G. Adrian

Preparation, Structure, Transport Properties and Photoemission Studies of the High-Tc-Superconductors YBa2Cu3O4-δ and La1-xSrxCuO4

Single-phase bulk-YBa2Cu3O9-δ was prepared by solid-state reaction under oxygen atmosphere as well as exclusively in air, observing a transition from semi- to superconducting behaviour depending on the preparation. Low temperature powder x-ray diffraction from 300K down to 10K was carried out to investigate the structure. The upper critical field is estimated to Bc2 (T=0) = 110T (Tc-midpoint), critical current densities of up to 180A/cm2 are determined at different temperatures in magnetic fields up to 7T. Photoemission experiments show, that grains are only coupled by semiconducting interfaces and reveal a strong hybridization between O-2p and Cu-3d states.

Irradiation induced disorder effects in heavy fermion systems

Abstract The variation of the electrical resistivity ϱ ( T ), Hall-effect R H ( T ), and magnetoresistivity Δϱ ( B , T ) of thin sputtered films of the heavy fermion systems CeCu 2 Si 2 , CeCu 6 and UPt 3 on lattice disorder introduced by irradiation with 25 MeV 16 O-ions is reported. All 3 systems exhibit sensitive dependences of their transport properties on lattice defects. In case of the Ce-based compounds the experiments show the destruction of the coherence of the heavy quasiparticle states by defects. The complex behavior observed for UPt 3 indicates the importance of bandstructure effects for this compound.

Superconductivity and normal state properties of YBa2(CU1−xNix)3O7−δ

We report the effect of Ni substituted for Cu in the high-Tc-superconductor YBa2Cu3O7 on bulk Tc, pressure dependence Tc(p) and thermopower S(T). As the coherence length of YBa2Cu3O7 is supposed to be rather short and highly anisotropic, we discuss effects of the random distribution of a small concentration of impurity atoms on the superconductive properties.

Preparation and electrical transport properties of Ce1−xLaxCu6-thin films

We report the preparation of Ce1-xLaxCu6 thin films and measurements of resisitivity ρ(T), Hall-effect RH(T), and magnetoresistivity Δρ(B,T). For CeCu6-films structures in ρ(T) and a T2-behavior below 0.18K, characteristic for Heavy Fermion systems, are observed. The formation and coherence of heavy quasiparticles is also reflected in strong temperature dependences of RH(T) and Δρ(B,T). These Heavy Fermion properties disappear at a La-concentration of x = 0.25. Increasing x further leads to a dilute Kondo impurity behavior.

Electrical and structural properties of YBa2Cu3O7 films on poly and single crystalline oxides of Cu and Ni

Abstract YBa2Cu3O7 films were deposited by inverted cylindrical magnetron sputtering on metallic substrates as e.g. Ni sheet covered with the natural oxide as barrier layer. These films grow c-axis oriented with a Tc,R=0 of about 80 K. For a better understanding of the growth procedure on metal oxides, thin YBa2Cu3O7 films were prepared on single crystalline NiO with the orientations (100), (110) and (111). These films are also c-axis oriented and exhibit Tc R = 0 =86K. So far (100) oriented NiO yields the best results, as can be seen from the rocking curve with FWHH= .4°, from channeling measurements and from the critical current density Jc,77K,B=0=105A/cm2. Even in a magnetic field of 12 T with B∥c-axis the critical current density at 50 K exceeds 105 A/cm2. On other substrate orientations Jc is much smaller. Similar investigations were performed on the Cu/Cu2O/YBa2Cu3O7-system. Films on polycrystalline Cu2O exhibit metallic R(T) behavior with Tc,R=0 ≈ 80 K. First results on single crystalline Cu2O substrates are quite promising.

Effects of lattice disorder on the heavy-fermion-systems CeCu2Si2, CeCu6, and UPt3

Abstract We report the influence of lattice defects on resistivity ρ(T) and Hall-effect R H (T) of the heavy fermion systems CeCu 2 Si 2 , CeCu 6 , and UPt 3 , introduced by heavy ion irradiation of thin films. In all cases the resulting drastic variations of the low temperature properties reveal a complete suppression of the coherent state at defect concentrations as low as 0.1dpa. Contrary to this the behavior at high defect concentrations is different for the Ce-systemy and for UPt 3 , indicating that in the latter compound f-d-hybrisation is not as important.

Evidence for second phase transitions in the superconducting states of heavy-fermion systems and high-TC superconductors

Abstract The anisotropic lower critical fields H c c1 ( T ) and H ab c1 ( T ) of polycrystalline YBa 2 Cu 3 O 7 , reported in this paper, consist of two separate parts which can be individually described by H c 1 ( T ) = H c 1 (0)(1− T 2 / T 2 cj with two critical temperatures T c,u · 90 K and T c,1 · 36 K. Substitution of Cu by Ni or Zn leads to monotonous reductions of these critical parameters. The experimental results closely resemble the H c1 ( T ) behaviour of the heavy-fermion system ( U Th)Be 13 which has been taken as evidence for a second phase transition. For Ce(Cu 1−x Ru x ) 2 Si 2 experimental indications for two transitions into the superconducting state have been observed in x ac ( T ) and specific heat data.

Comparison of impurity effects in heavy-fermion systems and high- Tc superconductors

Abstract Since electronic correlations are considered to be of crucial importance for the electronic properties of both heavy-fermion systems and high- T c superconductors we studied the influence of impurity atoms on the substitutional systems Ce 1− x La x Cu 6 and YBa 2 ( Cu 1− x T x ) 3 O 7− δ with T = Ni and Zn . For the heavy-fermion system the main effect is the dissapearance of the low temperature anomalies in resistivity ϱ( T ), magnetoresistivity Δϱ ( B , T ) and Hall-coefficient R H ( T ) associated with the formation of a coherent quasiparticle state. In case of YBa 2 (Cu 1−x T x ) 3 O 7 T c is linearly degraded with x with a slope of ( dT c d x ) x = 0 = 2.4 K % for Ni and -8 K/% for Zn.

Surface morphology and electrical transport properties of YBa2Cu3O7 films on substrates relevant for technical application

Abstract YBa 2 Cu 3 O 7 films deposited on Al 2 O 3 by laser ablation show c-axis oriented growth and critical temperatures T c as high as 90 K. However the critical current density J c is comparatively low. An improvement was achieved by using a ZrO 2 intermediate layer which acts as a diffusion barrier. Bilayer YBa 2 Cu 3 O 7 /ZrO 2 films prepared in-situ by laser ablation from a multi-target system under various conditions and with several buffer layer thicknesses exhibited J c -values up to 5·10 5 A/cm 2 at 77 K. The structure and the surfaces of the films were characterised by x-ray diffraction and scanning electron microscopy, respectively. The critical current density was measured as function of temperature, external magnetic field B, and relative orientation between B and the film surfaces.

Superconductivity and magnetic interactions in the pseudoternary bct system Ho(Rh1−xRux)4B4

Abstract We report temperature- and magnetic-field-dependent neutron diffraction experiments on polycrystalline samples of the pseudoternary bct Ho(Rh 1- x Ru x ) 4 B 4 system. The x -dependent variation of the electronic density of states at the Fermi level N ( E F ), which causes the well-known characteristic superconducting phase diagram, also leads to a change of the magnetic order from a non-linear antiferromagnet in coexistence with superconductivity for x x 0.80. A transition between these two magnetic structures which is accompanied by a destruction of superconductivity is also induced by an external field. The observed hysteresis in the H c2 ( T ) data below 0.6 K and magnetic relaxation phenomena in χ ac ( T ) for 0.40⩽ x ⩽0.70 are discussed on the basis of competing magnetic interactions.