Arun,
Motivation in your research field is one of those things that you need to find for yourself. In my experience the motivation and passion required to succeed within a research topic is a very personal thing and, in my opinion, one of the key lessons of graduate studies. I am not sure if you are at the beginning of your journey and trying to decide things or if you are part way in and feel a lack of motivation due to a blockage, the answer changes depending on which it is.
Scott
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