Topological surface electronic states in candidate nodal-line semimetal CaAgAs

Abstract

We investigate systematically the bulk and surface electronic structure of the candidate nodal-line semimetal CaAgAs by angle-resolved photoemission spectroscopy and density functional calculations. We observed a metallic, linear, non-k z -dispersive surface band that coincides with the high-binding-energy part of the theoretical topological surface state, proving the topological nontriviality of the system. An overall downshift of the experimental Fermi level points to a rigid-band-like p doping of the samples, due possibly to Ag vacancies in the as-grown crystals. DOI: 10.1103/PhysRevB.96.161112 The discovery of topologically nontrivial systems has dominated the field of condensed matter physics over the past decade. Unique nontrivial topological properties in these fermionic systems relate concepts from high-energy physics to various quasiparticle excitations in condensed matter, causing the systems to resist small perturbations due to the protection by particular topological invariants [1,2]. In the so-called topological semimetals, such nontrivialities appear as the touching of valence and conduction bands at isolated points, closed lines, or planes in momentum space. Such materials exhibit chiral anomaly [3,4], topological surface Fermi arcs [5][6][7][8][9][10][11][12][13], and/or superconducting zero modes [14][15][16], whose quasiparticle excitations correspond directly to the Dirac, Weyl, and Majorana fermions. These quasiparticles differ from the actual entities in high-energy physics, but obey the same underlying principles in quantum field theory, offering the opportunity to investigate the fundamental physical laws that govern a large subset of quantum condensed matter as well as creating a new approach for developing a broad range of low-power, high-efficiency spintronic and quantum computing devices [1,2,17].Topological nodal-line semimetals (NLSMs) exhibit novel topological properties that are manifested by surface states in the form of a drumlike membrane living in the continuous toroidal isosurface gap in three-dimensional (3D) momentum space. With strong spin-orbit coupling (SOC), the nodal lines in these materials are either protected by reflection or mirror symmetry, or gapped out due to the lack of such symmetries [18][19][20][21][22] realizing a clean, "hydrogen-atom-like" NLSM is therefore of urgent need. Single-crystalline CaAgAs is theoretically predicted to be among the best candidate NLSMs since its theoretical Fermi surface contains no more than a circular nodal contour linked by the topological surface state [36], which gives rise to the ultralow magnetoresistance found by transport measurements [37]. In this Rapid Communication, we systematically investigate the NLSM state of CaAgAs which is solely protected by mirror reflection symmetry through a comparison between angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. Our DFT calculations show that without SOC, a topological nodal ring enclosing appears in the first Bri...

Experimental observation of bulk nodal lines and electronic surface states in ZrB2

“…More importantly, we identify distinct surface states emanating from the bulk nodal lines, the surface signatures demonstrate the nontrivial topology of the nodal-line semimetal states in ZrB 2 .Compared with the previous nodal-line candidates CaAgAs, PbTaSe 2 , and the ZrSiS family[28][29][30][31][32][33] , ZrB 2 has the following advantages: (1) the whole band-crossing features forming the nodal lines are clearly resolved below the Fermi energy (E F ) in ZrB 2 , while the nodes of CaAgAs 28 , PbTaSe 2 29 ,…”

Pressure-induced superconductivity and topological phase transitions in the topological nodal-line semimetal SrAs3

“…In the search for nodal-line semimetals, several systems have been theoretically proposed since 2011 2,3 . However, only a few candidates including PbTaSe 2 13,14 , ZrSiX (X = S, Se, Te) 15,16 , CaAgX (X = P, As) 17,18 , and MB 2 (M = Ti, Zr) 19 have been verified experimentally. More recently, the CaP 3 family of materials (MAs 3 , for M = Ca, Ba, and SrX 3 for X = P, As) was proposed as another potential host of TNLSMs 20 .…”

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