Interplay between superconductivity and itinerant magnetism in underdoped Ba1−x_{1-x}Kx_xFe2_2As2_2 (x=x= 0.2) probed by the response to controlled point-like disorder

The response of superconductors to controlled introduction of point-like disorder is an important tool to probe their microscopic electronic collective behavior. In the case of iron-based superconductors (IBS), magnetic fluctuations presumably play an important role in inducing high temperature superconductivity. In some cases, these two seemingly incompatible orders coexist microscopically. Therefore, understanding how this unique coexistence state is affected by disorder can provide important information about the microscopic mechanisms involved. In one of the most studied pnictide family, hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BaK122), this coexistence occurs over a wide range of doping levels, 0.16~$\lesssim x \lesssim$~0.25. We used relativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairs that act as efficient point-like scattering centers. Upon increasing dose of irradiation, the superconducting transition temperature $T_c$ decreases dramatically. In the absence of nodes in the order parameter this provides a strong support for a sign-changing $s_{\pm}$ pairing. Simultaneously, in the normal state, there is a strong violation of the Matthiessen's rule and a decrease (surprisingly, at the same rate as $T_c$) of the magnetic transition temperature $T_{sm}$, which indicates the itinerant nature of the long-range magnetic order. Comparison of the hole-doped BaK122 with electron-doped Ba(Fe$_x$Co$_{1-x}$)$_2$As$_2$ (FeCo122) with similar $T_{sm}\sim$110~K, $x=$0.02, reveals significant differences in the normal states, with no apparent Matthiessen's rule violation above $T_{sm}$ on the electron-doped side. We interpret these results in terms of the distinct impact of impurity scattering on the competing itinerant antiferromagnetic and $s_{\pm}$ superconducting orders

Field Dependence of the Superconducting Basal Plane Anisotropy of TmNi2B2C

The superconductor TmNi2B2C possesses a significant four-fold basal plane anisotropy, leading to a square Vortex Lattice (VL) at intermediate fields. However, unlike other members of the borocarbide superconductors, the anisotropy in TmNi2B2C appears to decrease with increasing field, evident by a reentrance of the square VL phase. We have used Small Angle Neutron Scattering measurements of the VL to study the field dependence of the anisotropy. Our results provide a direct, quantitative measurement of the decreasing anisotropy. We attribute this reduction of the basal plane anisotropy to the strong Pauli paramagnetic effects observed in TmNi2B2C and the resulting expansion of vortex cores near Hc2.Comment: 8 pages, 6 figures, 1 tabl

Anisotropy Reversal of the Upper Critical Field at Low Temperatures and Spin-Locked Superconductivity in K2Cr3As3

We report the first measurements of the anisotropic upper critical field $H_{c2}(T)$ for K$_{2}$Cr$_{3}$As$_{3}$ single crystals up to 60 T and $T > 0.6$ K. Our results show that the upper critical field parallel to the Cr chains, $H_{c2}^\parallel (T)$, exhibits a paramagnetically-limited behavior, whereas the shape of the $H_{c2}^\perp (T)$ curve (perpendicular to the Cr chains) has no evidence of paramagnetic effects. As a result, the curves $H_{c2}^\perp (T)$ and $H_{c2}^\parallel(T)$ cross at $T\approx 4$ K, so that the anisotropy parameter $\gamma_H(T)=H_{c2}^\perp/H_{c2}^\parallel (T)$ increases from $\gamma_H(T_c)\approx 0.35$ near $T_c$ to $\gamma_H(0)\approx 1.7$ at 0.6 K. This behavior of $H_{c2}^\|(T)$ is inconsistent with triplet superconductivity but suggests a form of singlet superconductivity with the electron spins locked onto the direction of Cr chains

Structural optimization of framed structures using generalized optimality criteria

The application of a generalized optimality criteria to framed structures is presented. The optimality conditions, Lagrangian multipliers, resizing algorithm, and scaling procedures are all represented as a function of the objective and constraint functions along with their respective gradients. The optimization of two plane frames under multiple loading conditions subject to stress, displacement, generalized stiffness, and side constraints is presented. These results are compared to those found by optimizing the frames using a nonlinear mathematical programming technique

Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe2_{2}As2_{2}

The iron-based high temperature superconductors exhibit a rich phase diagram reflecting a complex interplay between spin, lattice, and orbital degrees of freedom [1-4]. The nematic state observed in many of these compounds epitomizes this complexity, by entangling a real-space anisotropy in the spin fluctuation spectrum with ferro-orbital order and an orthorhombic lattice distortion [5-7]. A more subtle and much less explored facet of the interplay between these degrees of freedom arises from the sizable spin-orbit coupling present in these systems, which translates anisotropies in real space into anisotropies in spin space. Here, we present a new technique enabling nuclear magnetic resonance under precise tunable strain control, which reveals that upon application of a tetragonal symmetry-breaking strain field, the magnetic fluctuation spectrum in the paramagnetic phase of BaFe$_{2}$As$_{2}$ also acquires an anisotropic response in spin-space. Our results unveil a hitherto uncharted internal spin structure of the nematic order parameter, indicating that similar to liquid crystals, electronic nematic materials may offer a novel route to magneto-mechanical control.Comment: 11 pages, 5 figure

Intrinsic pinning on structural domains in underdoped single crystals of Ba(Fe1−x_{1-x}Cox_x)2_2As2_2

Critical current density was studied in single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for the values of $x$ spanning the entire doping phase diagram. A noticeable enhancement was found for slightly underdoped crystals with the peak at $x = 0.058$. Using a combination of polarized-light imaging, x-ray diffraction and magnetic measurements we associate this behavior with the intrinsic pinning on structural domains in the orthorhombic phase. Domain walls extend throughout the sample thickness in the direction of vortices and act as extended pinning centers. With the increasing $x$ domain structure becomes more intertwined and fine due to a decrease of the orthorhombic distortion. This results in the energy landscape with maze-like spatial modulations favorable for pinning. This finding shows that iron-based pnictide superconductors, characterized by high values of the transition temperature, high upper critical fields, and low anisotropy may intrinsically have relatively high critical current densities.Comment: estimation of Jc correcte

Character of the structural and magnetic phase transitions in the parent and electron doped BaFe2As2 compounds

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At T_S = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at T_N = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe{1-x}Co{x})2As2 and Ba(Fe{1-x}Rh{x})2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve towards split secondorder transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co-doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magneto-elastic coupling in the form of an emergent Ising-nematic degree of freedom.Comment: 10 pages, 11 figures; accepted for publication in Phys. Rev.

Differential thermal analysis and solution growth of intermetallic compounds

To obtain single crystals by solution growth, an exposed primary solidification surface in the appropriate, but often unknown, equilibrium alloy phase diagram is required. Furthermore, an appropriate crucible material is needed, necessary to hold the molten alloy during growth, without being attacked by it. Recently, we have used the comparison of realistic simulations with experimental differential thermal analysis (DTA) curves to address both these problems. We have found: 1) complex DTA curves can be interpreted to determine an appropriate heat treatment and starting composition for solution growth, without having to determine the underlying phase diagrams in detail. 2) DTA can facilitate identification of appropriate crucible materials. DTA can thus be used to make the procedure to obtain single crystals of a desired phase by solution growth more efficient. We will use some of the systems for which we have recently obtained single-crystalline samples using the combination of DTA and solution growth as examples. These systems are TbAl, Pr$_7$Ni$_2$Si$_5$, and YMn$_4$Al$_8$.Comment: 17 pages, 8 figure

Effect of tensile stress on the in-plane resistivity anisotropy in BaFe2As2

The effect of uniaxial tensile stress and the resultant strain on the structural/magnetic transition in the parent compound of the iron arsenide superconductor, BaFe$_2$As$_2$, is characterized by temperature-dependent electrical resistivity, x-ray diffraction and quantitative polarized light imaging. We show that strain induces a measurable uniaxial structural distortion above the first-order magnetic transition and significantly smears the structural transition. This response is different from that found in another parent compound, SrFe$_2$As$_2$, where the coupled structural and magnetic transitions are strongly first order. This difference in the structural responses explains the in-plain resistivity anisotropy above the transition in BaFe$_2$As$_2$. This conclusion is supported by the Ginzburg-Landau - type phenomenological model for the effect of the uniaxial strain on the resistivity anisotropy