S. S.L. Zhang, A. A. Burkov, I. Martin, and O G. Heinonen. Spin-to-charge conversion in magnetic Weyl semimetals. ArXiv April 2019.
Weyl semimetals (WSMs) are a newly discovered class of quantum materials which can host a number of exotic bulk transport properties, such as the chiral magnetic effect, negative magneto-resistance, and the anomalous Hall effect. In this work, we investigate theoretically the spin-to-charge conversion in a bilayer consisting of a magnetic WSM and a normal metal (NM), where a charge current can be induced in the WSM by an spin current injection at the interface. We show that the induced charge current exhibits a peculiar anisotropy: it vanishes along the magnetization orientation of the magnetic WSM, regardless of the direction of the injected spin. This anisotropy originates from the unique band structure of magnetic WSMs and distinguishes the spin-to-charge conversion effect in WSM/NM structures from that observed in other systems, such as heterostructures involving heavy metals or topological insulators. The induced charge current depends strongly on injected spin orientation, as well as on the position of the Fermi level relative to the Weyl nodes and the separation between them. These dependencies provide additional means to control and manipulate spin-charge conversion in these topological materials.
L.-L. Wang, N. H. Jo, B. Kuthanazhi, Y. Wu, R. J. McQueeney, A. Kaminski, and P. C. Canfield. A Single Pair of Weyl Fermions in Half-metallic EuCd2As2 Semimetal. ArXiv January 2019.
While breaking time reversal symmetry, magnetism can introduce interesting topological states by combining with other crystalline symmetries to give rise to antiferromagnetic topological insulator and antiferromagnetic Dirac semimetal. For such a material, EuCd2As2, we find that the ferromagnetic configuration can host the ideal case of a single pair of Weyl points due to the half-metallic nature of the system. Analysis with low-energy effective Hamiltonian show that such ideal case is obtained in EuCd2As2 because the Dirac points are very close to the zone center and the ferromagnetic exchange splitting is large enough to push two Weyl points, one from each side of Gamma point to merge and annihilate. Furthermore, we show that by alloying Ba at Eu site, the ferromagnetic configuration can become the ground state and is more stable than the antiferromagnetic state to realize such an ideal case of a single pair of Weyl points.
N. Sirica, R. I. Tobey, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, B. Shen, D. A. Yarotski, P. Bowlan, S. A. Trugman, J.-X. Zhu, Y. M. Dai, A. K. Azad, N. Ni, X. G. Qiu, A. J. Taylor and R. P. Prasankumar. Tracking ultrafast photocurrents in the Weyl semimetal TaAs using THz emission spectroscopy. Physical Review Letters, 1222, 197401 (2019).
We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the non-centrosymmetric c-axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the ab-plane. This is consistent with static photocurrent experiments, and provides additional insight into their microscopic origin by way of the dynamical information gained from the emitted THz waveform. THz emission spectroscopy is thus a powerful, contact-free approach for distinguishing between injection and shift photocurrents by unraveling the polarization dependence, directionality, and intrinsic timescales that underlie their generation and decay.