Discovery of an unconventional charge modulation on the surface of charge-density-wave material TaTe₄

Abstract

Electronic systems with quasi-one-dimensional (Q1D) Fermi surface tend to form either a charge-density-wave (CDW) or a spin-density-wave ground state at low temperatures due to one-dimensional instabilities. Among various CDW states, surface CDWs are different from that within the bulk due to the reduced dimensionality. Here we report the systematic investigation of charge density modulation on the surface of in situ cleaved TaTe4 crystal by means of low temperature scanning tunneling microscopy/spectroscopy, corroborated with density functional theory calculation. Well-defined Q1D modulation (4a, 6c) accompanied with a periodic lattice distortion is clearly observed on the (010) cleaved surface, distinct from that of its bulk CDW (2a × 2a × 3c). Tunneling spectroscopic measurements reveal a partially-opened energy gap about 23 meV around Fermi level. Such gap shows similar spatial variation with the periodicity of surface modulation and diminishes subsequently as temperature rises, which indicates a novel surface-related CDW gap states. The surface modulation vectors fit well with the Fermi surface nesting vectors, derived from the calculated Fermi surfaces. Surprisingly, such surface modulation can be suppressed greatly by applying vertical magnetic field and a critical field about 9.05 T can be estimated from field-dependent data. Our results demonstrate that this unique CDW modulation is strongly related to Fermi surface nesting mediated electron–electron coupling due to the reduced dimensionality of the surface, and can be readily tuned by relatively small magnetic field.

Competition between Ta-Ta and Te-Te bonding leading to the commensurate charge density wave in TaTe4

“…Thus, the Fermi surface (FS)-driven mechanism behind the CDW formation [18] may not be at work here. More recently, a different 4a × 6c CDW has been shown to occur in the surface of the TaTe 4 crystals [19].…”

“…Previous theoretical work on these tetratellurides includes an early density functional theory (DFT) study by Bullet [20] noting the unusual strength of interchain interactions and a tight-binding study [16] proposing that FS nesting was indeed at work, although the reported FS does not provide compelling evidence for such claim. A more recent DFT study provided data concerning the electronic structure of TaTe 4 but the origin of the bulk CDW was not considered [19]. Angular resolved photoemission spectra of TaTe 4 exhibits a combination of 1D and 3D Fermi surface components [21]: The experiment does not clarify, though, if there are two decoupled Fermi surface components nor if the Fermi surface nesting plays a role in the structural modulation, although a recent theoretical study suggests that electron-phonon is the main source of the modulation [22].…”

“…Because of the renewed interest in these solids [4][5][6][7]19], we decided to carry out a first-principles DFT study for the average and modulated structures of TaTe 4 by means of an analysis of the FS, electron-hole response function, and phonon spectra of the nonmodulated structure and structural optimizations of the CDW phase. We focus on TaTe 4 although very similar arguments should apply for NbTe 4 .…”

Direct observation of discommensurate charge density wave modulation in the quasi-1D Weyl semimetal candidate NbTe$_4$

“…To our knowledge, there are no previous reports of the observation of the low-temperature CDW modulation of NbTe 4 by STM experiments. Recent experiment in the other family member of the tetrachalcogenides, TaTe 4 , reported on the observation of a new type of charge modulation, distinct from the ones reported by diffraction experiments, presumably coming from surface effects in this compound [6]. All of these observations motivate a thorough characterization of the low-temperature CDW states in NbTe 4 by direct imaging and spectroscopic techniques.…”

“…Transition-metal chalcogenides are a model family to study a large variety of quantum phenomena including charge density wave (CDW) formation, superconductivity, topological states and magnetism [1][2][3]. Among this family, the quasi-1-dimensional transition-metal tetrachalcogenides, with their representative members: NbTe 4 and TaTe 4 , have recently gained a renewed interest due to the possibility of hosting axionic states connecting different Weyl-points in the Fermi-surface through the formation of a CDW [4][5][6]. Therefore these compounds provide an ideal opportunity to study the connection between topological states of matter and correlated electronic states such as CDWs [7,8].…”