  Bound state and time evolution for single excitation in one dimensional XXZ
spin chain within non-Markovian reservoir are studied exactly. As for bound
state, a common feature is the localization of single excitation, which means
the spontaneous emission of excitation into reservoir is prohibited.
Exceptionally the pseudo-bound state can always be found, for which the single
excitation has a finite probability emitted into reservoir. We argue that under
limit $N\rightarrow \infty$ the pseudo-bound bound state characterizes an
equilibrium between the localization in spin chain and spontaneous emission
into reservoir. In addition, a critical energy scale for bound states is also
identified, below which only one bound state exists and it also is pseudo-bound
state. The effect of quasirandom disorder is also discussed. It is found in
this case that the single excitation is more inclined to locate at some spin
sites. Thus a many-body-localization like behavior can be found. In order to
display the effect of bound state and disorder on the preservation of quantum
information, the time evolution of single excitation in spin chain studied
exactly by numerically solving the evolution equation. A striking observation
is that the excitation can be stayed at its initial location with a probability
more than 0.9 when the bound state and disorder coexist. However if any one of
the two issues is absent, the information of initial state can be erased
completely or becomes mixed. Our finding shows that the combination of bound
state and disorder can provide an ideal mechanism for quantum memory.
