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Register a new userDefect trapping and phase separation in chemically doped bulk AgF2
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We report a computational survey of chemical doping of silver(II) fluoride, an oxocuprate analog. We find that the ground-state solutions exhibit strong tendency for localization of defects and for phase separation. The additional electronic states are strongly localized and the resulting doped phases exhibit insulating properties. Our results, together with previous insight from experimental attempts, indicate that chemical doping may not be a feasible way towards high-temperature superconductivity in bulk silver(II) fluoride.
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AgF2 is a layered material and a correlated charge transfer insulator with an electronic structure very similar to the parent compounds of cuprate high-Tc superconductors. It is also interesting for being a powerful oxidizer. Here we present a first principles computation of its electronic properties in a slab geometry including its work function for the (010) surface (7.76 eV) which appears to be one of the highest among known materials surpassing even that of fluorinated diamond (7.24 eV). We demonstrate that AgF2 will show a broken-gap type alignment becoming electron doped and promoting injection of holes in many wide band gap insulators if chemical reaction can be avoided. Novel junction devices involving p type and n type superconductors are proposed. The issue of chemical reaction is discussed in connection with the possibility to create flat AgF2 monolayers achieving high-Tc superconductivity. As a first step in this direction, we study the stability and properties of an isolated AgF2 monolayer.
Crystal and electronic structure, lattice dynamics and thermodynamic stability of little known mixed valent diamagnetic AgIAgIIIF4 beta form of AgF2 is thoroughly examined for the first time and compared with well known antiferromagnetic AgIIF2 alpha form within the framework of Density Functional Theory based methods, phonon direct method and quasiharmonic approximation. Computed equations of state, bulk moduli, electronic densities of states, electronic and phonon band structures including analysis of optically active modes and p T phase diagram of the alpha/beta system are presented. This study demonstrates that alpha is thermodynamically preferred over beta at all temperatures and pressures of its existance but simultaneously beta is dynamically stable in much broader pressure range. The beta phase is discussed in broader context of isostructural ternary metal fluorides and isolectronic oxides including NaCuO2, the reference compound for existence of CuIII species in high temperature oxocuprate superconductors.
We present a phenomenological model based on the thermodynamics of the phase separated state of manganites, accounting for its static and dynamic properties. Through calorimetric measurements on La$_{0.225}$Pr$_{0.40}$Ca$ _{0.375}$MnO$_{3}$ the low temperature free energies of the coexisting ferromagnetic and charge ordered phases are evaluated. The phase separated state is modeled by free energy densities uniformly spread over the sample volume. The calculations contemplate the out of equilibrium features of the coexisting phase regime, to allow a comparison between magnetic measurements and the predictions of the model. A phase diagram including the static and dynamic properties of the system is constructed, showing the existence of blocked and unblocked regimes which are characteristics of the phase separated state in manganites.
By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.
Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a surd5timessurd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors.
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