18,579 research outputs found
Explicit Construction of Spin 4 Casimir Operator in the Coset Model
We generalize the Goddard-Kent-Olive (GKO) coset construction to the
dimension 5/2 operator for and compute the fourth order
Casimir invariant in the coset model with the generic unitary minimal
series that can be viewed as perturbations of the
limit, which has been investigated previously in the realization of
free fermion model.Comment: 11 page
Yang-Baxter equation for the asymmetric eight-vertex model
In this note we study `a la Baxter [1] the possible integrable manifolds of
the asymmetric eight-vertex model. As expected they occur when the Boltzmann
weights are either symmetric or satisfy the free-fermion condition but our
analysis clarify the reason both manifolds need to share a universal invariant.
We also show that the free-fermion condition implies three distinct classes of
integrable models.Comment: Latex, 12 pages, 1 figur
Phonons in Nanocrystalline 57Fe
We measured the phonon density of states (DOS) of nanocrystalline Fe by resonant inelastic nuclear γ-ray scattering. The nanophase material shows large distortions in its phonon DOS. We attribute the high energy distortion to lifetime broadening. A damped harmonic oscillator model for the phonons provides a low quality factor, Qu, averaging about 5, but the longitudinal modes may have been broadened most. The nanocrystalline Fe also shows an enhancement in its phonon DOS at energies below 15 meV. The difference in vibrational entropy of the bulk and nanocrystalline Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies
Spin-Coupled Local Distortions in Multiferroic Hexagonal HoMnO3
Local structural measurements have been performed on hexagonal HoMnO3 in
order to ascertain the specific changes in bond distances which accompany
magnetic ordering transitions. The transition from paramagnetic to the
antiferromagetic (noncollinear) phase near ~70 K is dominated by changes in the
a-b plane Mn-Mn bond distances. The spin rotation transition near ~40 K
involves both Mn-Mn and nearest neighbor Ho-Mn interactions while the low
temperature transition below 10 K involves all interactions, Mn-Mn, Ho-Mn
(nearest and next nearest) and Ho-Ho correlations. These changes in bond
distances reveal strong spin-lattice coupling. The similarity in magnitude of
the change in J(Mn-Mn) and J(Ho-Mn) enhances the system frustration. The
structural changes are interpreted in terms of a model of competing spin order
and local structural distortions. Density functional calculations are used to
estimate the energies associated with ionic displacements. The calculations
also reveal asymmetric polarization of the charge density of Ho, O3 and O4
sites along the z-axis in the ferroelectric phase. This polarization
facilitates coupling between Ho atoms on neighboring planes normal to the
z-axis.Comment: 8 figure
Superconductivity from repulsion in LiFeAs: novel s-wave symmetry and potential time-reversal symmetry breaking
We analyze the structure of the pairing interaction and superconducting gap
in LiFeAs by decomposing the pairing interaction for various kz cuts into s-
and d-wave components and by studying the leading superconducting
instabilities. We use the ten orbital tight-binding model, derived from
ab-initio LDA calculations with hopping parameters extracted from the fit to
ARPES experiments. We find that the pairing interaction almost decouples
between two subsets, one consists of the outer hole pocket and two electron
pockets, which are quasi-2D and are made largely out of dxy orbital, and the
other consists of the two inner hole pockets, which are quasi-3D and are made
mostly out of dxz and dyz orbitals. Furthermore, the bare inter-pocket and
intra-pocket interactions within each subset are nearly equal. In this
situation, small changes in the intra-pocket and inter-pocket interactions due
to renormalizations by high-energy fermions give rise to a variety of different
gap structures. We find four different configurations of the s-wave gap
immediately below Tc: the one in which superconducting gap changes sign between
two inner hole pockets and between the outer hole pocket and two electron
pockets, the one in which the gap changes sign between two electron pockets and
three hole pockets, the one in which the gap on the outer hole pocket differs
in sign from the gaps on the other four pockets, and the one in which the gaps
on two inner hole pockets have one sign, and the gaps on the outer hole pockets
and on electron pockets have different sign. Different s-wave gap
configurations emerge depending on whether the renormalized interactions
increase attraction within each subset or increase the coupling between
particular components of the two subsets. We argue that the state with opposite
sign of the gaps on the two inner hole pockets has the best overlap with ARPES
data.Comment: 23 pages, 15 figure
Time-convolutionless reduced-density-operator theory of a noisy quantum channel: a two-bit quantum gate for quantum information processing
An exact reduced-density-operator for the output quantum states in
time-convolutionless form was derived by solving the quantum Liouville equation
which governs the dynamics of a noisy quantum channel by using a projection
operator method and both advanced and retarded propagators in time. The
formalism developed in this work is general enough to model a noisy quantum
channel provided specific forms of the Hamiltonians for the system, reservoir,
and the mutual interaction between the system and the reservoir are given.
Then, we apply the formulation to model a two-bit quantum gate composed of
coupled spin systems in which the Heisenberg coupling is controlled by the
tunneling barrier between neighboring quantum dots. Gate Characteristics
including the entropy, fidelity, and purity are calculated numerically for both
mixed and entangled initial states
Boundary Flows in general Coset Theories
In this paper we study the boundary effects for off-critical integrable field
theories which have close analogs with integrable lattice models. Our models
are the coset conformal field theories
perturbed by integrable boundary and bulk operators. The boundary interactions
are encoded into the boundary reflection matrix. Using the TBA method, we
verify the flows of the conformal BCs by computing the boundary entropies.
These flows of the BCs have direct interpretations for the fusion RSOS lattice
models. For super CFTs () we show that these flows are possible only for
the Neveu-Schwarz sector and are consistent with the lattice results. The
models we considered cover a wide class of integrable models. In particular, we
show how the the impurity spin is screened by electrons for the -channel
Kondo model by taking limit. We also study the problem using an
independent method based on the boundary roaming TBA. Our numerical results are
consistent with the boundary CFTs and RSOS TBA analysis.Comment: 22 pages, 3 postscript figure file
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