331 research outputs found
Spin-fluctuation mechanism of superconductivity in cuprates
The theory of superconductivity within the t-J model, as relevant for
cuprates, is developed. It is based on the equations of motion for projected
fermionic operators and the mode-coupling approximation for the self-energy
matrix. The dynamical spin susceptibility at various doping is considered as an
input, extracted from experiments. The analysis shows that the
superconductivity onset is dominated by the spin-fluctuation contribution. We
show that T_c is limited by the spin-fluctuation scale and shows a
pronounced dependence on the next-nearest-neighbor hopping t'. The latter can
offer an explanation for the variation of T_c among different families of
cuprates.Comment: Color figure
Quantum fluctuations in the effective pseudospin-1/2 model for magnetic pyrochlore oxides
The effective quantum pseudospin-1/2 model for interacting rare-earth
magnetic moments, which are locally described with atomic doublets, is studied
theoretically for magnetic pyrochlore oxides. It is derived microscopically for
localized Pr^{3+} 4f moments in Pr_2TM_2O_7 (TM = Zr, Sn, Hf, and Ir) by
starting from the atomic non-Kramers magnetic doublets and performing the
strong-coupling perturbation expansion of the virtual electron transfer between
the Pr 4f and O 2p electrons. The most generic form of the nearest-neighbor
anisotropic superexchange pseudospin-1/2 Hamiltonian is also constructed from
the symmetry properties, which is applicable to Kramers ions Nd^{3+}, Sm^{3+},
and Yb^{3+} potentially showing large quantum effects. The effective model is
then studied by means of a classical mean-field theory and the exact
diagonalization on a single tetrahedron and on a 16-site cluster. These
calculations reveal appreciable quantum fluctuations leading to quantum phase
transitions to a quadrupolar state as a melting of spin ice for the Pr^{3+}
case. The model also shows a formation of cooperative quadrupole moment and
pseudospin chirality on tetrahedrons. A sign of a singlet quantum spin ice is
also found in a finite region in the space of coupling constants. The relevance
to the experiments is discussed.Comment: 18 pages including 14 figures; Comparison with the magnetization
curve on Pr2Ir2O7 included; to appear in Phys. Rev.
Double dispersion of the magnetic resonant mode in cuprates
The magnetic excitation spectra in the vicinity of the resonant peak, as
observed by inelastic neutron scattering in cuprates, are studied within the
memory-function approach. It is shown that at intermediate doping the
superconducting gap induces a double dispersion of the peak, with an anisotropy
rotated between the downward and upward branch. Similar behavior, but with a
spin-wave dispersion at higher energies, is obtained for the low-doping case
assuming a large pairing pseudogap.Comment: 4 LaTeX pages, 4 figure
Theory of magnetic excitons in the heavy-fermion superconductor
We analyze the influence of unconventional superconductivity on the magnetic
excitations in the heavy fermion compound UPdAl. We show that it leads
to the formation of a bound state at energies well below 2 at the
antiferromagnetic wave vector {\textbf Q}=. Its signature is a
resonance peak in the spectrum of magnetic excitations in good agreement with
results from inelastic neutron scattering. Furthermore we investigate the
influence of antiferromagnetic order on the formation of the resonance peak. We
find that its intensity is enhanced due to intraband transitions induced by the
reconstruction of Fermi surface sheets. We determine the dispersion of the
resonance peak near {\textbf Q} and show that it is dominated by the magnetic
exciton dispersion associated with local moments. We demonstrate by a
microscopic calculation that UPdAl is another example in which the
unconventional nature of the superconducting order parameter can be probed by
means of inelastic neutron scattering and determined unambiguously.Comment: 6 pages, 4 figure
Neutron Scattering Studies of the Magnetic Fluctuations in YBa_2Cu_3O_{7-\delta}
Neutron scattering measurements have been made on the spin fluctuations in
YBa_2Cu_3O_{7-\delta} for different oxygen doping levels. Incommensurability is
clearly observed for oxygen concentrations of 6.6 and 6.7 and is suggested for
the 6.93. Measurements of the resonance for the O_{6.6} concentration show that
it exists in a broadened and less intense form at temperatures much higher than
T_c.Comment: 9 pages, 4 gif figures, Proceedings of Spectroscopies in Novel
Superconductors, Cape Cod, Massachusetts, Sept, 199
Coexistence of magnetism and superconductivity in a t-J bilayer
We investigate coexistence of antiferromagnetic and superconducting
correlations in bilayered materials using a two-dimensional t-J model with
couplings across the layers using variational Monte Carlo calculations. It is
found that the underdoped regime supports a coexisting phase, beyond which the
(d-wave) superconducting state becomes stable. Further, the effects of
interplanar coupling parameters on the magnetic and superconducting
correlations as a function of hole doping are studied in details. The magnetic
correlations are found to diminish with increasing interplanar hopping away
from half filling, while the exchange across the layers strengthens interplanar
antiferromagnetic correlations both at and away from half filling. The
superconducting correlations show more interesting features where larger
interplanar hopping considerably reduces planar correlations at optimal doping,
while an opposite behaviour, i.e. stabilisation of the superconducting state is
realised in the overdoped regime, with the interplanar exchange all the while
playing a dormant role.Comment: 8 pages, 9 figures, RevTex4, Submitted to Phys. Rev.
Two Distinct Electronic Contributions in the Fully Symmetric Raman Response of High Cuprates
We show by non resonant effect in HgBaCuO (Hg-1201)and by Zn
substitutions in YBaCuO (Y-123) compounds that the fully
symmetric Raman spectrum has two distinct electronic contributions. The
A response consists in the superconducting pair breaking peak at the
2 energy and a collective mode close to the magnetic resonance energy.
These experimental results reconcile the \textit{d-wave} model to the A
Raman response function in so far as a collective mode that is distinct from
the pair breaking peak is present in the A channel.Comment: 4 pages, 2 figure
Linear dependence of peak width in \chi(\bq, \omega) vs T_c for YBCO superconductors
It is shown that the momentum space width of the peak in the spin
susceptibility, Im, is linearly proportional to the
superconducting : with \AA. This relation is similar to the linear relation between incommensurate
peak splitting and in LaSrCuO superconductors, as first proposed by
Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity is
smaller than Fermi velocity or the spin-wave velocity of the parent compound
and remains the same for a wide doping range. This result points towards strong
similarities in magnetic state of YBCO and LaSrCuO.Comment: 5 pages, 3 figures, latex fil
Experimental investigation of the origin of the cross-over temperature in the cuprates
We investigate the cross-over temperature T* as a function of doping in
(Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_{y}, where the maximum Tc
(Tc^max) varies continuously by 30% between families (x) with minimal
structural changes. T* is determined by DC-susceptibility measurements. We find
that T* scales with the maximum Neel temperature TN^max of each family. This
result strongly supports a magnetic origin of T*, and indicates that three
dimensional interactions play a role in its magnitude.Comment: 4 pages, 4 figure
Spin-gap phase in nearly-half-filled one-dimensional conductors coupled with phonons
Asymptotic properties of nearly-half-filled one-dimensional conductors
coupled with phonons are studied through a renormalization group method. Due to
spin-charge coupling via electron-phonon interaction, the spin correlation
varies with filling as well as the charge correlation. Depending on the
relation between cut-off energy scales of the Umklapp process and of the
electron-phonon interaction, various phases appear. We found a metallic phase
with a spin gap and a dominant charge- density-wave correlation near half
filling between a gapless density-wave phase (like in the doped repulsive
Hubbard model) and a superconductor phase with a spin gap. The spin gap is
produced by phonon-assisted backward scatterings which are interfered with the
Umklapp process constructively or destructively depending on the character of
electron-phonon coupling.Comment: 14 pages, revtex, replaced 5 ps figures, published in PR
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