In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A(1g) superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.

Superconductivity in graphene has been highly sought after for its promise in various device applications and for general scientific interest. Ironically, the simple electronic structure of graphene, which is responsible for novel quantum phenomena, hinders the emergence of superconductivity. Theory predicts that doping the surface of the graphene effectively alters the electronic structure, thus promoting propensity towards Cooper pair instability (Profeta et al (2012) Nat. Phys. 8 131-4; Nandkishore et al (2012) Nat. Phys. 8 158-63) [1, 2]. Here we report the emergence of superconductivity at 7.4 K in Li-intercalated few-layer-graphene (FLG). The absence of superconductivity in 3D Li-doped graphite underlines that superconductivity in Li-FLG arises from the novel electronic properties of the 2D graphene layer. These results are expected to guide future research on graphene-based superconductivity, both in theory and experiments. In addition, easy control of the Li-doping process holds promise for various device applications.

We report a study on tuning the charge density wave (CDW) ferromagnet SmNiC2 to a weakly coupled superconductor by substituting La for Sm. X-ray diffraction measurements show that the doped compounds obey Vegard’s law, where La (Lu) alloying expands (shrinks) the lattice due to its larger (smaller) atomic size than Sm. In the series Sm1−xLaxNiC2, CDW transition (TCDW = 148 K) for SmNiC2 is gradually suppressed, while the ferromagnetic (FM) ordering temperature (TC) at 17 K slightly increases up to x = 0.3. For x > 0.3, TC starts to decrease and there is no signature that could be related with the CDW phase. Electrical resistivity, magnetic susceptibility and specific heat measurements point toward the possible presence of a FM quantum critical point (QCP) near x = 0.92, where the TC is extrapolated to zero temperature. Superconductivity in LaNiC2 (Tsc = 2.9 K) is completely suppressed with small amount of Sm inclusion near the proposed FM critical point, indicating a competition between the two ordered phases. The tunable lattice parameters via chemical substitution (La,Lu) and the ensuing change among the ordered phases of ferromagnetism, CDW and superconductivity underscores that SmNiC2 provides a rich avenue to study the rare example of a FM QCP, where the broken symmetries are intricately correlated.

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We studied two BaFe2−xNixAs2 (Ni-doped Ba-122) single crystals at two different doping levels (underdoped and optimally doped) using an optical spectroscopic technique. The underdoped sample shows a magnetic phase transition around 80 K. We analyze the data with a Drude-Lorentz model with two Drude components (D1 and D2). It is known that the narrow D1 component originates from electron carriers in the electron-pockets and the broad D2 mode is from hole carriers in the hole-pockets. While the plasma frequencies of both Drude components and the static scattering rate of the broad D2 component show negligible temperature dependencies, the static scattering rate of the D1 mode shows strong temperature dependence for the both samples. We observed a hidden quasi-linear temperature dependence in the scattering rate of the D1 mode above and below the magnetic transition temperature while in the optimally doped sample the scattering rate shows a more quadratic temperature dependence. The hidden non-Fermi liquid behavior in the underdoped sample seems to be related to the magnetic phase of the material.

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO substrate, an epitaxial polycrystalline SrRuO thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

SignificanceObservation of a quantum critical point (QCP) in correlated unconventional superconductors has been key to understanding of the intricate relationship between superconductivity and quantum criticality, which has been often hampered by the dome of superconducting (SC) state that veils the T=0 K quantum phase transition. This work demonstrates observation of a QCP in the Al-doped CrAs, where the QCP became completely detached from the dome of superconducting phase. The tuned QCP and its separation from the SC state imply that Cooper pair formation is not mediated solely by critical magnetic fluctuations in CrAs. This work illustrates the potential of using multiple non-thermal parameters to reveal the relationship between superconductivity and a hidden quantum critical point in classes of unconventional superconductors. AbstractThe origin of unconventional superconductivity and its relationship to a T=0 K continuous quantum phase transition (a quantum critical point, QCP), which is hidden inside the dome of a superconducting state, have long been an outstanding puzzle in correlated superconductors. The observation and tuning of the hidden QCP, which is critical in resolving the mystery, however, has been rarely reported due to lack of ideal systems. The helical antiferromagnet CrAs provides an example in which a dome of superconductivity appears at a pressure where its magnetic transition goes to zero temperature. Here we report the tuning of a projected critical point in CrAs via Al chemical doping (Al-CrAs) and separation of the magnetic critical point from the pressure-induced superconducting phase. When CrAs is doped with Al, its AFM ordering temperature T N increases from 260 K to 270 K. With applied pressure, T N decreases and extrapolates to zero Kelvin near 4.5 kbar, which is shifted from 8 kbar for undoped CrAs. A funnel of anomalously enhanced electron scattering and a non-Fermi liquid resistivity underscore an AFM QCP near 4.5 kbar in Al-CrAs. Pressure-induced superconductivity, in contrast, is almost independent of Al doping and forms a dome with essentially the identical maximum T c and same optimal pressure as in pure CrAs. The clear separation between the tuned AFM QCP and T c maximum in Al-CrAs suggests that superconductivity is independent of the AFM QCP, illustrating subtleties in the interplay between superconductivity and quantum criticality in correlated electron systems.

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