Session T5: Focus Session: Disorder in Fe-based Superconductors

Visualizing the Atomic-scale Influence on Superconductivity and Vortex Pinning of High-Energy Ion Irradiation in FeSeTe

The maximum sustainable supercurrent density, JC, may be greatly enhanced by preventing dissipative motion of quantized vortices. Irradiation of superconductors with heavy ions is often used to create nanoscale defects with deep pinning potential for the vortices and this approach holds great promise for high current applications of iron-based superconductivity. However, for these compounds virtually nothing is known directly about the atomic-scale interplay between the crystal damage from high-energy ions, the superconducting order parameter, and the vortex pinning processes. Here, we visualize the atomic-scale effects of irradiating FeSe$_{\mathrm{0.45}}$Te$_{\mathrm{0.55}}$ with 249 MeV Au ions and find two distinct forms of damage: compact regions of crystal disruption ascribable to the actual ion trajectory along with single atomic-site `point' defects. We show directly that the superconducting order is virtually annihilated within the former while it is strongly altered by the latter. Simultaneous atomically-resolved images of the crystal defects, the superconducting density-of-states, and the vortex cores, then reveal how the vortex pinning evolves with increasing field in irradiating FeSe$_{\mathrm{0.45}}$Te$_{\mathrm{0.55}}$.   

Experimentally tuning the ground state of BaFe$_{2}$As$_{2}$ by orbital differentiation

The role of structural parameters in layered systems, such as iron pnictides/chalcogenides (Fe-Pn/Ch), cuprates and heavy fermions, has become crucial for the understanding of their properties. In this talk, I will discuss this subject using a combination of macroscopic and microscopic techniques to study Ba$_{1-x}$Eu$_{x}$Fe$_{2-y}M_{y}$As$_{2}$ single crystals ($M =$ Co, Cu, Mn, Ni, and Ru). Interestingly, a close connection arises between the spin-density wave (SDW) phase suppression and local distortions in the structure. Furthermore, these changes are reflected at the Fermi surface by an increase of anisotropy and localization of the Fe $3d$ bands at the FeAs plane. Our results suggest that such increase in the planar ($xy$/$x^{2}-y^{2}$) orbital symmetry seems to be a favorable ingredient for the emergence of superconductivity in this class of materials.   

FORC Evidence for Splitting of the Magnetostructural Transition in Fe$_{1+y}$Te

Iron-based superconductors Fe$_{1+y}$Te and CaFe$_2$As$_2$ both have simultaneous magnetic and structural transitions, at 67K and 170K, respectively. We have investigated these transitions using the First-Order Reversal Curve (FORC) method, which provides a detailed mapping of any heterogeneities and irreversible switching events. In this work, electrical resistance FORC measurements have been performed in the presence of external magnetic fields to deconvolute the phase transition. In Fe$_{1+y}$Te the phase transition actually consists of two separate transitions, which is sensitive to the external field for H $>$ 2T, as revealed by the FORC distribution. The different responses of these separate transitions to the magnetic field suggest that, at a minimum, the coupling to the field is different for each phase. In contrast, CaFe$_2$As$_2$ shows no sign of a split transition, with or without the magnetic field. In both cases the magnetic field is shown to shift the distribution, suggesting a change in the energy landscape and highlighting the coupling between the magnetic and structural transition. This work has been supported by the NSF (DMR-1008791).   

Unexpected impact of magnetic disorder on multiband superconductivity

We analyze how the magnetic disorder affects the properties of the two-band $s_{\pm}$ and $s_{++}$ models, which are subject of hot discussions regarding iron-based superconductors and other multiband systems like MgB$_2$. We show that there are several cases when the transition temperature $T_c$ is not fully suppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory, but a saturation of $T_c$ takes place in the regime of strong disorder. These cases are: (1) the purely interband impurity scattering, (2) impurity scattering purely in one of the bands, (3) the unitary scattering limit. We show that the a transition between $s_{++}$ and $s_\pm$ states may occur with increasing magnetic disorder.   

Modulation of Pairing Symmetry with Bond Disorder in Iron-based Superconductors

We study a simple two-orbital t-J1-J2 model for iron-based superconductors in the presence of a bond disorder (via nearest-neighbor bond-dilution). By using Bogoliubov-de Gennes approach, we self-consistently calculate the local pairing amplitudes and the corresponding density of states, which demonstrate a change of dominant pairing symmetry from the s+- wave to d-wave as long as J2~J1. Although the system exhibits spatially inhomogeneous pairing in weak correlations with a given realization of disorder, it is still in sharp contrast to the case with potential disorder, where the superconducting islands and the insulating sea are both present in the strong disorder regime. Moreover, from the detailed examination of the pairing gap as well as the superfluid density, the superconducting transition here is suggested to be beyond the conventional Abrikosov-Gorkov consideration.   

Low-energy magnetic defects at nano- and meso-scale in Fe-based superconductors

In Fe-pnictides, ''ordered'' moments of Fe (0.8 - 1.04 $\mu_B$) measured by neutron scattering in the antiferromagnetic groundstate are half ($\sim$ 1.6 $\mu_B$) of that estimated from density-functional theory (DFT), while other experiments are closer to DFT -- a puzzle not yet understood. Structural and magnetic planar defects proliferate over differing length scales, and could be key to any moment description. Thus, we study via DFT the stability and magnetic properties of antiphase and domain boundaries, twins, and \textit{nano}twins, which exhibit low-moment states confined near defect boundaries. A single local-moment picture is thus inappropriate for long-range magnetic order. While the \textit{nano}scale defects are very low in energy, twins remain so at the mesoscale, where estimated distances between twin boundaries coincide with the observed magnetic correlation length. All these defects can be weakly mobile and/or have fluctuations that will lower assessed ``ordered'' moments from longer spatial and/or time averaging.   

NMR Investigations of Inhomogeneous glassy spin fluctuations in Doped BaFe$_2$As$_2$2

We present $^{75}$As and $^{31}$P NMR data in a series of Ba(Fe$_{1-x}$M$_x$)$_2$As$_2$ (M=Ni, Cu, Co) and BaFe$_2$(As$_{1-x}$P$_x$)$_2$ crystals that reveal the a large inhomogeneous distribution of glassy spin dynamics, as well as the coexistence of frozen antiferromagnetic domains in superconducting samples. In underdoped samples, the glassy dynamics turns on below temperatures on the order of 100K, persists in magnetic fields up to 30 Tesla, and is unrelated to a competition between antiferromagnetism and superconductivity. Rather, the glassy spin dynamics are driven by inhomogeneous nematic fluctuations.   

Competition between the spin fluctuations and disorder in an iron-pnictide superconductor

The evolution path of superconductivity with pressure in an optimally doped iron pnictide BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ is examined by resistance measurements. We find that the superconducting transition temperature T$_{\mathrm{c}}$ of this compound first increases with a maximum at around 5 GPa and then decreases with increasing pressure and eventually vanishes at around 12.5 GPa. The change of the strength of the spin fluctuations, derived from the analysis of the temperature-dependent resistance, behaves in a similar way to T$_{\mathrm{c}}$. After the destruction of superconductivity, the compound enters an insulating state due to the disorder-induced localization effect. These findings unveil that the superconductivity is controlled by the competition between the spin fluctuations and disorder and pin down the nature of the electron scattering and pairing in iron-based superconductors.   

Effects of Swift Particle Irradiations in (Ba,K)Fe$_{2}$As$_{2}$

Iron-based superconductors are believed to be good candidates for practical applications at high fields. Knowledge of the pinning mechanism of vortices is essential to achieve a large critical current density at high fields. Irradiations of swift particles are established way to introduce defects that pin vortices under the action of Lorentz force due to large current. We have systematically investigated the effect of swift particle irradiations in high-quality (Ba,K)Fe$_{2}$As$_{2}$ single crystals. We compare effects of different particles with different energies and densities, and discuss the origin of various features of the critical current density under magnetic field.   

Of substitution and doping: Spatial and electronic structure in iron pnictides

A highly intriguing aspect in iron-pnictide superconductors is the composition-dependent electronic structure, in particular the question if and how charge carriers are introduced to the system upon substitution of Ba by alkali metals or of Fe by other transition metals, \textit{TM}\@. We report on a systematic study of spatial structure and electronic states by x-ray diffraction and x-ray absorption, performed on a large number of compositions in the (Ba,K)(Fe,\textit{TM})$_2$As$_2$ family of compounds. The coherent combination of detailed structural information with an in-depth analysis of the electronic structure allows us to disentangle very sensitively ``doping'' effects from ``substitutional'' effects. This balance between substitution and doping turns out to be crucial for an understanding of magnetism and superconductivity in iron pnictides.   

Superconductivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ with and without artificial disorder over the entire doping range

Effects of electron irradiation on superconducting transition temperature and in-plane London penetration depth were studied in single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (x= 0.22, 0.34, 0.47, 0.56, 0.65, 0.80, 0.82, 0. 90, 0.92, 1.0). Electron irradiation introduces point - like disorder that gives insight into the superconducting gap structure by studying the effects of increasing scattering. We studied the entire superconducting ``dome'' and find distinctly different behaviours in underdoped, optimal, and overdoped compositions and unusual behaviour near x = 0.7 - 0.8.   

Vortex Pinning and Lower Critical Field Behavior of Overdoped Ba0.2K0. 8Fe2As2

We report on magneto-optical (MO) and Hall magnetometery measurements on single crystals of Ba$_{0.2}$K$_{0.8}$Fe$_2$As$_2$. The MO measurements demonstrate that the vortices prefer to stabilizing in the center of the sample upon penetration indicate of week pinning in these crystals. We determine the lower critical fields (H$_{c1}$) from the local magnetization using a Hall probe array. Below 2 K, an apparent sudden increase of H$_{c1}$ is observed which we associate with the surface barrier effect. The behavior of temperature dependence of Hc1 above 2 K can be understood in model of a nodal gap structure of overdoped Ba$_{0.2}$K$_{0.8}$Fe$_2$As$_2$.