Search

We report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above T c ≈ 600 mK , a truly inelastic response is detected in the superconducting state well below T c . This inelastic response is discussed in connection to the properties of the superconducting state.

Zero-field μ SR measurements have been performed on CeCu2Si2 single crystals. Antiferromagnetic order shows in an initial rapid damping of the μ + polarization. In a single crystal exhibiting antiferromagnetic order and superconductivity, there exists clear evidence of phase separation of both phenomena with a first order transition from antiferromagnetism at higher temperature to superconductivity at low temperature.

We investigated the magnetic properties of the two compounds YbCo4Ge2 and YbNi5Ge3 by means of magnetic susceptibility χ, specific heat Cp and electrical resistivity ρ measurements down to 0.5 K. Our results show that both compounds present a rather well localized, trivalent Yb state which undergoes a transition into an antiferromagnetic state at T N = 2.7 K (YbNi5Ge3) and T N = 2.5 K (YbCo4Ge2), respectively.

We report on extensive neutron diffraction on single crystalline CeCu 2 ( Si 1 - x Ge x ) 2 to determine the antiferromagnetic order in the regime with low Ge content, initially called A phase due to the unknown origin of the magnetic order. Starting from the x = 0.45 alloy we followed the magnetic order to lower Ge concentrations and to pure CeCu 2 Si 2 . For all Ge concentrations x ⩽ 1 an incommensurate magnetic order with a propagation vector close to τ = ( 0.25 0.25 0.5 ) is observed. The magnetic order is determined by the nesting properties of the Fermi surface.

We investigated the non-monotonous evolution of superconductivity with pressure ( P ) in a single crystal of CeCu 2 ( Si 0.9 Ge 0.1 ) 2 ( P = 0 : antiferromagnetic order at T N ≈ 1.2 K , superconductivity at T c ≈ 0.35 K ). Electrical resistivity measurements were performed at low temperatures T ⩾ 50 mK under nearly hydrostatic pressure conditions ( 0 P 4.5 GPa ) . The existence of two distinct superconducting domes is observed in the T – P phase diagram, one at 0 P 2.2 GPa and the other at 2.2 P 4.5 GPa . Surprisingly, our preliminary data suggest that the T – P phase diagram is more rich in structure than reported previously.

We present low temperature ( 0.4 K T 2 K ) heat capacity measurements under hydrostatic pressure ( p 2.1 GPa ) and in magnetic fields ( B ⩽ 8 T ) on a single crystal of the heavy-fermion compound CeCu2Si2. At ambient pressure, our A / S type single crystal orders antiferromagnetically (AFM) at T N ≈ 0.7 K and superconducts below a slightly lower temperature T c ≈ 0.5 K . Application of only a small pressure ( p ≈ 0.1 GPa ) suppresses the long-range AFM order and rapidly shifts the superconducting transition temperature to T c ≈ 0.62 K . Upon further increasing pressure T c ( p ) reveals a shallow minimum at p ≈ 1.6 GPa from where it starts to increase again, indicating the entrance into the high-pressure superconducting region.

We report here the magnetic susceptibility, specific heat and electrical resistivity on high-quality single crystals of YbIr 2 Si 2 . It crystallizes either in the I-type body-centered ThCr 2 Si 2 or P-type CaBe 2 Ge 2 structure. P-type sample orders magnetically below 0.7 K whereas the I-type system remains paramagnetic down to 40 mK and shows heavy fermion behavior. Low-temperature resistivity and specific heat of this compound has strong similarity with YbRh 2 Si 2 down to 400 mK. However, below 200 mK YbIr 2 Si 2 attains Fermi-liquid ground state in contrast to YbRh 2 Si 2 which shows non-Fermi-liquid behavior.