We investigated the elastic properties of the iron-based superconductor Ba(Fe 1Àx Co x ) 2 As 2 with eight Co concentrations. The elastic constant C 66 shows a large elastic softening associated with structural phase transition. C 66 was analyzed on the basis of the localized and itinerant pictures of Fe-3d electrons, which shows a strong electronlattice coupling and a possible mass enhancement in this system. The results are similar to those of unconventional superconductors, where the properties of the system are governed by quantum fluctuations associated with the zerotemperature critical point of long-range order, namely, the quantum critical point (QCP). In this system, the inverse of C 66 behaves just like the magnetic susceptibility in magnetic QCP systems. Although the QCPs of these existing superconductors are all ascribed to antiferromagnetism, our systematic studies on the canonical iron-based superconductor Ba(Fe 1Àx Co x ) 2 As 2 have revealed that there is a signature of ''structural quantum criticality'' in this material, which thus far has had no precedent. The elastic constant anomaly is suggested to concern with the emergence of superconductivity. These results highlight the strong electron-lattice coupling and effect of the band in this system, thus challenging the prevailing scenarios that focus on the role of iron 3d orbitals.

We report the magnetotransport and dc magnetic susceptibility of the polycrystalline samples of Pr2Ba4Cu7O 15−δ , to examine the effect of magnetic field on the superconducting phase of the metallic CuO double chain. The resistive critical magnetic field is estimated to be about 21 T at low temperatures from the resistive transition data. On the other hand, the corresponding critical field determined from the magnetization measurements gives rise to a very low value of ∼ 0.3 T at 2 K. These discrepancies in the magnetic response between the resistivity and magnetization data are caused by disappearance of the magnetically shielding effect even in relatively lower fields. In spite of the observation of the resistive drop associated with the superconducting transport currents, the suppression of the diamagnetic signal is probably related to the superconductivity of quasi one-dimensional CuO double-chain. The behavior of Seebeck coefficient in the superconducting Pr2Ba4Cu7O 15−δ is discussed on the basis of the double chain model from the density functional band calculation. PACS numbers: 74.25.Ha,74.90.+n

We have reported the effect of pressure on the magnetization, and transport properties in the nominal composition Pr2Ba4Cu7O 15−δ synthesized by a sol-gel technique. A reduction treatment of the as-sintered sample in vacuum causes higher superconductivity achieving Tc,on =∼ 30 K for δ = 0.94. Application of hydrostatic pressure on the oxygen depleted sample enhances its onset temperature up to 36 K at 1.2 GPa, indicating the nearly optimum doping level of the charge carrier in comparison to the pressure dependence of lower Tc samples with δ = 0.45. Seebeck coefficient of the superconducting sample shows a metallic conduction, followed by a clear drop below Tc,on and is in its temperature dependence below 100 K quite different from that of the non-superconducting one. This finding strongly suggests a dramatic change of the electronic state along the CuO double chain due to the reduction treatment for the appearance of superconductivity .

No abstract

We study the magnetization damping in the simplest of synthetic antiferromagnets, i.e. antiferromagnetically exchange-coupled spin valves in which applied magnetic fields tune the magnetic configuration to become noncollinear. We formulate the dynamic exchange of spin currents in a noncollinear texture based on the spin diffusion model with quantum mechanical boundary conditions at the interfaces and derive the Landau-Lifshitz-Gilbert equations coupled via the static interlayer non-local and the dynamic exchange interactions. We predict non-collinearity-induced additional damping that can be sensitively modulated by an applied magnetic field. The theoretical results compare favorably with published experiments.