Matthias Brinkmann

Residual resistivity and oxygen stoichiometry in Pr2−xCexCuO4+δ single crystals

Abstract Using an improved high-temperature annealing procedure we homogeneously vary the oxygen content of Pr 2− x Ce x CuO 4+δ single crystals and change the superconducting transition temperature in small steps. Minute changes of the oxygen concentration δ give rise to a strong increase of the residual resistivity but leave the Hall and the Seebeck coefficient nearly unchanged. A strong correlation of the change of the residual resistivity and the change of T c suggests that the point-defect scattering rate on oxygen interstitials or oxygen vacancies is an important parameter determining T c in the electron-doped high- T c systems.

Crystal growth of high-TC superconductors Pr2 − xCexCuO4 + δ with substitutions of Ni and Co for Cu

Abstract We report the crystal growth of the electron-doped high-TC superconducting Pr2 − xCexCuO4 + δ by a modified CuO self-flux flow method. We found that for the crystal growth process at different Ce concentrations the maximum growth temperature is the most important parameter and report about optimized crystal growth procedures for the Ce-concentration range x = 0−0.18. Using this flux growth method only Ni and Co atoms can be substituted for Cu in the Pr2 − xCex(Cu1 − yMy)O4 + δ crystals, the other substitutional elements that we have tested (M = Fe, Zn, Ga, and Sn) do not dissolve in the T′ phase. We introduce a new reduction treatment for single crystalline samples which ensures a homogeneous and variable oxygen stoichiometry over the whole crystal.

Electrical resistivity study of metallic Pr2−xCexCuO4+δ-single crystals over a broad concentration and temperature range

Abstract We report electrical resistivity measurements on single crystals of the electron doped high- T c superconductor Pr 2− x Ce x CuO 4+δ which have been made metallic in the concentration range 0.04 x x T towards a non-universal power law with increasing Ce-concentration. At high temperatures we always observe a cross-over to a linear temperature dependence. Measurements of the resistivity on single crystals with substitutions of Ni on the Cu-position revealed a change of the elastic as well as the inelastic scattering rate. The systematic development of the resistivity with changing charge carrier concentration in Pr 2− x Ce x CuO 4+δ is compared to that known for the hole doped high- T c , system La 2− x Sr x CuO 4+δ .

Resistance anisotropy in strongly anisotropic high-Tc superconductors

Abstract Non-superconducting metallic single crystals from the n-type high-Tc system Pr2−xCexCuO4 exhibit a two-dimensional weak-localization behavior at low temperatures. We use the negative magnetoresistance from the coherent backscattering of electrons in the CuO2 planes as a probe for the microscopic current paths, when the macroscopic current direction is along the c-axis. The anisotropy of the magnetoresistance for the magnetic field parallel and perpendicular to the CuO2 planes provides clear evidence that the voltage drop develops mainly in the (ab) planes. Hence metallic bridging defects between the CuO2 layers are of dominant importance for the c-axis conductivity. We demonstrate that the peculiar anisotropy of ϱc(T, H) at the superconducting transition of Bi(2212) crystals can be explained following the same lines.

Doping dependence of the ellipsometric spectra of Nd2−xCexCuO4−δ single crystals

Abstract We study the evolution with doping of the optical spectra of the Cu–O planes in Ln 2− x Ce x CuO 4− δ (Lnlanthanide, mostly Nd) with 0≤ x ≤0.21. We analyse in detail the spectral features that appear between 0.7 and 5.2 eV in room-temperature ellipsometric measurements. The intensity and energy position of the observed spectral features show systematic dependences with the composition of the samples. Already light doping causes relevant changes in the line shape of the charge-transfer excitation, located at 1.55 eV, due to the interaction of localized and mobile carriers, confirming the excitonic character of this transition. The additional shoulder that appears at 2 eV has band character. The doping-induced feature near 4.3 eV is assigned to an intraionic Cu transition. Its smooth shift with x gives the progressive Fermi level rise with doping. The systematics of all the results supports the picture of a doped insulator.

Intrinsic Josephson effects in various layered superconductors

Abstract The coherence lenghts of some layered superconductors are comparable to or smaller that the interlayer distance, i.e. the layered crystal structure is mapped to a periodic order parameter modulation. In our measurements on (Bi 1−x Pb x ) 2 Sr 2 CaCu 2 O 8+ϑ , IBi 2 Sr 2 CaCu 2 O 8 , Tl 2 Ba 2 Ca 2 Cu 3 O 10 , Pr 2−x Ce x CuO 4 +ϑ and κ-(BEDT-TTF) 2 Cu(NCS) 2 single crystals with transport perpendicular to the layers, we were able to detect the DC as well as the AC Josephson effects.

Crystal growth and characterization of Fe2 + xTi1 − x

Abstract Fe 2 Ti belongs to the hexagonal Laves phases, and single crystals are of great scientific interest with respect to their complicated magnetic properties. Up to now only small single crystals no larger than 10 mm 3 are known. In spite of the already known phase diagram, melt crystallization has unusual difficulties: (1) the congruent melting point is sensitive to very small changes in the concentration, (2) the solidified melt cracks during further cooling and (3) the melt is very aggressive against standard crucible materials. Therefore, different crucible-free growth methods were tried. Finally we got single crystals of 3 mm × 4 mm × 25 mm size by Czochralski growth from a levitated melt. Investigations of the susceptibility showed significant differences between single crystals and polycrystalline material.

Quantum and thermal creep in Pr1.85Ce0.15CuO4+δ single crystals

Abstract In order to investigate the vortex dynamics of a high-T c superconductor at fields up to B c2 , single crystals of the electron doped superconductor Pr 1.85 Ce 0.15 CuO 4+δ are oxygen doped in such a way that 6 T c2 (T=0) c in magnetic fields up to 7 T using high sensitivity torque magnetometry. For T ∼ 0.1 K the results are compared with a quantum creep theory for the firty limit. In the thermal regime data are analysed in terms of an activation energy U(j s ) = (U c /μ)[(j c /j s ) μ −1].

Magnetic irreversibility lines in Pr2−xCexCuO4−δ single crystals

We present mesurements of the ac-susceptibilty as a function of frequency and superimposed magnetic dcfields of electron doped high-Tc-superconductors Pr2−xCexCuO4−δ. The critical current density is derived from magnetic hysteresis loops. We observe a frequency shift of the irreversibility line similar to what is usually observed in p-type high-Tc-superconductors. For the critical current we observe a purely exponential field dependence over a broad temperature and field range.

Superconductivity of Pr2−x Ce x CuO4+δ- single crystals with Ni- and Co-substitution

We have measured electrical resistivity and Hall-coefficient on single crystals from the electron-doped high-Tc superconductor Pr2−xCexCuO4+δ with Cu replaced by Ni and Co partly. The impurities on the Cu-position lead to a strong increase of the residual resistivity of about 450 μΩcm/at.% substitution, Tc is suppressed at a rate of −20K/at.% substitution for both, Ni and Co. The Hall-coefficient RH(T) is strongly shifted towards positive values with increasing concentration of Ni and Co.