Condensed Matter Physics Seminar: Vivek Unikandanunni

Abstract: In many physical systems, the bath is often approximated as a passive background that acts as an energy sink. This approximation can break down when the bath has a structured spectrum. The phononic bath in solids provides a prominent example of such a structured bath and can, under appropriate conditions, influence the system’s dynamics on ultrafast (femto- to picosecond) timescales.
In this talk, I will show how structured baths give rise to surprising and counterintuitive behavior in both spin and lattice dynamics. I will begin with ultrafast spin dynamics, where experiments reveal clear signatures of memory effects arising from the interaction between spins and the phonon bath [1, 2]. These observations go beyond conventional descriptions based on the Landau–Lifshitz–Gilbert equation [3]. Rather than exhibiting simple precessional relaxation toward equilibrium, spins display nutation-like dynamics and non-monotonic evolution, reflecting the ability of the bath to temporarily store and return energy to the system.
I will then turn to non-magnetic systems, where the bath plays an even more pronounced role. In this case, a common acoustic bath mediates an effective interaction between phonons and THz photons when direct coherent coupling is weak. This interaction leads to level attraction, a hallmark of non-Hermitian physics, and provides a route to control hybrid excitations through the bath itself [4, 5].
Finally, I will discuss how structured baths modify lattice dynamics, leading to non-exponential decay and, in some cases, unexpectedly long-lived phonon modes [6]. These findings challenge the conventional view of the bath as purely dissipative and instead identify it as a key element for controlling ultrafast phenomena.
Overall, these experimental results point to a broader perspective: in complex materials, the bath does not simply act as an energy sink. Instead, the system and bath evolve together, giving rise to dynamics that extend beyond standard Markovian descriptions.
References
[1]. Unikandanunni, V., et al. "Inertial spin dynamics in epitaxial cobalt films." Physical review letters 129.23 (2022): 237201.
[2]. Hartmann, F., et al. "Intrinsic non-Markovian magnetisation dynamics." arXiv preprint arXiv:2512.07378 (2025).
[3]. Neeraj, K., et al. "Inertial spin dynamics in ferromagnets." Nature Physics 17.2 (2021): 245-250.
[4]. Unikandanunni, V., et al. “Observation and Coherent control of THz-dressed phonons in Perovskites”, In preparation.
[5]. Unikandanunni, V., et al. “Level Attraction in Terahertz Phononics”, In preparation.
[6]. Basini, Martina, et al. "Terahertz-driven parametric excitation of Raman-active phonons in LaAlO3." Physical review letters (In Press), arXiv preprint arXiv:2410.06748.