18535
The technique is simple and reliable , and the optical quality of the crystals is extremely high , as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35 % even at room temperature , suggesting a virtual absence of defects .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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