Angle-resolved two-photon photoemission of Mott insulator

Takami Tohyama

Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

The charge excitation across the Mott gap is controlled by both the lower Hubbard band (LHB) and the upper Hubbard band (UHB). It was theoretically proposed that in two-dimensional cuprates, the top of LHB (more precisely the Zhang-Rice singlet band) is at k=(π/2, π/2), while the bottom of UHB is at (π,0) and (0,π). The location of the top of LHB has clearly been observed by angle-resolved photoemission (ARPES). However, the bottom of UHB has not been directly observed yet. As a new technique to see directly the location of the bottom of UHB in the cuprates, we propose angle-resolved two-photon photoemission (AR-2PPES) measurement [1]. The AR-2PPES spectrum is calculated for small clusters by using the Lanczos and conjugate-gradient techniques. We consider two cases of the pump photon: One is the case that the pump photon induces an excitation from the LHB to UHB, while the other is an excitation from nonbonding oxygen band to UHB. In the former, we find two kinds of AR-2PPES spectra called "ω" and "2ω" processes, where the "ω" and "2ω" processes detect the states in UHB and LHB, respectively. However, the location of the bottom of UHB is hard to be identified by this "ω" process because of diffusive features in the spectra. In the latter case, we can clearly find the location of the bottom of UHB. In addition, new spectra that cannot be seen in the inverse photoemission spectrum emerge in the AR-2PPES with pump photon exciting an electron from the nonbonding to UHB. The origin of the new spectra is attributed to the spin excitation expected at half filling. These theoretical results will be useful for help for analyses of AR-2PPES experimental data in the near future. [1] H. Onodera et al, cond-mat/0503267.