Ground state properties of two spin models with exactly known ground states on the square lattice

We introduce a new two-dimensional model with diagonal four spin exchange and an exactly knownground-state. Using variational ansaetze and exact diagonalisation we calculate upper and lower bounds for the critical coupling of the model. Both for this model and for the Shastry-Sutherland model we study periodic systems up to system size 6x6.Comment: to appear in IJMPC 17, 12 pages, 7 figure

Finite size effects and magnetic order in the spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3}

High field electron spin resonance, nuclear magnetic resonance and magnetization studies addressing the ground state of the quasi two-dimensional spin-1/2 honeycomb lattice compound InCu{2/3}V{1/3}O{3} are reported. Uncorrelated finite size structural domains occurring in the honeycomb planes are expected to inhibit long range magnetic order. Surprisingly, ESR data reveal the development of two collinear antiferromagnetic (AFM) sublattices below ~ 20 K whereas NMR results show the presence of the staggered internal field. Magnetization data evidence a spin reorientation transition at ~ 5.7 T. Quantum Monte-Carlo calculations show that switching on the coupling between the honeycomb spin planes in a finite size cluster yields a Neel-like AFM spin structure with a substantial staggered magnetization at finite temperatures. This may explain the occurrence of a robust AFM state in InCu{2/3}V{1/3}O{3} despite an unfavorable effect of structural disorder.Comment: revised version, accepted as a Rapid Communication in Phys. Rev. B (2010

Quantum Phase Transition, O(3) Universality Class and Phase Diagram of Spin-1/2 Heisenberg Antiferromagnet on Distorted Honeycomb Lattice: A Tensor Renormalization Group Study

The spin-1/2 Heisenberg antiferromagnet on the distorted honeycomb (DHC) lattice is studied by means of the tensor renormalization group method. It is unveiled that the system has a quantum phase transition of second-order between the gapped quantum dimer phase and a collinear Neel phase at the critical point of coupling ratio \alpha_{c} = 0.54, where the quantum critical exponents \nu = 0.69(2) and \gamma = 1.363(8) are obtained. The quantum criticality is found to fall into the O(3) universality class. A ground-state phase diagram in the field-coupling ratio plane is proposed, where the phases such as the dimer, semi-classical Neel, and polarized phases are identified. A link between the present spin system to the boson Hubbard model on the DHC lattice is also discussed.Comment: 6 pages, 5 figures, published in Phys. Rev.

Magnetic Properties of the low dimensional spin system (VO)2_2P2_2O7_{7}: ESR and susceptibility

Experimental results on magnetic resonance (ESR) and magnetic susceptibility are given for single crystalline (VO)$_2$P$_2$O$_{7}$. The crystal growth procedure is briefly discussed. The susceptibility is interpreted numerically using a model with alternating spin chains. We determine $J$=51 K and $\delta$=0.2. Furthermore we find a spin gap of $\approx 6$meV from our ESR measurements. Using elastic constants no indication of a phase transition forcing the dimerization is seen below 300 K.Comment: 7 pages, REVTEX, 7 figure

beta-Cu2V2O7: a spin-1/2 honeycomb lattice system

We report on band structure calculations and a microscopic model of the low-dimensional magnet beta-Cu2V2O7. Magnetic properties of this compound can be described by a spin-1/2 anisotropic honeycomb lattice model with the averaged coupling \bar J1=60-66 K. The low symmetry of the crystal structure leads to two inequivalent couplings J1 and J1', but this weak spatial anisotropy does not affect the essential physics of the honeycomb spin lattice. The structural realization of the honeycomb lattice is highly non-trivial: the leading interactions J1 and J1' run via double bridges of VO4 tetrahedra between spatially separated Cu atoms, while the interactions between structural nearest neighbors are negligible. The non-negligible inter-plane coupling Jperp~15 K gives rise to the long-range magnetic ordering at TN~26 K. Our model simulations improve the fit of the magnetic susceptibility data, compared to the previously assumed spin-chain models. Additionally, the simulated ordering temperature of 27 K is in remarkable agreement with the experiment. Our study evaluates beta-Cu2V2O7 as the best available experimental realization of the spin-1/2 Heisenberg model on the honeycomb lattice. We also provide an instructive comparison of different band structure codes and computational approaches to the evaluation of exchange couplings in magnetic insulators.Comment: 11 pages, 10 figures, 2 tables: revised version, extended description of simulation result

Acoustic Faraday effect in Tb3_3Ga5_5O12_{12}

The transverse acoustic wave propagating along the [100] axis of the cubic Tb$_3$Ga$_5$O$_{12}$ (acoustic $c_{44}$ mode) is doubly degenerate. A magnetic field applied in the direction of propagation lifts this degeneracy and leads to the rotation of the polarization vector - the magneto-acoustic Faraday rotation. Here, we report on the observation and analysis of the magneto-acoustic Faraday-effect in Tb$_3$Ga$_5$O$_{12}$ in static and pulsed magnetic fields. We present also a theoretical model based on magnetoelastic coupling of 4$f$ electrons to both, acoustic and optical phonons and an effective coupling between them. This model explains the observed linear frequency dependence of the Faraday rotation angle