46 research outputs found
d_{x^2-y^2} Symmetry and the Pairing Mechanism
An important question is if the gap in the high temperature cuprates has
d_{x^2-y^2} symmetry, what does that tell us about the underlying interaction
responsible for pairing. Here we explore this by determining how three
different types of electron-phonon interactions affect the d_{x^2-y^2} pairing
found within an RPA treatment of the 2D Hubbard model. These results imply that
interactions which become more positive as the momentum transfer increases
favor d_{x^2-y^2} pairing in a nearly half-filled band.Comment: 9 pages and 2 eps figs, uses revtex with epsf, in press, PR
Theory for Electron-Doped Cuprate Superconductors: d-wave symmetry order parameter
Using as a model the Hubbard Hamiltonian we determine various basic
properties of electron-doped cuprate superconductors like
and for a
spin-fluctuation-induced pairing mechanism. Most importantly we find a narrow
range of superconductivity and like for hole-doped cuprates -
symmetry for the superconducting order parameter. The superconducting
transition temperatures for various electron doping concentrations
are calculated to be much smaller than for hole-doped cuprates due to the
different Fermi surface and a flat band well below the Fermi level. Lattice
disorder may sensitively distort the symmetry via
electron-phonon interaction
Superconducting instability in the Holstein-Hubbard model: A numerical renormalization group study
We have studied the d-wave pairing-instability in the two-dimensional
Holstein-Hubbard model at the level of a full fluctuation exchange
approximation which treats both Coulomb and electron-phonon (EP) interaction
diagrammatically on an equal footing. A generalized numerical renormalization
group technique has been developed to solve the resulting self-consistent field
equations. The -wave superconducting phase diagram shows an optimal T_c at
electron concentration ~ 0.9 for the purely electronic Hubbard system. The
EP interaction suppresses the d-wave T_c which drops to zero when the
phonon-mediated on-site attraction becomes comparable to the on-site
Coulomb repulsion . The isotope exponent is negative in this model
and small compared to the classical BCS value or compared
to typical observed values in non-optimally doped cuprate superconductors.Comment: 4 pages RevTeX + 3 PS figures include
Hybridization-induced superconductivity from the electron repulsion on a tetramer lattice having a disconnected Fermi surface
Plaquette lattices with each unit cell containing multiple atoms are good
candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita
to be favorable for spin-flucutation mediated superconductivity from electron
repulsion. Here we find an interesting example in a tetramer lattice where the
structure within each unit cell dominates the nodal structure of the gap
function. We trace its reason to the way in which a Cooper pair is formed
across the hybridized molecular orbitals, where we still end up with a T_c much
higher than usual.Comment: 4 pages, 6 figure
Superconductivity in quantum-dot superlattices composed of quantum wire networks
Based on calculations using the local density approximation, we propose
quantum wire networks with square and plaquette type lattice structures that
form quantum dot superlattices. These artificial structures are well described
by the Hubbard model. Numerical analysis reveals a superconducting ground state
with transition temperatures of up to 90 mK for the plaquette, which is
more than double the value of 40 mK for the square lattice type and is
sufficiently high to allow for the experimental observation of
superconductivity.Comment: 10 pages, 4 figure
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
Magnetic Properties of YBa_2Cu_3O_{7-\delta} in a self-consistent approach: Comparison with Quantum-Monte-Carlo Simulations and Experiments
We analyze single-particle electronic and two-particle magnetic properties of
the Hubbard model in the underdoped and optimally-doped regime of \YBCO by
means of a modified version of the fluctuation-exchange approximation, which
only includes particle-hole fluctuations. Comparison of our results with
Quantum-Monte Carlo (QMC) calculations at relatively high temperatures () suggests to introduce a temperature renormalization in order to
improve the agreement between the two methods at intermediate and large values
of the interaction .
We evaluate the temperature dependence of the spin-lattice relaxation time
and of the spin-echo decay time and compare it with the results
of NMR measurements on an underdoped and an optimally doped \YBCO sample. For
it is possible to consistently adjust the parameters of the Hubbard
model in order to have a good {\it semi-quantitative} description of this
temperature dependence for temperatures larger than the spin gap as obtained
from NMR measurements. We also discuss the case , which is more
appropriate to describe magnetic and single-particle properties close to
half-filling. However, for this larger value of the agreement with QMC as
well as with experiments at finite doping is less satisfactory.Comment: Final version, to appear in Phys. Rev. B (sched. Feb. 99
Stability of condensate in superconductors
According to the BCS theory the superconducting condensate develops in a
single quantum mode and no Cooper pairs out of the condensate are assumed. Here
we discuss a mechanism by which the successful mode inhibits condensation in
neighboring modes and suppresses a creation of noncondensed Cooper pairs. It is
shown that condensed and noncondensed Cooper pairs are separated by an energy
gap which is smaller than the superconducting gap but large enough to prevent
nucleation in all other modes and to eliminate effects of noncondensed Cooper
pairs on properties of superconductors. Our result thus justifies basic
assumptions of the BCS theory and confirms that the BCS condensate is stable
with respect to two-particle excitations
The 3-Band Hubbard-Model versus the 1-Band Model for the high-Tc Cuprates: Pairing Dynamics, Superconductivity and the Ground-State Phase Diagram
One central challenge in high- superconductivity (SC) is to derive a
detailed understanding for the specific role of the - and
- orbital degrees of freedom. In most theoretical studies an
effective one-band Hubbard (1BH) or t-J model has been used. Here, the physics
is that of doping into a Mott-insulator, whereas the actual high- cuprates
are doped charge-transfer insulators. To shed light on the related question,
where the material-dependent physics enters, we compare the competing magnetic
and superconducting phases in the ground state, the single- and two-particle
excitations and, in particular, the pairing interaction and its dynamics in the
three-band Hubbard (3BH) and 1BH-models. Using a cluster embedding scheme, i.e.
the variational cluster approach (VCA), we find which frequencies are relevant
for pairing in the two models as a function of interaction strength and doping:
in the 3BH-models the interaction in the low- to optimal-doping regime is
dominated by retarded pairing due to low-energy spin fluctuations with
surprisingly little influence of inter-band (p-d charge) fluctuations. On the
other hand, in the 1BH-model, in addition a part comes from "high-energy"
excited states (Hubbard band), which may be identified with a non-retarded
contribution. We find these differences between a charge-transfer and a Mott
insulator to be renormalized away for the ground-state phase diagram of the
3BH- and 1BH-models, which are in close overall agreement, i.e. are
"universal". On the other hand, we expect the differences - and thus, the
material dependence to show up in the "non-universal" finite-T phase diagram
(-values).Comment: 17 pages, 9 figure
An ARPES view on the high-Tc problem: phonons vs spin-fluctuations
We review the search for a mediator of high-Tc superconductivity focusing on
ARPES experiment. In case of HTSC cuprates, we summarize and discuss a
consistent view of electronic interactions that provides natural explanation of
both the origin of the pseudogap state and the mechanism for high temperature
superconductivity. Within this scenario, the spin-fluctuations play a decisive
role in formation of the fermionic excitation spectrum in the normal state and
are sufficient to explain the high transition temperatures to the
superconducting state while the pseudogap phenomenon is a consequence of a
Peierls-type intrinsic instability of electronic system to formation of an
incommensurate density wave. On the other hand, a similar analysis being
applied to the iron pnictides reveals especially strong electron-phonon
coupling that suggests important role of phonons for high-Tc superconductivity
in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538,
http://for538.wmi.badw.d