3,871 research outputs found
Dynamical Mean Field Theory for the Bose-Hubbard Model
The dynamical mean field theory (DMFT), which is successful in the study of
strongly correlated fermions, was recently extended to boson systems [Phys.
Rev. B {\textbf 77}, 235106 (2008)]. In this paper, we employ the bosonic DMFT
to study the Bose-Hubbard model which describes on-site interacting bosons in a
lattice. Using exact diagonalization as the impurity solver, we get the DMFT
solutions for the Green's function, the occupation density, as well as the
condensate fraction on a Bethe lattice. Various phases are identified: the Mott
insulator, the Bose-Einstein condensed (BEC) phase, and the normal phase. At
finite temperatures, we obtain the crossover between the Mott-like regime and
the normal phase, as well as the BEC-to-normal phase transition. Phase diagrams
on the plane and on the plane are
produced ( is the scaled hopping amplitude). We compare our results
with the previous ones, and discuss the implication of these results to
experiments.Comment: 11 pages, 8 figure
The Intersections of Music, Love, and Worship
When a right understanding of the doctrine of love infiltrates the art of music in the event of corporate worship, an opportunity arises for a unique synergism of knowledge, emotion and relationships. These qualities are paramount in the doctrine of love and practice of corporate worship, and they are especially encouraged by the powers of music. An exploration of these three aspects of love—knowledge, emotion and relationships—is here provided with special attention to the opportunity music and worship provide for intersections between them
Effective models for strong electronic correlations at graphene edges
We describe a method for deriving effective low-energy theories of electronic
interactions at graphene edges. Our method is applicable to general edges of
honeycomb lattices (zigzag, chiral, and even disordered) as long as localized
low-energy states (edge states) are present. The central characteristic of the
effective theories is a dramatically reduced number of degrees of freedom. As a
consequence, the solution of the effective theory by exact diagonalization is
feasible for reasonably large ribbon sizes. The quality of the involved
approximations is critically assessed by comparing the correlation functions
obtained from the effective theory with numerically exact quantum Monte-Carlo
calculations. We discuss effective theories of two levels: a relatively
complicated fermionic edge state theory and a further reduced Heisenberg spin
model. The latter theory paves the way to an efficient description of the
magnetic features in long and structurally disordered graphene edges beyond the
mean-field approximation.Comment: 13 pages, 9 figure
Z2 topological invariants in two dimensions from quantum Monte Carlo
We employ quantum Monte Carlo techniques to calculate the topological
invariant in a two-dimensional model of interacting electrons that exhibits a
quantum spin Hall topological insulator phase. In particular, we consider the
parity invariant for inversion-symmetric systems, which can be obtained from
the bulk's imaginary-time Green's function after an appropriate continuation to
zero frequency. This topological invariant is used here in order to study the
trivial-band to topological-insulator transitions in an interacting system with
spin-orbit coupling and an explicit bond dimerization. We discuss the
accessibility and behavior of this topological invariant within quantum Monte
Carlo simulations.Comment: 7 pages, 6 figure
A phason disordered two dimensional quantum antiferromagnet
We examine a novel type of disorder in quantum antiferromagnets. Our model
consists of localized spins with antiferromagnetic exchanges on a bipartite
quasiperiodic structure, which is geometrically disordered in such a way that
no frustration is introduced. In the limit of zero disorder, the structure is
the perfect Penrose rhombus tiling. This tiling is progressively disordered by
augmenting the number of random "phason flips" or local tile-reshuffling
operations. The ground state remains N\'eel ordered, and we have studied its
properties as a function of increasing disorder using linear spin wave theory
and quantum Monte Carlo. We find that the ground state energy decreases,
indicating enhanced quantum fluctuations with increasing disorder. The magnon
spectrum is progressively smoothed, and the effective spin wave velocity of low
energy magnons increases with disorder. For large disorder, the ground state
energy as well as the average staggered magnetization tend towards limiting
values characteristic of this type of randomized tilings.Comment: 5 pages, 7 figure
The European Union from an International Perspective: Sovereignty, Statehood, and Special Treatment
Dimerized Solids and Resonating Plaquette Order in SU(N)-Dirac Fermions
We study the quantum phases of fermions with an explicit SU(N)-symmetric,
Heisenberg-like nearest-neighbor flavor exchange interaction on the honeycomb
lattice at half-filling. Employing projective (zero temperature) quantum Monte
Carlo simulations for even values of N, we explore the evolution from a
weak-coupling semimetal into the strong-coupling, insulating regime.
Furthermore, we compare our numerical results to a saddle-point approximation
in the large-N limit. From the large-N regime down to the SU(6) case, the
insulating state is found to be a columnar valence bond crystal, with a direct
transition to the semimetal at weak, finite coupling, in agreement with the
mean-field result in the large-N limit. At SU(4) however, the insulator
exhibits a subtly different valence bond crystal structure, stabilized by
resonating valence bond plaquettes. In the SU(2) limit, our results support a
direct transition between the semimetal and an antiferromagnetic insulator.Comment: 5 pages, 6 figure
Finite-Temperature Dynamics and Thermal Intraband Magnon Scattering in Haldane Spin-One Chains
The antiferromagnetic spin-one chain is considerably one of the most
fundamental quantum many-body systems, with symmetry protected topological
order in the ground state. Here, we present results for its dynamical spin
structure factor at finite temperatures, based on a combination of exact
numerical diagonalization, matrix-product-state calculations and quantum Monte
Carlo simulations. Open finite chains exhibit a sub-gap band in the thermal
spectral functions, indicative of localized edge-states. Moreover, we observe
the thermal activation of a distinct low-energy continuum contribution to the
spin spectral function with an enhanced spectral weight at low momenta and its
upper threshold. This emerging thermal spectral feature of the Haldane spin-one
chain is shown to result from intra-band magnon scattering due to the thermal
population of the single-magnon branch, which features a large bandwidth-to-gap
ratio. These findings are discussed with respect to possible future studies on
spin-one chain compounds based on inelastic neutron scattering.Comment: 10 pages with 11 figures total (including Supplemental Material);
changes in v2: new Figs. S1 and S5, Fig. S3 expanded + related discussion +
many smaller modifications to match published versio
Dynamical Signatures of Edge-State Magnetism on Graphene Nanoribbons
We investigate the edge-state magnetism of graphene nanoribbons using
projective quantum Monte Carlo simulations and a self-consistent mean-field
approximation of the Hubbard model. The static magnetic correlations are found
to be short ranged. Nevertheless, the correlation length increases with the
width of the ribbon such that already for ribbons of moderate widths we observe
a strong trend towards mean-field-type ferromagnetic correlations at a zigzag
edge. These correlations are accompanied by a dominant low-energy peak in the
local spectral function and we propose that this can be used to detect
edge-state magnetism by scanning tunneling microscopy. The dynamic spin
structure factor at the edge of a ribbon exhibits an approximately linearly
dispersing collective magnonlike mode at low energies that decays into Stoner
modes beyond the energy scale where it merges into the particle-hole continuum.Comment: 4+ pages including 4 figure
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