48 research outputs found
Quantum Oscillations in Nodal Line Systems
We study signatures of magnetic quantum oscillations in three-dimensional
nodal line semimetals at zero temperature. The extended nature of the
degenerate bands can result in a Fermi surface geometry with topological genus
one, as well as a Fermi surface of electron and hole pockets encapsulating the
nodal line. Moreover, the underlying two-band model to describe a nodal line is
not unique, in that there are two classes of Hamiltonian with distinct band
topology giving rise to the same Fermi surface geometry. After identifying the
extremal cyclotron orbits in various magnetic field directions, we study their
concomitant Landau levels and resulting quantum oscillation signatures. By
Landau-fan-diagram analyses we extract the non-trivial Berry phase
signature for extremal orbits linking the nodal line.Comment: 8 pages, 9 figure
Artificial Staggered Magnetic Field for Ultracold Atoms in Optical Lattices
A time-dependent optical lattice with staggered particle current in the
tight-binding regime was considered that can be described by a time-independent
effective lattice model with an artificial staggered magnetic field. The low
energy description of a single-component fermion in this lattice at
half-filling is provided by two copies of ideal two-dimensional massless Dirac
fermions. The Dirac cones are generally anisotropic and can be tuned by the
external staggered flux \p. For bosons, the staggered flux modifies the
single-particle spectrum such that in the weak coupling limit, depending on the
flux \p, distinct superfluid phases are realized. Their properties are
discussed, the nature of the phase transitions between them is establised, and
Bogoliubov theory is used to determine their excitation spectra. Then the
generalized superfluid-Mott-insulator transition is studied in the presence of
the staggered flux and the complete phase diagram is established. Finally, the
momentum distribution of the distinct superfluid phases is obtained, which
provides a clear experimental signature of each phase in ballistic expansion
experiments.Comment: 14 pages, 5 figure
Propagation of collective pair excitations in disordered Bose superfluids
We study the effect of disorder on the propagation of collective excitations
in a disordered Bose superfluid. We incorporate local density depletion induced
by strong disorder at the meanfield level, and formulate the transport of the
excitations in terms of a screened scattering problem. We show that the
competition of disorder, screening, and density depletion induces a strongly
non-monotonic energy dependence of the disorder parameter. In three dimensions,
it results in a rich localization diagram with four different classes of
mobility spectra, characterized by either no or up to three mobility edges.
Implications on experiments with disordered ultracold atoms are discussed.Comment: 9 pages, 5 figure
Internal Josephson Oscillations for Distinct Momenta Bose-Einstein Condensates
The internal Josephson oscillations between an atomic Bose-Einstein
condensate (BEC) and a molecular one are studied for atoms in a square optical
lattice subjected to a staggered gauge field. The system is described by a
Bose-Hubbard model with complex and anisotropic hopping parameters that are
different for each species, i.e., atoms and molecules. When the flux per
plaquette for each species is small, the system oscillates between two
conventional zero-momentum condensates. However, there is a regime of
parameters in which Josephson oscillations between a vortex-carrying atomic
condensate (finite momentum BEC) and a conventional zero-momentum molecular
condensate may be realized. The experimental observation of the oscillations
between these qualitatively distinct BEC's is possible with state-of-the-art
Ramsey interference techniques.Comment: 8 pages, 6 figure
Competing Superconducting States for Ultracold Atoms in Optical Lattices with Artificial Staggered Magnetic Field
We study superconductivity in an ultracold Bose-Fermi mixture loaded into a
square optical lattice subjected to a staggered flux. While the bosons form a
superfluid at very low temperature and weak interaction, the interacting
fermions experience an additional long-ranged attractive interaction mediated
by phonons in the bosonic superfluid. This leads us to consider a generalized
Hubbard model with on-site and nearest-neighbor attractive interactions, which
give rise to two competing superconducting channels. We use the
Bardeen-Cooper-Schrieffer theory to determine the regimes where distinct
superconducting ground states are stabilized, and find that the non-local
pairing channel favors a superconducting ground state which breaks both the
gauge and the lattice symmetries, thus realizing unconventional
superconductivity. Furthermore, the particular structure of the single-particle
spectrum leads to unexpected consequences, for example, a dome-shaped
superconducting region in the temperature versus filing fraction phase diagram,
with a normal phase that comprises much richer physics than a Fermi-liquid.
Notably, the relevant temperature regime and coupling strength is readily
accessible in state of the art experiments with ultracold trapped atoms
Winding vector: how to annihilate two Dirac points with the same charge
The merging or emergence of a pair of Dirac points may be classified
according to whether the winding numbers which characterize them are opposite
( scenario) or identical ( scenario). From the touching point between
two parabolic bands (one of them can be flat), two Dirac points with the {\it
same} winding number emerge under appropriate distortion (interaction, etc),
following the scenario. Under further distortion, these Dirac points merge
following the scenario, that is corresponding to {\it opposite} winding
numbers. This apparent contradiction is solved by the fact that the winding
number is actually defined around a unit vector on the Bloch sphere and that
this vector rotates during the motion of the Dirac points. This is shown here
within the simplest two-band lattice model (Mielke) exhibiting a flat band. We
argue on several examples that the evolution between the two scenarios is
general.Comment: 5 pages, 6 figure
Spin- and band-ferromagnetism in trilayer graphene
We study the ground state properties of an ABA-stacked trilayer graphene. The
low energy band structure can be described by a combination of both a linear
and a quadratic particle-hole symmetric dispersions, reminiscent of monolayer-
and bilayer-graphene, respectively. The multi-band structure offers more
channels for instability towards ferromagnetism when the Coulomb interaction is
taken into account. Indeed, if one associates a pseudo-spin 1/2 degree of
freedom to the bands (parabolic/linear), it is possible to realize also a
band-ferromagnetic state, where there is a shift in the energy bands, since
they fill up differently. By using a variational procedure, we compute the
exchange energies for all possible variational ground states and identify the
parameter space for the occurrence of spin- and band-ferromagnetic
instabilities as a function of doping and interaction strength.Comment: 9 pages/ 8 figure