247 research outputs found
Field-induced gap in ordered Heisenberg antiferromagnets
Heisenberg antiferromagnets in a strong uniform magnetic field are
expected to exhibit a gapless phase with a global O(2) symmetry. In many real
magnets, a small energy gap is induced by additional interactions that can be
viewed as a staggered transverse magnetic field , where is a small
proportionality constant. We study the effects of such a perturbation,
particularly for magnets with long-range order, by using several complimentary
approaches: numerical diagonalizations of a model with long-range interactions,
classical equations of motion, and scaling arguments. In an ordered state at
zero temperature, the energy gap at first grows as and then may
dip to a smaller value, of order , at the quantum critical point
separating the ``gapless'' phase from the gapped state with saturated
magnetization. In one spatial dimension, the latter exponent changes to 4/5.Comment: 6 pages, 5 figure
Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2
Motivated by the recent discovery of the spin tube
[(CuCltachH)Cl]Cl, we investigate the properties of a frustrated
three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings.
We pay special attention to the evolution of the properties from weak to strong
inter-ring coupling and show on the basis of extensive density matrix
renormalization group and exact diagonalization calculations that the system
undergoes a first-order phase transition between a dimerized gapped phase at
weak coupling that can be described by the usual spin-chirality model and a
gapless critical phase at strong coupling that can be described by an effective
spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in
the gapped phase and slightly beyond. The implications for
[(CuCltachH)Cl]Cl are discussed, with the conclusion that this
system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio
Field Induced Staggered Magnetization and Magnetic Ordering in
We present a D NMR investigation of the gapped spin-1/2 compound . Our measurements reveal the presence of a magnetic
field induced transverse staggered magnetization (TSM) which persists well
below and above the field-induced 3D long-range magnetically ordered (FIMO)
phase. The symmetry of this TSM is different from that of the TSM induced by
the order parameter of the FIMO phase. Its origin, field dependence and
symmetry can be explained by an intra-dimer Dzyaloshinskii-Moriya interaction,
as shown by DMRG calculations on a spin-1/2 ladder. This leads us to predict
that the transition into the FIMO phase is not in the BEC universality class.Comment: 4 page
Uniform and staggered magnetizations induced by Dzyaloshinskii-Moriya interactions in isolated and coupled spin 1/2 dimers in a magnetic field
We investigate the interplay of Dzyaloshinskii-Moriya interactions and an
external field in spin 1/2 dimers. For isolated dimers and at low field, we
derive simple expressions for the staggered and uniform magnetizations which
show that the orientation of the uniform magnetization can deviate
significantly from that of the external field. In fact, in the limit where the
vector of the Dzyaloshinskii-Moriya interaction is parallel to the
external field, the uniform magnetization actually becomes {\it perpendicular}
to the field. For larger fields, we show that the staggered magnetization of an
isolated dimer has a maximum close to one-half the polarization, with a large
maximal value of in the limit of very small Dzyaloshinskii-Moriya
interaction. We investigate the effect of inter-dimer coupling in the context
of ladders with Density Matrix Renormalization Group (DMRG) calculations and
show that, as long as the values of the Dzyaloshinskii-Moriya and of the
exchange interaction are compatible with respect to the development of a
staggered magnetization, the simple picture that emerges for isolated dimers is
also valid for weakly coupled dimers with minor modifications. The results are
compared with torque measurements on
Cu(CHN)Cl.Comment: 8 pages, 9 figure
Quantum spin liquids: a large-S route
This paper explores the large-S route to quantum disorder in the Heisenberg
antiferromagnet on the pyrochlore lattice and its homologues in lower
dimensions. It is shown that zero-point fluctuations of spins shape up a
valence-bond solid at low temperatures for one two-dimensional lattice and a
liquid with very short-range valence-bond correlations for another. A
one-dimensional model demonstrates potential significance of quantum
interference effects (as in Haldane's gap): the quantum melting of a
valence-bond order yields different valence-bond liquids for integer and
half-integer values of S.Comment: Proceedings of Highly Frustrated Magnetism 2003 (Grenoble), 6 LaTeX
page
Condensation of magnons and spinons in a frustrated ladder
Motivated by the ever-increasing experimental effort devoted to the
properties of frustrated quantum magnets in a magnetic field, we present a
careful and detailed theoretical analysis of a one-dimensional version of this
problem, a frustrated ladder with a magnetization plateau at m=1/2. We show
that even for purely isotropic Heisenberg interactions, the magnetization curve
exhibits a rather complex behavior that can be fully accounted for in terms of
simple elementary excitations. The introduction of anisotropic interactions
(e.g., Dzyaloshinskii-Moriya interactions) modifies significantly the picture
and reveals an essential difference between integer and fractional plateaux. In
particular, anisotropic interactions generically open a gap in the region
between the plateaux, but we show that this gap closes upon entering fractional
plateaux. All of these conclusions, based on analytical arguments, are
supported by extensive Density Matrix Renormalization Group calculations.Comment: 15 pages, 15 figures. minor changes in tex
Structural Transition of Li2RuO3 Induced by Molecular-Orbit Formation
A pseudo honeycomb system Li2RuO3 exhibits a second-order-like transition at
temperature T=Tc=540 K to a low-T nonmagnetic phase with a significant lattice
distortion forming Ru-Ru pairs. For this system, we have calculated the band
structure, using the generalized gradient approximation (GGA) in both the high-
and low- T phases, and found that the results of the calculation can naturally
explain the insulating behavior observed in the low-T phase. The detailed
characters of the Ru 4d t2g bands obtained by the tight-binding fit to the
calculated dispersion curves show clear evidence that the structural transition
is driven by the formation of the Ru-Ru molecular-orbits, as proposed in our
previous experimental studies.Comment: 5 pages, 5 figures, 4 tables, submitted to J. Phys. Soc. Jp
Spin-1/2 frustrated antiferromagnet on a spatially anisotopic square lattice: contribution of exact diagonalizations
The phase diagram of a spin-1/2 model is investigated by means of
exact diagonalizations on finite samples. This model is a generalization of the
model on the square lattice with two different nearest-neighbor
couplings and may be also viewed as an array of coupled Heisenberg
chains. The results suggest that the resonnating valence bond state predicted
by Nersesyan and Tsvelik [Phys. Rev. B {\bf 67}, 024422 (2003)] for is realized and extends beyond the limit of small interchain coupling
along a curve nearly coincident with the line where the energy per spin is
maximum. This line is likely bordered on both side by a columnar dimer long
range order. This columnar order could extends for which correspond
to the model.Comment: 14 pages, 21 figures, final versio
Motion of Bound Domain Walls in a Spin Ladder
The elementary excitation spectrum of the spin-
antiferromagnetic (AFM) Heisenberg chain is described in terms of a pair of
freely propagating spinons. In the case of the Ising-like Heisenberg
Hamiltonian spinons can be interpreted as domain walls (DWs) separating
degenerate ground states. In dimension , the issue of spinons as
elementary excitations is still unsettled. In this paper, we study two
spin- AFM ladder models in which the individual chains are
described by the Ising-like Heisenberg Hamiltonian. The rung exchange
interactions are assumed to be pure Ising-type in one case and Ising-like
Heisenberg in the other. Using the low-energy effective Hamiltonian approach in
a perturbative formulation, we show that the spinons are coupled in bound
pairs. In the first model, the bound pairs are delocalized due to a four-spin
ring exchange term in the effective Hamiltonian. The appropriate dynamic
structure factor is calculated and the associated lineshape is found to be
almost symmetric in contrast to the 1d case. In the case of the second model,
the bound pair of spinons lowers its kinetic energy by propagating between
chains. The results obtained are consistent with recent theoretical studies and
experimental observations on ladder-like materials.Comment: 12 pages, 7 figure
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