Quantum Correlated Plasmons and Their Tunability in Undoped and Doped Mott-Insulator Cuprates

Plasmons, collective excitations of charge density, have attracted much interest in fundamental science and applications. While plasmons have been observed in metal-like phase of high-Tc superconductors, they have not been reported in the Mott-insulating form. Here, we report a new quantum correlated plasmons in ambipolar Y1–zLaz(Ba1–xLax)2Cu3Oy and optimal-doped YBa2Cu3O7–x using high-resolution spectroscopic ellipsometry. Interestingly, as functions of hole, electron-doping, and temperature, their dynamical loss-function and dielectric-functions exhibit formation of two plasmons at multiple ordered low- and high-photon energies due to on-site Cu3d Coulomb interaction and antibonding O2p–Cu3d. While the low-energy correlated plasmon transforms into a conventional-plasmon upon doping, the high-energy correlated plasmon remains. The intensity of these plasmons follows a mixture of singlet and triplet with decreasing temperature. Besides, their dephasing time decreases with increasing doping. Our result highlights the importance of charge-spin coupling in the correlated plasmons, which may potentially reveal a photon–electron interaction in the localized states of Mott-insulators.