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# Electronic structures of doped BaFe$$_2$$As$$_2$$ materials: virtual crystal approximation versus super-cell approach

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### Abstract

Employing virtual crystal approximation and super-cell methods for doping, we have performed a comparative study of the electronic structures of various doped BaFe$$_2$$As$$_2$$ materials by first principles simulations. Both of these methods give rise to a similar density of states and band structures in case of hole doping (K doping in Ba site) and iso-electronic P doping in As site. But in case of electron doped systems with higher doping concentration, electronic structures, calculated using virtual crystal approximation approach deviates from that of the super-cell method. On the other hand in case of iso-electronic Ru doping implemented by virtual crystal approximation, an extra shift of the chemical potential in electronic structure in comparison to super-cell method is observed and that shift can be used to predict the correct electronic structure within virtual crystal approximation as reflected in our calculated Fermi surfaces. But for higher Ru doping concentration, simple shifting of chemical potential does not work as the electronic structure calculated by virtual crystal approximation approach is entirely different from that of the calculated by super-cell formalism.

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### Nematic order in iron superconductors - who is in the driver's seat?

(2013)
Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the $$x$$ and $$y$$ directions in the Fe plane non-equivalent. This can happen because of (i) a tetragonal to orthorhombic structural transition, (ii) a spontaneous breaking of an orbital symmetry, or (iii) a spontaneous development of an Ising-type spin-nematic order - a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. The Landau theory of phase transitions dictates that the development of one of these orders should immediately induce the other two, making the origin of nematicity a physics realization of a "chicken and egg problem". The three scenarios are, however, quite different from a microscopic perspective. While in the structural scenario lattice vibrations (phonons) play the dominant role, in the other two scenarios electronic correlations are responsible for the nematic order. In this review, we argue that experimental and theoretical evidence strongly points to the electronic rather than phononic mechanism, placing the nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. We discuss different microscopic models for nematicity in the iron pnictides, and link nematicity to other ordered states of the global phase diagram of these materials -- magnetism and superconductivity. In the magnetic model nematic order pre-empts stripe-type magnetic order, and the same interaction which favors nematicity also gives rise to an unconventional $$s^{+-}$$ superconductivity. In the charge/orbital model magnetism appears as a secondary effect of ferro-orbital order, and the interaction which favors nematicity gives rise to a conventional $$s^{++}$$ superconductivity.
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### Superconductivity in single-layer films of FeSe with a transition temperature above 100 K

(2014)
Recently, interface has been employed to enhance superconductivity in the single-layer FeSe films grown on SrTiO3(001)(STO) with a possible Tc of ~ 80 K, which is nearly ten times of the Tc of bulk FeSe and is above the Tc record of 56 K for the bulk Fe-based superconductors. This work together with those on superconducting oxides interfaces revives the long-standing idea that electron pairing at a two-dimensional (2D) interface between two different materials is a potential path to high transition temperature (Tc) superconductivity. Subsequent angle-resolved photoemission spectroscopy (ARPES) measurements revealed different electronic structure from those of bulk FeSe with a superconducting-like energy gap closing at around 65K. However, previous ex situ electrical transport measurements could only detect the zero-resistance below ~30 K. Here we report the observation of high Tc superconductivity in the FeSe/STO system. By in situ 4-point probe (4PP) electrical transport measurement that can be conducted at an arbitrary position of the FeSe film on STO, we detected superconductivity above 100 K. Our finding makes FeSe/STO the exciting and ideal research platform for higher Tc superconductivity.
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### Magnetism and its microscopic origin in iron-based high-temperature superconductors

(2012)
High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising since these undoped states display dramatically different antiferromagnetic (AF) spin arrangements and N$$\rm \acute{e}$$el temperatures. Although there is general consensus that magnetic interactions are important for superconductivity, much is still unknown concerning the microscopic origin of the magnetic states. In this review, progress in this area is summarized, focusing on recent experimental and theoretical results and discussing their microscopic implications. It is concluded that the parent compounds are in a state that is more complex than implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials.
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Condensed matter