C.V. Tomy

Composites of superconducting YBa2Cu3O7−y with silver

Abstract In order to achieve better fabricability of the otherwise brittle ceramic superconductor YBa2Cu3O7−y, we have formed composites of this superconductor with metals such as Ag, Cu and Al. It is found that up to 50 wt.% of silver can be added to YBa2Cu3O7−ywithout degradation of its superconducting transition temperature. Such composites, sintered at about 850°C, can be easily rolled in the form of strips. Superconductivity with nearly the same Tc as in the parent compound is retained in the composites even after cold working, thus obviating the need to regenerate superconductivity by heat treatment. The superconducting properties of YBa2Cu3O7−y, however, degrade when composites are formed with Cu and Al.

Moment instabilities and magnetic ordering in Ce2Rh3Ge5 and Ce2Ir3Ge5

Abstract Magnetic behaviour of the four new Ce-based intermetallics, viz., Ce 2 T 3 X 5 ( T = Rh , Ir ; X = Si , Ge ) having orthorhombic U 2 Co 3 Si 5 -type structure has been studied as a function of temperature. Though Ce-ions in Ce 2 Rh 3 Si 5 are ‘trivalent’ as inferred from the cell constants, the magnetic susceptibility measurements, however, strongly suggest a nonmagnetic ground state (of Ce-ions) in this system. μ eff /Ce-ion and θ p as determined from high temperature Curie-Weiss behaviour, are 1.80μ B and 60K for Ce 2 Ir 3 Si 5 and 1.90μ B and 75K for Ce 2 Rh 3 Ge 5 . Ce 2 Ir 3 Ge 5 undergoes an antiferromagnetic ordering with T N = 10 K .

Superconductivity in YBa2(Cu1−xAgx)3O7−δ oxide system

Abstract The oxide YBa2Cu3O7−δ exhibits superconductivity in the liquid nitrogen temperature range. The Cu ion is in a mixed-valent state in this system and is thought to play a very important role in the occurrence of superconductivity. We have studied the effect of partial replacement of mixed-valent Cu by monovalent Ag on the superconductivity in this system. Although normal state magnetic susceptibility measurements indicate an increase in the Cu3+/Cu2+ ratio on Ag substitution, the superconducting transition temperature is observed to decreases. Creation of oxygen vacancies in the presence of Ag may be partly responsible for the reduction in Tc

Competition of Kondo Spin Fluctuations and RKKY Interactions in CeRh2Si2-xGex and CeM2X2 Compounds: a Kondo Necklace Problem

We present the results of our measurements of the lattice constants and magnetic susceptibility of the pseudo-ternary system CeRh2Si2-xGex which crystallizes in the tetragonal ThCr2Si2 structure. Both of the cell constants a and c increase linearly with x. The magnetic ordering temperature TN exhibits initially an enhancement with the increase in x (TNxa0=xa036.9xa0K for xxa0=xa00 and 38.3xa0K for xxa0=xa00.125) and then decreases as x continues to increase further. These results, along with those on the pressure dependence of TN in CeRh2Si2, can be understood on the basis of the Doniachs model of a Kondo necklace. We discuss also the applicability of this model to describe the strong correlation between the structural aspects and the ground-state properties of the whole series of Ce-based ternaries CeM2X2 (Mxa0=xa03d, 4d and 5d elements; Xxa0=xa0Si, Ge) of the ThCr2Si2 structure.

Spin‐reorientation phenomenon in pseudobinary (Pr1−xRx)Co5 compounds (R=Sm, Gd, Dy, Tb, Ho, and Er) as determined by ac susceptibility measurements

Spin‐reorientation transitions in (Pr1−xRx)Co5+δ, where R=Sm, Gd, Tb, Dy, Ho, and Er were determined by ac susceptibility measurements. The spin‐reorientation temperature TSR of PrCo5 decreases from 105 K to lower temperatures when Pr is partially replaced by Sm, Gd, or Er. This decrease arises from different causes: the decrease of TSR in (Pr1−xGdx)Co5 results from the dilution of nonaxial anisotropy on the Pr sublattice, while the decrease of TSR in (Pr1−xSmx)Co5 and (Pr1−xErx)Co5+δ is due to the positive second‐order Stevens coefficients of Sm and Er which yield uniaxial anisotropies. Rare earths of negative second‐order Stevens coefficients such as R=Tb and Dy give rise to axial to conical and conical to planar transitions for x≳0.3 and 0.4 in (Pr1−xTbx)Co5+0.1x and (Pr1−xDyx)Co5+0.2x, respectively. Magnetic anisotropy phase diagrams of these systems are constructed accordingly.

Structure, superconductivity and 151Eu Mössbauer studies on Eu(Ba2−xEux)Cu3O7+z oxide system

Abstract The compounds of the type Eu(Ba 2−x Eu x )Cu 3 O 7+z with x ranging from 0.0 to 0.5 have been prepared and investigated from structure and superconductivity view point. The compound with x = 0 is the well known 1:2:3 orthorhombic superconducting compound EuBa 2 Cu 3 O 7−y with mid-point T c of 87K. It is observed that in the Eu excess system, the orthorhombicity decreases with increasing x, and the compounds with x⩾0.2 appear tetragonal. The superconducting transition temperature decreases with increasing x: the sample with x = 0.4 is non-superconducting down to 4.2K. 151 Eu Mossbauer studies reveal that all the europium ions are in a trivalent non-magnetic state. The Eu at the regular site and also the one substituting at the divalent Ba site show nearly the same isomer shift. In this respect the two sites are very similar. The decrease in T c with increasing x is not due to the magnetism of the rare earth ion but, has to be attributed to the structure and electronic effects.

Magnetism and mixed valence in some R2M3X5 compounds: R = Ce, Eu; M = d metals, X = Si, Ge

Abstract Lattice parameters, magnetic susceptibility and LIII edge measurements have been performed as a function of temperature on a number of new materials of the chemical composition R2M3X5 (R = Ce, Eu; M = Ni, Rh, Pd, Ir; X = Si, Ge). Valence instabilities have been observed in Ce2Rh3Si5 and Ce2Ir3Si5. Eu2Ni3Si5 is mixed valent as revealed by LIII edge and Mossbauer measurements; and the valence of Eu varies between 2.75 and 2.60 in the temperature range 10–300 K. Other materials show magnetic ordering.

Superconducting behavior of tetragonal Ca1−xCdxBaLaCu3O7−δ

Abstract We report our attempts to substitute increasing amount of Cd for Ca in the 1113 high- T c phase CaBaLaCu 3 O 7− δ ( T c ≅66K). Single-phase superconducting Ca 1− x Cd x BaLaCu 3 O 7− δ can be synthesized for x ⩽0.6. However, the superconducting transition temperature falls from 66 to 21 K as x increases from 0 to 0.6. Attempts to synthesize Ca 0.2 Cd 0.8 BaLaCu 3 O 7− δ or CdBaLaCu 3 O 7− δ led to mixed-phase products with no evidence of any superconducting component indicating that the limit of stability for the superconducting phase with 1113-structure falls in the region 0.6 x 1− x Cd x BaLaCu 3 O 7− δ , the superconducting behaviour of one particular composition Ca 0.6 Cd 0.4 BaLaCu 3 O 7− δ has been studied in detail after annealing it in high p O 2 environment and reanneling it in ambient argon, and, lastly in ambient oxygen atmosphere. The results are compared with reference to the behaviour of Cd-free pure CaBaLaCu 3 O 7− δ which was also given identical annealing treatments.

Preparation of 2212 phase of Bi superconductor with Tc of 92 K

Abstract Preparation procedures of the 2212 phase of the bismuth superconductor which give T c ( R = 0) higher than 85 K, without tails in resistance vs temperature curves, are described. Heat treatments are done in air only. The highest value of T c obtained is 92 K. T c is dependent on the cooling rate. Subsequently, all these samples were treated in nitrogen ambient to see if removal of oxygen improved T c . This does not happen if the T c of the air treated sample is higher than 85 K, but the dependence on the cooling rate disappears. The characterizations are done using X-ray and resistivity measurements.

Infrared evidence for BaCo3 segregation during the preparation of high Tc superconductors

Abstract Using infrared spectroscopy, we show the presence of reprecipitated BaCO 3 in the slow cooled YBa 2 Cu 3 O 7−y systems, though undetectable in X-ray diffraction studies. However, the precipitation does not occur in samples quenched from 950 C and later reheated in flowing oxygen to regenerate superconductivity. The precipitation of BaCo 3 may explain the recently reported presence of carboxylic groups in YBa 2 Cu 3 O 7−y system observed in photoelectron and photoemission spectroscopy.