Speaker
Description
Nickel oxide, a type-II antiferromagnet with $T_{{N\acute{e}el}}=523.6$ K, is a model system not only for a charge-transfer insulator but also a strongly correlated oxide. Despite of long-standing research its electronic structure is still object of great experimental as well as theoretical interest. The strong electronic correlation in NiO causes the appearance of lower and upper Hubbard bands with an energetic separation $U$ which are formed by Ni 3$d$ electrons. On the other hand, the presence of oxygen ligands leads to the formation of a charge-transfer gap (CTG) of the energy size $\Delta < U$.
In this contribution we focus on the electron dynamics after exciting electrons across the CTG of NiO ultrathin films. These films of thicknesses from 1 to 20 monolayers (ML) are grown on Ag(001) via molecular beam epitaxy. Using two-photon photoemission spectroscopy (2PPE) we find that the CTG ($\Delta\simeq3.8$ eV) is formed in NiO films of 4 ML and beyond. Time-resolved measurements reveal an unexpectedly short electronic decay within less than 15 fs upon electron excitation across the CTG. This decay is coupled to the appearance of a coherent low-energy excitation which is long-living, thickness-, and temperature-dependent: periodic intensity modulations in the time-resolved 2PPE signal just above the vacuum cut-off are found for NiO film thicknesses $\geq 6$ ML. These intensity modulations with a frequency of about 4.1 THz (∼17 meV) are visible for temperatures below 470 K for 10 ML (560 K for 20 ML). Their dephasing times are as long as 300 to 450 fs at room temperature and decrease with increasing temperatures.
The observed intensity oscillations are discussed in the framework of fundamental excitations in solids such as excitons, phonons, or magnons, and lead to coherent population of multi-electron excitations ($dd$ excitations) within the NiO band gap that are subsequently probed.
