Protracting the Weyl phase by a giant negative lattice expansion in Bi doped Sm$_2$Ir$_2$O$_7$

Abstract

We show that the Weyl phase in $\rm Sm_2Ir_2O_7$ is protracted up to at least 2~\% alloying with Bi by an anomalous negative lattice expansion with $\rm \Delta a \sim- 0.01$\AA. With further doping, the magnetic ordering disappears and electrical resistivity decreases by orders of magnitude; the resistivity upturn remains but with $\rm 1/T$ dependence of Weyl phase changed to $\rm -lnT$ dependence characteristic of the Quadratic Band Touching (QBT). At the Weyl-QBT phase boundary, a new phase is evidenced whose resistivity scales as $\rm -T^{1/4}$ possibly due to proximity to a quantum critical point proposed several years ago [Phys. Rev. X 4, 041027 (2014)], but whose experimental evidence has remained elusive thus far.