Optical Controllable Spin-Polarization in Two Dimensional Altermagnets via Robust Spin-Momentum Locking Excitons

TL;DR

This work tackles the challenge of achieving robust, spin-polarized excitons in 2D semiconductors at room temperature without external fields. Using first-principles GW/BSE calculations and a 2D Wannier–Mott model, it predicts intrinsically spin-momentum-locked (SML) excitons in the 2D altermagnetic material V_{2}X_{2}O, driven by giant non-relativistic spin-splittings exceeding $1.2$ eV. In the monolayer, the lowest bright exciton at $E^{\mathrm{ex}}_1 \approx 2.39$ eV has a binding energy around $1.20$ eV and exhibits nearly 100% spin polarization under linear polarization, with dark excitons at $X$/$Y$ valleys; in vdW heterobilayers, stacking-tunable interlayer SML excitons (IX) show binding energies above $430$ meV and long radiative lifetimes, including $\sim$210 ns at room temperature. Collectively, these results establish 2D altermagnets as a new platform for all-optical opto-spintronics and altermagnetic exciton physics at practical temperatures.

Abstract

Spin-momentum locking (SML) excitons in two-dimensional semiconductors are appealing to programmable optical control of spin-polarized carriers in ultrafast spintronics. To address the current thirsty for long-lived excitons with zero-external-field stability and room-temperature spin-polarization, we hereby predict the existence of intrinsically SML excitons in altermagnetic V$_2 X_2$O ($X=$ S, Se) driven by giant non-relativistic spin-splittings ($>$ 1.2 eV). First-principles calculations reveal SML excitons with binding energies exceeding 1400 meV in monolayers and 430 meV in their van der Waals heterobilayers, along with stacking-dependent optical selection rules for tunable interlayer excitons. These remarkable physical properties, combined with their long radiative lifetimes, strongly suggest the feasibility of SML excitons with robust spin-polarization at room temperature. Our work provides a new paradigm for SML exciton physics via the novel altermagnetism, opening up new possibilities for all-optical manipulation in advanced opto-spintronics.

Figures (4)

• Figure 1: Schematic diagram of the generation of SML excitons at $K$ ($X$) and $K'$ ($Y$) valleys linear polarized light along $x/y$ direction ($\sigma^x/\sigma^y$), where the large exchange-induced spin-splittings for both valence and conduction bands ($\Delta^{\mathbin\uparrow\space\downarrow}_{\mathrm{VB}}$ and $\Delta^{\mathbin\uparrow\space\downarrow}_{\mathrm{CB}}$, over 1 eV in monolayer V$_2 X_2$O eV) would remarkably reduce the intra-valley spin-flipping, e.g. for conduction bands. (b) Atomic structure of monolayer V$_2 X_2$O with V1 (spin-up V), V2 (spin-down V), S/Se, O atoms colored in red, blue, orange and green, respectively.
• Figure 2: Calculated results for the V$_2$S$_2$O monolayer. The (a) spin-up and (b) spin-down band structures by PBE+U with a scissors operator ($\Delta_\mathrm{sc}=2.0$ eV, indicated by the y-axis break), projected onto the V1, V2 and S atoms with the weights proportional to the radii of the colored circles. Energy zero is at the top of valence band. The vertical arrows indicate the transitions of corresponding excitons. (c) Optical spectra Im($\varepsilon$) by BSE and IP for the light polarization along [110]. The vertical grey ticks indicate the excitonic energies with module square of dipole moment larger and smaller than $1\times10^{-2}$ are shown separately upon and below zero axis, respectively. The single-particle band gap after the rigid scissors operator ($E_\mathrm{g}+\Delta_\mathrm{sc}$) is indicated by the black vertical line. (d) Reciprocal-space distributions of optical transitions for selected SML excitons.
• Figure 3: The (a) geometry structures, (b) spin-resolved projected band structures by PBE+U with a scissors operator ($\Delta_\mathrm{sc}=2.0$ eV, indicated by the y-axis break) and (c) demonstration of optical selection rules of the interlayer excitations for AB and AB' stacked vdWHs. For a clear presentation, only the spin-up bands are projected onto the V1, S and Se atoms, while the spin-down bands are plotted with blue dashed lines for reference. The V1(S) and V1(Se) stands for the V1 (spin-up) atoms in the V$_2$S$_2$O and V$_2$Se$_2$O layers, respectively. The vertical arrows indicate the transitions of corresponding excitons.
• Figure 4: (a) The optical spectra Im($\varepsilon$) with the light polarization along [110] for AB and AB' vdWHs, color coded with red and blue, respectively. The single-particle band gaps of the whole vdWH after a rigid 2.0 eV scissors operator ($E_\mathrm{g}+\Delta_\mathrm{sc}$) are indicated by the vertical dotted lines. (b) The detailed optical spectra to show the low-lying excitons. The vertical ticks in red(blue) indicate the excitonic energies for AB(AB')-stacking, with module square of dipole moment larger and smaller than $1\times10^{-2}$ are shown separately upon and below zero axis, respectively. (c) The real-space excitonic wavefunction modulus truncated at 2% of their maximum values, and reciprocal-space distributions of optical transitions for the excitons $E1_1$@AB, $IX_1$@AB and $IX_3$@AB' in spin-up channel.