623 research outputs found
The two-dimensional frustrated Heisenberg model on the orthorhombic lattice
We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.Comment: 4 pages, 3 figures, accepted for publication in Journal of Physics:
Conference Serie
Structural distortion and frustrated magnetic interactions in the layered copper oxychloride [CuCl]LaNb(2)O(7)
We present a computational study of the layered copper oxychloride
[CuCl]LaNb(2)O(7) that has been recently proposed as a spin-1/2 frustrated
square lattice compound. Our results evidence an orbitally degenerate ground
state for the reported tetragonal crystal structure and reveal a
Jahn-Teller-type structural distortion. This distortion heavily changes the
local environment of copper -- CuO(2)Cl(2) plaquettes are formed instead of
CuO(2)Cl(4) octahedra -- and restores the single-orbital scenario typical for
copper oxides and oxyhalides. The calculated distortion is consistent with the
available diffraction data and the experimental results on the electric field
gradients for the Cu and Cl sites. The band structure suggests a complex
three-dimensional spin model with the interactions up to the fourth neighbors.
Despite the layered structure of (CuCl)LaNb(2)O(7), the spin system has
pronounced one-dimensional features. Yet, sizable interchain interactions lead
to the strong frustration and likely cause the spin-gap behavior. Computational
estimates of individual exchange couplings are in qualitative agreement with
the experimental data.Comment: 13 pages, 9 figures, 3 table
Ab initio modeling of Bose-Einstein condensation in Pb2V3O9
We apply density functional theory band structure calculations and quantum
Monte Carlo simulations to investigate the Bose-Einstein condensation in the
spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the
one-dimensional nature of this compound, we present a quasi-two-dimensional
model of spin dimers with ferromagnetic and antiferromagnetic interdimer
couplings. Our model is well justified microscopically and provides a
consistent description of the experimental data on the magnetic susceptibility,
high-field magnetization, and field vs. temperature phase diagram. The
Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9
is largely governed by intralayer interactions, whereas weak interlayer
couplings have a moderate effect on the ordering temperature. The proposed
computational approach is an efficient tool to analyze and predict high-field
properties of quantum magnets.Comment: 6 pages, 6 figures, 1 tabl
Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase: a hidden message from birds and trees
The green mineral dioptase Cu6Si6O18(H2O)6 has been known since centuries and
plays an important role in esoteric doctrines. In particular, the green
dioptase is supposed to grant the skill to speak with trees and to understand
the language of birds. Armed with natural samples of dioptase, we were able to
unravel the magnetic nature of the mineral (presumably with hidden support from
birds and trees) and show that strong quantum fluctuations can be realized in
an essentially framework-type spin lattice of coupled chains, thus neither
frustration nor low-dimensionality are prerequisites. We present a microscopic
magnetic model for the green dioptase. Based on full-potential DFT
calculations, we find two relevant couplings in this system: an
antiferromagnetic coupling J_c, forming spiral chains along the hexagonal c
axis, and an inter-chain ferromagnetic coupling J_d within structural Cu2O6
dimers. To refine the J_c and J_d values and to confirm the proposed spin
model, we perform quantum Monte-Carlo simulations for the dioptase spin
lattice. The derived magnetic susceptibility, the magnetic ground state, and
the sublattice magnetization are in remarkably good agreement with the
experimental data. The refined model parameters are J_c = 78 K and J_d = -37 K
with J_d/J_c ~ -0.5. Despite the apparent three-dimensional features of the
spin lattice and the lack of frustration, strong quantum fluctuations in the
system are evidenced by a broad maximum in the magnetic susceptibility, a
reduced value of the Neel temperature T_N ~ 15 K >> J_c, and a low value of the
sublattice magnetization m = 0.55 Bohr magneton. All these features should be
ascribed to the low coordination number of 3 that outbalances the
three-dimensional nature of the spin lattice.Comment: Dedicated to Stefan-Ludwig Drechsler on the occasion of his 60th
birthday (9 pages, 6 figures
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
Microscopic model of (CuCl)LaNb2O7: coupled spin dimers replace a frustrated square lattice
We present a microscopic model of the spin-gap quantum magnet (CuCl)LaNb2O7
that was previously suggested as a realization of the spin-1/2 frustrated
square lattice. Taking advantage of the precise atomic positions from recent
crystal structure refinement, we evaluate individual exchange integrals and
construct a minimum model that naturally explains all the available
experimental data. Surprisingly, the deviation from tetragonal symmetry leads
to the formation of spin dimers between fourth neighbors due to a Cu-Cl-Cl-Cu
pathway with a leading antiferromagnetic exchange J4 ~ 25 K. The total
interdimer exchange amounts to 12 - 15 K. Our model is in agreement with
inelastic neutron scattering results and is further confirmed by quantum
Monte-Carlo simulations of the magnetic susceptibility and the high-field
magnetization. Our results establish (CuCl)LaNb2O7 as a non-frustrated system
of coupled spin dimers with predominant antiferromagnetic interactions and
provide a general perspective for related materials with unusual
low-temperature magnetic properties.Comment: 4 pages, 4 figures, 1 table + supplementar
A Minimal Dissipation Type-Based Classification in Irreversible Thermodynamics and Microeconomics
We formulate the problem of finding classes of kinetic dependencies in irreversible thermodynamic and microeconomic systems for which minimal dissipation processes belong to the same type. We show that this problem is an inverse optimal control problem and solve it. The commonality of this problem in irreversible thermodynamics and microeconomics is emphasized.
- …