266 research outputs found
Dispersive Gap Mode of Phonons in Anisotropic Superconductors
We estimate the effect of the superconducting gap anisotropy in the
dispersive gap mode of phonons, which is observed by the neutron scattering on
borocarbide superconductors. We numerically analyze the phonon spectrum
considering the electron-phonon coupling, and examine contributions coming from
the gap suppression and the sign change of the pairing function on the Fermi
surface. When the sign of the pairing function is changed by the nesting
translation, the gap mode does not appear. We also discuss the suppression of
the phonon softening of the Kohn anomaly due to the onset of superconductivity.
We demonstrate that observation of the gap dispersive mode is useful for
sorting out the underlying superconducting pairing function.Comment: 7 pages, 12 figures, to be published in J. Phys. Soc. Jp
Charge-Stripe Order and Superconductivity in
A combined resistivity and hard x-ray diffraction study of superconductivity
and charge ordering in , as a function of Pt
substitution and externally applied hydrostatic pressure, is presented.
Experiments are focused on samples near the critical composition where competition and switching between charge order and
superconductivity is established. We show that charge order as a function of
pressure in is preempted - and hence
triggered - by a structural transition. Charge ordering appears uniaxially
along the short crystallographic (1,0,1) domain axis with a
modulation. Based on these results we
draw a charge-order phase diagram and discuss the relation between stripe
ordering and superconductivity.Comment: 8 pages, 4 figures: Accepted in Scientific Report
Otimização da interface/interfase de compósitos termoplåsticos de fibra de carbono/PPS pelo uso do poli(åcido âmico) do tipo BTDA/DDS
Hole Localization in Underdoped Superconducting Cuprates Near 1/8th Doping
Measurements of thermal conductivity versus temperature over a broad range of
doping in YBaCuO and HgBaCaCuO
(=1,2,3) suggest that small domains of localized holes develop for hole
concentrations near =1/8. The data imply a mechanism for localization that
is intrinsic to the CuO-planes and is enhanced via pinning associated with
oxygen-vacancy clusters.Comment: 4 pages, 4 eps fig.'s, to be published, Phys. Rev.
Anisotropic spin freezing in the S=1/2 zigzag ladder compound SrCuO2
Using magnetic neutron scattering we characterize an unusual low temperature
phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed
by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated
interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless
magnetic excitation spectrum indicating spin freezing on a time scale larger
than 200 picoseconds. While the frozen state has long range commensurate
antiferromagnetic order along the chains with the correlation length exceeding
200 lattice periods along the c-axis and a substantial correlation length of
60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2
lattice units are correlated along the b-axis which is the direction of the
frustrated interactions. The frozen magnetic moment of each Cu ion is very
small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature
dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue
that slow dynamics of stripe-like cooperative magnetic defects in tetragonal
a-c planes yield this anisotropic frozen state.Comment: 4 pages, LaTeX, submitted to PR
Structural instability associated with the tilting of CuO6 octahedra in La2-xSrxCuO4
Comprehensive inelastic neutron-scattering measurements were performed to
study the soft optical phonons in La2-xSrxCuO4 at x=0.10, 0.12 and 0.18. We
found at x=0.18 that the softening of Z-point phonon, suggesting incipient
structural transition from the low-temperature orthorhombic (LTO) to
low-temperature tetragonal (LTT) phase, breaks at Tc, which is consistent with
the previous report by Lee et al. for the optimally doped x=0.15 sample. As for
x=0.10 and 0.12, on the other hand, the softening continues even below Tc. It
is thus clarified that the breaking of soft phonon is characteristic of
La2-xSrxCuO4 in the optimally and overdoped regions. In the course of studying
the soft phonons, we discovered that a central peak remains above the LTO to
high-temperature tetragonal (HTT) phase transition at Ts1 and splits into
incommensurate components along the (1 1 0)HTT direction at higher
temperatures. This is a common feature for both x=0.12 and 0.18 and their
temperature dependences of the splitting 2d can be scaled by using a
renormalized temperature T/Ts1. In the high temperature limit, d saturates
around d ~ 0.12 r.l.u., which value is close to the splitting of incommensurate
magnetic signals. This implies that the incipient lattice modulation starts
appearing at very high temperature. Details of this modulation and its
relations with other properties are, however, not yet clarified.Comment: 7 pages, 5 eps figure
Magnetic Field Dependence of Electronic Specific Heat in Pr_{1.85} Ce_{0.15} CuO_4
The specific heat of electron-doped Pr_{1.85} Ce_{0.15} CuO_4 single crystals
is reported for the temperature range 2 - 10 K and magnetic field range 0 - 10
T. A non-linear magnetic field dependence is observed for the field range 0 - 2
T. Our data supports a model with lines of nodes in the gap function of these
superconductors. Theoretical calculations of the electronic specific heat for
dirty d-wave, clean d-wave, and s-wave symmetries are compared to our data.Comment: 10 pages Latex and 4 eps figures, submitted to Phys. Rev.
Specific Heat Study of the Magnetic Superconductor HoNi2B2C
The complex magnetic transitions and superconductivity of HoNi2B2C were
studied via the dependence of the heat capacity on temperature and in-plane
field angle. We provide an extended, comprehensive magnetic phase diagram for B
// [100] and B // [110] based on the thermodynamic measurements. Three magnetic
transitions and the superconducting transition were clearly observed. The 5.2 K
transition (T_{N}) shows a hysteresis with temperature, indicating the first
order nature of the transition at B=0 T. The 6 K transition (T_{M}), namely the
onset of the long-range ordering, displays a dramatic in-plane anisotropy:
T_{M} increases with increasing magnetic field for B // [100] while it
decreases with increasing field for B // [110]. The anomalous anisotropy in
T_{M} indicates that the transition is related to the a-axis spiral structure.
The 5.5 K transition (T^{*}) shows similar behavior to the 5.2 K transition,
i.e., a small in-plane anisotropy and scaling with Ising model. This last
transition is ascribed to the change from a^{*} dominant phase to c^{*}
dominant phase.Comment: 9 pages, 11 figure
Thermal Conductivity across the Phase Diagram of Cuprates: Low-Energy Quasiparticles and Doping Dependence of the Superconducting Gap
Heat transport in the cuprate superconductors YBaCuO and
LaSrCuO was measured at low temperatures as a function of
doping. A residual linear term kappa_{0}/T is observed throughout the
superconducting region and it decreases steadily as the Mott insulator is
approached from the overdoped regime. The low-energy quasiparticle gap
extracted from kappa_{0}/T is seen to scale closely with the pseudogap. The
ubiquitous presence of nodes and the tracking of the pseudogap shows that the
overall gap remains of the pure d-wave form throughout the phase diagram, which
excludes the possibility of a complex component (ix) appearing at a putative
quantum phase transition and argues against a non-superconducting origin to the
pseudogap. A comparison with superfluid density measurements reveals that the
quasiparticle effective charge is weakly dependent on doping and close to
unity.Comment: 12 pages, 9 figure
Acid epimerization of 20-keto pregnane glycosides is determined by 2D-NMR spectroscopy
Carbohydrates influence many essential biological events such as apoptosis, differentiation, tumor metastasis, cancer, neurobiology, immunology, development, host-pathogen interactions, diabetes, signal transduction, protein folding, and many other contexts. We now report on the structure determination of pregnane glycosides isolated from the aerial parts of Ceropegia fusca Bolle (Asclepiadaceae). The observation of cicatrizant, vulnerary and cytostatic activities in some humans and animals of Ceropegia fusca Bolle, a species endemic to the Canary Islands, encouraged us to begin a pharmacological study to determine their exact therapeutic properties. High resolution 1H-NMR spectra of pregnane glycosides very often display well-resolved signals that can be used as starting points in several selective NMR experiments to study scalar (J coupling), and dipolar (NOE) interactions. ROESY is especially suited for molecules such that ĎĎc ~ 1, where Ďc are the motional correlation times and Ď is the angular frequency. In these cases the NOE is nearly zero, while the rotating-frame Overhauser effect spectroscopy (ROESY) is always positive and increases monotonically for increasing values of Ďc. The ROESY shows dipolar interactions cross peaks even in medium-sized molecules which are helpful in unambiguous assignment of all the interglycosidic linkages. Selective excitation was carried out using a double pulsed-field gradient spin-echo sequence (DPFGSE) in which 180° Gaussian pulses are sandwiched between sine shaped z-gradients. Scalar interactions were studied by homonuclear DPFGSE-COSY and DPFGSE-TOCSY experiments, while DPFGSE-ROESY was used to monitor the spatial environment of the selectively excited proton. Dipolar interactions between nuclei close in space can be detected by the 1D GROESY experiment, which is a one-dimensional counterpart of the 2D ROESY method. The C-12 and C-17 configurations were determined by ROESY experiments
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