Magnetic structures and excitations in CePd2(Al,Ga)2 series: Development of the “vibron” states

Abstract

CePd 2 Al 2−x Ga x compounds crystallizing in the tetragonal CaBe 2 Ge 2 -type structure (space group P 4/nmm) and undergoing a structural phase transition to an orthorhombic structure (Cmme) at low temperatures were studied by means of neutron scattering. The amplitude-modulated magnetic structure of CePd 2 Al 2 is described by an incommensurate propagation vector k = (δ x , 1 2 + δ y ,0) with δ x = 0.06 and δ y = 0.04. The magnetic moments order antiferromagnetically within the ab planes stacked along the c axis and are arranged along the direction close to the orthorhombic a axis with a maximum value of 1.5(1) μ B /Ce 3+ . CePd 2 Ga 2 reveals a magnetic structure composed of two components: the first is described by the propagation vector, 0), and the second one propagates with k 2 = (0, 1 2 ,0). The magnetic moments of both components are aligned along the same direction-the orthorhombic [100] direction-and their total amplitude varies depending on the mutual phase of magnetic moment components on each Ce site. The propagation vectors k 1 and k 2 describe also the magnetic structure of substituted CePd 2 Al 2−x Ga x compounds, except the one with x = 0.1. CePd 2 Al 1.9 Ga 0.1 with magnetic structure described by k and k 1 stays on the border between pure CePd 2 Al 2 and the rest of the series. Determined magnetic structures are compared with other Ce 112 compounds. Inelastic neutron scattering experiments disclosed three nondispersive magnetic excitations in the paramagnetic state of CePd 2 Al 2 , while only two crystal field (CF) excitations are expected from the splitting of ground state J = 5 2 of the Ce 3+ ion in a tetragonal/orthorhombic point symmetry. Three magnetic excitations at 1.4, 7.8, and 15.9 meV are observed in the tetragonal phase of CePd 2 Al 2 . A structural phase transition to an orthorhombic structure shifts the first excitation up to 3.7 meV, while the other two excitations remain at almost the same energy. The presence of an additional magnetic peak is discussed and described within the Thalmeier-Fulde CF-phonon coupling (i.e., magnetoelastic coupling) model generalized to the tetragonal point symmetry. The second parent compound CePd 2 Ga 2 does not display any sign of additional magnetic excitation. The expected two CF excitations were observed. The development of magnetic excitations in the CePd 2 Al 2−x Ga x series is discussed and crystal field parameters determined.

Magnetoelastic hybrid excitations in CeAuAl 3

“…For instance, consistent with this mechanism evidence for a VBS in polycrystalline PrNi 2 vanishes as the optical phonon levels shift to lower energies under doping 27 . Putative evidence for vibronic states was also reported in cubic Ce 3 Pt 23 Si 11 28 as well as the tetragonal systems CePd 2 Al 2 10,29 and CeCuAl 3 11 . Moreover, vibronic excitations have even been proposed in rare-earth doped cuprates 30 , as well as geometrically frustrated oxides such as Tb 2 Ti 2 O 7 5,12 and Ho 2 Ti 2 O 7 31 , which underscores the wide-spread relevance.…”

“…For instance, formation of a vibronic bound state (VBS) between phonons and CEF excitations has been reported in normalCnormalenormalAnormall2 (19–22). Similar VBSs have also been proposed to exist in normalPnormalrnormalNnormali2 (23), normalCnormale3normalPnormalt23normalSnormali11 (24), normalCnormalenormalPnormald2normalAnormall2 (13, 25), and normalCnormalenormalCnormalunormalAnormall3 (14) as well as rare-earth doped cuprates (26) and geometrically frustrated oxides (8, 15, 27). However, as the phonon density of states must be large, it is generally believed that the VBS may be formed only with weakly dispersive optical phonons.…”

Neutron scattering study of the Tm2Ir2O7 pyrochlore iridate

“…Monochromatic neutron radiation with a 2.528 Å wavelength, and a multidetector with an angular range of 128 °and a step of 0.1 °between each of the 1280 3 He detection cells, were employed (see e.g. [31,32] for details). The Tm 2 Ir 2 O 7 powder sample was loaded in a cylindrical copper container to ensure good thermal contact, of 6 mm diam and 60 mm height mounted on a dilution insert stick.…”

High‐Pressure Studies of Correlated Electron Systems

“…[ 75–78 ] Similar VBSs have also been proposed for PrNi 2 , [ 79 ] Ce 3 Pt 23 Si 11 , [ 80 ] and CePd 2 Al 2 . [ 81,82 ] In contrast, an anticrossing, representing a different magneto‐elastic coupling, has been suggested between acoustic phonons and CEF excitations in Pr and PrAl 2 under an applied magnetic field as well as in the magnetically ordered state of VO 4 . While the latter are mediated by dipolar interactions, [ 83–88 ] a generic anticrossing in zero magnetic field that is mediated by quadrupolar interactions may have been observed in PrAlO 3 , [ 89 ] TbVO 4 , [ 90 ] and PrNi 5 .…”

“…Inspired by studies on CePd 2 Al 2 , neutron spectroscopy in polycrystalline as well as single‐crystal revealed a VBS. [ 105–107 ] Considering hybridization with optical phonons, which results in four doublets , , and , these studies suggested early on that the symmetry of lattice fluctuations results in a different character of VBSs in a tetragonal environment as compared to cubic systems such as CeAl 2 .…”