253 research outputs found
Bulk superconductivity in Bi4O4S3 revealed by specific heat measurement
Specific heat experiments on a well-characterized polycrystalline sample of
the BiS2 based superconductor Bi4O4S3 revealed that it shows a crear specific
heat anomaly at about Tc = 4.4 K, consistent with Tc from the resistivity and
dc susceptibility. This observation indicates the superconductivity of Bi4O4S3
to be bulk in nature
Disorder-driven quantum phase transition from antiferromagnetic metal to insulating state in multilayered high-Tc cuprate (Cu,C)Ba2Ca4Cu5Oy
We report on superconducting(SC) characteristics for oxygen-reduced Cu-based
five-layered high-temperature superconductor (Cu,C)Ba2Ca4Cu5Oy(Cu-1245(OPT)),
which includes five-fold outer planes (OP) and four-fold inner planes (IP).As a
result of the reduction of the carrier density, the bulk SC for Cu-1245 (OPT)
takes place at the nearly optimally-doped OP with Tc= 98 K that is different
from previously-reported Cu-1245(OVD) where IP plays a primary role for the
onset of SC. It gives an evidence that the carrier density of the
optimally-doped layer determines its bulk Tc.Static antiferromagnetic(AFM)
order is evidenced at IP's by zero-field Cu-NMR at low temperature,
irrespective of the SC transition at OP's below 98K. This AFM state at IP's is
characterized by a carrier localization at low temperatures due to disorder
effect, whereas the carrier densities in each layer are similar to Hg-1245(OPT)
where the AFM metallic state are realized in IP's. This finding reinforces the
phase diagram in which the AFM metallic phase exists between AFM insulator and
SC states for the case of ideally-flat CuO2 plane without disorder.Comment: 4 pages, 5 figure
Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study
We report the precise pressure dependence of FeSe from a resistivity
measurement up to 4.15 GPa. Superconducting transition temperature (T_c)
increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa.
The maximum T_c, which is determined by zero resistance, is 21 K at
approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also
measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using
77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in
the zero-resistance state. The pressure dependence of 1/T_1T just above T_c
shows a plateau as well as the pressure dependence of T_c, which gives clear
evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations
are suggested to contribute to the mechanism of superconductivity.Comment: 4pages, 6figures: to be published in J. Phys. Soc. Jpn. Vol.78 No.6
(2009
Synthesis and photoemission study of as-grown superconducting MgB2 thin films
As-grown superconducting thin films of MgB2 were prepared by molecular beam
epitaxy (MBE), and studied by X-ray and ultraviolet photoelectron spectroscopy
(XPS and UPS). Only films prepared at temperatures between 150 and 320 deg.
showed superconductivity. A best TC onset of 36 K with a sharp transition width
of -1 K was obtained although the film crystallinity was poor. The in-situ
photoelectron spectra obtained on the surfaces of the MBE grown MgB2 films were
free from dirt peaks. XPS revealed that the binding energy of the Mg 2p levels
in MgB2 is close to that of metallic Mg, and the binding energy of B 1s is
close to that of transition-metal diborides. The valence UP spectra showed a
clear Fermi edge although the density of states (DOS) at EF is low and the
major components of the valence band are located between 5 and 11 eV.Comment: ISS 2001 proceedin
Diamagnetism above Tc in underdoped Bi2.2Sr1.8Ca2Cu3O10+d
Single crystals of (Bi2223) with were grown by a traveling solvent floating
zone method in order to investigate the superconducting properties of highly
underdoped Bi2223.Grown crystals were characterized by X-ray diffraction, DC
susceptibility and resistivity measurements, confirming Bi2223 to be the main
phase.The crystals were annealed under various oxygen partial pressures to
adjust their carrier densities from optimally doped to highly underdoped.The
fluctuation diamagnetic component above the superconducting transition
temperature extracted from the anisotropic normal state
susceptibilities () and ()
was found to increase with underdoping, suggesting a decrease in the
superconducting dimensionality and/or increase in the fluctuating vortex liquid
region.Comment: 6 pages, 7 figures, corrected fig.4 and references, published in J.
Phys. Soc. Jpn. 79, 114711 (2010
Multigap Superconductivity in YC: A C-NMR Study
We report on the superconducting (SC) properties of YC with a
relatively high transition temperature K investigated by
C nuclear-magnetic-resonance (NMR) measurements under a magnetic field.
The C Knight shift has revealed a significant decrease below , suggesting a spin-singlet superconductivity. From an analysis of the
temperature dependence of the nuclear spin-lattice relaxation rate in
the SC state, YC is demonstrated to be a multigap superconductor that
exhibits a large gap at the main band and a
small gap at other bands. These results have
revealed that YC is a unique multigap s-wave superconductor similar to
MgB.Comment: 4 pages, 5 figure
Antiferromagnetism of SrFe2As2 studied by Single-Crystal 75As-NMR
We report results of 75As nuclear magnetic resonance (NMR) experiments on a
self-flux grown high-quality single crystal of SrFe2As2. The NMR spectra
clearly show sharp first-order antiferromagnetic (AF) and structural
transitions occurring simultaneously. The behavior in the vicinity of the
transition is compared with our previous study on BaFe2As2. No significant
difference was observed in the temperature dependence of the static quantities
such as the AF splitting and electric quadrupole splitting. However, the
results of the NMR relaxation rate revealed difference in the dynamical spin
fluctuations. The stripe-type AF fluctuations in the paramagnetic state appear
to be more anisotropic in BaFe2As2 than in SrFe2As2.Comment: 4 pages, 5 figures; discussion revised; accepted for publication in
J. Phys. Soc. Jp
An explanation for a universality of transition temperatures in families of copper oxide superconductors
A remarkable mystery of the copper oxide high-transition-temperature (Tc)
superconductors is the dependence of Tc on the number of CuO2 layers, n, in the
unit cell of a crystal. In a given family of these superconductors, Tc rises
with the number of layers, reaching a peak at n=3, and then declines: the
result is a bell-shaped curve. Despite the ubiquity of this phenomenon, it is
still poorly understood and attention has instead been mainly focused on the
properties of a single CuO2 plane. Here we show that the quantum tunnelling of
Cooper pairs between the layers simply and naturally explains the experimental
results, when combined with the recently quantified charge imbalance of the
layers and the latest notion of a competing order nucleated by this charge
imbalance that suppresses superconductivity. We calculate the bell-shaped curve
and show that, if materials can be engineered so as to minimize the charge
imbalance as n increases, Tc can be raised further.Comment: 15 pages, 3 figures. The version published in Natur
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