Lattice dynamical signature of charge density wave formation in underdoped ${\text{YBa}}_2{\text{Cu}}_3{\mathrm{O}}_{6+x}$

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high-temperature superconductor ${\text{YBa}}_2{\text{Cu}}_3{O}_{6+x}$ ($x=0.75$, 0.6, 0.55, and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa${}_2$${\text{Cu}}_3$O${}_{6.99}$, additional Raman-active modes appear upon cooling below $\sim$170–200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering thus has the potential to explore the evolution of this state under extreme conditions.

Figure 1

(a) X-ray diffuse scattering intensity around the (4.5 0 2.5) reciprocal space point. A schematic of the ortho-VIII ordering is shown in the inset (from Ref. [45]). (b) Temperature dependence of the half-width at half maximum (HWHM) of the O-superstructure peaks. (c) Temperature dependence of the integrated intensity of the O-superstructure peaks.

Figure 2

Room-temperature Raman spectra of detwinned optimally doped YBa${}_2$Cu${}_3$O${}_{6.99}$ and underdoped YBa${}_2$Cu${}_3$O${}_{6.75}$, YBa${}_2$Cu${}_3$O${}_{6.6}$, YBa${}_2$Cu${}_3$O${}_{6.55}$, and YBa${}_2$Cu${}_3$O${}_{6.45}$ single crystals in the $XX$ channel taken with a Neon laser line ($\lambda$ = 632.8 nm) at room temperature and $T=T_c$. The mode assignment corresponds to Ref. [58]. For clarity, the absolute intensity of the spectra was normalized and a vertical offset was added.

Figure 3

Detailed view of the $XX$ Raman spectrum of YBa${}_2$Cu${}_3$O${}_{6.55}$ at $T=T_c$.

Figure 4

Raman spectra of YBa${}_2$Cu${}_3$O${}_{6.75}$ obtained in various scattering geometries at $T=T_c$. The intensities have been normalized at 900 cm${}^{-1}$ and shifted vertically for clarity.

Figure 5

Comparison of the Raman spectra obtained at $T=T_c$ in $XX$ and $YY$ scattering geometries. The arrows indicate the strongly temperature dependent mode at 590 cm${}^{-1}$, associated with the O vibration within broken CuO chains.

Figure 6

Comparison of the Raman spectra obtained at $T=T_c$ and room temperature in $XZ$ and $ZZ$ scattering geometries on YBa${}_2$Cu${}_3$O${}_{6.55}$. For clarity, the $XZ$ spectra has been magnified 15 times, and a vertical offset has been applied between the two sets of data.

Figure 7

(a) $XX$ Raman spectra of YBa${}_2$Cu${}_3$O${}_{6.6}$ measured at $T=$ 60 K with red ($\lambda$ = 632.8 nm) and green ($\lambda$ = 514.5 nm) incident laser lights. (b) $XX$ Raman spectra of YBa${}_2$Cu${}_3$O${}_{6.75}$ measured at $T=$ 75 K with red ($\lambda$ = 632.8 nm), green ($\lambda$ = 532 nm), and blue ($\lambda$ = 488 nm) incident laser lights. For clarity, the intensities of the spectra have been normalized to that of the 340-cm${}^{-1}$ phonon.

Figure 8

Upper panel: Color map representation of the temperature dependence of the Raman intensity measured with $\lambda =632.8$ nm in the $XX$ channel for optimally doped YBa${}_2$Cu${}_3$O${}_{6.99}$, and underdoped YBa${}_2$Cu${}_3$O${}_{6.75}$, YBa${}_2$Cu${}_3$O${}_{6.6}$, YBa${}_2$Cu${}_3$O${}_{6.55}$, and YBa${}_2$Cu${}_3$O${}_{6.45}$. Lower panel: Raman spectra of the same compounds for selected temperatures.

Figure 9

Renormalization of the 340-${\text{cm}}^{\mathrm{-1}}$ “buckling” mode through $T_c$ for the different doping levels.

Figure 10

Temperature dependence of the integrated intensity of the modes at 340 cm${}^{-1}$ (upper panel) and 205, 560, and 610 cm${}^{-1}$ (lower panel) in YBa${}_2$Cu${}_3$O${}_{6.75}$ samples (green symbols), YBa${}_2$Cu${}_3$O${}_{6.6}$ (red symbols), and YBa${}_2$Cu${}_3$O${}_{6.55}$ samples (blue symbols).

Figure 11

Schematic phase diagram for the Y123 family. $T_{\mathrm{Raman}}$ corresponds to the appearance temperature of the new modes at 205, 560, and 610 cm${}^{-1}$, while $T_{\mathrm{x}\text{−}\mathrm{ray}}$ stands for the onset of the CDW peak seen in soft x-ray scattering experiments (Refs. [15, 16, 17]) in the same samples. Other lines are discussed in the text.