Session Y22: Focus Session: Fe-based Superconductors - Crystal Growth and New Materials

Single crystal growth and physical properties of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$ and BaCu$_2$Sb$_2$

We present the physical properties of self-flux-grown single crystals of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$, SrCu$_2$(As$_{0.84}$Sb$_{0.16}$)$_2$ and BaCu$_2$Sb$_2$ investigated by magnetic susceptibility $\chi $, specific heat $C_{\rm p}$ and electrical resistivity $\rho$ vs. temperature $T$ measurements. Contrasting structures occur for SrCu$_2$As$_2$ (ThCr$_2$Si$_2$-type), SrCu$_2$Sb$_2$ (CaBe$_2$Ge$_2$-type) and BaCu$_2$Sb$_2$ (a distorted intergrowth of ThCr$_2$Si$_2$-type and CaBe$_2$Ge$_2$-type unit cells). The $\chi(T) $ data for all these compounds exhibit weakly anisotropic diamagnetic behaviors. For $1.8 \leq T \leq 300$~K, the $\rho(T)$ data show metallic character and are well-described by the Bloch-Gr\"{u}neisen model, and the $C_{\rm p}(T)$ data are well-fitted by metallic $\gamma T$ plus Debye lattice contributions. From the low-$T$ $C_{\rm p}(T)$ data, we infer Sommerfeld coefficients $\gamma$ = 2.2--3.9 mJ/mol\,K$^2$ and Debye temperatures $\Theta_{\rm D}$ = 204--246 K\@. The electronic properties indicate that these compounds are $sp$ metals containing Cu in a nonmagnetic $3d^{10}$ electronic configuration.\footnote{D. J. Singh, Phys. Rev. B {\bf 79}, 153102 (2009).}   

Synthesis of Large Single Crystals Within the Zr - Fe - Si System

The synthesis of large single crystals of intermetallic silicides from a metal flux is challenging, owing to the poor solubility of silicon in many of the traditional fluxes at low temperatures. Accordingly, single crystal syntheses of several compounds within the Zr - Fe - Si system from a gallium flux were investigated. Zr$_4$Fe$_4$Si$_7$, a member of the well-known V-phase family of compounds, forms as silver lustrous rods. Successful growths yielded crystals with dimensions up to 12.1 mm by 0.14 mm by 0.14 mm, large enough to perform the first transport measurements of this compound. We grew for the first time single crystals of the hexagonal Laves phase ZrFe$_{1.5}$Si$_{0.5}$, which has previously only been reported in the polycrystalline form. Two previously unreported phases, including the silver lustrous rods Zr$_{2-x}$Fe$_4$Si$_{12-y}$, have been discovered as single crystals within this system. The composition of all phases was verified with powder and single crystal x-ray diffraction. Our group is currently investigating the growth of similar compounds using the methods developed for this system.   

Exploration of Superconductivity in Layered Structures

Superconductivity at high temperatures is of great interest in terms of both basic science and applications. Although there are many findings on the chemistry and physics of superconductors, there are a lot of questions that remain unanswered. We will review experimental routes to possibly finding and identifying new superconducting materials.   

High T$_{c}$ electron doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5 }$superconductors

In this talk, we will present the crystal structures and physical properties of two new iron arsenide superconductors, Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the ``10-3-8 phase") which crystallizes in the triclinic structure and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the ``10-4-8 phase") which crystallizes in the tetragonal structure. They are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to $T_{c}$ of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The anisotropic H$_{c2}$ measurement indicates the multiband superconductivity in these compounds. The often-cited empirical rule in the arsenide superconductor literature relating $T_{c }$to As-Fe-As bond angles does not explain the observed differences in $T_{c}$ of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase due to the two-channel interlayer interactions and the metallic nature of its intermediary Pt$_{4}$As$_{8}$ layer. The interlayer coupling is thus revealed as important in enhancing $T_{c}$ in the iron pnictide superconductors.   

Influence of Cr-doping on the magnetic structure of the FeAs strip compounds CaFe4As3 : a single crystal neutron diffraction study

CaFe$_4$As$_3$ offers the unique opportunity to modify the topology of the Fe$_2$As$_2$ layers, key in the understanding of superconductivity in the new iron pnictide superconductors, from infinite layers to strips of finite width linked a rectangular cross pattern. Bulk measurement on CaFe$_4$As$_3$ indicate a magnetic ordering at ~90K with a second transition seen at about 26K. Neutron diffraction allowed to ascribe the high temperature transition to a Spin Density Wave(SDW) with a propagation vector k=(0,$\delta$,0) eventually locking to $\delta$=3/8 at the lower transition. As expected a profound effect on the magnetic properties of CaFe$_4$As$_3$ can be obtained by chemically doping or applying pressure. We report here on the consequence of Cr-doping on the magnetic structure of CaFe$_4$As$_3$ derived from single crystal neutron diffraction.   

Anisotropic $H_{c2}$ curves determined up to 92 T and the signature of two-band superconductivity in the novel superconductor Ca$_{10}$(Pt$_{4}$As$_{8})$((Fe$_{1-x}$Pt$_{x})_{2}$As$_{2})_{5}$

The upper critical fields, $H_{c2}(T)$, of single crystals of the novel superconductor Ca$_{10}$(Pt$_{4}$As$_{8})$((Fe$_{1-x}$Pt$_{x})_{2}$As$_{2})_{5}$ with $x$=0.02 were determined over a wide range of temperatures down to $T$ = 1.42 K and magnetic fields up to $H $= 92 T. The measurements of anisotropic $H_{c2}(T)$ curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the ab-plane. Whereas a clear upward curvature in $H_{c2}^{c}(T)$ along H$\vert \vert $c is observed with decreasing temperature, the $H_{c2}^{ab}(T)$ along H$\vert \vert $ab shows a flattening at low temperatures. The rapid increase of the $H_{c2}^{c}(T)$ suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter, $H_{c2}^{ab}(T)$ /$ H_{c2}^{c}(T)$ , is $\sim $7 close to $T_{c}$ and decreases considerably to $\sim $1 with decreasing temperature, showing rather weak anisotropy at low temperatures.   

Direct measurement of the absolute value of the magnetic penetration depth in two-dimensional pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$[(Fe $_{1-x}$ Pt$_{x})_{2}$As$_{2}$]$_{5}$

We have measured the absolute value of the magnetic penetration depth $\lambda $ in a single crystal of the Ca$_{10}$(Pt$_{3}$As$_{8})$[(Fe $_{1-x}$ Pt$_{x})_{2}$As$_{2}$]$_{5}$ (``10-3-8'') superconductor using low temperature magnetic force microscopy (MFM). We directly probed local values of $\lambda $ in the ``10-3-8'' sample using Meissner response measurements and compared Meissner curves to those obtained in a Nb reference sample. The Meissner response measured at different locations on the sample shows similar behavior indicating homogeneity of the superconducting state, in accord with tunnel-diode resonator measurements. We also discuss larger values of $\lambda $ in 10-3-8 relative to $\lambda $ values measured in other pnictide systems.   

Local Displacements, Magnetoelastic Coupling, and Bonding in Spin-Ladder Iron Selenides

The spin-ladder ``1-2-3'' compounds BaFe$_2$Se$_3$ and KFe$_2$Se$_3$, built of double-chains of edge-sharing [FeSe$_4$] tetrahedra, are localized-moment antiferromagnetic semiconductors. Total neutron scattering of BaFe$_2$Se$_3$ reveals local iron displacements coupled to long-range ordered antiferromagnetism comprised of ferromagnetic Fe$_4$ plaquettes. The magnitude of the iron displacements follow the antiferromagnetic order parameter: a manifestation of magnetoelastic coupling. These local displacements are essential for properly understanding the electronic structure of these systems, as local structural modulations necessarily perturb the ground state wavefunctions. Furthermore, while iron displacements from magnetoelastic coupling in Fe$X_4$-based materials are hypothesized to be important in the emergence of superconductivity, the spin-ladders remain insulating down to 1.8 K, even upon hole doping by substitution of K for Ba. As with the copper oxide superconductors two decades ago, our results highlight the importance of reduced dimensionality spin-ladder compounds in the study of the coupling of spin, charge, and atom positions in superconducting materials.   

Spin glass and semiconducting behavior of the flux grown BaFe$_{2-\delta }$Se$_{3}$ crystals

In this talk, physical properties and crystal and electronic structures of BaFe$_{2-\delta }$Se$_{3}$ crystals, synthesized using tellurium flux, will be discussed. This phase is an iron-deficient derivative of the ThCr$_{2}$Si$_{2}$-type and its structure is made of double chains formed from edge-sharing FeSe$_{4}$ tetrahedra. The semiconducting BaFe$_{2-\delta }$Se$_{3}$ with \textit{$\delta $} $\approx $ 0.2 does not order magnetically, however, there is evidence for short-range magnetic correlations of spin glass type below 50 K in magnetization, heat capacity and neutron diffraction results. The semiconducting behavior of BaFe$_{2-\delta }$Se$_{3}$ is in line with the detrimental influence of iron deficiency to the superconductivity in $A_{x}$Fe$_{1.8}$Se$_{2}$ ($A$ = alkali metal) superconductors. The electronic structure calculations suggest that this compound can be considered as a low-dimensional (1D) ladder structure with a weak interchain coupling. Based on the survey of available data on BaFe$_{2}$Se$_{3}$ so far, lower concentrations of iron vacancies may lead to a long range antiferromagnetic order, whereas higher concentrations of iron vacancies may suppress long range order and then lead to a spin glass behavior.   

Ba$_{1-x}$K$_x$Mn$_2$As$_2$: An Antiferromagnetic Local Moment Metal

The syntheses of K-doped single crystalline $\rm{Ba_{0.984}K_{0.016}Mn_2As_2}$ and polycrystalline $\rm{Ba_{0.95}K_{0.05}Mn_2As_2}$ with the tetragonal ${\rm ThCr_2Si_2}$ structure are reported. Electrical resistivity, heat capacity, magnetic susceptibility, angle-resolved photoemission spectroscopy and neutron diffraction measurements and spin-polarized electronic structure calculations consistently establish that these hole-doped Ba$_{1-x}$K$_x$Mn$_2$As$_2$ samples are antiferromagnetic local-moment metals, in contrast to the parent BaMn$_2$As$_2$ [1-3] which is an antiferromagnetic local-moment semiconductor. This new class of materials bridges the gap between the iron pnictide and cuprate high $T_{\rm c}$ materials. Investigations of the phase diagram of the Ba$_{1-x}$K$_x$Mn$_2$As$_2$ system and other similar systems are underway.\\[4pt] [1] Y. Singh et al., Phys. Rev. B \textbf{79}, 094519 (2009).\\[0pt] [2] Y. Singh et al., Phys. Rev. B \textbf{80}, 100403 (2009).\\[0pt] [3] D. C. Johnston et al., Phys. Rev. B \textbf{84}, 094445 (2011).   

Role of ruthenium in iron-based superconductors and related materials

Ruthenium and iron share the same valence electron count, and form many isostructural compounds. However, the larger covalent radius and extent of the d-electrons of ruthenium lead to interesting and sometimes unexpected behavior when iron is partially or fully substituted by ruthenium. For example, ``doping'' layered iron compounds with ruthenium has been shown to produce superconductivity in some cases but not others, and ruthenium analogs of certain layered iron compounds do not form under similar conditions. We have investigated full and partial ruthenium substitution in several iron-based materials, including the superconducting 1111 and 122 families, and studied the effects on formation, crystal structures, and physical properties. Our new experimental findings and results from available literature will be used to discuss the unusual role that ruthenium plays in iron-based superconductors and related materials.   

Neutron Diffraction Studies on Ba$_{0.95}$K$_{0.05}$Mn$_{2}$As$_{2}$

There has been a great deal of interest in compounds, such as BaMn$_{2}$As$_{2}$, which are closely related to the iron pnictide superconductors. Although undoped BaMn$_{2}$As$_{2}$ is an antiferromagnetic (AF) insulator[1, 2], recent studies of lightly doped Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ have shown a striking change to metallic behavior for $x \quad >$ 0.01 and, therefore, may offer a bridge between the high $T_{c}$ cuprates and the iron-pnictide superconductors. We will present neutron diffraction measurements on polycrystalline Ba$_{0.95}$K$_{0.05}$Mn$_{2}$As$_{2}$ performed on the powder diffractometer at the Missouri Research Reactor. Our measurements reveal that the antiferromagnetism in the doped compound remains nearly the same as that found for undoped BaMn$_{2}$As$_{2}$, characterized by a G-type collinear AF spin structure below $T_{N }\sim $ 607(2) K with an ordered moment of 4.21(6) $\mu _{B}$/Mn ion at 14 K. Research at Ames Lab was supported by the USDOE, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. \\[4pt] [1] Y. Singh et al., Phys. Rev. B \textbf{80}, 100403 (2009). \\[0pt] [2] D. C. Johnston et al., Phys. Rev. B \textbf{84}, 094445 (2011).   

Growth of parent and electron doped NaFeAs

It has been found hole doping on Fe sites in BaFe$_{2}$As$_{2}$ does not induce superconductivity with Cr and Mn as dopants, but doping on Ba sites with K induces superconductivity as high as 38K. We have investigated hole doping with Titanium to be compared with other hole doping compounds. Single crystals of Titanium doped BaFe$_{2}$As$_{2}$ were grown by flux method. Transport and susceptibility measurements were done showing doping Titanium suppresses the Neel temperature but no superconductivity was found up to 4{\%} doping. Susceptibility measurements also showed spin glass behavior. Phase diagrams of temperature vs doping concentration have been constructed from transport and Susceptibility measurements.