Ultrafast Dynamics in High-Temperature Superconductors near a Quantum Critical Point

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Ultrafast time-resolved spectroscopy offers unique possibilities to study nonequilibrium dynamics of elementary excitations in condensed matter. This method has been particularly successful in studying the critical slowing down of relaxation processes at phase transitions, and tracing the evolution of the complex order parameter and the interplay between the lattice and electronic condensates. However, up to now, most studies have been performed only by varying temperature, where an electronic order is suppressed by thermal fluctuations. This approach does not allow investigating phase transitions between ground-state quantum phases (close to absolute zero) of a system. These quantum phase transitions are extremely important for understanding of high-Tc superconductors, where the coexistence of superconductivity with a magnetic order and its role in the strong enhancement of the superconducting transition temperature is widely debated. Nevertheless a detailed experimental picture of quantum phase transitions in high-Tc superconductors has not been obtained yet.Here we propose to utilize time-resolved terahertz spectroscopy to resolve this problem for the novel class of iron-based superconductors. Our approach relies on the synergy of ultrafast nonlinear spectroscopy and low-temperature high-pressure technology. It allows us to distinguish competing types of electronic order near the quantum critical point by comparing their ultrafast dynamics and spectral response. The experiments will be performed on two classes of superconducting materials: iron-based pnictides of 122 type (e.g. BaFe2As2) and iron telluride-selenides (FeTe1-xSex) which demonstrate a quantum criticality in the vicinity of the superconducting dome. Results of our studies will shed light on microscopic mechanisms leading to the formation of Cooper pairs and, in particular, clarify the role of magnetic and charge order fluctuations in this process.