High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsupercond...High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsuperconducting HEAs mostly under high pressure.Here we report the discovery of the superconductivity and temperature-driven polymorphism in(V0.5Nb0.5)3-xMoxAl0.5Ga0.5(0.2≤x≤1.4)HEAs,which are of a single body-centered cubic(bcc)structure for x=0.2 and a mixture of the bcc and A15 structures for higher x values.Upon annealing,the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibit bulk superconductivity.For the sample with x=0.2,the bcc polymorph is not superconducting down to1.8 K,whereas the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and estimated zero-temperature upper critical field Bc2(0)of 20.1 T,both of which are the highest among HEA superconductors.With increasing Mo content x,both Tc and Bc2(0)of the A15-type HEAs decrease,yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range.The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength.Furthermore,the valence electron count dependence of Tc,which is different from both the binary A15 and other structurally different HEA superconductors,suggests that Tc may be increased further by reducing the number of valence electrons.Our results not only uncover HEA superconductors of a new structural type,but also provide the first example of polymorphism-dependent superconductivity in HEAs.展开更多
基金supported by the National Key Research&Development Program of China(2017YFA0303002).
文摘High-entropy alloys(HEAs)are the focus of current research for their diverse properties,including superconductivity and structural polymorphism.However,the polymorphic transition has been observed only in nonsuperconducting HEAs mostly under high pressure.Here we report the discovery of the superconductivity and temperature-driven polymorphism in(V0.5Nb0.5)3-xMoxAl0.5Ga0.5(0.2≤x≤1.4)HEAs,which are of a single body-centered cubic(bcc)structure for x=0.2 and a mixture of the bcc and A15 structures for higher x values.Upon annealing,the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibit bulk superconductivity.For the sample with x=0.2,the bcc polymorph is not superconducting down to1.8 K,whereas the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and estimated zero-temperature upper critical field Bc2(0)of 20.1 T,both of which are the highest among HEA superconductors.With increasing Mo content x,both Tc and Bc2(0)of the A15-type HEAs decrease,yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range.The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength.Furthermore,the valence electron count dependence of Tc,which is different from both the binary A15 and other structurally different HEA superconductors,suggests that Tc may be increased further by reducing the number of valence electrons.Our results not only uncover HEA superconductors of a new structural type,but also provide the first example of polymorphism-dependent superconductivity in HEAs.
