Browsing by Author "Kasinathan, Deepa"

We have grown single crystals of EuFe(2)As(2), and investigated its electrical transport and thermodynamic properties. Electrical resistivity and specific-heat measurements clearly establish the intrinsic nature of magnetic phase transitions at 20 and 195 K. While the high-temperature phase transition is associated with the itinerant moment of Fe, the low-temperature phase transition is due to magnetic order of localized Eu moments. Band-structure calculations point out a very close similarity of the electronic structure with SrFe(2)As(2). Magnetically, the Eu and Fe(2)As(2) sublattices are nearly decoupled.

We studied the temperature-pressure phase diagram of EuFe2As2 by electrical resistivity measurements. The spin-density-wave transition at T-0 associated with the FeAs-layers is continuously suppressed with increasing pressure, while the antiferromagnetic ordering temperature of the Eu2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, rho(T), indicates the onset of superconductivity at T-c approximate to 29.5 K. Surprisingly, on further reducing the temperature, rho(T) is increasing again and exhibiting a maximum caused by the ordering of the Eu2+ moments, a behavior which is reminiscent of reentrant superconductivity as it is observed in the ternary Chevrel phases or in the rare-earth nickel borocarbides.

EuFe(2)As(2) shows a spin-density wave (SDW) type transition at 190 K and antiferromagnetic (AF) order below 20 K. Here, we have studied the effect of K substitution on the SDW transition at high temperature and AF Eu order at low temperature. 50% K substitution suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below T(c) = 32 K, as observed in the electrical resistivity, AC susceptibility, as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity. Below 10 K, short-range magnetic order of the Eu moments is suggested by a broad feature in the specific-heat data. Electronic structure calculations reveal very close similarity with the nonmagnetic superconductor Sr(0.5)K(0.5)Fe(2)As(2), but yield localized 4f magnetic moments for the remaining Eu atoms.

We report a systematic study of the influence of antiferromagnetic and ferromagnetic phases of Eu2+ moments on the superconducting phase upon doping the As site by isovalent P, which essentially acts like chemical pressure on EuFe2As2. Bulk superconductivity with transition temperatures of 22 and 28 K are observed for x = 0.16 and 0.20 samples, respectively. The Eu ions order antiferromagnetically for x <= 0.13, while bulk superconductivity coexists with Eu-antiferromagnetism for 0.13 < x < 0.22. In contrast, a crossover is observed for x >= 0.22 whereupon the Eu ions order ferromagnetically and superconductivity is fully suppressed. Density-functional-theory-based calculations reproduce the observed experimental findings consistently. We discuss in detail the coexistence of superconductivity and magnetism in a tiny region of the phase space and comment on the competition of ferromagnetism and superconductivity in the title compound.