Polarization-dependent infrared reflectivity study of Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ under pressure: Charge dynamics, charge distribution, and anisotropy

TL;DR

This study investigates how pressure affects the charge dynamics, distribution, and anisotropic optical properties of Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ using polarization-dependent infrared reflectivity, revealing increased hole concentration and persistent anisotropy.

Contribution

It provides new insights into pressure-induced charge transfer and anisotropic behavior in Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$, highlighting the evolution of hole concentration and electronic properties.

Findings

Optical conductivity along ladders increases with pressure.

Hole concentration in ladders rises and saturates at high pressure.

Material remains highly anisotropic under pressure.

Abstract

We present a polarization-dependent infrared reflectivity study of the spin-ladder compound Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ under pressure. The optical response is strongly anisotropic, with the highest reflectivity along the ladders/chains (\textbf{E}$\|$c) revealing a metallic character. For the polarization direction perpendicular to the ladder plane, an insulating behavior is observed. With increasing pressure the optical conductivity for \textbf{E}$\|$c shows a strong increase, which is most pronounced below 2000~cm$^{-1}$. According to the spectral weight analysis of the \textbf{E}$\|$c optical conductivity the hole concentration in the ladders increases with increasing pressure and tends to saturate at high pressure. At $\sim$7.5~GPa the number of holes per Cu atom in the ladders has increased by $\Delta \delta$=0.09 ($\pm$0.01), and the Cu valence in the ladders has reached the value +2.33. The optical data suggest that Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$ remains electronically highly anisotropic up to high pressure, also at low temperatures.