46 research outputs found
Resonant magnetic mode in superconducting 2-leg ladders
The spin dynamics of a doped 2-leg spin ladder is investigated by numerical
techniques. We show that a hole pair-magnon boundstate evolves at finite hole
doping into a sharp magnetic excitation below the two-particle continuum. This
is supported by a field theory argument based on a SO(6)-symmetric ladder.
Similarities and differences with the resonant mode of the high-T cuprates
are discussed.Comment: 5 pages, 5 figure
Quantum Disordered Regime and Spin Gap in the Cuprate Superconductors
We discuss the crossover from the quantum critical, , to the quantum
disordered regime in high-T materials in relation to the experimental data
on the nuclear relaxation, bulk susceptibility, and inelastic neutron
scattering. In our scenario, the spin excitations develop a gap
well above T, which is supplemented by the
quasiparticle gap below T. The above experiments yield consistent estimates
for the value of the spin gap, which increases as the correlation length
decreases.Comment: 14 pages, REVTeX v3.0, PostScript file for 3 figures is attached,
UIUC-P-93-07-06
Theory of Neutron Scattering in the Normal and Superconducting State of YBCO
We analyze neutron experiments on \ybco at various stoichiometries in the
superconducting state, within the context of a bi-layer theory which yields
good agreement with the normal state Cu-NMR and neutron data as a function of
\omega, q and T. A d-wave superconducting state exhibits peaks at q = ( \pi ,
\pi , \pi ) and sharp maxima as a function of \omega, at twice the gap
frequency. This behavior may have been observed experimentally. The counterpart
behavior for other choices of order parameter symmetry is discussed.Comment: uuencoded postscript file for the entire paper enclose
Principles of the Field Theory of High Temperature Superconductivity in Underdoped Copper-Oxides
Here I extend my last work about the origin of the pseudo-gaps in underdoped
cuprates (arXiv: cond-mat. 1011.3206), to include the mechanism of
superconductivity. This is done by adapting the formalism of the double
correlations in systems with nested Fermi surfaces to the semi one dimensional
system of strings of holes. It is proposed that magnetic interaction is crucial
for the establishment of the pseudogap and the high temperature
superconductivity. It is shown that superconductivity disturbs the completeness
of the strings of holes, and creates fluctuations in their shapes. This, in
turn, reduces the magnetic interaction and the pseudogap order.Comment: This paper has been withdrawn by the author. 27 page
Doping dependence of the resonance peak and incommensuration in high- superconductors
The doping and frequency evolutions of the incommensurate spin response and
the resonance mode are studied based on the scenario of the Fermi surface
topology. We use the slave-boson mean-field approach to the
model and including the antiferromagnetic fluctuation correction in the
random-phase approximation. We find that the equality between the
incommensurability and the hole concentration is reproduced at low frequencies
in the underdoped regime. This equality observed in experiments was explained
{\it only} based on the stripe model before. We also obtain the downward
dispersion for the spin response and predict its doping dependence for further
experimental testing, as well as a proportionality between the low-energy
incommensurability and the resonance energy. Our results suggest a common
origin for the incommensuration and the resonance peak based on the Fermi
surface topology and the d-wave symmetry.Comment: 5 pages, 4 PS figure
Neutron scattering and superconducting order parameter in YBa2Cu3O7
We discuss the origin of the neutron scattering peak at 41 meV observed in
YBaCuO below . The peak may occur due to spin-flip electron
excitations across the superconducting gap which are enhanced by the
antiferromagnetic interaction between Cu spins. In this picture, the experiment
is most naturally explained if the superconducting order parameter has -wave
symmetry and opposite signs in the bonding and antibonding electron bands
formed within a CuO bilayer.Comment: In this version, only few minor corrections and the update of
references were done in order to make perfect correspondence with the
published version. RevTeX, psfig, 5 pages, and 3 figure
Antiferromagnetic ordering in a 90 K copper oxide superconductor
Using elastic neutron scattering, we evidence a commensurate
antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-
cuprate (y=0.013, =93 K). As
in the Co-free system, the spin excitation spectrum is dominated by a magnetic
resonance peak at 41 meV but with a reduced spectral weight. The substitution
of Co thus leads to a state where AF and SC cohabit showing that the CuO
plane is a highly antiferromagnetically polarizable medium even for a sample
where T remains optimum.Comment: 3 figure
Spin Susceptibility in Underdoped
We report a comprehensive polarized and unpolarized neutron scattering study
of the evolution of the dynamical spin susceptibility with temperature and
doping in three underdoped single crystals of the \YBCO{6+x} high temperature
superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85}
(T_c = 87 K). Theoretical implications of these data are discussed, and a
critique of recent attempts to relate the spin excitations to the
thermodynamics of high temperature superconductors is given.Comment: minor revisions, to appear in PR
Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)
We report a detailed study, using neutron scattering, transport and
magnetization measurements, of the interplay between superconducting (SC) and
spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below
the same critical temperature. However, the SDW order grows with applied
magnetic field, whereas SC order is suppressed. Most importantly, the field
dependence of the SDW Bragg peak intensity has a cusp at zero field, as
predicted by a recent theory of competing SDW and SC order. This leads us to
conclude that there is a repulsive coupling between the two order parameters.
The question of whether the two kinds of order coexist or microscopically phase
separate is discussed.Comment: Version accepted for publication in Phys. Rev. B. Improved discussion
in connection with the muSR result
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