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Magnetotransport measurements of PtPb _4 single crystals with the T _c of ∼2.80 K reveal pronounced exotic topological nature. An anomalous negative longitudinal magnetoresistance (MR) related to the chiral anomaly and a linear-like transverse MR examined within the framework of quantum MR theory were studied. Results indicate the presence of chiral-anomaly-induced quantum transport associated with the Weyl states and show a Dirac-cone-like band dispersion in PtPb _4 . This work reveals the drastic impact of the concept that the surface electrons in a Weyl fermion state may dominate the normal-state magnetotransport in PtPb _4 , leading to the conclusion that PtPb _4 can be a Weyl-semimetal superconductor candidate.

Magnetotransport properties with a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed for quenched Fe _0.01 Bi _2 Te _3 single crystals. Large positive MR of ∼470% with a Hall mobility of ∼44 000 cm ^2 V ^−1 s ^−1 at 5 K and 6 T has been observed on a quenched Fe _0.01 Bi _2 Te _3 sample, in which the electrical parameters can be tuned by the quenching temperature T _q . The MR behaviors for the quenched samples show a crossover from a weak antilocalization-dominant MR to a linear and non-saturating MR at temperatures of T * ≈ 58−100 K, where the large MR at low temperatures possibly originates from the mechanism of topologically protected backscattering. On the contrary, the MR behaviors for the strain-released sample do not show such a distinct crossover, where only linear-like and non-saturating MR behaviors can be observed. Different electrical transports between the quenched and strain-released samples indicate that the band structure, as well as the surface Dirac electrons in Fe _0.01 Bi _2 Te _3 , can be modified by the lattice strain. Furthermore, it is found that the low-temperature magnetoconductivity can be well described by the weak-antilocalization transport formula, while the high-field linear-like MR at T > T * can be explained in terms of Abrikosov’s quantum transport of Dirac-cone states in quenched Fe _0.01 Bi _2 Te _3 single crystals.

Linear magnetoresistance (LMR) is usually observed in topological quantum materials and plausibly connected with the topologically nontrivial surface state with Dirac-cone-like linear dispersion because the frequently encountered large Hall resistivity can be trivially mixed into the LMR via charge inhomogeneity. Herein, by applying an optimal gate voltage to nodal-line semimetal ZrGeSe two-dimensional (2D) layers with specific thicknesses, we observe a giant nonsaturated LMR of 8 × 10 4 % at 2 K and a magnetic field of 9 T. This giant LMR is accompanied by a very small Hall resistivity, which is inconsistent with the charge inhomogeneity mechanism. Our systematic results confirm that the giant LMR is maximized when the topological semimetal is in the "even-metal" regime and suppressed upon evolution to the normal "odd-metal" regime. The "even-to-odd" transition is universal regardless of the thicknesses of the crystals. A comparison with Abrikosov's quantum LMR theory indicates that the observed LMR cannot be trivial.