The properties of magnetic insulators containing orbitally degenerate transition metal ions (Jahn-Teller ions) are discussed. Jahn-Teller effect in these causes structural phase transitions, lowers the lattice symmetry, and gives rise to an orbital ordering. Various interactions responsible for effects discussed: electron-lattice, quadrupole-quadrupole, exchange interactions. mutual ordering corresponding compounds interaction cases twofold threefold degeneracy is a field on structure...

УСПЕХИ ФИЗИЧЕСКИХ НАУК538.1 ЭФФЕКТ ЯНА-ТЕЛЛЕРА И МАГНЕТИЗМ: СОЕДИНЕНИЯ ПЕРЕХОДНЫХ МЕТАЛЛОВ К. И. Кугель, Д.

Mechanisms of electron phase separation in manganite-type oxide materials are analyzed using a simple Kondo-lattice model with intersite Coulomb repulsion between electrons. This predicts the instability magnetic (or charge) homogeneous ordering toward formation droplet structures (magnetic polarons) for wide parameter range diagram. Various types polarons examined. The transport properties and noise spectrum phase-separated also discussed.

The discovery of a flat two-dimensional crystal known as graphene has contradicted Landau–Peierls–Mermin–Wagner arguments that there is no stable form such crystals. Here, we show the "flat" shape arises due to microscopic buckling at smallest possible interatomic scale. We graphene, silicene, and other crystals are transverse short-range displacements appropriate atoms. distortions small various patterns, which describe in framework Ising model with competing interactions. when temperature...

Specific features of orbital and spin structure transition-metal compounds in the case face-sharing ${\text{MO}}_{6}$ octahedra are analyzed. In this geometry, we consider form spin-orbital Hamiltonian for ions with double $({e}_{g}^{\ensuremath{\sigma}})$ or triple $({t}_{2g})$ degeneracy. Trigonal distortions typical structures lead to splitting ${t}_{2g}$ orbitals into an ${a}_{1g}$ singlet ${e}_{g}^{\ensuremath{\pi}}$ doublet. For both doublets (${e}_{g}^{\ensuremath{\sigma}}$...

In the framework of a two-band model, we study phase-separation regime different kinds strongly correlated charge carriers as function energy splitting between two sets bands. The narrow (wide) band simulates more localized (more delocalized) type carriers. By assuming that internal chemical pressure on ${\text{CuO}}_{2}$ layer due to interlayer mismatch controls states, theoretical predictions are able reproduce phase separation at doping higher than 1/8 in experimental...

We study quasiparticle dynamics in graphene exposed to a linearly polarized electromagnetic wave of very large intensity. demonstrate that low-energy transport such system can be described by an effective time-independent Hamiltonian, characterized multiple Dirac points the first Brillouin zone. Around each point spectrum is anisotropic: velocity along polarization radiation significantly exceeds perpendicular direction. Moreover, some transverse oscillates as function find conductance p-n...

The electrical resistivity of epitaxial $({\mathrm{La}}_{0.5}{\mathrm{Pr}}_{0.5}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ films deposited on single crystalline ${\mathrm{LaAlO}}_{3}$ substrates was studied at temperature and magnetic fields ranging from 4.2 to 300 K 0 3 T, respectively. On cooling room zero field, the demonstrate first behavior typical charge-ordered (CO) insulating state, whereas below 40 they undergo transition a metal-like state with slowly decreasing resistivity....

The role of elastic interactions between Jahn-Teller ions in formation various orbital- and charge-ordered structures manganites related compounds is analyzed. It shown that such alone are often sufficient to reproduce the observed different regions phase diagram. A special attention focused on stripe at high doping levels.

The arrested nanoscale phase separation in a two-band Hubbard model for strongly correlated charge carriers is shown to occur particular range vicinity of the topological Lifshitz transition, where Fermi energy crosses bottom narrow band and new sheet surface related second comes into play. We determine diagram this as function two variables, carrier density shift between chemical potential band. In diagram, we first line quantum critical points transition find criteria electronic resulting...

We study an interplay between the orbital degeneracy and spin-orbit coupling giving rise to spin-orbital entangled states. As a specific example, we analyze interaction of electrons occupying triply degenerate single-ion $t_{2g}$ levels with trigonal vibronic modes (the $t\otimes T$ problem). A more general problem electron-lattice involving both tetragonal vibrations is also considered. It shown that result such crucially depends on occupation leading either suppression or enhancement...

The phase diagram for doped manganites and related compounds is analyzed in terms of the Kondo-lattice model taking into account an interplay between electrons localized due to lattice distortions those band states. It shown that number itinerant charge carriers can be significantly lower than implied by doping level. competition homogeneous (ferromagnetic or antiferromagnetic) phase-separated states discussed a strong tendency separation was revealed wide range.

A two-band model is used to study the phase separation in systems with different kinds of strongly correlated charge carrier, a special emphasis on cuprate superconductors near optimum doping. We show that such system can decompose into two metallic-like phases more and less localized carriers. This controlled by energy splitting between bands. In superconductors, this be related internal chemical pressure CuO2 layer due interlayer mismatch. The interplay surface nanoscale inhomogeneities...

Half-metallicity (full spin polarization of the Fermi surface) usually occurs in strongly correlated electron systems. We demonstrate that doping a spin-density wave insulator weak-coupling regime may also stabilize half-metallic states. In absence doping, is formed by four nested bands [i.e., each band characterized charge (electron or hole) and (up down) labels]. Of these bands, only two accumulate carriers introduced forming two-valley surface. Depending on parameters, polarizations...

A simple model is proposed to calculate the resistivity, magnetoresistance, and noise spectrum in nonmetallic phase-separated manganites containing small metallic droplets (magnetic polarons). The system taken be far from percolation transition into a state. It assumed that charge transfer occurs due electron tunneling one droplet another through insulating medium. As result of this tunneling, acquire or lose extra electrons forming metastable two-electron empty states. In framework model,...

A mechanism for electronic phase separation in iron pnictides is proposed. It based on the competition between commensurate and incommensurate spin-density-wave phases a system with an imperfect doping-dependent nesting of multi-sheeted Fermi surface. We model surface by two elliptical electron pockets three circular hole pockets. The interaction charge carrier band leads to formation order. spin density wave parent compound transforms when doping introduced. show that, certain parameter...

Abstract The anomalous Hall effect (AHE) arises from the interplay of spin-orbit interactions and ferromagnetic order is a potentially useful probe electron spin polarization, especially in nanoscale systems where direct measurement not feasible. While AHE rather well-understood metallic ferromagnets, much less known about relevance different physical mechanisms governing insulators. As insulators, but metals, lend themselves to gate-control understanding insulating state valuable point view...

We report the first experimental observation of superconductivity in Cd$_3$As$_2$ thin films without application external pressure. Surface studies suggest that observed transport characteristics are related to polycrystalline continuous part investigated with homogeneous distribution elements and Cd-to-As ratio close stoichiometric Cd$_3$As$_2$. The latter is also supported by Raman spectra studied films, which similar those single crystals. formation superconducting phase under study...

The effect of 16O-18O isotope exchange on the electric resistivity was studied for (La(1-y)Pr(y))0.7Ca0.3MnO3 ceramic samples. Depending y, this mixed perovskite exhibited different types low-temperature behavior ranging from ferromagnetic metal (FM) to charge ordered (CO) antiferromagnetic insulator. It found that at y=0.75, substitution 16O by 18O results in reversible transition a FM CO insulator zero magnetic field. applied field (H >= 2 T) transformed sample with again metallic state...