135 research outputs found
FERMIONIC SUPERFLUIDITY AND THE BEC-BCS CROSSOVER IN ULTRACOLD ATOMIC FERMI GASES
No full textIn our recent experiments at MIT, superfluidity has been observed in a strongly interacting atomic Fermi gas. These systems constitute a novel form of matter with model character: One can vary the temperature, density and dimensionality, the number of “spin up” versus “spin down” fermions, and, most remarkably, the interactions can be precisely controlled over an enormous range. This allows to study the crossover of a Bose-Einstein condensate of tightly bound molecules to a Bardeen-Cooper-Schrieffer superfluid of long-range Cooper pairs. Superfluidity in this crossover regime is demonstrated by setting the gas under rotation and observing ordered lattices of quantized vortices [1]. Thanks to its strong interactions, the gas is a high-temperature superfluid: Scaled to the density of electrons in a metal, superfluidity would occur already far above room temperature. A new regime is entered when the number of spin up versus spin down atoms is imbalanced. In this case, not every spin up atom can find a spin down partner. The ground state of such a system has been under debate for over 40 years. We observe the breakdown of superfluidity at a critical imbalance, the Chandrasekhar-Clogston limit [2]. The superfluid gives way to an intriguing, strongly-interacting Fermi gas with unequal spin populations [3, 4]
Atomic Physics: Neutral atoms put in charge
No full textAn 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
Two-photon pathway to ultracold ground state molecules of [superscript 23]Na [superscript 40]K
No full textWe report on high-resolution spectroscopy of ultracold fermionic [superscript 23]Na[superscript 40]K Feshbach molecules, and identify a two-photon pathway to the rovibrational singlet ground state via a resonantly mixed B[superscript 1]Π ~ c[superscript 3]Σ[superscript +]intermediate state. Photoassociation in a [superscript 23]Na–[superscript 40]K atomic mixture and one-photon spectroscopy on [superscript 23]Na[superscript 40]K Feshbach molecules reveal about 20 vibrational levels of the electronically excited c[superscript 3]Σ[superscript +]state. Two of these levels are found to be strongly perturbed by nearby B[superscript 1]Π levels via spin–orbit coupling, resulting in additional lines of dominant singlet character in the perturbed complex B[superscript 1}Π |v = 4〉 ~ c[superscript 3]∑[superscript +] v = 25〉, or of resonantly mixed character in B[superscript 1}Π |v = 12〉 ~ c[superscript 3]∑[superscript +] v = 35〉. The dominantly singlet level is used to locate the absolute rovibrational singlet ground state X[superscript 1]∑[superscript +] |v = 0, J = 0〉 via Autler–Townes spectroscopy. We demonstrate coherent two-photon coupling via dark state spectroscopy between the predominantly triplet Feshbach molecular state and the singlet ground state. Its binding energy is measured to be 5212.0447(1) cm[superscript −1], a thousand-fold improvement in accuracy compared to previous determinations. In their absolute singlet ground state, [superscript 23]Na[superscript 40]K molecules are chemically stable under binary collisions and possess a large electric dipole moment of 2.72 Debye. Our work thus paves the way towards the creation of strongly dipolar Fermi gases of NaK molecules.National Science Foundation (U.S.)United States. Air Force Office of Scientific Research (Presidential Early Career Award for Scientists and Engineers)United States. Army Research OfficeUnited States. Army Research Office. Multidisciplinary University Research InitiativeUnited States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeDavid & Lucile Packard Foundatio
Competition between Pairing and Ferromagnetic Instabilities in Ultracold Fermi Gases near Feshbach Resonances
No full textWe study the quench dynamics of a two-component ultracold Fermi gas from the weak into the strong interaction regime, where the short time dynamics are governed by the exponential growth rate of unstable collective modes. We obtain an effective interaction that takes into account both Pauli blocking and the energy dependence of the scattering amplitude near a Feshbach resonance. Using this interaction we analyze the competing instabilities towards Stoner ferromagnetism and pairing.United States. Army Research OfficeUnited States. Defense Advanced Research Projects Agency (DARPA) (OLE program)Swiss National Science FoundationNational Science Foundation (U.S.) (DMR-07-05472)National Science Foundation (U.S.) (PHY-06-53514)United States. Air Force Office of Scientific Research (MURI)Alfred P. Sloan Foundatio
Bound states of a localized magnetic impurity in a superfluid of paired ultracold fermions
No full textWe 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.United States. Army Research OfficeUnited States. Defense Advanced Research Projects Agency. Optical Lattice Emulator ProgramMIT-Harvard Center for Ultracold AtomsNational Science Foundation (U. S.) (Grant no. DMR-07-05472)National Science Foundation (U. S.) (Grant no. PHY-06-53514)United States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeUnited States. Air Force Office of Scientific Research. Young Investigator ProgramUnited States. Army Research Office. Multidisciplinary University Research Initiative. AtomtronicsAlfred P. Sloan Foundatio
Ultracold Fermionic Feshbach Molecules of [superscript 23]Na[superscript 40]K
No full textWe report on the formation of ultracold weakly bound Feshbach molecules of [superscript 23]Na[superscript 40]K, the first fermionic molecule that is chemically stable in its absolute ground state. The lifetime of the nearly degenerate molecular gas exceeds 100 ms in the vicinity of the Feshbach resonance. The measured dependence of the molecular binding energy on the magnetic field demonstrates the open-channel character of the molecules over a wide field range and implies significant singlet admixture. This will enable efficient transfer into the singlet vibrational ground state, resulting in a stable molecular Fermi gas with strong dipolar interactions.National Science Foundation (U.S.)United States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeUnited States. Air Force Office of Scientific Research. Presidential Early Career Award for Scientists and EngineersUnited States. Army Research OfficeUnited States. Defense Advanced Research Projects Agency. Optical Lattice Emulator ProgramDavid & Lucile Packard Foundatio
Ultracold Dipolar Gas of Fermionic [superscript 23]Na[superscript 40]K Molecules in Their Absolute Ground State
No full textWe report on the creation of an ultracold dipolar gas of fermionic [superscript 23]Na[superscript 40]Kmolecules in their absolute rovibrational and hyperfine ground state. Starting from weakly bound Feshbach molecules, we demonstrate hyperfine resolved two-photon transfer into the singlet X[superscript 1]Σ[superscript +]|v = 0,J = 0⟩ ground state, coherently bridging a binding energy difference of 0.65 eV via stimulated rapid adiabatic passage. The spin-polarized, nearly quantum degenerate molecular gas displays a lifetime longer than 2.5 s, highlighting NaK’s stability against two-body chemical reactions. A homogeneous electric field is applied to induce a dipole moment of up to 0.8 D. With these advances, the exploration of many-body physics with strongly dipolar Fermi gases of [superscript 23]Na[superscript 40]K molecules is within experimental reach.National Science Foundation (U.S.)United States. Air Force Office of Scientific Research. Presidential Early Career Award for Scientists and EngineersUnited States. Army Research OfficeUnited States. Army Research Office. Multidisciplinary University Research InitiativeUnited States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeDavid & Lucile Packard Foundatio
Positive representation of Inns of Court lawyers in Jacobean city comedy
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis examines representations of lawyers and law in examples of Jacobean city comedy, taking into account certain contemporary developments in the legal profession and the law in England. The period covered is 1598-1616. The thesis questions the conventional interpretation of city comedy as hostile to the legal profession. It suggests the topic is more complex than has been assumed, arguing that city comedy makes direct and indirect positive representation of Inns of Court lawyers, who are to be distinguished from attorneys (newly segregated in the Inns of Chancery), amateur quasi-lawyers, and university-educated civil lawyers. It is proposed that city comedy represents Inns of Court lawyers positively in two ways. Firstly, by means of legal content: representations of developments in the profession and the law demonstrate a wish to connect with the young lawyers and students of the Inns of Court, and reflect a contemporary drive by them for increased organization and regulation. Secondly, by means of literary form: ostensibly pejorative representations need not be taken at face value; instead, they may be found to be ironic. The main proposed contributions to knowledge are: that Inns of Court lawyers were a favoured part of the target audience of the private playhouses, making it questionable that they would be represented negatively in city comedy; that lawyers as represented in city comedy are not a single or a simple category; that representation of lawyers is inflected by the various forms and impulses of city comedy; and that city comedy incorporates some reflection of the increasing professionalization of legal practice in the period
Boiling a Unitary Fermi Liquid
No full textWe study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime, and short-range correlations of Fermi polarons at low temperatures T. In this regime, we observe a characteristic T^{2} dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high T, the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, ∝T^{-1/2}. In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins and yield the compressibility along with the effective mass of Fermi polarons.United States. National Science Foundation. (Award PHY-1734011)United States. National Science Foundation. (Award PHY-1506019)United States. Air Force. Office of Scientific Research (Grant FA9550-16-1-0324)United States. Air Force. Office of Scientific Research. Multidisciplinary University Research Initiative (Grant FA9550-14-1-0035)United States. Office of Naval Research (Grant N00014-17-1-2257
Spin transport in polaronic and superfluid Fermi gases
No full textWe 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.National Science Foundation (U.S.)United States. Army Research Office (DARPA OLE program)David & Lucile Packard FoundationAlfred P. Sloan FoundationUnited States. Air Force Office of Scientific Research (AFOSR-MURI)United States. Air Force Office of Scientific Research (AFOSR-YIP)United States. Army Research Office (ARO-MURI)United States. Office of Naval ResearchUnited States. Defense Advanced Research Projects Agency (DARPA YFA
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