The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Stewart, John (Ph.D., Physics) Probing a strongly interacting Fermi gas Thesis directed by Professor Dr. Deborah S. Jin This thesis presents experimental work probing a strongly interacting Fermi gas of atoms. The work presented here contributed to the demonstration that strongly interacting Fermi gas systems are accessing universal physics. The uni-versality of these systems means that we can use an atomic Fermi gas to gain an understanding of strongly interacting Fermions occurring elsewhere in nature, such as nuclear and neutron matter, high transition temperature superconduc-tors, and the quark-gluon plasma. Once the universality of these systems was verified, experiments were performed to extract the microscopic quantities of an
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Stewart, John (Ph.D., Physics) Probing a strongly interacting Fermi gas Thesis directed by Professor Dr. Deborah S. Jin This thesis presents experimental work probing a strongly interacting Fermi gas of atoms. The work presented here contributed to the demonstration that strongly interacting Fermi gas systems are accessing universal physics. The uni-versality of these systems means that we can use an atomic Fermi gas to gain an understanding of strongly interacting Fermions occurring elsewhere in nature, such as nuclear and neutron matter, high transition temperature superconduc-tors, and the quark-gluon plasma. Once the universality of these systems was verified, experiments were performed to extract the microscopic quantities of an
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Wild, Robert Johannes (Ph.D., Physics) Contact Measurements on a Strongly Interacting Bose Gas Thesis directed by Dr. Eric A. Cornell A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is potentially useful given the decreasing stability of BECs with increasing interactions. Successfully measuring the contact requires careful control of experimental parameters such as the magnetic field and the RF probe pulse. In this thesis I report on measurements of the contact, using RF spectroscopy, for an 85Rb atomic Bose-Einstein condensate (BEC). The measured contact exhibits beyond-mean-field behavior, the degree of which is dependent on the rates of change
The ability to study ultracold atomic Fermi gases holds the promise of significant advances in testing fundamental theories of many-body quantum physics. Of particular interest are strongly interacting Fermi gases in the BCS to BEC crossover that exhibit a transition to a superfluid state at temperatures near 0.2TF , where TF is the Fermi temperature. This transition, as a fraction of TF, is extremely high compared to any known superfluid or superconductor. These gases are also in a universal regime where the physics is independent of the details of the atomic interactions and is therefore relevant to fields as diverse as condensed matter, nuclear physics and astrophysics. In this thesis, I present an experimental probe of atomic gases that uses momentum-resolved RF spectroscopy to realize an analog of angle-resolved photoemission spectroscopy (ARPES) in materials. This measurement reveals the energy and momentum of single-particle states in the strongly interacting Fermi gas. In condensed matter, ARPES has proved to be one of the most powerful experimental techniques for studying the electronic structure of strongly correlated electron materials. The ability to perform analogous measurements in ultracold Fermi gases constitutes a significant advance in our ability to directly connect ultracold atomic gases to strongly correlated electron systems. Taking advantage of this new measurement technique, I investigate a long-standing problem in the field of strongly interacting fermions, namely whether a pseudogap state consisting of incoherent fermion pairs exists at temperatures above the critical temperature for superfluidity. The photoemission data I present provide strong evidence for this state and have implications for fundamental theories of strongly interacting Fermi gases and strongly correlated electron materials. I also discuss the experimental confirmation of recently predicted universal relations for strongly interacting Fermi gases, as well as some of the first experiments involving atomic Fermi gases with p-wave pairing.
Gases of neutral atoms cooled to nanoKelvin temperatures exhibit remarkable behaviour such as superfluidity in which particles can flow indefinitely with zero friction. Superfluids made of fermionic particles are especially interesting as they occur in a variety of settings from neutron stars to superconductors; however, many of their physical properties are difficult to predict theoretically. This thesis presents experimental measurements of the universal contact parameter for a Fermi gas with strong interactions which characterizes the many-body properties of the system and secondly the spin susceptibility which describes how the system responds to magnetic fields. These results further our understandings of strongly interacting quantum systems.
This thesis presents experiments probing the physics of strongly interacting fermionic atoms in the BCS-BEC crossover. Ultracold atom experiments bring the ability to arbitrarily tune interatomic interactions, which allows for unprecedented access to the regime of strongly interacting physics. The majority of cold atom experiments, however, are carried out in an atom trap that imprints an inhomogeneous density on the cloud of atoms. Many phenomena, especially the signatures of phase transitions, are significantly modified by this non-uniform density. In this thesis, I present a novel imaging technique that allows us to probe a region of nearly homogeneous density within a larger, inhomogenenous cloud. Using this technique, I present new results for strongly interacting fermionic atoms, including the first measurements of the contact and the occupied spectral function of a homogeneous Fermi gas, and the first direct observation of the "textbook" momentum distribution of an ideal Fermi gas.
Many-body fermion systems are important in many branches of physics, including condensed matter, nuclear, and now cold atom physics. In many cases, the interactions between fermions can be approximated by a contact interaction. A recent theoretical advance in the study of these systems is the derivation of a number of exact universal relations that are predicted to be valid for all interaction strengths, temperatures, and spin compositions. These equations, referred to as the Tan relations, relate a microscopic quantity, namely, the amplitude of the high-momentum tail of the fermion momentum distribution, to the thermodynamics of the many-body system. In this work, we provide experimental verification of the Tan relations in a strongly interacting gas of fermionic atoms by measuring both the microscopic and macroscopic quantities in the same system.
transition, critical parameters Abstract Strongly interacting Fermi gases provide a clean and controllable laboratory system for modeling strong interparticle interactions between fermions in nature, from high temperature superconductors to neutron matter and quark-gluon plasmas. Model-independent thermodynamic measurements, which do not require theoretical models for calibrations, are very important for exploring this important system experimentally, as they enable direct tests of predictions based on the best current non-perturbative many-body theories. At Duke University, we use all-optical methods to produce a strongly interacting Fermi gas of spin-1/2-up and spin-1/2-down 6 Li atoms that is magnetically tuned near a collisional (Feshbach) resonance. We conduct a series of measurements on the thermodynamic properties of this unique quantum gas, including the energy E, entropy S, and sound velocity c. Our model-independent measurements of E and S enable a precision study of the finite temperature thermodynamics. The E(S) data are directly compared to several recent predictions. The temperature in both the superfluid and normal fluid