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Abstract

The large-amplitude coherent mid-infrared excitation of apical oxy- gen oscillations in bilayer cuprates Y Ba 2 Cu 3 O 6+ x is known to pro- mote a short-lived superconducting-like state even far above the crit- ical temperature. Subsequent time-resolved x-ray diffraction exper- iments showed that the nonlinear coupling of the resonantly driven apical oxygen phonon mode to a set of Raman-active modes induces a transient crystal structure likely to favour this out-of-equilibrium superconductivity. 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-deficient Cu-O chains, respectively – was disregarded in these studies. The two modes were indeed excited simultaneously because the broadband driving pulses available (30 % ∆ E E bandwidth) didn’t allow to distinguish between them. Here, we present a mid-infrared pulsed light source sufficiently nar- rowband and tunable to drive separately the two near-degenerate api- cal oxygen phonon modes in the bilayer cuprate Y Ba 2 Cu 3 O 6 . 5 . By exploiting chirped pulse difference 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 ∆ E 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 μ J, could be kept constant even for different 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 first time, the effect of each of the two vibrational modes on the light-induced superconducting state. The data from pump-probe time-resolved THz-spectroscopy showed that the strength of the tran- sient superconducting coupling between bilayers above the equilibrium critical temperature – measured through the value of transient super- fluid density along the c-axis, ρ c – scales linearly with the amplitude of the driving electric field. According to the theory of nonlinear phononics, this implies that ρ c scales with the square root of the nor- mal coordinate Q R of the Raman mode associated with the reduction in the distance between bilayers. Futhermore, ρ c was found to assume a much higher value for the phonon at 19.28 THz that, between the two investigated, has the biggest spectral weight. These conclusions may help finding optimized routes for enhancing superconductivity with light and for making this transient states long- lived, in view of possible applications.