Auswahl der wissenschaftlichen Literatur zum Thema „Dirac nodal lines“

Topological semimetals are characterized by symmetry-protected band crossings, which can be preserved in different dimensions in momentum space, forming zero-dimensional nodal points, one-dimensional nodal lines, or even two-dimensional nodal surfaces. Materials harboring nodal points and nodal lines have been experimentally verified, whereas experimental evidence of nodal surfaces is still lacking. Here, using angle-resolved photoemission spectroscopy (ARPES), we reveal the coexistence of Dirac nodal surfaces and nodal lines in the bulk electronic structures of ZrSiS. As compared with previous ARPES studies on ZrSiS, we obtained pure bulk states, which enable us to extract unambiguously intrinsic information of the bulk nodal surfaces and nodal lines. Our results show that the nodal lines are the only feature near the Fermi level and constitute the whole Fermi surfaces. We not only prove that the low-energy quasiparticles in ZrSiS are contributed entirely by Dirac fermions but also experimentally realize the nodal surface in topological semimetals.

Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal lines that are found in several classes of unconventional intermetallic compounds. We investigated anisotropic electrodynamics of NbAs2 where the spin-orbit coupling (SOC) triggers energy gaps along the nodal lines. These gaps manifest as sharp steps in the optical conductivity spectra σ1(ω). This behavior is followed by the linear power-law scaling of σ1(ω) at higher frequencies, consistent with our theoretical analysis for dispersive Dirac nodal lines. Magneto-optics data affirm the dominant role of nodal lines in the electrodynamics of NbAs2.

Abstract Topological semimetals, in which conduction and valence bands cross each other at either discrete points or along a closed loop with symmetry protected in the momentum space, exhibited great potential in applications of optical devices as well as heterogeneous catalysts or antiferromagnetic spintronics, especially when the crossing points/lines matches Fermi level (E F). It is intriguing to find the ‘ideal’ topological semimetal material, in which has a band structure with Dirac band-crossing located at E F without intersected by other extraneous bands. Here, by using angle resolved photoemission spectroscopy, we investigate the band structure of the so-called ‘square-net’ topological material ZrGeS. The Brillouin zone (BZ) mapping shows the Fermi surface of ZrGeS is composed by a diamond-shaped nodal line loop at the center of BZ and small electron-like Fermi pockets around X point. The Dirac nodal line band-crossing located right at E F, and shows clearly the linear Dirac band dispersions within a large energy range >1.5 eV below E F, without intersected with other bands. The obtained Fermi velocities and effective masses along Γ–X, Γ–M and M–X high symmetry directions were 4.5–5.9 eV Å and 0–0.50 m e, revealing an anisotropic electronic property. Our results suggest that ZrGeS, as a promising topological nodal line semimetal, could provide a promising platform to investigate the Dirac-fermions related physics and the applications of topological devising.

LaAgSb2 is a Dirac semimetal showing charge density wave (CDW) order. Previous angle-resolved photoemission spectroscopy (ARPES) results suggest the existence of the Dirac-cone-like structure in the vicinity of the Fermi level along the Γ–M direction. This paper is devoted to a complex analysis of the electronic band structure of LaAgSb2 by means of ARPES and theoretical studies within the ab initio method as well as tight binding model formulation. To investigate the possible surface states, we performed the direct DFT slab calculation and the surface Green function calculation for the (001) surface. The appearance of the surface states, which depends strongly on the surface, points to the conclusion that LaSb termination is realized in the cleaved crystals. Moreover, the surface states predicted by our calculations at the Γ and X points are found by ARPES. Nodal lines, which exist along the X–R and M–A paths due to crystal symmetry, are also observed experimentally. The calculations reveal other nodal lines, which originate from the vanishing of spin–orbit splitting and are located at the X–M–A–R plane at the Brillouin zone boundary. In addition, we analyze the band structure along the Γ–M path to verify whether Dirac surface states can be expected. Their appearance in this region is not confirmed.

Searching for two-dimensional materials combining both magnetic order and topological order is of great significance for quantum devices and spintronic devices. Here, a class of two-dimensional transition metal borides, TM2B3 (TM = Ti–Ni), with high stability and stable antiferromagnetic (AFM) orders was predicted by using the first-principles method. The result shows that they possess large magnetic anisotropy energy and high critical temperature. Interestingly, Mn2B3 monolayer is confirmed to be AFM Dirac node line semimetal with several Dirac points near the Fermi level. Detailed analysis of the irreducible representations shows that the nodal lines are protected by the horizontal mirror symmetry Mz. Our findings provide an excellent platform for exploring topological and magnetic materials ready for the next generation of spintronic devices.

La réalisation de nouveaux matériaux bidimensionnels est un domaine en plein essor de la matière condensée, à la fois pour les aspects fondamentaux, avec les propriétés exotiques émergeant de la dimensionnalité réduite, et pour les applications technologiques potentielles, avec des promesses telles que des courants sans dissipation et des hétérostructures 2D plus performantes que la technologie actuelle à base de silicium à une fraction de la taille. Dans ce travail, nous avons adopté une approche expérimentale pour la réalisation et la caractérisation de matériaux prédits pour accueillir des lignes nodales de Dirac (DNL), qui malgré de nombreuses prédictions théoriques ont vu peu de réalisations expérimentales rapportées jusqu'à présent. Ces matériaux appartiennent à la classe récemment mise en évidence des semi-métaux topologiques, dont la spécificité est un croisement de bandes protégé par symétrie entre les bandes de valence et de conduction le long d'une ligne dans l'espace réciproque, avec une dispersion linéaire. Dans un premier temps, nous nous sommes concentrés sur Cu2Si, le premier matériau 2D dans lequel des DNL ont été mis en évidence lorsqu'il est préparé sur un substrat Cu(111). Après avoir reproduit avec succès les résultats existants, nous avons montré à l'aide de l'ARPES et du XPS que, contrairement aux attentes, la structure électronique et les DNL étaient préservées après le dépôt de Pb sur la surface. Nous avons ensuite étudié Cu2Si/Si(111), et constaté que malgré une structure atomique fortement liée, le substrat Si(111) interagit assez fortement avec les orbitales hors plan de la couche Cu2Si pour empêcher l'existence des lignes nodales. Nous nous sommes ensuite penchés sur le système 2D Cu2Ge, prédit pour accueillir la DNL, et avons tenté de le synthétiser en déposant du Ge sur Cu(111). En combinant nos résultats LEED, XPS et ARPES, nous avons constaté que toutes les mesures correspondaient étroitement à ce que l'on attendait d'une monocouche de Cu2Ge libre, ce qui montre l'absence presque totale d'interactions entre le substrat Cu(111) et la couche de Cu2Ge superficielle formée sur celui-ci. Il s'agit de la première réalisation expérimentale rapportée de Cu2Ge. Dans une étude miroir, nous avons déposé Cu sur Ge(111) et observé une structure de bande dissemblable. À l'aide du STM, nous avons expliqué ces différences par une structure atomique différente, résultant d'un substrat à forte interaction. Nous soulignons par ce travail l'influence du substrat, qu'il soit métallique ou semi-conducteur, sur les propriétés électroniques des systèmes 2D à DNL
The realization of new two-dimensional materials is a booming field of condensed matter, at once for the fundamental aspects, with the exotic properties emerging from the reduced dimensionality, and for the potential technological applications, with promises such as dissipationless currents and 2D heterostructures outperforming the current silicon-based technology at a fraction of the size. In this work, we took an experimental approach to the realization and characterization of materials predicted to host Dirac nodal lines (DNLs), which despite many theoretical predictions have seen few experimental realizations reported so far. These materials belong to the recently evidenced class of topological semimetals, whose specificity is a symmetry-protected band crossing of the valence and conduction bands along a line in momentum space, with linear dispersion. As a first step, we focused on Cu2Si, the first 2D material in which DNLs have been evidenced when prepared on a Cu(111) substrate. After successfully reproducing existing results, we showed using ARPES and XPS that contrary to expectations, the DNLs were preserved after deposition of Pb on the surface without any gap, and that a band splitting occurred. We followed by the investigation of Cu2Si/Si(111), and found that despite a strongly related atomic structure, the Si(111) substrate interacts strongly enough with the out-of-plane orbitals of the Cu2Si layer to prevent the existence of the nodal lines. We then looked at the 2D Cu2Ge system, predicted to host DNL, and attempted to synthesize it by depositing Ge on Cu(111). By combining our LEED, XPS and ARPES results we found that all measurements matched closely what was expected from a free-standing Cu2Ge monolayer, showing the almost complete absence of interactions between the Cu(111) substrate and the surface Cu2Ge layer grown on it. This is the first reported experimental realization of the two-dimensional Dirac nodal line semimetal Cu2Ge. In a mirroring study, we deposited Cu on Ge(111) and observed a dissimilar band structure. Helped by STM, we explained those differences by a different atomic structure, and by a strongly interacting substrate. We highlight through this work the influence of the substrate, whether metallic or semiconductor, on the electronic properties of 2D DNL systems