Low energy electrodynamics of strongly interacting disordered systems: quantum phase transitions and many-body localization

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Nontechnical description: It is hardly an exaggeration that most of what we know about physical systems comes from their response to perturbations at their characteristic frequencies. For instance, the fundamental tone of a plucked violin string depends on the length of the string, the tension in it, and its thickness. This is true from the acoustics of a violin to the energies of atoms. Unfortunately the natural frequency scales of many solid materials fall in a spectral range, which has been prohibitively difficult to access technically until recently. This project takes advantage of recent dramatic technical advances in THz and microwave spectroscopy to characterize the natural frequency scales of disordered solids. Material systems like superconductors, which can conduct electricity without resistance and various insulating states are being studied. The investigations performed herein give essential information to develop new materials with important technological implications. These technological developments are coupled to a broad initiative in education and outreach. The work is of particular educational value to students owing to the low frequency electrodynamics techniques that are employed and which are finding broad application in research and private industry. Public outreach activities in the form of the Johns Hopkins Physics Fair is also being realized. Technical description: Interactions, disorder, and their interplay is a central theme of modern condensed matter physics. This project is exploring two areas of intense recent interest where strong interactions and disorder conspire to create exotic low temperature states of quantum matter: the 2D superconductor insulator quantum phase transition and the phenomena of ``many-­body localization". They are being investigated by a number of novel low energy electrodynamic probes available in the PI's group. The 2D superconductor-­insulator transition (2D SIT) is a paradigmatic example of a quantum phase transition (QPT) -­ a topic of much interest in the condensed matter physics. Recent advances in low temperature microwave spectroscopies are being exploited to provide the first true dynamic information about this phase transition in thin superconducting (InO) films. The phenomena of many-­body localization is also being investigated. Recently Basko, Aleiner, and Altshuler demonstrated that for systems with strong enough disorder, localization can prevent energy or particle transport, so that the system fails to equilibrate and to be its own heat bath. This implies that (in the absence of delocalized degrees of freedom like phonons) there should be a finite temperature localization transition. We are investigating the relaxation dynamics of both electron glass systems and disordered Ising chains and among other things looking for changes in the THz relaxation as function of optical pump parameters.