26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

When does a Fermi puddle become a Fermi sea?

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral AIP | Atomic and Molecular Physics (ATMOP)

Description

How many grains are there in a heap of sand? A lone grain is far from enough, and adding a second or third makes little difference, yet beyond some vague threshold we instinctively call the collection of grains a "heap". The same question can be posed in the quantum realm: How many particles are needed to form a many-body quantum system?

This question is now experimentally accessible in ultracold-atom microtraps, where two-component Fermi gases can be assembled from the bottom up with single-particle control. In this talk, I address it theoretically for attractively interacting fermions confined in a quasi-two-dimensional harmonic trap, asking how the microscopic signatures of pairing evolve as particles are added one by one.

To do so, I use a method originally developed for high-precision few-body problems in nuclear, atomic, and molecular physics known as the explicitly correlated Gaussian (ECG) method. This computational technique combines a stochastic variational framework with Gaussian basis functions that explicitly depend on all interparticle distances in the problem, enabling very high accuracy. I model the interactions with a finite-range Gaussian potential, whose effective range is tuned to capture the experimentally relevant effects of tight transverse confinement. I calculate excitation spectra, density matrices, momentum distributions, and opposite-spin pair correlations as functions of increasing attractive interaction strength.

For balanced systems containing one, two, and three atoms per spin state, I find that time-reversed pairing in the ground state is strongest at momenta significantly below the Fermi momentum. In recent experiments on six atoms per spin state, pairing was observed to peak at the Fermi surface and to be strongly suppressed beneath it. Comparing these two regimes suggests that the Fermi sea — whose filled low-momentum states Pauli block pairing beneath the Fermi surface — emerges in the transition from six to twelve total atoms.