Session T22: Focus Session: Fe-based Superconductivity - Properties of 122 phases

Combined effects of chemical doping and pressure on physical properties of CaFe$_{2}$As$_{2}$

The AFe$_{2}$As$_{2}$ compounds (A = alkaline earth) are the most extensively studied materials among the FeAs-based superconductors, and CaFe$_{2}$As$_{2}$ is the most pressure sensitive among them. The structural/magnetic phase transition near 170K is suppressed rapidly under pressure and a nonmagnetic, collapsed tetragonal phase is stabilized. Depending on the hydrostaticity of the pressure medium, the superconducting phase may or may not be induced. In this talk we will present the combined effects of chemical doping and pressure on the structural/magnetic phase transition as well as the collapsed tetragonal structure phase transition in an attempt to better understand the interactions between these phases.   

First Principles Calculations of the Pnictide CaFe$_{2}$As$_{2}$ under Pressure

We carry out first principles calculations of the effects of pressure on the structural and magnetic properties of the pnictide CaFe$_{2}$As$_{2}$ and compare with experiments. Our PBE-GGA calculations accurately reproduce the experimentally observed structural and magnetic ordering at zero pressure. Enthalpic considerations show that antiferromagnetic orthorhombic phase is favored over the non-magnetic tetragonal phase at pressure P=0, while the ``collapsed'' tetragonal phase is favored for pressures greater than 0.4 GPa, in good agreement with experiments. The calculated pressure dependences of the lattice parameters and the Fe-As bond lengths agree with experimental trends. We will discuss the nature of bonding and antibonding orbitals near the Fermi surface and we will evaluate the interplanar magnetic exchange interaction J.   

Evidence for domain wall superconductivity in antiferromagnetic CaFe2As2

$^{75}$As nuclear magnetic resonance (NMR), resistivity, and magnetization measurements in the antiferromagnetic state of the iron-based superconductor parent compound CaFe$_2$As$_2$ exhibit anomalous features consistent with the collective freezing of domain walls. Below T* $\approx$ 10 K, the $^{75}$As NMR measurements reveal the presence of slow fluctuations of the hyperfine field, the resistivity shows an enhancement and subsequent suppression, and the bulk magnetization shows a sharp increase. These features in both the charge and spin response are strongly field dependent, are fully suppressed by H* $\approx$ 15 T, and suggest the presence of filamentary superconductivity nucleated at the antiphase domain walls.   

Local atomic structure of BaFe$_2$As$_2$ by neutron powder diffraction

All structures of iron-based superconductors (FeSC) have planar layers of iron, tetrahedrally coordinated by pnictogens or chalcogens, and the structural details of this layer impact the superconducting and magnetic properties. Local structural studies of the iron-coordinated layer show evidence for local distortions resulting from models that allow for overall reductions in local symmetry. BaFe$_2$As$_2$, a parent compound of several families of FeSC, undergoes a transition at T $\approx=$ 140K, resulting in the onset of AFM ordering following a small lattice distortion. Here we report the results of a by time of flight neutron diffraction study on BaFe$_2$As$_2$, analyzed using Rietveld and pair distribution function techniques. This work produces a comprehensive view of the local structure of BaFe$_2$As$_2$ as a function of temperature. The results are consistent with previous work showing stratification of the As-As bond length, and show that models accounting for an overall reduction in local symmetry provide the best fit to the experimental data. Further, the results show that local distortions are present up to room temperature. Details of the experiment and implications for the paramagnetic and magnetic states will be discussed.   

First principles study of uniaxial pressure-induced phase transitions in CaFe$_2$As$_2$ and BaFe$_2$As$_2$

We consider density functional theory methods to determine the equilibrium structures of CaFe$_2$As$_2$ and BaFe$_2$As$_2$ under the effect of uniaxial pressure. We compare the results with calculations for hydrostatic pressure as well as with available experimental results. In CaFe$_2$As$_2$, we observe a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase at intermediate uniaxial pressures. In contrast, for both uniaxial and hydrostatic pressure, BaFe$_2$As$_2$ shows two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We predict the critical transition pressures under uniaxial conditions to be much lower than those under hydrostatic conditions which implies that the systems are highly sensitive to uniaxial stress. We compare our results to available experimental measurements.   

Temperature-Pressure Phase Diagram of Lightly Hole-doped BaFe$_2$As$_2$

Chemical doping and application of pressure are the two common tools to tune the electronic structure of a material. Although electron-doping on Fe-site in BaFe$_2$As$_2$ gives superconductivity up to $\sim$ 22 K, it is puzzling that hole-doping does not. For this reason, we decided to carry out pressure studies on a few lightly Cr- or Mo-doped crystals of BaFe$_2$As$_2$. We have applied pressures of up to 2~GPa using a cylinder cell, and Fluorinert as pressure medium. Our preliminary findings reveal the shift of antiferromagnetic ordering temperatures to lower with pressure, and a down-turn in resistivity at low temperatures and pressures, which may be attributed to superconductivity.   

Magnetism, superconductivity, and the volume collapse transition in (Ca$_{0.67}$Sr$_{0.33})$Fe$_{2}$As$_{2}$ under pressure

The alkaline earth site of CaFe$_{2}$As$_{2}$ can be chemically substituted with Sr, forming a homogeneous solid solution series ending with SrFe$_{2}$As$_{2}$. It is found that (Ca$_{0.67}$Sr$_{0.33})$Fe$_{2}$As$_{2}$ exhibits a pressure-temperature phase diagram intermediate between the two end members of the series, shifting the phase lines for the suppression of magnetism, the development of superconductivity, and the occurrence of a volume collapse transition to higher pressures. The overall shift in the pressure-temperature phase diagram permits the study of each phase field, yielding valuable information about the correlations between local atomic structure, magnetism, superconductivity, and the volume collapse transition. Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344.   

Inducing structural collapse and superconductivity in CaFe$_{2}$As$_{2}$ by systematic substitutions of rare earths

Recently, we have reported structural collapse and 47 K superconductivity in CaFe$_{2}$As$_{2}$ by aliovalent rare earth substitutions for Ca atoms [1]. We will present the evolution of structural and superconducting properties in single crystals of CaFe$_{2}$As$_{2}$ by systematic substitutions of R (=La, Ce, Pr, and Nd) for Ca, causing electron doping that is indirect to FeAs layer. Effect of annealing, growth method, etching, and pressure on Ca$_{1-x}$R$_{x}$Fe$_{2}$As$_{2}$, indicating the intrinsic nature of this high Tc superconductivity, the highest in 122 Fe-based materials, will be discussed. Ref. [1] S. R. Saha \textit{et al}. arXiv:1105.4798.   

Defect-Associated Superconductivity in (Pr,Ca)Fe$_{2}$As$_{2}$

The superconductivity in rare earth doped CaFe$_{2}$As$_{2}$ remains puzzled. As reported before, there are two distinguishable superconductive transitions at 20 K and 49 K, respectively, and the 49 K superconductivity can be better modeled as Josephson-Junction-Arrays (JJA). The H- and T-dependencies of the ac/dc magnetization are further explored here. The data suggest that the effective lower-critical-fields at both $c$- and \textit{ab}-directions are below 1 Oe down to 5 K, in agreement with previous JJA assumption. The \textit{ac} susceptibility below the critical field is highly anisotropy, suggesting a thin-disk-like morphology of the JJA's. The associated weak-links, however, appear to be broken above 20 K by a \textit{dc} bias as small as 10 Oe. The \textit{ac} response at higher bias fields, therefore, will be dominated by the isolated superconducting islands of JJA. It is interesting to note, therefore, that the \textit{ac} susceptibility remains highly anisotropic at high fields, but is much weaker than that expected from the field-cooled magnetization. We interpret this as the result of sub-micron size, thin-disk-like local superconductivity, $i.e.$ Defect-associated superconductivity.   

Rare Earth Doping in the (Sr,Ca)Fe2As2 System

The (Sr,Ca)Fe2As2 system shows an unusual persistence of the Neel ordering temperature of $\sim $200 K up to a concentration of 70{\%} calcium. We present electrical transport, magnetic susceptibility and structural characterization data as a function of rare earth substitution into Sr0.3Ca0.7Fe2As2 single crystals, focusing on the resultant phase diagram and the comparisons of solubility limit of rare earth substitution as compared to end members SrFe2As2 and CaFe2As2.   

Microstructure and the non-bulk superconductivity up to 49K in electron-doped Rare-earth (Ca, R)Fe$_{2}$As$_{2}$ (R=La, Ce, Pr and Nd) Single Crystals

In an attempt to raise the Tc in the 122 family, we have carried out electron-doping and observed an onset Tc up to 49K in the single crystalline (Ca, R)Fe$_{2}$As$_{2}$ (R=La, Ce, Pr and Nd). The single crystals up to 5 x 5 mm size are grown from self-flux technique and the optimal doping parameters for different rare-earth elements will be reported. Magnetic and resistivity data suggest possible existence of two superconducting transitions in all the rare-earth electron doped (Ca, R)Fe$_{2}$As$_{2}$ samples: one starts at $\sim $40s K, and the other at $\sim $20K, with drastically different response to the field. Detailed single crystals diffraction analysis show that there are no significant difference in terms of atomic position, bond distances, angles and lattice parameters upon different rare earth doping; the defect-related local structure might be responsible for the observed high Tc in this system. The unusual superconducting phase appears to be filamentary or interfacial in nature, and the possible mechanism will be discussed.   

Codoping -- A way to enhance the upper critical field in iron-arsenic superconductors

Technological key features of iron-based superconductors are the high critical temperature $T_{c}$ of up to 55 K and the high tolerance against magnetic fields, which led so far to upper critical fields in the range of 75 T. Furthermore, the small $H_{c2}$-anisotropy between field applied along the $c$-direction and in the \textit{ab}-plane, in particular for the FeSe and \textit{AE}Fe$_{2}$As$_{2}$ (\textit{AE} = Ca, Sr, Ba) materials, is a prerequisite for several technical applications. Currently, different approaches (chemical substitutions, processing) are discussed how to increase $H_{c2}$ further. Here, we show a feasibility study for codoping of polycrystalline Sr- or BaFe$_{2}$As$_{2}$ samples, namely the simultaneous substitution of K on the Sr/Ba layer and of Co on the FeAs layer. The upper critical field was investigated by magnetoresistance in high pulsed magnetic fields up to 64~T. We find, that the extrapolated critical field $H_{c2}(T\to $0) is enhanced by 15{\%} for Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$ in comparison to Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$, although $T_{c}$ is almost identical in both materials. These results suggest that codoping is a promising route for the systematic optimization of iron-arsenic based superconductors for high-magnetic field and high-current applications.   

Influence of random point defects introduced by proton irradiation on the vortex pinning and dynamics of superconducting $122$ iron arsenides

Vortex matter in iron-arsenide superconductors exhibits a rich phenomenology that is still largely unexplored. One way to understand and manipulate the pinning mechanisms and the vortex dynamics in superconductors is by the artificial introduction of additional defects. In this work we explore the influence of the random point defects introduced by proton irradiation on the vortex dynamics and critical currents of 122 iron arsenide superconductors. Our comparison includes Ca1-xNaxFe2As2 and Co-doped BaFe2As2 single crystals. We find that the influence of random point defects on the creep rate (S) and the elastic to plastic crossover of the vortex dynamics show a strong dependence with intrinsic superconductor parameters such as the coherence length. We analyze the magnetic field -- temperature (H-T) vortex phase diagrams for the as-grown single crystals and the changes produced by the random point defects.   

Upper critical and irreversible fields of polycrystalline CeFeAsO$_{1-x}$F$_{x}$ superconductors

We have investigated the upper critical ($H_{c2})$ and irreversible ($H_{irr})$ fields of polycrystalline samples of Ce oxypnictide at different doping levels. $H_{c2}$ was obtained from temperature dependent resistivity measurements with increasing applied magnetic field. Critical field values as high as 150 Tesla were observed with a decreasing trend as the doping level shifts from a slightly under-doped state to the highly over-doped region. The irreversible fields were lower in this superconductor compared with other rare-earth oxypnictides, with values below 3 Tesla at 20 K. However, $H_{irr}$ was found to increase with increasing doping, opposite to that of $H_{c2}$. The origin of $H_{irr}$ was studied by determining the exponent `n' extracted from plots of log$_{10}(H_{irr})$ versus log$_{10}$(1-$T$/$T_{c})^{n}$. We found that $H_{irr}$ follows a 3D vortex lattice-melting model similar to the other low anisotropic iron-based superconductors.