[Paper Review] Colossal magneto-excitonic effects in 2D van der Waals magnetic semiconductor CrSBr

R. Komar, A. Łopion+11 more|arXiv (Cornell University)|Aug 30, 2024

2D Materials and Applications7 citations

TL;DR

The paper reports unprecedented magneto-excitonic effects in CrSBr, revealing ~100 meV magnetic-field-induced shifts in higher-energy excitonic transitions, supported by DFT calculations.

ABSTRACT

2D magnetic semiconductors, which intrinsically couple a rich landscape of magnetic orders with tightly bound electron-hole pairs (excitons), present an exciting platform to investigate the interplay between optical and magnetic phenomena at the atomic scale. In such systems, the strength of magneto-optical effects determines how deeply the magnetic properties can be revealed. Here, we report the observation of remarkably strong magneto-excitonic effects in the 2D magnetic semiconductor CrSBr that allow probing its magnetic order with unprecedented sensitivity. By investigating optical transitions above the fundamental exciton energy, we discover a massive spectral shift approaching 100 meV under applied magnetic fields - an order of magnitude larger than previously observed magneto-excitonic responses. Our comprehensive magneto-optical experiments accompanied by detailed DFT calculations indicate the possible origin of the transitions exhibiting such intriguing behavior. These findings open avenues for exploiting magneto-excitonic phenomena at newly accessible regimes, enabling novel opto-spintronic applications previously limited by weak magnetic responses.

Motivation & Objective

• Explore how magnetic order in CrSBr affects optical transitions beyond the fundamental exciton energy.
• Quantify the magneto-excitonic response in higher-energy excitonic states.
• Understand the origin of giant field-induced energy shifts via theoretical modeling.

Proposed method

• Perform magneto-optical spectroscopy (reflectance/photoluminescence) across 1.2–2.2 eV with fields along three crystallographic directions at 5 K.
• Identify and track excitonic resonances Omega1/2, Omega3, and Omega4 under varying B fields.
• Analyze polarization dependence to confirm exciton nature and anisotropy.
• Carry out DFT (PBE+U+SOC) calculations with noncollinear spin rotations to simulate field-induced changes in band structure.
• Compare experimental energy shifts with a 2x2 coupling model for Omega3 and Omega4.
• Assess temperature dependence up to near 185 K to connect to magnetic ordering regimes.

Experimental results

Research questions

• RQ1 What is the magnitude and anisotropy of magneto-excitonic shifts for CrSBr excitons under external magnetic fields?
• RQ2 How do higher-energy excitonic transitions (Omega3, Omega4) respond to magnetic order compared to fundamental excitons (Omega1/2)?
• RQ3 What is the possible microscopic origin of the colossal shifts observed in higher-energy transitions?
• RQ4 How does temperature influence the magneto-excitonic response and magnetic phase transitions in CrSBr?

Key findings

• A colossal magneto-excitonic effect with energy shifts approaching 100 meV in higher-energy excitons Omega3 and Omega4.
• The shifts are strongly field-dependent and mirror the magnetization behavior, saturating above ~2 T.
• Omega3 and Omega4 show mirror-like (coupled) behavior that can be described by a field-induced coupling model.
• DFT suggests strong dependence of interband transitions on Cr spin orientation, with spin splitting emerging in FM states.
• Exciton binding energy may compensate part of the electronic band-structure changes, explaining differences between theory and experiment for Omega1/2.
• Temperature elevates the onset field for saturation and broadens excitonic features, with a linear extrapolated critical temperature around 185 K.