Attoscience lights the way to superconductivity

Advances in attosecond soft X-ray spectroscopy by ICFO researchers have transformed materials analysis, particularly in the study of light-matter interactions and many-body dynamics, with promising implications for future technological applications.

X-ray absorption spectroscopy is an element-selective and electronic state-sensitive technique, and is one of the most widely used analytical techniques to study the structure of substances or substances. Until recently, this method required laborious wavelength scanning and did not provide ultrafast temporal resolution for studying electronic dynamics.

Over the past decade, the Attoscience and Ultrafast Optics group at ICFO, led by ICREA Professor at ICFO Jens Biegert h, has developed attosecond soft X-ray absorption spectroscopy into a new analytical tool without the need for scanning and with attosecond resolution.^[1,2]

A breakthrough in attosecond soft X-ray spectroscopy

Attosecond soft X-ray pulses with a duration of 23 to 165 ft and an associated coherent soft X-ray bandwidth of 120 to 600 eV^[3] Allowing the entire electronic structure of the material to be interrogated at once.

Combining the time resolution of real-time electronic motion detection and the coherent bandwidth that records where the change occurs provides an entirely new and powerful tool for solid-state physics and chemistry.

Exposing graphite to an ultrashort mid-infrared laser pulse gives rise to a highly conducting hybrid phase of photonic matter, in which optically excited electrons are strongly coupled to coherent photonic phonons. Observing such a strong multi-body state, which is optically excited, is made possible by studying the lifetimes of excited electronic states using a soft, attosecond X-ray pulse. Credit: ©ICFO

One of the most important processes is the interaction of light with matter, for example, to understand how solar energy is harvested in plants or how a solar cell converts sunlight into electricity.

A fundamental aspect of materials science is the possibility that the quantum state, or function, of a material or matter can be changed by light. Such research into the many-body dynamics of materials addresses fundamental challenges in contemporary physics, such as what triggers any quantum phase transition, or how material properties arise from microscopic interactions.

A recent study conducted by researchers from ICFO

In a recent study published in the magazine Nature CommunicationsICFO researchers Themis Sidiropoulos, Nicola Di Palo, Adam Summers, Stefano Severino, Maurizio Reduzzi, and Jens Bigert report observing a light-induced increase and control of conductivity in graphite by manipulating the multi-body state of the material.

Innovative measurement techniques

The researchers used light pulses with stable subcycle in the carrier phase and enveloped at 1850 nm to induce the hybrid state of the photonic material. They investigated the electronic dynamics using attosecond soft X-ray pulses with 165 km at the carbon K edge of graphite at 285 eV. Attosecond soft X-ray absorptiometry interrogated the entire electronic structure of the material at attosecond pump-probe delay steps. The pump at 1850 nm induced a high conductivity state in the material, which only exists due to photomatter interaction; Hence it is called light matter hybrid.

Researchers are interested in such conditions because they are expected to give rise to quantum properties of materials that exist in no other equilibrium state, and these quantum states can be switched at fundamental optical speeds of up to several terahertz.

However, it is largely unclear how exactly states emerge within materials. Hence, there is much speculation in recent reports about light-induced superconductivity and other topological phases. ICFO researchers have used attosecond pulses of soft X-rays for the first time to “look inside matter” and also show the state of matter with light.

“The requirements for coherent investigation, attosecond time resolution and attosecond synchronization between pump and probe are completely new and an essential requirement for such new investigations enabled by attosecond science,” notes the study's first author, Themis Sidiropoulos.

Electron dynamics in graphite

Unlike electron coils and twisted bilayers Graphene“Instead of manipulating the sample, we optically excite the material with a powerful pulse of light, thereby exciting the electrons to high-energy states and observing how these electrons relax” within the material, not only individually but As a complete system, monitor the interaction between charge carriers and the network itself.

To find out how the electrons in graphite relaxed after applying a strong pulse of light, they took a broad spectrum of Different energy levels. By observing this system, they were able to see that the energy levels of all charge carriers indicated that the photoconductivity of the material increased at some point, indicating signatures or memories of the superconducting phase.

Observation of coherent phonons

How were they able to see this? Well, actually, in a previous post, they observed the behavior of coherent (rather than random) phonons or collective excitation of atoms within a solid. Because graphite contains an array of very strong (high-energy) phonons, it can efficiently transfer large amounts of energy away from the crystal without damaging the material through the mechanical vibrations of the lattice. Because these coherent phonons move back and forth, like a wave, the electrons inside the solid appear to ride the wave, generating the signatures of artificial superconductivity that the team observed.

Implications and future prospects

The results of this study show promising applications in the field of photonic integrated circuits or optical computing, using light to manipulate electrons or control the properties of materials and manipulate them with light. As Jens Bigert concludes, “Many-body dynamics is at its core, and arguably one of the most challenging problems in contemporary physics. The results we have obtained here open up a new world of physics, offering new ways of investigating and manipulating interconnected phases of matter in real time, which They are crucial to modern technologies.

Reference: “Enhanced optical conductivity and many-body effects in strongly photo-catalyzed semi-metallic graphite” by TPH Sidiropoulos and N. Di Palo, D. E. Rivas, and A. Summers and S. Severino and M. Reduzzi and J. Biegert, November 16, 2023, Nature Communications.
doi: 10.1038/s41467-023-43191-5

Notes

1. “A sub-cycle driven, high-flux table-top soft Bodis, September 14, 2014, Optics Letters.
   doi:10.1364/OL.39.005383
2. “Fine structure spectroscopy of dispersive soft Barbara Buddis and Frank Coppins, May 19, 2018, optica.
   doi:10.1364/OPTICA.5.000502
3. “Attosecond lines in the water window: a new system for characterizing attosecond pulsation” by Seth L. Cosin, Nicola Di Palo, Barbara Bodis, Stefan M. Tishman, M. Reduzzi, M. DeVita, A. Jens Bigert, November 2, 2017, Physical review.
   doi: 10.1103/PhysRevX.7.041030