Superconductivity Induced in WB2 by the Formation of Metastable Planar Defects

The introduction of defects in a material can profoundly affect its mechanical and electronic properties. Defects often cause a deleterious effect on the superconducting properties of a material, decreasing the critical temperature and critical field. In this talk, we present our recent experimental and theoretical investigation on WB2 that starts superconducting above 50GPa, which we believe is due to the introduction of stacking fault and twin boundary defects. Our high-pressure X-ray diffraction data show that the ground state hP12 phase of WB2 persists up to 145GPa. Theoretically calculated Tc of the hP12 phase fails to explain the observed superconducting critical temperature of 17K at 70GPa. Our defect energy calculations show that stacking faults and twin boundaries can plausibly form under pressure in the hP12 phase. The local atomic arrangement of these planar defects resembles the arrangement of atoms in the P6/mmm MgB2. The P6/mmm WB2 show the highest calculated critical temperature among the competing phase of WB2. However, P6/mmm WB2 is thermodynamically stable only at pressures above 130GPa, much higher than the pressures where our sample displays superconducting behavior. The spatial projected density of states (DOS) at the planar defects mirrors the DOS of the bulk P6/mmm WB2 phase. Thus, the formation and percolation of superconducting stacking fault and twin boundary defects at high pressure can explain the observed Tc of WB2.