Itinerant Picture Research Articles

Topological magnons in CrI3 monolayers: an itinerant fermion description

Magnons dominate the magnetic response of ferromagnetic two-dimensional crystals such as CrI3. Because of the arrangement of Cr spins in a honeycomb lattice, magnons in CrI3 bear a strong resemblance with electrons in graphene. Neutron scattering experiments carried out in bulk CrI3 show the existence of a gap at the Dirac points, conjectured to have a topological nature. We propose a theory for magnons in CrI3 monolayers based on an itinerant fermion picture, with a Hamiltonian derived from first principles. We obtain the magnon dispersion for 2D CrI3 with a gap at the Dirac points with the same Berry curvature in both valleys. For CrI3 ribbons, we find chiral in-gap edge states. Analysis of the magnon wave functions in momentum space confirms their topological nature. Importantly, our approach does not require a spin Hamiltonian, and can be applied to insulating and conducting 2D materials with any type of magnetic order.

Microscopic Theory of Γ3 Quadrupole Ordering in Pr Compounds on the Basis of a j–j Coupling Scheme

Toward the understanding of incommensurate $\Gamma_3$ quadrupole ordering in PrPb$_3$, we develop a microscopic theory of multipole ordering in $f^2$-electron systems from an itinerant picture on the basis of a $j$-$j$ coupling scheme. For this purpose, we introduce the $\Gamma_7$-$\Gamma_8$ Hubbard model on a simple cubic lattice with the effective interactions that induce local $\Gamma_3$ states. By evaluating multipole susceptibility in a random phase approximation, we find that the hybridization between $\Gamma_7$ and $\Gamma_8$ orbitals plays a key role in the emergence of $\Gamma_3$ quadrupole ordering. We also emphasize that $\Gamma_3$ quadrupole ordering can be understood from the concept of {\it multipole nesting}, in which the Fermi surface region with large $\Gamma_8$ orbital density should be nested on the area with a significant $\Gamma_7$ component when the positions of the Fermi surfaces are shifted by the ordering vector. This concept cab be intuitively understood from the fact that local $\Gamma_3$ doublets are mainly composed of two singlets between $\Gamma_8$ and $\Gamma_7$ orbitals. Finally, we discuss the possible relevance of the present theory to the experimental results of PrPb$_3$ and point out some future problems in this direction of research.

Probing hole-doping of the weak antiferromagnet TiAu with first principles methods

We investigate hole-doping by Sc substitution for Ti in the weak antiferromagnet (wAFM) system TiScAu. Behavior reported so far fits a weak itinerant AFM picture, and experimentally leads to a quantum critical point at . Here we study supercells with several rational fractions of Ti replaced by Sc. We find, unexpectedly, definite local moment-like behavior, e.g. magnetic moments of the Ti atom comparable to or even larger than in the bulk persist even into the Ti-poor regime of this alloy system. Itinerant signatures persist, however, as the Ti projected density of states displays van Hove singularity peaks near the Fermi level in most cases, revealing a striking similarity to nonmagnetic bulk TiAu. The current picture of this system, midway between itinerant and local moment, will be provided and discussed in light of experimental observations.

Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

We studied the electron spin resonance (ESR) line width for localized moments within the framework of the Kondo lattice model. Only for a sufficiently small Kondo temperature can an ESR signal be observed for a Kondo impurity. On the other hand, for a Kondo lattice representing a heavy fermion compound, short-range ferromagnetic correlations (FM) between the localized moments are crucial to observe a signal. The spin relaxation rate (line width) and the static magnetic susceptibility are inversely proportional to each other. The FM enhance the susceptibility and hence reduce the line width. For most of the heavy fermion systems displaying an ESR signal, the FM order arises in the ab-plane from the strong lattice anisotropy. CeB6 is a heavy fermion compound with cubic symmetry having a Γ8 ground-quartet. Four transitions are expected for individual Ce ions with a Γ8 ground-multiplet, but only one has been observed. Antiferro-quadrupolar order (AFQ) arises below 4 K due to the orbital content of the Γ8-quartet. We addressed the effects of the interplay of AFQ and FM on the ESR line width and the phase diagram. It is usually difficult to distinguish among ESR resonances due to localized moments and conducting heavy electron spins, especially for anisotropic Ce and Yb compounds. However, for CeB6, an itinerant picture within the AFQ phase is necessary to explain the electron spin resonances. The longitudinal magnetic susceptibility has a quasi-elastic central peak of line width 1/T1 and inelastic peaks for the absorption/emission of excitations. The latter are measured via inelastic neutron scattering (INS) and provide insights into the magnetic order. We briefly summarize some of the INS results for CeB6 in the context of the picture that emerged from the ESR experiments.

Mean-field theory for multipole ordering in f-electron systems on the basis of a j-j coupling scheme

We develop a microscopic theory for multipole ordering, applicable to the system with plural numbers of f electrons per ion, from an itinerant picture on the basis of a j-j coupling scheme. For the purpose, by introducing the Γ8 Hubbard Hamiltonian as the minimum model to discuss the multipole ordering in f-electron systems, we describe the mean-field approximation in terms of the multipole operators. For the case of n=2, where n denotes the average f-electron number per ion, we analyze the model on a simple cubic lattice to obtain the multipole phase diagram. In particular, we find the order of non-Kramers Γ3 quadrupoles, O20 and O22, with different ordering vectors. We attempt to explain the phase diagram from the discussion on the interaction energy.

Resonant inelastic X-ray scattering spectra at the Ir L-edge in Na2IrO3

We analyze resonant X-ray scattering (RIXS) spectra in Na2IrO3 on the basis of the itinerant electron picture. Employing a multi-orbital tight-binding model on a honeycomb lattice, we find that the zigzag magnetic order is the most stable with sizable energy gap in the one-electron band within the Hartree–Fock approximation. We derive the RIXS spectra, which are connected to the generalized density–density correlation function. We calculate the spectra as a function of excitation energy ω, within the random phase approximation. The spectra consist of the peaks with ω<20meV, and of the peaks with 0.4<ω<0.8eV. The former peaks are composed of four bound states in the density–density correlation function, and may be identified as the magnetic excitations, while the latter peaks are composed of 16 bound states below the energy continuum of individual electron–hole pair excitations, and may be identified as the excitonic excitations. The calculated spectra agree qualitatively with the recent RIXS experiment.

Resonant inelastic x-ray scattering study of the spin and charge excitations in the overdoped superconductorLa1.77Sr0.23CuO4

We present a resonant inelastic x-ray scattering (RIXS) study of spin and charge excitations in overdoped La1.77Sr0.23CuO4 along two high-symmetry directions. The line shape of these excitations is analyzed and they are shown to be highly overdamped. Their spectral weight and damping are found to be strongly momentum dependent. Qualitative agreement between these observations and a calculated RPA susceptibility is obtained for this overdoped compound, implying that a significant contribution to the RIXS signal stems from a continuum of charge excitations. Furthermore, this suggests that the spin-excitations in the overdoped regime can be captured qualitatively by an itinerant picture. Our calculations also predict a new low-energy spin excitation branch to exist along the nodal direction near the zone center. With the energy resolution of the present experiment, this branch is not resolvable but we show that next generation of high-resolution spectrometers will be able to test this prediction.

Effects of stoichiometry and substitution in quasi-one-dimensional iron chalcogenideBaFe2S3

The effects of off-stoichiometry and elemental substitution on electronic properties of iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ are investigated. Resistivity and magnetization are revealed to be quite sensitive to the stoichiometry of Fe atoms, and 10% deficiency at Fe sites reduces the antiferromagnetic transition temperature by 40 K. The antiferromagnetic transition temperature decreases even faster and collapses to zero with hole doping through 10% K substitution at the Ba site, while the antiferromagnetic ordering phase remains with electron doping through 20% Co substitution at the Fe site. Such electron-hole asymmetry is opposite to two-dimensional iron-based superconductors, and can be explained on the basis of both itinerant and localized electronic pictures.

Correlation between stoichiometry, strain, and metal-insulator transitions of NdNiO3 films

The interplay of film stoichiometry and strain on the metal-insulator transition (MIT) and Hall coefficient of NdNiO3 films grown under different conditions is investigated. Unstrained lattice parameters and lattice mismatch strains are evaluated for films grown under a range of growth pressures and on different substrates. It is shown that both the temperature of the MIT and the Hall coefficient in the metallic phase are highly sensitive to film strain. In films grown with lower oxygen/total growth pressures, very large compressive in-plane strains can be obtained, which can act to suppress the MIT. Both the Hall coefficient and the temperature of the MIT are relatively insensitive to growth pressure, provided that films under the same strain are compared. The results support an itinerant picture of the transition that is controlled by the Ni eg bands, and that is relatively insensitive to changes in film stoichiometry.

ChemInform Abstract: Broken Symmetries in URu2Si2

AbstractReview: 52 refs.

Electronic evidence of an insulator-superconductor crossover in single-layer FeSe/SrTiO3 films.

In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator-superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator-superconductor crossover observed in the single-layer FeSe film grown on a SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator-superconductor crossover in FeSe/SrTiO3 film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator-superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.

Consistency between Itinerant and Local-Moment Pictures for Superconductivity in Alkaline Iron Selenide Superconductors

We study the superconducting pairing in an effective one-orbital model for alkaline iron-selenide superconductors. In both itinerant and local-moment pictures, we find that the nearest-neighbor hopping t1 plays a crucial role. The pairing symmetry changes from s-wave to d-wave as t1 is enhanced. For a reasonable t1 relevant to the experiment, the pairing symmetry is s-wave in both pictures, in agreement with the angle-resolved photo-emission. The results resolve the previous theoretical inconsistency in the two pictures.

Broken symmetries in URu2Si2

To resolve the nature of the hidden order below 17.5 K in the heavy-fermion compound URuSi, identifying which symmetries are broken below the hidden-order transition is one of the most important steps. Several recent experiments on the electronic structure have shown that the Fermi surface in the hidden-order phase is quite close to the result of band-structure calculations within the framework of itinerant electron picture assuming the antiferromagnetism. This provides strong evidence for the band folding along the axis with the ordering vector of , corresponding to broken translational symmetry. In addition to this, there is growing evidence for fourfold rotational symmetry breaking in the hidden-order phase from measurements of the in-plane magnetic anisotropy and the effective mass anisotropy in the electronic structure, as well as the orthorhombic lattice distortion. This broken fourfold symmetry gives a stringent constraint that the symmetry of the hidden-order parameter should belong to the degenerate -type irreducible representation. We also discuss a possibility that time reversal symmetry is also broken, which further narrows down the order parameter that characterizes the hidden order.

Temperature-induced change in the Fermi surface topology in the spin density wave phase of Sr(Fe1−xCox)2As2

We report electronic Raman scattering measurements of Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals in their magnetic - Spin Density Wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps as expected in a band-folding itinerant picture for a multiband system. The temperature dependence of the SDW gaps reveals an unusual evolution of the reconstructed electronic structure with at least one gap being activated only well below the magnetic SDW transition $T_N$. A comparison with temperature dependent Hall measurements allows us to assign this activated behavior to a change in the Fermi surface topology deep in the SDW phase, which we attribute to the disappearance of a hole-like Fermi pocket. Our results highlight the strong sensitivity of the low energy electronic structure to temperature in iron-arsenide superconductors.

Magnetic fluctuations and effective magnetic moments inγ-iron due to electronic structure peculiarities

Applying the local density and dynamical mean field approximations to paramagnetic \gamma-iron we revisit the problem of theoretical description of magnetic properties in a wide temperature range. We show that contrary to \alpha-iron, the frequency dependence of the electronic self-energy has a quasiparticle form for both, t_{2g} and e_g states. In the temperature range T=1200-1500 K, where \gamma-iron exist in nature, this substance can be nevertheless characterized by temperature-dependent effective local moments, which yield relatively narrow peaks in the real part of the local magnetic susceptibility. At the same time, at low temperatures \gamma-iron (which is realized in precipitates) is better described in terms of itinerant picture. In particular, the nesting features of the Fermi surfaces yield maximum of the static magnetic susceptibility at the incommensurate wave vector q_{max} belonging the direction q_X-q_W (q_X=(2\pi/a)(1,0,0),q_W=(2\pi/a)(1,1/2,0), a is a lattice parameter) in agreement with the experimental data. This state is found however to compete closely with the states characterized by magnetic wave vectors along the directions q_X-q_L-q_K, where q_L=(2\pi/a)(1/2,1/2,1/2), q_K=(2\pi/a)(3/4,3/4,0). From the analysis of the uniform magnetic susceptibility we find that contrary to \alpha-iron, the Curie-Weiss law is not fulfilled in a broad temperature range, although the inverse susceptibility is nearly linear in the moderate-temperature region (1200-1500 K). The non-linearity of the inverse uniform magnetic susceptibility in a broader temperature range is due to the density of states peak located close to the Fermi level. The effective exchange integrals in the paramagnetic phase are estimated on the base of momentum dependent susceptibility.

Electron spin resonance in CeB6

CeB6 is a cubic heavy fermion compound with antiferro-quadrupolar (AFQ) order below 4 K. The effects of the interplay of AFQ and ferromagnetic correlations on the phase diagram and the electron spin resonance linewidth are discussed. An itinerant picture within the AFQ phase is necessary to explain the electron spin resonances for CeB6.

Electron spin resonance in antiferro-quadrupolar-ordered CeB6

CeB${}_{6}$ is a cubic heavy-fermion compound with a ${\ensuremath{\Gamma}}_{8}$ ground quartet for which an ESR signal was observed. All other Ce or Yb compounds displaying an ESR signal have strong magnetic anisotropy and ferromagnetic correlations among the spins. The role of the ferromagnetic correlations is to narrow the resonance width rendering the signal observable. In CeB${}_{6}$ the orbital content of the ${\ensuremath{\Gamma}}_{8}$ quartet gives rise to an antiferro-quadrupolar-ordered phase below 4 K. Single ions with a ${\ensuremath{\Gamma}}_{8}$ ground multiplet are expected to display four transitions, however, only one has been observed. The following questions are addressed in this paper: (1) why is only one transition seen, (2) why is this transition observed if the Kondo temperature is comparable to the linewidth and the resonance frequency, and (3) are there ferromagnetic correlations between the Ce ions? The answer to these questions is associated with the antiferro-quadrupolar order. While for other Ce and Yb compounds with ESR signal it is difficult to distinguish if the resonance is due to localized spins or conducting heavy electron spins, an itinerant picture within the antiferro-quadrupolar phase is necessary for CeB${}_{6}$.

Electron doping evolution of the anisotropic spin excitations in BaFe2−xNixAs2

We use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe${}_{2\ensuremath{-}x}$Ni${}_{x}$As${}_{2}$ spanning from underdoped antiferromagnet to overdoped superconductor ($0.03\ensuremath{\le}x\ensuremath{\le}0.18$). In the undoped state, BaFe${}_{2}$As${}_{2}$ changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy ($\ensuremath{\le}\ensuremath{\sim}80$ meV) spin waves form transversely elongated ellipses in the $[H,K]$ plane of the reciprocal space. Upon Ni doping to suppress the static AF order and induce superconductivity, the $c$-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the $[H,K]$ plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping $x$, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at $x\ensuremath{\approx}0.1$, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron doping to decrease the superconducting transition temperature ${T}_{c}$, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials ($x\ensuremath{\le}0.1$), spin excitations in the overdoped side ($x=0.15$) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe${}_{2\ensuremath{-}x}$Ni${}_{x}$As${}_{2}$.

Ba1−xKxMn2As2: An Antiferromagnetic Local-Moment Metal

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature T(N)=625 K and a large ordered moment μ=3.9μ(B)/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high T(N) and large μ. Ba(1-x)K(x)Mn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba(1-x)K(x)Mn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high T(c) superconductivity.

Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2

Raman selection rules for electronic and magnetic excitations in BaFe 2 As 2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies. Raman spectra are composed of magnetic excitations with gradually decreasing intensity toward far above the SDW transition temperature ( T SDW ) and electronic excitations induced by the Brillouin zone folding below T SDW . The SDW gap has Dirac nodes, because many orbitals participate in the electronic states near the Fermi energy. Using a two-orbital band model the electronic excitations near the Dirac node and the anti-node are found to have different symmetries. Applying the symmetry difference to Raman scattering the nodal and anti-nodal electronic excitations are separately obtained. The low-energy spectra from the anti-nodal region have critical fluctuation just above T SDW and change into the gap structure by the first order transition at T SDW , while those from the nodal region gradually change into the SDW state. Magnetic excitations are observed as a very broad peak in all polarization configurations. The selection rule for two-magnon scattering from the stripe spin structure was obtained. Applying it to the two-magnon Raman spectra it is found that the magnetic exchange interaction energies are not presented by the short-range superexchange model, but the second derivative of the total energy of the stripe spin structure with respect to the moment directions. The selection rule and the peak energy are expressed by the two-magnon scattering process in an insulator, but the large spectral weight above twice the maximum spin wave energy is difficult to explain by the decayed spin wave. It may be explained by the electronic scattering of itinerant carriers with the magnetic self-energy in the localized spin picture or the particle–hole excitation model in the itinerant spin picture. The magnetic scattering spectra are compared to the insulating and metallic cuprate superconductors whose spins are believed to be localized.