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Register a new userEvidence for a Dirac nodal-line semimetal in SrAs$_{3}$
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Dirac nodal-line semimetals with the linear bands crossing along a line or loop, represent a new topological state of matter. Here, by carrying out magnetotransport measurements and performing first-principle calculations, we demonstrate that such a state has been realized in high-quality single crystals of SrAs3. We obtain the nontrivial pi Berry phase by analysing the Shubnikov-de Haas quantum oscillations. We also observe a robust negative longitudinal magnetoresistance induced by the chiral anomaly. Accompanying first-principles calculations identify that a single hole pocket enclosing the loop nodes is responsible for these observations.
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Topological nodal-line semimetals with exotic quantum properties are characterized by symmetry-protected line-contact bulk band crossings in the momentum space. However, in most of identified topological nodal-line compounds, these topological non-trivial nodal lines are enclosed by complicated topological trivial states at the Fermi energy ($E_F$), which would perplex their identification and hinder further applications. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide compelling evidence for the existence of Dirac nodal-line fermions in the monoclinic semimetal SrAs$_3$, which are close to $E_F$ and away from distraction of complex trivial Fermi surfaces or surface states. Our calculation indicates that two bands with opposite parity are inverted around emph{Y} near $E_F$, which results in the single nodal loop at the $Gamma$-emph{Y}-emph{S} plane with a negligible spin-orbit coupling effect. We track these band crossings and then unambiguously identify the complete nodal loop quantitatively, which provides a critical experimental support to the prediction of nodal-line fermions in the CaP$_3$ family of materials. Hosting simple topological non-trivial bulk electronic states around $E_F$ and no interfering with surface states on the natural cleavage plane, SrAs$_3$ is expected to be a potential platform for topological quantum state investigation and applications.
We instigate the angle-dependent magnetoresistance (AMR) of the layered nodal-line Dirac semimetal ZrSiS for the in-plane and out-of-plane current directions. This material has recently revealed an intriguing butterfly-shaped in-plane AMR that is not well understood. Our measurements of the polar out-of-plane AMR show a surprisingly different response with a pronounced cusp-like feature. The maximum of the cusp-like anisotropy is reached when the magnetic field is oriented in the $a$-$b$ plane. Moreover, the AMR for the azimuthal out-of-plane current direction exhibits a very strong four-fold $a$-$b$ plane anisotropy. Combining the Fermi surfaces calculated from first principles with the Boltzmanns semiclassical transport theory we reproduce and explain all the prominent features of the unusual behavior of the in-plane and out-of-plane AMR. We are also able to clarify the origin of the strong non-saturating transverse magnetoresistance as an effect of imperfect charge-carrier compensation and open orbits. Finally, by combining our theoretical model and experimental data we estimate the average relaxation time of $2.6times10^{-14}$~s and the mean free path of $15$~nm at 1.8~K in our samples of ZrSiS.
Dirac nodal line semimetals (DNLSs) host relativistic quasiparticles in their one-dimensional (1D) Dirac nodal line (DNL) bands that are protected by certain crystalline symmetries. Their novel low-energy fermion quasiparticle excitations and transport properties invite studies of relativistic physics in the solid state where their linearly dispersing Dirac bands cross at continuous lines with four-fold degeneracy. In materials studied up to now, the four-fold degeneracy, however, has been vulnerable to suppression by the ubiquitous spin-orbit coupling (SOC). Despite the current effort to discover 3D DNLSs that are robust to SOC by theory, positive experimental evidence is yet to emerge. In 2D DNLSs, because of the decreased total density of states as compared with their 3D counterparts, it is anticipated that their physical properties would be dominated by the electronic states defined by the DNL. It has been even more challenging, however, to discover robust 2D DNLSs against SOC because of their lowered symmetry; no such materials have yet been predicted by theory. By combining molecular beam epitaxy growth, STM, nc-AFM characterisation, with DFT calculations and space group theory analysis, here we reveal a novel class of 2D crystalline DNLSs that host the exact symmetry that protects them against SOC. The discovered quantum material is a brick phase 3-AL Bi(110), whose symmetry protection and thermal stability are imparted by the compressive vdW epitaxial growth on black phosphorus substrates. The BP substrate templates the growth of 3-AL Bi(110) nano-islands in a non-symmorphic space group structure. This crystalline symmetry protects the DNL electronic phase against SOC independent of any orbital or elemental factors. We theoretically establish that this intrinsic symmetry imparts a general, robust protection of DNL in a series of isostructural 2D quantum materials.
Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators (2DTIs) and their stacked three-dimensional (3D) equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose layered superconducting material CaSn as a WTI with nontrivial Z2 as well as nodal line semimetal protected by crystalline non-symmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunnelling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement and corresponding ab initio calculation strongly support the existence of TES in the non-symmorphic nodal line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.
The nodal line semimetals have attracted much attention due to their unique topological electronic structure and exotic physical properties. A genuine nodal line semimetal is qualified by the presence of Dirac nodes along a line in the momentum space that are protected against the spin-orbit coupling. In addition, it requires that the Dirac points lie close to the Fermi level allowing to dictate the macroscopic physical properties. Although the material realization of nodal line semimetals have been theoretically predicted in numerous compounds, only a few of them have been experimentally verified and the realization of a genuine nodal line semimetal is particularly rare. Here we report the realization of a genuine nodal line semimetal in LaSbTe. We investigated the electronic structure of LaSbTe by band structure calculations and angle-resolved photoemission (ARPES) measurements. Taking spin-orbit coupling into account, our band structure calculations predict that a nodal line is formed in the boundary surface of the Brillouin zone which is robust and lies close to the Fermi level. The Dirac nodes along the X-R line in momentum space are directly observed in our ARPES measurements and the energies of these Dirac nodes are all close to the Fermi level. These results constitute clear evidence that LaSbTe is a genuine nodal line semimetal,providing a new platform to explore for novel phenomena and possible applications associated with the nodal line semimetals.
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