Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates

The high-T_c copper-oxide (cuprate) superconductors are a particular class of strongly correlated systems in which the interplay between the Cu 3d and the O 2p states determines both the electronic structure close to the Fermi level as well as the high-energy properties related to the formation of Zhang–Rice singlets (i.e. hole shared among the four oxygen sites surrounding a Cu, and antiferromagnetically coupled to the Cu spin) and to the charge-transfer processes (see Fig. 1).
 

One of the unsolved issues of high-Tc superconductivity is whether the electronic many-body excitations at high-energy scales are involved in the condensate formation in the under- and over-doped regions of the superconducting dome. Solving this problem, that defies an explanation within the BCS theory, would provide a benchmark for unconventional models of high-T_c superconductivity. Conventional optical spectroscopies have been widely used to investigate high-Tc superconductors at equilibrium. Although these techniques have revealed a superconductivity-induced modification of the CuO₂-planes’ optical properties involving energy scales in excess of 1 eV, the identification of the high-energy electronic excitations involved in the onset of high-temperature superconductivity has remained elusive. The main reason is that, at equilibrium, this effect is hidden by the temperature-dependent narrowing of the Drude-like peak, describing the low-energy optical properties of the free carriers. Here we solve this problem adopting a non-equilibrium approach to disentangle the ultrafast modifications of the high-energy electronic excitations from the slower broadening of the Drude-like peak induced by the complete electron-phonon thermalization. The impulsive suppression of 2ΔSC is achieved by photoexciting superconducting Bi2 Sr2Ca0.92y0.08Cu2O8+δ crystals (y-Bi2212) through an ultrashort light pulse (pump). The supercontinuum spectrum produced by a nonlinear photonic crystal fibre is used to probe the high-energy (1.2–2.2 eV) modifications of the CuO2-plane’s optical properties, as a function of the delay from the excitation.

Figure 2: Energy- and time-resolved reflectivity on Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+δ. The dynamics of the reflectivity is measured over a broad spectral range. The two-dimensional scans of  δR/R(w,t) are reported for three different doping regimes (first column: Underdoped (UD), T_C = 83 K; second column: Optimally Doped (OP), T_C = 96 K; third column: Overdoped (OD), T_C = 86 K), in the normal (first row), pseudogap (second row) and superconducting phases (third row). The insets display schematically the position of each scan in the T-p phase diagram of Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+δ. The white lines (right axes) are the time traces at 1.5 eV photon energy.

In Fig. 2 we report the time- and frequency-resolved reflectivity variation (δR/R(w,t) = Rneq (w,t)/ Req (w,t)-1, where Rneq (w,t) and Req(w,t) are the non-equilibrium (pumped) and equilibrium (unpumped) reflectivities), for the normal (top row), pseudogap (middle row), and superconducting (bottom row) phases at three different dopings. While the δR/R(w,t) measured in the normal and pseudogap phases can be reproduced assuming an impulsive modification of the Drude model parameters, in the superconducting phase the structured variation of the reflectivity at high energies is accounted for only by assuming a doping-dependent modification of both the 1.5 eV and 2 eV oscillators. 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. Superconductivity-induced changes of the optical properties at high-energy scales seem to be a universal feature of high-temperature superconductors, suggesting that this issue will be decisive in understanding high-temperature superconductivity. The technique developed in this work will trigger the next generation of optical spectroscopies, where the temporal axis is added to the frequency axis, aimed at clarifying a variety of unsolved issues in solid-state and condensed-matter physics.