arXiv subjects

V. Yu. Irkhin

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

At least 109 records · Page 6Linked to original sources

arXiv2005

Spin and orbital ferromagnetism in strongly correlated itinerant electron systems

Spectra of one-electron and collective excitations in narrow-band ferromagnets with unquenched orbital moments are calculated in various theoretical models. The interaction of spin and orbital excitations with conduction electrons results in the damping of the former which, however, turns out to be rather small; therefore, apart from usual spin waves, well-defined orbitons can exist. Non-quasiparticle states occur in the electron energy spectrum near the Fermi energy due to this interaction. The criteria of stability of the saturated spin and orbital ferromagnetic ordering are considered. Possible effects of orbital ordering in magnetite and in colossal magnetoresistance manganites are discussed.

cond-mat.str-el↗

arXiv2005

Non-quasiparticle states in the core level spectra of ferromagnetic semiconductors and half-metallic ferromagnets

The Green's functions that determine x-ray spectra are calculated in the s-d exchange model of a saturated conducting ferromagnet in the presence of the core hole. A possibility to observe non-quasiparticle (NQP) states in the core level (x-ray absorption, emission and photoelectron) spectroscopy is demonstrated. It is shown that NQP contributions to resonant x-ray scattering spectra can be considerably enhanced by core hole effects in comparison with those to the density of states.

cond-mat.str-el↗

arXiv2005

Sum rules for X-ray magnetic circular dichroism spectra in strongly correlated ferromagnets

It is proven that the sum rules for X-ray magnetic dichroism (XMCD) spectra that are used to separate spin and orbital contributions to the magnetic moment are formally correct for an arbitrary strength of electron-electron interactions. However, their practical application for strongly correlated systems can become complicated due to the spectral density weight spreading over a broad energy interval. Relevance of incoherent spectral density for the XMCD sum rules is illustrated by a simple model of a ferromagnet with orbital degrees of freedom.

cond-mat.str-el↗

arXiv2004

Kondo resonance for orbitally degenerate systems

Formation of the Kondo state in general two-band Anderson model has been investigated within the numerical renormalization group (NRG) calculations. The Abrikosov-Suhl resonance is essentially asymmetric for the model with one electron per impurity (quarter filling case) in contrast with the one-band case. An external magnetic (pseudo-magnetic) field breaking spin (orbital) degeneracy leads to asymmetric splitting and essential broadening of the many-body resonance. Unlike the standard Anderson model, the ``spin up'' Kondo peak is pinned against the Fermi level, but not suppressed by magnetic field.

cond-mat.str-el↗

arXiv2004

Anomalous self-energy and Fermi surface quasi-splitting in the vicinity of a ferromagnetic instability

We discuss the low-temperature behavior of the electronic self-energy in the vicinity of a ferromagnetic instability in two dimensions within the two-particle self-consistent approximation, functional renormalization group and Ward-identity approaches. Although the long-range magnetic order is absent at T>0, the self-energy has a non-Fermi liquid form at low energies w<Δ_0 near the Fermi level, where Delta_0 is the ground-state spin splitting. The spectral function at temperatures T<Delta_0 has a two-peak structure with finite spectral weight at the Fermi level. The simultaneous inclusion of self-energy and vertex corrections shows that the above results remain qualitatively unchanged down to very low temperatures T<<Delta_0. It is argued, that this form of the spectral functions implies the quasi-splitting of the Fermi surface in the paramagnetic phase in the presence of strong ferromagnetic fluctuations.

cond-mat.str-el↗

arXiv2004

Nonquasiparticle states in half-metallic ferromagnets

Anomalous magnetic and electronic properties of the half-metallic ferromagnets (HMF) have been discussed. The general conception of the HMF electronic structure which take into account the most important correlation effects from electron-magnon interactions, in particular, the spin-polaron effects, is presented. Special attention is paid to the so called non-quasiparticle (NQP) or incoherent states which are present in the gap near the Fermi level and can give considerable contributions to thermodynamic and transport properties. Prospects of experimental observation of the NQP states in core-level spectroscopy is discussed. Special features of transport properties of the HMF which are connected with the absence of one-magnon spin-flip scattering processes are investigated. The temperature and magnetic field dependences of resistivity in various regimes are calculated. It is shown that the NQP states can give a dominate contribution to the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity. First principle calculations of the NQP-states for the prototype half-metallic material NiMnSb within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are presented.

cond-mat.str-el↗

arXiv2004

Exciton effects in a scaling theory of intermediate valence and Kondo systems

An interplay of the Kondo scattering and exciton effects (d-f Coulomb interaction) in the intermediate valence systems and Kondo lattices is demonstrated to lead to an essential change of the scaling behavior in comparison with the standard Anderson model. In particular, a marginal regime can occur where characteristic fluctuation rate is proportional to flow cutoff parameter. In this regime the "Kondo temperature" itself is strongly temperature dependent which may give a key to the interpretation of controversial experimental data for heavy fermion and related systems.

cond-mat.str-el↗

arXiv2004

Density-of-states picture and stability of ferromagnetism in the highly-correlated Hubbard model

The problem of stability of saturated and non-saturated ferromagnetism in the Hubbard model is considered in terms of the one-particle Green's functions. Approximations by Edwards and Hertz and some versions of the self-consistent approximations based on the 1/z-expansion are considered. The account of longitudinal fluctuations turns out to be essential for description of the non-saturated state. The corresponding pictures of density of states are obtained. "Kondo" density-of-states singularities owing to spin-flip processes are analyzed. The critical electron concentrations for instabilities of saturated ferromagnetism and paramagnetic state are calculated for various lattices. Drawbacks of various approximations are discussed. A comparison with the results of previous works is performed.

cond-mat.str-el↗

arXiv2002

New mechanism of first order magnetization process in many-sublattice rare-earth compounds

First-order field-induced spin reorientation transitions in multisublattice intermetallic compounds are considered within an anisotropic Heisenberg model. Unlike previous works, only leading-order anisotropy constants of the sublattices are included. The exchange interactions both between the rare-earth (RE) and transition metal (TM) sublattices and within the RE sublattice play an important role in the model. The latter interaction results in the occurrence of metastable states. One of the minima corresponds to the non-collinear magnetic structure where the RE subsystem is divided into two sublattices, and their moments deviate in opposite directions from the TM moment direction. The possible FOMP scenarios for R$_2$Fe$_{17}$, HoFe$_{11}$Ti, and TbMn$_6$Sn$_6$ systems are discussed.

cond-mat.mtrl-sci↗

arXiv2002

On the violation of the Fermi-liquid picture in two-dimensional systems owing to the Van-Hove singularities

We consider the two-dimensional t-t' Hubbard model with the Fermi level being close to the van Hove singularities. The phase diagram of the model is discussed. In a broad energy region the self-energy at the singularity points has a nearly-linear energy dependence. The corresponding correction to the density of states is proportional to ln^3(e). Both real- and imaginary part of the self-energy increase near the quantum phase transition into magnetically ordered or superconducting phase which implies violation of the Fermi-liquid behavior. The application of the results to cuprates is discussed.

cond-mat.str-el↗

arXiv2002

Robustness of the van Hove scenario for high-Tc superconductors

The pinning of the Fermi level to the van Hove singularity and the formation of flat bands in the two-dimensional t-t' Hubbard model is investigated by the renormalization group technique. The "van-Hove" scenario of non-Fermi-liquid behavior for high-Tc compounds can take place in a broad enough range of the hole concentrations. The results are in qualitative agreement with the recent ARPES data on La2CuO4.

cond-mat↗

arXiv2002

Temperature dependences of resistivity and magnetoresistivity for half-metallic ferromagnets

Peculiarities of transport properties of three- and two-dimensional half-metallic ferromagnets are investigated, which are connected with the absence of spin-flip scattering processes. The temperature and magnetic field dependences of resistivity in various regimes are calculated. The resistivity is proportional to T^{9/2} for T T*, T* being the crossover temperature for longitudinal scattering processes. The latter scale plays also an important role in magnetoresistance. The contribution of non-quasiparticle (incoherent) states to the transport properties is discussed. It is shown that they can dominate in the temperature dependence of the impurity-induced resistivity and in the tunnel junction conductivity.

cond-mat.str-el↗

arXiv2001

Metal-insulator transition in the Hubbard model: a simple description including the Kondo effect

The electron spectrum structure in the half-filled Hubbard model is considered in terms of the one-particle Green's functions within many-electron representation. A simple analytical generalization of the single-site Hubbard-III approximation is obtained, which takes into account the Fermi excitations (Kondo terms). The problem of the metal-insulator transition is investigated. The occurrence of a three-peak density-of-states structure including the "Kondo" peak at the Fermi level is discussed. A comparison with large-d calculations is performed.

cond-mat.str-el↗

arXiv2001

Effects of the Van Hove singularities on magnetism and superconductivity in the t-t' Hubbard model: a parquet approach

The t-t' Hubbard model for the Fermi level near the Van Hove singularity is considered within the renormalization group and parquet approaches. The interplay of ferromagnetic, antiferromagnetic, and superconducting channels is investigated, and the phase diagram of the model is constructed. In comparison with previous approaches, the account of ferromagnetic fluctuations suppresses superconducting pairing and, vice versa, the influence of the Cooper channel decreases the Curie temperature, so that the Stoner criterion is inapplicable even qualitatively.

cond-mat.str-el↗

arXiv2000

A supersymmetric mean-field approach to the infinite-U Hubbard model

A generalized supersymmetric representation of the Hubbard operator algebra is considered. This representation is applied to the infinite-U Hubbard model. A mean-field theory which takes into account both on-site and inter-site virtual boson-fermion transitions is developed. Unlike previous approaches, the mean-field theory considered is free from divergences. Possible application of these results to the ferromagnet-paramagnet transition, as well as to other problems is discussed.

cond-mat↗

arXiv2000

Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates $1/T_1(T)$ and $1/T_2(T)$ are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard $T$-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap $ω_0$ due to magnetic anisotropy, and for the 2D AFM case as $ω_0^{-1}$. The electron-magnon scattering processes yield $T^2\ln (T/ω_0)$ and $T^2/ω_0^{1/2}$-terms in $1/T_1$ for the 3D AFM and 2D FM cases, respectively. The two-magnon (``Raman'') contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed.

cond-mat.str-el↗

arXiv2000

The Kondo effect in periodic narrow-band systems

The Kondo divergences owing to interaction of current carriers with local moments in highly correlated electron systems are considered within the Hubbard and s-d exchange models with infinitely strong on-site interaction, the many-electron Hubbard representation being used. The picture of density of states containing a peak at the Fermi level is obtained. Various forms of the self-consistent approximation are used. The problem of the violation of analytical properties of the Green's function is discussed. Smearing of the "Kondo" peak owing to spin dynamics and finite temperatures is investigated.

cond-mat.str-el↗

arXiv2000

Quantitative Description of Thermodynamics of Layered Magnets in a Wide Temperature Region

The thermodynamics of layered antiferro- and ferromagnets with a weak interlayer coupling and/or easy-axis anisotropy is investigated. A crossover from an isotropic 2D-like to 3D Heisenberg (or 2D Ising) regime is discussed within the renormalization group (RG) analysis. Analytical results for the the (sublattice) magnetization and the ordering temperature are derived are obtained in different regimes. Numerical calculations on the base of the equations obtained yield a good agreement with experimental data on La2CuO4 and layered perovskites. Corresponding results for the Kosterlitz-Thouless and Curie (Néel) temperatures in the case of the easy-plane anisotropy are derived.

cond-mat.str-el↗