Electron Correlation and Spin Density Wave Order in Iron Pnictides

Abstract

We study the correlation effects on the electronic structure and spin density wave order in Fepnictides. Using the multiorbital Hubbard model and Gutzwiller projection, we show that correlation effects are essential to stabilize the metallic spin density wave phase for the intermediate correlation strengths appropriate for pnictides. We find that the ordered moments depend sensitively on the Hund's rule coupling J but weakly on the intraorbital Coulomb repulsion U , varying from 0.3µB to 1.5µB in the range J = 0.3 ∼ 0.8 eV for U = 3 ∼ 4 eV. We obtain the phase diagram and discuss the effects of orbital order and electron doping, the evolution of the Fermi surface topology with the ordered moment, and compare to recent experiments. PACS numbers: 71.27.+a, 74.70.Xa, 74.25.Ha, 74.25.Jb The iron pnictides have emerged recently as another class of high-T c superconductors [1] involving the transition metal d-electrons in addition to the cuprates. In the two most studied pnictide families, the 1111 (e.g. LaO 1−x F x FeAs) and the 122 series (e.g. Ba 1−x K x Fe 2 As 2 ), the iron valence is Fe 2+ . There are six electrons occupying five Fe 3d orbitals. Their direct overlap and via As 4p orbitals produce five energy bands with a total bandwidth around 4eV [2][3][4]. This is comparable to the on-site Coulomb repulsion U = 3 ∼ 4eV [4] typical for transition metals. Thus, the Fe-pnictides are multiorbital systems where the correlation strength is intermediate and comparable to the kinetic energy. In this intermediate correlation regime lies the challenge of the many-body physics responsible for unconventional electronic ground states and emergent phenomena in condensed matter and complex materials.That the correlation effects play an important role in pnictides can be seen from the fact that despite of the orbital degeneracy, the normal state behaves quite incoherently with an enhanced magnetic susceptibility in contrast to conventional Fermi liquids [5]. At low temperatures, the observed quasiparticle dispersion [6,7] exhibits a strong bandwidth reduction due to electron correlations, as shown in a first principle calculation that includes the interaction effects in the Gutzwiller approach [8]. Appropriate treatment of the electron correlation in this intermediate regime, especially its multiorbital nature, is essential for understanding the properties, including the high-T c superconductivity, of these materials.In this paper, we investigate the correlation effects on the spin density wave (SDW) order from which the high-T c superconductivity emerges with sufficient doping. At low temperatures, semi-metallic SDW order develops in the undoped pnictides with an ordering vector Q = (π, 0) connecting the geometric centers of the electron and hole Fermi surface (FS) pockets in the unfolded Brillouin zone (BZ) containing one Fe atom per unit cell [9,10]. The atomic states for Fe 2+ is predominantly S = 2 and S = 1 in the presence of Hund's rule coupling and crystal field splitting. However, the ordered Fe moments are much s...

Magnetic order in orbital models of the iron pnictides

“…94,95 Of course, another possible explanation for the stability of the (π, 0) AFM order involves strong-coupling physics beyond the nesting scenario. 65,68 We note that recent calculations using dynamical mean-field theory suggest that the correlation strength in these materials has been underestimated. 96 Further research is clearly needed to understand the complex magnetic physics of the pnictides.…”

“…A large number of different orbital tightbinding models have therefore been proposed involving between two or five of the Fe 3d orbitals, 42,43,48,51,52,54,56,59,60,64,65,[77][78][79] or also including the As 4p orbitals 45,80 and even orbitals from outside the FeAs planes. 81 Although the existence of superconductivity in these models has been extensively studied, 40,[44][45][46]48,51,53,56,60,63,64,66 the magnetic behaviour remains rather poorly understood.…”

Phase Diagram and Electronic Structure of Praseodymium and Plutonium

“…A possible way to compute the Gutzwiller solution is the following [27]: (i) Given ðR; λÞ, use Eqs. (32) and (33) to compute the Lagrange multipliers μ and η and the corresponding jΨ 0 i, which determines n (37)], which determine the left sides of Eqs. (38) and (39).…”

“…[20]. The GA approximation was, thereafter, extensively developed [21][22][23][24][25][26][27], and it has been formulated and implemented in combination with realistic electronic structure calculations such as the LDA þ GA approach [23,28], which has been applied successfully to many systems [29][30][31][32][33][34][35][36]. A third important many-body technique is the slave boson approach (SB) [37,38], which is, in principle, an exact reformulation of the quantum many-body problem for model Hamiltonians, and it reproduces the results of the GA at the saddle-point level [24,39].…”

High-energy pseudogap induced by Hund coupling in a degenerate Hubbard model

“…Moreover, as a result of the multiband nature of pnictides, the interplay of Coulomb and exchange interactions gives rise to phenomena not found in cuprates. The importance of Hund coupling in pnictides and chalcogenides was recently pointed out in several theoretical [2][3][4][5][6] and experimental studies. [7][8][9] Optical data on paramagnetic LaFePO (Ref.…”