An easy (not easy) 1st year undergraduate project:
Energy eigenvalues of a single spin1/2 system which Hamiltonian is given by
H =  k S^z
are /+ (1/2 k).
I got that using the spin operator S^z = 1/2 Sigma^z (let h_bar = 1), Hamiltonian is in diagonalized form:
( 1/2 k 0
0 1/2k )
So, Eigenvectors are given by (1, 0), (0, 1), spin up and spin down respectively.
How do I compute the energy eigenvalues, eigenvectors of an double identical spin1/2 system which Hamiltonian is given by
H =  J vec(S1) dot vec(S2)  k S1^z  k S2^z
J is a const. > 0
After going some references, Griffiths, Sakurai, Merzbacher, etc,
I have no idea how to begin,
I have the following problems in mind:
(1) if u u> represents both spins up, (1/sqrt 2)(u d> +/ d u>) represent one spin up, d d> represent both spins down. u u> and d d> should be eigenstates of the system (because they are ground states), for u d>, d u> states I am not sure.
(2) I can't use same spin matrices for S1^z, S2^z. But there is only one S^z matrix namely, 1/2 Sigma^z. Could S1^z be a tensor product of 1/2 Sigma^z with an identity matrix, and S2^2 the other way round? If it is so, I have no idea how to do the maths!
(3) Do vec(S1) and vec(S2) commute?
Anyone could give me some hints?
