single crystals*

The normal state in iron chalcogenides is metallic but highly unusual, with orbital and spin degrees of freedom partially itinerant or localized depending on temperature, a consequence of multi-orbital character and Hund's interaction. The unusual phenomena include, between others, the orbital-selective Mott phase (OSMP), documented by angular resolved photoemission (ARPES) [1], and C4 symmetry breaking of in-plane angular magnetoresistance (AMR) [2].

We study these phenomena using magnetization and magnetotransport measurements (resistivity, Hall effect, AMR) in Fe_1-yNi_yTe_0.65Se_0.35 single crystals (0xy orbital. We show that electron-dominated transport, observed at low T at large y, is replaced by hole-dominated transport at T > 180 K, suggesting direct link with the appearance of the d_z2 hole pockets at X points of the Brillouin zone in the OSMP phase, as recently reported [1]. The AMR shows the C₄ rotational symmetry of in-plane magnetocrystalline anisotropy at small y, replaced by C₂ symmetry at intermediate y, indicating development of spin nematicity induced by Ni doping, likely contributing to the suppression of superconductivity.

[1] J. Huang, et al., Commun. Phys. 5, 29 (2022).

[2] S. Liu, et al., Chin. Phys. Lett. 38, 087401 (2021).

[3] M. Rosmus et al., Supercond. Sci. Technol. 32, 105009 (2019).