Write Sz in the J angular momentum basis?

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SUMMARY

The discussion focuses on expressing the operator \( S_z \) in terms of the angular momentum basis \( \vec{J} = \vec{L} + \vec{S} \). The user seeks to operate \( S_z \) on states \( | \ell, s=1/2, j= \ell \pm 1/2, m \rangle \). It is confirmed that when using the \( z \) orientation for the problem, the basis elements are tensor products \( |j, m_j\rangle_z \otimes |s, m_s \rangle_z \), which remain eigenvectors of \( S_z \) with unchanged eigenvalues. In this broader basis, \( S_z \) acts like the identity matrix on the \( j \)-part.

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I'm working on a problem where I want to write the operator S_z down in terms of some operator(s) in the \vec{J} = \vec{L} + \vec{S} basis so that I can operate S_z on the states \mid \ell, s=1/2, j= \ell\pm1/2, m\rangle but I'm having trouble finding the correct combination of operators to do so.

Is this possible, and if so could anyone point me in the right direction? Thanks.
 
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If you're using z as the preferred orientation for your problem(Jz), and your orbital and spin AM are in the same direction, you are already working in a basis with elements that are tensor products \|j,m_j\rangle_z \otimes |s,m_s \rangle_z, which are still eigenvectors of Sz with the same eigenvalues. Sz in this "wider" basis constructed from Lz and Sz basis vectors simply acts like the unit matrix on the j-part.
 
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