Provide the full names and emails of unpaid research assistants.
full names = f_name, l_name; research assistant refers to the student who serves for research where the abbreviation is RA; unpaid research assistant refers to salary = 'free';
SELECT T2.f_name, T2.l_name, T2.email FROM RA AS T1 INNER JOIN student AS T2 ON T1.student_id = T2.student_id WHERE T1.salary = 'free'