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Since their theoretical discovery more than a half-century ago, vortices observed in bulk superconductors have carried a quantized value of magnetic flux determined only by fundamental constants. A recent experiment reported 'unquantized' quantum vortices carrying the same fraction of flux quantum in Ba$_{0.23}$K$_{0.77}$Fe$_2$As$_2$ in a small temperature range below its superconducting critical temperature ($T_C$). Here, we use scanning superconducting quantum interference device (sSQUID) microscopy with improved sensitivity to investigate the genesis of fractional vortices in Ba$_{0.23}$K$_{0.77}$Fe$_2$As$_2$. We report the direct observation of a single-flux quantum vortex splitting into two different fractions with increasing temperature. The flux of the two fractions has opposite dependence on temperature, while the total flux sums up to one flux quantum despite their spatial separation. Overall, our study shows the existence of different fractional vortices and their stability in temperature ranging from 0.1 to 0.99 $T_C$. Besides the implications of this observation for the fundamental question of quantum vorticity, the discovery of these objects paves the way for the new platform for anyon quasiparticles and applications for fractional fluxonics.

As-Based Zintl compounds Ba1-xKxCd2As2 crystallized in the CaAl2Si2-type structure (space group P3-m1) were prepared using solid-state reactions followed by hot-pressing. We have successfully substituted K for Ba up to x = 0.08, producing hole-carrier doping with concentrations up to 1.60*1020 cm-3. We have determined the band-gap value of non-doped BaCd2As2 to be 0.40 eV from the temperature dependence of the electrical resistivity. Both the electrical resistivity and the Seebeck coefficient decrease with hole doping, leading to a power factor value of 1.28 mW m-1 K-2 at 762 K for x = 0.04. A first-principles band calculation shows that the relatively large power factor mainly originates from the two-fold degeneracy of the bands comprising As px,y orbitals and from the anisotropic band structure at the valence-band maximum. The lattice thermal conductivity is suppressed by the K doping to 0.46 W m-1 K-1 at 773 K for x = 0.08, presumably due to randomness. The effect of randomness is compensated by an increase in the electronic thermal conductivity, which keeps the total thermal conductivity approximately constant. In consequence, the dimensionless figure-of-merit ZT reaches a maximum value of 0.81 at 762 K for x = 0.04., Comment: 7 pages, 9 figures

In general, magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors, in which superconductivity may coexist with magnetism, and only a few examples are known, when superconductivity itself induces spontaneous magnetism. The most known compounds are Sr$_2$RuO$_4$ and some noncentrosymmetric superconductors. Here, we report the finding of a narrow dome of a novel $s+is'$ superconducting (SC) phase with broken time-reversal symmetry (BTRS) inside the broad $s$-wave SC region of the centrosymmetric multiband superconductor Ba$_{\rm 1-x}$K$_{\rm x}$Fe$_2$As$_2$ ($0.7 \lesssim x \lesssim 0.85$). We observe spontaneous magnetic fields inside this dome using the muon spin relaxation ($\mu$SR) technique. Furthermore, our detailed specific heat study reveals that the BTRS dome appears very close to a change in the topology of the Fermi surface (Lifshitz transition). With this, we experimentally demonstrate the emergence of a novel quantum state due to topological changes of the electronic system., Comment: 28 pages, 11 figures

The interplay of high and low-energy mass renormalizations with band-shifts reflected by the positions of van Hove singularities (VHS) in the normal state spectra of the highest hole-overdoped and strongly correlated AFe$_2$As$_2$ (A122) with A = K, Cs is discussed phenomenologically based on ARPES data and GGA band-structure calculations with full spin-orbit coupling. The big increase of the Sommerfeld coefficient $\gamma$ from K122 to Cs122 is ascribed to an enhanced coupling to low-energy bosons in the vicinity of a quantum critical point to an unknown, yet incommensurate phase different from the commensurate Mott one. We find no sizeable increase in correlations for Cs122 in contrast to F. Eilers et al., PRL v. 116, 237003 (2016) [3]. The empirical (ARPES) VHS positions as compared with GGA-predictions point even to slightly weaker correlations in Cs122 in accord with low-$T$ magnetic susceptibility $\chi(T)$ data and a decreasing Wilson ratio $\propto \chi(0)/\gamma$., Comment: 8 pages, 7 figures, updated references, and a Note for arXiv-readers

Over the last years a lot of theoretical and experimental efforts have been made to find states with broken time reversal symmetry (BTRS) in multi-band superconductors. In particular, it was theoretically proposed that in the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system either an $s+is$ or an $s+id$ BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero field muon spin relaxation rate below the superconducting transition temperature for a high quality crystalline sample with $x \approx$ 0.73. This indicates that indeed the time reversal symmetry is broken in superconducting Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ at this doping level., Comment: 5 pages, 3 figures, and supplementary materials

We have investigated the superconducting gap of optimally doped Ba(Fe$_{0.65}$Ru$_{0.35}$)$_2$As$_2$ by angle-resolved photoemission spectroscopy (APRES) using bulk-sensitive 7 eV laser and synchrotron radiation. It was found that the gap is isotropic in the $k_x$-$k_y$ plane both on the electron and hole Fermi surfaces (FSs). The gap magnitudes of two resolved hole FSs show similar $k_z$ dependences and decrease as $k_z$ approaches $\sim$ 2$\pi$/$c$ (i.e., around the Z point) unlike the other Fe-based superconductors reported so far, where the superconducting gap of only one hole FS shows a strong $k_z$ dependence. This unique gap structure can be understood in the scenario that the $d_{z^2}$ orbital character is mixed into both hole FSs due to the finite spin-orbit coupling between almost degenerate FSs and is reproduced by calculations within the random phase approximation including the spin-orbit coupling.

Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0; Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors., Comment: 16 pages, 4 figures

Polarized inelastic neutron scattering experiments were performed to study magnetic excitations in the normal and superconducting phases of Co-underdoped BaFe$_2$As$_2$, which exhibits coexistence of antiferromagnetic order and superconductivity. In the normal state the antiferromagnetic order results in broadened spin gaps opening in all three spin directions that are reminiscent of the magnetic response in pure antiferromagnetic BaFe$_2$As$_2$. In particular longitudinal excitations exhibit a large gap. In the superconducting state we find two distinct resonance excitations, which both are anisotropic in spin-space, and which both do not appear in the longitudinal polarization channel. This behavior contrasts to previous polarized neutron results on samples near optimum or higher doping. The gap in the longitudinal fluctuations arising from the antiferromagnetic order seems to be sufficiently larger than twice the superconducting gap to suppress any interplay with the superconducting state. This suppressed low-energy weight of longitudinal fluctuations can explain the reduced superconducting transition temperature in underdoped BaFe$_2$As$_2$ and indicates that the coexistence of antiferromagnetism and superconductivity occurs locally.

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Ds), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1-xKxFe2As2 for 0.5

We study the intrinsic anisotropy of the superconducting state in KFe$_2$As$_2$, using small-angle neutron scattering to image the vortex lattice as the applied magnetic field is rotated towards the FeAs crystalline planes. The anisotropy is found to be strongly field dependent, indicating multiband superconductivity. Furthermore, the high field anisotropy significantly exceeds that of the upper critical field, providing further support for Pauli limiting in KFe$_2$As$_2$ for field applied in the basal plane. The effect of Pauli paramagnetism on the unpaired quasiparticles in the vortex cores is directly evident from the ratio of scattered intensities due to the longitudinal and transverse vortex lattice field modulation.

We investigate the in-plane anisotropy of Fe 3d orbitals occurring in a wide temperature and composition range of BaFe2(As1-xPx)2 system. By employing the angle-resolved photoemission spectroscopy, the lifting of degeneracy in dxz and dyz orbitals at the Brillouin zone corners can be obtained as a measure of the orbital anisotropy. In the underdoped regime, it starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. With increasing x, it well survives into the superconducting regime, but gradually gets suppressed and finally disappears around the non-superconducting transition (x = 0.7). The observed spontaneous in-plane orbital anisotropy, possibly coupled with anisotropic lattice and magnetic fluctuations, implies the rotational-symmetry broken electronic state working as the stage for the superconductivity in BaFe2(As1-xPx)2.

We have studied the anisotropy in the in-plane resistivity and the electronic structure of isovalent Ru-substituted BaFe$_2$As$_2$ in the antiferromagnetic-orthorhombic phase using well-annealed crystals. The anisotropy in the residual resistivity component increases in proportional to the Ru dopant concentration, as in the case of Co-doped compounds. On the other hand, both the residual resistivity and the resistivity anisotropy induced by isovalent Ru substitution is found to be one order of magnitude smaller than those induced by heterovalent Co substitution. Combined with angle-resolved photoemission spectroscopy results, which show almost the same anisotropic band structure both for the parent and Ru-substituted compounds, we confirm the scenario that the anisotropy in the residual resistivity arises from anisotropic impurity scattering in the magneto-structurally ordered phase rather than directly from the anisotropic band structure of that phase., Comment: 13 pages, 4 figures

Polarized neutron scattering experiments on the slightly overdoped superconductor Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ reveal broad magnetic resonance scattering peaking at approximately 15 meV. In spite of doping far beyond the suppression of magnetic order, this compound exhibits dispersive and anisotropic magnetic excitations. At energies below the resonance maximum, magnetic correlations polarized parallel to the layers but perpendicular to the propagation vector are reduced by a factor two compared to those in the two orthogonal directions; in contrast correlations at the peak maximum are isotropic., Comment: 5 pages, 5 figures

We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2 (BFAP) single crystals by small-angle neutron scattering (SANS). With the field along the c-axis, a nearly isotropic hexagonal VL was formed in the field range from 1 to 16 T, which is a record for this technique in the pnictides, and no symmetry changes in the VL were observed. The temperature-dependence of the VL signal was measured and confirms the presence of (non d-wave) nodes in the superconducting gap structure for measurements at 5 T and below. The nodal effects were suppressed at high fields. At low fields, a VL reorientation transition was observed between 1 T and 3 T, with the VL orientation changing by 45{\deg}. Below 1 T, the VL structure was strongly affected by pinning and the diffraction pattern had a fourfold symmetry. We suggest that this (and possibly also the VL reorientation) is due to pinning to defects aligned with the crystal structure, rather than being intrinsic., Comment: 9 pages, 9 figures

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure in the dispersion which is doping dependent where its energy peaks at the optimally-doped (OP) level (x~0.4) and decreases towards the underdoped (UD) and overdoped (OD) sides. It is also temperature-dependent and survives up to ~90K. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound which observed a similar bosonic mode associated with spin excitations. The relation between the mode energy ({\Omega}) and the SC transition temperature (Tc) deduced from our Laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE showing less doping and temperature dependence and may thus be of different origin., Comment: 5 pages, 3 figures

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of BaNi$_2$P$_2$ which shows a superconducting transition at $T_c$ $\sim$ 2.5 K. We observed hole and electron Fermi surfaces (FSs) around the Brillouin zone center and corner, respectively, and the shapes of the hole FSs dramatically changed with photon energy, indicating strong three-dimensionality. The observed FSs are consistent with band-structure calculation and de Haas-van Alphen measurements. The mass enhancement factors estimated in the normal state were $m^*$/$m_b$ $\leq$ 2, indicating weak electron correlation compared to typical iron-pnictide superconductors. An electron-like Fermi surface around the Z point was observed in contrast with BaNi$_2$As$_2$ and may be related to the higher $T_c$ of BaNi$_2$P$_2$., Comment: 6 figures

We have investigated the elastic constant C33 of Ba(Fe1-xCox)2As2 with eight different Co concentrations by ultrasonic measurement. We found remarkable elastic anomalies near the quantum critical point. We have studied them by measuring the electrical resistivity, heat capacity, and ultrasonic attenuation in addition to the elastic constant. These results have revealed that the inter-layer three-dimensional properties appearing in C33 to be possibly originated from the magnetic character of these materials. Our data about the elastic constant C33 highlight the importance of controlling the c-axis length in the emergence of superconductivity in iron-based superconductors.

Spin fluctuations in superconducting BaFe2(As1-xPx)2 (x=0.34, Tc = 29.5 K) are studied using inelastic neutron scattering. Well-defined commensurate magnetic signals are observed at ({\pi},0), which is consistent with the nesting vector of the Fermi surface. Antiferromagnetic (AFM) spin fluctuations in the normal state exhibit a three-dimensional character reminiscent of the AFM order in nondoped BaFe2As2. A clear spin gap is observed in the superconducting phase forming a peak whose energy is significantly dispersed along the c-axis. The bandwidth of dispersion becomes larger with approaching the AFM ordered phase universally in all superconducting BaFe2As2, indicating that the dispersive feature is attributed to three-dimensional AFM correlations. The results suggest a strong relationship between the magnetism and superconductivity., Comment: 5 pages, 5 figures

We carried out a comparative study of the in-plane resistivity and optical spectrum of doped BaFe2As2 and investigated the doping evolution of the charge dynamics. For BaFe2As2, charge dynamics is incoherent at high temperatures. Electron (Co) and isovalent (P) doping into BaFe2As2 increase coherence of the system and transform the incoherent charge dynamics into highly coherent one. On the other hand, charge dynamics remains incoherent for hole (K) doping. It is found in common with any type of doping that superconductivity with high transition temperature emerges when the normal-state charge dynamics maintains incoherence and when the resistivity associated with the coherent channel exhibits dominant temperature-linear dependence., Comment: 8 pages, 5 figures

We carried out combined transport and optical measurements for BaFe2As2 and five isostructural transition-metal (TM) pnictides. The low-energy optical conductivity spectra of these compounds are, to a good approximation, decomposed into a narrow Drude (coherent) component and an incoherent component. The iron arsenides, BaFe2As2 and KFe2As2, are distinct from other pnictides in their highly incoherent charge dynamics or bad metallic behavior with the coherent Drude component occupying a tiny fraction of the low-energy spectral weight. The fraction of the coherent spectral weight or the degree of coherence is shown to be well correlated with the TM-pnictogen bond angle and the electron filling of TM 3d orbitals, which are measures of the strength of electronic correlations. The iron arsenides are thus strongly correlated systems, and the doping into BaFe2As2 controls the strength of electronic correlations. This naturally explains a remarkable asymmetry in the charge dynamics of electron- and hole-doped systems, and the unconventional superconductivity appears to emerge when the correlations are fairly strong., Comment: 6 pages, 4 figures

We have investigated the superconducting(SC)-gap anisotropy for several Ba-doped KFe$_2$As$_2$ samples using laser-based angle-resolved photoemission spectroscopy. We show that the SC-gap anisotropy and node positions drastically change with a small amount of Ba doping. Our results totally exclude a possibility of $d$-wave symmetry and strongly suggest that both spin and orbital fluctuations are important for the paring interaction in the Ba-doped K122.

To gain insight into the unconventional superconductivity of Fe-pnictides with no electron pockets, we measure the thermal conductivity $\kappa$ and penetration depth $\lambda$ in the heavily hole-doped regime of Ba$_{1-x}$K$_x$Fe$_2$As$_2$. The residual thermal conductivity $(\kappa/T)_{T \rightarrow 0\,{\rm K}}$ and $T$-dependence of $\lambda$ consistently indicate the fully gapped superconductivity at $x=0.76$ and the (line) nodal superconductivity at higher hole concentrations. The magnitudes of $\frac{\kappa}{T}\cdot T_c|_{T \rightarrow 0\,{\rm K}}$ and $\frac{d\lambda}{d(T/T_c)}$ at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual non-monotonic x-dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range, suggesting a doping crossover of the gap function between the s-wave states with and without sign reversal between $\Gamma$-centered hole pockets., Comment: 5 pages, 3 figures

The nature of the pseudogap in high transition temperature (high-Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-Tc superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-Tc superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe2(As1-xPx)2. The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry., Comment: 14 pages, 10 figures

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurements on Ba(Fe_{1-x}Zn_x)_2As_2 (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around T~135 K, indicating antiferromagnetic order below the Neel temperature of T_N ~ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below T_N. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3d state ~10 eV below the Fermi level and do not participate in the formation of the FSs., Comment: 6 pages, 8 figures

We found that the transport Jc of the ex-situ PIT processed (Ba,K)Fe2As2 (Ba-122) wire with single Ag sheath can be significantly enhanced by repeating the combined process of rolling and heat treatment. The transport Jc (4.2 K and 10 T) of 4.4 x 103 A/cm2 (Ic =15.7 A) was obtained for a thin tape of 0.3 mm in thickness, which is the highest value reported so far for the PIT processed 122 wires and tapes with a Ag sheath and processed by a conventional route. The measurement by a hybrid magnet showed that Jc-H curve maintains very small field dependence up to the strong magnetic field of 28 T as expected from the previously reported high Hc2 value. The core of the thin tape shows dense grain structure with less cracks and voids and indicates the development of c-axis alignment, although it is still incomplete. The researches to elucidate the origin of Jc enhancement and to have further improvement of transport Jc are now ongoing. The process is simple using a Ag single sheath and, therefore, more realistic technique for long length wire production., Comment: 8 pages, 7 figures

In the iron pnictide superconductors, two distinct unconventional mechanisms of superconductivity have been put forth: One is mediated by spin fluctuations leading to the s+- state with sign change of superconducting gap between the hole and electron bands, and the other is orbital fluctuations which favor the s++ state without sign reversal. Here we report direct observation of peculiar momentum-dependent anisotropy in the superconducting gap from angle-resolved photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large anisotropy found only in the electron Fermi surface (FS) and the nearly isotropic gap on the entire hole FSs are together consistent with modified s+- gap with nodal loops, which can be theoretically reproduced by considering both spin and orbital fluctuations whose competition generates the gap modulation. This indicates that these two fluctuations are nearly equally important to the high-Tc superconductivity in this system.

Magnetic excitations in Ba(Fe0.94Co0.06)2As2 are studied by polarized inelastic neutron scattering (INS) above and below the superconducting transition. In the superconducting state we find clear evidence for two resonance-like excitations. At a higher energy of about 8 meV there is an isotropic resonance mode with weak dispersion along the c-direction. In addition we find a lower excitation at 4 meV that appears only in the c-polarized channel and whose intensity strongly varies with the L-component of the scattering vector. These resonance excitations behave remarkably similar to the gap modes in the antiferromagnetic phase of the parent compound BaFe2As2.

We investigated the anisotropy in the in-plane optical spectra of detwinned Ba(Fe1-xCox)2As2. The optical conductivity spectrum of BaFe2As2 shows appreciable anisotropy in the magnetostructural ordered phase, whereas the dc resistivity is almost isotropic at low temperatures. Upon Co doping, the resistivity becomes highly anisotropic, while the finite-energy intrinsic anisotropy is suppressed. It is found that anisotropy in resistivity arises from anisotropic impurity scattering from doped Co atoms, extrinsic in origin. Intensity of a specific optical phonon mode is also found to show striking anisotropy in the ordered phase. The anisotropy induced by Co impurity and that observed in the optical phonon mode are hallmarks of the highly polarizable electronic state in the ordered phase., Comment: 5 pages, 4 figures

We investigated the in-plane resistivity anisotropy for underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with improved quality. We demonstrate that the anisotropy in resistivity in the magnetostructural ordered phase arises from the anisotropy in the residual component which increases in proportion to the Co concentration $x$. This gives evidence that the anisotropy originates from the impurity scattering by Co atoms substituted for the Fe sites, rather than so far proposed mechanism such as the anisotropy of Fermi velocities of reconstructed Fermi surface pockets. As doping proceeds to the paramagnetic-tetragonal phase, a Co impurity transforms to a weak and isotropic scattering center., Comment: 12 pages, 3figures, published in PRL 110, 207001 (2013)

The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.

The electronic structure near the Fermi level (EF) of Ba1-xKxFe2As2 (BaK122 ; x = 0.2 - 0.7) is studied using laser ultrahigh-resolution angle-resolved photoemission spectroscopy(ARPES). For the optimally doped case of x = 0.4, we clearly observe two peaks below Tc in the ARPES spectra at a binding energies (BE) of 5 meV and 13meV. The former is assigned to a superconducting (SC) coherence peak since it appears and evolves below the bulk SC transition at Tc (= 36 K), accompanying a gap opening centered at EF. In contrast, the latter peak, which appears below ~ 90 K without any gap formation, is interpreted to be not directly related to a SC coherence peak. This high-BE peak is observed from x = 0.2 to 0.6, reduces in energy with overdoping (x > 0.4) and is absent for x = 0.7. The temperature(T)- and doping-dependent ARPES results suggest that the high-BE peak originates from coupling to a bosonic mode of energy ~ 8 meV., Comment: 8 pages, 5 figures

We have performed an angle-resolved photoemission study of the hole-overdoped iron pnictide superconductor KFe2As2, which shows a low Tc of ~4 K. Most of the observed Fermi surfaces show nearly two-dimensional shapes, while a band near the Fermi level shows a strong dispersion along the kz direction and forms a small three-dimensional hole pocket centered at the Z point, as predicted by band-structure calculation. However, hole Fermi surfaces of yz and zx orbital character centered at the \Gamma point of the two-dimensional Brillouin zone are smaller than those predicted by the calculation while the other hole Fermi surfaces of xy orbital character is much larger. Clover-shaped hole Fermi surfaces around the corner of the 2D BZ are also larger than those predicted by the calculation. These observations are consistent with the de Haas-van Alphen measurement and indicate orbital-dependent electron correlation effects. The effective masses of the energy bands show moderate to strong enhancement, partly due to electron correlation and partly due to energy shifts from the calculated band structure., Comment: 6 pages, 5 figures

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 \Delta_i(k) should appear on the different electronic bands i. Here we introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of \Delta_i(k) in such materials, focusing on LiFeAs. We identify the three hole-like bands assigned previously as \gamma, \alpha_2 and \alpha_1, and we determine the anisotropy, magnitude and relative orientations of their \Delta_i(k). These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

In order to examine to what extent the rigid-band-like electron doping scenario is applicable to the transition metal-substituted Fe-based superconductors, we have performed angle-resolved photoemission spectroscopy studies of Ba(Fe$_{1-x}$Ni$_{x}$)$_2$As$_2$ (Ni-122) and Ba(Fe$_{1-x}$Cu$_{x}$)$_2$As$_2$ (Cu-122), and compared the results with Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (Co-122). We find that Ni 3$\it{d}$-derived features are formed below the Fe 3$\it{d}$ band and that Cu 3$\it{d}$-derived ones further below it. The electron and hole Fermi surface (FS) volumes are found to increase and decrease with substitution, respectively, qualitatively consistent with the rigid-band model. However, the total extra electron number estimated from the FS volumes (the total electron FS volume minus the total hole FS volume) is found to decrease in going from Co-, Ni-, to Cu-122 for a fixed nominal extra electron number, that is, the number of electrons that participate in the formation of FS decreases with increasing impurity potential. We find that the N$\acute{\rm{e}}$el temperature $T_{\rm{N}}$ and the critical temperature $T_{\it{c}}$ maximum are determined by the FS volumes rather than the nominal extra electron concentration nor the substituted atom concentration., Comment: Total: 9 pages, 10 figures

We performed a Laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of Ba1-xKxFe2As2 (BaK) and precisely determined the doping evolution of the superconducting (SC) gaps in this compound. The gap size of the outer hole Fermi surface (FS) sheet around the Brillioun zone (BZ) center shows an abrupt drop with overdoping (for x > 0.6) while the inner and middle FS gaps roughly scale with Tc. This is accompanied by the simultaneous disappearance of the electron FS sheet with similar orbital character at the BZ corner. These results browse the different contributions of X2-Y2 and XZ/YZ orbitals to superconductivity in BaK and can be hardly completely reproduced by the available theories on iron-based superconductors.

The thermal conductivity of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of the RLT in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity was ten times larger. This independence of the RLT on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate for suppressing Tc to zero; the magnitude of the RLT, and its dependence on current direction and on magnetic field; the temperature dependence of the thermal conductivity., Comment: To appear in Physical Review Letters

We use angle-resolved photoemission spectroscopy to study twinned and detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic reconstruction are observed to occur at the structural (TS) and magnetic (TSDW) transitions. At TS, C4 rotational symmetry is broken in the form of an anisotropic shift of the orthogonal dxz and dyz bands. The magnitude of this orbital anisotropy rapidly develops to near completion upon approaching TSDW, at which temperature band folding occurs via the antiferromagnetic ordering wave vector. Interestingly, the anisotropic band shift onsetting at TS develops in such a way to enhance the nesting conditions in the C2 symmetric state, hence is intimately correlated with the long range collinear AFM order. Furthermore, the similar behaviors of the electronic reconstruction in NaFeAs and Ba(Fe1-xCox)2As2 suggests that this rapid development of large orbital anisotropy between TS and TSDW is likely a general feature of the electronic nematic phase in the iron pnictides, and the associated orbital fluctuations may play an important role in determining the ground state properties.

We investigated the transport properties of BaFe$_2$As$_2$ single crystals before and after annealing with BaAs powder. The annealing remarkably improves transport properties, in particular the magnitude of residual resistivity which decreases by a factor of more than 10. From the resistivity measurement on detwinned crystals we found that the anisotropy of the in-plane resistivity is remarkably diminished after annealing, indicative of dominant contributions to the charge transport from the carriers with isotropic and high mobility below magnetostructural transition temperature $T_{\rm s}$ and the absence of nematic state above $T_{\rm s}$. We found that the Hall resistivity shows strong non-linearity against magnetic field and the magnetoresistance becomes very large at low temperatures. These results give evidence for the manifestation of multiple carriers with distinct characters in the ordered phase below $T_{\rm s}$. By analyzing the magnetic field dependences, we found that at least three carriers equally contribute to the charge transport in the ordered phase, which is in good agreement with the results of recent quantum oscillation measurements., Comment: 20 pages, 8 figures, 2 tables, Published in Physical Review B

We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of a saddle-point singularity at the Z point, as well as the Gamma point. With antiferromagnetic ordering, both of these two van Hove singularities come down below the Fermi energy, leading to a topological change in the innermost Fermi surface around the kz axis from cylindrical to tear-shaped, as expected from first-principles calculation. These singularities may provide an additional instability for the Fermi surface of the superconductors derived from BaFe2As2., Comment: 14 pages, 4 figures, 1 table

An ordered phase showing remarkable electronic anisotropy in proximity to the superconducting phase is now a hot issue in the field of high-transition-temperature superconductivity. As in the case of copper oxides, superconductivity in iron arsenides competes or coexists with such an ordered phase. Undoped and underdoped iron arsenides have a magnetostructural ordered phase exhibiting stripe-like antiferromagnetic spin order accompanied by an orthorhombic lattice distortion; both the spin order and lattice distortion break the tetragonal symmetry of crystals of these compounds. In this ordered state, anisotropy of in-plane electrical resistivity is anomalous and difficult to attribute simply to the spin order and/or the lattice distortion. Here, we present the anisotropic optical spectra measured on detwinned BaFe2As2 crystals with light polarization parallel to the Fe planes. Pronounced anisotropy is observed in the spectra, persisting up to an unexpectedly high photon energy of about 2 eV. Such anisotropy arises from an anisotropic energy gap opening below and slightly above the onset of the order. Detailed analysis of the optical spectra reveals an unprecedented electronic state in the ordered phase., Comment: 7 pages, 7 figures, accepted to PNAS

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the iron-based superconductor PrFeAsO_{0.7} and examined the Fermi surfaces and band dispersions near the Fermi level. Heavily hole-doped electronic states have been observed due to the polar nature of the cleaved surfaces. Nevertheless, we have found that the ARPES spectra basically agree with band dispersions calculated in the local density approximation (LDA) if the bandwidth is reduced by a factor of ~2.5 and then the chemical potential is lowered by ~70 meV. Comparison with previous ARPES results on LaFePO reveals that the energy positions of the d_{3z^2-r^2}- and d_{yz,zx}-derived bands are considerably different between the two materials, which we attribute to the different pnictogen height as predicted by the LDA calculation.

We report the results of the angular-dependent magnetoresistance oscillations (AMROs), which can determine the shape of bulk Fermi surfaces in quasi-two-dimensional (Q2D) systems, in a highly hole-doped Fe-based superconductor KFe$_2$As$_2$ with $T_c \approx$ 3.7 K. From the AMROs, we determined the two Q2D FSs with rounded-square cross sections, corresponding to 12% and 17% of the first Brillouin zone. The rounded-squared shape of the FS cross section is also confirmed by the analyses of the interlayer transport under in-plane fields. From the obtained FS shape, we infer the character of the 3d orbitals that contribute to the FSs., Comment: 4 pages, 4 figures, accepted in Phys. Rev. Lett

A neutron scattering study of heavily hole-overdoped superconducting KFe$_2$As$_2$ revealed a well-defined low-energy incommensurate spin fluctuation at [$\pi(1\pm2\delta$),0] with $\delta$ = 0.16. The incommensurate structure differs from the previously observed commensurate peaks in electron-doped $A$Fe$_2$As$_2$ ($A$ = Ba, Ca, or Sr) at low energies. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)$_2$As$_2$ at high energies. A band structure calculation suggests interband scattering between bands around the $\Gamma$ and X points as an origin of this incommensurate peak. The perpendicular direction of the peak splitting can be understood within the framework of multiorbital band structure. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped samples, which can be responsible for the appearance of superconductivity in the heavily hole-doped samples., Comment: 4 pages, 4 figures

This paper has been withdrawn by the author due to some experimental mistakes. In this paper, we reported that C66, C44 and (C11-C12)/2 show remarkable softening toward the structural transition temperature TS. The data reported in this paper were acquired using the ultrasonic frequency lower than 25 MHz. Recently, we performed high-frequency measurements for the same system. We found that the anomaly of C44 and (C11-C12)/2 tend to disappear rapidly with increasing the frequency. On the other hand, C66 anomaly is still there at high frequencies. Therefore, we concluded that the observed anomalies in C44 and (C11-C12)/2 are not true. They would be ascribed to certain influence by the large softening of C66. So, we have checked our data through careful measurements by using ultrasonic frequency higher than 60 MHz, so far. Then, it has been found that C66 shows still nice softening toward TS, but that its temperature dependence is slightly different from the results of this paper. We have accumulated reliable data now. They will be reported in near future., Comment: This paper has been withdrawn

We have performed an angle-resolved photoemission study of the iron pnictide superconductor KFe2As2 with Tc 4 K. Most of the observed Fermi surfaces show almost two-dimensional shapes, while one of the quasi-particle bands near the Fermi level has a strong dispersion along the kz direction, consistent with the result of a band-structure calculation. However, hole Fermi surfaces \alpha and \zeta are smaller than those predicted by the calculation while other Fermi surfaces are larger. These observations are consistent with the result of a de Haas-van Alphen study and a theoretical prediction on inter-band scattering, possibly indicating many body effects on the electronic structure., Comment: 4 pages, 5 figures. Proceeding of the 9th International Conference on Spectroscopies in Novel Superconductors (SNS2010)

Understanding the mechanism and symmetry of electron pairing in iron-based superconductors represents an important challenge in condensed matter physics [1-3]. The observation of magnetic flux lines - "vortices" - in a superconductor can contribute to this issue, because the spatial variation of magnetic field reflects the pairing. Unlike many other iron pnictides, our KFe2As2 crystals have very weak vortex pinning, allowing small-angle-neutron-scattering (SANS) observations of the intrinsic vortex lattice (VL). We observe nearly isotropic hexagonal packing of vortices, without VL-symmetry transitions up to high fields along the fourfold c-axis of the crystals, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong temperature-dependence of the intensity down to T<

Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe$_2$As$_2$ is an unconventional superconductor with strong electron correlations., Comment: 6 pages, 6 figures, 1 table. Extended version to be published in Phys. Rev. B

We investigated the optical spectrum of Ba(Fe1-xCox)2As2 single crystals with various doping levels. It is found that the low-energy optical conductivity spectrum of this system can be decomposed into two components: a sharp Drude term and a broad "incoherent" term. For the compounds showing magnetic order, a gap appears predominantly in the "incoherent" component, while an s-wave like superconducting gap opens in both components for highly doped compounds. The Drude weight steadily increases as doping proceeds, consistent with electron doping in this system. On the other hand, the "incoherent" spectral weight is almost doping independent, but its spectral feature is intimately connected with the magnetism. We demonstrate that the presence of two distinct components in the optical spectrum well explains the doping and temperature dependences of the dc resistivity., Comment: 10 pages, 10 figures, to be published in Phys. Rev. B