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We report x-ray and optical absorption spectroscopy as well as hard x-ray photoemission spectroscopy on hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$ in which a Cu $3d$ spin-orbital liquid state is suggested from absence of the Jahn-Teller distortion of ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{6}$ octahedra and of the magnetic ordering. The experimental results and their cluster model analysis indicate that O $2p$ holes play a crucial role in realizing the spin-orbital liquid state of hexagonal ${\mathrm{Ba}}_{3}{\mathrm{CuSb}}_{2}{\mathrm{O}}_{9}$. These oxygen holes appear due to the ``reaction'' ${\mathrm{Sb}}^{5+}\ensuremath{\rightarrow}{\mathrm{Sb}}^{3+}\phantom{\rule{4pt}{0ex}}+$ two O $2p$ holes, with these holes being able to attach to Cu ions. The present work opens avenues towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.

We report on the synthesis and physical properties of cm-sized CoGeO3 single crystals grown in a high pressure mirror furnace at pressures of 80 bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with TN∼33.5 K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions, which reveals the presence of sizable orbital moments in CoGeO3.

Investigation of the recently synthesized series of isostructural compounds Ba4NbTM3O12 (TM=Mn, Rh, and Ir) with transition-metal trimers in a face-sharing geometry makes it possible to examine a tendency to the molecular orbitals (MO) formation going from 3d to 5d transition metal ions. Our ab initio calculations of electronic and magnetic properties describe the experimental findings and demonstrate gradual transition from the picture of localized electrons for Mn to the MO picture for Rh and especially for Ir. We also show that the often used criterion, according to which the metal-metal distance in a compound shorter than in a respective metal always gives MO, may break down in some cases. © 2020 American Physical Society. We are grateful to R.J. Cava for introducing us to trimer-based hexagonal perovskites and for various fruitful discussions. This research was supported by the Russian Foundation for Basic Researches (Grants No. RFBR 20-32-70019 and RFBR 20-32-90073) and the Russian Ministry of Science and High Education via program “Quantum” (Grant No. AAAA-A18-118020190095-4) and Contract No. 02.A03.21.0006 and the Deutche Forschungsgemeinschaft (DFG, German Reseach Foundation), Project No. 277146847-CRC 1238.

Significance It is shown that the long-standing puzzle of incommensurate crystal structure of A u T e 2 can be solved, if this material is considered as a negative charge-transfer system. Using modern computational methods, we demonstrate that charge redistribution associated with incommensurate modulations of crystal structure occurs not so much on Au, but predominantly on Te sites. This substantially reduces Coulomb energy costs for creating such a unique crystal structure. The same mechanism also explains superconductivity of doped A u T e 2 . Exploring different Au–Te compositions, we also discovered a previously unknown compound AuTe, which theoretically is very stable, and we predict its crystal structure.

In the present paper, the electronic, magnetic, and structural properties of a novel system FeZnMo3O8 with a polar crystal structure are investigated using GGA + U (Generalized gradient approximation taking into account Coulomb correlations) calculations. It is shown that Fe ions preferably occupy octahedral and Zn ions tetrahedral positions. This structural feature is caused by different ionic radii of these ions and not by the exchange coupling. The calculated exchange constants naturally explain magnetic structure observed in this material. © 2019, Pleiades Publishing, Inc.

To sum up, we show in the present paper that magnetic response in Nb$_2$O$_2$F$_3$ at the high-temperatures ($T>$90 K) is related to the orbital selective regime, when part of the electrons form molecular orbitals while other electrons have local magnetic moments. The charge disproportionation, which occurs at $T\sim$90 K is seen in the GGA calculations, but its degree ($\delta n \sim 0.1$ electron) is far from what one would expect from naive expectations based on the formal ionic valences. The mechanism of the charge ordering is argued to be related with a sizable kinetic energy gain due to formation of two molecular orbitals in short Nb$^{3+}$-Nb$^{3+}$ dimers caused by a strong nonlinearity of the distance dependence on electron hopping. We think that this mechanism of charge ordering, stabilized not by decrease of interaction energy, but rather by the gain in kinetic energy, may be operative in many other systems, especially consisting of structural dimers.

Recent discovery of the pyrite FeO2, which can be an important ingredient of the Earth’s lower mantle and which in particular may serve as an extra source of water in the Earth’s interior, opens new perspectives for geophysics and geochemistry, but this is also an extremely interesting material from physical point of view. We found that in contrast to naive expectations Fe is nearly 3+ in this material, which strongly affects its magnetic properties and makes it qualitatively different from well known sulfide analogue - FeS2. Doping, which is most likely to occur in the Earth’s mantle, makes FeO2 much more magnetic. In addition we show that unique electronic structure places FeO2 “in between” the usual dioxides and peroxides making this system interesting both for physics and solid state chemistry.

Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in $4d$ and $5d$ compounds. We argue that there is an important intersite effect -- an orbital-selective formation of covalent metal-metal bonds, which leads to an "exclusion" of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this novel mechanism analytically and numerically and show that it explains magnetic properties of not only several $4d-5d$ materials, including Nb$_2$O$_2$F$_3$ and Ba$_5$AlIr$_2$O$_{11}$, but can also be operative in $3d$ transition metal oxides, e.g. in CrO$_2$ under pressure. We also discuss the role of spin-orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behaviour.

A simple model of charge ordering is considered. It takes into account both the Coulomb repulsion at neighboring sites and the essential magnetic interactions. It is shown explicitly that at any deviation from half-filling ( n ≠1/2) the system is unstable with respect to phase separation into charge ordered regions with n =1/2 and metallic regions with smaller electron or hole density. Possible structure of this phase-separated state (metallic droplets in a charge-ordered matrix) is discussed. We estimate the parameters of these droplets and construct the phase diagram reproducing the main features observed in real manganites. The conductivity and noise spectrum for this phase-separated state are also discussed.

The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+ cations on cooling below 270 degrees C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a valence bond crystal, with a strong structural bond disproportionation. On warming, x-ray diffraction shows that discrete threefold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x rays show that disordered dimers survive at the nanoscale up to at least 650 degrees C. The high-temperature phase of Li2RuO3 is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analog of the resonating valence bond state often discussed in connection with high-T-c cuprates. (Less)

We show that in manganites close to half-doping, novel non-bipartite magnetic phases appear due to the interplay between double exchange, superexchange and orbital ordering. In considerable part of the phase diagram the ground state has a magnetic order that is intermediate between the canonical magnetic CE phase and a state that we identify as the recently observed Zener polaron state. The intermediate phase shows a type of charge ordering that breaks inversion symmetry and is therefore predicted to be ferroelectric.

We report on the observation of the Wohlleben effect (WE) in powders of Bi-based high-temperature superconductors consisting of isolated grains with sizes of order 1 mu m In these small grains the Josephson contacts with negative critical current (pi-junctions) generally involved to explain the WE must be able to carry a current density of order j approximate to 10(5)-10(6) A/cm(2) even close to T-c. Such high spontaneous currents can be explained, if such contacts are not ordinary tunneling Josephson junctions but superconductor-normal-metal-superconductor junctions and if the superconducting order parameter has d-wave symmetry.

A substitution of La3+ by Sr2+ in LaCoO3 induces holes in the low-spin ground state of the Co ions, which behave like magnetic impurities with a very high spin value (13 μB per hole). In this work, using single-crystal neutron spectroscopy, we prove that the charges introduced by strontium doping do not remain localized at the cobalt sites. Instead, each hole not only creates Co4+ in low-spin state, but it also transforms the six nearest neighboring Co3+ ions to the intermediate-spin state thereby forming a magnetic seven-site (heptamer) polaron. Spin-state polarons behave like magnetic nanoparticles embedded in an insulating nonmagnetic matrix. Therefore, lightly doped La1-xSrxCoO3 is a natural analog to artificial structures composed of ferromagnetic particles in insulating matrices.

The orbital structure of copper ions in high- T c cuprates is discussed in a localized picture. Experimental indications that the d z 2 -orbital becomes relevant, especially at high doping levels, are summarized. The possible mechanism of this effect, notably the crossing of d x 2 − y 2 and d z 2 -levels with doping, is identified and corresponding numerical calculations are carried out. It is shown that e.g. for La 2− x Sr x CuO 4 the crossing of these levels may occur at x ∼0.4. The consequences of this phenomenon for the stability of the system and for its superconductivity are briefly discussed.

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., Comment: to be published as a Topical Review in Journ.Magn.Magn.Mater

We report the multiferroic behaviour of MnWO$_4$, a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions MnWO$_4$ should exhibit ferroelectric polarization coexisting with the proper magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements., Comment: 5 pages, 3 figures

We consider the superstructures, which can be formed in spinels containing on B sites the transition-metal ions with partially filled t(2g) levels. We show that, when such systems are close to the itinerant state (e.g., have an insulator-metal transition), there may appear in them an orbitally driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that a similar phenomenon should be observed in NaTiO2 and possibly in some other systems.

Transition metal oxides with a perovskite-type structure constitute a large group of compounds with interesting properties. Among them are materials such as the prototypical ferroelectric system BaTiO(3), colossal magnetoresistance manganites and the high-T(c) superconductors. Hundreds of these compounds are magnetic, and hundreds of others are ferroelectric, but these properties very seldom coexist. Compounds with an interdependence of magnetism and ferroelectricity could be very useful: they would open up a plethora of new applications, such as switching of magnetic memory elements by electric fields. Here, we report on a possible way to avoid this incompatibility, and show that in charge-ordered and orbitally ordered perovskites it is possible to make use of the coupling between magnetic and charge ordering to obtain ferroelectric magnets. In particular, in manganites that are less than half doped there is a type of charge ordering that is intermediate between site-centred and bond-centred. Such a state breaks inversion symmetry and is predicted to be magnetic and ferroelectric.

The effect of partial 16 O→ 18 O isotope substitution on electrical and thermal resistivity and AC magnetic susceptibility was studied for (La 0.25 Pr 0.75 ) 0.7 Ca 0.3 MnO 3 ceramic samples. The measurements were performed for 11 samples with the molar percentage of 18 O varying from zero to 90%. For 0–40% enrichment by 18 O, the low-temperature resistivity had a metallic behavior. At higher 18 O content, the samples exhibited a percolation-like transition to the insulating state. Magnetic measurements demonstrated that the behavior typical of antiferromagnets arose only at 18 O enrichment exceeding 60%, whereas at lower 18 O content, we obtain an appreciable ferromagnetic contribution. The experimental data are interpreted in the picture of a percolation-type phase-separated system with the regions of ferromagnetic metal and antiferromagnetic charge ordered insulator.

An explanation is given for the charge order, orbital order and insulating state observed in half-doped manganese oxides, such as Nd$_{1/2}$Sr$_{1/2}$MnO$_{3}$. The competition between the kinetic energy of the electrons and the magnetic exchange energy drives the formation of effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological phase factor in the hopping, the chains are intrinsically insulating and orbital-ordered. Most surprisingly, the strong Coulomb interaction between electrons on the same Mn-ion leads to the experimentally observed charge ordering. For doping less than 1/2 the system is unstable towards phase separation into a ferromagnetic metallic and charge-ordered insulating phase., To appear in Phys. Rev. Lett., 4 pages, 4 figures

For certain ceramic samples of the Bi-based high-temperture superconductors, the dc-field-cooling signal becomes paramagnetic in fields below a few 100 mOe. This effect correlates with an anomaly in the low-field microwave absorption. The data are consistent with orbital paramagnetic moments due to the appearance of spontaneous supercurrents in fields smaller than the lower critical field ${\mathit{H}}_{\mathit{c}1\mathit{a}}$ parallel to the CuO planes.