CSU and NTU teams make progress related to high-temperature superconductivity

Recently, Special Researcher Jianjun Ying and others from the School of Physics, University of Science and Technology of China (USTC) and the Key Laboratory of Strongly Coupled Quantum Materials Physics, CAS, in collaboration with Professor Sun Jian's group at Nanjing University, have made important progress in the field of high pressure elemental superconductivity.

By means of ultra-high pressure technology, the research team found that elemental scandium has a superconducting transition temperature of up to 36 K at high pressure, which sets a new record for the highest transition temperature of elemental superconductivity. The research results were published online on June 22 as "Record High 36 K Transition Temperature to the Superconducting State of Elemental Scandium at a Pressure of 260 GPa "The results were published online in Physical Review Lett. 130, 256002 (2023).

Jianjun Ying, Distinguished Research Fellow of the School of Physics, CSU, is the first author and co-corresponding author of the related article, and Xianhui Chen and Jian Sun, Professor of Nanjing University, are co-corresponding authors of the above article. The related work was supported by funds from the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and a guiding project in Anhui Province.

Elemental superconductors provide one of the simplest and cleanest material platforms for the study of superconductivity. Since the discovery of superconductivity in elemental mercury by the Dutch scientist Onis in 1911, more and more elements have been found to have superconductivity. In total, more than 50 elements have now been found to be superconducting under atmospheric or high-pressure environments. However, most elements have low superconducting transition temperatures, with the highest previous elemental superconducting transition temperature of 26 K being achieved by elemental titanium at high pressures.

Early studies found that elemental scandium undergoes four structural phase transitions under pressure. Above 23 GPa, the Sc-I phase transforms into the Sc-II phase, and the superconducting transition temperature of the Sc-II phase reaches a maximum of nearly 20 K around 100 GPa, where its relatively high superconducting transition temperature is thought to result from the gradual transfer of electrons from 4s to 3d orbitals. Due to the limitations of early high-pressure experimental techniques, studies on the superconductivity of elemental scandium at higher pressures are still scarce.

Phase diagram of the evolution of the superconducting transition temperature of elemental scandium with pressure.

To address this issue, the research team conducted a transport study of elemental scandium at ultra-high pressures to determine its superconducting phase diagram at high pressures. The high voltage electrical transport measurements revealed that in the Sc-II phase, the superconducting transition temperature (Tc) increases rapidly with increasing pressure, which is consistent with earlier reports. In contrast, after entering the Sc-III phase, Tc remains almost constant with pressure. When entering the Sc-IV phase, Tc continues to increase again with increasing pressure, reaching up to 28 K.

When the system finally enters the Sc-V phase at high pressure, its superconducting transition temperature suddenly rises to 36 K and remains almost constant with pressure. The team then explored the physical origin of the large increase in superconducting transition temperature at high pressure through first-principles calculations.

The calculations showed that the strong coupling between d-electrons and intermediate frequency phonons in the Sc-V phase is the most important reason for its high Tc. These results suggest that the superconducting transition temperature of elemental scandium under pressure is closely related to its structure, and the 36 K superconducting transition temperature found in the Sc-V phase not only sets a new record for elemental superconducting transition temperature, but also provides a new idea for finding high-temperature superconducting materials in simple systems.