Seminar on 11 November 2024
Title: Exploring Altermagnetism in Orthorhombic Pnma structure through Group Theory and DFT Calculations
Speaker: Suman Rooj (IISER Bhopal)
Time: 3:30 pm IST
Venue: Seminar Room 363C
Abstract: Magnetism stands as a cornerstone in condensed matter physics, playing a crucial role in various technological advancements. Magnetic solids are typically divided into two broad categories: ferromagnet (FM) and antiferromagnet (AFM). FM exhibits a net magnetization in direct space, leading to intriguing phenomena stemming from the electronic spin split band structure in the reciprocal space that breaks the time reversal (T) symmetry in the non-relativistic (NR) limit. On the contrary, in a conventional AFM, due to the compensating antiparallel magnetic ordering, resulting in negligible small macroscopic magnetization, implying the absence of spin split band structure, thereby preserving the T symmetry in reciprocal space in the NR limit. However, recent research has sparked a growing interest in exploring the T reversal symmetry-breaking macroscopic phenomena and the existence of NR spin-splitting [1], characteristics typically associated with FM, in compounds with vanishingly small magnetization, which is characteristic of an AFM. This abundant and previously unexplored magnetic phase has been termed Altermagnet (AM) [2]. Intriguingly, we identify two centrosymmetric materials, bulk orthorhombic BiFeO3 (BFO) and CaMnO3 (CMO), that exhibit such NR spin-splitting phenomena. Through Density Functional Theory (DFT), we unveil an insulating state in both compounds with a preferred AFM order. Our DFT calculations and the magnetic space group (MSG) symmetry analysis in the NR limit uncover a substantial spin splitting phenomena observed only in the ky − kz plane in the Brillouin zone (BZ) for both compounds. In contrast, the spin degeneracy is maintained in the kx − ky and kx − kz planes, respectively. Using the invariant theory, we also formulate the governing Hamiltonian for the spin split pair of bands near the high symmetry Γ point. Finally, we extend our findings to encompass two possible MSGs falling within the Pnma space group that support such NR spin-splitting phenomena along with the probable form of their Hamiltonian [3]. In this talk, I will briefly provide an overview of altermagnetism and its distinctions from the other two conventional magnetic phases. Further, I will discuss our DFT findings and symmetry analysis on two representative altermagnetic materials.
References:
[1] L. D. Yuan, Z. Wang, J. W. Luo, E. I. Rashba, and A. Zunger, "Giant momentum-dependent spin splitting in centrosymmetric lowZ antiferromagnets," Phys. Rev. B 102, 014422 (2020).
[2] L. Smejkal, J. Sinova, and T. Jungwirth, "Emerging Research Landscape of Altermagnetism," Phys. Rev. X 12, 040501 (2022).
[3] S. Rooj, S. Saxena, and N. Ganguli, "Exploring Altermagnetism in Orthorhombic Pnma structure through Group Theory and DFT Calculations," arXiv:2406.06232 (2024).