Putative magnetic quantum criticality in (Sr1−xLax)3Ir2O7

Abstract

Sr1−xLax)3Ir2O7 undergoes a bulk insulator-to-metal transition (IMT) at x ≈ 0.04. Through careful analysis of previously published data (x = 0.053, 0.061, 0.076), we find an extended region below the Debye temperature in which the resistivity appears to scale linearly with temperature. Meanwhile resonant (in)elastic x-ray scattering data (x = 0.065) suggest a possible crossover from quantum paramagnetic to quantum critical phenomenology between 100 and 200 K. We put this into context with other results, and propose a possible phase diagram as a function of doping.The properties of a system proximate to a quantum critical point (QCP) at g = g c generally fit the following scenario. At low temperatures, and for g < g c , the ground state may exhibit some form of long-range order (LRO) above its lower critical dimension. In antiferromagnets for instance, this corresponds to Néel order. This order is destroyed by classical thermal fluctuations, which dictate the scaling of thermodynamic properties in the vicinity of some critical temperature T c . Above the transition, quasiparticles may still be welldefined on intermediate length scales, even though LRO has disappeared. This corresponds to a so-called thermal disordered regime. At sufficiently high temperatures (T ∼ |g − g c | νz ), these quasiparticles are replaced by a critical continuum of excitations. This continuum is thermally excited; which leads to a characteristic ω/T scaling of the spin fluctuations in the vicinity of the critical wavevector, and unconventional power-law temperature dependences of physical observables. If instead g > g c , then the ground state is disordered and characterized by well-defined quasiparticle excitations. The properties of the system are dictated primarily by the magnitude of a singlet-triplet gap ∆ ∼ (g − g c ) νz , which exists at all wavevectors. We refer to this as a quantum paramagnetic state, although it is also known as quantum disordered behavior in the literature. A crossover to quantum critical behavior typically occurs around T ∼ ∆ (Fig. 2d). Detailed reviews of quantum phase transitions (QPTs) are given in Refs. 1 and 2, among others.A number of the cuprates have been proposed -albeit with some controversy -to undergo QPTs as a function of doping. These include the (hole-doped) hightemperature superconductor La 2−x Sr x CuO 4 (LSCO), in which a magnetic QCP may lie underneath the superconducting dome. 3-5 Some similarities can be drawn between LSCO and the electron-doped perovskite iridate (Sr 1−x La x ) 2 IrO 4 . This material, like LSCO, is an insulator and easy-plane antiferromagnet below the Néel temperature at low carrier doping. It undergoes an insulatorto-metal transition (IMT) at x ≈ 0.04, 6,7 with evidence of a pseudogap and hole-like Fermi surface in the metallic phase x ≥ 0.05, 8,9 along with possible spin density wave (SDW) order. 10 In contrast with LSCO, however, nanoscale electronic phase separation can be observed well into the metallic regime. 7 Moreover, experimental evidence of a QPT in this syst...