The Justice Department has dropped charges against a Chinese-American professor accused of sharing technology with China, after officials misidentified key evidence in the case. In May, FBI agents stormed the home of Temple University physics professor Xi Xiaoxing — an American citizen — and arrested him on accusations of sending protected blueprints for a device called a pocket heater to scientists in China. But after Xi had been placed on administrative leave, lost his chairmanship of the physics department and faced what he described to The New York Times as a “nightmare,” it turned out the blueprints didn’t actually show a pocket heater. The case is seen as part of a widening crackdown on U.S. citizens of Chinese descent.