1. Relativistic Quantum Mechanics
Lecture 7
Books Recommended:
Lectures on Quantum Field Theory by Ashok Das
Advanced Quantum Mechanics by Schwabl
Relativistic Quantum Mechanics by Greiner
Quantum Field Theory by Mark Srednicki

2. Spin of Dirac Particle
We define 4-dim matrix which is generalization
Of Pauli matrices
This is related to

3. As and thus
Note that

4. Also

5. With above, free Dirac Hamiltonian gives

6. Orbital angular momentum is
We calculate

7. To find spin angular momentum, we note that
Thus

8. Total angular momentum
Defining spin angular momentum
In particular