High Harmonic Spectroscopy Reveals Quantum Phases

Superconductors are materials that exhibit the ability to conduct electricity without any resistance. This phenomenon is observed in materials when they are cooled below the so-called superconductor transition temperature, often at very low temperatures (a few degrees above the absolute 0). Among these materials, there are the so-called high-temperature superconductors, which behave as superconductors at temperatures above 77K (the boiling point of liquid nitrogen). These materials are showing to be essential in the development of new electronic and information processing devices as well as optical quantum computers and even for improving the efficiency of electrical transmission lines.

Novel Methodology to Investigate Phase Transitions in High-Temperature Superconductors

However, researchers have observed that high-temperature superconductivity is closely linked to the control of their microscopic dynamics. So far, detecting the different microscopic quantum phases in these complex materials has proven quite challenging. The physical processes of these dynamic states remain incomplete due to their wide array of quantum states, and the current methods for exploring their dynamics at microscopic scales lack sensitivity. Therefore, researchers need new tools to better understand the dynamic evolution of these types of superconductors.

In an international study, ICFO researchers Utso Bhattacharya, Ugaitz Elu, Tobias Grass, Piotr T. Grochowski, Themistoklis Sidiropoulos, Tobias Steinle, and Igor Tyulnev, led by ICREA Professors Jens Biegert and Maciej Lewenstein, collaborated with ICMAB-CSIC researchers Jordi Alcalà and Anna Palau, and Marcelo Ciappina from the Guangdong Technion-Israel Institute of Technology to propose a new methodology. They based this methodology on the use of High Harmonic Spectroscopy (HHS) to investigate the transitions between the different phases of YBCO, a well-known high-temperature superconductor. This study represents a major scientific breakthrough as it marks the first time researchers have used a highly non-linear and non-perturbative diagnostics/detection methodology to understand the behavior of strongly correlated materials.

In view of the experimental results obtained, the researchers have also gone beyond and present a new theoretical model to identify the connection between the measured optical spectra and the transition between the different quantum states of the YBCO: strange metal, pseudogap, and superconductor. The study has been recently published in the journal PNAS.

Researchers mounted thin YBCO films (100 nm) on a micro-refrigerator. They verified the superconducting quality of the films. Then, using ultrashort infrared laser pulses, they generated high harmonics in the samples, cooled to 77K inside a vacuum chamber.

High Harmonic Spectroscopy Reveals Quantum Phase Transitions

High Harmonics are the high-energy photons that the electrons of a system emit when placed in a strong laser field. These emitted photons have a frequency many times that of the driving laser field.

When the laser pulses hit the surface, the researchers recorded the reflected radiation with a spectrograph to study the harmonic spectrum, which contains the imprints of this nonlinear optical response, and found a connection with the phase transitions.

The researchers saw these experimental results in the lab and lacked a theory that could explain their observations, so they developed a new strong-field quasi-Hubbard model to shed light on the connections between the measured high harmonics and the formation of Cooper pairs, that is, the paired electrons that are responsible for the superconducting phase.

When using this new theoretical model, the theoretical calculations of the high harmonic spectra obtained matched the experimental data. “The model faithfully reproduces the functional form of the measurement data over the entire temperature range and for several orders of magnitude of harmonic amplitude”, the authors highlighted. This new approach, as they noted, has permitted a theoretical connection between the measurements and the underlying microscope dynamics providing a “powerful new methodology to study the quantum phase transitions” in correlated materials.

Finally, the team highlights that their work provides a “first striking example” of how researchers can use High Harmonic Spectroscopy to distinguish correlated phases of matter. They also consider that it paves the way toward a “refined understanding of the physical processes that occur inside high-temperature superconductors”.

Reference article

Alcalà J, Bhattacharya U, Biegert J, Ciappina M, Elu U, Graß T, Grochowski PT, Lewenstein M, Palau A, Sidiropoulos TPH, Steinle T, Tyulnev I. (2022) High-harmonic spectroscopy of quantum phase transitions in a high-Tc superconductor. PNAS;119(40):e2207766119. doi: 10.1073/pnas.2207766119.