Origin of structural and magnetic transitions in BaFe$_{2-x}$Ru x As 2 materials

Singh

2015

New J. Phys.

A three-orbital itinerant-electron model involving d xz , d yz and d xy Fe 3d orbitals is proposed for iron pnictides towards understanding the ( , 0 π ) ordered magnetism and magnetic excitations in these materials. It is shown that this model at half filling yields a gapped ( , 0 π ) magnetic state, and simultaneously reproduces several experimentally observed features such as the electronic structure, spin excitations, as well as the ferro orbital order between the d xz and d yz orbitals.interaction strength U c for ( , 0 π ) ordering. However, spin wave dispersion in this model does not agree in detail with INS measurements [27,28]. Moreover, it does not include d xy Fe orbital which contributes some portions of the electron pockets. Similarly, three-orbital models with one-third filling (i.e. two electrons in three orbitals) [29], two-third filling (i.e. four electrons in three orbitals) [30,31], and four-orbital model at half-filling [32] can reproduce the desired FS structure, but spin wave excitations in these models have not been investigated yet. More realistic five-orbital models [33][34][35], which yield the FS topology similar to experimental findings, were proposed aimed at investigation of pairing instabilities, but spin excitations were not studied. Although anisotropic spin wave excitation was obtained in a recent study [36] for a five-orbital model [37], investigation of spin excitations over the entire BZ was not carried out.In this context, we present and investigate a three-orbital itinerant-electron model in this paper. We find that this minimal model simultaneously yields the correct FS structure as well as spin wave dispersion consistent with INS experiments. Moreover, ferro orbital order of appropriate sign is also obtained in this model.The organization of this paper is as follows. The importance of ferromagnetic (F) spin couplings on the experimentally measured spin wave dispersion in the ( , 0 π ) state is briefly discussed in section 2. Then in section 3, it is shown that no F spin coupling is generated in the two-band model with FS nesting [24] ,although this model yields correct FS topology. A third d xy Fe orbital is therefore necessary to overcome this shortcoming, and a three-orbital model having d xz , d yz and d xy Fe 3d orbitals is presented in section 4 highlighting the FS and DOS. The investigation of magnetic excitations and orbital ordering in the ( , 0 π ) magnetic state is carried out in section 5. Finally conclusions are discussed in section 6. Spin wave energy at the ferromagnetic zone boundary and ferromagnetic spin couplingIn order to highlight the effect of induced F spin couplings on the spin wave dispersion and spin wave energy at the FZB, we consider the case of single band Hubbard model with nearest-neighbor (NN) hopping t and nextnearest-neighbor (NNN) hopping t′. In this case, the spin wave dispersion in the strong coupling limit [38] is given by ( ) J J J b q J J q q H U n n U J n n J J a a a a S S

Section: Orbital Orderingmentioning

confidence: 63%

Electronic structure, spin excitations, and orbital ordering in a three-orbital model for iron pnictides

Singh

2015

New J. Phys.

“…A number of authors addressed to the fact that density funtional theory within local density approximation (LDA) as well as generalized gradient approximation (GGA) was unable to achieve the experimental value of z As (fractional co-ordinate of As) fairly accurately [16,25,26,37,38,49]. In fact the optimized value of z As is about 0.1Å smaller than that of the experimental z As .…”

Section: Theory and Computational Methodsmentioning

confidence: 99%

“…It should be mentioned that the highest transition temperature (T c ) till now for Fe-based superconductors is 109K [20]. Correlation of superconducting transition temperature and other physical properties with structural parameters is well established [21][22][23][24][25][26]. Fe-As(pnictides)or Fe-Se(chalcogenides) planes are back bone structures of these superconductors where As atoms are placed above and below the Fe plane.…”

Section: Introductionmentioning

confidence: 99%

“…Height of this As atom from Fe plane is defined as 'anion height' closely related to z As i.e., the fractional co-ordinate of As atom. It is also evident from literature that, as far as veracity of calculation of electronic structure is concerned, the role of structural parameters specially z As is instrumental [25][26][27]. Fe-based superconductors are broadly classified into six families among them 122 family is the most studied family, both experimentally as well as theoretically.…”

Section: Introductionmentioning

confidence: 99%

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Electronic structures of doped BaFe2As2 materials: virtual crystal approximation versus super-cell approach

Sen

2016

Eur. Phys. J. B

Self Cite

Employing virtual crystal approximation and super-cell methods for doping, we have performed a comparative study of the electronic structures of various doped BaFe2As2 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.

“…Avci et al, displayed the appearance of an additional C 4 (tetragonal) phase well inside the orthorhombic phase in Ba 1−x Na x Fe 2 As 2 system at the boundary between superconductivity and SDW (stripe antiferromagnetism) which is different from the previous observations of re-entrant tetragonal phase in electron-doped compounds [15] and has many important consequences. The role of various structural parameters specially z As (fractional z co-ordinate of As atom) or anion height (height of the As atom from the Fe plane) in superconductivity, magnetism and structural transition is indispensable [20,22,[27][28][29][30][31]. However the interrelationship among superconductivity, magnetic order, nematic order, orbital order and structural transition is under thorough investigations both theoretically as well as experimentally [3,5,[32][33][34].…”

Section: Introductionmentioning

confidence: 99%

Stoner factors of doped 122 Fe-based superconductors: First principles results

Sen

Computational Materials Science

2017

Self Cite

A comprehensive study on the evolution of Stoner factor with doping concentration for various doped 122 systems (like BaFe2As2, SrFe2As2) of Fe-based superconductors is presented. Our first principles electronic structure calculations reveal that for Co/Ru (electron or iso-electronic) doping at Fe sites or P doping at As sites result in a reduction of Stoner factor with increasing doping concentration. On the contrary, in case of Na/K (hole) doping at the Ba sites, Stoner factor is enhanced for higher doping concentrations. This may be considered as an indicator of elevation of "magnetic fluctuation" in these systems. We find that the Stoner factor uniquely follows the variation of the pnictide height zAs/Fe-As bond length with various kinds of doping. Our calculated Fermi surfaces explicate the diversities in the behaviour of Stoner factors for various doped 122 systems ; larger degree of Fermi surface nesting, larger the value of Stoner factor and vice versa.