Quantum limit transport and destruction of the Weyl nodes in TaAs

Abstract

Weyl fermions are a recently discovered ingredient for correlated states of electronic matter. A key difficulty has been that real materials also contain non-Weyl quasiparticles, and disentangling the experimental signatures has proven challenging. Here we use magnetic fields up to 95 T to drive the Weyl semimetal TaAs far into its quantum limit, where only the purely chiral 0th Landau levels of the Weyl fermions are occupied. We find the electrical resistivity to be nearly independent of magnetic field up to 50 T: unusual for conventional metals but consistent with the chiral anomaly for Weyl fermions. Above 50 T we observe a two-order-of-magnitude increase in resistivity, indicating that a gap opens in the chiral Landau levels. Above 80 T we observe strong ultrasonic attenuation below 2 K, suggesting a mesoscopically textured state of matter. These results point the way to inducing new correlated states of matter in the quantum limit of Weyl semimetals.

Emergence of gapped bulk and metallic side walls in the zeroth Landau level in Dirac and Weyl semimetals

“…1(b)]. This threshold scale agrees qualitatively with the observed field strength above which the anomalous conductivity disappears in TaAs [7]. A parallel analysis can be applied to the remaining eight pairs of W2 nodes that share a similar node structure, but are off the k z = 0 plane.…”

“…Surprisingly, it was found that the anticipated negative longitudinal magnetoresistance started to break down at large magnetic field (B ∼ 50 T), implying a gap opening and the loss of chiral anomaly. A second surprise is that the exponential rise in resistivity saturates at low temperature and the saturated resistivity decreases at even higher field strength (B ∼ 80 T) [7]. Gap opening has also been suggested in other Weyl materials such as TaP [8] and previous numerical studies also support similar ideas [9,10].…”

“…The presence of metallic side walls could potentially explain a number of surprising observations about the magnetoresistance of TaAs [7]. It was observed that the longitudinal resistance became thermally activated at 50 T, but saturated at low temperature.…”

“…Suppose initially the W1 nodes have metallic surfaces and W2 nodes are gapped. Reference [7] shows evidence for a bulk phase transition at 80 T, which can cause changes in parameters for W2 nodes such that the edge gap is reduced and μ moves from inside to outside the gap. This may explain the resistivity drop which is unexpected because phase transitions typically involve gap openings in the bulk.…”

“…Since its proposal, significant experimental progress has been made to observe the chiral anomalous effect in solid-state systems including Dirac [2][3][4] and Weyl [5,6] semimetals. This excitement has been pushed further by a recent transport experiment on Weyl semimetal TaAs in the extreme quantum limit [7]. Surprisingly, it was found that the anticipated negative longitudinal magnetoresistance started to break down at large magnetic field (B ∼ 50 T), implying a gap opening and the loss of chiral anomaly.…”