593 research outputs found
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
Pulse profile modelling of thermonuclear burst oscillations - II: Handling variability
Pulse profile modelling is a relativistic ray-tracing technique that can be used to infer masses, radii, and geometric parameters of neutron stars. In a previous study, we looked at the performance of this technique when applied to thermonuclear burst oscillations from accreting neutron stars. That study showed that ignoring the variability associated with burst oscillation sources resulted in significant biases in the inferred mass and radius, particularly for the high count rates that are nominally required to obtain meaningful constraints. In this follow-on study, we show that the bias can be mitigated by slicing the bursts into shorter segments where variability can be neglected, and jointly fitting the segments. Using this approach, the systematic uncertainties on the mass and radius are brought within the range of the statistical uncertainty. With about 106 source counts, this yields uncertainties of approximately 10 per cent for both the mass and radius. However, this modelling strategy requires substantial computational resources. We also confirm that the posterior distributions of the mass and radius obtained from multiple bursts of the same source can be merged to produce outcomes comparable to that of a single burst with an equivalent total number of counts
Electronic Collective Modes and Superconductivity in Layered Conductors
A distinctive feature of layered conductors is the presence of low-energy
electronic collective modes of the conduction electrons. This affects the
dynamic screening properties of the Coulomb interaction in a layered material.
We study the consequences of the existence of these collective modes for
superconductivity. General equations for the superconducting order parameter
are derived within the strong-coupling phonon-plasmon scheme that account for
the screened Coulomb interaction. Specifically, we calculate the
superconducting critical temperature Tc taking into account the full
temperature, frequency and wave-vector dependence of the dielectric function.
We show that low-energy plasmons may contribute constructively to
superconductivity. Three classes of layered superconductors are discussed
within our model: metal-intercalated halide nitrides, layered organic materials
and high-Tc oxides. In particular, we demonstrate that the plasmon contribution
(electronic mechanism) is dominant in the first class of layered materials. The
theory shows that the description of so-called ``quasi-two-dimensional
superconductors'' cannot be reduced to a purely 2D model, as commonly assumed.
While the transport properties are strongly anisotropic, it remains essential
to take into account the screened interlayer Coulomb interaction to describe
the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure
Substitution Effect by Deuterated Donors on Superconductivity in -(BEDT-TTF)Cu[N(CN)]Br
We investigate the superconductivity in the deuterated BEDT-TTF molecular
substitution system
-[(h8-BEDT-TTF)(d8-BEDT-TTF)]Cu[N(CN)]Br, where h8
and d8 denote fully hydrogenated and deuterated molecules, respectively.
Systematic and wide range ( = 0 -- 1) substitution can control chemical
pressure finely near the Mott boundary, which results in the modification of
the superconductivity. After cooling slowly, the increase of
observed up to 0.1 is evidently caused by the chemical pressure
effect. Neither reduction of nor suppression of
superconducting volume fraction is found below 0.5. This demonstrates
that the effect of disorder by substitution is negligible in the present
system. With further increase of , both and superconducting
volume fraction start to decrease toward the values in = 1.Comment: J. Phys. Soc. Jp
Bhjet: a public multi-zone, steady state jet + thermal corona spectral model
Accreting black holes are sources of major interest in astronomy, particular
those launching jets because of their ability to accelerate particles, and
dramatically affect their surrounding environment up to very large distances.
The spatial, energy and time scales at which a central active black hole
radiates and impacts its environment depend on its mass. The implied
scale-invariance of accretion/ejection physics between black hole systems of
different central masses has been confirmed by several studies. Therefore,
designing a self-consistent theoretical model that can describe such systems,
regardless of their mass, is of crucial importance to tackle a variety of
astrophysical sources. We present here a new and significantly improved version
of a scale invariant, steady-state, multi-zone jet model, which we rename
bhjet, resulting from the efforts of our group to advance the modelling of
black hole systems. We summarise the model assumptions and basic equations, how
they have evolved over time, and the additional features that we have recently
introduced. These include additional input electron populations, the extension
to cyclotron emission in near-relativistic regime, an improved multiple inverse
Compton scattering method, external photon seeds typical of AGN and a
magnetically-dominated jet dynamical model as opposed to the pressure-driven
jet configuration present in older versions. In this paper, we publicly release
the code on Github and, in order to facilitate the user's approach to its many
possibilities, showcase a few applications as a tutorial.Comment: 28 pages, 16 figures, accepted for publication on MNRAS, code
available at https://github.com/matteolucchini1/BHJet/, updated to the latest
version of the manuscrip
Patient-Specific Prosthetic Fingers by Remote Collaboration - A Case Study
The concealment of amputation through prosthesis usage can shield an amputee
from social stigma and help improve the emotional healing process especially at
the early stages of hand or finger loss. However, the traditional techniques in
prosthesis fabrication defy this as the patients need numerous visits to the
clinics for measurements, fitting and follow-ups. This paper presents a method
for constructing a prosthetic finger through online collaboration with the
designer. The main input from the amputee comes from the Computer Tomography
(CT) data in the region of the affected and the non-affected fingers. These
data are sent over the internet and the prosthesis is constructed using
visualization, computer-aided design and manufacturing tools. The finished
product is then shipped to the patient. A case study with a single patient
having an amputated ring finger at the proximal interphalangeal joint shows
that the proposed method has a potential to address the patient's psychosocial
concerns and minimize the exposure of the finger loss to the public.Comment: Open Access articl
Isotope effect in superconductors with coexisting interactions of phonon and nonphonon mechanisms
We examine the isotope effect of superconductivity in systems with coexisting
interactions of phonon and nonphonon mechanisms in addition to the direct
Coulomb interaction. The interaction mediated by the spin fluctuations is
discussed as an example of the nonphonon interaction. Extended formulas for the
transition temperature Tc and the isotope-effect coefficient alpha are derived
for cases (a) omega_np omega_D, where omega_np is
an effective cutoff frequency of the nonphonon interaction that corresponds to
the Debye frequency omega_D in the phonon interaction. In case (a), it is found
that the nonphonon interaction does not change the condition for the inverse
isotope effect, i.e., mu^* > lambda_ph/2, but it modifies the magnitude of
alpha markedly. In particular, it is found that a giant isotope shift occurs
when the phonon and nonphonon interactions cancel each other largely. For
instance, strong critical spin fluctuations may give rise to the giant isotope
effect. In case (b), it is found that the inverse isotope effect occurs only
when the nonphonon interaction and the repulsive Coulomb interaction, in total
effect, work as repulsive interactions against the superconductivity. We
discuss the relevance of the present result to some organic superconductors,
such as kappa-(ET)2Cu(NCS)2 and Sr2RuO4 superconductors, in which inverse
isotope effects have been observed, and briefly to high-Tc cuprates, in which
giant isotope effects have been observed.Comment: 4 pages, 2 figures, (with jpsj2.cls, ver.1.2), v2:linguistic
correction
Electronic correlation in the infrared optical properties of the quasi two dimensional -type BEDT-TTF dimer system
The polarized optical reflectance spectra of the quasi two dimensional
organic correlated electron system -(BEDT-TTF)Cu[N(CN)],
Br and Cl are measured in the infrared region. The former shows the
superconductivity at 11.6 K and the latter does the
antiferromagnetic insulator transition at 28 K. Both the
specific molecular vibration mode of the BEDT-TTF molecule and
the optical conductivity hump in the mid-infrared region change correlatively
at 38 K of -(BEDT-TTF)Cu[N(CN)]Br, although
no indication of but the insulating behaviour below 50-60 K are found in -(BEDT-TTF)Cu[N(CN)]Cl. The
results suggest that the electron-molecular vibration coupling on the
mode becomes weak due to the enhancement of the itinerant
nature of the carriers on the dimer of the BEDT-TTF molecules below ,
while it does strong below because of the localized carriers on
the dimer. These changes are in agreement with the reduction and the
enhancement of the mid-infrared conductivity hump below and , respectively, which originates from the transitions between the upper
and lower Mott-Hubbard bands. The present observations demonstrate that two
different metallic states of -(BEDT-TTF)Cu[N(CN)]Br are
regarded as {\it a correlated good metal} below including the
superconducting state and {\it a half filling bad metal} above . In
contrast the insulating state of -(BEDT-TTF)Cu[N(CN)]Cl
below is the Mott insulator.Comment: 8 pages, 7 figure
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