59 research outputs found
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
Substitution effect of Zn and Cu in MgB2 on Tc and structure
The investigation of Zn substitution in MgB2 polycrystalline samples shows
that about 0.1 Zn can be substituted in the structure of Mg deficient samples
while it can not be substituted in stoichiometric MgB2. As a result of the
isovalent Zn substitution into the Mg position of Mg deficient samples the a
and c lattice constants increase by about 0.17% and 0.2% respectively.
Susceptibility measurements show a slight lowering of Tc by about 0.5 K for
0.05 Zn, which decreases to 0.2 K for 0.1 Zn substitution. Investigation of the
pressure derivative of Tc shows dp/dTc = -0.15 K/kbar independent of Zn
content. Substitution of Cu leads to multiphase samples without any changes of
lattice constants, which indicates that Cu does not enter the structure of
MgB2. The Cu substitution broadens the superconducting transition considerably,
while the onset remains unchanged.Comment: 13 pages, 5 figure
Directional point-contact spectroscopy of MgB2 single crystals in magnetic fields: two-band superconductivity and critical fields
The results of the first directional point-contact measurements in MgB2
single crystals, in the presence of magnetic fields up to 9 T either parallel
or perpendicular to the ab planes, are presented. By applying suitable magnetic
fields, we separated the partial contributions of the sigma and pi bands to the
total Andreev-reflection conductance. Their fit with the BTK model allowed a
very accurate determination of the temperature dependency of the gaps
(Delta_sigma and Delta_pi), that resulted in close agreement with the
predictions of the two-band models for MgB2. We also obtained, for the first
time with point-contact spectroscopy, the temperature dependence of the
(anisotropic) upper critical field of the sigma band and of the (isotropic)
upper critical field of the pi band.Comment: 2 pages, 2 figures, proceedings of M2S-HTSC-VII conference, Rio de
Janeiro (May 2003
Anisotropic properties of MgB2 by torque magnetometry
Anisotropic properties of superconducting MgB2 obtained by torque
magnetometry are compared to theoretical predictions, concentrating on two
issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc
from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly,
from the evaluation of torque vs angle curves it is concluded that the
anisotropy of the penetration depth gamma_lambda has to be substantially higher
at low temperature than theoretical estimates, at least in fields higher than
0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref
adde
Single crystal growth and properties of MgB2 and Mg(B1-xCx)2
Single crystals of MgB2 and Mg(B1-xCx)2 have been grown using cubic anvil
technique. Tc values vary in a wide range (39-9 K) with carbon content varying
from 0 up to 16%. Using SiC as the precursor leads to C and not to Si
substituted crystals. Micro-hardness measurements performed on MgB2 single
crystals give average value of 1100 kg/mm2.Comment: 6 pages, 3 figures. submitted to Physica C (M2S-Rio proceedings
MgB2 and Mg1-xAlxB2 single crystals: high-pressure growth and physical properties
Single crystals of MgB2 have been grown with a high-pressure cubic anvil
technique. They grow via the peritectic decomposition of the MgNB9 ternary
nitride. The crystals are of a size up to 2x1x0.1mm3 with a weight up to 230
micrograms. Typically they have transition temperatures between 38 and 38.6 K
with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that
the MgB2 phase is stable at least up to 2190C at high hydrostatic pressure in
the presence of Mg vapor under high pressure. Substitution of aluminum for
magnesium in single crystals leads to stepwise decrease of Tc. This indicates a
possible appearance of superstructures or phases with different Tc's. The upper
critical field decreases with Al doping.Comment: 6 pages, 2 figures. submitted to Physica C (M2S-Rio proceedings).
Text and Fig.2 revise
Large oxygen-isotope effect in Sr_{0.4}K_{0.6}BiO_{3}: Evidence for phonon-mediated superconductivity
Oxygen-isotope effect has been investigated in a recently discovered
superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite
structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with
^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K,
corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope
exponent is very close to that for a similar bismuthate superconductor
Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c
dependencies of alpha_{O} observed in bismuthates and cuprates suggest that
bismuthates should belong to conventional phonon-mediated superconductors while
cuprates might be unconventional supercondutors.Comment: 9 pages, 5 figure
Scanning Tunneling Spectroscopy on Single Crystal MgB2
We report on the results of scanning tunneling spectroscopy measurements on
single crystals of Mg2. Tunneling was performed both parallel and perpendicular
to the crystalline c-axis. In the first case, a single superconducting gap
(Delta_pi = 2.2 meV) associated with the pi-band is observed. Tunneling
parallel to the ab-plane reveals an additional, larger gap (Delta_sigma ~ 7
meV) originating in the highly two-dimensional sigma-band. Vortex imaging in
the pi-band was performed with the field and tunnel current parallel to the
c-axis. The vortices have a large core size compared to estimates based on Hc2,
and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. A comparison to specific heat measurements is performed.Comment: 12 pages, 7 figs. Submitted to Physica
Grand-Canonical Ensemble of Random Surfaces with Four Species of Ising Spins
The grand-canonical ensemble of dynamically triangulated surfaces coupled to
four species of Ising spins (c=2) is simulated on a computer. The effective
string susceptibility exponent for lattices with up to 1000 vertices is found
to be . A specific scenario for models is
conjectured.Comment: LaTeX, 11 pages + 1 postscript figure appended, preprint LPTHE-Orsay
94/1
Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals
The angular and temperature dependence of the upper critical field Hc2 in
MgB2 was determined from torque magnetometry measurements on single crystals.
The Hc2 anisotropy gamma_H was found to decrease with increasing temperature,
in disagreement with the anisotropic Ginzburg-Landau theory, which predicts
that the gamma_H is temperature independent. This behaviour can be explained by
the two band nature of superconductivity in MgB2. An analysis of measurements
of the reversible torque in the mixed state yields a field dependent effective
anisotropy gamma_eff, which can be at least partially explained by different
anisotropies of the penetration depth and the upper critical field. It is shown
that a peak effect in fields of about 0.85 Hc2 is a manifestation of an
order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2
for H//c correlates with the intermediate strength of thermal fluctuations in
MgB2, as compared to those in high and low Tc superconductors.Comment: 12 pages, 8 figures, Physica C in print (invited paper for a special
issue on MgB2
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