Transport of Spin and Mass at Normal-Superfluid Interfaces in the Unitary Fermi Gas

Transport in strongly interacting Fermi gases provides a window into the non-equilibrium behavior of strongly correlated fermions. In particular, the interface between a strongly polarized normal gas and a weakly polarized superfluid at finite temperature presents a model for understanding transport at normal-superfluid and normal-superconductor interfaces. An excess of polarization in the normal phase or a deficit of polarization in the superfluid brings the system out of equilibrium, leading to transport currents across the interface. We implement a phenomenological mean-field model of the unitary Fermi gas, and investigate the transport of mass and spin across the interface under non-equilibrium conditions. We calculate the spin current and show how it can be understood in terms of the threshold for creating excitations in the superfluid. We find that a large net (mass) current acts to dilute excess polarization in the normal region, and show that the net current results primarily from reverse Andreev reflection.Comment: 13 pages, 13 figure

Strongly interacting Fermi gases : non-equilibrium dynamics and dimensional crossover

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 163-176).Experiments using ultracold atomic gases address fundamental problems in many-body physics. This thesis describes experiments on strongly-interacting gases of fermionic atoms, with a focus on non-equilibrium physics and dimensionality. One of the fundamental dissipative processes in two-component gases is the transport of spin due to relative motion between the two spin components. We generate spin transport in strongly-interacting Fermi gases using a spin dipole excitation and measure the transport coefficients describing spin drag and spin diffusion. For resonant interactions, we observe strong suppression of spin transport, with the spin transport coefficients reaching quantum-mechanical limits. Dimensionality plays an important role in the formation of bound states between pairs of particles. We tune the dimensionality of a Fermi gas from three to two dimensions (2D) using an optical lattice potential and observe the evolution of the pair binding energy using radio-frequency spectroscopy. The binding energy increases as the lattice depth increases, approaching the 2D limit. Gases with resonant interactions, which have no two-body bound state in three dimensions, show a large binding energy determined by the confinement energy of the lattice wells. The themes of non-equilibrium dynamics and dimensionality come together in the study of soliton excitations in superfluid Fermi gases. We create a planar defect in the superfluid order parameter of an elongated Fermi gas using detuned laser light. This defect moves through the gas as a solitary wave, or soliton, without dispersing. We measure the oscillation period of the soliton and find it to exceed the predictions of mean-field theory by an order of magnitude.by Ariel T. Sommer.Ph.D

Evolution of Fermion Pairing from Three to Two Dimensions

We follow the evolution of fermion pairing in the dimensional crossover from 3D to 2D as a strongly interacting Fermi gas of $^6$Li atoms becomes confined to a stack of two-dimensional layers formed by a one-dimensional optical lattice. Decreasing the dimensionality leads to the opening of a gap in radio-frequency spectra, even on the BCS-side of a Feshbach resonance. The measured binding energy of fermion pairs closely follows the theoretical two-body binding energy and, in the 2D limit, the zero-temperature mean-field BEC-BCS theory.Comment: 5 pages, 4 figure

Spin-Injection Spectroscopy of a Spin-Orbit Coupled Fermi Gas

The coupling of the spin of electrons to their motional state lies at the heart of recently discovered topological phases of matter. Here we create and detect spin-orbit coupling in an atomic Fermi gas, a highly controllable form of quantum degenerate matter. We reveal the spin-orbit gap via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states. For energies within the spin-orbit gap, the system acts as a spin diode. To fully inhibit transport, we open an additional spin gap, thereby creating a spin-orbit coupled lattice whose spinful band structure we probe. In the presence of s-wave interactions, such systems should display induced p-wave pairing, topological superfluidity, and Majorana edge states

Long-Lived Ultracold Molecules with Electric and Magnetic Dipole Moments

We create fermionic dipolar $^{23}$Na$^6$Li molecules in their triplet ground state from an ultracold mixture of $^{23}$Na and $^6$Li. Using magneto-association across a narrow Feshbach resonance followed by a two-photon STIRAP transfer to the triplet ground state, we produce $3\,{\times}\,10^4$ ground state molecules in a spin-polarized state. We observe a lifetime of $4.6\,\text{s}$ in an isolated molecular sample, approaching the $p$-wave universal rate limit. Electron spin resonance spectroscopy of the triplet state was used to determine the hyperfine structure of this previously unobserved molecular state.Comment: 5 pages, 5 figure

Universal Spin Transport in a Strongly Interacting Fermi Gas

Transport of fermions is central in many elds of physics. Electron transport runs modern technology, de ning states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. 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 [4, 5]. It has been established that even above the super uid transition such gases ow as an almost perfect uid with very low viscosity [3, 6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin di usivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin di usivity approaches a minimum value set by ħ/m, the quantum limit of di usion, where ħ is the reduced Planck's constant and m the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7{9].National Science Foundation (U.S.)United States. Office of Naval ResearchUnited States. Army Research Office (DARPA OLE programme)Alfred P. Sloan FoundationUnited States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeUnited States. Army Research Office. Multidisciplinary University Research InitiativeUnited States. Defense Advanced Research Projects Agency. Young Faculty AwardDavid & Lucile Packard Foundatio

Mitochondrial physiology

As the knowledge base and importance of mitochondrial physiology to evolution, health and disease expands, the necessity for harmonizing the terminology concerning mitochondrial respiratory states and rates has become increasingly apparent. The chemiosmotic theory establishes the mechanism of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force provides the framework for developing a consistent theoretical foundation of mitochondrial physiology and bioenergetics. We follow the latest SI guidelines and those of the International Union of Pure and Applied Chemistry (IUPAC) on terminology in physical chemistry, extended by considerations of open systems and thermodynamics of irreversible processes. The concept-driven constructive terminology incorporates the meaning of each quantity and aligns concepts and symbols with the nomenclature of classical bioenergetics. We endeavour to provide a balanced view of mitochondrial respiratory control and a critical discussion on reporting data of mitochondrial respiration in terms of metabolic flows and fluxes. Uniform standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of data repositories of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery

Mitochondrial physiology

As the knowledge base and importance of mitochondrial physiology to evolution, health and disease expands, the necessity for harmonizing the terminology concerning mitochondrial respiratory states and rates has become increasingly apparent. The chemiosmotic theory establishes the mechanism of energy transformation and coupling in oxidative phosphorylation. The unifying concept of the protonmotive force provides the framework for developing a consistent theoretical foundation of mitochondrial physiology and bioenergetics. We follow the latest SI guidelines and those of the International Union of Pure and Applied Chemistry (IUPAC) on terminology in physical chemistry, extended by considerations of open systems and thermodynamics of irreversible processes. The concept-driven constructive terminology incorporates the meaning of each quantity and aligns concepts and symbols with the nomenclature of classical bioenergetics. We endeavour to provide a balanced view of mitochondrial respiratory control and a critical discussion on reporting data of mitochondrial respiration in terms of metabolic flows and fluxes. Uniform standards for evaluation of respiratory states and rates will ultimately contribute to reproducibility between laboratories and thus support the development of data repositories of mitochondrial respiratory function in species, tissues, and cells. Clarity of concept and consistency of nomenclature facilitate effective transdisciplinary communication, education, and ultimately further discovery