In my humble opinion, this conclusion should be substinate with some calculations as the statement its self, even if likely true, should be checked out to be true
> I don't have access to the actual paper, but it's here.
> https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301
See https://arxiv.org/abs/1612.04055 ... but as already stated: it is negative effective mass, which is also stated in the article as well