C. Allgeier

Bio: C. Allgeier is an academic researcher from Ludwig Maximilian University of Munich. The author has contributed to research in topics: Magnetic susceptibility & Superconductivity. The author has an hindex of 6, co-authored 8 publications receiving 162 citations.

Correlation between the magnitude of the superconducting transition temperature and the normal-state magnetic susceptibility in Bi2Sr2CaCu2O8+y and Ti2Ba2CuO6+y as a function of oxygen content

Abstract: The static magnetic susceptibility of Bi 2 Sr 2 CaCu 2 O 8+ y and Tl 2 Ba 2 CuO 6+ y is studied as a function of oxygen content y over the temperature range 2–900 K. With increasing oxygen (hole) content the superconducting transition temperature T c ( y ) passes through a maximum for the Bi-compound and falls monotonically for the Tl-system, reaching a maximal value of c ≅92 K in both systems. As the oxygen content is varied, a correlation is observed between the magnitude of T c ( y ) and the temperature dependence of the normal-state magnetic susceptibility. The fact that this same correlation can also be seen in the published data on La 2- x Sr x CuO 4 and YBa 2 Cu 2 O 6+ y implies that it may be a universal property of hole-doped oxide superconductors.

Transport, structuralm and magnetic properties of the single-copper-oxygen layer Bi2Sr2−xLaxCuOy system

Abstract: The Bi2Sr2−xLaxCuO6+y was studied in the rigime 0.3 ≤ x ≤ 1.0. Measurements of the magnetic susceptibility, electrical resistivity, superconducting transition temperature, crystal structure via neutron and X-ray diffraction, and oxygen concentration y are presented. We find that stable single-phase specimens can be prepareed in the range 0.3

Specific heat and thermal expansion of the Bi and Tl high temperature superconductors near Tc

Abstract: We present measurements of the specific heat and of the thermal expansion of the most prominent phases of the Bi and Tl high temperature superconductors (Bi-2212, Bi-2223, T1-2212, T1-2223) in the neighbourhood of T c . In all these systems we observe small but sharp anomalies which have very little similarity with the mean field jump found in the conventional superconductors. The analysis of these anomalies shows clear evidence for the presence of strong fluctuations of the order parameter. For temperatures more than ≈ 5 K away from T c , 2D Gaussian fluctuations are found, while within ± 5 K of T c the fluctuation contribution is best fitted by critical fluctuations. The shape of the thermal expansion anomalies is similar to that of the specific heat. Combination of both measurements predicts dT c /dp ≈ +0.2 K/kbar for Bi-and T1-2223 systems.

Dependence of the magnetization of superconducting yttrium and thallium compounds on oxygen content and high hydrostatic pressure

Abstract: In YBa2Cu3O7 removing oxygen leads to a reduction in both Tc and the Pauli paramagnetism, accompanied by strengthening of the magnetism in the Cu O planes. For single- and polycrystalline YBa2Cu3O7 we find Tc to normally increase with pressure; however, for one single-crystal we observe ∂Tc/∂P = 0. For polycrystalline Tl2CaBa2Cu2O8 Tc(P) shows a broad maximum at 25 kbar.

Electrical resistivity and magnetization studies of the high-temperature superconductor Tl2Ca2Ba2Cu3O10

Abstract: The magnetization of polycrystalline samples of Tl 2 Ca 2 Ba 2 Cu 3 O 10 showing zero resistivity at temperatures as high as 120.5 K have been studied in the temperature range 4–300 K and in magnetic fields up to 60 kOe. In fields below 50 Oe the diamagnetic properties are dominated by supercurrents flowing across weak-link grain boundaries in the sintered material. H cl (4.2 K) takes on the value ∼900 Oe. The samples are dense and homogeneous enough to remain nearly perfectly diamagnetic to temperatures up to ∼100 K. From the high-field magnetic hysteresis loop we estimate the intragrain critical supercurrent density to be ∼7 × 10 6 A/cm 2 at 4 K. The magnetic susceptibility in the normal state can be analyzed in terms of a sizeable Curie-Weiss term plus a constant; the constant takes on an almost identical value per mol Cu to that found previously for superconducting La 1.85 Sr 0.15 CuO 4 and YBa 2 Cu 3 O 7 .

Invited review “sol-gel” preparation of high temperature superconducting oxides

Abstract: This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Variation of the superconducting properties of Bi2Sr2CaCu2O8+x with oxygen content

Abstract: We have mapped the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) in the ( p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C. Equilibrium conditions are frozen by quenching the samples into liquid gallium. The DC diamagnetic transitions remain sharp, and span an unusual temperature range for the 2212 phase (51.5 K to 94.4 K). This variation is attributed to a measured 2.5% change of the oxygen content ( Δδ ⋍ 0.2 ) in the structure. At the same time, we do not observe any significant variation of the lattice parameters. The Meissner effect, measured in an external magnetic field of 20 Oe, increases with T c . The slope of the normal-state susceptibility ∂ χ / ∂T becomes markedly less negative with decreasing oxygen concentration. It vanishes for the optimum doping that leads to T c ⋍ 94 K .

Magnetization and resistivity study on the 130 K superconductor in the HgBaCaCuO system

Abstract: Samples containing superconducting HgBa 2 Ca 2 Cu 3 O 8+ x (Hg-1223) (a=3.851 (3) A , c=15.847 (9) A ) as the dominant phase have been investigated by resistivity and magnetization measurements in external magnetic fields up to 50 kOe. The specimens exhibit critical temperatures T c ≈ 133 K, as measured by susceptibility and specific-heat experiments, with typical field-cooling susceptibilities χ FC > 20% of - 1/4 π at low temperatures. In zero field, the electrical resitance is zero below T = 129 K. From magnetization-loop, M ( H ), measurements and the broadening of the resistive transition in external magnetic fields, H , we estimate the location of the irreversibility boundary H irr ( T ) in the magnetic phase diagram of the material. Estimates of the critical-current densities j c in a magnetic field for T = 10 K and 75 K suggest that the intragrain critical-current densities are of comparable order of magnitude to those routinely observed in YBa 2 Cu 3 O 7 at those temperatures.

Correlation between the magnitude of the superconducting transition temperature and the normal-state magnetic susceptibility in Bi2Sr2CaCu2O8+y and Ti2Ba2CuO6+y as a function of oxygen content

Abstract: The static magnetic susceptibility of Bi 2 Sr 2 CaCu 2 O 8+ y and Tl 2 Ba 2 CuO 6+ y is studied as a function of oxygen content y over the temperature range 2–900 K. With increasing oxygen (hole) content the superconducting transition temperature T c ( y ) passes through a maximum for the Bi-compound and falls monotonically for the Tl-system, reaching a maximal value of c ≅92 K in both systems. As the oxygen content is varied, a correlation is observed between the magnitude of T c ( y ) and the temperature dependence of the normal-state magnetic susceptibility. The fact that this same correlation can also be seen in the published data on La 2- x Sr x CuO 4 and YBa 2 Cu 2 O 6+ y implies that it may be a universal property of hole-doped oxide superconductors.