Martin Wood Complex, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU
Max McGinley email@example.com
What can we learn by measuring nonlinear response in many-body systems?
Most conventional experimental probes used in condensed matter systems can be interpreted through the framework of linear response. There are however cases where linear response functions alone are insufficient to discriminate between different physical scenarios. In these situations, nonlinear response functions could provide a valuable resource to sharpen our understanding. I will discuss two proposals along these lines.
The first  uses third-order response functions to clearly pinpoint the origin of a broad spectrum of excitations observed in linear response functions to a continuum of excitations, as opposed to, e.g., finite lifetime effects of non dispersive levels. The second proposal  concerns the measurement of scattering matrices in integrable 1D systems where the initial state is the quasiparticle vacuum. In this setting, I will argue that a component of the response function is linearly growing in time, and by determining the slope of this growth the scattering matrix can be computed.
 Y. Wan and N. Armitage, Phys. Rev. Lett. 122, 257401 (2019)
 MF, S. Gopalakrishnan, R. Vasseur, S. A. Parameswaran, F. Essler, in preparation