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Due to competing long range ferromagnetic order, the transition metals Fe, Co and Ni are not superconductors at ambient pressure. While superconductivity was observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to be discovered in Co and Ni under any experimental conditions. Here, we report emergence of superconductivity in the recently discovered high-density nonmagnetic face centered cubic phase in Co thin films below a transition temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact spectroscopy and resistance, and four-probe measurements of resistance at ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of Co within first-principles density functional theory, and show that its superconductivity below 5 K originates from anomalous softening of zone-boundary phonons and their enhanced coupling with electrons upon biaxial strain.
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The observation of superconductivity in the layered transition metal oxide NaxCoO2 y H2O (K. Takada et al., Nature 422, 53 (2003)) has caused a tremendous upsurge of scientific interest due to its similarities and its differences to the copper based high-temperature superconductors. Two years after the discovery, we report the fabrication of single-phase superconducting epitaxial thin films of Na0.3CoO2 x 1.3 D2O grown by pulsed laser deposition technique. This opens additional roads for experimental research exploring the superconducting state and the phase diagram of this unconventional material.
We report the high-field superconducting properties of thin, disordered Re films via magneto-transport and tunneling density of states measurements. Films with thicknesses in the range of 9 nm to 3 nm had normal state sheet resistances of $sim$0.2 k$Omega$ to $sim$1 k$Omega$ and corresponding transition temperatures in the range of 6 K to 3 K. Tunneling spectra were consistent with those of a moderate coupling BCS superconductor. Notwithstanding these unremarkable superconducting properties, the films exhibited an extraordinarily high upper critical field. We estimate their zero-temperature $H_{c2}$ to be more than twice the Pauli limit. Indeed, in 6 nm samples the estimated reduced critical field $H_{c2}/T_csim$ 5.6 T/K is among the highest reported for any elemental superconductor. Although the sheet resistances of the films were well below the quantum resistance $R_Q=h/4e^2$, their $H_{c2}$s approached the theoretical upper limit of a strongly disordered superconductor for which $k_Fellsim1$.
We investigated the effect of alloying on the upper critical field $H_{c2}$ in 12 $MgB_2$ films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant $H_{c2}$ enhancement in C-alloyed films, and an anomalous upward curvature of $H_{c2}(T)$. Record high values of $H_{c2}^{perp}(4.2) simeq 35T$ and $H_{c2}|(4.2) simeq 51T$ were observed perpendicular and parallel to the ab plane, respectively. The temperature dependence of $H_{c2}(T)$ is described well by a theory of dirty two-gap superconductivity. Extrapolation of the experimental data to T=0 suggests that $H_{c2}|(0)$ approaches the paramagnetic limit of $sim 70T$.
This paper has been withdrawn by the author due to journal requirement.
In this article, we studied the role of oxygen in Pr$_{2}$CuO$_{4pmdelta}$ thin films fabricated by polymer assisted deposition method. The magnetoresistance and Hall resistivity of Pr$_{2}$CuO$_{4pmdelta}$ samples were systematically investigated. It is found that with decreasing the oxygen content, the low-temperature Hall coefficient ($R_H$) and magnetoresistance change from negative to positive, similar to those with the increase of Ce-doped concentration in R$_{2-x}$Ce$_{x}$CuO$_{4}$ (R= La, Nd, Pr, Sm, Eu). In addition, $T_c$ versus $R_H$ for both Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$ and Pr$_{2}$CuO$_{4pmdelta}$ samples can coincide with each other. We conclude that the doped electrons induced by the oxygen removal are responsible for the superconductivity of $T^prime$-phase parent compounds.
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