More Like this

1. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials

   https://doi.org/10.1038/s41524-020-00462-9  

   Wang, Hua; Qian, Xiaofeng (December 2020, npj Computational Materials)

   null (Ed.)

   Abstract Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ± k points. Taking $${\cal{P}}{\cal{T}}$$ P T -symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi 2 Te 4 as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks $${\cal{P}}{\cal{T}}$$ P T symmetry and enables large NSC response in bilayer AFM MnBi 2 Te 4 , which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM- z MnBi 2 Te 4 as a tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials. 

   more » « less
2. Van der Waals opto-spintronics

   https://doi.org/10.1038/s41928-024-01167-3  

   Gish, J Tyler; Lebedev, Dmitry; Song, Thomas W; Sangwan, Vinod K; Hersam, Mark C (May 2024, Nature Electronics)

   Van der Waals materials with long-range magnetic order show a range of correlated phenomena that could be of use in the development of optoelectronic and spintronic applications. Magnetically ordered van der Waals semiconductors with spin-polarized currents are, in particular, sensitive to external stimuli such as strain, electrostatic fields, magnetic fields and electromagnetic radiation. Their combination of two-dimensional magnetic order, semiconducting band structure and weak dielectric screening means that these materials could be used to create novel atomically thin opto-spintronic devices. Here we explore the development of van der Waals opto-spintronics. We examine the interplay between optical, magnetic and electronic excitations in van der Waals magnetic semiconductors, and explore the control of their magnetization via external stimuli. We consider fabrication and passivation strategies for the practical handling and design of opto-spintronic devices. We also explore potential opto-spintronic device architectures and applications, which include magnonics, quantum transduction, neuromorphic computing and non-volatile memory. 

   more » « less
3. Role of weak interlayer coupling in ultrafast exciton-exciton annihilation in two-dimensional rhenium dichalcogenides

   https://doi.org/10.1103/PhysRevB.101.174309  

   Sim, Sangwan; Lee, Doeon; Lee, Jekwan; Cha, Myungjun; Cha, Soonyoung; Heo, Wonhyeok; Cho, Sungjun; Shim, Wooyoung; Lee, Kyusang; Yoo, Jinkyoung; et al (May 2020, Physical review)

   Strong interactions between excitons are a characteristic feature of two-dimensional (2D) semiconductors, determining important excitonic properties, such as exciton lifetime, coherence, and photon-emission efficiency. Rhenium disulfide (ReS2), a member of the 2D transition-metal dichalcogenide (TMD) family, has recently attracted great attention due to its unique excitons that exhibit excellent polarization selectivity and coherence features. However, an in-depth understanding of exciton-exciton interactions in ReS2 is still lacking. Here we used ultrafast pump-probe spectroscopy to study exciton-exciton interactions in monolayer (1L), bilayer (2L), and triple layer ReS2. We directly measure the rate of exciton-exciton annihilation, a representative Auger-type interaction between excitons. It decreases with increasing layer number, as observed in other 2D TMDs. However, while other TMDs exhibit a sharp weakening of exciton-exciton annihilation between 1L and 2L, such behavior was not observed in ReS2. We attribute this distinct feature in ReS2 to the relatively weak interlayer coupling, which prohibits a substantial change in the electronic structure when the thickness varies. This work not only highlights the unique excitonic properties of ReS2 but also provides novel insight into the thickness dependence of exciton-exciton interactions in 2D systems. 

   more » « less
4. Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

   https://doi.org/10.1038/s41467-021-22777-x  

   Zhang, Xiaoqian; Lu, Qiangsheng; Liu, Wenqing; Niu, Wei; Sun, Jiabao; Cook, Jacob; Vaninger, Mitchel; Miceli, Paul F.; Singh, David J.; Lian, Shang-Wei; et al (December 2021, Nature Communications)

   Abstract While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe 2 , a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature ( T C ) up to 300 K, an atomic magnetic moment of ~0.21  $${\mu }_{{\rm{B}}}$$ μ B /Cr and perpendicular magnetic anisotropy (PMA) constant ( K u ) of 4.89 × 10 5  erg/cm 3 at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer ( T C  ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices. 

   more » « less
5. Moiré magnetism and moiré excitons in twisted CrSBr bilayers

   https://doi.org/10.1073/pnas.2413326121  

   Liu, Junyi; Zhang, Xu; Lu, Gang (January 2025, Proceedings of the National Academy of Sciences)

   Moiré excitons and moiré magnetism are essential to semiconducting van der Waals magnets. In this work, we perform a comprehensive first-principles study to elucidate the interplay of electronic excitation and magnetism in twisted magnetic CrSBr bilayers. We predict a twist-induced quantum phase transition for interlayer magnetic coupling and estimate the critical twist angle below which moiré magnetism with mixed ferromagnetic and antiferromagnetic domains could emerge. Localized one-dimensional moiré excitons are stable if the interlayer coupling is ferromagnetic and become unstable if the coupling turns to antiferromagnetic. Exciton energy modulation by magnons is estimated and dependence of exciton oscillator strength on the twist angle and interlayer coupling is analyzed. An orthogonally twisted bilayer is revealed to exhibit layer-dependent, anisotropic optical transitions. Electric field is shown to induce net magnetic moments in moiré excitons, endowing them with exceedingly long lifetimes. Our work lays the foundation for using magnetic moiré bilayers in spintronic, optoelectronic, and quantum information applications. 

   more » « less