Change in the optical spectrum of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ with applied uniaxial strain

We measured the optical reflectivity spectra under applied uniaxial strain for bulk single crystals of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$, which exhibits orbital ordering at 130 K. We found that, with compressive strain, particularly, along the $b$ axis, the reflectivity spectrum at room temperature changes almost in an isotropic manner as if it changes to that at lower temperatures. This experimental result indicates a strong coupling between the orbital degree of freedom and the strain in this compound.

Figure 1

(a) Arrangement of the V ions in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ at room temperature. (b) Schematic of the V trimers below $T_c$. (c) Reflectivity spectra at various temperatures, (d) Temperature dependence of the strain (the shrinkage of the crystal measured by a strain gauge) along the $a,b$, and $c$ axes for ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$.

Figure 2

(a) and (b) Change in reflectivity spectra when compressive ($\mathrm{\Delta }L<0$) or tensile ($\mathrm{\Delta }L>0$) strain is applied along the $b$ axis (a) with $b$ polarization and (b) with $c$ polarization. (c) and (d) Change in reflectivity spectra with temperature $R\left(200\mathrm{K}\right)/R\left(300\mathrm{K}\right)-1$ and $-\left\{R\right(200\mathrm{K})/R(200\mathrm{K})-1\}$, (c) with $b$ polarization and (d) with $c$ polarization.

Figure 3

(a) and (b) Change in reflectivity spectra when compressive ($\mathrm{\Delta }L<0$) or tensile ($\mathrm{\Delta }>0$) strain is applied along the $a$ axis (a) with $b$ polarization and (b) with $a$ polarization. (c) and (d) Change in reflectivity spectrum when strain is applied along the $c$ axis (c) with $b$ polarization and (d) with $c$ polarization.

Figure 4

(a) Change in reflectivity spectrum with $b$ polarization when compressive ($L<0$) or tensile ($L>0$) strain is applied along the $b$ axis on the $ab$ plane. The solid lines are the results with the same polarization and strain directions but on the $bc$ plane [shown in Fig. 1]. (b) Hypothetical change in reflectivity spectra with $b$ polarization when compressive strain is applied only along the $a,b$, or $c$ axis whereas no strain is applied along the other two axes. The two lines for $R_{bb}$ were calculated using the two different data sets to demonstrate the consistency (see the text). (c) Schematic of the measurement with the directions of strain and polarization.