The Fermi surface of a non-interacting gas is spherical — all momentum states are filled up to the Fermi momentum, and the Fermi momentum is the same in every direction. The sphere reflects the isotropy of free space: nothing distinguishes one direction from another, so the boundary of the occupied region must be the same in every direction.
Long-range anisotropic interactions break this isotropy. Dipolar interactions — the force between objects with permanent electric or magnetic dipole moments — prefer specific orientations. Head-to-tail alignment is attractive; side-by-side alignment is repulsive. In a Fermi gas of dipolar particles, this anisotropy modifies the energy of each momentum state depending on its direction relative to the dipole axis. States aligned with the dipole feel a different interaction energy than states perpendicular to it. The Fermi surface deforms from a sphere into an ellipsoid or a more complex shape, directly reflecting the symmetry of the interaction.
Biswas, Eppelt, Tian, and collaborators (arXiv 2602.22447, February 2026) observe this deformation directly in a degenerate Fermi gas of sodium-potassium polar molecules, measuring Fermi surface distortions up to 7% — more than twice the largest previously observed deformation, achieved with magnetic atoms at much higher densities.
The key is the strength of the electric dipole moment compared to the magnetic dipole moment. Polar molecules have electric dipole moments that create interaction energies orders of magnitude larger than the magnetic dipole moments of atoms. The stronger interaction produces larger deformation at lower density. The experiment works with 8,000 molecules at 0.23 times the Fermi temperature — deep in the quantum degenerate regime, where the Fermi surface is sharp enough for the distortion to be measured.
The technical challenge is that polar molecules are fragile. Inelastic collisions — where the collision energy converts into rotational or vibrational excitation, ejecting the molecules from the trap — limit the lifetime of the gas. The authors use a double microwave shielding technique: two microwave fields dress the molecular states to create a repulsive barrier at short range, suppressing inelastic losses by a factor of three compared to single microwave shielding while preserving the elastic dipolar scattering that drives the Fermi surface deformation.
The experiment demonstrates continuous tuning of the interaction symmetry from axial (U(1), where the interaction looks the same for any rotation around the dipole axis) to biaxial (C2, where only 180-degree rotation is a symmetry). This is achieved by adjusting the polarization of the microwave fields. The Fermi surface deformation follows the interaction symmetry — axial interactions produce an ellipsoidal Fermi surface, biaxial interactions produce a more complex distortion with different deformation in two perpendicular transverse directions.
The Fermi surface is usually treated as a given — the boundary condition that everything else in condensed matter is built on top of. Here it is the observable. The cold gas has no crystal lattice, no phonons, no impurities — just the dipolar interaction and the Fermi statistics. The shape of the Fermi surface is the pure many-body response to the anisotropy of the force between particles, measured directly.