Direct optical detection of Weyl fermion chirality in a topological semimetal

Abstract

A Weyl semimetal (WSM) is a novel topological phase of matter [1][2][3][4][5][6][7][8][9][10][11][12][13], in which Weyl fermions (WFs) arise as pseudo-magnetic monopoles in its momentum space. The chirality of the WFs, given by the sign of the monopole charge, is central to the Weyl physics, since it directly serves as the sign of the topological number [5,12] and gives rise to exotic properties such as Fermi arcs [5,9,11] and the chiral anomaly [12][13][14][15][16]. Despite being the defining property of a WSM, the chirality of the WFs has never been experimentally measured. Here, we directly detect the chirality of the WFs by measuring the photocurrent in response to circularly polarized mid-infrared light. The resulting photocurrent is determined by both the chirality of WFs and that of the photons. Our results pave the way for realizing a wide range of theoretical proposals [12,13,[17][18][19][20][21][22][23][24][25][26] for studying and controlling the WFs and their associated quantum anomalies by optical and electrical means. More broadly, the two chiralities, analogous to the two valleys in 2D materials [27,28], lead to a new degree of freedom in a 3D crystal with potential novel pathways to store and carry information.infrared pump and a soft X-ray probe, which is technically very challenging. On the other hand, optical experiments on WSMs have remained very limited [36,37], although they are promising approaches to achieve these goals [12]. In this paper, we detect the chirality of the WFs in the WSM TaAs by measuring its mid-infrared photocurrent response. Circularly polarized light induced photocurrents, also called the circular photogalvanic effect (CPGE), have been previously measured in other systems [29-31] but have not been experimentally studied in WSMs.We first discuss the theoretical picture of the CPGE for optical transitions from the lower part of the Weyl cone to the upper part [17]. There are two independent factors important for the CPGE here. The first is the chirality selection rule (Figs. 1c,d). For a right circularly polarized (RCP) light propagating along +ẑ and a χ = +1 WF, the optical transition is allowed on the +k z side but forbidden on the −k z side [17]. The second is the Pauli blockade, which is only present when chemical potential is away from the Weyl node. FIG. 4: Detection and manipulation of chiral Weyl fermions by optical means. a, Our calculations (see SI.II.3) showĴ THY = χ W 1klight ×ĉ. b, By measuring the direction of the currentJ and knowing the polarization and propagation direction of the light, we obtainĴ EXP =k light ×ĉ from data. By comparing theory and data, we obtain χ W 1 = +1. The χ W 1 = +1 determined by the photocurrent agrees with that predicted by first-principles ( Fig. 1j), further confirming our experimental detection of WF chirality. c-e, Comparison between the chirality degree of freedom of the WFs and valley degree of freedom in gapped Dirac system. c, In a gapped Dirac system, an optical excitation with a particular handedness can only populate ...

Towards properties on demand in quantum materials

“…Berry phase effects also underlie the phenomenon of 'shift currents'-an optically induced charge separation arising from asymmetry in the electronic wavefunctions-which provides a new paradigm for designing high-performance optical-frequency conversion and photovoltaic materials 28,29 . Recent studies of transition metal monopnictide-based Weyl semimetals (discussed in 'Topological phenomena under control') that exhibit the requisite band topology have revealed giant second-order nonlinear optical responses 30,31 . Another promising device concept is an optically pumped low-threshold laser based on a monolayer of the prototypical TMD material WSe 2 32 .…”

Photocurrent in gyrotropic Weyl semimetals

“…However, in this case the equation for the photocurrent loses its universality. Recently Qiong Ma et al [10] have observed a circular photocurrent in the TaAs crystal under excitation by CO 2 laser radiation.…”

Berry curvature dipole in Weyl semimetal materials: An ab initio study

“…Because Weyl points are monopole sources or drains of the Berry curvature of Bloch wave functions in momentum space, a WSM can exhibit an anomalous Hall effect when breaking the time-reversal symmetry (TRS) [17][18][19] or a spin Hall effect [20], as a linear response to an external electric field. Recent theoretical [21][22][23][24][25][26][27][28][29][30] and experimental [31][32][33][34] studies have revealed giant nonlinear optical responses in inversion-symmetry-breaking WSMs, such as the photocurrent from the circular photogalvanic effect (CPGE), second harmonic generation (SHG), and nonlinear Hall effect. These nonlinear effects can be much stronger in WSMs than traditional electro-optic materials owing to the large Berry curvature [22,35,36].…”

“…Corresponding nonlinear susceptibilities in several materials promise a nonlinear Hall effect in the dc field limit, which is within the experimentally detectable range. [31][32][33][34] studies have revealed giant nonlinear optical responses in inversion-symmetry-breaking WSMs, such as the photocurrent from the circular photogalvanic effect (CPGE), second harmonic generation (SHG), and nonlinear Hall effect. These nonlinear effects can be much stronger in WSMs than traditional electro-optic materials owing to the large Berry curvature [22,35,36].…”