Is it necessary to send a "thank you" email to reseach commitee after on-campus interview?
In my experience this is somewhat uncommon, but not unheard of.  When a candidate did thank everyone they met with it made a positive impression on me and at least some other faculty I talked to.  So if you'd like to send such a note I'd say go ahead and do so, but you should not feel obligated.  I'd suggest you keep the note very short though and not say anything weird.