Superconducting Order Parameter in Bi-Layer Cuprates: Occurrence of π\pi Phase Shifts in Corner Junctions

We study the order parameter symmetry in bi-layer cuprates such as YBaCuO, where interesting $\pi$ 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. $\pi$ 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 $T_c$ are discussed.Comment: 5 pages, RevTeX 3.0, 9 figures (available upon request

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, $z=1$, behavior, characterized by temperature independent $T_1T/T_{\rm 2G}$ and linear $T_1T$, 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, $z=1$, regime. The crossover to the $z=2$ 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 La2−x_{2-x}Srx_xCaCu2_2O6_6

A neutron scattering study of nonsuperconducting La$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (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, $z\!=\!1$, to the quantum disordered regime in high-T$_c$ 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 $\Delta\!\sim\!1/\xi$ well above T$_c$, which is supplemented by the quasiparticle gap below T$_c$. 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 YBa$_2$Cu$_3$O$_{6+x}$ 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 $[(1\pm\delta)\pi,\pi]$ and $[\pi,(1\pm\delta)\pi]$ at low energies with $\delta$ initially increasing with doping at low dopings and then saturating at higher dopings. These incommensurate peaks are suppressed, and the parameter $\delta$ is reduced with increasing energy. Eventually it converges to the $[\pi,\pi]$ 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 $T_c$ 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 $T_c$ superconductors.Comment: 22 pages, 18 figures, submitted to PR

Glassy nature of stripe ordering in La(1.6-x)Nd(0.4)Sr(x)CuO(4)

We present the results of neutron-scattering studies on various aspects of crystalline and magnetic structure in single crystals of La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have reexamined the degree of stripe order in an x=0.12 sample. Measurements of the width for an elastic magnetic peak show that it saturates at a finite value below 30 K, corresponding to a spin-spin correlation length of 200 A. A model calculation indicates that the differing widths of magnetic and (previously reported) charge-order peaks, together with the lack of commensurability, can be consistently explained by disorder in the stripe spacing. Above 30 K, the width of the nominally elastic signal begins to increase. Interpreting the signal as critical scattering from slowly fluctuating spins, the temperature dependence of the width is consistent with renormalized classical behavior of a 2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering measurements show that incommensurate spin excitations survive at and above 50 K, where the elastic signal is neglible. We also report several results related to the LTO-to-LTT transition.Comment: 13 pp, 2-col. REVTeX, 11 figures embedded with psfig; expanded discussion of T-dep. of magnetic peak width; version to appear in Phys. Rev. B (01Jun99