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Applying a radian phase shift other than 2π is a key issue for superconducting circuits, such as flux qubits. The magnetic flux is useful when generating a phase shift. However, a quantized magnetic flux accompanying a trapped vortex in a superconductor does not have a phase shifter function. The magnetic flux generated by an external field generates noise. In this study, we propose a phase bias system that does not require an external field during operation. We confirm the phase shift of a direct current superconducting interference device (SQUID) placed on an ultrathin superconducting Nb bilayer with a through-hole by cooling it to a temperature below the superconducting transition temperature with an external field. Although the cause of the phase shift in our system is unclear, we believe that it may be caused by a fractional quantum in the bilayer. When the SQUID is replaced by a qubit, the phase shift can be applied to a phase bias.

An ultrathin superconducting bilayer can admit a monotonic magnetic field when one layer has a slit even in a Meissner state. The rotation of the superconducting quantum phase by a vector potential for the magnetic field is compensated by a phase shift accompanying an inter-layer phase difference mode which grows from the slit. This phase difference mode may grow to form a fractional vortex, and it may show a close relationship with experimentally observed fractional vortices.

We report the experimental formation of a fractional vortex generated by using a thin superconducting bi-layer in the form of a niobium bi-layer, observed as a magnetic flux distribution image taken by a scanning superconducting quantum interference device (SQUID) microscope. Thus, we demonstrated that multi-component superconductivity can be realized by an s-wave conventional superconductor, because, in these superconductors, the magnetic flux is no longer quantized as it is destroyed by the existence of an inter-component phase soliton (i-soliton).

We developed a minimal method of forming and detecting a fractional magnetic flux quantum in a two-component superconductor fabricated using an ultra-thin niobium bilayer that sandwiches a Josephson barrier. The fractional quantum created by cooling a bilayer disk in the magnetic field survived after turning off the magnetic field. The survived quantum was confirmed by the change in the critical current of a superconducting quantum interference device (SQUID) placed on the bilayer disk. By replacing a flux qubit with the SQUID, we can use the fractional flux quantum as the flux bias for the flux qubit. This method enables the investigation of fractional quantization and the related topology.

An ultra-thin superconducting bi-layer is an artificial multi-component superconductor in which the magnetic flux can be fractionally quantized. The fractionally quantized state is energetically unfavorable, so it is difficult to realize experimentally. We propose a new method to create fractional quantized states by the reaction of a unit quantized state with an externally injected inter-layer phase-difference soliton. Simulation shows that the unit quantized state splits the soliton and the soliton fragments produce fractional quanta by dissociation of the unit quantum.

The sliding motions of inter-band phase difference solitons ( i -solitons) cause time-dependent superconducting phase variations in two-band superconductors. Two different voltages cannot exist at the same spatial location, although the conventional AC Josephson effect demands the presence of a voltage difference between the two bands at the same location, if the time-variation of the phase generates the voltage, for which a physical interpretation is not possible. Instead of the conventional AC Josephson effect, a non-linear Klein-Gordon equation (sine-Gordon equation) can enable the dissipationless movement of i -solitons, without voltage generation.

There is a current-induced massless mode of an interband phase difference in two-band superconductors. For a thin wire, the externally applied current always invokes a finite interband phase difference when the end of the wire is terminated by a natural boundary condition, i.e., where the total current is specified but the other parameters are left as free and a finite interband phase difference is allowed. This condition can be realized by the normal state region formed by the shrinking of a cross section of the wire where the critical current density is lower than that of the other region of the wire. The interband interaction in the wire cannot completely prevent the emergence of the interband phase difference, though it reduces it somewhat. Instead, boundary conditions determine the presence of the interband phase difference. By reverting the normal state into the superconducting state at the shrunken region by decreasing the current, we may trap a rotation of integral multiples of 2π radians of the interband phase difference in the wire. After switching off the current, this rotation of integral multiples of 2π radians, which continuously spreads over the whole wire, is separated into several interband phase difference solitons (i-solitons), where one i-soliton locally generates a 2π interband phase difference.

A basic design is proposed for a classical bit element of a superconducting circuit that mimics a frustrated multiband superconductor and is composed of an array of π-Josephson junctions (π-junction). The phase shift of π provides the lowest energy for one π-junction, but neither a π nor a zero phase shift gives the lowest energy for an assembly of π-junctions. There are two chiral states that can be used to store one bit information. The energy scale for reading and writing to memory is of the same order as the junction energy, and is thus in the same order of the driving energy of the circuit. In addition, random access is also possible.

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.

Multiband superconductors can have several types of domains that are inhibited in conventional single-band superconductors. These domains are phase domains and chiral domains and their domain wall are an interband phase difference soliton. In a superconductor with an odd number of electronic bands (five or more) and with positive interband Josephson interactions, we find other types of domains with different interband phase differences. We call these domains configuration domains because pseudo-order parameters for each band are dispersed in the complex plain and several configurations, which have several local minima. Fractional vortices serve as hubs for phase difference solitons (configuration domain walls). The divergence of the number of configurations with local minima would pose a serious problem for the stability of superconductivity.

We investigate multi-component superconductors, in relation to iron pnictides, by using the Ginzburg–Landau theory. We show that a three-band superconductor exhibits several significant properties that are not found in single-band or two-band superconductors. The frustrating pairing interaction among Fermi surfaces may lead to a time-reversal symmetry broken pairing state. In fact, we have a solution with time-reversal symmetry breaking, that is, a chiral solution when there is such a frustration. The Ginzburg–Landau equation for three-component superconductors leads to a double sine-Gordon equation. A kink solution exists to this equation that results in the existence of fractional-quantum flux vortices on the domain wall.

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.

The discovery of superconductivity in MgB2 and, more recently, in iron pnictides, revived the interest in two-component and other exotic superconductors, including in the field of vortex matter, as was shown recently by the discovery of “type 1.5” superconductivity. However, even some “older” exotic cuprates grown by high-pressure techniques showed interesting anomalies related to the interplay between Josephson and magnetic coupling and/or two-gap superconductivity. Here, we will present two such examples of exotic vortex matter: magnetically-coupled pancake-vortex molecules in super-multilayered cuprates, and vortex molecules composed of fractional flux quanta glued by an interband phase difference soliton, and will discuss the major differences between MgB2 and such multilayer cuprates in terms of the relative strength of the interband interaction and, respectively, in terms of the coherence and penetration lengths.

Transport properties of (Cu, C)Ba 2 CuO x [(Cu, C)-1201] thin films have been characterized by in situ temperature dependence of resistivity, without breaking vacuum from the deposition to the measurement. In in situ transport properties measurements, the obtained results reveal that (Cu, C)Ba 2 CuO x films exhibit T c > 20 K on the cased of conductivity at 290 K ( σ [290 K] ) > 4 × 10 2 S/cm and temperature coefficient of resistivity (TCR) > 1.5 × 10 −3 K −1 , and doping level of them should be in between under-doped and optimally-doped states. Their results suggest that there would be possible to further increases of T c , and XPS data suggest that (Cu, C)-system should have the excellent dopability in their charge reservoir and the possibility of low anisotropy.

We observed the disappearance of the Meissner effect and the specific heat jump in Ba0.2K0.8Fe2As2. A crossover of the type of superconducting order parameter can be a source of these phenomena. A quantum phase disorder provides a possible explanation.

There are four topologically different paths to interchange two single-component winding vortices in a two-band superconductor having a weak inter-band interaction, where each band has its own superconducting component and its own phase, θν, other than the ordinary quantum phase, Θ (ν is the index of the band and Θ is defined at the region of θ1=θ2.). The acquired phase for the interchange of two single-winding vortices depends on the path, and three values, Θsoliton, −Θsoliton and 0, are observed. Θsoliton is the phase-slip accompanying the inter-band phase difference soliton (i-soliton). The interchange requires the creation and annihilation of the i-soliton and gives rise to an ambiguity in the statistics.

HgBa 2 Ca n −1 Cu n O y (Hg:12( n − 1) n ) phases have the same T c independent of n and the same vortex melting line for n ⩾ 6. In these materials, there are two types of pancake pairs: those separated by the thick block of ( n − 2) inner CuO 2 planes, which are weakly coupled by the magnetic interaction, and, respectively, those separated by the thin HgBa 2 O x charge reservoir layer, which are much stronger coupled by the Josephson coupling. The vortex melting line common to all Hg:12( n − 1) n phases ( n ⩾ 6), separates two new vortex phases, pancake-molecule-glass, and pancake-molecule-liquid.

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 multilayered cuprates of HgBa 2 Ca n −1 Cu n O y (Hg-system) and Ba 2 Ca n −1 Cu n O 2 n (O, F) (F-system) ( n = 5–9) have been synthesized under high-pressure to investigate the variation of T c . The a - and c -parameters showed reasonable behavior: as the n increases from 5 to 9 the a -parameter approached that of an infinite layer CaCuO 2 (IL) and the average interval of c (or c /2) was close to the c -parameter of the IL. All the samples showed high T c values of about 100 and 80 K for Hg- and F-system, respectively. In addition, the T c was almost constant for n ⩾ 5 for both systems. We have shown that the T c variations can be explained using inhomogeneous charge distribution model as follows. The high T c value is maintained because the charge reservoir layer supplies enough carriers for superconductivity to the adjacent CuO 2 planes (OP) even for large n . The T c is independent of n because the carrier concentration of OP is insensitive to n for large n .

Vortex images of superconducting material Tl-1223 thin film taken with a scanning SQUID microscope are presented. Among the vortices images, coexistence of vortex–antivortex pairs is observed at low field region. Short distances of the pairs indicate the existence of strong pinning centers in this material.

The microwave power dependence of high quality Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films grown on 1'' diameter LSAT substrates using the Amorphous Phase Epitaxy method has been investigated in this paper. We have performed surface resistance (R/sub S/) measurements of Tl-1223 thin films on LSAT using the Hakki-Coleman sapphire dielectric resonator technique in the temperature range 15-94 K. High accuracy in measurements was achieved by using the multi-frequency Transmission Mode Q-Factor Technique. The Tl-1223 films exhibited low surface resistance comparable to YBCO films. When RF power levels were increased the onset of nonlinearity was observed at 0 dBm for all measured temperatures (17, 50, 70 and 84 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.

Epitaxial Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub y/ and Tl(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ thin films have been prepared on CeO/sub 2/ buffered sapphire substrate by an ex situ process with high reproducibility. Microstructure analysis of the surface of the Tl-2212 films showed well connected and smaller sized grains of 1 /spl mu/m or less. In the case of Tl-1223 film, plate-like crystals with pits and pin holes could be observed. The superconducting critical temperatures of both films are around 96 K. Critical current density measured by ac susceptibility technique is as high as 1 MA/cm/sup 2/ at 77 K. The microwave surface resistance of these films was measured over a wide range of temperatures by a HTS-sapphire-HTS resonator method at 38 GHz on films with area 10 mm /spl times/ 10 mm. The temperature dependence of surface resistance of Tl-2212 film follows very closely to that of the best YBCO films on MgO substrate, whereas the Tl-1223 exhibits slightly higher values resulting from an inferior surface morphology.

By very short time rf sputtering in certain deposition conditions we have grown three-dimensional Ag nanodots on the substrate prior to the growth of Tl-based superconducting films. These nanodots create pinning centers, leading to an increase in the critical current density about 10 times. From the frequency dependence of the critical current density we estimated also the activation energy for the flux jumps, which resulted to be several times higher. The rate of the thermally-activated flux jumps decreases several orders of magnitude. We suggest that our method can be used for the reduction of thermal noise in high-T/sub c/ dc SQUID's.

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.

Here we report on the successful synthesis of Er FeAsO-based superconductor by hydrogen doping using high-pressure (HP) synthesis technique. The Er FeAsO 1− y target pellets were sandwiched between La FeAsO 0.8 H 0.8 pellets, which act as a hydrogen source. Hydrogen released from the hydrogen source soaks into the target pellets during HP synthesis and the target pellets crystallizes with the tetragonal crystal structure with the lattice constants a = 3.8219 A and c = 8.2807 A. The hydrogen doped sample Er FeAsO 1− y (H) thus obtained shows superconducting critical temperatures ( T c ) of 44.5 K and 42.5 K determined by onsets of resistive and diamagnetic transitions, respectively.

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.

A change of electronic structure and chemical bond nature of high pressure synthesized (Cu,Tl)Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [(Cu,Tl)-1223] and TlBa/sub 2/Ca/sub 2/Cu/sub 3/O/sub 9-/spl delta// [Tl-1223] with reduction-annealing, which leads to a significant rise of their superconducting transition temperature T/sub c/ above 130 K, has been investigated by in-situ photoemission spectroscopy. Their intrinsic surface nature has been achieved by fracturing in vacuum. In (Cu,Tl)-1223, a Fermi edge is observed for the first time. Trivalent Tl and heavily over-doped states of Cu-O clusters are dominant in as-grown specimens of both systems. The annealing resulted in remarkable shifts of photoemission spectra of Ba and Tl core levels towards higher binding energy with an increase of the annealing temperature, which means a selective oxygen-depletion around their charge-reservoirs and a decrease of valency of Tl. In contrast, there are quite little shape-changes and movements in valence band spectra and core signals of Cu and O. These results reveal that, in the high temperature-treated specimens, major portions of the electrons left should be absorbed into the cations in their charge reservoir, which consequently should cause a much more moderate change of the hole-concentration of the CuO/sub 2/ planes. It suggests that the utilization of this charge-redistribution mechanism in over-doped specimens should be an advantageous way to optimize the electronic structure of the 1223 phases for high T/sub c/.

We have found that (Ba, K)Fe 2 As 2 superconductor (a transition temperature, T c ∼ 38 K) shows an inverse Iron isotope effect ( α Fe = −0.18 ± 0.03, where T c ∼ M − α Fe and M is the iron isotope mass), i.e. the sample containing the larger iron mass depicts higher T c . Systematic studies using three types of Fe-isotopes ( 54 Fe, natural Fe and 57 Fe) reveal a clear inverse shift on T c by measurements of temperature dependent magnetization and resistivity. The inverse isotope effect that is the first case in high- T c superconductors strongly suggests that superconducting mechanism of the iron-based system is not explained by conventional BCS theory mediated by phonons.

Two-component superconductivity based on the two-band superconductor has a functional topology such as an inter-band phase difference soliton ( i -soliton) to realize topological electronics (topolonics). Many gauge field theories are applied to investigate the topology of two-band superconductivity. To ease experimental and electronics applications, these theories should be refined. Weinberg–Salam theory and SU(2) (two-dimensional special unitary symmetry) gauge field theory are proper starting points. An effective extra force field because of the crystal structure and inter-band Josephson interaction, rather than spontaneous symmetry breaking, simplifies the conventional gauge field theory.

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.

The bi-axially-oriented thin films of Cu/sub 1-x/Tl/sub x/(Ba,Sr)/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10-y/(Cu/sub 1-x/Tl/sub x/-1223: x=0.2-1.0) were prepared by an amorphous phase epitaxy (APE) technique on SrTiO/sub 3/ substrates. The amorphous phase was deposited by sputtering on the substrate and annealed in a thallium atmosphere at /spl sim/885/spl deg/C for /spl sim/50 min. Their T/sub c/ values were 100-113 K and higher than those of the YBa/sub 2/Cu/sub 3/O/sub 7-y/ (Y-123, that is, Cu-1212) system. Their J/sub c/ values were 2/spl times/10/sup 7/ A/cm/sup 2/ (77 K, 0 T) and 4/spl times/10/sup 5/ A/cm/sup 2/ (77 K, 10 T) at their maximum for x/spl sim/0.5. The J/sub c/ values were higher than the maximum values of Y-123 thin films by a factor of 2. The high J/sub c/ values are consistent with the low superconducting anisotropy (/spl gamma/=/spl xi//sub ab///spl xi//sub c/=2.5-4) of Cu/sub 1-x/Tl/sub x/-1223 and its high T/sub c/.

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.

Pinning properties of superconducting material Tl-1223 thin film are investigated. Vortex images are compared between a pristine and high-energy gold ion irradiated sample. This comparison implies which pinning center is effective for this material between large pinning centers and columnar defect. Large pinning centers are better for this material.

The advent of user-oriented pulsed magnetic fields combined with low temperature capability has opened up a new perspective on low-dimensional, and in particular, organic conductor physics. Opportunities include: the application of Fermiology methods both below and above the magnetic field induced phase transitions in the density wave materials; the quest for the quantum limit in bulk low-dimensional materials; and the search for electron interaction effects in the quasi-one-dimensional limit. This paper will focus on several such applications and directions in the (BEDT—TTF) 2 MHg(SCN) 4 quasi-two-dimensional and (TMTSF) 2 ClO 4 quasi-one-dimensional organic conductors.

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.