Observation of dimension-crossover of a tunable 1D Dirac fermion in topological semimetal NbSixTe2

Abstract

Condensed matter systems in low dimensions exhibit emergent physics that does not exist in three dimensions. When electrons are confined to one dimension (1D), some significant electronic states appear, such as charge density wave, spin-charge separations, and Su-Schrieffer-Heeger (SSH) topological state. However, a clear understanding of how the 1D electronic properties connects with topology is currently lacking. Here we systematically investigated the characteristic 1D Dirac fermion electronic structure originated from the metallic NbTe2 chains on the surface of the composition-tunable layered compound NbSixTe2 (x = 0.40 and 0.43) using angle-resolved photoemission spectroscopy. We found the Dirac fermion forms a Dirac nodal line structure protected by the combined $$\widetilde {M_y}$$ M y ̃ and time-reversal symmetry T and proves the NbSixTe2 system as a topological semimetal, in consistent with the ab-initio calculations. As x decreases, the interaction between adjacent NbTe2 chains increases and Dirac fermion goes through a dimension-crossover from 1D to 2D, as evidenced by the variation of its Fermi surface and Fermi velocity across the Brillouin zone in consistence with a Dirac SSH model. Our findings demonstrate a tunable 1D Dirac electron system, which offers a versatile platform for the exploration of intriguing 1D physics and device applications.

Plasmons in a two-dimensional nonsymmorphic nodal-line semimetal

“…where a i,j and b i,j are the particle operators for A and B sites in a unit cell labeled by the index (i, j). We suppress spin indices in the model, because the spin-orbit coupling strength in NbSi x Te 2 is found to be weak [33,34,36]. This 1D model is similar to the famous SSH model [56].…”

“…In order to capture the nonsymmorphic nodal line in NbSi x Te 2 family materials, a minimal lattice model was proposed by some of the current authors in Ref. [36]. In this section, we briefly introduce this model and discuss its main features.…”

“…Recently, a series of theoretical and experimental works have established a good TSM state in the NbSi x Te 2 family materials [31][32][33][34][35][36]. These materials were first synthesized in the 1990s [37].…”

“…While the specific band dispersion varies with n, all these members in 2D feature the nonsymmorphic nodal line [34]. It was found that the low-energy bands that make the nodal line are mainly from an array of NbTe 2 chains, which can be well captured by a 2D Su-Schrieffer-Heeger (SSH) like model [36]. Evidently, the NbSi x Te 2 family materials offer a promising platform for studying the physics of 2D nodal-line TSMs.…”

“…However, plasmons associated with nodalline states were mainly studied in 3D systems, and the line is often assumed to have the shape of a perfect ring in Brillouin zone (BZ) [53][54][55]. This is not the case for the nonsymmorphic nodal line in 2D NbSi x Te 2 , which traverses the BZ along a straight path and exhibits strong anisotropy [34,36]. Thus, it is desirable to find out how this nonsymmorphic nodal-line state impacts the plasmon properties.…”