Effects of hole doping on magnetic and lattice excitations in ${\text{Sr}}_2$${\text{Ir}}_{1-x}$${\text{Ru}}_x$${\text{O}}_4$ ($x=0–0.2$)

Raman scattering is employed to explore the effects of hole doping in single crystals of ${\text{Sr}}_2$${\text{Ir}}_{1-x}$${\text{Ru}}_x$${\text{O}}_4$ ($x=0$, 0.01, 0.03, 0.05, 0.1, and 0.2). Introducing a few percentages of holes has a strong impact on magnetic excitations and lattice dynamics. With increasing $x$ the well structured two-magnon continuum turns into diffusive magnetic scattering. Furthermore one- and two-phonon scatterings are rapidly suppressed. Remarkably, the two Ir(Ru)-O-Ir(Ru) bond angle modes with different Ir(Ru)${\text{O}}_6$ octahedral rotations coexist and compete upon hole doping. This is ascribed to the difference of electronic properties between Ir${}^{4+}$ and Ru${}^{4+}$ ions. The doping and temperature dependence of the bond angle modes suggests that an electronically phase separated state develops upon Ru doping.

Figure 1

Raman spectra of ${\text{Sr}}_2$${\text{Ir}}_{1-x}$${\text{Ru}}_x$${\text{O}}_4$ ($x=0$, 0.03, 0.1, and 0.2) at $T=7$ K (lower panel) and 287 K (upper panel) in ($xu$) polarization. The insets depict the eigenvectors of the 188, 278, and 392 cm${}^{-1}$ modes. The relative amplitude of the vibrations is given by the arrows. The green balls stand for the Sr ions, the grey ones for Ir(Ru) ions, and the red ones for the oxygen ions

Figure 2

Frequencies, full widths at half maximum, and integrated intensities of the phonons at 392 and 278 cm${}^{-1}$ as a function of Ru doping.

Figure 3

(Upper panel) Raman spectra of ${\text{Sr}}_2$${\text{Ir}}_{1-x}$${\text{Ru}}_x$${\text{O}}_4$ ($x=0$, 0.03, 0.1, and 0.2) measured in a wide-frequency range at $T=7$ K. (Lower panels) Doping dependence of the magnetic continua obtained by subtracting the phonon peaks.

Figure 4

Normalized integrated intensities of two-magnon scattering, one-phonon scattering at 730 cm${}^{-1}$ ($I_1$), two-phonon scattering at 1470 cm${}^{-1}$ ($I_2$), and the ratio ($I_2/I_1$) as a function of $x$.

Figure 5

Comparison of the undoped Raman spectra for ($xx$) and ($zz$) polarization at $T=5$ and 300 K.

Figure 6

(a) Comparison of Raman spectra at $T=5$ and 300 K in ($zz$) polarization. (b) Temperature dependence of the 563 cm${}^{-1}$ mode with a sketch of its normal mode displacement. (c)–(e) Temperature dependence of frequency, linewidth, and normalized intensity of the 563 cm${}^{-1}$ mode, respectively.

Figure 7

Temperature dependence of Raman spectra of $x=0.05$ (upper panel) and $x=0.2$ (lower panel) in the temperature range of $T=7–287$ K measured in ($xu$) polarization.

Figure 8

Temperature dependence of the frequency, linewidth, and normalized integrated intensities of the 278 cm${}^{-1}$ mode for (a) $x=0.05$ and (b) $x=0.2$. The integral intensities are corrected by the Bose-Einstein thermal factor. The solid lines are fits to an anharmonic model.

Figure 9

Temperature dependence of the relative intensity of the 278 cm${}^{-1}$ to the 254 cm${}^{-1}$ mode for $x=0.05$ and $x=0.2$. The ratio is normalized by the value at $T=7$ K.