1,708 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
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
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
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
Finite Projective Spaces, Geometric Spreads of Lines and Multi-Qubits
Given a (2N - 1)-dimensional projective space over GF(2), PG(2N - 1, 2), and
its geometric spread of lines, there exists a remarkable mapping of this space
onto PG(N - 1, 4) where the lines of the spread correspond to the points and
subspaces spanned by pairs of lines to the lines of PG(N - 1, 4). Under such
mapping, a non-degenerate quadric surface of the former space has for its image
a non-singular Hermitian variety in the latter space, this quadric being {\it
hyperbolic} or {\it elliptic} in dependence on N being {\it even} or {\it odd},
respectively. We employ this property to show that generalized Pauli groups of
N-qubits also form two distinct families according to the parity of N and to
put the role of symmetric operators into a new perspective. The N=4 case is
taken to illustrate the issue.Comment: 3 pages, no figures/tables; V2 - short introductory paragraph added;
V3 - to appear in Int. J. Mod. Phys.
Heat Transport and the Nature of the Order Parameter in Superconducting
Recent thermal conductivity data on the heavy fermion superconductor
have been interpreted as offering support for an model of the order
parameter as opposed to an model. In this paper, we analyze this issue
from a theoretical standpoint including the detailed effects of Fermi surface
and gap anisotropy. Our conclusion is that although current data put strong
constraints on the gap anisotropy, they cannot definitively distinguish between
these two models. Measurements on samples of varying quality could be decisive
in this regard, however.Comment: 8 pages, revtex, 15 uunencoded postscript figure
Pseudogap in the microwave response of YBa_2Cu_3O_{7-x}
The in-plane and out-of-plane surface impedance and microwave conductivity
components of one and the same YBa_2Cu_3O_{7-x} (0.07\le x\le 0.47) single
crystal are determined in the wide ranges of temperature T and carrier
concentration p in CuO_2 planes. The following features of the superfluid
density n_s(T,p)\propto\lambda_{ab}^{-2}(T,p) are observed at T<Tc/2 and
0.078\le p\le 0.16: (i) n_s(0,p) depends linearly on p, (ii) the derivative
|dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately
doped regions (0.10<p\le 0.16); however, it rapidly increases with p further
lowering and (iii) the latter finding is accompanied by the linear
low-temperature dependence \Delta n_s(T)\propto(-T) changing to \Delta
n_s(T)\propto(-\sqrt{T}). For optimum oxygen content the temperature dependence
of the normalized imaginary part of the c-axis conductivity
\lambda_c^2(0)/\lambda_c^2(T) is found to be strikingly similar to that of
\lambda_{ab}^2(0)/\lambda_{ab}^2(T) and becomes more convex with p lowering.
\lambda_c^{-2}(0,p) values are roughly proportional to the normal state
conductivities \sigma_c(T_c,p) along the c-axis. All these properties can be
treated in the framework of d-density wave order of pseudogap.Comment: 7 pages, 9 figures, presented at EUCAS 2003 (September 14-18),
submitted to SUS
AIOCJ: A Choreographic Framework for Safe Adaptive Distributed Applications
We present AIOCJ, a framework for programming distributed adaptive
applications. Applications are programmed using AIOC, a choreographic language
suited for expressing patterns of interaction from a global point of view. AIOC
allows the programmer to specify which parts of the application can be adapted.
Adaptation takes place at runtime by means of rules, which can change during
the execution to tackle possibly unforeseen adaptation needs. AIOCJ relies on a
solid theory that ensures applications to be deadlock-free by construction also
after adaptation. We describe the architecture of AIOCJ, the design of the AIOC
language, and an empirical validation of the framework.Comment: Technical Repor
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
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