Strong Exciton-Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr

Resonance Raman scattering and anomalous anti-Stokes phenomena in CrSBr

CrSBr, a van der Waals material, stands out as an air-stable magnetic semiconductor with appealing intrinsic properties such as crystalline anisotropy, quasi-1D electronic characteristics, layer-dependent antiferromagnetism, and non-linear optical effects. We investigate the differences between the absorption and emission spectra, focusing on the origin of the emission peak near 1.7 eV observed in the photoluminescence spectrum of CrSBr. Our findings are corroborated by excitation energy-dependent Raman experiments. Additionally, we explore the anti-Stokes Raman spectra and observe an anomalously high anti-Stokes-to-Stokes intensity ratio of up to 0.8, which varies significantly with excitation laser power and crystallographic orientation relative to the polarization of the scattered light, highlighting the unique vibrational and electronic interactions in CrSBr. Lastly, we examine stimulated Raman scattering and calculate the Raman gain in CrSBr, which attains a value of 1 × 108 cm GW-1, nearly four orders of magnitude higher than that of previously studied three-dimensional systems.

Synthesis of Two-Dimensional Antiferromagnetic Semiconductor CuFeS2 with a High Néel Temperature

Two-dimensional (2D) antiferromagnetic (AFM) semiconductors have garnered significant attention due to their promising applications in spintronics and intriguing magneto-optical performance. However, low Néel temperatures (TN) and inadequate air stability constrain their application. Here, we report the synthesis of a 2D semiconducting CuFeS2 (CFS) exhibiting AFM order. It features a high TN (473.0 ± 0.4 K) and remains stable in air for at least 2 weeks. The semiconductor characteristics are confirmed via electrical measurements. AFM order and a high magnetic phase transition temperature are demonstrated by Raman and second-harmonic generation (SHG) spectra. This work provides a new 2D AFM semiconductor with a high Néel temperature, offering a novel platform for studying unconventional physical phenomena and spintronics.

Magnon-mediated exciton-exciton interaction in a van der Waals antiferromagnet

Excitons are fundamental excitations that govern the optical properties of semiconductors. Interactions between excitons can lead to various emergent phases of matter and large nonlinear optical responses. In most semiconductors, excitons interact via exchange interactions or phase-space filling. Correlated materials that host excitons coupled to other degrees of freedom could offer pathways for controlling these interactions. Here we demonstrate magnon-mediated interactions between excitons in CrSBr, an antiferromagnetic semiconductor. These interactions manifest as the dependence of the exciton energy on the exciton density via a magnonic adjustment of the spin canting angle. Our study demonstrates the emergence of quasiparticle-mediated interactions in correlated quantum materials, leading to large nonlinear optical responses and potential device concepts such as magnon-mediated quantum transducers.

Lattice thermal conductivity in CrSBr: the effects of interlayer interaction, magnetic ordering and external strain

With the continuous development of digital information and big data technologies, the ambient temperature and heat generation during the operation of magnetic storage devices play an increasingly crucial role in ensuring data security and device stability. In this study, we conducted a thorough investigation on in-plane lattice thermal conductivity of the van der Waals (vdWs) magnetic semiconductor CrSBr from bulk to monolayer using first-principles calculations and phonon Boltzmann transport equation. Our findings indicated that CrSBr show strong anisotropic thermal transport behaviors and layer number and magnetic ordering dependent lattice thermal conductivity. The lowest thermal conductivity is observed in y direction of antiferromagnetic CrSBr bilayer at all temperatures. Through the analysis of phonon spectra, phonon lifetime, heat capacity, scattering probability, phonon-phonon interaction strength, we demonstrated that out of plane acoustic phonon modes soften, the shift of Cr-Br antisymmetrical stretching vibrations, and large phonon band gap are the main factors. These results offer a comprehensive insight into phonon transport phenomena in vdWs magnetic materials.

Large exciton binding energy in a bulk van der Waals magnet from quasi-1D electronic localization

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid-state systems and are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence supporting electronic localization and weak dielectric screening as mechanisms contributing to the amplified exciton binding energy. Furthermore, we report that surface doping enables broad tunability of the band gap offering promise for engineering of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

Probing the Band Splitting near the Γ Point in the van der Waals Magnetic Semiconductor CrSBr

This study investigates the electronic band structure of chromium sulfur bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis on the PL data robustly unveils energy splittings, band gaps, and excitonic transitions across different thicknesses of CrSBr, from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the Néel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal splitting in both the conduction and valence bands but also highlight a significant advance in our understanding of the interplay between the optical, electronic, and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals.

CrSBr: An Air-Stable, Two-Dimensional Magnetic Semiconductor

The discovery of magnetic order at the 2D limit has sparked new exploration of van der Waals magnets for potential use in spintronics, magnonics, and quantum information applications. However, many of these materials feature low magnetic ordering temperatures and poor air stability, limiting their fabrication into practical devices. In this Mini-Review, we present a promising material for fundamental studies and functional use: CrSBr, an air-stable, two-dimensional magnetic semiconductor. Our discussion highlights experimental research on bulk CrSBr, including quasi-1D semiconducting properties, A-type antiferromagnetic order (TN = 132 K), and strong coupling between its electronic and magnetic properties. We then discuss the behavior of monolayer and few-layer flakes and present a perspective on promising avenues for further studies on CrSBr.