26 research outputs found
Water Content and Superconductivity in Na0.3CoO2*yH2O
We report here the correlation between the water content and
superconductivity in Na0.3CoO2*yH2O under the influences of elevated
temperature and cold compression. The x-ray diffraction of the sample annealed
at elevated temperatures indicates that intergrowths exist in the compound at
equilibrium when 0.6 < y < 1.4. Its low-temperature diamagnetization varies
linearly with y, but is insensitive to the intergrowth, indicative of quasi-2D
superconductivity. The Tc-onset, especially, shifts only slightly with y. Our
data from cold compressed samples, on the other hand, show that the water-loss
non-proportionally suppresses the diamagnetization, which is suggestive of weak
links.Comment: 10 pages, 10 figures; submitted to Physica C (August 13, 2003
Thermodynamic properties in the normal and superconducting states of Na(x)CoO(2)*yH(2)O powder measured by heat capacity experiments
The heat capacity of superconducting Na(x)CoO(2)*yH(2)O was measured and the
data are discussed based on three different models: The thermodynamic
Ginzburg-Landau model, the BCS theory, and a model including the effects of
line nodes in the superconducting gap function. The electronic heat capacity is
separated from the lattice contribution in a thermodynamically consistent way
maintaining the entropy balance of superconducting and normal states at the
critical temperature. It is shown that for a fully gapped superconductor the
data can only be explained by a reduced (about 55 %) superconducting volume
fraction. The data are compatible with 100 % superconductivity in the case
where line nodes are present in the superconducting gap function.Comment: Revised, 19 pages, 3 figure
Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O
We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on
the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under
hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure
at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the
decrease of T_c is accompanied by a weakening of the spin correlations at a
finite wave vector and a reduction of the density of states (DOS) at the Fermi
level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly
due to a reduction of the DOS. These results indicate that the
electronic/magnetic state of Co is primarily responsible for the
superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a
T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling
evidence for the presence of line nodes in the superconducting gap function.Comment: published on 19, Sept. 2007 on Phys. Rev.
Field-Induced Resistive Switching in Metal-Oxide Interfaces
We investigate the polarity-dependent field-induced resistive switching
phenomenon driven by electric pulses in perovskite oxides. Our data show that
the switching is a common occurrence restricted to an interfacial layer between
a deposited metal electrode and the oxide. We determine through impedance
spectroscopy that the interfacial layer is no thicker than 10 nm and that the
switch is accompanied by a small capacitance increase associated with charge
accumulation. Based on interfacial I-V characterization and measurement of the
temperature dependence of the resistance, we propose that a field-created
crystalline defect mechanism, which is controllable for devices, drives the
switch.Comment: 4 pages, 3 figure
Effect of Hydrostatic Pressure on the Superconductivity in NaxCoO2.yH2O
The effect of hydrostatic pressure on the superconducting transition
temperature of Na{0.35}CoO{2}.yH{2}O was investigated by ac susceptibility
measurements up to 1.6 GPa. The pressure coefficient of T{c} is negative and
the dependence T{c}(p) is nonlinear over the pressure range investigated. The
magnitude of the average dlnT{c}/dp=-0.07 GPa^{-1} is comparable to the
pressure coefficient of electron-doped high-T{c} copper oxide superconductors
with a similar value of T{c}. Our results provide support to the assumption of
two-dimensional superconductivity in Na{0.35}CoO{2}.yH{2}O, which is similar to
the cuprate systems, and suggest that intercalation of larger molecules may
lead to an enhancement of T{c}.Comment: Revised Manuscrip
Unconventional Superconductivity and Electron Correlations in Cobalt Oxyhydrate NaCoOHO
We report a precise Co nuclear quadrupolar resonance (NQR) measurement
on the recently discovered cobalt oxyhydrate NaCoOHO superconductor from =40 K down to 0.2 K. We find that in the
normal state the spin-lattice relaxation rate follows a Curie-Weiss
type temperature () variation, , with =-42 K,
suggesting two-dimensional antiferromagnetic spin correlations. Below =3.9
K, decreases with no coherence peak and follows a dependence with
2.2 down to 2.0 K but crosses over to a
variation below =1.4 K, which suggests non s-wave superconductivity. The
data in the superconducting state are most consistent with the existence of
line nodes in the gap function.Comment: submitted for publication in June '0
Raman Phonons and Ageing-Related Surface Disorder in NaxCoO2
The polarized Raman spectra from ab and ac surfaces of single crystal NaxCoO2
(x~0.7), parent compound of recently discovered superconductor NaxCoO2.yH2O,
are reported and discussed. The crystals were hexagon platelets of typical size
3x3x0.1 mm. Three of the five (A1g+E1g+3E2g) Raman active phonons were
unambiguously identified at 458 (E1g), 494(E2g) and 574 (A1g) cm-1. The spectra
from ab and ac surfaces differ significantly and provide evidence that within
hours after preparation the ac surface, unlike the ab one, is strongly
disordered. Within several days the disorder extends over the ab surface too.Comment: 4 pages, 1 table, 4 figures. Physica C (accepted 26 September 2003