No abstract

The nature of strongly interacting Fermi gases and magnetism is one of the most important and studied topics in condensed-matter physics. Still, there are many open questions. A central issue is under what circumstances strong short-range repulsive interactions are enough to drive magnetic correlations. Recent progress in the field of cold atomic gases allows one to address this question in very clean systems where both particle numbers, interactions and dimensionality can be tuned. Here we study fermionic few-body systems in a one dimensional harmonic trap using a new rapidly converging effective-interaction technique, plus a novel analytical approach. This allows us to calculate the properties of a single spin-down atom interacting with a number of spin-up particles, a case of much recent experimental interest. Our findings indicate that, in the strongly interacting limit, spin-up and spin-down particles want to separate in the trap, which we interpret as a microscopic precursor of one-dimensional ferromagnetism in imbalanced systems. Our predictions are directly addressable in current experiments on ultracold atomic few-body systems.We are interested in obtaining numerically exact eigensolutions for the system of trapped atoms with arbitrary strong, zero-range interaction between different spin components. We use New J. Phys. 16 (2014) 063003 E J Lindgren et al New J. Phys. 16 (2014) 063003 E J Lindgren et al 3 New J. Phys. 16 (2014) 063003 E J Lindgren et al 5 New J. Phys. 16 (2014) 063003 E J Lindgren et al 7 4 We thank the group of Selim Jochim for a comparison of our results to their unpublished data for = − − g 1.3 1 .

No-Core Gamow Shell Model (NCGSM) is applied for the first time to study selected well-bound and unbound states of helium isotopes. This model is formulated on the complex energy plane and, by using a complete Berggren ensemble, treats bound, resonant, and scattering states on equal footing. We use the Density Matrix Renormalization Group method to solve the manybody Schrödinger equation. To test the validity of our approach, we benchmarked the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for 3 H and 4 He nuclei. We also performed ab initio NCGSM calculations for the unstable nucleus 5 He and determined the ground state energy and decay width, starting from a realistic N 3 LO chiral interaction.

The search for a resonant four-neutron system has been revived thanks to the recent experimental hints reported in Phys. Rev. Lett. 116, 052501 (2016) [1]. The existence of such a system would deeply impact our understanding of nuclear matter and requires a critical investigation. In this work, we study the existence of a four-neutron resonance in the quasi-stationary formalism using ab initio techniques with various two-body chiral interactions. We employ the No-Core Gamow Shell Model and the Density Matrix Renormalization Group method, both supplemented by the use of natural orbitals and a new identification technique for broad resonances. We demonstrate that while the energy of the four-neutron system may be compatible with the experimental value, its width must be larger than the reported upper limit, supporting the interpretation of the experimental observation as a reaction process too short to form a nucleus.