Effective quantum dimer model for trimerized kagome antiferromagnet

An effective spin-orbit Hamiltonian is derived for a spin-1/2 trimerized kagome antiferromagnet in the second-order of perturbation theory in the ratio of two coupling constants. Low-energy singlet states of the obtained model are mapped to a quantum dimer model on a triangular lattice. The quantum dimer model is dominated by dimer resonances on a few shortest loops of the triangular lattice. Characteristic energy scale for the dimer model constitutes only a small fraction of the weaker exchange coupling constant.Comment: 7 pages, 3 figure

Interband proximity effect and nodes of superconducting gap in Sr2RuO4

The power-law temperature dependences of the specific heat, the nuclear relaxation rate, and the thermal conductivity suggest the presence of line nodes in the superconducting gap of Sr2RuO4. These recent experimental observations contradict the scenario of a nodeless (k_x+ik_y)-type superconducting order parameter. We propose that interaction of superconducting order parameters on different sheets of the Fermi surface is a key to understanding the above discrepancy. A full gap exists in the active band, which drives the superconducting instability, while line nodes develop in passive bands by interband proximity effect.Comment: 4 pages, 1 figur

High field properties of geometrically frustrated magnets

Above the saturation field, geometrically frustrated quantum antiferromagnets have dispersionless low-energy branches of excitations corresponding to localized spin-flip modes. Transition into a partially magnetized state occurs via condensation of an infinite number of degrees of freedom. The ground state below the phase transition is a magnon crystal, which breaks only translational symmetry and preserves spin-rotations about the field direction. We give a detailed review of recent works on physics of such phase transitions and present further theoretical developments. Specifically, the low-energy degrees of freedom of a spin-1/2 kagom\'e antiferromagnet are mapped to a hard hexagon gas on a triangular lattice. Such a mapping allows to obtain a quantitative description of the magnetothermodynamics of a quantum kagom\'e antiferromagnet from the exact solution for a hard hexagon gas. In particular, we find the exact critical behavior at the transition into a magnon crystal state, the universal value of the entropy at the saturation field, and the position of peaks in temperature- and field-dependence of the specific heat. Analogous mapping is presented for the sawtooth chain, which is mapped onto a model of classical hard dimers on a chain. The finite macroscopic entropies of geometrically frustrated magnets at the saturation field lead to a large magnetocaloric effect.Comment: 22 pages, proceedings of YKIS2004 worksho

Magnon pairing in quantum spin nematic

Competing ferro- and antiferromagnetic exchange interactions may lead to the formation of bound magnon pairs in the high-field phase of a frustrated quantum magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior to the onset of a conventional one-magnon instability. We develop an analytical approach to study the zero-temperature properties of the magnon-pair condensate, which is a bosonic analog of the BCS superconductors. Representation of the condensate wave-function in terms of the coherent bosonic states reveals the spin-nematic symmetry of the ground-state and allows one to calculate various static properties. Sharp quasiparticle excitations are found in the nematic state with a small finite gap. We also predict the existence of a long-range ordered spin-nematic phase in the frustrated chain material LiCuVO4 at high fields.Comment: 5 pages, final versio

Monte Carlo study of first-order transition in Heisenberg fcc antiferromagnet

Nearest-neighbor Heisenberg antiferromagnet on a face-centered cubic lattice is studied by extensive Monte Carlo simulations in zero magnetic field. The parallel tempering algorithm is utilized, which allows to overcome a slow relaxation of the magnetic order parameter and fully equilibrate moderate size clusters with up to N ~ 7*10^3 spins. By collecting energy and order parameter histograms on clusters with up to N ~ 2*10^4 sites we accurately locate the first-order transition point at T_c=0.4459(1)J.Comment: 5 pages, 5 figure

Order and excitations in large-SS kagom\'e-lattice antiferromagnets

We systematically investigate the ground-state and the spectral properties of antiferromagnets on a kagom\'{e} lattice with several common types of the planar anisotropy: $XXZ$, single-ion, and out-of-plane Dzyaloshinskii-Moriya. Our main focus is on the role of nonlinear, anharmonic terms, which are responsible for the quantum order-by-disorder effect and for the corresponding selection of the ground-state spin structure in many of these models. The $XXZ$ and the single-ion anisotropy models exhibit a quantum phase transition between the ${\bf q}\!=\!0$ and the $\sqrt{3}\times\!\sqrt{3}$ states as a function of the anisotropy parameter, offering a rare example of the quantum order-by-disorder fluctuations favoring a ground state which is different from the one selected by thermal fluctuations. The nonlinear terms are also shown to be crucial for a very strong near-resonant decay phenomenon leading to the quasiparticle breakdown in the kagom\'{e}-lattice antiferromagnets whose spectra are featuring flat or weakly dispersive modes. The effect is shown to persist even in the limit of large spin values and should be common to other frustrated magnets with flat branches of excitations. Model calculations of the spectrum of the $S=5/2$ Fe-jarosite with Dzyaloshinskii-Moriya anisotropy provide a convincing and detailed characterization of the proposed scenario.Comment: 17 pages, 13 figures, published version. Recipient of the PRB beauty award (Editors' Suggestion

Quantum Selection of Order in an XXZXXZ Antiferromagnet on a Kagom\'e Lattice

Selection of the ground state of the kagom\'e-lattice $XXZ$ antiferromagnet by quantum fluctuations is investigated by combining non-linear spin-wave and real-space perturbation theories. The two methods unanimously favor ${\bf q}=0$ over $\sqrt{3}\times\sqrt{3}$ magnetic order in a wide range of the anisotropy parameter $0\leq \Delta\leq 0.72$. Both approaches are also in an accord on the magnitude of the quantum order-by-disorder effect generated by topologically non-trivial, loop-like spin-flip processes. A tentative $S-\Delta$ phase diagram of the model is proposed.Comment: 5 pages, 4 figures + 6.2 pages, 4 figures supplemental, minor changes, accepted versio