298 research outputs found
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
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
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
The spin gap in malachite Cu2(OH)2CO3 and its evolution under pressure
We report on the microscopic magnetic modeling of the spin-1/2 copper mineral
malachite at ambient and elevated pressures. Despite the layered crystal
structure of this mineral, the ambient-pressure susceptibility and
magnetization data can be well described by an unfrustrated
quasi-one-dimensional magnetic model. Weakly interacting antiferromagnetic
alternating spin chains are responsible for a large spin gap of 120K. Although
the intradimer Cu-O-Cu bridging angles are considerably smaller than the
interdimer angles, density functional theory (DFT) calculations revealed that
the largest exchange coupling of 190K operates within the structural dimers.
The lack of the inversion symmetry in the exchange pathways gives rise to
sizable Dzyaloshinskii-Moriya interactions which were estimated by
full-relativistic DFT+U calculations. Based on available high-pressure crystal
structures, we investigate the exchange couplings under pressure and make
predictions for the evolution of the spin gap. The calculations evidence that
intradimer couplings are strongly pressure-dependent and their evolution
underlies the decrease of the spin gap under pressure. Finally, we assess the
accuracy of hydrogen positions determined by structural relaxation within DFT
and put forward this computational method as a viable alternative to elaborate
experiments
Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisites CuBi(SeOOX
We investigate the antiferromagnetic canting instability of the spin-1/2
kagome ferromagnet, as realized in the layered cuprates
CuBi(SeOOX (X=Br, Cl, and I). While the local canting can be
explained in terms of competing exchange interactions, the direction of the
ferrimagnetic order parameter fluctuates strongly even at short distances on
account of frustration which gives rise to an infinite ground state degeneracy
at the classical level. In analogy with the kagome antiferromagnet, the
accidental degeneracy is fully lifted only by non-linear 1/S corrections,
rendering the selected uniform canted phase very fragile even for spins-1/2, as
shown explicitly by coupled-cluster calculations. To account for the observed
ordering, we show that the minimal description of these systems must include
the microscopic Dzyaloshinsky-Moriya interactions, which we obtain from
density-functional band-structure calculations. The model explains all
qualitative properties of the kagome francisites, including the detailed nature
of the ground state and the anisotropic response under a magnetic field. The
predicted magnon excitation spectrum and quantitative features of the
magnetization process call for further experimental investigations of these
compounds.Comment: 21 pages, 6 figure
Uniform spin chain physics arising from NCN bridges in CuNCN: surprises on the way from copper oxides to their nitride analogs
We report on the unexpected uniform spin chain physics in CuNCN, the
insulating nitride analog of copper oxides. Based on full-potential band
structure calculations, we derive the relevant microscopic parameters, estimate
individual exchange couplings, and establish a realistic spin model of this
compound. The structure of CuNCN contains chains of edge-sharing CuN(4)
squares. As a surprise, in contrast to analogous [CuO(2)] chains in
"edge-sharing" cuprates, the leading magnetic interactions J ~ 2500 K run
perpendicular to the structural [CuN(2)] chains via bridging NCN groups. The
resulting spin model of a uniform chain is in agreement with the experimentally
observed temperature-independent magnetic susceptibility below 300 K. The
nearest-neighbor and next-nearest-neighbor interactions along the structural
[CuN(2)] chains are J(1) ~ -500 K and J(2) ~ 100 K, respectively. Despite the
frustrating nature of J(1) and J(2), we assign the anomaly at 70 K to
long-range magnetic ordering, which is likely collinear with antiparallel and
parallel arrangement of spins along the 'c' and 'a' directions, respectively.
The pronounced one-dimensionality of the spin system should lead to a reduction
in the ordered moment and to a suppression of the transition anomaly in the
specific heat, thus impeding the experimental observation of the long-range
ordering. Our results suggest CuNCN as a promising material for ballistic heat
transport within spin chains, while the sizable bandwidth W ~ 3 eV may lead to
a metal-insulator transition and other exotic properties under high pressure.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.
Spiral ground state in the quasi-two-dimensional spin-1/2 system Cu2GeO4
We apply density functional theory band structure calculations, the
coupled-cluster method, and exact diagonalization to investigate the
microscopic magnetic model of the spin-1/2 compound Cu2GeO4. The model is
quasi-two-dimensional, with uniform spin chains along one direction and
frustrated spin chains along the other direction. The coupling along the
uniform chains is antiferromagnetic, J 130 K. The couplings along the
frustrated chains are J1 -60 K and J2 80 K between nearest neighbors and
next-nearest neighbors, respectively. The ground state of the quantum model is
a spiral, with the reduced sublattice magnetization of 0.62 mu_B and the pitch
angle of 84 deg, both renormalized by quantum effects. The proposed spiral
ground state of Cu2GeO4 opens a way to magnetoelectric effects in this
compound.Comment: Extended version: 8 pages, 5 figures, 1 tabl
Square-lattice magnetism of diaboleite Pb2Cu(OH)4Cl2
We report on the quasi-two-dimensional magnetism of the natural mineral
diaboleite Pb2Cu(OH)4Cl2 with a tetragonal crystal structure, which is closely
related to that of the frustrated spin-1/2 magnet PbVO3. Magnetic
susceptibility of diaboleite is well described by a Heisenberg spin model on a
diluted square lattice with the nearest-neighbor exchange of J~35 K and about
5% of non-magnetic impurities. The dilution of the spin lattice reflects the
formation of Cu vacancies that are tolerated by the crystal structure of
diaboleite. The weak coupling between the magnetic planes triggers the
long-range antiferromagnetic order below TN~11 K. No evidence of magnetic
frustration is found. We also analyze the signatures of the long-range order in
heat-capacity data, and discuss the capability of identifying magnetic
transitions with heat-capacity measurements.Comment: 10 pages, 10 figures + Supplementary Informatio
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