The chemical influence in the phase separation phenomenon that occurs in perovskite manganites is discussed by means of ab initio calculations. Supercells have been used to simulate a phase separated state, that occurs at Ca concentrations close to the localized itinerant crossover. We have first considered a model with two types of magnetic ordering coexisting within the same compound. This is not stable. However, a non-isotropic distribution of chemical dopants is found to be the ground state. This leads to regions in the system with different effective concentrations, that would always accompany the magnetic phase separation at the same nanometric scale, with hole-rich regions being more ferromagnetic in character and hole-poor regions being in the antiferromagnetic region of the phase diagram, as long as the system is close to a phase crossover.

The series of V spinels [A2+] V2 O4 (A = Cd, Mn, Zn, Mg) provides an opportunity to tune the V-V distance continuously, in the frustrated pyrochlore lattice of the spinel. This system has been shown to approach the metallic state when V-V distance is reduced. The proximity to the transition leads to a dimerized structure in ZnV2 O4 caused by lattice instabilities. A different manner to tune the V − V distance of this structure is to fix the A2+ cation (in our case, Zn) and apply pressure. We have analyzed the evolution of the electronic structure of the system in the dimerized state. Such structure prevents the system to present a metallic phase at moderate pressures. We have also calculated the transport properties in a semiclassical approach based on Boltzmann transport theory. Our results support the validity of this structural distortion by providing a nice fit with experimental measurements.

We have synthesized Sr6Co5O15, a quasi-one-dimensional oxide, measured its magnetic properties, and calculated its electronic structure by ab initio techniques. We have found strong evidence for its electronic and magnetic behavior not to follow the trend of its structural series. The magnetic coupling inside the CoO3 chains is not purely ferromagnetic, and the long-range coupling inside the chains is very weak. The Co moments are slightly canted due to their large orbital angular momenta being oriented along each particular quantization axis, which is different for each Co4+ atom in the structure. Our thermopower calculations are in agreement with the experiment, supporting our model of the magnetic ground state of the compound.

The quasi-one-dimensional cobalt oxide Sr6Co5O15 is studied using first-principles electronic-structure calculations and Boltzmann transport theory. We have been able to describe the electronic structure, characterized by the structural one-dimensionality and a particular type of charge ordering, with unexpected electronic structure of the different Co atoms. The origin of the large unquenched misaligned orbital angular momenta comes out naturally from a correct description of the different crystal-field environments. The evolution with the on-site Coulomb repulsion (U) of the electronic structure and the transport properties is discussed, with a best agreement with experiment found for the smallest value of U that allows to converge the correct in-chain ferrimagnetic ground state.

Electronic structure calculations were performed on the compound CoS_2, an itinerant ferromagnet whose magnetic properties can be understood in terms of spin fluctuation theory. We have identified nesting features in the Fermi surface of the compound, active for long wavelength spin fluctuations. The electronic structure of the material is close to a half-metal. We show the importance of introducing spin-orbit coupling (SOC) in the calculations, that partially destroys the half-metallicity of the material. By means of transport properties calculations, we have quantified the influence of SOC in the conductivity at room temperature, with an important decrease comparing to the GGA alone conductivity. SOC also helps to understand the negative 0 of the material, whose conductivity varies by a few percent with the introduction of small perturbations in the states around the Fermi level.

Electronic structure calculations have been performed on the compound CoS2, an itinerant ferromagnet whose magnetic properties can be understood in terms of spin fluctuation theory. We have identified nesting features in the Fermi surface of the compound, active for long wavelength spin fluctuations. The electronic structure of the material is close to a half-metal. We show the importance of introducing spin–orbit coupling (SOC) in the calculations, which partially destroys the half-metallicity of the material. By means of transport properties calculations, we have quantified the influence of SOC in the conductivity at room temperature. Analyzing the effect of SOC helps in understanding the negative magnetoresistance of the material, whose conductivity varies within a few per cent with the introduction of small perturbations in the states around the Fermi level.

We have synthesized Sr_6Co_5O_15, a quasi-one-dimensional oxide, measured its magnetic properties and calculated its electronic structure by ab initio techniques. We have found strong evidence for its electronic and magnetic behavior not to follow the trend of its structural series. The magnetic coupling inside the CoO_3 chains is not purely ferromagnetic, the long-range coupling inside them is very weak. The Co moments are slightly canted due to their large orbital angular momenta being oriented along each particular quantization axis, that is different for each Co^4+ atom in the structure. Our thermopower calculations are in agreement with the experiment, supporting our model of the magnetic ground state of the compound.

We report a study of the dopant distribution in the lattice of La1-xCaxMnO3, at concentration x = 0.375, in the ferromagnetic region of the phase diagram. The symmetry of the compound allows for four possible inequivalent Ca2+ distributions. We calculate within the LDA+U approximation (by full-potential ab initio methods) the optimized geometries for each structure and the influence of the dopant homogeneity on the electronic structure of the material. We have analyzed the electronic structure for each of the distributions of Ca2+ considered, using the density of states and the partial Mn density of states. The system is half-metallic for all configurations, due to the large Hund's rule exchange on the Mn3+ cations. The analysis shows that the electronic structure remains largely invariant under different Ca2+ distributions in the lattice. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electronic structure calculations on the low dimensional spin-1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti3+ (d1) ions have one t2g orbital occupied (dyz) with a large hopping integral along the b direction of the crystal. The most important magnetic coupling is Ti-Ti along the b axis. The transition temperature (Tc) has a linear evolution with pressure, and at about 10 GPa this Tc is close to room temperature, leading to a room temperature spin-Peierls insulator-insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures, with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.

We report high-pressure diffraction and magnetization measurements to demonstrate that the partial collapse of electronic gap at high-pressure insulator to metal transition reported in TiOCl (C. Kuntscher et al. Phys. Rev. B 74 184402 (2006).) corresponds to a Ti3+–Ti3+ dimerization at room temperature within the space group P21/m. The shortest Ti–Ti distance is comparable to that of the Ti metal, but a Peierls-like distortion prevents a metallic behaviour.

Electronic structure calculations were performed for ZnV2O4, a material close to a metal-insulator transition. Structural optimization leads to the formation of V-V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained. (C) 2008 Elsevier B. V. All rights reserved.

We report x-ray diffraction and magnetization measurements under pressure combined with ab initio calculations to show that high-pressure TiOCl corresponds to an enhanced Ti3+-Ti3+ dimerized phase existing already at room temperature. Our results demonstrate the formation of a metal-metal bond between Ti3+ ions along the b axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures.

We report specific heat, resistivity and susceptibility measurements at different temperatures, magnetic fields, and pressures to provide solid evidence of CoS2 being a marginal Fermi liquid. The presence of a tricritical point in the phase diagram of the system provides an opportunity to test the spin fluctuation theory with a high limit of accuracy. A magnetic field suppresses the amplitude of the spin fluctuations and recovers conventional Fermi liquid behavior, connecting both states continuously.

Through a detailed study of the specific heat, resistivity, and susceptibility under pressure in polycrystalline and single-crystal CoS2, we demonstrate that the thermodynamic and transport properties of this system are governed by spin-density fluctuations. We give the correct (P, T) coordinates of its tricritical point and discuss the origin of the first-order magnetic phase transition in this system. Our results highlight the importance of having clean single crystals in order to extract definitive conclusions about the intrinsic properties of itinerant weak ferromagnets.

On the basis of experimental thermoelectric power results and ab initio calculations, we propose that a metal–insulator transition takes place at high pressure (approximately 6GPa) in MgV2O4.

Ab initio calculations analyze the properties of the G-type antiferromagnetic Sr2Co2O5 as a high-spin system. The description of the electronic structure is given; all the Co atoms are in a high-spin state with a small unquenched orbital angular momentum. Thermal analysis on a polycrystalline sample shows a magnetic phase transition at high temperatures.