24 research outputs found
Local density of states induced by anisotropic impurity scattering in a d-wave superconductor
We study a single impurity effect on the local density of states in a d-wave
superconductor accounting for the momentum-dependent impurity potential. We
show that the anisotropy of the scattering potential can alter significantly
the spatial dependence of the quasiparticle density of states in the vicinity
of the impurity.Comment: 8 pages, revtex4, 14 figure
Effect of anisotropic impurity scattering on a density of states of a d-wave superconductor
We discuss the effect of an anisotropic impurity potential on the critical
temperature, local density of states in the vicinity of a single impurity, and
the quasiparticle density of states for a finite impurity concentration in a
d-wave superconductor. Different scattering regimes are concerned.Comment: 3 pages, revtex4, 4 figure
The Critical Temperature of an Anisotropic Superconductor in the Presence of a Homogeneous Magnetic Field and Impurities
The effect of a homogeneous magnetic field and nonmagnetic impurities on the
critical temperature of an anisotropic superconductor has been investigated.
The role of these pair-breakers in relation to the anisotropy of the order
parameter is clarified.Comment: 7 pages, RevTeX, 1 PostScript figur
Combined potential and spin impurity scattering in cuprates
We present a theory of combined nonmagnetic and magnetic impurity scattering
in anisotropic superconductors accounting for the momentum-dependent impurity
potential. Applying the model to the d-wave superconducting state, we obtain a
quantitative agreement with the initial suppression of the critical temperature
due to Zn and Ni substitutions as well as electron irradiation defects in the
cuprates. We suggest, that the unequal pair-breaking effect of Zn and Ni may be
related to a different nature of the magnetic moments induced by these
impurities.Comment: 5 pages, 3 tables, RevTex, to be published in Phys. Rev.
Weak anisotropic impurity scattering in unconventional superconductors
The effect of weak anisotropic (momentum-dependent) impurity scattering in
unconventional superconductors has been investigated. It is shown that the
anisotropic scattering can lead either to a small reduction or a small
enhancement of the isotropic pair-breaking effect. The influence of the
anisotropy of the scattering potential becomes significant for the order
parameters with large Fermi surface average values. In that case an unexpected
enhancement (up to 10%) of the critical temperature over the critical
temperature in the absence of impurities is predicted for a small impurity
concentration.Comment: 12 pages, RevTeX, 3 PostScript figure
Isotope effect in impure high T_c superconductors
The influence of various kinds of impurities on the isotope shift exponent
\alpha of high temperature superconductors has been studied. In these materials
the dopant impurities, like Sr in La_{2-x}Sr_xCuO_4, play different role and
usually occupy different sites than impurities like Zn, Fe, Ni {\it etc}
intentionally introduced into the system to study its superconducting
properties.
In the paper the in-plane and out-of-plane impurities present in layered
superconductors have been considered. They differently affect the
superconducting transition temperature T_c. The relative change of isotope
shift coefficient, however, is an universal function of T_c/T_{c0} (T_{c0}
reffers to impurity free system) {\it i.e.} for angle independent scattering
rate and density of states function it does not depend whether the change of
T_c is due to in- or out-of-plane impurities. The role of the anisotropic
impurity scattering in changing oxygen isotope coefficient of superconductors
with various symmetries of the order parameter is elucidated. The comparison of
the calculated and experimental dependence of \alpha/\alpha_0, where \alpha_0
is the clean system isotope shift coefficient, on T_c/T_{c0} is presented for a
number of cases studied.
The changes of \alpha calculated within stripe model of superconductivity in
copper oxides resonably well describe the data on
La_{1.8}Sr_{0.2}Cu_{1-x}(Fe,Ni)_xO_4, without any fitting parameters.Comment: 8 pages, 6 figures, Phys. Rev. B67 (2003) accepte
Effect of anisotropic impurity scattering in superconductors
We discuss the weak-coupling BCS theory of a superconductor with the
impurities, accounting for their anisotropic momentum-dependent potential. The
impurity scattering process is considered in the t-matrix approximation and its
influence on the superconducting critical temperature is studied in the Born
and unitary limit for a d- and (d+s)-wave superconductors. We observe a
significant dependence of the pair-breaking strength on the symmetry of the
scattering potential and classify the impurity potentials according to their
ability to alter T_c. A good agreement with the experimental data for Zn doping
and oxygen irradiation in the overdoped cuprates is found.Comment: 31 pages, RevTex, 15 PostScript figure
Role of anisotropic impurity scattering in anisotropic superconductors
A theory of nonmagnetic impurities in an anisotropic superconductor including
the effect of anisotropic (momentum-dependent) impurity scattering is given. It
is shown that for a strongly anisotropic scattering the reduction of the
pair-breaking effect of the impurities is large. For a significant overlap
between the anisotropy functions of the scattering potential and that of the
pair potential and for a large amount of anisotropic scattering rate in
impurity potential the superconductivity becomes robust vis a vis impurity
concentration. The implications of our result for YBCO high-temperature
superconductor are discussed.Comment: 14 pages, RevTeX, 5 PostScript figures, to be published in Phys. Rev.
B (December 1, 1996
Magnetic-Field Variations of the Pair-Breaking Effects of Superconductivity in (TMTSF)2ClO4
We have studied the onset temperature of the superconductivity Tc_onset of
the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction
of the magnetic field H. We compare the results of two samples with nearly the
same onset temperature but with different scattering relaxation time tau. We
revealed a complicated interplay of a variety of pair-breaking effects and
mechanisms that overcome these pair-breaking effects. In low fields, the linear
temperature dependences of the onset curves in the H-T phase diagrams are
governed by the orbital pair-breaking effect. The dips in the in-plane
field-angle phi dependence of Tc_onset, which were only observed in the
long-tau sample, provides definitive evidence that the field-induced
dimensional crossover enhances the superconductivity if the field direction is
more than about 19-degrees away from the a axis. In the high-field regime for
H//a, the upturn of the onset curve for the long-tau sample indicates a new
superconducting state that overcomes the Pauli pair-breaking effect but is
easily suppressed by impurity scatterings. The Pauli effect is also overcome
for H//b' by a realization of another state for which the maximum of
Tc_onset(phi) occurs in a direction different from the crystalline axes. The
effect on Tc_onset of tilting the applied field out of the conductive plane
suggests that the Pauli effect plays a significant role in determining
Tc_onset. The most plausible explanation of these results is that (TMTSF)2ClO4
is a singlet superconductor and exhibits Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) states in high fields.Comment: 12 pages, 10 figures. To be published in J. Phys. Soc. Jpn. (vol.77,
2008
Magnetic field effects on the density of states of orthorhombic superconductors
The quasiparticle density of states in a two-dimensional d-wave
superconductor depends on the orientation of the in-plane external magnetic
field H. This is because. in the region of the gap nodes, the Doppler shift due
to the circulating supercurrents around a vortex depend on the direction of H.
For a tetragonal system the induced pattern is four-fold symmetric and, at zero
energy, the density of states exhibits minima along the node directions. But
YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes
two-fold symmetric with the position of the minima occuring when H is oriented
along the Fermi velocity at a node on the Fermi surface. The effect of impurity
scattering in the Born and unitary limit is discussed.Comment: 24 pages, 11 Figure