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Manipulation of localized excitons in CrPS $_4$ by temperature and magnetic field

This study combines theoretical calculations and experimental measurements to characterize the localized excitonic transitions in the layered antiferromagnetic semiconductor CrPS $_4$ , revealing how temperature and magnetic fields modulate these excitons to provide optical signatures of magnetic phase transitions and potential pathways for all-optical magnetic control.

Original authors: Dipankar Jana, Swagata Acharya, Amit Pawbake, Dmitrii Litvinov, Aljoscha Soll, Zdenek Sofer, Clement Faugeras, Dimitar Pashov, Mark van Schilfgaarde, Kostya S. Novoselov, Marek Potemski, Maciej Kopers

Published 2026-08-05

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Dipankar Jana, Swagata Acharya, Amit Pawbake, Dmitrii Litvinov, Aljoscha Soll, Zdenek Sofer, Clement Faugeras, Dimitar Pashov, Mark van Schilfgaarde, Kostya S. Novoselov, Marek Potemski, Maciej Koperski

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where light doesn't just illuminate objects but can also talk to the invisible magnetic forces holding them together. This is the playground of condensed matter physics, a field where scientists study how electrons behave in solid materials. In this story, we are looking at a special class of materials called "van der Waals magnets." Think of these like stacks of ultra-thin playing cards, where each card is a single layer of atoms. Unlike the magnets on your fridge that stick to metal, these materials have a secret life: their magnetic order (how their tiny internal compasses align) is deeply intertwined with how they absorb and emit light.

The key characters in this drama are "excitons." You can picture an exciton as a temporary dance partner pair: an electron gets kicked up to a higher energy level, leaving a hole behind, and the two are held together by an electric attraction, dancing around each other before they eventually recombine. In these magnetic materials, this dance is sensitive to the temperature and the magnetic field. If the magnetic order of the material changes, the dance steps change too. Scientists are fascinated by this because if we can read the light these materials emit, we might be able to "see" their magnetic state without touching it, potentially leading to new ways to control magnetic information with light.

The paper you are about to read dives deep into a specific material called CrPS4 (Chromium Phosphorus Sulfide). For a long time, scientists were a bit confused about this material. They knew it was a semiconductor (a material that conducts electricity under certain conditions) and that it was magnetic, but they couldn't agree on its exact properties. Some thought it had a certain energy gap, while others thought it was different. They also saw some mysterious, narrow lines of light being emitted and weren't sure if these were just standard electron jumps or something more exotic involving a "spin flip" (where an electron's internal compass suddenly turns around).

In this study, the researchers decided to settle the score by combining two powerful tools: super-computer simulations and real-world experiments. They used advanced theories to calculate exactly how the electrons should behave in CrPS4 and then tested those predictions by shining lasers on the material at different temperatures and magnetic fields.

Here is what they found. First, they confirmed that CrPS4 is a "direct-gap" semiconductor with a specific energy gap of 2.48 eV when it is in its magnetic, ordered state. But the real magic happened with the light. The researchers discovered that the mysterious, narrow lines of light they saw were actually a mix of two different types of electron dances. Most of the light came from "spin-allowed" transitions, where the electron's internal compass stays pointing the same way. However, there was also a specific, weaker signal that came from a "spin-flip" transition, where the compass actually flips.

What makes this discovery so cool is how the material reacts to heat and magnets. When the researchers warmed up the material, the light didn't just get dimmer; the energy of the light shifted in a very specific way. This shift acted like a thermometer for the magnetic order, clearly showing the exact moment the material switched from being magnetically ordered to disordered (at about 38 K). Even more interesting, when they applied a magnetic field, the light shifted again, but it didn't split apart like you might expect if the electrons were flipping their spins randomly. This told the scientists that the main light-emitting dances were indeed "spin-allowed" and very stable.

The team also noticed something strange about the timing of the light. At very cold temperatures, the light faded away quickly. But as they warmed it up, the light lasted longer and actually got brighter. They figured out that the excitons were getting "stuck" in little magnetic traps at low temperatures. As the temperature rose, they gained enough energy to escape these traps and join the "bright" crowd, leading to that surge in light.

In short, this paper acts like a detective story. It solved the mystery of what kind of semiconductor CrPS4 really is, identified exactly which electron dances are happening, and showed that we can use light to track the magnetic state of the material with high precision. The researchers suggest that because these excitons are so sensitive to magnetic changes, they could be used as tiny, all-optical sensors to detect magnetic order in the future, opening up new doors for controlling magnetic materials with light.

Technical Summary: Manipulation of Localized Excitons in CrPS4 by Temperature and Magnetic Field

Problem Statement
Layered van der Waals magnetic semiconductors offer a platform for studying the interplay between excitonic phenomena and magnetic order. CrPS4, a layered A-type antiferromagnet, exhibits rich excitonic features, yet their microscopic origins and connection to magnetic ordering remain incompletely understood. Previous studies have reported conflicting values for the material's bandgap (ranging from 1.4 eV to 2.4 eV) and debated the nature of sub-bandgap optical transitions. While some features near 1.4 eV were attributed to spin-flip transitions within the Tanabe–Sugano framework, their lack of Zeeman shifts and persistence above the Néel temperature ( $T_N \approx 38$ K) challenged this assignment. A comprehensive understanding of the electronic structure, the nature of localized excitons (spin-allowed vs. spin-flip), and their response to external perturbations is required to utilize these materials as optical probes for magnetic order.

Methodology
The study employs a synergistic approach combining advanced computational many-body theory with temperature- and magnetic-field-dependent photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy.

Computational Framework: The electronic structure was calculated using a self-consistent ab initio many-body perturbation theory approach, specifically the quasi-particle self-consistent $GW $($ QSGW$) and the extended $QSG\hat{W}$ methods. The latter includes ladder electron-hole vertex corrections crucial for magnetic systems with partially filled $d$ states. To capture spin-flip transitions and atomic multiplet effects missing from standard MBPT, the authors utilized Dynamical Mean-Field Theory (DMFT) with an exact-diagonalization (ED) impurity solver. This combination allowed for the calculation of both spin-allowed excitonic transitions and spin-flip atomic multiplet transitions localized on Cr $^{3+}$ ions.
Experimental Setup: Experiments were conducted on bulk CrPS4 crystals (thickness $\approx 500$ $\mu$ m) and exfoliated flakes ( $\approx 10$ –$20$ $\mu$ m). PL and PLE spectra were measured at temperatures ranging from 5 K to 70 K and under magnetic fields up to 14 T applied perpendicular to the sample plane. Time-resolved PL measurements were performed to analyze exciton decay dynamics.

Key Contributions and Results

Electronic Structure and Bandgap:
Theoretical calculations establish CrPS4 as a direct-gap semiconductor with a bandgap of 2.48 eV in the antiferromagnetic (AFM) phase. The study resolves previous contradictions by distinguishing between the fundamental bandgap and sub-bandgap localized transitions.
Nature of Excitonic Transitions:
The authors identify and classify several sub-bandgap transitions near 1.4 eV:
- Spin-Allowed Transitions: The dominant features ( $X_1$ , $X_2$ , $F_0$ ) are identified as spin-allowed transitions ( $\Delta m_s = 0$ ) between exchange-split levels of the $^4A_2$ ground state and $^2E$ , $^2T_1$ excited states. The lowest-energy spin-allowed transition is optically dark.
- Spin-Flip Transition: A distinct feature marked by an asterisk (*) at $\approx 1.38$ eV is identified as a spin-flip transition ( $\Delta m_s = 1$ ) between the $m'_s = \pm 1/2$ excited states and the $m_s = \pm 3/2$ ground state. This transition disappears above $T_N$ , consistent with DMFT predictions.
- High-Energy Transitions: A transition at 1.97 eV ( $F_1$ ) is attributed to the $^2T_2 \to ^4A_2$ transition.
Temperature-Dependent Behavior:
- Energy Shifts: All excitonic features near 1.4 eV exhibit a systematic blueshift as temperature increases from 5 K to 50 K. This is attributed to the widening of the bandgap upon the transition from the AFM to the paramagnetic (PM) phase. Theoretical calculations predict the bandgap increases from 2.48 eV (AFM) to 2.66 eV (PM).
- Intensity and Dynamics: A simultaneous enhancement of PL intensity and decay time is observed for low-energy excitons as temperature rises. This is attributed to thermally activated transfer of excitons from magnetically induced localized (dark) states to optically bright states. The decay dynamics follow a stretched exponential function, suggesting a distribution of trap depths.
- Absence of Stokes Shift: The coincidence of excitation and emission energies indicates no significant Stokes shift for the dominant transitions.
Magnetic Field Dependence:
- Blueshift and Saturation: Low-energy excitons ( $X_2$ ) exhibit a weak blueshift with increasing magnetic field up to the critical saturation field ( $B_c \approx 8$ T), where the system transitions to a ferromagnetic state. Above $B_c$ , the energy becomes field-independent.
- Redshift of High-Energy Exciton: The high-energy $F_1$ exciton shows a pronounced redshift with increasing field but vanishes at a threshold of $\approx 4$ T.
- Absence of Zeeman Splitting: Crucially, none of the observed spin-allowed transitions exhibit a Zeeman shift or splitting. This implies that the ground and excited states possess identical $g$ -factors and spin projection quantum numbers ( $g' = g, m'_s = m_s$ ), confirming their spin-allowed nature. In contrast, the spin-flip transition (*LO) shows distinct magnetic field dependence.

Significance
The paper demonstrates that CrPS4 serves as a sensitive probe for magnetic order in layered van der Waals antiferromagnets. By combining parameter-free theoretical calculations with experimental spectroscopy, the authors successfully disentangle spin-allowed and spin-flip excitonic transitions, correcting previous misassignments based on the Tanabe–Sugano framework alone. The characteristic energy shifts and intensity changes of these excitons provide clear optical signatures of magnetic phase transitions (AFM-to-PM and AFM-to-FM). These findings suggest that excitonic transitions can be used for all-optical sensing and light-driven control of magnetic order in such materials, offering a pathway to explore spin-exciton coupling in low-dimensional magnets.

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