Electron-quasiparticle interaction in the melt-spun cubic RECu₅ (RE - heavy rare earth)

Up to the present there exist very few experimental data on the transport, magnetic properties and crystalline electric field (CEF) for the intermetallic compounds of the cubic RECu₅ (RE = rare earths) series. One reason is the difficulty in obtaining single-phase samples due to the proximity of two different congruently melting RECu₅ phases. The compounds with light RE(e.g. RE from La to Gd) crystallise in the simple hexagonal CaCu₅-type structure, while the heavy rare earths compounds exhibit the cubic AuBe₅-type structure or also hexagonal one. In order to overcome the problem of obtaining of the single phase samples we have used the melt spinning technique as the successful method for sample preparation. Further, we have performed a systematic study of the transport, magnetic and point-contact properties of RECu₅ (RE = Tb, Dy, Ho, Er, Tm) with the stress on CEF effects. In order to study the CEF contribution to the interaction of conduction electrons with other quasiparticles presented in the compound (the electron-quasiparticle interaction function) directly we have used the point contact (PC) spectroscopy. Thus we demonstrated the possibility of PCS study of the samples prepared in the form of the melt-spun ribbons.
    In Fig. 4 the measured characteristic PC spectra of the heterocontacts between RECu₅ and Cu in the ballistic regime at a temperature 1.5 K are presented. Hered²V/dI²(U)is directly proportional to the electron-quasiparticle interaction function. We observed always a broad maximum situated at about 18 - 19 meV. In this energy range there is the main peak which is characteristic for the electron-phonon interaction in pure Cu-Cu contacts. However, the contribution of one metal to the heterocontact PC spectrum is inversely proportional to the Fermi velocity of this metal. The homocontact measurements were not always possible. We have shown in the TmCu₅-TmCu₅ case that this maximum is present at the same energy position and it has smaller intensity. Therefore, it is connected with the scattering of conduction electrons on phonons of RECu₅ and Cu.

Figure 4: Characteristic PC spectra of RECu₅-Cu heterocontacts (RE = Tb, Dy, Ho, Er and Tm) in the ballistic regime at 4.2 K.

    The intensity of this broad maximum reflects the dominating phonon contribution to the PC electron-quasiparticle interaction function. Moreover, we observed a small shift of the position of this maximum to higher energy with increasing rare earth atomic number. This effect is similar to the RENi₅ compounds (RE = La, Y) due to increasing dimension of unit cells.
    Main differences between characteristic PC spectra for different compounds have been observed in the low energy range (usually below 12 meV). The low energy structures are characterized by the presence of maxima or shoulders. These structures are well reproducible. This low energy part is mainly responsible for the different behaviour of different compounds.
    In order to distinguish between the phonon and the CEF origin of peaks and to study the influence of transition temperatures we measured the temperature behaviour of the characteristic PC spectrum of the same PC's. With increasing temperature the broad maximum at 18 - 19 meV does not change its position, shape and intensity. This is an additional confirmation of its phonon origin. A difference has been found only in the low energy region of the PC spectrum below 12 meV. In Fig. 5, such a characteristic behaviour of the PC spectrum is presented as example for the case of DyCu₅. Due to the complicated magnetic behaviour (two magnetic transitions) one could see the shift of maxima to lower energies with increasing temperature and disappearing above T_C. This is a clear relation between magnetic ordering and this structure. We could identify our structure as connected with the excitations of 4f Dy electrons from the ground CEF level to the higher excited CEF levels. Moreover, the shift of some CEF peaks with changing temperature is due to the Zeeman splitting of CEF levels by the molecular field arising during the transition into the magnetically ordered state.
 

Figure 5: Characteristic PC spectra of DyCu₅-Cu heterocontacts in ballistic regime at 4.2 K

    For TmCu₅ the observed low energy PC structure (peak at 2.2 meV) was compared with the measurements by inelastic neutron scattering (INS). This peak is connected with the excitations of 4f Tm electrons from singlet (or non-magnetic doublet) ground CEF level to second excited CEF level. We have not seen the transition to the first excited CEF level. Moreover, as from INS follows the existence of a group of CEF levels at energies between 8 and 14 meV (broad maximum without detail identification of CEF level), we suppose that our observed shoulder at 7.8 meV and the small anomaly at 12 meV are connected with the transition from ground CEF level to this group of CEF states with higher energies.
    In case of Dy, Ho and Tm we have measured the temperature dependencies of electrical resistivity r(T), magnetization (Dy, Ho) and susceptibility (Dy, Ho). The low temperature parts of r(T) have shown the typical CEF feature (maximum) corresponding to the thermal population of first excited CEF level. Moreover, the information about magnetic phase transitions have been obtained.

M. Reiffers
S. Ilkovič (University of Prešov, Prešov, Slovakia), B. Idzikowski, IMP Poznaň, Poland), K.-H. Müller (IFW Dresden, Dresden, Germany)