BCS-BEC crossover in a (t2g)4 excitonic magnet

Nitin Kaushal, Rahul Soni, Alberto Nocera, Gonzalo Alvarez, Elbio Dagotto

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13 Scopus citations

Abstract

The condensation of spin-orbit-induced excitons in t2g4 electronic systems is attracting considerable attention. At large Hubbard U, antiferromagnetism was proposed to emerge from the Bose-Einstein Condensation (BEC) of triplons (Jeff=1). Here, we show that even at intermediate U regimes, the spin-orbit exciton condensation is possible leading also to staggered magnetic order. The canonical electron-hole excitations (excitons) transform into local triplon excitations at large U, and this BEC strong coupling regime is smoothly connected to the intermediate U excitonic insulator region. We solved the degenerate three-orbital Hubbard model with spin-orbit coupling (λ) in one dimension using the density matrix renormalization group, while in two dimensions we use the Hartree-Fock approximation (HFA). Employing these techniques, we provide the full λ versus U phase diagrams for both one- and two-dimensional lattices. Our main result is that at intermediate Hubbard U, increasing λ at fixed U the system transitions from an incommensurate spin-density-wave metal to a Bardeen-Cooper-Schrieffer (BCS) excitonic insulator, with coherence length rcoh of O(a) and O(10a) in 1d and 2d, respectively, with a being the lattice spacing. Further increasing λ, the system eventually crosses over to the BEC limit (with rcoha).

Original languageEnglish
Article number245147
JournalPhysical Review B
Volume101
Issue number24
DOIs
StatePublished - Jun 15 2020

Funding

N.K., R.S., and E.D. were supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. A.N. was supported by the Canada First Research Excellence Fund. G.A. was supported by the Scientific Discovery through Advanced Computing (SciDAC) program funded by the U.S. DOE, Office of Science, Advanced Scientific Computing Research and Basic Energy Sciences, Division of Materials Sciences and Engineering. Part of this work was conducted at the Center for Nanophase Materials Sciences, sponsored by the Scientific User Facilities Division (SUFD), BES, DOE, under contract with UT-Battelle.

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