  11Li is a Borromean nucleus, where two out of three objects as 9Li + n and
two neutrons independently do not form bound systems. Two neutrons should form
a di-neutron cluster in the nuclear field generated by the 9Li core nucleus. We
treat di-neutron clustering by solving the two neutron relative wave function
precisely by using the bare nucleon-nucleon interaction so that the spatial
clustering structure is obtained quantitatively within the whole 11Li nucleus.
This di-neutron clustering is an essential dynamics to form the halo structure
by making a compact di-neutron cluster, which distributes loosely around the
9Li core. The di-neutron clustering is a new concept and is a general
phenomenon in neutron skin and neutron halo nuclei. This quantitative
description of di-neutron clustering has made it necessary to consider another
important deuteron-like tensor correlation, which is caused by strong tensor
interaction in the nucleon-nucleon interaction. The tensor interaction
originates from pion exchange and known to provide large attraction to form the
4He nucleus. The unique feature of the tensor correlation is to make highly
correlated deuteron-like excitation, which interferes with shell model like
structure in a unique way. This dynamical effect removes the magic number
effect and makes easy the participation of the s-wave neutrons. Hence, there
are pairing and deuteron-like tensor correlations in addition to the mean field
structure in 9Li. The combined system of two additional neutrons with the
correlated 9Li provides the halo phenomenon, in which the di-neutron clustering
develops with the help of large s-wave component caused by the deuteron-like
tensor correlation. In this lecture note, we would like to introduce these two
new concepts in a systematic manner and fill a gap between the halo phenomenon
and the microscopic reason for this interesting phenomenon.
