  By the analytical coupled cluster method (CCM), we study both the ground
state and lowest-lying excited-state properties of the alternating bond diamond
chain. The numerical exact diagonalization (ED) method is also applied to the
chain to verify the accuracy of CCM results. The ED results show that the
ground-state phase diagram contains two exact spin cluster solid ground states,
namely, the tetramer-dimer (TD) state and dimer state, and the ferrimagnetic
long-range-ordered state. We prove that the two exact spin cluster solid ground
states can both be formed by CCM. Moreover, the exact spin gap in the TD state
can be obtained by CCM. In the ferrimagnetic region, we find that the CCM
results for some physical quantities, such as the ground-state energy, the
sublattice magnetizations, and the antiferromagnetic gap, are comparable to the
results obtained by numerical methods. The critical line dividing the TD state
from the ferrimagnetic state is also given by CCM and is in perfect agreement
with that determined by the ED method.
