Does a husband sleeping in a separate room from his wife—under the pretext of studying and research—constitute abandonment and a sin?

There is no sin upon you for sleeping in your office room, and this is not among the forbidden abandonment. However, it is better for you to sleep with your wife in one bed, for the completeness of cohabitation with kindness and the perfection of showing benevolence to her. Al-Nawawi mentioned that it is better for spouses to share one bed if there is no excuse for either of them, and this is evident from the actions of the Messenger of Allah, peace and blessings be upon him.