Topic: Emergent Phenomena in Quantum Matter: Spin, Orbital and Topological Dynamics
Speaker name: Mr. Sobhan Subhra Mishra
Time/Date: 12:00-1:00PM/19th August 2025
Venue: SV Narsaiah Auditorium
Abstract: Quantum materials represent a broad class of topological and magnetic systems where subtle quantum effects govern their macroscopic behavior. The rich interplay among orbital, spin, and charge degrees of freedom gives rise to multiple coexisting and competing quantum phases. Terahertz radiation, with its low photon energy and ultrafast temporal resolution, offers a powerful tool to probe the dynamics associated with angular momentum and complex electronic band structures in these systems.
In this talk, I will demonstrate how ultrafast time-resolved terahertz emission spectroscopy can be used to probe spin and orbital dynamics in magnetic heterostructures, with a particular focus on the different transport mechanisms involved. I will share our recent breakthrough in achieving long-range ballistic transport and optical control of orbital angular momentum over tens of nanometers within magnetic heterostructures[1]. Additionally, I will introduce a reconfigurable chiral spintronic terahertz emission technique in a synthetic antiferromagnet (FM/NM/FM) system, which enables us to isolate spin information from individual ferromagnetic layers[2].
I will also discuss how terahertz emission spectroscopy can reveal topological dynamics in the 3D Dirac semimetal Cd₃As₂. By breaking time-reversal symmetry using circularly polarized light, we observe helicity-dependent chiral THz emission as a signature of the emergent Floquet Weyl state. Moreover, by tuning the Fermi level, we demonstrate electrical control over the Floquet band evolution[3].
Understanding the synergy between the unique electronic properties of quantum materials and the spectral and temporal characteristics of terahertz offers exciting possibilities, not only for enhancing our grasp of fundamental physics but also for advancing the next generation of ultrafast technologies.
References:
1. Mishra, S. S., Lourembam, J., Lin, D. J. X. & Singh, R. Active ballistic orbital transport in Ni/Pt heterostructure. Nat. Commun. 15, 4568 (2024).
2. Agarwal, P. et al. Reconfigurable Chiral Spintronic THz Emitters. Adv. Opt. Mater. 12, 2303128 (2024).
3. Mishra, Sobhan Subhra, Thomas CaiWei Tan, Manoj Gupta, Faxian Xiu, and Ranjan Singh. “Electrically tunable Floquet Weyl photon emission from Dirac semimetal Cd3As2.” arXiv preprint arXiv:2501.16498 (2025) (Under Review, Nature Materials).
About Speaker : Mr. Sobhan Subhra Mishra is currently a Research Fellow in the TeraX Lab at Nanyang Technological University (NTU), Singapore, under the supervision of Professor Ranjan Singh. He recently completed his Ph.D. at NTU, where his research focused on terahertz spectroscopy of quantum materials. His doctoral work explored the various forms of angular momentum as well as topological dynamics in novel topological and quantum matter systems through THz transmission and emission spectroscopy. His current research continues to investigate ultrafast dynamics in quantum materials, with a particular interest in emerging material platforms such as altermagnets and two-dimensional quantum materials.