Researchers use terahertz light pulses to uncover superconducting disorders

A team of researchers from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, and Brookhaven National Laboratory in the United States has demonstrated a new way to study disorder in superconductors using terahertz light pulses.

By applying techniques used in nuclear magnetic resonance to terahertz spectroscopy, the team was for the first time able to follow the evolution of disorder in the transport properties up to the superconducting transition temperature. Their work is published in Nature Physics.

The importance of disorder in physics is matched only by the difficulty of studying it: the extraordinary properties of high-temperature superconductors, for example, are strongly influenced by changes in the chemical composition of the solid.

Techniques that allow the measurement of such disorder and its effect on electronic properties, such as scanning tunneling microscopy, only work at very low temperatures and do not affect these physical properties near the transition temperature.

Superconductivity is a quantum phenomenon that allows electric current to flow without resistance and is one of the most important phenomena in condensed matter physics due to its revolutionary technological implications.

Many materials that become superconducting at so-called “high temperatures” (approximately -170°C), such as the well-known cuprate superconductors, derive their remarkable properties from chemical doping that introduces disorder, but it is still unclear how this chemical change affects their superconducting properties.

In superconductors, and more generally in condensed matter, disorder is typically studied by experiments with precise spatial resolution, such as using extremely sharp metallic tips. However, the sensitivity of these experiments limits their applicability to liquid helium temperatures, which are well below the superconducting transition, preventing the study of many fundamental questions related to the transition itself.

Taking inspiration from techniques of multidimensional spectroscopy, originally developed for nuclear magnetic resonance and later adapted to visible and ultraviolet light frequencies by chemists studying molecular and biological systems, the MPSD researchers extended this class of techniques into the terahertz frequency range, where collective modes of solids resonate.

This technique involves sequentially exciting the material of interest using multiple intense terahertz pulses, typically in a collinear geometry where the pulses travel along the same direction.

To investigate the cuprate superconductor La1.83Sr0.17CuO4, an opaque material that transmits minimal light, the team extended traditional methods to implement two-dimensional terahertz spectroscopy (2DTS) in a non-collinear geometry for the first time, allowing the researchers to isolate specific terahertz nonlinearities by radiation direction.

With this angle-resolved 2DTS technique, the researchers observed the restoration of superconducting transport in the cuprate after excitation with a terahertz pulse, a phenomenon they call the “Josephson echo.”

Remarkably, these Josephson echoes revealed that the disorder in the superconducting transport is significantly lower than the corresponding disorder observed in the superconducting gap measured by spatially resolved techniques such as scanning microscopy experiments.

Furthermore, the versatility of the angle-resolved 2DTS technique enabled the research team to measure disorder near the superconducting transition temperature for the first time, and found that disorder is stable up to a relatively warm 70% of the transition temperature.

As well as improving our understanding of the enigmatic properties of cuprate superconductors, the researchers emphasise that these initial experiments open the door to many exciting future directions: In addition to broadly applying angle-resolved 2DTS to other superconductors and quantum materials, the ultrafast nature of 2DTS also allows it to be applied to transient states in materials that are too short-lived for traditional disorder probes.