5 research outputs found
Non-local corrections to dynamical mean-field theory from the two-particle self-consistent method
Theoretical methods that are accurate for both short-distance observables and
long-wavelength collective modes are still being developed for the Hubbard
model. Here, we benchmark against published diagrammatic quantum Monte Carlo
results an approach that combines local observables from dynamical mean-field
theory (DMFT) with the two-particle self-consistent theory (TPSC). This method
(TPSC+DMFT) is relevant for weak to intermediate interaction, satisfies the
local Pauli principle and allows us to compute a spin susceptibility that
satisfies the Mermin-Wagner theorem. The DMFT double occupancy determines the
spin and charge vertices through local spin and charge sum rules. The TPSC
self-energy is also improved by replacing its local part with the local DMFT
self-energy. With this method, we find improvements for both spin and charge
fluctuations and for the self-energy. We also find that the accuracy check
developed for TPSC is a good predictor of deviations from benchmarks. TPSC+DMFT
can be used in regimes where quantum Monte Carlo is inaccessible. In addition,
this method paves the way to multi-band generalizations of TPSC that could be
used in advanced electronic structure codes that include DMFT.Comment: 15 pages, 19 figures. Changes from v1: added reference
An Improved Two-Particle Self-Consistent Approach
The two-particle self-consistent approach (TPSC) is a method for the one-band
Hubbard model that can be both numerically efficient and reliable. However,
TPSC fails to yield physical results deep in the renormalized classical regime
of the bidimensional Hubbard model where the spin correlation length becomes
exponentially large. We address the limitations of TPSC with improved
approaches that we call TPSC+ and TPSC+SFM. In this work, we show that these
improved methods satisfy the Mermin-Wagner theorem and the Pauli principle. We
also show that they are valid in the renormalized classical regime of the 2D
Hubbard model, where they recover a generalized Stoner criterion at zero
temperature in the antiferromagnetic phase. We discuss some limitations of the
TPSC+ approach with regards to the violation of the f-sum rule and conservation
laws, which are solved within the TPSC+SFM framework. Finally, we benchmark the
TPSC+ and TPSC+SFM approaches for the one-band Hubbard model in two dimensions
and show how they have an overall better agreement with available diagrammatic
Monte Carlo results than the original TPSC approach
Spin Hall conductivity in the Kane-Mele-Hubbard model at finite temperature
The Kane-Mele model is known to show a quantized spin Hall conductivity at
zero temperature. Including Hubbard interactions at each site leads to a
quantum phase transition to an XY antiferromagnet at sufficiently high
interaction strength. Here, we use the two-particle self-consistent approach
(TPSC), which we extend to include spin-orbit coupling, to investigate the
Kane-Mele-Hubbard model at finite temperature and half-filling. TPSC is a weak
to intermediate coupling approach capable of calculating a frequency- and
momentum-dependent self-energy from spin and charge fluctuations. We present
results for the spin Hall conductivity and correlation lengths for
antiferromagnetic spin fluctuations for different values of temperature,
spin-orbit coupling and Hubbard interaction. The vertex corrections, which here
are analogues of Maki-Thompson contributions, show a strong momentum dependence
and give a large contribution in the vicinity of the phase transition at all
temperatures. Their inclusion is necessary to observe the quantization of the
spin Hall conductivity for the interacting system in the zero temperature
limit. At finite temperature, increasing the Hubbard interaction leads to a
decrease of the spin Hall conductivity. This decrease can be explained by
band-gap renormalization from scattering of electrons on antiferromagnetic spin
fluctuations.Comment: 11 pages, 8 figure
Analysis of the magnetic and magnetocaloric properties of ALaFeMnO<SUB>6</SUB> (A = Sr, Ba, and Ca) double perovskites
International audienceIn previous studies, we have reported that double perovskite La2NiMnO6 presents non-negligible potential for room temperature magnetocaloric tasks. With the aim of improving even further the cooling performances and the working temperature range of double perovskites, we report the magnetic and magnetocaloric properties of La2MnFeO6 and ALaMnFeO6 (A = Sr, Ba, and Ca) compounds. X-ray diffraction and Rietveld refinement show that La2MnFeO6 (LMFO) and CaLaMnFeO6 (CLMFO) samples crystallize in an orthorhombic structure with the P n m a space group. However, a rhombohedral structure with the R 3 ? C space group is obtained for BaLaMnFeO6 (BLMFO) and SrLaMnFeO6 (SLMFO) samples. Substituting La by Ba or Sr in LMFO leads to a clear increase of the Curie temperature (Tc) compared to LMFO from 150 K for BLMFO up to 350 K for SLMFO. Moreover, CLMFO shows the smallest Tc down to 70 K. Ferromagnetic-like behavior is observed for SLMFO and BLMFO, while CLMFO's magnetism resembles that of LMFO. A clear connection between the structural parameters and the magnetic properties of these doped LMFO samples is unveiled as the highest Tc and the largest magnetization are observed for SLMFO which also shows bond angles closest to 180° and the smallest bond lengths, thus optimizing the superexchange interaction. The partial substitution of Sr for La leads, in fact, to a significant magnetocaloric effect over a wide operating temperature range extending beyond 300 K. For some optimal growth conditions, its entropy change varies slowly over an unusually large temperature range, which is of clear interest from a practical point of view