H. Spille

Magnetic order in a Kondo lattice: A neutron scattering study of CeCu2Ge2

Elastic and inelastic neutron scattering studies of the Kondo lattice CeCu2Ge2 were performed. AtTN=4.1 K an incommensurate magnetic order develops with an ordering wave vectorq0=(0.28, 0.28, 0.54) and an ordered moment µs=0.74 µB. The crystalline electric field splits the 4f1-J-multiplet of the Ce ion into a ground state doublet and a quartet at 191 K. The wave function of the ground state yields an ordered moment of 1.54µB. Thus, due to the onset of the formation of a Kondo singlet the magnetic moment is considerably reduced. The magnetic relaxation rate Λ was investigated via quasielastic neutron scattering. The temperature dependence of Λ(T) is characteristic of heavy-fermion systems with a high temperature square root dependence and a limiting low temperature value, yielding a Kondo temperatureTK≈10K. The quasielastic component of the scattered neutron intensities persists down to the lowest temperatures, well belowTN. This quasielastic line is regarded as a characteristic feature of heavy-fermion systems and corresponds to the enhanced value of the linear term of the specific heat.

Specific heat, resistivity and neutron scattering studies in the Kondo lattice CeNi2Ge2

Abstract Specific heat, resistivity and inelastic neutron scattering experiments are reported, which demonstrate that CeNi 2 Ge 2 exhibits a heavy-fermion groundstate which is neither magnetic nor superconducting. The specific heat coefficient γ( T ) reaches 350 mJ/K 2 mol and exhibits a peak at 0.4 K showing that coherence effects become important at low temperatures. The magnetic relaxation rates as measured via the line width in inelastic neutron scattering experiments are strongly enhanced and deviate from a Korringa law behaviour. The Kondo temperature T K ≈ 30 K was determined from the magnetic relaxation rates and from the specific heat data.

Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu2Si2☆

Abstract Polycrystalline samples of CeCu2Si2 exhibit bulk superconductivity carried by “heavy fermions”. While usually Tc ≊ (0.55 ± 0.15) K, a few samples showed considerably lower transition temperatures (Tc ⩾ 0.11 K), which could, however, be raised to usual values by proper heat treatment. We found that lower Tcs are accompanied by both slightly reduced values of the spin-fluctuation temperature of the Ce3+ ions and the phenomenon of “gapless superconductivity”. In agreement with recent results by Aliev et al. [5], a CeCu2Si2 single crystal did not become superconducting above 20 mK, but showed a 20% deficiency in Cu occupation.

Magnetic properties of the high-Tc superconductor La1.85Sr0.15CuO4

For the high-Tc superconductor La1.85Sr0.15CuO4, we present results for the low-field magnetization and the lower and upper critical fieldsBc1(T) andBc2(T). ForBc1(0) we find a value of about 20 mT. Extrapolating our resistive data for the transition midpoint taken in fields up to 12 T, we deduce a value of about 50 T forBc2(0). TheBc2 values obtained by inductive measurements are significantly lower. We explain this by a largeBc2 anisotropy due to the layered structure of these compounds. Losses in the Meissner state observed in magnetization and susceptibility measurements disappear after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample.

Is Nd2−xCexCuO4 an electron-superconductor?

We present results from electron energy-loss spectroscopy on Nd2−xCexCuO4. There is no clear evidence for Cu1+ but for holes at the oxygen sites. The valency of Ce is found to be close to 3.4. Thus our results do not corroboraten-type superconductivity in this compound.

Observation of a Thermal Expansion Anomaly at 36 K in La2CuO4: Evidence for a Structural Phase Transition

We report results for the thermal expansion, α(T), of La2CuO4 and La1.85Sr0.15CuO4. Both compounds exhibit surprisingly similar anomalies in α(T) near 36 K. While for the Sr-doped compound this anomaly is shown to indicate bulk superconductivity, for La2CuO4 it has to be attributed to a structural phase transition. Our results suggest that the onset of superconductivity, both in minor parts of the volume in La2CuO4 and in the bulk of La1.85Sr0.15CuO4, is accompanied and possibly even triggered by this structural transition.

Anomalous Hc1(T) behavior of single crystalline YBa2Cu3O7-δ

Abstract We report dc-magnetization measurements M(T,H) on a single crystal of YBa2Cu3O7-δ using a SQUID magnetometer. From the M(T,H) data we determined the lower critical field Hc1(T). For both orientations of the field, parallel and perpendicular to the c-axis, we find remarkable anomalous enhancements of H∥c1(T) and H∥c1(T) for T 40K.

Spin relaxation dynamics in heavy-fermion systems

Abstract The magnetic relaxation rates Γ as determined via inelastic neutron scattering experiments are reported for the ternary compounds CeM 2 X 2 (M  Ni, Cu, Ag, Au, Ru; X  Si, Ge). Strongly enhanced values of Γ and deviations from a Korringa-type of behaviour were found. A close correlation between the unit cell volume and the 4f-local spin / conduction electron hybridization is demonstrated.

AN INELASTIC NEUTRON SCATTERING STUDY ON THE HEAVY FERMION SYSTEMS CeCu2Ge2 AND CeAg2Ge2

An inelastic neutron scattering study is performed on the new magnetically ordered heavy-fermion systems CeCu2Ge2 and CeAg2Ge2. The temperature dependence of the quasielastic line can be interpreted in terms of a Kondo type of behavior. In addition crystalline electric field transition energies are reported.

Low temperature specific heat of YBa2Cu3O7−δ

For T 2 Cu 3 O 7−gd (δ = 0.13 and 0.99) can be decomposed into a nuclear term αT −2 and a linear term γT (γ = 20–30 mJ/KZ mole), which is strongly depressed by increasing the temperature as well as the external magnetic field. We attribute the γT term to a spin-glass type freezing mechanism. The large a-value (54 μJ/K mole) found for δ = 0.13 indicates considerable Zeeman splitting of nuclear spin states in the absence of both long-range magnetic order and an external magnetic field.