Spin-polarized triplet excitonic insulators in Ta3X8 (X=I, Br) monolayers

Abstract

Bose-Einstein condensation of spin-polarized triplet excitons can give rise to an intriguing spin supercurrent, enabling experimental detection of exciton condensation. In this work, we predict that Ta3X8 (X=I, Br) ferromagnetic monolayers are spin-polarized triplet excitonic insulators (EIs), based on the systematic first-principles GW calculations coupled with the Bethe-Salpeter equation (GW+BSE). The single-particle calculations of spin-polarized band structures reveal that these monolayers are bipolar magnetic semiconductors, where the highest valence band and the lowest conduction band possess opposite spin polarization. The two low-energy bands, primarily originating from Ta $d_{z^2}$ orbitals, are almost flat. The same-orbital parity and opposite-spin natures of the band-edge states effectively suppress dielectric screening, promoting the emergence of the EI state. The GW+BSE calculations reveal that the binding energy of the lowest-energy exciton is 1.499 eV for Ta3I8 monolayer and 1.986 eV for Ta3Br8 monolayer. Since both values exceed the respective GW band gaps, these results indicate a strong excitonic instability in these monolayers. A wavefunction analysis confirms that the lowest-energy exciton is a tightly bound Frenkel-like state, exhibiting a spin-polarized triplet nature with $S_z=1$. Our findings establish a valuable material platform for investigating spin-polarized triplet EIs, offering promising potential for spintronic applications.

References in corpus (30)

• Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer WS2
• Excitons in atomically thin transition metal dichalcogenides
• Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
• Ab initio calculation of excitonic effects in the optical spectra of semiconductors
• Bose-Einstein Condensation of Excitons in Bilayer Electron Systems
• Dielectric screening in two-dimensional insulators: Implications for excitonic and impurity states in graphane
• Quasiparticle band structure based on a generalized Kohn-Sham scheme
• Topological kagome magnets and superconductors
• Signatures of exciton condensation in a transition metal dichalcogenide
• Bipolar Magnetic Semiconductors: A New Class of Spintronics Materials
• Discovery of charge density wave in a correlated kagome lattice antiferromagnet
• Gate-Tuned Spontaneous Exciton Insulator in Double-Quantum Wells
• Exciton band structure in two-dimensional materials
• Observation of topological flat bands in the kagome semiconductor Nb${}_3$Cl${}_8$
• Evidence for equilibrium excitons and exciton condensation in monolayer WTe2
• Spin-Triplet Excitonic Insulator: The Case of Graphone
• Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8
• Half Excitonic Insulator: A Single-Spin Bose-Einstein Condensate
• Realizing intrinsic excitonic insulator by decoupling exciton binding energy from the minimum band gap
• Spin superconductor in ferromagnetic graphene
• Excitonic Condensate in Flat Valence and Conduction Bands of Opposite Chirality
• Excitonic Instability and Electronic Property of Two-dimensional AlSb Limit
• Robust high-temperature topological excitonic insulator of transition-metal carbides (MXenes)
• Observation of flat and weakly dispersing bands in a van der Waals semiconductor Nb3Br8 with breathing kagome lattice
• VASP2KP: kp models and Lande g-factors from ab initio calculations
• Excitonic Instability in Ta2Pd3Te5 Monolayer
• Two elementary band representation model, Fermi surface nesting, and surface topological superconductivity in $A$V${}_3$Sb${}_5$ ($A=\text{K, Rb, Cs}$)
• One-Dimensional Magnetic Excitonic Insulators
• Manipulation of topology by electric field in breathing kagome lattice
• Identifying topological excitonic insulators via bulk-edge correspondence