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Room temperature ultrafast terahertz emission and spin current generation in a two-dimensional superlattice (Fe₃GeTe₂/CrSb)₃

Peiyan Li¹, Shanshan Liu², Faxian Xiu², Xiaojun Wu^1,*

¹School of Electronic and Information Engineering, Beihang University, Beijing 100191, China

²State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China

Future information technologies, such as high-speed data recording, quantum computation, or spintronics, call for ultrafast control of spins. Terahertz (THz) electromagnetic radiation can couple spin dynamics on their intrinsic energy scale of magnetic excitations and offer response rates scaling up to terabits per second, which stimulates new concepts for THz spintronics. These concepts require the generation and detection of spin currents on the picosecond timescale to become practical. A feasible scheme is spintronic THz emission, where spin currents are generated in a ferromagnet after femtosecond laser excitation and then converted into charge currents. This process can be controlled by customizing magnetic fields, structural and applying nonlinear electric fields, etc., which are quantitatively analyzed by recording the electric field waveform of emitted THz radiation following the decaying charge current. This way is efficiently and widely employed in THz spintronics, and further development greatly relies on exploring novel two-dimensional (2D) magnetic materials and structures.
Fe₃GeTe₂ (FGT), a van der Waals (vdW) layered ferromagnetic metal, exhibits the most stable and controllable magnetism among the recently observed intrinsic 2D magnets, making it the chosen material for this study. However, to use FGT for exploring 2D THz spintronic applications at room temperature, key challenges consist of two parts: (1) the generation of ultrafast spin currents, which currently only occur in materials with room temperature magnetic ordering[1]; (2) sensitively detecting ultrafast spin currents in atomically thin 2D magnetic materials.

In this work, we eliminate these two limitations and experimentally demonstrate the optically triggered ultrafast THz spin currents at room temperature without external magnets in a 2D ferromagnetic/antiferromagnetic superlattice (Fe₃GeTe₂/CrSb)₃ (abbreviated as (FGTCS)₃) [3], whose exhibits a Curie temperature of 206 K. As depicted in Fig. 1A, we excited the (FGT/CS)₃ superlattice with femtosecond optical pulses and measured the emitted THz radiation. The photoconductive antenna is utilized to solely capture vertically polarized THz waves. The corresponding THz temporal waveform is illuminated in Fig. 1B, where the pump fluence is 3.75×10^-5 mJ/cm2. To investigate the origin of this THz emission from the (FGT/CS)₃ superlattice, samples of FGT-only film (10 nm) and CrSb (CS)-only film (4 nm) are also measured under identical conditions. The time-domain THz emission signal of the superlattice sample is approximately 10 times stronger than that of the CS-only film. Meanwhile, the THz signal from the FGT film is barely detectable. The corresponding Fourier transformation results are shown in Fig. 1B. These results indicate that the predominant THz emission from the superlattice is not the consequence of CS-only or FGT-only films.

To further explore the radiation mechanism, we examined the dependence of the emitted signal on the sample azimuth angle θ and the laser polarization α in (FGT/CS)₃ superlattice. Figure 1C shows the detected THz electric-field peak amplitude as a function of the θ, which can be fitted well by a sinusoidal function with a period of 360°. Here, the laser polarization α is fixed along the y-axis (α = 0°) and only vertically polarized THz can be collected by the photoconductive antenna. The THz electric-field peak reaches its maximum value at θ = 60° and its minimum value at θ = 240°. The laser polarization dependence is illustrated in Fig. 1D, where θ is fixed at 60°. When the polarization of the laser α is varied from 0° to 180°, the correlation between the THz electric field and the laser polarization angle demonstrates a cosine oscillation with a small amplitude, in addition to a significant nonzero offset. The fitting curve is based on the cosine function which corresponds well with the experimental data. The small amplitude polarization-dependent contribution is convergent with the total radiation of CS-only films. Notably, from Fig. 1C and 1D, we obtain the variation of laser-polarization-independent THz radiation part with increasing sample azimuth from 0° to 360°, which exhibits twofold rotational symmetry (Fig. 1E) and is only slightly less than that of total THz radiation. These outcomes indicate that the most of THz emission in (FGT/CS)₃ superlattice can be attributed to the external-magnetic-field-free spin-to-charge conversion [2]. We also flipped the sample 180° left and right, showing the electric field of THz radiation to reversed, while the polarity remains unchanged when flipping up and down (Fig.1F-G), which is also corresponded to the spintronic THz emission.

In conclusion, we report experimental observations of ultrafast spin currents in the vdW superlattice (FGT/CS)₃, which highlights THz emission spectroscopy as a powerful tool for characterizing ultrafast spin dynamics. This advance creates unique opportunities not only in the burgeoning field of 2D spintronic devices but also for other spin systems with atomic scale.

Figure 1: (A). Temporal THz waveforms of the THz emission from (FGT/CS)₃ superlattice, CS, and FGT at room temperature. (B) Corresponding Fourier-transformed spectra of (B). (C) Radiated THz amplitudes as a function of the sample azimuth. (D) Dependence of laser-dependent components on laser polarization. (E) The laser-polarization-independent component shows twofold rotational symmetry. (F) Schematic diagrams of sample flip experiments. (G) The transient THz waveforms from the superlattice were measured by flipping 180° in both left-right and up-down directions.

References
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