Titanium sets new record for elemental superconductivity

Titanium metal is an essential raw material in the field of high technology. Because of its light weight, low density, great mechanical strength, and excellent performance such as corrosion resistance, it has a wide range of even irreplaceable application values in extreme environments such as space, ocean and deep earth. Now, elemental titanium metal shows current outstanding performance at elevated pressure, showing the highest superconducting transition temperature of Tc 26 K in known element superconductors. Here are the Titanium Pipes.

The team of Jin Changqing, Key Laboratory of Physics under Extreme Conditions, Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, has been conducting research on the preparation and functional regulation of new materials under extreme conditions under elevated pressure for a long time. They have designed and developed a joint experimental facility of extreme pressure, low temperature, strong field and laser heating with independent intellectual property rights, which can conduct ultrahigh pressure and elevated temperature synthesis and joint characterization of in-situ physical properties. Using the above advanced extreme condition techniques, they have successively revealed novel structure-activity associations of a series of functional materials under extreme conditions, including associations, topologies, polymers and other emerging functional material systems (PNAS 105, 7115(2008); ​jACS 132, 4876(2010); PNAS 108, 24(2011); ​jACS 133, 7892(2011); ​aS 110, 17263(2013); ​nature Commun. 5, 3731(2014); ​adv. Mater. 29, 1700715(2017); ​angew. Chem. Int. Ed. 56, 1(2017); NPG Asia Mater. 11, 60(2019).

They have recently used extreme pressure synthesis techniques to experimentally discover the first 4d transition metal hydrogen-rich high temperature superconductor Tc 71K zirconium based superconductor (Sci.Bull. 67, 907 (2022)). The first hafnium-based superconductor (Mater Today Phys 27, 100826(2022)) of 5d transition metal hydrogen-rich high-temperature superconductor Tc 83K was discovered. They independently discovered a calcium-based hydrogen-abundant elevated temperature superconductor (Nature Commun. 13, 2863 (2022)) over 210K, which became the first 2-element elevated temperature superconducting material with Tc over 200K after sulfur hydrogen and rare earth hydride, further expanding the category of high temperature superconducting materials.

The team of Changqing Jin and Xiancheng Wang recently made fresh progress in the research of elemental superconductivity. The experiment found that titanium superconductivity with transition temperature Tc>26 K under elevated pressure, which broke the record of the highest transition temperature of elemental superconductivity (Figure 1a). ​Elevated voltage in-situ electrical characterization revealed that the Tc of titanium increased from 2 K@18 GPa to ~10 K@99 GPa with the increase of pressure. When the pressure of ~108 GPa is near, the Tc rapidly rises to ~20 K. Then the Tc increases slowly with the pressure, and the superconducting temperature reaches the maximum of 26.2K at 248 GPa. As the pressure increases significantly, the superconducting Tc decreases slightly (FIG. 1b). ​Fig. 1c is a detailed phase diagram of elevated voltage superconductivity of titanium metal, covering the range of atmospheric pressure up to 310GPa (1GPa ~ 10,000 pressure). The 3.1 million pressure achieved in this experiment is the highest pressure reported thus far that any known material can possess and maintain superconductivity properties. According to the superconducting transition changes with the applied magnetic field, it is estimated that the upper critical field μ0Hc2 (0) of the superconducting phase of 26 K titanium metal is about 30 Tesla, and the corresponding superconducting coherent length of Ginzburg Landau is 32 A (Fig. 2 a, b).

The theoretical calculation by Professor Changfeng Chen from University of Nebraska shows that with the increase of pressure, the 4s orbital energy level which overlaps with the 3d orbital near the Fermi level moves up more, resulting in the gradual transfer of electrons from the 4s orbital to the 3d orbital. Above the pressure of 180 GPa, the band near the Fermi level is dominated by 3d electrons (FIG. 3), indicating that the elevated temperature superconductivity of titanium at elevated pressure is closely related to 3d electrons with electron correlation properties. In collaboration with the APS team of the United States and the team of Liu Haozhe of Beijing High Pressure Science and Technology Center, the high pressure synchrotron radiation X-ray diffraction also revealed the series crystal structure phase transformation of titanium metal at elevated pressure. They are the Tiα phase (0-9 GPa) with hexagonal dense stack structure, the hexagonal Tiω phase (9-116 GPa), the distorted hexagonal dense stack structure Tiγ phase (116-140 GPa), the distorted body-centered cubic structure Tiδphase (140-243 GPa), the simple body-centered cubic structure Tiβphase (>243) GPa). These phases have the same or close coordination configurations. Combined with the chemical loading design, it is possible to reproduce the high-voltage superconducting phase in the form of metastable phase under near-normal conditions. The atmospheric pressure interception of metastable superconducting phase has a precedent. As early as in 1965, Matthias and Geballe et al. found that NbGe had a superconducting temperature of 17K. In 1973, the superconducting temperature of metastable phase of Nb3Ge atmospheric pressure thin film was raised to 23K, which was the highest record of known superconducting materials at that time. This highest temperature record remained until the discovery of copper based elevated temperature superconductivity in 1986. Based on the characteristics of elevated pressure metastable phase of Ti metal, combined with thin film stress and rapid cooling techniques, it is possible to reproduce the structure of elevated temperature superconducting phase at atmospheric pressure. If thin-film metastable superconductivity can be realized, based on Josephson effect, Ti elementary 26K elevated temperature superconductivity will have an essential application prospect in the design and construction of quantum circuits. NbTi alloy is a widely used alloy superconducting material (accounting for more than 90% of the high electric application of low-temperature superconducting materials), but its superconducting temperature is only about 10K, and the upper critical field is about 15Tesla. Compared with NbTi alloy superconducting materials, the superconducting temperature and upper critical field of titanium superconductor are doubled, which has potential application prospects in the field of extreme working conditions and strong current.

This work reveals that through the combined action of electro-acoustic coupling and electron correlation effect, higher superconducting transition temperature can be achieved in materials with simple components such as single element, which will make the processing and application of superconducting materials relatively simple.

The above work is published in Nat.Commun. 13,5411(2022). Changling Zhang and Xin He, PhD students, are co-first authors, and Xiancheng Wang, Changfeng Chen and Changqing Jin are co-corresponding authors. The research was supported by the Innovative Research Group of NSFC, the Ministry of Science and Technology and the leading special project of the Chinese Academy of Sciences.

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