Dual CFT on Nariai limit for Kerr-Newman-dS and Kerr-Sen-dS black holes

In this work, we study the Kerr-Newman-de Sitter black hole in the Nariai limit. Nariai limit denotes the coincidence between event horizon and the cosmological horizon. We show that the near-horizon Kerr-Newman-de Sitter black hole in Nariai limit is a fiber over AdS$_2$ with an appropriate coordinate transformation, instead of fiber over dS$_2$. Hence, we can compute the associated central charge and CFT temperature by using the Kerr/CFT method. It is remarkably exhibited that through Cardy's growth of states, the Bekenstein-Hawking entropy on cosmological horizon is reproduced. Moreover, we show that the radial equation of the quantum scalar field on this charged rotating background in Nariai limit can be portrayed in quadratic Casimir operator form with $SL(2,R)\times SL(2,R)$ isometry. We also compute the corresponding thermodynamic quantities from CFT to find the absorption cross-section and real-time correlator. We also extend the dual CFT computation in Nariai limit for Kerr-Sen-dS black hole solution. The results show that Nariai/CFT correspondence is also valid for Kerr-Sen-dS solution.