Magnetic properties of Fe-pnictides superconductors as a function of pressure and doping
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Magnetic properties of Fe-pnictides superconductors as a function of pressure and doping Gianni Profeta1, Nicola Colonna2 and Alessandra Continenza3 1
Dipartimento di Fisica Universita’ degli Studi dell’Aquila and CNR-Spin – Via Vetoio 10. 67100 L’Aquila (ITALY) 2 SISSA, via Bonomea, 265 - 34136 Trieste (ITALY) 3 CNISM -Dipartimento di Fisica Università degli Studi dell’Aquila Via Vetoio 10 - 67100 L’Aquila (ITALY) ABSTRACT We present a first principles study of the electronic and magnetic properties of Fe-based pnicitdes superconductors as a function of pressure and doping. We show that the magnetic phase and a local magnetic moment persists at doping level quite larger than what found in experiments and the pressure phase diagram consists of a paramagnetic, antiferromagnetic and non-magnetic phases. Although this result calls for the inclusion of long-wavelength or local fluctuations of iron magnetic moment and non-hydrostatic pressure effects, in order to improve the theoretical description of real experimental conditions, recent photoemission experiments[1] reconcile these DFT results, showing a local magnetic moment on Fe site different from zero in the paramagnetic, antiferromagnetic and the superconducting phase. INTRODUCTION In recent years superconductivity has come back on stage, thanks also to new classes of materials that reopened the quest for high transition temperatures and unconventional pairing mechanisms. After the cuprates, fullerenes, and MgB2, superconductivity has been discovered in iron pnictides and the palette of materials and pairing candidates is still expanding, with, e.g., alkali-doped aromatic hydrocarbons. These new materials are entering a mature stage where real progress is made. However, the new Fe-based superconductors are every day reserving surprises and new developments that continuously challenge current understanding. The competition and, more often, the coexistence of superconductivity and magnetism in these compounds enriches the wealth of issues that have to be addressed and explained, while new experimental evidences make the field more and more challenging. In particular, the issue we would like to address in this paper is related to the magnitude of the Fe-magnetic moment and thus to the presence of a possible magnetic order (short or long ranged) in different conditions. Ever since the first days of the Fe-based superconductors discovery, it has always been remarked the inadequacy of Density Functional Theory (DFT) to correctly describe the magnetic moment in these compounds [2]: while in fact DFT-based results accurately predict the so-called stripe-antiferromagnetic (sAFM) as more stable magnetic phase over other possible choices (check board and ferromagnetic), the magnitude of the magnetic moment localized on the Fe-atoms is greatly overestimated with respect to experimental findings (about 2 µB compared with less than 1 µB, respectively). This problem has been often associated to the fact that the description of the high-temperature non magnetic (PM) phase seems t
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