The significance of disorder in physics is as great as the challenges that come with studying it. For instance, the outstanding characteristics of high-temperature superconductors are heavily influenced by changes in the chemical makeup of the material. Current techniques that measure this disorder and its effects on electronic properties, like scanning tunneling microscopy, only function at very low temperatures and do not provide insights into the behavior close to the transition temperature. Recently, researchers from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Germany, along with Brookhaven National Laboratory in the United States, have introduced a novel method for investigating disorder in superconductors using terahertz pulses of light.
The significance of disorder in physics is as great as the challenges that come with studying it. For instance, the outstanding characteristics of high-temperature superconductors are heavily influenced by changes in the chemical makeup of the material. Current techniques that measure this disorder and its effects on electronic properties, like scanning tunneling microscopy, only function at very low temperatures and do not provide insights into the behavior close to the transition temperature. Recently, researchers from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Germany, along with Brookhaven National Laboratory in the United States, have introduced a novel method for investigating disorder in superconductors using terahertz pulses of light. By adapting nuclear magnetic resonance techniques for terahertz spectroscopy, the team successfully tracked the changes in disorder affecting transport properties right up to the superconducting transition temperature, marking a first in this field. Their findings, reported by the Cavalleri group, have been published in Nature Physics.
Superconductivity, a quantum phenomenon that allows electrical currents to flow without resistance, is one of the most crucial phenomena in condensed matter physics due to its profound technological implications. Many materials that exhibit superconductivity at so-called ‘high temperatures’ (around -170°C), like the widely studied cuprate superconductors, owe their unique properties to chemical doping, which introduces disorder. Nonetheless, the precise influence of these chemical changes on their superconducting characteristics remains to be clarified.
In the context of superconductors and condensed matter systems in general, disorder is usually examined with experiments that require high spatial resolution, often relying on very sharp metallic tips. However, the sensitivity required for these experiments limits their use to temperatures achievable only with liquid helium, which is far below the necessary superconducting transition, thus hindering the exploration of several key questions related to said transition.
Inspired by ‘multi-dimensional spectroscopy’ methods originally developed for nuclear magnetic resonance and later modified for use in visible and ultraviolet optical frequencies, MPSD researchers have successfully adapted these methodologies to the terahertz frequency domain, where solid materials resonate. This approach involves exciting a sample with multiple strong terahertz pulses in a sequential manner, typically in a collinear layout where the pulses move in the same direction. To analyze the cuprate superconductor La1.83Sr0.17CuO4—which is an opaque material with poor light transmission—the team innovatively employed two-dimensional terahertz spectroscopy (2DTS) in a non-collinear arrangement for the first time, allowing them to focus on specific terahertz nonlinearities based on their direction of emission.
With this angle-resolved 2DTS technique, the researchers found that the superconducting transport in the cuprate was rejuvenated following excitation by the terahertz pulses—a phenomenon they referred to as ‘Josephson echoes’. Interestingly, these echoes indicated that the disorder associated with superconducting transport was considerably less than what was observed for the superconducting gap through spatially resolved techniques like scanning microscopy. Additionally, the flexibility of the angle-resolved 2DTS method allowed the team to explore disorder levels close to the superconducting transition temperature for the first time, revealing that it remained stable up to a relatively warm 70% of this transition temperature.
In addition to enhancing the understanding of the puzzling characteristics of cuprate superconductors, the researchers highlight that these initial experiments pave the way for numerous exciting future possibilities. Beyond applying angle-resolved 2DTS techniques to other superconductors, the quick nature of 2DTS also makes it suitable for studying transient states of matter that are too brief for standard disorder probing techniques.
