Jordan-Wigner Approach to Dynamic Correlations in 2D Spin-1/2 Models

We discuss the dynamic properties of the square-lattice spin-1/2 XY model obtained using the two-dimensional Jordan-Wigner fermionization approach. We argue the relevancy of the fermionic picture for interpreting the neutron scattering measurements in the two-dimensional frustrated quantum magnet Cs_2CuCl_4.Comment: Presented at 12-th Czech and Slovak Conference on Magnetism, Ko\v{s}ice, 12-15 July 200

Quantum phase transition in the Plaquette lattice with anisotropic spin exchange

I study the influence of anisotropic spin exchange on a quantum phase transition in the Plaquette lattice driven by the purely quantum effect of singlet formation. I study the influence of i) a Dzyaloshinskii-Moriya exchange and ii) four spin exchange on the transition point by evaluating spin--spin correlations and the spin gap with exact diagonalization. The results point to a stabilization of the Neel-like long range order when the Dzyaloshinskii-Moriya exchange is added, whereas the four-spin exchange might stabilize the singlet order as well as the Neel-like order depending on its strength.Comment: LaTeX article with 4 pages and 3 figures, prepared with material for the ICM 200

The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field

Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF) Cs2CuCl4 have been measured as a function of magnetic field using neutron scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D incommensurate ordering. Fields greater than Bc =1.66 T, but less than the field (~8 T) required to fully align the spins, are observed to decouple the chains, and the system enters a disordered intermediate-field phase (IFP). The IFP excitations are in agreement with the predictions of Muller et al. for the 1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published in PRL, e-mail comments to [email protected]

Theoretical Analysis of the Reduction of Neel Temperature in La2_{2}(Cu1−x_{1-x}Zn(or Mg)x)_x)O4_{4}

Using Tyablikov's decoupling approximation, we calculate the initial suppression rate of the Neel temperature, $R_{IS}=-lim_{x-> 0} T^{-1}_{N} dT_{N}/dx$, in a quasi two-dimensional diluted Heisenberg antiferromagnet with nonmagnetic impurities of concentration $x$. In order to explain an experimental fact that $R^{(Zn)}_{IS}=3.4$ of the Zn-substitution is different from $R^{(Mg)}_{IS}=3.0$ of the Mg-substitution, we propose a model in which impurity substitution reduces the intra-plane exchange couplings surrounding impurities, as well as dilution of spin systems. The decrease of 12% in exchange coupling constants by Zn substitution and decrease of 6% by Mg substitution explain those two experimental results, when an appropriate value of the interplane coupling is used.Comment: 2 pages, 3 figure

Finite-temperature perturbation theory for quasi-one-dimensional spin-1/2 Heisenberg antiferromagnets

We develop a finite-temperature perturbation theory for quasi-one-dimensional quantum spin systems, in the manner suggested by H.J. Schulz (1996) and use this formalism to study their dynamical response. The corrections to the random-phase approximation formula for the dynamical magnetic susceptibility obtained with this method involve multi-point correlation functions of the one-dimensional theory on which the random-phase approximation expansion is built. This ``anisotropic'' perturbation theory takes the form of a systematic high-temperature expansion. This formalism is first applied to the estimation of the N\'eel temperature of S=1/2 cubic lattice Heisenberg antiferromagnets. It is then applied to the compound Cs$_2$CuCl$_4$, a frustrated S=1/2 antiferromagnet with a Dzyaloshinskii-Moriya anisotropy. Using the next leading order to the random-phase approximation, we determine the improved values for the critical temperature and incommensurability. Despite the non-universal character of these quantities, the calculated values are different by less than a few percent from the experimental values for both compounds.Comment: 11 pages, 6 figure

Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions: β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ (M = Ga, Cr, Fr; Υ = C₅H₅N)

We report high-field magnetotransport measurements on β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ, where M =Ga, Cr and Fe and Υ = C₅H₅N. We observe similar Shubnikov–de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area ≈ 8.5% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are ∼mₑ, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of β" — (BEDT — TTF)₄ [(H₃O)M(C₂O₄)₃]Υ salts

Suppression of orbital ordering by chemical pressure in FeSe1-xSx

We report a high-resolution angle-resolved photo-emission spectroscopy study of the evolution of the electronic structure of FeSe1-xSx single crystals. Isovalent S substitution onto the Se site constitutes a chemical pressure which subtly modifies the electronic structure of FeSe at high temperatures and induces a suppression of the tetragonal-symmetry-breaking structural transition temperature from 87K to 58K for x=0.15. With increasing S substitution, we find smaller splitting between bands with dyz and dxz orbital character and weaker anisotropic distortions of the low temperature Fermi surfaces. These effects evolve systematically as a function of both S substitution and temperature, providing strong evidence that an orbital ordering is the underlying order parameter of the structural transition in FeSe1-xSx. Finally, we detect the small inner hole pocket for x=0.12, which is pushed below the Fermi level in the orbitally-ordered low temperature Fermi surface of FeSe.Comment: Latex, 5 pages, 4 figure

Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations

Na$_2$IrO$_3$, a honeycomb 5$d^5$ oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal-metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na$_2$IrO$_3$, the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin-orbit interactions, this strong dependence on distortions of the Ir$_2$O$_2$ plaquettes singles out the honeycomb 5$d^5$ oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ