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Odd-parity superconductivity is recognized as a key ingredient to realize topological superconductivity. In Bi2Se3-based bulk topological superconductors, the odd-parity pairing, which spontaneously breaks the threefold rotational symmetry of the crystal, leading to a subsidiary nematic order, was discovered. The nematic superconducting state can be identified by thermodynamic measurement, nuclear magnetic resonance measurement, ac- $$\chi$$ measurement, magnetic torque measurement or magnetoresistance measurement. Here, the single crystal of topological superconductor Pd $$_x$$ Bi2Te3 was grown. Its upper critical fields were obtained by magnetoresistance measurement in various magnetic field orientations parallel to the basal plane. The in-plane upper critical fields clearly demonstrate a twofold symmetry that break the threefold crystal symmetry. This provides experimental evidence that Pd-doped Bi2Te3 is a nematic topological superconductor similar to the Cu-, Sr-, and Nb-doped Bi2Se3.

We report the superconductivity of a new quaternary compound ThMo$_2$Si$_2$C, synthesized with the arc-melting technique. The compound crystallizes in a tetragonal CeCr$_2$Si$_2$C-type structure with cell parameters of $a$ = 4.2296 \r{A}and $c$ = 5.3571 \r{A}. An interlayer Si-Si covalent bonding is suggested by the atomic distance. The electrical resistivity and magnetic susceptibility measurements indicate a Pauli-paramagnetic metal with dominant electron-electron scattering in the normal-state. Bulk superconductivity at 2.2 K is demonstrated with a dimensionless specific-heat jump of ${\Delta}C/\gamma_{\rm n}T$ = 0.98. The superconducting parameters of the critical magnetic fields, coherence length, penetration depth, and superconducting energy gap are given., Comment: 14 pages, 6 figures, 3 tables. Accepted for publication in SCIENCE CHINA Physics, Mechanics & Astronomy

We have synthesized a new series of layered oxyselenides Bi 2 LnO 4 Cu 2 Se 2 (Ln=Nd, Sm, Eu, Dy, Er, Yb). Their crystal structures and physical properties were studied through X-ray diffraction, electric transport measurements, bulk magnetization and first-principle calculation. All these compounds have a tetragonal structure with space group I4/mmm. They exhibit hole-type metallic behaviours which is also verified by the DFT calculation. The new Bi 2 LnO 4 -type block in these compounds may give people some enlightenment in synthesizing new iron-based superconductors or other layered compounds.

Recently, An electron-doped 12442-type iron-based superconductor BaTh$_2$Fe$_4$As$_4$(N$_{0.7}$O$_{0.3}$)$_2$ has been successfully synthesized with high-temperature solid-state reactions on basis of a structural design. The inter-block-layer charge transfer between the constituent units of "BaFe$_2$As$_2$" and "ThFeAsN$_{0.7}$O$_{0.3}$" was found to be essential to stabilize the target compound. Dominant electron-type conduction and bulk superconducting transition at ~22 K were demonstrated., Comment: 16 pages in total. 5 figures, 1 table (main article); 6 figures, 3 tables (Supporting Information)

The relationship between the doping position of Nb in Bi2Se3 and its superconductivity is investigated by XPS, magnetic measurement and transport measurement. It is found that for NbxBi2Se3, the main impurity phase comes from BiNbSe3. Superconducting volume fraction of Nb doped Bi2Se3 increases with the ratio of Nb inserted in the van der Waals layers. The van der Waals-layer-position doped Nb may lead to superconductivity in Bi2Se3. This work may stimulate further investigation on topological superconductivity mechanism of NbxBi2Se3.

We use neutron powder diffraction to study on the non-superconducting phases of ThFeAsN$_{1-x}$O$_x$ with $x=0.15, 0.6$. In our previous results on the superconducting phase ThFeAsN with $T_c=$ 30 K, no magnetic transition is observed by cooling down to 6 K, and possible oxygen occupancy at the nitrogen site is shown in the refinement(H. C. Mao \emph{et al.}, EPL, 117, 57005 (2017)). Here, in the oxygen doped system ThFeAsN$_{1-x}$O$_x$, two superconducting region ($0\leqslant x \leqslant 0.1$ and $0.25\leqslant x \leqslant 0.55$) have been identified by transport experiments (B. Z. Li \emph{et al.}, J. Phys.: Condens. Matter 30, 255602 (2018)). However, within the resolution of our neutron powder diffraction experiment, neither the intermediate doping $x=0.15$ nor the heavily overdoped compound $x= 0.6$ shows any magnetic order from 300 K to 4 K. Therefore, while it shares the common phenomenon of two superconducting domes as most of 1111-type iron-based superconductors, the magnetically ordered parent compound may not exist in this nitride family., Comment: 4 pages, 4 figures

ThFeAsN$_{1-x}$O$_x$ ($0\leq x\leq0.6$) system with heavy electron doping has been studied by the measurements of X-ray diffraction, electrical resistivity, magnetic susceptibility and specific heat. The non-doped compound exhibits superconductivity at $T_\mathrm{c}^\mathrm{onset}=30$ K, which is possibly due to an internal uniaxial chemical pressure that is manifested by the extremely small value of As height with respect to the Fe plane. With the oxygen substitution, the $T_\mathrm{c}$ value decreases rapidly to below 2 K for $0.1\leq x\leq0.2$, and surprisingly, superconductivity re-appears in the range of $0.25\leq x\leq0.5$ with a maximum $T_\mathrm{c}^\mathrm{onset}$ of 17.5 K at $x=0.3$. For the normal-state resistivity, while the samples in intermediate non-superconducting interval exhibit Fermi liquid behavior, those in other regions show a non-Fermi-liquid behavior. The specific heat jump for the superconducting sample of $x=0.4$ is $\Delta C/(\gamma T_\mathrm{c})=0.89$, which is discussed in terms of anisotropic superconducting gap. The peculiar phase diagram in ThFeAsN$_{1-x}$O$_x$ presents additional ingredients for understanding the superconducting mechanism in iron-based superconductors., Comment: 10 pages, 7 figures

The recently synthesized ThFeAsN iron-pnictide superconductor exhibits a $T_c$ of 30 K, the highest of the 1111-type series in absence of chemical doping. To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the ${}^{75}$As Knight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multi-band $s^{\pm}$-wave gap symmetry in the dirty limit, while the gap-to-$T_c$ ratio $\Delta/k_\mathrm{B}T_c$ hints at a strong-coupling scenario. Pressure modulates the geometrical parameters, thus reducing $T_c$, as well as $T_m$, the temperature where magnetic-relaxation rates are maximized, both at the same rate of approximately -1.1 K/GPa. This decrease of $T_c$ with pressure is consistent with band-structure calculations, which relate it to the deformation of the Fe 3$d_{z^2}$ orbitals., Comment: 6 pages, 4 figures

We have investigated the superconducting ground state of the newly discovered superconductor ThFeAsN with a tetragonal layered crystal structure using resistivity, magnetization, heat capacity, and transverse-field muon-spin rotation ($\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$) measurements. Our magnetization and heat-capacity measurements reveal an onset of bulk superconductivity with ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A nonlinear magnetic-field dependence of the specific heat coefficient $\ensuremath{\gamma}(H)$ has been found in the low-temperature limit, which indicates that there is a nodal energy gap. Our analysis of the $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ results shows that the temperature dependence of the superfluid density is better described by a two-gap model either isotropic $s+s$ wave or $s+d$ wave than a single-gap isotropic $s$-wave model for the superconducting gap, consistent with other Fe-based superconductors. The combination of $\ensuremath{\gamma}(H)$ and $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ results suggest that the ($s+d$)-wave model is the most consistent candidate for the gap structure of ThFeAsN. The observation of two gaps in ThFeAsN suggests a multiband nature of the superconductivity possibly arising from the $d$ bands of Fe ions. Furthermore, from our $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ study we have estimated the magnetic penetration depth in the polycrystalline sample of ${\ensuremath{\lambda}}_{L}(0)=375\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, superconducting carrier density ${n}_{s}=4.97\ifmmode\times\else\texttimes\fi{}{10}^{27}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$, and carrier's effective-mass ${m}^{*}=2.48{m}_{e}$. We compare the results of our present paper with those reported for the Fe-pnictide families of superconductors.

We report neutron diffraction and transport results on the newly discovered superconducting nitride ThFeAsN with $T_c=$ 30 K. No magnetic transition, but a weak structural distortion around 160 K, is observed cooling from 300 K to 6 K. Analysis on the resistivity, Hall transport and crystal structure suggests this material behaves as an electron optimally doped pnictide superconductors due to extra electrons from nitrogen deficiency or oxygen occupancy at the nitrogen site, which together with the low arsenic height may enhance the electron itinerancy and reduce the electron correlations, thus suppress the static magnetic order., 4 pages, 4 figures, Accepted by EPL

Here we report effects of pressure on the recently discovered iron-based superconductor ThFeAsN, the first FeAs-based superconductor containing an actinide element, by in situ high-pressure resistance and synchrotron X-ray diffraction measurements. Our results show that the superconducting transition temperature (T C ) decreases monotonously with increasing pressure and then vanishes at 25.4 GPa. The analysis of the As anion height above the Fe layer ( ), as well as the bond angle of As-Fe-As (α ), for the pressurized ThFeAsN sample found that its T C vs. or α follows the same universal curve describing most of the iron-based superconductors. The results indicate that the lattice parameters also control the superconductivity of the ThFeAsN superconductor, similar to other FeAs-based superconductors, as long as the “superconducting gene” is a FeAs layer. Furthermore, we demonstrate that the ambient-pressure ThFeAsN is an “intrinsic” superconductor, while its lattice structure is not in the optimal state of the FeAs-based superconductors.

We report the first nitrogen-containing iron-pnictide superconductor ThFeAsN, which is synthesized by a solid-state reaction in an evacuated container. The compound crystallizes in a ZrCuSiAs-type structure with the space group P4/nmm and lattice parameters a=4.0367(1) {\AA} and c=8.5262(2) {\AA} at 300 K. The electrical resistivity and dc magnetic susceptibility measurements indicate superconductivity at 30 K for the nominally undoped ThFeAsN., Comment: 6 pages, 4 figures, 1 table

We report the antimony (Sb) doping effect in a prototype system of iron-based supercon-ductors LaFeAsO1-yFy (y=0, 0.1, 0.15). X-ray powder diffraction indicates that the lattice pa-rameters increase with Sb content within the doping limit. Rietveld structural refinements show that, with the partial substitution of Sb for As, while the thickness of the Fe2As2 layers increases significantly, that of the La2O2 layers shrinks simultaneously. So a negative chemical pressure is indeed "applied" to the superconducting-active Fe2As2 layers, in con-trast to the effect of positive chemical pressure by the phosphorus doping. Electrical resis-tance and magnetic susceptibility measurements indicate that, while the Sb doping hardly influences the SDW anomaly in LaFeAsO, it recovers SDW order for the optimally-doped sample of y=0.1. In the meantime, the superconducting transition temperature can be raised up to 30 K in LaFeAs1-xSbxO1-yFy with x=0.1 and y=0.15. The Sb doping effects are discussed in term of both J1-J2 model and Fermi Surface (FS) nesting scenario., Comment: 7 pages, 4 figures, 1 table. to be published in Science in China Series G

We report the synthesis and physical properties of a nickel-based arsenide ThNiAsN. The new compound crystallizes in a ZrCuSiAs-type structure (space group ) with lattice parameters and . Bulk superconductivity at is demonstrated by the measurements of electrical resistivity, dc magnetic susceptibility, and heat capacity. The upper critical field at zero temperature, , and the specific-heat jump at T c are 6.1 T and , respectively. These two physical quantities, together with the T c value, are the highest among the Ni-based ZrCuSiAs-type materials. We discuss this enhanced superconductivity in terms of the internal chemical pressure in ThNiAsN.

We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. The P-for-As substitution results in shrinkage of lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with the maximum $T_c$ of 30 K emerges at $x$=0.32, coinciding with a magnetic quantum critical point (QCP) which is evidenced by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The $T_c$ values were found to decrease with further P doping, and no superconductivity was observed down to 2 K for $x\geq$ 0.77. The appearance of superconductivity in the vicinity of QCP hints to the superconductivity mechanism in iron-based arsenides., 9 pages, 4 figures; more data; to appear in Journal of Physics: Condensed Matter

We have studied a quinary Fe-based superconductor ${\text{Sr}}_{2}{\text{VFeAsO}}_{3}$ by the measurements of x-ray diffraction, x-ray absorption, M\"ossbauer spectrum, resistivity, magnetization, and specific heat. This apparently undoped oxyarsenide is shown to be self-doped via electron transfer from the ${\text{V}}^{3+}$ ions. We observed successive magnetic transitions within the ${\text{VO}}_{2}$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the ${\text{VO}}_{2}$ planes are discussed based on mixed valence of ${\text{V}}^{3+}$ and ${\text{V}}^{4+}$.

We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped $\text{Eu}{({\text{Fe}}_{0.89}{\text{Co}}_{0.11})}_{2}{\text{As}}_{2}$ single crystal. We observed a resistivity drop at ${T}_{c}\ensuremath{\sim}21\text{ }\text{K}$, which shifts toward lower temperatures under external fields, suggesting a superconducting transition. The upper critical fields near ${T}_{c}$ show large anisotropy, in contrast with those of other ``122'' FeAs-based superconductors. Low-field magnetic susceptibility data also show evidence of superconductivity below 21 K. Instead of the expected zero resistance below ${T}_{c}$, however, a resistivity re-entrance appears at 17 K under zero field, coincident with the magnetic ordering of ${\text{Eu}}^{2+}$ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the ${\text{Eu}}^{2+}$ spins is proposed. External magnetic fields easily change the helimagnetism into ferromagnetism with fully polarized ${\text{Eu}}^{2+}$ spins, accompanying by disappearance of the resistivity re-entrance. Therefore, superconductivity coexists with ferromagnetic state of ${\text{Eu}}^{2+}$ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$.

We have studied Ni-substitution effect in ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Ni}}_{x}\text{AsO}$ $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03\ensuremath{\le}x\ensuremath{\le}0.06$ with the maximum ${T}_{c}$ of 6.5 K at $x=0.04$, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered ${T}_{c}$ in comparison with ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system is discussed.

We have studied ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}{)}_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance, and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}{)}_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of ${\mathrm{Eu}}^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing the coexistence of superconductivity and ferromagnetism in the ferroarsenide family.

We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$\leq$\emph{x}$\leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $x\geq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $x\geq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.

We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe$_2$As$_2$ layers are squeezed while the La$_2$O$_2$ layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at $\sim$10 K for the optimally-doped LaFeAs$_{1-x}$P$_{x}$O ($x=0.25\sim0.3$). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency., 5 pages, 6 figures, more data presented

Ternary iron arsenide ${\text{EuFe}}_{2}{\text{As}}_{2}$ with ${\text{ThCr}}_{2}{\text{Si}}_{2}$-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall, and specific-heat measurements. The compound undergoes two magnetic phase transitions at about 200 and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower, and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an $A$-type antiferromagnetic ordering of ${\text{Eu}}^{2+}$ moments. Comparing with the physical properties of the isostructural compounds ${\text{BaFe}}_{2}{\text{As}}_{2}$ and ${\text{SrFe}}_{2}{\text{As}}_{2}$, we expect that superconductivity could be induced in ${\text{EuFe}}_{2}{\text{As}}_{2}$ through appropriate doping.

We report the measurements of anisotropic magnetization and magnetoresistance on single crystals of EuFe$_2$As$_2$, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin-density-wave transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu$^{2+}$ spins ($J=S=7/2$). The latter AFM state exhibits metamagnetic transition under magnetic fields. Upon applying magnetic field with $H\parallel c$ at 2 K, the magnetization increases linearly to 7.0 $\mu_{B}$/f.u. at $\mu_{0}H$=1.7 T, then keeps at this value of saturated Eu$^{2+}$ moments under higher fields. In the case of $H\parallel ab$, the magnetization increases step-like to 6.6 $\mu_{B}$/f.u. with small magnetic hysteresis. A metamagnetic phase was identified with the saturated moments of 4.4 $\mu_{B}$/f.u. The metamagnetic transition accompanies with negative in-plane magnetoresistance, reflecting the influence of Eu$^{2+}$ moments ordering on the electrical conduction of FeAs layers. The results were explained in terms of spin-reorientation and spin-reversal based on an $A$-type AFM structure for Eu$^{2+}$ spins. The magnetic phase diagram has been established., Comment: 10 pages, 8 figures. accepted for publication in New Journal of Physics as a special issue article

Stabilization of cobalt oxyhydrate superconductor was realized by post-treatment of the as-prepared superconductor in concentrated NaOH solution; this finding is expected to promote future experimental research on the intriguing material.

Stabilization of cobalt oxyhydrate superconductor was realized by post-treatment of the as-prepared superconductor in concentrated NaOH solution; this finding is expected to promote future experimental research on the intriguing material.

We report Zn-doping effect in the parent and F-doped LaFeAsO oxy-arsenides. Slight Zn doping in LaFe$_{1-x}$Zn$_{x}$AsO drastically suppresses the resistivity anomaly around 150 K associated with the antiferromagnetic (AFM) spin density wave (SDW) in the parent compound. The measurements of magnetic susceptibility and thermopower confirm further the effect of Zn doping on AFM order. Meanwhile Zn doping does not affect or even enhances the $T_c$ of LaFe$_{1-x}$Zn$_{x}$AsO$_{0.9}$F$_{0.1}$, in contrast to the effect of Zn doping in high-$T_c$ cuprates. We found that the solubility of Zn content ($x$) is limited to less than 0.1 in both systems and further Zn doping (i.e., $x$ $\geq$ 0.1) causes phase separation. Our study clearly indicates that the non-magnetic impurity of Zn$^{2+}$ ions doped in the Fe$_2$As$_2$ layers affects selectively the AFM order, and superconductivity remains robust against the Zn doping in the F-doped superconductors., Comment: 7 figures, 13 pages; revised version with more data

We demonstrate that, by the P/As substitution—without doping of charge carriers—in a FeAs-layer-based parent compound, superconductivity can be universally introduced. The maximum superconducting critical temperature ( T c ) of BaFe 2 ( As 1 - x P x ) 2 achieves 30 K. The P doping in LnFeAsO system (Ln = La and Sm) produces superconductivity below 11 K. The normal-state resistivity obeys linear temperature dependence and the normal-state Hall coefficient shows strong temperature dependence. These non-Fermi liquid behaviors suggest magnetic quantum criticality. The maximum T c values in different systems correlates strongly with the diagonal bondangle of Fe–As–Fe, implying the important role of the next-nearest-neighbor magnetic exchange coupling in iron pnictide superconductors.

The isovalent substitution effect of Ru in CeFe1−xRuxAsO (0 ⩽ x ⩽ 1) has been systematically studied by powder X-ray diffraction, electrical resistivity, magnetization, and specific heat measurements. The antiferromagnetic (AFM) ordering of both d and 4f electrons are suppressed upon Ru doping, followed by Pauli paramagnetism (d electrons) and local moment paramagnetism (4f electrons) with strong ferromagnetic fluctuation, respectively. Neither superconductivity above 2 K nor pronounced Kondo screening are observed in the substitution phase diagram. Combined with published results of the cerium-based quaternary compounds CeMXO(M = Fe, Ru; X = P, As), our data suggest that the end member CeRuAsO is on the verge of becoming an FM Kondo lattice. Meanwhile, the ground state of 4f electrons in the quaternary CeMXO system should be determined by both the interlayer d-f Kondo coupling (JKondo) and the intralayer Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction (JRKKY ), which are both very sensitive to the change in crystal structure.

The results of x-ray diffraction, electrical resistivity, magnetic susceptibility, and (57)Fe and (155)Gd Mössbauer spectroscopy studies of polycrystalline samples of nonsuperconducting GdFeAsO and superconducting Gd(0.84)Th(0.16)FeAsO are reported. The superconductor Gd(0.84)Th(0.16)FeAsO has the onset transition temperature of 54.4(1) K. We find that Fe and Gd magnetic moments in GdFeAsO order antiferromagnetically at 132.7(1) K and 4.1(1) K, respectively. In Gd(0.84)Th(0.16)FeAsO, Fe atoms carry no magnetic moment down to 1.9 K whereas Gd magnetic moments order below 2.9(1) K. This is taken as evidence for the coexistence of Gd magnetic order and superconductivity. The Debye temperatures of GdFeAsO and Gd(0.84)Th(0.16)FeAsO are, respectively, 409(4) and 389(3) K.

Following the discovery of superconductivity in an iron-based arsenide LaO1-xFxFeAs with a superconducting transition temperature (Tc) of 26 K[1], Tc was pushed up surprisingly to above 40 K by either applying pressure[2] or replacing La with Sm[3], Ce[4], Nd[5] and Pr[6]. The maximum Tc has climbed to 55 K, observed in SmO1-xFxFeAs[7, 8] and SmFeAsO1-x[9]. The value of Tc was found to increase with decreasing lattice parameters in LnFeAsO1-xFx (Ln stands for the lanthanide elements) at an apparently optimal doping level. However, the F- doping in GdFeAsO is particularly difficult[10,11] due to the lattice mismatch between the Gd2O2 layers and Fe2As2 layers. Here we report observation of superconductivity with Tc as high as 56 K by the Th4+ substitution for Gd3+ in GdFeAsO. The incorporation of relatively large Th4+ ions relaxes the lattice mismatch, hence induces the high temperature superconductivity., 4 pages, 3 figures