Holstein polaron in a pseudospin-$1$ quantum spin Hall system: first and second order topological phase transitions
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2024
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| author | Bhattacharyya, Kuntal Lahiri, Srijata Islam, Mijanur Basu, Saurabh |
| author_facet | Bhattacharyya, Kuntal Lahiri, Srijata Islam, Mijanur Basu, Saurabh |
| contents | We theoretically propose the occurrence of a quantum spin Hall (QSH) and a second order topological phase transition (TPT) driven by electron-phonon (e-p) coupling in a pseudospin-$1$ fermionic system on an $α$-$T_3$ lattice. Our model is formulated in the spirit of the Kane-Mele model modified by the Holstein Hamiltonian. The Lang-Firsov approach is employed to describe polarons reasonably well in the anti-adiabatic (high frequency) limit and to obtain an effective electronic Hamiltonian. It is shown that the system possesses topologically nontrivial phases up to a critical e-p coupling, $λ_c$ and are characterized by the helical QSH edge states along with a non-zero $\mathbb{Z}_2$ invariant for a certain range of $α$. The topological phase vanishes beyond $λ_c$ and is accompanied by a bulk gap closing transition at $λ_c$, manifesting a TPT. We observe a more intriguing phenomenon for higher values of $α$, where the system exhibits TPTs supported by two distinct gap closing transitions at $λ_{c_1}$ and $λ_{c_2}$, while a slim region at slightly lower values hosts a semi-metallic signature below $λ_{c_1}$. Subsequently, to explore more intricate features, we introduce a time reversal symmetry breaking magnetic field to trigger the formation of a second order topological phase. The magnetic field, by construction causes a boundary dependent gapping out of the edge states, consequently giving rise to robust corner modes in a tailored open boundary conditions. We justify the formation of the higher order phase by employing an appropriate invariant, namely the projected spin Chern number. Finally, we show that the e-p coupling significantly influences the corner modes (and also the real space energy bandstructure), corroborating a higher order TPT as we tune $λ$ beyond a critical value for a given value of $α$. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2408_14367 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Holstein polaron in a pseudospin-$1$ quantum spin Hall system: first and second order topological phase transitions Bhattacharyya, Kuntal Lahiri, Srijata Islam, Mijanur Basu, Saurabh Mesoscale and Nanoscale Physics We theoretically propose the occurrence of a quantum spin Hall (QSH) and a second order topological phase transition (TPT) driven by electron-phonon (e-p) coupling in a pseudospin-$1$ fermionic system on an $α$-$T_3$ lattice. Our model is formulated in the spirit of the Kane-Mele model modified by the Holstein Hamiltonian. The Lang-Firsov approach is employed to describe polarons reasonably well in the anti-adiabatic (high frequency) limit and to obtain an effective electronic Hamiltonian. It is shown that the system possesses topologically nontrivial phases up to a critical e-p coupling, $λ_c$ and are characterized by the helical QSH edge states along with a non-zero $\mathbb{Z}_2$ invariant for a certain range of $α$. The topological phase vanishes beyond $λ_c$ and is accompanied by a bulk gap closing transition at $λ_c$, manifesting a TPT. We observe a more intriguing phenomenon for higher values of $α$, where the system exhibits TPTs supported by two distinct gap closing transitions at $λ_{c_1}$ and $λ_{c_2}$, while a slim region at slightly lower values hosts a semi-metallic signature below $λ_{c_1}$. Subsequently, to explore more intricate features, we introduce a time reversal symmetry breaking magnetic field to trigger the formation of a second order topological phase. The magnetic field, by construction causes a boundary dependent gapping out of the edge states, consequently giving rise to robust corner modes in a tailored open boundary conditions. We justify the formation of the higher order phase by employing an appropriate invariant, namely the projected spin Chern number. Finally, we show that the e-p coupling significantly influences the corner modes (and also the real space energy bandstructure), corroborating a higher order TPT as we tune $λ$ beyond a critical value for a given value of $α$. |
| title | Holstein polaron in a pseudospin-$1$ quantum spin Hall system: first and second order topological phase transitions |
| topic | Mesoscale and Nanoscale Physics |
| url | https://arxiv.org/abs/2408.14367 |