- Milan Klicpera: Magnetic structures and excitations in CePd2(Al,Ga)2 compounds – a vibron state in CePd2Al2
- 15. 3. 2017, 14:30
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
CeT2X2 compounds, where T = transition element d-metal and X = p-metal, form relatively large family of intermetallics with a variety of interesting physical properties. The electronic ground state in these compounds strongly depends on the competition between the RKKY and Kondo interaction, but also on the influence of the crystal electric field on the single 4f electron of Ce. Ce 122 compounds generally crystallize in two centro-symmetric tetragonal structure types: ThCr2Si2 (space group I4/mmm) and CaBe2Ge2 (P4/nmm). The former one is adopted by compounds revealing exotic electronic properties as heavy-fermion behavior, valence-fluctuations, non-Fermi-liquid behavior or pressure induced superconductivity. The later structure type is not related to such phenomena, usually. However, compounds crystallizing in CaBe2Ge2-type of tetragonal structure frequently undergo structural phase transition (or structural distortion) to lower symmetry structure at low temperature.
CePd2Al2 crystallizes in CaBe2Ge2-type structure and reveals both structural instability and interesting physical properties. It undergoes structural phase transition from tetragonal to orthorhombic structure (Cmme) at around 13 K [1] and transition to antiferromagnetic state below 2.7 [2]. The energy spectrum of CePd2Al2 contains three magnetic peaks which cannot be described within a standard crystal field model for Ce3+ ion [1]. The presence of an additional peak is ascribed to a strong interaction between crystal field excitations and phonons leading to a new quasi-bound state, so called vibron. Such an explanation was proposed by Thalmeier and Fulde [3] for cubic CeAl2 compound exhibiting two magnetic excitations instead of one and generalized for tetragonal point symmetry [1,4]. Only several compounds revealing an additional magnetic excitation have been reporter, e.g. CeCuAl3 [4], PrNi2 [5] or recently Tb2Ti2O7 [6,7]. On the other end of investigated series, CePd2Ga2 undergoes the same structural phase transition as its isostructural and isoelectronic Al analog (Tstruc = 120 K) [8] and orders antiferromagnetically below 2.3 K [8].
We present the results of our recent neutron scattering study on both magnetic structures and magnetic excitations in CePd2(Al,Ga)2 series. The magnetic structure of CePd2Al2 is described by the incommensurate propagation vector k = (0.06, 0.54, 0) and magnetic moments aligned ≈ along orthorhombic a-axis. CePd2Ga2 reveals a magnetic structure composed of two components described by the propagation vectors k1 = (0.5, 0.5, 0) and k2 = (0, 0.5, 0). The magnetic moments of both components are aligned along the same direction - the orthorhombic [100] direction. The propagation vectors k1 and k2 describe also the magnetic structure of substituted CePd2Al2−xGax compounds, except the one with x = 0.1 staying on the border between pure CePd2Al2 and the rest of the series. The inelastic neutron scattering confirmed three nondispersive magnetic excitations in spectrum of CePd2Al2. Surprisingly, the first excitation is strongly influenced by structural phase transition (distortion) as the excitation energy shifts from 1.4 meV to 3.7 meV, while another two excitations (at 7.8 and 15.9) remain at almost the same energy. Al-rich compounds reveal very similar energy spectra as CePd2Al2. The spectra of CePd2Ga2 and Ga-rich compounds contain two magnetic excitations described by standard crystal field model.
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