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

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

Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisites Cu3Bi(SeO3)2O2X (X = Br, Cl)

We investigate the antiferromagnetic canting instability of the spin-1/2 kagome ferromagnet, as realized in the layered cuprates Cu3Bi(SeO3)2O2X (X = Br, Cl). 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 nonlinear 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

Hidden magnetic order in CuNCN

We report a comprehensive experimental and theoretical study of the quasi-one-dimensional quantum magnet CuNCN. Based on magnetization measurements above room temperature as well as muon spin rotation and electron spin resonance measurements, we unequivocally establish the localized Cu+2-based magnetism and the magnetic transition around 70 K, both controversially discussed in the previous literature. Thermodynamic data conform to the uniform-spin-chain model with a nearest-neighbor intrachain coupling of about 2300 K, in remarkable agreement with the microscopic magnetic model based on density functional theory band-structure calculations. Using exact diagonalization and the coupled-cluster method, we derive a collinear antiferromagnetic order with a strongly reduced ordered moment of about 0.4 mu_B, indicating strong quantum fluctuations inherent to this quasi-one-dimensional spin system. We re-analyze the available neutron-scattering data, and conclude that they are not sufficient to resolve or disprove the magnetic order in CuNCN. By contrast, spectroscopic techniques indeed show signatures of long-range magnetic order below 70 K, yet with a rather broad distribution of internal field probed by implanted muons. We contemplate the possible structural origin of this effect and emphasize peculiar features of the microstructure studied with synchrotron powder x-ray diffraction.Comment: 17 pages, 17 figures, 1 tabl

Ground-state properties of the triangular-lattice Heisenberg antiferromagnet with arbitrary spin quantum number s

We apply the coupled cluster method to high orders of approximation and exact diagonalizations to study the ground-state properties of the triangular-lattice spin-s Heisenberg antiferromagnet. We calculate the fundamental ground-state quantities, namely, the energy e0, the sublattice magnetization Msub, the in-plane spin stiffness ρs and the in-plane magnetic susceptibility χ for spin quantum numbers s=1/2,1,…,smax, where smax=9/2 for e0 and Msub, smax=4 for ρs and smax=3 for χ . We use the data for s≥3/2 to estimate the leading quantum corrections to the classical values of e0, Msub, ρs, and χ. In addition, we study the magnetization process, the width of the 1/3 plateau as well as the sublattice magnetizations in the plateau state as a function of the spin quantum number s

The spin-half XXZ antiferromagnet on the square lattice revisited: A high-order coupled cluster treatment

We use the coupled cluster method (CCM) to study the ground-state properties and lowest-lying triplet excited state of the spin-half {\it XXZ} antiferromagnet on the square lattice. The CCM is applied to it to high orders of approximation by using an efficient computer code that has been written by us and which has been implemented to run on massively parallelized computer platforms. We are able therefore to present precise data for the basic quantities of this model over a wide range of values for the anisotropy parameter Δ in the range −1≤Δ1) regimes, where Δ→∞ represents the Ising limit. We present results for the ground-state energy, the sublattice magnetization, the zero-field transverse magnetic susceptibility, the spin stiffness, and the triplet spin gap. Our results provide a useful yardstick against which other approximate methods and/or experimental studies of relevant antiferromagnetic square-lattice compounds may now compare their own results. We also focus particular attention on the behaviour of these parameters for the easy-axis system in the vicinity of the isotropic Heisenberg point (Δ=1), where the model undergoes a phase transition from a gapped state (for Δ>1) to a gapless state (for Δ≤1), and compare our results there with those from spin-wave theory (SWT). Interestingly, the nature of the criticality at Δ=1 for the present model with spins of spin quantum number s=12 that is revealed by our CCM results seems to differ qualitatively from that predicted by SWT, which becomes exact only for its near-classical large-s counterpart

Influence of an inter-chain coupling on spiral ground-state correlations in frustrated spin-1/2 J1-J2 Heisenberg chains

We investigate the influence of an inter-chain coupling on the spiral ground state correlations of a spin-1/2 Heisenberg model consisting of a two-dimensional array of coupled chains with nearest (J1) and frustrating next-nearest neighbor (J2) in-chain exchange couplings. Using the coupled-cluster method we calculate the transition point between the commensurate and the incommensurate (spiral) ground state as well as the quantum pitch angle of the spiral ground state. In addition, we provide a simple empirical formula which describes the relation between the quantum pitch angle and the frustration parameter J2/J1.Comment: 9 pages, 11 figure

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

DEVELOPING A HABITAT MANAGEMENT PLAN FOR PRIVATE LAND

ABSTRACT. Ray Conway has ope.rated for 17 years the 4,400 acre Sugarloaf Rancb for qu.t11 and livestock production. This is his most. recent of several similar endeavors. .raclual changes in goals over time have occurred, and a decision was made to change the management orientation to include deer, turkey, dove, band .. tal1ed pigeon. and nongame.habitatwhfle maintaining a viable quail hunting base, all compatible with existing live$toekgrazing. Svoboda (1980) descrilled a good wildlife planning process for private landowners. The present paper descrilles practtca1 application of Svoboda's technique to the Sugarloaf Ranch. Each step of the process ts described and the resulting plan fs briefly outlined. This plan can serve as a guide to habitat development tasks that are practical and economic in the view of a rancher with over 40 years experience in private-land wildlife management. INTRODUCTION