Semiclassical instability of inner-extremal regular black holes

The construction of black hole spacetimes that are regular (singularity-free) is plagued by the ``mass inflation'' instability, a classical perturbation instability induced surface gravity at inner horizon and characterized exponentially diverging stress energy there. Recently, class ``inner-extremal'' holes was proposed possesses vanishing therefore avoids mass inflation, while still maintaining separation nonzero outer-horizon gravity. However, when semiclassical effects taken into account, it found an inner-horizon remains for generic inner-extremal formed from collapse. This divergence analyzed perspective both effective Hawking temperature renormalized stress-energy tensor, its origin genericity examined in detail.