930 research outputs found
Extremely strong-coupling superconductivity and anomalous lattice properties in the beta-pyrochlore oxide KOs2O6
Superconducting and normal-state properties of the beta-pyrochlore oxide
KOs2O6 are studied by means of thermodynamic and transport measurements. It is
shown that the superconductivity is of conventional s-wave type and lies in the
extremely strong-coupling regime. Specific heat and resistivity measurements
reveal that there are characteristic low-energy phonons that give rise to
unusual scattering of carriers due to strong electron-phonon interactions. The
entity of the low-energy phonons is ascribed to the heavy rattling of the K ion
confined in an oversized cage made of OsO6 octahedra. It is suggested that this
electron-rattler coupling mediates the Cooper pairing, resulting in the
extremely strong-coupling superconductivity.Comment: 17 pages (only 4 pages included here. go to
http://hiroi.issp.u-tokyo.ac.jp/Published%20papers/K-SC6.pdf for full pages),
to be published in PR
Magnetic phase transitions in SmCoAsO
Magnetization, x-ray diffraction and specific-heat measurements reveal that
SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition
attributed to the Co ions, emerges at TC=57 K with a small saturation moment of
0.15muB/Co. Reorientation of the Co moment to an antiferromagnetic state is
obtained at TN2=45 K. The relative high paramagnetic effective moment Peff=1.57
MuB/Co indicates an itinerant ferromagnetic state of the Co sublattice. The
third magnetic transition at TN1=5 K is observed clearly in the specific-heat
study only. Both magnetic and 57Fe Mossbauer studies show that substitution of
small quantities of Fe for Co was unsuccessful.Comment: 10pages text+Figures: comments welcome ([email protected]
A second phase transition and superconductivity in the beta-pyrochlore oxide KOs2O6
Another phase transition that is probably of first order is found in the
beta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition
temperature Tc of 9.6 K. It takes place at Tp=7.5 K in the superconducting
state in a zero magnetic field. By applying magnetic fields of up to 140 kOe,
the Tc gradually decreased to 5.2 K, while Tp changed little, eventually
breaking through the Hc2 line at approximately 65 kOe in the H-T diagram. Both
the normal-state resistivity and Hc2 change slightly but significantly across
the second phase transition. It is suggested that the transition is associated
with the rattling of potassium ions located in an oversized cage of osmium and
oxide ions.Comment: 10 pages including 6 figures; to be published in the Proceedings of
HFM2006(J. Phys.: Condens. Matter
Synthesis of SmFeAsO by an Easy and Versatile Route and its Physical Property Characterization
We report synthesis, structure, electrical transport and heat capacity of
SmFeAsO. The title compound is synthesized by one-step encapsulation of
stoichiometric FeAs, Sm, and Sm2O3 in an evacuated (10-5 Torr) quartz tube by
prolong (72 hours) annealing at 1100oC. The as synthesized compound is
crystallized in tetragonal structure with P4/nmm space group having lattice
parameters a = 3.93726(33) A and c = 8.49802(07) A. The resistance (R-T)
measurements on the compound exhibited ground state spin-density-wave
(SDW)-like metallic steps below 140 K. Heat capacity CP(T) measurements on the
title compound, showed an anomaly at around 140 K, which is reminiscent of the
SDW ordering of the compound. At lower temperatures the CP(T) shows a clear
peak at around 4.5 K. At lower temperature below 20 K, Cp(T) is also measured
under an applied field of 7 Tesla. It is concluded that the CP(T) peak at 4.5 K
is due to the anti-ferromagnetic(AFM) ordering of Sm3+ spins. These results are
in confirmation with ordering of Sm in Sm2-xCexCuO4.Comment: 9 pages Text + Figs Contact Author ([email protected]
Non-magnetic pair-breaking effect on La(Fe_{1-x}Zn_{x})AsO_{0.85} studied by NMR and NQR
As and La NMR and nuclear quadrupole resonance (NQR) studies
on Zn-substituted LaFeAsO have been performed to investigate the
Zn-impurity effects microscopically. Although superconductivity in
LaFeAsO disappears by 3% Zn substitution, we found that NMR/NQR
spectra and NMR physical quantities in the normal state are hardly changed,
indicating that the crystal structure and electronic states are not modified by
Zn substitution. Our results suggest that the suppression of superconductivity
by Zn substitution is not due to the change of the normal-state properties, but
due to strong non-magnetic pair-breaking effect to superconductivity.Comment: 5 pages, 4 figures, This paper was chosen as "Paper of Editors'
Suggestion
Large and homogeneous mass enhancement in the rattling-induced superconductor KOsO
We have determined the Fermi surface in KOsO ( = 9.6 K and
32 T) via de Haas-van Alphen (dHvA) oscillation measurements and
a band structure calculation. We find effective masses up to 26(1) (
is the free electron mass), which are unusually heavy for compounds where the
mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved
mass enhancement parameters are large but fairly homogeneous,
concentrated in the range 5 -- 8. We discuss origins of the large homogeneous
mass enhancement in terms of rattling motion of the K ions.Comment: Minor revisions, Fig.2a modifie
Upper critical field and de Haas-van Alphen oscillations in KOsO measured in a hybrid magnet
Magnetic torque measurements have been performed on a KOsO single
crystal in magnetic fields up to 35.3 T and at temperatures down to 0.6 K. The
upper critical field is determined to be 30 T. De Haas-van Alphen
oscillations are observed. A large mass enhancement of (1+) = = 7.6 is found. It is suggested that, for the large upper critical
field to be reconciled with Pauli paramagnetic limiting, the observed mass
enhancement must be of electron-phonon origin for the most part.Comment: 4 pages, 4 figures, published versio
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