When a material enters its superconducting phase, the electron spectrum presents an energetic gap at the Fermi level. This direct signature of the formation of electronic Cooper pairs can be observed by different methods. Among these, ARPES (Angle Resolved Photoelectron Spectroscopy) is one of the most direct and powerful: this technique allows mapping the Fermi surface and brought the conclusive proof that copper oxide superconductors exhibit an unconventional electron coupling. In particular, ARPES experiments show that the electron gap is strongly anisotropic: maximum along the direction of the copper-oxygen (antinodal direction) and minimal at 45 ° (nodal direction) [1]. Recently, time-resolved ARPES (TR-ARPES) has been developed to explore the electron dynamics [2]. A team of researchers from LSI, LPS and UPMC used this method to map the excitation and relaxation of the superconducting condensate at the picosecond scale.

The dynamics of the superconducting gap of a cuprate (Bi₂Sr₂CaCu₂O_8+δ) in its superconducting state (T -15s) induces some electronic excitation. A second probe laser pulse (E=6 eV) photoionizes the electrons. The angular and energy analysis of the emitted electrons allows reconstituting the density of the occupied electronic states. A quasi-instantaneous closure of the gap is then observed just after the pulsed laser excitation, indicating a separation of the Cooper pairs by electronic excitation. For an illumination of 50 μJ / cm², a complete closure of the gap can be observed in a few picoseconds (10^-12 s). During the electronic relaxation, a progressive reopening of the gap is obtained in a few tens of picoseconds (10^-12 s).

Due to the instantaneous electron pairs breaking, the closing of the gap takes place within the duration of the pump laser pulse. Conversely, the reopening of the gap requires that the excited electrons dissipate their excess energy into the crystal lattice by the excitations of phonons. After a few picoseconds, the "hot" electrons have cooled down below the condensation temperature, the Cooper pairs become stable again and the gap is restored. However, this process does not occur equally in all crystallographic directions, as shown by the mappings of the reciprocal space allowed by the TR-ARPES technique. It is observed that the energy density threshold for a complete gap closure is function of the direction of observation of the electrons: it is in the direction of the copper-oxygen bonds that the gap is more robust and shows a higher perturbation upon the laser excitation. This marked anisotropy is important information about the unconventional pairing mechanism in copper oxide superconductors.

Interestingly, an additional measure of the low-frequency electrodynamics shows that Cooper pairs, with angular momentum oriented in the directions of the Cu-O bonds, remain, but they are no longer able to be the carriers of a super-current [3]. The moderate photo-excitation of the laser pump pulse alters only the macroscopic phase coherence between the paired electrons. The condensation of the Cooper pairs, intrinsically linked to superconductivity, can thus be studied and manipulated by laser excitation. The technique of TR-Femto-ARPES thus appears as a powerful tool to decipher high temperature superconductivity.