Due to its excellent comprehensive properties, niobium-titanium alloy superconductors have been widely used among thousands of known superconductors. At present, they are the key materials for superconducting magnets in medical nuclear magnetic resonance and large scientific research equipment. Niobium-titanium alloy is a typical binary alloy of transition elements. Previous studies on high entropy alloy superconductors (TaNb)0.67(HfZrTi)0.33 composed of multiple transition metal elements found that under ultra-high pressure (1 million atmospheres or more is called ultra-high pressure, 1 million atmospheres = 100 GPa), the alloy exhibits extremely stable superconductivity. Since niobium and titanium are the main components of this high entropy alloy, studying the superconductivity of niobium-titanium alloy under ultra-high pressure can deepen the understanding of the microscopic mechanism of superconductivity in high entropy alloys.
The superconductivity of niobium-titanium alloy superconductors under ultra-high pressure was systematically studied. The results show that niobium-titanium alloy can still maintain zero-resistance superconductivity at a pressure of up to 261.7GPa, indicating that niobium-titanium alloy is the superconductor with the strongest pressure resistance among the known superconductors. This pressure is the highest superconducting pressure reported so far. Under this pressure, the transition temperature of the superconducting niobium-titanium alloy superconductor increases from 9.6K at normal pressure to 19.1K. The results of the strong magnetic field and high-pressure magnetoresistance experiments carried out in Hefei show that at a pressure of 211GPa and a temperature of 1.8K, its critical magnetic field increases from 15.4T to 19T, which is the highest superconducting transition temperature and the highest critical magnetic field found in transition metal alloy superconductors. The results of the synchrotron radiation high-pressure XRD experiment carried out at the Shanghai Synchrotron Radiation Facility show that at a pressure of 200GPa, the crystal structure basically does not change, but the volume is compressed by about 43%.
The above studies show that alloy superconductors composed of transition metal elements have stable superconducting properties and can withstand large deformations under high pressure. This is in sharp contrast to the high sensitivity of superconductivity to volume changes in copper oxide and iron-based superconductors. This is also significantly different from the behavior of superconductors of late transition metal elements (where the d electrons in the valence electron shell are in full shells), where the superconducting transition temperature decreases with volume compression.
