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We investigated the nature of the quasi-particle state in the vortex core by means of the flux-flow Hall effect measurements at 15.8 GHz. We measured the flux-flow Hall effect in cuprate superconductors, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{y}$ and YBa$_{2}$Cu$_{3}$O$_{y}$ single crystals, whose equilibrium $B$-$T$ phase diagrams were different. As a result, we found that the Hall angle is independent of the magnetic field, and reaches an order of unity at low temperatures in BSCCO. However, in YBCO, the angle increases with increasing magnetic field even at low temperatures. We understood that this difference in the magnetic field dependence of the Hall angle is due to the difference in the influence of the pinning, which originated from the difference in the vortex state (liquid vs. solid) between the two materials. However, as a common feature, both materials showed a large tangent of the Hall angle at low temperatures, which was larger by an order of magnitude than those obtained in the effective viscous drag coefficient measurements. We discussed the origin of the discrepancy both in terms of the possible nonlinearity of the viscous drag force and possible hidden dissipation mechanisms. The unexpectedly large Hall angle of the vortex motion in cuprates revealed in our flux-flow Hall effect study poses a serious question on the fundamental understanding of the motion of the quantized vortex in superconductors, and it deserves further investigation.

Compelling efforts to improve the critical temperature ($T_{c}$) of superconductors have been made through high-pressure application. Understanding the underlying mechanism behind such improvements is critically important, however, much remains unclear. Here we studied ortho-III YBa$_{2}$Cu$_{3}$O$_{6.73}$ (YBCO) using x-ray scattering under hydrostatic-pressure (HP) up to ~6.0 GPa. We found the reinforced oxygen order (OO) of YBCO under HP, revealing an oxygen rearrangement in the Cu-O layer, which evidently shows the charge transfer phenomenon between the CuO$_{2}$ plane and Cu-O layer. Concurrently, we also observed no disorder-pinned charge density wave (CDW) signature in CuO$_{2}$ plane under HP. This indicates that the oxygen rearrangement modifies the quenched disorder state in the CuO$_{2}$ plane. Using these results, we appropriately explain why pressure-condition can achieve higher $T_{c}$ compared with the optimal $T_{c}$ under ambient pressure in YBa$_{2}$Cu$_{3}$O$_{6+x}$. As an implication of these results, finally, we have discussed that the change in disorder could make it easier for YBa$_{2}$Cu$_{3}$O$_{6+x}$ to undergo a transition to the nematic order under an external magnetic field., Comment: 24 pages, 6 figures

A central issue in the quest to understand the superconductivity in cuprates is the nature and origin of the pseudogap state, which harbours anomalous electronic states such as Fermi arc, charge density wave (CDW), and $d$-wave superconductivity. A fundamentally important, but long-standing controversial problem has been whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature $T^*$. Electronic nematicity, a fourfold ($C_4$) rotational symmetry breaking, has emerged as a key feature inside the pseudogap regime, but the presence or absence of a nematic phase transition and its relationship to the pseudogap remain unresolved. Here we report thermodynamic measurements of magnetic torque in the underdoped regime of orthorhombic YBa$_2$Cu$_3$O$_y$ with a field rotating in the CuO$_2$ plane, which allow us to quantify magnetic anisotropy with exceptionally high precision. Upon entering the pseudogap regime, the in-plane anisotropy of magnetic susceptibility increases after exhibiting a distinct kink at $T^*$. Our doping dependence analysis reveals that this anisotropy is preserved below $T^*$ even in the limit where the effect of orthorhombicity is eliminated. In addition, the excess in-plane anisotropy data show a remarkable scaling behaviour with respect to $T/T^*$ in a wide doping range. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which is distinct from the CDW transition that accompanies translational symmetry breaking. This suggests that nematic fluctuations near the pseudogap phase boundary have a potential link to the strange metallic behaviour in the normal state, out of which high-$T_c$ superconductivity emerges., Comment: 6 pages, 4 figures. An updated manuscript with SI will appear in Nature Physics

To investigate the origin of the enhanced Tc ({\approx} 110 K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+{\delta} (Bi-2223), its underdoped single crystals are a critical requirement. Here, we demonstrate the first successful in-plane resistivity measurements of heavily underdoped Bi-2223 (zero-resistivity temperatures {\approx} 20~35 K). Detailed crystal growth methods, the annealing process, as well as X-ray diffraction (XRD) and magnetic susceptibility measurement results are also reported., Comment: 4 pages, 4 figures, 27th International Symposium on Superconductivity, ISS 2014, to appear in Physics Procedia

The evolution of the electronic state through the reduction annealing has been investigated in electron-doped Pr_1.3-x_La_0.7_Ce_x_CuO_4+delta_ (x=0.10) single crystals with the so-called T' structure. From the ab plane and c axis electrical resistivity measurements in magnetic fields, it has been found that, through the reduction annealing, the strongly localized state of carriers accompanied by the antiferromagnetic (AF) pseudogap in the as-grown crystal changes to a metallic state bringing about the Kondo effect without AF pseudogap and to a superconducting state. These results are able to be understood in terms of a model based on the strong electron correlation. The complete removal of excess oxygen in the T'-cuprates is expected to result in the appearance of superconductivity in a wide range of the Ce concentration including the parent compound of x=0., Comment: 5 pages, 3 figures, J. Phys. Soc. Jpn. in press

We have investigated the dielectric response in an antiferromagnetic dimer-Mott insulator beta'-(BEDT-TTF)2ICl2 with square lattice, compared to a spin liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the dielectric constant shows a peak structure obeying Curie-Weiss law with strong frequency dependence. We found an anisotropic ferroelectricity by pyrocurrent measurements, which suggests the charge disproportionation in a dimer. The ferroelectric actual charge freezing temperature is related to the antiferromagnetic interaction, which is expected to the charge-spin coupled degrees of freedom in the system., Comment: 5 pages, 4 figures, to be published in Phys. Rev. B

We report detailed studies of the upper critical field and low-temperature specific heat in the two-gap superconductor Lu$_{2}$Fe$_{3}$Si$_{5}$. The anisotropy of the upper critical field suggests that the active band is quasi-one-dimensional. Low-temperature specific heat in magnetic fields reveals that the virtual $H_{c2}$ in the passive band is almost isotropic. These results strongly indicate that the two bands have two different anisotropies, similar to the typical two-gap superconductor MgB$_{2}$, and their interplay may be essential to the two-gap superconductivity in Lu$_{2}$Fe$_{3}$Si$_{5}$., Comment: 5 pages, 5 figures

We have performed inelastic neutron scattering measurements on the single-layer cuprate Bi2+xSr2-xCuO6+y (Bi2201) with x=0.2, 0.3, 0.4 and 0.5, a doping range that spans the spin-glass (SG) to superconducting (SC) phase boundary. The doping evolution of low energy spin fluctuations was found to be characterized by a change of incommensurate modulation wave vector from the tetragonal [110] to [100]/[010] directions, while maintaining a linear relation between the incommensurability and the hole concentration, delta p. In the SC regime, the spectral weight is strongly suppressed below 4 meV. Similarities and differences in the spin correlations between Bi2201 and the prototypical single-layer system La2-xSrxCuO4 are discussed., Comment: 5 page,4 figures

We report electrical resistivity measurements and neutron diffraction studies under magnetic fields of Bi$_{1.75}$Pb$_{0.35}$Sr$_{1.90}$Cu$_{0.91}$Fe$_{0.09}$O$_{6+y}$, in which hole carriers are overdoped. This compound shows short-range incommensurate magnetic correlation with incommensurability $\delta=0.21$, whereas a Fe-free compound shows no magnetic correlation. Resistivity shows an up turn at low temperature in the form of $ln(1/T)$ and shows no superconductivity. We observe reduction of resistivity by applying magnetic fields (i.e., a negative magnetoresistive effect) at temperatures below the onset of short-range magnetic correlation. Application of magnetic fields also suppresses the Fe induced incommensurate magnetic correlation. We compare and contrast these observations with two different models: 1) stripe order, and 2) dilute magnetic moments in a metallic alloy, with associated Kondo behavior. The latter picture appears to be more relevant to the present results., Comment: 7 pages, 5 figures

We have performed high-resolution angle-resolved photoemission spectroscopy on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining the Fermi surface and energy band dispersion, we solve long-standing controversial issues as to the anomalous electronic states of Y-based high-Tc cuprates. We unambiguously identified surface-bilayer-derived bonding and antibonding bands, together with their bulk counterparts. The surface bands are highly overdoped (hole concentration x = 0.29), showing no evidence for the gap opening or the dispersion anomaly in the antinodal region, while the bulk bands show a clear dx2-y2-wave superconducting gap and the Bogoliubov quasiparticle-like behavior with a characteristic energy scale of 50-60 meV indicative of a strong electron-boson coupling in the superconducting state. All these results suggest that the metallic and superconducting states coexist at the adjacent bilayer of Y123 surface., Comment: Accepted for publication in Phys. Rev. B

We have performed detailed magnetic and thermal hysteresis experiments in the normal-state magnetization of La1.85Sr0.15CuO4 single crystal. Using a combination of in-field and in-zero-magnetic-field measurements at different stages of thermal history of the sample, we identified subtle effects associated with the presence of magnetic signatures which resemble those below the superconducting transition temperature (Tc=36 K) but survive up to 250 K., Comment: One file including text and figures

We report detailed bulk magnetization measurements of the normal state in the high transition temperature (high-Tc) superconductor La2-xSrxCuO4. A magnetic order in the form of hysteresis in the low field magnetization was observed at temperatures well above Tc but below the pseudogap temperature. The order arises from the interaction of magnetic domains, and the doping (x) dependence of its onset and strength broadly follows that of Tc(x)., Comment: More data and discussion, better signal to noise

We have measured the thermal conductivity parallel to the a-axis of the Zn-free and 1% Zn-substituted SrCu$_{2-x}$Zn$_x$(BO$_3$)$_2$ in magnetic fields up to 14 T, in order to examine the thermal conductivity due to the multi-triplet magnons. It has been found that the thermal conductivity peak observed in the spin gap state is suppressed by the substitution of Zn for Cu in high magnetic fields above 6 T, while it is not changed in low magnetic fields below 6 T. The results suggest that the thermal conductivity peak in the spin-gap state of SrCu$_2$(BO$_3$)$_2$ is composed of not only thermal conductivity due to phonons but also that due to the multi-triplet magnons in high fields above 6 T., Comment: 7 pages, 2 figures

We have succeeded in growing large-size single-crystals of Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$ with $0 \le x \le 1.67$ and measured the magnetic susceptibility, specific heat and magnetization curve, in order to study the magnetic ground state in the edge-sharing CuO$_2$ chain as a function of hole concentration and magnetic field. In $0 \le x \le 1.3$, it has been found that an antiferromagnetically ordered phase with the magnetic easy axis along the b-axis is stabilized and that a spin-flop transition occurs by the application of magnetic fields parallel to the b-axis. The antiferromagnetic transition temperature decreases with increasing $x$ and disappears around $x =$ 1.4. Alternatively, a spin-glass phase appears around $x = 1.5$. At $x = 1.67$ where the hole concentration is $\sim$ 1/3 per Cu, it appears that a spin-gap state is formed owing to the formation of spin-singlet pairs. No sign of the coexistence of an antiferromagnetically ordered state and a spin-gap one suggested in Ca$_{1-x}$CuO$_2$ has been found in Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$., Comment: 13 pages, 12 figures, 1 table

We have measured the thermal conductivity of a TlCuCl$_3$ single crystal in magnetic fields up to 14 T. It has been found that the temperature dependence of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which is suppressed by the application of magnetic fields up to 7 T. The peak is concluded to be attributable to the enhancement of the thermal conductivity due to phonons because of the formation of a spin-gap state. In high magnetic fields above 7 T, on the other hand, another sharp peak appears around 4 K and this is enhanced with increasing magnetic field. This peak is regarded as being attributable to the enhancement of the thermal conductivity due to magnons and/or phonons because of the drastic extension of the mean free path of magnons and/or phonons in the Bose-Einstein condensed state., Comment: 4 pages, 4 figures

We have measured the thermal conductivity in the $ab$-plane of La$_{2-x}$Sr$_x$CuO$_4$ ($x =$ 0.10, 0.115, 0.13) in magnetic fields up to 14 T parallel to the c-axis and also parallel to the $ab$-plane. By the application of magnetic fields parallel to the c-axis, the thermal conductivity has been found to be suppressed at low temperatures below the temperature $T_\kappa$ which is located above the superconducting transition temperature and is almost independent of the magnitude of the magnetic field. The suppression is marked in $x =$ 0.10 and 0.13, while it is small in $x =$ 0.115. Furthermore, no suppression is observed in the 1 % Zn-substituted La$_{2-x}$Sr$_x$Cu$_{0.99}$Zn$_{0.01}$O$_4$ with $x =$ 0.115. Taking into account the experimental results that the temperature dependence of the relative reduction of the thermal conductivity is quite similar to the temperature dependence of the intensity of the incommensurate magnetic Bragg peak corresponding to the static stripe order and that the Zn substitution tends to stabilize the static order, it is concluded that the suppression of the thermal conductivity in magnetic fields is attributed to the development of the static stripe order. The present results suggest that the field-induced magnetic order in La$_{2-x}$Sr$_x$CuO$_4$ originates from the pinning of the dynamical stripes of spins and holes by vortex cores., Comment: 8 pages, 5 figures. Fig. 1 has been modified

We report the transport properties in the vortex liquid states induced by quantum fluctuations at low temperature in the layered organic superconductor kappa-(BEDT-TTF)_{2} Cu(NCS)_{2}. A steep drop of the resistivity observed below about 1 K separates the liquid state into two regions. In the low resistance state at lower temperature, a finite resistivity with weak temperature dependence persists down to 100 mK at least. The finite resistivity in the vortex state at T ~= 0 K indicates the realization of quantum vortex liquid assisted by the strong quantum fluctuations instead of the thermal one. A possible origin for separating these liquid states is a remnant vortex melting line at the original position, which is obscured and suppressed by the quantum fluctuations. A non-linear behavior of the in-plane resistivity appears at large current density in only the low resistance state, but not in another vortex liquid state at higher temperature, where the thermal fluctuations are dominant. The transport properties in the low resistance state are well understood in the vortex slush concept with a short-range order of vortices. Thus the low resistance state below 1 K is considered to be a novel quantum vortex slush state., Comment: 7 pages, 5 figures

We investigate the influence of disorders in terminal ethylene groups of BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br. Magnetization measurements are performed under different cooling-processes. The second peak in the magnetization hysteresis curve is observed for all samples studied, and the hysteresis width of the magnetization becomes narrower by cooling faster. In contradiction to the simple pinning effect of disorder, this result shows the suppression of the vortex pinning force by introducing more ethylene-disorders. The ethylene-disorder domain model is proposed for explaining the observed result. In the case of the system containing a moderate number of the ethylene-disorders, the disordered molecules form a domain structure and it works as an effective pinning site. On the contrary, an excess number of the ethylene-disorders may weaken the effect of the domain structure, which results in the less effective pinning force on the vortices., Comment: 6 pages, 6 figures

We report micro-Raman measurements that reveal a low-energy soft mode at 16-19 meV in MgB_2 which becomes sharper in the superconducting state. With a boson spectrum based on this mode we reproduce, within Eliashberg theory, the T_c, the T-shape of the NMR relaxation rate as well as unusual dip-hump features in available tunneling data. We compare MgB_2 to cuprates, borocarbides and UPd_2Al_3, where we identify soft bosonic modes at energies which scale with their T_c suggesting that all these materials are very strong coupling superconductors to soft bosonic modes, that are possibly precursors of quantum instabilities., Comment: 4 pages and 5 figures, Submitted to PRL

Dynamics of vortices reflects the electronic states of quasiparticles in the core. To understand this, we investigated the following three issues. (1) We investigated the complex surface impedance, Zs, of YBa2Cu3Oy as a function of magnetic field, H. The total features were well expressed by the Coffey-Clem model. From the data, we estimated the viscosity and pinning frequency, which were found to be independent of frequency. In particular, the obtained viscosity definitely shows that the core of vortex of YBa2Cu3Oy is moderately clean. This result suggests that new physics will show up, for the physics of quantum moderately clean vortex core is unknown at all. (2) An anomaly found in the surface reactance at the first order transition (FOT) of vortex lattice was investigated in Bi2Sr2CaCu2Oy with various doping levels. As a result, the anomaly was found only in the samples exhibiting the FOT. On the other hand, we did not observe the anomaly in YBa2Cu3Oy. These suggest that the anomaly is due to the change in the electronic states of the vortices characteristic of materials with very strong anisotropy. (3) We measured H dependence of both the thermal conductivity \kappa(H) and Zs(H) in exactly the same pieces of crystal. We could not find any anomaly in Zs(H) even at the onset of the plateau. This result suggests that the origin of the plateau in \kappa(H) is not a drastic phase transition but is rather gradual crossover., Comment: 6 pages, 5 figures, Proceedings of Plasma2000(Sendai), to be published in Physica C