E. Faulhaber

Crystal field excitations in CeCu2Ge2: Revisited employing a single crystal and inelastic neutron scattering

The intermetallic compound, CeCu2Ge2, is the counterpart of the heavy-fermion superconductor CeCu2Si2. CeCu2Ge2 is a magnetically ordering (TN = 4.1K) Kondo lattice with a moderate Sommerfeld coefficient of 140 mJ/ molK2. Earlier inelastic neutron measurements on a polycrystalline sample revealed a doublet ground state and a quasi-quartet excited state at 16.5 meV, although a splitting of the 4f1 (J = 5/2) ground state multiplet into 3 doublets is expected from the point symmetry of the Ce3+ ions. We performed detailed inelastic neutron scattering experiments on a single crystal at the thermal triple-axis spectrometer PUMA at FRM II for different crystallographic directions. From our results we infer that the quasi-quartet, in fact, consists of two doublets at 17.0 and 18.3 meV which exhibit a strong directional dependence of their transition matrix elements to the ground state doublet. Finally, we will present a new set of crystal field parameters.

Peculiarities of the antiferromagnetism in CeCu2Si2

We report on a detailed investigation of the magnetism in A-type CeCu2Si2 by neutron scattering, thermodynamic and transport measurements. The linewidths of the magnetic peaks broaden well below the Neel temperature TN ≈ 800mK pointing to a finite domain size/correlation length. In contrast, within the resolution of the experiments the magnetic order is static. Complementary measurements of the electrical resistivity observed a superconducting transition at T< 450mK well below TN. However, no anomaly has been detected in heat capacity measurements. All results indicate that A-type single crystals exhibit parasitic superconductivity limiting the size of the antiferromagnetically ordered regions. The observed behavior can be understood by the slight off-stoichiometry of A-type CeCu2Si2.

Do antiferromagnetism and superconductivity coexist in 2% and 10% Ge doped CeCu2Si2?

Doping the heavy-fermion compound CeCu2Si2 with Ge allows for studying the unconventional superconducting state in the presence of a more stabilized antiferromagnetic phase. We performed heat capacity and elastic neutron scattering measurements on single crystals of 2% and 10% Ge doped CeCu2Si2 at different temperatures and magnetic fields. The zero field superconducting and magnetic transition temperatures in 2% Ge doped CeCu2Si2 are still close to those in pure CeCu2Si2, whereas Tc and Tn differ by an order of magnitude in 10% Ge doped CeCu2Si2. Our neutron scattering results show that the magnetic Bragg peak intensity decreases with decreasing temperature below Tc in CeCu2(Si0.98 Ge0.02)2. However, in CeCu2(Si0.9Ge0.1)2, it stays constant in the superconducting regime down to lowest temperatures. We conclude that, while a phase separation between magnetic and superconducting volumes takes place in 2% Ge doped CeCu2Si2, antiferromagnetism and superconductivity coexist on the microscopic scale in 10% doped CeCu2Si2.

Characteristics of the magnetic order in CeCu2Si2 revealed by neutron spin-echo measurements

In antiferromagnetically ordered A-type CeCu2Si2 the linewidth of the magnetic Bragg peaks, measured by elastic neutron scattering, is larger than the resolution limit for T < Tc,ρ, with Tc,ρ identified by resistivity measurements as the onset of a very broad transition to the superconducting state. In contrast to that, the linewidth in A/S-type CeCu2Si2, where bulk superconductivity (SC) expels magnetic order, is resolution-limited. This suggests that filamentary SC limits the size of magnetically ordered regions in A-type crystals. We present neutron spin-echo measurements which yield significantly faster exponential decay rates of the normalized intermediate scattering function S(AF,t)/S(AF,0) in A-type CeCu2Si2 at T < Tc,ρ than at Tc,ρ < T < TN as well as in A/S-type CeCu2Si2 at T < Tc. We conclude that filamentary SC also causes reduced lifetimes and enhanced fluctuation rates of the magnetic order in A-type CeCu2Si2.

Magnetic phases in CeCu2(Si1-xGex)2 near the tetracritical point

We report on neutron diffraction experiments on single crystals of CeCu2(Si1-xGex)2CeCu2(Si1-xGex)2 with x=0.18x=0.18, 0.45. The aim was to investigate the magnetic order in the vicinity of the tetracritical point at x≈0.25x≈0.25. The magnetic structure factors of the observed antiferromagnetic satellite peaks at lowest temperatures scale onto each other for both samples. This indicates identical magnetic low-TT structures below and above x≈0.25x≈0.25 and suggests no change in the itinerancy of the magnetic order.

Magnetic phases in near the tetracritical point

We report on neutron diffraction experiments on single crystals of CeCu2(Si1-xGex)2CeCu2(Si1-xGex)2 with x=0.18x=0.18, 0.45. The aim was to investigate the magnetic order in the vicinity of the tetracritical point at x≈0.25x≈0.25. The magnetic structure factors of the observed antiferromagnetic satellite peaks at lowest temperatures scale onto each other for both samples. This indicates identical magnetic low-TT structures below and above x≈0.25x≈0.25 and suggests no change in the itinerancy of the magnetic order.

Single crystal X-ray diffraction analysis of CeCu2Si2

The rare-earth copper silicide, CeCu2Si2, is a heavy-fermion superconductor which was first reported by Steglich et al. [1] in 1979. An unusual type of magnetic ordering, the so-called ’’A-phase’’ [2], a superconducting ’’S-phase’’ as well as the coexistence of Aand S-phase can be observed for different crystals at temperatures T < 1 K. These properties depend strongly on small variations of the chemical composition. For Si-rich crystals A-phase behaviour was observed whereas the Si-deficient S-phase exhibits only superconductivity. The A/S-phase properties can be found for a composition in between. All phases are meant to crystallise within the ThCr2Si2-type of structure with tetragonal space group symmetry (139) I4/mmm. Three different single crystals, A-, Sand A/S-phase, were fluxgrown by amodified Bridgman technique [3]. The crystals were then investigated by means of single crystal X-ray diffraction measurements at ambient conditions. Structure refinement and electron density determination of the experimental data were performed to determine the exact crystal structure and composition of the crystals. The tetragonal structure was found for all three crystals. Significant differences were found for the unit cell parameter cwhich is increased for the A-phase. An unusual high electron density was found between the Si atoms which varied for the different crystals. The extinction parameter was comparably high for the S-phase crystal which supposes a very high quality crystal.