Pairing Strength and Quadrupole-Soft Tin Isotopes

Abstract: Background: Understanding the experimental $B(E2)$ values for Sn isotopes around $^{110}$Sn has been a significant challenge in nuclear structure studies for over a decade. Both experimental data and many, though not all, calculations suggest a picture of the light Sn isotopes as being quadrupole-soft, that is, spherical, yet easy to deform. Purpose: To investigate the delicate interplay of quadrupole deformation and pairing correlations in these nuclides. In particular, by using slightly enhanced pairing, we ask: can we generate spherical mean-field solutions that describe the data? Method: First, we apply the standard spherical Skyrme HFBCS-QRPA calculation with default pairing parameters, allowing us to identify nuclides that are unstable against quadrupole deformation among Sn isotopes. Next, we moderately enhance the pairing strength to reproduce the experimental binding energy in the deformation-unstable isotopes. Result: Within our choice of Skyrme parameters and use of density-independent pairing, this moderate adjustment sufficiently stabilizes the HFBCS ground states against deformation, ensuring a successful QRPA calculation and, more importantly, leading to more realistic properties for the quadrupole $2^+$ states. Conclusion: Careful attention to the sensitive interplay of pairing and shell effects in deformation-soft nuclides can be crucial to their correct descriptions. This sensitivity can be exploited to optimize the treatment of pairing in phenomenological approaches such as the present Skyrme-QRPA.