32 research outputs found
Superconducting Order Parameter in Bi-Layer Cuprates: Occurrence of Phase Shifts in Corner Junctions
We study the order parameter symmetry in bi-layer cuprates such as YBaCuO,
where interesting phase shifts have been observed in Josephson junctions.
Taking models which represent the measured spin fluctuation spectra of this
cuprate, as well as more general models of Coulomb correlation effects, we
classify the allowed symmetries and determine their associated physical
properties. phase shifts are shown to be a general consequence of
repulsive interactions, independent of whether a magnetic mechanism is
operative. While it is known to occur in d-states, this behavior can also be
associated with (orthorhombic) s-symmetry when the two sub-band gaps have
opposite phase. Implications for the magnitude of are discussed.Comment: 5 pages, RevTeX 3.0, 9 figures (available upon request
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Polarization Dependence of the Spin-Density-Wave Excitations in Single-Domain Chromium
A polarised neutron scattering experiment has been performed on a single-Q, single domain sample of Cr in a magnetic field of 4 T in the transverse spin-density-wave phase. It is confirmed that the longitudinal fluctuations are enhanced for energy transfers E {lt} 8 meV similarly as in the longitudinal spin-density-wave phase. The spin wave modes with deltaS parallel and perpendicular to Q are isotropic within the E-range investigated
Essential Role of the Cooperative Lattice Distortion in the Charge, Orbital and Spin Ordering in doped Manganites
The role of lattice distortion in the charge, orbital and spin ordering in
half doped manganites has been investigated. For fixed magnetic ordering, we
show that the cooperative lattice distortion stabilize the experimentally
observed ordering even when the strong on-site electronic correlation is taken
into account. Furthermore, without invoking the magnetic interactions, the
cooperative lattice distortion alone may lead to the correct charge and orbital
ordering including the charge stacking effect, and the magnetic ordering can be
the consequence of such a charge and orbital ordering. We propose that the
cooperative nature of the lattice distortion is essential to understand the
complicated charge, orbital and spin ordering observed in doped manganites.Comment: 5 pages,4 figure
Toward a Unified Magnetic Phase Diagram of the Cuprate Superconductors
We propose a unified magnetic phase diagram of cuprate superconductors. A new
feature of this phase diagram is a broad intermediate doping region of
quantum-critical, , behavior, characterized by temperature independent
and linear , where the spin waves are not completely
absorbed by the electron-hole continuum. The spin gap in the moderately doped
materials is related to the suppression of the low-energy spectral weight in
the quantum disordered, , regime. The crossover to the regime, where
T_1T/T_{\rm 2G}^2 \simeq \mbox{const}, occurs only in the fully doped
materials.Comment: 14 pages, REVTeX v2.1, PostScript file for 3 figures attached,
UIUC-P-93-06-04
Structural and Magnetic Instabilities of LaSrCaCuO
A neutron scattering study of nonsuperconducting
LaSrCaCuO (x=0 and 0.2), a bilayer copper oxide without CuO
chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a
doping dependent transition temperature. The predominant structural
modification below the transition is an in-plane shift of the apical oxygen. In
the doped sample, the orthorhombic superstructure is strongly disordered, and a
glassy state involving both magnetic and structural degrees of freedom develops
at low temperature. The spin correlations are commensurate.Comment: published versio
Optical Investigations of Charge Gap in Orbital Ordered La1/2Sr3/2MnO4
Temperature and polarization dependent electronic structure of La1/2Sr3/2MnO4
were investigated by optical conductivity analyses. With decreasing
temperature, for E//ab, a broad mid-infrared (MIR) peak of La1/2Sr3/2MnO4
becomes narrower and moves to the higher frequency, while that of
Nd1/2Sr3/2MnO4 nearly temperature independent. We showed that the MIR peak in
La1/2Sr3/2MnO4 originates from orbital ordering associated with CE-type
magnetic ordering and that the Jahn-Teller distortion has a significant
influence on the width and the position of the MIR peak.Comment: 10 pages, 4 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
Convergence of energy-dependent incommensurate antiferromagnetic neutron scattering peaks to commensurate resonance in underdoped bilayer cuprates
The recently discovered coexistence of incommensurate antiferromagnetic
neutron scattering peaks and commensurate resonance in underdoped
YBaCuO is calling for an explanation. Within the t-J model, the
doping and energy dependence of the spin dynamics of the underdoped bilayer
cuprates in the normal state is studied based on the fermion-spin theory by
considering the bilayer interactions. Incommensurate peaks are found at
and at low energies with
initially increasing with doping at low dopings and then saturating at
higher dopings. These incommensurate peaks are suppressed, and the parameter
is reduced with increasing energy. Eventually it converges to the
resonance peak. Thus the recently observed coexistence is
interpreted in terms of bilayer interactions.Comment: 15 pages, Revtex, five figures are included, accepted for publication
in Phys. Rev.
Spin correlations in the algebraic spin liquid - implications for high Tc superconductors
We propose that underdoped high superconductors are described by an
algebraic spin liquid (ASL) at high energies, which undergoes a spin-charge
recombination transition at low energies. The spin correlation in the ASL is
calculated via its effective theory - a system of massless Dirac fermions
coupled to a U(1) gauge field. We find that without fine tuning any parameters
the gauge interaction strongly enhances the staggered spin correlation even in
the presence of a large single particle pseudo-gap. This allows us to show that
the ASL plus spin-charge recombination picture can explain many highly unusual
properties of underdoped high superconductors.Comment: 22 pages, 18 figures, submitted to PR
Resonant x-ray diffraction study of the magnetoresistant perovskite Pr0.6Ca0.4MnO3
We report a x-ray resonant diffraction study of the perovskite
Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the
electronic structure of the Mn atoms. We discuss the resonant x-ray spectra
measured above and below the charge and orbital ordering phase transition
temperature (TCOO = 232 K), and present a detailed analysis of the energy and
polarization dependence of the resonant scattering. The analysis confirms that
the structural transition is a transition to an orbitally ordered phase in
which inequivalent Mn atoms are ordered in a CE-type pattern. The Mn atoms
differ mostly by their 3d orbital occupation. We find that the charge
disproportionation is incomplete, 3d^{3.5-\delta} and 3d^{3.5+\delta} with
\delta\ll0.5 . A revised CE-type model is considered in which there are two Mn
sublattices, each with partial e_{g} occupancy. One sublattice consists of Mn
atoms with the 3x^{2}-r^{2} or 3y^{2}-r^{2} orbitals partially occupied, the
other sublattice with the x^{2}-y^{2} orbital partially occupied.Comment: 15 pages, 15 figure