Organic materials are believed to be potential superconductor with high transition temperature (TC). Organic superconductors mainly have two families: the quasi-one dimensional (TMTSF)2X and two dimensional (BEDT-TTF)2X (Ref. 1 and 2), in which TMTSF
is tetramethyltetraselenafulvalene (C10H12Se4) and BEDT-TTF or ET is bis(ethylenedithio)tetrathiafulvalene (C10H8S8). One key feature of the organic superconductors is that they have {pi}-molecular orbitals, and the {pi}-electron can delocalize throughout the crystal giving rise to metallic conductivity due to a {pi}-orbital overlap between adjacent molecules. The introduction of charge into C60 solids and graphites with {pi}-electron networks by doping to realize superconductivity has been extensively reported3,4. Very recently, superconductivity in alkali-metal doped picene with {pi}-electron networks was reported5. Here we report the discovery of superconductivity in potassium doped Phenanthrene with TC~5 K. TC increases with increasing pressure, and the pressure of 1 GPa leads to an increase of 20% in TC, suggesting that the potassium doped phenanthrene shows unconventional superconductivity. Both phenanthrene and picene are polycyclic aromatic hydrocarbons, and contain three and five fused benzene rings, respectively. The ribbon of fused benzene rings is part of graphene. Therefore, the discovery of superconductivity in K3Phenanthrene produces a novel broad class of superconductors consisting of fused hydrocarbon benzene rings with {pi}-electron networks. The fact that TC increases from 5 K for KxPhenanthrene with three benzene rings to 18 K for Kxpicene with five benzene rings suggests that such organic hydrocarbons with long benzene rings is potential superconductor with high TC.
The recent discovery of superconductivity in oxypnictides with the critical temperature (TC) higher than McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer (BCS) theory) has generated great excitement. Theoretical
calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, while strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. However, superconductivity and magnetism in pnictide superconductors show a strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotopic mass on Tc and the spin-density wave (SDW) transition temperature (TSDW) in SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The results show that oxygen isotope effect on TC and TSDW is very little, while the iron isotope exponent alpha=-dlnTc/dlnM is about 0.35, being comparable to 0.5 for the full isotope effect. Surprisingly, the iron isotope exchange shows the same effect on TSDW as TCc These results indicate that electron-phonon interaction plays some role in the superconducting mechanism, but simple electron-phonon coupling mechanism seems to be rather unlikely because a strong magnon-phonon coupling is included. Sorting out the interplay between the lattice and magnetic degrees of freedom is a key challenge for understanding the mechanism of high-TC superconductivity.
We prepared the samples K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ (Ln=Sm, Nd and La) with ThCr$_2$Si$_2$-type structure. These samples were characterized by X-ray diffraction, resistivity, susceptibility and thermoelectric power (TEP). Substitution of Ln (Ln=La,
Nd and Sm) for K in K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system raises the superconducting transition temperature to 34-36 K. The TEP measurements indicate that the TEP of K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ is positive, being similar to the case of the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system with p-type carrier. In the K$_{1-x}$Ln$_{x}$Fe$_2$As$_2$ system, the superconducting $KFe_2As_2$ with $T_csim 3$ K is the parent compound, and no structural and spin-density wave instabilities exist in this system.
We synthesized the samples $Ba_{1-x}M_xFe_2As_2$ (M=La and K) with $ThCr_2Si_2$-type structure. These samples were systematically characterized by resistivity, thermoelectic power (TEP) and Hall coefficient ($R_H$). $BaFe_2As_2$ shows an anomaly in r
esistivity at about 140 K. Substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observed. Potassium doping leads to suppression of the anomaly in resistivity and induces superconductivity at 38 K as reported by Rotter et al.cite{rotter}. The Hall coefficient and TEP measurements indicate that the TEP is negative for $BaFe_2As_2$ and La-doped $BaFe_2As_2$, indicating n-type carrier; while potassium doping leads to change of the sign in $R_H$ and TEP. It definitely indicates p-type carrier in superconducting $Ba_{1-x}K_xFe_2As_2$ with double FeAs layers, being in contrast to the case of $LnO_{1-x}F_xFeAs$ with single FeAs layer. A similar superconductivity is also observed in the sample with nominal composition $Ba_{1-x}K_xOFe_2As_2$.
Since the discovery of superconductivity in the cuprates two decades ago, it has been firmly established that the CuO_2 plane is consequential for high T_C superconductivity and a host of other very unusual properties. A new family of superconductors
with the general composition of LaFeAsO_(1-x)F_x has recently been discovered but with the conspicuous lacking of the CuO_2 planes, thus raising the tantalizing questions of the different pairing mechanisms in these oxypnictide superconductors. Intimately related to pairing in a superconductor are the superconducting gap, its value, structure, and temperature dependence. Here we report the observation of a single gap in the superconductor SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but dramatically different from that of the pseudogap behavior in the cuprate superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, t-J model, and the like, originally designed for cuprates.
The magnetic fluctuations associated with a quantum critical point (QCP) are widely believed to cause the non-Fermi liquid behaviors and unconventional superconductivities, for example, in heavy fermion systems and high temperature cuprate supercondu
ctors. Recently, superconductivity has been discovered in iron-based layered compound $LaO_{1-x}F_xFeAs$ with $T_c$=26 Kcite{yoichi}, and it competes with spin-density-wave (SDW) ordercite{dong}. Neutron diffraction shows a long-rang SDW-type antiferromagnetic (AF) order at $sim 134$ K in LaOFeAscite{cruz,mcguire}. Therefore, a possible QCP and its role in this system are of great interests. Here we report the detailed phase diagram and anomalous transport properties of the new high-Tc superconductors $SmO_{1-x}F_xFeAs$ discovered by uscite{chenxh}. It is found that superconductivity emerges at $xsim$0.07, and optimal doping takes place in the $xsim$0.20 sample with highest $T_c sim $54 K. While $T_c$ increases monotonically with doping, the SDW order is rapidly suppressed, suggesting a QCP around $x sim$0.14. As manifestations, a linear temperature dependence of the resistivity shows up at high temperatures in the $x<0.14$ regime, but at low temperatures just above $T_c$ in the $x>0.14$ regime; a drop in carrier density evidenced by a pronounced rise in Hall coefficient are observed, which mimic the high-$T_c$ cuprates. The simultaneous occurrence of order, carrier density change and criticality makes a compelling case for a quantum critical point in this system.
