Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe 2 As 2 , a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy ⌬ = 9.8͑4͒ meV. The in-plane spin-wave velocity v ab and out-of-plane spin-wave velocity v c measured at 12 meV are 280͑150͒ and 57͑7͒ meV Å, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At T N = 136͑1͒ K, the gap closes and quasielastic scattering is observed above T N , indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the L direction becomes "rodlike," characteristic of uncorrelated out-of-plane spins, attesting to the twodimensionality of the system.

Ternary Ba-Fe-As system has been studied to determine a primary solidification field of the BaFe 2 As 2 phase. We found that the BaFe 2 As 2 phase most likely melts congruently and primarily solidifies either in the FeAs excess or Ba x As 100−x excess liquid. Knowing the primary solidification field, we have performed the vertical Bridgman growth using the starting liquid composition of Ba 15 Fe 42.5 As 42.5 .Large single crystals of the typical size 10x4x2 mm 3 were obtained and their quality was confirmed by X-ray Laue and neutron diffraction.

The spin dynamics in single crystal, electron-doped Ba͑Fe 1−x Co x ͒ 2 As 2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound ͑x = 0.06͒ that share a remarkable similarity with those in the paramagnetic state of the parent compound ͑x =0͒. In the overdoped superconducting compound ͑x = 0.14͒, magnetic excitations show a gaplike behavior, possibly related to a topological change in the hole Fermi surface ͑Lifshitz transition͒ while the imaginary part of the spin susceptibility Љ prominently resembles that of the overdoped cuprates. For the heavily overdoped, nonsuperconducting compound ͑x = 0.24͒ the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.

The magnetic properties of the hyperkagome system of Tb 3 Ga 5 O 12 have been investigated by neutron scattering. Evidence of antiferromagnetic long-range order of the Tb moments at T N Յ 0.35 K in zero field is provided. With the application of magnetic field in the paramagnetic phase, ferromagnetic peaks initially appear at nuclear Bragg reflections. With the field higher than 3 Tesla, antiferromagnetic Bragg peaks appear as well indicating that the low-temperature magnetic phase extends well beyond the phase boundary, into the paramagnetic phase, with field. The new magnetic symmetry sets in at the metamagnetic transition in this system.When physical systems have access to more than one ground state, collective effects invoked from the competition or frustration of the interactions may emerge leading to exotic phases. 1 Even in crystals with pristine order, the presence of residual entropy at low temperatures is evidence of frustration inherent to the lattice. 2 In a typical crystal, atoms along with their magnetic moments, if present, are expected to follow certain symmetry rules. In magnetism, the Curie-Weiss, CW , temperature 3 defines an approximate scale at which point long-range ordering should appear. However, magnetic spinels, pyrochlores, and garnets defy this classic notion precisely because of their crystal symmetry. [4][5][6] If spins reside at the corners of triangular or tetrahedral lattices, the pairwise spin interactions cannot be simultaneously satisfied if they are to be aligned antiferromagnetically. 7 Such geometric frustration gives rise to a macroscopic degenerate ground-state manifold. In quantum spin systems, exotic collective effects may give rise to a resonating valence bond state. 8 In classical spin systems, frustration may lead to a spin-liquid state where the spins fluctuate continuously down to absolute zero, 9 or to a spin-ice state as in the paradigmatic hexagonal ice. 10,11 In the rare-earth ͑RE͒ garnets with Ia3d symmetry, the classical RE spin resides at the corners of triangles organized in two interpenetrating sublattices forming a hyperkagome structure 1 as seen in Fig. 1͑a͒. In some garnets such as the Gd 3 Ga 5 O 12 with a Heisenberg spin, magnetic long-range order is suppressed all the way down to ϳ0 K. 12,13 However, in garnets of the Tb family such as Tb 3 Al 5 O 12 , the paramagnetic ͑PM͒ state makes way to static antiferromagnetic ͑AF͒ order. 14 Intriguing is the Ising-like nature of the Tb spin with local axes that point in three orthogonal directions in the triangle, 15 producing a multiaxis magnet with potentially different responses to external perturbations such as an applied magnetic field. 14 The low-energy physics is dominated by two crystal-field ground-state singlets of this non-Kramers ion, while magnetic interactions which mixes them give rise to Néel order in the absence of an external magnetic field. 16,17 Thus, long-range dipolar interactions and nearestneighbor exchange interactions play an important role in determining the magnetic properties, and ...

Angle resolved photoemission spectroscopy of Ba(Fe1−xCox)2As2 (x = 0.06, 0.14, and 0.24) shows that the width of the Fe 3d yz/zx hole band depends on the doping level. In contrast, the Fe 3d x 2 − y 2 and 3z 2 − r 2 bands are rigid and shifted by the Co doping. The Fe 3d yz/zx hole band is flattened at the optimal doping level x = 0.06, indicating that the band renormalization of the Fe 3d yz/zx band correlates with the enhancement of the superconducting transition temperature. The orbital-dependent and doping-dependent band renormalization indicates that the fluctuations responsible for the superconductivity is deeply related to the Fe 3d orbital degeneracy.PACS numbers:

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In the heavily-electron-doped regime of the Ba(Fe,Co)2As2 superconductor, three hole bands at the zone center are observed and two of them reach the Fermi level. The larger hole pocket at the zone center is apparently nested with the smaller electron pocket around the zone corner. However, the (pi,0) Fermi surface reconstruction reported for the hole-doped case is absent in the heavily-electron-doped case. This observation shows that the apparent Fermi surface nesting alone is not enough to enhance the antiferromagnetic correlation as well as the superconducting transition temperature.

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