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

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.

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 Cu3_3Bi(SeO3)2_3)_2O2_2X

We investigate the antiferromagnetic canting instability of the spin-1/2 kagome ferromagnet, as realized in the layered cuprates Cu$_3$Bi(SeO$_3)_2$O$_2$X (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

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