Uncovering a Pressure-Tuned Electronic Transition in ${\mathrm{Bi}}_{1.98}{\mathrm{Sr}}_{2.06}{\mathrm{Y}}_{0.68}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ using Raman Scattering and X-Ray Diffraction

We report pressure-tuned Raman and x-ray diffraction data of ${\mathrm{Bi}}_{1.98.}{\mathrm{Sr}}_{2.06}{\mathrm{Y}}_{0.68}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ revealing a critical pressure at 21 GPa with anomalies in electronic Raman background, electron-phonon coupling $\lambda$, spectral weight transfer, density dependent behavior of phonons and magnons, and a compressibility change in the $c$ axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the spectral changes suggest that the critical pressure at 21 GPa is related to the critical point at optimal doping.

Figure 1

(a) Raman spectra of the $B_{1g}$ phonon, O-Bi phonon in the Bi-O block layer, and the apical oxygen phonon at $\sim 20\mathrm{K}$ with pressure. (b) Slope of the linear electronic background from $200–300{\mathrm{cm}}^{-1}$ at $\sim 20\mathrm{K}$. Spectra at each pressure were normalized to the intensity at $850{\mathrm{cm}}^{-1}$. (c) The full Raman vertex, using a normal state electronic background, was fit to 20 K data to derive the electron-phonon coupling parameter $\lambda$. Procedure in Ref. 25.

Figure 2

Raman spectra of the two-magnon peak at $\sim 20\mathrm{K}$ with pressure (3 GPa top curve, 32 GPa bottom curve). Since the low frequency side of the two-magnon peak coincides with the 2nd order Raman peak of diamond, background spectra taken from the sample-free region were subtracted from the sample spectra. The light intensity and optical focusing were kept constant throughout this set of measurements; no other normalization was necessary. The inset graphs the ratio of the integrated spectral weight below $1000{\mathrm{cm}}^{-1}$ to that above $1000{\mathrm{cm}}^{-1}$ for each pressure point.

Figure 3

Peak positions of the two-magnon peak (a), $B_{1g}$ phonon (b), and apical phonon (c), at low (20 K) and high (300 K) temperatures plotted against the $c$ axis lattice density and pressure. The data was taken with different pressure, temperature pathways indicated by the marker styles. Inset of (a) shows the derivative of a fit to all the data points (dotted curve). Insets of (b) and (c) show the same derivative for the phonons. Solid lines in (b) and (c) are guides to the eye, following the low pressure dependence.

Figure 4

(a) The $c$ axis and $5(a+b)/2$ axis lattice constants plotted versus pressure. Solid lines are fits to the Birch-Murnaghan equation of state between 0–20 GPa. Dotted curves are polynomial fits to the entire data range between 0–35 GPa. A second derivative of this polynomial fit for the $c$ axis is plotted in the inset. (b) Plot of $2c/(a+b)$.