Coherent Terahertz Control and Ultrafast Spectroscopy of Layered Antiferromagnets

This thesis presents new insights into the strong interactions among electronic, lattice, spin, and orbital degrees of freedom in layered magnetic materials, as well as their emergent properties. Using a suite of spectroscopic techniques, both in equilibrium and out-of-equilibrium settings, several important findings have been made. In a family of transition metal thiophosphates, a novel bound state resulting from electronic transitions between d-orbitals and Raman-active phonons was observed in NiPS3, using femtosecond transient absorption spectroscopy. Furthermore, this phonon symmetry was employed to identify a new magnetostrictive effect in FePS3 through coherent phonon spectroscopy. These and other observations point to strong interactions between spin and lattice degrees of freedom in this system. This coupling has been harnessed to actively control the magnetic structure. Specifically, intense, tailored terahertz pulses were used to displace the lattice along particular phonon directions, inducing a new magnetic order characterized by net magnetization. This effect is notably more efficient and exhibits an increasingly longer lifetime near the phase transition point, highlighting the key role played by critical fluctuations. Finally, second harmonic generation, linear dichroism, and Raman spectroscopy were employed to discover a new type-II multiferroic phase that persists down to the atomic monolayer limit in NiI2.

Nominated as an outstanding PhD thesis by Massachusetts Institute of Technology Provides an extended introduction to ultrafast science in condensed matter physics Presents cutting-edge spectroscopic techniques for uncovering hidden interactions in quantum materials

Autorentext

Dr. Batyr Ilyas earned his B.S. degree in Physics from Nazarbayev University in Astana, Kazakhstan, and his Ph.D. in Physics from the Massachusetts Institute of Technology (MIT) in Cambridge, MA. At MIT, his research focused on using tailored intense terahertz pulses to manipulate the magnetic properties of layered antiferromagnets via nonlinear phononics, as well as probing their ultrafast dynamics. In 2024, he joined the University of California, Berkeley, and Lawrence Berkeley National Laboratory as a Postdoctoral Researcher. His current work investigates local optical, electronic, and lattice dynamics in correlated electron systems using scanning probe microscopy techniques coupled with external laser sources.

Inhalt

Introduction.- Ultrafast phenomena in quantum materials.- Layered magnets A highly tunable platform of magnetism.- THz field induced metastable magnetization near criticality in FePS35 Time of flight detection of terahertz phonon polariton.- Coherent detection of hidden magnetostriction effect.- Magnetically brightened dark electron-phonon bound states.- Discovery of monolayer van der Waals multiferroic.