### Session A35: Focus Session: Iron Pnictides and Other Novel Superconductors I: Synthesis and New Materials

 Monday, March 16, 2009 8:00AM - 8:12AM A35.00001: Superconductivity in layered pnictides BaIr$_{2}$P$_{2}$ and BaRh$_{2}$P$_{2}$ Daigorou Hirai , Tomohiro Takayama , Ryuji Higashinaka , Hiroko Aruga-Katori , Hidenori Takagi The exploration of new superconductors, triggered by the discovery of LaFeAs(O,F), has concentrated mostly on Fe-based pnictides. A variety of non-Fe pnictides, isostructural to Fe pnictide superconductors, have been known for a long time but not yet fully explored in terms of possible superconductivity. Further exploration of non-Fe pnitide superconductors is important for understanding the key factors in realizing the high $T_{c}$ in the Fe pnictides. We report new Ir and Rh pnictide superconductors, isostructural to BaFe$_{2}$As$_{2}$, BaRh$_{2}$P$_{2}$ and BaIr$_{2}$P$_{2}$, with $T_{c}$ = 1.0 and 2.1 K respectively. This discovery demonstrates the presence of superconductivity over a surprisingly broad range of transition metal compounds with ThCr$_{2}$Si$_{2}$-type structure from Fe to Ir. Monday, March 16, 2009 8:12AM - 8:24AM A35.00002: Superconductivity and enhanced susceptibility in SrFe$_2$As$_2$ single crystals S. R. Saha , N. P. Butch , K. Kirshenbaum , J. Paglione Single crystals of SrFe$_2$As$_2$ grown using a self-flux solution method were characterized via x-ray, transport, magnetization and specific heat studies, revealing a superconducting transition at 21 K which appears far below the magnetostructural transition at 198 K as evidenced by transitions in resistivity and susceptibility. We present experiments which probe the nature of this phase and its relation to the enhancement of magnetic susceptibility in superconducting samples. Monday, March 16, 2009 8:24AM - 8:36AM A35.00003: Growth and properties of single crystalline F-doped and Co-doped NdFeAsO. M. E. Tillman , S. Kim , R. Prozorov , M. A. Tanatar , S. L. Bud'ko , P. C. Canfield We report the results of single crystal growth of F-doped and Co-doped, NdFeAsO.~ A high pressure furnace using a 19mm truncation edge length cubic anvil capable of reaching 3.3 GPa and 2000 C was used to grow crystals of doped NdFeAsO from on- and off-stoichiometry melts.~ Single crystals with linear dimensions of $>$1mm can be grown using this technique. Anisotropic transport, thermodynamic and spectroscopic properties [1-3] have been measured and the results of these measurements as well as crystal size and doping level will be correlated to growth profile and initial stoichiometry. [1] T. Kondo et al., Phys. Rev. Lett. \textbf{101}, 147003 (2008), [2] C. Liu, et al., arXiv:0806.2147, [3] C. Martin et al., arXiv: 0807.0776. Monday, March 16, 2009 8:36AM - 8:48AM A35.00004: Transport, magnetic and thermal properties of $M$FePO $M$ = La, Pr, and Nd single crystals Ryan Baumbach , James Hamlin , Lei Shu , Diego Zocco , Nicole Crisosto , M. Brian Maple The recent discovery of $T_{c}$ values near 26 K in the compound LaFeAsO$_{1-x}$F$_{x}$ induced a torrent of publications on what are now recognized as a new class of Fe-based high temperature superconductors. To date, the phosphorus based versions of these compounds have recieved little attention due to their comparitively low $T_{c}$ values. In this work we report the low temperature electrical resistivity, magnetic susceptibility, and specific heat data of single crystalline PrFePO and NdFePO. We also report the effect of annealing on the properties of LaFePO, PrFePO, and NdFePO. A systematic comparison of the occurence of superconductivity in the series $M$FePO and $M$FeAsO (where $M$ is a lanthenide) points to a possible difference in the origin of the superconductivity in these two series of compounds. Monday, March 16, 2009 8:48AM - 9:00AM A35.00005: New ternary phosphide superconductors with the ThCr2Si2 parent structure Nicholas Berry , Cigdem Capan , Gabriel Seyfarth , Zachary Fisk Compounds with the ThCr2Si2 structure have been studied extensively for over 20 years for their interesting magnetic, superconducting, and heavy fermion properties. We have grown single crystals of new ternary phosphides in this structure in the form of AX2P2, with A being an Alkaline Earth Metal and X a transition metal. We have characterized the properties of these new materials with X-ray, heat capacity, resistivity, and susceptibility measurements and have discovered new superconductors. Monday, March 16, 2009 9:00AM - 9:12AM A35.00006: Magnetic, Thermodynamic, and Transport Properties of Layered Arsenides BaRu$_{2}$As$_{2}$ and SrRu$_{2}$As$_{2}$ R. Nath , Y. Singh , D.C. Johnston As part of our effort to search for novel superconductors related to the FeAs high-$T$ superconductors, we have synthesized polycrystalline samples of BaRu$_{2}$As$_{2}$ and SrRu$_{2}$As$_{2}$. The magnetic, transport and thermodynamic properties of the samples were investigated by means of magnetic susceptibility $\chi(T)$, electrical resistivity $\rho(T)$, and heat capacity $C_{p}(T)$ measurements. The temperature dependence of $\rho$ indicates metallic character for both compounds with a residual resistivity ratio $\rho$(310 K)/$\rho$(2 K) of 17 and 7 for the Ba and Sr compounds, respectively. The $C_{p}(T)$ results indicate a low density of states at the Fermi level with the low-$T$ Sommerfeld coefficient $\gamma \simeq 4.9$ and $4.13$ mJ/mole K$^{2}$ for the Ba and Sr compounds, respectively. The Debye temperature $\Theta_{D}$ was estimated to be $270$ K and $260$ K for the Ba and Sr compounds, respectively. The $\chi (T)$ was found to be diamagnetic with a small absolute value for both the compounds. No evidence for superconductivity, a spin density wave, or a structural transition was observed from the $C_{p}(T)$ and $\rho(T)$ measurements down to $2$ K. However, the $\chi(T)$ data for SrRu$_{2}$As$_{2}$ exhibit a cusp at $\sim 190$ K, possibly an indication of a structural and/or magnetic transition.\\ \noindent * Supported by the USDOE under contact No. DE-AC02-07CH11358. Monday, March 16, 2009 9:12AM - 9:24AM A35.00007: Robust Ferromagnetism in Ultrathin Films of CeFeAsO Lifeng Yin , Zheng Gai , Thomas Zac Ward , Paul C. Snijders , Jian Shen The mechanism of magnetic order in iron-oxypnictides has attracted lots of research efforts, but the conclusions are so distinct: local moment ground state with antiferromagnetic fluctuation vs. itinerant ground state with ferromagnetic fluctuation, although a spin-density-wave (SDW) ground state is generally observed in polycrystalline parent compounds. Because single-crystal growth of iron-oxypnictides seems very tough, the fabrication of epitaxial thin films by pulsed laser deposition (PLD) technique should be an important solution for basic research. Here we report a successfully fabrication of c-axis oriented high quality CeFeAsO(001) heteroepitaxial ultrathin film, which shows an atomic flat surface with (4$\times$1) reconstruction at initial stage. We find a striking in-plane ferromagnetic ground state with a small Fe moment in CeFeAsO ultrathin film, followed by the development of spin glass like order with thickness. Therefore, the robust ferromagnetism in single crystalline parent compound thin film implies that the superconductivity in 1111 system should be mediated by ferromagnetic spin fluctuation. Monday, March 16, 2009 9:24AM - 9:36AM A35.00008: Reaction Kinetics of the Formation of SmFeAsO$_{1-x}$F$_x$ High \textit{T}$_{\rm c}$ Superconductor Probed by High Energy X-ray Diffraction S. Das , Y. Singh , R. Nath , G.E. Rustan , A.I. Goldman , J.Q. Yan , A. Kreyssig , D.C. Johnston , R.W. McCallum , M.J. Kramer The recent discoveries of superconductivity with $T_{\rm c}$ up to 55 K in a new class of compounds $R$FeAsO$_{1-x}$F$_x$ ($R$ = rare earth), have attracted much attention. It is difficult to synthesize single phase high quality samples that are needed to study the intrinsic properties of these materials. The samples reported are made at high temperatures $T$ $\sim$~1100~$^{\circ}$C and usually contain impurity phases along with the $R$FeAsO$_{1- x}$F$_x$ phase. Information on the kinetics of formation of the superconducting phase would be helpful. Here we report the results of high-energy x-ray diffraction measurements versus temperature, carried out at the Advanced Photon Source. We determined the kinetics of the reaction between the appropriate mixtures of SmAs, ${\rm Fe_2O_3}$, Fe, and ${\rm SmF_3}$ to give nominal compositions of SmFeAsO and SmFeAsO$_{1-x}$F$_x$, respectively. Our results indicate that the SmFeAsO$_{1-x}$F$_x$ phase starts forming at $T~\sim$~750~$^{\circ}$C and its formation ceases by $\sim$~1000~$^{\circ}$C\@. \noindent $^*$Supported by the USDOE under Contract Nos.\ DE- AC02-07CH11358 and DE-AC02-06CH11357. Monday, March 16, 2009 9:36AM - 9:48AM A35.00009: Growth and Properties of SmFeAsO1-xFx thin films using pulsed laser deposition Sanghan Lee , Chad Folkman , Seung Hyub Baek , David Felker , Mark Rzchowski , Chang Beom Eom , Jianyi Jiang , Eric Hellstrom The discovery of iron pnictide superconductors has been gaining interest due to their highest transition temperatures (Tc) among transition metal compound except cuprate systems. In iron pnictide system, Tc has been significantly enhanced by replacing elements. The growth of epitaxial thin film of this interesting compound is desirable for fundamental understanding of superconductivity and potential device applications. So far, there is no report with successful growth of epitaxial thin films with the same Tc of the bulk materials. Although several reports show Tc of thin film, the resistivity dose not reach zero down to at 4K. We have employed pulsed laser deposition for the growth of iron pnictides, SmFeAsO1-xFx, thin films on various single crystal substrates followed by ex--situ post annealing. In this talk, we will discuss the structural and superconducting properties of SmFeAsO1-xFx thin film including Tc, Jc and Hc2. Monday, March 16, 2009 9:48AM - 10:00AM A35.00010: Superconducting Fe-Based Compounds (A$_{1-x}$Sr$_{x })$Fe$_{2}$As$_{2}$ with A = K and Cs with Transition Temperatures up to 37 K Kalyan Sasmal , Bing Lv , Bernd Lorenz , Arnold M. Guloy , Feng Chen , Yu-Yi Xue , Ching-Wu Chu New high-Tc Fe-based superconducting compounds, AFe$_{2}$As$_{2}$ with A = K, Cs, K/Sr, and Cs/Sr, were synthesized. The Tc of KFe$_{2}$As$_{2}$ and CsFe$_{2}$As$_{2}$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50{\%}--60{\%} Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent p-type analogs of the n-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (Fe$_{2}$As$_{2})$ layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental A layers provides new avenues to superconductivity at higher Tc. Monday, March 16, 2009 10:00AM - 10:12AM A35.00011: The superconductivity in alkaline containing iron arsenide system Changqing Jin , X.C. Wang , Q.Q. Liu , Y.X. Lv , W.B. Gao , L.X. Yang , R.C. Yu , F.Y. Li We report superconducting properties of LiFeAs that crystallize into 111 type structure showing superconducting transition at Tc 18K (http://xxx.lanl.gov/abs/0806.4688 (arXiv:0806.4688)). We will present the update experimental results on characterizations of superconductivity. The possible role of magnetic order to this system will be discussed. Monday, March 16, 2009 10:12AM - 10:24AM A35.00012: Superconductivity in Sr-122 Iron Arsenide System by Yttrium Doping Shen V. Chong , Kazuo Kadowaki Superconductivity in SrFe$_{2}$As$_{2}$ by interlayer electron doping is achieved for the first time with the substitution of 30 to 50 atomic percent of the strontium with yttrium (Sr$_{1-x}$Y$_{x}$Fe$_{2}$As$_{2})$. In the optimum doped Y = 0.4 sample, Hall effect measurements indicate the injection of extra electrons into the system with negative Hall coefficient below room temperature down to $T_{c}$, and an electron charge carrier density reaching up to 10$^{21}$ cm$^{-3}$ above 200 K. Temperature dependent magnetization measurements indicate the suppression of the magnetic orderings observed in the parent compound at 60 and 210 K upon yttrium doping; while resistivity versus temperature ($R-T)$ measurements show the anomaly above 200 K still persists in the superconducting samples. A maximum onset $T_{c}$ at 26.4 K based on $R-T$ measurement was observed in Sr$_{0.6}$Y$_{0.4}$Fe$_{2}$As$_{2}$. No superconductivity was observed below 0.3 and above 0.5 Y-doping. Monday, March 16, 2009 10:24AM - 10:36AM A35.00013: High-pressure growth of SmFeAsO$_{1-x}$ single crystals without fluorine doping Hyun-Sook Lee , J.H. Park , J.Y. Lee , Hu-Jong Lee , N.H. Sung , J.Y. Kim , B.K. Cho , T.Y. Koo , C.U. Jung We synthesized fluorine-free SmFeAsO$_{1-x}$ single crystals for a nominal value of $x$=0.15 at a pressure of 3.3 GPa and at a temperature of 1350-1450 $^{\circ}$C by using a self-flux method. Plate-shaped single crystals were obtained with the lateral size of a few-150 $\mu$m. Single crystals showed the superconducting transition at about 53.5 K with a narrow resistive transition width of 0.5 K. The synchrotron-irradiated X-ray diffractometry (XRD) peaks and the high-resolution scanning transmission electron microscopy (HR-STEM) images indicate the good crystallinity of our single crystals, which have a well-defined layered tetragonal structure. The chemical composition of the crystals was examined with the electron-probe X-ray microanalysis. A sharp transition, a low residual resistivity, and a large residual resistivity ratio indicate high quality of our single crystals. Fluorine-free single crystals of Fe-based superconductors are expected to provide an easier route to find the optimum conditions for the crystal formation in this family of materials. Monday, March 16, 2009 10:36AM - 10:48AM A35.00014: Growth of superconducting FeSe films Michio Naito , Shinya Agatsuma , Shinya Ueda The recently discovered Fe arsenide and chalcogenide superconductors have provided the superconducting community with a great surprise that Fe-based compounds are not ferromagnetic but superconducting with high Tc. The superconducting Fe arsenides and chalcogenides are also interested from the viewpoint of superconducting electronics. One can see good lattice compatibility between the superconducting Fe family and the existing III-V and II-VI semiconducting family (GaAs, ZnSe). All-epitaxial super-semiconductor multilayer structures may be ideal for superconducting electronics and spintronics. Toward this goal, we have attempted to grow epitaxial thin films of the superconducting Fe family. Of this family, tetragonal $\alpha$-FeSe seems to be the easiest to grow thin films. We employed two approaches for FeSe film growth: post-annealing and MBE growth. In the post-annealing, precursor films of Fe are annealed at 500 - 600 $^{o}$C with Se vapor in an evacuated quartz tube. Annealing with elemental Se produced semiconducting FeSe$_{2}$ whereas annealing with FeSe polycrystalline pellets produced superconducting FeSe with $T_{c}$(onset) $\sim$ 10 K. In the MBE growth, we attempted the growth similar to GaAs growth, namely with the vapor rich in Se, expecting self-limiting adsorption of Se. MBE films so far obtained with the growth temperature of 330 $^{o}$C are nonsuperconducting hexagonal $\beta$-FeSe. Monday, March 16, 2009 10:48AM - 11:00AM A35.00015: \textit{In situ} X-ray Synchrotron Diffraction Study of the Synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$ R.W. McCallum , J.-Q. Yan , G.E. Rustan , E.D. Mun , S. Das , R.C. Nath , Youwen Xu , S.L. Bud'ko , K.W. Dennis , D.C. Johnston , P.C. Canfield , M.J. Kramer , A. Kreyssig , T.A. Lograsso , A.I. Goldman The reaction path for the synthesis of LaFeAsO and LaFeAsO$_{1-x}$F$_{x}$ by nominally solid state reaction was studied by \textit{in situ} x-ray synchrotron diffraction technique and Differential Thermal Analysis (DTA) in the temperature interval 100\r{ }C $\le$ T $\le$ 1150\r{ }C. Starting materials were LaAs, Fe$_{2}$O$_{3}$, Fe and for the F containing materials LaF3. The results show that the synthesis is characterized by three temperature intervals: (1) below 400\r{ }C, Fe$_{2}$O$_{3}$ gradually transforms to Fe$_{3}$O$_{4}$. (2) In the temperature interval 400\r{ }C $<$ T $<$ 800\r{ }C, multiple intermediate reactions take place resulting in the formation of La$_{2}$O$_{3}$ and Fe --As compounds. (3) above 800\r{ }C, reaction leads to the formation of LaFeAsO. Possible reaction paths and the difference between F-free and F-doped samples will be discussed in the talk.