Mott transition is a metal-insulator transition (MIT) due to strong electron correlations. When the ratio between the on-site Coulomb repulsion U and the bandwidth W is larger than a certain critical value, the electron hopping between neighboring sites is prohibited. One could realize Mott transition by controlling either the filling or the bandwidth of the Hubbard bands. In filling control, partially-filled Hubbard band is created by doping, as in the cuprate high temperature superconductors. In bandwidth control, W is tuned against U by applying physical or chemical pressure. So far, there is no direct observation of the electronic structure evolution across the bandwidth-control Mott transition (BCMT). NiS2−xSex is a prototypical BCMT system. Due to its non-half-filled multiband nature, its BCMT is more sophisticated than those described by the half-filled single-band Hubbard model, and a consistent andcomprehensive understanding has not been achieved.



Figure 1: 3-dimensional electronic structure of NiS2-xSex and its evolution across the bandwidth-control Mott transition.

With the help of soft X-ray angle-resolved photoemission spectroscopy, we reveal the 3-dimensional electronic structure of NiS2-xSex (Fig. 1(a)-1(c)). We have presented the first direct demonstration and main electronic characters of the BCMT in this multiband non-half-filled system. We found that the Fermi surface volume is unaffected by sulfur doping in the PM and AFM phase, and the bare bandwidth is just moderately narrowed. The increased correlations transfer the spectral weight into higher binding energies, reduce the coherent bandwidth (Fig. 1(f)), suppress the coherent spectral weight (Fig. 1(g)), and eventually lead to the Mott transition that is characterized by a divergent effective quasiparticle mass and a depleted coherent weight at EF. Moreover, the insulating phase is characterized by finite incoherent spectral weight at EF without opening a charge gap. These results deepen our understanding of Mott transition in general.

The photoemission data were taken at the Advanced Resonant Spectroscopies (ADRESS) beam line of the Swiss Light Source (SLS). This work is supported in part by the National Science Foundation of China and National Basic Research Program of China (973 Program) under the grant Nos. 2012CB921400, 2011CB921802, 2011CBA00112.

Ref.
H. C. Xu, Y. Zhang, M. Xu, R. Peng, X. P. Shen, V. N. Strocov, M. Shi, M. Kobayashi, T. Schmitt, B. P. Xie, and D. L. Feng*, Direct Observation of the Bandwidth Control Mott Transition in the NiS2−xSex Multiband System, Physical Review Letters 112, 087603 (2014). link