Recent News : Research Highlight

Research Highlights from Ultrafast and Terahertz spectroscopy (UFTS) group led by Prof. N. Kamaraju in collaboration with Prof. Thiruppathaiah Setti’s group from SNBNCBS Kolkata (Single crystals), Prof. Shreeganesh Prabhu’s group in TIFR Mumbai, Prof. Sidhartha Lal, and Prof. Chiranjib Mitra of the Department of Physical Sciences, IISER Kolkata.

Authors: Sidhanta Sahu, Anupama Chauhan, Poulami Ghosh, Sayan Routh, Ruturaj Puranik, Thirupathaiah Setti, Siddhartha Lal, Shriganesh Prabhu, Chiranjib Mitra, N. Kamaraju.

Congratulations to all the authors, especially Sidhanta Sahu as this is his first research article contributing to his thesis work. Journal Link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.111.L140414

Spin-orbit entangled excitons (SOEEs) are a recently discovered class of quasiparticles in correlated quantum materials, where the spin and orbital degrees of freedom of electrons become intrinsically intertwined. In the layered van der Waals antiferromagnet NiPS 3 , these unique excitons play a central role in the material's electronic and magnetic properties. However, despite extensive equilibrium studies, the nonequilibrium dynamics of SOEEs following ultrafast optical excitation—and their coupling to magnetic correlations—have remained largely unexplored.

In this work, we employed femtosecond pump-probe spectroscopy to investigate the nonequilibrium dynamics of spin-orbit entangled excitons in NiPS 3 over timescales extending from picoseconds to nanoseconds. The measurements reveal two distinct relaxation pathways that exhibit pronounced and contrasting temperature dependences across the exciton dissociation temperature and the antiferromagnetic Néel transition. While the ultrafast relaxation reflects the evolution of the photoexcited excitonic state, the slower relaxation channel captures the influence of long-lived magnetic correlations, providing direct evidence of the intimate coupling between excitonic and spin degrees of freedom far from equilibrium. By resolving the evolution of spin-orbit entangled excitons across multiple temporal and temperature scales, this work provides new insight into the interplay between excitonic excitations and antiferromagnetic order in a two-dimensional quantum magnet. The findings establish ultrafast optical spectroscopy as a powerful tool for uncovering many-body interactions in van der Waals materials and pave the way for manipulating coupled excitonic and magnetic states on ultrafast timescales, with potential implications for future quantum and opto-spintronic technologies.

Highlights of the study  Exciton binding energy: ~132 meV (from Rothwarf–Taylor analysis)  Spin-wave gap: ~1.1 meV (from critical dynamics)  Critical exponent: , consistent with the 2D XY universality class  Main conclusion: Ultrafast spectroscopy directly reveals the dynamical coupling between spin-orbit entangled excitons and magnetic order in NiPS 3 .

Credits: High-quality NiPS3 single crystals were provided by Prof. Thirupathaiah Setti's group at SNBNCBS, Kolkata. The ultrafast optical pump–probe experiments were carried out in Prof. N. Kamaraju’s group at the Department of Physical Sciences (DPS), IISER Kolkata. The temperature- dependent magnetization (M–T) measurements were performed by Prof. Shriganesh Prabhu's group at TIFR, Mumbai). The phenomenological theory was developed with the help from Prof. Siddhartha Lal at DPS, IISER Kolkata. The low-temperature measurements were carried out with the help of Prof. Chiranjib Mitra.

Reference: S. Sahu, A. Chauhan, P. Ghosh, S. Routh, R. Puranik, S. Thirupathaiah, S. Lal, S. Prabhu, C. Mitra, and N. Kamaraju, Ultrafast dynamics of spin-orbit entangled excitons and magnetic correlations in the van der Waals antiferromagnet NiPS 3 , Phys. Rev. B 114, 014413 (2026).

#Research Highlight

Posted on: July 28th, 2026