The formation of Cooper pairs by electron pairing and the realization of long-range coherence by Cooper pairs are two essential processes for materials to enter the superconducting state. In conventional superconductors, due to their high superfluid density, electron pairing and long-range coherence occur at the same temperature, and the corresponding superconducting energy gap opening temperature coincides with the superconducting critical temperature, but in two-dimensional superconductors with lower superfluid density such as copper basides high-temperature superconductors, the electron pairing temperature may be higher than the coherence temperature, producing a pre-pairing behavior that leads to the formation of a pseudo-energy gap, which is usually considered to correspond to the superconducting rise corresponding to the rise and fall. In iron-based superconductors, there is still no consensus on the existence of similar prepairing as well as pseudo-energy gap behavior, and there are few relevant studies and reports. Monolayer FeSe/SrTiO3 (FeSe/STO) thin films have received extensive attention and research due to their demonstration of potentially high superconducting critical temperatures and unique electronic structures, but the determination of their superconducting critical temperatures is still debated. The superconducting critical temperature (Tc) obtained from the established electrical transport and magnetic measurements, despite the differences, most of the Tc measured by transport (Tonset in the range of 40-50 K) are significantly lower than the spectroscopically obtained 65 K energy gap closure temperature. This in turn leads to a series of important questions related to monolayer FeSe/STO films, what is the temperature of superconducting electron pairing, what is the true superconducting transition temperature, whether there is a pseudo-energy gap and whether the superconducting transition temperature can be further increased. In-depth study of these questions is extremely important for exploring superconductors with higher Tc and for understanding the mechanism of iron-based high-temperature superconductivity.
Xu Yu, Hongtao Rong, and Ding Song Wu, PhD students in Xingjiang Zhou’s research group at the State Key Laboratory of Superconductivity, Institute of Physics, Chinese Academy of Sciences/Beijing State Research Center for Condensed Matter Physics, and Associate Researchers Qingyan Wang and Lin Zhao have conducted systematic electronic structure and superconductivity studies of prepared high-quality monolayer FeSe/STO superconducting films using high-resolution angle-resolved photoelectron spectroscopy, and found spectroscopic evidence of superconducting pairing at 83 K in monolayer FeSe/STO films.
Using a self-developed molecular beam epitaxy (MBE) system, they grew ultra-high quality monolayer FeSe/STO superconducting thin films by optimizing the film preparation conditions, which enabled them to not only significantly observe the energy band splitting in monolayer FeSe/STO in high-resolution angle-resolved photoelectron spectroscopy measurements, but also to observe for the first time the strong superconductivity-induced The Bogoliubov backbend energy band, which can extend even to 100 meV below the Fermi energy level, was observed for the first time (Figure 1). In angle-resolved photoelectron spectroscopy measurements, superconductivity has two remarkable spectroscopic features: one is the opening of the superconducting energy gap near the Fermi energy level, and the other is the formation of the superconducting-induced Bogoliubov backbend energy band. The observation of strong Bogoliubov back-bending energy bands in monolayer FeSe/STO films provides a new and more intrinsic spectroscopic feature beyond the energy gap for the study of its superconducting pairing temperature.
Fig. 1. Clear energy band splitting and strong Bogoliubov backbending energy bands are observed in monolayer FeSe/STO films.
Their study found that the conventional method of obtaining energy gaps by spectral line symmetry is no longer applicable for monolayer FeSe/STO films with significant energy band cleavage. They used a new analytical method to obtain a reliable energy gap and the evolution of the spectral weight with temperature near the Fermi energy level (Fig. 2).
Figure 2. Evolution of the energy gap with correlation spectral weight with temperature extracted from the energy spectrum curves in Fig. 1 Cut 1.
In particular, the Bogoliubov back-bending energy band was found to persist up to 83 K by direct observation and analysis, providing strong spectroscopic evidence for the existence of superconducting pairing in monolayer FeSe/STO films at 83 K (Fig. 3).
Figure 3. Evolution of the energy band structure, energy spectrum curves and associated spectral weight with temperature along Cut2 in Figure 1.
By analyzing the spectroscopic behavior of monolayer FeSe/STO films in the superconducting pairing temperature region, they further found that the temperature region of superconducting pairing can be further divided into two regions, 64-83 K and below 64 K (Fig. 4).
Figure 4. The various temperature-induced variations in monolayer FeSe/STO films are summarized into three temperature regions.
These results indicate the presence of superconducting pairing temperatures up to 83 K in monolayer FeSe/STO films. There are two possibilities for the understanding of the two temperature regions found. One is that 83 K corresponds directly to the superconducting transition temperature Tc; the other is that 64-83 K is the pre-pairing region with superconducting rise and pseudo-energy gap behavior, while below 64 K the paired electrons are long-range coherent into the superconducting state. Further experiments are needed to determine which scenario corresponds exactly. However, either possibility is of great importance for the realization of high TCM in iron-based superconductors and the understanding of the related superconductivity mechanism.
Related findings were published in the recent Nature Communications, Yu Xu et al., Spectroscopic evidence of superconductivity pairing at 83 K in single-layer FeSe/ SrTiO3 films, Nature Communications 12, 2840 (2021). The above research work was supported by grants from the National Natural Science Foundation of China, the Ministry of Science and Technology, and the Chinese Academy of Sciences.
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