Niobium titanium alloy superconductor is widely used in thousands of known superconductors due to its excellent comprehensive properties and is the key material in medical nuclear magnetic resonance and superconducting magnets for large scientific devices. Niobium titanium alloy is a typical binary alloy composed of transition group elements. In the previous study on the high entropy alloy superconductor (TaNb)0.67(HfZrTi)0.33 composed of multiple transition metal elements, it was found that under ultra high pressure (the pressure above million atmospheres is ultra high pressure, 1 million atmospheres =100 GPa), The alloy exhibits an unusually stable superconductivity [PNAS 114(2017)13144]. Because niobium and titanium are the main components of this high entropy alloy, the study of the superconductivity of niobium titanium alloy under ultra high pressure can deepen the understanding of the micromechanism of superconductivity of high entropy alloy.
The superconductivity of niobium titanium alloy superconductor under ultra high pressure was studied systematically. It is found that niobium titanium alloy maintains a zero-resistance superconductivity at a pressure of up to 261.7GPa, which indicates that niobium titanium alloy is the most pressure-resistant superconductor known among all superconductors. This pressure is the highest pressure for superconductivity that has been reported. At this pressure, the transition temperature of superconductivity of the niobium-titanium alloy superconductor increases from 9.6K to 19.1K, and the critical magnetic field increases from 15.4T to 19T at 211GPa and 1.8K. This is the highest superconducting transition temperature and the highest critical magnetic field found in the transition metal element alloy superconductor.
It is revealed that the superconductivity of alloy superconductors composed of transition metal elements can resist large deformation and exist stably under high pressure, which is in sharp contrast to the high sensitivity of copper oxide and iron based superconductors to volume change. It is also significantly different from the behavior that the superconducting transition temperature decreases with volume compression in the post-transition metal element superconductors (full shell of d electrons in valence electrons).
