Local magnetic moments in iron and nickel at ambient and Earth's core conditions

Influence of shock waves on bifunctional nickel particles: Enhancing magnetic properties and supercapacitor applications

In the present study, shock-wave impact experiments were conducted to investigate the structural properties of nickel metal powder when exposed to shock waves. Both X-ray diffractometry and scanning electron microscopy were used to evaluate the structural and surface morphological changes in the shock-loaded samples. Notably, the experimental results revealed variations in lattice parameters and cell structures as a function of the number of shock pulses and the increasing volume. The transition occurred from P2 (100 shocks) to P3 (200 shocks). Remarkably, P5 (400 shocks) exhibited attempts to return to its initial state, and intriguingly, P4 displayed characteristics reminiscent of the pre-shock condition. Additionally, significant morphological changes were observed with an increase in shock pulses. Magnetic measurements revealed an increase in magnetic moment for P2, P3, and P4, but a return to the original state was observed for P5. Moreover, the capacitance exhibited an upward trend with increasing shock pulses, except for P5, where it experienced a decline. These findings underscore the significant impact of even mild shock waves on the physical and chemical characteristics of bifunctional nickel particles. This research sheds light on the potential applications of shock wave-induced structural changes in enhancing the magnetic properties and supercapacitor performance of nickel particles.

Rigorous Screened Interactions for Realistic Correlated Electron Systems

We derive a widely applicable first-principles approach for determining two-body, static effective interactions for low-energy Hamiltonians with quantitative accuracy. The algebraic construction rigorously conserves all instantaneous two-point correlation functions in a chosen model space at the level of the random phase approximation, improving upon the traditional uncontrolled static approximations. Applied to screened interactions within a quantum embedding framework, we demonstrate these faithfully describe the relaxation of local subspaces via downfolding high-energy physics in molecular systems, as well as enabling a systematically improvable description of the long-range plasmonic contributions in extended graphene.

Heavy fermions vs doped Mott physics in heterogeneous Ta-dichalcogenide bilayers

Controlling and understanding electron correlations in quantum matter is one of the most challenging tasks in materials engineering. In the past years a plethora of new puzzling correlated states have been found by carefully stacking and twisting two-dimensional van der Waals materials of different kind. Unique to these stacked structures is the emergence of correlated phases not foreseeable from the single layers alone. In Ta-dichalcogenide heterostructures made of a good metallic "1H"- and a Mott insulating "1T"-layer, recent reports have evidenced a cross-breed itinerant and localized nature of the electronic excitations, similar to what is typically found in heavy fermion systems. Here, we put forward a new interpretation based on first-principles calculations which indicates a sizeable charge transfer of electrons (0.4-0.6 e) from 1T to 1H layers at an elevated interlayer distance. We accurately quantify the strength of the interlayer hybridization which allows us to unambiguously determine that the system is much closer to a doped Mott insulator than to a heavy fermion scenario. Ta-based heterolayers provide therefore a new ground for quantum-materials engineering in the regime of heavily doped Mott insulators hybridized with metallic states at a van der Waals distance.

Oxygen-Driven Enhancement of the Electron Correlation in Hexagonal Iron at Earth's Inner Core Conditions

Earth's inner core (IC) consists of mainly iron with some light elements. Understanding its structure and related physical properties has been elusive as a result of its required extremely high pressure and temperature conditions. The phase of iron, elastic anisotropy, and density-velocity deficit at the IC have long been questions of great interest. Here, we find that the electron correlation effect is enhanced by oxygen and modifies several important features, including the stability of iron oxides. Oxygen atoms energetically stabilize hexagonal-structured iron at IC conditions and induce elastic anisotropy. Electrical resistivity is much enhanced in comparison to pure hexagonal close-packed (hcp) iron as a result of the enhanced electron correlation effect, supporting the conventional thermal convection model. Moreover, our calculated seismic velocity shows a quantitative match with geologically observed preliminary reference Earth model (PREM) data. We suggest that oxygen is the essential light element to understand and model Earth's IC.

Computational prediction of new magnetic materials

The discovery of new magnetic materials is a big challenge in the field of modern materials science. We report the development of a new extension of the evolutionary algorithm USPEX, enabling the search for half-metals (materials that are metallic only in one spin channel) and hard magnetic materials. First, we enabled the simultaneous optimization of stoichiometries, crystal structures, and magnetic structures of stable phases. Second, we developed a new fitness function for half-metallic materials that can be used for predicting half-metals through an evolutionary algorithm. We used this extended technique to predict new, potentially hard magnets and rediscover known half-metals. In total, we report five promising hard magnets with high energy product (|BH|_MAX), anisotropy field (H_a), and magnetic hardness (κ) and a few half-metal phases in the Cr-O system. A comparison of our predictions with experimental results, including the synthesis of a newly predicted antiferromagnetic material (WMnB₂), shows the robustness of our technique.

Role of pressure on electronic, magnetic and structural properties at iron's Curie temperature: a DFT + DMFT study

We use the combination of density functional theory and dynamical mean-field theory to compute the Curie temperature of the iron body-centered cubicαphase and probe its pressure dependence. Our calculations reveal thatTCshows a decrease which is very weak over a domain of pressures that is much larger than the stability domain of theαphase. This is consistent with the experimental results. We highlight the importance of the Hund's couplingJnot only on the electronic and magnetic properties but also on the structural properties. Lastly, we analyze the electronic and magnetic properties under pressure and discuss the evolution of magnetic moments in both phases in relation to the change of Curie temperature.

Itinerant magnetism of chromium under pressure: a DFT+DMFT study

We consider electronic and magnetic properties of chromium, a well-known itinerant antiferromagnet, by a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We find that electronic correlation effects in chromium, in contrast to its neighbors in the periodic table, are weak, leading to the quasiparticle mass enhancement factorm*/m≈ 1.2. Our results for local spin-spin correlation functions and distribution of weights of atomic configurations indicate that the local magnetic moments are not formed. Similarly to previous results of DFT at ambient pressure, the non-uniform magnetic susceptibility as a function of momentum possesses close to the wave vectorQ_H= (0, 0, 2π/a) (ais the lattice constant) sharp maxima, corresponding to Kohn anomalies. We find that these maxima are preserved by the interaction and are not destroyed by pressure. Our calculations qualitatively capture a decrease of the Néel temperature with pressure and a breakdown of itinerant antiferromagnetism at pressure of ∼9 GPa in agreement with experimental data, although the Néel temperature is significantly overestimated because of the mean-field nature of DMFT.

Resistivity Exponents in 3D Dirac Semimetals From Electron-Electron Interaction

We study the resistivity of three-dimensional semimetals with linear dispersion in the presence of on-site electron-electron interaction. The well-known quadratic temperature dependence of the resistivity of conventional metals is turned into an unusual T^{6} behavior. An analogous change affects the thermal transport, preserving the linearity in T of the ratio between thermal and electrical conductivities. These results hold from weak coupling up to the nonperturbative region of the Mott transition. Our findings yield a natural explanation for the hitherto not understood large exponents characterizing the temperature dependence of transport experiments on various topological semimetals.

Characteristic Timescales of the Local Moment Dynamics in Hund's Metals

We study the characteristic timescales of the fluctuating local moments in Hund's metal systems for different degrees of correlation. By analyzing the dynamical spin susceptibility in the real-time domain, we determine the timescales controlling oscillation and damping of on-site fluctuations-a crucial factor for the detection of local moments with different experimental probes. We apply this procedure to different families of iron pnictides or chalcogenides, explaining the material trend in the discrepancies reported between experimental and theoretical estimates of their magnetic moments.

Electronic correlations and transport in iron at Earth's core conditions

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 total calculated thermal conductivity of this phase is 220 Wm⁻¹ K⁻¹ with the EES reduction not exceeding 20%. The EES and electron-lattice scattering are intertwined resulting in breaking of the Matthiessen’s rule with increasing EES. In the hexagonal close-packed iron the EES is also not increased by thermal disorder and remains weak. Our main finding thus holds for the both likely iron phases in the inner core.

The heat and electrical conductivity of Earth’s core matter represent key input quantities for geophysical models of the Earth’s core evolution and geodynamo. Here, the authors show how the scattering due to interactions between electrons has a relatively weak impact on the electrical and thermal conductivities of iron at core conditions.

Ultrafast Electron Correlations and Memory Effects at Work: Femtosecond Demagnetization in Ni

Experimental observations of the ultrafast (less than 50 fs) demagnetization of Ni have so far defied theoretical explanations particularly since its spin-flipping time is much less than that resulting from spin-orbit and electron-lattice interactions. Through the application of an approach that benefits from spin-flip time-dependent density-functional theory and dynamical mean-field theory, we show that proper inclusion of electron correlations and memory (time dependence of electron-electron interaction) effects leads to demagnetization at the femtosecond scale, in good agreement with experimental observations. Furthermore, our calculations reveal that this ultrafast demagnetization results mainly from spin-flip transitions from occupied to unoccupied orbitals implying a dynamical reduction of exchange splitting. These conclusions are found to be valid for a wide range of laser pulse amplitudes. They also pave the way for ab initio investigations of ultrafast charge and spin dynamics in a variety of quantum materials in which electron correlations may play a definitive role.

Electronic correlation effects and local magnetic moments in L10phase of FeNi

We study the electronic and magnetic properties of L10phase of FeNi, a perspective rare-earth-free permanent magnet, by using a combination of density functional and dynamical mean-field theory. Although L10FeNi has a slightly tetragonally distorted fcc lattice, we find that magnetic properties of its constituent Fe atoms resemble those in pure bcc Fe. In particular, our results indicate the presence of well-localized magnetic moments on Fe sites, which are formed due to Hund's exchange. At the same time, magnetism of Ni sites is much more itinerant. Similarly to pure bcc Fe, the self-energy of Fe 3d states is found to show the non-Fermi-liquid behavior. This can be explained by peculiarities of density of Fe 3d states, which has pronounced peaks near the Fermi level. Our study of local spin correlation function and momentum dependence of particle-hole bubble suggests that the magnetic exchange in this substance is expected to be of RKKY-type, with iron states providing local-moment contribution, and the states corresponding to nickel sites (including virtual hopping to iron sites) providing itinerant contribution.

Electronic correlations in dense iron: from moderate pressure to Earth's core conditions

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). We review in detail two particularly interesting regimes: first, a moderate pressure range up to 60 GPa and, second, the ultra-high pressure of about 360 GPa expected inside the solid inner core of Earth. Electronic correlations in iron under the moderate pressure of several tens GPa are discussed in the first section. DMFT-based methods predict an enhancement of electronic correlations at the pressure-induced body-centered cubic α to hexagonal close-packed [Formula: see text] phase transition. 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 [Formula: see text]-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 [Formula: see text]-Fe by many-electron effects; its experimental signatures are analyzed. The 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 ([Formula: see text], [Formula: see text] and face-centered-cubic [Formula: see text]) 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 [Formula: see text]-Fe and an almost perfect Fermi liquid in the case of [Formula: see text]-Fe. A recent active discussion on the electronic state and transport properties of [Formula: see text]-Fe at the EIC conditions is reviewed in details. Estimations for the dynamical many-electron contribution to the relative phase stability are presented. 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 relevant for studies of iron-base systems as well as perspective directions for further development.

Spectral functions of Sr2IrO4: theory versus experiment

The spin-orbit Mott insulator Sr₂IrO₄ has attracted a lot of interest in recent years from theory and experiment due to its close connection to isostructural high-temperature copper oxide superconductors. Despite not being superconductive, its spectral features closely resemble those of the cuprates, including Fermi surface and pseudogap properties. In this article, we review and extend recent work in the theoretical description of the spectral function of pure and electron-doped Sr₂IrO₄ based on a cluster extension of dynamical mean-field theory ('oriented-cluster DMFT') and compare it to available angle-resolved photoemission data. Current theories provide surprisingly good agreement for pure and electron-doped Sr₂IrO₄, both in the paramagnetic and antiferromagnetic phases. Most notably, one obtains simple explanations for the experimentally observed steep feature around the M point and the pseudo-gap-like spectral feature in electron-doped Sr₂IrO₄.

Thermal Conductivity and Electrical Resistivity of Solid Iron at Earth's Core Conditions from First Principles

We compute the thermal conductivity and electrical resistivity of solid hcp Fe to pressures and temperatures of Earth's core. We find significant contributions from electron-electron scattering, usually neglected at high temperatures in transition metals. Our calculations show a quasilinear relation between the electrical resistivity and temperature for hcp Fe at extreme high pressures. We obtain thermal and electrical conductivities that are consistent with experiments considering reasonable error. The predicted thermal conductivity is reduced from previous estimates that neglect electron-electron scattering. Our estimated thermal conductivity for the outer core is 77±10  W m^{-1} K^{-1} and is consistent with a geodynamo driven by thermal convection.

Thermoelectricity in correlated narrow-gap semiconductors

We review many-body effects, their microscopic origin, as well as their impact on thermoelectricity in correlated narrow-gap semiconductors. Members of this class-such as FeSi and FeSb₂-display an unusual temperature dependence in various observables: insulating with large thermopowers at low temperatures, they turn bad metals at temperatures much smaller than the size of their gaps. This insulator-to-metal crossover is accompanied by spectral weight-transfers over large energies in the optical conductivity and by a gradual transition from activated to Curie-Weiss-like behaviour in the magnetic susceptibility. We show a retrospective of the understanding of these phenomena, discuss the relation to heavy-fermion Kondo insulators-such as Ce₃Bi₄Pt₃ for which we present new results-and propose a general classification of paramagnetic insulators. From the latter, FeSi emerges as an orbital-selective Kondo insulator. Focussing on intermetallics such as silicides, antimonides, skutterudites, and Heusler compounds we showcase successes and challenges for the realistic simulation of transport properties in the presence of electronic correlations. Further, we explore new avenues in which electronic correlations may contribute to the improvement of thermoelectric performance.

Unconventional slowing down of electronic recovery in photoexcited charge-ordered La1/3Sr2/3FeO3

The coupling of ordered electronic phases with lattice, spin, and orbital degrees of freedom are of central interest in strongly correlated systems. Their interplay has been intensively studied from femtosecond to picosecond time scales, while their dynamics beyond nanoseconds are usually assumed to follow lattice cooling. Here, we report an unusual slowing down of the recovery of an electronic phase across a first-order phase transition. Following optical excitation, the recovery time of both transient optical reflectivity and X-ray diffraction intensity from the charge-ordered superstructure in a La_1/3Sr_2/3FeO₃ thin film increases by orders of magnitude as the sample temperature approaches the phase transition temperature. In this regime, the recovery time becomes much longer than the lattice cooling time. The combined experimental and theoretical investigation shows that the slowing down of electronic recovery corresponds to the pseudo-critical dynamics that originates from magnetic interactions close to a weakly first-order phase transition.

Unusual electronic behavior can emerge in complex oxides due to strong coupling between charge, magnetic and lattice degrees of freedom. Zhu et al. observe separation of electronic and lattice equilibration times in La_1/3Sr_2/3FeO₃ as magnetic interactions make the recovery of charge order much slower than lattice relaxation.