Snapshots of the retarded interaction of charge carriers with ultrafast fluctuations in cuprates

Abstract

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...

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