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We have successfully synthesized three quasi-2D geometrically frustrated magnetic compounds (\alpha-MCr_2O_4, M=Ca, Sr, Ba) using the spark-plasma-sintering technique. All these members of the \alpha-MCr_2O_4 family consist of the stacking planar triangular lattices of Cr$^{3+}$ spins (${\rm S}=3/2$), separated by non-magnetic alkaline earth ions. Their corresponding magnetic susceptibility, specific heat, dielectric permittivity and ferroelectric polarization are systematically investigated. A long-range magnetic ordering arises below the N\'{e}el temperature (around 40K) in each member of the \alpha-MCr_2O_4 family, which changes to the quasi-120\degree proper-screw-type helical spin structure at low temperature. A very small but confirmed spontaneous electric polarization emerges concomitantly with this magnetic ordering. The direction of electric polarization is found within the basal triangular plane. The multiferroicity in \alpha-MCr_2O_4 can not be explained within the frameworks of the magnetic exchange striction or the inverse Dzyaloshinskii-Moriya interaction. The observed results are more compatible with the newly proposed Arima mechanism that is associated the d-p hybridization between the ligand and transition metal ions, modified by the spin-orbit coupling. The evolution of multiferroic properties with the increasing inter-planar spacing (as M changes from Ca to Ba) reveals the importance of interlayer interaction in this new family of frustrated magnetic systems., Comment: 10 pages, 8 figures, published online in Physical Review B

We have carried out a systematic study of the PbO-type compound FeSe_{1-x}Te_x (x = 0~1), where Te substitution effect on superconductivity is investigated. It is found that superconducting transition temperature reaches a maximum of Tc=15.2K at about 50% Te substitution. The pressure-enhanced Tc of FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe., Comment: Some typing errors are corrected; we take out one figures, now the paper has 14 pages, 5 figures

The recent discovery of superconductivity with relatively high transition temperature Tc in the layered iron-based quaternary oxypnictides La[ O1-xFx] FeAs was a real surprise. The excitement generated can be seen by the number of subsequent works published within a very short period of time. Although there exists superconductivity in alloy that contains Fe element, LaOMPn (with M= Fe, Ni; and Pn=P and As) is the first system where Fe-element plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to ask whether there exists other Fe based planar compounds that exhibit superconductivity. Here we report the observation of superconductivity with zero resistance transition temperature at 8K in the PbO-type alpha-FeSe compound. Although FeSe has been studied quite extensively, a key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. What is truly striking, is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Furthermore, FeSe is, compared with LaOFeAs, much easier to handle and fabricate. In view of the abundance of compounds with PbO type structure, this result opens a new route to the search for unconventional superconductors., Comment: PDF format, 10 pages, 5 figures