Understanding the complexities of superconducting disorder

Superconducting disorder refers to disruptions within the structure of superconducting materials, which can affect their ability to conduct electricity without resistance. Understanding these complexities is crucial for advancing technologies that rely on superconductivity, such as quantum computing and medical imaging. By studying how disorder impacts superconductors, scientists aim to improve their performance, stability, and potential applications in various fields, driving innovation in energy and technology.

The significance of disorder in physics is equaled only by the challenges in studying it. For instance, the extraordinary characteristics of high-temperature superconductors are heavily influenced by fluctuations in their chemical composition. Methods like scanning tunneling microscopy, which measure this disorder and its effects on electronic properties, are limited to very low temperatures and cannot capture the physics near the transition temperature. However, a group of researchers has now introduced a novel approach to investigating disorder in superconductors using terahertz light pulses.

The significance of disorder in physics is rivaled only by the challenges of studying it. For instance, high-temperature superconductors exhibit remarkable properties that are heavily influenced by variations in their chemical makeup.

Techniques like scanning tunneling microscopy, which can measure disorder and its effects on electronic properties, are limited to very low temperatures and cannot capture the physics near the superconducting transition temperature.

Now, researchers from the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Germany and Brookhaven National Laboratory in the United States have introduced a novel way to study disorder in superconductors using terahertz light pulses. By adapting methods used in nuclear magnetic resonance to terahertz spectroscopy, they were able to trace disorder in transport properties up to the superconducting transition temperature for the first time. This work, led by the Cavalleri group, has been published in Nature Physics.

Superconductivity, a quantum phenomenon allowing electrical currents to flow without resistance, is crucial in condensed matter physics due to its transformative technological potential. Many high-temperature superconductors, like cuprates, owe their unique properties to chemical doping, which introduces disorder, although its precise effect on superconductivity remains unclear.

Typically, disorder in superconductors and other condensed matter systems is studied using experiments with high spatial resolution, such as with sharp metallic tips. However, the sensitivity of these techniques limits them to liquid helium temperatures, preventing the study of fundamental questions related to the transition.

Inspired by 'multi-dimensional spectroscopy' originally developed for nuclear magnetic resonance and later applied to optical frequencies, MPSD researchers adapted this approach to the terahertz frequency range, where collective modes of solids resonate. This involved sequentially exciting materials with intense terahertz pulses. To study the cuprate superconductor La1.83Sr0.17CuO4, the team implemented two-dimensional terahertz spectroscopy (2DTS) in a non-collinear geometry for the first time, isolating specific terahertz nonlinearities based on their emission direction.

Using this angle-resolved 2DTS technique, they discovered that superconducting transport in the cuprate could be restored after terahertz pulse excitation, a phenomenon they called 'Josephson echoes.' Surprisingly, these echoes indicated that the disorder in superconducting transport was lower than the disorder measured in the superconducting gap using spatially resolved methods like scanning microscopy. Moreover, the angle-resolved 2DTS allowed the team to measure disorder near the superconducting transition temperature, remaining stable up to 70% of the transition temperature.

This breakthrough offers new insights into the properties of cuprate superconductors, and the researchers believe their experiments pave the way for future exploration. Beyond investigating other superconductors and quantum materials, the ultrafast nature of 2DTS makes it suitable for studying transient states of matter that are too short-lived for conventional techniques.

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Source: sciencedaily‍