Magnetic properties and magnetic structures ofTbBaMn2O5.75: Possible observation of unconventional polaron trimers

Abstract

The magnetism of the oxygen deficient, A-site layered perovskite TbBaMn 2 O 5.75 has been studied by magnetic susceptibility vs. temperature measurements and neutron powder diffraction. Four magnetic phases have been identified. Above 457 K a charge melted state exists where ferromagnetic double exchange interactions between the Mn ions dominate. Below 457 K charge ordering occurs and the exchange interactions become primarily antiferromagnetic. The magnetic moment in this phase is too large to be consistent with a charge ordering of Mn 3+ and Mn 4+ but could be explained by having 2 e g electrons being shared among 3 Mn centers, causing them to act as a single paramagnetic unit. This suggests that S = 11/2 (Mn 3.33+ ) 3 polaron trimers may be present along with Mn 3+ . Below 203 K long range ordering of the moments on the Mn ions occurs. Neutron powder diffraction data at 52 K reveal a magnetic structure containing an ↑↑↑↓ pattern of spins which is consistent with the formation of tripolarons. Below 11 K the Tb 3+ ions also undergo long range magnetic ordering.

Spin polarization of electrons by ultraintense lasers

“…To describe the spin polarization dynamics of electrons in the rotating electric field (indeed in any arbitrary electromagnetic field) we need to use a proper non-precessing spin polarization basis ζ for which dζ/dτ = 0 [24], defined in the rest frame of the electron, where τ = t/γ is the proper time. Spin-up and spin-down electron states, defined with respect to that basis, do not mix.…”

Extended locally constant field approximation for nonlinear Compton scattering

“…Returning now to the low-s behaviour, we also need to make sure that the rate stays positive. (This is not fulfilled even by the full QED "rate" dP/dϕ because of quantum interference effects, which frequently give negative contributions [30]. However, our interest is not in directly approximating QED observables, which are readily calculable by other means [31,32], but in generating an improved rate suitable for eventual implementation in Monte Carlo (MC) codes.)…”

Completeness and orthonormality of the Volkov states and the Volkov propagator in configuration space

“…(50) and (51). In fact, in order to follow the method outlined below, it is more convenient to start by assuming a finite quantization volume, to be a large cube of side L and volume V = L 3 centered at the origin of the coordinates.…”

“…It is worth pointing out, in fact, that for optical laser facilities as those mentioned above, the parameter ξ 0 exceeds unity already at laser intensities of the order of 10 18 W/cm 2 . Apart from the pioneering papers cited above and by also referring the reader to the reviews [13][14][15][16][17], we mention here several investigations on nonlinear Compton scattering [18][19][20][21][22][23][24][25][26][27][28][29][30][31], on nonlinear Breit-Wheeler pair production [18,[32][33][34][35][36][37][38][39], on nonlinear Bethe-Heitler pair production [40][41][42][43][44][45][46], on electron-positron annihilation [47], and on higher-order processes like nonlinear double Compton scattering [48][49][50][51] and trident pair production [52,53]. In order to calculate a second-order process like nonlinear double Compton scattering, the corresponding Feynman propagator in a plane-wave field (Volkov propagator) has to be employed…”