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Superheating and supercooling effects are characteristic kinetic processes in firstorder phase transitions, and asymmetry between them is widely observed. In materials where electronic and structural degrees of freedom are coupled, a wide, asymmetric hysteresis may occur in the transition between electronic phases. Structural defects are known to seed heterogeneous nucleation of the phase transition, hence reduce the degree of superheating and supercooling. Here we show that in the metal-insulator transition of single-crystal VO 2 , a large kinetic asymmetry arises from the distinct spatial extension and distribution of two basic types of crystal defects: point defects and twin walls.Nanometer-thick twin walls are constantly consumed but re-generated during the transition to the metal phase, serving as dynamical heterogeneous nucleation seeds and eliminating superheating; On the other hand, the transition back to the insulator phase relies on nucleation at point defects because twinning is structurally forbidden in the metal phase, leading to a large supercooling. By controlling the formation, location and extinction of these defects, the kinetics of the phase transition might be externally modulated, offering possible routes toward new memory and logic device technologies.3

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