arXiv subjects

V. Yu. Irkhin

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

At least 91 records · Page 5Linked to original sources

arXiv2011

Ferromagnetism, spiral magnetic structures and phase separation in the two-dimensional Hubbard model

The quasistatic approximation and equation-of-motion decoupling for the electron Green's functions are applied to trace the effect of electronic dispersion and electron correlations on the ferromagnetism of two-dimensional itinerant-electron systems. It is found that next-nearest-neighbor hopping t' is of crucial importance for ferromagnetism formation yielding the magnetic phase diagram which is strongly asymmetric with respect to half-filling. At small t' in the vicinity of half-filling the ferromagnetic phase region is restricted by the spin-density wave instability, and far from half-filling by one-particle (spin-polaron) instability. At t' close to t/2 ferromagnetism is stabilized at moderate Hubbard U due to substantial curvature of the Fermi surface which passes in the vicinity of the van Hove singularity points. The results obtained are of possible importance for high-T_c compounds and layered ruthenates.

cond-mat.str-el↗

arXiv2011

The Kondo effect in the presence of Van Hove singularities: A numerical renormalization group study

A numerical renormalization group (NRG) investigation of the one-centre $t-t'$ Kondo problem is performed for the square lattice with account of logarithmic Van Hove singularities (VHS) in the electron density of states. The magnetic susceptibility, entropy and specific heat are calculated. The temperature dependences of the thermodynamic properties in the presence of VHS turn out to be non-trivial. For finite $t'$ inverse logarithm of the corresponding Kondo temperature $T_K$ demonstrates a crossover from the square-root to standard linear dependence on the $s-d$ exchange coupling. The low-temperature behavior of magnetic susceptibility and linear specific heat are investigated, and the Wilson ratio is obtained. For $t' -> 0$ the Fermi-liquid behavior is broken.

cond-mat.str-el↗

arXiv2011

The Neel temperature and sublattice magnetization for the stacked triangular-lattice antiferromagnet with a weak interlayer coupling

The quantum Heisenberg antiferromagnet on the stacked triangular lattice with the intralayer nearest-neighbor exchange interaction J and interlayer exchange J' is considered within the non-linear $σ$-model with the use of the renormalization group (RG) approach. For J' << J the asymptotic formula for the Neel temperature $T_{Neel}$ and sublattice magnetization are obtained. RG turns out to be insufficient to describe experimental data since it does not take into account the $\mathcal{Z}_2$-vortices. Therefore $T_{Neel}$ is estimated using the Monte-Carlo result for the 2D correlation length [10] which has a Kosterlitz-type behavior near the temperature $T_{KT}$ where the vortices are activated.

cond-mat.stat-mech↗

arXiv2011

A Modern Model Description of Magnetism

A comparison is given of the fundamental works of S. V. Vonsovsky on the many-electron polar and s-d(f) exchange models with the subsequent development of the theory of magnetism of transition and rare earth metals and their compounds. Special attention is given to the derivation of different many-electron models (by Heisenberg, Hubbard, Anderson) and to the interrelation between them. Among the problems considered in this review, of most importance are the many-electron approaches used in the description of systems of d and f electrons, the atomic representation of X-operators, and the problem of strong itinerant magnetism and formation of local moments. The application of these concepts to highly correlated systems, in particular, to the half-metallic ferromagnets and the Kondo lattices is discussed.

cond-mat.str-el↗

arXiv2011

Ferromagnetic instability and finite-temperature properties of two-dimensional electron systems with van Hove singularities

We study a ferromagnetic tendency in the two-dimensional Hubbard model near van Hove filling by using a functional renormalization-group method. We compute temperature dependences of magnetic susceptibilities including incommensurate magnetism. The ferromagnetic tendency is found to occur in a dome-shaped region around van Hove filling with an asymmetric property: incommensurate magnetism is favored near the edge of the dome above van Hove filling whereas a first-order-like transition to the ferromagnetic ground state is expected below van Hove filling. The dome-shaped phase diagram is well captured in the Stoner theory by invoking a smaller Coulomb interaction. Triplet p-wave superconductivity tends to develop at low temperatures inside the dome and extends more than the ferromagnetic region above van Hove filling.

cond-mat.str-el↗

arXiv2010

Magnetic fluctuations and self--energy effects in two--dimensional itinerant systems with van Hove singularity of electronic spectrum

We investigate a competition of tendencies towards ferromagnetic and incommensurate order in two-dimensional fermionic systems within functional renormalization group technique using temperature as a scale parameter. We assume that the Fermi surface (FS) is substantially curved, lies in the vicinity of van Hove singularity points and perform an account of the self-energy corrections. It is shown that the character of magnetic fluctuations is strongly asymmetric with respect to the Fermi level position relatively to van Hove singularity (VHS). For the Fermi level above VHS we find at low temperatures dominant incommensurate magnetic fluctuations, while below VHS level we find indications for ferromagnetic ground state. In agreement with the Mermin-Wagner theorem, at finite temperatures and in small magnetic fields we obtain rather small magnetization, which appears to be a power-law function of magnetic field. It is found that the FS curvature is slightly increased by correlation effects, and the renormalized bandwidth decreases at sufficiently low temperatures.

cond-mat.str-el↗

arXiv2010

Ideas by S.V. Vonsovsky and Modern Model Treatment of Magnetism

A review of fundamental works by Shubin and Vonsovsky on the formulation of the polar and s-d(f) exchange models is given. Their ideas are compared with subsequent developments in the theory of magnetism in d- and f-metals and their compounds. Modern approaches including different slave-boson and slave-fermion representations, formation of exotic quasiparticles etc. are discussed. Internal connections between various many-electron models (the Heisenberg, Hubbard, t-J, Anderson Hamiltonians) are presented. Description of anomalous rare-earth and actinide compounds (Kondo lattices, systems with heavy fermions and non-Fermi-liquid behavior) within the framework of the s-d(f) exchange model and related models is considered.

cond-mat.str-el↗

arXiv2010

Ferromagnetism in the Highly-Correlated Hubbard Model

The Hubbard model with strong correlations is treated in the many-electron representation of Hubbard's operators. The regions of stability of saturated and non-saturated ferromagnetism in the n-U plane for the square and simple cubic lattices are calculated. The role of the bare density of states singularities for the magnetic phase diagram is discussed. A comparison with the results of previous works is performed.

cond-mat.str-el↗

arXiv2009

Incommensurate magnetic order and phase separation in the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping

We consider the ground state magnetic phase diagram of the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping in terms of electronic density and interaction. We treat commensurate ferro- and antiferromagnetic, as well as incommensurate (spiral) magnetic phases. The first-order magnetic transitions with changing chemical potential, resulting in a phase separation (PS) in terms of density, are found between ferromagnetic, antiferromagnetic and spiral magnetic phases. We argue that the account of PS has a dramatic influence on the phase diagram in the vicinity of half-filling. The results imply possible interpretation of the unusual behavior of magnetic properties of one-layer cuprates in terms of PS between collinear and spiral magnetic phases. The relation of the results obtained to the magnetic properties of ruthenates is also discussed.

cond-mat.str-el↗

arXiv2008

Universal properties of frustrated spin systems: 1/N-expansion and renormalization group approaches

We consider a quantum two-dimensional O(N)xO(2)/O(N-2)xO(2) nonlinear sigma model for frustrated spin systems and formulate its 1/N-expansion which involves fluctuating scalar and vector fields describing kinematic and dynamic interactions, respectively. The ground state phase diagram of this model is obtained within the 1/N-expansion and 2+εrenormalization group approaches. The temperature dependence of correlation length in the renormalized classical and quantum critical regimes is discussed. In the region of the symmetry broken ground state ρ_in<ρ_out, χ_in<χ_out (rho_in,out and chi_in,out are the in- and out-of-plane spin stiffnesses and susceptibilities), where the mass M_μof the vector field can be arbitrarily small, physical properties at finite temperatures are universal functions of rho_in,out, chi_in,out, and temperature T. For small M_μthese properties show a crossover from low- to high temperature regime at T \sim M_μ. For ρ_in>ρ_out or χ_in>χ_out finite-temperature properties are universal functions only at sufficiently large M_μ. The high-energy behaviour in the latter regime is similar to the Landau-pole dependence of the physical charge e in quantum electrodynamics, with mass M_μplaying a role of e^{-1}. The application of the results obtained to the triangular-lattice Heisenberg antiferromagnet is considered.

cond-mat.str-el↗

arXiv2008

Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation

We investigate magnetic properties of a frustrated Heisenberg antiferromagnet with a face-centered cubic (FCC) lattice and exchange interactions between the nearest- and next-nearest neighbours, J1 and J2. In a collinear phase with the wave vector Q = (pi,pi,pi) the equations of the self-consistent spin-wave theory for the sublattice magnetization and the average short range order parameter are obtained and numerically solved. The dependence of the Neel temperature T_N on the ratio J2/J1 is obtained. It is shown, that at strong enough frustration there is a wide temperature region above T_N with strong short range magnetic order. Application of this result to description of structural phase transition between alpha and gamma-phase of Fe is considered.

cond-mat.str-el↗

arXiv2008

Spin fluctuations and ferromagnetic order in two-dimensional itinerant systems with Van Hove singularities

The quasistatic approach is used to analyze the criterion of ferromagnetism for two-dimensional (2D) systems with the Fermi level near Van Hove (VH) singularities of the electron spectrum. It is shown that the spectrum of spin excitations (paramagnons) is positively defined when the interaction between electrons and paramagnons, determined by the Hubbard on-site repulsion U, is sufficiently large. Due to incommensurate spin fluctuations near the ferromagnetic quantum phase transition, the critical interaction Uc remains finite at VH filling and exceeds considerably its value obtained from the Stoner criterion. A comparison with the functional renormalization group results and mean-field approximation which yields a phase separation is also performed.

cond-mat.str-el↗

arXiv2007

Half-metallic ferromagnets: From band structure to many-body effects

A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.

cond-mat.mtrl-sci↗

arXiv2007

Spectral functions of 2D systems with coupling of electrons to collective or localized spin degrees of freedom

The spectral properties of itinerant 2D systems with (nearly) ferromagnetic ground state are studied within the spin-fermion and the classical s-d exchange models. While the former model describes the effect of collective magnetic excitations on the electronic properties, the latter one considers the effect of local moments. We use the equation of motion approach combined with the 1/M and 1/z expansions (M is the number of spin components, z is the coordination number) to investigate spectral functions. In both the models the spectrum splitting occurs at low temperatures T in the renormalized classical regime due to strong magnetic fluctuations. For the interaction J between electronic and magnetic degrees of freedom comparable to the bandwidth (the intermediate-coupling regime) the 1/z expansion predicts full splitting of electronic spectrum below some temperature T_s. At the same time, the spectrum remains only partly split in the 1/M expansion with the quasiparticle structure which becomes fully coherent at low T.

cond-mat.str-el↗

arXiv2007

Magnetic order and spin fluctuations in low-dimensional insulating systems

The current theoretical and experimental situations are reviewed for low-dimensional insulating systems with a low magnetic transition temperature TM and pronounced short-range magnetic order above this temperature. Both the standard and self-consistent spin-wave theories are shown to be insufficient to quantitatively describe the experimental data on these systems. Field-theoretical approaches taking into account the contribution of the spin-fluctuation excitations (neglected in the spin-wave theories) to the thermodynamic properties of ferro- and antiferromagnets are discussed.

cond-mat.str-el↗

arXiv2006

Single-particle spectrum for a model of fermions interacting with two-level local excitations on a lattice: A dynamical CPA approach

The problem of motion of a single electron interacting with a periodic lattice of two-level systems is investigated within a spinless fermion model. The Green's function is calculated in a single-site dynamical coherent potential approximation which is equivalent to DMFT. The picture of one-electron density of states is obtained for various values of the tunnel splitting and coupling between the electron and two-level system. The occurrence of a band splitting with increase of the coupling is demonstrated.

cond-mat.str-el↗

arXiv2005

Role of the d-f Coulomb interaction in intermediate valence and Kondo systems: a numerical renormalization group study

Using numerical renormalization group method the temperature dependences of the magnetic susceptibility $χ(T)$ and specific heat $C(T)$ are obtained in the single-impurity Anderson model with inclusion of the $d-f$ Coulomb interaction. It is shown that the exciton effects owing to this interaction can change considerably the dependence $C(T)$ in comparison with the standard Anderson model at not too low temperatures, whereas the dependence $χ(T)$ remains universal. The renormalization of the effective hybridization parameter and $f$-level position, which is connected with the $d-f$ interaction, is calculated, a satisfactory agreement with the Hartree-Fock approximation being derived.

cond-mat.str-el↗

arXiv2005

Spin-polarized scanning tunneling microscopy of half-metallic ferromagnets: Non-quasiparticle contributions

The role of the many-body (spin-polaronic) effects in the scanning tunneling spectroscopy of half-metallic ferromagnets (HMF) is considered. It is shown that the non-quasiparticle (NQP) states exist in the majority or minority spin gap in the presence of arbitrary external potential and, in particular, at the surfaces and interfaces. Energy dependence of the NQP density of states is obtained in various models of HMF, an important role of the hybridization nature of the energy gap being demonstrated. The corresponding temperature dependence of spin polarization is calculated. It is shown that the NQP states result in a sharp bias dependence of the tunneling conductance near zero bias. Asymmetry of the NQP states with respect to the Fermi energy provides an opportunity to separate phonon and magnon peaks in the inelastic spectroscopy by STM.

cond-mat.str-el↗