17,548 research outputs found
Vector Positronium States in QED3
The homogeneous Bethe-Salpeter equation is solved in the quenched ladder
approximation for the vector positronium states of 4-component quantum
electrodynamics in 2 space and 1 time dimensions. Fermion propagator input is
from a Rainbow approximation Dyson-Schwinger solution, with a broad range of
fermion masses considered. This work is an extension of earlier work on the
scalar spectrum of the same model. The non-relativistic limit is also
considered via the large fermion mass limit. Classification of states via their
transformation properties under discrete parity transformations allows
analogies to be drawn with the meson spectrum of QCD.Comment: 24 pages, 2 encapsulated postscript figure
Interplay between electronic topology and crystal symmetry: Dislocation-line modes in topological band-insulators
We elucidate the general rule governing the response of dislocation lines in
three-dimensional topological band insulators. According to this rule, the lattice topology, represented by
dislocation lines oriented in direction with Burgers vector , combines with the electronic-band topology, characterized by the
band-inversion momentum , to produce gapless propagating
modes when the plane orthogonal to the dislocation line features a band
inversion with a nontrivial ensuing flux . Although it has already been discovered by Y. Ran
{\it et al.}, Nature Phys. {\bf 5}, 298 (2009), that dislocation lines host
propagating modes, the exact mechanism of their appearance in conjunction with
the crystal symmetries of a topological state is provided by the rule . Finally, we discuss possible
experimentally consequential examples in which the modes are oblivious for the
direction of propagation, such as the recently proposed
topologically-insulating state in electron-doped BaBiO.Comment: Main text + supplementary material, published versio
Finite Grand Unified Theories and the Quark Mixing Matrix
In N = 1 super Yang-Mills theories, under certain conditions satisfied by the
spectrum and the Yukawa couplings, the beta functions will vanish to all orders
in perturbation theory. We address the generation of realistic quark mixing
angles and masses in such finite Grand Unified Theories. Working in the context
of finite SUSY SU(5), we present several examples with realistic quark mixing
matrices. Non-Abelian discrete symmetries are found to be important in
satisfying the conditions for finiteness. Our realistic examples are based on
permutation symmetries and the tetrahedral symmetry . These examples
enable us to address questions such as the decay rate of the proton in finite
GUTs.Comment: 16 pages, LaTeX, typos correcte
Highly Frustrated Magnetic Clusters: The kagome on a sphere
We present a detailed study of the low-energy excitations of two existing
finite-size realizations of the planar kagome Heisenberg antiferromagnet on the
sphere, the cuboctahedron and the icosidodecahedron. After highlighting a
number of special spectral features (such as the presence of low-lying singlets
below the first triplet and the existence of localized magnons) we focus on two
major issues. The first concerns the nature of the excitations above the
plateau phase at 1/3 of the saturation magnetization Ms. Our exact
diagonalizations for the s=1/2 icosidodecahedron reveal that the low-lying
plateau states are adiabatically connected to the degenerate collinear
``up-up-down'' ground states of the Ising point, at the same time being well
isolated from higher excitations. A complementary physical picture emerges from
the derivation of an effective quantum dimer model which reveals the central
role of the topology and the intrinsic spin s. We also give a prediction for
the low energy excitations and thermodynamic properties of the spin s=5/2
icosidodecahedron Mo72Fe30. In the second part we focus on the low-energy
spectra of the s>1/2 Heisenberg model in view of interpreting the broad
inelastic neutron scattering response reported for Mo72Fe30. To this end we
demonstrate the simultaneous presence of several broadened low-energy ``towers
of states'' or ``rotational bands'' which arise from the large discrete spatial
degeneracy of the classical ground states, a generic feature of highly
frustrated clusters. This semiclassical interpretation is further corroborated
by their striking symmetry pattern which is shown, by an independent group
theoretical analysis, to be a characteristic fingerprint of the classical
coplanar ground states.Comment: 22 pages Added references Corrected typo
Symmetry-protected topological phases of alkaline-earth cold fermionic atoms in one dimension
We investigate the existence of symmetry-protected topological phases in
one-dimensional alkaline-earth cold fermionic atoms with general half-integer
nuclear spin I at half filling. In this respect, some orbital degrees of
freedom are required. They can be introduced by considering either the
metastable excited state of alkaline-earth atoms or the p-band of the optical
lattice. Using complementary techniques, we show that SU(2) Haldane topological
phases are stabilised from these orbital degrees of freedom. On top of these
phases, we find the emergence of topological phases with enlarged SU(2I+1)
symmetry which depend only on the nuclear spin degrees of freedom. The main
physical properties of the latter phases are further studied using a
matrix-product state approach. On the one hand, we find that these phases are
symmetry-protected topological phases, with respect to inversion symmetry, when
I=1/2,5/2,9/2,..., which is directly relevant to ytterbium and strontium cold
fermions. On the other hand, for the other values of I(=half-odd integer),
these topological phases are stabilised only in the presence of exact
SU(2I+1)-symmetry
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