Resonating valence bond wave function for the two-dimensional fractional spin liquid

The unconventional low-lying spin excitations, recently observed in neutron scattering experiments on Cs2CuCl4, are explained with a spin liquid wave function. The dispersion relation as well as the wave vector of the incommensurate spin correlations are well reproduced within a projected BCS wave function with gapless and fractionalized spin-1/2 excitations around the nodes of the BCS gap function. The proposed wave function is shown to be very accurate for one-dimensional spin-1/2 systems and remains similarly accurate in the two-dimensional model corresponding to Cs2CuCl4, thus representing a good ansatz for describing spin fractionalization in two dimensions.     

Generalized percolation models for frustrated spin systems

Summary A brief review on how to study frustrated spin models by mapping them into generalized percolation models is given. The percolation models associated to a number of deterministic frustrated models are discussed with particular attention on the properties of critical clusters. Paper presented at the I Internatioanl Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Resonating valence bond phase in the triangular lattice quantum dimer model

We study the quantum dimer model on the triangular lattice, which is expected to describe the singlet dynamics of frustrated Heisenberg models in phases where valence bond configurations dominate their physics. We find, in contrast to the square lattice, that there is a truly short ranged resonating valence bond phase with no gapless excitations and with deconfined, gapped, spinons for a finite range of parameters. We also establish the presence of crystalline dimer phases.     

Resonating valence bond mechanism of the H2 dissociation on Pd surface

Resonating valence bond theory combined with DFT calculations permit to build a simple model for the dissociation of H₂ on palladium surface. Based on analysis of the electron transfer and total energy for different geometries of the Pd₂–H₂ and Pd₅–H₂ systems it is found that the predissociative state corresponds to a tilted molecule adsorbed on the surface.     

Resonating valence bond states in doped square-lattice antiferromagnets: finite cluster calculations

We discuss some general features of the resonating valence bond (RVB) ansatz for the ground state of quantums=1/2 Heisenberg antiferromagnets (AFMs). For finite clusters of up to 16 spins on the square lattice we compare the exact ground state with a short-range correlated RVB trial wave function. Since in the pure square-lattice AFM the short-range correlated RVB state differs significantly from the real ground state we discuss different mechanisms that favour the RVB state. In particular, we study the influence of anisotropy, disorder and frustration, which could be relevant for slightly doped high-T _c superconducting materials. Furthermore, we discuss the influence of holes on the realization of a RVB state. We found that exchange disorder and, in particular, frustration and holes can favour a short-range correlated magnetic state, which is well described by a RVB state.     

The signs of the valence bond wave functions in an alternant hydrocarbon

The infrared spectrum of ¹²CD₃F from 1040 to 1200 cm^-1 has been analysed using a new program which diagonalizes the complete interaction hamiltonian matrix for the v ₂ and v ₅ ro-vibrational levels. The program fits simultaneously the microwave, laser Stark and diode spectra and over 800 transitions in the Nicolet FTIR spectrum. A comparison of the calculated and observed infrared spectra shows the phases of the dipole moment derivatives to be of opposite sign. Several new Stark spectra have been assigned.     

Correlated spin networks in frustrated systems

We introduce a network model for frustrated spin systems based on highly correlated spin fluctuations, to quantify and visualize their ordering. This model shows that networks of strongly correlated but non-contiguous spins exist at low temperatures on a triangular Ising lattice with competing nearest-neighbor interactions. This finding is consistent with chaotic renormalization-group trajectories previously reported for frustrated hierarchical lattices.     

Magnetic-field induced spin-Peierls instability in strongly frustrated quantum spin lattices

For a class of frustrated antiferromagnetic spin lattices (in particular, the square-kagomé and kagomé lattices) we discuss the impact of recently discovered exact eigenstates on the stability of the lattice against distortions. These eigenstates consist of independent localized magnons embedded in a ferromagnetic environment and become ground states in high magnetic fields. For appropriate lattice distortions fitting to the structure of the localized magnons the lowering of magnetic energy can be calculated exactly and is proportional to the displacement of atoms leading to a spin-Peierls lattice instability. Since these localized states are present only for high magnetic fields, this instability might be driven by magnetic-field. The hysteresis of the spin-Peierls transition is also discussed.     

Magnetic properties of strongly frustrated and correlated systems

While consequences of frustration of magnetic interactions are much studied in localized spin systems, much less studies have been performed on frustrated metallic systems. However, several effects of strong geometrical frustration in metallic correlated system have also been experimentally observed in rare-earth or transition metal compounds: coexistence of magnetic and non-magnetic sites in ordered magnetic structure, heavy fermion behaviour and anomalous Hall effect due to spin chirality are consequences of frustration. An overview of the experimental observations and of the proposed models is given. Other interesting effects due to magnetic frustration in metallic systems, which have been predicted theoretically, are also reviewed.     

Empirical magnetic structure solution of frustrated spin systems

Frustrated magnetism plays a central role in the phenomenology of exotic quantum states. However, since the magnetic structures of frustrated systems are often aperiodic, there has been the problem that they cannot be determined by using traditional crystallographic techniques. Here we show that the magnetic component of powder neutron scattering data is actually sufficiently information-rich to drive magnetic structure solution for frustrated systems, including spin ices, spin liquids, and molecular magnets. Our methodology employs ab?initio reverse Monte?Carlo refinement, making informed use of an additional constraint that minimizes variance in local spin environments. The atomistic spin configurations obtained in this way not only reflect a magnetic structure "solution" but also reproduce the full three-dimensional magnetic scattering pattern.     

Spin dynamics of geometrically frustrated spin systems observed by muon spin relaxation

μSR is shown to be a sensitive probe of fluctuating internal magnetic fields in geometrically frustrated magnets. The usefulness of μSR in these systems is illustrated in the case of pyrochlores, in which the antiferromagnetically coupled ions occupy a lattice of corner sharing tetrahedra. Remarkably, one observes a type of spin freezing in Y₂Mo₂O₇ and Tb₂Mo₂O₇ which is similar to that seen in conventional spin glasses, even though there is no detectable structural disorder. Unlike ordinary spin glasses these geometrically frustrated antiferromagnets display unusual low temperature spin dynamics which persist down to the lowest accessible temperatures.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

原子的价壳层激发态波函数和跃迁特性

本文探讨了快电子碰撞下判别原子跃迁多极性的方法,指出了从实验上由广义振子强度比和光学振子强度比及中间耦合系数的归一化特性来确定中间耦合系数的方法.然后以氩原子为例,计算了其价壳层激发的中间耦合系数,并在2500 eV入射能量下实验测量了氩原子跃迁到3p54s[3/2]1和3p54s′[1/2]1的广义振子强度比,在此基础上阐明了氩原子价壳层激发的多极性,从实验上获得了氩原子3p54s[3/2]1和3p54s′[1/2]1的中间耦合系数.本文得出了LS耦合的波函数足以描述氩原子的光子或快带电粒子碰撞激发过程的结论,这是由于氩原子基态的LS耦合特性在快电子激发过程中选择了激发态波函数中的单态成份决定的.并指明该结论可以推广到描述其它原子的光子或快带电粒子的碰撞激发过程.     

自旋粒子的相对论波函数

从相对论波动方程和Lorentz变换理论出发，讨论了自旋粒子的相对论波函数，并给出了求相对论粒子高自旋态的方法。     

First and second order transition of frustrated Heisenberg spin systems

Starting from the hypothesis of a second order transition we have studied modifications of the original Heisenberg antiferromagnet on a stacked triangular lattice (STA-model) by the Monte Carlo technique. The change is a local constraint restricting the spins at the corners of selected triangles to add up to zero without stopping them from moving freely (STAR-model). We have studied also the closely related dihedral and trihedral models which can be classified as Stiefel models. We have found indications of a first order transition for all three modified models instead of a universal critical behavior. This is in accordance with the renormalization group investigations but disagrees with the Monte Carlo simulations of the original STA-model favoring a new universality class. For the corresponding x-y antiferromagnet studied before, the second order nature of the transition could also not be confirmed. Received 17 May 1999 and Received in final form 30 July 1999     

Projected spin functions for many electron systems

The krypton potentials of Barker et al., Buck et al., Gough et al. (Maitland and Smith (n-6)) and Thakkar and Smith (MIST 2C and 6B) are compared as to their ability to predict dilute gas properties (second virial, viscosity, self-diffusion and thermal conductivity coefficients and thermal diffusion factors). The potentials of Barker et al. and Buck et al. and Gough et al. (MS) accurately predict these properties almost equally. The MIST potentials do not fare quite so well. The isotopic thermal diffusion factor, the dilute gas property most sensitive to the shape of the potential, is best described by the Buck et al. and MS potentials. An analysis of this property suggests the correctness of the slope of the repulsive wall of these potentials.     

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr2O4

The spin-lattice coupling plays an important role in strongly frustrated magnets. In ZnCr2O4, an excellent realization of the Heisenberg antiferromagnet on the pyrochlore network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at T(c)=12.5 K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr2O4 tracks the nearest-neighbor spin correlations above T(c). The splitting of the phonon triplet below T(c) provides a way to measure the spin-Peierls order parameter.