In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity ρ, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility χ, 75 As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient αc is negative from 7 to 300 K, whereas αa (the a-axis thermal expansion coefficient) is positive over this T range. The ρ(T ) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy EF. The band calculations exhibit an extremely sharp peak in the density of states D(E ≈ EF) arising from a flat d x 2 −y 2 band, where the x and y axes are along the a and b axes of the Co square lattice, respectively. A comparison of the Sommerfeld coefficient of the electronic specific heat with χ(T → 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The χ(T ) does have a large magnitude, but also exhibits a broad maximum at ≈ 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we suggest that metallic SrCo2As2 has a gapless quantum spin-liquid ground state.

Vortices in superconductors driven at microwave frequencies exhibit a response related to the interplay between the vortex viscosity, pinning strength, and flux creep effects. At the same time, the trapping of vortices in superconducting microwave resonant circuits contributes excess loss and can result in substantial reductions in the quality factor. Thus, understanding the microwave vortex response in superconducting thin films is important for the design of such circuits, including superconducting qubits and photon detectors, which are typically operated in small, but non-zero, magnetic fields. By cooling in fields of the order of 100 µT and below, we have characterized the magnetic field and frequency dependence of the microwave response of a small density of vortices in resonators fabricated from thin films of Re and Al, which are common materials used in superconducting microwave circuits. Above a certain threshold cooling field, which is different for the Re and Al films, vortices become trapped in the resonators. Vortices in the Al resonators contribute greater loss and are influenced more strongly by flux creep effects than in the Re resonators. This different behavior can be described in the framework of a general vortex dynamics model.

The controlled motion of objects through narrow channels is important in many fields. We have fabricated asymmetric weak-pinning channels in a superconducting thin-film strip for controlling the dynamics of vortices. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of uniformwidth channels. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is quite different, with substantial asymmetries in both the static depinning and dynamic flux flow. This vortex ratchet effect has a rich dependence on magnetic field and driving force amplitude.PACS numbers: 74.25. Qt, 74.25.Sv, 74.25.Op Recently there has been much interest in developing artificial ratchets for generating directed motion using tailored asymmetries [1]. Such ratchets could be used as pathways for producing net transport of matter at the nanoscale. In addition, artificial ratchets can serve as model systems for understanding similar ratchet phenomena in biological systems while allowing for experimental control over many of the ratchet parameters [2]. A variety of ratchets have been considered, but one particular type that has been implemented in several different systems is the rocking ratchet, where a spatial asymmetry is engineered into the potential energy landscape governing particle motion and an external control variable can be adjusted to tilt this potential. The application of an oscillatory drive of the control variable with zero mean can result in the net motion of particles through the potential because of the different rates for overcoming the barriers in the two directions through the ratchet.Implementations of ratchets in solid-state devices include asymmetric structures of electrostatic gates above a two-dimensional electron gas [3], and arrays of Josephson junctions with asymmetric critical currents [4]. Structures have also been developed for producing a ratchet effect with vortices in superconducting thin films involving either asymmetric arrangements of pinning centers [5,6] or asymmetric magnetic pinning structures [7]. In this Communication, we describe a vortex ratchet using two-dimensional guides to generate asymmetric channels for vortex motion. In our structures, the potential asymmetries arise from differences in the interaction strength between vortices and the channel walls, resulting in a substantial ratchet effect for the motion of vortices through the channels. Our design is related to a previous vortex ratchet proposal [8], although our ratchet is in a somewhat different parameter regime.Nanoscale channels for guiding vortices through superconducting films with a minimal influence from pin- ning have been developed for studies of vortex matter in confined geometries, including experiments on melting [9], commensurabili...

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe2As2 are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior. PACS numbers: 74.70.Xa, 75.30.Fv, 75.30.Kz The role of chemical substitution and its effects on structure, magnetism and superconductivity have become central issues in studies of the iron-pnictide superconductors. [1][2][3][4] This is particularly true for transition-metal (M ) substitution on Fe sites, resulting, nominally, in electron doping of the FeAs layers. When low concentrations of Co, [5,6] Ni, [7,8] Rh,[9, 10] Pt [11] and Pd [9,10] replace Fe, the structural transition temperature (T S ) and the antiferromagnetic (AFM) transition temperature (T N ) are both suppressed to lower values and split with T S > T N . [5-7, 9, 12-14] When the structural and magnetic transitions are suppressed to sufficiently low temperatures, superconductivity emerges below T c and coexists with antiferromagnetism over some range of concentration. Moreover, for Co, Rh and Ni substitutions in BaFe 2 As 2 , neutron diffraction measurements manifest a distinct suppression of the magnetic order parameter in the superconducting regime (T < T c ), which clearly indicates competition between AFM order and superconductivity. [13][14][15][16][17] Cu substitution in BaFe 2 As 2 , in contrast, suppresses the magnetic and structural transitions, but does not support superconductivity [2,8] except, perhaps, below 2 K over a very narrow range in composition.[18] This dichotomy between Co and Ni substitutions and that of Cu is also realized in quaternary fluoroarsenides.[19] However, for Co/Cu co-substitutions in BaFe 2 As 2 , at a fixed non-superconducting Co concentration, the addition of Cu first promotes and then suppresses T c .[18] It has been suggested that previously neglected impurity effects play an important role in this behavior. [8,20] The effects of impurity scattering are also neglected in a simple rigid-band picture for M substitutions, which, at least for Co substitution in BaFe 2 As 2 , seems to adequately account for the evolution of angleresolved photoemission spectroscopy (ARPES) [21], Hall effect, and thermoelectric power (TEP) measurements with concentration.[22] The rigid-band model has also been used successfully to model the suppression of the AFM transition temperature and ordered moment in Ba(Fe 1−x Co x ) 2 As 2 for "u...

We study the dynamics of vortices in an asymmetric (i.e., consisting of triangular cells) ring channel driven by an external ac current I in a Corbino setup. The asymmetric potential rectifies the motion of vortices and induces a net vortex flow without any unbiased external drive, i.e., the ratchet effect. We show that the net flow of vortices strongly depends on vortex density and frequency of the driving current. Depending on the density, we distinguish a "single-vortex" rectification regime (for low density, when each vortex is rectified individually) determined by the potential-energy landscape inside each cell of the channel (i.e., "hard" and "easy" directions) and "multi-vortex", or "collective", rectification (high density case) when the inter-vortex interaction becomes important. We analyze the average angular velocity ω of vortices as a function of I and study commensurability effects between the numbers of vortices and cells in the channel and the role of frequency of the applied ac current. We have shown that the commensurability effect results in a stepwise ω − I curve. Besides the "integer" steps, i.e., the large steps found in the single vortex case, we also found "fractional" steps corresponding to fractional ratios between the numbers of vortices and triangular cells. We have performed preliminary measurements on a device containing a single weak-pinning circular ratchet channel in a Corbino geometry and observed a substantial asymmetric vortex response.