Understanding angle dependence in iron pnictides Belén Valenzuela Universidad Autónoma de Madrid (UAM) and Instituto de Ciencias Materiales de Madrid (ICMM-CSIC) In collaboration with: María J. Calderón and Elena Bascones (ICMM-CSIC)

High Tc superconductors: Pnictides High Tc SC based on As-Fe layers: but the As atom is not exactly on the iron plane… J. Zhao, e al. arXiv:0806.2528 Neither structural nor magnetic transition, lower Tc Structural and magnetic transition, higher Tc T.M. McQueen, et al. arXiv:0805.2149

θ Experimentally and in first principle calculations the angle of the Fe-(As, P) bond to the vertical (θ) is crucial for the superconducting, magnetic and structural properties of the iron pnictides. El pseudogap se ve en todos los cupratos y experimentalmente lo que se ve es una reduccion de la densidad de estados o una reduccion En los procesos de scattering. ARPES NODAL ANTINODAL θ

Angle dependence in First-principle calculations: LDA It is a condensate of Cooper pairs. Experimentos de fotoemisión revelan picos en el espectro de excitaciones indicando la presencia de Cuasipartículas que predice la teoría BCS. Simetría d-wave no trivial. Sin embargo experimentos de Raman y ARPES muy recientes Nos dicen que parece que hay dos escalas una nodal y otra antinodal en la zona superconductora infradopada. V. Vildosola, e al. arXiv:0806.2528

Angle dependence in experiments La superconductividad es una inestabilidad del estado normal y por lo tanto para entender el origen de la superconductividad necesitamos Entender el estado normal del que surge. RVB: el pseudogap (J-tx) se forma cuando los spines forman singletes y este spin gap reduce el Escattering entre los huecos que da lugar a un pico de Drude mas estrecho en la fase del pseudogap. Pares de Cooper se forman En T* y coherencia a Tc. Angle related with electron correlation. Stronger correlation -> higher Tc Angle depends on doping J. Zhao, e al. arXiv:0806.2528

But: The Fe crystal field environment due to As atoms is nearly tetrahedral θtetra=π/2-54.7º and the order of the energy levels is But: Is this relevant for the energy levels? Our answer-> Yes, it can even change the order of the energy levels deg deg This angle varies! Iron - environment T.M. McQueen, et al. arXiv:0805.2149

Angle dependence of the Crystal Field: As environment: When crystal field is calculated for values of the angle different from θtetra a new harmonic appears that splits the energy levels. This information is encoded in ř: E Order of the energy levels usually used in the literature As +Fe environment: a richer scenario appears… Poster

Further doping dependences in the poster Tight-binding for two levels, dxz and dyz, angle-dependence of the hopping txyz,yz Further doping dependences in the poster θtetra θ

Tight-binding for two levels: topology and bandwidth changes with angle! Regular tetrahedron Regular tetrahedron Squashed tetrahedron Squashed tetrahedron

Message: angle matters! Crystal field can change for different angles with two possible origins: It is not a perfect tetrahedron As+Fe environment Tight Binding for two levels: Topology depends on angle: crucial for nesting Bandwidth depends on angle: crucial for electron-correlation effects