Speaker: Prof. Zhiqiang Mao
Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70018
Abtract: Recent discoveries of three dimensional (3D) topological Dirac semimetals (DSM) in Na3Bi [1,2] and Cd3As2 [3-5] and Weyl semimetals (WSM) in monopnictides TX (T=Ta/Nb, X=As/P) [6-9] and photonic crystals have generated immense interests since they represent new topological states of quantum matter. Both classes of materials feature relativistic fermions with linearly dispersing excitations. WSMs can be seen as evolving from DSMs in the presence of the breaking of time reversal symmetry (TRS) or space inversion symmetry. WSMs caused by the loss of space inversion symmetry have been experimentally realized in non-centrosymmetric crystals of TX [8-11]. In this talk, I will first give a brief introduction to this emerging area and then present our recent studies on TaP and Sr1-yMn1-zSb2 (y, z < 0.1). TaP is one of the predicted WSMs. We observed two fundamental transport properties predicted for a Weyl state, i.e. the chiral anomaly-induced negative magnetoresistance and the Berry phase. Furthermore, we have generalized the Lifshitz-Kosevich (LK) formula for multiple-band Shubnikov-de Haas (SdH) oscillations and extracted the Berry phases of for multiple Fermi pockets in TaP through the direct fits of the modified LK formula to the SdH oscillations . For Sr1-yMn1-zSb2, our work showed the two-dimensional Sb layers of this material host relativistic fermions; remarkable signatures of relativistic fermions, including light effective quasiparticle mass, high carrier mobility, a Berry phase and valley polarized interlayer conduction were all revealed from quantum transport measurements . Another distinct aspect of this materials lies in its ferromagnetism. Coupling between ferromagnetism and relativistic fermion transport has also been observed. The combination of relativistic fermion behavior and ferromagnetism make this material a promising candidate for exploring the long-sought magnetic Weyl semimetal.
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