s-Wave interaction in a two-species Fermi-Fermi mixture at a narrow Feshbach resonance

Efficient Creation of Ultracold Ground State ^{6}Li^{40}K Polar Molecules

We report the creation of ultracold ground state ^{6}Li^{40}K polar molecules with high efficiency. Starting from weakly bound molecules, stimulated Raman adiabatic passage is adopted to coherently transfer the molecules to their singlet rovibrational ground state |X^{1}Σ^{+},v=0,J=0⟩. By employing a singlet stimulated Raman adiabatic passage pathway and low-phase-noise narrow-linewidth lasers, we observed a one-way transfer efficiency of 96(4)%. Held in an optical dipole trap, the lifetime of the ground state molecules is measured to be 5.0(3) ms. The large permanent dipole moment of LiK is confirmed by applying a dc electric field on the molecules and performing Stark shift spectroscopy of the ground state. With recent advances in the quantum control of collisions, our work paves the way for exploring quantum many-body physics with strongly interacting ^{6}Li^{40}K molecules.

Exploring Ultracold Collisions in ^{6}Li-^{53}Cr Fermi Mixtures: Feshbach Resonances and Scattering Properties of a Novel Alkali-Transition Metal System

We investigate ultracold collisions in a novel mixture of ^{6}Li and ^{53}Cr fermionic atoms, discovering more than 50 interspecies Feshbach resonances via loss spectroscopy. Building a full coupled-channel model, we unambiguously characterize the ^{6}Li-^{53}Cr scattering properties and yield predictions for other isotopic pairs. In particular, we identify various Feshbach resonances that enable the controlled tuning of elastic s- and p-wave ^{6}Li-^{53}Cr interactions. Our studies thus make lithium-chromium mixtures emerge as optimally suited platforms for the experimental search of elusive few- and many-body regimes of highly correlated fermionic matter, and for the realization of a new class of ultracold polar molecules possessing both electric and magnetic dipole moments.

Investigation of Feshbach resonances in ultracold 40K spin mixtures

Magnetically tunable Feshbach resonances are an indispensable tool for experiments with atomic quantum gases. We report on 37 thus far unpublished Feshbach resonances and four further probable Feshbach resonances in spin mixtures of ultracold fermionic ⁴⁰K with temperatures well below 100 nK. In particular, we locate a broad resonance at B = 389.7G with a magnetic width of 26.7 G. Here 1 G = 10^-4 T. Furthermore, by exciting low-energy spin waves, we demonstrate a means to precisely determine the zero crossing of the scattering length for this broad Feshbach resonance. Our findings allow for further tunability in experiments with ultracold ⁴⁰K quantum gases.

Stability of a frequency-comb-based transfer-lock using a passive Fabry-Perot resonator

We report on a transfer-lock laser frequency stabilization that utilizes a frequency comb (FC) and a radio frequency counter referenced to a GPS frequency standard to compensate for the frequency drifts of two lasers, which are locked to a single passive Fabry-Perot resonator (FPR). The method requires only one optical phase lock with the FC and allows transfer locking of lasers at wavelengths beyond the usable range of the FC. To attain a large frequency tuning range for the lasers, we implement optical serrodyning. We further demonstrate an efficient scheme to suppress residual amplitude modulation, thereby improving the stability of the Pound-Drever-Hall lock used in this case. The absolute frequency stability was found to be better than 2×10^-13 on timescales up to 300 s. Hence, together with the frequency stability on short timescales provided by the FPR, this scheme facilitates coherent Raman spectroscopy as needed for an example for the production of ultracold dipolar heteronuclear molecules.

Observation of an Orbital Interaction-Induced Feshbach Resonance in (173)Yb

We report on the experimental observation of a novel interorbital Feshbach resonance in ultracold (173)Yb atoms. This opens up the possibility of tuning the interactions between the (1)S(0) and (3)P(0) metastable state, both possessing zero total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via interorbital cross thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy. Our measurements are well described by a generalized two-channel model of the orbital-exchange interactions.

Observation of a strong atom-dimer attraction in a mass-imbalanced Fermi-Fermi mixture

We investigate a mixture of ultracold fermionic K40 atoms and weakly bound Li6K40 dimers on the repulsive side of a heteronuclear atomic Feshbach resonance. By radio-frequency spectroscopy we demonstrate that the normally repulsive atom-dimer interaction is turned into a strong attraction. The phenomenon can be understood as a three-body effect in which two heavy K40 fermions exchange the light Li6 atom, leading to attraction in odd partial-wave channels (mainly p wave). Our observations show that mass imbalance in a fermionic system can profoundly change the character of interactions as compared to the well-established mass-balanced case.

Quasiparticle lifetime in ultracold fermionic mixtures with density and mass imbalance

We show that atomic Fermi mixtures with density and mass imbalance exhibit a rich diversity of scaling laws for the quasiparticle decay rate beyond the quadratic energy and temperature dependence of conventional Fermi liquids. For certain densities and mass ratios, the decay rate is linear, whereas in other cases, it exhibits a plateau. Remarkably, this plateau extends from the deeply degenerate to the high temperature classical regime of the light species. Many of these scaling laws are analogous to what is found in very different systems, including dirty metals, liquid metals, and high temperature plasmas. The Fermi mixtures can in this sense span a whole range of seemingly diverse and separate physical systems. Our results are derived in the weakly interacting limit, making them quantitatively reliable. The different regimes can be detected with radio-frequency spectroscopy.

Realization of a resonant Fermi gas with a large effective range

We have measured the interaction energy and three-body recombination rate for a two-component Fermi gas near a narrow Feshbach resonance and found both to be strongly energy dependent. Even for de Broglie wavelengths greatly exceeding the van der Waals length scale, the behavior of the interaction energy as a function of temperature cannot be described by atoms interacting via a contact potential. Rather, energy-dependent corrections beyond the scattering length approximation are required, indicating a resonance with an anomalously large effective range. For fields where the molecular state is above threshold, the rate of three-body recombination is enhanced by a sharp, two-body resonance arising from the closed-channel molecular state which can be magnetically tuned through the continuum. This narrow resonance can be used to study strongly correlated Fermi gases that simultaneously have a sizable effective range and a large scattering length.

Observation of collective atomic recoil motion in a degenerate fermion gas

We demonstrate collective atomic recoil motion with a dilute, ultracold, degenerate fermion gas in a single spin state. By utilizing an adiabatically decompressed magnetic trap with an aspect ratio different from that of the initial trap, a momentum-squeezed fermion cloud is achieved. With a single pump pulse of the proper polarization, we observe, for the first time, multiple wave-mixing processes that result in distinct collective atomic recoil motion modes in a degenerate fermion cloud. Contrary to the case with Bose condensates, no pump-laser detuning asymmetry is present.

Hydrodynamic expansion of a strongly interacting fermi-fermi mixture

We report on the expansion of an ultracold Fermi-Fermi mixture of (6)Li and (40)K under conditions of strong interactions controlled via an interspecies Feshbach resonance. We study the expansion of the mixture after release from the trap and, in a narrow magnetic-field range, we observe two phenomena related to hydrodynamic behavior. The common inversion of the aspect ratio is found to be accompanied by a collective effect where both species stick together and expand jointly despite of their widely different masses. Our work constitutes a major experimental step for a controlled investigation of the many-body physics of this novel strongly interacting quantum system.