Electric Control of Fermi Arc Spin Transport in Individual Topological Semimetal Nanowires

The exotic topological surface states of Dirac or Weyl semimetals, namely Fermi arcs, are predicted to be spin polarized, while their spin polarization nature is still not revealed by transport measurements. Here, we report the spin-polarized transport in a Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanowire employing the ferromagnetic electrodes for spin detection. The spin-up and spin-down states can be changed by reversing the current polarity, showing the spin-momentum locking property. Moreover, the nonlocal measurements show a high fidelity of the spin signals, indicating the topological protection nature of the spin transport. As tuning the Fermi level away from the Dirac point by gate voltages, the spin signals gradually decrease and finally are turned off, which is consistent with the fact that the Fermi arc surface state has the maximum ratio near the Dirac point and disappears above the Lifshitz transition point. Our results should be valuable for revealing the transport properties of the spin-polarized Fermi arc surface states in topological semimetals.

Figure 1

The local spin transport. (a) The transmission electron microscopy image of a typical ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanowire. (b) The device diagram of the spin detection via local measurement. (c) The SEM image of a typical device (false color) with gold electrodes in yellow and a cobalt electrode in blue. Scale bar: 500 nm. (d),(e) The magnetic hysteretic loop measured at 2 K with a direct current of 100 and $-100\mathrm{nA}$, respectively. The background from the bulk states was subtracted. The electron spin $\mathit{S}$ is right-angle locked with the electron momentum $k_e$. (f) The temperature dependence of the spin signal in another similar device. The $\mathrm{\Delta }H$ is defined as the window height at zero magnetic field of the hysteresis loop.

Figure 2

The nonlocal spin transport at 2 K. (a),(b) The magnetic hysteretic loop measured using the nonlocal configuration depicted in the inset. A direct current $\pm 100\mathrm{nA}$ was applied. $k_e^{\prime }$ denotes the electron momentum direction in the nonlocal channel. Apparently the nonlocal spin signals do not change the sign with reversing the source-drain current direction.

Figure 3

The field effect of spin signals at 2 K. (a),(b) The $V_g$ tuning of the magnetic hysteretic loops for (a) local and (b) nonlocal measurements. (c) The gate voltage dependence of the window height of the magnetic hysteretic loop extracted from (a),(b). The error bars represent the standard deviations over multiple measurements. (d) The mechanism of the gate-tuned on-off effect of the spin signal. Top: the bulk bands schematic diagram. Bottom: the schematic of surface states. The blue and red dots correspond to different chirality and the arrows denote the spins locked to the momentum. The brown shaded circles denote the Fermi energy pockets. When the Fermi level is above the Lifshitz transition, the two Fermi pockets emerge together, and there are trivial surface states.

Figure 4

The Shubnikov–de Haas oscillations at different gate voltages. (a),(d),(g) The SdH oscillations measured with the magnetic field along the nanowire with [112] crystal direction. (b),(e),(h) The fast-Fourier-transform (FFT) analysis. The dual-frequency pattern can be found at high gate voltages in (h). (c),(f),(i) The Fermi surfaces of the ${\mathrm{Cd}}_3{\mathrm{As}}_2$ at different gate voltages. The $k_z$ axis points towards the [001] direction, along which there are two Dirac points distribution. Under a magnetic field along the [112] direction, the maximum cross section (dark brown) of the Fermi surfaces corresponds to the single SdH frequency. When the Fermi level is above the Lifshitz transition point, the two Fermi surfaces merge together and a minimum cross section (light brown) emerges, see in (i), corresponding to the additional smaller frequency observed.