arXiv subjects

K. I. Kugel

Publications and source records attributed to K. I. Kugel.

68 records · Page 4Linked to original sources

arXiv2006

Jahn-Teller distortions and phase separation in doped manganites

A "minimal model" of the Kondo-lattice type is used to describe a competition between the localization and metallicity in doped manganites and related magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant charge carriers can be significantly lower than that implied by the doping level x. A strong tendency to the phase separation is demonstrated for a wide range of intermediate doping concentrations vanishing at low and high doping. The phase diagram of the model in the x-T plane is constructed. At low temperatures, the system is in a state with a long-range magnetic order: antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS) state. At high temperatures, there can exist two types of the paramagnetic (PM) state with zero and nonzero density of the itinerant electrons. In the intermediate temperature range, the phase diagram includes different kinds of the PS states: AF-FM, FM-PM, and PM with different content of itinerant electrons. The applied magnetic field changes the phase diagram favoring the FM ordering. It is shown that the variation of temperature or magnetic field can induce the metal-insulator transition in a certain range of doping levels.

cond-mat.str-el↗

arXiv2004

The evolution with temperature of magnetic polaron state in an antiferromagnetic chain with impurities

The thermal behavior of a one-dimensional antiferromagnetic chain doped by donor impurities was analyzed. The ground state of such a chain corresponds to the formation of a set of ferromagnetically correlated regions localized near impurities (bound magnetic polarons). At finite temperatures, the magnetic structure of the chain was calculated simultaneously with the wave function of a conduction electron bound by an impurity. The calculations were performed using an approximate variational method and a Monte Carlo simulation. Both these methods give similar results. The analysis of the temperature dependence of correlation functions for neighboring local spins demonstrated that the ferromagnetic correlations inside a magnetic polaron remain significant even above the Néel temperature $T_N$ implying rather high stability of the magnetic polaron state. In the case when the electron-impurity coupling energy $V$ is not too high (for $V$ lower that the electron hopping integral $t$), the magnetic polaron could be depinned from impurity retaining its magnetic structure. Such a depinning occurs at temperatures of the order of $T_N$. At even higher temperatures ($T \sim t$) magnetic polarons disappear and the chain becomes completely disordered.

cond-mat.str-el↗

arXiv2003

Spin-dependent transport in phase-separated manganites

Starting from the assumption that ferromagnetically correlated regions exist in manganites even in the absence of long-range magnetic order, we construct a model of charge transfer due to the spin-dependent tunnelling of charge carriers between such regions. This model allows us to analyze the temperature and magnetic field dependence of resistivity, magnetoresistance, and magnetic susceptibility of phase-separated manganites in the temperature range corresponding to non-metallic behavior. The comparison of theoretical and experimental results reveals the main characteristics of the phase-separated state.

cond-mat.str-el↗

arXiv2003

Magnetic polarons in doped 1D antiferromagnetic chain

The structure of magnetic polarons (ferrons) is studied for an 1D antiferromagnetic chain doped by non-magnetic donor impurities. The conduction electrons are assumed to be bound by the impurities. Such a chain can be described as a set of ferrons at the antiferromagnetic background. We found that two types of ferrons can exist in the system. The ground state of the chain corresponds to the ferrons with the sizes of the order of the localization length of the electron near the impurity. The ferrons of the second type produce a more extended distortion of spins in the chain. They are stable within a finite domain of the system parameters and can be treated as excitations above the ground state. The ferrons in the excited states can appear in pairs only. The energy of the excited states decreases with the growth in density of impurities. This can be interpreted as a manifestation of an attractive interaction between ferrons.

cond-mat.str-el↗

arXiv2003

Nanoscale phase separation in manganites

We study the possibility of nanoscale phase separation in manganites in the framework of the double exchange model. The homogeneous canted state of this model is proved to be unstable toward the formation of small ferromagnetic droplets inside an antiferromagnetic insulating matrix. For the ferromagnetic polaronic state we analyze the quantum effects related to the tails of electronic wave function and a possibility of electron hopping in the antiferromagnetic background. We find that these effects lead to the formation of the threshold for the polaronic state.

cond-mat.str-el↗

arXiv2002

Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution

The effect of $^{16}$O $\to$ $^{18}$O isotope substitution on electrical resistivity and magnetic susceptibility of Sm$_{1-x}$Sr$_x$MnO$_3$ manganites is analyzed. It is shown that the oxygen isotope substitution drastically affects the phase diagram at the crossover region between the ferromagnetic metal state and that of antiferromagnetic insulator (0.4 $< x <$ 0.6), and induces the metal-insulator transition at for $x$ = 0.475 and 0.5. The nature of antiferromagnetic insulator phase is discussed.

cond-mat.str-el↗

arXiv2001

Elastic interaction and superstructures in manganites and other Jahn-Teller systems

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

cond-mat.str-el↗

arXiv2001

Why stripes? Spontaneous formation of inhomogeneous structures due to elastic interactions

We argue that elastic interactions between ions in different valence states can play an essential role in stabilization of stripes(or 2D "sheets")in doped oxides. These interactions are in general long-range and anisotropic (attractive in certain directions and repulsive in others). This can naturally give rise to stripe-like structures in insulating materials. We illustrate this general idea with certain specific examples and show that the situation can be described by the Ising model with anisotropic interactions. The case of anisotropic impurities, relevant e.g. for manganites, is also briefly discussed.

cond-mat.str-el↗

arXiv2001

Approximate Ginzburg-Landau solution for the regular flux-line lattice. Circular cell method

A variational model is proposed to describe the magnetic properties of type-II superconductors in the entire field range between $H_{c1}$ and $H_{c2}$ for any values of the Ginzburg-Landau parameter $κ>1/\sqrt{2}$. The hexagonal unit cell of the triangular flux-line lattice is replaced by a circle of the same area, and the periodic solutions to the Ginzburg-Landau equations within this cell are approximated by rotationally symmetric solutions. The Ginzburg-Landau equations are solved by a trial function for the order parameter. The calculated spatial distributions of the order parameter and the magnetic field are compared with the corresponding distributions obtained by numerical solution of the Ginzburg-Landau equations. The comparison reveals good agreement with an accuracy of a few percent for all $κ$ values exceeding $κ\approx 1$. The model can be extended to anisotropic superconductors when the vortices are directed along one of the principal axes. The reversible magnetization curve is calculated and an analytical formula for the magnetization is proposed. At low fields, the theory reduces to the London approach at $κ\gg 1$, provided that the exact value of $H_{c1}$ is used. At high fields, our model reproduces the main features of the well-known Abrikosov theory. The magnetic field dependences of the reversible magnetization found numerically and by our variational method practically coincide. The model also refines the limits of some approximations which have been widely used. The calculated magnetization curves are in a good agreement with experimental data on high-T$_c$ superconductors.

cond-mat.supr-con↗

arXiv2000

First integrals of Ginzburg-Landau equations and stability criteria for vortex-free state in unconventional superconductors

The first integrals of the Ginzburg-Landau equations for a vortex-free state of superconductors with different mixed symmetries of the order parameter are found. The general boundary conditions for the order parameter at the ideal interface between the superconductor and vacuum are derived. Based on these integrals and boundary conditions, we analyze the stability criteria for vortex-free state in unconventional superconductors. The threshold field above which the Abrikosov vortices can enter the superconductor is found to be higher or equal to the thermodynamic critical field for all states under study.

cond-mat.supr-con↗

arXiv2000

Resistivity and 1/f Noise in Non-Metallic Phase Separated Manganites

A simple model is proposed to calculate resistivity, magnetoresistance, and noise spectrum in non-metallic phase-separated manganites containing small metallic droplets (magnetic polarons). The system is taken to be far from the percolation transition into a metallic state. It is assumed that the charge transfer occurs due to electron tunneling from one droplet to another through the insulating medium. As a result of this tunneling, the droplets acquire or lose extra electrons forming metastable two-electron and empty states. In the framework of this model, explicit expressions for dc conductivity and noise power of the system are derived. It is shown that the noise spectrum has 1/f form in the low-frequency range.

cond-mat.str-el↗

arXiv2000

Phase separation in systems with charge ordering

A simple model of charge ordering is considered. It is shown explicitly that at any deviation from half-filling ($n \neq 1/2$) 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. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. An analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of manganites and other substances with charge ordering.

cond-mat.str-el↗

arXiv1998

Metal-insulator transition induced by 16O -18O oxygen isotope exchange in colossal negative magnetoresistance manganites

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 on y, this mixed perovskite exhibited different types of low-temperature behavior ranging from ferromagnetic metal (FM) to charge ordered (CO) antiferromagnetic insulator. It was found that at y=0.75, the substitution of 16O by 18O results in the reversible transition from a FM to a CO insulator at zero magnetic field. The applied magnetic field (H >= 2 T) transformed the sample with 18O again to the metallic state and caused the increase in the FM transition temperature Tc of the 16O sample. As a result, the isotope shift of Tc at H = 2 T was as high as 63 K. Such unique sensitivity of the system to oxygen isotope exchange, giving rise even to the metal-insulator transition, is discussed in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the CO and FM phases.

cond-mat.str-el↗

arXiv1998

Spin and orbital excitations in magnetic insulators with Jahn-Teller ions

The elementary excitations of a model Hamiltonian that captures the low energy behavior of a simple two-fold degenerate Hubbard Hamiltonian with Hund's rule coupling, is studied. The phase diagram in the mean-field limit and in a two-site approach reveals a rich variety of phases where both the orbital and the spin degrees of freedom are ordered. We show that besides usual spin waves (magnons) there exist also orbital waves (orbitons) and, most interestingly, in a completely ferromagnetically coupled system, a combined spin-orbital excitation which can be visualized as a bound state of magnons and orbitons. For a completely degenerate system the bound states are found to be the lowest lying elementary excitations, both in one- and two-dimensions. Finally we extend our treatment to almost-degenerate systems. This can serve as an example that the elementary excitations in orbitally degenerate strongly correlated electron systems in general carry both spin and orbital character.

cond-mat.str-el↗