Analysis of diverging effective mass near quantum critical point of x=0.3 in BaFe₂(As_1-xP_x)₂

The quantum critical point (QCP) is the metal-insulator (or superconductor)-transition point at T=0 K and has been observed in many superconducting materials. To reveal the identity of the QCP is most important for clarifying the superconductor mechanism in compound superconductors. A particular phenomenon is a diverging effective mass (DEM) near the QCP. For BaFe₂(As_1-xP_x)₂, the effective mass of quasiparticle in metal diverges near the QCP of x=0.3 and the inverse effective mass is linearly proportional to carrier density [1]. This was observed by measurements of heat capacity [1], penetration depth [2], the dHvA effect [1], and superconducting critical field [3]. Here, we demonstrate the linear behavior in inverse effective mass and reveal that the QCP has the maximum carrier density, through a fitting of the DEM near x=0.3 using m*/m=1/(1-ρ⁴) and ρ=(0.29/x) in the extended Brinkman-Rice picture [4]. the DEM's physical meaning indicates that the extent of a non-metallic phase increases with increasing x from the QCP. [1] PRL 110, 257002 (2013), [2] Science 336 (2012) 1154, [3] arXiv:1705.00695v1, [4] arXiv:1710.07754.