Strong Mass Renormalization at a Local Momentum Space in Multiorbital ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$

We have studied the mass renormalization in ${\mathrm{Ca}}_{2-x}{\mathrm{Sr}}_x{\mathrm{RuO}}_4$ ($x=0.2$) using high-resolution angle-resolved photoemission spectroscopy. We observed precise band dispersions near the Fermi level ($E_F$) and the corresponding Fermi surfaces. A characteristic flat band with $\sim 4\mathrm{meV}$ dispersion accompanying sharp quasiparticle (QP) peaks shows up in a limited momentum region around ($\pi$, 0). The QP peak rapidly evolves below the crossover temperature $T^{*}\simeq 20\mathrm{K}$, which agrees well with the mass enhancement behavior indicated by thermal, magnetic, and transport properties. We discuss the origin of the mass renormalization in relation to the local flat band at ($\pi$, 0) possibly derived from the $\gamma$ ($d_{xy}$) band.

Figure 1

Mapping of ARPES spectral weight at $E_F$ from ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ (a) and ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$ (b). Three FSs composed of $\alpha$, $\beta$, and $\gamma$ bands in $x=2.0$ are respectively depicted by red, blue, and green markers. FSs of $x=0.2$ (b) are shown by black markers. $(0,\pm \pi )\mathrm{\text{−}}(\pi ,0)$ lines in (b) show the first Brillouin zone boundary of $x=0.2$. The flat band region around ($\pi$, 0) is shown by black broken lines.

Figure 2

ARPES image of ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$ at 4 K along 1–7 momentum cuts, together with the schematic BZ and FSs. $k_0$ shows the intersection of each cut and the $(0,\pi )\mathrm{\text{−}}(\pi ,0)$ line. Respective bands are labeled by $A$, $B$, and $C$.

Figure 3

(a) ARPES image of ${\mathrm{Ca}}_{1.8}{\mathrm{Sr}}_{0.2}{\mathrm{RuO}}_4$ at 4 K along $\Gamma \mathrm{\text{−}}(\pi ,\pi )$ with ${\mathit{k}}_0=(\pi /2,\pi /2)$. Markers show the peak positions for $A$, $B$ and $C$ bands obtained from MDCs. (b1) ARPES image at 4 K along $\Gamma \mathrm{\text{−}}(\pi ,0)$. (b2) Magnified ARPES spectrum near $E_F$ divided by the Gaussian convoluted FD function. (b3) and (b4) correspond to the EDCs and those divided by the convoluted FD function showing the flat band, respectively. Blue markers indicate the spectral peak position, while the broken line is a guide for eyes. (c1) ARPES image at 4 K along the cut slightly off $\Gamma \mathrm{\text{−}}(\pi ,0)$. (c2) The corresponding ARPES image near $E_F$ divided by the convoluted FD function, (c3) EDCs, and (c4) those divided by the convoluted FD function.

Figure 4

(a) $T$-dependence of the spectrum which is $k$ integrated along $(0.6\pi ,0)\mathrm{\text{−}}(\pi ,0)$. Thin curve shows the spectrum at 4 K integrated along $(0.3\pi ,0.3\pi )\mathrm{\text{−}}(0.7\pi ,0.7\pi )$. Inset shows the $T$ dependence of $I_{\mathrm{peak}}(E_F)$ and $C_e/T$ [2] (see text). (b) $T$ dependence of the spectrum divided by the experimentally broadened FD function at each $T$. Each spectrum is fitted by a Lorentzian with a linear background.