The recently discovered quaternary arsenide oxide superconductor La[O1-xFx]FeAs with the superconducting critical transition temperature (Tc) of 26 K [1], has been quickly expanded to another high-Tc superconducting system beyond copper oxides by the replacement of La with other rare earth elements, such as Sm, Ce, and Pr etc. [2-4], and the Pr[O1-xFx]FeAs has become to be the first non-cuprate superconductor that holding a Tc above 50 K. All these arsenide (including phosphide) superconductors formed in a same tetragonal layered structure with the space group P4/nmm which has an alternant stacked Fe-As layer and RO (R = rare earth metals) layer. Here we report the discovery of another superconductor in this system, the neodymium-arsenide Nd[O1-xFx]FeAs with an resistivity onset Tc of 51.9 K, which is the second non-cuprate compound that superconducts above 50 K.
Here we report the superconductivity in the iron-based oxyarsenide Sm[O1-xFx]FeAs, with the onset resistivity transition temperature at 55.0 K and Meissner transition at 54.6 K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides.
Since the discovery of copper oxide superconductor in 1986 [1], extensive efforts have been devoted to the search of new high-Tc superconducting materials, especially high-Tc systems other than cuprates. The recently discovered quaternary superconductor La[O1-xFx]FeAs with the superconducting critical transition Tc of 26 K [2], which has a much simple layered structure compared with cuprates, has attracted quick enthusiasm and is going to become a new high-Tc system [3-6]. Here we report the discovery of bulk superconductivity in the praseodymium-arsenide oxides Pr[O1-xFx]FeAs with an onset dr…
High temperature superconductors with a Tc above 40 K have been found to be strongly correlated electron systems and to have a layered structure. Guided by these rules, Kamihara et al. discovered a Tc up to 26 K in the layered La(O1-xFx)FeAs. By replacing La with tri-valence rare-earth elements RE of smaller ionic radii, Tc has subsequently been raised to 41-52 K. Many theoretical models have been proposed emphasizing the important magnetic origin of superconductivity in this compound system and a possible further Tc-enhancement in RE(O1-xFx)FeAs by compression. This later prediction appears t…
We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 hours and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96 %. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950 °C, 900 °C, 850 °C, 800 °C, 750 °C and 700 °C. It is found th…
The layered compound LaOBiSSe was successfully synthesized by a two-step solid-state reaction method, which is isostructural with the quaternary compound LaOBiS2 indicated by the powder X-ray diffraction characterization, and the crystal lattice parameters are slightly expanded by the substitution of Se atoms. Electrical transport measurement shows that LaOBiSSe exhibits semiconducting behavior similar with the parent compound LaOBiS2, while by F-doping the samples can be much more easily turned to be metallic and superconducting at low temperature with a small amount of F. Under the optimal c…
We report on the coexistence of magnetic and superconducting states in CeFeAsO1-xFx for x=0.06(2), characterized by transition temperatures T_m=30 K and T_c=18 K, respectively. Zero and transverse field muon-spin relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the CeFeAsO1-xFx phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies.
The Phase diagram of SmFeAsO1-xFx in terms of x is exhibited in this study. SmFeAsO1-xFx from x = 0 to x = 0.3 were prepared by low temperature sintering with slow cooling. The low temperature sintering suppresses the formation of the amorphous FeAs, which is inevitably produced as an impurity by using high temperature sintering. Moreover, slow cooling is effective to obtain the high fluorine concentration. The compositional change from feedstock composition is quite small after this synthesis. We can reproducibly observe a record superconducting transition for an iron based superconductor at …
The recently discovered superconducting - spin density wave materials, containing Fe and As, have raised huge interest. However most materials prepared to date, suffer from a varying degree of content of foreign Fe-As phases, Fe2As, FeAs2 and FeAs, which can lead to wrong conclusions concerning the properties of these materials. We show here that Mossbauer Spectroscopy is able to determine quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of seven samples of superconducting or spin density wave materials, prepared in three different laboratories…
We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induce…
In high-transition temperature (high-Tc) copper oxides, it is generally believed that antiferromagnetism plays a fundamental role in the superconducting mechanism because superconductivity occurs when mobile electrons or holes are doped into the antiferromagnetic parent compounds. The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems [RO1-xFxFeAs] has generated enormous interest because these materials are the first noncopper oxide superconductors with Tc exceeding 50 K. The parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies near 150…
LaFeAsO1-xFx thin films were deposited successfully on (001)-oriented LaAlO3 and MgO substrates from stoichiometric LaFeAsO1-xFx polycrystalline targets with fluorine concentrations up to x = 0.25 by PLD. Room temperature deposition and post annealing of the films yield nearly phase pure films with a pronounced c-axis texture and a strong biaxial in-plane orientation. Transport measurements show metallic resistance and onset of superconductivity at 11 K. Hc2(T) was determined by resistive measurements and yield Hc2 values of 3 T at 3.6 K for B||c and 6 T at 6.4 K for B||ab.
Raman spectra of polycrystalline NdFeAsO1-xFx (x=0.0, 0.1, 0.2) compound have been systematically investigated as functions of temperature and fluorine concentration. Scanning electron microscopic and Raman microscopic characterization demonstrates that the polycrystalline samples mainly contain two phases, i.e. superconductor NdFeAsO1-xFx compound and a MnP-type FeAs phase, with dissimilar characteristic Raman bands. It was found that fluorine doping leads to structure disorder in the insulator Nd-O layers and high temperature coefficient of Fe-As vibrational mode.
In layered single band systems, the interlayer conductivity depends on the orientation of the in-plane magnetic field and takes maximum values when the magnetic field is perpendicular to flat regions of the Fermi surface. Extending this known results to multi-band systems, we propose an experiment to extract information about their Fermi surface topology. We discuss application of the formula to a FeAs-based superconductor, LaFeAsO$_{1-x}$F$_x$. We show that the magnetically ordered state in the parent compound is clearly distinguished from the paramagnetic state by the oscillation period in t…
Polycrystalline samples of RO1-xFxFeAs (0 < x < 0.25) (R = La, Sm, Gd) were investigated by wavelength-dispersive X-ray spectroscopy (WDX) in the electron microscope to determine the composition of the samples, in particular the fluorine content. It was found that the measured fluorine content can deviate considerably from the initial weight. In the lanthanum compound LaO1-xFxFeAs, we found good agreement mainly for x > 0.05, but for x < 0.05 the fluorine hardly goes into the sample. For the samarium compound we measured less than half the fluorine in the sample as initially weighed at all flu…
We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carrier…
The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0.30), which shows that static magnetism persists well into the superconducting regime. The analogy with the cuprates is q…
Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_…
We examined doping effect of 3d transition metal elements (TM: Cr, Mn, Co, Ni, and Cu) at the Fe site of a quaternary fluoroarsenide CaFeAsF, an analogue of 1111-type parent compound LaFeAsO. The anomaly at ~120 K observed in resistivity (rho) vs. temperature (T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co- and Ni-doping (CaFe1-xTMxAsF, TM = Co, Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22 K) and at x = 0.05 for Ni (12 K). These optimal doping levels may be understood by considering tha…
We present measurements of the temperature and field dependencies of the magnetization M(T,H) of Nd(O0.89F0.11)FeAs at fields up to 33T, which show that superconductivity with the critical temperature Tc ~ 51K cannot coexist with antiferromagnetic ordering. Although M(T,H) at 55 < T < 140K exhibits a clear Curie-Weiss temperature dependence corresponding to the Neel temperature TN ~ 11-12K, the behavior of M(T,H) below Tc is only consistent with either paramagnetism of weakly interacting magnetic moments or a spin glass state. We suggest that the anomalous magnetic behavior of an unusual high-…
We have synthesized a quaternary fluoroarsenide SrFeAsF with the ZrCuSiAs-type structure (P4/nmm, a = 0.3999 and c = 0.8973 nm), which is composed of an alternately stacked (FeAs)- and (SrF)+ layers, analogous to the FeAs-based superconductor LaFeAsO. SrFeAsF shows metallic type conduction with the anomaly at ~180 K. The partial replacement of the Fe with Co suppresses the anomaly and induces the superconductivity, while the maximal Tc (4 K for SrFe0.875Co0.125AsF) is much lower than that of the Co-substituted LaFeAsO. Replacement of (LaO)+ layers with (SrF)+ layers results in a enlargement of…
The recently discovered high-temperature superconductivity in doped quaternary iron oxypnictides correlates experimentally with a magnetic instability. We have used first-principles calculations to determine a magnetic phase diagram of ReO$_{1-δ}$FeAs (Re=La--Dy) as a function of the doping $δ$, of the FeAs in-plane lattice constant $a$, and of the distance between the Fe and As planes, that is qualitatively consistent with recent experimental findings on the doping, internal (chemical) and external pressure dependence. The existence of a tricritical point (TCP) in the phase diagram suggests n…
The superconducting gap in FeAs-based superconductor SmFeAs(O1-xFx) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other "gap-like" features vary appreciably for different contacts. The determined gap value of 2D = 13.34 +/-0.47 meV for SmFeAs(O0.85F0.15) gives 2D/kBTC = 3.68, close to the BCS prediction of 3.53. The superconducting gap decreases with temperature and vanishes at TC, in a manner similar to the BCS behavior but dramaticall…
The recent discovery of superconductivity in the iron based superconductors with very high upper critical fields presents a new possibility for practical applications, but fabricating fine-wire is a challenge because of mechanically hard and brittle powders and the toxicity and volatility of arsenic. In this paper, we report the synthesis and the physical characterization of iron pnictide wires and bulks prepared by the powder-in-tube method (PIT). A new class of high-Tc iron pnictide composite wires, such as LaFeAsO1-xFx, SmFeAsO1-xFx and Sr1-xKxFeAs, has been fabricated by the in situ PIT te…
By means of first-principle FLAPW-GGA calculations, we have investigated the electronic properties of the newly discovered layered quaternary systems SrFeAsF and CaFeAsF as parent phases for a new group of oxygen-free FeAs superconductors. The electronic bands, density of states, Fermi surfaces, atomic charges, together with Sommerfeld coefficients and molar Pauli paramagnetic susceptibility have been evaluated and discussed in comparison with oxyarsenide LaFeAsO - a parent phase for a new class of high-temperature (Tc about 26-56K) oxygen-containing FeAs superconductors. Similarity of our dat…
Study of superconductivity in layered iron-based materials was initiated in 2006 by Hosono's group, and boosted in 2008 by the superconducting transition temperature, Tc, of 26 K in LaFeAsO1-xFx. Since then, enormous researches have been done on the materials, with Tc reaching as high as 55 K. Here, we review briefly experimental and theoretical results on atomic and electronic structures and magnetic and superconducting properties of FeAs-based superconductors and related compounds. We seek for clues for unconventional superconductivity in the materials.
High resolution photoemission measurements have been carried out on non-superconducting LaOFeAs parent compound and various superconducting R(O1-xFx)FeAs (R=La, Ce and Pr) compounds. We found that the parent LaOFeAs compound shows a metallic character. Through extensive measurements, we have identified several common features in the electronic structure of these Fe-based compounds: (1). 0.2 eV feature in the valence band; (2). A universal 13~16 meV feature; (3). A clear Fermi cutoff showing zero leading-edge shift in the superconducting state;(4). Lack of superconducting coherence peak(s); (5)…
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 whet…
A phenomenological bond order model predicting doping curves [Tc vs doped charge, c] of cuprates is found to have wider applicability. In this model Tc is dependent on the density of electronic pair crystals [EC] in a covalently bonding layer structure and on a layer Isolation factor, f [ECI model]. Characteristic doping curve events such as optima are correlated with a select number of EC with pair repeats corresponding to multiples of lattice parameters such as c=2/3x4=0.167, where 3x4 represent pair periodicity of 3a0 and 4b0. At these EC all doped charge is converted into pairs according t…
The discovery of high temperature superconductivity in La[O1-xFx]FeAs at the beginning of this year [1] has generated much excitement and has led to the rapid discovery of similar compounds with as high as 55 K transition temperatures [2]. The high superconducting transition temperatures are seemingly incompatible with the electron-phonon driven pairing of conventional superconductors, resulting in wide speculation as to the mechanism and nature of the superconductivity in these materials. Here we report results of the first scanning tunneling microscopy study of the 32 K superconductor (Sr1-x…
