71 research outputs found
Classical spin models of the windmill lattice and their relevance for Pb Cu Te 2 O 6
We investigate classical Heisenberg models on the distorted windmill lattice and discuss their applicability to the spin-1/2 spin liquid candidate PbCuTe2O6. We first consider a general Heisenberg model on this lattice with antiferromagnetic interactions Jn (n=1,2,3,4) up to fourth neighbors. Setting J1=J2 (as approximately realized in PbCuTe2O6) we map out the classical ground-state phase diagram in the remaining parameter space and identify a competition between J3 and J4 that opens up interesting magnetic scenarios. Particularly, these couplings tune the ground states from coplanar commensurate or non-coplanar incommensurate magnetically ordered states to highly degenerate ground-state manifolds with subextensive or extensive degeneracies. In the latter case, we uncover an unusual classical spin liquid defined on a lattice of corner-sharing octahedra. We then focus on the particular set of interaction parameters Jn that has previously been proposed for PbCuTe2O6 and investigate the system's incommensurate magnetic ground-state order and finite-temperature multistage ordering mechanism. We perform extensive finite-temperature simulations of the system's dynamical spin structure factor and compare it with published neutron scattering data for PbCuTe2O6 at low temperatures. Our results demonstrate that thermal fluctuations in the classical model can largely explain the signal distribution in the measured spin structure factor but we also identify distinct differences. Our investigations make use of a variety of different analytical and numerical approaches for classical spin systems, such as Luttinger-Tisza, classical Monte Carlo, iterative minimization, and molecular dynamics simulations
Coupled spin-lattice fluctuations in a compound with orbital degrees of freedom: the Cr based dimer system Sr3Cr2O8
We report on an extended fluctuation regime in the spin dimer system Sr3Cr2O8
based on anomalies in Raman active phonons and magnetic scattering. The
compound has two characteristic temperatures, TS = 275 K, related to a
Jahn-Teller transition with structural distortions and orbital ordering and a
second, T*= 150 K, which is due to further changes in the orbital sector. Below
TS quasielastic scattering marks strong fluctuations and in addition phonon
anomalies are observed. For temperatures below T* we observe an exponential
decrease of one phonon linewidth and determine a gap of the orbital
excitations. At low temperatures the observation of two- and three-magnon
scattering allows the determination of the spin excitation gap.Comment: 7 pages, 4 figures, 1 tabl
Magnetic Structure and Interactions in the Quasi-1D Antiferromagnet CaVO
CaVO is a spin-1 antiferromagnet, where the magnetic vanadium ions
are arranged on quasi-one-dimensional (1D) zig-zag chains with potentially
frustrated antiferromagnetic exchange interactions. High temperature
susceptibility and single-crystal neutron diffraction measurements are used to
deduce the non-collinear magnetic structure, dominant exchange interactions and
orbital configurations. The results suggest that at high temperatures
CaVO behaves as a Haldane chain, but at low temperatures, orbital
ordering lifts the frustration and it becomes a spin-1 ladder.Comment: 5 pages, 4 figure
From confined spinons to emergent fermions: Observation of elementary magnetic excitations in a transverse-field Ising chain
We report on spectroscopy study of elementary magnetic excitations in an
Ising-like antiferromagnetic chain compound SrCoVO as a function of
temperature and applied transverse magnetic field up to 25 T. An optical as
well as an acoustic branch of confined spinons, the elementary excitations at
zero field, are identified in the antiferromagnetic phase below the N\'{e}el
temperature of 5 K and described by a one-dimensional Schr\"{o}dinger equation.
The confinement can be suppressed by an applied transverse field and a quantum
disordered phase is induced at 7 T. In this disordered paramagnetic phase, we
observe three emergent fermionic excitations with different transverse-field
dependencies. The nature of these modes is clarified by studying spin dynamic
structure factor of a 1D transverse-field Heisenberg-Ising (XXZ) model using
the method of infinite time evolving block decimation. Our work reveals
emergent quantum phenomena and provides a concrete system for testifying
theoretical predications of one-dimension quantum spin models.Comment: 8 pages and 6 figure
Physical realization of a quantum spin liquid based on a novel frustration mechanism
Unlike conventional magnets where the magnetic moments are partially or
completely static in the ground state, in a quantum spin liquid they remain in
collective motion down to the lowest temperatures. The importance of this state
is that it is coherent and highly entangled without breaking local symmetries.
Such phenomena is usually sought in simple lattices where antiferromagnetic
interactions and/or anisotropies that favor specific alignments of the magnetic
moments are "frustrated" by lattice geometries incompatible with such order
e.g. triangular structures. Despite an extensive search among such compounds,
experimental realizations remain very few. Here we describe the investigation
of a novel, unexplored magnetic system consisting of strong ferromagnetic and
weaker antiferromagnetic isotropic interactions as realized by the compound
CaCrO. Despite its exotic structure we show both
experimentally and theoretically that it displays all the features expected of
a quantum spin liquid including coherent spin dynamics in the ground state and
the complete absence of static magnetism.Comment: Modified version accepted in Nature Physic
Non-Abelian statistics in light scattering processes across interacting Haldane chains
The Haldane state is constructed from a product of local singlet dimers
in the bulk and topological states at the edges of a chain. It is a fundamental
representative of topological quantum matter. Its well-known representative,
the quasi-one-dimensional SrNiVO shows both conventional as well as
unconventional magnetic Raman scattering. The former is observed as one- and
two-triplet excitations with small linewidths and energies corresponding to the
Haldane gap and the exchange coupling along the chain,
respectively. Well-defined magnetic quasiparticles are assumed to be stabilized
by interchain interactions and uniaxial single-ion anisotropy. Unconventional
scattering exists as broad continua of scattering with an intensity that
shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also
been observed in Kitaev spin liquids and could point to a non-Abelian symmetry.
As the ground state in the bulk of SrNiVO is topologically trivial,
we suggest its fractionalization to be due to light-induced interchain exchange
processes. These processes are supposed to be enhanced due to a proximity to an
Ising ordered state with a quantum critical point. A comparison with
SrCoVO, the analogue to our title compound, supports these
statements.Comment: 3 figures, 1 tabl
Field-induced effects in the spin liquid candidate PbCuTeO
PbCuTeO is considered as one of the rare candidate materials for a
three-dimensional quantum spin liquid (QSL). This assessment was based on the
results of various magnetic experiments, performed mainly on polycrystalline
material. More recent measurements on single crystals revealed an even more
exotic behavior, yielding ferroelectric order below , accompanied by distinct lattice distortions, and a somewhat
modified magnetic response which is still consistent with a QSL. Here we report
on low-temperature measurements of various thermodynamic, magnetic and
dielectric properties of single crystalline PbCuTeO in magnetic fields
. The combination of these various probes allows us to
construct a detailed - phase diagram including a ferroelectric phase for
and a -induced magnetic phase at
. These phases are preceded by or coincide with a structural
transition from a cubic high-temperature phase into a distorted non-cubic
low-temperature state. The phase diagram discloses two quantum critical points
(QCPs) in the accessible field range, a ferroelectric QCP at =
and a magnetic QCP at = . Field-induced
lattice distortions, observed in the state at and which are
assigned to the effect of spin-orbit interaction of the Cu-ions, are
considered as the key mechanism by which the magnetic field couples to the
dielectric degrees of freedom in this material
Spin liquid and ferroelectricity close to a quantum critical point in PbCuTe2O6
Geometrical frustration among interacting spins combined with strong quantum
fluctuations destabilize long-range magnetic order in favour of more exotic
states such as spin liquids. By following this guiding principle, a number of
spin liquid candidate systems were identified in quasi-two-dimensional
(quasi-2D) systems. For 3D, however, the situation is less favourable as
quantum fluctuations are reduced and competing states become more relevant.
Here we report a comprehensive study of thermodynamic, magnetic and dielectric
properties on single crystalline and pressed-powder samples of PbCuTeO,
a candidate material for a 3D frustrated quantum spin liquid featuring a
hyperkagome lattice. Whereas the low-temperature properties of the powder
samples are consistent with the recently proposed quantum spin liquid state, an
even more exotic behaviour is revealed for the single crystals. These crystals
show ferroelectric order at , accompanied by
strong lattice distortions, and a modified magnetic response -- still
consistent with a quantum spin liquid -- but with clear indications for quantum
critical behaviour.Comment: 59 pages, 15 figures, This version of the article has been accepted
for publication, after peer review but is not the Version of Record and does
not reflect post-acceptance improvements, or any corrections. The Version of
Record is available onlin
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