Temperature-dependent generalized ellipsometry of the metal-insulator phase transition in low-symmetry charge-transfer salts

Abstract

Determining the optical and electronic properties of strongly anisotropic materials with symmetries below orthorhombic remains challenging; generalized ellipsometry is a powerful technique in this regard. Here, we employ Mueller matrix spectroscopic and temperature-dependent ellipsometry to determine the frequency dependence of six components of the dielectric-function tensor of the two-dimensional charge-transfer salt $α$-(BEDT-TTF)$_2$I$_3$ across its metal-insulator transition. Our results offer valuable insights into temperature-dependent changes of the components of the spectroscopic dielectric-function tensor across the metal-insulator transition. This advanced method allows extension to other electronic transitions.

Figures (3)

• Figure 1: (a) Unit cell of $\alpha$-(BEDT-TTF)$_2$I$_3$. $\alpha$, $\beta$ and $\gamma$ are the triclinic unit-cell angles, and crystallographic axes are indicated as $a$, $b$ and $c$kobayashi1984crystalkakiuchi2007charge. (b) Schematic representation of the orientation of orthogonal auxiliary frame with respect to laboratory frame (x,y,z), defined by Euler angles $\phi$, $\theta$ and $\psi$.
• Figure 2: (a) Experimental (colored solid lines and dots) and best match calculated (black dashed lines) Mueller matrix elements for $\alpha$-(BEDT-TTF)$_2$I$_3$ versus sample azimuth rotation at 2.35 eV for three different angles of incidence ($55^\circ$, $60^\circ$, and $65^\circ$). The sample was rotated azimuthally from $0^\circ$ to $360^\circ$ in steps of $15^\circ$. Note all the Mueller matrix elements are normalized to M$_\mathrm{11}$. (b) Real (solid lines, left axis) and imaginary (dashed lines, right axis) parts of the dielectric-function tensor $\varepsilon_\mathrm{11}$, $\varepsilon_\mathrm{22}$ (upper panel), $\varepsilon_\mathrm{33}$ (lower panel) of $\alpha$-(BEDT-TTF)$_2$I$_3$. Due to the availability of only one crystal face for measurements, the uncertainty in the out-of-plane components is greater than that of the in-plane components. Nevertheless, the level of sensitivity is adequate to support the conclusion (see supplement document Table-S1).
• Figure 3: Real (left panels) and imaginary (right panels) parts of the dielectric functions $\varepsilon_\mathrm{11}$, $\varepsilon_\mathrm{22}$, $\varepsilon_\mathrm{33}$ of $\alpha$-(BEDT-TTF)$_2$I$_3$ vs frequency, for different temperatures across metal-insulator transition.