Bismuth under intense laser pulses: a Fano-like interference description.

New time-resolved experiments by Misochko et al published in a recent issue of J. Phys.: Condens. Matter [1] have furthered our understanding of the complex behaviour of Bi far from equilibrium. There has been a long-standing interest in the dynamical path from a coherent to a stochastic state. In 1955, Fermi, Pasta, and Ulam (FPU) [2] numerically tested the ergodicity hypothesis of statistical mechanics. By considering a chain of classical harmonic oscillators coupled with a quadratic nonlinearity they investigated how the energy in one mode spreads to the rest. The study was performed on the fastest computing machine at the time (MANIAC I at Los Alamos). Surprisingly, instead of a gradual, continuous flow of energy from the first vibrational mode to the higher modes, FPU found that the system cycled periodically, i.e. after a while the first mode was revived. Nowadays, the availability of intense femtosecond laser pulses in small-scale laboratories enables the generation and study of coherent phonon states in real materials. Particularly interesting for applications are the large-amplitude phonons, as they can offer a unique way to manipulate lattice structures and shapes [3]. However, understanding large-amplitude coherent dynamics is a difficult task, as it involves both electronic and ionic degrees of freedom departing from their ground state. Bi is an interesting system to probe. It is a semimetal that exhibits a delicate coupling between its electronic and ionic subsystems. This is evidenced by its α-arsenic A7 crystal structure, which is an almost perfect cubic structure with a slight elongation along the (111) diagonal (figure 1). The origin of this distortion is the Peierls effect, which is most commonly encountered in one-dimensional systems. In the report of Misochko et al [1], two coherent modes of different symmetries are excited and studied: a fully symmetric A1g mode, where atomic motion occurs along the Peierls distortion direction, and a degenerate Eg mode, where motion occurs in the perpendicular plane (figure 1). Various time-resolved studies on the Bi response obtained that low-amplitude coherent phonon dynamics showed the expected decay and thermalization. However, for abovethreshold excitations, the recurrent dynamics of coherent phonons reminds one of the celebrated FPU paradox, showing a collapse and revival in the A1g mode [4]. Such nonlinear behaviour brings into attention both lattice anharmonicity [5] and dynamical screening effects [6]