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”.
Acknowledgements
We thank V. Kunets from MMR Technologies and X. Menino at Institut de Ciencies Fotoniques for technical support and Dr. K. Dewhurst and Dr. S. Sharma for help with the ELK code. J.B., U.E., T.P.H.S., T.S., and I.T. acknowledge financial support from the European Research Council (ERC) for ERC Advanced Grant “TRANSFORMER” (grant 788218) and ERC Proof of Concept Grant “miniX” (grant 840010). J.B. and group acknowledge support from FET -OPEN “PETACom” (grant 829153), FET-OPEN “OPTOlogic” (grant 899794), EIC-2021-PATHFINDEROPEN “TwistedNano” (grant 101046424), Laserlab-Europe (grant 654148), Marie Sklodowska-Curie ITN “smart-X” (grant 860553), Plan Nacional PID-PID2020-112664GB-I00-210901, AGAUR for 2017 SGR 1639, “Severo Ochoa” (grant SEV-2015-0522), Fundació Cellex Barcelona, the CERCA Programme/Generalitat de Catalunya, and the Alexander von Humboldt Foundation for the Friedrich Wilhelm Bessel Prize. U.B., T.G., P.T.G., and M.L. acknowledge funding from the ERC for ERC Advanced Grant NOQIA; Agencia Estatal de Investigación (R&D project CEX2019-000910-S, funded by MCIN/AEI/10.13039/501100011033, Plan National FIDEUA PID2019-106901GB-I00, FPI, QUANTERA MAQS PCI2019-111828-2, Proyectos de I+D+I “Retos Colaboración” RTC2019-007196-7); Fundació Cellex; Fundació Mir-Puig; Generalitat de Catalunya through the CERCA program; AGAUR grant 2017 SGR 134; QuantumCAT U16-011424, cofunded by ERDF Operational Program of Catalonia grant 2014-2020; EU Horizon 2020 FET-OPEN OPTOLogic (grant 899794); National Science Centre, Poland (Symfonia grant 2016/20/W/ST4/00314); Marie Skłodowska-Curie grant STREDCH 101029393; “La Caixa” Junior Leaders fellowships (ID100010434); and EU Horizon 2020 under Marie Skłodowska-Curie grant agreement 847648 (LCF/BQ/PI19/11690013, LCF/BQ/PI20/11760031, LCF/BQ/PR20/11770012, and LCF/BQ/PR21/11840013). P.T.G. acknowledges the Polish National Science Center grants 2018/31/N/ST2/01429 and 2020/36/T/ST2/00065 and is supported by the Foundation for Polish Science. Center for Theoretical Physics of the Polish Academy of Sciences is a member of the National Laboratory of Atomic, Molecular and Optical Physics. A.P. and J.A. acknowledge support from the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (grant SEV-2015-0496), SuMaTe project (RTI2018-095853-B-C21) cofinanced by the European Regional Development Fund, and the Catalan Government grant 2017-SGR-1519. Supported by EU COST action NANOCOHYBRI CA16218. M.C. acknowledges financial support from the Guangdong Province Science and Technology Major Project (Future functional materials under extreme conditions – 2021B0301030005).
We also acknowledge the Scientific Services at Institut de Ciència de Materials de Barcelona and ICN2.
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