Photo-induced Enhancement of Critical Temperature in Spin-Fermion Systems Demonstrates Nonthermal Mechanism

The interplay between competing magnetic states represents a fundamental challenge in condensed matter physics, and recent experiments suggest light can surprisingly stabilise these systems, even boosting the temperatures at which order emerges. Sankha Subhra Bakshi from the Indian Institute of Science Education and Research-Kolkata, along with colleagues, now demonstrates a mechanism behind this photo-induced enhancement of magnetic order. The team investigates a model system where competing magnetic tendencies arise from the behaviour of both electrons and localized spins, revealing that brief pulses of light create a population of energetic electrons which resist returning to their usual energy levels. This sustained energy input effectively shifts the balance between competing magnetic states, favouring a more ordered arrangement and raising the temperature at which this order appears, offering potential pathways towards controlling magnetism and other complex electronic phenomena.

Light Stabilizes Magnetic Order in Electrons and Spins

This research reveals how light can enhance magnetic order in materials where competing interactions typically suppress it. Scientists investigated a model system, demonstrating that photoexcitation creates a persistent population of charge carriers that alters the balance between competing magnetic tendencies. The study shows that light can effectively convert antiferromagnetic interactions into ferromagnetism, increasing the temperature at which magnetic order emerges.

Researchers combined a Landau-Lifshitz-Gilbert-Brown dynamics approach for localized spins with a mean-field treatment of itinerant electrons, allowing them to model how spin interactions dynamically change. This approach demonstrates a nonthermal mechanism for stabilizing ordered phases, creating a long-lived nonequilibrium carrier population that resists thermalization and reshapes the magnetic order.

Dissipation and Nonequilibrium Population Dynamics Demonstrated

Supplementary material provides further evidence supporting the main findings and details the robustness of the results. This material addresses the sensitivity of the enhanced critical temperature to dissipation and explores the dynamics of the electron population when the system is excited with light.

Researchers varied the dissipation rate and observed its effect on the magnetic order. The enhancement of magnetic order is not strongly sensitive to the dissipation rate within a reasonable range, suggesting the observed enhancement is a genuine physical effect. The study also explored the behavior of the electron population, tracking the occupation of electronic states before and after excitation.

They found that the electron population does not follow a simple thermal distribution, but instead exhibits a non-thermal distribution with an effective temperature, indicating that the light creates a state far from equilibrium. The upper-band population shows a clear response to the light, rising quickly and settling to a steady-state value that increases with light intensity, confirming that the light drives the system out of equilibrium.

Light Induces Magnetic Order in Materials

This research demonstrates a nonthermal mechanism by which light can enhance magnetic ordering in correlated materials. Specifically, photoexcitation generates a long-lived population of carriers that alters the balance between competing magnetic interactions. This work establishes a general framework for understanding how nonequilibrium carrier populations can stabilize ordered phases, with implications extending beyond ferromagnetism to potentially include charge-density-wave order and superconductivity.

The authors acknowledge that their model is a simplification, omitting orbital degrees of freedom and beyond-mean-field fermionic correlations. However, they emphasize that the developed methodology, particularly the explicit calculation of nonequilibrium electronic stiffness, provides a concrete basis for assessing how carrier redistribution influences exchange pathways. Future research may extend this approach to more complex photoexcited correlated systems, including Mott insulators and materials with strong electron-phonon coupling.