Infrared properties of the quasi-one-dimensional superconductor $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ under pressure

Pressure-dependent optical properties in the mid-infrared range are presented for the quasi-one-dimensional superconductor $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ at room temperature. From the monotonic changes of the spectra for pressures below 12 GPa a pressure-induced dimensional crossover and the relevance of electron-phonon coupling can be inferred. The major changes in the spectra above 12 GPa, in particular the development of new excitations, may be due to a redistribution of charge with a possible relation to structural changes. Furthermore, a possible relation between the interchain coupling and the superconducting order is printed out.

Figure 1

(Color online) (a) Room-temperature reflectivity spectra $R_{s-d}$ of $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ inside the diamond anvil cell at $P=1.7\mathrm{GPa}$ for the polarization $\mathbf{E}$ parallel and perpendicular to the chain direction $b$. The blue lines are fits of the reflectivity spectra with the Drude-Lorentz model. Inset: Enlargement of the low-frequency range of the $\mathbf{E}\perp b$ reflectivity spectrum showing three sharp optical phonon modes. (b) Optical conductivity spectra obtained from the Drude-Lorentz fits of the reflectivity data. Inset: Geometries for the sample and reference measurements.

Figure 2

(Color online) Room-temperature reflectivity spectra of $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ as a function of pressure for the polarization $\mathbf{E}$ parallel and perpendicular to the $b$ axis. Insets: Enlargements of the low-frequency reflectivity for $\mathbf{E}\perp b$.

Figure 3

(Color online) Pressure-dependent optical conductivity spectra of $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ at room temperature for the polarization $\mathbf{E}$ parallel to the chains, obtained by Drude-Lorentz fits of the reflectivity data.

Figure 4

Peak positions of (a) the three optical phonons for $\mathbf{E}\perp b$ as a function of pressure, and (b) the new excitations evolving for $\mathbf{E}\perp b$ at $P>12\mathrm{GPa}$. The lines are polynomial fits of the data.