Davide Bossini

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A major research effort in the area of condensed matter physics aims to optically manipulate the
macroscopic properties of quantum materials [1]. Remarkable critical phenomena have been
reported, such as the laser-induced metal-insulator transition [2], metamagnetic phase
transitions [3], and even superconductivity [4]. The dynamical control of the order parameter has
been achieved in magnetic materials [5], even via coherent processes, which rely on the optical
generation of coherent collective eigenmodes [6,7]. In my talk, I will introduce the main light-
matter interaction mechanisms enabling the optical activation of coherent quasiparticles, with a
particular focus on magnetic excitations. I will discuss in details two aspects. First, the generation
of collective spin excitations, namely spin waves or magnons, both at the center and at the
edges of the Brillouin zone, while avoiding the absorption of light by the lattice and the
electrons [8-9]. The regime of quantum spin dynamics triggered by the activation of high-energy
magnons will be discussed as well [9-10]. Second, I will address the coupling between spins and
electrons at the ultrashort timescale. Mechanisms allowing to drive spin waves both at the
center and at the edges of the Brillouin zone, via mixed electronic and magnonic transitions, will
be the topic of discussion [11-12]. I will then report on our very recent successful attempts to
couple coherent THz magnons to charges at the picosecond timescale and our ongoing efforts
to establish a manipulation of the spin-orbit coupling, via the excitation of coherent THz
phonons [13]. In the final part of my talk, I will then present novel concepts to manipulate
quantum materials, i.e. the dynamical modification of the spectrum and dispersion relation. I will
show an unprecedented scheme to couple zone-edge and zone-centre modes [14], which
results in the renormalization of the frequency and amplitude of the zone-centre magnons [15].
The observed regime of strongly nonlinear coherent spin dynamics is elusive of the conventional
picture provided by linear spin wave theory. The development of a quantitative theoretical
understanding of the observations, in collaboration with theorists, will be presented as well. In
conclusion, I will briefly show extremely novel results concerning an alternative route to modify
the magnon spectrum, which does not involve zone-edge magnons at all. The concepts
discussed in my talk provide basic tools for a dynamical engineering of macroscopic coherent
states of quantum materials, operating at the intrinsic characteristic time scales of the collective
eigenmodes. The approaches presented here can be applied to a wide variety of systems and
can be generalized to lattice excitations.
[1] Advances in Physics 65, 58–238 (2016).
[2] Nature 449, 72–74 (2007).
[3] Physical Review Letters 116, 097401 (2016).
[4] Science 331, 189–191 (2011).
[5] Physical Review Letters 99, 047601 (2007).
[6] Nature 435, 655 (2005).
[7] Nature Photonics 5, 31–34 (2010).
[8] Physical Review B 89, 060405(R) (2014).
[9] Nature Communications 7, 10645 (2016).
[10] Physical Review B 100, 024428 (2019).
[11] Physical Review Letters 127, 077202 (2021).
[12] Nature Physics 14, 370–374 (2018).
[13] ArXiv (2023) doi:10.48550/arxiv.2310.08411.
[14] Laser Photonics Rev. 2301152, (2024).
[15] ArXiv (2023) doi: 10.48550/arXiv:2310.19667v1

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Uni. Konstanz
Seminar Room of the Institute of Physics II
Contact: Erwann/Matteo