31,604 research outputs found
Anderson localisation in tight-binding models with flat bands
We consider the effect of weak disorder on eigenstates in a special class of
tight-binding models. Models in this class have short-range hopping on periodic
lattices; their defining feature is that the clean systems have some energy
bands that are dispersionless throughout the Brillouin zone. We show that
states derived from these flat bands are generically critical in the presence
of weak disorder, being neither Anderson localised nor spatially extended.
Further, we establish a mapping between this localisation problem and the one
of resonances in random impedance networks, which previous work has suggested
are also critical. Our conclusions are illustrated using numerical results for
a two-dimensional lattice, known as the square lattice with crossings or the
planar pyrochlore lattice.Comment: 5 pages, 3 figures, as published (this version includes minor
corrections
On the origin of surface states in a correlated local-moment film
The electronic quasiparticle structure of a ferromagnetic local moment film
is investigated within the framework of the s-f model. For the special case of
a single electron in an otherwise empty energy band being exchange coupled to a
fully ordered localised spin system the problem can be solved exactly and, for
the spin-down electron, some marked correlation effects can be found. We extend
our model to incorporate the influence of the surface on the electronic
structure. Therefore we modify the hopping integrals in the vicinity of the
surface. This leads to the existence of surface states, both for the spin-up
and the spin-down spectral density of states. The interplay between the
modification of the hopping integrals and the existence of surface states and
correlation effects is discussed in detail.Comment: 9 pages, 9 figures, accepted for publication in European Physical
Journal
Ferromagnetic spin-polaron on complex lattices
We present a simpler derivation of the exact solution of a spin-polaron in a
ferromagnet and generalize it to complex lattices and/or longer range exchange
interactions. As a specific example, we analyze a two-dimensional MnO-like
lattice (as in the ferromagnetic layers in LaMnO) and discuss the
properties of the resulting spin-polaron in various regimes. At strong
couplings the solution is reminiscent of the Zhang-Rice singlet, however the
electronic wavefunction involved in the singlet is dependent on the momentum of
the singlet, and multiple bands may appear.Comment: 12 pages, 7 figure
A Diabatic Surface Hopping Algorithm based on Time Dependent Perturbation Theory and Semiclassical Analysis
Surface hopping algorithms are popular tools to study dynamics of the
quantum-classical mixed systems. In this paper, we propose a surface hopping
algorithm in diabatic representations, based on time dependent perturbation
theory and semiclassical analysis. The algorithm can be viewed as a Monte Carlo
sampling algorithm on the semiclassical path space for piecewise deterministic
path with stochastic jumps between the energy surfaces. The algorithm is
validated numerically and it shows good performance in both weak coupling and
avoided crossing regimes
Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices
We consider ultracold bosons in a 2D square optical lattice described by the
Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is
applied to the system, which shakes the lattice along one of the diagonals. The
effect of the shaking is to renormalize the nearest-neighbor hopping
coefficients, which can be arbitrarily reduced, can vanish, or can even change
sign, depending on the shaking parameter. It is therefore necessary to account
for higher-order hopping terms, which are renormalized differently by the
shaking, and introduce anisotropy into the problem. We show that the
competition between these different hopping terms leads to finite-momentum
condensates, with a momentum that may be tuned via the strength of the shaking.
We calculate the boundaries between the Mott-insulator and the different
superfluid phases, and present the time-of-flight images expected to be
observed experimentally. Our results open up new possibilities for the
realization of bosonic analogs of the FFLO phase describing inhomogeneous
superconductivity.Comment: 7 pages, 7 figure
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