Expectation values of spin operators in changing magnetic field

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SUMMARY

The discussion centers on calculating expectation values of spin operators in a changing magnetic field, drawing parallels to the Larmor precession problem. The participant suggests using a spin state proportional to the vector ##\left(\begin{array}{c}1\\0\end{array}\right)##, representing spin up along the z-axis. However, it is clarified that at time t = 0, the spin state should be an eigenstate of the Hamiltonian H, aligned with the magnetic field direction defined by angles ##\theta## and ##\phi##. This approach ensures accurate representation of the system's dynamics under the influence of the magnetic field.

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Homework Statement


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Homework Equations





The Attempt at a Solution


I have totally no idea how to solve this question. But I find it somehow similar to the Larmor precession problem. Therefore I try to solve my problem by referring to that.
Are there any mistakes if I do it like this?

p.s. since the matrix elements are quite complicated, in order to save time, I use "c" to stand for cosine, "e" to stand for exponential that repeat over and over again.
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It looks like you're taking the spin state of the particle at time t = 0 to be proportional to ##\left(\begin{array}{c}1\\0\end{array}\right)##. That would correspond to spin up along the z-axis. But at t = 0 the spin state is an eigenstate of the Hamiltonian H for a magnetic field in the direction specified by ##\theta## and ##\phi##. In particular, it's the state that corresponds to "spin up" along the ##\theta##, ##\phi## direction (or, positive energy eigenvalue for H).
 

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