M. Reiffers

Electronic and magnetic properties of heavy fermion CeCu4Al

The studies of the magnetic susceptibility, specific heat, electrical resistivity, x-ray photoemission spectra (XPS) and the band structure calculations for CeCu4Al are reported. CeCu4Al is paramagnetic and follows the Curie?Weiss law with ?eff = 2.53??B/f.u. and ?p = ?10?K. The experimental value of ?eff is close to the calculated one for a free Ce3+ ion (2.54??B), thus indicating the presence of well-localized magnetic moments carried by the stable Ce3+ ions. Below the Fermi energy the total density of states contains mainly the d states of Cu atoms, which hybridize with the Ce f electronic states. The analysis of the Ce 3d and 4d XPS spectra indicates the localized character of the Ce f states. The determined electronic specific heat coefficient ? = 210?mJ?K?2?mol?1 confirms the heavy fermion character of the CeCu4Al compounds.

Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy

We have investigated the intermediate valence narrow-gap semiconductor SmB6 at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of SmB6, did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero - bias conductance are in the diffusive Maxwell regime. They had spectra with only small non-linearities. Contacts with small zero - bias conductance are in the tunnelling regime. They had larger anomalies, but still indicating a finite 45 % residual quasiparticle density of states at the Fermi level at low temperatures of T = 0.1 K. The density of states derived from the tunelling spectra can be decomposed into two energy-dependent parts with Eg = 21 meV and Ed = 4.5 meV wide gaps, respectively.

Point-contact spectroscopy of the electron-phonon interaction in single-crystal LaB6

Point-contact spectra of single-crystal LaB6 are obtained, yielding the energy positions of all the phonon modes up to 160 meV. A relatively strong anisotropy of the spectra is observed. It is related to the anisotropy of the phonon system. The point-contact electron-phonon interaction function and the point-contact electron-phonon interaction parameter are compared with published calculated data and a qualitative agreement is found. From the measured spectra the temperature dependence of the LaB6 electrical resistivity and heat capacity are calculated.

Pressure-Induced Localization of 4f Electrons in the Intermediate Valence Compound SmB6

We report high-pressure studies of X-ray diffraction and 11 B-nuclear magnetic resonance (NMR) in the intermediate- valence compound SmB6. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequencyQ, obtained from the 11 B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship betweenQ and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f -holes may be responsible for the long-range magnetic order under pressure.

Anomalous magnetoresistance of carbon-dopedEuB6: Possible role of nonferromagnetic regions

M. Bat’ková, ∗ I. Bat’ko, K. Flachbart, K. Jurek, E. S. Konovalova, J. Kováč, M. Reiffers, V. Sechovský, N. Shitsevalova, E. Šantavá, and J. Šebek Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague, Czech Republic Institute for Problems of Material Science, NASU, 252680 Kiev, Ukraine Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Praha 2, Czech Republic (Dated: February 1, 2008)

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB6 single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ∼ 7 and ∼4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB6 at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.

Point-contact spectroscopy of the electron-phonon interaction in LaNi5

The electron-phonon interaction in LaNi5 is studied by point-contact spectroscopy. The spectrum corresponding to the electron-phonon interaction function g (the phonon density of states modulated by the matrix element of the electron-phonon interaction) is found. The functiong is used to analyze the point-contact spectra of PrNi5, where besides the electron-phonon contribution there is also an interaction between the conduction electrons and the crystal electric field levels of the Pr3+ ion.

POINT-CONTACT PROPERTIES OF YBa2Cu3O7−δ AND SmBa2Cu3O7−δ

The I-V characteristics and their derivatives on the point-heterocontacts of Y(Sm)Ba2Cu3O7−δCu(Ag) are presented. Three types of dependences were observed. The observed deviations from classical tunneling behavior are explained. The phonon energies were determined from the classical PC tunneling spectra.

Crystal-Field Effect on the Electrical Magnetoresistivity of PrNi5

It is shown that the observed decrease of the magnitude of the measured magnetic field dependences of the electrical magnetoresistivity of PrNi5 up to 7 T in the temperature range 11–16 K is a manifestation of the occupation of the first excited crystal-field (CEF) energy level of Pr-ions. Fitting the experimental curves magnetic field and temperature dependences of the molecular field parameter are obtained. The role of the magnetic excitons is discussed.

Crystal structure and physical properties of the novel stannide Yb3Pd2Sn2

The crystal structure and the physical properties at low temperatures of the novel stannide Yb3Pd2Sn2 have been studied. The compound crystallizes in a new structure type, space group Pbcm, with Yb occupying four different positions in the lattice. Measurements of DC susceptibility and 170Yb Mossbauer suggest a close to divalent Yb-ions behaviour, ruling out a scenario of heterogeneous mixed valence for the Yb-ions.