[Paper Review] Spin-correlated exciton-polaritons in a van der Waals magnet

Florian Dirnberger, Rezlind Bushati+5 more|arXiv (Cornell University)|Mar 11, 2022

Strong Light-Matter Interactions4 citations

TL;DR

This study demonstrates strong light-matter coupling between spin-correlated excitons in the van der Waals antiferromagnet NiPS₃ and photons in a microcavity, resulting in a novel class of polaritonic quasiparticles. The key finding is the emergence of magnetically correlated exciton-polaritons with suppressed long-range interactions, enabling cavity quantum electrodynamics control of strongly correlated electronic states.

ABSTRACT

Strong coupling between light and elementary excitations is emerging as a powerful tool to engineer the properties of solid-state systems. Spin-correlated excitations that couple strongly to optical cavities promise control over collective quantum phenomena such as magnetic phase transitions, but their suitable electronic resonances have yet to be found. Here we report strong light-matter coupling in $ extrm{NiPS}_3$, a van der Waals antiferromagnet with highly correlated electronic degrees of freedom. A previously unobserved class of polaritonic quasiparticles emerges from the strong coupling between its spin-correlated excitons and the photons inside a microcavity. Detailed spectroscopic analysis in conjunction with a microscopic theory provides unique insights into the origin and interactions of these exotic magnetically coupled excitations. Our work introduces van der Waals magnets to the field of strong light-matter physics and provides a path towards the design and control of correlated electron systems via cavity quantum electrodynamics.

Motivation & Objective

• To explore strong light-matter coupling in van der Waals magnets, specifically NiPS₃, a correlated antiferromagnetic insulator.
• To identify and characterize a new class of polaritons arising from the hybridization of spin-correlated excitons and cavity photons.
• To investigate the origin and interactions of spin-correlated excitons in strongly correlated systems using spectroscopic and theoretical methods.
• To demonstrate that these exciton-polaritons serve as a probe for understanding the magnetic and electronic correlations in NiPS₃.
• To establish a platform for cavity quantum electrodynamics control of correlated electron phenomena in 2D magnetic materials.

Proposed method

• Fabricated high-quality microcavities using 30 pairs of Si₃N₄/SiO₂ distributed Bragg reflectors grown by plasma-enhanced chemical vapor deposition.
• Exfoliated and transferred thin flakes of NiPS₃ (30–300 nm thick) onto SiO₂/Si or Bragg mirror substrates using a PDMS-assisted transfer technique.
• Employed a 35 nm silver top mirror to form a high-finesse cavity with quality factors ~100.
• Performed angle-integrated and angle-resolved reflectance and photoluminescence spectroscopy at 4 K to map polariton dispersions and Rabi splitting.
• Used a coupled oscillator model and nonlinear spectroscopy with variable laser power to extract Rabi splitting energy dependence on polariton density.
• Applied the saturation model Ω(nₚ)/Ω₀ = 1/√(1 + nₚ/nₛ) to extract saturation density nₛ from power-dependent Rabi splitting data.

Experimental results

Research questions

• RQ1 Can spin-correlated excitons in a van der Waals magnet like NiPS₃ exhibit strong light-matter coupling with cavity photons?
• RQ2 What are the unique spectroscopic signatures and dispersion characteristics of the resulting polaritonic quasiparticles?
• RQ3 How do the interactions between spin-correlated excitons differ from those in conventional semiconductors, particularly in terms of long-range correlations?
• RQ4 What is the microscopic origin of the observed excitons in NiPS₃, and how does it differ from standard band-structure excitons?
• RQ5 Can cavity quantum electrodynamics be used to probe and control the magnetic and correlated electronic order in NiPS₃?

Key findings

• A previously unobserved class of polaritons emerges from strong coupling between spin-correlated excitons in NiPS₃ and cavity photons, exhibiting hybrid light-matter character.
• The excitonic emission in NiPS₃ shows a narrow linewidth of ~350 μeV and strong linear polarization (83%), indicating high coherence and anisotropic dipole moments.
• Three distinct excitonic resonances (X₁, X₂, X₃) were identified below the charge transfer gap via optical absorption spectroscopy.
• The Rabi splitting energy was measured to decrease with increasing excitation power, following the saturation model with a fitted saturation density nₛ of ~1.5 × 10¹³ cm⁻².
• Long-range excitonic interactions are severely suppressed due to the tightly bound and spatially localized nature of the spin-correlated excitons.
• Microscopic modeling confirms that the spin-correlated excitons in NiPS₃ originate from localized d-orbital transitions coupled to magnetic order, distinct from conventional band-to-band excitons.