Orbital-selective Mottness in ladder iron chalcogenides*

Simultaneously active charge, spin, and orbital degrees of freedom induce complex ordered states. Predicting these states is a challenge unless in ladders and chains. Fortunately, many real materials have this geometry. Here, I will review results using density matrix renormalization group (DMRG) applied to multiorbital Hubbard models. I focus on two aspects: (1) First, the “orbital selective Mott phase” (OSMP). This state occurs when orbitals with different crystal fields and hoppings are affected by the Hubbard and Hund couplings. Some orbitals become gapped and half-filled, while others do not, creating a localized and metallic mixture. Within OSMP, spin ``block’’ states were found, with FM islands, AFM coupled [1]. Powder neutron scattering for BaFe₂Se₃ confirmed their existence [2]. Dynamical DMRG predicts a combination of spin acoustic and optical modes for single crystals [3]. Even exotic “block spirals” were discussed [4]. (2) Second, in multiorbital ladders and chains, pairing was observed [5] as in BaFe₂X₃ (X=S,Se) at high pressure [6]. Singlet pairing was also found in degenerate two-orbital Hubbard models [7]. RVB states explain this pairing, with the orbital acting as an effective leg ladder index. In summary, computational tools applied to multiorbital interacting models in low dimensions have unveiled a remarkably rich landscape of exotic states.

[1] J. Rincon et al., PRL 112, 106405 (2014); J. Herbrych et al., PRL 123, 027203 (2019); P. Bradraj et al., PRB 102, 035149 (2020).
[2] M. Mourigal et al., PRL 115, 047401 (2015).
[3] J. Herbrych et al., Nat. Comm. 9, 3736 (2018); PRB 102, 115134 (2020).
[4] J. Herbrych et al., PNAS 117, 16226 (2020).
[5] N. Patel et al., PRB 94, 075119 (2016); PRB 96, 024520 (2017).
[6] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); J. Ying et al., PRB 95, 241109(R) (2017).
[7] N. Patel et al., npj QM 5, 27 (2020).