Two universality classes for the many-body localization transition

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We provide a systematic comparison of the many-body localization transition in spin chains with nonrandom quasiperiodic vs.

• random fields.
• We find evidence that these belong to two separate universality classes: one dominated by "intrinsic" intra-sample randomness, and the second dominated by external inter-sample quenched randomness.
• We show that the effects of inter-sample variations are strongly growing, but not yet dominant, at the system sizes probed by exact-diagonalization studies on random models.