# LITHIUM Lab

Principal Investigators: Tarik Yefsah and Christophe Salomon **Selected Publications**

**Universal Loss Dynamics in a Unitary Bose Gas**

Ulrich Eismann, Lev Khaykovich, Sébastien Laurent, Igor Ferrier-Barbut, Benno S. Rem, Andrew T. Grier, Marion Delehaye, Frédéric Chevy, Christophe Salomon, Li-Chung Ha, and Cheng Chin

Phys. Rev. X **6**, 021025 (2016) — arXiv:1505.04523

**Critical Velocity and Dissipation of an ultracold Bose-Fermi Counterflow**

M. Delehaye, S. Laurent, I. Ferrier-Barbut, S. Jin, F. Chevy, C. Salomon

Phys. Rev. Lett. **115**, 265303 (2015) — arXiv:1510.06709

**A Mixture of Bose and Fermi Superfluids**

I. Ferrier-Barbut, M. Delehaye, S. Laurent, A.T. Grier, M. Pierce, B.S. Rem, F. Chevy, C. Salomon

Science **345**, 1035 (2014) — arXiv:1404.2548

**Lifetime of the Bose Gas with Resonant Interactions**

B. S. Rem, A. T. Grier, I. Ferrier-Barbut, U. Eismann, T. Langen, N. Navon, L. Khaykovich, F. Werner, D. S. Petrov, F. Chevy, C. Salomon

Phys. Rev. Lett. **110**, 163202 (2013) — arXiv:1212.5274

**Dynamics and Thermodynamics of the Low-Temperature Strongly Interacting Bose Gas**

Nir Navon, Swann Piatecki, Kenneth Günter, Benno Rem, Trong Canh Nguyen, Frédéric Chevy, Werner Krauth, and Christophe Salomon

Phys. Rev. Lett. **107**, 135301 (2011) — arXiv:1103.4449

**Fermi-Liquid Behavior of the Normal Phase of a Strongly Interacting Gas of Cold Atoms**

S. Nascimbène, N. Navon, S. Pilati, F. Chevy, S. Giorgini, A. Georges, and C. Salomon

Phys. Rev. Lett. **106**, 215303 (2011) — arXiv:1012.4664

**The Equation of State of a Low-Temperature Fermi Gas with Tunable Interactions**

N. Navon, S. Nascimbène, F. Chevy, C. Salomon

Science **328**, 729-732 (2010) — arXiv:1004.1465

**Exploring the thermodynamics of a universal Fermi gas**

S. Nascimbène, N. Navon, K. J. Jiang, F. Chevy & C. Salomon

Nature **463**, 1057-1060 (2010) — arXiv:0911.0747

**Collective Oscillations of an Imbalanced Fermi Gas: Axial Compression Modes and Polaron Effective Mass**

S. Nascimbène, N. Navon, K. J. Jiang, L. Tarruell, M. Teichmann, J. McKeever, F. Chevy, and C. Salomon

Phys. Rev. Lett. **103**, 170402 (2009) — arXiv:0907.3032

## Strongly interacting Fermi and Bose gases

This experiment is dedicated to the physics of quantum degenerate mixtures of bosonic and fermionic gases. Thanks to the great versatility offered by laser cooling and trapping techniques, this system allows for the experimental study of a broad range of different physical phenomena, from strongly correlated fermionic superfluids, to counterflowing mixtures of superfluids.

## News from the lab

October 9th, 2017 — Sébastien Laurent PhD Thesis Defense

Congratulations to Sébastien for defending his PhD thesis on the “Dynamics and Stability of a Bose-Fermi Mixture”!

March 10th, 2017 — Our work `Connecting few-body inelastic decay to quantum correlations in a many-body system: A weakly coupled impurity in a resonant Fermi gas’ published in Physical Review Letters

Sébastien Laurent, Matthieu Pierce, Marion Delehaye, Tarik Yefsah, Frédéric Chevy, Christophe Salomon

Phys. Rev. Lett. **118**, 103403 (2017) — arXiv:1612.08279

We study three-body recombination in an ultracold Bose-Fermi mixture. We first show theoretically that, for weak inter-species coupling, the loss rate is proportional to Tan’s contact. Second, using a 7Li/6Li mixture we probe the recombination rate in both the thermal and dual superfluid regimes. We find excellent agreement with our model in the BEC-BCS crossover. At unitarity where the fermion-fermion scattering length diverges, we show that the loss rate is proportional the 4/3 power of the fermionic density. This unusual exponent signals non-trivial two-body correlations in the system. Our results demonstrate that few-body losses can be used as a quantitative probe of quantum correlations in many-body ensembles.

May 20th, 2016 — Our work `Universal Loss Dynamics in a Unitary Bose Gas’ published in Physical Review X

*in collaboration with Bar-Ilan University and Chicago University*

Ulrich Eismann, Lev Khaykovich, Sébastien Laurent, Igor Ferrier-Barbut, Benno S. Rem, Andrew T. Grier, Marion Delehaye, Frédéric Chevy, Christophe Salomon, Li-Chung Ha, and Cheng Chin

Phys. Rev. X **6**, 021025 (2016) — arXiv:1505.04523

The low temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here we first present a theoretical model that describes the dynamic competition between two-body evaporation and three-body re-combination in a harmonically trapped unitary atomic gas above the condensation temperature. We identify a universal magic trap depth where, within some parameter range, evaporative cooling is balanced by recombination heating and the gas temperature stays constant. Our model is developed for the usual three-dimensional evaporation regime as well as the 2D evaporation case. Experiments performed with unitary 133 Cs and 7 Li atoms fully support our predictions and enable quantitative measurements of the 3-body recombination rate in the low temperature domain. In particular, we measure for the first time the Efimov inelasticity parameter η * = 0.098(7) for the 47.8-G d-wave Feshbach resonance in 133 Cs. Combined 133 Cs and 7 Li experimental data allow investigations of loss dynamics over two orders of magnitude in temperature and four orders of magnitude in three-body loss. We confirm the 1/T 2 temperature universality law up to the constant η *.

May 8th, 2016 — Marion Delehaye PhD Thesis Defense

Congratulations to Marion for defending her PhD thesis on the “Mixture of Superfluids”!

December 31st, 2015 — Our work `Critical Velocity and Dissipation of an Ultracold Bose-Fermi Counterflow’ published in Physical Review Letters

M. Delehaye, S. Laurent, I. Ferrier-Barbut, S. Jin, F. Chevy, C. Salomon

Phys. Rev. Lett. **115**, 265303 (2015) — arXiv:1510.06709

We study the dynamics of counterflowing bosonic and fermionic lithium atoms. First, by tuning the interaction strength we measure the critical velocity vc of the system in the BEC-BCS crossover in the low temperature regime and we compare it to the recent prediction of Castin et al., C. R. Phys. 16, 241 (2015). Second, raising the temperature of the mixture slightly above the superfluid transitions reveals an unexpected phase locking of the oscillations of the clouds induced by dissipation.

October 31st, 2014 — Igor Ferrier-Barbut PhD Thesis Defense

Congratulations to Igor for defending his PhD thesis on the “Mixtures of Bose and Fermi Superfluids”!

August 29th, 2014 — Our work `A mixture of Bose and Fermi Superfluids’ published in Science

I. Ferrier-Barbut, M. Delehaye, S. Laurent, A.T. Grier, M. Pierce, B.S. Rem, F. Chevy, C. Salomon

Science **345**, 1035 (2014) — arXiv:1404.2548

Superconductivity and superfluidity of fermionic and bosonic systems are remarkable many-body quantum phenomena. In liquid helium and dilute gases, Bose and Fermi superfluidity has been observed separately, but producing a mixture in which both the fermionic and the bosonic components are superfluid is challenging. Here we report on the observation of such a mixture with dilute gases of two lithium isotopes, lithium-6 and lithium-7. We probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping below a certain critical velocity. Using high-precision spectroscopy of these modes, we observe coherent energy exchange and measure the coupling between the two superfluids. Our observations can be captured theoretically using a sum-rule approach that we interpret in terms of two coupled oscillators.

December 17th, 2013 — Benno Rem PhD Thesis Defense

Congratulations to Benno for defending his PhD thesis entitled “The Road to the Unitary Bose Gas”!

April 19th, 2013 — Our work `Lifetime of the Bose Gas with Resonant Interactions’ published in Physical Review Letters

*in collaboration with L. Khaykovich and D.S Petrov*

B. S. Rem, A. T. Grier, I. Ferrier-Barbut, U. Eismann, T. Langen, N. Navon, L. Khaykovich, F. Werner, D. S. Petrov, F. Chevy, C. Salomon

Phys. Rev. Lett. **110**, 163202 (2013) — arXiv:1212.5274

We study the lifetime of a Bose gas at and around unitarity using a Feshbach resonance in lithium 7. At unitarity, we measure the temperature dependence of the three-body decay coefficient L3. Our data follow a L3=λ3/T2 law with λ3=2.5(3)stat(6)syst×10−20 (μK)2 cm6 s−1 and are in good agreement with our analytical result based on zero-range theory. Varying the scattering length a at fixed temperature, we investigate the crossover between the finite-temperature unitary region and the previously studied regime where |a| is smaller than the thermal wavelength. We find that L3 is continuous across the resonance, and over the whole a<0 range our data quantitatively agree with our calculation.

September 19th, 2011 — Our work `Dynamics and Thermodynamics of the Low-Temperature Strongly Interacting Bose Gas‘ published in Physical Review Letters

*in collaboration with S. Piatecki and W. Krauth (ENS)*

Nir Navon, Swann Piatecki, Kenneth Günter, Benno Rem, Trong Canh Nguyen, Frédéric Chevy, Werner Krauth, and Christophe Salomon

Phys. Rev. Lett. **107**, 135301 (2011) — arXiv:1103.4449

We measure the zero-temperature equation of state of a homogeneous Bose gas of Li7 atoms by analyzing the *in situ* density distributions of trapped samples. For increasing repulsive interactions our data show a clear departure from mean-field theory and provide a quantitative test of the many-body corrections first predicted in 1957 by Lee, Huang, and Yang [Phys. Rev. **106**, 1135 (1957).]. We further probe the dynamic response of the Bose gas to a varying interaction strength and compare it to simple theoretical models. We deduce a lower bound for the value of the universal constant ξ>0.44(8) that would characterize the universal Bose gas at the unitary limit.

September 9th, 2011 — Nir Navon PhD Thesis Defense

Congratulations to Nir for defending his PhD thesis entitled “Thermodynamics of ultracold Bose and Fermi gases“!

May 27th, 2011 — Our work `Fermi-Liquid Behavior of the Normal Phase of a Strongly Interacting Gas of Cold Atoms‘ published in Physical Review Letters

*in collaboration with S. Pilati, S. Giorgini and A. Georges*

S. Nascimbène, N. Navon, S. Pilati, F. Chevy, S. Giorgini, A. Georges, and C. Salomon

Phys. Rev. Lett. **106**, 215303 (2011) — arXiv:1012.4664

We measure the magnetic susceptibility of a Fermi gas with tunable interactions in the low-temperature limit and compare it to quantum Monte Carlo calculations. Experiment and theory are in excellent agreement and fully compatible with the Landau theory of Fermi liquids. We show that these measurements shed new light on the nature of the excitations of the normal phase of a strongly interacting Fermi gas.