I am a PhD student studying in a field that I enjoy. Furthermore, I enjoy the area within the field that I am specializing in. However, my specialty generally has two major application arenas, which in some ways are disjoint. I came to the program knowing I would be working in one of them, sub-specialty A, despite having no prior experience in it. I did have experience in the other, sub-specialty B, but the faculty in my department who work in sub-specialty B either didn't have openings or didn't see me as a fit for their group (i.e. they didn't support my application).
I arrived planning to give sub-specialty A a fair shake, as I honestly knew little about it and for all I knew I'd enjoy it. After a semester, I am finding it hard to stay interested in the research, and this feeling is compounded by the frustration of feeling like I have to start over in the field. It's also tough watching my peers work on problems of sub-specialty B that I find so much more interesting than my work in sub-specialty A.
I know a PhD is the start, not the end, of a career, and that what you do during it is not what you have to do for the rest of your life, but, that being said, I do have to spend 4-6 years working with this material.
How does one handle this sort of situation? Is it appropriate to bring up my concerns with my adviser? Could trying to bring in a co-adviser from sub-specialty B be a feasible way to bridge the gap between the sub-specialties? (NOTE: It's worth mentioning that this is literally the only problem I am having with my PhD; the school is great, the cohort is great, and my adviser is great) Have other people been in these circumstances? If so, what did you do about it?