We discuss the role of dynamical many-electron effects in the physics of iron and iron-rich solid alloys under applied pressure on the basis of recent ab initio studies employing the dynamical mean-field theory (DMFT). Electronic correlations in iron in the moderate pressure range up to 60 GPa are discussed in the first section. DMFT-based methods predict an enhancement of electronic correlations at the pressure-induced transition from body-centered cubic (bcc) alpha-Fe to hexagonal close-packed (hcp) epsilon-Fe. In particular, the electronic effective mass, scattering rate and electron-electron contribution to the electrical resistivity undergo a step-wise increase at the transition point. One also finds a significant many-body correction to the epsilon-Fe equation of state, thus clarifying the origin of discrepancies between previous DFT studies and experiment. An electronic topological transition is predicted to be induced in epsilon-Fe by many-electron effects; its experimental signatures are analyzed. Next section focuses on the geophysically relevant pressure-temperature regime of the Earth's inner core (EIC) corresponding to the extreme pressure of 360 GPa combined with temperatures up to 6000 K. The three iron allotropes (bcc, hcp and face-centered-cubic) previously proposed as possible stable phases at such conditions are found to exhibit qualitatively different many-electron effects as evidenced by a strongly non-Fermi-liquid metallic state of bcc-Fe and an almost perfect Fermi liquid in the case of hcp-Fe. A recent active discussion on the electronic state and transport properties of hcp-Fe at the EIC conditions is reviewed in details. We also discuss the impact of a Ni admixture, which is expected to be present in the core matter. We conclude by outlining some limitation of the present DMFT-based framework and perspective directions for further development.
The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO (R=Ce, Pr, Nd, Sm, Gd and Tb) and RFe2As2 (R=Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magnetic-fiel…
The transport properties of iron under Earth's inner core conditions are essential input for the geophysical modelling but are poorly constrained experimentally. Here we show that the thermal and electrical conductivity of iron at those conditions remains high even if the electron-electron-scattering (EES) is properly taken into account. This result is obtained by ab initio simulations taking into account consistently both thermal disorder and electronic correlations. Thermal disorder suppresses the non-Fermi-liquid behavior of the body-centered cubic iron phase, hence, reducing the EES; the t…
The electronic state and transport properties of hot dense iron are of the utmost importance to geophysics. Combining the density functional and dynamical mean field theories we study the impact of electron correlations on electrical and thermal resistivity of hexagonal close-packed $ε$-Fe at Earth's core conditions. $ε$-Fe is found to behave as a nearly perfect Fermi liquid. The quadratic dependence of the scattering rate in Fermi liquids leads to a modification of the Wiedemann-Franz law with suppression of the thermal conductivity as compared to the electrical one. This significantly increa…
Some Bravais lattices have a particular geometry that can slow down the motion of Bloch electrons by pre-localization due to the band-structure properties. Another known source of electronic localization in solids is the Coulomb repulsion in partially filled d- or f-orbitals, which leads to the formation of local magnetic moments. The combination of these two effects is usually considered of little relevance to strongly correlated materials. Here we show that it represents, instead, the underlying physical mechanism in two of the most important ferromagnets: nickel and iron. In nickel, the van…
Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of minerals that altogether constitute $\sim 95$ vol\% of the Earth's lower mantle: ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate (MgSiO$_3$) perovskite. Despite great strides made in the past decade, the existence of intermediate-spin (IS) state in ferrous iron (Fe$^{2+}$) (with total electron spin $S=1$) and its possible role in the pressure-induced spin crossover in these lower-mantle minerals still remain controversial. Using density functional theory $+$ self-consist…
Rare-earth iron garnets $R_{3}$Fe$_{5}$O$_{12}$ are fascinating insulators with very diverse magnetic phases. Their strong potential in spintronic devices has encouraged a renewal of interest in the study of their low temperature spin structures and spin wave dynamics. A striking feature of rare-earth garnets with $R$-ions exhibiting strong crystal-field effects like Tb-, Dy-, Ho-, and Er-ions is the observation of low-temperature non-collinear magnetic structures featuring "umbrella-like" arrangements of the rare-earth magnetic moments. In this study, we demonstrate that such umbrella magneti…
Properties of iron oxides at the extreme conditions are of essential importance in condensed matter physics and Geophysics. The recent discovery of a new type of iron oxide, Fe4O5, at high pressure and high temperature of Earth's deep interior attracts great interests. In this paper, we report the electronic structure and the magnetic properties of Fe4O5 predicted by the density functional theory plus dynamic mean field theory (DFT+DMFT) approach. We find that Fe4O5 stays metallic from ambient pressure to high pressure. The magnetic moments of iron atoms at the three different crystallographic…
We introduce a cluster DMFT (Dynamical Mean Field Theory) approach to study the normal state of the iron pnictides and chalcogenides. In the regime of moderate mass renormalizations, the self-energy is very local, justifying the success of single site DMFT for these materials and for other Hunds metals. We solve the corresponding impurity model with CTQMC (Continuous Time Quantum Monte-Carlo) and find that the minus sign problem is not severe in regimes of moderate mass renormalization.
Detwinning of magnetic (nematic) domains in Fe-based superconductors has so far only been obtained through mechanical straining, which considerably perturbs the ground state of these materials. The recently discovered non-mechanical detwinning in EuFe$_{2}$As$_{2}$ by ultra-low magnetic fields offers an entirely different, non-perturbing way to achieve the same goal. However, this way seemed risky due to the lack of a microscopic understanding of the magnetically driven detwinning. Specifically, the following issues remained unexplained: (i) ultra-low value of the first detwinning field of $\s…
Recent discovery of pyrite FeO$_2$, which can be an important ingredient of the Earth's lower mantle and which in particular may serve as an extra source of water in the Earth's interior, opens new perspectives for geophysics and geochemistry, but this is also an extremely interesting material from physical point of view. We found that in contrast to naive expectations Fe is nearly 3+ in this material, which strongly affects its magnetic properties and makes it qualitatively different from well known sulfide analogue - FeS$_2$. Doping, which is most likely to occur in the Earth's mantle, makes…
Strength of correlations in pnictides and its assessment by theoretical calculations and spectroscopy experiments
本文采用LDA+DMFT（Local Density Approximation combined with Dynamical Mean-场论）计算方法，计算了新型高T_c铁氧磷属化合物LaFeAsO的光谱性质。采用第一性原理约束密度泛函理论的Wannier函数方法，计算了铁3d电子的平均库仑排斥参数U=3-4 eV和Hund交换参数J=0.8 eV.利用这些参数进行的DMFT计算得到了适度关联的电子结构，其有效电子质量增加值m^*~2与实验X射线谱和光电子能谱一致。磷属元素化合物的实验谱与金属铁的相应谱吻合较好，而与莫特绝缘体FeO的谱有很大差异，证实了中等关联强度的结论。
The spin crossover of iron in Fe$^{3+}$-bearing bridgmanite, the most abundant mineral of the Earth's lower mantle, is by now a well-established phenomenon, though several aspects of this crossover remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA + U$_{sc}$ calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted configurations in super-cells containing up to 80 atoms. Vibrational/thermal effects on the crossover are addressed within the quas…
Using nuclear quadrupole resonance, the phase diagram of 1111 $R$FeAsO$_{1-x}$F$_x$ ($R$$=$La, Ce, Sm) iron pnictides is constructed as a function of the local charge distribution in the paramagnetic state, which features low-doping-like (LD-like) and high-doping-like (HD-like) regions. Compounds based on magnetic rare earths (Ce, Sm) display a unified behavior, and comparison with La-based compounds reveals the detrimental role of static iron $3d$ magnetism on superconductivity, as well as a qualitatively different evolution of the latter at high doping. It is found that the LD-like regions f…
Iron oxide is one of the most important components in Earth's mantle. Recent discovery of the stable presence of Fe5O6 at Earth's mantle environment stimulates significant interests in the understanding of this new category of iron oxides. In this paper, we report the electronic structure and magnetic properties of Fe5O6 calculated by the density functional theory plus dynamic mean field theory (DFT+DMFT) approach. Our calculations indicate that Fe5O6 is a conductor at the ambient pressure with dominant Fe-3d density of states at the Fermi level. The magnetic moments of iron atoms at three non…
