5,460 research outputs found
Dielectric screening of surface states in a topological insulator
Hexagonal warping provides an anisotropy to the dispersion curves of the
helical Dirac fermions that exist at the surface of a topological insulator. A
sub-dominant quadratic in momentum term leads to an asymmetry between
conduction and valence band. A gap can also be opened through magnetic doping.
We show how these various modifications to the Dirac spectrum change the
polarization function of the surface states and employ our results to discuss
their effect on the plasmons. In the long wavelength limit, the plasmon
dispersion retains its square root dependence on its momentum,
, but its slope is modified and it can acquire a weak
dependence on the direction of . Further, we find the existence
of several plasmon branches, one which is damped for all values of
, and extract the plasmon scattering rate for a representative
case.Comment: 11 pages, 8 figure
Equation of State of the Fermionic 2D Hubbard Model
We present results for the equation of state of the two-dimensional Hubbard
model on an isotropic square lattice as obtained from a controlled and
numerically exact large-cluster dynamical mean field simulation. Our results
are obtained for large but finite systems and are extrapolated to infinite
system size using a known finite size scaling relation. We present the energy,
entropy, double occupancy and nearest-neighbour spin correlations extrapolated
to the thermodynamic limit and discuss the implications of these calculations
on pseudogap physics of the 2D-Hubbard model away from half filling. We find a
strong behavioural shift in energy below a temperature which becomes more
pronounced for larger clusters. Finally, we provide reference calculations and
tables for the equation of state for values of doping away from half filling
which are of interest to cold atom experiments.Comment: 8 pages 6 figures - See Source for Supplementary Material File
Impact of Electron-Phonon Coupling on Near-Field Optical Spectra
The finite momentum transfer () longitudinal optical response
of graphene has a peak at an energy
. This corresponds directly to a quasiparticle peak in the
spectral density at momentum relative to the Fermi momentum . Inclusion
of coupling to a phonon mode at results, for , in
a constant electron-phonon renormalization of the bare bands by a mass
enhancement factor and this is followed by a phonon kink at
where additional broadening begins. Here we study the corresponding
changes in the optical quasiparticle peaks which we find to continue to
directly track the renormalized quasiparticle energies until is large
enough that the optical transitions begin to sample the phonon kink region of
the dispersion curves where linearity in momentum is lost in the renormalized
Dirac Fermion dispersion curves and the correspondence to a single
quasiparticle energy is lost. Nevertheless there remains in
features analogous to the phonon kinks of the
dispersion curves which are observable through variation of and .Comment: 6 pages, 5 figure
Signatures of Fermi surface reconstruction in Raman spectra of underdoped cuprates
We have calculated the Raman B and B spectra as a function of
temperature, as well as doping, for the underdoped cuprates, using a model
based on the resonating valence-bond spin-liquid. We discuss changes in
intensity and peak position brought about by the presence of a pseudogap and
the implied Fermi surface reconstruction, which are elements of this model.
Signatures of Fermi surface reconstruction are evident as a sharp rise in the
doping dependence of the antinodal to nodal peak ratio which occurs below the
quantum critical point. The temperature dependence of the B polarization
can be used to determine if the superconducting gap is limited to the Fermi
pocket, as seen in angle resolved photoemission spectroscopy, or extends
beyond. We find that the slope of the linear low energy B spectrum
maintains its usual d-wave form, but with an effective gap which reflects the
gap amplitude projected on the Fermi pocket. Our calculations capture the main
qualitative features revealed in the extensive data set available on the
HgBaCuO (Hg-1201) cuprate.Comment: 13 pages, 14 figure
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