Doping dependence of the upper critical field in La_{2-x}Sr_xCuO_4 from specific heat

The low-temperature specific heat of La_{2-x}Sr_xCuO_4 (LSCO) single crystals in magnetic field H up to 12 T has been examined over a wide range of doping (0.063=< p =<0.238). From this we have mapped the upper critical field H_{c2} of LSCO across the entire superconducting diagram. It is found that the H_{c2} shows a doping dependence similar to that of the critical temperature T_c. We have discussed the implications of the result and proposed that there may be an effective superconducting energy scale responsible for the H_{c2} behavior in the underdoped region.Comment: 6 pages,3 figures,1 tabl

BaFe2Se2O as an Iron-Based Mott Insulator with Antiferromagnetic Order

A new compound with a quasi-two-dimensional array of FeSe3O tetrahedra and an orthorombic structure, namely BaFe2Se2O, has been successfully fabricated. Experimental results show that this compound is an insulator and has an antiferromagnetic (AF) transition at 240 K. Band structure calculation reveals the narrowing of Fe 3d bands near the Fermi energy, which leads to the localization of magnetism and the Mott insulating behavior. The large distances between the Fe atoms perhaps are responsible for the characters. Linear response calculation further indicates a strong in-plane AF exchange $J$, this can account for the enhanced magnetic susceptibility (which has a maximum at about 450 K) above the Neel temperature.Comment: submitted to PRL on 2 May 2012, resubmitted to PRB on 31 May 2012, and accepted by PRB on 5 July 201