Both (Sorry). References to a study (or more accurately the paper summarizing the results, see note at bottom) generally use the present tense, while references to actual researchers generally use past. And unless your research subject is English language, you will see variations.
To explain the examples you gave:
Firstly, "are" in this context means "exist". So one could rewrite the sentence as "there exist studies that...". Sencondly, the studies* still exist, and they still currently (as of writing) summarize opinions on mining work. Incidentally, they will always summarize opinions on mining work; it's just that the opinions will eventually no longer be current as of time of reading. Thus, because the studies still exist, we use the present tense.
Zhou and Chaovalit are the actors here. They are (presumably) no longer performing sentiment classification, thus the action is in the past, and we use past tense. Actually, if Zhou and Chaovalit are continuing to work on this subject, we can use the past tense as we are not referring to their current work at all, but their past, published work (and findings, see note below)
*One thing to note: "Study"(singular form of "studies") as a noun, in this context, can refer to both the actual study itself(the research, testing, test subjects, etc.) and to the report(s) detailing the study and its results. The second option is probably intended here.