50 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
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
Hysteresis loops and adiabatic Landau-Zener-St\"uckelberg transitions in the magnetic molecule V
We have observed hysteresis loops and abrupt magnetization steps in the
magnetic molecule V, where each molecule comprises a pair of identical spin
triangles, in the temperature range 1-5 K for external magnetic fields with
sweep rates of several Tesla/ms executing a variety of closed cycles. The
hysteresis loops are accurately reproduced using a generalization of the Bloch
equation based on direct one-phonon transitions between the instantaneous
Zeeman-split levels of the ground state (an doublet) of each spin
triangle. The magnetization steps occur for and they are explained
in terms of adiabatic Landau-Zener-St\"{u}ckelberg transitions between the
lowest magnetic energy levels as modified by inter-triangle anisotropic
exchange of order 0.4 K.Comment: 4 pages, 3 figure
Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality
Quantum spin liquids are long-range entangled states of matter with emergent
gauge fields and fractionalized excitations. While candidate materials, such as
the Kitaev honeycomb ruthenate -RuCl, show magnetic order at low
temperatures , here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low and frequencies to long-lived
fractionalized fermionic quasiparticles at higher and . This
crossover is akin to the presence of spinon continua in quasi-1D spin chains.
It is further shown to go hand in hand with persistent typicality down to very
low . This aspect, which has also been observed in the spin-1/2 kagome
Heisenberg antiferromagnet, is a signature of proximate spin liquidity and
emergent gauge degrees of freedom more generally, and can be the basis for the
numerical study of many finite- properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2
We report the microscopic magnetic model for the spin-1/2 Heisenberg system
CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization
plateau. Recent neutron diffraction experiments on this compound [M. Hase et
al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order,
inconsistent with the previously suggested phenomenological magnetic model of
isolated dimers and spin chains. Based on extensive density-functional theory
band structure calculations, exact diagonalizations, quantum Monte Carlo
simulations, third-order perturbation theory, as well as high-field
magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2
are accounted for by a frustrated quasi-2D magnetic model featuring four
inequivalent exchange couplings: the leading antiferromagnetic coupling J_d
within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2,
forming magnetic tetramers, and a ferromagnetic coupling J_it between the
tetramers. Based on comparison to the experimental data, we evaluate the ratios
of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30,
with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the
frustration and triggers long-range antiferromagnetic ordering. The proposed
model accounts correctly for the different magnetic moments localized on
structurally inequivalent Cu atoms in the ground-state magnetic configuration.
We extensively analyze the magnetic properties of this model, including a
detailed description of the magnetically ordered ground state and its evolution
in magnetic field with particular emphasis on the 1/2-magnetization plateau.
Our results establish remarkable analogies to the Shastry-Sutherland model of
SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material
realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.Comment: 16 pages, 13 figures, 2 tables. Published version with additional QMC
dat
Ferrimagnetism of the magnetoelectric compound CuOSeO probed by Se NMR
We present a thorough Se NMR study of a single crystal of the
magnetoelectric compound CuOSeO. The temperature dependence of the
local electronic moments extracted from the NMR data is fully consistent with a
magnetic phase transition from the high-T paramagnetic phase to a low-T
ferrimagnetic state with 3/4 of the Cu ions aligned parallel and 1/4
aligned antiparallel to the applied field of 14.09 T. The transition to this
3up-1down magnetic state is not accompanied by any splitting of the NMR lines
or any abrupt modification in their broadening, hence there is no observable
reduction of the crystalline symmetry from its high-T cubic \textit{P}23
space group. These results are in agreement with high resolution x-ray
diffraction and magnetization data on powder samples reported previously by Bos
{\it et al.} [Phys. Rev. B, {\bf 78}, 094416 (2008)]. We also develop a mean
field theory description of the problem based on a microscopic spin Hamiltonian
with one antiferromagnetic ( K) and one ferromagnetic
( K) nearest-neighbor exchange interaction
Hindered magnetic order from mixed dimensionalities in CuPO
We present a combined experimental and theoretical study of the spin-1/2
compound CuPO that features a network of two-dimensional (2D)
antiferromagnetic (AFM) square planes, interconnected via one-dimensional (1D)
AFM spin chains. Magnetic susceptibility, high-field magnetization, and
electron spin resonance (ESR) data, as well as microscopic density-functional
band-structure calculations and subsequent quantum Monte-Carlo simulations,
show that the coupling 40 K in the layers is an order of magnitude
larger than 4 K in the chains. Below 8 K, CuPO
develops long-range order (LRO), as evidenced by a weak net moment on the 2D
planes induced by anisotropic magnetic interactions of Dzyaloshinsky-Moriya
type. A striking feature of this 3D ordering transition is that the 1D moments
grow significantly slower than the ones on the 2D layers, which is evidenced by
the persistent paramagnetic ESR signal below . Compared to typical
quasi-2D magnets, the ordering temperature of CuPO 0.2
is unusually low, showing that weakly coupled spins sandwiched between 2D
magnetic units effectively decouple these units and impede the long-range
ordering.Comment: 11 pages, 12 figures, 1 table; published version with few additional
citations added and misprints fixe
The quantum origins of skyrmions and half-skyrmions in Cu2OSeO3
The Skyrme-particle, the , was introduced over half a century ago
and used to construct field theories for dense nuclear matter. But with
skyrmions being mathematical objects - special types of topological solitons -
they can emerge in much broader contexts. Recently skyrmions were observed in
helimagnets, forming nanoscale spin-textures that hold promise as information
carriers. Extending over length-scales much larger than the inter-atomic
spacing, these skyrmions behave as large, classical objects, yet deep inside
they are of quantum origin. Penetrating into their microscopic roots requires a
multi-scale approach, spanning the full quantum to classical domain. By
exploiting a natural separation of exchange energy scales, we achieve this for
the first time in the skyrmionic Mott insulator CuOSeO. Atomistic ab
initio calculations reveal that its magnetic building blocks are strongly
fluctuating Cu tetrahedra. These spawn a continuum theory with a skyrmionic
texture that agrees well with reported experiments. It also brings to light a
decay of skyrmions into half-skyrmions in a specific temperature and magnetic
field range. The theoretical multiscale approach explains the strong
renormalization of the local moments and predicts further fingerprints of the
quantum origin of magnetic skyrmions that can be observed in CuOSeO,
like weakly dispersive high-energy excitations associated with the Cu
tetrahedra, a weak antiferromagnetic modulation of the primary ferrimagnetic
order, and a fractionalized skyrmion phase.Comment: 5 pages, 3 figure
Establishing the fundamental magnetic interactions in the chiral skyrmionic Mott insulator Cu2OSeO3 by terahertz electron spin resonance
The recent discovery of skyrmions in CuOSeO has established a new
platform to create and manipulate skyrmionic spin textures. We use high-field
electron spin resonance (ESR) spectroscopy combining a terahertz free electron
laser and pulsed magnetic fields up to 64 T to probe and quantify its
microscopic spin-spin interactions. Besides providing direct access to the
long-wavelength Goldstone mode, this technique probes also the high-energy part
of the excitation spectrum which is inaccessible by standard low-frequency ESR.
Fitting the behavior of the observed modes in magnetic field to a theoretical
framework establishes experimentally that the fundamental magnetic building
blocks of this skyrmionic magnet are rigid, highly entangled and weakly coupled
tetrahedra.Comment: 5 pages, 3 Figure
Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations
NaIrO, a honeycomb 5 oxide, has been recently identified as a
potential realization of the Kitaev spin lattice. The basic feature of this
spin model is that for each of the three metal-metal links emerging out of a
metal site, the Kitaev interaction connects only spin components perpendicular
to the plaquette defined by the magnetic ions and two bridging ligands. The
fact that reciprocally orthogonal spin components are coupled along the three
different links leads to strong frustration effects and nontrivial physics.
While the experiments indicate zigzag antiferromagnetic order in NaIrO,
the signs and relative strengths of the Kitaev and Heisenberg interactions are
still under debate. Herein we report results of ab initio many-body electronic
structure calculations and establish that the nearest-neighbor exchange is
strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the
Heisenberg contribution is significantly weaker and antiferromagnetic. The
calculations further reveal a strong sensitivity to tiny structural details
such as the bond angles. In addition to the large spin-orbit interactions, this
strong dependence on distortions of the IrO plaquettes singles out the
honeycomb 5 oxides as a new playground for the realization of
unconventional magnetic ground states and excitations in extended systems.Comment: 13 pages, 2 tables, 3 figures, accepted in NJ