Altermagnets are characterized by non-relativistic alternating spin splitting in the band structure and collinear compensated magnetic moments in real space. They combine the advantages of both ferromagnetic and antiferromagnetic order, exhibiting time-reversal symmetry-breaking magneto responses, vanishing stray fields, and high-frequency spin dynamics. Consequently, altermagnets hold great potential for various research fields, especially for developing spintronic devices such as high-density magnetic memories and terahertz nano-oscillators. Furthermore, altermagnetism is found in a broad spectrum of materials, including metals, semiconductors, insulators, and superconductors, thereby stimulating widespread interest in functional material research. In this paper, we provide an overview of recent experimental progress in altermagnets, focusing particularly on observations of lifted spin degeneracy via spectroscopic techniques and the resultant spin transport phenomena. The non-relativistic alternating lifting of spin degeneracy is a pivotal aspect of altermagnets. Substantial effort has been dedicated to directly probing the spin splitting with altermagnetic symmetry in reciprocal space. Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for investigating the electronic energy bands of materials, making it well-suited for probing spin splitting in altermagnets. Pioneering measurements were recently achieved in altermagnet candidates, such as α-MnTe and CrSb, using soft X-ray ARPES. Besides, spin-resolved ARPES can directly distinguish spin-up and spin-down bands at lower photon energies, which has been employed for various altermagnets, including layered Rb-doped V2Te2O and K-doped V2Se2O. Another crucial technique in element-specific spectroscopy and microscopy is X-ray magnetic circular dichroism (XMCD). Characteristic XMCD data of altermagnets have been theoretically calculated and experimentally explored. Although band splitting cannot be directly resolved by this method, the broken time-reversal symmetry and the presence of alternating spin polarization can be indirectly inferred in conjunction with theoretical models. The XMCD technology sparked a natural interest in utilizing photoelectron emission microscopy to image altermagnetic domains. It can also be detected in angle-resolved photoemission spectra, offering a direct way to observe time-reversal symmetry breaking in the band structure. Functionalizing the interaction between spin and charge constitutes a core focus of spintronics, and it is explored under various external stimuli such as magnetic or electric fields, optical or thermal excitation, and strain. The spin-split band structure of altermagnets enables time-reversal symmetry-breaking responses, such as anomalous Hall and Nernst effect, magneto-optical Kerr effect, giant/tunneling magnetoresistance, non-relativistic charge-spin conversion, and unconventional piezomagnetism. The modulation of altermagnetism by multiple methods, such as electric current and strain, is also discussed. Overall, exploring altermagnetic candidates in various material systems is highly significant, as it opens new horizons in various fields, including spintronics, magnonics, ultrafast photonics, phononics, superconductivity, topology, and multiferroicity. Specifically, altermagnetic semiconductors such as α-MnTe permit the coexistence of high-temperature magnetism and semiconducting properties, a long-pursued goal in dilute magnetic semiconductor research. For altermagnetic insulators, the chiral-split magnon bands would advance the research of spin waves. Altermagnetism is also proposed to be compatible with spin-triplet superconductors, providing a platform for exploring unconventional superconductors with zero stray fields and developing field-free superconducting diode effects. Moreover, experimental investigations of organic and van der Waals altermagnetic candidates would pave the way to flexible and easily stackable spintronic devices.
Bai et al. (Tue,) studied this question.