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The extremely neutron-rich nucleus 19 C has attracted much attention for its exotic properties. The most interesting is that the ground state of 19 C behaves like a one-neutron halo. In order to understand this peculiar characteristic, we apply the complex momentum representation method to explore the weakly bound structure of 19 C. We have calculated the single-particle energies for the bound and resonant states together with their evolutions to deformation, and checked the occupation probabilities of major configurations in the level occupied by the valance neutron. The result suggests that 19 C is a prolate halo formed by a dominant s-wave configuration.

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Electron-positron pair (EPP) creation under the Gaussian and super-Gaussian potential wells are studied by the computational quantum field theory (CQFT). We find that the EPP creation rate decreases, while the positron spectra have better monochromaticity when the potential well is wider and gentler. The phenomenon is explained from the Feshbach resonance by the complex scaling method (CSM). The width of the Feshbach resonance is narrower in the wider and gentler potential well, and the narrower Feshbach resonance can lead to lower creation rate and better energy monochromaticity. This study indicates that the width of the Feshbach resonance plays an important role in the EPP creation, and the Gaussian-type potential well has an advantage in tuning the Feshbach resonance width.