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Modeling of LaO 1-x F x FeAs from the Strong Coupling Perspective: the Magnetic Order and Pairing Channels. Adriana Moreo Dept. of Physics and ORNL University of Tennessee, Knoxville, TN, USA. Collaborators: M.Daghofer, J. Riera., E. Arrigoni, D. Scalapino and E. Dagotto.
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Modeling of LaO1-xFxFeAs from the Strong Coupling Perspective: the Magnetic Order and Pairing Channels. Adriana Moreo Dept. of Physics and ORNL University of Tennessee, Knoxville, TN, USA. Collaborators: M.Daghofer, J. Riera., E. Arrigoni, D. Scalapino and E. Dagotto. Supportedby NSF grants DMR-0706020.
Quaternary oxypnictides: LnOMPn (Ln: La, Pr; M:Mn, Fe, Co, Ni; Pn: P, As). Fe –As planes. La-O planes. Fe form a square lattice. F replaces O and introduces e- in Fe. F doped LaOFeAs
Experimental Properties • Tc up to 28K (above 50K replacing La by other rare earths). Chen et al., cond-mat:0803.0128 • Bad metal or semiconductor in undoped regime. • Anomalous specific heat. Suggest nodes in SC state. • Anomaly in resistivity at 150K. Dong et al., cond-mat: 0803.3426
Parent compound • Long range magnetic order. • Small order parameter: suggests small or intermediate U and J. De la Cruz et al., cond-mat: 0804.0795. See also McGuire et al., cond-mat:0804.0796; Dong et al., cond-mat:0803.3426 and others.
Doped compound • No magnetic order (neutrons, NMR, Mossbauer). • Nodal Gap (specific heat,NMR) or large ungapped regions (integrated PES). • Two gaps (NMR). • Unconventional mechanism, singlet pairing (mSR, NMR). • Hole doping also occurs. (La1-xSrxOFeAs). La1-xSrxOFeAs Wen et al., cond-mat:0803.3021.
Theory • Band Structure: 3d Fe orbitals are important. (LDA) • dxz and dyz most important close to eF. (Korshunov et al., cond-mat:0804.1793). • Metallic state. • Possible itinerant magnetic order. Singh et al., cond-mat: 0803.0429; Xu et al., cond-mat:0803.1282; Giovannetti et al., cond-mat: 0804.0866.
Fermi Surface LDA • Two hole pockets at G point. • Two electron pockets at M. • dxz and dyz orbitals (with some dxy hybridization). ARPES Liu et al., cond-mat: 0806.2147 NdFeAsO1-xFx Singh et al., cond-mat: 0803.0429
Interactions: strong or weak electronic correlations? • X-ray spectra: weak correlation (Kurmaev et al., cond.-mat.: 0805.0668). • DMFT : strong correlation (Haule et al., cond-mat: 0803.1279). • RPA: U~3, J=0 (Raghu et al., cond.-mat: 0804.1113). • RPA and mean field calculations including Coulomb and Hund interactions are predicting the expected magnetic order and all possible variations of the order parameter.
Numerical Simulations (Daghofer et al., cond-mat:0805.0148). • Relevant degrees of freedom need to be identified. • Construct the minimal model. • Exact diagonalization in a small cluster. • Very successful with the cuprates: found magnetic order and correct pairing symmetry.
Minimum Model • Consider the Fe-As planes. • Two d orbitals dxz and dyz based on LDA and experimental results. • Consider electrons hopping between Fe ions through a double exchange process Fe-As-Fe. Square Fe lattice. • Interactions: Coulomb and Hund.
Hoppings Obtain from Slate-Koster overlap integrals between Fe-d and As-p orbitals and Fe-As-Fe double exchange hopping.
Hoppings pds=1 pdp=-0.2 ddp=-pdp ddd=0.1pdp The hoppings are obtained in terms of the lattice parameters and the pd overlap Integrals.
Numerical Results: no doping • U<1 for metal in undoped case. • If J=U/4, U<1 to reproduce experimental order parameter in parent compound. • S(k) peaks at (0,p) and (p,0).
Electron Doping Spin singlet J=U/4 Spin triplet
Symmetry of the Pairing Operator According to D4h point group See Y. Wan and Q-H Wang, cond-mat:0805.0923. Sixteen possible pairing operators.
Band Structure gap Y M singlet X G triplet node Fermi Surface
Two Gaps • Experimental indications of two gaps. • Experiments indicate “d-wave” gap. Y. Wang et al., cond-mat: 0806.1986; K. Matano et al., cond-mat: 0806.0249; F. Hunte et al., cond-mat:0804.0485.
Singlet (from numerical simulations) • No gap on e- pockets. • Nodes on h-pockets. • Two gaps. • “d-wave”
Conclusions • U and J have to be small or intermediate. • The above does not mean “weak coupling” since the ground state in the undoped case appears to be a magnetically highly correlated state. • In the region of parameter space explored the favored singlet pairing state has “d-wave” symmetry. • Hole FS develop gaps with nodes. No gaps on electron FS. • The spin singlet pairing operator that we obtained does not disagree with any of the experimental results currently available.
U=1., J=0.25, pdp=-0.5 Symmetry breaking for (0,p) U=0.5, J=0.125, pdp=-0.2