The phase diagrams of iron-based superconductors: theory and experiments

Phase diagrams play a primary role in the understanding of materials properties. For iron-based superconductors (Fe-SC), the correct definition of their phase diagrams is crucial because of the close interplay between their crystallo-chemical and magnetic properties, on one side, and the possible coexistence of magnetism and superconductivity, on the other. The two most difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin of the structural transformation taking place during cooling and its relationship with magnetism; 2) the correct description of the region where a crossover between the magnetic and superconducting electronic ground states takes place. Hence a proper and accurate definition of the structural, magnetic and electronic phase boundaries provides an extremely powerful tool for material scientists. For this reason, an exact definition of the thermodynamic phase fields characterizing the different structural and physical properties involved is needed, although it is not easy to obtain in many cases. Moreover, physical properties can often be strongly dependent on the occurrence of micro-structural and other local-scale features (lattice micro-strain, chemical fluctuations, domain walls, grain boundaries, defects), which, as a rule, are not described in a structural phase diagram. In this review, we critically summarize the results for the most studied 11-, 122- and 1111-type compound systems, providing a correlation between experimental evidence and theory

Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides

In this work we study the effect of the rare earth element in iron oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out Density Functional Theory calculations of the band structure, which evidence the multiband character of these compounds and the presence of Dirac cones along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other hand, we explore transport behavior by means of resistivity, Hall resistance and magnetoresistance measurements, which confirm the dominant role of Dirac cones. By combining our theoretical and experimental approaches, we extract information on effective masses, scattering rates and Fermi velocities for different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of Physics

Stability and electronic properties of the LaNiO2_2/SrTiO3_3 interface

Infinite-layer nickelate thin films materialize an intriguing new platform for high-temperature unconventional superconductivity, with LaNiO$_2$/SrTiO$_3$ as reference setup. We discuss the relative stability of the elementary interfaces of this system and determine the corresponding electronic band structure. We find substantial changes compared to the bulk, in particular in relation to the 5$d$ orbital contributions to the low-energy physics which can be totally replaced by purely Ni-3$d$ flat bands. The $d^9$ configuration characteristic of cuprates can thus be supplemented by an extra interfacial ingredient destabilizing the normal non-superconducting state in these heterostructures.Comment: 6 pages, 2 tables, 3 figures + Supplemental Material. v2: updated refs., extra figure + improved discussion of interfacial structure. v3: new appendi

Macroscopic polarization and band offsets at nitride heterojunctions

Ab initio electronic structure studies of prototypical polar interfaces of wurtzite III-V nitrides show that large uniform electric fields exist in epitaxial nitride overlayers, due to the discontinuity across the interface of the macroscopic polarization of the constituent materials. Polarization fields forbid a standard evaluation of band offsets and formation energies: using new techniques, we find a large forward-backward asymmetry of the offset (0.2 eV for AlN/GaN (0001), 0.85 eV for GaN/AlN (0001)), and tiny interface formation energies.Comment: RevTeX 4 pages, 2 figure

Geometric effects in the infinite-layer nickelates

Geometric effects in the infinite-layer nickelates $R$NiO$_2$ associated with the relative size of the $R$-site atom are investigated via first-principles calculations. We consider, in particular, the prospective YNiO$_2$ material to illustrate the impact of these effects. Compared to LaNiO$_2$, we find that the La $\to$ Y substitution is equivalent to a pressure of 19 GPa and that the presence of topotactic hydrogen can be precluded. However, the electronic structure of YNiO$_2$ departs from the cuprate-like picture due to an increase in both self-doping effect and $e_g$ hybridization. Furthermore, we find that geometric effects introduce a quantum critical point in the $R$NiO$_2$ series. This implies a $P4/mmm \leftrightarrow I4/mcm$ structural transformation associated to a $A_3^+$ normal mode, according to which the oxygen squares undergo an in-plane rotation around Ni that alternates along $c$. We find that such a $A_3^+$-mode instability has a generic character in the infinite-layer nickelates and can be tuned via either the effective $R$-site atom size or epitaxial strain.Comment: 6 pages, 4 figures, 4 table

Non-linear macroscopic polarization in III-V nitride alloys

We study the dependence of macroscopic polarization on composition and strain in wurtzite III-V nitride ternary alloys using ab initio density-functional techniques. The spontaneous polarization is characterized by a large bowing, strongly dependent on the alloy microscopic structure. The bowing is due to the different response of the bulk binaries to hydrostatic pressure, and to internal strain effects (bond alternation). Disorder effects are instead minor. Deviations from parabolicity (simple bowing) are of order 10 % in the most extreme case of AlInN alloy, much less at all other compositions. Piezoelectric polarization is also strongly non-linear. At variance with the spontaneous component, this behavior is independent of microscopic alloy structure or disorder effects, and due entirely to the non-linear strain dependence of the bulk piezoelectric response. It is thus possible to predict the piezoelectric polarization for any alloy composition using the piezoelectricity of the parent binaries.Comment: RevTex 7 pages, 7 postscript figures embedde