QUANTUM MATERIALS AND DEVICES SEMINAR: Non-Hermitian quantum phenomena in correlated systems

DIPC Seminars

Norio Kawakami, Department of Physics, Kyoto University
Hybrid Seminar: Donostia International Physics Center
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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__ [1]: 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__ [2]: 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__ [3]: 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. [1] M. Nakagawa, N. Kawakami and M. Ueda, _Phys. Rev. Lett. **121** , 203001 (2018)_ [2] 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_ [3] 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. Cazalilla ZOOM: YouTube: __