432 research outputs found

### 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

### 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

### 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

### Order from structural disorder in $XY$ pyrochlore antiferromagnet $\rm Er_2Ti_2O_7$

Effect of structural disorder is investigated for an $XY$ pyrochlore
antiferromagnet with continuous degeneracy of classical ground states. Two
types of disorder, vacancies and weakly fluctuating exchange bonds, lift
degeneracy selecting the same subset of classical ground states. Analytic and
numerical results demonstrate that such an "order by structural disorder"
mechanism competes with the effect of thermal and quantum fluctuations. Our
theory predicts that a small amount of nonmagnetic impurities in
$\rm{Er_2Ti_2O_7}$ will stabilize the coplanar $\psi_3$ ($m_{x^2-y^2}$)
magnetic structure as opposed to the $\psi_2$ ($m_{3z^2-r^2}$) state found in
pure material

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