244 research outputs found
Comment on "Superradiant Phase Transitions and the Standard Description of Circuit QED"
A Comment on the Letter by O. Viehmann, J. von Delft, and F. Marquardt [Phys.
Rev. Lett. {\bf 107}, 113602 (2011)]
Superradiant phase transitions with three-level systems
We determine the phase diagram of identical three-level systems
interacting with a single photonic mode in the thermodynamical limit () by accounting for the so-called diamagnetic term and the inequalities
imposed by the Thomas-Reich-Kuhn (TRK) oscillator strength sum rule. The key
role of transitions between excited levels and the occurrence of first-order
phase transitions is discussed. We show that, in contrast to two-level systems,
in the three-level case the TRK inequalities do not always prevent a
superradiant phase transition in presence of a diamagnetic term
Variational Monte-Carlo investigation of SU() Heisenberg chains
Motivated by recent experimental progress in the context of ultra-cold
multi-color fermionic atoms in optical lattices, we have investigated the
properties of the SU() Heisenberg chain with totally antisymmetric
irreducible representations, the effective model of Mott phases with
particles per site. These models have been studied for arbitrary and
with non-abelian bosonization [I. Affleck, Nuclear Physics B 265, 409 (1986);
305, 582 (1988)], leading to predictions about the nature of the ground state
(gapped or critical) in most but not all cases. Using exact diagonalization and
variational Monte-Carlo based on Gutzwiller projected fermionic wave functions,
we have been able to verify these predictions for a representative number of
cases with and , and we have shown that the opening of
a gap is associated to a spontaneous dimerization or trimerization depending on
the value of m and N. We have also investigated the marginal cases where
abelian bosonization did not lead to any prediction. In these cases,
variational Monte-Carlo predicts that the ground state is critical with
exponents consistent with conformal field theory.Comment: 9 pages, 10 figures, 3 table
Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime
We investigate theoretically the quantum vacuum properties of a chain of
superconducting Josephson atoms inductively coupled to a transmission line
resonator. We derive the quantum field Hamiltonian for such circuit-QED system,
showing that, due to the type and strength of the interaction, a quantum phase
transition can occur with a twice degenerate quantum vacuum above a critical
coupling. In the finite-size case, the degeneracy is lifted, with an energy
splitting decreasing exponentially with increasing values of , where
is the dimensionless vacuum Rabi coupling per artificial atom. We determine
analytically the ultrastrong coupling asymptotic expression of the two
degenerate vacua for an arbitrary number of artificial atoms and of resonator
modes. In the ultrastrong coupling regime the degeneracy is protected with
respect to random fluctuations of the transition energies of the Josephson
elements.Comment: Published PRL version (with Supplementary Material
Discrete nonlinear Schrödinger equations for periodic optical systems : pattern formation in \chi(3) coupled waveguide arrays
Discrete nonlinear Schrödinger equations have
been used for many years to model the propagation of light in optical architectures whose refractive index profile is modulated periodically
in the transverse direction. Typically, one considers a modal decomposition of the electric field
where the complex amplitudes satisfy a coupled
system that accommodates nearest neighbour
linear interactions and a local intensity dependent term whose origin lies in the Ï
(3) contribution to the medium's dielectric response.
In this presentation, two classic continuum
configurations are discretized in ways that have
received little attention in the literature: the
ring cavity and counterpropagating waves. Both
of these systems are defined by distinct types of
boundary condition. Moreover, they are susceptible to spatial instabilities that are ultimately
responsible for generating spontaneous patterns
from arbitrarily small background disturbances.
Good agreement between analytical predictions
and simulations will be demonstrated
Double symmetry breaking and 2D quantum phase diagram in spin-boson systems
The quantum ground state properties of two independent chains of spins
(two-levels systems) interacting with the same bosonic field are theoretically
investigated. Each chain is coupled to a different quadrature of the field,
leading to two independent symmetry breakings for increasing values of the two
spin-boson interaction constants and . A phase diagram is
provided in the plane (,) with 4 different phases that can
be characterized by the complex bosonic coherence of the ground states and can
be manipulated via non-abelian Berry effects. In particular, when
and are both larger than two critical values, the fundamental
subspace has a four-fold degeneracy. Possible implementations in
superconducting or atomic systems are discussed
An Additive Schwarz Method Type Theory for Lions's Algorithm and a Symmetrized Optimized Restricted Additive Schwarz Method
International audienceOptimized Schwarz methods (OSM) are very popular methods which were introduced by P.L. Lions in [27] for elliptic problems and by B. Després in [8] for propagative wave phenomena. We give here a theory for Lions' algorithm that is the genuine counterpart of the theory developed over the years for the Schwarz algorithm. The first step is to introduce a symmetric variant of the ORAS (Optimized Restricted Additive Schwarz) algorithm [37] that is suitable for the analysis of a two-level method. Then we build a coarse space for which the convergence rate of the two-level method is guaranteed regardless of the regularity of the coefficients. We show scalability results for thousands of cores for nearly incompressible elasticity and the Stokes systems with a continuous discretization of the pressure
Chiral spin liquids in triangular lattice SU(N) fermionic Mott insulators with artificial gauge fields
We show that, in the presence of a artificial gauge field per
plaquette, Mott insulating phases of ultra-cold fermions with symmetry
and one particle per site generically possess an extended chiral phase with
intrinsic topological order characterized by a multiplet of low-lying
singlet excitations for periodic boundary conditions, and by chiral edge states
described by the Wess-Zumino-Novikov-Witten conformal field theory
for open boundary conditions. This has been achieved by extensive exact
diagonalizations for between and , and by a parton construction
based on a set of Gutzwiller projected fermionic wave-functions with flux
per triangular plaquette. Experimental implications are briefly
discussed.Comment: 5+2 pages, 4 figures, 2 table
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