Probing the Origin of Chiral Charge Density Waves in the Two-Dimensional Limits

Abstract

Chirality generates spontaneous symmetry breaking and profoundly influences the topology, charge, and spin orders of materials. The chiral charge density wave (CDW) exhibits macroscopic chirality in the achiral crystal during the spontaneous electronic phase transitions. However, the mechanism of chiral CDW formation is shrouded in controversy. In this work, we report that two-dimensional H-phase TaS2 synthesized by molecular-beam epitaxy (MBE) shows a predominantly chiral CDW phase. Scanning tunneling microscopy (STM) imaging of the CDW reconstruction spots reveals a clockwise or anticlockwise intensity variation along the STM-imaged spots. First-principles calculations further show that the rotational symmetry of the momentum-dependent electron-phonon coupling is broken, giving rise to chirality. Our work provides new insights into the physical origin of the chiral charge-ordered states, shedding light on a general ordering rule in chiral CDWs.

Emerged charge-density wave featuring quasi-one-dimensional chains in Ta-intercalated bilayer TaS$_{2}$ with coexisting superconductivity

“…The calculated phonon dispersion (ω qν ) of Ta 2 S 4 exhibits the most negative energy at 2/3ΓM (Sec. S2 [27]), indicating CDW instabilities in 3×3 supercells, consistent with recent experiment [28], which further validates our methods. When Ta atoms are intercalated into the interlayer spacing, they tend to occupy the octahedral sites, located at the midpoints between the two nearest Ta atoms of adjacent monolayers [9].…”

“…To unveil the phonon softening for Ta 3 S 4 , we computed its χ q and χ qν , and the results were shown in Figs. The above finding is similar to the cases of monolayer TaS 2 [28] and NbSe 2 [34], where the intensive EPC matrix elements lead to the imaginary phonons at 2/3ΓM. To gain more insights into the formation of the imaginary phonons, we then tend to the reason of the large χ qν=1 at M points (χ q=M,ν=1 ).…”

“…Indeed, our calculations demonstrate that the phonon softening at M points are triggered by EPC, mainly arising from intensive EPC matrix elements. The phonon softening at branch ν and momentum q arising from EPC, can be quantified by generalized static electronic susceptibility (χ qν ) [28,[30][31][32], associated with real part of phonon self-energy due to EPC:…”

Atomic Visualization of the 3D Charge Density Wave Stacking in 1T-TaS2 by Cryogenic Transmission Electron Microscopy

“…Experimental investigations on 3D CDW order mainly include electron and X-ray diffractions, − angle-resolved photoemission spectroscopy (ARPES), − and scanning tunneling microscopy (STM). ,,, The first two techniques capture the global average signals of PLD, which are insensitive to local SOD stacking displacements. While both ARPES and STM are surface-sensitive, they cannot unveil the bulk structure along the c -axis.…”