Narrowband phonon pumping for the investigation of light-induced superconductivity

The large-amplitude coherent mid-infrared excitation of apical oxy- gen oscillations in bilayer cuprates Y Ba2CuO6+x is known to promote a short-lived superconducting-like state even far above the critical temperature. Subsequent time-resolved x-ray di raction experiments showed that the nonlinear coupling of the resonantly driven apical oxy- gen phonon mode to a set of Raman-active modes induces a transient crystal structure likely to favour this out-of-equilibrium superconduc- tivity. However, the splitting of the apical oxygen vibrations into lower- and higher- frequency modes at 16.5 and 19.3 THz { corresponding to the oscillations of the apical oxygen atoms in the oxygen-rich and oxygen-de cient Cu-O chains, respectively { was disregarded in these studies. The two modes were indeed excited simultaneously because the broadband driving pulses available (30 % DeltaE/E bandwidth) didn't allow to distinguish between them. Here, we present a mid-infrared pulsed light source su ciently nar- rowband and tunable to drive separately the two near-degenerate api- cal oxygen phonon modes in the bilayer cuprate Y BaCuO6:5. By exploiting chirped pulse di erence frequency generation in GaSe non- linear crystal, we managed to produce carrier-envelope phase stable pulses, tunable between 16 THz and 23 THz, with a minimum relative bandwidth DeltaE/E of 2% The bandwidth of these pulses scaled linearly with their time duration, which can be chosen, by the amount of chirp imprinted on the generating near-infrared pulses, between 200 fs and 1 ps. The energy of the mid-infrared pulses around 8 J could be kept constant even for di erent pulse durations by adjusting the spot sizes of the generating NIR pulses in the GaSe nonlinear crystal to maintain the gain coefficient.By means of this source, we were able to clearly distinguish, possi- bly for the rst time, the e ect of each of the two vibrational modes on the light-induced superconducting state. The data from pump-probe time-resolved THz-spectroscopy suggested that the Josephson cou- pling energy between bilayers in the non-equilibrium superconducting state scales with the real part of the dielectric constant epsilon1, i.e, with the strength of the mechanical displacement of the lattice atoms involved in the coherent transport between bilayers along the c-axis of the crystal. This conclusion may help nding optimized routes for enhancing superconductivity with light and for making this transient states long-lived, in view of possible applications.

Studio della superconduttivita` indotta ad alte temperature tramite l'eccitazione del materiale YBCO con impulsi laser a banda stretta in grado di eccitare in maniera selettiva i modi vibrazionali del cristallo associati con l'oscillazione degli atomi di ossigeno nella catena Cu-O nella regione tra le coppie di piani superconduttivi.