Orbital-selective formation of local moments inα-iron: First-principles route to an effective model

Abstract

We revisit a problem of theoretical description of α-iron. By performing LDA+DMFT calculations in the paramagnetic phase we find that Coulomb interaction and, in particular Hund exchange, yields the formation of local moments in eg electron band, which can be traced from imaginary time dependence of the spin-spin correlation function. This behavior is accompanied by non-Fermi-liquid behavior of eg electrons and suggests using local moment variables in the effective model of iron. By investigating orbital-selective contributions to the Curie-Weiss law for Hund exchange I = 0.9 eV we obtain an effective value of local moment of eg electrons 2p = 1.04µ B . The effective bosonic model, which allows to describe magnetic properties of iron near the magnetic phase transition, is proposed.

Magnetism of iron and nickel from rotationally invariant Hirsch-Fye quantum Monte Carlo calculations

“…7,8,[27][28][29][30][31][32] However, an agreement was achieved only in terms of the reduced temperature T /T C , while the calculated Curie temperature T C was found to be almost twice larger than the experimental value of 1043 K (Ref. 33).…”

Mechanism for theα→εphase transition in iron

“…In particular, the localization of electrons in the non-bonding e g state but not in the bonding t 2g state is a key point 19 for understanding paramagnetic bcc iron. However, DFT ab initio techniques alone can handle FM bcc iron and the thermodynamical fundamentals of the α → ǫ transition 20 : The Generalized Gradient Approximation (GGA) correctly captures the basic Stoner mechanism for the reduction of magnetism, under the increase of atom coordination during the α → ǫ transition, as well as the larger spin polarization of e g orbitals with respect to t 2g in the α phase.…”

“…In order to describe α iron from an ab-initio point of view, Dynamical Mean Field Theory 12,19 in combination with Density Functional Theory (DFT) or other advanced schemes 13 have been used. In particular, the localization of electrons in the non-bonding e g state but not in the bonding t 2g state is a key point 19 for understanding paramagnetic bcc iron.…”

First-Principles Theory of Momentum Dependent Local Ansatz Approach to Correlated Electron System

“…The firstprinciples MLA holds the high momentum and total-energy resolutions in the numerical calculations of the ground-state properties such as the momentum distribution function (MDF) and the groundstate energy because all the physical quantities in the MLA are expressed analytically and they are calculated with use of the Laplace transformation which transforms the 6-fold energy integrals into the 2-fold time integrals. We also point out that the present theory is the first which quantitatively explains the mass enhancement factor m * /m of Fe at zero temperature because most of the LDA+DMFT calculations for m * /m are limited to the finite temperature case 29) and the zero-temperature calculations of the LDA+DMFT with use of the three-body theory failed in the quantitative explanation of experimental data of bcc Fe. 30) Furthermore the method has an advantage that it allows us to calculate any static physical quantities because we know the wavefunction itself.…”

“…The calculated average mass enhancement factor m * /m = 1.65 is consistent with the experimental data obtained by the low-temperature specific heat [31][32][33] and the angle resolved photoemission spectroscopy (ARPES), 30) as well as the recent result of the LDA+DMFT calculations. 29) Preliminary results of the present work which were based on the lowest order calculations have been published as a proceedings. 34) In the following section, we present the first-principles MLA based on the tight-binding (TB) LDA+U Hamiltonian.…”