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arXiv:2310.10153 · Physical review. B./Physical review. B

Photoinduced η -pairing correlation in the Hubbard ladder

Ryota Ueda, Kazuhiko Kuroki, Tatsuya Kaneko

Published Feb 12, 2024• 5 citations
arXivHTMLPDF

Abstract

We investigate the staggered correlation of the on-site pairs, the so-called $\ensuremath{\eta}$-pairing correlation, induced by pump electric fields in the Hubbard model on the ladder lattice. Employing the time-evolution method based on exact diagonalization, we compute the photoinduced $\ensuremath{\eta}$-pairing correlation with different strengths of the interchain hopping. When the pump field is polarized along the chain direction, the $\ensuremath{\eta}$-pairing correlation is noticeably induced in the weakly coupled double-chain region, where the pairing property is similar to the $\ensuremath{\eta}$ pairing in the single-chain Hubbard model. On the other hand, the pump field polarized along the rung direction prominently induces the $\ensuremath{\eta}$-pairing correlation in the strong interchain coupling regime, where the pair correlations developing across rungs strongly contribute to the $\ensuremath{\eta}$ pairing. Based on the local rung approximation, which is valid in the strong interchain coupling regime, we discuss the origin of the photoinduced pair.

DOI
10.1103/physrevb.109.075122

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We investigate the staggered correlation of the on-site pairs, the so-called \ensuremathη\ensuremath{\eta}\ensuremathη-pairing correlation, induced by pump electric fields in the Hubbard model on the ladder lattice. Employing the time-evolution method based on exact diagonalization, we compute the photoinduced \ensuremathη\ensuremath{\eta}\ensuremathη-pairing correlation with different strengths of the interchain hopping. When the pump field is polarized along the chain direction, the \ensuremathη\ensuremath{\eta}\ensuremathη-pairing correlation is noticeably induced in the weakly coupled double-chain region, where the pairing property is similar to the \ensuremathη\ensuremath{\eta}\ensuremathη pairing in the single-chain Hubbard model. On the other hand, the pump field polarized along the rung direction prominently induces the \ensuremathη\ensuremath{\eta}\ensuremathη-pairing correlation in the strong interchain coupling regime, where the pair correlations developing across rungs strongly contribute to the \ensuremathη\ensuremath{\eta}\ensuremathη pairing. Based on the local rung approximation, which is valid in the strong interchain coupling regime, we discuss the origin of the photoinduced pair.
Employing the density-matrix renormalization group technique in the matrix-product-state representation, we investigate the photoexcited superconducting correlations induced by the η-pairing mechanism in the half-filled Hubbard chain.We estimate the characteristic pair correlation function and verify the accuracy of our numerical results by comparison with exact-diagonalization data for small systems.The optimal parameter set of the pump that most enhances the η-pair correlations, is calculated in the strong-coupling regime.For such a pump, we explore the possibility of quasi-long-range order.
Photoinduced carrier dynamics in a correlated electron system on a coupled two-leg ladder lattice are studied. The two-leg ladder Hubbard model is analyzed utilizing the exact diagonalization method based on the Lanczos algorithm in finite-size clusters. In order to reveal the transient carrier dynamics after photoirradiation, we calculate the low-energy components of the hole kinetic energy, the pair-field correlation function, the optical conductivity spectra, and other characteristics. It is shown that the photoinduced metallic-like state appears in a half-filled Mott insulating state, while the low-energy carrier motion is suppressed by photoirradiation in the case of hole-doped metallic states. These photoinduced changes in the electron dynamics are associated with changes in the carrier-pair coherence; they are not attributed to a naive thermalization, and are associated with a ladder-lattice effect instead. Based on the numerical results, optical control of hole pairs using double-pulse pumping is demonstrated. Implications for recent optical pump-probe experiments are presented.
Real-time dynamics techniques have proven increasingly useful in theoretically and experimentally understanding strongly correlated systems. By employing unbiased time-resolved exact diagonalization, we study pump dynamics in the two-dimensional plaquette Hubbard model, where distinct hopping integrals th{t}_{h}th​ and ${t}_{h}^{\ensuremath{'}}$ are present within and between plaquettes. In the intermediate coupling regime, a significant enhancement of ddd-wave superconductivity is observed and compared with that obtained by simply examining expectation values with the eigenstates of the Hamiltonian. Our work provides a further understanding of superconductivity in the Hubbard model, extends the description of the pairing amplitude to the frequency-anisotropy plane, and offers a promising approach for experimentally engineering emergent out-of-equilibrium states.
By employing unbiased numerical methods, we show that pulse irradiation can induce unconventional superconductivity even in the Mott insulator of the Hubbard model. The superconductivity found here in the photoexcited state is due to the η-pairing mechanism, characterized by staggered pair-density-wave oscillations in the off-diagonal long-range correlation, and is absent in the ground-state phase diagram; i.e., it is induced neither by a change of the effective interaction of the Hubbard model nor by simple photocarrier doping. Because of the selection rule, we show that the nonlinear optical response is essential to increase the number of η pairs and thus enhance the superconducting correlation in the photoexcited state. Our finding demonstrates that nonequilibrium many-body dynamics is an alternative pathway to access a new exotic quantum state that is absent in the ground-state phase diagram, and also provides an alternative mechanism for enhancing superconductivity.
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By employing the time-dependent exact diagonalization method, we investigate the photoexcited states of the excitonic insulator in the extended Falicov-Kimball model (EFKM). We here show that the pulse irradiation can induce the interband electron-electron pair correlation in the photoexcited states, while the excitonic electron-hole pair correlation in the initial ground state is strongly suppressed. We also show that the photoexcited states contain the eigenstates of the EFKM with a finite number of interband electron-electron pairs, which are responsible for the enhancement of the electron-electron pair correlation. The mechanism found here is due to the presence of the internal SU(2) pairing structure in the EFKM and thus it is essentially the same as that for the photoinduced $\ensuremath{\eta}$ pairing in the repulsive Hubbard model reported recently [T. Kaneko et al., Phys. Rev. Lett. 122, 077002 (2019)]. This also explains why the nonlinear optical response is effective to induce the electron-electron pairs in the photoexcited states of the EFKM. Furthermore, we show that, unlike the $\ensuremath{\eta}$ pairing in the Hubbard model, the internal SU(2) structure is preserved even for a nonbipartite lattice when the EFKM has the direct-type band structure, in which the pulse irradiation can induce the electron-electron pair correlation with momentum $\mathbit{q}=\mathbit{0}$ in the photoexcited states. We also discuss briefly the effect of a perturbation that breaks the internal SU(2) structure.