Publications

@article{Sommer2011,

title = {Universal spin transport in a strongly interacting Fermi gas},

author = {A Sommer and M Ku and G Roati and M W Zwierlein},

doi = {10.1038/nature09989},

issn = {00280836 14764687},

year  = {2011},

date = {2011-01-01},

journal = {Nature},

volume = {472},

number = {7342},

abstract = {Transport of fermions, particles with half-integer spin, is central to many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin is being explored as a new carrier of information. Neutrino transport energizes supernova explosions following the collapse of a dying star, and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics. Even above the superfluid transition, such gases flow as an almost perfect fluid with very low viscosity when interactions are tuned to a scattering resonance. In this hydrodynamic regime, collective density excitations are weakly damped. Here we experimentally investigate spin excitations in a Fermi gas of 6 Li atoms, finding that, in contrast, they are maximally damped. A spin current is induced by spatially separating two spin components and observing their evolution in an external trapping potential. We demonstrate that interactions can be strong enough to reverse spin currents, with components of opposite spin reflecting off each other. Near equilibrium, we obtain the spin drag coefficient, the spin diffusivity and the spin susceptibility as a function of temperature on resonance and show that they obey universal laws at high temperatures. In the degenerate regime, the spin diffusivity approaches a value set by [planck]/m, the quantum limit of diffusion, where [planck]/m is Plancks constant divided by and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state. ? 2011 Macmillan Publishers Limited. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Sommer2011a,

title = {Spin transport in polaronic and superfluid Fermi gases},

author = {A Sommer and M Ku and M W Zwierlein},

doi = {10.1088/1367-2630/13/5/055009},

issn = {13672630},

year  = {2011},

date = {2011-01-01},

journal = {New Journal of Physics},

volume = {13},

abstract = {We present measurements of spin transport in ultracold gases of fermionic 6Li in a mixture of two spin states at a Feshbach resonance. In particular, we study the spin-dipole mode, where the two spin components are displaced from each other against a harmonic restoring force. We prepare a highly imbalanced, or polaronic, spin mixture with a spin-dipole excitation and we observe strong, unitarity-limited damping of the spin-dipole mode. In gases with small spin imbalance, below the Pauli limit for superfluidity, we observe strongly damped spin flow even in the presence of a superfluid core. This indicates strong mutual friction between superfluid and polarized normal spins, possibly involving Andreev reflection at the superfluid-normal interface. ? IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Vernier2011,

title = {Bound states of a localized magnetic impurity in a superfluid of paired ultracold fermions},

author = {E Vernier and D Pekker and M W Zwierlein and E Demler},

doi = {10.1103/PhysRevA.83.033619},

issn = {10502947 10941622},

year  = {2011},

date = {2011-01-01},

journal = {Physical Review A - Atomic, Molecular, and Optical Physics},

volume = {83},

number = {3},

abstract = {We consider a localized impurity atom that interacts with a cloud of fermions in the paired state. We develop an effective scattering length description of the interaction between an impurity and a fermionic atom using their vacuum scattering length. Treating the pairing of fermions at the mean-field level, we show that the impurity atom acts like a magnetic impurity in the condensed matter context, and leads to the formation of a pair of Shiba bound states inside the superconducting gap. In addition, the impurity atom can lead to the formation of deeply bound states below the Fermi sea. ? 2011 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wu2011,

title = {Strongly interacting isotopic Bose-Fermi mixture immersed in a Fermi sea},

author = {C -H Wu and I Santiago and J W Park and P Ahmadi and M W Zwierlein},

doi = {10.1103/PhysRevA.84.011601},

issn = {10502947 10941622},

year  = {2011},

date = {2011-01-01},

journal = {Physical Review A - Atomic, Molecular, and Optical Physics},

volume = {84},

number = {1},

abstract = {We have created a triply quantum-degenerate mixture of bosonic K41 and two fermionic species K40 and Li6. The boson is shown to be an efficient coolant for the two fermions, spurring hopes for the observation of fermionic superfluids with imbalanced masses. We observe multiple heteronuclear Feshbach resonances, in particular a wide s-wave resonance for the combination K41-K40, opening up studies of strongly interacting isotopic Bose-Fermi mixtures. For large imbalance in the local densities of different species, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath. ? 2011 American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Schirotzek2009,

title = {Observation of Fermi polarons in a tunable Fermi liquid of ultracold atoms},

author = {A Schirotzek and C -H Wu and A Sommer and M W Zwierlein},

doi = {10.1103/PhysRevLett.102.230402},

issn = {00319007 10797114},

year  = {2009},

date = {2009-01-01},

journal = {Physical Review Letters},

volume = {102},

number = {23},

abstract = {We have observed Fermi polarons, dressed spin-down impurities in a spin-up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid, coexisting with a Fermi sea. ? 2009 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2009a,

title = {Atomic physics: Neutral atoms put in charge},

author = {M Zwierlein},

doi = {10.1038/462584a},

issn = {00280836 14764687},

year  = {2009},

date = {2009-01-01},

journal = {Nature},

volume = {462},

number = {7273},

abstract = {An elegant experiment shows that atoms subjected to a pair of laser beams can behave like electrons in a magnetic field, as demonstrated by the appearance of quantized vortices in a neutral superfluid. ? 2009 Macmillan Publishers Limited. All rights reserved.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Baym2007,

title = {Coherence and clock shifts in ultracold fermi gases with resonant interactions},

author = {G Baym and C J Pethick and Z Yu and M W Zwierlein},

doi = {10.1103/PhysRevLett.99.190407},

issn = {00319007 10797114},

year  = {2007},

date = {2007-01-01},

journal = {Physical Review Letters},

volume = {99},

number = {19},

abstract = {Using arguments based on sum rules, we derive a general result for the average shifts of rf lines in Fermi gases in terms of interatomic interaction strengths and two-particle correlation functions. We show that near an interaction resonance shifts vary inversely with the atomic scattering length, rather than linearly as in dilute gases, thus accounting for the experimental observation that clock shifts remain finite at Feshbach resonances. ? 2007 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Schunck2007,

title = {Superfluid Expansion of a Rotating Fermi Gas},

author = {C H Schunck and M W Zwierlein and A Schirotzek and W Ketterle},

year  = {2007},

date = {2007-01-01},

journal = {Physical Review Letters},

volume = {98},

number = {5},

pages = {50404},

publisher = {APS},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Schunck2007a,

title = {Pairing without superfluidity: The ground state of an imbalanced fermi mixture},

author = {C H Schunck and Y Shin and A Schirotzek and M W Zwierlein and W Ketterle},

doi = {10.1126/science.1140749},

issn = {00368075 10959203},

year  = {2007},

date = {2007-01-01},

journal = {Science},

volume = {316},

number = {5826},

abstract = {We used radio-frequency spectroscopy to study pairing in the normal and superfluid phases of a strongly interacting Fermi gas with imbalanced spin populations. At high spin imbalances, the system does not become superfluid even at zero temperature. In this normal phase, full pairing of the minority atoms was observed. Hence, mismatched Fermi surfaces do not prevent pairing but can quench the superfluid state, thus realizing a system of fermion pairs that do not condense even at the lowest temperature.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shin2006,

title = {Observation of Phase Separation in a Strongly Interacting Imbalanced Fermi Gas},

author = {Y Shin and M W Zwierlein and C H Schunck and A Schirotzek and W Ketterle},

year  = {2006},

date = {2006-01-01},

journal = {Physical Review Letters},

volume = {97},

pages = {30401},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2006a,

title = {Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases},

author = {Martin W Zwierlein and Christian H Schunck and André Schirotzek and Wolfgang Ketterle},

year  = {2006},

date = {2006-01-01},

journal = {Nature},

volume = {442},

pages = {54--58},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2006c,

title = {Fermionic superfluidity with imbalanced spin populations},

author = {Martin W Zwierlein and André Schirotzek and Christian H Schunck and Wolfgang Ketterle},

url = {http://www.sciencemag.org/cgi/doi/10.1126/science.1122318},

doi = {10.1126/science.1122318},

issn = {00368075},

year  = {2006},

date = {2006-01-01},

journal = {Science},

volume = {311},

number = {5760},

pages = {492--496},

abstract = {We established superfluidity in a two-state mixture of ultracold fermionic atoms with imbalanced state populations. This study relates to the long-standing debate about the nature of the superfluid state in Fermi systems. Indicators for superfluidity were condensates of fermion pairs and vortices in rotating clouds. For strong interactions, near a Feshbach resonance, superfluidity was observed for a broad range of population imbalances. We mapped out the superfluid regime as a function of interaction strength and population imbalance and characterized the quantum phase transition to the normal state, known as the Pauli limit of superfluidity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Schunck2005,

title = {Feshbach resonances in fermionic Li6},

author = {C H Schunck and M W Zwierlein and C A Stan and S M F Raupach and W Ketterle and A Simoni and E Tiesinga and C J Williams and P S Julienne},

doi = {10.1103/PhysRevA.71.045601},

issn = {10502947 10941622},

year  = {2005},

date = {2005-01-01},

journal = {Physical Review A - Atomic, Molecular, and Optical Physics},

volume = {71},

number = {4},

abstract = {Feshbach resonances in Li6 were experimentally studied and theoretically analyzed. In addition to two previously known s-wave resonances, three p-wave resonances were found. Four of these resonances are narrow and yield a precise value of the singlet scattering length. The position of the broad s-wave resonance near 83mT is mostly sensitive to the triplet potential. It was previously determined in a molecule-dissociation experiment for which we, here, discuss systematic shifts. ? 2005 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2005,

title = {Vortices and superfluidity in a strongly interacting Fermi gas},

author = {M W Zwierlein and J R Abo-Shaeer and A Schirotzek and C H Schunck and W Ketterle},

year  = {2005},

date = {2005-01-01},

journal = {Nature},

volume = {435},

pages = {1047--1051},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2005b,

title = {Formation dynamics of a fermion pair condensate},

author = {M W Zwierlein and C H Schunck and C A Stan and S M F Raupach and W Ketterle},

doi = {10.1103/PhysRevLett.94.180401},

issn = {00319007 10797114},

year  = {2005},

date = {2005-01-01},

journal = {Physical Review Letters},

volume = {94},

number = {18},

abstract = {The dynamics of pair condensate formation in a strongly interacting Fermi gas close to a Feshbach resonance was studied. We employed a phase-shift method in which the delayed response of the many-body system to a modulation of the interaction strength was recorded. The observable was the fraction of condensed molecules in the cloud after a rapid magnetic field ramp across the Feshbach resonance. The measured response time was slow compared to the rapid ramp, which provides final proof that the molecular condensates reflect the presence of fermion pair condensates before the ramp. ? 2005 The American Physical Society.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stan2004,

title = {Observation of Feshbach Resonances between Two Different Atomic Species},

author = {C A Stan and M W Zwierlein and C H Schunck and S M F Raupach and W Ketterle},

year  = {2004},

date = {2004-01-01},

journal = {Physical Review Letters},

volume = {93},

pages = {143001},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2004,

title = {Condensation of Pairs of Fermionic Atoms near a Feshbach Resonance},

author = {M W Zwierlein and C A Stan and C H Schunck and S M F Raupach and A J Kerman and W Ketterle},

year  = {2004},

date = {2004-01-01},

journal = {Physical Review Letters},

volume = {92},

pages = {120403},

abstract = {We have observed Bose-Einstein condensation of pairs of fermionic atoms in an ultracold $^6$Li gas at magnetic fields above a Feshbach resonance, where no stable 6Li2 molecules would exist in vacuum. We accurately determined the position of the resonance to be 822$pm$3 G. Molecular Bose-Einstein condensates were detected after a fast magnetic field ramp, which transferred pairs of atoms at close distances into bound molecules. Condensate fractions as high as 80% were obtained. The large condensate fractions are interpreted in terms of preexisting molecules which are quasistable even above the two-body Feshbach resonance due to the presence of the degenerate Fermi gas.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Gupta2003b,

title = {Radio-frequency spectroscopy of ultracold fermions},

author = {S Gupta and Z Hadzibabic and M W Zwierlein and C A Stan and K Dieckmann and C H Schunck and E G M {Van Kempen} and B J Verhaar and W Ketterle},

doi = {10.1126/science.1085335},

issn = {00368075},

year  = {2003},

date = {2003-01-01},

journal = {Science},

volume = {300},

number = {5626},

abstract = {Radio-frequency techniques were used to study ultracold fermions. We observed the absence of mean-field "clock" shifts, the dominant source of systematic error in current atomic clocks based on bosonic atoms. This absence is a direct consequence of fermionic antisymmetry. Resonance shifts proportional to interaction strengths were observed in a three-level system. However, in the strongly interacting regime, these shifts became very small, reflecting the quantum unitarity limit and many-body effects. This insight into an interacting Fermi gas is relevant for the quest to observe superfluidity in this system.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hadzibabic2003,

title = {Fifty-fold improvement in the number of quantum degenerate fermionic atoms},

author = {Z Hadzibabic and S Gupta and C A Stan and C H Schunck and M W Zwierlein and K Dieckmann and W Ketterle},

year  = {2003},

date = {2003-01-01},

journal = {Physical Review Letters},

volume = {91},

pages = {160401},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2003a,

title = {Spectroscopic Insensitivity to Cold Collisions in a Two-State Mixture of Fermions},

author = {M W Zwierlein and Z Hadzibabic and S Gupta and W Ketterle},

issn = {00319007},

year  = {2003},

date = {2003-01-01},

journal = {Physical Review Letters},

volume = {91},

number = {25},

abstract = {The absence of the mean-field shift of radio frequency transitions in a fully decohered, interacting binary mixture of fermions was demonstrated. This was explained by proving the invariance of the interaction energy under coherent Hilbert space rotations. The absence of mean-field energy in a purely coherent superposition state. The results show that there is no spectroscopic shift even after decoherence has led to measurable mean-field energies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zwierlein2003b,

title = {Observation of Bose-Einstein Condensation of Molecules},

author = {M W Zwierlein and C A Stan and C H Schunck and S M F Raupach and S Gupta and Z Hadzibabic and W Ketterle},

issn = {00319007},

year  = {2003},

date = {2003-01-01},

journal = {Physical Review Letters},

volume = {91},

number = {25},

abstract = {Bose-Einstein condensation (BCS) of tightly bound pairs (molecules) to BCS superfluidity of Cooper pairs was observed. Condensates were observed after evaporative cooling at various magnetic fields between 720 and 820 G. The results show Bose-Einstein condensates of up to 900 000 molecules by evaporative cooling of a spin mixture of fermionic lithium atoms. This condensate realizes the limit of tightly bound fermion pairs in the crossover between BCS superfluidity and BCS.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Hadzibabic2002a,

title = {Two species mixture of quantum degenerate Bose and Fermi gases},

author = {Z Hadzibabic and C A Stan and K Dieckmann and S Gupta and M W Zwierlein and A Goerlitz and W Ketterle},

year  = {2002},

date = {2002-01-01},

journal = {Physical Review Letters},

volume = {88},

pages = {160401},

keywords = {},

pubstate = {published},

tppubtype = {article}

}