# From Molecules to Cooper Pairs: Experiments in the BEC-BCS Crossover. ebook

## by Markus Bartenstein,Munchen Fraunhofer ZV

23 Questions on BEC-BCS crossover BEC of molecules: excellent starting point for exploring the crossover Q1. .For T Tc, generalized Cooper pairs are broken, hence loss of superfluidity. At higher T, the step smoothes and shifts towards smaller 1/k F a 1000

23 Questions on BEC-BCS crossover BEC of molecules: excellent starting point for exploring the crossover Q1: Lifetime of molecules? Q2: interaction between molecules? Q3: What happens in strongly correlated regime: unitarity: k F a 1? Q4: Can we measure the excitation gap? . At higher T, the step smoothes and shifts towards smaller 1/k F a 1000. 31 Critical temperature in in BEC- BCS crossover Harmonic trap Perali et a. Cond-mat Homogeneous case Sa de Melo, Randéria, Engelbrecht, PRL 71, (1993) BCS 1/k F a BEC At B c, 1/k F a in trap.

urn:arXiv:cond-mat/0412712.

This crossover between BCS-type superfluidity and the BEC limit has long been of theoretical interest . This allows us to explore the crossover from a Bose-Einstein condensate of molecules to a effer superfluid of loosely bound pairs.

This crossover between BCS-type superfluidity and the BEC limit has long been of theoretical interest, motivated in part by the discovery of high-temperature superconductors. In atomic Fermi gas experiments superfluidity has not yet been demonstrated; however, long-lived molecules consisting of locally paired fermions have been reversibly created. The crossover is associated with a new form of superfluidity that may provide insights into erature superconductors.

Our results clearly show a crossover by increasing the neutron density.

In particular, results obtained on an ultracold spin-mixture of 6Li atoms in a series of experiments performed in Innsbruck is presented . We find that Cooper pairing enhances the recombination rate in contrast to the decrease of this rate accompanying Bose-Einstein condensation in a Bose gas. This trend is characteristic for all interval of temperatures T

Using a two-channel model, we show that the number of closed-channel molecules in a two-component Fermi gas close to a Feshbach resonance is directly related to the derivative of the energy of the gas with respect to the inverse scattering length

Using a two-channel model, we show that the number of closed-channel molecules in a two-component Fermi gas close to a Feshbach resonance is directly related to the derivative of the energy of the gas with respect to the inverse scattering length. We extract this quantity from the fixed-node Monte Carlo equation of state and we compare to the number of closed-channel molecules measured in the Rice experiment with lithium.

This thesis presents experiments probing physics in the crossover between BoseEinstein condensation (BEC) and BCS . Here pairs of fermionic atoms have some properties of diatomic molecules and some properties of Cooper pairs.

This thesis presents experiments probing physics in the crossover between BoseEinstein condensation (BEC) and BCS superconductivity using an ultracold gas of atomic fermions. Scattering resonances in these ultracold gases (known as Feshbach resonances) provide the unique ability to tune the fermion-fermion interactions. I present studies of a normal Fermi gas at a Feshbach resonance and the work required to cool the gas to temperatures where superfluidity in the crossover could be observed.

Fermi gas Bose-Einstein condensation Cooper pairing chemistry .

Fermi gas Bose-Einstein condensation Cooper pairing chemistry Superconductor AMO High-Tc He-3 6 Li M. Holland. 9 rf In the molecule picture Pairing gap measurement: RF excitation State 3 is initially unpopulated f atom E 23 f molecule E 23 +E b +2E K atomsmolecules EbEb Excitation rate.