4,157 research outputs found
Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model
The BCS-BEC crossover within the two-dimensional attractive Hubbard model is
studied by using the Cellular Dynamical Mean-Field Theory both in the normal
and superconducting ground states. Short-range spatial correlations
incorporated in this theory remove the normal-state quasiparticle peak and the
first-order transition found in the Dynamical Mean-Field Theory, rendering the
normal state crossover smooth. For smaller than the bandwidth, pairing is
driven by the potential energy, while in the opposite case it is driven by the
kinetic energy, resembling a recent optical conductivity experiment in
cuprates. Phase coherence leads to the appearance of a collective Bogoliubov
mode in the density-density correlation function and to the sharpening of the
spectral function.Comment: 5 pages, 4 figure
Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of BiSrCaCuO
There is an ongoing debate about the nature of the bosonic excitations
responsible for the quasiparticle self energy in high temperature
superconductors -- are they phonons or spin fluctuations? We present a careful
analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV
in angle resolved photoemission data using Eliashberg theory for a d-wave
superconductor. Starting from the assumption that nodal quasiparticles are not
coupled to the magnetic resonance, the sharp structure at meV
can be assigned to phonons. We find that not only can we account for the shifts
of the kink energy seen on oxygen isotope substitution but also get a
quantitative estimate of the fraction of the area under the electron-boson
spectral density which is due to phonons. We conclude that for optimally doped
BiSrCaCuO phonons contribute % and
non-phononic excitations %.Comment: 6 pages, 3 figure
Anomalous superconductivity and its competition with antiferromagnetism in doped Mott insulators
Proximity to a Mott insulating phase is likely to be an important physical
ingredient of a theory that aims to describe high-temperature superconductivity
in the cuprates. Quantum cluster methods are well suited to describe the Mott
phase. Hence, as a step towards a quantitative theory of the competition
between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the
cuprates, we use Cellular Dynamical Mean Field Theory to compute zero
temperature properties of the two-dimensional square lattice Hubbard model. The
d-wave order parameter is found to scale like the superexchange coupling J for
on-site interaction U comparable to or larger than the bandwidth. The order
parameter also assumes a dome shape as a function of doping while, by contrast,
the gap in the single-particle density of states decreases monotonically with
increasing doping. In the presence of a finite second-neighbor hopping t', the
zero temperature phase diagram displays the electron-hole asymmetric
competition between antiferromagnetism and superconductivity that is observed
experimentally in the cuprates. Adding realistic third-neighbor hopping t''
improves the overall agreement with the experimental phase diagram. Since band
parameters can vary depending on the specific cuprate considered, the
sensitivity of the theoretical phase diagram to band parameters challenges the
commonly held assumption that the doping vs T_{c}/T_{c}^{max} phase diagram of
the cuprates is universal. The calculated ARPES spectrum displays the observed
electron-hole asymmetry. Our calculations reproduce important features of
d-wave superconductivity in the cuprates that would otherwise be considered
anomalous from the point of view of the standard BCS approach.Comment: 13 pages, 7 figure
Reading In English By Children In Korea: Frequency, Effectiveness, And Barriers
A study of the English non-textbook reading of fourth graders in Korea revealed that about 80% had done at least some reading, confirming that there is enthusiasm for English reading. About half, however, had read only five books or fewer. Non-readers said that the reason they did not read in English was the difficulty of English texts. Those who read more did better on a test of English spelling and vocabulary
Short-range spin correlations and induced local spin-singlet amplitude in the Hubbard model
In this paper, from the microscopic Hubbard Hamiltonian we extract the local
spin-singlet amplitude due to short-range spin correlations, and quantify its
strength near half-filling. As a first application of the present approach, we
study a problem of the energy dispersion and its d-wave modulation in the
insulating cuprates, SrCuOCl and CaCuOCl.
Without any adjustable parameters, most puzzling issues are naturally and
quantitatively explained within the present approach.Comment: 6 pages, 3 figure
Verifying multi-partite mode entanglement of W states
We construct a method for verifying mode entanglement of N-mode W states. The
ideal W state contains exactly one excitation symmetrically shared between N
modes, but our method takes the existence of higher numbers of excitations into
account, as well as the vacuum state and other deviations from the ideal state.
Moreover, our method distinguishes between full N-party entanglement and states
with M-party entanglement with M<N, including mixtures of the latter. We
specialize to the case N=4 for illustrative purposes. In the optical case,
where excitations are photons, our method can be implemented using linear
optics.Comment: 11 pages, 12 figure
On the optical conductivity of Electron-Doped Cuprates I: Mott Physics
The doping and temperature dependent conductivity of electron-doped cuprates
is analysed. The variation of kinetic energy with doping is shown to imply that
the materials are approximately as strongly correlated as the hole-doped
materials. The optical spectrum is fit to a quasiparticle scattering model;
while the model fits the optical data well, gross inconsistencies with
photoemission data are found, implying the presence of a large, strongly doping
dependent Landau parameter
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
site
J/psi hadron interaction in vacuum and in QGP
Motivated by the recent lattice data that will survive up to
1.6, we calculate the thermal width of at finite temperature in
perturbative QCD. The inputs of the calculation are the parton quarkonium
dissociation cross sections at the NLO in QCD, which were previously obtained
by Song and Lee, and a gaussian charmonium wave function, whose size were
fitted to an estimate by Wong by solving the schrodinger equation for
charmonium in a potential extracted from the lattice at finite temperature. We
find that the total thermal width above 1.4 becomes larger than 100 to 200
MeV, depending on the effective thermal masses of the quark and gluon, which we
take it to vary from 600 to 400 MeV.Comment: 4 pages, Talk at Quark Matter 200
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