Itinerant Band Research Articles

Magnetic Form Factor ofα-Ce: Towards Understanding the Magnetism of Cerium

A favored interpretation of the gamma <--> alpha phase transition in cerium postulates the transformation of the localized 4f state in gamma-Ce to a weakly correlated itinerant 4f band in alpha-Ce. However, results of high-energy neutron inelastic scattering measurements, presented here, show clearly that the magnetic susceptibility response from alpha-Ce follows the Ce3+ form factor despite the large, 30-fold, increase in its spectral width relative to that in gamma-Ce. This observation provides, for the first time, indisputable evidence for the localized character of the 4f state in the alpha phase. The present findings appear consistent with recent calculations combining dynamical mean-field theory with the local density approximation that indicate a strongly correlated 4f state in alpha-Ce. The localized 4f state is also fundamental to the Kondo volume collapse theories for the gamma <--> alpha phase transition in cerium.

First Observation of de Haas–van Alphen Effect in PuIn3

We observed de Haas–van Alphen (dHvA) oscillation in a paramagnetic plutonium compound, PuIn 3 . In the present paper, we report the first observation of dHvA effect in plutonium compounds. One detected dHvA branch named γ corresponds to a closed band-27 electron Fermi surface, where three kinds of closed Fermi surfaces are calculated on the basis of the 5 f -itinerant band model. The cyclotron mass of branch γ was determined as 4.8 m 0 . The dHvA signals are found to be extremely reduced as a function of time due to point defects produced by α-decay of 239 Pu.

Itinerant 5 f Electrons and the Fermi Surface Properties in an Enhanced Pauli Paramagnet NpGe3

We succeeded in growing a high-quality single crystal of an enhanced Pauli paramagnet, NpGe 3 , by the Bi-flux method, and observed the de Haas–van Alphen (dHvA) effect. The topology of a Fermi surface is well explained by the relativistic linear augmented-plane-wave (LAPW) band calculations based on the 5 f itinerant band model. The Fermi surface consists of a nearly spherical electron-Fermi surface with necks along the direction, forming a hollow ball, centered at the R point, which is derived from the single band. The cyclotron effective mass is in the range from 2.6 to 16 m 0 , which is enhanced approximately 3.5 times from the corresponding band mass.

Carrier-induced ferromagnetism in concentrated and diluted local-moment systems

For modeling the magnetic properties of concentrated and diluted magnetic semiconductors, we use the Kondo-lattice model. The magnetic phase diagram is derived by inspecting the static susceptibility of itinerant band electrons, which are exchange coupled to localized magnetic moments. It turns out that rather low band occupations favour a ferromagnetic ordering of the local moment systems due to an indirect coupling mediated by a spin polarization of the itinerant charge carriers. The disorder in diluted systems is treated by adding a CPA-type concept to the theory. For almost all moment concentrations x, ferromagnetism is possible, however, only for carrier concentrations n distinctly smaller than x. The charge carrier compensation in real magnetic semiconductors (in Ga_{1-x}Mn_{x}As by e.g. antisites) seems to be a necessary condition for getting carrier induced ferromagnetism.

Orbital and spin correlations in Ca $\mathsf{_{2-x}}$Sr$\mathsf{_x}$RuO$\mathsf{_4}$: A mean field study

The alloy Ca$_{2-x}$Sr$_x$RuO$_4$ exhibits a complex phase diagram with peculiar magnetic metallic phases. In this paper some aspects of this alloy are discussed based on a mean field theory for an effective Kugel-Khomskii model of localized orbital and spin degrees of freedom. This model results from an orbital selective Mott transition which in the three-band system localized two orbitals while leaving the third one itinerant. Special attention is given to the region around a structure quantum phase transition at $ x \approx 0.5 $ where the crystal lattice changes from tetragonal to orthorhombic symmetry while leaving the system metallic. This transition yields, a change from ferromagnetic to antiferromagnetic spin correlations. The complete mean field phase diagram for this transition is given including orbital and spin order. The anisotropy of spin susceptibility, a consequence of spin-orbit coupling and orbital correlation, is a tell-tale sign of one of these phases. In the predominantly antiferromagnetic phase we describe a metamagnetic transition in a magnetic field and show that coupling of the itinerant band to the localized degrees of freedom yields an anomalous longitudinal magnetoresistance transition. Both phenomena are connected with the evolution of the ferromagnetic and antiferromagnetic domains in the external magnetic field and agree qualitatively with the experimental findings.

Localized character of 4f electrons in CeRhx (x=2,3) and CeNix (x=2,5).

We have measured Ce 4f spectral weights of extremely alpha-like Ce transition metal intermetallic compounds CeRhx (x=2,3) and CeNix (x=2,5) by using the bulk-sensitive resonant photoemission technique at the Ce M5(3d(5/2)-->4f) edge. High energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Kondo resonance tails at the Fermi level, which can be well described by the Gunnarsson-Schönhammer calculation based on the Anderson impurity Hamiltonian. On the other hand, the itinerant 4f band description shows big discrepancies, which implies that Ce 4f electrons retain localized characters even in extremely alpha-like compounds.

Theory of superconductivity coexisting with the antiferromagnetic state in UPd2Al3

We investigate the superconductivity (SC) coexisting withthe antiferromagnetic (AF) state observed in UPd2Al3.Referring to the band calculation, we consider the band structure with adual nature, that is, localized electrons, which mainly construct the AFstate, and itinerant electrons, which form the heavy fermion state andthe unconventional SC at lower temperatures. Within the random-phaseapproximation (RPA), we estimate the AF transition temperature TN, andcalculate the dynamical spin susceptibility. It possesses a collective spin wave around Q = (0, 0, 0.5) belowTN.In this situation, we evaluate the Eliashberg equation for the remaining itinerant band withinthe RPA. The most favourable even-parity pairing symmetry is cos(kx + 2ky) − cos(kx − 2ky) modulated by acos(kz)-like function. Thisis the symmetry dxywhich, in addition, possesses a tiny gap around the AF zone boundary.

Two Pseudogaps in Underdoped Cuprates

Cuprate superconductivity gaps are investigated applying the interband pairing scheme to the itinerant band and a defect "band" which is created by hole doping. A pseudogap is naturally present in this model at underdoping. Vibronic hybridization of bands renormalizes the pseudogap and two superconductivity gaps. Now two pseudogaps of electronic and vibronic origin are present in the underdoped normal state as recently observed. Entering the optimally doped region (overlapping bands) there appears a joint normal state pseudogap of vibronic origin.

Calculations of the thermoelectric power for the (Gd1xRx)nMm alloys

Disordered substitutional alloys (Gd1–xRx)mMn represent a very interesting alloy family showing both well defined (no orbital contribution) randomly distributed localized spins (Gd) and itinerant electron bands with a population depending on the concentration x. Because of the complicated many-body task, only the paramagnetic state was considered. The influence of the external uniform magnetic field was also discussed. The main contribution to the thermoelectric power (TEP) has a magnetic origin and is caused by scattering of the conducting electrons on the stochastic distributions of the large Gd spin (S = 7/2ħ) over the lattice sites. The many-body approximations used for our models are based on the alloy analogy approximations for the fictitious electronic alloys. Band structure calculations yielding informations on the widths and positions of the itinerant bands were performed. The results on TEP as a function of alloy concentration x fit well the experimental results.

Vibronic Effects in Cuprate Superconductors with a Two-Component Spectrum Created by Doping

A model [1] of the electron spectrum of a cuprate superconductor involves a new band borning with hole doping above the itinerant band. Vibronic coupling of these components leads to the renormalization of the gaps created by the interband pair transfer interaction. The phonon softening is maximal at the under/overdoped separation line where the bands begin to overlap. The vibronic hybridization determines the normal state joint pseudogap at optimal doping and one of the two pseudogaps of the underdoped material. The larger pseudogap changes its nature with doping.

CUPRATE SUPERCONDUCTIVITY ON INTERACTING ELECTRONIC COMPONENTS PREPARED BY DOPING

A model of a "defect" band which expands by doping above the itinerant hole band is used to describe the cuprate superconductivity by the interband scattering of pairs. Bands overlap reached by progressive doping determines the under/optimal-doping separation line and the reset of the Fermi-liquid behaviour. A two-band scheme with the interband scattering of intraband pairs reproduces the observed peculiar behaviour of gaps. A pseudogap appears as the minimal excitation energy of the itinerant band at underdoping. Doping forms the electronic structure of cuprates on which the interband superconductivity can occur.

Electronic structure, magnetic, and cohesive properties ofLixMn2O4:Theory

The volume dependent electronic structure of the spinel-type lithium manganese oxides ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4},$ $x=0,0.5,1,$ is studied ab initio by employing a full-potential electronic structure method. The electronic structure, total energies, open-circuit voltage, and magnetic moments were obtained for various spin configurations of ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ in the cubic spinel structure and the low-temperature orthorhombic structure. The effect of magnetic ordering on the band structure and structural stability has been investigated and an antiferromagnetic ordering proved to be the ground state of the ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ spinels. Our calculations show that the manganese majority ${t}_{2g} d$ band is filled for all ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ compounds studied, and the filling of the minority ${t}_{2g}$ band is expected in the lithiation process. The lithium intercalation potential, bulk modulus, magnetic moments, and optical properties are calculated within the itinerant band approach and are found to be in good agreement with available experimental data, indicating, that the density-functional theory provides reliable electronic structure of the ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$ system. The effect of the orthorhombic distortion on electronic structure and magnetism of ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ was investigated, and our calculations do not show a substantial charge ordering at the structural transition from the cubic spinel to the orthorhombic structure, as proposed earlier. Instead, the low-temperature orthorhombic structure is found to possess the lowest energy via a Jahn-Teller distortion driven by the d band.

Pseudogap and superconductivity gaps in a two-band model with doping-determined components

A model incorporating an itinerant band and a ‘defect band’ which develops together with doping is postulated to interpolate the two-component scenario of cuprate superconductivity. The calculated gap-like characteristics reproduce the dependencies on doping of the observed type. A pseudogap excitation is naturally present in the underdoped region. It transforms into the largest of two superconductivity gaps on passing the under/optimal doping border corresponding to the onset of bands overlap.

Ground-state and excited-state properties ofLaMnO3from full-potential calculations

The ground state and excited state properties of the perovskite LaMnO$_3$ are calculated based on the generalized-gradient-corrected relativistic full-potential method. The electronic structure, magnetism and energetics of various spin configurations for LaMnO$_3$ in the ideal cubic perovskite structure and the experimentally observed distorted orthorhombic structure are obtained. The excited state properties such as the optical, magneto-optical, x-ray photoemission (XPS), bremsstrahlung isochromat (BIS), x-ray absorption near edge structure (XANES) spectra are calculated and found to be in excellent agreement with available experimental results. The present results suggest that the correlation effect is not significant in LaMnO$_3$ and the presence of ferromagnetic coupling within the $ab$ plane as well as the antiferromagnetic coupling perpendicular to this plane can be explained through the itinerant band picture. As against earlier expectations, our calculations show that the Mn 3$d$ $t_{2g}$ as well as the $e_{g}$ electrons are present in the whole valence band region. In particular significantly large amounts of $t_{2g}$ electrons are present in combination with the $e_{g}$ electrons at the top of the valence band against the common expectation of presence of only pure $e_{g}$ electrons. We have calculated the hyperfine field parameters for the A-type antiferromagnetic and the ferromagnetic phases of LaMnO$_3$ and compared the findings with the available experimental results. The role of the orthorhombic distortion on electronic structure, magnetism and optical anisotropy are analyzed.

Magnetic and Fermi Surface Properties of UFeGa5

We succeeded in growing a high-quality single crystal of UFeGa 5 by the self-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization and de Haas-van Alphen (dHvA) oscillation. Many dHvA branches were observed, ranging from 5.4×10 5 to 1.4×10 8 Oe. These branches are in good agreement with theoretical ones based on 5 f -itinerant RLAPW band calculations. The cyclotron mass is relatively large, ranging from 2.4 to 9.9 m 0 .

Two-band high- Tc superconductivity with a pseudogap

The cuprate superconductors pseudogap is interpreted in a two-band model as the minimal quasiparticle excitation energy of the itinerant band. In the underdoped region the chemical potential lies out of the bands overlap in the upper doping created band. Further doping transforms this pseudogap to one of the superconductivity gaps. An illustration using a simple interpolative model with a chosen parameter set is given.

Stabilization of itinerant (band) magnetism in FeAl by Ga substitution for Al

Band structure calculations of FeGa1−xAlx have been performed, to further investigate the stability of ferromagnetism in FeAl. The Stoner parameter increases by about 20% at the FeGa end. This is also confirmed by our spin-polarized calculations. We conclude that Ga substitution for Al is likely to stabilize the elusive (or illusive) ferromagnetic state in FeAl.

Superconductivity and high‐Tc superconductors

Superconductivity and high‐T_<i>c</i> superconductors

Electronic structure of the strongly hybridized ferromagnetCeFe2

We report on results from high-energy spectroscopic measurements on CeFe2, a system of particular interest due to its anomalous ferromagnetism with an unusually low Curie temperature and small magnetization compared to the other rare earth-iron Laves phase compounds. Our experimental results indicate very strong hybridization of the Ce 4f states with the delocalized band states, mainly the Fe 3d states. In the interpretation and analysis of our measured spectra, we have made use of two different theoretical approaches: The first one is based on the Anderson impurity model, with surface contributions explicitly taken into account. The second method consists of band-structure calculations for bulk CeFe2. The analysis based on the Anderson impurity model gives calculated spectra in good agreement with the whole range of measured spectra, and reveals that the Ce 4f -- Fe 3d hybridization is considerably reduced at the surface, resulting in even stronger hybridization in the bulk than previously thought. The band-structure calculations are ab initio full-potential linear muffin-tin orbital calculations within the local-spin-density approximation of the density functional. The Ce 4f electrons were treated as itinerant band electrons. Interestingly, the Ce 4f partial density of states obtained from the band-structure calculations also agree well with the experimental spectra concerning both the 4f peak position and the 4f bandwidth, if the surface effects are properly taken into account. In addition, results, notably the partial spin magnetic moments, from the band-structure calculations are discussed in some detail and compared to experimental findings and earlier calculations.

Systematics for trivalent and divalent rare-earth sulphides

Abstract The electronic structures of the rare-earth monosulphides are calculated with the self-interaction corrected local-spin-density approximation. This scheme allows part of the rare-earth f shell to be localized. Both trivalent and divalent configurations of the rare earths are considered, and the systematics in lattice constants and electronic densities of states are investigated. The scheme is found to be accurate in describing the properties of these systems and reveals that two kinds of f electrons in fact are present in the rare-earth monosulphides, namely both localized atomic-like f electrons and itinerant band f electrons, which hybridize with the normal conduction bands. The stability of the divalent configuration directly correlates with the number of band f electrons.