Ferromagnetic Fluctuations in the Heavily Overdoped Regime of Single-Layer High-Tc Cuprate Superconductors.

To investigate proposed ferromagnetic fluctuations in the so-called single-layer Bi-2201 and La-214 high-Tc cuprates, we performed magnetization and electrical resistivity measurements using single-layer Tl-2201 cuprates Tl2Ba2CuO6+δ and La-214 La2-xSrxCuO4 in the heavily overdoped regime. Magnetization of Tl2Ba2CuO6+δ and La2-xSrxCuO4 exhibited the tendency to be saturated in high magnetic fields at low temperatures, suggesting the precursor behavior toward the formation of a ferromagnetic order. It was found that the power of temperature n obtained from the temperature dependence of the electrical resistivity is ~4/3 and ~5/3 for Bi-2201 and La2-xSrxCuO4, respectively, and is ~4/3 at high temperatures and ~5/3 at low temperatures in Tl2Ba2CuO6+δ. These results suggest that two- and three-dimensional ferromagnetic fluctuations exist in Bi-2201 and La2-xSrxCuO4, respectively. In Tl2Ba2CuO6+δ, it is suggested that the dimension of ferromagnetic fluctuations is two at high temperatures and three at low temperatures, respectively. The dimensionality of ferromagnetic fluctuations is understood in terms of the dimensionality of the crystal structure and the bonding of atoms in the blocking layer.

Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor Bi2Sr2CaCu2O8+δ as a Candidate of a Topological Superconductor.

One of the key challenges in condensed-matter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen-Cooper-Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures ( Tc) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi2Sr2CaCu2O8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to Tc of Bi2212 (85 K). The small Fermi energy and strong spin-orbit coupling of Bi(110), together with the large pairing gap and high Tc, make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

Development of Ferromagnetic Fluctuations in Heavily Overdoped (Bi,Pb)_{2}Sr_{2}CuO_{6+δ} Copper Oxides.

We demonstrate the presence of ferromagnetic (FM) fluctuations in the superconducting and nonsuperconducting heavily overdoped regimes of high-temperature superconducting copper oxides, using (Bi,Pb)_{2}Sr_{2}CuO_{6+δ} (Bi-2201) single crystals. Magnetization curves exhibit a tendency to be saturated in high magnetic fields at low temperatures in the heavily overdoped crystals, which is probably a precursor phenomenon of a FM transition at a lower temperature. Muon spin relaxation detects the enhancement of spin fluctuations at high temperatures below 200 K. Correspondingly, the ab-plane resistivity follows a 4/3 power law in a wide temperature range, which is characteristic of metals with two-dimensional FM fluctuations due to itinerant electrons. As the Wilson ratio evidences the enhancement of spin fluctuations with hole doping in the heavily overdoped regime, it is concluded that two-dimensional FM fluctuations reside in the heavily overdoped Bi-2201 cuprates, which is probably related to the decrease in the superconducting transition temperature in the heavily overdoped cuprates.

A Mössbauer effect study of single crystals of the non-superconducting parent compound Fe1.09Te and the superconductor FeSe0.4Te0.6.

The results of a (57)Fe Mössbauer spectroscopy study between 2.0 and 297 K of the parent compound Fe1.09Te and the superconductor FeSe0.4Te0.6 are reported. It is shown that in both compounds the magnitude of the quadrupole splitting increases with decreasing temperature and is well described by a T(3/2) power-law relation. The presence of incommensurate spin-density-wave antiferromagnetism in Fe1.09Te is demonstrated, with the Néel temperature T(N) = 71.1(6) K. A theoretical prediction (Zhang et al 2009 Phys. Rev. B 79 012506) of the Fe magnetic moment at the 2c sites being significantly larger than that at the 2a sites in the parent compound is confirmed experimentally by showing that these moments at 4.4 K are, respectively, 3.20(4) and 1.78(3) µ(B). The absence of magnetic order in FeSe0.4Te0.6 down to 2.0 K is confirmed. The Debye temperatures of Fe1.09Te and FeSe0.4Te0.6 are found to be 290(1) and 233(1) K, respectively.