QUANTUM MATERIALS AND DEVICES SEMINAR: Non-Hermitian quantum phenomena in correlated systems
Recently, non-Hermitian (NH) quantum phenomena have attracted a great deal of attention. We here address the following three issues in this context.
1. Non-Hermitian Kondo Effect :
We first investigate the Kondo effect in an open quantum system motivated by recent experiments with ultracold alkaline-earth atoms. Due to inelastic collisions and the associated atom losses, this system is described by a NH extension of the Kondo problem. We show that the non-Hermiticity induces anomalous reversion of renormalization- group, leading to a unique quantum phase transition. Furthermore, by exactly solving the non-Hermitian Kondo Hamiltonian, we obtain the critical line consistent with the renormalization-group flow.
2. Non-Hermitian Tomonaga-Luttinger Liquids :
We next demonstrate the universal properties of dissipative Tomonaga-Luttinger (TL) liquids by calculating correlation functions and performing finite-size scaling analysis of a non-Hermitian XXZ spin chain as a prototypical model in one-dimensional open quantum many-body systems. Our analytic calculation is based on effective field theory with bosonization, finite-size scaling approach in conformal field theory, and the Bethe-ansatz solution. We uncover that the model in the massless regime with weak dissipation belongs to the universality class characterized by the complex-valued TL parameter, which is related to a complex generalization of c=1 conformal field theory.
3. Non-Hermitian Fermionic Superfluidity :
Finally, we address a NH BCS superfluidity with a complex-valued interaction arising from inelastic scattering between fermions. We find unconventional phase transitions unique to NH systems: the superfluidity shows reentrant behavior with increasing dissipation, as a consequence of non-diagonalizable exceptional points in the quasiparticle Hamiltonian for weak attractive interactions. For strong attractive interactions, the superfluid gap is enhanced by dissipation due to an interplay between the BCS-BEC crossover and the quantum Zeno effect.
 M. Nakagawa, N. Kawakami and M. Ueda, Phys. Rev. Lett. 121, 203001 (2018)
 K. Yamamoto, M. Nakagawa, M. Tezuka, M. Ueda, and N.Kawakami, Phys. Rev. B 105, 205125 (2022); K. Yamamoto and N. Kawakami, arXiv:2207.04395
 K. Yamamoto, M. Nakagawa, K. Adachi, K. Takasan, M. Ueda and N. Kawakami, Phys. Rev. Lett. 123, 123601 (2019); K. Yamamoto, M. Nakagawa, N. Tsuji, M. Ueda, and N. Kawakami, Phys. Rev. Lett. 127, 055301 (2021）
Host: Miguel A. CazalillaZOOM: https://dipc-org.zoom.us/j/97943380475YouTube: https://youtu.be/fRp546YNuzg
Hybrid Seminar: Donostia International Physics Center
Norio Kawakami, Department of Physics, Kyoto University