxial with smooth surface and excellent crystallinity. Superconducting transition temperatures ( T c ) of Tl-2212 and 1223 are around 95 and 104 K, respectively. Critical current density ( J c ) at 77 K measured on 5×5 mm films by inductive technique is as high as 2 MA/cm 2 . Surface resistance R s of Tl-2212 films measured by a dielectric resonator technique at 38 GHz is comparable to the best YBCO films.

We pointed out a soliton can be present in a multi-band superconductor [Phys. Rev. Lett. 88 (2002) 017002]. We named it i -soliton. It connects two different coherent states having different phases. Simultaneously the time reversal symmetry breaks down. The i -soliton traps a fractional flux inside a superconducting loop. These properties may realize a fractional flux quantum device without Josephson junction in a real space. It is beyond a single flux quantum device. The i -soliton would be found in the multi-layer cuprate superconductor having crystallographically nonequivalent CuO 2 planes in one unit cell of which representative is Cu x Ba 2 Ca 3 Cu 4 O y (Cu-1234).

Samples of (Cu 0.6 C 0.4 )Ba 2 Ca 3 Cu 4 O y ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba 2 Ca n −1 Cu n O y series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. T c decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and J c has shown a fish-tail effect.

Anomalous suppression of Tc has been found in an overdoped region of TlBa2Ca2Cu3O9−δ superconductors. A heavily overdoped sample (Tc≃100 K) was synthesized under high pressure. The Tc was measured with decreasing the oxygen content until the sample becomes optimally doped state with a Tc≃131 K. A dip of Tc was observed before the Tc reaches to an optimal value i.e. in an overdoped region. This behavior is quite strange because Tc is known to change monotonically in an overdoped region. Some possible explanations for this anomalous behavior will be presented.

As the temperature is lowered below Tc (107 K), a remarkable increase of the thermal conductivity, having a broad maximum, was observed. The behavior of the temperature dependence of the enhancement below Tc seems to be correlated to specific characteristics relating to a coexistence of magnetic and superconducting state in this compound.

Annealing effect of the irreversibility fields ( H irr ) in (Cu,C)Ba 2 Ca 2 Cu 3 O y ((Cu,C)-1223) and (Cu,C)Ba 2 Ca 3 Cu 4 O y ((Cu,C)-1234) have been investigated. The samples prepared under high pressure were post-annealed in nitrogen gas in the temperature range of 400–500 °C. H irr of (Cu,C)-1234 was improved after annealing at 450 °C while T c remained almost constant. This suggests that carrier density of five-coordinated CuO 2 planes or conductivity of charge reservoir layers is changed by annealing while keeping that of four-coordinated CuO 2 planes almost constant. Both samples show H irr ≃4 T at 90 K by the post-annealing, which is quite attractive for practical application.

We report a coexistence of superconductivity and antiferromagnetism in HgBa 2 Ca 4 Cu 5 O y with T c =108 K trough Cu-NMR measurements. This compound is composed of two kinds of CuO 2 planes in a unit cell, three inner planes (IP) and two outer planes (OP). The OP is optimally-doped, undergoing a superconducting transition at T c =108 K, whereas the IP is strongly underdoped, doing an antiferromagnetic transition below T N ∼60 K.

A new Tc enhancement method, a combination of alloying of Cu- and Tl-system and selective reduction for hole-doping has been developed for Cu 1−x Tl x Ba 2 Ca 3 Cu 4 O 12−y (Cu 1−x Tl x -1234) and Cu 1−x Tl x Ba 2 Ca 2 Cu 3 O 10−y (Cu 1−x Tl x -1223) system. The method is very effective for the Tc enhancement up to 132 K for Cu 1−x Tl x -1223 and 126 K for Cu 1−x Tl x -1234( x =0.4∼0.9). A new concept of band degeneration effect is proposed as the Tc enhancement mechanism for a common optimum hole-doping in every CuO 2 layers.

In order to study the difference in the isotope shilt of intramolecular vibration between two organic superconductors which are κ-ET2Cu[N(CN)2]Br and κ-ET2Cu(NCS)2, infrared reflectivity spectra for both deuterated and undeuterated single crystals of the former were measured and compared with those for the latter. A difference was found in the frequency of CC stretching mode coupled to the charge carriers, suggesting the difference in the electron-molecular vibration or the bare frequency of CC stretching mode between the two superconductors.

Low-temperature magnetic and electric properties of a series of isostructural organic conductors (BEDT-TTF)2MHg(SCN)4 (M=K, NH4 and Rb) were studied by electron spin resonance (ESR) and electric resistance measurement. In the case of both M=K and Rb salt, we observed an anomaly around 10∼20 K in the temperature dependence of ESR g factor and intensity as well as electric resistance, suggesting a phase transition to some magnetic ground state. On the other hand, no such anomaly was observed in the superconducting NH4 salt. These results suggest that the ground state of the former is not a simple metal but have some magnetic character, which prevents the appearance of superconductivity.

Measurements of infrared (IR) reflectivity and Raman scattering have been performed on Ba 1−x Rb x BiO 3 . The oscillator strength of the Bi-Bi stretching mode decreases with increasing Rb concentration, while the frequency change is small. The phonons detected by the Raman scattering experiment have the same frequencies as the LO phonons, although the system has the center of symmetry in the average structure. It is suggested that the coherence length of the charge density wave shorter than the incident laser wavelength 5145A gets shorter with increasing the Rb concentration.