147 research outputs found
Emergence of One-Dimensional Physics from the Distorted Shastry-Sutherland Lattice
Motivated by the on-going investigation of SrCu(BO) under
pressure, we study a variant of the two-dimensional Shastry-Sutherland (SS)
spin-1/2 model with two types of dimers. Combined with the frustration of the
SS model, this modification induces, in a large parameter range, a dimensional
reduction at low energies, with nearly decoupled effective S=1 Haldane chains
forming along one of the diagonals of the lattice. We also present evidence
that the intermediate plaquette solid phase of the undistorted SS model remains
stable in a finite region of the phase diagram.Comment: 4 pages, 5 figure
Master Equations for pulsed magnetic fields: Application to magnetic molecules
We extend spin-lattice relaxation theory to incorporate the use of pulsed
magnetic fields for probing the hysteresis effects and magnetization steps and
plateaus exhibited, at low temperatures, by the dynamical magnetization of
magnetic molecules. The main assumption made is that the lattice degrees of
freedom equilibrate in times much shorter than both the experimental time scale
(determined by the sweep rate) and the typical spin-lattice relaxation time. We
first consider the isotropic case (a magnetic molecule with a ground state of
spin well separated from the excited levels and also the general isotropic
Heisenberg Hamiltonian where all energy levels are relevant) and then we
include small off-diagonal terms in the spin Hamiltonian to take into account
the Landau-Zener-St\"{u}ckelberg (LZS) effect. In the first case, and for an
magnetic molecule we arrive at the generalized Bloch equation recently
used for the magnetic molecule \{V\} in Phys. Rev. Lett. 94, 147204 (2005).
An analogous equation is derived for the magnetization, at low temperatures, of
antiferromagnetic ring systems. The LZS effect is discussed for magnetic
molecules with a low spin ground state, for which we arrive at a very
convenient set of equations that take into account the combined effects of LZS
and thermal transitions. In particular, these equations explain the deviation
from exact magnetization reversal at observed in \{V\}. They
also account for the small magnetization plateaus (``magnetic Foehn effect''),
following the LZS steps, that have been observed in several magnetic molecules.
Finally, we discuss the role of the Phonon Bottleneck effect at low
temperatures and specifically we indicate how this can give rise to a
pronounced Foehn effect.Comment: 10 pages, 4 figure
Classical spin liquid instability driven by off-diagonal exchange in strong spin-orbit magnets
We show that the off-diagonal exchange anisotropy drives Mott insulators with
strong spin-orbit coupling to a classical spin liquid regime, characterized by
an infinite number of ground states and Ising variables living on closed or
open strings. Depending on the sign of the anisotropy, quantum fluctuations
either fail to lift the degeneracy down to very low temperatures, or select
non-collinear magnetic states with unconventional spin correlations. The
results apply to all 2D and 3D tri-coordinated materials with bond-directional
anisotropy, and provide a consistent interpretation of the suppression of the
x-ray magnetic circular dichroism signal reported recently for
-LiIrO under pressure
Microscopic theory of the nearest-neighbor valence bond sector of the spin-1/2 kagome antiferromagnet
The spin-1/2 Heisenberg model on the kagome lattice, which is closely
realized in layered Mott insulators such as ZnCu(OH)Cl, is one of
the oldest and most enigmatic spin-1/2 lattice model. While the numerical
evidence has accumulated in favor of a quantum spin liquid, the debate is still
open as to whether it is a spin liquid with very short-range correlations
(some kind of Resonating Valence Bond spin liquid), or an algebraic spin-liquid
with power-law correlations. To address this issue, we have pushed the program
started by Rokhsar and Kivelson in their derivation of the effective quantum
dimer model description of Heisenberg models to unprecedented accuracy for the
spin-1/2 kagome, by including all the most important virtual singlet
contributions on top of the orthogonalization of the nearest-neighbor valence
bond singlet basis. Quite remarkably, the resulting picture is a competition
between a spin liquid and a diamond valence bond crystal with a 12-site
unit cell, as in the DMRG simulations of Yan, Huse and White. Furthermore, we
found that, on cylinders of finite diameter , there is a transition between
the spin liquid at small and the diamond valence bond crystal at
large , the prediction of the present microscopic description for the 2D
lattice. These results show that, if the ground state of the spin-1/2 kagome
antiferromagnet can be described by nearest-neighbor singlet dimers, it is a
diamond valence bond crystal, and, a contrario, that, if the system is a
quantum spin liquid, it has to involve long-range singlets, consistent with the
algebraic spin liquid scenario.Comment: 11 pages, 14 figures. Revised and extended version. Results are
untouched, implications have been clarified and better put in contex
Frustrated magnetism and resonating valence bond physics in two-dimensional kagome-like magnets
We explore the phase diagram and the low-energy physics of three Heisenberg
antiferromagnets which, like the kagome lattice, are networks of corner-sharing
triangles but contain two sets of inequivalent short-distance resonance loops.
We use a combination of exact diagonalization, analytical strong-coupling
theories, and resonating valence bond approaches, and scan through the ratio of
the two inequivalent exchange couplings. In one limit, the lattices effectively
become bipartite, while at the opposite limit heavily frustrated nets emerge.
In between, competing tunneling processes result in short-ranged spin
correlations, a manifold of low-lying singlets (which can be understood as
localized bound states of magnetic excitations), and the stabilization of
valence bond crystals with resonating building blocks.Comment: Published versio
Quantum dimer model for the spin-1/2 kagome Z2 spin liquid
We revisit the description of the low-energy singlet sector of the spin-1/2
Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer
model. With the help of exact diagonalizations of appropriate finite-size
clusters, we show that the embedding of a given process in its kagome
environment leads to dramatic modifications of the amplitudes of the elementary
loop processes, an effect not accessible to the standard approach based on the
truncation of the Hamiltonian to the nearest-neighbour valence-bond basis. The
resulting parameters are consistent with a Z spin liquid rather than with a
valence-bond crystal, in agreement with the last density matrix renormalization
group results.Comment: Potential terms of the effective QDM include
Highly Frustrated Magnetic Clusters: The kagome on a sphere
We present a detailed study of the low-energy excitations of two existing
finite-size realizations of the planar kagome Heisenberg antiferromagnet on the
sphere, the cuboctahedron and the icosidodecahedron. After highlighting a
number of special spectral features (such as the presence of low-lying singlets
below the first triplet and the existence of localized magnons) we focus on two
major issues. The first concerns the nature of the excitations above the
plateau phase at 1/3 of the saturation magnetization Ms. Our exact
diagonalizations for the s=1/2 icosidodecahedron reveal that the low-lying
plateau states are adiabatically connected to the degenerate collinear
``up-up-down'' ground states of the Ising point, at the same time being well
isolated from higher excitations. A complementary physical picture emerges from
the derivation of an effective quantum dimer model which reveals the central
role of the topology and the intrinsic spin s. We also give a prediction for
the low energy excitations and thermodynamic properties of the spin s=5/2
icosidodecahedron Mo72Fe30. In the second part we focus on the low-energy
spectra of the s>1/2 Heisenberg model in view of interpreting the broad
inelastic neutron scattering response reported for Mo72Fe30. To this end we
demonstrate the simultaneous presence of several broadened low-energy ``towers
of states'' or ``rotational bands'' which arise from the large discrete spatial
degeneracy of the classical ground states, a generic feature of highly
frustrated clusters. This semiclassical interpretation is further corroborated
by their striking symmetry pattern which is shown, by an independent group
theoretical analysis, to be a characteristic fingerprint of the classical
coplanar ground states.Comment: 22 pages Added references Corrected typo
Quantum spin liquid in the semiclassical regime
Quantum spin liquids have been at the forefront of correlated electron
research ever since their original proposal in 1973, and the realization that
they belong to the broader class of intrinsic topological orders, along with
the fractional quantum Hall states. According to received wisdom, quantum spin
liquids can arise in frustrated magnets with low spin , where strong quantum
fluctuations act to destabilize conventional, magnetically ordered states. Here
we present a magnet that has a quantum spin liquid ground state already
in the semiclassical, large- limit. The state has both topological and
symmetry related ground state degeneracy, and two types of gaps, a `magnetic
flux' gap that scales linearly with and an `electric charge' gap that drops
exponentially in . The magnet is described by the spin- version of the
spin-1/2 Kitaev honeycomb model, which has been the subject of intense studies
in correlated electron systems with strong spin-orbit coupling, and in optical
lattice realizations with ultracold atoms. The results apply to both integer
and half-integer spins
Theoretical investigation of dynamic properties of magnetic molecule systems as probed by NMR and pulsed fields experiments
In this dissertation we theoretically investigate static and especially dynamic properties of magnetic molecules (MM\u27s), as probed by the nuclear spin lattice relaxation rate 1/T1 (first part) and pulsed fields measurements of the magnetization M( t) (second part). In the first part, we provide a general first-principles account for 1/T1, which incorporates the decay of spin fluctuations and the corresponding broadening of the discrete magnetic energy levels of MM\u27s. This is achieved by including the interaction of the electronic moments with the local deformation of the host lattice (phonons), in the Markovian regime and employing the quantum regression theorem. Within this framework, we provide a rigorous interpretation of a number of 1/ T1 experimental findings in MM\u27s. We also provide an extensive account of the model spin-1/2 tetramer V12 by analyzing magnetic susceptibility and 1/T1 data. The second part focuses on phenomena manifested in pulsed fields measurements of M(t), such as hysteresis loops and Landau-Zener-Stuckelberg (LZS) steps. First, we give a theoretical analysis of the low-T hysteresis loops and LZS steps at B ≈ 0 observed in the magnetic molecule V6. The loops are successfully reproduced by employing a generalization of the standard Bloch equation which in turn reveals the one-phonon acoustic processes as the dominant source of relaxation in this system. The origin of the US steps is attributed to the presence in V 6 of a weak intra-molecular anisotropic exchange. The small deviation from the quantum-mechanical prediction of exact magnetization reversals at B ≈ 0 is attributed to the role of the phonon heat bath (dissipative US problem). Second, we provide a general, first-principles account of all dynamic phenomena manifested in pulsed fields experiments, by extending the standard spin-lattice relaxation theory to include time-dependent (pulsed) fields. This theory accounts for: (i) hysteresis effects (including the generalized Bloch equation used for V6), (ii) the effects associated with the dissipative US problem, in the adiabatic regime and in particular, (iii) the so-called magnetic Foehn effect. We also discuss how the phonon bottleneck effect (typically occurring at T ≲ 1 K) can give rise to an enhanced Foehn effect
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