https://escholarship.org/content/qt7511z1pj/qt7511z1pj.pdf?t=p0uvj3

Quantum theory of nonequilibrium processes, I

Self-consistent Green's function method for nuclei and nuclear matter

Production of deuterons and pions in a transport model of energetic heavy ion reactions

Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, such as peculiar band dispersions or orbital couplings, is, however, unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. Whereas fermion pairing characterizes the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly correlated Fermi gases, pointing to the existence of a ferromagnetic instability. Can short-range repulsion alone bring a system into the ferromagnetic phase? The question is explored by investigating the spin dynamics in a resonantly interacting ultracold Fermi gas, and a Stoner-like ferromagnetic instability is observed.

Exploring the ferromagnetic behaviour of a repulsive Fermi gas through spin dynamics

Operator expectation values, self-energies, cutting rules, and higher-order processes in quantum many-body theory

The Galitskii-Feynman T matrix, which sums the infinite ladder series in a many-fermion system for both particle-particle and hole-hole scattering, is studied in detail for a family of realistic He-He interactions. The structure of the S-wave bound-state singularity, reported previously, and its dependence on the bare interaction are documented at length. Special attention is devoted to the T matrix in the scattering region, where the c.m. energy of the interacting pair is positive. In particular, the on-energy-shell T matrix in this region is parametrized in terms of real “effective” phase shifts incorporating many-body effects. The critical behavior discussed previously in the bound-state region manifests itself clearly in the zero-energy limit of these phase shifts for the S wave. Below (above) a certain critical density, which is a function of both temperature and c.m. momentum, this limit approaches the value 0(−π) radians. A generalized Levinson's theorem relates this behavior to the existence of fermion-fermion pairing. An especially striking feature of these many-body phase shifts is the cusp behavior exhibited at the Fermi surface in the lowtemperature limit, which turns out to arise essentially from the structure of the particle and hole occupation probabilities. Throughout this study the temperature dependence of the T matrix is particularly emphasized.

/pdf/a-study-of-the-galitskii-feynman-t-matrix-for-liquid-3-he-4eqzl2lt1p.pdf

A study of the Galitskii-Feynman T matrix for liquid 3 He

The low-energy atom-atom scattering properties are studied for various combinations of He isotopes interacting via a family of semiphenomenological potentials. Some of the potentials bind the (4He)2 molecule and some do not, and it is observed that molecular beam scattering measurements at energies currently accessible cannot resolve the difference between the two types of potentials. Our results are discussed within the framework of possible measurements to resolve this discrepancy. We present a method for solving the Schrodinger equation which is uniformly applicable for both bound and scattering state solutions and which is particularly suited to the types of potentials describing interactions for the rare gas atoms.

/pdf/low-energy-he-he-interactions-with-phenomenological-o5ypddf50b.pdf

Low-energy He-He interactions with phenomenological potentials

/pdf/composite-pairs-and-effective-two-body-scattering-in-a-many-2n9deyl6dz.pdf

Composite pairs and effective two-body scattering in a many-body medium

/pdf/singularities-in-the-galitskii-feynman-t-matrix-u8sb1vdsve.pdf

Singularities in the Galitskii-Feynman T matrix

Quasiparticle pairing in a many-body medium, with pairwise boundary-condition model interactions, is discussed within a formalism which treats two-particle and two-hole interactions simultaneously. It is found that the possibility of two sorts of bound pairs one predominantly particle-like, which corresponds at zero density to a free bound state, and the other predominantly hole-like, which appears only above a certain critical density, and show that they form a continuous spectrum.

/pdf/quasiparticle-pairing-at-arbitrary-densities-a-soluble-model-3kcz0fgbaq.pdf

M. R. Strayer

Papers

A study of the Galitskii-Feynman T matrix for liquid 3 He

Low-energy He-He interactions with phenomenological potentials

Composite pairs and effective two-body scattering in a many-body medium

Singularities in the Galitskii-Feynman T matrix

Quasiparticle pairing at arbitrary densities: A soluble model