Spatial structure of Cooper pairs in nuclei

We discuss the spatial structure of the Cooper pair in dilute neutron matter and neutron-rich nuclei by means of the BCS theory and the Skyrme-Hartree-Fock-Bogioliubov model, respectively. The neutron pairing in dilute neutron matter is close to the region of the BCS-BEC crossover in a wide density range, giving rise to spatially compact Cooper pair whose size is smaller than the average interaparticle distance. This behavior extends to moderate low density ($\sim 10^{-1}$ of the saturation density) where the Cooper pair size becomes smallerst ($\sim 5$ fm). The Cooper pair in finite nuclei also exhibits the spatial correlation favoring the coupling of neutrons at small relative distances $r \lesim 3$ fm with large probability. Neutron-rich nuclei having small neutron separation energy may provide us opportunity to probe the spatial correlation since the neutron pairing and the spatial correlation persists also in an area of low-density neutron distribution extending from the surface to far outside the nucleus.