Xiaoyang Zhu1
Columbia University1
An exciton is a quasi-particle consisting of an electron and a hole bound by the Coulomb potential. Compared to their 3D counterparts, the exciton binding energy in 2D can be order(s) of magnitude higher, a result of both spatial/quantum confinement and reduced dielectric screening. As bosonic particles, many-body interaction of 2D excitons can lead to interesting quantum phases, such as exciton lattices, gases, plasmas, and Bose-Einstein condensates. In this lecture, we will discuss our efforts in exploring the coupling of 2D excitons to other degrees of freedom, particularly magnetic order, in 2D van der Waals layered materials. We focus on CrSBr, a 2D A-type antiferromagnet with an excitonic transition at ~ 1.35 eV. Using second harmonic generation, we establish ferromagnetically (FM) ordered monolayers with a Curie temperature <i>T<sub>C</sub></i> = 146 K and antiferromagnetically (AFM) coupled bilayer or multilayers with Néel temperatures of <i>T<sub>N</sub></i> = 132-148 K. The embodiment of both magnetic and semiconducting properties in this material allows the magnetic control of interlayer electronic coupling, as manifested in tunable excitonic transitions. Excitonic transitions in bilayer CrSBr and above is abruptly modified when the magnetic order is changed from paramagnetic to AFM as <i>T </i>is lowered below <i>T<sub>N</sub></i> or as it is switched from layered AFM to the field-induced FM state. The magnetic-excitonic coupling is attributed to the spin-allowed interlayer hybridization of electron and hole orbitals in the FM, but not AFM state. This magneto-exciton coupling allows the detection of coherent magnons, i.e., spin waves from optical transitions. We launch the spin waves from across gap excitonic transition and detect the spin waves from oscillations in exciton energies. The spin waves are found at 24 and 34 GHz, corresponding to the in-phase and out-of-phase modes, espectively. They possess long coherent times (≥ 6 ns) and long transport distances (≥ 10 µm). We will discuss the prospects of using these optically accessible spin waves as quantum interconnects.