Cuprate Superconductor Dynamics: Ultrafast Excitations Relax Via 100, 300 Fs and 1, 5ps Channels

Understanding how energy influences the behaviour of complex materials remains a central challenge in physics, and recent work addresses this question in the context of superconductivity. Alessandra Milloch, Francesco Proietto, and Naman Agarwal, alongside colleagues at institutions including Universit`a Cattolica del Sacro Cuore and KU Leuven, investigate the ultrafast response of a cuprate superconductor to energy input. Their experiments demonstrate that the speed and nature of electronic changes within the material depend primarily on the amount of energy delivered, rather than the specific method of excitation. This finding establishes extreme-ultraviolet pulses from free-electron lasers as a powerful new tool for probing and controlling the behaviour of materials on incredibly short timescales, potentially paving the way for advances in understanding and manipulating complex electronic states.

Ultrafast Carrier Dynamics in High-Tc Superconductors

Scientists investigated the behaviour of the high-temperature superconductor Bi₂Sr₂CaCu₂O₈₊δ using ultrafast optical techniques to understand how excited electrons, known as carriers, behave and how the superconducting state responds to strong pulses of light. Measurements were performed at both room temperature and below the critical temperature where superconductivity emerges, allowing the team to determine the timescales and mechanisms governing the relaxation of these excited carriers. Experiments at room temperature revealed that the material’s response to light varies depending on the energy of the light pulse, with the decay of the excited state following multiple pathways at different rates. Oscillations in the signals suggest the presence of coherent vibrations within the material’s crystal structure, contributing to the overall relaxation process and providing insights into its dynamic properties. Lowering the temperature introduced a “boson bottleneck” effect, where the relaxation of excited carriers slows down due to a limited number of available final states. This observation provides valuable information about the fundamental processes governing superconductivity and confirms the reliability of the experimental setup and analysis methods.

EUV Pump-Probe Mapping of Superconducting Dynamics

Scientists pioneered a new approach to studying ultrafast dynamics in materials by employing a free-electron laser (FEL) to deliver extreme-ultraviolet (EUV) light alongside near-infrared laser pulses. This technique was applied to the cuprate superconductor Bi₂Sr₂CaCu₂O₈₊δ, utilizing systematic variation of EUV energy and FEL pulse intensity to map excitation pathways and their impact on the material. The experimental setup enabled direct driving of ultrafast dynamics using EUV light, achieving the necessary energy density to perturb the superconducting condensate and observe transient changes in its properties. Measurements were conducted at room temperature, allowing detailed analysis of excitation pathways and their influence on the material’s electronic structure. The team discovered that both near-infrared and extreme-ultraviolet excitations lead to remarkably similar ultrafast dynamics, primarily governed by the amount of energy absorbed rather than the specific microscopic excitation pathway. This establishes EUV free-electron laser spectroscopy as a powerful new tool for quantum materials research, opening avenues for exploring the intrinsic timescales of electronic correlations with unprecedented precision.

EUV Drives and Controls Cuprate Dynamics

Scientists achieved a breakthrough by demonstrating that extreme ultraviolet (EUV) light can effectively drive and control dynamics within the cuprate superconductor Bi₂Sr₂CaCu₂O₈₊δ. Experiments reveal that excitations, whether initiated by EUV or near-infrared light, relax through remarkably similar channels, characterized by fast components spanning 100 to 300 femtoseconds and slower components lasting 1 to 5 picoseconds. Experiments conducted at 30K and room temperature demonstrate that EUV excitation, even at core-level resonances, produces dynamics that converge with those induced by near-infrared excitation. The team meticulously controlled excitation intensity and accounted for energy-dependent absorption to accurately assess the excitation volume, demonstrating a universal non-equilibrium response spanning a wide range of excitation energies. The measurements confirm that EUV-induced dynamics closely resemble those observed with near-infrared excitation, establishing EUV free-electron laser spectroscopy as a powerful frontier tool for quantum materials research and a cornerstone for investigating nonequilibrium phenomena in complex quantum systems.

Energy Density Drives Ultrafast Superconductor Dynamics

This research demonstrates a surprising universality in the behaviour of high-temperature superconductors when excited by light. Scientists achieved comparable ultrafast dynamics in a cuprate superconductor using both extreme ultraviolet (EUV) and near-infrared laser pulses, despite accessing fundamentally different electronic states. The resulting response is primarily governed by the amount of energy absorbed, revealing a dominant role for energy density in driving the initial stages of relaxation. Below the superconducting transition temperature, both excitation methods reveal a delayed signal linked to the recombination of electrons and holes, and the recovery of the superconducting condensate, indicating that the superconducting state maintains coherence even under high-energy excitation.

However, EUV excitation requires a significantly higher energy density due to its limited penetration depth, leading to more localized excitation and a weaker signal. The findings establish EUV pulses as a powerful new tool for probing and controlling the behaviour of quantum materials on ultrafast timescales, extending the capabilities of existing optical techniques. Future work aims to clarify whether the observed dynamics directly couple to the condensate or proceed through indirect scattering processes, laying the groundwork for next-generation pump-probe studies and exploring correlated phases using soft X-ray and attosecond spectroscopies.