Kim, Soyeun | Search NASA

DOE OSTI2024

Emergent symmetry in TbTe 3 revealed by ultrafast reflectivity under anisotropic strain

Here, 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. Our study presents an ultrafast approach to assess the stability of phases and order parameter dynamics in strained systems.

Kim, Soyeun↗

DOE OSTI2024

Disorder and diffuse scattering in single-chirality (TaSe 4 ) 2 ⁢I crystals

The quasi-one-dimensional chiral compound (TaSe 4 ) 2 I has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature $T$ CDW ≈263K, (TaSe 4 ) 2 I exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector ∼(0.05,0.05,0.11), oblique to the TaSe 4 chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for (TaSe 4 ) 2 I. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate (TaSe 4 ) 2 I crystals have uniform handedness, supported by direct imaging and helicity-dependent terahertz emission spectroscopy. Our single-crystal x-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent (TaSe 4 ) 2 I crystal structure. Further, the short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why (TaSe 4 ) 2 I behaves as an electronic semiconductor at temperatures above and below $T$ CDW , despite a metallic band structure calculated from density functional theory of the ideal structure.

36 MATERIALS SCIENCE↗