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 .     

Incoherent Kondo ground state in CeCu1.54Si1.46

Measurements of the magnetic susceptibility, magnetization, specific heat and electrical resistivity on a new Kondo lattice compound CeCu_1.54Si_1.46 have revealed an antiferromagnetic phase transition at 6.9 K. The analysis of the specific heat demonstrates that this compound is a moderately heavy electron system with strong spin fluctuations. Based on the resistivity result, we maintain that the coherence between Kondo states at Ce sites is hindered by the disorder in the Cu and Si sublattice in this non-stoichiometric compound.     

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.     

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.     

On the anomalous H dependence of the amplitude of de Haas-van Alphen oscillations in CeCu2Si2

The de Haas-van Alphen effect is studied in heavy-fermion antiferromagnets near the spin-flip transition is studied. It is shown that the strong increase occurring in the amplitude of oscillations near the spin-flip point in an increasing magnetic field, as observed experimentally in CeCu₂Si₂, can be explained by strong single-site correlations and magnetic ordering in the subsystem of localized electronic states. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 270–275 (25 February 1998)     

Temperature dependence of the cyclotron resonance in electron inversion layers on Si

A study has been made of the adsorption of sulphur on the (110) face of molybdenum using LEED and AES combined with a radioactive tracer technique employing ³⁵S. The content of the surface unit mesh can thus be determined precisely. By applying the 2D space groups, information can be obtained concerning the distribution of the adsorbed atoms. The hypothesis of a high symmetry has been assumed. In this case the observed patterns correspond to structures made of adjacent rows of occupied sites separated by one row of vacancies.     

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.     

Temperature dependence of a single Kondo impurity

Recent advances in scanning tunneling microscopy have allowed the observation of the Kondo effect for individual magnetic atoms. One hallmark of the Kondo effect is a strong temperature-induced broadening of the Kondo resonance. In order to test this prediction for individual impurities, we have investigated the temperature dependent electronic structure of isolated Ti atoms on Ag(100). We find that the Kondo resonance is strongly broadened in the temperature range T = 6.8 K to T = 49.0 K. These results are in good agreement with theoretical predictions for Kondo impurities in the Fermi liquid regime, and confirm the role of electron-electron scattering as the main thermal broadening mechanism.     

Pressure-induced valence crossover in superconducting CeCu2Si2

Measurement of the Ce valence in the heavy fermion CeCu(2)Si(2) is reported for the first time under pressure and at low temperature (T=14 K) in proximity of the superconducting region. CeCu(2)Si(2) is considered as a strong candidate for a new type of pairing mechanism related to critical valence fluctuations which could set in at high pressure in the vicinity of the second superconducting dome. A quantitative estimate of the valence in this pressure region was achieved from the measurements of the Ce L(3) edge in the high-resolution partial-fluorescence yield mode and subsequent analysis of the spectra within the Anderson impurity model. While a clear increase of the Ce valence is found, the weak electron transfer and the continuous valence change under pressure suggests a crossover regime with the hypothetical valence line terminating at a critical end point T(cr) close to zero.     

Temperature dependence of the antiferromagnetic resonance linewidth in MnF2

The temperature dependent contribution to the antiferromagnetic resonance linewidth in MnF₂ has been measured at K-band frequencies (～ 23 GHz) between 5° and 40°K. It can be closely approximated by a T⁴ power law except at the highest temperatures.     

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.     

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.     

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).     

Electron spin resonance investigation of the Heavy-Fermion compound CeCu2(Si1−x Ge x )2

The Kondo-lattice system CeCu₂(Si_1?x Ge _x )₂ exhibits an alloying induced transition from a coherent Fermi-liquid (x=0) with strongly enhanced effective masses to an antiferromagnetically ordered heavy-fermion system (x=1). This transition is studied by Gd³⁺ ESR in oriented powder samples. The temperature dependence of the ESR line width follows a characteristic pattern which allows one to distinguish between the different ground states. The results obtained in polycrystalline CeCu₂Si₂ are in good agreement with the measurements performed in single crystals. Finally we compare our results with⁶³Cu-NMR data obtained from CeCu₂Si₂ and CeCu₂Ge₂.     

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.     

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