368 research outputs found
Spin-Peierls transition with strong structural fluctuations in the vanadium oxide VOSbO
We report on the magnetic susceptibility and electron spin resonance
measurements on polycrystalline samples of the vanadium oxide
VOSbO, a quasi-one dimensional S=1/2 Heisenberg system. We show
that the susceptibility vanishes at zero temperature, as in a gapped system,
and we argue that this is due to a spin-Peierls transition with strong
structural fluctuations.Comment: 5 pages, 4 figure
Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsOF
The electrical resistivity, crystalline structure and electronic properties
calculated from the experimentally measured atomic positions of the compound
SmFeAsOF have been studied up to pressures ~20GPa. The
correlation between the pressure dependence of the superconducting transition
temperature (Tc) and crystallographic parameters on the same sample shows
clearly that a regular FeAs tetrahedron maximizes Tc, through
optimization of carrier transfer to the FeAs planes as indicated by the
evolution of the electronic band structures.Comment: 15pages, 4 figure
Pancreatico-jejunostomy decreases post-operative pancreatic fistula incidence and severity after central pancreatectomy
BACKGROUNDS: Central pancreatectomy (CP) is an alternative to pancreaticoduodenectomy and distal pancreatectomy in benign tumours of pancreatic isthmus management. It is known for a high post-operative pancreatic fistula (POPF) rate. The purpose of this study was to compare POPF incidence between pancreatico-jejunostomy (PJ) and pancreatico-gastrostomy (PG).
METHODS: Fifty-eight patients (mean age 53.9 ± 1.9 years) who underwent a CP in four French University Hospitals from 1988 to 2011 were analysed. The distal pancreatic remnant was either anastomosed to the stomach (44.8%, n = 25) or to a Roux-en-Y jejunal loop (55.2%, n = 35) with routine external drainage allowing a systematic search for POPF. POPF severity was classified according to the International Study Group on Pancreatic Fistula (ISGPF) and Clavien-Dindo classifications.
RESULTS: The groups were similar on sex ratio, mean age, ASA score, pancreas texture, operative time and operative blood loss. Mean follow-up was 36.2 ± 3.9 months. POPF were significantly more frequent after PG (76.9 versus 37.5%, P = 0.003). PG was associated with significantly higher grade of POPF both when graded with ISGPF classification (P = 0.012) and Clavien-Dindo classification (P = 0.044). There was no significant difference in post-operative bleeding (0.918) and delayed gastric emptying (0.877) between the two groups. Hospital length of stay was increased after PG (23.6 ± 3.5 days versus 16.5 ± 1.9 days, P = 0.071). There was no significant difference in incidence of long-term exocrine (3.8 versus 19.2%, P = 0.134) and endocrine (7.7 versus 9.4%, P = 0.575) pancreatic insufficiencies.
CONCLUSION: PG was associated with a significantly higher POPF incidence and severity in CP. We recommend performing PJ especially in older patients to improve CP outcomes
Thermal Conductivity Anisotropy in Superconducting
Recent thermal conductivity measurements on single crystals by
Lussier et al. indicate the existence of a strong b--c anisotropy in the
superconducting state. We calculate the thermal conductivity in various
unconventional candidate states appropriate for the ``B phase" and
compare with experiment, specifically the and states
predicted in some Ginzburg-Landau analyses of the phase diagram. For the
simplest realizations of these states over spherical or ellipsoidal Fermi
surfaces, the normalized conductivity is found, surprisingly, to be
completely isotropic. We discuss the effects of inelastic scattering and
realistic Fermi surface anisotropy, and deduce constraints on the symmetry
class of the ground state.Comment: 4 postscript pages, UFL102
Amorphous Bi2Pb0.6Sr2Ca2Cu3Ox obtained by melt-spinning and its superconductivity after crystallisation
Previous quenching experiments on 2212 bismuth containing high T c oxides have been extended to the 2223 compound with lead addition. Amorphous Bi2Pb0.6Sr2Ca2Cu3Ox was prepared by a modified melt-spinning technique and samples in the form of small tapes and wires were obtained. The subsequent evolution of the amorphous oxide during annealing was monitored by differential scanning calorimetry (DSC). The glass transition was found to occur at Tg = 680 K and the onset of crystallisation at T = 730 K. After oxygen annealing, magnetic susceptibility measurements showed evidence of two superconducting phases with transition temperatures at Tc =110 K and Tc = 75 K respectively
Thermodynamic phase diagram of Fe(Se0.5Te0.5) single crystals in fields up to 28 tesla
International audienceWe report on specific heat Cp , transport, Hall probe, and penetration depth measurements performed on Fe Se0.5Te0.5 single crystals Tc 14 K . The thermodynamic upper critical field Hc2 lines has been deduced from Cp measurements up to 28 T for both H c and H ab, and compared to the lines deduced from transport measurements up to 55 T in pulsed magnetic fields . We show that this thermodynamic Hc2 line presents a very strong downward curvature for TâTc which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/Tc 0.99 for H ab which is the consequence of a very small value of the coherence length c 0 4 Ă
and ab 0 15 Ă
, confirming the strong renormalization of the effective mass as compared to DMFT calculations previously observed in ARPES measurements A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger, Phys. Rev. Lett. 104, 097002 2010 . Hc1 measurements lead to ab 0 =430 50 nm and c 0 =1600 200 nm and the corresponding anisotropy is approximatively temperature independent 4 , being close to the anisotropy of Hc2 for TâTc. The temperature dependence of both T2 and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system
Anomalous dependence of the c-axis polarized Fe B phonon mode with Fe and Se concentrations in FeTeSe
We report an investigation of the lattice dynamical properties in a range of FeTeSe compounds, with special emphasis on the c-axis polarized vibration of Fe with B symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se and excess Fe concentrations. In parent compound FeTe, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature . The situation smoothly evolves towards a regular anharmonic behavior as Te is substituted for Se and long range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the B phonon at low temperatures. We performed Density Functional Theory (DFT) ab-initio calculations within the local density approximation (LDA) to calcuate the phonon frequencies including magnetic polarization and Fe non-stoichiometry in the Virtual Crystal Approximation (VCA). We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low energy magnetic fluctuations that can not be treated accurately within these approaches. As recently revealed by neutron scattering and -SR studies, these phenomena occur in the temperature range where anomalous decay of the B phonon is observed, and suggests a peculiar coupling of this mode with local moments and spin fluctuations in FeTeSe
On the low temperature properties and specific anisotropy of pure anisotropically paired superconductors
Dependences of low temperature behavior and anisotropy of various physical
quantities for pure unconventional superconductors upon a particular form of
momentum direction dependence for the superconducting order parameter (within
the framework of the same symmetry type of superconducting pairing) are
considered. A special attention is drawn to the possibility of different
multiplicities of the nodes of the order parameter under their fixed positions
on the Fermi surface, which are governed by symmetry. The problem of an
unambiguous identification of a type of superconducting pairing on the basis of
corresponding experimental results is discussed. Quasiparticle density of
states at low energy for both homogeneous and mixed states, the low temperature
dependences of the specific heat, penetration depth and thermal conductivity,
the I-V curves of SS and NS tunnel junctions at low voltages are examined. A
specific anisotropy of the boundary conditions for unconventional
superconducting order parameter near for the case of specular reflection
from the boundary is also investigated.Comment: 20 page
Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)Bi03
Thermodynamic (specific heat, reversible magnetization, tunneling
spectroscopy) and transport measurements have been performed on high quality
(K,Ba)BiO single crystals. The temperature dependence of the magnetic field
corresponding to the onset of the specific heat anomaly presents a
clear positive curvature. is significantly smaller than the field
for which the superconducting gap vanishes but is closely related to
the irreversibility line deduced from transport data. Moreover, the temperature
dependence of the reversible magnetization present a strong deviation from the
Ginzburg--Landau theory emphasazing the peculiar nature of the superconducting
transition in this material.Comment: 4 pages, 4 figures, 28 reference
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