High-harmonic spectroscopies reveal that fast energy transfer within 20 fs triggers ultrafast magnetic phase transition in Ni.

By correlating time- and angle-resolved photoemission and time-resolved transverse magneto-optical Kerr effect measurements, both at extreme ultraviolet wavelengths, we uncover the universal nature of the ultrafast photoinduced magnetic phase transition in Ni. This allows us to explain the ultrafast magnetic response of Ni at all laser fluences-from a small reduction of the magnetization at low laser fluences, to complete quenching at high laser fluences. Both probe methods exhibit the same demagnetization and recovery timescales. The spin system absorbs the energy required to proceed through a magnetic phase transition within 20 fs after the peak of the pump pulse. However, the spectroscopic signatures of demagnetization of the material appear only after ≈200  fs and the subsequent recovery of magnetization on timescales ranging from 500 fs to >70  ps. We also provide evidence of two competing channels with two distinct timescales in the recovery process that suggest the presence of coexisting phases in the material.

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Electronic structure, formation energies, transition levels, and concentration of intrinsic defects in wurtzite ZnO are investigated by the projector augmented wave method in the generalized gradient approximation. Interstitials, vacancies, and antisites at different charge states are considered. Convergence of the formation energies of various intrinsic point defects is carefully checked, and comparison with earlier results is made and discussed. Even though there exists a difference for the calculated formation energies of certain defects, our calculations also show that oxygen and zinc vacancies are the dominant intrinsic donor and acceptor defects in ZnO, indicating a consistency among results by different methods. The oxygen vacancy is not expected to be the main source of strong n-type conductivity in the unintentionally doped ZnO, due to its deep level in the bandgap, but it must be the origin of the experimentally observed visible photoluminescence band centred between 2.3 and 2.5 eV.