Isotope effect and characteristic features of the phase diagram for cobaltites with spin-state transitions

The effect of ¹⁶O → ¹⁸O oxygen isotope substitution has been studied for (Pr_1−y Eu _y )_0.7Ca_0.3CoO₃ cobaltites (0.12 < y < 0.26). A pronounced isotope shift has been found for the spinstate transition temperature, which increases with the oxygen isotope mass. In contrast, the ferromagnetic transition temperature has slightly lower values for the samples with heavier oxygen. The observed phenomena and constructed phase diagram confirm the results reported previously for (Pr_{1 − y} Sm _y )_0.7Ca_0.3CoO₃ in [G. Y. Wang, X. H. Chen, T. Wu, et al., Phys. Rev. B 74, 165113 (2006)]. The measurements of the specific heat have been performed for (Pr_{1 − y} Eu _y )_0.7Ca_0.3CoO₃ with the main emphasis on the analysis of the isotope effect. The contributions to the isotope effect coming from the lattice and magnetic components of the specific heat have been separated. The mechanisms underlying the large isotope effect are discussed.     

Projective Hausdorff gaps

Todor?evi? (Fund Math 150(1):55–66, 1996) shows that there is no Hausdorff gap (A, B) if A is analytic. In this note we extend the result by showing that the assertion “there is no Hausdorff gap (A, B) if A is coanalytic” is equivalent to “there is no Hausdorff gap (A, B) if A is ${{\bf \it{\Sigma}}^{1}_{2}}$ ”, and equivalent to ${\forall r \; (\aleph_1^{L[r]}\,< \aleph_1)}$ . We also consider real-valued games corresponding to Hausdorff gaps, and show that ${\mathsf{AD}_\mathbb{R}}$ for pointclasses Γ implies that there are no Hausdorff gaps (A, B) if ${{\it{A}} \in {\bf \it{\Gamma}}}$ .     

Multiferroics: Different ways to combine magnetism and ferroelectricity

Multiferroics — materials which are simultaneously (ferro)magnetic and ferroelectric, and often also ferroelastic — attract now considerable attention, both because of the interesting physics involved and as they promise important practical applications. In this paper, I give a survey of microscopic factors determining the coexistence of these properties, and discuss different possible routes to combine them in one material. In particular, the role of the occupation of d-states in transition metal perovskites is discussed, possible role of spiral magnetic structures is stressed, and the novel mechanism of ferroelectricity in magnetic systems due to combination of site-centred and bond-centred charge ordering is presented. Microscopic nature of multiferroic behaviour in several particular materials, including magnetite Fe₃O₄, is discussed.     

Crystal field splitting in correlated systems with negative charge-transfer gap

Special features of the crystal field splitting of d-levels in the transition metal compounds with small or negative charge-transfer gaps Δ(CT) are considered. We show that in this case the Coulomb term and the covalent contribution to the t(2g)-e(g) splitting have different signs. In order to check theoretical predictions we carried out ab initio band structure calculations for Cs(2)Au(2)Cl(6), in which the charge-transfer gap is negative, so that the d-electrons predominantly occupy low-lying bonding states. For these states the e(g)-levels lie below the t(2g) ones, which demonstrates that at least in this case the influence of the p-d covalency on the total value of the crystal field splitting is stronger than the Coulomb interaction (which would lead to the opposite level order). We also show that the states in the conduction band are made predominantly of p-states of ligands (Cl), with a small admixture of d-states of Au.     

Hall Effect in a Doped Mott Insulator: DMFT Approximation

JETP Letters - In the framework of dynamical mean-field theory, we analyze the Hall effect in a doped Mott insulator as a parent cuprate superconductor. We consider the partial filling (hole...     

Spin-state transition in LaCoO3: direct neutron spectroscopic evidence of excited magnetic states

A gradual spin-state transition occurs in LaCoO3 around T approximately 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at approximately 0.6 meV appears, whose intensity increases with temperature, following the bulk magnetization. Within a model including crystal-field interaction and spin-orbit coupling, we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g approximately 3, the second interpretation is definitely favored.     

Thermoelectric power of HoBaCo2O5.5: possible evidence of the spin blockade in cobaltites

Thermoelectric power measurements have been performed for an ordered oxygen-deficient perovskite, HoBaCo2O5.5, in which the alternative layers of CoO6 octahedra and of [CoO(5)](2) bipyramids are occupied by Co3+ species. The T-dependent Seebeck coefficient S shows a clear change of the conduction regime at the metal-insulator (MI) transition (T(MI) approximately 285 K). The sign change of S from S<0 to S>0 can be explained assuming that a spin state transition occurs at T(MI). In the metallic state, Co2+ e(g) electrons are moving in a broad band on the background of high or intermediate spin Co3+ species. In contrast, the insulating behavior may result from the Co3+ spin state transition to a low-spin Co3+ occurring in the octahedra. In this phase the transport would occur by hopping of the low-spin Co(4+)t(2g) holes, whereas the high-spin Co2+ electrons become immobilized due to a spin blockade.     

Superconductivity in SnO: a nonmagnetic analog to Fe-based superconductors?

We discovered that under pressure SnO with α-PbO structure, the same structure as in many Fe-based superconductors, e.g., β-FeSe, undergoes a transition to a superconducting state for p?6 GPa with a maximum Tc of 1.4 K at p=9.3 GPa. The pressure dependence of Tc reveals a domelike shape and superconductivity disappears for p?16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.     

Insulator-metal transition in V3O5

The results of a study of the insulator-metal phase transition in V₃O₅ occuring at T = 450 K are presented. Conductivity, thermopower, magnetic, structural and optical data are reported. On a basis of the data obtained we conclude that the high-temperature phase is a poor metal with strong electron correlations and localized magnetic moments. The metal-non-metal transition is presumably driven by the spatial ordering of V³⁺ and V⁴⁺ ions.     

Temperature dependence of exchange integrals in magnetic insulators with Jahn-Teller ions

The exchange interaction in magnetic insulators with Jahn-Teller ions (Cu²⁺, Mn³⁺ etc.) is discussed on the basis of degenerate Hubbard model with phonons. It is shown that the interaction with a phonon mode lifting degeneracy modifies exchange and results, in particular, in reduction of exchange integrals with increasing temperature. This may account for the temperature dependence of exchange integrals, observed experimentally in K₂CuCl₄.2H₂O and in similar compounds.