Novel measures of thermalization and time-evolution of strongly correlated, disordered, and topological systems by nonlinear THz spectroscopy

Non-Technical Abstract:

If a physical system is shaken, the rate that it returns to equilibrium is an important measure of its physics. In materials, we are used to measuring such rates in the context of their response to small electric and magnetic fields. For instance, the rate that current decays in a metal after an electric field impulse can be related to the width of its low-frequency “Drude” response in its optical conductivity.

The rate that polarization decays after polling a liquid with an electric field corresponds to the width of the broad peak in the Debye relaxational functional form. In contrast, the energy relaxation rate is a fundamental rate that governs many processes, but which is unfortunately not measured straightforwardly via conventional electrodynamic measurements using small fields.

This proposal concerns the development of measurement tools to probe materials with large THz range fields that can push materials far from equilibrium allowing new parameters to be measured. The systems of interest in this proposal possess unconventional features with regards to their interactions or band structures that will affect energy relaxation in interesting ways.

The team will investigate the rates of energy relaxation in a variety of materials with interesting quantum mechanical correlations. This project also includes a broad initiative in education and outreach. The work is of particular educational value in training students with unique skills to prepare them as the work force in high-tech industries.

The research team will play active roles in the Johns Hopkins Physics Fair- an outreach activity which brings hundreds of people each year through Hopkins' labs during a Saturday event and exposes them to various physics demonstrations and activities. The team will also give demonstration shows at the Physics Fair and work with under-resourced local schools.

Technical Abstract:

This is a project to investigate fundamental aspects of energy relaxation, time-evolution, and thermalization in strongly correlated, topological, and disordered systems by new nonlinear “pump-probe” THz spectroscopies. The research team will exploit recent developments in the form of THz range 2D coherent spectroscopy (and its relatives) to get new information about energy relaxation in correlated and topological metals, as well as disordered electron glasses.

Measurements will emphasize the extended THz range (here 0.1 - 40 THz [0.4 - 165 meV]) where generally responses target the low energy emergent degrees of freedom, but experiments will use a full complement of photon energies up through the near infrared. This topic of THz range dynamics and nonlinear response of quantum materials is an effectively wide-open area, in which there has been much interest and some theory, but very few experiments.

Among other aspects, the team will investigate the rates of energy relaxation in the regime of “Plankian dissipation” in the cuprate superconductors. It has been conjectured that their anomalous properties reflect an intrinsic quantum mechanical bound on the rate of relaxation that arises from maximally allowed inelastic scattering. This has been inferred from the resistivity experiments, among others.

The team will shed light on the origin of this remarkable property by probing the interplay of energy relaxation with this bounding scale. Another project that will confront a long-standing challenge in condensed matter physics is the understanding of materials with both strong interactions and strong disorder. The team will investigate the issue of thermalization and time-evolution in the “electron- glass” state of doped semiconductors on the insulating side of the 3D metal-insulator transition.

The team will use both the novel technique of THz 2D coherent and THz pump-probe spectroscopy. The mechanism of energy relaxation, both internal to the electron sub-system and coupling to the lattice are expected to be very different in such glasses and the results of this study may have relevance to the lack of thermalization in many-body localized systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.