arXiv subjects

M. I. Katsnelson

Publications and source records attributed to M. I. Katsnelson.

At least 397 records · Page 22Linked to original sources

arXiv2002

Many-spin effects and tunneling splittings in Mn12 magnetic molecules

We calculate the tunneling splittings in a Mn$_{12}$ magnetic molecule taking into account its internal many-spin structure. We discuss the precision and reliability of these calculations and show that restricting the basis (limiting the number of excitations taken into account) may lead to significant error (orders of magnitude) in the resulting tunneling splittings for the lowest energy levels, so that an intuitive picture of different decoupled energy scales does not hold in this case. Possible routes for further development of the many-spin model of Mn$_{12}$ are discussed.

cond-mat.mes-hall↗

arXiv2002

Electronic structure and magnetic properties of correlated metals: A local self-consistent perturbation scheme

In the framework of ab initio dynamical mean field theory for realistic electronic structure calculations a new perturbation scheme which combine the T-matrix and fluctuating exchange approximations has been proposed. This method is less computationally expensive than numerically exact quantum Monte Carlo technics and give an adequate description of the electronic structure and exchange interactions for magnetic metals. We discuss a simple expression for the exchange interactions corresponding to the neglecting of the vertex corrections which becomes exact for the spin-wave stiffness in the local approximation. Electronic structure, correlation effects and exchange interactions for ferromagnetic nickel have been 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↗

arXiv2002

Peculiarities of anharmonic effects in the lattice thermodynamics of fcc metals

Explicit expressions for anharmonic contributions to the thermodynamic properties with allowance for higher-order phonon-phonon interactions for closed-packed crystals are given, and the calculations for some fcc metals near the melting (Ir, Rh) and martensite phase transition (Ca, Sr) points are carried out. A detailed comparison of anharmonic and electron contributions to the heat capacity of these metals is carried out. The computational results for high-temperature heat capacity agree well with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2001

Observation of Hubbard Bands in $γ$-Manganese

We present angle-resolved photoemission spectra of the $γ$-phase of manganese as well as a theoretical analysis using a recently developed approach that combines density functional and dynamical mean field methods (LDA+DMFT). The comparison of experimental data and theoretical predictions allows us to identify effects of the Coulomb correlations, namely the presence of broad and undispersive Hubbard bands in this system.

cond-mat.str-el↗

arXiv2001

Quantum oscillations without quantum coherence

We study numerically the damping of quantum oscillations and the increase of entropy with time in model spin systems decohered by a spin bath. In some experimentally relevant cases, the oscillations of considerable amplitude can persist long after the entropy has saturated near its maximum, i.e. when the system has been decohered almost completely. Therefore, the pointer states of the system demonstrate non-trivial dynamics. The oscillations exhibit slow power-law decay, rather than exponential or Gaussian, and may be observable in experiments.

quant-ph↗

arXiv2001

Length scales coupling for nonlinear dynamical problems in magnetism

The dynamics of real magnets is often governed by several interacting processes taking place simultaneously at different length scales. For dynamical simulations the relevant length scales should be coupled, and the energy transfer accurately described. We show that in this case the micromagnetic theory is not always reliable. We present a coarse-graining approach applicable to nonlinear problems, which provides a unified description of all relevant length scales, allowing a smooth, seamless coupling. The simulations performed on model systems show that the coarse-graining approach achieves nearly the precision of all-atom simulations.

cond-mat.mtrl-sci↗

arXiv2001

Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules

We have studied the effect of local Coulomb interactions on the electronic structure of the molecular magnet Mn12-acetate within the LDA+U approach. The account of the on-site repulsion results in a finite energy gap and an integer value of the molecule's magnetic moment, both quantities being in a good agreement with the experimental results. The resulting magnetic moments and charge states of non-equivalent manganese ions agree very well with experiments. The calculated values of the intramolecular exchange parameters depend on the molecule's spin configuration, differing by 25-30% between the ferrimagnetic ground state and the completely ferromagnetic configurations. The values of the ground-state exchange coupling parameters are in reasonable agreement with the recent data on the magnetization jumps in megagauss magnetic fields. Simple estimates show that the obtained exchange parameters can be applied, at least qualitatively, to the description of the spin excitations in Mn12-acetate.

cond-mat.mes-hall↗

arXiv2001

Nature of insulating state in NaV$_{2}$O$_{5}$ above charge-ordering transition: a cluster DMFT study

The nature of insulating state driven by electronic correlations in the quarter-filled ladder compound $α'$NaV$_{2}$O$_{5}$ is investigated within a cluster dynamical mean-field approach. An extended Hubbard model with first-principle tight-binding parameters have been used. It is shown that the insulating state in the charge-disordered phase of this compound is formed due to the transfer of spectral density and dynamical charge fluctuations where for the latter, the role of inter-site Coulomb interaction is found to be of crucial importance.

cond-mat.str-el↗

arXiv2001

Magnetism of correlated systems: beyond LDA

A novel approach to electronic correlations in magnetic crystals which takes into account a dynamical many-body effects is present. In order to to find a frequency dependence of the electron self energy, an effective quantum-impurity many-particle problem need to be solved within the dynamical mean-field theory. The numerically exact QMC-scheme and the spin-polarized fluctuation exchange approximation are used for the self-consistent solution of this single-site many-particle problem. The calculations of effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals have been discussed.

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↗

arXiv2001

Finite-temperature magnetism of transition metals: an LDA+DMFT approach

We present an ab initio quantum theory of the finite temperature magnetism of iron and nickel. A recently developed technique which combines dynamical mean-field theory with realistic electronic structure methods, successfully describes the many-body features of the one electron spectra and the observed magnetic moments below and above the Curie temperature.

cond-mat.str-el↗

arXiv2000

Propagation of local decohering action in distributed quantum systems

We study propagation of the decohering influence caused by a local measurement performed on a distributed quantum system. As an example, the gas of bosons forming a Bose-Einstein condensate is considered. We demonstrate that the local decohering perturbation exerted on the measured region propagates over the system in the form of a decoherence wave, whose dynamics is governed by elementary excitations of the system. We argue that the post-measurement evolution of the system (determined by elementary excitations) is of importance for transfer of decoherence, while the initial collapse of the wave function has negligible impact on the regions which are not directly affected by the measurement.

quant-ph↗

arXiv2000

Neel state of antiferromagnet as a result of a local measurement in the distributed quantum system

Single-site measurement in a distributed macroscopic antiferromagnet is considered; we show that it can create antiferromagnetic sublattices at macroscopic scale. We demonstrate that the result of measurement depends on the symmetry of the ground state: for the easy-axis case the Neel state is formed, while for the easy-plane case unusual ``fan'' sublattices appear with unbroken rotational symmetry, and a decoherence wave is generated. For the latter case, a macroscopically large number of measurements is needed to pin down the orientation of the sublattices, in spite of the high degeneracy of the ground state. We note that the type of the final state and the appearance of the decoherence wave are governed by the degree of entanglement of spins in the system.

quant-ph↗

arXiv2000

One-dimensional spinless fermion model with competing interactions beyond half-filling

An accurate numerical consideration of 1D spinless fermion model with next-nearest neighbour (NNN) interactions is carried out for the electron concentrations 4/7. It is shown that depending on the parameters of the model it can be either Luttinger liquid or bipolaron liquid. In the former case competing interactions can result in a smooth behavior of one-electron distribution function at the Fermi surface with a divergence in the second derivative with respect to the quasimomentum (and not in the first one, as usual). In this connection, uncommon photoemission spectra for some 1D conductors can be explained qualitatively.

cond-mat.str-el↗

arXiv2000

Mechanisms of decoherence in weakly anisotropic molecular magnets

Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules, such as V15, are studied. We show that an important decohering factor is the rapid thermal fluctuation of dipolar interactions between magnetic molecules. A model is proposed to describe the influence of this source of decoherence. Based on the exact solution of this model, we show that at relatively high temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not suppressed, and, in principle, can be detected experimentally. Therefore, these molecules may be suitable prototype systems for study of physical processes taking place in quantum computers.

cond-mat.mtrl-sci↗

arXiv2000

Thermal expansion and the equation of state of Ir and Rh

The simplest anharmonic characteristics of Ir and Rh are discussed in the framework of a previously developed simple pseudopotential model which describes the elastic moduli, phonon spectra and the lattice heat capacity in the harmonic approximation of these metals succesfully. The microscopic Gruneisen parameters, the dependences of the elastic moduli on pressure, the coefficient of thermal expansion and the equations of state at the finite temperatures have been calculated. The ab initio calculations of the energy-band structure and the equation of state for Ir at T=0 have been done to test the model for adequacy at high pressures. The values of different contributions (zero-point oscillations, quasiharmonic, etc.) in the considered thermodynamic characteristics of Ir and Rh are discussed.

cond-mat.mtrl-sci↗