1,980 research outputs found
Microwave Conductivity due to Impurity Scattering in a d-wave Superconductor
The self-consistent t-matrix approximation for impurity scattering in
unconventional superconductors is used to interpret recent measurements of the
temperature and frequency dependence of the microwave conductivity of YBCO
crystals below 20K. In this theory, the conductivity is expressed in terms of a
fequency dependent single particle self-energy, determined by the impurity
scattering phase shift which is small for weak (Born) scattering and approaches
for unitary scattering. Inverting this process, microwave
conductivity data are used to extract an effective single-particle self-energy
and obtain insight into the nature of the operative scattering processes. It is
found that the effective self-energy is well approximated by a constant plus a
linear term in frequency with a small positive slope for thermal quasiparticle
energies below 20K. Possible physical origins of this form of self-energy are
discussed.Comment: 5 pages, 4 figure
Cubic Curves, Finite Geometry and Cryptography
Some geometry on non-singular cubic curves, mainly over finite fields, is
surveyed. Such a curve has 9,3,1 or 0 points of inflexion, and cubic curves are
classified accordingly. The group structure and the possible numbers of
rational points are also surveyed. A possible strengthening of the security of
elliptic curve cryptography is proposed using a `shared secret' related to the
group law. Cubic curves are also used in a new way to construct sets of points
having various combinatorial and geometric properties that are of particular
interest in finite Desarguesian planes.Comment: This is a version of our article to appear in Acta Applicandae
Mathematicae. In this version, we have corrected a sentence in the third
paragraph. The final publication is available at springerlink.com at
http://www.springerlink.com/content/xh85647871215644
Impurity-Induced Quasiparticle Transport and Universal Limit Wiedemann-Franz Violation in d-Wave Superconductors
Due to the node structure of the gap in a d-wave superconductor, the presence
of impurities generates a finite density of quasiparticle excitations at zero
temperature. Since these impurity-induced quasiparticles are both generated and
scattered by impurities, prior calculations indicate a universal limit (\Omega
-> 0, T -> 0) where the transport coefficients obtain scattering-independent
values, depending only on the velocity anisotropy v_f/v_2. We improve upon
prior results, including the contributions of vertex corrections and Fermi
liquid corrections in our calculations of universal limit electrical, thermal,
and spin conductivity. We find that while vertex corrections modify electrical
conductivity and Fermi liquid corrections renormalize both electrical and spin
conductivity, only thermal conductivity maintains its universal value,
independent of impurity scattering or Fermi liquid interactions. Hence, low
temperature thermal conductivity measurements provide the most direct means of
obtaining the velocity anisotropy for high T_c cuprate superconductors.Comment: 22 pages, 6 figures; revised version to be published in Phys Rev
Theory of Thermal Conductivity in YBa_2Cu_3O_{7-\delta}
We calculate the electronic thermal conductivity in a d-wave superconductor,
including both the effect of impurity scattering and inelastic scattering by
antiferromagnetic spin fluctuations. We analyze existing experiments,
particularly with regard to the question of the relative importance of
electronic and phononic contributions to the heat current, and to the influence
of disorder on low-temperature properties. We find that phonons dominate heat
transport near T_c, but that electrons are responsible for most of the peak
observed in clean samples, in agreement with a recent analysis of Krishana et
al. In agreement with recent data on YBa_2(Cu_1-xZn_x)_3O_7-\delta the peak
position is found to vary nonmonotonically with disorder.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Effect of electron irradiation on superconductivity in single crystals of Ba(FeRu)As (0.24)
A single crystal of isovalently substituted Ba(FeRu)As
() was sequentially irradiated with 2.5 MeV electrons up to a maximum
dose of electrons/cm^2. The electrical resistivity was
measured \textit{in - situ} at 22 K during the irradiation and \textit{ex -
situ} as a function of temperature between subsequent irradiation runs. Upon
irradiation, the superconducting transition temperature, , decreases and
the residual resistivity, , increases. We find that electron
irradiation leads to the fastest suppression of compared to other types
of artificially introduced disorder, probably due to the strong short-range
potential of the point-like irradiation defects. A more detailed analysis
within a multiband scenario with variable scattering potential strength shows
that the observed vs. is fully compatible with pairing,
in contrast to earlier claims that this model leads to a too rapid a
suppression of with scattering
Anisotropic Optical Conductivity of Nd2-xCexCuO4 Thin Films
Opticcal conductivity spectra of Nd2-xCexCuO4 thin films,
measured by the reflectance-transmittance method (R-T method) which has been
proposed to investigate far-infrared spectroscopy, are investigated based on
the anisotropic pairing model. Precise measurements of the frequency-dependent
conductivity enable us to examine quantitatively the nature of the
superconducting gap through infrared properties in the electron-doped high-Tc
superconductors. We show that the behavior of optical conductivity
is consistent with the anisotropic superconducting gap and is well explained by
the formula for d-wave pairing in the low-energy regime of the far-infrared
region. Our results suggest that the electron-doped cuprate superconductors
Nd2-xCexCuO4 have nodes in the superconducting gap.Comment: 4 pages, 3 figure
Finite Density of States in a Mixed State of d_x^2-y^2+id_xy Superconductor
We have calculated the density of states of quasiparticles in a
d_x^2-y^2+id_xy superconductor, and show that in the mixed state the
quasiparticle spectrum remains gapless because of the Doppler shift by
superflow. It was found that if the d_{xy} order gap
as suggested by experiments, then thermal conductivity in accord with experimental data at lowest temperatures. This is an
appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999).
We now also discuss the disorder effects and analyze the H log H crossover at
small fields. We argue that H log H regime is present and disorder effect is
dominant as the field-induced seconary gap is small at small fields.Comment: This is an appended version of the paper published in Phys. Rev. {\bf
B 59}, 6024, (1999). We now also discuss the disorder effects and analyze the
H log H crossover at small fields. 3 pages, Latex file with 2 eps figure
file
d-Wave Model for Microwave Response of High-Tc Superconductors
We develop a simple theory of the electromagnetic response of a d- wave
superconductor in the presence of potential scatterers of arbitrary s-wave
scattering strength and inelastic scattering by antiferromagnetic spin
fluctuations. In the clean London limit, the conductivity of such a system may
be expressed in "Drude" form, in terms of a frequency-averaged relaxation time.
We compare predictions of the theory with recent data on YBCO and BSSCO
crystals and on YBCO films. While fits to penetration depth measurements are
promising, the low temperature behavior of the measured microwave conductivity
appears to be in disagreement with our results. We discuss implications for
d-wave pairing scenarios in the cuprate superconductors.Comment: 33 pages, plain TeX including all macros. 16 uuencoded, compressed
postscript figures are appended at the en
On Invariant Notions of Segre Varieties in Binary Projective Spaces
Invariant notions of a class of Segre varieties \Segrem(2) of PG(2^m - 1,
2) that are direct products of copies of PG(1, 2), being any positive
integer, are established and studied. We first demonstrate that there exists a
hyperbolic quadric that contains \Segrem(2) and is invariant under its
projective stabiliser group \Stab{m}{2}. By embedding PG(2^m - 1, 2) into
\PG(2^m - 1, 4), a basis of the latter space is constructed that is invariant
under \Stab{m}{2} as well. Such a basis can be split into two subsets whose
spans are either real or complex-conjugate subspaces according as is even
or odd. In the latter case, these spans can, in addition, be viewed as
indicator sets of a \Stab{m}{2}-invariant geometric spread of lines of PG(2^m
- 1, 2). This spread is also related with a \Stab{m}{2}-invariant
non-singular Hermitian variety. The case is examined in detail to
illustrate the theory. Here, the lines of the invariant spread are found to
fall into four distinct orbits under \Stab{3}{2}, while the points of PG(7,
2) form five orbits.Comment: 18 pages, 1 figure; v2 - version accepted in Designs, Codes and
Cryptograph
Extinction of impurity resonances in large-gap regions of inhomogeneous d-wave superconductors
Impurity resonances observed by scanning tunneling spectroscopy in the
superconducting state have been used to deduce properties of the underlying
pure state. Here we study a longstanding puzzle associated with these
measurements, the apparent extinction of these resonances for Ni and Zn
impurities in large-gap regions of the inhomogeneous BSCCO superconductor. We
calculate the effect of order parameter and hopping suppression near the
impurity site, and find that these two effects are sufficient to explain the
missing resonances in the case of Ni. There are several possible scenarios for
the extinction of the Zn resonances, which we discuss in turn; in addition, we
propose measurements which could distinguish among them.Comment: 10 pages, 8 figure
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