2,103 research outputs found
Unconventional properties of superconducting cuprates
We present an explanation of the unusual peak/dip/hump features observed in
photoemission experiments on Bi2212 at . We argue that these
features arise from the interaction of the fermionic quasi-particles with
overdamped spin fluctuations. We show that the strong spin-fermion interaction
combined with the feedback effect on the spin damping due to superconductivity
yields a Fermi-liquid form of the fermionic spectral function for where is the maximum value of the superconducting gap, and a
non-Fermi-liquid form for . In the Fermi-liquid regime,
the spectral function displays a quasiparticle peak at
; in the non-Fermi-liquid regime it possesses a broad
maximum (hump) at . In between the two regimes, the
spectral function has a dip at . We argue that our
theory also explains the tunneling data for the superconducting density of
states.Comment: 4 pages, RevTeX, 4 eps figures embedded in the tex
Fidelity susceptibility and general quench near an anisotropic quantum critical point
We study the scaling behavior of fidelity susceptibility density at or close to an anisotropic quantum critical point characterized by two
different correlation length exponents and along
parallel and perpendicular spatial directions, respectively. Our studies show
that the response of the system due to a small change in the Hamiltonian near
an anisotropic quantum critical point is different from that seen near an
isotropic quantum critical point. In particular, for a finite system with
linear dimension () in the parallel (perpendicular)
directions, the maximum value of is found to increases in a
power-law fashion with for small , with an exponent depending
on both and and eventually crosses over to a scaling
with for . We
also propose scaling relations of heat density and defect density generated
following a quench starting from an anisotropic quantum critical point and
connect them to a generalized fidelity susceptibility. These predictions are
verified exactly both analytically and numerically taking the example of a
Hamiltonian showing a semi-Dirac band-crossing point.Comment: 6 pages, 6 pigure
Thermally fluctuating superconductors in two dimensions
We describe the different regimes of finite temperature dynamics in the
vicinity of a zero temperature superconductor to insulator quantum phase
transition in two dimensions. New results are obtained for a low temperature
phase-only hydrodynamics, and for the intermediate temperature quantum-critical
region. In the latter case, we obtain a universal relationship between the
frequency-dependence of the conductivity and the value of the d.c. resistance.Comment: Presentation completely revised; 4 pages, 2 figure
Quasi-freestanding and single-atom thick layer of hexagonal boron nitride as a substrate for graphene synthesis
We demonstrate that freeing a single-atom thick layer of hexagonal boron
nitride (hbn) from tight chemical bonding to a Ni(111) thin film grown on a
W(110) substrate can be achieved by intercalation of Au atoms into the
interface. This process has been systematically investigated using
angle-resolved photoemission spectroscopy, X-ray photoemission and absorption
techniques. It has been demonstrated that the transition of the hbn layer from
the "rigid" into the "quasi-freestanding" state is accompanied by a change of
its lattice constant. Using chemical vapor deposition, graphene has been
successfully synthesized on the insulating, quasi-freestanding hbn monolayer.
We anticipate that the in situ synthesized weakly interacting graphene/hbn
double layered system could be further developed for technological applications
and may provide perspectives for further inquiry into the unusual electronic
properties of graphene.Comment: in print in Phys. Rev.
Quantum Monte Carlo simulation in the canonical ensemble at finite temperature
A quantum Monte Carlo method with non-local update scheme is presented. The
method is based on a path-integral decomposition and a worm operator which is
local in imaginary time. It generates states with a fixed number of particles
and respects other exact symmetries. Observables like the equal-time Green's
function can be evaluated in an efficient way. To demonstrate the versatility
of the method, results for the one-dimensional Bose-Hubbard model and a nuclear
pairing model are presented. Within the context of the Bose-Hubbard model the
efficiency of the algorithm is discussed.Comment: 11 pages, 8 figure
Quantum entanglement of spin-1 bosons with coupled ground states in optical lattices
We examine particle entanglement, characterized by pseudo-spin squeezing, of
spin-1 bosonic atoms with coupled ground states in a one-dimensional optical
lattice. Both the superfluid and Mott-insulator phases are investigated
separately for ferromagnetic and antiferromagnetic interactions. Mode
entanglement is also discussed in the Mott insulating phase. The role of a
small but nonzero angle between the polarization vectors of counter-propagating
lasers forming the optical lattice on quantum correlations is investigated as
well.Comment: 18 pages, 8 figures. To be published in Journal of Physics
Phases of the 2D Hubbard model at low doping
We show that the planar spiral phase of the 2D Hubbard model at low doping,
x, is unstable towards a noncoplanar spin configuration. The novel equilibrium
state we found at low doping is incommensurate with the inverse pitch of the
spiral varying as x^(1/2), but nevertheless has a dominant peak in the
susceptibility at (\pi,\pi). Relevance to the NMR and neutron scattering
experiments in La_2-xSr_xCuO_4 is disccussed.Comment: 12 pages, emtex v.3.
Field-tuned quantum critical point of antiferromagnetic metals
A magnetic field applied to a three-dimensional antiferromagnetic metal can
destroy the long-range order and thereby induce a quantum critical point. Such
field-induced quantum critical behavior is the focus of many recent
experiments. We investigate theoretically the quantum critical behavior of
clean antiferromagnetic metals subject to a static, spatially uniform external
magnetic field. The external field does not only suppress (or induce in some
systems) antiferromagnetism but also influences the dynamics of the order
parameter by inducing spin precession. This leads to an exactly marginal
correction to spin-fluctuation theory. We investigate how the interplay of
precession and damping determines the specific heat, magnetization,
magnetocaloric effect, susceptibility and scattering rates. We point out that
the precession can change the sign of the leading \sqrt{T} correction to the
specific heat coefficient c(T)/T and can induce a characteristic maximum in
c(T)/T for certain parameters. We argue that the susceptibility \chi =\partial
M/\partial B is the thermodynamic quantity which shows the most significant
change upon approaching the quantum critical point and which gives experimental
access to the (dangerously irrelevant) spin-spin interactions.Comment: 12 pages, 8 figure
Superfluid-Insulator transition of ultracold atoms in an optical lattice in the presence of a synthetic magnetic field
We study the Mott insulator-superfluid transition of ultracold bosonic atoms
in a two-dimensional square optical lattice in the presence of a synthetic
magnetic field with p/q (p and q being co-prime integers) flux quanta passing
through each lattice plaquette. We show that on approach to the transition from
the Mott side, the momentum distribution of the bosons exhibits q precursor
peaks within the first magnetic Brillouin zone. We also provide an effective
theory for the transition and show that it involves q interacting boson fields.
We construct, from a mean-field analysis of this effective theory, the
superfluid ground states near the transition and compute, for q=2,3, both the
gapped and the gapless collective modes of these states. We suggest experiments
to test our theory.Comment: 4 pages, 4 figs; v
Non-equilibrium Gross-Pitaevskii dynamics of boson lattice models
Motivated by recent experiments on trapped ultra-cold bosonic atoms in an
optical lattice potential, we consider the non-equilibrium dynamic properties
of such bosonic systems for a number of experimentally relevant situations.
When the number of bosons per lattice site is large, there is a wide parameter
regime where the effective boson interactions are strong, but the ground state
remains a superfluid (and not a Mott insulator): we describe the conditions
under which the dynamics in this regime can be described by a discrete
Gross-Pitaevskii equation. We describe the evolution of the phase coherence
after the system is initially prepared in a Mott insulating state, and then
allowed to evolve after a sudden change in parameters places it in a regime
with a superfluid ground state. We also consider initial conditions with a "pi
phase" imprint on a superfluid ground state (i.e. the initial phases of
neighboring wells differ by pi), and discuss the subsequent appearance of
density wave order and "Schrodinger cat" states.Comment: 16 pages, 11 figures; (v2) added reference
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