CM/AMO Seminar with Professor Luis Balicas

Abstract

Topological semimetals such as Weyl and Dirac systems are three-dimensional phases of matter characterized by topology and symmetry protected gapless electronic excitations. These three- dimensional analogs of graphene have generated a lot of interest recently given that their quasiparticles display properties akin to those of relativistic and chiral fermions in particle physics. Their unconventional electronic structures are predicted to lead to protected surface states and to unconventional responses to applied electric and magnetic fields. In the past few years, we have studied a few of these compounds [1-11] under high magnetic fields, with the goal of i) extracting their electronic structure at the Fermi level in order to ii) compare it with theoretical predictions, and of iii) exposing their transport properties which are expected to be unconventional due to their “topological” character. Here, after a broad introduction, we will focus on the magnetoresistivity and the Hall-effect of the type-I Weyl semimetal TaAs in attempt to address the strong controversy surrounding its anomalous transport properties in relation to its bulk topological character. For fields and currents along the basal plane, we observe a very pronounced planar Hall effect (PHE) upon field rotation with respect to the crystallographic axes at temperatures as high as T = 100 K [10]. Parametric plots of the PHE signal as a function of the longitudinal magnetoresistivity (LMR) collected at T = 10 K lead to concentric traces as reported for Na3Bi and GdBiPt suggesting that both the negative LMR and the PHE observed in TaAs are intrinsically associated to the axial anomaly among its Weyl nodes. For fields nearly along the a axis we also observe hysteresis as one surpasses the quantum limit, where the magnetic torque indicates a change in regime as the field increases, i.e., from paramagnetism and diamagnetism due to Weyl fermions above and below the Weyl node(s), respectively, to a paramagnetic one associated with the field-independent n=0 Landau level. Hysteresis coupled to the overall behavior of the torque would be consistent with a topological phase transition associated with the suppression of the Weyl dispersion at the quantum limit. This transition leads to the suppression of the negative LMR confirming that it is associated to the Weyl dispersion [10].