Chiral spin-order in some purported Kitaev spin-liquid compounds

We examine recent magnetic torque measurements in two compounds, $\gamma$-Li$_2$IrO$_3$ and RuCl$_3$, which have been discussed as possible realizations of the Kitaev model. The analysis of the reported discontinuity in torque, as an external magnetic field is rotated across the $c-$axis in both crystals, suggests that they have a translationally-invariant chiral spin-order of the from $\ne 0$ in the ground state and persisting over a very wide range of magnetic field and temperature. An extra-ordinary $|B|B^2$ dependence of the torque for small fields, beside the usual $B^2$ part, is predicted due to the chiral spin-order, and found to be consistent with experiments upon further analysis of the data. Other experiments such as inelastic scattering and thermal Hall effect and several questions raised by the discovery of chiral spin-order, including its topological consequences are discussed.Comment: Clearer figures of the experimental data provided. Also clearer exposition and comment on related recent wor

Collective Modes in the Loop Current Ordered Phase of Cuprates

Recently two branches of weakly dispersive collective modes have been discovered in under-doped cuprates by inelastic neutron scattering. Polarization analysis reveals that the modes are magnetic excitations. They are only visible for temperatures below the transition temperature to a broken symmetry phase which was discovered earlier and their intensity increases as temperature is further decreased. The broken symmetry phase itself has symmetries consistent with ordering of orbital current loops within a unit-cell without breaking translational symmetry. In order to calculate the collective modes of such a state we add quantum terms to the Ashkin-Teller (AT) model with which the classical loop current order has been described. We derive that the mean field ground state of the quantum model is a product over all unit-cells of linear combination of the four possible classical configurations of the loop current order in each unit-cell. The collective modes are calculated by using a generalized Holstein-Primakoff boson representation of orbital moment operators and lead to three branches of gapped weakly dispersive collective modes. The experimental results are consistent with the two lower energy branches; the third mode is at a higher energy than looked for by present neutron scattering experiments and might also be over-damped. Implications of the discovery of the collective modes are discussed.Comment: 16 pages, 6 figure

Collective Modes in the Loop Ordered Phase of Cuprates

We show that the two branches of collective modes discovered recently in under-doped Cuprates with huge spectral weight are a necessary consequence of the loop-current state. Such a state has been shown in earlier experiments to be consistent with the symmetry of the order parameter competing with superconductivity in four families of Cuprates. We also predict a third branch of excitations and suggest techniques to discover it. Using parameters to fit the observed modes, we show that the direction of the effective moments in the ground state lies in a cone at an angle to the c-axis as observed in experiments

Staggered Flux Phase in a Model of Strongly Correlated Electrons

We present numerical evidence for the existence of a staggered flux (SF) phase in the half-filled two-leg t-U-V-J ladder, with true long-range order in the counter-circulating currents. The density-matrix renormalization-group (DMRG) / finite-size scaling approach, generalized to describe complex-valued Hamiltonians and wavefunctions, is employed. The SF phase exhibits robust currents at intermediate values of the interaction strength.Comment: Version to appear in Phys. Rev. Let

A Theory of the Pseudogap State of the Cuprates

The phase diagram for a general model for Cuprates is derived in a mean-field approximation. A phase violating time-reversal without breaking translational symmetry is possible when both the ionic interactions and the local repulsions are large compared to the energy difference between the Cu and O single-particle levels. It ends at a quantum critical point as the hole or electron doping is increased. Such a phase is necessarily accompanied by singular forward scattering such that, in the stable phase, the density of states at the chemical potential, projected to a particular point group symmetry of the lattice is zero producing thereby an anisotropic gap in the single-particle spectrum. It is suggested that this phase occupies the "pseudogap" region of the phase diagram of the cuprates. The temperature dependence of the single-particle spectra, the density of states, the specific heat and the magnetic susceptibility are calculated with rather remarkable correspondence with the experimental results. The importance of further direct experimental verification of such a phase in resolving the principal issues in the theory of the Cuprate phenomena is pointed out. To this end, some predictions are provided.Comment: 41 pages, 8 figure

Only Fermi-Liquids are Metals

Any singular deviation from Landau Fermi-liquid theory appears to lead, for arbitrarily small concentration of impurities coupling to a non-conserved quantity, to a vanishing density of states at the chemical potential and infinite resistivity as temperature approaches zero. Applications to copper-oxide metals including the temperature dependence of the anisotropy in resistivity, and to other cases of non Fermi-liquids are discussed.Comment: 11 pages,revtex, 1 Postscript figur

Light Scattering from Nonequilibrium Concentration Fluctuations in a Polymer solution

We have performed light-scattering measurements in dilute and semidilute polymer solutions of polystyrene in toluene when subjected to stationary temperature gradients. Five solutions with concentrations below and one solution with a concentration above the overlap concentration were investigated. The experiments confirm the presence of long-range nonequilibrium concentration fluctuations which are proportional to $(\nabla T)^2/k^4$, where $\nabla T$ is the applied temperature gradient and $k$ is the wave number of the fluctuations. In addition, we demonstrate that the strength of the nonequilibrium concentration fluctuations, observed in the dilute and semidilute solution regime, agrees with theoretical values calculated from fluctuating hydrodynamics. Further theoretical and experimental work will be needed to understand nonequilibrium fluctuations in polymer solutions at higher concentrations.Comment: revtex, 16 pages, 7 figures. J. Chem. Phys., to appea

Universality of the single-particle spectra of cuprate superconductors

All the available data for the dispersion and linewidth of the single-particle spectra above the superconducting gap and the pseudogap in metallic cuprates for any doping has universal features. The linewidth is linear in energy below a scale $\omega_c$ and constant above. The cusp in the linewidth at $\omega_c$ mandates, due to causality, a "waterfall", i.e., a vertical feature in the dispersion. These features are predicted by a recent microscopic theory. We find that all data can be quantitatively fitted by the theory with a coupling constant $\lambda_0$ and an upper cutoff at $\omega_c$ which vary by less than 50% among the different cuprates and for varying dopings. The microscopic theory also gives these values to within factors of O(2).Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let

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आँध्रप्रदेश की परुषकवचि (क्रस्टेशियाई) मात्स्यिकी संपदाए

Effective Lorentz Force due to Small-angle Impurity Scattering: Magnetotransport in High-Tc Superconductors

We show that a scattering rate which varies with angle around the Fermi surface has the same effect as a periodic Lorentz force on magnetotransport coefficients. This effect, together with the marginal Fermi liquid inelastic scattering rate gives a quantitative explanation of the temperature dependence and the magnitude of the observed Hall effect and magnetoresistance with just the measured zero-field resistivity as input.Comment: 4 pages, latex, one epsf figure included in text. Several revisions and corrections are included. Major conclusions are the sam