Researchers from ICFO, ICMAB-CSIC and Guangdong Technion-Israel Institute of Technology have developed a new methodology to investigate and measure the quantum phase transitions of a high-temperature superconductor by using High Harmonic spectroscopy.
Exploring Superconductors
Superconductors are materials that exhibit the ability to conduct electricity without any resistance. This phenomenon occurs in materials when they cool below the so-called superconductor transition temperature, often at very low temperatures (a few degrees above absolute zero). Among these materials are the so-called high-temperature superconductors, which function as superconductors at temperatures above 77K (the boiling point of liquid nitrogen). Researchers have found these materials to be crucial in developing new electronic and information processing devices, as well as optical quantum computers, and even in improving the efficiency of electrical transmission lines.
Challenges in Understanding High-Temperature Superconductivity
However, scientists have observed that high-temperature superconductivity closely relates to controlling their microscopic dynamics. So far, detecting the various microscopic quantum phases in these complex materials has proven quite challenging. Not only do these dynamic states have incomplete physical processes due to their wide array of quantum states, but the current methods used to explore their dynamics at microscopic scales lack sensitivity. Therefore, researchers need new tools to better understand the dynamic evolution of these types of superconductors.
Now, 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, in collaboration with ICMAB-CSIC researchers Jordi Alcalà and Anna Palau, and Marcelo Ciappina, from the Guangdong Technion-Israel Institute of Technology, proposed a new methodology based on the use of High Harmonic spectroscopy (HHS) to investigate the transitions between the different phases of YBCO, a copper oxide cuprate material which is a well-known high-temperature superconductor. This study represents a major scientific breakthrough since it is the first time that highly non-linear and non-perturbative diagnostics/detection methodology is used to understand the behavior of strongly correlated materials.
Experimental and Theoretical Innovations
In light of the experimental results, the researchers surpassed expectations and presented a new theoretical model to identify the connection between the measured optical spectra and the transition between the different quantum states of YBCO: strange metal, pseudogap, and superconductor. The study, recently published in the journal PNAS, showcases their findings.
In their experiment, the researchers used 100nm thick films of YBCO mounted on a micro-refrigerator. They first characterized the superconducting properties of the YBCO films and confirmed their quality. Then, using ultra-short infrared laser pulses, they induced high harmonics generation in the material samples, which they placed inside a vacuum chamber and cooled to a temperature of 77K.
High Harmonics are the high-energy photons emitted by the electrons of a system when it experiences a strong laser field. These emitted photons have a frequency many times that of the driving laser field.
Upon hitting the surface, they recorded the reflected radiation with a spectrograph to study the harmonic spectrum. This spectrum contains the imprints of this nonlinear optical response and has a connection with the phase transitions.
Theoretical Modeling
Observing these experimental results in the lab and the absence of a theory that could explain what they were observing, the researchers 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 responsible for the superconducting phase.
When applying 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 allowed 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 emphasizes that their work offers a “first striking example” of how High Harmonic Spectroscopy can distinguish correlated phases of matter. They also believe that it paves the way for a “refined understanding of the physical processes occurring inside high-temperature superconductors”.
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