The superconducting Kondo lattice: CeCu2Si2

Studying electric, magnetic, and thermolectric properties of Ce_xLa_1–xCu₂Si₂ alloys we have determined the variation of two principal parameters-T _k andT _RKKY -with concentrationx in the range 0<x1. The magnetic phase diagram of Ce_xLa_1-xCu₂Si₂ alloys has been found to be similar to that proposed by Doniach for the one-dimensional Kondo-necklace model. The anomalous low temperature properties of nonmagnetic Kondon lattices, including heavy fermion superconductivity, are related to the formate of the narrow Abrikosov-Suhl resonance in the vicinity of the Fermi level in concentrated Kondo systems withT _K T _RKKY .     

Growth of large superconducting CeCu2Si2single crystals

Large CeCu₂Si₂ single crystals have been grown with the cold boat technique. The transition temperature to the superconducting state is strongly dependent on the copper concentration in the crystal. Annealing under copper atmosphere shifts the transition temperature to higher values.     

Feedback spin resonance in superconducting CeCu2Si2 and CeCoIn5

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn5 and CeCu2Si2 are magnetic excitons originating from superconducting quasiparticles. The wave vector Q of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet d_{x;{2}-y;{2}} state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn5. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from Q which can be checked experimentally. Our analysis reveals the similar origin of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.     

Crystallographic investigations and comparison of superconducting and nonsuperconducting CeCu2Si2

Two samples of CeCu _x Si₂ withx=1.8 (non superconducting) andx=2.2 (superconducting) have been investigated by neutron powder diffraction. Both samples were characterized crystallographically and then their impurity content and lattice site occupation were determined. Anisotropic thermal vibrations of the Cu and Si atoms is detected at low temperatures. A relationship between the structural parameterz (defining the distance Ce to Si) and the occurance of superconductivity is suggested.     

Pressure-induced valence and crystal field shifts in YbCu2Si2 and TmTe by neutron scattering

We present the quasielastic and inelastic neutron scattering of YbCu₂Si₂ and TmTe at high pressure and compare the results to those at ambient pressure performed at the same samples. In both cases we found variations of the quasielastic line width due to a pressure-induced 4f valence shift. The observations are shown to be in a gross quantitative agreement with theory as based on the Anderson model and therefore comply with the common picture of a dynamic 4f valence fluctuation. A measured variation of the crystal field parameters in YbCu₂Si₂ indicates a dominant contribution of the conduction electrons to the crystal field rather than the applicability of the Point Charge Model. As a by-product, very general theoretical expressions are provided for the pressure variation of crystal field parameters.This work is faithfully dedicated to Dieter Wohlleben whose tireless scientific truth and enthusiastic visions of the rules of nature will inspire us all forever.     

Nature of the A phase in CeCu2Si2

Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.     

A crossover from the magnetic Kondo compound CeCu5 to the heavy Fermion compounds CeCu4Al and CeCu3Al2

We present measurements on the isostructural series CeCu₅, CeCu₄Al and CeCu₃Al₂, which demonstrate the supression of the antiferromagnetic phase transition at 4 K in CeCu₅, due to the substitution of Cu by Al in CeCu₄Al and CeCu₃Al₂. This substitution, however, is linked to a dramatic enhancement of properties reflecting the electronic density of states.     

Pressure-induced valence changes in EuS and SmTe

Abstract

X-ray absorption spectroscopy was applied to study the pressure-induced valence changes in EuS and SmTe, which are divalent semiconductors of NaCl-type structure at ambient pressure. In both systems the Eu-L_III and Sm-L_III thresholds exhibit the onset of intermediate valencies of the rare earth ions at 15 GPa and 4 GPa, respectively. In EuS, one observes only a small increase of the valency within the investigated pressure range (36 GPa), while in SmTe a full transition to trivalency is observed above 20 GPa. The transition to the CsCl-Type high-pressure phase has no significant influence on the valence in both systems.     

Quasiparticle band structure of CeCu2Si2 and CeAl3

The band structure for the ground state is obtained for CeCu₂Si₂ and CeAl₃ within the local-density approximation (LDA). Subsequently, the quasiparticle band structure is calculated on the basis of the Kondo Lattice Ansatz (KLA) for the Cerium 4f state using the LDA potential parameters for all other electrons. The KLA requires the specification of both the symmetry of the scattering channel, which is taken from the crystal-field (CF) ground state, and the energy dependence of the scattering phase shift, for which a Kondo resonance is assumed. Different results are obtained for two choices of the CF ground-state parameters for CeCu₂Si₂. Based on our results the low temperature specific heat is discussed.     

Transport properties of LaCu2Si2 and CeCu2Si2 between 1.5K and 300K

We report measurements of the electrical resistivity, the thermal conductivityk and the thermoelectric powerS between 1.5K and 300K on the anomalous CeCu₂Si₂ compound and on LaCu₂Si₂ as reference compound. For LaCu₂Si₂ the temperature dependences of andS are in accord with those found in otherd band metals. For CeCu₂Si₂ the observed resistivity (220 µ cm at 200K) leads to a very short electronic mean free path which is of the order of the Ce-Ce spacing. Correspondingly,k is almost identical with the phonon contributionk _p . Below 20K, resistivity and thermoelectric power strongly suggest Fermi liquid behavior with a degeneracy temperature between 20K and 40K. Above 200K, both andS decrease proportionally to –ln(T/1 K).Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/KölnPart of this work will be presented at the Int. Conf. on Rare Earths in the Metallic State, St. Pierre de Chartreuse, Sept. 1978     

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu2Si2 versus YbRh2Si2

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu₂Si₂, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu₂Si₂. In YbRh₂Si₂, superconductivity appears to be suppressed at T???10?mK by AF order (T_N?=?70?mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at T_A slightly above 2?mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at T_c?=?2?mK. Like the pressure – induced QCP in CeRhIn₅, the magnetic field – induced one in YbRh₂Si₂ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-T unconventional heavy – fermion superconductors and other families of unconventional superconductors with higher T_cs, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.     

Pressure tuning of the interplay of magnetism and superconductivity in CeCu2Si2

We carried out specific-heat and ac-susceptibility experiments under hydrostatic pressure to investigate the interplay of spin-density-wave antiferromagnetism (A) and superconductivity (S) in single-crystalline AS-type CeCu(2)Si(2). We find evidence for a line of magnetic-field- and pressure-tuned quantum critical points in the normal state in the zero-temperature magnetic field-pressure plane. Our analysis suggests an extension of this line into the superconducting state and corroborates the close connection of the underlying mechanisms leading to the formation of the antiferromagnetic and the superconducting states in AS-type CeCu(2)Si(2).     

Temperature dependence of the Kondo resonance and its satellites in CeCu2Si2

We present high-resolution photoemission spectroscopy studies on the Kondo resonance of the strongly correlated Ce system CeCu2Si2. By exploiting the thermal broadening of the Fermi edge we analyze position, spectral weight, and temperature dependence of the low-energy 4f spectral features, whose major weight lies above the Fermi level E(F). We also present theoretical predictions based on the single-impurity Anderson model using an extended noncrossing approximation, including all spin-orbit and crystal field splittings of the 4f states. The excellent agreement between theory and experiment provides strong evidence that the spectral properties of CeCu2Si2 can be described by single-impurity Kondo physics down to T approximately 5 K.     

Structural properties and band structure of heavy fermion systems: CeCu2Si2 and LaCu2Si2

The structure of CeCu₂Si₂, isotypic with ThCr₂Si₂, has been refined in a single-crystal study. The atomic parameters were used in self-consistent LMTO band structure calculations for CeCu₂Si₂ and isostructural LaCu₂Si₂. The results are analysed in terms of energy levels, charge distribution and in particular Fermi surface properties. The Ce-4f levels are situated mainly aboveE _F , except near the Γ-point. The density-of-states atE _F is large and heavily concentrated around the Ce-4f band. This makes local Ce 4f fluctuations possible, while interaction with the rest of the band structure is small. The comparison with LaCu₂Si₂ shows that the additional Cef electron is partly promoted into the non-f bands. Large enhancement factors are required to bring band results into agreement with observed specific heat and magnetic critical field. Strong Fermi surface anisotropies are pointed out for both compounds suggesting new experiments on single crystals.     

Mixed valence on dilute Eu in YCu2Si2

We report measurements of electrical resistivity, thermopower, thermal conductivity, magnetic susceptibility, Mößbauer andL _III x-ray absorption of intermediate valent Eu in Y_1–x Eu _x Cu₂Si₂ alloys withx=0.03,x=0.07 andx=0.10. A qualitative comparison of the physical properties of dilute Eu in YCu₂Si₂ with concentrated Eu in EuCu₂Si₂ shows only slight differences; the dominant feature is a very strong variation of the Eu valence with temperature, from about 2.8 at 4 K to about 2.45 at 800 K, in the concentrated compound as well as in the dilute alloys.