Researchers at the University of California, Irvine have recently made groundbreaking advancements in understanding how superconductivity is enhanced at the atomic level in an iron-based material, specifically iron selenide (FeSe) when layered on a strontium titanate (STO) substrate. This discovery, published in Nature, could pave the way for more efficient superconducting materials with applications ranging from quantum computing to medical technologies.
Superconductivity, the phenomenon where certain materials can conduct electricity without resistance at low temperatures, has long been a subject of intense study due to its potential in revolutionizing industries like energy transmission and computing. One of the major challenges has been achieving superconductivity at higher temperatures. Iron-based superconductors like FeSe have shown promise, but the mechanisms that allow for enhanced superconductivity in these materials have remained elusive.
At UC Irvine, scientists utilized advanced spectroscopy tools located in the UC Irvine Materials Research Institute (IMRI) to analyze the atomic-level vibrations of the material. This technique allowed the researchers to observe new phonons—quasiparticles that carry thermal energy—specifically at the interface of the FeSe film and the STO substrate. These phonons, which arise from the out-of-plane vibrations of oxygen atoms at the interface and in the apical oxygens of STO, play a critical role in enhancing superconductivity. According to lead author Xiaoqing Pan, UC Irvine Distinguished Professor of materials science and engineering, the phonons couple with electrons due to the spatial overlap of the electron and phonon wave functions at the interface. This strong electron-phonon coupling, the researchers believe, is a key mechanism in raising the transition temperature of FeSe to superconductivity.
FeSe is known to become superconductive at 65 Kelvin (roughly -340 degrees Fahrenheit), a temperature higher than many other superconductors. The new findings reveal that this transition temperature is highly sensitive to the uniformity of the FeSe/STO interface. In particular, greater homogeneity at the interface results in a higher superconducting transition temperature. This insight underscores the importance of carefully engineered interfaces in achieving enhanced superconductivity in ultrathin materials.
Through their vibrational spectroscopy technique, the researchers were able to closely observe the interlayer spacing between the FeSe film and the STO substrate. This spacing was found to correlate with the superconducting gap, which is crucial in determining the strength of the electron-phonon coupling. This relationship between the spacing and superconductivity highlights the importance of atomic-level precision in controlling the properties of superconducting materials.
The collaboration between experimental observations and theoretical simulations played a vital role in this discovery. Co-author Ruqian Wu, UC Irvine Distinguished Professor of physics and astronomy, emphasized how the ultrahigh spatial and energy resolutions of IMRI’s state-of-the-art instruments allowed for precise experimental data. This data, when combined with theoretical models, enabled the team to identify the atomic contributions to the enhancement of the superconducting transition temperature. The precision of these instruments was key to deepening the understanding of superconductivity at complex, heterogeneous interfaces.
The findings from this study are significant not only for fundamental physics but also for practical applications. Pan highlighted that these results are a crucial step toward developing scalable fabrication methods for superconducting materials that could be used in a range of technologies, including quantum computers, mass transportation systems utilizing magnetic levitation, and advanced medical diagnostic and treatment devices. Achieving higher temperature superconductivity with iron-based materials like FeSe could help make these technologies more practical and widespread, ultimately benefiting society in diverse fields.
Source: University of California, Irvine