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We discuss the temperature (T ) dependence of the generalized density of states G(ω,T ) and of the phonon intensities of the filled skutterudite compound YbFe4Sb12 investigated by inelastic neutron scattering in an extensive range of energy-momentum phase space. The inelastic response has been measured at T =1.7, 50, 100, 200, and 290 K at a thermal time-of-flight spectrometer with an incident wavelength of 1.3 Å. The experimentally determined signal is compared with T -dependent ab initio powder averaged lattice dynamics calculations. An additional set of T -dependent spectra is calculated with thermal displacement parameters extracted from neutron diffraction data within the incoherent approximation. G(ω,T ) shows a nonuniform intensity variation with T due to the specific thermal displacement parameters of Yb, Fe, and Sb. We demonstrate that this temperature variation can be well reproduced by the two simulation approaches. In particular, the ab initio powder averaged lattice dynamics calculation is capable of reproducing the momentum and energy characteristics of the phonon form factors of YbFe4Sb12. [ABSTRACT FROM AUTHOR]

A microscopic magnetic model of the low-pressure modification of CdVO3 is established, based on density functional theory (DFT) band-structure calculations, magnetization measurements, and quantum Monte Carlo simulations. This compound is a rare example of a quasi-one-dimensional spin- 1/2 system showing exclusively ferromagnetic exchange. The spin lattice of CdVO3 entails zigzag chains with an effective intrachain coupling J ≃ -90 K and interchain couplings of Jc ≃ - 18 K and Ja ≃ -3 K. Quantum fluctuations are partially suppressed by the sizable interchain coupling Jc that leads to an intermediate regime between one-dimensional and two-dimensional ferromagnetic systems. Apart from the peculiar spin model, CdVO3 features an unusual mechanism of ferromagnetic superexchange. The couplings largely originate from Cd 5s states mediating hoppings between half-filled and empty 3d states of V+4. [ABSTRACT FROM AUTHOR]

The vibrational dynamics of the ternary Ca1-xFe4Sb12, Ba1-xFe4Sb12, Sr1-xFe4Sb12, and Yb1-xFe4Sb12 compounds is studied for its temperature dependence, dispersion relations, and material properties by experiments and ab initio powder-averaged lattice-dynamics (PALD) calculations. The experimental techniques used are high-resolution inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). Vibrational properties of polycrystalline material have been mapped out in an extensive energy-momentum phase space facilitating the study of powder-averaged phonon-dispersion relations within the first and second Brillouin zones. Qualitatively, the experimental dynamic structure factor S(Q,ω) matches, to a high degree, PALD calculations allowing different schemes of collective vibrational eigenmodes to be evaluated. From the PALD calculations, quantitative values of velocity of sound, Debye temperatures, and elastic, bulk, and Young's moduli are computed. The variation of the generalized density of states G(ω) with temperature is studied experimentally in the range from 2 to 600 K. No particular anomalies in the collective dynamics are observed apart from a global softening of G(ω) upon heating. [ABSTRACT FROM AUTHOR]

We report a systematic study of the influence of antiferromagnetic and ferromagnetic phases of Eu2+ moments on the superconducting phase upon doping the As site by isovalent P, which essentially acts like chemical pressure on EuFe2As2. Bulk superconductivity with transition temperatures of 22 and 28 K are observed for x=0.16 and 0.20 samples, respectively. The Eu ions order antiferromagnetically for x ≤ 0.13, while bulk superconductivity coexists with Eu-antiferromagnetism for 0.13 < x<0.22. In contrast, a crossover is observed for x≥0.22 whereupon the Eu ions order ferromagnetically and superconductivity is fully suppressed. Density-functional-theory-based calculations reproduce the observed experimental findings consistently. We discuss in detail the coexistence of superconductivity and magnetism in a tiny region of the phase space and comment on the competition of ferromagnetism and superconductivity in the title compound. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of single-crystalline and polycrystalline FeGa3 are systematically investigated over a wide temperature range. At low temperatures, below 20 K, previously not known pronounced peaks in the thermal conductivity (400-800 W K-1 m-1) with corresponding maxima in the thermopower (in the order of -16000 μV K-1) were found in single-crystalline samples. Measurements in single crystals along [100] and [001] directions indicate only a slight anisotropy in both the electrical and thermal transports. From susceptibility and heat-capacity measurements, a magnetic or structural phase transition was excluded. Using density functional theory based calculations, we have revisited the electronic structure of FeGa3 and compared the magnetic (including correlations) and nonmagnetic electronic densities of states. Thermopower at fixed carrier concentrations is calculated using semiclassical Boltzmann transport theory, and the calculated results match fairly with our experimental data. The inclusion of strong electron correlations treated in a mean field manner (by LSDA + U) does not improve this comparison, rendering strong correlations as the sole explanation for the low-temperature enhancement unlikely. Eventually, after a careful review, we assign the peaks in the thermopower as a manifestation of the phonon-drag effect, which is supported by thermopower measurements in a magnetic field. [ABSTRACT FROM AUTHOR]

A combined theoretical and experimental study of the natural Cu2+mineral callaghanite is presented. Its crystal structure features well separated Cu2(OH)6 structural dimers with weakly bonded carbonate groups and water molecules in between. Susceptibility, field-dependent magnetization and specific-heat measurements reveal a compound with a small spin gap of about 7 K. The observed magnetic properties are well described by a model of isolated antiferromagnetic spin dimers. Possible ferromagnetic interactions between the dimers amount to -1.5 K, at most. Different flavors of electronic structure calculations have been employed to locate the magnetic dimers in the crystal structure, i.e., to determine whether they coincide with the structural dimers or not. Calculations of the coupling between the structural dimers clearly show that magnetic and structural dimers are the same. For the intradimer coupling, however, the computational results confirmed a coupling strength close to zero but the sign of the coupling could not be determined unambiguously. Based on this finding, we then discuss how the reliability of the numerical methods depends on the characteristics of exchange pathways and on structural features of the compound in general. Eventually, we try to provide a minimum coupling strength that is needed for a reliable computational description. [ABSTRACT FROM AUTHOR]

We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba1-xKx(Fe1-yCoy)2As2 with x/2 ≈ y. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac-susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of x/2 ≈ y ⩽ 0.13, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low-temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for x/2 ≈ y ⩽ 0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum Tc of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x/2 ≈ y ⩾ 0.25, a tetragonal nonmagnetic ground state is observed. Our DFT calculations yield a significant reduction of the Fe 3d density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density. [ABSTRACT FROM AUTHOR]

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 120 K. 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 190 K 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 [ABSTRACT FROM AUTHOR]

Magnetic susceptibility measurements and a microscopic magnetic model of the mineral clinoclase Cu3(AsO4)(OH)3 are reported. This spin-1/2 material can be well described as a combination of two nonequivalent spin dimers with the sizable magnetic couplings of J≃700 K and JD2≃300 K but only small interdimer couplings. Based on density functional theory calculations, we pinpoint the location of dimers in the rather complex crystal structure. Counterintuitively, the largest exchange coupling operates between the structural Cu2O6 dimers. Additionally, we investigate magnetostructural correlations in Cu2O6 structural dimers by considering the influence of the hydrogen position on the magnetic coupling. To evaluate accurate exchange couplings, we establish the hydrogen positions that were not known so far and analyze the pattern of hydrogen bonding. [ABSTRACT FROM AUTHOR]

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-½ 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 nonmagnetic 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. [ABSTRACT FROM AUTHOR]

We apply neutron diffraction, high-resolution synchrotron x-ray diffraction, magnetization measurements, electronic structure calculations, and quantum Monte-Carlo simulations to unravel the structure and magnetism of (CuCl)LaTa²O7. Despite the pseudo-tetragonal crystallographic unit cell, this compound features an orthorhombic superstructure, similar to the Nb-containing (CuX)LaNb²O7 with X = CI and Br. The spin lattice entails dimers formed by the antiferromagnetic fourth-neighbor coupling J4, as well as a large number of nonequivalent interdimer couplings quantified by an effective exchange parameter Jeff. In (CuCl)LaTa²O7, the interdimer couplings are sufficiently strong to induce the long-range magnetic order with the Néel temperature TN ≃ 7 K and the ordered magnetic moment of 0.53μB, as measured with neutron diffraction. This magnetic behavior can be accounted for by Jeff/J4 ≃ 1.6 and J4 ≃ 1.6 K. We further propose a general magnetic phase diagram for the (CuCl)LaNb²O7-type compounds, and explain the transition from the gapped spin-singlet (dimer) ground state in (CuCl)LaNb²O7 to the long-range antiferromagnetic order in (CuCl)LaTa²O7 and (CuBr)LaNb²O7 by an increase in the magnitude of the interdimer couplings Jeff/J4, with the (CuCl)LaM²O7 (M = Nb, Ta) compounds lying on different sides of the quantum critical point that separates the singlet and long-range-ordered magnetic ground states. [ABSTRACT FROM AUTHOR]

We report on 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&mgr;B, indicating strong quantum fluctuations inherent to this quasi-one-dimensional spin system. We reanalyze 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 the strong anisotropic reflection broadening observed with synchrotron powder x-ray diffraction on stoichiometric samples of CuNCN, as well as possible deviations from the ideal CuNCN stoichiometry. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuCl)LaNb2O7. the Br analog of the spin-gap quantum magnet (CuCl)LaNb2O7. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high-resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr)LaNb207. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the ab plane, their arrangement along the c direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the c lattice parameter that was not observed in (CuCl)LaNb207. (CuBr)LaNb207 undergoes long-range antiferromagnetic ordering at TN = 32 K, which is nearly 70% of the leading exchange coupling J4 ≃ 48 K. The Br compound does not show any experimental signatures of low-dimensional magnetism because the underlying spin lattice is three-dimensional. The coupling along the c direction is comparable to the couplings in the ab plane, even though the shortest Cu-Cu distance along c (11.69 &Adeg;) is threetimes larger than nearest-neighbor distances in the ab plane (3.55 &Adeg;). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the ab plane and antiparallel spins along c is verified experimentally and confirmed by the microscopic analysis. [ABSTRACT FROM AUTHOR]

Recently, we reported [M. Wagner et al., J. Mater. Res. 26, 1886 (2011)] transport measurements on the semiconducting intermetallic system Ruln3 and its substitution derivatives RuIn3-xAx (A = Sn, Zn). Higher values of the thermoelectric figure of merit (zT = 0.45) compared to the parent compound were achieved by chemical substitution. Here, using density functional theory based calculations, we report on the microscopic picture behind the measured phenomenon. We show in detail that the electronic structure of the substitution variants of the intermetallic system Ruln3-xAx (A = Sn, Zn) changes in a rigid-band-like fashion. This behavior makes possible the fine tuning of the substitution concentration to take advantage of the sharp peaklike features in the density of states of the semiconducting parent compound. Trends in the transport properties calculated using the semiclassical Boltzmann transport equations within the constant scattering time approximation are in good agreement with the former experimental results for Ruin3-xSnx. Based on the calculated thermopower for the &rgr;-doped systems, we reinvestigated the Zn-substituted derivative and obtained ZnO-free RuIn3-xZnx. The new experimental results are consistent with the calculated trend in thermopower and yield a large zT value of 0.8. [ABSTRACT FROM AUTHOR]

Based on density functional theory band-structure calculations, quantum Monte Carlo simulations, and high-field magnetization measurements, we address the microscopic magnetic model of BaAg2Cu[VO4]2 that was recently proposed as a spin-1/2 anisotropic triangular lattice system. We show that the actual physics of this compound is determined by a peculiar superposition of ferromagnetic and antiferromagnetic uniform spin chains with nearest-neighbor exchange couplings of J(1)a ⋍ --19 K and J(1)a ⋍ 9.5K, respectively. The two chains featuring different types of the magnetic exchange perfectly mimic the specific heat of a triangular spin lattice, while leaving a clear imprint on the magnetization curve that is incompatible with the triangular-lattice model. Both ferromagnetic and antiferromagnetic spin chains run along the crystallographic a direction, and slightly differ in the mutual arrangement of the magnetic CuO4 plaquettes and nonmagnetic VO4 tetrahedra. These subtle structural details are, therefore, crucial for the ferromagnetic or antiferromagnetic nature of the exchange couplings, and put forward the importance of comprehensive microscopic modeling for a proper understanding of quantum spin systems in transition-metal compounds. [ABSTRACT FROM AUTHOR]

In a joint experimental and theoretical study, we investigate the isostructural collapse from the ambient pressure tetragonal phase to a collapsed tetragonal phase for nonsuperconducting metallic SrFe2As2 and SrFe1.8Ru0.2As2. The crystallographic details have been studied using x-ray powder diffraction up to 20 GPa pressure in a diamond anvil cell. The structural phase transition occurs at 10 and 9 GPa for SrFe2As2 and SrFe1.8Ru0.2As2, respectively. The changes in the unit cell dimensions are highly anisotropic with a continuous decrease of the c lattice parameter with pressure, while the a-axis length increases until the transition to a collapsed tetragonal phase and then continues to decrease. Across the phase transition, we observe volume reductions of 5% and 4% for SrFe2As2 and SrFe1.8Ru0.2As2, respectively. We are able to discern that Ru substitution on the Fe site acts like "chemical pressure" to the system. Density-functional theory-based calculations of the electronic structure and electron localizability indicator are consistent with the experimental observations. Detailed analysis of the electronic structure in k space and real space reveals As 4pz interlayer bond formation as the driving force of the c/a collapse with a change in the As-As bond length of about 0.35 Å. [ABSTRACT FROM AUTHOR]

The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are investigated by low-temperature neutron powder diffraction and density functional theory band structure calculations. In spite of the random distribution of the Mn3+ and Fe3+ cations, this centrosymmetric compound undergoes a transition into an incommensurate antiferromagnetically ordered state below TN≃220 K. The magnetic structure is characterized by the propagation vector k=[0,β,0] with β≃0.14 and the P221211′(0β0)0s0s magnetic superspace symmetry. It comprises antiferromagnetic helixes propagating along the b axis. The magnetic moments lie in the ac plane and rotate about π(1+β)≃204.8-deg angle between the adjacent magnetic atoms along b. The spiral magnetic structure arises from the peculiar frustrated arrangement of exchange couplings in the ab plane. The antiferromagnetic coupling along the c axis cancels the possible electric polarization and prevents ferroelectricity in BiMnFe2O6. [ABSTRACT FROM AUTHOR]

We report on the crystal structure and magnetic behavior of the spin-1/2 compound AgVOAsO4. Magnetic susceptibility, high-field magnetization, and electron spin resonance measurements identify AgVOAsO4 as a gapped quantum magnet with a spin gap Δ ≃ 13 K and a saturation field μ0Hs ≃ 48.5 T. Extensive band structure calculations establish the microscopic magnetic model of spin chains with alternating exchange couplings J ≃ 40 K and J' ≃ 26 K. However, the precise evaluation of the spin gap emphasizes the role of interchain couplings which are frustrated due to the peculiar crystal structure of the compound. The unusual spin model and the low energy scale of the exchange couplings make AgVOAsO4 a promising candidate for an experimental investigation of Bose-Einstein condensation in high magnetic fields. [ABSTRACT FROM AUTHOR]

We report the crystal structure, magnetization measurements, and band-structure calculations for the spin-1/2 quantum magnet CaCu2(SeO3)2Cl2. The magnetic behavior of this compound is well reproduced by a uniform spin-1/2 chain model with the nearest-neighbor exchange of about 133 K. Due to the peculiar crystal structure, spin chains run in the direction almost perpendicular to the structural chains. We find an exotic regime of frustrated interchain couplings owing to two inequivalent exchanges of 10 K each. Peculiar superexchange paths grant an opportunity to investigate bond-randomness effects under partial Cl-Br substitution. [ABSTRACT FROM AUTHOR]

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 a reduced sublattice magnetization of 0.62 μB and a pitch angle of 84°, both renormalized by quantum effects. The proposed spiral ground state of Cu2GeO4 opens a way to magnetoelectric effects in this compound. [ABSTRACT FROM AUTHOR]

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh2As2 (SC transition temperature TSC∼0.37 K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism TN within an SC phase, with TN

We have investigated the magnetic ground state of CuFeAs and CuFeSb by means of 57Fe -Mössbauer spectroscopy, muon spin rotation/relaxation (μSR), neutron diffraction, and electronic structure calculations. Both materials share the 111-LiFeAs crystal structure and are closely related to the class of iron-based superconductors. In both materials there is a considerable occupancy of the Cu site by Fe, which leads to ferromagnetic moments, which are magnetically strongly coupled to the regular Fe site magnetism. Our study shows that CuFeAs is close to an antiferromagnetic instability, whereas a ferromagnetic ground state is observed in CuFeSb, supporting theoretical models of anion height driven magnetism. [ABSTRACT FROM AUTHOR]

In this joint experimental and theoretical work magnetic properties of the Cu2+ mineral szenicsite Cu3(MoO4)(OH)4 are investigated. This compound features isolated triple chains in its crystal structure, where the central chain involves an edge-sharing geometry of the CuO4 plaquettes, while the two side chains feature a corner-sharing zigzag geometry. The magnetism of the side chains can be described in terms of antiferromagnetic dimers with a coupling larger than 200 K. The central chain was found to be a realization of the frustrated antiferromagnetic J1-J2 chain model with J1~68 K and a sizable second-neighbor coupling J2. The central and side chains are nearly decoupled owing to interchain frustration. Therefore, the low-temperature behavior of szenicsite should be entirely determined by the physics of the central frustrated J1-J2 chain. Our heat-capacity measurements reveal an accumulation of magnetic entropy at low temperatures and suggest a proximity of the system to the Majumdar-Ghosh point of the antiferromagnetic J1-J2 spin chain, J2/J1=0.5. [ABSTRACT FROM AUTHOR]

Cuprate materials, such as those hosting high-temperature superconductivity, represent a famous class of materials where the correlations between the strongly entangled charges and spins produce complex phase diagrams. Several years ago, the Zhang-Rice singlet was proposed as a natural quasiparticle in hole-doped cuprates. The occurrence and binding energy of this quasiparticle, consisting of a pair of bound holes with antiparallel spins on the same CuO4 plaquette, depends on the local electronic interactions, which are fundamental quantities for understanding the physics of the cuprates. Here, we employ state-of-the-art resonant inelastic x-ray scattering (RIXS) to probe the correlated physics of the CuO4 plaquettes in the quasi-one-dimensional chain cuprate Li2CuO2. By tuning the incoming photon energy to the O K edge, we populate bound states related to the Zhang-Rice quasiparticles in the RIXS process. Both intra- and interchain Zhang-Rice singlets are observed and their occurrence is shown to depend on the nearest-neighbor spin-spin correlations, which are readily probed in this experiment. We also extract the binding energy of the Zhang-Rice singlet and identify the Zhang-Rice triplet excitation in the RIXS spectra. [ABSTRACT FROM AUTHOR]

Spatial inversion symmetry in crystal structures is closely related to the superconducting (SC) and magnetic properties of materials. Recently, several theoretical proposals that predict various interesting phenomena caused by the breaking of the local inversion symmetry have been presented. However, experimental validation has not yet progressed owing to the lack of model materials. Here we present evidence for antiferromagnetic (AFM) order in CeRh&#95;{2}As&#95;{2} (SC transition temperature T&#95;{SC}∼0.37  K), wherein the Ce site breaks the local inversion symmetry. The evidence is based on the observation of different extents of broadening of the nuclear quadrupole resonance spectrum at two crystallographically inequivalent As sites. This AFM ordering breaks the inversion symmetry of this system, resulting in the activation of an odd-parity magnetic multipole. Moreover, the onset of antiferromagnetism T&#95;{N} within an SC phase, with T&#95;{N}

The natural mineral azurite Cu(3)(CO(3))(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material., (© 2011 American Physical Society)

Essentially all undoped cuprates exhibit a quasiplanar, fourfold Cu-O coordination responsible for the magnetically active antibonding 3d(x(2)-y(2)) like state. Here, we present an electronic structure study for CuSb(2)O(6) that reveals, in contrast, a half-filled 3d(3z(2)-r(2)) orbital. This hitherto unobserved ground state originates from a competition of in- and out-of-plaquette orbitals where the strong Coulomb repulsion drives the surprising and unique orbital ordering. This, in turn, gives rise to an unexpected quasi-one-dimensional magnetic behavior. Our results provide a consistent explanation of recent thermodynamical and neutron diffraction measurements.

Cs2AgF4 was proposed to be an orbitally ordered ferromagnet based on recent neutron scattering data. Here, we report a detailed electronic structure study within the local spin density approximation also including strong Coulomb repulsion U. We investigate the influence of an orthorhombic distortion of the Ag environment and the importance of the on-site Coulomb repulsion. We find good quantitative agreement with both the experimentally observed exchange coupling and structural changes. Thus, our results strongly support that Cs2AgF4 is a strongly correlated charge-transfer insulator where the ferromagnetism is driven by orbital order.