13,895 research outputs found
Dimer states in atomic mixtures
A mixture of heavy atoms in a Mott state and light spin-1/2 fermionic atoms
is studied in an optical lattice. Inelastic scattering processes between both
atomic species excite the heavy atoms and renormalize the tunneling rate as
well as the interaction of the light atoms. An effective Hamiltonian for the
latter is derived that describes tunneling of single fermions, tunneling of
fermionic pairs and an exchange of fermionic spins. Low energy states of this
Hamiltonian are a N\'eel state for strong effective repulsion, dimer states for
moderate interaction, and a density wave of paired fermions for strong
effective attraction.Comment: 10 pages, 3 figure, extended versio
Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions
We consider a disordered d--wave superconductor in two dimensions. Recently,
we have shown in an exact calculation that for a lattice model with a
Lorentzian distributed random chemical potential the quasiparticle density of
states at the Fermi level is nonzero. As the exact result holds only for the
special choice of the Lorentzian, we employ different methods to show that for
a large class of distributions, including the Gaussian distribution, one can
establish a nonzero lower bound for the Fermi level density of states. The fact
that the tails of the distributions are unimportant in deriving the lower bound
shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR
Short note on the density of states in 3D Weyl semimetals
The average density of states in a disordered three-dimensional Weyl system
is discussed in the case of a continuous distribution of random scattering. Our
result clearly indicate that the average density of states does not vanish,
reflecting the absence of a critical point for a metal-insulator transition.
This calculation supports recent suggestions of an avoided quantum critical
point in the disordered three-dimensional Weyl semimetal. However, the
effective density of states can be very small such that the
saddle-approximation with a vanishing density of states might be valid for
practical cases.Comment: 5 pages, 2 figures, minor changes, additional supplemen
Lattice symmetries, spectral topology and opto-electronic properties of graphene-like materials
The topology of the band structure, which is determined by the lattice
symmetries, has a strong influence on the transport properties. Here we
consider an anisotropic honeycomb lattice and study the effect of a
continuously deformed band structure on the optical conductivity and on
diffusion due to quantum fluctuations. In contrast to the behavior at an
isotropic node we find super- and subdiffusion for the anisotropic node. The
spectral saddle points create van Hove singularities in the optical
conductivity, which could be used to characterize the spectral properties
experimentally.Comment: 9 pages, 6 figures. Slightly extended version, e.g. Eq.(12) include
Optical conductivity of graphene in the presence of random lattice deformations
We study the influence of lattice deformations on the optical conductivity of
a two-dimensional electron gas. Lattice deformations are taken into account by
introducing a non-abelian gauge field into the Eucledian action of
two-dimensional Dirac electrons. This is in analogy to the introduction of the
gravitation in the four-dimensional quantum field theory. We examine the effect
of these deformations on the averaged optical conductivity. Within the
perturbative theory up to second order we show that corrections of the
conductivity due to the deformations cancel each other exactly. We argue that
these corrections vanish to any order in perturbative expansion.Comment: 9 pages, 9 figure
Floating Wigner molecules and possible phase transitions in quantum dots
A floating Wigner crystal differs from the standard one by a spatial
averaging over positions of the Wigner-crystal lattice. It has the same
internal structure as the fixed crystal, but contrary to it, takes into account
rotational and/or translational symmetry of the underlying jellium background.
We study properties of a floating Wigner molecule in few-electron
spin-polarized quantum dots, and show that the floating solid has the lower
energy than the standard Wigner crystal with fixed lattice points. We also
argue that internal rotational symmetry of individual dots can be broken in
arrays of quantum dots, due to degenerate ground states and inter-dot Coulomb
coupling.Comment: 6 pages incl 3 figure
A renormalized Gross-Pitaevskii Theory and vortices in a strongly interacting Bose gas
We consider a strongly interacting Bose-Einstein condensate in a spherical
harmonic trap. The system is treated by applying a slave-boson representation
for hard-core bosons. A renormalized Gross-Pitaevskii theory is derived for the
condensate wave function that describes the dilute regime (like the
conventional Gross-Pitaevskii theory) as well as the dense regime. We calculate
the condensate density of a rotating condensate for both the vortex-free
condensate and the condensate with a single vortex and determine the critical
angular velocity for the formation of a stable vortex in a rotating trap.Comment: 13 pages, 5 figures; revision and extension, figure 2 adde
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