IMX Colloquium - Time domain THz spectroscopy: a powerful tool to study magnets and superconductors

I will divide my talk into two parts, which illustrate the main scientific questions we are tackling by using time domain THz spectroscopy.

1) Generating ultra-fast spin currents is important in the context of ultra-fast magnetic memories. In the first part of the talk I will discuss our work on using antiferromagnets as efficient emitters of sub-picosecond spin current pulses, despite the net-zero magnetisation. I will present our work on the optically induced picosecond spin transfer from magnetically compensated magnets, including antiferromagnets and ferromagnetic alloys, to Pt using time-domain THz emission spectroscopy. We will focus on three studies in antiferromagnetic insulators KCoF3 and KNiF3 [1], in antiferromagnetic metal FeRh [2], and rare earth-transition metal alloy CoGd [3]. Through our studies, we are able to shed light on the microscopy of spin transfer at picosecond timescales and identify key figures of merit for its efficiency.

[1] Nature Communications 14, 538 (2023)

[2] Nature Communications 15, 4958 (2024)

[3] Advanced Optical Materials 2500056 (2025)

 

2)The ambition of superconducting spintronics is to integrate magnetic and superconducting elements in a unique device that combines permanent storage and low power logic characteristics. Recently, new discoveries have shown new routes for superconductivity and magnetism to co-operate, enabling novel device concepts based on the interplay between spin, charge and superconducting phase coherence. [1]. Understanding the timescales of this interplay is extremely important. For this reason, the first question we are trying to answer is how fast a superconductor responds to a fast perturbation, such as spin or charge injection or a fast electric field or magnetic field pulse. In this part of the talk I will discuss our very recent experiments in which we apply THz time domain spectroscopy to study the dynamics of NbN superconducting thin films after pumping with a femtosecond IR pulse. Our measurements uncover a pronounced lengthening of the superconductivity quenching time when the absorbed optical energy is close to the condensation energy, which constitutes a non-equilibrium analogue of critical slowing down [2].

[1] Nature Materials 17, 499 (2018)

[2] arXiv:2603.12473

Bio: Chiara Ciccarelli is Professor of Physics at the Cavendish Laboratory, University of Cambridge. She completed her undergraduate and master studies at Tor Vergata University in Rome. After her PhD in Cambridge she held a Junior Research Fellowship at Gonville and Caius College. In 2017 she started her research group at the Cavendish Laboratory with a Winton Advanced Research Fellowship. She is a Royal Society University Research Fellow since October 2017. In 2023 she was nominated Wohlfarth Lecturer by the Institute of Physics and has been awarded an ERC consolidator grant.