134 research outputs found
Disorder Effect on the Vortex Pinning by the Cooling Process Control in the Organic Superconductor -(BEDT-TTF)Cu[N(CN)]Br
We investigate the influence of disorders in terminal ethylene groups of
BEDT-TTF molecules (ethylene-disorders) on the vortex pinning of the organic
superconductor -(BEDT-TTF)Cu[N(CN)]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 figure
Real space imaging of the metal - insulator phase separation in the band width controlled organic Mott system -(BEDT-TTF)Cu[N(CN)]Br
Systematic investigation of the electronic phase separation on macroscopic
scale is reported in the organic Mott system
-(BEDT-TTF)Cu[N(CN)]Br. Real space imaging of the phase
separation is obtained by means of scanning micro-region infrared spectroscopy
using the synchrotron radiation. The phase separation appears near the Mott
boundary and changes its metal-insulator fraction with the substitution ratio
in
-[(-BEDT-TTF)(-BEDT-TTF)]Cu[N(CN)]Br,
of which band width is controlled by the substitution ratio between the
hydrogenated BEDT-TTF molecule (-BEDT-TTF) and the deuterated one
(-BEDT-TTF). The phase separation phenomenon observed in this class of
organics is considered on the basis of the strongly correlated electronic phase
diagram with the first order Mott transition.Comment: 10 pages, 8 figure
Evidence for structural and electronic instabilities at intermediate temperatures in -(BEDT-TTF)X for X=Cu[N(CN)]Cl, Cu[N(CN)]Br and Cu(NCS): Implications for the phase diagram of these quasi-2D organic superconductors
We present high-resolution measurements of the coefficient of thermal
expansion of the quasi-twodimensional
(quasi-2D) salts -(BEDT-TTF)X with X = Cu(NCS), Cu[N(CN)]Br
and Cu[N(CN)]Cl. At intermediate temperatures (B), distinct anomalies
reminiscent of second-order phase transitions have been found at
K and 45 K for the superconducting X = Cu(NCS) and Cu[N(CN)]Br salts,
respectively. Most interestingly, we find that the signs of the uniaxial
pressure coefficients of are strictly anticorrelated with those of
. We propose that marks the transition to a spin-density-wave
(SDW) state forming on minor, quasi-1D parts of the Fermi surface. Our results
are compatible with two competing order parameters that form on disjunct
portions of the Fermi surface. At elevated temperatures (C), all compounds show
anomalies that can be identified with a kinetic, glass-like
transition where, below a characteristic temperature , disorder in the
orientational degrees of freedom of the terminal ethylene groups becomes frozen
in. We argue that the degree of disorder increases on going from the X =
Cu(NCS) to Cu[N(CN)]Br and the Cu[N(CN)]Cl salt. Our results
provide a natural explanation for the unusual time- and cooling-rate
dependencies of the ground-state properties in the hydrogenated and deuterated
Cu[N(CN)]Br salts reported in the literature.Comment: 22 pages, 7 figure
Impurity Effect on the In-plane Penetration Depth of the Organic Superconductors -(BEDT-TTF) ( = Cu(NCS) and Cu[N(CN)]Br)
We report the in-plane penetration depth of single
crystals -(BEDT-TTF) ( Cu(NCS) and Cu[N(CN)]Br) by
means of the reversible magnetization measurements under the control of
cooling-rate. In = Cu(NCS), as an
extrapolation toward = 0 K does not change by the cooling-rate within the
experimental accuracy, while is slightly reduced. On the other
hand, in = Cu[N(CN)]Br, indicates a distinct
increase by cooling faster. The different behavior of
on cooling-rate between the two salts is quantitatively explained in terms of
the local-clean approximation (London model), considering that the former salt
belongs to the very clean system and the later the moderate clean one. The good
agreement with this model demonstrates that disorders of ethylene-group in
BEDT-TTF introduced by cooling faster increase the
electron(quasiparticle)-scattering, resulting in shorter mean free path.Comment: 8 pages, 9 figure
Pairing Symmetry Competition in Organic Superconductors
A review is given on theoretical studies concerning the pairing symmetry in
organic superconductors. In particular, we focus on (TMTSF)X and
-(BEDT-TTF)X, in which the pairing symmetry has been extensively
studied both experimentally and theoretically. Possibilities of various pairing
symmetry candidates and their possible microscopic origin are discussed. Also
some tests for determining the actual pairing symmtery are surveyed.Comment: 16 pages, 8 figures, to be published in J. Phys. Soc. Jpn., special
issue on "Organic Conductors
The Dependence of the Superconducting Transition Temperature of Organic Molecular Crystals on Intrinsically Non-Magnetic Disorder: a Signature of either Unconventional Superconductivity or Novel Local Magnetic Moment Formation
We give a theoretical analysis of published experimental studies of the
effects of impurities and disorder on the superconducting transition
temperature, T_c, of the organic molecular crystals kappa-ET_2X and beta-ET_2X
(where ET is bis(ethylenedithio)tetrathiafulvalene and X is an anion eg I_3).
The Abrikosov-Gorkov (AG) formula describes the suppression of T_c both by
magnetic impurities in singlet superconductors, including s-wave
superconductors and by non-magnetic impurities in a non-s-wave superconductor.
We show that various sources of disorder lead to the suppression of T_c as
described by the AG formula. This is confirmed by the excellent fit to the
data, the fact that these materials are in the clean limit and the excellent
agreement between the value of the interlayer hopping integral, t_perp,
calculated from this fit and the value of t_perp found from angular-dependant
magnetoresistance and quantum oscillation experiments. If the disorder is, as
seems most likely, non-magnetic then the pairing state cannot be s-wave. We
show that the cooling rate dependence of the magnetisation is inconsistent with
paramagnetic impurities. Triplet pairing is ruled out by several experiments.
If the disorder is non-magnetic then this implies that l>=2, in which case
Occam's razor suggests that d-wave pairing is realised. Given the proximity of
these materials to an antiferromagnetic Mott transition, it is possible that
the disorder leads to the formation of local magnetic moments via some novel
mechanism. Thus we conclude that either kappa-ET_2X and beta-ET_2X are d-wave
superconductors or else they display a novel mechanism for the formation of
localised moments. We suggest systematic experiments to differentiate between
these scenarios.Comment: 18 pages, 5 figure
Late pleistocene sedimentation history of the Shirshov Ridge, Bering Sea
The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper
Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was
the Yukon–Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon
River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing
sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous
with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we
distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The
results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and
glacial cycles in Alaska
Suppression of superconductivity by non-magnetic disorder in organic superconductor -(BEDT-TTF)Cu(NCS)
The suppression of superconductivity by nonmagnetic disorder is investigated
systematically in the organic superconductor
-(BEDT-TTF)Cu(NCS). We introduce a nonmagnetic disorder arising
from molecule substitution in part with deuterated BEDT-TTF or BMDT-TTF for
BEDT-TTF molecules and molecular defects introduced by X-ray irradiation. A
quantitative evaluation of the scattering time is carried out
by de Haas-van Alphen (dHvA) effect measurement. A large reduction in with a linear dependence on is found in the
small-disorder region below 1 10
s in both the BMDT-TTF molecule-substituted and X-ray-irradiated
samples. The observed linear relation between and is in agreement with the Abrikosov-Gorkov (AG) formula, at least in the
small-disorder region. This observation is reasonably consistent with the
unconventional superconductivity proposed thus far for the present organic
superconductor. A deviation from the AG formula, however, is observed in the
large-disorder region above 1 10
s, which reproduces the previous transport study (J. G. Analytis {\it et
al.}: Phys. Rev. Lett. {\bf 96} (2006) 177002). We present some interpretations
of this deviation from the viewpoints of superconductivity and the inherent
difficulties in the evaluation of scattering time.Comment: 11 pages, 6 figure
Radiochronological age of a uranium metal sample from an abandoned facility
A piece of scrap uranium metal bar buried in the dirt floor of an old, abandoned metal rolling mill was analyzed using multi-collector inductively coupled plasma mass spectroscopy (MC-ICP-MS). The mill rolled uranium rods in the 1940s and 1950s. Samples of the contaminated dirt in which the bar was buried were also analyzed. The isotopic composition of uranium in the bar and dirt samples were both the same as natural uranium, though a few samples of dirt also contained recycled uranium; likely a result of contamination with other material rolled at the mill. The time elapsed since the uranium metal bar was last purified can be determined by the in-growth of the isotope {sup 230}Th from the decay of {sup 234}U, assuming that only uranium isotopes were present in the bar after purification. The age of the metal bar was determined to be 61 years at the time of this analysis and corresponds to a purification date of July 1950 {+-} 1.5 years
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