Experimental studies of structurally incommensurate crystal phases

Infrared phonon anomaly and magnetic excitations in single-crystal Cu3Bi(SeO3)2OMiller1, Stephens2, Martin3 et al. 2012Phys. Rev. B

“…4,5 Another possibility is that an incommensurate transition occurs in Cu 3 Bi(SeO 3 ) 2 O 2 Cl below 115 K. Incommensurate phases acquire satellite x-ray diffraction lines around the Bragg peaks of the symmetric phase. 19 We did not observe satellite peaks, but it is possible they exist and were too weak to be detected in the present powder x-ray diffraction measurements. We plan to perform more sensitive measurements (e.g., single crystal or neutron powder diffraction) to resolve this issue.…”

Soft vibrational mode associated with incommensurate orbital order in multiferroicCaMn7O12

“…[33][34][35] In such materials, the frequency (or energy) of a phonon decreases as the transition temperature T c is approached from above, and it reaches zero at T c , where the corresponding movement of the atoms freezes in. The resultant lattice distortion breaks at least one of the structural symmetries of the high-temperature phase, and the distortion amplitude continuously increases from zero upon further cooling below T c .…”

“…32 This frequency defines a distinct type of excitations involving the amplitude of the order parameter, which is commonly referred to as the amplitude excitations, or "amplitudons". [33][34][35] They are always Raman-active below T c and belong to the A g totally symmetric representation. 34 Since all of these attributes are consistent with what we find, we conclude that the soft mode in CaMn 7 O 12 is the amplitude excitation of the order parameter, which has a combined orbital and lattice character.…”

“…[33][34][35] They are always Raman-active below T c and belong to the A g totally symmetric representation. 34 Since all of these attributes are consistent with what we find, we conclude that the soft mode in CaMn 7 O 12 is the amplitude excitation of the order parameter, which has a combined orbital and lattice character. 17 To our knowledge, this is the first observation of amplitude excitations associated with an orbital order.…”

“…The first category consists of insulators, for which the most invoked mechanisms commonly involve some type of competition, e.g., between long-and short-range lattice interactions. 34 The second category consists of metallic charge-density-wave (CDW) materials, 35 in which the phase transition is driven by Fermi-surface (FS) nesting in conjunction with electron-phonon coupling; the CDW wave vector and phonon (Kohn) anomalies 36,37 are closely related to the nesting wave vector 2k F . Since CaMn 7 O 12 is highly insulating 29 both above and below T o with no resistivity anomaly at T o (Fig.…”