The adjacency matrix of a non-oriented connected graph is symmetric. Hence, hence its spectrum is real.
If the graph is bipartite, then the spectrum of its adjacency matrix is symmetric about $0$0. A few lower bounds on the smallest eigenvalue are known in the literature, but I could not find any upper bound. Hence my question: What is known about this? Do there exist graphs whose adjacency matrix is positive semidefinitesemi-definite?