Precision measurement of $^{11}$Li moments: influence of halo neutrons on the $^{9}$Li core

The electric quadrupole moment and the magnetic moment of the $^{11}$Li halo nucleus have been measured with more than an order of magnitude higher precision than before, |Q|=33.3(5) mb and $\mu$=3.6712(3)$\mu\scriptstyle_\textrm{N}$, revealing a 8.8(1.5)% increase of the quadrupole moment relative to that of $^{9}$Li. This result is compared to various models that aim at describing the halo properties. In the shell model an increased quadrupole moment points to a significant occupation of the 1$d$ orbits, whereas in a simple halo picture this can be explained by relating the quadrupole moments of the proton distribution to the charge radii. Advanced models so far fail to reproduce simultaneously the trends observed in the radii and quadrupole moments of the lithium isotopes.