  Our understanding of nuclear properties in the vicinity of 100Sn, suggested
to be the heaviest doubly magic nucleus with equal numbers of protons (Z=50)
and neutrons (N=50), has been a long-standing challenge for experimental and
theoretical nuclear physics. Contradictory experimental evidence exists on the
role of nuclear collectivity in this region of the nuclear chart. Using
precision laser spectroscopy, we measured the ground-state electromagnetic
moments of indium (Z=49) isotopes approaching the N=50 neutron number down to
101In, and nuclear charge radii of 101-131In spanning almost the complete range
between the two major neutron closed-shells at N=50 and N=82. Our results for
both nuclear charge radii and quadrupole moments reveal striking parabolic
trends as a function of the neutron number, with a clear reduction toward these
two neutron closed-shells, thus supporting a doubly magic character of 100Sn.
Two complementary nuclear many-body frameworks, density functional theory and
ab initio methods, elucidate our findings. A detailed comparison with our
experimental results exposes deficiencies of nuclear models, establishing a
benchmark for future theoretical developments.
