Superconductor CeCu Research Articles

Growth and characterisation of CeCu 2Si 2 single crystals

The heavy fermion superconductor CeCu 2Si 2 shows interesting low temperature properties and a complex B– T phase diagram. Crystal growth is complicated due to the high vapour pressure of Cu and Si at the melting point (about 1540°C), the peritectic melting behaviour and a lack of crucible materials. We have grown single crystals of CeCu 2Si 2 from a levitated melt using the Nacken Kyropoulos technique. We present ultrasonic measurements, which show the low temperature properties of different CeCu 2Si 2 single crystals.

Interplay of magnetism and superconductivity in CeCu 2Si 2 under hydrostatic pressure.

One point of interest of the heavy fermion superconductor CeCu 2Si 2 is the apparent coexistence of superconductivity with a phase of magnetic origin, usually called A-phase. We have studied the behaviour of the A-phase under hydrostatic pressure. We find that it is progressively weakened, and has almost disappeared at a pressure of 3.2 kbar. We have also measured the upper critical field and find that close to the A-phase boundary, a maximum in the H c2( T) curve appears. We have analysed the H c2 data taking into account a temperature dependent susceptibility χ( T). In order to reproduce the behaviour expected for antiferromagnetic order, the exchange integral must be negative, implying that in CeCu 2Si 2, H c2 is enhanced by the Jaccarino–Peter effect

Transport measurements of the heavy fermion superconductor CeCu 2Si 2 under hydrostatic pressure in helium

Transport measurements on a single crystal of the heavy fermion superconductor CeCu 2Si 2 have been performed under pressure in a diamond anvil cell with helium as pressure transmitting medium. Probably because of the very hydrostatic conditions, the superconducting transition is shown to remain narrow under pressure. The temperature dependence of the upper critical field, H c2 , is analyzed assuming strong coupling. The effective mass is found to decrease consistently with other reports, but the expected sharp increase of T c under pressure is not found in the sample here.

Enhancement of Superconducting Transition Temperature in CeCu2Ge2under High Pressures

Enhancement of superconducting transition temperature T c is confirmed at around 16 GPa in the pressure-induced superconductor CeCu 2 Ge 2 . The superconducting H c - T c phase diagrams at the low- T c and high- T c pressure regions indicates that the heavy quasi particles form the singlet Cooper pairs. At both low- T c and high- T c pressure regions, Fermi liquid behavior of ρ∼ A T 2 is observed only at low temperatures below 1 K in the normal state induced by the external field above H c 2 . At 21 GPa where the superconductivity disappears down to 50 mK, the temperature region following ρ∼ A T 2 increases drastically up to 40 K and the coefficient A decreases rapidly. These results indicate that the strongly correlated electrons are responsible essentially for the appearance of superconductivity in this system.

Non-fermi liquid behavior in Ce(Cu(in1− xNi x) 2Ge 2 and CeCu 2(Si 1− yGe y) 2

The heavy-fermion compounds Ce(Cu 1− x Ni x ) 2Ge 2 and CeCu 2(Si 1− y Ge y ) 2 were explored using electrical transport, magnetic susceptibility, and neutron scattering measurements. The hybridization of CeCu 2Ge 2 can be changed by substituting either Cu by Ni or Ge by Si. From the local type of antiferromagnetism in CeCu 2Ge 2 to the heavy-fermion superconductor CeCu 2Si 2, the system reveal a rich magnetic phase diagram with up to three distinct phase transitions below 5 K. In the case of substituting Cu by Ni, the system shows a transition from a local-type of ordering to a band-like magnetism of heavy-fermions. For the concentration x ≈ 0.8 the magnetic order is supressed and systematic deviations from the Fermi-liquid behavior are observed in the temperature dependence of the resistivity.

Precise determination of the pressure dependence of Tc in the heavy-fermion superconductor CeCu 2Si 2

We report new experimental results on the pressure dependence of the superconducting transition temperature, T c ( p), for the heavy-fermion superconductor CeCu 2Si 2. Two characteristic pressures, p 1 = 3.0 GPa and p 2 = 7.7 GPa, discriminate three regimes. A fairly linear decrease is observed in the intermediate regime. Data suggest that T c at zero pressure is strongly reduced relatively to this regime due to an unknown mechanism.

Fermi Surface of CeCu2Si2within the Local Density Approximation

Very recently, the de Haas-van Alphen effect in the heavy fermion superconductor CeCu 2 Si 2 was observed by M. Hunt et al . We report the band structure calculation and Fermi surface of CeCu 2 Si 2 within the local density approximation. The slender arms of the hole sheet can explain the angular dependence of the measurements. The mass enhancement factor is up to 6 in the measured field.

Phase transitions in CeCu 2Si 2

We report the first measurements of the de Haas-van Alphen effect in the heavy-fermion superconductor CeCu 2 2Si 2 . In addition to providing information on the quasiparticle band structure and effective masses, the measurements reveal a complex phase diagram in the B-T plane. By observing magnetic oscillations on either side of a magnetic phase transition, we detect in one instance a minor modification of the Fermi surface, although the nature of the magnetic or other ordering is not determined. We conclude that the superconducting state in CeCu 2 Si 2 formes out of a fully developed coherent Fermi liquid.

Static magnetic ordering of CeCu 2.1Si 2 found by muon spin relaxation

Zero- and longitudinal-field muon spin relaxation measurements on a poly-crystal sample of a heavy-fermion superconductor CeCu 2.1 Si 2 ( T c = 0.7 K) have revealed an onset of static magnetic ordering below T ∼ 0.8 K. The line shapes of the observed spectra in zero field indicate a wide distribution of static random local fields at muon sites, suggesting that the ordering is either spin glass or incommensurate spin-density-wave state. The observed width of the random local field at T = 0.05 K corresponds to a small averaged static moment of the order of 0.1μ B per formula unit.

Resonant Photoemission Study of CeCu2and CeCu2Si2

The energy distribution of the 4 f partial density of states has been measured on CeCu 2 and CeCu 2 Si 2 by means of the 4 d -4 f resonant photoemission. In CeCu 2 , states with the 4 f character are distributed all over the valence band forming a major peak at 2.3 eV below the Fermi edge; the Ce 5 d band exists near the Fermi level where the fractional density of the states with the 4 f character is low. In CeCu 2 Si 2 , the 4 f band spreads over the range within 3.5 eV from the Fermi edge with a few fine structures and does not form a prominent peak; the 4 f partial density of states at the Fermi level is much higher than those in CeCu 2 and CeCu 6 . Such aspects suggest a large hopping matrix element between the 4 f level and the conduction states, as is consistent with the heavy-fermion superconductivity in CeCu 2 Si 2 .

Thin-film preparation, superconductivity and transport properties of the heavy-fermion system CeCu 2Si 2

The preparation and superconducting and transport properties of thin films of the heavy fermion superconductor CeCu 2Si 2 are reported. Superconductivity with onset temperatures above 600 mK was observed. The resistively measured temperature dependence of the upper critical field could be self-consistently fitted by Werthamer-Helfand-Hohenberg-theory with a Maki parameter α ≈ 20 and a spin-orbit scattering parameter λ SO ≈ 6. The electrical resistivity was measured up to 300 K revealing characteristic structures slightly more pronounced than is known from bulk material and a clear T 2 behavior below 2 K. The formation of heavy quasiparticles at low temperature appears to be also the origin of the measured unusual temperature dependences of the Hall-effect and magnetoresistance.

Calorimetric investigation under high pressure of the heavy fermion superconductor CeCu 2Si 2

The heavy fermion compound CeCu 2Si 2 is commonly regarded as a Kondo lattice system. Though it has been shown that the heavy mass quasiparticles participate in its superconductivity below ∼ 0.7 K, a detailed understanding of the interdependence of the superconducting and the Kondo lattice parameters is still to be developed. The application of pressure is one useful approach to study this problem. In this paper we present results of specific heat measurements between 0.3 K and 2 K under pressures up to 5.9 kbar. While in our sample T c hardly changes, the normal state specific heat, which is exclusively of electronic origin in the present temperature range, is rapidly decreased in a monotonous way, qualitatively corresponding to the expected rise of the Kondo temperature with pressure. In contrast to this behaviour, a strong nonlinear change of the jump Δc(T c) passing through a maximum near 3 kbar is observed. We suggest that this reflects changes of the Kondo lattice coherence structure in the quasiparticle density of states near E F.

Heavy fermion superconductivity

Abstract We discuss the phenomenon of heavy-fermion superconductivity by investigating the exemplary system CeCu 2 Si 2 in its normal and superconducting states. Like the non-superconducting compound CeAl 3 , CeCu 2 Si 2 behaves as a Kondo lattice. We find no evidence for triplet superconductivity in CeCu 2 Si 2 .

Note on Upper Critical Field of Heavy Fermion Superconductivity

Effects of Fermi liquid parameters on upper critical field H c2 are discussed theoretically. The Maki parameter which characterizes behavior of H c2 in the presence of the Pauli paramagnetic effect is found to be renormalized by electron-electron interaction. Experimental data on a “heavy Fermion” superconductor CeCu 2 Si 2 are analyzed.