Moment Formation Research Articles

Imaging magnetic transition of magnetite to megabar pressures using quantum sensors in diamond anvil cell

High-pressure diamond anvil cells have been widely used to create novel states of matter. Nevertheless, the lack of universal in-situ magnetic measurement techniques at megabar pressures makes it difficult to understand the underlying physics of materials’ behavior at extreme conditions, such as high-temperature superconductivity of hydrides and the formation or destruction of the local magnetic moments in magnetic systems. Here, we break through the limitations of pressure on quantum sensors by modulating the uniaxial stress along the nitrogen-vacancy axis and develop the in-situ magnetic detection technique at megabar pressures with high sensitivity (~1μT/Hz\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1{{{\\rm{\\mu }}}}{{{\\rm{T}}}}/\\sqrt{{{{\\rm{Hz}}}}}$$\\end{document}) and sub-microscale spatial resolution. By directly imaging the magnetic field and the evolution of magnetic domains, we observe the macroscopic magnetic transition of Fe3O4 in the megabar pressure range from ferrimagnetic (α-Fe3O4) to weak ferromagnetic (β-Fe3O4) and finally to paramagnetic (γ-Fe3O4). The scenarios for magnetic changes in Fe3O4 characterized here shed light on the direct magnetic microstructure observation in bulk materials at high pressure and contribute to understanding magnetism evolution in the presence of numerous complex factors such as spin crossover, altered magnetic interactions and structural phase transitions.

The impact of heat treatment process on the drilling characteristics of Al–5Si–1Cu–Mg alloy produced by sand casting

Aluminum–Silicon (Al–Si) based materials are commonly preferred in engineering studies requiring high mechanical performance as an alternative to steel materials. These alloys are especially preferred in the automotive industry, aerospace components and heavy machinery parts. In post-casting processes, machining operations are of great importance for high geometric precision, surface quality and longer fatigue life in mechanical environments. In this research, the microstructural, mechanical and machining characteristics of the Al–5Si–1Cu–Mg material produced by sand casting method in both as-cast (AC) and heat-treated (HTed) (Solid solution, quenching and aging-SQA) states were experimentally investigated. Microstructural examinations were carried out with optical microscope and SEM images. Mechanical properties were determined by tensile and hardness tests. Then, drilling experiments were performed in the CNC vertical machining center with 8 mm diameter uncoated HSS (High Speed Steel) cutting tools at constant cutting conditions (i.e., cutting speed-V: 125 m/min, feed rate-f: 0.05 mm/rev and depth of cut-DoC: 15 mm). In microstructural investigations, it was determined that the microstructure of the Al–5Si–1Cu–Mg material in AC state consists of α-Al, eutectic Si, β-Fe (β-Al5FeSi) and π-Fe (π-Al8Mg3FeSi6) intermetallics. After the SQA, the existing phases generally exhibited a spherical structure, and it was seen that the β phase in the microstructure transformed into the Ɵ (Al7FeCu2) phase. SQA improved the hardness, yield and tensile durability of the material, whereas decreasing the elongation to fracture. SQA process improved the machining characteristics of the material by decreasing thrust force, moment, surface roughness and built-up edge (BUE) formation. The machined subsurface structure of HTed alloy under constant cutting conditions was determined to be more stable and smooth and the machined surface microhardness of HTed alloy was higher compared to as-cast alloy due to the effect of solid precipitation hardening. In addition, shorter and more broken chips occurred in the machining of HTed alloys due to the effect of low elongation to fracture.

Calculation on power factor and figure of merit in a single-layer NiBr2

Abstract Implementation of the classical Boltzmann transport theory has been performed to calculate the power factor, as well as the figure of merit in a single-layer NiBr2 after performing density functional theory. Those two properties were formulated from the Seebeck coefficient, electron thermal conductivity, and electrical conductivity. An in-plane ferromagnetic formation of magnetic moments in the Ni atoms was applied in the primitive cell. We found a small power factor, but a high figure of merit at the Fermi level near critical temperature. Since the efficiency of thermoelectric materials is represented by the figure of merit, a single-layer NiBr2 is a strong candidate for thermoelectric materials which can be applied in future devices.

Local magnetic moment formation and Kondo screening in the presence of Hund exchange: Two-band Hubbard model analysis

Local magnetic moment formation and Kondo screening in the presence of Hund exchange: Two-band Hubbard model analysis

Temperature dependence on magnetic moment formation at a Ce impurity in Gd, Tb, and Dy: An intermediate valence model

Based on the influence of the Ce intermediate valence on magnetic properties in metallic systems, we theoretically study the local magnetic moment formation at a Ce impurity diluted in Gd, Tb, and Dy rare earth metals at finite temperatures. To investigate this point, our model considers the functional integral formalism, coherent potential approximation, and extracting relative charge from the Ce 4f-resonance and transferring it to the 5d Ce band at the temperature range from 0K to Curie critical temperature. As a result, our self-consistent calculation, focusing on temperature dependence, shows that the intermediate valence model explains the regimes of the local magnetic moments and magnetic hyperfine fields. Moreover, our results for the temperature dependence on magnetic hyperfine fields at the Ce site in Gd and Tb agree with experimental reports.

Electronic correlations and intrinsic magnetism of interstitial quasi-atomic states in Li8Au electride.

We investigate the electronic sub-system of a recently designed Li8Au superconducting electride to reveal its many-body correlated nature and magnetic properties. Using maximally localized Wannier functions (MLWFs) to describe the interstitial anion electron (IAE) states, it was found that these states are partially occupied with a population of 1.5e- and have negligible hybridization with the almost completely filled p-Au states. The averaged interaction screened Hubbard parameter U for quasi-atomic IAE states evaluated by the constrained random-phase approximation (CRPA) method is 2 eV, comparable to the width of the electride band suggesting moderate electronic correlations. Using dynamical mean field theory (DMFT) approach we found that IAEs in Li8Au electride behave as magnetic centers and possess their own well localised magnetic moments of 0.5μB per quasi-atomic IAE. The obtained results deepen the understanding of the significance of many-body effects in the IAE subsystem of electronic states and reveal the mechanism for the formation of intrinsic magnetic moments on IAEs, which behave like ferromagnetic quasi-atoms in the Li8Au electride. Overall, the observed correlation effects in Li8Au emphasize their importance in materials with excess electrons confined in cavities.

Theoretical approach to defect-induced magnetism in oxygen-deficient γ-Ga2O3 films

Gallium oxide (Ga2O3) is an emerging material with several known polymorphs that exhibit many interesting properties for applications. In this work is theoretically studied and discussed the unconventional ferromagnetism observed in oxygen-deficient thin films of Ga2O3-x (0.42 < x < 0.60) grown on sapphire substrates. The formation of the magnetic moments is demonstrated in good agreement with experimental magnetization measurements using first-principles calculations based on the density functional theory performed for a standard crystal structure of γ-Ga2O3 with oxygen vacancies in different configurations. The random distribution of pairs of oxygen vacancies and rearrangements of them within the spinel-like cubic structure containing tetrahedral and octahedral sites for increasing temperatures are consistent with long-range persistent non-collinear magnetic order up to temperatures that may be exceed the temperature at which full conversion of γ-Ga2O3 to β-Ga2O3 takes place. Our present results also reveal that magnetic irreversibility observed from 10 K to room temperature are possibly determined by the intrinsic disorder in the structure of tetrahedral and octahedral sites that allows numerous possible paths for the migration of oxygen vacancies with low values of activation energies and migration barriers, as indicated by the previously published reports.

Higher-order multipoles in metaphotonic structures: reviewing symmetric approaches and constructing a comprehensive theoretical method

This research explores localized electric fields in two-dimensional (2D) photonic crystals, focusing on higher-order multipoles. Novel formulas are developed to accurately describe and analyze these complex electromagnetic modes. By considering various crystal element arrangements, we investigate the formation and interactions of electric, magnetic, and toroidal moments. We examine the effects of symmetry on the localized electromagnetic fields in 2D periodic structures and explain how bound states in the continuum (BICs) arise and behave in different symmetry classes. It is important to point out that every singularity point that occurs in multipolar series expansions depends on a BIC point, and it inspires us to study the characteristics of them. Furthermore, we analyze a variety of BICs and use perturbed and degeneracy breaking to investigate high-Q-factor quasi-BICs. The results provide a thorough understanding of how symmetry and multipolar interactions affect potential applications in cutting-edge metasurfaces and photonic devices (metaphotonic).

Derivation of Key Characteristics of Columns in Frames with Sway

The paper paper examines columns of elastic frames that are able to displace laterally under axial load. Columns on the same level (story) of such frames may contribute to lateral support (stability) or require lateral support (supported columns). In the former case, maximum moments will occur at column ends, but may occur between column ends in the latter case. The column response, including the formation of moments and shears, is often complicated, often calling for approximate analysis methods and information that presently may not be readily available. Main attention of the paper is paid to providing results that will be helpful in providing, in a rather simple manner, improved understanding of column mechanics, and that may be helpful also as a supplement to full second-order analyses. Towards this goal, the major objective of the paper is threefold: (i) to identify characteristic points, or behavioral“landmarks” in the axial load-moment solution space, through the study of rotationally restrained elastic columns and two-column panels with sidesway; (ii) to derive simple, novel, closed-form expressions for these, thereby providing a tool simplifying the establishment of the variation of end and maximum moments versus axial load; (iii) to identify to what extent isolated (single) column analyses may adequately represent horizontally interacting columns, and potential unwinding, in frames.

Heavy quasiparticles and cascades without symmetry breaking in twisted bilayer graphene

Among the variety of correlated states exhibited by twisted bilayer graphene, cascades in the spectroscopic properties and in the electronic compressibility occur over larger ranges of energy, twist angle and temperature compared to other effects. This suggests a hierarchy of phenomena. Using a combined dynamical mean-field theory and Hartree calculation, we show that the spectral weight reorganisation associated with the formation of local moments and heavy quasiparticles can explain the cascade of electronic resets without invoking symmetry breaking orders. The phenomena reproduced here include the cascade flow of spectral weight, the oscillations of remote band energies, and the asymmetric jumps of the inverse compressibility. We also predict a strong momentum differentiation in the incoherent spectral weight associated with the fragile topology of twisted bilayer graphene.

Electron correlation e ects in paramagnetic cobalt

We study the influence of Coulomb correlations on spectral and magnetic properties of fcc cobalt using a combination of density functional theory and dynamical mean-field theory. The computed uniform and local magnetic susceptibilities obey the Curie–Weiss law, which, as we demonstrate, occurs due to the partial formation of local magnetic moments. We find that the lifetime of these moments in cobalt is significantly less than in bcc iron, suggesting a more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained electron self-energies exhibit a quasiparticle shape with the quasiparticle mass enhancement factor m*/m ~ 1.8, corresponding to moderately correlated metal. Finally, our calculations reveal that the static magnetic susceptibility of cobalt is dominated by ferromagnetic correlations, as evidenced by its momentum dependence.

Coulomb correlations in a germanide iron-based superconductor: The example of YFe2Ge2

We report results of a detailed theoretical study of the effect of Coulomb correlations on the structural, electronic, and magnetic properties of the non-pnictide iron-based superconductor ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$. In particular, by employing the density functional theory plus dynamical mean-field theory method ($\mathrm{DFT}+\mathrm{DMFT}$) we perform a structural optimization of the unit-cell volume and the internal coordinate ${z}_{\mathrm{Ge}}$ of Ge. Our results for the lattice parameters obtained by $\mathrm{DFT}+\mathrm{DMFT}$ show a much better agreement with experiment as compared to those calculated by DFT implying the importance of correlation effects. We show that ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$ is a moderately correlated system with electron-electron correlations being strong enough to renormalize the low-energy band structure but not sufficient to form Hubbard bands, in close analogy to the iron-based pnictides and chalcogenides. The analysis of the local spin-spin correlation function and momentum-dependent spin susceptibility $\ensuremath{\chi}(\mathbf{q})$ indicates the correlation induced formation of short-lived local moments in the Fe ${t}_{2g}$ states and the presence of competing spin fluctuations, in agreement with experiment.

Electron Correlation Effects in Paramagnetic Cobalt

We study the influence of Coulomb correlations on spectral and magnetic properties of fcc cobalt using a combination of density functional theory and dynamical mean-field theory. The computed uniform and local magnetic susceptibilities obey the Curie–Weiss law, which, as we demonstrate, occurs due to the partial formation of local magnetic moments. We find that the lifetime of these moments in cobalt is significantly less than in bcc iron, suggesting a more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained electron self-energies exhibit a quasiparticle shape with the quasiparticle mass enhancement factor m*/m ~ 1.8, corresponding to moderately correlated metal. Finally, our calculations reveal that the static magnetic susceptibility of cobalt is dominated by ferromagnetic correlations, as evidenced by its momentum dependence.

Models of the electromechanical system of the floating wave power station

The structure of the electromechanical system (EMS) of a floating wave power plant (FWPP) is developed, which, in addition to generating electricity, protects coastal structures or individual objects on the high seas. The mechanisms of formation of moments on the FWPP shaft are analyzed. Problems are formulated to determine the relationship between the capacity of an electric accumulator and the volume of the water basin of an energy concentrator. Mathematical and simulation models of EMS components are developed. The power ratios of the EMS components and the rated power of the FWPP under the expected operating conditions are determined. Keywords: model, energy converter, electromechanical system. kryukov@nntu.ru

Coulomb Correlations and Electronic Structure of CuCo2S4: a DFT + DMFT Study

By employing a combined method of density functional theory and dynamical mean-field theory (DFT + DMFT) we study the spectral properties and local moment formation in the normal state of a Co-based superconductor CuCo2S4. The obtained quasiparticle mass enhancement m{text{*/}}m sim 1.2 is smaller than that in iron-based parent compounds indicating weak correlations in the Co 3d states. We show that the mass enhancement is accompanied by an insignificant renormalization and shift of the bands in the vicinity of the Fermi level. This subtle modification is sufficient to induce a substantial transformation of one of the Fermi surface sheets and occurs due to the presence of shallow Fermi surface pockets in the electronic structure of CuCo2S4. The calculated local and fluctuating moments are smaller than those in iron-based superconductors questioning the importance of spin fluctuations for understanding the pairing mechanism of CuCo2S4.

Synchrotron radiation techniques and their application to actinide materials

The actinides comprise a 15-member group of metallic radioactive elements occupying the bottom row of the periodic table. Electron localization accompanied by the formation of large magnetic moments due to the strong Coulomb repulsion is balanced by hybridization with neighboring-atom electronic states. This hybridization promotes an opposite tendency toward itinerancy and the emergence of complex behavior. This review shows how x-ray synchrotron radiation techniques provide a variety of powerful tools to unravel the complexity of actinide materials. Experimental techniques, theoretical background, and applications to actinide materials are covered in detail.

Instanton gas approach to the Hubbard model

In this paper, we consider a path integral formulation of the Hubbard model based on a Hubbard-Stratonovich transformation that couples the auxiliary field to the local electronic density. This decoupling is known to have a saddle-point structure that shows a remarkable regularity: The field configuration at each saddle point can be understood in terms of a set of elementary field configurations localized in space and imaginary time which we coin instantons. The interaction between instantons is short ranged. Here, we formulate a classical partition function for the instanton gas that has predictive power. For a given set of physical parameters, we can predict the distribution of instantons and show that the instanton number is sharply defined in the thermodynamic limit, thereby defining a unique dominant saddle point. Decoupling in the charge channel conserves SU(2) spin symmetry for each field configurations. Hence, the instanton approach provides an SU(2) spin-symmetric approximation to the Hubbard model. It fails, however, to capture the magnetic transition inherent to the Hubbard model on the honeycomb lattice despite being able to describe local moment formation. In fact, the instanton itself corresponds to local moment formation and concomitant short-ranged antiferromagnetic correlations. This aspect is also seen in the single particle spectral function that shows clear signs of the upper and lower Hubbard bands. Our instanton approach bears remarkable similarities to local dynamical approaches, such as dynamical mean-field theory, in the sense that it has the unique property of allowing for local moment formation without breaking the SU(2) spin symmetry. In contrast to local approaches, it captures short-ranged magnetic fluctuations. Furthermore, it also offers possibilities for systematic improvements by taking into account fluctuations around the dominant saddle point. Finally, we show that the saddle point structure depends upon the choice of lattice geometry. For the square lattice at half filling, the saddle-point structure reflects the itinerant to localized nature of the magnetism as a function of the coupling strength. The implications of our results for Lefschetz thimble approaches to alleviate the sign problem are also discussed.

Transition from Pauli paramagnetism to Curie-Weiss behavior in vanadium

We study electron correlations and their impact on magnetic properties of bcc vanadium by a combination of density functional and dynamical mean-field theory. The calculated uniform magnetic susceptibility {in bcc structure} is of Pauli type at low temperatures, while it obeys the Curie-Weiss law at higher temperatures. Thus, we qualitatively reproduce the experimental temperature dependence of magnetic susceptibility without introducing the martensitic phase transition. Our results for local spin-spin correlation function and local susceptibility reveal that the Curie-Weiss behavior appears due to partial formation of local magnetic moments, which originate from $t_{2g}$ states and occur due to local spin correlations caused by Hund's rule coupling. At the same time, the fermionic quasiparticles remain well-defined, while the formation of local moments is accompanied by a deviation from the Fermi-liquid behavior. In particular, the self-energy of the $t_{2g}$ states shows the non-analytic frequency dependence, which is a characteristic of the spin-freezing behavior, while the quasiparticle damping changes approximately linearly with temperature in the intermediate temperature range $200$--$700$~K. By analyzing the momentum dependence of static magnetic susceptibility, we find incommensurate magnetic correlations, which may provide a mechanism for unconventional superconductivity at low temperatures.

Local magnetic moments due to loop currents in metals

We present Hartree-Fock calculations on a simple model to obtain the conditions of formation of local magnetic moments due to loop currents ${L}_{o}$ and spin-loop currents ${L}_{s}$ and compare them to the conditions of formation of local spin moments $M$ which were given long ago in a similar approximation by Anderson. A model with three degenerate orbitals sitting on an equilateral triangle, with on-site and nearest-neighbor repulsions $U$ and $V$, respectively, and intersite kinetic energy, hybridizing with conduction electrons with a parameter $\mathrm{\ensuremath{\Delta}}$ is investigated. ${L}_{o}$ and ${L}_{s}$ are promoted by large $V/\mathrm{\ensuremath{\Delta}}$ and their magnitude is relatively unaffected by $U/\mathrm{\ensuremath{\Delta}}$. Spin-magnetic moments $M$ promoted by large $U/\mathrm{\ensuremath{\Delta}}$ on the other hand are adversely affected by $V/\mathrm{\ensuremath{\Delta}}$. In this model, ${L}_{o}$ for $V$ multiplied by the number of neighbors is approximately the same as the $M$ promoted by $U$ in Anderson's local model for $M$. ${L}_{o}$ and ${L}_{s}$ are degenerate if exchange interactions and Hund's rule are neglected but ${L}_{o}$ is favored when they are included. Many of the qualitative results are visible in an expression for the Hartree-Fock ground state energy derived as a function of small ${L}_{o}, {L}_{s}$, and $M$. Numerical minimization of the Hartree-Fock energy is presented for larger values. We also briefly discuss the connection and differences of the interaction-generated orbital currents and spin currents discussed here and generalized to a lattice with the topological states in metals and semiconductors.

Transition from band insulator to Mott insulator and formation of local moment in the half-filled ionic SU(N) Hubbard model

We investigate the local moment formation in the half-filled SU($N$) Hubbard model under a staggered ionic potential. As the Hubbard $U$ increases, the charge fluctuations are suppressed and eventually frozen when $U$ is above a critical value $U_c$, marking the development of well-defined local moment with integer $m$ fermions on the A-sublattice and $(N-m)$ fermions on the B-sublattice, respectively. We obtain an analytical solution for $U_c$ for the paramagnetic ground state within the variational Gutzwiller approximation and renormalized mean field theory. For large $N$, $U_c$ is found to depend on $N$ linearly with fixed $m/N$, but sublinearly with fixed $m$. The local moment formation is accompanied by a peculiar phase transition from the band insulator to the Mott insulator, where the ionic potential and quasiparticle weight are renormalized to zero simultaneously. Inside the Mott phase, the low energy physics is described by the SU($N$) Heisenberg model with conjugate representations, which is widely studied in the literature.