Itinerant Band Research Articles

Heavy Fermion State in YbIr2Zn20

We measured the high-field magnetization and de Haas–van Alphen (dHvA) oscillations for YbIr 2 Zn 20 with a cubic crystal structure, together with the electrical resistivity and magnetic susceptibility. The magnetic susceptibility χ with an effective magnetic moment of Yb 3+ becomes temperature-independent at low temperatures, with a broad peak at T χmax =7.4 K for H ∥<110>. The corresponding magnetization indicates a metamagnetic transition at H m =120 kOe, consistent with a T χmax vs H m relation in the Ce- and U-based heavy fermion compounds. The large cyclotron masses of 4–27 m 0 are detected in the dHvA experiment, and are found to be reduced at magnetic fields higher than H m =120 kOe. The resistivity follows the Fermi liquid relation ρ=ρ 0 + A T 2 under magnetic field, and the \(\sqrt{A}\) value is also found to have a maximum at H m as a function of magnetic field. From the present experimental results, together with the results of 4 f -itinerant energy band calculations, the 4 f electrons are found to contribute to the heavy fermion state in YbIr 2 Zn 20 .

Nodal superconductivity and antinodal pseudogap in cuprate superconductors

According to recent experimental findings the leading pairing resides in the nodal (FS arcs) momentum region of hole doped cuprates. The pseudogap is an antinodal feature. A corresponding multiband model of the electronic background evolving with doping serves the usually presented phase diagram. The pairing is due by the pair-transfer between overlapping nodal defect (polaron) band and the itinerant band. A bare gap vanishing with extended doping between the antinodal defect subband and the itinerant band top leads to the formation of the pseudogap as a perturbative band-structure effect. The calculated behaviour of two superconducting gaps and of the pseudogap on the whole doping scale is in qualitative agreement with the observations. Arguments to include cuprates into the class of multiband–multigap superconductors are given by these results.

De Haas–van Alphen Effect and Fermi Surface Properties in High-Quality Single Crystals YbCu2Si2 and YbCu2Ge2

We succeeded in growing high-quality single crystals of a valence fluctuating compound YbCu 2 Si 2 and a divalent compound YbCu 2 Ge 2 . The magnetic susceptibility of YbCu 2 Si 2 follows the Curie–Weiss law with Yb 3+ at high temperatures, but reveals a broad peak around 40 K for H ∥[100], which is due to the formation of a 4 f -itinerant heavy fermion state at lower temperatures. This is also reflected in the temperature dependence of Hall coefficient, thermoelectric power and thermal expansion. The corresponding de Haas–van Alphen (dHvA) branches are approximately explained by the 4 f -itinerant LDA band model, and the 4 f -itinerant LDA+ U model is found to be much applicable to the dHvA data. The cyclotron effective masses of main Fermi surfaces are relatively large, being 30–40 m 0 , which is consistent with the electronic specific heat coefficient γ=150 mJ/(K 2 ·mol). These results indicate that the localized 4 f electrons at high temperatures become itinerant at low temperatures, forming a narrow ...

Ground-state electronic structure of actinide monocarbides and mononitrides

The self-interaction corrected local spin-density approximation is used to investigate the ground-state valency configuration of the actinide ions in the actinide monocarbides, AC A=U,Np,Pu,Am,Cm, and the actinide mononitrides, AN. The electronic structure is characterized by a gradually increasing degree of f electron localization from U to Cm, with the tendency toward localization being slightly stronger in the more ionic nitrides compared to the more covalent carbides. The itinerant band picture is found to be adequate for UC and acceptable for UN, while a more complex manifold of competing localized and delocalized f-electron configurations underlies the ground states of NpC, PuC, AmC, NpN, and PuN. The fully localized 5f-electron configuration is realized in CmC f 7 ,C mNf 7 , and AmN f 6 . The observed sudden increase in lattice parameter from PuN to AmN is found to be related to the localization transition. The calculated valence electron densities of states are in good agreement with photoemission data.

MULTIBAND MODEL OF CUPRATE SUPERCONDUCTIVITY

A minimal model for the description of cuprate superconductor characteristics on doping scale (hole and electron) is developed. The leading interband pairing channel couples an itinerant band and defect states created by doping. Bare gaps between them are supposed and become closed by extended doping. Band overlap conditions determine special points in the phase diagram. Nodal and antinodal momentum regions are distinguished. Illustrative calculations have been made using a mean-field pair-transfer multiband Hamiltonian and corresponding free-energy expansions. The results are self-consistent and demonstrate that the elaborated approach is able to reproduce characteristic features of cuprate superconductors as, e.g., the doping dependence of Tc, superconducting gaps and pseudogaps, supercarrier density and effective mass, coherence length and penetration depth, critical magnetic fields and some other properties. Interband pairing scheme is suggested to be an essential aspect of cuprate superconductivity.

Neutron scattering from α-Ce at epithermal neutron energies

Neutron scattering data, using neutrons of incident energies as high as 2 eV, on α-Ce and α-Ce-like systems such as CeRh2, CeNi2, CeFe2, CeRu2, and many others that point clearly to the substantially localized 4f electronic state in these systems are reviewed. The present interpretation is contrary to the widely held view that the 4f electrons in these systems form a narrow itinerant electron 4f band.

Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films

Epitaxial thin films of cobalt-doped ZnO (Co : ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10−4–105 Ω cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co : ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co : ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.

The Fermi surface and f-valence electron count of UPt3

Combining old and new de Haas-van Alphen (dHvA) and magnetoresistance data, we arrive at a detailed picture of the Fermi surface of the heavy fermion superconductor UPt3. Our work was partially motivated by a new proposal that two 5f valence electrons per formula unit in UPt3 are localized by correlation effects—agreement with previous dHvA measurements of the Fermi surface was invoked in its support. Comprehensive comparison with our new observations shows that this ‘partially localized’ model fails to predict the existence of a major sheet of the Fermi surface, and is therefore less compatible with experiment than the originally proposed ‘fully itinerant’ model of the electronic structure of UPt3. In support of this conclusion, we offer a more complete analysis of the fully itinerant band structure calculation, where we find a number of previously unrecognized extremal orbits on the Fermi surface.

Pressure-Induced Superconductivity in Antiferromagnet CePd5Al2

A layered compound CePd 5 Al 2 , which is an analog of a heavy fermion superconductor NpPd 5 Al 2 , is found to be an antiferromagnet with an easy-axis along the tetragonal [001] direction but becomes superconductive in the pressure region from 9 to 12 GPa, with the superconducting transition temperature, T sc = 0.57 K and the upper critical field at 0 K, H c2 (0) = 2.5 kOe, at 10.8 GPa. The quasi-two dimensional Fermi surfaces are also theoretically calculated for CePd 5 Al 2 on the basis of the 4 f -itinerant band model.

Multiband superconductivity of doped cuprates

Numerous indications point to the multiband nature of the superconductivity in doped cuprates. The electronic structure of these compounds evolves with doping. Based on experimental findings, a simple model for the description of cuprate superconductor characteristics on the doping is developed. The leading interband pairing channel between an itinerant band and defect states created by doping is postulated. Bare gaps between them are supposed and to be closed by extended doping. Band overlap conditions determine the phase diagram critical points. The supercarrier density varies with the disposition of the chemical potential and bands. Nodal and antinodal momentum regions are distinguished. In the case of hole doping the relevant regime lies near the top of the oxygen band. Illustrative calculations have been made using a mean-field multiband Hamlitonian. Manifestations of various gaps and pairing strength characteristics on the doping level are discussed. A nonmonotonic dependence of the critical coherence length is calculated. The coherence of noncritical mode is also investigated. The supercarrier effective mass diminishes moderately out of underdoping. The penetration depth curve is of the observed type. The isotope effect on supercarrier density is predicted. In the case of electron doping the events concentrate near the bottom of the upper Hubbard band. The gross features of the phase diagram with two pseudogaps remind the hole doping case. The pairing strength and the phase coherence develop simultaneously. The agreement of the results of our model calculations with experimental findings suggests that multiband pairing is an essential aspect of cuprate superconductivity.

Spin-resolved photoelectron spectroscopy of rare-earth overlayers on rare-earth and d-metal substrates

Spin- and angle-resolved photoelectron spectroscopy was applied for studies of electronic and magnetic structures of Eu/Gd and Ce/Fe. Ferromagnetic coupling of 4f moments of Eu and Gd was found in the 1 ML Eu/Gd(0 0 0 1) system with high net Eu magnetization at low temperatures reflected by a spin polarization of ∼15% of the Eu 4f state. In case of the 1 ML Ce/Fe(1 1 0) system the antiparallel orientation of the Ce 4f spins with respect to the magnetization direction of the Fe substrate was observed. Very different shapes of the spin-up and spin-down Ce 4f spectral weights can be explained within periodic Anderson model by spin-dependent hybridization between Ce localized 4f and itinerant valence band states.

Quenched magnetic moment in Mn-doped amorphous Si films

The absence of a Mn local moment was observed in Mn-doped amorphous silicon $(a\text{\ensuremath{-}}{\mathrm{Mn}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x})$ films. The magnetic susceptibility obeys the Curie-Weiss law for a wide range of $x$ ($5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ up to 0.175) but with extremely small moment. Magnetization measurements suggest that this behavior occurs because only a small percentage of Mn (${\mathrm{Mn}}^{2+}$ states with $J=S=5∕2$) contribute to the magnetization. Thus, the magnetic moments are quenched for the majority of Mn atoms, contrary to the general belief of the existence of a localized Mn moment in Si. X-ray absorption spectroscopy suggests that the quenching of Mn moments is attributed to the formation of an itinerant but Anderson-localized impurity band, forming at $x$ as low as $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$.

Magnetic Properties of the Extended Periodic Anderson Model

We study magnetic properties of the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. By combining dynamical mean-field theory with the numerical renormalization group we calculate the sublattice magnetization and the staggered susceptibility to determine the phase diagram in the particle-hole symmetric case. We find that two kinds of magnetically ordered states compete with the Kondo insulating state at zero temperature, which induces non-monotonic behavior in the temperature-dependent magnetization. It is furthermore clarified that a novel magnetic metallic state is stabilized at half filling by the competition between Hund's coupling and the hybridization.

Quantum phase transitions in the extended periodic Anderson model

We investigate quantum phase transitions in the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. We calculate zero and finite temperature properties of the system using the combination of dynamical mean-field theory and the numerical renormalization group. At half filling, a first-order phase transition between a Mott insulating state and a Kondo insulating state occurs in the strong-coupling regime. We furthermore find that a metallic state is stabilized in the weak-coupling regime. This state should be adiabatically connected to the orbital selective Mott state with one orbital-localized and the other itinerant. The effect of hole doping is also addressed.

Kink Structures of Spin-Polarized Electrons due to Electron–Magnon Scattering in Ferromagnetic State

We propose a model composed of spin-polarized itinerant electrons and bosonic spin-wave excitations, and study renormalization of the spin-polarized itinerant electron bands due to electron-magnon scattering. Spin-polarized kink structures are predicted in the normal state quasiparticle dynamics of ferromagnetic superconductor as UGe2. It is suggested that the angle-resolved photoemission experiment may be helpful in this respect.

Electrical and magnetic properties of nanocrystalline Fe 100−xNi x alloys

The electrical and magnetic properties of nanocrystalline binary Fe 100− x Ni x alloys, where x ranges from 0 to 100, prepared by a combination of aqueous and solid-state reduction processes have been studied vis-à-vis their microstructure. The microstructural studies indicate the formation of near-equilibrium phases in the alloys with crystallite size in the range 20–40 nm. The crystallite size in the case of pure Fe and Ni, however, is in the range 40–80 nm. The electrical transport in the temperature range 20–300 K exhibits a typical ferromagnetic metallic behavior in all the cases and the absolute resistivity of nanocrystalline Fe 100− x Ni x alloys decreases monotonically with increasing Ni content. The saturation magnetization of the alloys on the other hand decreases progressively with Ni addition towards that of pure Ni value. The coercivity of alloys is found to be independent of temperature in the range 5–300 K except in the case of pure Ni wherein it increases from 30 Oe at 300 K to 65 Oe at 5 K. The electrical and magnetic properties of the nanocrystaline Fe–Ni alloys do not follow the predictions of simple itinerant band model for alloys. The temperature dependence of saturation magnetization in all the cases has a T 3/2 Bloch variation while the average atomic moment of the alloys has an effective medium composition dependence.

Thermodynamic properties of the two-orbital Hubbard model

We investigate an infinite-dimensional two-orbital Hubbard model with different bandwidths by means of the self-energy functional approach. By calculating the quasi-particle weight and the entropy at finite temperatures, we discuss how thermal fluctuations affect the stability of an intermediate phase, called the orbital-selective Mott phase, which has localized as well as itinerant bands.

Decrease of the coherence temperature with low Rh doping in [formula omitted

Comparative study of U ( Ru 1 - x Rh x ) 2 Si 2 ( x = 0 and 4%) through resistivity, magnetic susceptibility, and Hall coefficient measurements reveals that the coherence temperature, T coh , of the Rh 4% doped sample clearly decreases by about 10–20% as compared with that of undoped one located around 50 K. Furthermore, T * , a characteristic temperature below which the resistivity shows the T 2 dependence, is also found to decrease by almost the same factor. These findings suggest that the low Rh doping sensitively destroys the itinerant band gap due to the hidden order (HO) through the reduction of the Fermi surface volume in the quasi-particle band.

Shubnikov-de Hass effect across a metamagnetic transition in high quality single crystals of Sr4Ru3O10

We performed a detailed electrical transport study at very high magnetic fields and low temperatures in high quality single crystals of Sr4Ru3O10. When the field is applied along the inter-plane direction Shubnikov-de Hass oscillations are observed showing that a metamagnetic transition leads to a quite non-trivial evolution of the geometry of the Fermi surface. It can be ascribed either that localized and itinerant bands coexist in this system, the former leading to the ferromagnetic response and the latter to itinerant metamagnetic behavior, or that itinerant metamagnetism emerges from a Stoner-like ferromagnetic ground state.

STUDY OF MAGNETIZATION IN MANGANITE SYSTEM IN THE PRESENCE OF LATTICE DISTORTION

The manganites of the type Re1-xAxMnO3( Re = La , Nd , A = Ca , Sr , Ba ) are believed to be half metallic magnets exhibiting colossal magnetoresistance (CMR). The manganite system is described by a simple model Hamiltonian consisting of the hopping of the itinerant d-electrons in the doubly degenerate egband of Mn ions which is split by a static J-T lattice distortion due to band Jahn-Teller (J-T) effect. An external magnetic field results in further Zeeman splitting of the same egband. The ferromagnetism is assumed to originate from the exchange interaction between the spins of the localized core t2gelectrons. The J-T split itinerant (eg) bands are assumed to hybridize rather strongly with the on-site localized (t2g) levels. In the model under consideration the magnetization (m) as well as the lattice strain (e) are expected to depend on the model parameters of the system: i.e. the position of the localized level (d) with respect to the Fermi level, the strength of hybridization (v), the magnetic exchange coupling constant (g1), J-T coupling constant (g), external magnetic field (b) and the impurity concentration (x). The equations for the magnetization and the lattice distortion are solved self-consistently. The effect of different interactions on the quasiparticle bands and the density of states (DOS) are analyzed in detail to understand the evolution of the physical properties of the system on switching the interactions. The temperature dependence of the magnetization due to the localized electrons and that induced in the conduction band are studied.