Fermi surface,magnetic, and superconducting properties in actinide compounds

The de Haas–van Alphen effect, which is a powerful method to explore Fermi surface properties, has been observed in cerium, uranium, and nowadays even in neptunium and plutonium compounds. Here, we present the results of several studies concerning the Fermi surface properties of the heavy fermion superconductors UPt₃ and NpPd₅Al₂, and of the ferromagnetic pressure-induced superconductor UGe₂, together with those of some related compounds for which fascinating anisotropic superconductivity, magnetism, and heavy fermion behavior has been observed.     

de Haas–van Alphen effect in strongly correlated electron systems

We present the Fermi surface properties in strongly correlated electron systems of rare earth and uranium compounds via de Haas–van Alphen experiments. The conduction electrons with large cyclotron effective masses over 100m₀ (m₀: rest mass of an electron) are detected in CeRu₂Si₂, CeCoIn₅ and UPt₃. These electrons move slowly in the crystal. The topology of the Fermi surface and the cyclotron mass are compared to those of energy band calculations.     

Anisotropic superconductivity in UPt3

A consistent picture of conventional superconductivity in UPt₃ is presented which explains qualitatively the gap anisotropy, the dependence of T_c on crystal perfection, the low value of the specific heat discontinuity at T_c and the induced magnetic form factor. The model obtained is based on results of a local density band calculation for UPt₃ which are in excellent agreement with photoemission experiments and on estimates of the strength and anisotropy of the electron-phonon interaction. The key feature is anisotropy of the superconducting energy gap which arises from the variation of the electronic mass on the Fermi surface.     

Electronic structure of UPd3 — A localized f compound

Various experiments on UPd₃, the analogue of the heavy fermion superconductor, UPt₃, have ascertained that there are two f electrons per U which are localized in a magnetic singlet state. Recently, both photoemission (PES) and de Haas-van Alphen (dHvA) experiments have been reported on UPd₃. To complement this experimental work, local density energy band calculations have been performed on UPd₃ where the f electrons have been treated as core states. The resulting density of states is found to be in good agreement with photoemission data. The theoretical Fermi surface is found to be more complex than current dHvA data indicate, but one can still unambiguously assign theoretical extremal orbits to the experimental data. Thus again, the data is consistent with a local f² configuration. Since the band calculations can explain the dHvA data in heavy fermion UPt₃ with the f electrons treated as band states, one finds that the Kohn-Sham ansatz for treating the f electrons as Bloch states breaks down between these two cases. This finding is confirmed by recent U(Pd_xPt_3-x) alloy experiments which show a sudden decrease in the specific heat coefficient when alloying these two compounds.     

Photoemission study of UFe2 and UPt3 using synchrotron radiation

The U 5f density of states in UPt₃ and UFe₂ has been determined experimentally using synchrotron radiation. Cross section variations were exploited to separate the valence band contributions. By means of difference spectra, the variations in the U 5f density at the Fermi level have been measured.     

Microwave surface resistance measurements on the heavy fermion superconductor UPt3

The relative change of the microwave surface resistance has been measured as a function of temperature in the superconducting and normal states of UPt₃ using a cavity perturbation technique. With our frequency range of ∼9–38 GHz, the microwave photon energy (ℏω) is ∼20–95% of the estimated zero-temperature (BCS) energy gap (2Δ₀) of UPt₃. The surface resistance shows a change in slope as the signature of T_c and has a significantly weaker temperature dependence in the superconducting state than would be predicted by the Mattis–Bardeen theory. At higher frequencies, the slope tends to change by a smaller amount. No indication of collective modes at any of the frequencies was observed down to the lowest temperatures measured (∼0.1 K).     

The study of the energy band structures of TiC,VC, Ti4C3 and V4C3 by the LMTO-ASA method

The energy band structures of TiC, VC, Ti₄C₃ and V₄C₃ have been studied by the linear muffin-tin-orbital method (LMTO). The influence of vacancies on the density of states (DOS) in the 2s C-band is a uniform reduction and a narrowing of this DOS. The splitting of the peak in the low-energy part of the 2p C, 3d Me-band (Me = Ti, V) is observed, while in the neighbourhood of the Fermi energy two peaks of DOS appear which consist of 3d Me- and vacancy states. Under the influence of vacancies the electronic charge in the carbon atomic sphere is shifted partly into the vacancy sphere. For Ti₄C₃, the increase of the Fermi energy and DOS at this energy is observed, while in the case of V₄C₃ these values decrease. In the presence of vacancies a contraction of crystal lattice is observed while the bulk moduli of TiC and VC diminish which is most pronounced for VC. The results are compared with previous data obtained by the LCAO coherent potential and APW calculations and with the experimental data concerning the DOS at the Fermi energy.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.     

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni₂MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

Electronic, magnetic and transport properties of Co2TiZ (Z=Si, Ge and Sn): A first-principle study

We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co₂TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co₂TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μ_B, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co₂TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.     

Theoretical study of p-d hybridization in Co perovskite

The real-space recursion method and unrestricted Hartree-Fock approximation have been applied to calculate the density of states of various Co perovskite, CeCoO₃, SrCoO₃ and Sr_1−xCe_xCoO₃. We have studied the magnetically ordered states of these Co perovskites in an enlarged double cell, and find its various magnetic structures due to the occupancy of 3d band and its interaction with neighboring Co ions. In this study, we have studied the p-d hybridization of the three Co perovskites, we find t_2g electrons are localized and the flat e_g band is responsible for the itinerant behavior, and although the rare earth elements itself contribute little to the DOS at the Fermi energy, the DOS at Fermi energy and the magnetic moment changed consequently because of different valence of Co ions in these compounds and p-d hybridization effect is very important.     

Valence and core state modifications in Pt3Ti

XPS data for the valence band, the Pt 4? states, and the Ti 2p states are presented for the intermetallic Pt₃Ti. Relative to the Pt valence band, the Pt₃Ti band shows a decrease in the density of states just below the Fermi level and a shift of the centroid to higher (binding) energy. Ti 2p and Pt 4? binding energies showed relatively large shifts with respect to the pure metals. These changes in the valence band density of states and core level binding energies are interpreted as arising from hybridization of the d-orbitals in both metals to form strong intermetallic bonds.     

Electronic structure and optical properties of LaNiO3: First-principles calculations

Using a first-principles method, we investigate the electronic structure and optical properties of rhombohedral LaNiO₃. The total density of states shows that there is no band gap and bulk LaNiO₃ is metallic. There is a strong hybridization between Ni and O orbits near the Fermi level, suggesting that the metallic nature of LaNiO₃ mainly originates from Ni 3d states and La atoms have no noticeable contribution to this. The absorption coefficient of LaNiO₃ is one order of magnitude less than that of nickel in the lower energy region (0–5 eV), and the interband optical transitions are mainly derived from O 2p and Ni 3d states. In reflectivity spectrum of LaNiO₃, there are three main reflectance peaks located at 0 eV, 15.6 eV and 22.9 eV, respectively. In the visible–ultraviolet energy range, the reflectivity of LaNiO₃ remarkably decreases with the increasing photon energy and the value is always smaller than that of nickel in the region.     

Crystal structure, electronic and elastic properties for novel Hf3AlN and Zr3AlN ceramics explored by first principles studies

Investigations into crystal structure, electronic and elastic properties of M₃AlN (M=Hf, Zr) had been conducted by plane-wave pseudopotential calculations. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of these two compounds. The charge density distributions and density of states indicate that there exist relatively soft Al-M and strong N-M covalent bonds, which might be contributed to layered chemical bonding character of M₃AlN. By analyzing Cauchy pressure and the bulk modulus to C₄₄ ratio, Hf₃AlN was predicted to be more ductile than Zr₃AlN.     

Cr adsorption induced magnetism in Bi2Se3 film by proximity effects

Based on first-principles calculations within density functional theory, we studied the effects of Cr adsorption on the electronic and magnetic properties of Bi₂Se₃ topological insulators employing spin–orbit coupling (SOC) self-consistently. Cr atom induces a spin-polarization with total net magnetic moments of 2.157 μ_B (spin up). There is a p-d hybridization between the Cr 3d states and the nearest neighbor Se 4p states. A peak of density of states appears at Fermi level. The electronic structures change and the energy levels split near the Fermi level. No gap opening has been found at the Dirac point of the surface state from the bottom surface.     

Anomalous hall effect in some uranium intermetallic compounds

We report Hall effect measurements on the uranium intermetallic compounds UAl₂, U_0.97La_0.03Al₂, U_0.93La_0.07Al₂, U_0.85La_0.15Al₂, UPt₃ and UPd₃. We find in all of these substances, a large positive Hall effect at high temperatures and maxima or minima at low temperatures. We have compared our data with the results of magnetic skew scattering theories and found qualitative agreement at high temperatures in the cases of U_1-xLa_xAl₂ and UPt₃, but not in the case of UPd₃.     

First-principles study of the structural, electronic, and magnetic properties of InCCo3 and InNCo3

Spin-polarized calculations were performed to investigate the structural, elastic, electronic, and magnetic properties of InCCo₃ and InNCo₃. The deviation of our calculated lattice parameters and equilibrium volume from experimental results is less than 0.8% and 2.5%, respectively. The obtained values of elasticity moduli C_ij, bulk modulus B, and shear modulus G are discussed. From the calculated band structure and the total and partial densities of states, we have concluded that these compounds are electrical conductors; moreover, they are bonded by a mixture of covalent, ionic, and metallic bonds. Our calculations show that InCCo₃ has nonmagnetic properties, while InNCo₃ could have a magnetic behaviour, with an average magnetic moment about 0.54 μ_B/atom, which is mostly derived from d electrons of the cobalt atoms in the energy range from −5 eV up to the Fermi level.     

Band structure and high pressure study of Rh3Sc, Rh3Y and Rh3La

The electronic structure of the Rhodium based intermetallic compounds (A₃B) such as Rh₃Sc, Rh₃Y and Rh₃La are studied by the Self Consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) method. In the present work, an attempt has been made to understand why the compounds namely Rh₃Y and Rh₃La crystallize in hexagonal structure, rather than the cubic structure, where as some of the similar rhodium based A₃B compounds namely Rh₃Ti, Rh₃Zr, Rh₃Hf, Rh₃V, Rh₃Nb, Rh₃Ta and Rh₃Sc are found to stabilize in cubic structure. In this work a prediction has been made about the structural phase transition in Rh₃Y and Rh₃La, from Hexagonal phase to Cubic phase. A report of the lattice constant, bulk moduli, cohesive energy and electronic specific heat coefficient is made and is compared with the available experimental data. Band structure and density of states histograms are also plotted. An electronic topological transition is predicted in Rh₃La, which may lead to the changes in the Fermi surface topology and hence changes the physical properties of Rh₃La.     

Surface states of charge carriers in epitaxial films of the topological insulator Bi2Te3

The galvanomagnetic properties of p-type bismuth telluride heteroepitaxial films grown by the hot wall epitaxy method on oriented muscovite mica substrates have been investigated. Quantum oscillations of the magnetoresistance associated with surface electronic states in three-dimensional topological insulators have been studied in strong magnetic fields ranging from 6 to 14 T at low temperatures. The cyclotron effective mass, charge carrier mobility, and parameters of the Fermi surface have been determined based on the results of analyzing the magnetoresistance oscillations. The dependences of the cross-sectional area of the Fermi surface S(k _F), the wave vector k _F, and the surface concentration of charge carriers n _s on the frequency of magnetoresistance oscillations in p-type Bi₂Te₃ heteroepitaxial films have been obtained. The experimentally observed shift of the Landau level index is consistent with the value of the Berry phase, which is characteristic of topological surface states of Dirac fermions in the films. The properties of topological surface states of charge carriers in p-type Bi₂Te₃ films obtained by analyzing the magnetoresistance oscillations significantly expand fields of practical application and stimulate the investigation of transport properties of chalcogenide films.