Suppression of magnetism under pressure in FeS: A DFT+DMFT study

Abstract

We investigate the evolution of the magnetic properties in FeS under pressure, and show that these cannot be explained solely in terms of the spin state transition from a high to low spin state due to an increase of the crystal field. Using a combination of density functional theory and dynamical mean field theory (DFT+DMFT), our calculations show that at normal conditions the Fe 2+ ions are in the 3d 6 high spin (S = 2) state, with some admixture of a 3d 7 L (S = 3/2) configuration, where L stands for the ligand hole. Suppressing the magnetic moment by uniform compression is related to a substantial increase in electron delocalization and occupation of several lower spin configurations. The electronic configuration of Fe ions cannot be characterized by a single ionic state, but only by a mixture of the 3d 7 L, 3d 8 L 2 , and 3d 9 L 3 configurations at pressures ∼ 7.5 GPa. The local spin-spin correlation function shows well-defined local magnetic moment, corresponding to a large lifetime in the high spin state at normal conditions. Under pressure FeS demonstrates a transition to a mixed state with small lifetimes in each of the spin configurations.

Role of temperature and Coulomb correlation in the stabilization of the CsCl-type phase in FeS under pressure

“…The on-site Hubbard U = 6 eV and Hund's intra-atomic exchange J H = 0.95 eV were estimated in QE using constrained GGA calculations [46]. Note that these values agree well with the results of previous calculations of U for other Fe sulfides and oxides at high pressure [23,47] on the same Wannier functions which were applied to construct a small noninteracting Hamiltonian used in the subsequent DFT+DMFT calculations [35,45].…”

Nanocrystalline Iron Monosulfides Near Stoichiometry

“…Sometimes called the “DFT + U” method 37 , 38 , this methodology provides results for structural quantities, like lattice constants, that agree well with experimental measurements for the related iron sulfur compounds of troilite and iron pyrite 8 , 12 , 39 . The presence of an insulating state in our DFT + U calculations for troilite is consistent with calculations from the much more computationally expensive dynamical mean field theory (DMFT) methods, that also find an insulating ground state, but one with a much narrower gap 40 , 41 . Because the ground states of troilite and pyrrhotite are magnetic, all of the calculations reported here are spin-polarized.…”

“…With no defects, the troilite unit cell maintains its P-62c symmetry and is insulating with a band gap of 0.71 eV. This band gap is larger than that reported in recent DMFT calculations 40 , 41 , but similar to that reported in another DFT + U paper 8 . Once we introduce a single neutral iron vacancy, the structure distorts from hexagonal into a monoclinic cell resembling a pyrrhotite phase with a = 5.937 Å, b = 5.991 Å, c = 11.655 Å, and unit cell volume = 359.52 Å 3 , shown in Fig.…”

Pressure effects on the FeSe-based superconductors

“…但是, 当采用活塞-圆筒压腔在较好静水压环境 下测试高压μSR时, 人们发现FeSe在~1 GPa开始形成 长程反铁磁序, 并且磁有序温度T N 随压力增加而单调 升高 [42][43][44] . 随后, 利用活塞-圆筒压腔测试FeSe单晶样 品的电阻率时, 发现压力诱导的反铁磁序在T N 会造成 明显的电阻率异常 [45] , 这使得人们可以通过测试高压 [44,51] , 形成反铁磁序的同时也伴随着四方-正交结构相变 [52] , 与FeAs基超导体类似; 同时, 高温超导毗邻长程反铁 磁有序相, 与FeAs基超导体系的相图也类似, 表明反 铁磁临界涨落与高温超导具有紧密联系 [53,54] . 这引起 了我们对统一理解铁基高温超导机理问题的思考.…”