A new platform to uncover fundamental physics Ultracold gases provide an effective framework for investigating a number of physical phenomena, including superfluidity and ferro-magnetism. The PoLiChroM project aims to create a new ultra-cold mixture of two fermionic species of different atoms, enabling researchers to investigate many-body physics and elusive quantum phases, as Dr Matteo Zaccanti explains A great deal of research in contemporary physics has focused on the phenomena of superfluidity and magnetism. These are ubiquitous in Nature, and they show up in a wealth of fermionic environments, including quark and nuclear matter and solid state materials. Ultracold Fermi gases represent a unique platform to experimentally tackle these phenomena, as they allow researchers to probe the validity of theoretical predictions and investigate elusive many-body regimes in a clean and controlled environment. Experimentalists have been able to make degenerate quantum gases of fermionic atoms: firstly lithium 6 (6Li) and potassium 40 (40K), more recently also other fermionic species such as Ytterbium, Dysprosium, Erbium and Chromium. In some cases, researchers have even been able to combine two different species, realizing Fermi-Fermi mixtures. Now Dr. M. Zaccanti aims to take a step further by developing a new model system in the PoLiChroM project: “I want to combine lithium (6Li) and chromium (53Cr) fermionic atoms together in a new Fermi mixture, with which we can look at many-body physics. By investigating such a system, we hope to uncover elusive superfluid and magnetic phases, whose existence has been debated for decades, but which have not yet been observed in physical systems.”
New model system These phenomena have so far been investigated mainly in homonuclear mixtures, mixtures of the same atomic species. Developing a new model system combining ultracold 6Li and 53Cr atoms is a technically challenging task. “While 6Li atoms have been produced and explored in many labs, 53Cr is a little known species, and almost nothing is known about the collisional properties between chromium
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and lithium atoms,” says Dr Zaccanti. Researchers will use sophisticated techniques to cool the mixture and bring it to quantum degeneracy; Dr Zaccanti plans to investigate three main topics. “The first step would be to investigate the few-body properties of this mixture,” he explains. “I would like to investigate the physics and collisional properties of a three-body system realised by two identical chromium atoms, each of which interacts via a shortrange interaction with a third 6Li atom.” The goal of this few-body physics study is to uncover for the first time the existence of a weakly-bound, three-body bound state, the existence of which was theoretically predicted by the Russian physicists Kartavstev and Malykh, but which has not yet been observed experimentally. This is an unusual, universal bound state in three-body physics, which Dr Zaccanti says holds
details of the specific system would be inessential for describing the properties of such a bound state.” In particular, realising these objects would open up the possibility to tune the three-body interaction, a potentially revolutionary tool for ultracold atoms. “With ultra-cold gases, the fact that the system is very dilute makes three-body collisions rare, and the system is usually dominated by two-body collisions,” explains Dr Zaccanti. Researchers have long been able to tune two-body collisions, the strength of interaction between two atoms, building on a feature called a Feshbach resonance. “A Feshbach resonance is realised in experiments by changing – via an external magnetic field – the energy detuning between two colliding atoms (the scattering threshold) and the binding energy of a molecular state, thanks to the Zeeman effect,” continues Dr Zaccanti.
What I want to do is to combine lithium 6 and chromium 53 together, from which we can then look at many-body physics, and the elusive quantum phases arising in Fermi-Fermi mixtures, with a certain
mass and population imbalance several points of interest. “We want to investigate whether this new trimer state features universal properties – in the sense that no matter what fermionic particles you choose to bind together, the binding energy and the shape of the trimer wave-function will be determined solely by the mass ratio between the two species and by the strength of the two-body interaction, encoded into the value of the scattering length,” he outlines. “This would be universal behaviour in the sense that the features of this three-body state would be characterised by only a few parameters, and microscopic
The scattering length associated with an ultracold collision between two atoms is resonantly enhanced as the energy of the two-body bound state approaches the scattering threshold, and it eventually diverges once the molecular and scattering states become degenerate. In this way, experimentalists have been able to control the strength of the interaction between two atoms, now Dr Zaccanti aims to achieve something similar with three-body interactions. “Similarly to what is done for two-body collisions, we will use an external magnetic field to tune the energy
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