Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES
E. Perfetto, D. Sangalli, A. Marini, G. Stefanucci
DOI 10.1103/PhysRevMaterials.3.124601 · Physical Review Materials
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Abstract
We consider a ground-state band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a nonequilibrium Green's function treatment on the Konstantinov-Perel' contour. We present the first ab initio band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states. We also show that NEQ-EI states can be generated by currently available pump pulses. Peculiar fingerprints of the NEQ-EI phase in time-resolved angle-resolved photoemission spectroscopy spectra are highlighted: (i) during the pump-driving, the system goes through a BEC-BCS crossover and (ii) concomitantly the excitonic spectral structure undergoes a convex-to-concave shape transition; (iii) attosecond pulses shone after the pump-driving at different times tdelay produce a photocurrent that oscillates in tdelay with a pump-tunable frequency—we relate this phenomenon to the ac response of an exotic Josephson junction.
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