Environmental Engineering Reference
In-Depth Information
The missing ingredients in the model presented here to describe
Pr have a negligible influence on the pressure-induced ordered struc-
ture, and most of the observations made in this phase were explained
by Jensen et al. (1987) utilizing only the information obtained from the
zero-pressure studies of Houmann et al. (1979). Because the ordered
moments in the antiferromagnetic phase are parallel to Q , the change
of the ground state affects primarily the longitudinal excitations, and
the low-energy optical branch close to the ordering wave-vector is par-
ticularly strongly modified. Fig. 7.15 shows the experimental excitation
energies of the optical modes in the ΓM-direction at 5.5 K, in the pres-
ence of a uniaxial pressure of 1.28 kbar, compared with the predictions
of the RPA theory.
Under the conditions of the measurements, the analysis shows that
the induced moments
J
=
J η ( Q )
cos( Q · R i + ϕ )
(7 . 4 . 7 a )
are so small that the effective ( J = 1)-model is adequate to describe
the excitations, and the value of the third harmonic of the longitudi-
nally ordered moments is only a few per cent of
. A full account
of the structure would require specifying two phase constants, one for
J η ( Q )
Fig. 7.15. The dispersion rela-
tions for the optical excitations in
the antiferromagnetic phase of Pr at
5.5 K under an applied uniaxial pres-
sure of 1.28 kbar. The ΓM direction
shown is perpendicular to the pres-
sure axis. The circles depict the ex-
perimental results obtained from in-
elastic neutron scattering, with solid
and open symbols indicating the lon-
gitudinal and transverse branches re-
spectively. The solid lines are the
calculated RPA energies for the exci-
tations, whereas the dashed lines in-
dicate longitudinal modes of weaker
intensity. The thin lines are the ex-
perimental dispersion relations in
unstressed Pr, as in Fig. 7.1
Search WWH ::




Custom Search