Observation of Fermi-Arc Spin Texture in TaAs

We have investigated the spin texture of surface Fermi arcs in the recently discovered Weyl semimetal TaAs using spin- and angle-resolved photoemission spectroscopy. The experimental results demonstrate that the Fermi arcs are spin polarized. The measured spin texture fulfills the requirement of mirror and time-reversal symmetries and is well reproduced by our first-principles calculations, which gives strong evidence for the topologically nontrivial Weyl semimetal state in TaAs. The consistency between the experimental and calculated results further confirms the distribution of chirality of the Weyl nodes determined by first-principles calculations.

Figure 1

(a) Schematic of a WSM with spin-polarized Fermi arcs on its surfaces connecting the projections of two Weyl nodes with opposite chirality. The red and blue colors of the bulk Weyl cones and the corresponding projection points on the surfaces represent opposite chirality of the Weyl nodes. The red arrows on the surfaces indicate the spin texture of the Fermi arcs. (b) Schematic illustration of the SARPES measurement geometry. The incident beam of light and the outgoing electrons entering the detector define the outgoing plane. The incident light and the hemispherical analyzer form a 45° angle. The reciprocal space along $k_x$ and $k_y$ is explored by varying the angles $\psi$ and $\theta$, respectively. (c) Crystal structure of TaAs. (d) Bulk and projected (001) surface BZs of TaAs. (e) 3D intensity plot of the photoemission data recorded by conventional ARPES measurements, showing the FSs and band dispersions along high-symmetry lines of TaAs. Red and yellow solid circles indicate the locations of the bulk Weyl nodes $W1$ projected onto the (001) surface BZ, with red and yellow representing opposite chirality.

Figure 2

(a) Photoemission intensity plot at $E_F$ of the (001) surface recorded at $h\nu =54$ (outside the green box) and 36 eV (inside the green box) recorded by conventional ARPES systems at the Dreamline and SIS beam lines. Red and yellow solid circles indicate the locations of the bulk Weyl nodes projected onto the (001) surface BZ, with red and yellow representing opposite chirality. The red and yellow arrows indicate the direction of spin polarization on the inner and outer FSs at the high-symmetry lines, respectively. The in-plane spin-polarization $x$ and $y$ axes are defined along the $\overline{\mathrm{\Gamma }}\text{−}\overline{X}$ and $\overline{\mathrm{\Gamma }}\text{−}\overline{Y}$ directions, respectively, and the out-of-plane spin-polarization $z$ axis is defined as the outward normal direction of the sample surface. (b) and (c) Spin-integrated energy–momentum intensity cuts along $C1$ and $C3$ [indicated in (a)], respectively. The data were taken with the spin-resolved system at the COPHEE end station. (d)–(f) Spin-resolved intensity along $C1$ projected on the $x$, $y$, and $z$ directions, respectively, measured at $h\nu =36\mathrm{eV}$ with left-hand circular ($C\text{−}$) polarization. The red and blue symbols are the intensity of spin-up and spin-down states, respectively. (g)–(i) Corresponding angle-resolved spin polarizations in the $x$, $y$, and $z$ directions, respectively. (j)–(l) Same as (g)–(i) but measured along $C2$. (m) Same as (h) but measured with linear horizontal (LH) polarization. (n) Same as (h) but measured at $h\nu =40\mathrm{eV}$. (o) Spin polarization along $C3$ projected on the $x$ direction measured at $h\nu =36\mathrm{eV}$ with LH polarization. All the spectra are measured at a binding energy of 20 meV, indicated as white dashed lines in (b) and (c).

Figure 3

(a) Spin-integrated FS map near $\overline{\mathrm{\Gamma }}\text{−}\overline{Y}$ recorded with the spin-resolved system at the COPHEE end station. The red arrows indicate the direction of measured in-plane spin polarizations of the Fermi arc $b2$ at $C4$. (b) Corresponding theoretical spin texture of surface states, with white lines indicating the locations of the SARPES measurements, labeled as $C1$, $C3$, $C4$, and $C5$. Red and yellow dashed circles indicate the Weyl nodes $W1$ with negative and positive chirality, respectively. (c)–(e) Angle-resolved spin polarizations along $C4$ in the $x$, $y$, and $z$ directions, respectively. (f)–(h) Same as (c)–(e), but along $C5$. All the spectra in Fig. 3 were measured at $h\nu =36\mathrm{eV}$ with $C\text{−}$ polarization and at a binding energy of 20 meV.

Figure 4

(a) Schematic of the spin-polarized surface states in a 3D TI. The blue arrows indicate the spin polarizations of the surface states. (b) Schematic of a DSM with spin-polarized Fermi arcs on its surface connecting projections of two bulk Dirac nodes. The black color of Dirac cones indicates that one Dirac node is the degeneracy of two Weyl nodes with opposite chirality. (c) Schematic of a WSM with spin-polarized Fermi arcs on its surface connecting projections of two bulk Weyl nodes. The red and blue colors represent opposite chirality. For clarity, only surface states on the top and bottom surfaces are indicated.