minigap

A minigap is a small energy gap that can appear in the electronic spectrum of mesoscopic systems. It typically occurs in a region that is proximitized by a superconductor or limited by finite size, and is smaller than the intrinsic gap of the parent material. The minigap signals a suppression of low-energy states near the Fermi level.

In superconducting hybrids such as SNS (superconductor–normal–superconductor) or NSN structures, a minigap opens in the local density of states of the normal region due to the superconducting proximity effect. It reflects the presence of coherent Andreev reflections at the interfaces and the formation of Andreev bound states.

Its magnitude depends on geometry and transport regime. In diffusive SNS junctions the minigap scales with the Thouless energy E_Th = ħD/L^2 (D: diffusion constant, L: length of the normal region). For very short junctions, the minigap can approach the parent superconducting gap Δ; for longer junctions it is of order E_Th. The minigap can be phase-dependent and may close as the superconducting phase difference across the junction approaches π.

Experimentally, minigaps have been observed in semiconductor–superconductor hybrids (for example InAs or InSb nanowires, graphene, and metallic nanostructures) using tunneling spectroscopy, scanning tunneling microscopy, or transport measurements. They influence subgap conductance and the spectrum of subgap states.

The term is distinct from the bulk gap Δ of the superconductors involved. The minigap is often described within theoretical frameworks for superconducting proximity effect, such as the Usadel equations in diffusive systems or the Eilenberger equations in ballistic systems.