44 research outputs found
Entanglement scaling and spatial correlations of the transverse field Ising model with perturbations
We study numerically the entanglement entropy and spatial correlations of the
one dimensional transverse field Ising model with three different
perturbations. First, we focus on the out of equilibrium, steady state with an
energy current passing through the system. By employing a variety of
matrix-product state based methods, we confirm the phase diagram and compute
the entanglement entropy. Second, we consider a small perturbation that takes
the system away from integrability and calculate the correlations, the central
charge and the entanglement entropy. Third, we consider periodically weakened
bonds, exploring the phase diagram and entanglement properties first in the
situation when the weak and strong bonds alternate (period two-bonds) and then
the general situation of a period of n bonds. In the latter case we find a
critical weak bond that scales with the transverse field as =
, where is the strength of the strong bond, of the weak bond
and the transverse field. We explicitly show that the energy current is not
a conserved quantity in this case.Comment: 9 pages, 12 figures, version accepted in PR
A practical method to detect, analyse and engineer higher order Van Hove singularities in multi-band Hamiltonians
We present a practical method to detect, diagnose and engineer higher order
Van Hove singularities in multiband systems, with no restrictions on the number
of bands and hopping terms. The method allows us to directly compute the Taylor
expansion of the dispersion of any band at arbitrary points in momentum space,
using a generalised extension of the Feynman Hellmann theorem, which we state
and prove. Being fairly general in scope, it also allows us to incorporate and
analyse the effect of tuning parameters on the low energy dispersions, which
can greatly aid the engineering of higher order Van Hove singularities. A
certain class of degenerate bands can be handled within this framework. We
demonstrate the use of the method, by applying it to the Haldane model.Comment: 19 page
Dirac fermion time-Floquet crystal: manipulating Dirac points
We demonstrate how to control the spectra and current flow of Dirac electrons
in both a graphene sheet and a topological insulator by applying either two
linearly polarized laser fields with frequencies and or a
monochromatic (one-frequency) laser field together with a spatially periodic
static potential(graphene/TI superlattice). Using the Floquet theory and the
resonance approximation, we show that a Dirac point in the electron spectrum
can be split into several Dirac points whose relative location in momentum
space can be efficiently manipulated by changing the characteristics of the
laser fields. In addition, the laser-field controlled Dirac fermion band
structure -- Dirac fermion time-Floquet crystal -- allows the manipulation of
the electron currents in graphene and topological insulators. Furthermore, the
generation of dc currents of desirable intensity in a chosen direction occurs
when applying the bi-harmonic laser field which can provide a straightforward
experimental test of the predicted phenomena.Comment: 9 pages, 7 figures, version that will appear in Phys. Rev.
Effect of paramagnetic fluctuations on a Fermi-surface topological transition in two dimensions
We study the Fermi-surface topological transition of the pocket-opening type in a two-dimensional Fermi liquid. We find that the paramagnetic fluctuations in an interacting Fermi liquid typically drive the transition first order at zero temperature. We first gain insight from a calculation using second-order perturbation theory in the self-energy. This is valid for weak interaction and far from instabilities. We then extend the results to stronger interaction, using the self-consistent fluctuation approximation. Experimental signatures are given in light of our results
Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons
We discuss the form of the damping of magnetic excitations in a metal near a
ferromagnetic instability. The paramagnon theory predicts that the damping term
should have the form with (the
Landau damping). However, the experiments on uranium metallic compounds UGe
and UCoGe showed that tends to a constant value at vanishing .
A non-zero is impossible in systems with one type of carriers
(either localized or itinerant) because it would violate the spin conservation.
It has been conjectured recently that a non-zero in UGe and
UCoGe may be due to the presence of both localized and itinerant electrons in
these materials, with ferromagnetism involving predominantly localized spins.
We present microscopic analysis of the damping of near-critical localized
excitations due to interaction with itinerant carriers. We show explicitly how
the presence of two types of electrons breaks the cancellation between the
contributions to from self-energy and vertex correction insertions
into the spin polarization bubble and discuss the special role of the
Aslamazov-Larkin processes. We show that increases with both
in the paramagnetic and ferromagnetic regions, but in-between it has a peak at
. We compare our theory with the available experimental data.Comment: 8 pages including Supplemental Material, version published in Phys.
Rev. Let
Correlated Fermions on a Checkerboard Lattice
A model of strongly correlated spinless fermions hopping on a checkerboard
lattice is mapped onto a quantum fully-packed loop model. We identify a large
number of fluctuationless states specific to the fermionic case. We also show
that for a class of fluctuating states, the fermionic sign problem can be
gauged away. This claim is supported by numerically evaluating the energies of
the low-lying states. Furthermore, we analyze in detail the excitations at the
Rokhsar-Kivelson point of this model thereby using the relation to the height
model and the single-mode approximation.Comment: 4 Pages, 3 Figures; v4: updated version published in Phys. Rev.
Lett.; one reference adde
How to test the "quantumness" of a quantum computer?
We discuss whether, to what extent and how a quantum computing device can be
evaluated and simulated using classical tools.Comment: Submitted 12.10.201