A comparison was made between four low-temperature properties of LaCu2Si2and CeCu2Si2. Whereas LaCu2Si2behaves like a normal metal, CeCu2Si2shows (i) low-temperature anomalies typical of “unstable 4f shell” behavior and (ii) a transition into a superconducting state atT c ≃ 0.5 K. Our experiments demonstrate for the first time that superconductivity can exist in a metal in which many-body interactions, probably magnetic in origin, have strongly renormalized the properties of the conduction-elec-tron gas.

/pdf/superconductivity-in-the-presence-of-strong-pauli-2ybleidu3b.pdf

Superconductivity in the presence of strong Pauli paramagnetism : CeCu2Si2

Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.

https://iopscience.iop.org/article/10.1088/0034-4885/44/1/001/pdf

Valence fluctuation phenomena

Quantum phase transitions arise in many-body systems because of competing interactions that promote rivaling ground states. Recent years have seen the identification of continuous quantum phase transitions, or quantum critical points, in a host of antiferromagnetic heavy-fermion compounds. Studies of the interplay between the various effects have revealed new classes of quantum critical points and are uncovering a plethora of new quantum phases. At the same time, quantum criticality has provided fresh insights into the electronic, magnetic, and superconducting properties of the heavy-fermion metals. We review these developments, discuss the open issues, and outline some directions for future research.

http://science.sciencemag.org/content/sci/329/5996/1161.full.pdf

Heavy Fermions and Quantum Phase Transitions

Measurements are reported on the electrical resistivity, susceptibility, dc-Meissner effect and specific heat of the tetragonal U-compound URu2Si2. At T N ≃17 K, an antiferromagnetic phase transition is observed. This magnetically ordered state appears to coexist with superconductivity, which is found below T c ≃1.5 K. An analysis of the upper critical field data reveals rather unusual properties of both the normal-and the superconducting state. The effective mass of the quasiparticles at low temperatures is substantially enhanced (m*≃50 m 0), though the measured low-temperature specific heat coefficient γ does not exceed 65 mJ/K2mol.

Superconductivity and Magnetic Order in a Strongly Interacting Fermi-System: URu2Si2

A review is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and non-magnetic metals, with emphasis on the magnetic properties. Included are the properties of compounds in which the non-rare-earth component is a 4d or 5d transition element. Special consideration is given to the properties of pseudo-binary compounds. Results of magnetisation measurements, neutron diffraction and neutron scattering are discussed together with results derived from NMR, ESR and Mossbauer effect spectroscopy. An evaluation is given of the relevance of the experimental results with respect to different types of exchange interactions in this class of intermetallics. Special consideration is also given to the influence of crystal field effects on the magnetic properties and, furthermore, to the occurrence of intermediate valences in several of the compounds of Ce, Sm, Eu and Yb.

https://iopscience.iop.org/article/10.1088/0034-4885/42/8/003/pdf

Intermetallic compounds of rare earths and non-magnetic metals

We report on specific heat measurements of (La1−x
Ce
 x
 )Al2 samples, with 1.5 a/o≦x≦100 a/o, performed in magnetic fields of up to 5 T between 0.3 and 10 K. In the Ce rich alloys, and especially in CeAl2, aλ-type peak of an antiferromagnetic phase transition, and at lower temperatures spin waves and very large electronic contributions are clearly visible. In higher magnetic fields, that is when antiferromagnetic order can be suppressed, the specific heat of the alloys exhibits a broadened Schottky peak. All these phenomena add up tok ln 2, i.e. to the correct entropy change per single Ce3+ ion in itsΓ
7 crystal field ground state. We interpret experimental results as an interplay between cooperative magnetism and the single-ion Kondo effect which describes a gradual turning off of one magnetic moment. The broadening of the Schottky peak is directly related to the Kondo temperatureT

 K
 , which we determine with a simple “resonance level model”.T

 K
 increases by an order of magnitude whenx increases from 1.5 a/o to 100 a/o. This is interpreted as caused by a lattice contraction. A quadraticx dependence of the Neel temperature suggests that (forT≲T

 K
 ) stable Ce moments can only exist through pair interactions. The very large (and almost field independent) specific heat term linear in temperature with a coefficientγ=135 mJ/K2 mole for CeAl2 is attributed to the Kondo effect—still present in the antiferromagnetically ordered state. Our evaluation of the experimental data is backed by a molecular field theory for a simplified antiferromagnetic structure combined with the simplest possible Kondo theory.

Specific heat of concentrated kondo systems: (La, Ce)Al2 and CeAl2

— We review the experimental situation of the unusual compound CeAl2 based on inelastic neutron scattering and specific heat measurements. The most interesting aspect of CeAl2 is its low temperature behavior. Below 3.9 K, a complicated antiferromagnetic structure coexists with a dynamic interaction between Ce moments and conduction electrons. It is argued that this dynamic interaction is due to the Kondo effect rather than to valence fluctuations. The experimental situation can be phenomenologically well described with the aid of a

https://hal.archives-ouvertes.fr/docs/00/21/88/89/PDF/ajp-jphyscol197940C5106.pdf

Antiferromagnetic ordering between unstable 4f shells in CeAl2

Dilute (La1−x
Nd
 x
 )Sn3 alloys with 0.01<x<0.15 are characterized by determination of the superconducting transition temperatureT

 c
 as well as low temperature measurements of the specific heat and inelastic neutron scattering. As an important result of these experiments we found that the Nd impurities in LaSn3 exist as stable trivalent ions, in contrast to all other light rare earths. In addition, specific heat and neutron scattering results revealed the scheme of crystal field (CF) levels of Nd3+. Using this level scheme, the concentration dependence of the superconducting transition temperature,T

 c
 (x), could be quantitatively fitted up tox≃9 at % by the theory of Keller and Fulde [J. Low. Temp. Phys.4, 289 (1971)]. From the relatively high initial slope ofT

 c
 (x) we inferred that — besides isotropic spin exchange — other pair-breaking processes are important. In contrast toT

 c
 (x), the reduced specific heat jumpsΔC/ΔC
0 as a function ofT

 c
 /T

c0 (whereT

c0 andΔC
0 refer to LaSn3) were found to lie considerably below the theoretical curve for isolated Nd3+ ions. This is explained by Nd—Nd interactions resulting in a mean Zeeman splitting (<k

 B
 
T

 c
 ) of the CF ground state, which could be directly observed in the form of broadened Schottky humps in the normal state specific heats.

C. D. Bredl

Papers

Specific heat of concentrated kondo systems: (La, Ce)Al2 and CeAl2

Antiferromagnetic ordering between unstable 4f shells in CeAl2

Low temperature properties of (La, Nd)Sn3 alloys

Specific heat of CeAl2 in high magnetic fields

Specific heat of a Kondo system showing magnetic order