W. Lieke

Superconductivity in CeCu2Si2 (invited)

The specific heat, dc magnetization, and resistivity have been measured on a number of samples of CeCu2Si2, which was previously found to assume a novel superconducting state below 0.5 K [F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979)]. By comparing the transition temperatures of samples which have undergone different heat treatments, we demonstrate that small fractions of spurious phases hinder the onset of superconductivity but are gradually removed with annealing. With a very pure sample obtained in this way, we have fully confirmed all the characteristics of the superconducting phase transition reported before on less pure samples. However, even after this heat treatment, the samples show clear signs of striking inhomogeneities, manifested by a static Meissner effect of bulk samples representing typically only a few percent of the volume. An increase of this Meissner signal up to 60 vol% is observed after powdering the samples. In addition to measurements in the superconducting state, the intrin...

Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu2Si2☆

Abstract Polycrystalline samples of CeCu2Si2 exhibit bulk superconductivity carried by “heavy fermions”. While usually Tc ≊ (0.55 ± 0.15) K, a few samples showed considerably lower transition temperatures (Tc ⩾ 0.11 K), which could, however, be raised to usual values by proper heat treatment. We found that lower Tcs are accompanied by both slightly reduced values of the spin-fluctuation temperature of the Ce3+ ions and the phenomenon of “gapless superconductivity”. In agreement with recent results by Aliev et al. [5], a CeCu2Si2 single crystal did not become superconducting above 20 mK, but showed a 20% deficiency in Cu occupation.

Low-temperature transport properties of Kondo lattices

Abstract The low-temperature behavior of the resistivity, thermal conductivity and thermoelectric power (TEP) is compared for three Kondo lattices, i.e. antiferromagnetic CeAl 2 , enhanced Pauli paramagnetic CeAl 3 and superconducting CeCu 2 Si 2 . For the latter two compounds, coherence leads to a hitherto not observed change of sign in the TEP below T = 0.5 K .

Evidence for two magnetic phase transitions in CeAl2: Resolution of high pressure results

Abstract Zero-pressure thermal-expansion and specific-heat experiments on CeAl2 show that two phase transitions having pressure dependences of the transition temperature, t p =T −1 N ( d T N d p ) p=0> , of opposite sign occur near T N ⋍3.8 K . Comparison with earlier high-pressure neutron-diffraction results reveals an antiferromagnetic nature of both transitions: The one with tp 0.

Specific heat and thermal expansion of CeCu2Si2 at low temperature

Specific-heat and thermal-expansion measurements on CeCu2Si2 samples of varying stoichiometry as well as on dilute (Ce, T)Cu2Si2 alloys (T:Y,La) indicate that the observed suppression of superconductivity in samples with “high” unit-cell volume is associated with a reduction of the Kondo temperature and the occurrence of another cooperative phenomenon, probably magnetic in origin.

Magnetization and specific heat of CeAl3

Abstract Measurements of the high-field dc magnetization and low-field ac susceptibility as well as specific-heat experiments covering almost four decades in temperature characterize CeAl 3 as a “dense Kondo system” with T k ≈ 5 K. From these results, the low-temperature value of the “Wilson ratio”, R (0) = 2.9, is deduced.

The magnetic phase diagram of CeAl2

Abstract We give a discussion of the magnetic field-temperature phase diagram and of the temperature-pressure phase diagram of CeAl 2 . Experimental results of sound velocity, thermal expansion and specific heat measurements are used to determine the different magnetically ordered phases.