Density Functional Calculations Near Ferromagnetic Quantum Critical Points

Abstract

We discuss the application of the density functional theory in the local density approximation (LDA) near a ferromagnetic quantum critical point. The LDA fails to describe the critical fluctuations in this regime. This provides a fingerprint of a materials near ferromagnetic quantum critical points: overestimation of the tendency to magnetism in the local density approximation. This is in contrast to the typical, but not universal, tendency of the LDA to underestimate the tendency to magnetism in strongly Hubbard correlated materials. We propose a method for correcting the local density calculations by including critical spin fluctuations. This is based on (1) Landau expansion for the free energy, evaluated within the LDA, (2) lowest order expansion of the RPA susceptibility in LDA and (3) extraction of the amplitude of the relevant (critical) fluctuations by applying the fluctuation-dissipation theorem to the difference between a quantum-critical system and a reference system removed from the quantum critical point. We illustrate some of the aspects of this by the cases of Ni3Al and Ni3Ga, which are very similar metals on opposite sides of a ferromagnetic quantum critical point. LDA calculations predict that Ni3Ga is the more magnetic system, but we find that due to differences in the band structure, fluctuation effects are larger in Ni3Ga, explaining the fact that experimentally it is the less magnetic of the two materials.

Gap symmetry and structure of Fe-based superconductors

“…In FeBS, the same effect is present due to the nonmagnetic interband scattering [118]. Since the Hebel-Slichter peak is not present in this scenario even in a clean sample, see Equation (30), the pair-breaking effect is more subtle: it changes exponential behavior below T c to a more power-law like one. If the impurity-induced bound state lies at the Fermi level (Section 3.4), the relaxation rate acquires a low-temperature linear-T Korringa-like term over a range of temperatures corresponding to the impurity bandwidth [261].…”

“…The corresponding physics is, therefore, largely missing when DFT is used in a "local" approximation. This belongs to two major classes: (i) on-site Coulomb fluctuations, also called Hubbard correlations, which are included in a very limited way on the level of the Stoner magnetic interaction (reflecting the first Hund's rule), and (ii) quantum critical fluctuations; examples of such are long range ferromagnetic fluctuations in nearly ferromagnetic metals [30]. The hallmark of the former is underestimation of the tendency to magnetism in a DFT calculation, of the latter -overestimation.…”

Compositional disorder and its influence on the structural, electronic, and magnetic properties ofMgC(Ni1−xCox)3Joseph1, Singh2 2005Phys. Rev. B4110View full textAdd to dashboardCite

“…According to Mazin and others, 61 the overestimation of the tendency of metals towards ferromagnetism within the LDA can be used as an indicator of critical fluctuations in a material. However, for this to be an effective indicator, competing states, such as anti-ferromagnetism need to be ruled out in each material.…”

Borderline Magnetism: How Adding Mg to Paramagnetic CeCo3 Makes a 450-K Ferromagnet with Large Magnetic Anisotropy

“…While Ce-3a site is surrounded by six Co-6c (first nearest) and twelve Co-18h (second nearest) neighbors, the Ce-6c site has six first nearest (Co-18h) and only three (Co-3b) second nearest neighbors. Numerous prior studies [37][38][39][40][41] on the weak itinerant magnetic systems have shown that the choice of functional can be very important in predicting the correct magnetic ground state. Hence, first a comparative analysis of LDA and GGA functional for CeCo 3 was performed.…”