Chirality locking charge density waves in a chiral crystal

Abstract

In Weyl semimetals, charge density wave (CDW) order can spontaneously break the chiral symmetry, gap out the Weyl nodes, and drive the material into the axion insulating phase. Investigations have however been limited since CDWs are rarely seen in Weyl semimetals. Here, using scanning tunneling microscopy/spectroscopy (STM/S), we report the discovery of a novel unidirectional CDW order on the (001) surface of chiral crystal CoSi – a unique Weyl semimetal with unconventional chiral fermions. The CDW is incommensurate with both lattice momentum and crystalline symmetry directions, and exhibits an intra unit cell π phase shift in the layer stacking direction. The tunneling spectrum shows a particle-hole asymmetric V-shaped energy gap around the Fermi level that modulates spatially with the CDW wave vector. Combined with first-principle calculations, we identify that the CDW is locked to the crystal chirality and is related by a mirror reflection between the two enantiomers of the chiral crystal. Our findings reveal a novel correlated topological quantum state in chiral CoSi crystals and raise the potential for exploring the unprecedented physical behaviors of unconventional chiral fermions.

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

“…A chiral charge density wave (CDW) is present when the mirror and rotation symmetries of the modulated electron waves are broken. A recently discovered material, AV 3 Sb 5 (A = K, Rb, and Cs), demonstrates the intertwinements of chiral CDW with nontrivial band topology, a giant anomalous Hall effect, and Kagome superconductivity. − With its intrinsic broken-symmetry state, geometrical chirality is predicted to give rise to the emergence of topological Weyl fermions in some materials, providing the opportunity to tune Berry curvature and topological properties by manipulating chiral CDWs. − In Weyl semimetals, CDW formation breaks the chiral symmetry and removes the Weyl nodes, turning the semimetal into an axion insulation state. − The chiral charge order can potentially serve as a strong precursor for odd-parity Cooper pairing in view of the fact that it is formed under odd parity. − …”

“…In other words, chiral CDW exists in the 2D limit and may not require interlayer interactions to exist. Our work provides new insight into the nature of chiral CDW, which is significant in exploring the interplays between CDW and unconventional superconductivity, orbital magnetism, and topological bands. ,, …”

“…Our work provides new insight into the nature of chiral CDW, which is significant in exploring the interplays between CDW and unconventional superconductivity, orbital magnetism, and topological bands. 1,5,8 Using MBE, we grew monolayer 1H-TaS 2 with a predominant chiral CDW phase and investigated its formation mechanism by STM and first-principles simulations. Our theoretical analyses indicate that lattice distortions in the monolayer TaS 2 crystal break the rotational symmetry of the q-EPC, leading to CDW chirality.…”

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

“…In polycrystalline thin films of cubic B20 materials, the control over the selective growth of homochiral cubic B20 grains is difficult, and the final products generally have heterochiral growth domains with intergrowth regions of twins. , Although internal interfaces such as twin boundaries are found between the adjacent grains with opposite crystal chirality in polycrystalline cubic B20 thin films, these internal interfaces are generally surrounded by other grain boundaries and do not extend throughout the bulk of the samples. In a cleaved (001) surface of cubic B20 single crystals, a structural domain wall boundary is occasionally formed between the crystal enantiomers at half step edges (step height ∼ a /2), due to presence of a 2-fold screw axis perpendicular to the surface. , These domains can be transformed into each other by a glide-mirror symmetry operation. In contrast to the twinned internal interface, these domain boundaries in cubic B20 bulk crystals are irregularly oriented and do not extend throughout the bulk of the material, as these interfaces are only found at the step edges of a cleaved surface.…”

“…Although internal interfaces between the crystallites of opposite chirality can form during the growth of cubic B20 materials, − these interfaces are irregularly oriented, generally surrounded by other grain boundaries and do not extend throughout the bulk of the crystal. Possible WSMs with atomically sharp and crystallographically oriented internal interfaces would enable a deeper understanding of topological features and correlated phenomena at internal interfaces.…”