Overdamped Antiferromagnetic Strange Metal State in ${\mathrm{Sr}}_3{\mathrm{IrRuO}}_7$

The unconventional electronic ground state of ${\mathrm{Sr}}_3{\mathrm{IrRuO}}_7$ is explored via resonant x-ray scattering techniques and angle-resolved photoemission measurements. As the Ru content approaches $x=0.5$ in ${\mathrm{Sr}}_3({\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{O}}_7$, intermediate to the $J_{\mathrm{eff}}=1/2$ Mott state in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ and the quantum critical metal in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, a thermodynamically distinct metallic state emerges. The electronic structure of this intermediate phase lacks coherent quasiparticles, and charge transport exhibits a linear temperature dependence over a wide range of temperatures. Spin dynamics associated with the long-range antiferromagnetism of this phase show nearly local, overdamped magnetic excitations and an anomalously large energy scale of 200 meV—an energy far in excess of exchange energies present within either the ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ or ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ solid-solution end points. Overdamped quasiparticle dynamics driven by strong spin-charge coupling are proposed to explain the incoherent spectral features of the strange metal state in ${\mathrm{Sr}}_3{\mathrm{IrRuO}}_7$.

Figure 1

(a) Schematic electronic phase diagram of ${\mathrm{Sr}}_3({\mathrm{Ir}}_{1-x}{\mathrm{Ru}}_x{)}_2{\mathrm{O}}_7$. (b) Scattering intensity of an $x=0.48$ sample collected at the (1,0,24) Bragg position above and below $T_N$. (c) $1/(\chi -{\chi }_0)$ as a function of temperature for $x=0.51$, with $H_{ab}=100\mathrm{Oe}$. The solid line is the result of a Curie-Weiss fit as detailed in the text. (d) $ab$-plane resistivity plotted as a function of temperature for $x=0.5$. Paramagnetic metal (PM-M), Paramagnetic insulator (PM-I), canted antiferromagnetic insulator (CAF-I), and antiferromagnetic metal (AF-M) regions are marked in the phase diagram.

Figure 2

(a),(b) Photoemission spectra collected along high-symmetry directions for $x=0.5$. Red symbols show the dispersions of the holelike bands near $E-E_F=0$, extracted from peaks in momentum-distribution cuts (see Supplemental Material [17]). (c) Map of incoherent spectral weight for $x=0.5$ at $E_F$. Dashed white square shows the Brillouin zone of the $Bbcb$ unit cell, while the high-symmetry directions are indicated with respect to the smaller tetragonal unit cell. Red dashed circles highlight features reminiscent of diffuse holelike pockets centered around the $X$ points. The arrows indicate the momentum cut directions in (a) and (b). (d) Representative energy distribution curves collected at $k_F$, at positions indicated in (a) and (b).

Figure 3

Representative RIXS spectra for $x=0.17$ sample collected at the magnetic zone center (a) and the zone boundary (b) and for the $x=0.48$ sample at the zone center (c) and boundary (d). All spectra were collected at $T=40\mathrm{K}$. Solid red line is a fit to the data utilizing the spectral components discussed in the text in addition to a linear background term.

Figure 4

(a) Characteristic energies ${\omega }_0$ for the $M$ modes in $x=0$ (reproduced from Kim et al. [11]), $x=0.17$, and $x=0.48$. Solid line is a fit using an effective mean-field bond operator model as discussed in the text. (b) Inverse lifetimes $\gamma /2$ of the $M$ modes across the Brillouin zone in both the underdamped $x=0.17$ and overdamped $x=0.48$ samples.