Interactions in Ultracold Mixtures of Dy and K atoms

Ultracold quantum matter is created in the laboratory by cooling atomic ensembles down to extremely low temperatures just a few billionth of a degree above absolute zero. Under such extreme conditions, matter fully unveils its fundamental quantum character and exhibits intriguing quantum phenomena. With superb experimental access and unique control of interactions, such systems serve as test bed for understanding quantum many-body systems, to benchmark advanced theoretical descriptions, and to identify new situations with no traditional counterparts. Mixtures of two different atomic species play a very important role in the field, as the combination of different atoms allows experimentalists to create systems with novel properties. Many different mixtures have already been realized in the laboratory, but most of them are based on two different alkali atoms, for the simple reason that the necessary cooling and trapping techniques are well established for this class. In order to overcome the limitations of these conventional systems and to open up possible new applications, new mixed-species systems are of great interest. In the present project, we combine a magnetic lanthanide atom with an alkali atom and thus create the first representative of a whole new class of mixed-species systems. Our choice is a mixture of dysprosium (Dy) and potassium (K), for which individually laser cooling and trapping techniques are well developed and compatible with each other. Moreover, both species offer a rich choice of different isotopes with both bosonic and fermionic character. The combination of two fermionic isotopes is of particular interest for the creation of novel superfluid states of quantum matter with analogies to high-Tc superconducting materials. The central goal of this project is to gain fundamental understanding of the unknown interaction properties of Dy-K mixtures and thus to prepare the ground for all possible future applications. Of particular interest is the tunability of interactions that arises from magnetic-field dependent resonance phenomena (so-called Feshbach resonances). As a function of an applied magnetic field, we will measure the thermalization between both species and losses related to the formation of molecules. These studies will be complemented by spectroscopic measurements on the molecular states near resonances. In a first stage of the experiments, we will focus on the combination of two fermionic isotopes with the special motivation to create novel superfluid states. We will then extend our studies to other isotopic combinations in view of other possible applications with bosonic and fermionic quantum matter. We expect that our project will enable a broad variety of future experiments on strongly interacting quantum gases.

In quantum physics, all particles can be divided into two classes: Bosons can condense into a single quantum state, if the temperature is extremely low. These particles then lose their identity and behave collectively. In contrast, fermions behave as individualists and avoid each other, which in physics is known as the "Pauli principle". This fundamental principle stabilizes various kinds of matter, ranging from atoms, solid-state materials, and even neutron stars. Fermions have a fundamental role as building blocks of matter. Playing a little trick, fermionic systems can circumvent the Pauli principle. If two fermions get closely together to form a pair, they can jointly act as a boson. Then they again can form quantum condensates, as it is well known from paired electrons, so-called Cooper pairs, in superconductors. Analogous mechanisms are present in ultracold quantum gases, prepared by methods of laser cooling and evaporative cooling in the temperature range of billionths degrees above absolute zero. For pair formation, a tunable interaction is needed, which can be achieved by magnetically controlled resonances of quantum-mechanically interacting atoms. Such systems offer unique possibilities, to explore the basic principles underlying superfluidity and superconductors, using well-accessible model systems. Up to now, superfluid fermionic Gases have been realized only with mixtures of two different spin states (like electrons in superconductor) of the same atomic species. In principle, this pairing trick could be also accomplished by combining different fermionic elements. If they exhibit a large difference in their masses, theoretical work predicts the occurrence of novel superfluid states. However, in the world of ultracold quantum gases, no combination of elements is known that offers the required interaction properties, and the expected exotic states of matter have remained elusive. The goal of the completed research project was to understand the basic interaction properties in mixtures of fermionic dysprosium atoms with fermionic potassium atoms and to identify suited conditions for controlled pair formation. Indeed, a strong quantum-mechanical resonance was found and characterized, which fulfills all necessary conditions. With this finding, the research project has reached a central goal and prepared the ground for future experiments on exotic regimes of superfluidity and superconductivity.