Magnetic exciton

On the other hand, Mitani et al. (1975) described, from their magnetic circular absorption results and their Faraday rotation spectra, a magnetic exciton model for the low energy absorption band of EuS. 

The experimental situation is more favourable if we consider magnetic exciton bands. They result from magnetic CEF excitations coupled by an interionic (Heisenberg) exchange... 

Fig. 29. Mixing of f, and magnetic excitons and the [101] transverse acoustic phonons in a magnetic field. A model parameter to(q )IA = 3 where q = V2 jrla was used (Thalmeier and Fulde 1975).

Experimentally this effect has been found in dhcp Pr (Houmann et al. 1979) and in PrAlj (Purwins et al. 1976). The latter has cubic site symmetry and the ground and first excited states are Fj (OK) and 7 (27.4K). It orders ferromag-netically at = 33 K. At low temperatures (F T ) only three of the field-split F3-F4 magnetic excitons are seen (fig. 30) and those with 7+ polarization show a strong anti-crossing effect with a transverse acoustic phonon mode in the [001] direction. Detailed model calculations for this mixed-mode spectrum were performed by Aksenov et al. (1981) using an equation of motion approach. 

X Y(T) impurity or dopant concentration Yoshida function O) (q) frequency magnetic exciton dispersion... 

Here x (q, co) and the single ion dynamical susceptibility u( >) are tensors in both sublattice (A, B) and transverse xy) Cartesian coordinates. The poles of eq. (84) determine the collective excitations of 5f-local moments. In the paramagnetic phase they are given by the magnetic exciton dispersion... 

The physical origin and nature of magnetic excitons is illustrated in the inset of fig. 38 and described in the caption. When is approached the exciton energy at Q (PM zone boundary or AF zone center) becomes soft as a precursor to the appearance of the AF moment according to... 

Fig. 39. INS spectrum for various temperatures above and below 7 = 1.8 K (Sato et al., 2001). The momentum transfer was varied around the AF point q = 0,0, qi). The solid line is the result of a calculation using eq. (87) from which the magnetic exciton energy ct> (q) is determined for every q. The result is shown in the inset for 7 = 2.5 K (crosses) and 7 = 0.15 K (circles).

Fig. 41. Calculated tunneling DOS Ng using isotropic Eliashberg equations and the retarded potential of eq. (89). Nn is the DOS in the normal state, (a) The various lines correspond to temperatures T/Tq = 0.25,0.6,0.9,0.95 from top to bottom at eV/A = 2. (b) Enlarged strong coupling anomaly due to magnetic exciton exchange for T = 0.25...

Here the dimensionless interaction parameter Xaf = (/( f>o)/( l ) describes the strength of the AF influence on A(k). It is interesting to note that despite the presence of a node line the above order parameter is fully symmetric (Aig) because it is invariant under all symmetry transformations of the magnetic unit cell. Thus in a strict sense UPd2Al3 cannot be called a superconductor with unconventional gap symmetry, although the novel magnetic exciton mechanism certainly is unconventional. Various possible nodal stractures in addition to eq. (90) have been discussed by Thalmeier and MaM (2002) in the context of magnetothermal properties and experiments were performed by Watanabe et al. (2004). 

This energy separation has also been derived from inelastic neutron scattering determinations of the magnetic excitation spectrum of Pr. The excitations in a singlet ground state system are known as magnetic excitons and their dispersion is given by... 

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