Passage: In Stuttgart, Barton steals iridium needed to stabilize the Tesseract's power while Loki causes a distraction, leading to a brief confrontation with Rogers, Stark, and Romanoff that ends with Loki's surrender. While Loki is being escorted to S.H.I.E.L.D., Thor, his adoptive brother, arrives and frees him, hoping to convince him to abandon his plan and return to Asgard. After a confrontation with Stark and Rogers, Thor agrees to take Loki to S.H.I.E.L.D.'s flying aircraft carrier, the Helicarrier. Upon arrival, Loki is imprisoned while Banner and Stark attempt to locate the Tesseract.

Question: What are Banner and Stark searching for?
Tesseract