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This article states that light can "slow down by a factor of a few hundred" when it strikes graphene. I've read elsewhere that the index of refraction of graphene is about 2.65 + 1.27i. According to the article shouldn't the refractive index be a few hundred? How is this possible? What is the phase velocity of light in graphene?

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It is a mistake in the David L. Chandler MIT News article. The research paper "Efficient plasmonic emission by the quantum Čerenkov effect from hot carriers in graphene" doi: 10.1038/ncomms11880, states that "plasmons in graphene can have an exceptionally slow phase velocity, down to a few hundred times slower than the speed of light". But plasmons are not light waves, but collective excitations of electrons (propagating at Fermi velocity). Chandler misunderstood the nature of plasmons because of the use of the term "Čerenkov effect" which is not the same as "Čerenkov light". The paper has discovered that the carriers in graphene with speed larger than that of the graphene plasmons emit "Čerenkov plasmons".

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