Periodic Reporting for period 2 - SuperCoolMix (Novel Superfluids in Ultracold Fermionic Mixtures)
Reporting period: 2023-07-01 to 2024-12-31
Superfluidity and superconductivity represent macroscopic quantum phenomena of great fundamental importance and technological relevance. The interest cuts across various disciplines of physics, from low-temperature physics, condensed-matter systems, and high-Tc superconductivity to neutron stars and primordial matter.
The physics of strongly interacting fermions is at the heart of such states of matter. A key mechanism is the pairing of fermionic constituents to form composite bosons, which eventually condense at sufficiently low temperatures. Standard theory, developed decades ago, describes conventional superconductivity by electrons forming so-called Cooper pairs. However, in exotic superfluids with strong interactions this picture fails or is at least incomplete. The intricate mechanisms of superfluidity in these regimes of quantum matter challenge our understanding.
Our experimental approach is based on quantum simulation. In the laboratory, we use ultracold atomic quantum gases in optical traps to realize model systems of degenerate atomic quantum matter. Our building blocks are fermionic atoms, i.e. the fermionic isotopes of lithium (Li), potassium (K), and dysprosium (Dy). With these constituents, we create ultracold two-species mixtures, where strong interactions and pairing can be present between different species. The formation of pairs can be controlled by a magnetic field based on the Feshbach resonance phenomenon.
A key point in this project is the theoretical prediction that mass imbalance favors unconventional regimes of superfluidity. In contrast to the widely investigated ultracold fermionic spin mixtures (systems with equal masses), the regions in the phase diagram for unconventional phases are much larger and appear at temperatures that can be realistically achieved in experiments. This is our main motivation for working with quantum mixtures of fermionic K and Dy, Li and K, and Li and Dy.
The experiments will address the following main questions: Can one realize an asymmetric superfluid of ultracold atoms? What is the nature of pairs in such a superfluid? What is the role of few-body interactions? What is the role of population imbalance and mass imbalance? What kind of trapping technology is needed to realize exotic superfluid states, and how can one probe the system?
The physics of strongly interacting fermions is at the heart of such states of matter. A key mechanism is the pairing of fermionic constituents to form composite bosons, which eventually condense at sufficiently low temperatures. Standard theory, developed decades ago, describes conventional superconductivity by electrons forming so-called Cooper pairs. However, in exotic superfluids with strong interactions this picture fails or is at least incomplete. The intricate mechanisms of superfluidity in these regimes of quantum matter challenge our understanding.
Our experimental approach is based on quantum simulation. In the laboratory, we use ultracold atomic quantum gases in optical traps to realize model systems of degenerate atomic quantum matter. Our building blocks are fermionic atoms, i.e. the fermionic isotopes of lithium (Li), potassium (K), and dysprosium (Dy). With these constituents, we create ultracold two-species mixtures, where strong interactions and pairing can be present between different species. The formation of pairs can be controlled by a magnetic field based on the Feshbach resonance phenomenon.
A key point in this project is the theoretical prediction that mass imbalance favors unconventional regimes of superfluidity. In contrast to the widely investigated ultracold fermionic spin mixtures (systems with equal masses), the regions in the phase diagram for unconventional phases are much larger and appear at temperatures that can be realistically achieved in experiments. This is our main motivation for working with quantum mixtures of fermionic K and Dy, Li and K, and Li and Dy.
The experiments will address the following main questions: Can one realize an asymmetric superfluid of ultracold atoms? What is the nature of pairs in such a superfluid? What is the role of few-body interactions? What is the role of population imbalance and mass imbalance? What kind of trapping technology is needed to realize exotic superfluid states, and how can one probe the system?
We have identified a low-field Feshbach resonance in the Fermi-Fermi mixture of Dy and K, which was previously unknown. The magnetically tuned resonance turned out to be very beneficial for accurate interaction control in this mixture, and will serve as an important tool in our future experiments. We have thoroughly characterized the resonance by interspecies thermalization, binding energy measurements, and magnetic moment spectroscopy, and thus gained a detailed understanding of elastic and inelastic interactions near this resonance.
We have demonstrated the formation of ultracold weakly bound “Feshbach” molecules of Dy and K and prepared a pure molecular sample in an optical dipole trap. The DyK molecules are bosons and, with measured phase-space densities close to one, conditions are close to molecular Bose-Einstein condensation. We identified an unexpected loss mechanism, which limits the lifetime of the molecular sample in the optical trap. The trap light itself can induce losses, which we interpret as a consequence of the extremely high density of molecular states in the DyK molecules. We have also shown that these losses can be suppressed by choosing the particular wavelength of the optical trap.
For probing hydrodynamic behavior, we have studied the two-species dipole mode spectrum in the Dy-K mixture. By variation of the interaction strength across the Feshbach resonance, we investigated the resonant crossover from collisionless to hydrodynamic behavior. We observed a fast damping mode, which was not seen in earlier experiments and which provides direct information on the interspecies friction. A universal transport coefficient can be derived from these measurements.
In our experiments on Li-K mixtures, we addressed a long-standing question concerning the polaronic regime, where a minority of particles (K atoms) is immersed in a large Fermi sea of majority atoms (Li). We verified essential predictions of Landau’s Fermi-liquid theory concerning the sign (i.e. the attractive or repulsive nature) of mediated interactions. Changing K isotopes (from fermionic to bosonic), we could also demonstrate that this sign depends on the quantum statistics of the impurity atoms.
Various technological developments have been made: Shaping optical potentials by digital-mirror devices, pushing optical dipole traps further into the infrared, realizing species-selective momentum changes by Raman transitions, and improved atomic beam sources.
We have demonstrated the formation of ultracold weakly bound “Feshbach” molecules of Dy and K and prepared a pure molecular sample in an optical dipole trap. The DyK molecules are bosons and, with measured phase-space densities close to one, conditions are close to molecular Bose-Einstein condensation. We identified an unexpected loss mechanism, which limits the lifetime of the molecular sample in the optical trap. The trap light itself can induce losses, which we interpret as a consequence of the extremely high density of molecular states in the DyK molecules. We have also shown that these losses can be suppressed by choosing the particular wavelength of the optical trap.
For probing hydrodynamic behavior, we have studied the two-species dipole mode spectrum in the Dy-K mixture. By variation of the interaction strength across the Feshbach resonance, we investigated the resonant crossover from collisionless to hydrodynamic behavior. We observed a fast damping mode, which was not seen in earlier experiments and which provides direct information on the interspecies friction. A universal transport coefficient can be derived from these measurements.
In our experiments on Li-K mixtures, we addressed a long-standing question concerning the polaronic regime, where a minority of particles (K atoms) is immersed in a large Fermi sea of majority atoms (Li). We verified essential predictions of Landau’s Fermi-liquid theory concerning the sign (i.e. the attractive or repulsive nature) of mediated interactions. Changing K isotopes (from fermionic to bosonic), we could also demonstrate that this sign depends on the quantum statistics of the impurity atoms.
Various technological developments have been made: Shaping optical potentials by digital-mirror devices, pushing optical dipole traps further into the infrared, realizing species-selective momentum changes by Raman transitions, and improved atomic beam sources.
The realization of an asymmetric superfluid in a two-species mixture of ultracold fermionic atoms lies beyond the current state of the art. The availability of such a unique model system will facilitate its use as a quantum simulator for phases of strongly interacting fermionic quantum matter and will help us to answer important fundamental questions. The experiments will elucidate the nature of pairing, from the formation and condensation of molecules to exotic states of finite-momentum pairs in symmetry-broken states. It will also allow us to study collective behavior, in particular hydrodynamic properties of the superfluid or degenerate states close to superfluidity.
The system will also allow us to address many related questions in currently uncharted terrain, such as the collisional stability of mixtures near resonant interactions, the properties of the polaronic phase, where the mixture is strongly interacting, but non-superfluid, or in general the role of few-body interactions involving particles of different masses.
The system will also allow us to address many related questions in currently uncharted terrain, such as the collisional stability of mixtures near resonant interactions, the properties of the polaronic phase, where the mixture is strongly interacting, but non-superfluid, or in general the role of few-body interactions involving particles of different masses.