1,201 research outputs found
On bipartite Rokhsar-Kivelson points and Cantor deconfinement
Quantum dimer models on bipartite lattices exhibit Rokhsar-Kivelson (RK)
points with exactly known critical ground states and deconfined spinons. We
examine generic, weak, perturbations around these points. In d=2+1 we find a
first order transition between a ``plaquette'' valence bond crystal and a
region with a devil's staircase of commensurate and incommensurate valence bond
crystals. In the part of the phase diagram where the staircase is incomplete,
the incommensurate states exhibit a gapless photon and deconfined spinons on a
set of finite measure, almost but not quite a deconfined phase in a compact
U(1) gauge theory in d=2+1! In d=3+1 we find a continuous transition between
the U(1) resonating valence bond (RVB) phase and a deconfined staggered valence
bond crystal. In an appendix we comment on analogous phenomena in quantum
vertex models, most notably the existence of a continuous transition on the
triangular lattice in d=2+1.Comment: 9 pages; expanded version to appear in Phys. Rev. B; presentation
improve
Double point contact in Quantum Hall Line Junctions
We show that multiple point contacts on a barrier separating two laterally
coupled quantum Hall fluids induce Aharonov-Bohm (AB) oscillations in the
tunneling conductance. These quantum coherence effects provide new evidence for
the Luttinger liquid behavior of the edge states of quantum Hall fluids. For a
two point contact, we identify coherent and incoherent regimes determined by
the relative magnitude of their separation and the temperature. We analyze both
regimes in the strong and weak tunneling amplitude limits as well as their
temperature dependence. We find that the tunneling conductance should exhibit
AB oscillations in the coherent regime, both at strong and weak tunneling
amplitude with the same period but with different functional form.Comment: 4 pages, 3 figures; new version, edited text, 2 new references;
figure 2 has been edited; new paragraph in page 1 and minor typos have been
correcte
Artificial electric field in Fermi Liquids
Based on the Keldysh formalism, we derive an effective Boltzmann equation for
a quasi-particle associated with a particular Fermi surface in an interacting
Fermi liquid. This provides a many-body derivation of Berry curvatures in
electron dynamics with spin-orbit coupling, which has received much attention
in recent years in non-interacting models. As is well-known, the Berry
curvature in momentum space modifies naive band dynamics via an artificial
magnetic field in momentum space. Our Fermi liquid formulation completes the
reinvention of modified band dynamics by introducing in addition an "artificial
electric field", related to Berry curvature in frequency and momentum space. We
show explicitly how the artificial electric field affects the renormalization
factor and transverse conductivity of interacting U(1) Fermi liquids with
non-degenerate bands. Accordingly, we also propose a method of momentum
resolved Berry's curvature detection in terms of angle resolved photoemission
spectroscopy (ARPES)
Unconventional magnetism in imbalanced Fermi systems with magnetic dipolar interactions
We study the magnetic structure of the ground state of an itinerant Fermi
system of spin-\nicefrac{1}{2} particles with magnetic dipole-dipole
interactions. We show that, quite generally, the spin state of particles depend
on its momentum, i.e., spin and orbital degrees of freedom are entangled and
taken separately are not ``good'' quantum numbers. Specifically, we consider a
uniform system with non-zero magnetization at zero temperature. Assuming the
magnetization is along -axis, the quantum spin states are -dependent
linear combinations of eigenstates of the Pauli matrix. This leads
to novel spin structures in \textit{momentum space} and to the fact that the
Fermi surfaces for ``up'' and ``down'' spins are not well defined. The system
still has a cylindrical axis of symmetry along the magnetization axis. We also
show that the self energy has a universal structure which we determine based on
the symmetries of the dipolar interaction and we explicitly calculated it in
the Hartree-Fock approximation. We show that the bare magnetic moment of
particles is renormalized due to particle-particle interactions and we give
order of magnitude estimates of this renormalization effect. We estimate that
the above mentioned dipolar effects are small but we discuss possible scenarios
where this physics may be realized in future experiments.Comment: 10 pages, 6 figures(2 subfigures); 4 appendices. Version published in
Physical Review
Ice: a strongly correlated proton system
We discuss the problem of proton motion in Hydrogen bond materials with
special focus on ice. We show that phenomenological models proposed in the past
for the study of ice can be recast in terms of microscopic models in close
relationship to the ones used to study the physics of Mott-Hubbard insulators.
We discuss the physics of the paramagnetic phase of ice at 1/4 filling (neutral
ice) and its mapping to a transverse field Ising model and also to a gauge
theory in two and three dimensions. We show that H3O+ and HO- ions can be
either in a confined or deconfined phase. We obtain the phase diagram of the
problem as a function of temperature T and proton hopping energy t and find
that there are two phases: an ordered insulating phase which results from an
order-by-disorder mechanism induced by quantum fluctuations, and a disordered
incoherent metallic phase (or plasma). We also discuss the problem of
decoherence in the proton motion introduced by the lattice vibrations (phonons)
and its effect on the phase diagram. Finally, we suggest that the transition
from ice-Ih to ice-XI observed experimentally in doped ice is the
confining-deconfining transition of our phase diagram.Comment: 12 pages, 9 figure
Mapping the magneto-structural quantum phases of Mn3O4
We present temperature-dependent x-ray diffraction and temperature- and
field-dependent Raman scattering studies of single crystal Mn3O4, which reveal
the novel magnetostructural phases that evolve in the spinels due to the
interplay between strong spin-orbital coupling, geometric frustration, and
applied magnetic field. We observe a structural transition from tetragonal to
monoclinic structures at the commensurate magnetic transition at T2=33K, show
that the onset and nature of this structural transition can be controlled with
an applied magnetic field, and find evidence for a field-tuned quantum phase
transition to a tetragonal incommensurate or spin glass phase.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Lett; typos correcte
Raman scattering studies of temperature- and field-induced melting of charge order in (La,Pr,Ca)MnO
We present Raman scattering studies of the structural and magnetic phases
that accompany temperature- and field-dependent melting of charge- and
orbital-order (COO) in La0.5Ca0.5MnO3 and La0.25Pr0.375Ca0.375MnO3. Our results
show that thermal and field-induced COO melting in La0.5Ca0.5MnO3 exhibits
three stages in a heterogeneous melting process associated with a structural
change: a long-range, strongly JT distorted/COO regime; a coexistence regime;
and weakly JT distorted/PM or FM phase. We provide a complete structural phase
diagram of La0.5Ca0.5MnO3 for the temperature and field ranges 6<=T<=170 K and
0<=H<=9 T. We also investigate thermal and field-induced melting in
La0.25Pr0.375Ca0.375MnO3 to elucidate the role of disorder in melting of COO.
We find that while thermal melting of COO in La0.25Pr0.375Ca0.375MnO3 is quite
similar to that in La0.5Ca0.5MnO3, the field-induced transition from the COO
phase to the weakly JT-distorted/FM phase in La0.25Pr0.375Ca0.375MnO3 is very
abrupt, and occurs at significantly lower fields (H~2 T at T~0 K) than in
La0.5Ca0.5MnO3 (H~30 T at T=0 K). Moreover, the critical field H_c increases
with increasing temperature in La0.25Pr0.375Ca0.375MnO3 in contrast to
La0.5Ca0.5MnO3. To explain these differences, we propose that field-induced
melting of COO in La0.25Pr0.375Ca0.375MnO3 is best described as the
field-induced percolation of FM domains, and we suggest that Griffiths phase
physics may be an appropriate theoretical model for describing the unusual
temperature- and field- dependent transitions observed in
La0.25Pr0.375Ca0.375MnO3.Comment: 14 pages, 8 figures, to be published in PR
Theory of the nodal nematic quantum phase transition in superconductors
We study the character of an Ising nematic quantum phase transition (QPT)
deep inside a d-wave superconducting state with nodal quasiparticles in a
two-dimensional tetragonal crystal. We find that, within a 1/N expansion, the
transition is continuous. To leading order in 1/N, quantum fluctuations enhance
the dispersion anisotropy of the nodal excitations, and cause strong scattering
which critically broadens the quasiparticle (qp) peaks in the spectral
function, except in a narrow wedge in momentum space near the Fermi surface
where the qp's remain sharp. We also consider the possible existence of a
nematic glass phase in the presence of weak disorder. Some possible
implications for cuprate physics are also discussed.Comment: 9 page, 4 figures, an error in one of expressions corrected and a new
author was added. New references and footnotes are added and this is the
version to appear in PR
Monopole Condensation in full QCD using the Schroedinger Functional
We use a lattice thermal partition functional to study Abelian monopole
condensation in full QCD with staggered fermions. We present
preliminary results on and lattices.Comment: Lattice2002(topology). 3 pages, 3 figure
Chiral Symmetry Breaking and Confinement Beyond Rainbow-Ladder Truncation
A non-perturbative construction of the 3-point fermion-boson vertex which
obeys its Ward-Takahashi or Slavnov-Taylor identity, ensures the massless
fermion and boson propagators transform according to their local gauge
covariance relations, reproduces perturbation theory in the weak coupling
regime and provides a gauge independent description for dynamical chiral
symmetry breaking (DCSB) and confinement has been a long-standing goal in
physically relevant gauge theories such as quantum electrodynamics (QED) and
quantum chromodynamics (QCD). In this paper, we demonstrate that the same
simple and practical form of the vertex can achieve these objectives not only
in 4-dimensional quenched QED (qQED4) but also in its 3-dimensional counterpart
(qQED3). Employing this convenient form of the vertex \emph{ansatz} into the
Schwinger-Dyson equation (SDE) for the fermion propagator, we observe that it
renders the critical coupling in qQED4 markedly gauge independent in contrast
with the bare vertex and improves on the well-known Curtis-Pennington
construction. Furthermore, our proposal yields gauge independent order
parameters for confinement and DCSB in qQED3.Comment: 8 pages, 6 figure
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