Probing many-body localization in a disordered quantum magnet

Abstract

Excitations in disordered systems are typically categorized as localized or delocalized, depending on whether they entail disturbances extending throughout the system or are confined to small, generally nanometer scale, subsystems. Such categorization is impossible to achieve using traditional spectroscopy where the response to a weak oscillating (ac) electromagnetic probe is measured as a function of frequency. However, the localized excitations can be separated from each other as well as the delocalized continuum by measuring a spectral "hole" in the ordinary response while a large amplitude pump is imposed at a fixed frequency. Localized excitations will result in a very sharp "hole," and any residual couplings to other excitations, both localized and extended, will determine its detailed shape. This technique probes incoherent lifetime effects as well as coherent mixing or quantum interference phenomena, describable in terms of the Fano effect. Here we show that in a disordered Ising magnet, LiHo0.045Y0.955F4, the quality factor Q for spectral holes, the ratio of the drive frequency to their width, can be as high as 100,000. In addition, we can tune the dynamics of the quantum degrees of freedom by sweeping the quantum mixing parameter through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing is associated with a dissipationless response at the drive frequency. The identification of such a point where localized degrees of freedom are minimally mixed with their environment in a dense and disordered dipolar coupled spin system implies control over the bath coupling of qubits emerging from strongly interacting many-body systems.

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