Atomic-Scale Photon Mapping Revealing Spin-Current Relaxation

A nanoscopic understanding of spin-current dynamics is crucial for controlling the spin transport in materials. However, gaining access to spin-current dynamics at an atomic scale is challenging. Therefore, we developed spin-polarized scanning tunneling luminescence spectroscopy (SP STLS) to visualize the spin relaxation strength depending on spin injection positions. Atomically resolved SP STLS mapping of gallium arsenide demonstrated a stronger spin relaxation in gallium atomic rows. Hence, SP STLS paves the way for visualizing spin current with single-atom precision.

Figure 1

Spin injection and photon detection by spin-polarized scanning tunneling luminescence spectroscopy (SP STLS). (a) Experimental setup showing three key processes in SP STLS: tunneling (I), relaxation (II), and luminescence (III). Fe, iron; $p$-GaAs, $p$-type gallium arsenide; $P_e^0$ ($P_e$), initial (final) spin polarization of electrons in $p$-GaAs; $P_{\mathrm{ph}}$, circular polarization of luminescence; $E_{\mathrm{ph}}$, photon energy; ${\sigma }^{+}$ (${\sigma }^{-}$), clockwise (anticlockwise) circularly polarized light. (b) Schematic of an optical system for measuring $P_{\mathrm{ph}}$ and $E_{\mathrm{ph}}$. (c) Spectroscopy of ${\sigma }^{+}$ and ${\sigma }^{-}$ photons (set point: 1.60 V, 150 pA). The inset shows the optical transitions (dotted arrows) between the electronic states of $p$-GaAs (black bars), which is known to occur in the ratio $3:1$ [3, 31, 32]. The numbers shown above or below each electronic state indicate their angular momentum. $V_{\mathrm{bias}}$, bias voltage; $E_F$, Fermi level of $p$-GaAs; cps, counts per second. (d) Magnetic field dependence of normalized luminescence spectra (set point: 1.60 V, 150 pA).

Figure 2

Atomic-scale SP STLS mapping reveals spin-current relaxation. (a) Atomic-scale $P_{\mathrm{ph}}$ mapping in response to the spin-down injection (top) with an STM topography and the schematic of the atomic structure [11] (bottom). The $|P_e|$ values were estimated using $P_e=-P_{\mathrm{ph}}/0.458$ to determine the remaining spin polarization in the conduction band minimum (set point: 1.6 V, 150 pA). (b) Three-dimensional topography of $p$-GaAs with schematic spatial distributions of the densities of states for individual electronic states (blue and pink). ${\mathrm{Ga}}_r$, gallium atomic row (set point: 1.60 V, 150 pA). (c) Band structure of $p$-GaAs showing three valleys of bulk states ($\mathrm{\Gamma }$, $L$, and $X_b$), one valley of a surface state ($X_s$), and the corresponding possible spin relaxation pathways. AB, acceptor band. (d) Tunnel conductance ($dI/d{V}_{\mathrm{bias}}$) spectra around the band gap of $p$-GaAs. Red spectrum was spatially averaged in the surface and black spectrum was taken at ${\mathrm{Ga}}_r$, in which the individual spectra were taken in different tip conditions. Arrows indicate the valley edges. Set points: 1.60 V, 10 pA for red spectrum; 1.60 V, 150 pA for black spectrum; lock-in: 731 Hz, 20 mV for red spectrum; 817 Hz, 10 mV for black spectrum.

Figure 3

Bias control of the spin-current relaxation. (a) Bias dependence of the $P_e$ mapping taken along the $[1\overline{1}0]$ direction in response to the spin-down injection (top) with schematics of the atomic structure (bottom). Corresponding STM topography is in Fig. 2 (set point: 150 pA). (b) Bias dependence of $P_e$ averaged in ${\mathrm{Ga}}_r$ (blue) or in the arsenic atomic row (${\mathrm{As}}_r$, red) (set point: 150 pA).