Discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si (1611.09578v2)

Abstract: Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the {\Gamma} point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a new platform to study the novel physical properties in two-dimensional Dirac materials and provide new opportunities to realize high-speed low-dissipation devices.

• The paper reveals that monolayer Cu₂Si hosts two distinct Dirac nodal lines confirmed by DFT and ARPES measurements.
• The study shows that mirror reflection symmetry protects these nodal lines, with minimal spin–orbit coupling effects preserving their structure.
• The research underlines the potential of 2D Dirac nodal line fermions for innovative, low-power nanoscale electronic applications.

Experimental Realization of Two-Dimensional Dirac Nodal Line Fermions in Monolayer Cu₂Si

The paper presented in the paper investigates the existence and characteristics of Dirac nodal line fermions in a two-dimensional (2D) material, specifically monolayer Cu₂Si. This research is significant as it bridges the gap between the understanding of topological nodal line semimetals predominantly in three-dimensional systems and the burgeoning field of 2D materials that have remarkable implications for nano-scaled electronic applications.

Research Background

Topological materials have attracted immense interest in condensed matter physics due to their extraordinary physical properties governed by nontrivial topological band structures. The paper builds on the established concepts of topological insulators and topological semimetals, where the latter exhibit band structures with valence and conduction bands meeting at discrete points or extended lines, known as Dirac/Weyl semimetals and nodal line semimetals, respectively. The challenge and novelty lie in the realization of these structures in 2D materials, which have been less explored compared to their three-dimensional counterparts.

Methodological Approach

The authors employed a combination of theoretical calculations and advanced experimental techniques. The theoretical framework involves first-principles calculations based on density functional theory (DFT) to predict the electronic band structure, while experimental observations are executed through angle-resolved photoemission spectroscopy (ARPES). The ARPES measurements confirm the theoretical predictions by directly observing the band structures intrinsic to monolayer Cu₂Si prepared on Cu(111) substrates.

Key Findings

1. Dirac Nodal Line Formation: The paper reveals that monolayer Cu₂Si hosts two distinct Dirac nodal lines, forming concentric loops around the $\Gamma$ point in the Brillouin Zone. This configuration arises from the linear crossing of bands characteristic of Dirac fermions.
2. Symmetry Protection: The nodal lines in Cu₂Si are protected by mirror reflection symmetry. The perturbation analysis indicates that breaking this symmetry, for instance, through buckling or substrate effects, leads to gapping of these nodal lines.
3. Two-Dimensional Characteristics: The ARPES data, consistent over varying photon energies, affirm the two-dimensional nature of Cu₂Si’s electronic structure. The experimentally measured Fermi surface matches with theoretical predictions, reinforcing the existence of these nodal lines.
4. Minimal Spin-Orbit Coupling (SOC) Effects: The paper notes that the intrinsic SOC does not induce significant gaps at the nodal lines, thereby preserving the Dirac nodal structure in practical settings.

Implications and Future Prospects

The realization of Dirac nodal line fermions in a two-dimensional material opens new avenues for research and application. Such materials can play pivotal roles in the development of high-speed, low-power electronic devices. Additionally, the symmetry-protected nature of these nodal lines suggests stability against certain perturbations, essential for practical applications.

Future research could focus on exploring other 2D materials capable of hosting similar topological features and investigating the potential of manipulating these properties through substrate interactions, alloying, or external fields. Moreover, examining the interplay between SOC and symmetry-breaking perturbations could further reveal new topological phases within reduced-dimensional systems.

In conclusion, the paper successfully demonstrates the potential for integrating topological concepts into 2D materials, thus paving the way for advancements in next-generation electronic devices that harness the distinctive properties of Dirac nodal line fermions.