Raman scattering through surfaces having biaxial symmetry

Magnetic Raman scattering in two-leg spin ladder materials and the relationship between the anisotropic exchange integrals are analyzed by P. J. Freitas and R. R. P. Singh in Phys. Rev. B, {\bf 62}, 14113 (2000). The angular dependence of the two-magnon scattering is shown to provide information for the magnetic anisotropy in the Sr_14Cu_24O_41 and La_6Ca_8Cu_24O_41 compounds. We point out that the experimental results of polarized Raman measurements at arbitrary angles with respect to the crystal axes have to be corrected for the light ellipticity induced inside the optically anisotropic crystals. We refer quantitatively to the case of Sr_14Cu_24O_41 and discuss potential implications for spectroscopic studies in other materials with strong anisotropy.Comment: To be published as a Comment in Phys. Rev.

Sliding Density-Wave in Sr_{14}Cu_{24}O_{41} Ladder Compounds

We used transport and Raman scattering measurements to identify the insulating state of self-doped spin 1/2 two-leg ladders of Sr_{14}Cu_{24}O_{41} as a weakly pinned, sliding density wave with non-linear conductivity and a giant dielectric response that persists to remarkably high temperatures

Magnetic order in lightly doped La_{2-x}Sr_{x}CuO_{4}

We study long wavelength magnetic excitations in lightly doped La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferromagnetic (AF) phase. The second magnetic resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state and led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the N\'{e}el temperature in x = 0.01.Comment: 5 pages, 5 figure

Resonant two-magnon Raman scattering in two-dimensional and ladder-type Mott insulators

We investigate the resonant two-magnon Raman scattering in the two-dimensional (2D) and ladder-type Mott insulators by using a half-filled Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the model can reproduce the experimental features in the 2D that the Raman intensity is enhanced when the incoming photon energy is not near the absorption edge but well above it. In the ladder-type Mott insulators, the Raman intensity is found to resonate with absorption spectrum in contrast to the 2D system. The difference between 2D and the ladder systems is explained by taking into account the fact that the ground state in 2D is a spin-ordered state while that in ladder is a spin-gapped one.Comment: REVTeX4, 3 pages, 3 figures, Proceedings for ISS2002 (Yokohama, November 2002). To be published in Physica

Derivation of the generalized Non Linear Sigma Model in the presence of the Dzyaloshinskii-Moriya interaction

We derive the long-wavelength non-linear sigma model for a two-dimensional Heisenberg system in the presence of the Dzyaloshinskii-Moriya and pseudodipolar interactions. We show that the system is a non-conventional easy-axis antiferromagnet, displaying an anomalous coupling between the magnetic field and the staggered order parameter. Our results are in good agreement with recent experimental data for undoped La2CuO4 compounds.Comment: Proceedings of SCES05, to appear on Physica

First-Order Type Effects in YBa2_2Cu3_3O6+x_{6+x} at the Onset of Superconductivity

We present results of Raman scattering experiments on tetragonal ${\rm (Y_{1-y}Ca_{y})Ba_{2}Cu_{3}O_{6+x}}$ for doping levels $p(x,y)$ between 0 and 0.07 holes/CuO$_2$. Below the onset of superconductivity at $p_{\rm sc1} \approx 0.06$, we find evidence of a diagonal superstructure. At $p_{\rm sc1}$, lattice and electron dynamics change discontinuously with the charge and spin properties being renormalized at all energy scales. The results indicate that charge ordering is intimately related to the transition at $p_{\rm sc1}$ and that the maximal transition temperature to superconductivity at optimal doping $T_{c}^{\rm max}$ depends on the type of ordering at $p>p_{\rm sc1}$.Comment: 4 pages, 4 figure

Crystal structure and high-field magnetism of La2CuO4

Neutron diffraction was used to determine the crystal structure and magnetic ordering pattern of a La2CuO4 single crystal, with and without applied magnetic field. A previously unreported, subtle monoclinic distortion of the crystal structure away from the orthorhombic space group Bmab was detected. The distortion is also present in lightly Sr-doped crystals. A refinement of the crystal structure shows that the deviation from orthorhombic symmetry is predominantly determined by displacements of the apical oxygen atoms. An in-plane magnetic field is observed to drive a continuous reorientation of the copper spins from the orthorhombic b-axis to the c-axis, directly confirming predictions based on prior magnetoresistance and Raman scattering experiments. A spin-flop transition induced by a c-axis oriented field previously reported for non-stoichiometric La2CuO4 is also observed, but the transition field (11.5 T) is significantly larger than that in the previous work

Charge collective modes in an incommensurately modulated cuprate

We report the first measurement of collective charge modes of insulating Sr14Cu24O41 using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from the ladder-containing planar substructures. The observed ladder modes (E vs. Q) are found to be dispersive for momentum transfers along the "legs" but nearly localized along the "rungs". Dispersion and peakwidth characteristics are similar to the charge spectrum of 1D Mott insulators, and we show that our results can be understood in the strong coupling limit (U >> t_{ladder}> t_{chain}). The observed behavior is in marked contrast to the charge spectrum seen in most two dimensional cuprates. Quite generally, our results also show that momentum-tunability of inelastic scattering can be used to resolve mode contributions in multi-component incommensurate systems.Comment: 4+ pages, 5 figure

Magnon bands of N-leg integer-spin antiferromagnetic systems in the weak interchain-coupling regime

Using the exact results of the O(3) nonlinear sigma model (NLSM) and a few quantitative numerical data for integer-spin antiferromagnetic (AF) chains, we systematically estimate all magnon excitation energies of N-leg integer-spin AF ladders and tubes in the weak-interchain-coupling regime. Our method is based on a first-order perturbation theory for the strength of the interchain coupling. It can deal with any kind of interchain interactions, in principle. We confirm that results of the perturbation theory are in good agreement with those of a quantum Monte Carlo simulation and with our recent study based on a saddle-point approximation of the NLSM [Phys. Rev. B 72, 104438 (2005)]. Our theory further supports the existence of a Haldane (gapped) phase even in a d-dimensional (d\geq 2) spatially anisotropic integer-spin AF model, if the exchange coupling in one direction is sufficiently strong compared with those in all the other directions. The strategy in this paper is applicable to other N-leg systems consisting of gapped chains which low-energy physics is exactly or quantitatively known.Comment: 11 pages, 4 figures, Revtex, published version, see also cond-mat/0506049 (PRB72, 104438 (2005)