Excitons at the phase transition of 2D hybrid perovskites by Ziegler, JD, Lin, K-Q, Meisinger, B, Zhu, X, Kober-Czerny, M, Nayak, PK, Vona, C, Taniguchi, T, Watanabe, K, Draxl, C, Snaith, HJ, Lupton, JM, Egger, DA, Chernikov, A