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When a superconductor has multiple bands and there is a phase difference of π between Δ 1 and Δ 2 where Δ i is a gap parameter on a band indexed by i , the junction formed with another superconductor rotates the relative phase between Δ 1 and Δ 2 near the interface to lower the Josephson coupling energy. This means that the order parameter is no longer a real number but is a complex number. This phase-instability is demonstrated with a simple model describing the two-band superconductor in terms of the Ginzburg–Landau theory. Due to this effect there is a phase shift between the two bulk superconductors forming the junction. Using a quantum interferometer we will be able to detect this shift experimentally.

Specific heat anomaly was studied on Cu x Ba 2 Ca 3 Cu 4 O y . ΔC/ T c , a height of peak of the specific heat at the superconducting transition ( T c ) divided by T c , is less than 60% of that of optimally doped YBa 2 Cu 3 O 7-δ . This suggests that a large density of states (DOS) remains to prohibit the creation of Bardeen-Cooper-Schrieffer (BCS) pairs at T c . It is much larger than the residual DOS at 0 K. We interpret that the “ missing DOS ” start to become BCS pairs at a temperature lower than T c , as proposed by an NMR study [Phys. Rev. B 61 (2000) 9707], though we have not yet seen clear evidence of this.

Electron spin resonance (ESR) study of halogen-doped C 60 was carried out by varying the doping conditions such as concentration and annealing time. Temperature dependence of the ESR linewidth of C 60 Br x different from that of K x C 60 was obtained. The electronic properties of C 60 Br x and K x C 60 were discussed.

Frequency shift and broadening of a soft mode phonon in MEM(TCNQ) 2 were clearly observed by far infrared spectroscopy below 30 cm -1 , as increasing temperature from 4 K to a spin-Peierls transition temperature. The soft mode phonon frequency at 4 K is 8.5 cm -1 . Taking account of the frequency of 8.5 cm -1 and its mode of motion, it is considered that the dynamical displacement of the soft mode phonon lies almost in a TCNQ plane which contains the static shift associated with the spin-Peierls transition. In addition, two holding phonon bands, whose frequencies with 19.5 and 24.5 cm -1 do not vary with the temperature, were clearly observed only below the spin-Peierls transition temperature.

Chiral superconductivity emerges in three-band superconductors when interband interactions are much smaller than intraband interactions and either one or three interband interactions are positive. The chiral ground states are demonstrated using the phenomenological Ginzburg–Landau free energy modeling of three-component superconductivity based on a three-band superconductor. The origin of chiral superconductivity in three-band superconductors is different from that of the conventional time reversal symmetry breaking states such as triplet superconductivity and d + i s wave.

Recently reported inverse isotope effect is investigated on the basis of the two-band and two-channel attractive model by using the weak coupling Bardeen–Cooper–Schrieffer (BCS) scheme. If we assume the several attractive or repulsive interactions for Cooper pairs, which works in different energy ranges, the isotope exponent α takes values even in the range of α 0.5, depending on the strength and the range of interactions. We apply the theory to recently observed remarkable inverse isotope effect α≈-0.18 for the Fe-pnictide (Ba,K)Fe 2 As 2 superconductor. The large inverse isotope effect naturally originates from the competition between the magnetically induced pair interaction and the inter-band electron–phonon coupling. It evidently indicates that the superconductivity in Fe pnictides arises from the magnetically induced attractive interaction.

The emergence of superconductivity in Fe-based oxypnictides has attracted much attention. To determination of the cause of the inhibition of superconductivity in the nonsuperconducting oxypnictides is crucial for further increasing superconducting transition temperature. Non-superconducting oxypnictides can also be useful for the future development of the Josephson device using these materials. Thus, the transport properties of non-superconducting oxypnictides should be studied. A typical phenomenon indicative of the temperature dependence of resistivity is the appearance of a peak at around 150K. The formation of a spin density wave (SDW) is strongly suggested at around this temperature. Below this temperature, the reported temperature dependence of resistivity varies. Some of reported ones show logarithmic divergence and others show no divergence. The common tendency is a large magnetoresistance below the temperature giving the peak of resistance. It remains to be determined which tendency reflects an intrinsic property. To answer this question, we prepare two types of samples with the same nominal composition and almost the same synthesis conditions. With these samples, we measured magnetoresistance and conclude that both tendencies reflect the true electronic property of non-superconducting oxypnictides. Polycrystalline samples were prepared by the highpressure synthesis method described in ref. 3. We selected two samples designated #080515 and #080520 for the transport study. The lattice parameters of #080515 and #080520 are a 1⁄4 3:967 A and c 1⁄4 8:570 A, a 1⁄4 3:967 A and c 1⁄4 8:576 A, respectively. The oxygen content of #080515 estimated by neutron diffraction analysis was NdFeAsO0:95, which is larger than the nominal value. The difference is probably due to the possible oxidation of the starting material (Nd, Fe, or As) or oxygen gain during the synthesis. Although the oxygen content of the sample #080520 was not estimated, judging from the lattice parameters, the difference in oxygen content between the two samples is small. For resistivity measurement, we adopted the four-terminal method. Using a Quantum Design Physical Property Measurement System (PPMS), we applied a magnetic field of up to 14 T perpendicular to the electric current, which is a 7.5Hz square wave excitation of 1mA. The voltage drop parallel to the excitation current was measured, and resistance was calculated. We show the X-ray diffraction (XRD) patterns of #080515 and #080520 in Fig. 1. These two samples are almost identical in their XRD patterns. Figure 2 shows the temperature dependence of resistivity. In contrast to that shown in the XRD pattern, the absolute values and temperature dependences of the resistivities of these two samples are markedly different. The difference results from the small difference in oxygen content between the samples. The resistivity of #080515 is 60% larger than that of #080520 at 300K. Although the resistivity of #080515 has a divergent tendency at a low temperature, that of #080520 show no clear upturn at a low temperature. In both samples, we show the peak of the resistivity at around 140K. Above this temperature, the temperature dependences of the resistivities of the two samples are similar. Below 140K, magnetoresistance becomes prominent, although the position of the peak itself is insensitive to magnetic field. We plot ðRH R0Þ=ð 0H R0Þ, where 0H is the magnetic field, RH is the resistance under the magnetic field, and R0 is the resistance under a zero magnetic field, in Fig. 1. ðRH R0Þ=ð 0H R0Þ is almost qualitatively equivalent to each other within an error of 10%. The general tendency of the magnetoresistance obtained by this measurement is consistent with those in other reports. There is a peak at around 150K and below it an abnormal magnetoresistance is observed. Two typical temperature dependences below 150K are reproduced. #080515 shows a logarithmic-like upturn at a low temperature but #080520 shows no such an upturn. Our major finding is the common temperature dependence of ðRH R0Þ=ð 0H R0Þ in these two samples. Because magnetoresistance sharply starts at around 150K near the peak position, it is easy to imagine that it relates to the origin of the 150K anomaly. This anomaly is usually interpreted as an indication of the emergence of SDW triggered by structural phase transition. Anti-ferromagnetic instability was also pointed out by band calculation. In te ns ity [ ar bi tr ar y un it]

We have proposed a T c vs n relationship for multilayered high- T c superconductors from the analysis of HgBa 2 Ca n -1 Cu n O y grain-aligned samples containing n =6 to 16 phases. The sample showed a large and sharp superconducting transition at 105 K in its susceptibility vs temperature curve. This means that multilayered HgBa 2 Ca n -1 Cu n O y can maintain a high T c at least up to n ≈16, otherwise many transitions would have been observed. It is generally believed that the highest T c decreases with n for n ≥4–5 for multilayered superconductors. We conclude that the highest T c is almost constant above about n =5 if the factors reducing T c , such as disorder, are eliminated for multilayered superconductors. This behavior can be explained using an inhomogeneous charge distribution model in multilayered cuprates.

We report the anomalous T c behavior in a five-layered compound of the cuprate superconductor (Cu,C)Ba 2 Ca 4 Cu 5 O y . The T c decreases once from 94.7 K (as-synthesized) to 93.3 K (annealed at 460 °C under N 2 flow), and then increases to 99.0 K (annealed at 530 °C under N 2 flow). We argue that this T c change reflects the CuO 2 planes governing the bulk T c , shifting from the three inner CuO 2 planes to the two outer CuO 2 planes. A model to explain this behavior is proposed, drawing two bell-shaped curves against the oxygen content (doping state), and the T c values change along the curves from that of the inner CuO 2 plane to that of outer one.