Electronic structure in a transition metal dipnictide TaAs2

The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently,TaAs2, a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the (2‾01) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface ofTaAs2are trivial bulk bands. The absence of symmetry-protected surface state on the (2‾01) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR ofTaAs2.

Experimental observation of drumhead surface states in SrAs3

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 first-principles calculations, we report the observation of a topological nodal-loop (TNL) in SrAs3, whereas CaAs3 exhibits a topologically trivial state. Our data reveal that surface projections of the bulk nodal-points are connected by clear drumhead surface states in SrAs3. Furthermore, our magneto-transport and magnetization data clearly suggest the presence (absence) of surface states in SrAs3 (CaAs3). Notably, the observed topological states in SrAs3 are well separated from other bands in the vicinity of the Fermi level. RAs3 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.

Distinct multiple fermionic states in a single topological metal

Among the quantum materials that have recently gained interest are the topological insulators, wherein symmetry-protected surface states cross in reciprocal space, and the Dirac nodal-line semimetals, where bulk bands touch along a line in k-space. However, the existence of multiple fermion phases in a single material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic material Hf₂Te₂P and discover properties, which are unique in a single topological quantum material. We experimentally observe weak topological insulator surface states and our calculations suggest additional strong topological insulator surface states. Our first-principles calculations reveal a one-dimensional Dirac crossing—the surface Dirac-node arc—along a high-symmetry direction which is confirmed by our ARPES measurements. This novel state originates from the surface bands of a weak topological insulator and is therefore distinct from the well-known Fermi arcs in semimetals.

The existence of multiple topological phases in a single material, although theoretically possible, has not been verified. Here, the authors observe weak topological insulator surface states and a one-dimensional Dirac-node crossing surface state in a single metallic material Hf₂Te₂P.

Observation of the spin-polarized surface state in a noncentrosymmetric superconductor BiPd

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here we report a systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higher-binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperature-dependent and spin-resolved ARPES measurements complemented by our first-principles calculations demonstrate the existence of the spin-polarized surface states at high-binding energy. The absence of such spin-polarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. Our direct experimental observation of spin-polarized surface states in BiPd provides critical information that will guide the future search for topological superconductivity in noncentrosymmetric materials.

Observation of Dirac-like semi-metallic phase in NdSb

The search of new topological phases of matter is one of the new directions in condensed matter physics. Recent experimental realizations of Dirac semimetal phases pave the way to look for other exotic phases of matter in real materials. Here we present a systematic angle-resolved photoemission spectroscopy (ARPES) study of NdSb, a potential candidate for hosting a Dirac semi-metal phase. Our studies reveal two hole-like Fermi surface pockets present at the zone center ([Formula: see text]) point as well as two elliptical electron-pockets present in the zone corner (X) point of the Brillouin zone (BZ). Interestingly, Dirac-like linearly dispersive states are observed about the zone corner (X) point in NdSb. Our first-principles calculations agree with the experimentally observed bands at the [Formula: see text] point. Moreover, the Dirac-like state observed in NdSb may be a novel correlated state, not yet predicted in calculations. Our study opens a new direction to look for Dirac semi-metal states in other members of the rare earth monopnictide family.