59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)

We report an NMR investigation of the superconductivity in BaFe(2)As(2) induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Delta 560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.Comment: Accepted for publication in J. Phys. Soc. Jpn. (4 pages

Field-Induced Magnetostructural Transitions in Antiferromagnetic Fe1+yTe1-xSx

The transport and structural properties of Fe1+yTe1-xSx (x=0, 0.05, and 0.10) crystals were studied in pulsed magnetic fields up to 65 T. The application of high magnetic fields results in positive magnetoresistance effect with prominent hystereses in the antiferromagnetic state. Polarizing microscope images obtained at high magnetic fields showed simultaneous occurrence of structural transitions. These results indicate that magnetoelastic coupling is the origin of the bicollinear magnetic order in iron chalcogenides.Comment: 5 pages, 5 figures, accepted for publication in Journal of the Physical Society of Japa

Iron pnictides as a new setting for quantum criticality

Two major themes in the physics of condensed matter are quantum critical phenomena and unconventional superconductivity. These usually occur in the context of competing interactions in systems of strongly-correlated electrons. All this interesting physics comes together in the behavior of the recently discovered iron pnictide compounds that have generated enormous interest because of their moderately high-temperature superconductivity. The ubiquity of antiferromagnetic ordering in their phase diagrams naturally raises the question of the relevance of magnetic quantum criticality, but the answer remains uncertain both theoretically and experimentally. Here we show that the undoped iron pnictides feature a novel type of magnetic quantum critical point, which results from a competition between electronic localization and itinerancy. Our theory provides a mechanism to understand the experimentally-observed variation of the ordered moment among the undoped iron pnictides. We suggest P substitution for As in the undoped iron pnictides as a means to access this new example of magnetic quantum criticality in an unmasked fashion. Our findings point to the iron pnictides as a much-needed new setting for quantum criticality, one that offers a new set of control parameters.Comment: (v3) New abstract, more explanatory material, accepted for PNA

High precision fundamental constants at the TeV scale

This report summarizes the proceedings of the 2014 Mainz Institute for Theoretical Physics (MITP) scientific program on "High precision fundamental constants at the TeV scale". The two outstanding parameters in the Standard Model dealt with during the MITP scientific program are the strong coupling constant $\alpha_s$ and the top-quark mass $m_t$. Lacking knowledge on the value of those fundamental constants is often the limiting factor in the accuracy of theoretical predictions. The current status on $\alpha_s$ and $m_t$ has been reviewed and directions for future research have been identified.Comment: 57 pages, 24 figures, pdflate

Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition

Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high temperature superconductors. Similarly, the new iron-based high temperature superconductors exhibit a tetragonal to orthorhombic structural transition (i.e. a broken C4 symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here we present an angle resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant dxz and dyz character, which is consistent with anisotropy observed by other probes. For compositions x>0, for which the structural transition (TS) precedes the magnetic transition (TSDW), an anisotropic splitting is observed to develop above TSDW, indicating that it is specifically associated with TS. For unstressed crystals, the band splitting is observed close to TS, whereas for stressed crystals the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.Comment: final version published in PNAS, including supplementary informatio

Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx

We have investigated the fluorine-doping dependence of lattice constants, transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly and monotonically decreases the in-plane lattice parameter. In the normal state, electrical resistivity at low temperature is proportional to the square of temperature and the electronic specific heat coefficient has large value, indicating the existence of moderate electron-electron correlation in this system. Hall coefficient has large magnitude, and shows large temperature dependence, indicating the low carrier density and multiple carriers in this system. Temperature dependence of the upper critical field suggests that the system is a two gap superconductor. The F-doping dependence of these properties in this system are very weak, while in the FeAs system (LaFeAsO), the F doping induces the large changes in electronic properties. This difference is probably due to the different F-doping dependence of the lattice in these two systems. It has been revealed that a pure effect of electron doping on electronic properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp