POLICHROM

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 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

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 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.

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

Investigating the ferromagnetic instability of a repulsive Fermi Gas with an artificially created ferromagnet of ultracold 6LI atoms. In arXiv:1605.07850 Valtolina et al expored the ferromagnetic properties of a repulsive Fermi mixture of ultracold 6Li atoms initialized in a fully ferromagnetic configuration (panel a) by investigating collective spin dynamics and spin diffusion as a function of temperature and interaction strength. The softening of the spin dipole collective mode followed by a sudden jump of the spin dipole mode frequency for critical values of repulsion (panel b) points to the occurrence of the ferromagnetic instability of the Fermi liquid towards a ferromagnetic state. 3D render of the completed design of the dual-species vacuum setup of the experiment and its construction will begin very soon.

detuning between the energy of three colliding atoms and that of the trimer bound state,” he explains. It will therefore be possible to not only tune the scattering length between lithium and chromium atoms, but also to tune the effective interaction between two chromium atoms and one lithium atom, which represents a qualitatively new knob in ultra-cold gas experiments. “The novelty of this new chromium-chromium-lithium trimer is that the presence of such a state close to the free atom scattering threshold will not dramatically enhance inelastic losses of atoms from the trap,” says Dr Zaccanti.

Trimer state

This chromium-chromium-lithium trimer state predicted by Kartavstev and Malykh has the potential to allow resonant tuning of three-body interactions, but only in elastic channels. This could open up new perspectives in ultracold gas research, believes Dr Zaccanti. “I think this really has the potential to lead to an evolutionary step forward in the few-body physics that one can explore with ultra-cold gases,” he says. Certain conditions have to be met however; this trimer state requires a specific mass ratio between the heavy and light components. “Below 8.2, there are no trimer states present. Above 13.6, there will be an infinite number of trimer states, but these will be Efimov states, which are completely different trimers,” explains Dr Zaccanti. “They will cause completely different behaviour in the system. You will eventually be able to observe threebody resonances, but they will be characterised by strong inelastic decay.”

The trimer state that Dr Zaccanti is looking for is a very weakly bound trimer state, which occurs only in fermionic gases with a mass ratio in the required range. This state has a completely different three-body wave function which will not cause any loss of atoms, while still allowing the resonant tuning of threebody interactions. “The really interesting thing with such trimer states, and this three-body resonance, is that it’s mainly an elastic process. So the lifetime of the ultra-cold mixture isn’t limited – this allows us to think that this phenomenon could be extended to a many-body environment,” says Dr Zaccanti. This could allow researchers to explore new many-body systems, in the presence of resonant three-body interactions, another area that the project is investigating. “We plan to investigate how the presence of such an additional few-body property will affect the many-body ground state of a 6Li-53Cr mixture,” continues Dr Zaccanti.

Exotic superfluidity and normal many-body quantum phases

Researchers are pursuing theoretical research into how the situation will change when moving from homonuclear mixtures to a heteronuclear one, such as a 6Li-53Cr mixture. The mass asymmetry between two atomic species in this type of system, on top of a population imbalance, can encourage the emergence and enhance the observability of exotic superfluid phases; Dr Zaccanti says the existence of certain superfluid regimes in imbalanced

Full Project Title

Superfluidity and ferromagnetism of unequal mass fermions with 2- and 3-body resonant interactions (PoLiChroM)

Project Objectives

• Produce the first mixture of ultracold fermionic Chromium and Lithium atoms worldwide • Unveil yet unexplored or not yet observed few-body phenomena in such systems • Explore the superfluid and normal phases of mass-imbalanced Fermi mixtures • Study ferromagnetic behavior of itinerant fermion systems • Quantum simulation of strongly correlated fermions • Benchmark theoretical models

Project Funding

Project budget 1.495.000 euros ERC Starting Grant.

Project Partners

Giacomo Roati, INO-CNR at LENS, University of Florence (experimentalist) • Dmitry Petrov, CNRS, Laboratoire de Physique Théorique et Modèles Statistiques, Orsay (theorist)

Contact Details

Project Coordinator, Doctor Matteo Zaccanti INO-CNR, LENS and University of Florence, Via Nello Carrara 1, 50019 Sesto Fiorentino (Florence), Italy T: +39 055 457 2474 E: zaccanti@lens.unifi.it W: http://quantumgases.lens.unifi.it/exp/crli

arXiv:1605.07850 - Evidence for ferromagnetic instability in a repulsive Fermi gas of ultracold atoms - G. Valtolina, F. Scazza, A. Amico, A. Burchianti, A. Recati, T. Enss, M. Inguscio, M. Zaccanti, G. Roati

Dr Matteo Zaccanti

Matteo Zaccanti obtained his PhD in 2008 at LENS, University of Florence (Italy). After about three years as a senior post-doc and Lise-Meitner fellow in the group of Prof. R. Grimm at IQOQI, Innsbruck (Austria), in 2012 he became an INO-CNR researcher in Florence. He currently works on ultracold Fermi gases in the INO-CNR labs in LENS and University of Florence, within the group of Prof. M. Inguscio. Dr Zaccanti is the Principal Investigator of the PoLiChroM project and the co-PI of the Lithium team, led by Dr. G. Roati. His research interests range from few-body systems, atom interferometry and disordered Bose gases, to strongly interacting Fermi gases. systems was theoretically predicted in the ‘60s. “One is the Fulde-Ferrell-LarkinOvchinnikov (FFLO) phase, where a superfluid, or a superconductor in some sense, can still exist in populationimbalanced systems if the order parameter characterising the superfluid phase is no longer homogenous over the entire system, but presents oscillations in density,” he says. Experimental observations of this FFLO phase have proved elusive however, an area of great interest to Dr Zaccanti. “The large mass-ratio of a lithiumchromium mixture, together with the unique control of three-body interactions, could be very favourable for the investigation of this elusive superfluid phase,” he says.

This would represent an important breakthrough in superfluid research. Dr Zaccanti aims to investigate the low temperature properties of the phase diagram of such a lithium-chromium Fermi mixture by scanning both the interaction strength and the relative population of 6Li-53Cr atoms. “With the beneficial effects of an increased mass ratio, plus the resonant tuning of threebody interactions, we hope to enhance the probability of getting this superfluid phase in experiments,” he outlines. Exploring the phase diagram of such a mixture could reveal other novel phases. “For instance, if a three-body bound state is the ground state of the threebody system, for some magnetic field values, then one could expect that at the many-body level a populationimbalanced lithium-chromium mixture could prefer – even at extremely low temperatures – to form a very exotic phase, comprised of a Fermi gas of CrCrLi trimers, instead of forming a superfluid phase,” says Dr Zaccanti. Another key feature of lithium-chromium mixtures is the extraordinary suppression of three-body inelastic collisions that currently limit the lifetime of homonuclear systems in the regime of strong atom-atom repulsive interaction. This few-body property can allow for other kinds of investigation. “It will allow us to study the many-body physics of a repulsive Fermi gas, free from inelastic decay and undesired spurious effects. This is very much related to the phenomenon of ferromagnetism,” outlines Dr Zaccanti.

There has been growing interest in the search for ferromagnetic behaviour in Fermi gases over recent years, building on the Stoner model of ferromagnetism. “The Stoner model was developed in 1938 by Edmund Stoner, it tried to provide a description of the ferromagnetic behaviour of transition metals in the simplest possible terms. Namely, you take the metal as being a free electron gas, a free Fermi gas with particles that interact via a short range repulsive interaction. This models a screening of Coulomb interactions between electrons in the metal,” continues Dr Zaccanti. “Ultracold Fermi gases, on the repulsive side of a Feshbach resonance, allow you in principle to have the cleanest possible environment to test the validity of the predictions of the Stoner model. However, in homonuclear systems the dynamics of the gas are complicated by the instability of the repulsive gas against three-body decay, which renders the observation of the magnetic properties of the mixture extremely challenging. We have recently made some important steps forward in the investigation of the Stoner model with an ultracold Fermi gas of 6Li atoms. Our ultimate goal is to perform a thorough investigation of the ferromagnetic phases of a repulsive lithium-chromium mixture.”

Image of a recently developed home-made and inexpensive high power laser source @425nm: up to 800mW of blue light for laser cooling of Chromium.