Negative Magnetoresistance in Topological Semimetals of Transition-Metal Dipnictides with Nontrivial Z2 Indices

Abstract

Negative magnetoresistance (NMR) induced by the Adler-Bell-Jackiw anomaly is regarded as the most prominent quantum signature of Weyl semimetals when electrical field E is collinear with the external magnetic field B. In this article, we report universal NMR in nonmagnetic, centrosymmetric transition metal dipnictides MPn2 (M=Nb and Ta; Pn=As and Sb), in which the existence of Weyl fermions can be explicitly excluded. Using temperature-dependent magnetoresistance, Hall and thermoelectric coefficients of Nernst and Seebeck effects, we determine that the emergence of the NMR phenomena in MPn2 is coincident with a Lifshitz transition, corresponding to the formation of unique electron-hole-electron (e-h-e) pockets along the I − L − I direction. First-principles calculations reveal that, along the I − L − I line, the dxy and d x 2 −y 2 orbitals of the transition metal form tilted nodal rings of band crossing well below the Fermi level. Strong spin-orbital coupling gaps all the crossing points and creates the characteristic e-h-e structure, making MPn2 a topological semimetal with Z2 indices of [0;(111)]. By excluding the weak localization contribution of the bulk states, we conclude that the universal NMR in MPn2 may have an exotic origin in topological surface states, which appears in pairs with opposite spin-momentum locking on nontrivial surfaces.

Optical signatures of Dirac nodal lines in NbAs 2

“…2(b), left panel, we display the 10 K optical conductivity for both polarizations of light. The Drude conductivity in both σ a 1 and σ b 1 feature at least two free-carrier components, consistent with multiple Fermi pockets revealed by quantum oscillation measurements in NbAs 2 [31][32][33]. The most striking feature is the sharp double-step in σ b 1 (green arrows), followed by σ 1 (ω) ∼ ω relation over an extended frequency range.…”

“…Furthermore, the Dirac linear dispersion perpendicular to the lines is established by √ B-spaced Landau-levels (LLs) in magneto-optics, previously observed only for nodal-points [28][29][30]. The nodal-lines discovered through unusual response functions here also naturally explain the exotic magnetoresistance (MR) properties of NbAs 2 [31][32][33]. plane passes through the line.…”

“…We now discuss the implication of massive Dirac nodallines for the unusual magneto-transport properties of NbAs 2 . Gigantic MR (> 10 5 %) in nonmagnetic NbAs 2 has been observed [31][32][33]41] and explained as a cooperation of perfect electron-hole compensation and highmobility carriers, leading to a ∼ B 2 increase. However, high-field MR measurements clearly deviate from the B 2 dependence starting at ∼10 T and linearly increase with B without saturation [32].…”

Hidden Weyl points in centrosymmetric paramagnetic metals

“…Recently, transition metal dipnictides of the type AB 2 (A ∈ {Ta, Nb}, B ∈ {As, Sb}) have gained a lot of attention [12][13][14][15][16][17][18][19] for their giant magnetoresistance. These materials are semimetals, but without a direct closure of the band gap.…”

Magnetoresistance and Shubnikov-de Haas oscillation in YSb

“…Recently, much attention has been paid on the extremely large MR (XMR) around 10 5 % to 10 6 %. The XMR has been detected in several nonmagnetic materials, such as TX (T=Ta/Nb, X=As/P) 7-15 , TX 2 (T=Ta/Nb, X=As/Sb) [16][17][18][19][20][21][22][23][24][25] , LaX(X=Sb/Bi) [26][27][28][29][30] , Cd 3 As 2 31,32 , and WTe 2 33-39 etc.…”