arXiv subjects

V. Yu. Irkhin

Publications and source records attributed to V. Yu. Irkhin.

124 records · Page 7Linked to original sources

arXiv1999

Electron spectrum, thermodynamics and transport in antiferromagnetic metals at low temperatures

Electron spectrum of 2D and 3D antiferromagnetic metals is calculated with account of spin-fluctuation corrections within perturbation theory in the s-f exchange model. Effects of the interaction of conduction electrons with spin waves in thermodynamic and transport properties are investigated. At lowest temperatures $T T^{*}, in 2D and ``nested'' 3D systems the picture corresponds to a marginal Fermi liquid (TlnT-contributions to specific heat and nearly T-linear dependence of resistivity). Frustrations in the spin system in the 3D case are demonstrated to lead to similar results. The Kondo contributions to electronic properties are analyzed and demonstrated to be strongly suppressed. The incoherent contributions to transport properties in the presence of impurity scattering are considered. In particular, in the 2D case T-linear terms in resistivity are present up to T=0, and thermoelectric power demonstrates the anomalous T\lnT-dependence.

cond-mat.str-el↗

arXiv1999

Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics

A scaling consideration of the Kondo lattices is performed with account of singularities in the spin excitation spectral function. It is shown that a non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$ coupling constant occurs naturally for complicated magnetic structures with several magnon branches. This may explain the fact that a NFL behavior takes place often in the heavy-fermion systems with peculiar spin dynamics. Another kind of a NFL-like state (with different critical exponents) can occur for simple antiferromagnets with account of magnon damping, and for paramagnets, especially with two-dimensional character of spin fluctuations. The mechanisms proposed lead to some predictions about behavior of specific heat, resistivity, magnetic susceptibility, and anisotropy parameter, which can be verified experimentally.

cond-mat.str-el↗

arXiv1999

Calculation of Neel temperature for S=1/2 quasi-one-dimensional Heisenberg antiferromagnets

Isotropic S=1/2 quasi-one-dimensional antiferromagnets are considered within the bosonization method. The 1/zp-corrections to the interchain mean-field theory (zp is the number of nearest neighbors in transverse to chain directions) are obtained for the ground-state sublattice magnetization and Neel temperature T_N. The corrections to T_N make up about 25% of mean-feld value, while those to ground-state sublattice magnetization are small enough (especially in the three-dimensional case). The fluctuation corrections obtained improve considerably the agreement with the experimental data for magnetic-chain compounds KCuF3, Sr2CuO3 and Ca2CuO3.

cond-mat↗

arXiv1999

Orientational phase transitions in anisotropic rare-earth magnets at low temperatures

Orientational phase transitions are investigated within the Heisenberg model with single-site anisotropy. The temperature dependence of the cone angle is calculated within the spin-wave theory. The role of the quantum renormalizations of anisotropy constants is discussed. A comparison with the experimental data on the cone-plane orientational transition in holmium is performed.

cond-mat↗

arXiv1999

Scaling theory of magnetic ordering in the Kondo lattices with anisotropic exchange interactions

The lowest-order scaling consideration of the magnetic state formation in the Kondo lattices is performed within the $s-f$ model with inclusion of anisotropy for both the $f-f$ coupling and $s-f$ exchange interaction. The Kondo renormalizations of the effective transverse and longitudinal $s-f$ coupling parameters, spin-wave frequency, gap in the magnon spectrum and ordered moment are calculated in the case of both ferro- and antiferromagnets. The anisotropy-driven change of the scaling behavior (e.g., critical value of $g$ for entering the strong-coupling region and the corresponding critical exponents) is investigated numerically for N=2 and analytically in the large-N limit. The dependence of the effective Kondo temperature on the bare $s-f$ coupling parameter $g$ weakens in the presence of anisotropy. The relative anisotropy parameters for both the $s-f$ and $f-f$ coupling are demonstrated to decrease during the renormalization process. The role of next-nearest exchange interactions for this effect in the antiferromagnet is discussed.

cond-mat.str-el↗

arXiv1999

On the self-consistent spin-wave theory of layered Heisenberg magnets

The versions of the self-consistent spin-wave theories (SSWT) of two-dimensional (2D) Heisenberg ferro- and antiferromagnets with a weak interlayer coupling and/or magnetic anisotropy, that are based on the non-linear Dyson-Maleev, Schwinger, and combined boson-pseudofermion representations, are analyzed. Analytical results for the temperature dependences of (sublattice) magnetization and short-range order parameter, and the critical points are obtained. The influence of external magnetic field is considered. Fluctuation corrections to SSWT are calculated within a random-phase approximation which takes into account correctly leading and next-leading logarithmic singularities. These corrections are demonstrated to improve radically the agreement with experimental data on layered perovskites and other systems. Thus an account of these fluctuations provides a quantitative theory of layered magnets.

cond-mat.str-el↗

arXiv1998

Charge screening and magnetic anisotropy in RCo5 compounds

An analysis of magnetic anisotropy in RCo5 compounds is performed with account of screening of ion point charges by conduction electrons. A crucial role of non-uniform distribution of screening electrons (the terms containing derivatives of charge density) is demonstrated. Influence of anisotropy of screening, that is connected with the anisotropy of the Fermi surface, on the magnetic anisotropy sign is discussed.

cond-mat↗

arXiv1998

The Kosterlitz-Thouless and magnetic transition temperatures in layered magnets with a weak easy-plane anisotropy

The two-dimensional (2D) Heisenberg magnet with a weak easy-plane anisotropy is considered. A renormalization group (RG) analysis in this model is performed for both quantum and classical cases. A crossover from the Heisenberg to 2D XY model is discussed. The magnetic transition owing to the interlayer coupling is considered. Analytical results for the Kosterlitz-Thouless and Curie (Neel) temperatures are derived with account of two-loop corrections. The results are compared with experimental data, e.g. on K2CuF4, and turn out to provide a quantitative description, unlike the standard one-loop results.

cond-mat↗

arXiv1998

Charge screening and magnetic anisotropy in metallic rare-earth systems

The calculation of magnetic anisotropy constants is performed beyond the point charge model for a continuous charge density distribution of screening conduction electrons. An important role of the non-uniform electron density, in particular, of the Friedel oscillations, in the formation of crystal field is demonstrated. Such effects can modify strongly the effective ion (impurity) charge and even change its sign. This enables one to justify the anion model, which is often used at discussing experimental data on hydrogen-containing systems. Possible applications to the pure rare-earth metals and RCo5 compounds are discussed. The deformation of magnetic structure near the interstitial positive muon owing to the strong local anisotropy, and the corresponding contribution to the dipole field at the muon are considered.

cond-mat↗

arXiv1998

Quantum phase transitions and thermodynamic properties in highly anisotropic magnets

The systems exhibiting quantum phase transitions (QPT) are investigated within the Ising model in the transverse field and Heisenberg model with easy-plane single-site anisotropy. Near QPT a correspondence between parameters of these models and of quantum phi^4 model is established. A scaling analysis is performed for the ground-state properties. The influence of the external longitudinal magnetic field on the ground-state properties is investigated, and the corresponding magnetic susceptibility is calculated. Finite-temperature properties are considered with the use of the scaling analysis for the effective classical model proposed by Sachdev. Analytical results for the ordering temperature and temperature dependences of the magnetization and energy gap are obtained in the case of a small ground-state moment. The forms of dependences of observable quantities on the bare splitting (or magnetic field) and renormalized splitting turn out to be different. A comparison with numerical calculations and experimental data on systems demonstrating magnetic and structural transitions (e.g., into singlet state) is performed.

cond-mat↗

arXiv1997

On the derivation of the t-J model: electron spectrum and exchange interactions in narrow energy bands

A derivation of the t-J model of a highly-correlated solid is given starting from the general many-electron Hamiltonian with account of the non-orthogonality of atomic wave functions. Asymmetry of the Hubbard subbands (i.e. of ``electron'' and ``hole''cases) for a nearly half-filled bare band is demonstrated. The non-orthogonality corrections are shown to lead to occurrence of indirect antiferromagnetic exchange interaction even in the limit of the infinite on-site Coulomb repulsion. Consequences of this treatment for the magnetism formation in narrow energy bands are discussed. Peculiarities of the case of ``frustrated'' lattices, which contain triangles of nearest neighbors, are considered.

cond-mat.str-el↗

arXiv1997

Thermodynamics of isotropic and anisotropic layered magnets: renormalization group approach and 1/N expansion

The O(N) model of layered antiferro- and ferromagnets with a weak interlayer coupling and/or easy-axis anisotropy is considered. A renormalization group (RG) analysis in this model is performed, the results for N=3 being expected to agree with those of the 1/M expansion in the CP^{M-1} model at M=2. The quantum and classical cases are considered. A crossover from an isotropic 2D-like to 3D Heisenberg (or 2D Ising) regime is investigated within the 1/N expansion. Analytical results for the temperature dependence of the (sublattice) magnetization are obtained in different regimes. The RG results for the ordering temperature are derived. In the quantum case they coincide with the corresponding results of the 1/N expansion. The numerical calculations on the base of the equations obtained yield a good agreement with experimental data on the layered perovskites La2CuO4, K2NiF4 and Rb2NiF4, and the Monte Carlo results for the anisotropic classical systems.

cond-mat↗

arXiv1997

Scaling picture of magnetism formation in the anomalous f-systems: interplay of the Kondo effect and spin dynamics

Formation of magnetically ordered state in the Kondo lattices is treated within the degenerate $s-f$ exchange and Coqblin-Schrieffer models. The Kondo renormalizations of the effective coupling parameter, magnetic moment and spin excitation frequencies are calculated within perturbation theory. The results of one-loop scaling consideration of the magnetic state in Kondo lattices are analyzed. The dependence of the critical values of the bare model parameters on the type of the magnetic phase and space dimensionality is investigated. Renormalization of the effective Kondo temperature by the interatomic exchange interactions is calculated. An important role of the character of spin dynamics (existence of well-defined magnon excitations, presence of magnetic anisotropy etc.) is demonstrated. The regime of strongly suppressed magnetic moments, which corresponds to magnetic heavy-fermion system, may occur in a rather narrow parameter region only. At the same time, in the magnetically ordered phases the renormalized Kondo temperature depends weakly on the bare coupling parameter in some interval. The critical behavior, corresponding to the magnetic transition with changing the bare $s-f$ coupling parameter, is investigated. In the vicinity of the strong coupling regime, the spectrum of the Bose excitations becomes softened. Thus on the borderline of magnetic instability the Fermi-liquid picture is violated in some temerature interval due to scattering of electrons by these bosons. This may explain the fact that a non-Fermi-liquid behavior often takes place in the heavy-fermion systems near the onset of magnetic ordering.

cond-mat.str-el↗

arXiv1997

1/N Expansion for Critical Exponents of Magnetic Phase Transitions in CP^{N-1} Model at 2<d<4

Critical exponents in the CP^{N-1} model, which describes localized-moment ferro- and antiferromagnets (N=2 in the Heisenberg model), are calculated from two-particle Green's functions to first order in 1/N. For d=2+εthe results agree with earlier renormalization group calculations. For d=3 the leading 1/N-corrections turn out to be very large at N=2. For d=4-εthe 1/N-corrections are small at any N and insufficient to describe correctly the magnetic phase transition.

cond-mat↗

arXiv1997

Critical behavior and the Neel temperature of quantum quasi-two-dimensional Heisenberg antiferromagnets

The nonlinear sigma-model and its generalization on N-component spins, the O(N) model, are considered to describe thermodynamics of a quantum quasi-two-dimensional (quasi-2D) Heisenberg antiferromagnet. A comparison with standard spin-wave approaches is performed. The sublattice magnetization, Neel temperature and spin correlation function are calculated to first order of the 1/N-expansion. A description of crossover from a 2D-like to 3D regime of sublattice magnetization temperature dependence is obtained. The values of the critical exponents derived are β=0.36, η=0.09. An account of the corrections to the standard logarithmic term of the spin-wave theory modifies considerably the value of the Neel temperature. The thermodynamic quantities calculated are universal functions of the renormalized interlayer coupling parameter. The renormalization of interlayer coupling parameter turns out to be considerably temperature dependent. A good agreement with experimental data on La2CuO4 is obtained. The application of the approach used to the case of a ferromagnet is discussed.

cond-mat↗

arXiv1997

On the Self-Consistent Spin-Wave Theory of Two-Dimensional Magnets With Impurities

The self-consistent spin-wave theory is applied to investigate the magnetization distribution around the impurity in isotropic and easy-axis two-dimensional ferro- and antiferromagents. The temperature dependences of host magnetization disturbance and impurity magnetization are calculated. The short-range order in the isotropic case is investigated. Importance of dynamic and kinematic interactions of spin waves is demonstrated.

cond-mat↗