Phase separation in the vicinity of the surface of κ\kappa-(BEDT-TTF)2_2Cu[N(CN)2_2]Br by fast cooling

Partial suppression of superconductivity by fast cooling has been observed in the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br by two means: a marked sample size effect on the magnetic susceptibility and direct imaging of insulating regions by scanning microregion infrared reflectance spectroscopy. Macroscopic insulating regions are found in the vicinity of the crystalline surface after fast cooling, with diameters of 50--100 $\mu$m and depths of a few $\mu$m. The very large in-plane penetration depth reported to date ($\sim$ 24--100 $\mu$m) can be explained by the existence of the insulating regions.Comment: Several rhetoric alternations to avoid misleadings. 6 pages, 3 figures. to be publihsed in Phys. Rev.

Field-induced staggered magnetic moment in the quasi-two-dimensional organic Mott insulator κ\kappa-(BEDT-TTF)2_{2}Cu[N(CN)2_{2}]Cl

We investigated the magnetism under a magnetic field in the quasi-two-dimensional organic Mott insulator $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl through magnetization and $^{13}$C-NMR measurements. We found that in the nominally paramagnetic phase (i.e., above N\'eel temperature) the field-induced local moments have a staggered component perpendicular to the applied field. As a result, the antiferromagnetic transition well defined at a zero field becomes crossover under a finite field. This unconventional behavior is qualitatively reproduced by the molecular-field calculation for Hamiltonian including the exchange, Dzyaloshinsky-Moriya (DM), and Zeeman interactions. This calculation also explains other unconventional magnetic features in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl reported in the literature. The present results highlight the importance of the DM interaction in field-induced magnetism in a nominally paramagnetic phase, especially in low-dimensional spin systems.Comment: 11 pages, 12 figures, selected for Editors' Suggestion

Recent developments in the determination of the amplitude and phase of quantum oscillations for the linear chain of coupled orbits

De Haas-van Alphen oscillations are studied for Fermi surfaces (FS) illustrating the model proposed by Pippard in the early sixties, namely the linear chain of orbits coupled by magnetic breakdown. This FS topology is relevant for many multiband quasi-two dimensional (q-2D) organic metals such as $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ and $\theta$-(BEDT-TTF)$_4$CoBr$_4$(C$_6$H$_4$Cl$_2$) which are considered in detail. Whereas the Lifshits-Kosevich model only involves a first order development of field- and temperature-dependent damping factors, second order terms may have significant contribution on the Fourier components amplitude for such q-2D systems at high magnetic field and low temperature. The strength of these second order terms depends on the relative value of the involved damping factors, which are in turns strongly dependent on parameters such as the magnetic breakdown field, effective masses and, most of all, effective Land\'{e} factors. In addition, the influence of field-dependent Onsager phase factors on the oscillation spectra is considered.Comment: arXiv admin note: text overlap with arXiv:1304.665

Periodic Oscillations of Josephson-Vortex Flow Resistance in Oxygen-Deficient Y1Ba2Cu3Ox

We measured the Josephson vortex flow resistance as a function of magnetic field applied parallel to the ab-planes using annealed Y1Ba2Cu3Ox intrinsic Josephson junctions having high anisotropy (around 40) by oxygen content reduction. Periodic oscillations were observed in magnetic fields above 45-58 kOe, corresponding to dense-dilute boundary for Josephson vortex lattice. The observed period of oscillations, agrees well with the increase of one fluxon per two junctions ($H_{p}$\textit{=$\Phi$}$_{0}$\textit{/2Ls}), may correspond to formation of a triangular lattice of Josephson vortices as has been reported by Ooi et al. for highly anisotropic (larger than 200) Bi-2212 intrinsic Josephson junctions.Comment: 5 pages, 4 figure

Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-2D organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br: quest for intrinsic scattering contributions

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. According to their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous rho(T) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a strongly temperature-dependent behavior at T > T* to an only weakly T-dependent rho(T) at T < T* is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, (iii) both variants reveal a rho(T) proportional to AT^2 dependence at low temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent coefficients A and upper bounds for the T^2 behavior measured by T_0. The latter result is inconsistent with the T^2 dependence originating from coherent Fermi-liquid excitations.Comment: 8 pages, 6 figure

Theory of the beta-type Organic Superconductivity under Uniaxial Compression

We study theoretically the shift of the superconducting transition temperature (Tc) under uniaxial compression in beta-type organic superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order to clarify the electron correlation, the spin frustration and the effect of dimerization. The transfer integrals are calculated by the extended Huckel method assuming the uniaxial strain and the superconducting state mediated by the spin fluctuation is solved using Eliashberg's equation with the fluctuation-exchange approximation. The calculation is carried out on both the dimerized (one-band) and nondimerized (two-band) Hubbard models. We have found that (i) the behavior of Tc in beta-(BEDT-TTF)2I3 with a stronger dimerization is well reproduced by the dimer model, while that in weakly dimerized beta-BDA-TTP salts is rather well reproduced by the two-band model, and (ii) the competition between the spin frustration and the effect induced by the fluctuation is important in these materials, which causes nonmonotonic shift of Tc against uniaxial compression.Comment: 18 pages, 16 figures, 2 tabl

Transport criticality of the first-order Mott transition in a quasi-two-dimensional organic conductor, κ\kappa-(BEDT-TTF)2_{2}Cu[N(CN)2_{2}]Cl

An organic Mott insulator, $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, was investigated by resistance measurements under continuously controllable He gas pressure. The first-order Mott transition was demonstrated by observation of clear jump in the resistance variation against pressure. Its critical endpoint at 38 K is featured by vanishing of the resistive jump and critical divergence in pressure derivative of resistance, $|\frac{1}{R}\frac{\partial R}{\partial P}|$, which are consistent with the prediction of the dynamical mean field theory and have phenomenological correspondence with the liquid-gas transition. The present results provide the experimental basis for physics of the Mott transition criticality.Comment: 4 pages, 5 figure

Coronal Temperature Diagnostic Capability of the Hinode/X-Ray Telescope Based on Self-Consistent Calibration

The X-Ray Telescope (XRT) onboard the Hinode satellite is an X-ray imager that observes the solar corona with unprecedentedly high angular resolution (consistent with its 1" pixel size). XRT has nine X-ray analysis filters with different temperature responses. One of the most significant scientific features of this telescope is its capability of diagnosing coronal temperatures from less than 1 MK to more than 10 MK, which has never been accomplished before. To make full use of this capability, accurate calibration of the coronal temperature response of XRT is indispensable and is presented in this article. The effect of on-orbit contamination is also taken into account in the calibration. On the basis of our calibration results, we review the coronal-temperature-diagnostic capability of XRT

Nature of bonding and electronic structure in MgB2, a boron intercalation superconductor

Chemical bonding and electronic structure of MgB2, a boron-based newly discovered superconductor, is studied using self-consistent band structure techniques. Analysis of the transformation of the band structure for the hypothetical series of graphite - primitive graphite - primitive graphite-like boron - intercalated boron, shows that the band structure of MgB2 is graphite-like, with pi-bands falling deeper than in ordinary graphite. These bands possess a typically delocalized and metallic, as opposed to covalent, character. The in-plane sigma-bands retain their 2D covalent character, but exhibit a metallic hole-type conductivity. The coexistence of 2D covalent in-plane and 3D metallic-type interlayer conducting bands is a peculiar feature of MgB2. We analyze the 2D and 3D features of the band structure of MgB2 and related compounds, and their contributions to conductivity.Comment: 4 pages in revtex, 3 figures in 4 separate EPS file