2,276 research outputs found
Kac and New Determinants for Fractional Superconformal Algebras
We derive the Kac and new determinant formulae for an arbitrary (integer)
level fractional superconformal algebra using the BRST cohomology
techniques developed in conformal field theory. In particular, we reproduce the
Kac determinants for the Virasoro () and superconformal () algebras.
For there always exist modules where the Kac determinant factorizes
into a product of more fundamental new determinants. Using our results for
general , we sketch the non-unitarity proof for the minimal series;
as expected, the only unitary models are those already known from the coset
construction. We apply the Kac determinant formulae for the spin-4/3
parafermion current algebra ({\em i.e.}, the fractional superconformal
algebra) to the recently constructed three-dimensional flat Minkowski
space-time representation of the spin-4/3 fractional superstring. We prove the
no-ghost theorem for the space-time bosonic sector of this theory; that is, its
physical spectrum is free of negative-norm states.Comment: 33 pages, Revtex 3.0, Cornell preprint CLNS 93/124
Unconventional electronic Raman spectra of borocarbide superconductors
Borocarbide superconductors, which are thought to be conventional BCS-type
superconductors, are not so conventional in several electronic Raman
properties. Anisotropic gap-like features and finite scattering strength below
the gap were observed for the NiBC ( = Lu, Y) systems in our
previous study. The effects of Co-doping on the 2 gap-like features and
the finite scattering strength below and above the gap are studied in = Lu
(B = B) system. In superconducting states, Co-doping strongly suppresses
the 2 peak in both B and B symmetries. Raman
cross-section calculation which includes inelastic scattering shows a
relatively good fit to the features above the 2 peak, while it does not
fully account for the features below the peak.Comment: 2 pages, 1 figur
Low-Lying States of the Six-Dimensional Fractional Superstring
The fractional superstring Fock space is constructed in terms of
\bZ_4 parafermions and free bosons. The bosonization of the \bZ_4
parafermion theory and the generalized commutation relations satisfied by the
modes of various parafermion fields are reviewed. In this preliminary analysis,
we describe a Fock space which is simply a tensor product of \bZ_4
parafermion and free boson Fock spaces. It is larger than the Lorentz-covariant
Fock space indicated by the fractional superstring partition function. We
derive the form of the fractional superconformal algebra that may be used as
the constraint algebra for the physical states of the FSS. Issues concerning
the associativity, modings and braiding properties of the fractional
superconformal algebra are also discussed. The use of the constraint algebra to
obtain physical state conditions on the spectrum is illustrated by an
application to the massless fermions and bosons of the fractional
superstring. However, we fail to generalize these considerations to the massive
states. This means that the appropriate constraint algebra on the fractional
superstring Fock space remains to be found. Some possible ways of doing this
are discussed.Comment: 69 pages, LaTeX, CLNS 91/112
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
Effect of Ordering on Spinodal Decomposition of Liquid-Crystal/Polymer Mixtures
Partially phase-separated liquid-crystal/polymer dispersions display highly
fibrillar domain morphologies that are dramatically different from the typical
structures found in isotropic mixtures. To explain this, we numerically explore
the coupling between phase ordering and phase separation kinetics in model
two-dimensional fluid mixtures phase separating into a nematic phase, rich in
liquid crystal, coexisting with an isotropic phase, rich in polymer. We find
that phase ordering can lead to fibrillar networks of the minority polymer-rich
phase
Two-spinon dynamic structure factor of the one-dimensional S=1/2 Heisenberg antiferromagnet
The exact expression derived by Bougourzi, Couture, and Kacir for the
2-spinon contribution to the dynamic spin structure factor
of he one-dimensional =1/2 Heisenberg antiferromagnet at is evaluated
for direct comparison with finite-chain transition rates () and an
approximate analytical result previously inferred from finite- data, sum
rules, and Bethe-ansatz calculations. The 2-spinon excitations account for
72.89% of the total intensity in . The singularity structure
of the exact result is determined analytically and its spectral-weight
distribution evaluated numerically over the entire range of the 2-spinon
continuum. The leading singularities of the frequency-dependent spin
autocorrelation function, static spin structure factor, and -dependent
susceptibility are determined via sum rules.Comment: 6 pages (RevTex) and 5 figures (Postscript
Temperature-dependent Raman spectroscopy in BaRuO systems
We investigated the temperature-dependence of the Raman spectra of a
nine-layer BaRuO single crystal and a four-layer BaRuO epitaxial film,
which show pseudogap formations in their metallic states. From the polarized
and depolarized spectra, the observed phonon modes are assigned properly
according to the predictions of group theory analysis. In both compounds, with
decreasing temperature, while modes show a strong hardening, (or
) modes experience a softening or no significant shift. Their different
temperature-dependent behaviors could be related to a direct Ru metal-bonding
through the face-sharing of RuO. It is also observed that another
mode of the oxygen participating in the face-sharing becomes split at low
temperatures in the four layer BaRuO. And, the temperature-dependence of
the Raman continua between 250 600 cm is strongly correlated to
the square of the plasma frequency. Our observations imply that there should be
a structural instability in the face-shared structure, which could be closely
related to the pseudogap formation of BaRuO systems.Comment: 8 pages, 6 figures. to be published in Phys. Rev.
BCS theory for s+g-wave superconductivity borocarbides Y(Lu)NiBC
The s+g mixed gap function \Delta_k=\Delta {[(1-x)-x\sin^4\theta\cos4\phi]}
(x: weight of g-wave component) has been studied within BCS theory. By suitable
consideration of the pairing interaction, we have confirmed that the
coexistence of s- and g-wave, as well as the state with equal s and g
amplitudes (i.e., x=1/2) may be stable. This provides the semi-phenomenological
theory for the s+g-wave superconductivity with point nodes which has been
observed experimentally in borocarbides YNi_2B_2C and possibly in LuNi_2B_2C.Comment: 5 pages, 3 figure
Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals
We discuss the irreversible magnetic properties of self-flux grown
Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering
the whole phase diagram from the underdoped to the overdoped regime, x=0.038,
0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized
by a magneto-optical method and show excellent spatial uniformity of the
superconducting state. The overall behavior closely follows classical Bean
model of the critical state. The field-dependent magnetization exhibits second
peak at a temperature and doping - dependent magnetic field, Hp. The evolution
of this fishtail feature with doping is discussed. Magnetic relaxation is
time-logarithmic and unusually fast. Similar to cuprates, there is an apparent
crossover from collective elastic to plastic flux creep above Hp. At high
fields, the field dependence of the relaxation rate becomes doping independent.
We discuss our results in the framework of the weak collective pinning and show
that vortex physics in iron-based pnictide crystals is much closer to high-Tc
cuprates than to conventional s-wave (including MgB2) superconductors.Comment: for the special issue of Physica C on iron-based pnictide
superconductor
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