Structural, electronic and magnetic properties of weakly correlated metal Sr 2 CrTiO 6 : a first principles study.

Structural, electronic and magnetic behaviour of a less-explored material Sr ₂ CrTiO ₆ has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t _{2 g} density of states around the Fermi energy. The Cr ⁴⁺ ion in Sr ₂ CrTiO ₆ , which is a d ² system, shows a reverse splitting of the t _{2 g} orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d _xz and d _yz orbitals, while the d _xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t _{2 g} bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner's criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d _xz / d _yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d _xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr ₂ CrTiO ₆ belongs to the class of weakly correlated metals, while the shifting of O 2 p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr ₂ CrTiO ₆ .
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