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State-of-the-art theoretical methods fail in describing the optical absorption spectrum, band gap, and optical onset of Cu(2)O. We have extended a recently proposed self-consistent quasiparticle approach, based on the GW approximation, to the calculation of optical spectra, including excitonic effects. The band structure compares favorably with our present angle-resolved photoemission measurements. The excitonic effects based on these realistic band structure and screening provide a reliable optical absorption spectrum, which allows for a revised interpretation of its main structures.

Magnetization dynamics in alloys of ferrimagnetic CoGd have been studied in the vicinity of the magnetization and angular momentum compensation point as a function of alloy composition and temperature. In agreement with standard mean-field treatments of the dynamics of the total magnetization we observe an increase of the precessional frequency and the effective damping parameter near the angular momentum compensation point. We demonstrate the consistency of the magnetization dynamics extracted from frequency domain methods such as ferromagnetic resonance and time resolved laser pump-probe measurements. DOI: 10.1103/PhysRevB.74.134404 PACS number͑s͒: 75.40.Gb, 75.50.Gg, 76.50.ϩg Transition metal ͑TM͒ rare earth ͑RE͒ ferrimagnets are ideal canonical systems to probe magnetization dynamics. Typically, TM-RE alloys are nearly amorphous materials. The TM sublattice is antiferromagnetically ͑AF͒ coupled to the RE sublattice. When the coupling is strong, as, e.g., in CoGd, there are two transition temperatures, the magnetization compensation temperature T M where M Gd = M Co , and the angular momentum compensation temperature T L , where M Gd / ␥ Gd = M Co / ␥ Gd , and ␥ is the gyromagnetic ratio. These temperatures are sensitive functions of the relative concentration. At the magnetic compensation temperature, applied magnetic fields cannot couple to the magnetization to alter its energy since M Gd − M Co = M eff = 0. Angular momentum is quenched at the angular momentum compensation point, where the AF coupled sublattices gyrate 180°out of phase about the magnetic field. Studying the dynamics in ferrimagnetic systems are complicated by these tightly coupled AF sublattices. As T L is approached from low temperatures, the phenomenological mean-field damping parameter ␣ eff which governs how fast the system as a whole dissipates energy increases quickly, and the gyromagnetic frequency changes sign as the angular momentum of the dominant sublattice changes from Gd to Co. An ideal ferrimagnet should dissipate angular momentum instantaneously at T L .1,2 CoGd was chosen for this study because T M and T L are very close to each other, and the intrinsic orbital moment of Gd is essentially zero, thereby eliminating additional loss channels due to spin-orbit coupling. 3 We compare experimental results obtained by a frequency domain method used to study the dynamics of the total magnetization of M eff -namely, ferromagnetic resonance ͑FMR͒-to time domain ultrafast laser pump/probe experiments.The most straight forward method to excite magnetization dynamics uses strong magnetic field pulses that couple directly to the magnetization ͑spin͒.4,5 These field pulses are typically produced by external sources. However, these methods cannot excite the magnetization at the magnetization compensation point in a ferrimagnet since there is no net magnetic moment one can couple to. Another method to excite spin-systems employs ultrashort laser pulses that alter the magnetic system by heating across a critical temperature ͑Curie, Néel, ...

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Synchrotron radiation time structure is becoming a common tool for studying dynamic properties of materials. The main limitation is often the wide time domain the user would like to access with pump-probe experiments. In order to perform photoelectron spectroscopy experiments over time scales from milliseconds to picoseconds it is mandatory to measure the time at which each measured photoelectron was created. For this reason the usual CCD camera-based two-dimensional detection of electron energy analyzers has been replaced by a new delay-line detector adapted to the time structure of the SOLEIL synchrotron radiation source. The new two-dimensional delay-line detector has a time resolution of 5 ns and was installed on a Scienta SES 2002 electron energy analyzer. The first application has been to characterize the time of flight of the photoemitted electrons as a function of their kinetic energy and the selected pass energy. By repeating the experiment as a function of the available pass energy and of the kinetic energy, a complete characterization of the analyzer behaviour in the time domain has been obtained. Even for kinetic energies as low as 10 eV at 2 eV pass energy, the time spread of the detected electrons is lower than 140 ns. These results and the time structure of the SOLEIL filling modes assure the possibility of performing pump-probe photoelectron spectroscopy experiments with the time resolution given by the SOLEIL pulse width, the best performance of the beamline and of the experimental station.

We report here on the electronic structure of electron-doped half-metallic ferromagnetic perovskites such Sr2−xLaxFeMoO6 (x=0-0.6) as obtained from high-resolved valence-band photoemission spectroscopy (PES). By comparing the PES spectra with band structure calculations, a distinctive peak at the Fermi level (EF ) with predominantly (Fe+Mo) t ↓ 2g character has been evidenced for all samples, irrespectively of the x values investigated. Moreover, we show that the electron doping due to the La substitution provides selectively delocalized carriers to the t ↓ 2g metallic spin channel. Consequently, a gradual rising of the density of states at the EF has been observed as a function of the La doping. By changing the incoming photon energy we have shown that electron doping mainly rises the density of states of Mo parentage. These findings provide fundamental clues for understanding the origin of ferromagnetism in these oxides and shall be of relevance for tailoring oxides having still higher TC .

The spin transition in LaCoO_{3} has been investigated using density-functional theory in combination with dynamical mean-field theory employing continuous time quantum Monte Carlo and exact diagonalization impurity solvers. Calculations on the experimental rhombohedral atomic structure with two Co sites per unit cell show that an independent treatment of the Co atoms results in a ground state with strong charge fluctuations induced by electronic correlations. Each atom shows a contribution from either a d^{5} or a d^{7} state in addition to the main d^{6} state. These states play a relevant role in the spin transition which can be understood as a low spin-high spin (LS-HS) transition with significant contributions (~10%) to the LS and HS states of d^{5} and d^{7} states, respectively. We report spectra as well as optical conductivity data for all cases. A thermodynamic analysis reveals a significant kinetic energy gain through introduction of charge fluctuations, which in addition to the potential energy reduction lowers the total energy of the system.

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