Session V39: Focus Session: Iron Based Superconductors: Lattice Probes & Irradiation

Determination of the Nature of the Tetragonal to Orthorhombic Phase Transition in SrFe$_{2}$As$_{2}$ by Measurement of the Local Order Parameter

SrFe$_{2}$As$_{2}$ is the end-member for a series of the recently discovered iron-pnictide superconductors. There is controversy over whether the tetragonal to orthorhombic phase transition it exhibits is first or second order and we suggest that much of the problem stems from the macroscopic measurements used to make this determination. Instead we use transmission electron microscopy to monitor the order parameter on a local scale and this demonstrates that the transformation is first order and that the orthorhombic phase grows as needle domains. From these observations, we classify the phase transition as martensitic so that the rearrangement of atoms required to form the orthorhombic phase forms is effected via the passage of transformation dislocations which nucleate at the edge of the sample or at defective regions and these then move into the bulk. This helps to resolve some apparent inconsistencies seen in x-ray diffraction investigations into this compound.   

Charge carrier localization induced by excess Fe in the Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system

Iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ is the simplified version of Fe-based superconductors. Its non-superconducting parent compound Fe$_{1+y}$Te exhibits an AFM structure distinct from those seen in undoped FeAs compounds. Understanding of the superconducting properties of this system has been considered critical. We have investigated the effect of Fe nonstoichiometry on properties of the Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. Together with neutron scattering studies and recent DFT calculations, our results suggest that such weak charge carrier localization originates from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets.   

Study of local structural distortions in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ superconductors

In iron pnictides [1] presence of orbital degrees of freedom on the Fe site is one of the notable differences from cuprate superconductors (SC). The presence of an electron in a particular orbital directly influences the positions of the nearest neighbor ions and couples strongly to the lattice. The orbital degrees of freedom potentially strongly enhance the electron-phonon coupling, and could play a central role in the SC pairing mechanism. Understanding how this orbital occupancy evolves as a function of doping and temperature is of crucial importance. Probing orbital occupancy directly is difficult as the orbital order is expected to be ferro (identical from site to site), and thus not amenable to conventional scattering experiments. Doping towards the SC state destroys long range magnetic order, but the presence of short range ferro-orbital order is suspected. Results will be presented of a systematic x-ray total scattering based atomic pair distribution function study, to map out the local structure across the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ [2], and investigate the importance of the orbital degree of freedom in this new class of materials. [1] Y. Kamihara \textit{et al.}, J. Am. Chem. Soc. 130, 3296 (2008). [2] Y.J. Uemura, Nature Materials 8, 253 (2009). Supported by the DOE under Contract. No. DE-AC02-98CH10886.   

Phonon probe of structural variations in FeSe$_{1-x}$Te$_{x}$

In the iron chalcogenide system FeSe$_{1-x}$Te$_{x }$the superconducting compositions are produced by isovalent substitution of Te for Se. The electronic structure and magnetic properties of FeSe$_{1-x}$Te$_{x}$ also show an unusual sensitivity to the crystal structure modification with chalcogen substitution. We present the evolution of Raman active phonons in single crystals of FeSe$_{1-x}$Te$_{x}$ with x = 0.0, 0.5, 0.75, and 1.0 as measured with polarized Raman spectroscopy at room temperature. Lattice dynamics calculations of the studied compositions yield a simple eigenvector for the A$_{1g}$ and B$_{1g}$ modes involving vibrations along the c-axis of only Se(Te) and Fe atoms, respectively. Remarkably, the A$_{1g}$ mode exhibits a single mode behavior thus pointing to the importance of the average structure rather than local one for the properties of iron chalcogenides. The atomic disorder associated with Te for Se substitution produces a Raman band around 150 cm$^{-1}$ that persists also in the composition with x =1 thus reflecting the stoichiometric deviation in the actual structure FeTe$_{0.92}$ and the band is absent in FeSe.   

The Vortex Lattice of KFe$_{2}$As$_{2}$

Small angle neutron scattering (SANS) studies of the vortex lattice in the FeAs-based superconductors are currently being hindered by the strong vortex pinning that seems almost omnipresent in these materials. An investigation of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ [1] gave only a ring of diffraction intensity, not well-defined Bragg peaks, showing a disordered vortex structure. It is likely that local inhomogeneities in these doped compounds are responsible for the significant vortex pinning. KFe$_{2}$As$_{2}$ belongs to the 122 class of pnictide superconductors and is intrinsically superconducting (with a T$_{c}$ of 3.8 K), allowing high quality crystals to be grown that exhibit extremely low vortex lattice pinning. Our SANS measurements of KFe$_{2}$As$_{2}$ are the first to observe sharp Bragg peaks from a well-ordered vortex lattice. It is now the underlying physics, not pinning, that controls the vortex structure. We present the vortex lattice structure observed as a function of temperature and field, and discuss what the temperature dependence of the diffracted intensity implies for the superconducting order parameter. [1] M.R. Eskildsen et al. Phys. Rev. B 79, 100501(R) (2009)   

Origin of conductance asymmetry in the Andreev reflection spectra between normal metals and Fe-As based superconductors

Large conductance asymmetry is ubiquitously observed in point-contact Andreev reflection spectra between Fe-As based superconductors and metallic tips. We investigated the origin of this in point contacts between high quality epitaxial thin films of BaFe$_{1.84}$Co$_{0.16}$As$_{2}$ and sharp silver tips. In our experiments a large conductance asymmetry has been consistently observed in all the PCAR spectra. We discuss the possible origin of this conductance asymmetry.   

Point-contact spectroscopy of the iron chalcogenide superconductors: interplay between multiband superconductivity and magnetism

We report point-contact conductance measurements on the iron chalcogenide superconductors, Fe$_{1+y}$Te$_{1-x}$Se$_{x}$. The excess Fe atoms are known to occupy the interstitial sites in the Te-Se plane, affecting the superconductivity as well as the magnetism in this family. For a compound having nominal values of y=0 and x=0.45, a single superconducting transition is observed at 14.2 K. In the superconducting state, BTK-like double peak structures due to Andreev reflection are observed. However, the peak position of different point contacts falls to a wide voltage range, 1.5 -- 4 mV. Additional multiple humps are sometimes observed in a much higher bias voltage range, 8 -- 15 mV. Most strikingly, conductance enhancement persists well above T$_{c}$. We will present possible interpretations of these experimental observations in terms of multiband superconductivity and the interplay between superconductivity and magnetism.   

Point contact Andreev reflection spectroscopic (PCARS) studies on 122-type iron-based superconductors

PCARS is applied to investigate the superconducting gap in iron pnictide single crystal superconductors of the AFe$_2$As$_2$ (A=Ba, Sr) family with two categories of G(V) curves observed [1]: one where Andreev reflection (AR) is present for (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$, and the other without AR but a V$^{2/3}$ shape for Sr$_{0.6}$Na$_{0.4}$Fe$_2$As$_2$ and Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$. The latter is also observed in the nonsuperconducting parent compound BaFe$_2$As$_2$. Mesoscopic phase-separated coexistence of magnetic and superconducting orders is considered to explain distinct behaviors. A gap size $\sim$3.0-4.0 meV with $2\Delta_0/k_BT_c \sim$2.0-2.6 is observed for PCARS on Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. For the Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$, G(V) curves typically display a zero-bias conductance peak, sometimes with a V-shape background. [1] Xin Lu \textit{et al.,} arXiv:0910.4230   

Effect of Heavy-ion Irradiation in Co-doped BaFe$_{2}$As$_{2}$

The critical current density, $J_{c}$, at low temperatures and fields reaches 1x10$^{6}$ A/cm$^{2}$ in the optimally doped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ with $T_{c}\sim $24 K. In order to further improve the current carrying capability, we have irradiated swift heavy-ions into Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. Even with the same ion species and energy as those formed columnar defects in cuprate superconductors, columnar defects are not always created. By irradiating heavier ions like Au and Xe, we have successfully confirmed the creation of columnar defects and resulting enhancement of $J_{c}$. up to 5x10$^{6}$ A/cm$^{2}$ in the case of Au irradiation with a matching field of 20 kG. High-resolution TEM observations reveal the presence of discontinuous columnar defects with diameters of 2-5 nm, which are much smaller that those in cuprate superconductors. Interestingly, the lattice image is sustained in the defect region in contrast to amorphous formation in cuprates. Vortex dynamics in pristine and irradiated Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ will also be discussed.   

High-energy heavy ion and proton irradiation of iron-pnictide superconductors

We report on magnetization measurements on Ba$_{0.6}$K$_{0.4}$ Fe$_{2}$As$_{2}$ single crystals irradiated with 6 MeV protons followed by 1.4 GeV Pb ions to a dose matching field of 2.0 Tesla. We see a systematic increase of the critical current at all temperatures and fields with J$_{c}$ increasing nearly a factor of ten at 20 K. In addition, we report on magnetization measurements on Ba$_{0.6}$K$_{0.4}$ Fe$_{2}$As$_{2}$ single crystals irradiated with 1.4GeV Pb ions to dose matching fields of 0.1T and 1.0T. Here, we see a systematic increase of both the irreversibility line and the critical current with increasing irradiation dose. Our results show that both proton and heavy ions are good candidates for increasing vortex pinning in these materials.   

T$_{c}$ suppression in NdFeAs(OF) single crystal by Kondo-like scattering induced by $\alpha $-particle irradiation

We investigated the suppression of $T_{c}$ in a thin NdFeAs(OF) single crystal by uniform disorder induced by irradiation with 2MeV $\alpha $-particles with the fluence up to 5.25$\times $10$^{16}$cm$^{-2}$. Our results indicate that irradiation defects produce both nonmagnetic and magnetic scattering, resulting in a significant Kondo-like excess resistance $\Delta \rho $(T) $\propto $\textit{ lnT} above $T_{c}$ over 2 decades in $T$. Despite very high densities of irradiation defects, the dose at which $T_{c}$ is suppressed to zero is far larger than that required for cuprates and similar to that found for the s-wave two-band superconductor MgB$_{2}$. The observed resilience of multiband superconductivity in pnictides to strong magnetic and nonmagnetic disorder presents a challenge to the existing theories.   

Substantial enhancement of critical currents by heavy-ion irradiation in single crystals of Co and Ni doped 122 pnictide superconductors: a magneto-optical study

Single crystals of Ba(Fe$_{1-x}$T$_{x})_{2}$As$_{2}$ (T=Co, Ni) were irradiated with 1.4 GeV Pb ions at different fluencies to produce pinning centers of controlled density. Magneto-optical imaging utilizing the Faraday effect in iron-garnet ferrimagnetic films has been used to map the distribution of the magnetic induction in the irradiated samples. The results show a substantial enhancement of the apparent critical current densities at all temperatures as revealed by the much larger Bean penetration fields that scale with the density of the irradiation-induced defects. Magneto-optical observations are supported by the conventional magnetic measurements revealing a large increase of the hysteretic magnetization. Given the quite three-dimensional nature of iron-based pnictide superconductors, our results suggest that irradiation with heavy ions is a very effective way to enhance the current-currying capabilities of these novel superconducting materials.   

Effect of irradiation-induced defects on the phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

We present a study of the changes of T$_{c}$ and of the upper critical field of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ that are induced by irradiation with 9 MeV protons and 1.4 GeV Pb-ions. Irradiation to a fluence of 2x10$^{15}$ protons/cm$^{2}$ creates sparse individual point defects and their clusters. These are sufficient to increase vortex pinning, but do not alter the phase diagram in a noticeable way. In contrast, heavy-ion irradiation to a dose matching field of 2 T induces a suppression of T$_{c}$ by $\sim $1K and a reduction of the jump of the specific heat, $\Delta $C, at T$_{c}$ by $\sim $50 {\%}. Furthermore, the upper critical field slopes increase to enormously high values of dH$_{c2}^{c}$/dT=-12.8 T/K and dH$_{c2}^{ab}$/dT=-21 T/K, corresponding to a low anisotropy of $\Gamma $ $\sim $ 1.6. Such behavior is not expected for a d-wave superconductor, but is consistent with pair-breaking by non-magnetic scattering centers in a superconductor with s$_{\pm }$ gap symmetry.