Unveiling the nature of the bosonic excitations that mediate the formation of Cooper pairs is a key issue for understanding unconventional superconductivity. A fundamental step toward this goal would be to identify the relative weight of the electronic and phononic contributions to the overall frequency (Ω) dependent bosonic function, Π(Ω). We perform optical spectroscopy on Bi2Sr2Ca0.92Y0.08Cu2O 8+δ crystals with simultaneous time-and frequency-resolution; this technique allows us to disentangle the electronic and phononic contributions by their different temporal evolution. The strength of the interaction (λ∼1.1) with the electronic excitations and their spectral distribution fully account for the high critical temperature of the superconducting phase transition. [7]. Inelastic neutron and X-ray scattering experiments found evidence for both QP-phonon anomalies [8] and bosonic excitations attributed to spin fluctuations [7,9] and loop currents [10]. Dip features in tunnelling experiments have been used to alternatively support the scenarios of dominant electron-phonon interactions [11] or antiferromagnetic spin fluctuations [12]. The frequency-dependent dissipation of the Drude optical conductivity, σ (ω), measured by equilibrium optical spectroscopies, has been interpreted [13][14][15] as the coupling of electrons to bosonic excitations, in which the separation of the phononic and electronic contributions is impeded by their partial coexistence on the same energy scale (<90 meV).We disentangle the electronic and phononic contributions to Π(Ω) through a non-equilibrium optical spectroscopy, in which the femtosecond time-resolution is combined with an energy-resolution smaller than 10 meV, over a wide photon energy range (0.5-2 eV). Our approach is based on the widely-used assumption [16,17] that, after the interaction between a superconductor and a short laser pulse (1.55 eV photon energy), the effective electronic temperature (T e ) relaxes toward its equilibrium value through energy exchange with the different degrees of freedom that linearly contribute to Π(Ω). In a more formal description, the total bosonic function is given by Π(Ω)=Π be (Ω)+Π SCP (Ω)+Π lat (Ω) where Π be refers to the bosonic excitations of electronic origin at the effective temperature T be , Π SCP to the small fraction of strongly-coupled phonons (SCPs) at T SCP [16] and Π lat to all other * Electronic address: s.f.p.dalconte@tue.nl † Electronic address: claudio.giannetti@unicatt.it

In strongly correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high-energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high-energy scale physics associated with Mott-like excitations (|E−EF|>1 eV) is involved in the condensate formation. Here, we report the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV, and the onset of HTSC. This is revealed by a novel optical pump-supercontinuum-probe technique that provides access to the dynamics of the dielectric function in Bi2Sr2Ca0.92Y0.08Cu2O8+δ over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc).

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive as electronic correlations are expected to greatly accelerate the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and frequency range of transient reflectivity measurements up to an unprecedented level, enabling us to directly observe the ∼16 fs build-up of the e ective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.A fter almost 30 years of intensive experimental and theoretical efforts to understand the origin of high-temperature superconductivity in copper oxides, a consensus about the microscopic process responsible for the superconducting pairing is still lacking. The large Coulomb repulsion U 1 eV between two electrons occupying the same lattice site is believed to have fundamental consequences for the normal state of these systems 1 , and it is not clear whether a BCS-like bosonic glue that mediates the electron interactions and eventually leads to pairing can still be defined [2][3][4] . The fundamental issue can be reduced to the question whether the electronic interactions are essentially unmediated and instantaneous, or whether the low-energy physics, including superconductivity, can be effectively described in terms of interactions among the fermionic charge carriers mediated by the exchange of bosons. The problem can be rationalized by considering the Hubbard model, in which the instantaneous virtual hopping of holes into already occupied sites (with an energy cost of U ) inherently favours an antiferromagnetic (AF) coupling J = 4t 2 h /U between neighbouring sites, where t h is the nearestneighbour hopping energy. As a consequence, antiferromagnetic fluctuations with a high-energy cutoff of 2J U naturally emerge as a candidate 5 for mediating the low-energy electronic interactions, on a characteristic retarded timescale of the order ofh/2J .In principle, time-resolved optical spectroscopy 6 may be used to prove the existence of an effective retarded boson-mediated interaction, provided that the temporal resolution is of the order of the inverse bosonic-fluctuation scale (for example,h/2J for AF fluctuations) and the optical properties are probed over a sufficiently broad frequency range, to extract the dynamics of the electron-boson coupling. Recent advances in ultrafast optical spectroscopy have succeeded in separately fulfilling these requirements. For example, high-temporal-resolution (<15 fs) experiments 7,8 have been carried out to investigate the...

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system’s initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system’s excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.

The impulsive modification by ultrashort light pulses (∼100 fs) of electronic, magnetic and structural properties in complex materials, has recently opened the field of photo-induced phase-transitions (PIPT)(1). Previous studies have focused on the insulator-to-metal PIPT in perovskite manganites (2-4) or vanadium dioxide (5,6), in which the transformation of both the electronic and structural properties occurs at once. On the contrary no evidence of optical control of a purely electronic phase transition has been reported so far.The superconducting-to-normal state phase-transition (SNPT) is one of the most important examples of electronic phase transitions in solid-state systems. At zero temperature the BCS theory (7) explains the formation of the superconducting (SC) electronic phase in terms of the macroscopic condensation of the Cooper pairs, originated from the phonon-mediated coupling of two electrons. The excitation spectrum is characterized by the opening of the superconducting gap Δk, representing half the energy necessary to break a Cooper pair and create two excitations with wavevectors +k and -k, i.e. quasiparticles (QP) (7). At finite temperature the main role of the electronic distribution is to block states otherwise available to form the superconducting condensate.As the temperature of the system is increased, the electronic distribution causes the continuous closing of the SC gap Δk(T) up to the point where Δk(TC)=0 and the second-order SNPT takes place.