Frustrated magnets in three dimensions: a nonperturbative approach

Frustrated magnets exhibit unusual critical behaviors: they display scaling laws accompanied by nonuniversal critical exponents. This suggests that these systems generically undergo very weak first order phase transitions. Moreover, the different perturbative approaches used to investigate them are in conflict and fail to correctly reproduce their behavior. Using a nonperturbative approach we explain the mismatch between the different perturbative approaches and account for the nonuniversal scaling observed.Comment: 7 pages, 1 figure. IOP style files included. To appear in Journal of Physics: Condensed Matter. Proceedings of the conference HFM 2003, Grenoble, Franc

The Coupled Electronic-Ionic Monte Carlo Simulation Method

Quantum Monte Carlo (QMC) methods such as Variational Monte Carlo, Diffusion Monte Carlo or Path Integral Monte Carlo are the most accurate and general methods for computing total electronic energies. We will review methods we have developed to perform QMC for the electrons coupled to a classical Monte Carlo simulation of the ions. In this method, one estimates the Born-Oppenheimer energy E(Z) where Z represents the ionic degrees of freedom. That estimate of the energy is used in a Metropolis simulation of the ionic degrees of freedom. Important aspects of this method are how to deal with the noise, which QMC method and which trial function to use, how to deal with generalized boundary conditions on the wave function so as to reduce the finite size effects. We discuss some advantages of the CEIMC method concerning how the quantum effects of the ionic degrees of freedom can be included and how the boundary conditions can be integrated over. Using these methods, we have performed simulations of liquid H2 and metallic H on a parallel computer.Comment: 27 pages, 10 figure

Theory of Impurity Effects on the Spin Nematic State

The effect of magnetic bond disorder in otherwise antiferro nematic ordered system is investigated. We introduced triangular-shaped ferromagnetic bond disorder in the S=1 bilinear-biquadratic model on a triangular lattice. It is shown that the coupling between the impurity magnetic moment and nonmagnetic excitation in the bulk yields single-moment anisotropy and long-range anisotropic interaction between impurity magnetic moments. This interaction can induce unconventional spin-freezing phenomena observed in triangular magnet, NiGa2S4.Comment: 19 pages, 14 figure

Flat Energy-Histogram Simulation of the Phase Transition in an Ising Fully Frustrated Lattice

We show in this paper the results on the phase transition of the so-called fully frustrated simple cubic lattice with the Ising spin model. We use here the Monte Carlo method with the flat energy-histogram Wang-Landau technique which is very powerful to detect weak first-order phase transition. We show that the phase transition is clearly of first order, providing a definite answer to a question raised 25 years ago.Comment: Submitted for publicatio

Spin-1/2 Ising-Heisenberg model with the pair XYZ Heisenberg interaction and quartic Ising interactions as the exactly soluble zero-field eight-vertex model

The spin-1/2 Ising-Heisenberg model with the pair XYZ Heisenberg interaction and quartic Ising interactions is exactly solved by establishing a precise mapping relationship with the corresponding zero-field (symmetric) eight-vertex model. It is shown that the Ising-Heisenberg model with the ferromagnetic Heisenberg interaction exhibits a striking critical behavior, which manifests itself through re-entrant phase transitions as well as continuously varying critical exponents. The changes of critical exponents are in accordance with the weak universality hypothesis in spite of a peculiar singular behavior to emerge at a quantum critical point of the infinite order, which occurs at the isotropic limit of the Heisenberg interaction. On the other hand, the Ising-Heisenberg model with the antiferromagnetic Heisenberg interaction surprisingly exhibits less significant changes of both critical temperatures as well as critical exponents upon varying a strength of the exchange anisotropy in the Heisenberg interaction.Comment: 11 pages, 9 figure

Exact evidence for the spontaneous antiferromagnetic long-range order in the two-dimensional hybrid model of localized Ising spins and itinerant electrons

The generalized decoration-iteration transformation is adopted to treat exactly a hybrid model of doubly decorated two-dimensional lattices, which have localized Ising spins at their nodal lattice sites and itinerant electrons delocalized over pairs of decorating sites. Under the assumption of a half filling of each couple of the decorating sites, the investigated model system exhibits a remarkable spontaneous antiferromagnetic long-range order with an obvious quantum reduction of the staggered magnetization. It is shown that the critical temperature of the spontaneously long-range ordered quantum antiferromagnet displays an outstanding non-monotonic dependence on a ratio between the kinetic term and the Ising-type exchange interaction.Comment: 8 pages, 6 figure

Critical behavior of frustrated systems: Monte Carlo simulations versus Renormalization Group

We study the critical behavior of frustrated systems by means of Pade-Borel resummed three-loop renormalization-group expansions and numerical Monte Carlo simulations. Amazingly, for six-component spins where the transition is second order, both approaches disagree. This unusual situation is analyzed both from the point of view of the convergence of the resummed series and from the possible relevance of non perturbative effects.Comment: RevTex, 10 pages, 3 Postscript figure

^{17}O and ^{51}V NMR for the zigzag spin-1 chain compound CaV2O4

$^{51}$V NMR studies on CaV2O4 single crystals and $^{17}$O NMR studies on $^{17}$O-enriched powder samples are reported. The temperature dependences of the $^{17}$O NMR line width and nuclear spin-lattice relaxation rate give strong evidence for a long-range antiferromagnetic transition at Tn = 78 K in the powder. Magnetic susceptibility measurements show that Tn = 69 K in the crystals. A zero-field $^{51}$V NMR signal was observed at low temperatures (f $\approx$ 237 MHz at 4.2 K) in the crystals. The field swept spectra with the field in different directions suggest the presence of two antiferromagnetic substructures. Each substructure is collinear, with the easy axes of the two substructures separated by an angle of 19(1) degree, and with their average direction pointing approximately along the b-axis of the crystal structure. The two spin substructures contain equal number of spins. The temperature dependence of the ordered moment, measured up to 45 K, shows the presence of an energy gap Eg in the antiferromagnetic spin wave excitation spectrum. Antiferromagnetic spin wave theory suggests that Eg lies between 64 and 98 K.Comment: 11 pages, 14 figures. v2: 2 new figures; version published in Phys. Rev.

Contributions of rotation, expansion and line broadening to the morpho-kinematics of the inner CSE of oxygen-rich AGB star R Hya

We use archival ALMA observations of the CO(2-1) and SiO(5-4) molecular line emissions of AGB star R Hya to illustrate the relative contributions of rotation, expansion and line broadening to the morpho-kinematics of the circumstellar envelope (CSE) within some ~0.5 arcsec from the centre of the star. We give evidence for rotation and important line broadening to dominate the inner region, within ~100 mas from the centre of the star. The former is about an axis that projects a few degrees west of north and has a projected rotation velocity of a few km/s. The latter occurs within some 50-100 mas from the centre of the star, the line width reaching two to three times its value outside this region. We suggest that it is caused by shocks induced by stellar pulsations and convective cell ejections. We show the importance of properly accounting for the observed line broadening when discussing rotation and evaluating the radial dependence of the rotation velocity.Comment: 8 pages, 8 figure