Pressure-induced quantum critical point in the heavily hydrogen-doped iron-based superconductor LaFeAsO

An iron-based superconductor LaFeAsO$_{1-x}$H$_x$ (0 $\leq x \leq$ 0.6) undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x$=0.6) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the $d_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.

一种铁基超导体LaFeAsO$_{1 - x}$H$_x$（0 $\leq x \leq$ 0.6）在氢掺杂时会经历两个反铁磁（AF）相。我们利用核磁共振技术在压力下研究了第二个反铁磁相（$x$ = 0.6）。在高达2吉帕的压力下，基态是一个具有大能隙的自旋密度波态；然而，能隙在4.0吉帕时关闭，这表明存在一个压力诱导的量子临界点。有趣的是，在2到4吉帕的压力之间，有能隙激发与无能隙的磁涨落共存。这种共存可归因于$d_{xy}$轨道提升到费米能级，即压力下的利弗席兹相变。