4,704 research outputs found
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
Spin-S Kagome quantum antiferromagnets in a field with tensor networks
Spin- Heisenberg quantum antiferromagnets on the Kagome lattice offer,
when placed in a magnetic field, a fantastic playground to observe exotic
phases of matter with (magnetic analogs of) superfluid, charge, bond or nematic
orders, or a coexistence of several of the latter. In this context, we have
obtained the (zero temperature) phase diagrams up to directly in the
thermodynamic limit thanks to infinite Projected Entangled Pair States (iPEPS),
a tensor network numerical tool. We find incompressible phases characterized by
a magnetization plateau vs field and stabilized by spontaneous breaking of
point group or lattice translation symmetry(ies). The nature of such phases may
be semi-classical, as the plateaus at th, th and
th of the saturated magnetization (the latter followed by a
macroscopic magnetization jump), or fully quantum as the spin-
-plateau exhibiting coexistence of charge and bond orders. Upon
restoration of the spin rotation symmetry a finite compressibility
appears, although lattice symmetry breaking persists. For integer spin values
we also identify spin gapped phases at low enough field, such as the
(topologically trivial) spin liquid with no symmetry breaking, neither spin nor
lattice.Comment: 5 pages, 3 figures, 1 table + supplemental materia
From the triangular to the kagome lattice: Following the footprints of the ordered state
We study the spin-1/2 Heisenberg model in a lattice that interpolates between
the triangular and the kagome lattices. The exchange interaction along the
bonds of the kagome lattice is J, and the one along the bonds connecting kagome
and non-kagome sites is J', so that J'=J corresponds to the triangular limit
and J'=0 to the kagome one. We use variational and exact diagonalization
techniques. We analyze the behavior of the order parameter for the
antiferromagnetic phase of the triangular lattice, the spin gap, and the
structure of the spin excitations as functions of J'/J. Our results indicate
that the antiferromagnetic order is not affected by the reduction of J' down to
J'/J ~ 0.2. Below this value, antiferromagnetic correlations grow weaker, a
description of the ground state in terms of a Neel phase renormalized by
quantum fluctuations becomes inadequate, and the finite-size spectra develop
features that are not compatible with antiferromagnetic ordering. However, this
phase does not appear to be connected to the kagome phase as well, as the
low-energy spectra do not evolve with continuity for J'-> 0 to the kagome
limit. In particular, for any non-zero value of J', the latter interaction sets
the energy scale for the low-lying spin excitations, and a gapless triplet
spectrum, destabilizing the kagome phase, is expected.Comment: 9 pages, 10 Figures. To be published in PR
Quantum Effects and Broken Symmetries in Frustrated Antiferromagnets
We investigate the interplay between frustration and zero-point quantum
fluctuations in the ground state of the triangular and Heisenberg
antiferromagnets, using finite-size spin-wave theory, exact diagonalization,
and quantum Monte Carlo methods. In the triangular Heisenberg antiferromagnet,
by performing a systematic size-scaling analysis, we have obtained strong
evidences for a gapless spectrum and a finite value of the thermodynamic order
parameter, thus confirming the existence of long-range N\'eel order.The good
agreement between the finite-size spin-wave results and the exact and quantum
Monte Carlo data also supports the reliability of the spin-wave expansion to
describe both the ground state and the low-energy spin excitations of the
triangular Heisenberg antiferromagnet. In the Heisenberg model, our
results indicate the opening of a finite gap in the thermodynamic excitation
spectrum at , marking the melting of the antiferromagnetic
N\'eel order and the onset of a non-magnetic ground state. In order to
characterize the nature of the latter quantum-disordered phase we have computed
the susceptibilities for the most important crystal symmetry breaking
operators. In the ordered phase the effectiveness of the spin-wave theory in
reproducing the low-energy excitation spectrum suggests that the uniform spin
susceptibility of the model is very close to the linear spin-wave prediction.Comment: Review article, 44 pages, 18 figures. See also PRL 87, 097201 (2001
The frustrated Heisenberg antiferromagnet on the honeycomb lattice: A candidate for deconfined quantum criticality
We study the ground-state (gs) phase diagram of the frustrated spin-1/2
-- antiferromagnet with on the
honeycomb lattice, using coupled-cluster theory and exact diagonalization
methods. We present results for the gs energy, magnetic order parameter,
spin-spin correlation function, and plaquette valence-bond crystal (PVBC)
susceptibility. We find a N\'eel antiferromagnetic (AFM) phase for , a collinear striped AFM phase for , and a paramagnetic PVBC phase for . The transition at
appears to be of first-order type, while that at is
continuous. Since the N\'eel and PVBC phases break different symmetries our
results favor the deconfinement scenario for the transition at
Spontaneous Collapse of Supersymmetry
It is shown that, if generators of supersymmetry transformations
(supercharges) can be defined in a spatially homogeneous physical state, then
this state describes the vacuum. Thus, supersymmetry is broken in any thermal
state and it is impossible to proceed from it by ``symmetrization'' to states
on which an action of supercharges can be defined. So, unlike the familiar
spontaneous breakdown of bosonic symmetries, there is a complete collapse of
supersymmetry in thermal states. It is also shown that spatially homogeneous
superthermal ensembles are never supersymmetric.Comment: 22 pages, amslatex, minor changes in text, two references adde
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