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arXiv:2009.05913 · Physical Review A

Spin-polarized fermions with p -wave interactions

Furkan Çağrı Top, Yair Margalit, Wolfgang Ketterle

Published Oct 15, 2021• 17 citations
arXivHTMLPDF

Abstract

We have created quantum degenerate Fermi gases of $^{6}\mathrm{Li}$ atoms at unprecedented high densities exceeding $1/{\ensuremath{\lambda}\phantom{\rule{-0.5em}{0ex}}\ensuremath{-}}^{3}$ (where $\ensuremath{\lambda}$ is the wavelength of resonant light). In this regime, new optical properties are predicted. Spin-polarized Fermi gases are usually regarded as noninteracting due to the weakness of $p$-wave interactions. Here we study the properties of ``$p$-wave matter,'' where elastic and inelastic $p$-wave collisions determine the dynamics of the system. We characterize thermalization, evaporative cooling, and inelastic two-body and three-body collisions. $P$-wave dipolar relaxation creates a metastable mixture of the lowest and highest hyperfine states.

DOI
10.1103/physreva.104.043311

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We have created quantum degenerate Fermi gases of 6Li^{6}\mathrm{Li}6Li atoms at unprecedented high densities exceeding 1/\ensuremathλ\ensuremath−31/{\ensuremath{\lambda}\phantom{\rule{-0.5em}{0ex}}\ensuremath{-}}^{3}1/\ensuremathλ\ensuremath−3 (where \ensuremathλ\ensuremath{\lambda}\ensuremathλ is the wavelength of resonant light). In this regime, new optical properties are predicted. Spin-polarized Fermi gases are usually regarded as noninteracting due to the weakness of ppp-wave interactions. Here we study the properties of ``ppp-wave matter,'' where elastic and inelastic ppp-wave collisions determine the dynamics of the system. We characterize thermalization, evaporative cooling, and inelastic two-body and three-body collisions. PPP-wave dipolar relaxation creates a metastable mixture of the lowest and highest hyperfine states.
Observation of a broad state-to-state spin-exchange collision near a p-wave Feshbach resonances of 6^66Li atoms
The study of state-to-state spin-exchange collisions in the vicinity of ppp-wave Feshbach resonances offer great opportunities to explore many-body interactions and novel quantum phases. Here, we report the observation of a spin-exchange collision near a ppp-wave Feshbach resonance within a mixture of the lowest and third-lowest hyperfine states of 6^66Li atoms. The spin-exchange interaction is observed over a range of ten gausses and produces a pair of atoms in the second-lowest hyperfine states that are captured by a deep optical dipole trap. We apply a coupled-channel method to calculate the scattering properties of this system. We find that the ppp-wave resonance exhibits a low inelastic collision rate and a broad resonance profile, which is due to the modification by the accompanying spin-exchange collisions. These findings open up new possibilities for the creation of long-lived, strongly interacting ppp-wave Fermi gases.
Abstract Spin mixtures of degenerate fermions are a cornerstone of quantum many-body physics, enabling superfluidity, polarons, and rich spin dynamics through s -wave scattering resonances. Combining them with strong, long-range dipolar interactions provides highly flexible control schemes promising even more exotic quantum phases. Recently, microwave shielding gave access to spin-polarized degenerate samples of dipolar fermionic molecules, where tunable p -wave interactions were enabled by field-linked resonances available only by compromising the shielding (due to experimental limitations). Here, we study the scattering properties of a fermionic dipolar spin mixture and show that a universal s -wave resonance is readily accessible without compromising the shielding. We develop a universal description of the tunable s -wave interaction and weakly bound tetratomic states based on the microwave-field parameters. The s -wave resonance paves the way to stable, controllable and strongly-interacting dipolar spin mixtures of deeply degenerate fermions and supports favorable conditions to reach this regime via evaporative cooling.
This thesis presents studies of quantum degenerate atomic gases of fermionic 6Li and bosonic 7Li. Degeneracy is reached by evaporative cooling of 7Li in a strongly confining magnetic trap. Since at low temperatures direct evaporative cooling is not possible for a polarized fermionic gas, 6Li is sympathetically cooled by thermal contact with 7Li. In a first series of experiments both isotopes are trapped in their low-field seeking higher hyperfine states. A Fermi degeneracy of is achieved for 105 fermions. For more than ~300 atoms, the 7Li condensate collapses, due to the attractive interatomic interaction in this state. This limits the degeneracy reached for both species. To overcome this limit, in a second series of experiments 7Li and 6Li atoms are transferred to their low field seeking lower hyperfine states, where the boson-boson interaction is repulsive but weak. The inter-isotope collisions are used to thermalize the mixture. A 7Li Bose-Einstein condensate (BEC) of 104 atoms immersed in a Fermi sea is produced. The BEC is quasi-one-dimensional and the thermal fraction can be negligible. The measured degeneracies are . The temperature is measured using the bosonic thermal fraction, which vanishes at the lowest temperatures, limiting our measurement sensitivity. In a third series of experiments, the bosons are transferred into an optical trap and their internal state is changed to , the lowest energy state. A Feshbach resonance is detected and used to produce a BEC with tunable atomic interactions. When the effective interaction between atoms is tuned to be small and attractive, we observe the formation of a matter-wave bright soliton. Propagation of the soliton without spreading over a macroscopic distance of 1.1~mm is observed.
The work described in this thesis can be divided into two parts. The first part is concerned with the production of degenerate Fermi gases and Bose-Fermi mixtures. The second part of the thesis describes studies of strongly interacting spin-mixtures of ultracold fermions. We have cooled a magnetically trapped spin-polarized 6Li Fermi gas into quantum degeneracy by sympathetic cooling with bosonic 23Na. In order to do this, we have converted our previously developed apparatus for Bose-Einstein condensation of 23Na into a two-species cooling facility. We have produced 6Li samples with up to 7 × 107 atoms at temperatures below 0.5TF , and up to 3× 107 atoms at temperatures as low as 0.05TF . Further, we have also produced stable quantum degenerate mixtures of 23Na and 6Li, with several million atoms of each species. In a second set of experiments, we have studied strongly interacting spin-mixtures of 6Li. The strength of interactions was resonantly controlled by applying a magnetic field and tuning to “Feshbach resonances”, which occur when the total energy of the colliding particles coincides with the energy of a bound molecular state. These studies are relevant for the continuing quest for a superfluid transition in a Fermi gas. By studying the inelastic decay of the cloud, we have experimentally identified two Feshbach resonances for the inter-state interactions of the two lowest ground states of 6Li. Most importantly, we found that the lifetime of the strongly interacting system is long enough to observe a superfluid transition in the future. The effects of the coherent collisions in 6Li spin-mixtures were studied using radiofrequency (RF) spectroscopy. We demonstrated the absence of mean field “clock shifts” in a two-state mixture of fermions, and have introduced a three-level method to observe the interaction energy shifts. In the strongly interacting regime, near a Feshbach resonance, we have observed universal behavior of the interaction energy, independent of the value of the scattering length. Finally, we have theoretically studied the effects of Pauli blocking during a free expansion of a degenerate Fermi gas. We have concluded that a strongly interacting normal gas will undergo a hydrodynamic expansion even at zero temperature, due to inefficiency of the Pauli suppression of collisions. Therefore, the already observed hydrodynamic expansion cannot be conclusively interpreted as a signature of superfluidity. Thesis Supervisor: Wolfgang Ketterle Title: John D. MacArthur Professor of Physics
Mixtures of ultracold gases with long-range interactions are expected to open new avenues in the study of quantum matter. Natural candidates for this research are spin mixtures of atomic species with large magnetic moments. However, the lifetime of such assemblies can be strongly affected by the dipolar relaxation that occurs in spin-flip collisions. Here we present experimental results for a mixture composed of the two lowest Zeeman states of ^{162}Dy atoms, that act as dark states with respect to a light-induced quadratic Zeeman effect. We show that, due to an interference phenomenon, the rate for such inelastic processes is dramatically reduced with respect to the Wigner threshold law. Additionally, we determine the scattering lengths characterizing the s-wave interaction between these states, providing all necessary data to predict the miscibility range of the mixture, depending on its dimensionality.
Elastic and inelastic properties of weakly bound sss- and ppp-wave molecules of fermionic atoms that collide with a third atom are investigated. Analysis of calculated collisional properties of sss-wave dimers of fermions in different spin states permit us to compare and highlight the physical mechanisms that determine the stability of sss-wave and ppp-wave molecules. In contrast to sss-wave molecules, the collisional properties of ppp-wave molecules are found to be largely insensitive to variations of the ppp-wave scattering length, and these collisions will usually result in short molecular lifetimes. We also discuss the importance of this result for both theories and experiments involving degenerate Fermi gases.