The mechanism of high-temperature superconductivity in the newly discovered iron-based superconductors is unresolved. We use spectroscopic imaging-scanning tunneling microscopy to study the electronic structure of a representative compound CaFe1.94Co0.06As2 in the "parent" state from which this superconductivity emerges. Static, unidirectional electronic nanostructures of dimension eight times the inter-iron-atom distance a(Fe-Fe) and aligned along the crystal a axis are observed. In contrast, the delocalized electronic states detectable by quasiparticle interference imaging are dispersive along the b axis only and are consistent with a nematic alpha2 band with an apparent band folding having wave vector q vector congruent with +/-2pi/8a(Fe-Fe) along the a axis. All these effects rotate through 90 degrees at orthorhombic twin boundaries, indicating that they are bulk properties. As none of these phenomena are expected merely due to crystal symmetry, underdoped ferropnictides may exhibit a more complex electronic nematic state than originally expected.

If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δ(i)(k) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δ(i)(k) in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α(2), and α(1), and we determine the anisotropy, magnitude, and relative orientations of their Δ(i)(k). These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

Iron-based high-temperature superconductivity develops when the 'parent' antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms 1. But their impact on atomic-scale electronic structure, although in theory rather complex 2-13 , is unknown experimentally. What is known is that a strong transport anisotropy 14-25 with its resistivity maximum along the crystal b axis 14-25 , develops with increasing concentration of dopant atoms 14,20-25 ; this 'nematicity' vanishes when the parent phase disappears near the maximum superconducting T c. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom and the transport nematicity has therefore become a pivotal focus 7,8,12,22,23 of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe 1−x Co x) 2 As 2 generates a dense population of identical anisotropic impurity states. Each is ∼8 Fe-Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b axis. These data provide direct support for the recent proposals 7,8,12,22,23 that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density 14,20-25 and for the occurrence of the highest resistivity along the b axis 14-25. Commensurate antiferromagnetism with orthorhombic crystal symmetry exists in the phase 'parent' to the superconductivity in most underdoped FeAs materials. The a axis unit cell is fractionally longer than that of the b axis, with the Fe spins aligning in an anti-parallel fashion along the a axis but parallel along the b axis. Doping is usually achieved by substitution of x transition-metal atoms per Fe. When x ∼ 4 ± 1%, the superconductivity appears, reaching its maximum T c usually near x c ∼ 8 ± 1% (Fig. 1a), where the antiferromagnetic/orthorhombic phase disappears. A broken discrete rotational symmetry of both the crystal structure

Topological surface states in PbTaSe2 show fully gapped superconductivity, making it a potential topological superconductor.

We show that all the measured phenomena comprise the predicted QPI "fingerprint" of a self-energy due to antiferromagnetic spin-fluctuations, thereby distinguishing them as the predominant electron-boson interaction. 1The microscopic mechanism for Cooper pairing in iron-based high-temperature superconductors has not been identified definitively [1][2][3] . Among the complicating features in these superconductors is the multiband electronic structure (see Fig. 1a). However, it is believed widely that the proximity to spin order [1][2][3][4][5] 2Each type of electron-boson interaction should produce a characteristic electronicrepresenting its effect on every non-interacting electronic state k with momentum ħk and energy ħω. Thus, the interacting Green's function obtained by first visualizing scattering interference patterns in real-space (r-space) images of the tip-sample differential tunneling conductance dI/dV(r,ω=eV)≡g(r,ω) using spectroscopic-imaging scanning tunneling microscopy, and then Fourier transforming g(r,ω) to obtain the power spectral density g(q,ω) 11 . The g(q,ω) can then be used to reveal the electron dispersion k(ω) because elastic scattering of electrons from −k(ω) to +k(ω) results in high intensity at q(ω)=2k(ω) in g (q,ω). Sudden changes in the energy evolution k(ω) due to Σ(k,ω) can then be determined, in principle 19 , using such data. 3In a conventional single band s-wave superconductor with isotropic energy gap magnitude Δ, it has been well-established that coupling to an optical phonon with frequency Ω can lead to a renormalization of the electronic spectra at energy Δ+Ω (ħ=1)due to a singularity in the momentum independent self-energyThis classic case is illustrated in Fig. 1c,d through a model spectral function A(k,ω)∝ImG(k,ω) and the associated density of states N(ω)=∫ dk A(k,ω).In Fig. 1c, the "free" dispersion of a hole-like band is represented by the red dashed line, while the renormalized dispersion k(ω) due to Σ(ω) is highlighted by the locus of maxima in A(k,ω). These effects can be understood from the conservation of energy and momentum during scattering processes (Fig. 1b), where the flat dispersion of an optical phonon presents constraints only on energy without any momentum dependence. 4In developing our new approach to "fingerprinting" different electron-boson interactions using QPI, we use the realization that the kinematic constraints for a multiband electronic system coupled to resonant AFSF with a sharp momentum structure should result in a strongly momentum-dependent (anisotropic) self-energy. This is because, given a fermionic dispersion ) , ( n k k ω for different bands n and a spectrum of spin fluctuations whose intensity is strongly concentrated at (Q,Ω), the renormalization due to the self-energy at a point ) , ( n k k ω will be most intense when that point can be connected to another pointThis is the constraint from conservation of both energy and momentum in the electron- (2) can be satisfied and thus where the strongest self-energy effect due to coupling to A...

Spin waves can be used as information carriers with low energy dissipation. The excitation and propagation of spin waves along reconfigurable magnonic circuits is the subject of much interest in the field of magnonic applications. Here we experimentally demonstrate an effective excitation of spin waves in reconfigurable magnetic textures at frequencies as high as 15 GHz and wavelengths as short as 80 nm from Ni80Fe20 (Py) nanodisk–film hybrid structures. Most importantly, we demonstrate these spin wave modes, which were previously confined within a nanodisk, can now couple to and propagate along a nanochannel formed by magnetic domain walls at zero magnetic bias field. The tunable high-frequency, short-wavelength, and propagating spin waves may play a vital role in energy efficient and programmable magnonic devices at the nanoscale.

In thin-film La 0.6 Ca 0.4 MnO 3 , conducting-tip and magnetic force microscopy reveal a pattern of nanoscale phase separation that is reproducible across cooling runs. This pattern represents the intersection of buried three-dimensional filamentary ferromagnetic metallic pathways with the sample surface. As an interlayer in current-perpendicular-to-the-plane trilayer devices, this phase-separated material magnetically decouples ferromagnetic metallic La 0.7 Ca 0.3 MnO 3 electrodes which switch sharply. This yields sharp two-state low-field magnetoresistance that is also reproducible across cooling runs. The reproducibility and the magnitude of the resistance jump are linked to highly resistive ͑ϳ10 −12 ⍀ m 2 ͒ constrained domain walls in the pathways of the phase-separated interlayer. Phase separation is normally associated with high-field colossal magnetoresistance and, therefore, its exploitation here to produce low-field effects is unusual.The manganites rose to prominence in the 1990s with the discovery 1 of colossal magnetoresistance ͑CMR͒, i.e., large changes in electrical resistance driven by large magnetic fields. Interest persists for two reasons. First, the ferromagnetic metallic ͑FMM͒ phase is attractive for spintronics 2 because the conduction-electron-spin polarization 3,4 approaches 100 % and is therefore much higher than the values recorded for common magnetic metals such as cobalt ͑Ref. 5͒ ϳ40 %. Second, manganites of appropriate composition display complex magnetic and electronic phase-separation phenomena over a wide range of length scales. 6,7 Here, using epitaxial thin films, we investigate and exploit both the spin-polarized FMM phase and phase separation.High-field magnetoresistance 1,8,9 ͑MR͒ in chemically single-phase manganites is circumstantially 2 associated with magnetic and electronic phase separation. 6,7 Phase separation strongly influences MR effects, but phase separation is not a prerequisite for MR effects, which could ͑in principle͒ arise via the interconversion of very different homogeneous phases. In practice, phase separation arises near first-order phase transitions due to nucleation and pinning by imperfections and is associated with pronounced MR effects showing either large high-field jumps, 8,9 e.g., 10 10 %, or continuous responses. 1 Continuous MR responses, such as the prototypical CMR effect, 1 are so large ͑ϳ10 5 % at 6 T͒ that even a small applied magnetic field H produces a substantial MR ͑ϳ50 % at 10 mT͒. Therefore, low-field memory effects arising due to phase separation can be large and subtle. 10 Low-field MR in manganites has not hitherto been associated with phase separation. Large low-field effects arise when H serves to align and misalign magnetic domains on either side of grain boundaries, 11 tunnel barriers, 12,13 and nanoconstrictions. 14-16 Small low-field effects in nominally single-domain unpatterned continuous crystals 17 are a manifestation of the high-field CMR effect; when the magnetization switches to align with H, the local magnetic order is e...

Dirac nodal line semimetals represent a new state of quantum matters in which the electronic bands touch to form a closed loop with linear dispersion. Here, we report a combined study on ZrSiS by density functional theory calculation, scanning tunnelling microscope (STM) and magneto-transport measurements. Our STM measurements reveal the spectroscopic signatures of a diamond-shaped Dirac bulk band and a surface band on two types of cleaved surfaces as well as a spin-polarized surface band at G at E∼0.6 eV on S-surface, consistent with our band calculation. Furthermore, we find the surface termination does not affect the surface spectral weight from the Dirac bulk bands but greatly affect the surface bands due to the change in the surface orbital composition. From our magnetotransport measurements, the primary Shubnikov-de-Haas frequency is identified to stem from the hole-type quasi-two-dimensional Fermi surface between Γ and X. The extracted non-orbital magnetoresistance (MR) contribution D(θ, H) yields a nearly H-linear dependence, which is attributed to the intrinsic MR in ZrSiS. Our results demonstrate the unique Dirac line nodes phase and the dominating role of Zr-d orbital on the electronic structure in ZrSiS and the related compounds.