R. Schefzyk

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.

Observation of a Thermal Expansion Anomaly at 36 K in La2CuO4: Evidence for a Structural Phase Transition

We report results for the thermal expansion, α(T), of La2CuO4 and La1.85Sr0.15CuO4. Both compounds exhibit surprisingly similar anomalies in α(T) near 36 K. While for the Sr-doped compound this anomaly is shown to indicate bulk superconductivity, for La2CuO4 it has to be attributed to a structural phase transition. Our results suggest that the onset of superconductivity, both in minor parts of the volume in La2CuO4 and in the bulk of La1.85Sr0.15CuO4, is accompanied and possibly even triggered by this structural transition.

Transition from Kondo to intermediate valence in Ce1−xYxAl2

Abstract High-temperature specific heat measurements and an extensive study of the thermal-expansion coefficient on Ce 1− x Y x Al 2 reveal for x ⩽ 0.6 trivalent Ce-ions, which are subject to Kondo and crystal-field effects. A continuous transition into an intermediate valent state occurs for higher Y concentrations.

Intermediate valence and Kondo effect in Ce1−xScxAl2

Results of lattice parameter, thermal expansion, thermopower and magnetic susceptibility measurements are reported for Ce1 − xScxAl2, a system exhibiting a solubility gap over a wide composition range. Two isostructural phases are identified within the gap. For the Sc-rich systems, both phases contain intermediate valent (IV) Ce ions, whereas in the Ce-rich systems (x < 0.5), a low-volume IV phase coexists with a high-volume phase containing Ce3+ ions. The temperature-induced overall valence change of the IV ions amounts to 15–20%. A factor-of-three increase of the spinfluctuation temperature is found upon increasing the Sc concentration from x = 0.5 to 0.95. For the Ce-rich systems, we have discovered a hitherto not known “Kondo anomaly” in the temperature dependence of the thermal expansivity.

Heavy fermion effects in Ce(Cu1−xNix)2Ge2

Abstract We report results of the specific heat, dc susceptibility, thermal expansion, thermopower and resistivity for a number of Ce(Cu1−xNix)2Ge2 alloy. Upon increasing x, a non-linear increase of the heavy-fermion band width T∗ and a stron non-monotonic Neel temperature TN(x) are found ( T N = 4.1 K for x = 0 and T N K for x ≧ 0.75 ).

Negative thermal expansion in the Kondo system CexLa1−xAl2

Abstract Measurements of the thermal expansion, α( T ), between 0.05 and 370 K are reported for the Kondo system Ce x La 1− x Al 2 with 0.07 ⩽ x ⩽ 1. As the most intriguing result we find for all samples a negative α( T ) anomaly at sufficiently low temperatures. Our data suggest this anomaly to be a single-ion property. A natural explanation is furnished by a local Fermi-liquid picture.

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.

Thermal expansion of La1−x CexB6 single crystals

Abstract The single-ion Kondo effect in La 1− x Ce x B 6 (0 x ⩽ 1) is shown to cause a pronounced positive maximum in the thermal expansion coefficient near T K , and an additional negative peak at lower temperatures. An antiferroquadrupolar ordering transition is found for x > 0.8, whereas antiferromagnetic order seems to exist for x ≳ 0.5.

Phenomenology of Heavy Fermion Systems

Local heavy fermions are formed at the sites of independent Kondo ions as substantiated by the discovery of a pronounced low temperature anomaly in the thermal expansion of certain dilute and moderately concentrated Ce alloys. In periodic Kondo lattice systems like CeAl3 a coherent heavy fermion band structure is developed at low T, one hallmark of which is a maximum in the Sommerfeld coefficient γ(T) near 0.4 K. This distinct phenomenon can even show up in the presence of long-range magnetic order as in CeCu2Ge2 and is found to have a strong influence on the superconducting properties of CeCu2Si2. According to its normal-state behavior, UBe13 appears to be closely related to the Ce-based Kondo lattices. A number of the superconducting properties of pure and Th-doped UBe13 can be well understood in terms of an anisotropic pairing interaction, which seems to track the anisotropy of the Fermi surface. This leads to a reinterpretation of the phase diagram of the superconducting states in U1-xThxBe13.