Investigation of the magnetocaloric effect in correlated metallic systems with Van Hove singularities in the electron spectrum

Abstract

Abstract⎯In this work, the magnetic contribution to the isothermal entropy change upon switching on a magnetic field has been investigated in correlated metallic ferromagnets within the Hubbard nondegenerate model. The analytical expression for obtained in the mean-field approximation depends substantially on the electronic structure (density of electron states), which presents new ways to increase the absolute value of relative to the known result obtained within the Heisenberg model. The temperature dependence of has been calculated at different values of the Coulomb interaction U and the number of electrons n for the Bethe infinite-dimensional lattice and square lattice with allowance for transfer integrals in the first (t) and the second (t') coordination shells. It has been found that the presence of Van Hove singularities in the electronic spectrum near the Fermi level makes it possible to considerably increase at a fixed magnetic field. The possibility of first-order magnetic phase transitions depending on the model parameters has been analyzed.

Investigation of magnetocaloric effect: Stoner approximation vs DMFT

“…For both methods maximum value of ∆S (T ) (∆S max ) is observed at the Curie temperature T C . In the MFA ∆S max quite significantly decreases as U grows [15]. Similar but much weaker decrease of ∆S max is found for DMFT results.…”

“…The MFA calculations here were carried out as described in our earlier work [15]. For the DMFT calculations we employed the CT-QMC impurity solver [22][23][24][25].…”

“…where c † iσ /c iσ is electron creation/annihilation operator, σ is spin projection, U is local Coulomb interaction parameter, t i j = −t(t ) for nearest (next-nearest) neighbor hopping transfer integral. Recently investigation of of MCE for the Hubbard model within the mean-field (Stoner) approximation (MFA) for the second [15] and first [16] order magnetic phase transitions was carried out. Next natural step is to take into account the electron-electronic correlations more accurately within dynamical mean-field theory (DMFT) [17].…”

Giant density-of-states van Hove singularities in the face-centered cubic lattice

“…. Such a singularity is analogous to that for the square lattice in the next-nearest-neighbour hopping approximation at t ′ = 0.5t when the flat band in the vicinity of van Hove lines k x = 0 and k y = 0 is formed [30].…”

Giant Van Hove Density of States Singularities and Anomalies of Electron and Magnetic Properties in Cubic Lattices

“…Including the next-nearest hopping separates the logarithmic DOS peak from the Fermi level corresponding to half-filling and eventually results in the formation of the van Hove line ΓX (X= (0, π)), Σ direction, and in the change of the topology of the spectrum at τ = 1/2 [27]. With further increase of t ′ , the local maximum of the spectrum is formed at the Γ point, which yields a jump in DOS doubling its value, whereas the van Hove saddle point (denote it as Σ * ) migrates along the direction ΓM (M being (π, π)) from the Γ point (at τ < 1/2 the saddle point is positioned at the X point) [28], see DOS plots at different τ in Fig 1b . Explicit expression for the square lattice DOS within the nearest and nextnearest neighbour hopping approximation has the form…”