1,173 research outputs found

### Nernst effect and diamagnetic response in a stripe model of superconducting cuprates

We examine the possibility that the experimentally observed enhancement of
superconducting (SC) fluctuations above the SC transition temperature in the
underdoped cuprates is caused by stripes -- an intrinsic electronic
inhomogeneity, common to hole-doped cuprates. By evaluating the strengths of
the diamagnetic response and the Nernst effect within the striped SC model, we
find results that are qualitatively consistent with the experimental
observations. We make a prediction for anisotropic thermopower in detwinned
samples that can be used to further test the proposed scenario.Comment: 5 pages, 3 figures; final version published in Europhysics Letter

### Effects of doping on thermally excited quasiparticles in the high-$T_c$ superconducting state

The physical properties of low energy superconducting quasiparticles in high-
$T_c$ superconductors are examined using magnetic penetration depth and
specific heat experimental data. We find that the low energy density of states
of quasiparticles of La$_{2-x}$Sr$_x$CuO$_4$ scales with $(x-x_c)/T_c$ to the
leading order approximation, where $x_c$ is the critical doping concentration
below which $T_c=0$. The linear temperature term of the superfluid density is
renormalized by quasiparticle interactions and the renormalization factor times
the Fermi velocity is found to be doping independent.Comment: 3 pages, 3 figures, minor change to the content, fig1 is reploted, to
appear in Phys Rev

### The Critical Hopping Parameter in O(a) improved Lattice QCD

We calculate the critical value of the hopping parameter, $\kappa_c$, in O(a)
improved Lattice QCD, to two loops in perturbation theory. We employ the
Sheikholeslami-Wohlert (clover) improved action for Wilson fermions.
The quantity which we study is a typical case of a vacuum expectation value
resulting in an additive renormalization; as such, it is characterized by a
power (linear) divergence in the lattice spacing, and its calculation lies at
the limits of applicability of perturbation theory.
The dependence of our results on the number of colors $N$, the number of
fermionic flavors $N_f$, and the clover parameter $c_{SW}$, is shown
explicitly. We compare our results to non perturbative evaluations of
$\kappa_c$ coming from Monte Carlo simulations.Comment: 11 pages, 2 EPS figures. The only change with respect to the original
version is inclusion of the standard formulae for the gauge fixing and ghost
parts of the action. Accepted for publication in Physical Review

### Low Temperature Superfluid Response of High-Tc Superconductors

We have reviewed our theoretical and experimental results of the low
temperature superfluid response function of high temperature superconductors
(HTSC). In clean high-Tc materials the in-plane superfluid density rho_s^{ab}
varies linearly with temperature. The slope of this linear T term is found to
scale approximately with 1/Tc which, according to the weak coupling BCS theory
for a d-wave superconductor, implies that the gap amplitude scales
approximately with Tc. A T^5 behavior of the out-of-plane superfluid density
rho_s^c for clean tetragonal HTSC was predicted and observed experimentally in
the single layer Hg-compound HgBa_2CuO_{4+delta}. In other tetragonal high-Tc
compounds with relatively high anisotropy, such as Hg_2Ba_2Ca_2Cu_3O_{8+delta},
rho_s^c varies as T^2 due to disorder effects. In optimally doped
YBa_2Cu_3O_{7-delta}, rho_s^c varies linearly with temperature at low
temperatures, but in underdoped YBa_2Cu_3O_{7-delta}, rho_s^c varies as T^2 at
low temperatures; these results are consistent with our theoretical
calculations.Comment: 26 pages, 8 figure

### Universal optimal hole-doping concentration in single-layer high-temperature cuprate superconductors

We argue that in cuprate physics there are two types, hole content per
CuO$_2$ plane ($P_{pl}$) and the corresponding hole content per unit volume
($P_{3D}$), of hole-doping concentrations for addressing physical properties
that are two-dimensional (2D) and three-dimensional (3D) in nature,
respectively. We find that superconducting transition temperature ($T_c$)
varies systematically with $P_{3D}$ as a superconducting \textquotedblleft
$dome$\textquotedblright with a universal optimal hole-doping concentration
$P_{3D}^{opt.}$ = 1.6 $\times$ 10$^{21}$ cm$^{-3}$ for single-layer high
temperature superconductors. We suggest that $P_{3D}^{opt.}$ determines the
upper bound of the electronic energy of underdoped single-layer high-$T_c$
cuprates.Comment: 8 pages, 4 figures; added references ;accepted for the publication in
Supercond. Sci. Technol ; Ref. 13 is revise

### Penetration Depth Measurements in MgB_2: Evidence for Unconventional Superconductivity

We have measured the magnetic penetration depth of the recently discovered
binary superconductor MgB_2 using muon spin rotation and low field
$ac$-susceptibility. From the damping of the muon precession signal we find the
penetration depth at zero temperature is about 85nm. The low temperature
penetration depth shows a quadratic temperature dependence, indicating the
presence of nodes in the superconducting energy gap.Comment: 4 pages 3 figure

### Lattice effects in the La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4}$ compounds

Systematic Raman studies on several cuprates (YBa$_{\rm 2}$Cu$_{\rm
3}$O$_{\rm x}$, YBa$_{\rm 2}$Cu$_{\rm 4}$O$_{\rm 8}$ or Bi$_{\rm 2}$Sr$_{\rm
2}$CaCu$_{\rm 2}$O$_{\rm 8}$) have shown that at optimal doping the compounds
are at the edge of lattice instability; once this level is exceeded, by means
of doping or applying external hydrostatic pressure, the changes in the
transition temperature are accompanied by spectral modifications. There are
strong indications that the reduction in T$_{\rm c}$ is correlated with a
separation into nanoscale phases, which involve the oxygen atoms of the
CuO$_{\rm 2}$ planes. In this work, modifications with doping in the Raman
spectra of the La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4}$ compound are presented,
which show that spin or charge ordering is coupled with lattice distortions in
the whole doping region.Comment: 6 pages, 6 figure

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