Stripe Antiferromagnetism and Chiral Superconductivity Achieved in tWSe at -Point Van Hove Singularity

Researchers are increasingly focused on understanding correlated electronic states in twisted bilayer materials, and a new study published in Nature Physics details the surprising link between antiferromagnetism and superconductivity in twisted tungsten diselenide (tWSe). Erekle Jmukhadze and Sam Olin, both from Binghamton University, alongside Allan H MacDonald from the University of Texas at Austin, and Wei-Cheng Lee from Binghamton University et al, demonstrate how competing charge and spin orders within this moiré material can give rise to a chiral superconducting state. This finding is significant because it suggests a novel pathway to engineer unconventional superconductivity through the manipulation of magnetic order, potentially paving the way for future topological quantum technologies? Their work combines density functional theory with path-integral calculations to reveal that antiferromagnetic interactions can break time-reversal symmetry, ultimately inducing this unusual superconducting behaviour in tWSe. The investigation focused on understanding the interplay between antiferromagnetism and superconductivity in tWSe2 when the Fermi level is near the M-point van Hove singularity and the applied displacement field is minimal. Researchers constructed a moiré model directly from DFT calculations, avoiding reliance on fitting procedures that assume smooth spatial variations, and applied this model to explore the phase diagram of tWSe2. Hartree-Fock calculations, utilising gate-screened Coulomb potentials, were performed to identify competing spin and charge orders within the moiré structure.

The team constructed a t, J, U model, based on the observed antiferromagnetic ordering patterns and strong onsite Hubbard repulsion in the narrow moiré bands, to explain the origin of superconductivity in tWSe2. This model proposes a two-band framework and suggests that the second-neighbor superexchange interaction, J2, facilitates the formation of intra-layer Cooper pairs. By performing full moiré continuum model mean-field calculations, concentrating on a moiré band hole filling factor of ν = 1, the study provides insights into the delicate interplay between band topology and electronic correlations that govern the preferred order in TMD homobilayer moiré materials. DFT and Path-Integrals for Twisted Bilayer Heterostructures offer The team directly obtained tight-binding models from first-principles calculations, meticulously including virtual hoppings via all but the selected orbitals through the path-integral method, thereby eliminating the need for empirical fitting parameters. The study pioneered a technique to ‘integrate out’ irrelevant orbitals using a path-integral formalism, significantly reducing computational cost while maintaining accuracy comparable to large-scale DFT calculations. Researchers began with a partition function expressed in terms of Grassmann variables and Matsubara frequencies, then leveraged the path integral to eliminate the degrees of freedom associated with orbitals beyond the chosen two, resulting in an effective action and Green’s function for the remaining orbitals. This process yielded an effective Hamiltonian, enabling the study of moiré band structures with unprecedented efficiency. Experiments employed this model to investigate twisted bilayer WSe₂ at twist angles of 2.7°, revealing that layer antiferromagnetism and stripe spin-density-waves compete closely with quantum anomalous Hall insulator states in terms of mean-field energy. Hartree-Fock calculations demonstrated that the stripe SDW state is insulating for dielectric constants below ε 14, potentially explaining observed insulating behaviour with zero magnetization. Furthermore, the team showed that the layer antiferromagnetic state remains metallic due to preserved C₂y rotational symmetry, proposing that superconductivity can emerge from this metallic state via antiferromagnetic spin fluctuation pairing. The approach reveals that repulsive interactions suppress onsite pairing components, favouring isotropic A₁g states, while chiral superconductivity remains robust. Topological States and Competing Orders in Moiré Heterostructures Scientists have discovered that the layer-dependent Hamiltonians of parallel-stacked MoTe₂ and WSe₂ homobilayer moiré structures exhibit topologically non-trivial behaviour in both real and momentum space, supporting integer and fractional anomalous Hall states, alongside antiferromagnetic and superconducting states. Researchers argue that antiferromagnetic spin interactions on the next-neighbor bond can induce a time-reversal symmetry breaking chiral superconducting state. The work constructs a t, J, U model to explain the origin of superconductivity in tWSe₂, utilising a two-band model that qualitatively captures the underlying physics. Measurements confirm that competition between the stripe SDW state and superconductivity underlies the first-order transition observed at small displacement fields within this system. The study specifies stacking by fixing the lateral shift ‘d’ of the top layer metal atom relative to the bottom layer, allowing metal ions to move freely along the c-axis and chalcogenide ions to move freely in all directions. Varying ‘d’ over one unit cell of the untwisted lattice, with 0 ≤ d₁, d₂ 1, a 9×9 grid was used for (d₁, d₂). The Vienna ab initio Simulation Package (VASP) was employed for structural relaxation and band structure calculations, adopting the LDA functional with spin-orbit coupling (LDA+SO) for all first-principles calculations. Following structural relaxation, a tight-binding (TB) model including the d orbitals of metal ions and the p orbitals of chalcogenide ions was constructed using WANNIER90 for each displacement ‘d’.

The team obtained an effective two-level model in the basis of Y±₂₂ orbitals at metal sites in opposite layers, resulting in a bilayer moiré material continuum model. The valence band maximum (VBM) at K (K’) is strongly spin-polarized due to strong spin-orbit coupling, with the orbital angular momentum aligning with the spin moment.

Results demonstrate that the layer separations are minimized at the equilibrium metal-on-chalcogenide (MX) and chalcogenide-on-metal (XM) stacking points, with the layer potential difference (∆t − ∆b) being zero at AA stacking and largest in magnitude at MX and XM stackings. For MoTe₂, the team obtained (Vm, ψ, wT) = (10.5meV, −89.948°, 11.1meV) and for WSe₂, (11.2meV, −89.83°, 13.7meV), with Vm and wT significantly larger than previous estimates without vertical relaxation. Relaxation unlocks WSe2 moiré bilayer ground states Scientists have demonstrated a theoretical framework incorporating local lattice relaxation to model the electronic structures of moiré bilayers. This improved model significantly enhances accuracy compared to those neglecting relaxation, while maintaining a computational efficiency comparable to large-scale density functional theory calculations. The approach constructs tight-binding models directly from first-principles calculations, accurately accounting for inter-orbital interactions through a path-integral method, thereby eliminating the need for empirical fitting parameters. Researchers focused on twisted bilayer WSe2, revealing that layer antiferromagnetic and stripe spin-density-wave states are the most competitive ground states alongside quantum anomalous Hall insulator states. Furthermore, the study proposes that superconductivity can arise from the layer antiferromagnetic metallic state, driven by antiferromagnetic spin fluctuations, with a pairing mechanism favouring chiral superconducting states over conventional isotropic pairings. The authors acknowledge limitations in the treatment of triplet pairing components, which were found to be smaller in their scenario, and suggest future work could explore moiré models near the Γ point or for conduction bands, potentially expanding the number of orbitals included in the effective model to study a wider range of transition metal dichalcogenide heterostructures. 👉 More information 🗞 Stripe antiferromagnetism and chiral superconductivity in tWSe 🧠 ArXiv: https://arxiv.org/abs/2601.20836 Tags: