To begin with, even though Ran was originally in my department, why did she go to Yumemi's research lab?

Please ask her yourself.

She said "I want to broaden my views", but there's still so much more she can learn from me.

She wants to broaden her views...is that it?

That girl has been disagreeing with my study method for a long time. I wonder if that's the reason?

And what kind of method is that?

Sleep learning.

That's rather questionable.