156 research outputs found
Microscopic analysis of the magnetic form factor in low-dimensional cuprates
We analyze the magnetic form factor of Cu in low-dimensional quantum
magnets by taking the metal-ligand hybridization into account explicitly. In
this analysis we use the form of magnetic Wannier orbitals, derived from the
first-principles calculations, and identify the contributions of different
atomic sites. Having performed local density approximation calculations for
cuprates with different types of ligand atoms, we discuss the influence of the
on-site Coulomb correlations on the structure of the magnetic orbital. The
typical composition of Wannier functions for copper oxides, chlorides and
bromides is defined and related to features of the magnetic form factor. We
propose easy-to-use approximations of the partial orbital contributions to the
magnetic form factor in order to give a microscopic explanation for the results
obtained in previous first-principles studies.Comment: 5 pages, 4 figure
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
Hybridization and spin-orbit coupling effects in quasi-one-dimensional spin-1/2 magnet Ba3Cu3Sc4O12
We study electronic and magnetic properties of the quasi-one-dimensional
spin-1/2 magnet Ba3Cu3Sc4O12 with a distinct orthogonal connectivity of CuO4
plaquettes. An effective low-energy model taking into account spin-orbit
coupling was constructed by means of first-principles calculations. On this
basis a complete microscopic magnetic model of Ba3Cu3Sc4O12, including
symmetric and antisymmetric anisotropic exchange interactions, is derived. The
anisotropic exchanges are obtained from a distinct first-principles numerical
scheme combining, on one hand, the local density approximation taking into
account spin-orbit coupling, and, on the other hand, projection procedure along
with the microscopic theory by Toru Moriya. The resulting tensors of the
symmetric anisotropy favor collinear magnetic order along the structural chains
with the leading ferromagnetic coupling J1 = -9.88 meV. The interchain
interactions J8 = 0.21 meV and J5 = 0.093 meV are antiferromagnetic. Quantum
Monte Carlo simulations demonstrated that the proposed model reproduces the
experimental Neel temperature, magnetization and magnetic susceptibility data.
The modeling of neutron diffraction data reveals an important role of the
covalent Cu-O bonding in Ba3Cu3Sc4O12.Comment: 11 pages, 12 figure
Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO
We apply moderate-high-energy inelastic neutron scattering (INS) measurements
to investigate Yb crystalline electric field (CEF) levels in the
triangular spin-liquid candidate YbMgGaO. Three CEF excitations from the
ground-state Kramers doublet are centered at the energies = 39,
61, and 97\,meV in agreement with the effective \mbox{spin-1/2} -factors and
experimental heat capacity, but reveal sizable broadening. We argue that this
broadening originates from the site mixing between Mg and Ga
giving rise to a distribution of Yb--O distances and orientations and, thus, of
CEF parameters that account for the peculiar energy profile of the CEF
excitations. The CEF randomness gives rise to a distribution of the effective
spin-1/2 -factors and explains the unprecedented broadening of low-energy
magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO,
although a distribution of magnetic couplings due to the Mg/Ga disorder may be
important as well.Comment: Accepted in Phys. Rev. Let
Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO
We apply moderate-high-energy inelastic neutron scattering (INS) measurements
to investigate Yb crystalline electric field (CEF) levels in the
triangular spin-liquid candidate YbMgGaO. Three CEF excitations from the
ground-state Kramers doublet are centered at the energies = 39,
61, and 97\,meV in agreement with the effective \mbox{spin-1/2} -factors and
experimental heat capacity, but reveal sizable broadening. We argue that this
broadening originates from the site mixing between Mg and Ga
giving rise to a distribution of Yb--O distances and orientations and, thus, of
CEF parameters that account for the peculiar energy profile of the CEF
excitations. The CEF randomness gives rise to a distribution of the effective
spin-1/2 -factors and explains the unprecedented broadening of low-energy
magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO,
although a distribution of magnetic couplings due to the Mg/Ga disorder may be
important as well.Comment: Accepted in Phys. Rev. Let
Nearest-neighbor resonating valence bonds in YbMgGaO4
Since its proposal by Anderson, resonating valence bonds (RVB) formed by a
superposition of fluctuating singlet pairs have been a paradigmatic concept in
understanding quantum spin liquids (QSL). Here, we show that excitations
related to singlet breaking on nearest-neighbor bonds describe the high-energy
part of the excitation spectrum in YbMgGaO4, the effective spin-1/2 frustrated
antiferromagnet on the triangular lattice, as originally considered by
Anderson. By a thorough single-crystal inelastic neutron scattering (INS)
study, we demonstrate that nearest-neighbor RVB excitations account for the
bulk of the spectral weight above 0.5 meV. This renders YbMgGaO4 the first
experimental system where putative RVB correlations restricted to nearest
neighbors are observed, and poses a fundamental question of how complex
interactions on the triangular lattice conspire to form this unique many-body
state.Comment: To be published in Nature Communication
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
Magnetism of coupled spin tetrahedra in ilinskite-type KCuO(SeO)Cl
Synthesis, thermodynamic properties, and microscopic magnetic model of
ilinskite-type KCuO(SeO)Cl built by corner-sharing Cu
tetrahedra are reported, and relevant magnetostructural correlations are
discussed. Quasi-one-dimensional magnetic behavior with the short-range order
around 50\,K and the absence of long-range order down to at least 2\,K is
observed experimentally and explained in terms of weakly coupled spin ladders
(tubes) with a complex topology formed upon fragmentation of the tetrahedral
network. This fragmentation is rooted in the non-trivial effect of the SeO
groups that render the Cu--O--Cu superexchange strongly ferromagnetic.Comment: 9 pages, 7 figure
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