The higher order topological insulator (HOTI) has enticed enormous research interests owing to its novelty in supporting gapless states along the hinges of the crystal. Despite several theoretical predictions, enough experimental confirmation of HOTI state in crystalline solids is still lacking. It has been well known that interplay between topology and magnetism can give rise to various magnetic topological states including HOTI and Axion insulator states. Here using the high-resolution angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles calculations, we report a systematic study on the electronic band topology across the magnetic phase transition in EuIn2As2 which possesses an antiferromagnetic ground state below 16 K. Antiferromagnetic EuIn2As2 has been predicted to host both the Axion insulator and HOTI phase. Our experimental results show the clear signature of the evolution of the topological state across the magnetic transition. Our study thus especially suited to understand the interaction of higher-order topology with magnetism in materials.

the topological nodal-line semimetal (tnS) is a unique class of materials with a one dimensional line node accompanied by a nearly dispersionless two-dimensional surface state. However, a direct observation of the so called drumhead surface state within current nodal-line materials is still elusive. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with firstprinciples calculations, we report the observation of a topological nodal-loop (tnL) in SrAs 3 , whereas caAs 3 exhibits a topologically trivial state. our data reveal that surface projections of the bulk nodalpoints are connected by clear drumhead surface states in SrAs 3. furthermore, our magneto-transport and magnetization data clearly suggest the presence (absence) of surface states in SrAs 3 (caAs 3). notably, the observed topological states in SrAs 3 are well separated from other bands in the vicinity of the fermi level. RAs 3 where R = Ca, Sr, thus, offers a unique opportunity to realize an archetype nodal-loop semimetal and establish a platform for obtaining a deeper understanding of the quantum phase transitions. Experimental discoveries of non-trivial topological states in semimetals such as the Dirac 1-4 , Weyl 5-8 , and nodal-line 9-12 semimetals have greatly expanded the family of available topological materials beyond topological insulators 13-18. In the case of the node line/loop semimetals the valence and conduction bands touch along lines/loops in the Brillouin zone and disperse linearly in directions perpendicular to these lines. The density of states at the Fermi energy in an NLS is greater than that of a Dirac or Weyl semimetal, providing a more favorable condition for investigating exotic non-trivial phases and realistic material platforms for developing applications. Note that, the NLSs are not robust against spin-orbit coupling or other perturbations and require crystal symmetries for their protection. To date, several structural classes of NLSs such as PbTaSe 2 19 , LaN 20 , Cu 3 PdN 21 , and ZrSiS-type 10,11,22-27 materials have been reported with associated space group symmetries that protect the nodal-line state. However, the nodal-loop states in PbTaSe 2 19 , and Cu 3 PdN 21 lie in the vicinity of other metallic bands, LaN requires multiple symmetries for protection, while in the ZrSiX-type systems the topological states lie above the Fermi level. It is highly desirable, therefore, to find materials which require minimum symmetry protections without the presence of other nearby bands that interfere in isolating topological features within the electronic spectrum. It has been recently shown that time-reversal symmetry (TRS) with a center of inversion symmetry (CIS) is sufficient, in principle, to protect a nodal-line state 28-30. APn 3 (A = Ca, Sr, Ba, Eu; Pn = P, As) family of compounds has been identified as a potential material class to host such a minimal symmetry protected NLS when SOC is excluded 29,30. Among these, CaP 3 and CaAs 3 are the only members of this series to have a triclinic cryst...

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Initiated by the discovery of topological insulators, topologically non-trivial materials, more specifically topological semimetals and metals have emerged as new frontiers in the field of quantum materials. In this work, we perform a systematic measurement of EuMg2Bi2, a compound with antiferromagnetic transition temperature at 6.7 K, observed via electrical resistivity, magnetization and specific heat capacity measurements. By utilizing angle-resolved photoemission spectroscopy in concurrence with first-principles calculations, we observe Dirac cones at the corner and the zone center of the Brillouin zone. From our experimental data, multiple Dirac states at Γ and K points are observed, where the Dirac nodes are located at different energy positions from the Fermi level. Our experimental investigations of detailed electronic structure as well as transport measurements of EuMg2Bi2 suggest that it could potentially provide a platform to study the interplay between topology and magnetism.

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