1,426 research outputs found
G(2) quivers
We present, in explicit matrix representation and a modernity befitting the community, the classification of the finite discrete subgroups of G2 and compute the McKay quivers arising therefrom. Of physical interest are the classes of Script N = 1 gauge theories descending from M-theory and of mathematical interest are possible steps toward a systematic study of crepant resolutions to smooth G2 manifolds as well as generalised McKay Correspondences. This writing is a companion monograph to hep-th/9811183 and hep-th/9905212, wherein the analogues for Calabi-Yau three- and four-folds were considered
A Two-dimensional Superconductor in a Tilted Magnetic Field - new states with finite Cooper-pair momentum
Varying the angle Theta between applied field and the conducting planes of a
layered superconductor in a small interval close to the plane-parallel field
direction, a large number of superconducting states with unusual properties may
be produced. For these states, the pair breaking effect of the magnetic field
affects both the orbital and the spin degree of freedom. This leads to pair
wave functions with finite momentum, which are labeled by Landau quantum
numbers 0<n<\infty. The stable order parameter structure and magnetic field
distribution for these states is found by minimizing the quasiclassical free
energy near H_{c2} including nonlinear terms. One finds states with coexisting
line-like and point-like order parameter zeros and states with coexisting
vortices and antivortices. The magnetic response may be diamagnetic or
paramagnetic depending on the position within the unit cell. The structure of
the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at Theta=0 is reconsidered.
The transition n->\infty of the paramagnetic vortex states to the FFLO-limit is
analyzed and the physical reason for the occupation of higher Landau levels is
pointed out.Comment: 24 pages, 11 figure
Super-Hubbard models and applications
We construct XX- and Hubbard- like models based on unitary superalgebras
gl(N|M) generalising Shastry's and Maassarani's approach of the algebraic case.
We introduce the R-matrix of the gl(N|M) XX model and that of the Hubbard model
defined by coupling two independent XX models. In both cases, we show that the
R-matrices satisfy the Yang--Baxter equation, we derive the corresponding local
Hamiltonian in the transfer matrix formalism and we determine the symmetry of
the Hamiltonian. Explicit examples are worked out. In the cases of the gl(1|2)
and gl(2|2) Hubbard models, a perturbative calculation at two loops a la Klein
and Seitz is performed.Comment: 26 page
Quantized bulk fermions in the Randall-Sundrum brane model
The lowest order quantum corrections to the effective action arising from
quantized massive fermion fields in the Randall-Sundrum background spacetime
are computed. The boundary conditions and their relation with gauge invariance
are examined in detail. The possibility of Wilson loop symmetry breaking in
brane models is also analysed. The self-consistency requirements, previously
considered in the case of a quantized bulk scalar field, are extended to
include the contribution from massive fermions. It is shown that in this case
it is possible to stabilize the radius of the extra dimensions but it is not
possible to simultaneously solve the hierarchy problem, unless the brane
tensions are dramatically fine tuned, supporting previous claims.Comment: 25 pages, 1 figure, RevTe
Quantum geometrodynamics: whence, whither?
Quantum geometrodynamics is canonical quantum gravity with the three-metric
as the configuration variable. Its central equation is the Wheeler--DeWitt
equation. Here I give an overview of the status of this approach. The issues
discussed include the problem of time, the relation to the covariant theory,
the semiclassical approximation as well as applications to black holes and
cosmology. I conclude that quantum geometrodynamics is still a viable approach
and provides insights into both the conceptual and technical aspects of quantum
gravity.Comment: 25 pages; invited contribution for the Proceedings of the seminar
"Quantum Gravity: Challenges and Perspectives", Bad Honnef, Germany, April
200
Influence of orbital pair breaking on paramagnetically limited states in clean superconductors
Paramagnetic pair breaking is believed to be of increasing importance in many
layered superconducting materials such as cuprates and organic compounds.
Recently, strong evidence for a phase transition to the
Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state has been obtained for the first
time. We present a new theory of competing spin and orbital pair breaking in
clean superconducting films or layers. As a general result, we find that the
influence of orbital pair breaking on the paramagnetically limited phase
boundary is rather strong, and its neglect seldom justified. This is
particularly true for the FFLO state which can be destroyed by a very small
orbital contribution. We discuss the situation in YBa_2Cu_3O_7 which has two
coupled conducting Cu-O layers per unit cell. As a consequence, an intrinsic
orbital pair breaking component might exist even for applied field exactly
parallel to the layers.Comment: 19 pages, 5 figures, submitted to PR
The spectral gap for some spin chains with discrete symmetry breaking
We prove that for any finite set of generalized valence bond solid (GVBS)
states of a quantum spin chain there exists a translation invariant
finite-range Hamiltonian for which this set is the set of ground states. This
result implies that there are GVBS models with arbitrary broken discrete
symmetries that are described as combinations of lattice translations, lattice
reflections, and local unitary or anti-unitary transformations. We also show
that all GVBS models that satisfy some natural conditions have a spectral gap.
The existence of a spectral gap is obtained by applying a simple and quite
general strategy for proving lower bounds on the spectral gap of the generator
of a classical or quantum spin dynamics. This general scheme is interesting in
its own right and therefore, although the basic idea is not new, we present it
in a system-independent setting. The results are illustrated with an number of
examples.Comment: 48 pages, Plain TeX, BN26/Oct/9
Tomato: a crop species amenable to improvement by cellular and molecular methods
Tomato is a crop plant with a relatively small DNA content per haploid genome and a well developed genetics. Plant regeneration from explants and protoplasts is feasable which led to the development of efficient transformation procedures.
In view of the current data, the isolation of useful mutants at the cellular level probably will be of limited value in the genetic improvement of tomato. Protoplast fusion may lead to novel combinations of organelle and nuclear DNA (cybrids), whereas this technique also provides a means of introducing genetic information from alien species into tomato. Important developments have come from molecular approaches. Following the construction of an RFLP map, these RFLP markers can be used in tomato to tag quantitative traits bred in from related species. Both RFLP's and transposons are in the process of being used to clone desired genes for which no gene products are known. Cloned genes can be introduced and potentially improve specific properties of tomato especially those controlled by single genes. Recent results suggest that, in principle, phenotypic mutants can be created for cloned and characterized genes and will prove their value in further improving the cultivated tomato.
Orientifolds of K3 and Calabi-Yau Manifolds with Intersecting D-branes
We investigate orientifolds of type II string theory on K3 and Calabi-Yau
3-folds with intersecting D-branes wrapping special Lagrangian cycles. We
determine quite generically the chiral massless spectrum in terms of
topological invariants and discuss both orbifold examples and algebraic
realizations in detail. Intriguingly, the developed techniques provide an
elegant way to figure out the chiral sector of orientifold models without
computing any explicit string partition function. As a new example we derive a
non-supersymmetric Standard-like Model from an orientifold of type IIA on the
quintic Calabi-Yau 3-fold with wrapped D6-branes. In the case of supersymmetric
intersecting brane models on Calabi-Yau manifolds we discuss the D-term and
F-term potentials, the effective gauge couplings and the Green-Schwarz
mechanism. The mirror symmetric formulation of this construction is provided
within type IIB theory. We finally include a short discussion about the lift of
these models from type IIB on K3 to F-theory and from type IIA on Calabi-Yau
3-folds to M-theory on G_2 manifolds.Comment: 82 pages, harvmac, 5 figures. v2: references added. v3: T^6
orientifold corrected, JHEP versio
Transport properties of strongly correlated metals:a dynamical mean-field approach
The temperature dependence of the transport properties of the metallic phase
of a frustrated Hubbard model on the hypercubic lattice at half-filling are
calculated. Dynamical mean-field theory, which maps the Hubbard model onto a
single impurity Anderson model that is solved self-consistently, and becomes
exact in the limit of large dimensionality, is used. As the temperature
increases there is a smooth crossover from coherent Fermi liquid excitations at
low temperatures to incoherent excitations at high temperatures. This crossover
leads to a non-monotonic temperature dependence for the resistance,
thermopower, and Hall coefficient, unlike in conventional metals. The
resistance smoothly increases from a quadratic temperature dependence at low
temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar
a/e^2 (where "a" is a lattice constant) associated with mean-free paths less
than a lattice constant. Further signatures of the thermal destruction of
quasiparticle excitations are a peak in the thermopower and the absence of a
Drude peak in the optical conductivity. The results presented here are relevant
to a wide range of strongly correlated metals, including transition metal
oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure
- …