We have performed high-resolution angle-resolved photoemission spectroscopy on Fe-based superconductor LiFeAs (T(c)=18 K). We reveal multiple nodeless superconducting (SC) gaps with 2Δ/k(B)T(c) ratios varying from 2.8 to 6.4, depending on the Fermi surface (FS). We also succeeded in directly observing a gap anisotropy along the FS with magnitude up to ~30%. The anisotropy is fourfold symmetric with an antiphase between the hole and electron FSs, suggesting complex anisotropic interactions for the SC pairing. The observed momentum dependence of the SC gap offers an excellent opportunity to investigate the underlying pairing mechanism.

The emergent behavior of spin liquids that are born out of geometrical frustration makes them an intriguing state of matter. We show that in the quantum kagome antiferromagnet ZnCu3(OH)6SO4 several different correlated, yet fluctuating states exist. By combining complementary local-probe techniques with neutron scattering, we discover a crossover from a critical regime into a gapless spin-liquid phase with decreasing temperature. An additional unconventional instability of the latter phase leads to a second, distinct spin-liquid state that is stabilized at the lowest temperatures. We advance such complex behavior as a feature common to different frustrated quantum magnets.

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor ðTl; RbÞ y Fe 2Àx Se 2 (T c ¼ 32 K). We determined the orbital characters and the k z dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4p z and Fe 3d xy orbitals to the small electron pocket. We argue that the emergence of Se 4p z states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.

Impurities, which are unavoidable in real materials, may play an important role in the magnetism of frustrated spin systems with a spin-liquid ground state. We address the impurity issue in quantum kagome antiferromagnets by investigating ZnCu3(OH)6SO4 (Zn-brochantite) by means of muon spin spectroscopy. We show that muons dominantly couple to impurities, originating from Cu-Zn intersite disorder, and that the impurity spins are highly correlated with the kagome spins, allowing us to probe the host kagome physics via a Kondo-like effect. The low-temperature plateau in the impurity susceptibility suggests that the kagome spin-liquid ground state is gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and a non-trivial field dependence.PACS numbers: 75.10. Kt, 76.75.+i,75.30.Hx The two-dimensional Heisenberg quantum kagome antiferromagnet (QKA), the paradigm of geometrical frustration, has been in the focus of attention for several years [1,2]. Theoretical studies have lately converged on a spin-liquid (SL) ground state [3][4][5][6][7][8], most likely with a finite gap to magnetic spinon excitations [5][6][7]. Although early experiments, on the contrary, spoke in favor of a gapless ground state [9][10][11][12], experimental evidence of a gapped SL has also recently been presented [13,14]. The confusion about the gap likely stems from the fact that all known QKA representatives contain a significant amount of defects that may affect their ground state, on top of other perturbations, such as magnetic anisotropy [15][16][17][18][19] and exchange interactions beyond the nearest neighbors [10,[19][20][21][22]. In particular, it is difficult to discern the magnetic response of the impurity spins from that of the kagome spins, which makes identification of their influence on the ground state elusive [23][24][25][26][27][28].In herbertsmithite, the most extensively studied QKA representative to date [29], a sizable amount (5-10%) of Cu-Zn intersite disorder is present [9,30,31]. The general consensus is that these defects contribute significantly to the bulk magnetic response, but only at low energies (E 0.7 meV) and at low temperatures (T /J 1/20) [14,[32][33][34]. Even though the defects are often described as quasi-free spin-1/2 impurities [29], their relation with the kagome spins is yet unsettled [13,14,35,36]. It is important to resolve this issue, because a strong coupling would mean that defects could be intimately involved in the selection of a particular ground state of the QKA.We provide a unique perspective on the impurity problem by investigating another QKA representative, the recently synthesized ZnCu 3 (OH) 6 SO 4 (Zn-brochantite) [12], which bears a resemblance to herbertsmithite in many respects. Despite a sizable average intraplane exchange interaction of J = 65 K [12], it remains magnetically disordered down to at least T /J = 1/3000 [37]. Moreover, just like herbertsmithite [32], it exhibits scalefree magnetic fluctuations at high T 's that are reminiscent of a cr...

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