Low Carrier Density Metal Realized in Candidate Line-Node Dirac Semimetals CaAgP and CaAgAs

Topological semimetals are characterized by protected crossings between conduction and valence bands. These materials have recently attracted significant interest because of the deep connections to high-energy physics, the novel topological surface states, and the unusual transport phenomena. While Dirac and Weyl semimetals have been extensively studied, the nodal-line semimetal remains largely unexplored due to the lack of an ideal material platform. In this paper, we report the magneto-transport properties of two nodal-line semimetal candidates CaAgAs and CaCdGe. First, our single crystalline CaAgAs supports the first "hydrogen atom" nodal-line semimetal, where only the topological nodal-line is present at the Fermi level. Second, our CaCdGe sample provides an ideal platform to perform comparative studies because it features the same topological nodal line but has a more complicated Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through our systematic magneto-transport and first-principles band structure calculations, we show that our CaTX compounds can be used to study, isolate, and control the novel topological nodal-line physics in real materials.

Section: Introductionmentioning

confidence: 99%

Magnetotransport properties of the single-crystalline nodal-line semimetal candidatesCaTX(T=Ag,Cd;X=As,Ge)

Emmanouilidou¹,

Shen²,

Deng³

et al. 2017

“…Even for a "conventional" s-wave state, there exists a flat band of neutral Majorana bound states on each surface at the projected location of the FS. Finally, note that while Ca 3 P 2 [10], CaAgP and CaAgAs [12] materials have a four-fold degenerate nodal line (i.e. a Dirac loop), our conclusion can be generalized to these materials.…”

mentioning

confidence: 62%

“…Nodal-line semimetals are a new class of topological materials that has recently been proposed [1][2][3][4][5][6][7][8][9] and observed in Ca 3 P 2 [10], TlTaSe 2 [11], CaAgP and CaAgAs [12]. A Weyl-loop material is the simplest example of these systems which at a particular filling display a two-fold degenerate one-dimensional Fermi ring in the bulk (which becomes toroidal upon doping), with massless Dirac-like quasiparticles.…”

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confidence: 99%

Topological surface superconductivity in doped Weyl loop materials

Wang

Nandkishore

2017

We study surface superconductivity involving the `drumhead' surface states of (doped) Weyl loop materials. The leading weak coupling instability in the bulk is toward a chiral superconducting order, which fully gaps the Fermi surface. In this state the surface also becomes superconducting, with $p+ip$ symmetry. We show that the surface SC state is "topological" as long as it is fully gapped, and the system traps Majoarana modes wherever a vortex line enters or exits the bulk. In contrast to true 2D $p+ip$ superconductors, these Majorana zero modes arise even in the "strong pairing" regime where the chemical potential is entirely above/below the drumhead. We also consider conventional $s$-wave pairing, and show that in this case the surface hosts a flat band of charge neutral Majorana fermions, whose momentum range is given by the projection of the bulk Fermi surface. Weyl loop materials thus provide access to new forms of topological superconductivity.Comment: 9 pages, 5 figure

“…The nodal lines lie on the mirror/glide plane. The spinless nodal-line semimetals have been reported in CaAgP and CaAgAs, which are noncentrosymmetric 36,37 . In general, the protection by the mirror symmetry can coexist with the topological protection.…”

Section: Introductionmentioning

confidence: 99%

Universal phase transition and band structures for spinless nodal-line and Weyl semimetals

Okugawa

Murakami

2017

We study a general phase transition between spinless topological nodal-line semimetal and Weyl semimetal phases. We classify topological nodal lines into two types based on their positions and shapes, and their phase transitions depends on their types. We show that a topological nodalline semimetal becomes the Weyl semimetal by breaking time-reversal symmetry when the nodal lines enclose time-reversal invariant momenta (type-A nodal lines). We also discuss an effect of crystallographic symmetries determining the band structure of the topological nodal-line semimetals. Thanks to protection by the symmetries, the topological nodal-line semimetals can transition into spinless Weyl semimetals or maintain the nodal lines in many crystals after inversion symmetry is broken.