Emergent s-Wave Interactions between Identical Fermions in Quasi-One-Dimensional Geometries

Abstract

Orbital degrees of freedom play an essential role in metals, semiconductors, and strongly confined electronic systems. Experiments with ultracold atoms have used highly anisotropic confinement to explore low-dimensional physics, but they typically eliminate orbital degrees of freedom by preparing atoms in the motional ground states of the strongly confined directions. Here, we prepare multiband systems of spin-polarized fermionic potassium (K40) in the quasi-one-dimensional (q1D) regime and quantify the strength of atom-atom correlations using radio-frequency spectroscopy. The activation of orbital degrees of freedom leads to a new phenomenon: a low-energy scattering channel that has even particle-exchange parity along the q1D axis, as if the underlying interactions weres-wave. This emergent exchange symmetry is enabled by orbital singlet wave functions in the strongly confined directions, which also confer high-momentum components to low-energy q1D collisions. We measure both the q1D odd-wave and even-wave “contact” parameters for the first time and compare them to theoretical predictions of one-dimensional many-body models. The strength and spatial symmetry of interactions are tuned by ap-wave Feshbach resonance and by transverse confinement strength. Near resonance, the even-wave contact approaches its theoretical unitary value, whereas the maximum observed odd-wave contact remains several orders of magnitude below its unitary limit. Low-energy scattering channels of multi-orbital systems, such as those found here, may provide new routes for the exploration of universal many-body phenomena.

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Plain Language Summary

The defining characteristic of particles known as fermions is that when they trade places with each other, their overall wave function acquires a minus sign. This “exchange antisymmetry” has numerous consequences, ranging from the stability of matter to the way electrons pair up in superconductors. Surprisingly, we find that fermions colliding in a quasi-1D geometry appear to circumvent this exchange symmetry.

The quasi-1D world in our experiments is created by capturing ultracold atoms in highly elongated optical traps. Since these fermionic particles have thermal energies far less than one quantum of excitation in the strongly confined direction, they are free to move only along the long, weakly confined axis of the trap.

We quantify the nature of collisions by measuring the atom-atom correlations at short interparticle distances. Under the right conditions, two fermions share one quantum of transverse motion in a way that satisfies exchange antisymmetry, known as a singlet Bell state. We then observe the correlation signature of exchange-symmetric wave functions along the quasi-1D direction. This new collisional mode is enabled by a nonequilibrium occupation of the excited transverse state, which was at first unintentional—a fortuitous impurity—and later controlled in our experimental protocol.

This finding points to new combinations of dimensionality and scattering symmetry. While in 3D, collisions always obey fermionic symmetry, in lower dimensions this is more flexible. The mechanism discovered here in 1D might also be extended to 2D geometries or to different types of interactions.