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WASHINGTON — When the Justice Department arrested the chairman of Temple University’s physics department this spring and accused him of sharing sensitive American-made technology with China, prosecutors had what seemed like a damning piece of evidence: schematics of sophisticated laboratory equipment sent by the professor, Xi Xiaoxing, to scientists in China.The schematics, prosecutors said, revealed the design of a device known as a pocket heater. The equipment is used in semiconductor research, and Dr. Xi had signed an agreement promising to keep its design a secret.But months later, long after federal agents had led Dr. Xi away in handcuffs, independent experts discovered something wrong with the evidence at the heart of the Justice Department’s case: The blueprints were not for a pocket heater.