Magnetic proximity coupling to defects in a two-dimensional semiconductor

TL;DR

This study addresses how to read the Néel order in a two-dimensional antiferromagnet by leveraging magnetic proximity to defect-active states in a neighboring TMDC. The authors demonstrate magnetic proximity interaction in a CrPS4/WSe2 heterostructure, where spin-polarized charge transfer enabled by a type-II band alignment allows optical readout of the CrPS4 surface-layer magnetization via defect states in WSe2. Density functional theory supports the type-II alignment and reveals anisotropy from CrPS4's monoclinic structure, explaining polarization signatures that are largely independent of excitation polarization. The work offers a route to ultrafast optical memory and quantum-information processing using AFM 2D magnets and TMDC heterostructures, with surface-layer magnetism detectable despite a zero net moment in the bulk.

Abstract

The ultrathin structure and efficient spin dynamics of two-dimensional (2D) antiferromagnetic (AFM) materials hold unprecedented opportunities for ultrafast memory devices, artificial intelligence circuits, and novel computing technology. For example, chromium thiophosphate (CrPS4) is one of the most promising 2D A-type AFM materials due to its robust stability in diverse environmental conditions and net out-of-plane magnetic moment in each layer, attributed to anisotropy in crystal axes (a and b). However, their net zero magnetic moment poses a challenge for detecting the Neel state that is used to encode information. In this study, we demonstrate the detection of the Neel vector by detecting the magnetic order of the surface layer by employing defects in tungsten diselenide (WSe2). These defects are ideal candidates for optically active transducers to probe the magnetic order due to their narrow linewidth and high susceptibility to magnetic fields. We observed spin-polarized charge transfer in the heterostructure of bulk CrPS4 and single-layer WSe2 indicating type-II band alignment as supported by density functional theory (DFT) calculations. In the A-type AFM regime, the intensity of both right-handed and left-handed circularly polarized light emanating from the sample remains constant as a function of the applied magnetic field, indicating a constant polarized transition behavior. Our results showcase a new approach to optically characterizing the magnetic states of 2D bulk AFM material, highlighting avenues for future research and technological applications.

Figures (9)

• Figure 1: Low-temperature polarization-resolved photoluminescence (PL) of the CrPS$_4$ and WSe$_2$ heterostructure: In (a), a schematic illustrates the magnetic proximity interaction in a 2D system. Optical selection rules for CrPS$_4$ and WSe$_2$ are shown in (b) and (c), respectively. In (d), the first sample co-polarization resolved PL of the heterostructure reveals multiple peaks between 1.2-1.4 eV attributed to CrPS$_4$ due to vibrational progression. The peak centered around 1.65 eV corresponds to intrinsic defect states PL in WSe$_2$, and the 1.725 eV peak indicates the excitonic transition in WSe$_2$ at 1.8 K, (e) shows the second sample co-polarization resolved PL of defect-based bound excitons of WSe$_2$ within the CrPS$_4$ and WSe$_2$ heterostructure at 1.8 K .
• Figure 2: Magnetic field-dependent PL of the heterostructure in Faraday's geometry: (a) and (b) shows the PL contribution from CrPS$_4$ at high magnetic fields of $\pm$ 8.5 Tesla, while (d) and (e) show the PL contribution of intrinsic defects and excitons from WSe$_2$ under similar magnetic conditions. Additionally, (c) and (f) present the PL contribution of defect-based bound excitons from WSe$_2$ excited by linear polarized (L) light at high magnetic fields of $\pm$ 8 Tesla.
• Figure 3: The degree of circular polarization ($\rho$) and Zeeman splitting measurements as a function of magnetic fields are shown. (a) illustrates the $\rho$ measurement of one of the peaks of CrPS$_4$ centered around 1.355 eV at varying magnetic fields, with arrows indicating the direction of the magnetic sweep. (b) presents the $\rho$ measurement of the intrinsic defect peak of WSe$_2$ centered around 1.65 eV, while (c) displays a color plot illustrating the $\rho$ of multiple defect-based bound excitons in WSe$_2$. (d) is a zoomed-in plot of (a), focusing on specific details. (e) shows the Zeeman splitting of the defect peak of WSe$_2$ centered around 1.65 eV. Additionally, (f) provides a schematic demonstrating the spin-polarized charge transfer effect in the heterostructure due to type-II band alignment.
• Figure 4: (a) Weighted intensities of the co-polarization component of one of the CrPS$_4$ peaks at 1.355 eV during the backward sweep direction, providing insights into the polarization behavior of the emitted light. (b) A zoomed-in image of (a), focusing specifically on a range corresponding to the A-type AFM state in CrPS$_4$, allowing for a detailed examination of the polarization characteristics within this magnetic regime. (c) The crystal structure of CrPS$_4$, as determined by DFT, offers a visual representation of its atomic arrangement and symmetry properties. (d) Electronic structures of CrPS$_4$ across varying layer thicknesses, ranging from monolayer to bulk, along with comparisons to that of WSe$_2$ with and without spin-orbit coupling, further confirming the type-II band alignment of CrPS$_4$ and WSe$_2$ heterostructure.
• Figure S1: The co-polarization resolved photoluminescence (PL) of the heterostructure at room temperature is shown, the peak centered around 1.35 eV corresponds to bulk CrPS$_4$, while the peak centered around 1.65 eV is attributed to the excitonic transition in WSe$_2$. Notably, the negligible polarization contrast and energy degeneracy observed in the excitonic transition indicate the absence of a magnetic proximity effect. This lack of effect can be attributed to the paramagnetic behavior exhibited by CrPS$_4$ at room temperature.