Caffeine is great for concentration, improving attention, and focus. The downside is increased heart rate, blood pressure, and anxious / jittery feelings. 

L-Theanine is a relaxant that can reduce heart rate and blood pressure, promote alpha brain waves (associated with relaxation) and provide balance for the brain to avoid jitters. 

But combined, caffeine and L-theanine work better together. 

You can concentrate on your work more thoroughly, be more creative, and achieve more success with the absence of headaches, crashes, and anxiety. The combination gives you the power to perform better than taking either alone.