Ultrafast measurements under anisotropic strain reveal near equivalence of competing charge orders in TbTe${}_3$

We report ultrafast reflectivity measurements of the dynamics of the order parameter of the charge density wave (CDW) in TbTe${}_3$ under anisotropic strain. We observe an increase in the frequency of the amplitude mode with increasing tensile strain along the $a$-axis (which drives the lattice into $a>c$, with $a$ and $c$ the lattice constants), and similar behavior for tensile strain along $c$ ($c>a$). This suggests that both strains stabilize the corresponding CDW order and further support the near equivalence of the CDW phases oriented in $a$- and $c$-axis, in spite of the orthorhombic space group. The results were analyzed within the time-dependent Ginzburg-Landau framework, which agrees well with the reflectivity dynamics.