(Johns Hopkins University)
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If a physical system is perturbed from equilibrium, the rate that it equilibrates is an important measure of its physics. In condensed matter physics, we are used to measuring such rates in the context of linear response to electromagnetic 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 the optical conductivity. The rate that polarization decays after polling a liquid with an E field corresponds to the width of the broad peak in the Debye relaxational functional form. In contrast, the rate of energy relaxation is a fundamental rate that governs many processes in solids, but which is unfortunately not measured straightforwardly via conventional electrodynamic linear response. However quite generically, this rate can be measured in various non-linear chi3 spectroscopies. I will discuss recent technical developments in the form of THz range 2D coherent spectroscopy (and its relatives) that allow us to get new information about energy relaxation in correlated and topological metals, as well as disordered electron glasses. I will discuss a number of systems and phenomena in which unconventional dynamics and energy relaxation govern their low energy behavior.
In a first example , I will concentrate on the long-standing challenge in condensed matter physics of understanding of materials with both strong interactions and strong disorder. We 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. In phosphorus doped silicon, we observe  — despite the intrinsically disordered nature of these materials — coherent excitations and strong photon echoes that provide us with a powerful method for the study of their decay processes. We extract the energy relaxation and decoherence rates close to the metal–insulator transition. We observe that both rates are linear in excitation frequency with a slope close to unity. The energy relaxation timescale counterintuitively increases with increasing temperature, and the coherence relaxation timescale has little temperature dependence below 25 K, but increases as the material is doped towards the metal–insulator transition. Here we argue that these features imply that the system behaves as a well-isolated electronic system on the timescales of interest, and relaxation is controlled by electron–electron interactions. Our observations constitute a distinct phenomenology, driven by the interplay of strong disorder and strong electron–electron interactions, which we dub the marginal Fermi glass. In a second example , I will discuss the rates of energy relaxation in the regime of “Plankian dissipation” in the cuprate superconductors. It is remarkable that still after thirty years, the properties of the enigmatic normal state physics of the cuprates is not understood. It has been conjectured that their anomalous properties reflects 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. However it has been conjectured that the inelastic lifetime of quasiparticles is an avatar of a more basic many-body timescale: the equilibration time. I will discuss our recent work which compares the rate of energy relaxation to momentum relaxation in the cuprate strange metal regime.
 Fahad Mahmood et al., "Observation of a marginal Fermi glass”, Nature Physics 2021
 D. Barbalas et al., to be submitted 2022
N. Peter Armitage has been at Johns Hopkins University since 2006. He received his B.S. in Physics from Rutgers University in 1994 and his Ph.D. from Stanford University in 2002. He is a physicist whose research centers on material systems which exhibit coherent quantum effects at low temperatures, like superconductors and "quantum" magnetism. Dr. Armitage's principal scientific interest is understanding how is it that large ensembles of strongly interacting, but fundamentally simple particles like electrons in solids act collectively to exhibit complex emergent quantum phenomena. He is exploiting (and developing) recent technical breakthroughs using very low frequency microwave and THz range radiation to probe these systems at their natural frequency scales. The material systems of interest require novel measurement techniques as their relevant frequencies typically fall between the range of usual optical and electronic methods.
He has been the recipient of a DARPA Young Faculty Award, an NSF Career Award, a Sloan Research Fellowship, was a three time Kavli Frontiers Fellow, the Spicer Award from the Stanford Synchrotron Radiation Laboratory, the McMillan Award from the University of Illinois and 2016 Genzel Prize. He was also the co-chair of the 2014 Gordon Research Conference in Correlated Electron Systems.
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