Formation Of Local Magnetic Moments Research Articles

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.

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.

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.

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.

Effect of local magnetic moments on spectral properties and resistivity near interaction- and doping-induced Mott transitions

We study the effect of the formation and screening of local magnetic moments on the temperature- and interaction dependencies of spectral functions and resistivity in the vicinity of the metal-insulator transition. We use the dynamical mean-field theory for the strongly correlated Hubbard model and associate the peculiarities of the above mentioned properties with those found for the local charge $\chi_c$ and spin $\chi_s$ susceptibilities. We show that at half filling the maximum of resistivity at a certain temperature $T^*$ corresponds to the appearance of central quasiparticle peak of the spectral function and entering the metallic regime with well defined fermionic quasiparticles. At the same time, the temperature of the crossover to the regime with screening of local magnetic moments, determined by the minimum of double occupation, is smaller than the temperature scale $T^*$ and coincides at half filling with the boundary $T_{\beta=1}(U)$ corresponding to the exponent of resistivity $\beta\equiv d\ln \rho/d\ln T=1$. Away from half filling we find weak increase of the temperature of the beginning and completion of the screening (i.e. Kondo temperature) of local magnetic moments, while the unscreened local magnetic moments exist only up to few percents of doping. In the low temperature regime $T<T_{\beta=1}$ simultaneous presence of itinerant and localized degrees of freedom yields almost linear temperature dependence of scattering rate and resistivity.

Understanding the magnetic response of the Quantum Spin Liquid compound 1[formula omitted]-TaS[formula omitted

1T-tantalum disulphide (1T-TaS2) is a promising material to realize quantum spin liquid state. Here, we take a closer look on the temperature dependent magnetic susceptibility of 1T-TaS2 at low magnetic fields to understand the formation of local magnetic moment in this system and its response as a function of temperature and field. We found that the susceptibility increases with reducing temperature even in the nearly commensurate charge density wave (CDW) phase, which exhibits metallic conductivity and hence is expected to exhibit temperature independent Pauli paramagnetic susceptibility. It indicates that local moment starts forming in the nearly commensurate CDW phase itself, which is well above the temperature where evidence of Mott insulating state appears in resistivity measurement. In the commensurate CDW phase, susceptibility significantly deviates from the Curie–Weiss law. Below T = 10 K, temperature dependence of susceptibility indicates the formation of random singlet state in 1T-TaS2.

Spin dynamics of itinerant electrons: Local magnetic moment formation and Berry phase

The state-of-the-art theoretical description of magnetic materials relies on solving effective Heisenberg spin problems or their generalizations to relativistic or multi-spin-interaction cases that explicitly assume the presence of local magnetic moments in the system. We start with a general interacting fermionic model that is often obtained in ab initio electronic structure calculations and show that the corresponding spin problem can be introduced even in the paramagnetic regime, which is characterized by a zero average value of the magnetization. Further, we derive a physical criterion for the formation of the local magnetic moment and confirm that the latter exists already at high temperatures well above the transition to the ordered magnetic state. The use of path-integral techniques allows us to disentangle spin and electronic degrees of freedom and to carefully separate rotational dynamics of the local magnetic moment from Higgs fluctuations of its absolute value. It also allows us to accurately derive the topological Berry phase and relate it to a physical bosonic variable that describes dynamics of the spin degrees of freedom. As the result, we demonstrate that the equation of motion in the case of a large magnetic moment takes a conventional Landau-Lifshitz form that explicitly accounts for the Gilbert damping due to itinerant nature of the original electronic model.

Local magnetic moment formation and Kondo screening in the half-filled single-band Hubbard model

We study the formation of local magnetic moments in the strongly correlated Hubbard model within dynamical mean-field theory and associate peculiarities of the temperature dependence of local charge ${\ensuremath{\chi}}_{c}$ and spin ${\ensuremath{\chi}}_{s}$ susceptibilities with different stages of local moment formation. The local maximum of the temperature dependence of the charge susceptibility ${\ensuremath{\chi}}_{c}$ is associated with the beginning of local magnetic moment formation, while the minimum of the susceptibility ${\ensuremath{\chi}}_{c}$ and double occupation, as well as the low-temperature boundary of the plateau of the effective local magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}^{2}=T{\ensuremath{\chi}}_{s}$ temperature dependence are connected with the full formation of local moments. We also obtain the interaction dependence of the Kondo temperature ${T}_{K}$, which is compared to the fingerprint criterion of Chalupa et al. [Phys. Rev. Lett. 126, 056403 (2021)]. Near the Mott transition the two criteria coincide, while further away from the Mott transition the fingerprint criterion somewhat overestimates the Kondo temperature. The relation of the observed features to the behavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge susceptibility and divergences of irreducible vertices is discussed.

Correlation strength, orbital-selective incoherence, and local moments formation in the magnetic MAX-phaseMn2GaC

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn$_2$GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically-ordered phases of Mn$_2$GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of Mn$_2$GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn$_2$GaC, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that Mn$_2$GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by the Hund's exchange interaction, and not the Coulomb interaction.

Environmental screening and ligand-field effects to magnetism in CrI3 monolayer

We study the microscopic origin of magnetism in suspended and dielectrically embedded CrI3 monolayer by down-folding minimal generalized Hubbard models from ab initio calculations using the constrained random phase approximation. These models are capable of describing the formation of localized magnetic moments in CrI3 and of reproducing electronic properties of direct ab initio calculations. Utilizing the magnet force theorem, we find a multi-orbital super-exchange mechanism as the origin of magnetism in CrI3 resulting from an interplay between ferro- and anti-ferromagnetic Cr-Cr d coupling channels, which is decisively affected by the ligand p orbitals. We show how environmental screening, such as resulting from encapsulation with hexagonal boron nitride, affects the Coulomb interaction in the film and how this controls its magnetic properties. Driven by a non-monotonic interplay between nearest and next-nearest neighbor exchange interactions we find the magnon dispersion and the Curie temperature to be non-trivially affected by the environmental screening.

Unraveling 5f correlated electronic states in paramagnetic PuSn3

Plutonium-based compounds establish an ideal platform for exploring the interplay between long-standing itinerant-localized 5f states and strongly correlated electronic states. In this paper, we exhaustively investigate the 5f correlated electronic states of PuSn3 dependence on temperature by means of a combination of the density functional theory and the embedded dynamical mean-field theory. It is found that the spectral weight of narrow 5f band grows significantly and remarkable quasiparticle multiplets appear around the Fermi level at low temperature. A striking c–f hybridization and prominent valence state fluctuations indicate the advent of coherence and itinerancy of 5f states. It is predicted that a 5f localized to itinerant crossover is induced by temperature accompanied by the change in Fermi surface topology. Therefore itinerant 5f states are inclined to take in active chemical bonding, suppressing the formation of local magnetic moment of Pu atoms, which partly elucidates the intrinsic feature of paramagnetic PuSn3. Furthermore, the 5f electronic correlations are orbital selective, which manifested themselves in differentiated band renormalizations and electron effective masses. Consequently, the convincing results remain crucial to our understanding of plutonium-based compounds and promote ongoing research.

Spontaneous Time-Reversal Symmetry Breaking at Individual Grain Boundaries in Graphene.

Graphene grain boundaries (GBs) have attracted interest for their ability to host nearly dispersionless electronic bands and magnetic instabilities. Here, we employ quantum transport and universal conductance fluctuation measurements to experimentally demonstrate a spontaneous breaking of time-reversal symmetry across individual GBs of chemical vapor deposited graphene. While quantum transport across the GBs indicate spin-scattering-induced dephasing and hence formation of local magnetic moments, below T≲4 K we observe complete lifting of time-reversal symmetry at high carrier densities (n≳5×10^{12} cm^{-2}) and low temperature (T≲2 K). An unprecedented thirtyfold reduction in the universal conductance fluctuation magnitude with increasing doping density further supports the possibility of an emergent frozen magnetic state at the GBs. Our experimental results suggest that realistic GBs of graphene can be a promising resource for new electronic phases and spin-based applications.

Fingerprints of the Local Moment Formation and its Kondo Screening in the Generalized Susceptibilities of Many-Electron Problems.

We identify the precise hallmarks of the local magnetic moment formation and its Kondo screening in the frequency structure of the generalized charge susceptibility. The sharpness of our identification even pinpoints an alternative criterion to determine the Kondo temperature of strongly correlated systems on the two-particle level, which only requires calculations at the lowest Matsubara frequency. We showcase its strength by applying it to the single impurity and the periodic Anderson model as well as to the Hubbard model. Our results represent a significant progress for the general understanding of quantum field theory at the two-particle level and allow for tracing the limits of the physics captured by perturbative approaches for correlated systems.

Magnetic fluctuations and superconducting pairing in ε -iron

We study Coulomb correlation effects and their role in superconductivity of $\varepsilon$-iron under pressure from 12 to 33 GPa by using a combination of density functional and dynamical mean-field theory. Our results indicate a persistence of the Fermi-liquid behavior below the temperature $\sim$1000 K. The Coulomb correlations are found to substantially renormalize the density of states, reducing the distance from the peak to the Fermi level to 0.4 eV compared to 0.75 eV obtained in DFT calculations. We find significant antiferromagnetic correlations, which are accompanied by the formation of short-lived local magnetic moments. We use the obtained results as a starting point for construction of the multi-band Bethe-Salpeter equation, which eigenvalues indicate that antiferromagnetic spin fluctuations may result in the superconducting pairing in $\varepsilon$-Fe. Moreover, the tendency to superconducting instability becomes weaker with the increase of pressure, which may explain the disappearance of superconductivity at $\sim$30 GPa.

Dynamical resilience to disorder: The dilute Hubbard model on the Lieb lattice

In itinerant systems, electron-electron interactions may lead to the formation of local magnetic moments and their effective exchange coupling, which in turn gives rise to long-range magnetic order. Therefore, when moment formation is weakened, such as in the single-band Hubbard model on a square lattice with the on-site repulsion being randomly switched off on a fraction $x$ of sites, magnetic order is suppressed beyond some critical $x_c$, which was found to lie below the classical percolation threshold, $x_c^\text{(perc,sq)}$. Here we study dilute magnetism in flat band systems, namely in the Hubbard model on a `Lieb' lattice. Interestingly, we show that magnetic order persists to $x$ almost twice as large as the classical percolation threshold for the lattice, thus emphasizing the central role of electron itinerancy to the magnetic response. The analysis of the orbital-resolved order parameters reveals that the contribution of the four-fold coordinated `d' sites to magnetism is dramatically affected by dilution, while the localized `p' states of the flat band provide the dominant contribution to long-range correlations. We also examine the transport properties, which suggest the existence of an insulator-to-metal transition in the same range of the critical magnetic dilution.

Formation of localized magnetic states in a large-spin Fermi system

We extend the Anderson impurity model to a large-spin Fermi system with spin $f$=3/2, stimulated by the realization of large-spin ultracold Fermi atoms. The condition required for the spontaneous formation of local magnetic moments is examined and the ground state mean-field magnetic phase diagram is explored carefully. We find that the spin-3/2 Fermi system involves magnetic phase regions I, II, and III that correspond to one, two, and three particle/hole occupation, respectively. In addition, it is observed that all the three magnetic phases have four-fold degenerate ground states. Finally, the phase transition between the three phases seems to be of the first-order.

Electronic Structure and Magnetic Properties of Strongly Correlated Transition Metal Compounds

Different classes of compounds based on transition metals belong to strongly correlated compounds due to strong interactions of d and f electrons with each other and with itinerant electronic states. This results in a number of interesting phenomena, including metal–insulator and various magnetic spin transitions, “heavy fermion” compounds, interplay between magnetic order and superconductivity, formation of local magnetic moments, anomalies of transport properties, etc. Recent results in this field based on applications of ab initio approaches and dynamical mean-field theory are reviewed in this paper.

Electronic and magnetic properties of defect complexes in Eu-doped GaN: First-principles calculations

Various forms of defect complexes are studied in Eu-doped GaN by first-principles calculations with the framework of density functional theory. We consider defect complexes formed by dopant Eu and vacancies or insterstitals with the neutral charge states in GaN, i.e., EuGa-VGa, EuGa-VN, EuGa-Ni, and EuGa-Oi, where EuGa represents the Eu site substituting for Ga site, and VGa, VN, Ni and Oi are Ga vacancies, N vacancies, interstitial N and interstitial O, respectively. The formation energies are calculated for all defect configurations. The formation energies of native Ga vacancies are found to be higher compared to those of N vacancies, while the tetrahedral interstitials are energetically more stable than the octahedral interstitial. The Eu-4f electrons introduce a magnetic moment of about 6.15 μB in the case of single dopant Eu, i.e., EuGa, which is probably suppressed in the presence of octahedral interstitial N around Eu. And Ga vacancies and octahedral interstitial O can contribute the magnetic moments to the systems, while other intrinsic defects do almost not promote the formation of local magnetic moments.