471 research outputs found
Phase separation in the vicinity of the surface of -(BEDT-TTF)Cu[N(CN)]Br by fast cooling
Partial suppression of superconductivity by fast cooling has been observed in
the organic superconductor -(BEDT-TTF)Cu[N(CN)]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 m and
depths of a few m. The very large in-plane penetration depth reported to
date ( 24--100 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 -(BEDT-TTF)Cu[N(CN)]Cl
We investigated the magnetism under a magnetic field in the
quasi-two-dimensional organic Mott insulator
-(BEDT-TTF)Cu[N(CN)]Cl through magnetization and
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
-(BEDT-TTF)Cu[N(CN)]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
-(BEDT-TTF)Cu(NCS) and
-(BEDT-TTF)CoBr(CHCl) 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 (\textit{=}\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, -(BEDT-TTF)Cu[N(CN)]Cl
An organic Mott insulator, -(BEDT-TTF)Cu[N(CN)]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, , 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
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