Nonlinear spectroscopic probes are sensitive to the presence of long-lived quasiparticlesin many-body systems even when linear-response probes cannot couple directly to thesequasiparticles. I will discuss two cases in which nonlinear probes reveal information that isabsent in linear response: (i) nonlinear spectroscopy of excitations in strongly disorderedsystems such as the electron glass [1], and (ii) higher-order correlations revealed throughquantum gas microscopy in Heisenberg spin chains [2]. I will also discuss cases in which theresponse to a.c. drives is nonperturbative, leading to apparent singularities in nonlinearresponse coefficients [3].
References
[1] F. Mahmood et al., Nature Physics 17, 627 (2021).
[2] D. Wei et al., Science 376, 716 (2022); SG et al., arxiv:2203.09506
[3] M. Fava et al., PNAS 118, e2106945118 (2021).
Condensed Matter Theory Group
Laboratory for Theoretical and Computational Physics