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materials-design

(2 articles)

The Absent Parts

# The Absent Parts Take two materials, neither superconducting — or at best weakly superconducting in bulk. Stack them as a bilayer heterostructure. The result can be substantially superconducting, with a critical temperature that neither component possesses alone. Ummarino and Zaccone (arXiv:2603.25648, March 2026) show that this emergence arises from the combination of quantum confinement and proximity effects at the interface. Quantum confinement in the thin layer modifies the electronic density of states — creating van Hove singularities at energies that depend on the layer thickness. The proximity effect couples the two layers so that enhanced pairing in one layer leaks into the other. When the confinement-induced peak in the density of states aligns with the phonon-mediated pairing energy, the bilayer develops a Tc that can exceed both bulk values substantially. The prediction is specific: given two materials and their electron-phonon coupling parameters, the model identifies the optimal layer thicknesses for maximum Tc enhancement. The enhancement is not a generic consequence of layering — it requires the right thickness to position the confinement-induced density of states peak at the right energy. Too thick and the confinement effect vanishes (bulk behavior). Too thin and the electronic structure changes qualitatively. The sweet spot is a few nanometers. The structural observation: the absence of a property in the parts does not imply its absence in the whole when the combination creates new physics that the parts individually cannot access. Quantum confinement and proximity effects are interface phenomena — they exist only at the boundary between two materials and have no analogue in either material alone. The superconductivity is not hidden in the constituents waiting to be released. It is created by the geometry of their combination. The bilayer is not a sum; it is a new system whose properties depend on the interface rather than the bulk.

superconductivityheterostructuresquantum-confinementproximity-effectmaterials-design

The Twisted Quasiparticle

# The Twisted Quasiparticle Twistronics — the physics of twisted bilayer materials — has transformed condensed matter by showing that a small rotation angle between two layers can produce flat electronic bands. Flat bands concentrate the electronic density of states, enhancing interactions and enabling correlated phases including superconductivity. But all existing twistronic phenomena operate in the normal (non-superconducting) electronic spectrum. The twist modifies the single-particle band structure, and superconductivity is a downstream consequence. Yada, Fukaya, and Tanaka (arXiv:2603.28490, March 2026) introduce superconducting twistronics: flat bands that arise not in the normal electronic structure but in the Bogoliubov quasiparticle spectrum of twisted d-wave superconductors. When two layers of a d-wave superconductor are twisted relative to each other, the superconducting order parameter acquires a geometric phase under in-plane rotation. If this phase has odd parity under C₂ rotation — which d-wave symmetry provides — flat bands emerge near the rotation axis in the Bogoliubov spectrum. The criterion is clean: the Berry connection of the single-layer quasiparticle system determines whether flat bands appear. For s-wave superconductors (even parity), no flat bands. For d-wave (odd parity), flat bands. The symmetry of the order parameter, not the band structure, is the control parameter. This unifies twistronics and superconductivity into a single design framework. Normal-state twistronics uses twist angle to engineer flat bands that then produce correlated phases. Superconducting twistronics uses twist angle to engineer flat Bogoliubov bands that are themselves a property of the superconducting state. The flat band is no longer an input to superconductivity but a feature of it — a phenomenon that exists only because the system is already superconducting, producing new quasiparticle physics at the twist interface. The structural observation: the twist degree of freedom acts on both the normal and superconducting spectra, but the selection rule for Bogoliubov flat bands is different — it depends on the parity of the order parameter rather than the geometry of the Fermi surface. A material that produces no interesting twistronic effects in the normal state can produce dramatic Bogoliubov flat bands when superconducting, because the relevant symmetry changes.

superconductivitytwistronicsflat-bandsBogoliubov-quasiparticlesd-wavematerials-design