Effect of onsite Coulomb repulsion on thermoelectric properties of full-Heusler compounds with pseudogaps

Abstract

Electronic structure calculations using local density and generalized gradient(LDA/GGA) approximations for the full Heusler compound Fe2VAl show that it is a pseudo-gap (negative gap) system with very small density of states (DOS) at the Fermi level but rapidly rising DOS away from it, a feature that makes this compound a promising thermoelectric material. Thermopower (S) measurements in nominally pure and n-doped Fe2VAl give indeed large values of S (∼-150 µV/K at 200 K). To improve on the inadequacy of LDA/GGA in handling d-electron systems and to understand the origin of large thermopowers measured, we have carried out electronic structure calculations using GGA+U method with several values of the on-site Coulomb interaction parameter U including the ones calculated using constrained density functional theory (DFT). For the latter, we found Fe2VAl to be a narrow band gap semiconductor with a gap of 0.55 eV. With the calculated band structures, we have studied the carrier concentration and temperature dependence of S using Boltzmann transport equation in constant relaxation time approximation for both the pseudo-gap and gapped cases.Comparison between theory and experiment suggests that neither the pseudo-gap nor the finite gap (0.55 eV) model can explain all the transport properties consistently. Therefore treatment of U beyond simple mean-field approach (done in GGA+U) and/or inclusion of defect induced changes in the host electronic structure might be important in understanding the experiments.

Semimetallic behavior in Heusler-type Ru2TaAl and thermoelectric performance improved by off-stoichiometry

“…A similar sign reversal in S has also been reported in the isostructural Fe 2−x V 1+x Al system [13,29,30]. From previous various investigations [31][32][33][34][35][36][37], the replacement of Fe by V in Fe 2 VAl may cause the modification of electronic structure, the presence of the impurity state within the pseudogap, and/or the accompanying other disorder effects. Those would play different roles on the type of carriers and thus lead to the sign change in S. Based on the similarity between Ru 2 TaAl and Fe 2 VAl, it is realistic to associate the observed sign reversal in S of Ru 1.95 Ta 1.05 Al with similar origins.…”

Ru2NbGa : A Heusler-type compound with semimetallic characteristics

“…4, the band structures of Ru 2 NbGa indicate that it is a semimetal with a negative band gap (so-called pseudogap) of −0.53 eV and narrow-band semiconductor with a band gap of 0.07 eV, using PBE and HSE06 functionals, respectively. The results of these two functionals are somewhat controversial and such phenomena have been also reported in the prototype of Fe 2 VAl [56][57][58][59]. As mentioned above, the DFT results obtained strongly depend on the exchange correlation functional used and thus a correct procedure to resolve this discrepancy is highly desirable.…”

Half-metallicity in new Heusler alloys Mn2ScZ (Z = Si, Ge, Sn)

“…62,63 At present there are large number of studies in case of Heusler alloys in which calculation of U value and its effect on the electronic and magnetic properties are reported. [64][65][66] A clear implication of U calculation and its inuence on Heusler alloys to predict exact ground state properties can be found from the studies carried out by Rai et al 67,68 The effect of electron correlation in Heusler alloys are evident from the above literature and it is expected to have signicant impact on alloys where highly localized d orbitals based transition metals Mn and Sc are present whose exact ground state properties are yet unexplored. Hence, in view of importance of DFT+U calculation in the material research, an effort have been made to investigate the half metallic behaviour of highly localized Mn and Sc based sample alloys Mn 2 ScZ (Z ¼ Si, Ge and Sn) by adding U correction for 3d electrons of Mn and Sc.…”