8,489 research outputs found
One-dimensional itinerant ferromagnets with Heisenberg symmetry and the ferromagnetic quantum critical point
We study one-dimensional itinerant ferromagnets with Heisenberg symmetry near
a ferromagnetic quantum critical point. It is shown that the Berry phase term
arises in the effective action of itinerant ferromagnets when the full SU(2)
symmetry is present. We explicitly demonstrate that dynamical critical exponent
of the theory with the Berry term is in the sense of
expansion, as previously discovered in the Ising limit. It appears,
however, that the universality class at the interacting fixed point is not the
same. We point out that even though the critical theory in the Ising limit can
be obtained by the standard Hertz-Millis approach, the Heisenberg limit is
expected to be different. We also calculate the exact electron Green functions
and near the transition in a range of temperature, which
can be used for experimental signatures of the associated critical points.Comment: Replaced with final version accepted in PRB; minor changes from the
previous versio
The color-singlet contribution to e^+ e^- ->J/psi + X at the endpoint
Recent observations of the J/psi spectrum produced in e^+e^- collisions at
the Upsilon(4S) resonance are in conflict with fixed-order calculations using
Non-Relativsitic QCD effective theory (NRQCD). One problem is an enhancement in
the cross section when the J/psi has maximal energy, due to large perturbative
corrections (Sudakov logarithms). In a recent paper, the Sudakov logarithms in
the color-octet contribution were summed by combining NRQCD with the
Soft-Collinear Effective Theory. However to be consistent, the color-singlet
contributions must also be summed in the endpoint region which was not done in
that paper. In this paper, we sum the leading and next-to-leading logarithms in
the color-singlet contribution to the J/psi production cross section. We find
that the color-singlet cross section is suppressed near endpoint compared to
the fixed order NRQCD prediction.Comment: 17 pages, 7 figure
Bose-Hubbard model on a star lattice
We analyze the Bose-Hubbard model of hardcore bosons with nearest neighbor
hopping and repulsive interactions on a star lattice using both quantum Monte
Carlo simulation and dual vortex theory. We obtain the phase diagram of this
model as a function of the chemical potential and the relative strength of
hopping and interaction. In the strong interaction regime, we find that the
Mott phases of the model at 1/2 and 1/3 fillings, in contrast to their
counterparts on square, triangular, and Kagome lattices, are either
translationally invariant resonant valence bond (RVB) phases with no
density-wave order or have coexisting density-wave and RVB orders. We also find
that upon increasing the relative strength of hopping and interaction, the
translationally invariant Mott states undergo direct second order
superfluid-insulator quantum phase transitions. We compute the critical
exponents for these transitions and argue using the dual vortex picture that
the transitions, when approached through the tip of the Mott lobe, belong to
the inverted XY universality class.Comment: 10 pages, 18 figures, minor changes, two references adde
Nematic domains and resistivity in an itinerant metamagnet coupled to a lattice
The nature of the emergent phase near a putative quantum critical point in
the bilayer ruthenate SrRuO has been a recent subject of intensive
research. It has been suggested that this phase may possess electronic nematic
order(ENO). In this work, we investigate the possibility of nematic domain
formation in the emergent phase, using a phenomenological model of electrons
with ENO and its coupling to lattice degrees of freedom. The resistivity due to
the scattering off the domain walls is shown to closely follow the ENO
parameter. Our results provide qualitative explanations for the dependence of
the resistivity on external magnetic fields in SrRuO.Comment: 4 pages, 4 figures, published versio
Superfluid-Insulator transitions of bosons on Kagome lattice at non-integer fillings
We study the superfluid-insulator transitions of bosons on the Kagome lattice
at incommensurate filling factors f=1/2 and 2/3 using a duality analysis. We
find that at f=1/2 the bosons will always be in a superfluid phase and
demonstrate that the T_3 symmetry of the dual (dice) lattice, which results in
dynamic localization of vortices due to the Aharanov-Bohm caging effect, is at
the heart of this phenomenon. In contrast, for f=2/3, we find that the bosons
exhibit a quantum phase transition between superfluid and translational
symmetry broken Mott insulating phases. We discuss the possible broken
symmetries of the Mott phase and elaborate the theory of such a transition.
Finally we map the boson system to a XXZ spin model in a magnetic field and
discuss the properties of this spin model using the obtained results.Comment: 10 pages, 8 figures, a few typos correcte
U-Spin Tests of the Standard Model and New Physics
Within the standard model, a relation involving branching ratios and direct
CP asymmetries holds for the B-decay pairs that are related by U-spin. The
violation of this relation indicates new physics (NP). In this paper, we assume
that the NP affects only the Delta S = 1 decays, and show that the NP operators
are generally the same as those appearing in B -> pi K decays. The fit to the
latest B -> pi K data shows that only one NP operator is sizeable. As a
consequence, the relation is expected to be violated for only one decay pair:
Bd -> K0 pi0 and Bs -> Kbar0 pi0.Comment: 12 pages, latex, no figures. References changed to follow MPL
guidelines; info added about U-spin breaking and small NP strong phases;
discussion added about final-state pi-K rescattering; analysis and
conclusions unaltere
Critical Current of the Spin-Triplet Superconducting Phase in SrRuO
There have been two different proposals for the spin-triplet order parameter
of the superconducting phase in SrRuO; an -wave order parameter and
the multigap model where some of the bands have the line node. In an effort to
propose an experiment that can distinguish two cases, we study the behavior of
the supercurrent and compute the critical current for these order parameters
when the sample is a thin film with the thickness where is
the coherence length. It is found that the supercurrent behaves very
differently in two models. This will serve as a sharp test for the
identification of the correct order parameter.Comment: 4 pages, 1 figur
Valence Bond Solids and Their Quantum Melting in Hard-Core Bosons on the Kagome Lattice
Using large scale quantum Monte Carlo simulations and dual vortex theory we
analyze the ground state phase diagram of hard-core bosons on the kagome
lattice with nearest neighbor repulsion. In contrast to the case of a
triangular lattice, no supersolid emerges for strong interactions. While a
uniform superfluid prevails at half-filling, two novel solid phases emerge at
densities and . These solids exhibit an only partial
ordering of the bosonic density, allowing for local resonances on a subset of
hexagons of the kagome lattice. We provide evidence for a weakly first-order
phase transition at the quantum melting point between these solid phases and
the superfluid.Comment: 4 pages, 7 figure
Is there New Physics in B Decays ?
Rare decays of the meson are sensitive to new physics effects. Several
experimental results on these decays have been difficult to understand within
the standard model (SM) though more precise measurements and a better
understanding of SM theory predictions are needed before any firm conclusions
can be drawn. In this talk we try to understand the present data assuming the
presence of new physics. We find that the data points to new physics of an
extended Higgs sector and we present a two higgs doublet model with a 2-3
flavor symmetry in the down type quark sector that can explain the deviations
from standard model reported in several rare B decays.Comment: 8 pages, Talk presented at Theory Canada II, Perimeter Institute,
Waterloo, Canada. New references added and update
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