First-Principles Calculations of the Electronic Structure of Tetragonal alpha-FeTe and alpha-FeSe Crystals: Evidence for a Bicollinear Antiferromagnetic Order
Abstract:
By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal -FeTe is in a bicollinear antiferromagnetic order, in which the Fe local moments ( 2:5B) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This novel bicollinear order results from the interplay among the nearest, the next-nearest, and the next-next-nearest neighbor superexchange interactions, mediated by Te 5p band. In contrast, the ground state of -FeSe is in a collinear antiferromagnetic order, similar to those in LaFeAsO and BaFe2As2. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors.
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The microscopic origin of the magnetic state is an important issue under hot debate. In particular, there are two scenarios in describing this magnetic ordered state. One suggests that there are no local moments and the collinear AFM order is entirely induced by the Fermi surface nesting [8,9]; the other suggests that the collinear AFM order is driven by exchange AFM interactions between the nearest neighbor and next-nearest neighbor FeFe fluctuating local moments.
To resolve the above problem, we have performed the first-principles calculations on the electronic structures of tetragonal -FeTe and -FeSe. We find that the nonmagnetic electronic band structures of both -FeTe and -FeSe, especially the Fermi surfaces, are similar to those of LaFeAsO and BaFe2As2 [9,10,14]. It is thus expected that all these materials would adopt the similar magnetic order if the magnetic order is induced by the Fermi surface nesting. However, we find that the ground state of -FeTe is in a novel bicollinear AFM order [Fig. 1(a)], while that of -FeSe is in a conventional collinear AFM order. To our knowledge, this novel bicollinear AFM order has never been found in real materials.
To quantify the magnetic interactions, we assume that the energy differences between these magnetic orderings are predominantly contributed from the interactions between the Fe moments with spin S~, which can be effectively modeled by the following frustrated Heisenberg model with the nearest, the next-nearest, and the nextnext-nearest neighbor couplings J1, J2, and J3,
.........
From the calculated energy data, we find that for -FeTe J1 ¼ 2:1 meV=S2, J2 ¼ 15:8 meV=S2, and J3 ¼ 10:1 meV=S2, while for -FeSe J1 ¼ 71 meV=S2, J2 ¼ 48 meV=S2, and J3 ¼ 8:5 meV=S2.
To resolve the above problem, we have performed the first-principles calculations on the electronic structures of tetragonal -FeTe and -FeSe. We find that the nonmagnetic electronic band structures of both -FeTe and -FeSe, especially the Fermi surfaces, are similar to those of LaFeAsO and BaFe2As2 [9,10,14]. It is thus expected that all these materials would adopt the similar magnetic order if the magnetic order is induced by the Fermi surface nesting. However, we find that the ground state of -FeTe is in a novel bicollinear AFM order [Fig. 1(a)], while that of -FeSe is in a conventional collinear AFM order. To our knowledge, this novel bicollinear AFM order has never been found in real materials.
To quantify the magnetic interactions, we assume that the energy differences between these magnetic orderings are predominantly contributed from the interactions between the Fe moments with spin S~, which can be effectively modeled by the following frustrated Heisenberg model with the nearest, the next-nearest, and the nextnext-nearest neighbor couplings J1, J2, and J3,
.........
From the calculated energy data, we find that for -FeTe J1 ¼ 2:1 meV=S2, J2 ¼ 15:8 meV=S2, and J3 ¼ 10:1 meV=S2, while for -FeSe J1 ¼ 71 meV=S2, J2 ¼ 48 meV=S2, and J3 ¼ 8:5 meV=S2.
Hello, my name is Dr Abderrahmane Reggad. I'm a 51 year old and I am a researcher in materials' sciences. 
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