1,499 research outputs found
Spectral properties of coupled cavity arrays in one dimension
Spectral properties of coupled cavity arrays in one dimension are
investigated by means of the variational cluster approach. Coupled cavity
arrays consist of two distinct "particles," namely, photons and atomiclike
excitations. Spectral functions are evaluated and discussed for both particle
types. In addition, densities of states, momentum distributions and spatial
correlation functions are presented. Based on this information, polariton
"quasiparticles" are introduced as appropriate, wave vector and filling
dependent linear combinations of photon and atomiclike particles. Spectral
functions and densities of states are evaluated for the polariton
quasiparticles, and the weights of their components are analyzed.Comment: 17 pages, 16 figures, version as publishe
Renormalized SO(5) symmetry in ladders with next-nearest-neighbor hopping
We study the occurrence of SO(5) symmetry in the low-energy sector of
two-chain Hubbard-like systems by analyzing the flow of the running couplings
(-ology) under renormalization group in the weak-interaction limit. It is
shown that SO(5) is asymptotically restored for low energies for rather general
parameters of the bare Hamiltonian. This holds also with inclusion of a
next-nearest-neighbor hopping which explicitly breaks particle-hole symmetry
provided one accounts for a different single-particle weight for the
quasiparticles of the two bands of the system. The physical significance of
this renormalized SO(5) symmetry is discussed.Comment: Final version: to appear in Phys. Rev. Lett., sched. Mar. 9
Extended self-energy functional approach for strongly-correlated lattice bosons in the superfluid phase
Among the various numerical techniques to study the physics of strongly
correlated quantum many-body systems, the self-energy functional approach (SFA)
has become increasingly important. In its previous form, however, SFA is not
applicable to Bose-Einstein condensation or superfluidity. In this paper we
show how to overcome this shortcoming. To this end we identify an appropriate
quantity, which we term , that represents the correlation correction of the
condensate order parameter, as it does the self-energy for the Green's
function. An appropriate functional is derived, which is stationary at the
exact physical realizations of and of the self-energy. Its derivation is
based on a functional-integral representation of the grand potential followed
by an appropriate sequence of Legendre transformations. The approach is not
perturbative and therefore applicable to a wide range of models with local
interactions. We show that the variational cluster approach based on the
extended self-energy functional is equivalent to the "pseudoparticle" approach
introduced in Phys. Rev. B, 83, 134507 (2011). We present results for the
superfluid density in the two-dimensional Bose-Hubbard model, which show a
remarkable agreement with those of Quantum-Monte-Carlo calculations.Comment: 1 additional figure showing the region close to the tip of the Mott
lobe, minor changes in the tex
Variational Cluster Perturbation Theory for Bose-Hubbard models
We discuss the application of the variational cluster perturbation theory
(VCPT) to the Mott-insulator--to--superfluid transition in the Bose-Hubbard
model. We show how the VCPT can be formulated in such a way that it gives a
translation invariant excitation spectrum -- free of spurious gaps -- despite
the fact that if formally breaks translation invariance. The phase diagram and
the single-particle Green function in the insulating phase are obtained for
one-dimensional systems. When the chemical potential of the cluster is taken as
a variational parameter, the VCPT reproduces the dimension dependence of the
phase diagram even for one-site clusters. We find a good quantitative agreement
with the results of the density-matrix renormalization group when the number of
sites in the cluster becomes of order 10. The extension of the method to the
superfluid phase is discussed.Comment: v1) 10 pages, 6 figures. v2) Final version as publishe
Weak phase separation and the pseudogap in the electron-doped cuprates
We study the quantum transition from an antiferromagnet to a superconductor
in a model for electron- and hole-doped cuprates by means of a variational
cluster perturbation theory approach. In both cases, our results suggest a
tendency towards phase separation between a mixed
antiferromagnetic-superconducting phase at low doping and a pure
superconducting phase at larger doping. However, in the electron-doped case the
energy scale for phase separation is an order of magnitude smaller than for
hole doping. We argue that this can explain the different pseudogap and
superconducting transition scales in hole- and electron-doped materials.Comment: Final version, accepted for publication in Europhysics Letter
Interrelation of Superconducting and Antiferromagnetic Gaps in High-Tc Compounds: a Test Case for a Microscopic Theory
Recent angle resolved photoemission (ARPES) data, which found evidence for a
d-wave-like modulation of the antiferromagnetic gap, suggest an intimate
interrelation between the antiferromagnetic insulator and the superconductor
with its d-wave gap. This poses a new challenge to microscopic descriptions,
which should account for this correlation between, at first sight, very
different states of matter. Here, we propose a microscopic mechanism which
provides a definite correlation between these two different gap structures: it
is shown that a projected SO(5) theory, which aims at unifying
antiferromagnetism and d-wave superconductivity via a common symmetry principle
while explicitly taking the Mott-Hubbard gap into account, correctly describes
the observed gap characteristics. Specifically, it accounts for both the
dispersion and the order of magnitude difference between the antiferromagnetic
gap modulation and the superconducting gap.Comment: 8 pages, 5 figure
Phase diagram and single-particle spectrum of CuO layers within a variational cluster approach to the 3-band Hubbard model
We carry out a detailed numerical study of the three-band Hubbard model in
the underdoped region both in the hole- as well as in the electron-doped case
by means of the variational cluster approach. Both the phase diagram and the
low-energy single-particle spectrum are very similar to recent results for the
single-band Hubbard model with next-nearest-neighbor hoppings. In particular,
we obtain a mixed antiferromagnetic+superconducting phase at low doping with a
first-order transition to a pure superconducting phase accompanied by phase
separation. In the single-particle spectrum a clear Zhang-Rice singlet band
with an incoherent and a coherent part can be seen, in which holes enter upon
doping around . The latter is very similar to the coherent
quasi-particle band crossing the Fermi surface in the single-band model. Doped
electrons go instead into the upper Hubbard band, first filling the regions of
the Brillouin zone around . This fact can be related to the enhanced
robustness of the antiferromagnetic phase as a function of electron doping
compared to hole doping.Comment: 14 pages, 15 eps figure
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