Anomalous transition with modulation around band degeneracies

A resonance transition occurs between two states when the system is under an oscillatory driving field with a proper frequency. It is commonly believed that frequency must be matched at the resonance transition. Here we show that the above condition is not necessarily satisfied when the system under driving encloses nontrivial band degeneracies, such as a Dirac point. We consider a kagome lattice under the modulation of an artificial gauge field and show the inequivalence of the modulation frequency and energy difference between states at transitions. Meanwhile, our analytical formula reveals that the transitions exhibit chirality around the Dirac cone. When the system Hamiltonian under modulation circles around a higher-order charged band degeneracy, the fundamental one-photon transition is absent. These peculiar properties can be explained with the effective Hamiltonian under n-fold rotational symmetry.

Automated summary

A resonance transition occurs between two states under an oscillatory driving field with a proper frequency. The conventional belief that the frequency must match at the resonance transition is challenged when the system under driving has nontrivial band degeneracies. We study a kagome lattice under the modulation of an artificial gauge field and find the inequivalence between the modulation frequency and energy difference at transitions. The transitions exhibit chirality around the Dirac cone, and the fundamental one-photon transition is absent when the system Hamiltonian circles around a higher-order charged band degeneracy.