Exciton-polariton condensate in the van der Waals magnet CrSBr

summary

The gist

The gist: The study demonstrates a coherent macroscopic quantum phase, the bosonic condensation of exciton-polaritons, emerging in a CrSBr flake embedded in a fully tunable cryogenic open optical

In short

The study demonstrated a coherent macroscopic quantum phase, or bosonic condensation of exciton-polaritons, in CrSBr embedded in a tunable cavity. This condensate exhibits new non-linearity driven by incoherent magnons, and its behavior is highly sensitive to magnetic order. The system shows attractive interactions in the antiferromagnetic (AFM) phase and repulsive interactions in the forced ferromagnetic (FM) phase.

Key concepts

Exciton-Polariton Condensate: This refers to a macroscopic quantum state where light particles (photons) and matter particles (excitons) become strongly coupled into new quasiparticles called polaritons. When enough are excited, these polaritons condense into a single coherent quantum phase, similar to a Bose-Einstein condensate.
Magnetic Order Control: The magnetic state of the CrSBr material can be controlled by external factors like strain or pressure. This control shifts the material between an antiferromagnetic (AFM) state and a forced ferromagnetic (FM) state. This magnetic change directly alters how excitons interact, leading to different non-linear behaviors in the polariton system.
Tunable Non-linearity: The paper investigates how the interactions within the polariton condensate change based on whether the underlying material is in an AFM or FM magnetic order. This sensitivity allows researchers to potentially engineer controllable quantum fluids of light and develop optomagnonic devices where spin magnetism influences light-matter interactions.

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This episode discusses

The paper

Exciton-polariton condensate in the van der Waals magnet CrSBr · Read on arXiv

Institute of Physics, Faculty V, Carl von Ossietzky University Oldenburg · Department of Physics and Astronomy, University of Exeter Department of Physics and Astronomy, Xiamen University Malaysia School of Physics and Optoelectronic Engineering Beijing University of Technology Technion-Israel Institute of Technology Guangdong Technion-Israel Institute of Technology Guangdong Provincial Key Laboratory Materials and Technologies for Energy Conversion Institute for Applied Physics Abbe Center of Photonics Fraunhofer-Institute for Applied Optics and Precision Engineering Max Planck School of Photonics Department of Inorganic Chemistry Faculty of Chemical Technology University of Chemistry and Technology Prague School of Mathematical and Physical Sciences University Department

Van der Waals magnets are an emergent material class of paramount interest for fundamental studies in coupling light with matter excitations, which are uniquely linked to their underlying magnetic properties. Among these materials, the semiconducting magnet CrSBr is possibly a first playground where we can study simultaneously the interaction of photons, magnons, and excitons at the quantum level. Here we demonstrate a coherent macroscopic quantum phase, the bosonic condensation of exciton-polaritons, emerging in a CrSBr flake embedded in a fully tunable cryogenic open optical cavity. The Bose condensate is characterized by a highly non-linear threshold-like behavior, macroscopic occupation of the ground state and coherence manifests distinctly via its first and second order quantum correlations. We find that the condensate's non-linearity is highly susceptible to the magnetic order in CrSBr. Specially, it can encounter a sign change from attractive to repulsive interactions when the intrinsic antiferromagnetic order transforms to the forced ferromagnetic order. Our findings open a route towards magnetically controllable quantum fluids of light, and optomagnonic devices where spin magnetism is coupled to on-chip Bose-Einstein condensates.

DOI: 10.21203/rs.3.rs-5699844/v2

Transcript

Introduction to the show: ident: Quantum Radio. Generated commentary on the latest quantum physics and condensed matter papers.

Kai: I'm Kai, and with me are Mira and Lev, guest researcher.

Mira: Today's paper: "Exciton-polariton condensate in the van der Waals magnet CrSBr".

Kai: The gist: The study demonstrates a coherent macroscopic quantum phase, the bosonic condensation of exciton-polaritons, emerging in a CrSBr flake embedded in a fully tunable cryogenic open optical cavity,

Mira: First, who's behind it and why it matters.

Paper summary: Kai: So to recap this paper on "Exciton-polariton condensate in the van der Waals magnet CrSBr," they're claiming they've found a coherent macroscopic quantum phase—the bosonic condensation of these exciton-polaritons.

Mira: The core thesis is that this condensation emerges within a CrSBr flake embedded in a fully tunable cryogenic open optical cavity, and what’s really intriguing is the discovery of an entirely new way to tune polariton non-linearity by exciting incoherent magnons.

Lev: The authors are setting up this system to investigate the interplay between light-matter coupling and magnetic order in CrSBr, which they see as a potential playground for studying photons, magnons, and excitons at the quantum level simultaneously.

Kai: They start by looking at the material itself—CrSBr—which has specific properties because it’s stable and semiconducting, and below its Néel temperature there’s an A-type interlayer antiferromagnetic order.

Mira: In that AFM phase, excitons are strongly confined within the individual layers due to spin-forbidden interlayer charge transfer, which sets up the initial conditions for their behavior.

Lev: They then explore how this magnetic order can be manipulated using strain or pressure, and also by applying out-of-plane magnetic fields which can force a parallel spin configuration of staggered ferromagnetic order.

Kai: The key finding is that the exciton energy shifts drastically based on the interlayer hybridization, and when external magnetic fields are introduced, the excitonic landscape gets modulated by magnons.

Mira: This modulation is what leads to the emergence of these incoherent magnons in the system, which then drive a new kind of tunable polariton non-linearity.

Lev: So they're not just looking at a static effect; they’re showing how the magnetic order actively shapes the dynamics and interactions of this light fluid.

Conclusion: Kai: Thinking about the title, "Exciton-polariton condensate in the van der Waals magnet CrSBr," it really sums up exactly what they achieved: linking light, matter, and magnetism in a single material.

Mira: It’s important to remember that this work isn't just about finding a new phase; it's about showing how magnetic control can be coupled to these quantum fluids of light.

Lev: What this means for the field is that we can now think about building devices where spin magnetism and on-chip Bose-Einstein condensates are directly linked.

Kai: The implications are in creating optomagnonic devices where you use spin control to influence the behavior of polaritons on a chip.

Mira: It’s about moving beyond just looking at static interactions and showing how these dynamic magnetic excitations can actively drive non-linear effects in light matter systems.

Lev: If we can reliably control that non-linearity via magnons, it opens up new avenues for designing quantum fluid devices where the magnetic state dictates the fluid's response.

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