Quasi-Static Strain Governing Ultrafast Spin Dynamics

The quasi-static strain (QSS) is the product induced by the lattice thermal expansion after ultrafast photo-excitation. Although the QSS and thermal effects are barely distinguishable in time, they should be treated separately because of their different fundamental actions to ultrafast spin dynamics. By employing ultrafast Sagnac interferometry and the magneto-optical Kerr effect, we demonstrate quantitatively the existence of QSS and the decoupling of two effects counteracting each other in typical polycrystalline Co and Ni films. The Landau-Lifshitz-Gilbert and Kittel equations considering a magnetoelastic energy term showed that QSS, rather than the thermal energy, in ferromagnets plays a governing role in ultrafast spin dynamics. This demonstration provides an essential way to analyze ultrafast photo-induced phenomena.

Статический и динамический вклады в расщепление основного состояния Eu-=SUP=-2+-=/SUP=- в SrMoO-=SUB=-4-=/SUB=-

The EPR spectrum of an Eu^2+ impurity center in a SrMoO_4 single crystal in the temperature range T = 1.8, 111–300 K has been studied, and the temperature changes in the spin Hamiltonian parameters describing the EPR spectrum of odd europium isotopes have been determined. It is shown that small temperature changes in the diagonal parameters of the spin Hamiltonian (for odd Eu^2+ isotopes) $$b_{2}^{0}$$ ( T ) = b _2( F ) + b _2( L ) and $$P_{2}^{0}$$ ( T ) = P _2( F ) + P _2( L ) are explained by the compensation of spin–phonon contributions b _2( F ) and P _2( F ) by the contributions of the lattice thermal expansion b _2( L ) and P _2( L ). The quantities b _2( L ) and P _2( L ) that are dependent on the static lattice parameters at a given temperature, are estimated in terms of the superposition Newman model. Then, the spin–phonon b _2( F ) and P _2( F ) contributions determined by the lattice ion vibrations are separated. An analysis shows that $$b_{2}^{0}$$ ( F ) and $$P_{2}^{0}$$ ( F ) > 0, b _2( L ) and P _2( L ) < 0, and the temperature behavior of the spin–phonon contribution is well described by G. Pfister’s model of local vibrations.

Thermal Shifts and Electron-Phonon Coupling Parameters for the Three Luminescence Lines of 5DJ→7FJ in SrFCl:Sm2+ Crystal

The thermal blue shifts of three spectral lines E1(5D0→7F0), E2(5D0→7F1), and E3(5D1→7F0) in SrFCl:Sm2+ crystal are investigated by a complete expression consisting of both the static contribution due to lattice thermal expansion and the vibrational contribution owing to electron-phonon interaction. The obtained true electron-phonon coupling parameters α′ in both sign and magnitude are considerably different from the apparent electron-phonon coupling parameters α acquired in the previous paper by considering only the vibrational contribution. It is suggested that differing from the simple expression including only the vibrational contribution (many authors thought that it cannot be used to explain the thermal blue shifts), the complete expression containing both the vibrational and static contributions is effective in the studies of thermal shift (whether red shift or blue shift) and true electron-phonon coupling parameter for a spectral line in crystals.

Multi-temperature structure of thermoelectric Mg2Si and Mg2Sn

A multi-temperature structural study of Mg2Si and Mg2Sn was carried out from 100 to 700 K using synchrotron X-ray powder diffraction. The temperature dependence of the lattice parameters can be expressed as a = 6.3272 (4) + 6.5 (2) × 10−5 T + 4.0 (3) × 10−8 T 2 Å and a = 6.7323 (7) + 8.5 (4) × 10−5 T + 3.8 (5) × 10−8 T 2 Å for Mg2Si and Mg2Sn, respectively. The atomic displacement parameters (ADPs) are reported and analysed using a Debye model for the averaged U iso giving Debye temperatures of 425 (2) K for Mg2Si and 243 (2) K for Mg2Sn. The ADPs are considerably smaller for Mg2Si than for Mg2Sn reflecting the weaker chemical bonding in the Mg2Sn structure. Following the heating, an annealing effect is observed on the lattice parameters and peak widths in both structures, presumably due to changes in the crystal defects, but the lattice thermal expansion is almost unchanged by the annealing. This work provides accurate structural parameters which are of importance for studies of Mg2Si, Mg2Sn and their solid solutions.

Lattice thermal expansion of Pu1−yAmyO2−xplutonium–americium mixed oxides

Plutonium–americium mixed oxides, Pu1−yAmyO2−x, with various Am contents (y= 0.018, 0.077, 0.21, 0.49, 0.80 and 1.00) were studiedin situby high-temperature X-ray diffraction. In this study, the lattice thermal expansion of the six compounds subjected to heat treatments up to 1773 K under reconstituted air (N2+ 21% O2+ ∼5 vpm H2O) was investigated. The materials remained monophasic throughout the experiments and, depending upon the americium content, the lattice parameter of the face-centred cubic phase deviated from linear lattice expansion at elevated temperatures as a result of the progressive reduction of Am4+to Am3+.