In recent years, compressed hydrogen-based compounds such as SH3, CaH6, and LaH10 have continuously refreshed the superconducting critical temperature (Tc) record with Tc above 200 K, keeping the hope for room-temperature superconductivity alive. Despite being a long way off, a plethora of unprecedented ternary hydrides at high pressures offers a new opportunity to search for a room-temperature superconductor with improved superconductivity. The advancement of theoretical and experimental techniques, such as the combination of machine learning methods and crystal structure prediction, as well as the fine processing of diamond anvils, is a step toward achieving room-temperature superconductivity in the near future.
Since the discovery of superconductivity for Hg with a Tc of 4.2 K in 1911, improving the critical temperature (Tc) of superconductors has been of particular interest in the field of condensed matter physics. Moreover, the quest for room-temperature superconductivity has been listed as one of the 30 most important physical problems of this century [1]. Based on conventional superconducting theory [2], Ashcroft [3] proposed that metallic hydrogen-rich compounds are potential room-temperature superconductors due to their lightest weight and high Debye temperature, which is intimately related to high-temperature superconductivity. In line with this strategy, over the last 8 years, experimental groups inspired by theoretical predictions [4], [5], [6], [7] have synthesized several hydrogen-based superconductors with Tc of 203 K in SH3 [8] and 200-260 K in metal polyhydrides (CaH6, LaH10, et al.) [9], [10], [11], [12], [13] within cage-like hydrogen frameworks that are available at pressures above 150 GPa (1 GPa = 109 Pa). Given the strong superconductivity in hydrogen-based compounds, the key challenge in this enticing field is to further improve the Tc of these hydrides as well as their stability at relatively low (even ambient) pressures.
Despite the fact that almost all binary hydrides with various hydrogen concentrations have been investigated theoretically and experimentally, there is a lack of new hydrides with Tc greater than 260 K in LaH10 in the experimentally accessible pressure regime. In an effort to broaden the horizon of hydrides, the current focus has shifted to ternary systems with larger phase space (Fig. 1), which could provide a better search platform for high-temperature superconductors [14]. Some of our recent findings [15], [16] indicated that by introducing an additional element into the binary hydrides, the superconductivity of the parent binary hydride system could be greatly enhanced.
Fig. 1. Estimated number of binary, ternary, and quaternary hydride compounds. |
On March 8th, 2023, Dias et al. claimed to have synthesized a new ternary lutetium-nitrogen-hydrogen compound with a maximum Tc of 294 K (21 ℃) at near-ambient pressure of 1 GPa (Fig. 2) via a chemical reaction between Lu and H2/N2 gas mixture at a pressure of 2 GPa and temperature of 65 ℃ [17]. The recovered specimen showed a weird color change from blue to pink and then to red under compression. They also carried out experimental characterization, such as temperature-dependent resistance, magnetic susceptibility, and heat capacity measurements, to verify its elementary superconducting properties of zero electrical resistance, Messner effect, and second-order phase transition. These comprehensive characterizations appear to be self-consistent and have attracted extraordinary attention. If true, the discovery of the room-temperature superconducting Lu-N-H compound at such a low pressure will be one of the most important scientific breakthroughs of this century. However, this work is doomed to be met with more skepticism because its authors have previously been accused of academic misconduct, and independent experiments are required to confirm its validity. As a matter of fact, room-temperature superconductivity (288 K) at 267 GPa was also reported in an indeterminate carbonaceous sulfur hydride by Dias et al. [18], but the authenticity of these results was questioned and could not be reproduced by other research groups, eventually leading to the retraction of this paper from Nature. Back in 2017, Ranga Dias and Isaac Silvera claimed the synthesis of the first-ever sample of solid metallic hydrogen at 495 GPa [19], but when other scientists questioned the sample’s legitimacy, they responded that the sample is lost, and diamonds failed in their diamond cell.
Fig. 2. Some high-temperature superconducting hydrides (Data of CeH9 and CeH10 are from Ref. [20]) under high pressures and the newly reported Lu-N-H compound. |
Several ternary hydrides with high Tc have recently been synthesized or predicted but the study of ternary superconducting hydrides is still facing great challenges due to the complexity of the composition of multiple systems. On the one hand, due to the limitations of computing power and current simulation methods, it is still impractical to theoretically conduct extensive searches for all ternary hydrides and evaluate their superconducting properties. On the other hand, experimental work is also extremely expensive and time-consuming due to the difficulties in synthesizing and characterizing structure and superconductivity under extreme conditions. To address these issues, we are attempting to develop theoretical and experimental techniques, such as the combination of the machine learning method and crystal structure prediction, as well as the fine processing of diamond anvil to achieve room-temperature superconductivity in the tantalizing hydrogen-based compounds. Finally, based on the great successes and breakthroughs already made in hydrogen-based compounds under high pressure, we continue to believe that ternary hydrides are the best hope for achieving room-temperature superconductivity in the near future.
