278 research outputs found
Mass enhancement, correlations, and strong coupling superconductivity in the beta-pyrochlore KOs2O6
To assess electron correlation and electron-phonon coupling in the recently
discovered beta-pyrochlores KOs2O6 and RbOs2O6, we have performed specific heat
measurements in magnetic fields up to 14 T. We present data from high quality
single crystalline KOs2O6, showing that KOs2O6 is a strong coupling
superconductor with a coupling parameter lambda_ep \approx 1.0 to 1.6 (RbOs2O6:
lambda_ep \approx 1). The estimated Sommerfeld coefficient of KOs2O6, gamma=76
to 110 mJ/(mol K^2), is twice that of RbOs2O6 [gamma=44 mJ/(mol K^2)]. Using
strong-coupling corrections, we extract useful thermodynamic parameters of
KOs2O6. Quantifying lambda_ep allows us to determine the mass enhancement over
the calculated band electronic density of states. A significant contribution in
addition to the electron-phonon term of lambda_c=1.7 to 4.3 is deduced. In an
effort to understand the origin of the enhancement mechanism, we also
investigate an unusual energetically low-lying phonon. There are three phonon
modes per RbOs2O6, suggestive of the phonon source being the rattling motion of
the alkali ion. This dynamic instability of the alkali ions causes large
scattering of the charge carriers which shows up in an unusual temperature
dependence of the electrical resistivity.Comment: Accepted for publication in PR
Synthesis of superconducting pyrochlore RbOs2O6
RbOs2O6, the third superconducting pyrochlore oxide (known so far), has been
synthesized by encapsulation and by high pressure techniques. Suitable post
chemical treatment of the as-prepared sample allowed us to eliminate the
impurity phases. Bulk superconductivity with Tc=6.4 K was observed in
magnetisation and specific heat measurements. The transition temperature of
RbOs2O6 was found to be the same for both preparation methods. Structural
investigations showed that Rb atoms occupy the 8b site in the pyrochlore
lattice with a lattice parameter of 10.1137(1) A.Comment: 12 pages, 5 figures, correction to one author name and Figure
Spin fluctuations, magnetic long-range order and Fermi surface gapping in NaxCoO2
In this study an extended low energy phase diagram for NaxCoO2 is
experimentally established with emphasis on the high x range. It is based on
systematic heat capacity studies on both polycrystalline and single crystalline
samples and on uSR measurements. Main features are the existence of mass
enhancement, spin fluctuations without long-range order, and magnetic order
with associated Fermi surface gapping. The latter is seen in the electronic
density of states (DOS) and suppression of nuclear specific heat. While there
is agreement between the band structure and the low energy DOS in the low x
range, in the high x range (x > 0.6) the thermodynamically determined DOS is
approximately three times that deduced from the angle-resolved photoemission
spectroscopy (ARPES)-measured band dispersion or local-density approximation
(LDA) calculations.Comment: 9 pages, 5 figure
Superconductivity in Geometrically Frustrated Pyrochlore RbOs2O6
We report the basic thermodynamic properties of the new geometrically
frustrated beta-pyrochlore bulk superconductor RbOs2O6 with a critical
temperature Tc = 6.4 K. Specific heat measurements are performed in magnetic
fields up to 12 T. The electronic density of states at the Fermi level in the
normal state results in gamma = (33.7 \pm 0.2) mJ/mol_f.u./K^2. In the
superconducting state, the specific heat follows conventional BCS-type behavior
down to 1 K, i.e. over three orders of magnitude in specific heat data. The
upper critical field slope at Tc is 1.2 T/K, corresponding to a Maki-parameter
alpha = 0.64 \pm 0.1. From the upper critical field mu0 Hc2 \approx 6 T at 0 K,
we estimate a Ginzburg-Landau coherence length xi \approx 7.4 nm. RbOs2O6 is
the second reported metallic AB2O6 type pyrochlore compound after KOs2O6, and
one of only three pyrochlore superconductors in addition to Cd2Re2O7 and
KOs2O6
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
Superconductivity and Rattling under High Pressure in the beta-Pyrochlore Oxide RbOs2O6
Rattling-induced superconductivity in the beta-pyrochlore oxide RbOs2O6 is
investigated under high pressures up to 6 GPa. Resistivity measurements in a
high-quality single crystal show that the superconducting transition
temperature Tc increases gradually from 6.3 K at ambient pressure to 8.8 K at
3.5 GPa, surprisingly remains almost constant at 8.8 \pm 0.1 K in a wide
pressure range between 3.5 (Po) and 4.8 GPa, and suddenly drops to 6.3 K at Ps
= 4.9 GPa, followed by a gradual decrease with further pressure increase. Two
anomalies in the temperature dependence of the normal-state resistivity are
observed at Po Ps, revealing the presence of two high-pressure
phases corresponding to the changes in Tc. The rattling of the Rb ion inside a
cage made of Os and O atoms may be slightly and seriously modified in these
high-pressure phases that probably have cages of reduced symmetry,
respectively, so that electron-rattler interactions that govern the
superconducting and transport properties of beta-RbOs2O6 are significantly
affected.Comment: arXiv admin note: text overlap with arXiv:1009.035
Thermal Perception of Ventilation Changes in Full-Face Motorcycle Helmets: Subject and Manikin Study
We report the effects of full-face motorcycle helmet ventilation systems on heat, airflow, noise, and comfort perception for ventilation changes on the scalp. Eight subjects (aged 28.0 ± 5.4 years) underwent two experimental trials at ambient temperatures of 23.7 ± 0.4°C or 27.5 ± 0.3°C. In each trial, the thermally equilibrated subjects underwent two examination phases, during which four different helmets were assessed at wind speeds of 39.2 ± 1.9 km h−1 and 59.3 ± 1.4 km h−1. Vent-induced heat loss in the scalp ranged from −6.1 to 6.1 W, corresponding to vents being closed or opened, respectively. Perception of vent-induced changes was assessed immediately after the change. We find that the vent-induced heat loss, the subject, and the helmet are the most important response factors. In addition, comparison of two helmets with similar vent-induced heat loss suggests that internal airflow patterns may be important in explaining the observed perception difference
NaxCoO2: Enhanced low-energy excitations of electrons on a 2D triangular lattice
To elucidate the low-energy excitation spectrum of correlated electrons on a
2D triangular lattice, we have studied the electrical resistance and specific
heat down to 0.5 K and in magnetic fields up to 14 T, in NaxCoO2 samples with a
Na content ranging from x \approx 0.5 to 0.82. Two distinct regimes are
observed: for x from about 0.6 to x \approx 0.75 the specific heat is strongly
enhanced, with a pronounced upturn of C/T below about 10 K, reaching 47 mJ/(mol
K^2). This enhancement is suppressed in a magnetic field indicative of strong
low-energy spin fluctuations. At higher Na content the fluctuations are reduced
and mu-SR data confirm the SDW ground state below 22 K and the much reduced
heat capacity is field independent.Comment: Accepted in Physica
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