# Investigation of Feshbach Resonances in ultra-cold 40 K spin mixtures

## TL;DR

This paper reports the discovery of new Feshbach resonances in ultracold 40 K fermionic gases, including a broad resonance at 389.6 G, and introduces a novel method to precisely locate the zero crossing of the scattering length.

## Contribution

The study identifies twenty new Feshbach resonances and demonstrates a new technique using spin waves to accurately determine the zero crossing in 40 K gases.

## Key findings

- Discovered 20 new Feshbach resonances in 40 K
- Located a broad resonance at 389.6 G with 26.4 G width
- Developed a method to precisely find the zero crossing of scattering length

## Abstract

Magnetically-tunable Feshbach resonances are an indispensable tool for experiments with atomic quantum gases. We report on twenty thus far unpublished Feshbach resonances and twenty one further probable Feshbach resonances in spin mixtures of ultracold fermionic 40 K with temperatures well below 100 nK. In particular, we locate a broad resonance at B=389.6 G with a magnetic width of 26.4 G. Here 1 G=10^-4 T. Furthermore, by exciting low-energy spin waves, we demonstrate a novel means to precisely determine the zero crossing of the scattering length for this broad Feshbach resonance. Our findings allow for further tunability in experiments with ultracold 40 K quantum gases.