Two-dimensional fluctuations and competing phases in the stripe-like antiferromagnet BaCoS$_{2}$

Abstract

By means of a combined x-ray diffraction, magnetic susceptibility and specific heat study, we investigate the interplay between orthorhombic distortion and stripe-like antiferromagnetic (AFM) order in the Mott insulator BaCoS2 at TN = 290 K. The data give evidence of a purely electronic AFM transition with no participation of the lattice. The observation of large thermal fluctuations in the vicinity of TN and a Schottky anomaly unveils competing ground states within a minute ∼1 meV energy range that differ in the orbital and spin configurations of the Co ions. This interpretation suggests that the stripe-like order results from a spontaneous symmetry breaking of the geometrically frustrated pristine tetragonal phase, which offers an ideal playground to study the driving force of multi-orbital Mott transitions without the participation of the lattice.

Order from disorder phenomena in BaCoS2

“…As a matter of fact, the two symmetry-lowering routes, P4/ nmm → Cmme and P4/nmm → Pba2, correspond to different Jahn-Teller-like distortions involving the d xz -d yz doublet and the E g phonon mode of the P4/nmm structure at the M point, which is found to have imaginary frequency by ab initio calculations 27 . However, latest high-accuracy X-ray diffraction data 18 confirm the Cmme orthorhombic structure even at room temperature, thus supporting our assumption.…”

“…We note that for all orbital configurations the energy gain due to a finite d is tiny with respect to d = 0. For instance, the lowest-energy orbital configuration xyyx reaches a minimum at about d xyyx min $ À0:5%, not far from the experimental value d exp $ À0:8% 25 , which reduces to d exp $ À0:4% under high pressure synthesis 18 . However, the energy gain with respect to d = 0 is less than 3 K. This result suggests that, despite the hypothetical P4/nmm structure of BaCoS 2 being inherently unstable to an orthorhombic Jahn-Teller distortion, the latter plays almost no role in stabilising the AFS magnetic order in contrast to naïve expectations.…”

“…The energy difference between AFS-a and AFS-b, i.e., O0 and O3, is about 20 K, and gives a measure of the spin-exchange spatial anisotropy in the a and b directions due to the orthorhombic distortion. This small value implies that the Néel transition temperature T N ≃ 290K is largely insensitive to the orthorhombic distortion that exists also above T N 18,19 . In particular, the energy difference between C-type and G-type stacking, O0 and O1 in Table 3, remains the same tiny value found in the tetragonal phase.…”

“…Inelastic neutron scattering (INS) experiments show that magnetic excitations below T N have pronounced two-dimensional (2D) character 16,17 implying strong quantum and thermal fluctuations that join with magnetic frustration to further hamper magnetic order. In spite of all that, the Néel temperature of BaCoS 2 is rather large, between 290K 18 and 305K 19 , which is highly surprising. Indeed, a direct estimate of the spin exchange constants by neutron diffraction has been recently attempted in doped tetragonal BaCo 0.9 Ni 0.1 S 1.9 subject to an uniaxial strain 17 .…”

“…Another startling property is the anomalously broad peak of the magnetic susceptibility at T N 18,19 , which suggests a transition in the Ising universality class rather than the expected Heisenberg one 19 . A possible reason of this behaviour might be spin-orbit coupling 19 .…”