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.     

Non-uniform doping across the Fermi surface of NbS<Subscript>2</Subscript> intercalates

Magnetic ordering of the first row transition metal intercalates of NbS₂ due to coupling between the conduction electrons and the intercalated ions has been explained in terms of Fermi surface nesting. We use angle-resolved photoelectron spectroscopy to investigate the Fermi surface topology and the valence band structure of the quasi-two-dimensional layer compounds Mn_1/3NbS₂ and Ni_1/3NbS₂. Charge transfer from the intercalant species to the host layer leads to non-uniform, pocket selective doping of the Fermi surface. The implication of our results on the nesting properties are discussed.     

Determination of the g-factor of electrons in N-type silicon surface inversion layers

The g-factor of conduction electrons in the surface inversion layer on a silicon (100) surface has been determined using the tilted magnetic field method developed by Fang and Stiles.The value of (m¹/m₀)·g at the fixed magnetic field was independent of surface carrier density n_s, whereas it had a sharp peak at about 97 koe. At strong magnetic field limit the value was constant and 0.4. If we take the effective mass of conduction electrons in the inversion layer on the (100) surface as 0.2m₀, the g-factor is about two which is the same as that for conduction electrons in bulk silicon.     

Nuclear Quadrupole spin-lattice relaxation in Rhombohedral Arsenic

The temperature dependence of the spin-lattice relaxation time T₁ in rhombohedral arsenic has been measured by nuclear quadrupole resonance. The relaxation time is inversely proportional to the temperature and of a magnitude which indicates that the relaxation results from the Fermi contact interaction of the conduction electrons and holes and the arsenic nuclei. The density of electrons and holes at the site of the nucleus, averaged over the Fermi surface is approximately 2.6 × 10²¹ carriers cm^?3.     

Relations between the Ruderman-Kittel-Kasuya-Yosida interaction electron concentration and crystal structure

The Ruderman-Kittel-Kasuya-Yosida interaction between localized magnetic spins on the basis of perfectly free conduction electron is calculated as a function of the Fermi wave vector of the conduction electrons. The expression of the interaction according to us converges rapidly, furthermore is strongly related to the detailed crystal structure. The q-dependence of exchange interaction j(q) between conduction- and localized magnetic electron is also taken into account. The result of the calculation is compared successfully with the change of magnetic structures, with θ_p and T_c or T_N of compounds having the CsCl structure, but less successfully with gadolinium compounds having the NaCl structure. These facts are discussed in conjunction with the difference of the RKKY interaction in the cscl and the NaCl structure.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Theory of the anisotropic magnetoresistance in copper

The motion of the guiding center of magnetic circulation generates a charge transport. The application of kinetic theory to the motion gives a modified Drude formula for the magnetoconductivity: σ=e²n_cτ/M^*, where M^? is the magnetotransport mass distinct from the cyclotron mass, n_c the density of the conduction electrons, and τ the relaxation time. The density n_c depends on the applied magnetic field direction relative to copper's face-centered-cubic lattice, when the Fermi surface of copper is nonspherical with necks. The anisotropic magnetoresistance of copper is calculated with the assumption of the necks representing by spheres of radius a centered at the eight singular points on the ideal Fermi surface. A good fit with experiments is obtained.     

Mossbauer effect measurements in the dysprosium-silicon and dysprosium-gallium equiatomic compounds

Mössbauer effect measurements have been performed in dysprosium-silicon and dysprosium-gallium equiatomic compounds. The experimental results were compared with crystal field calculations based on a point charge model. The calculated second order crystal field parameters were found to be predominant. The magnetic moment of the ground state is highly anisotropic and its high value (9.8 μ_B) is in good agreement with the observed magnetic hyperfine field which is similar for both the pure and the diluted compounds.The same magnitude of the quadrupolar interactions in these compounds as well as in the Dy metal may be attributed to the predominant contribution of the 4f electrons.     

Phase transitions in a ferromagnetic semiconductor. III

We investigate the ferromagnetic phase transition in a heavily doped semiconductor or semimetal containing magnetic ions. The coupling between conduction electrons and magnetic ions is treated in the molecular field approximation. We show that the system undergoes a transition to an ordered state for arbitrarily small concentrations of the magnetic ions and the conduction electrons. In the limitJ/? _F?1 we obtain the Ruderman-Kittel result for the Curie temperature, and forJ/? _F?1 a generalization of Thompson’s strong coupling result. (J is the exchange coupling constant between conduction electrons and magnetic ions and? _F is the kinetic Fermi energy at absolute zero.) The intermediate range is evaluated numerically.     

s-f exchange interaction in metallic GdxY1−xS and GdxLa1−xS

We compare ESR spectra of insulating EuS with those of metallic GdS. ESR investigations of the diluted metallic compounds Gd_xY_1?xS and Gd_xLa_1?xS prove the existence of an exchange interaction between conduction electrons and localized 4f moments in these sulphides. Since a bottleneck occurs, we attribute this interaction to conduction electrons with s character at the Fermi level.     

Electronic structures of the LaBO3 (B = Co, Fe, Al) perovskite oxides related to their catalysis

The discrete-variational (DV) X α molecular orbital method has been applied to investigate the bulk and surface electronic structures of perovskite oxides such as LaCoO₃, LaFeO₃ and LaAlO₃. The calculated XPS spectra for these oxides are in good agreement with the experimental ones and only the LaCoO₂ exhibited a rather high electron density of states near the Fermi level (E_F). The catalytic behavior of these oxides is discussed on the basis of their electronic structures; the marked catalysis by LaCoO₃ is associated with electron occupation of crystal field d states near E_F and with the buildup of surface charge so as to enhance the electron transfer between a surface cation and an interacting molecule.     

Pressure induced phase transition in rare earth mono-antimonides

Pressure induced structural phase transition of mono-antimonides of lanthanum, cerium, praseodymium and neodymium (LnSb, Ln=La, Ce, Pr and Nd) has been studied theoretically using an inter-ionic potential with modified ionic charge which parametrically includes the effect of Coulomb screening by the delocalized f electrons of rare earth (RE) ion. The anomalous structural properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of Ln ion with the p orbital of neighbouring antimonide ion. All the four compounds are found to undergo from their initial NaCl (B₁) phase to body centered tetragonal (BCT) phase at high pressure and agree well with the experimental results. The body centered tetragonal phase is viewed as distorted CsCl structure and is highly anisotropic with c/a=0.82. The transition pressure of LnSb compounds is observed to increase with decreasing lattice constant in NaCl phase. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-Sb) distances at high pressure. The calculated values of elastic constants are also reported.     

Kinetics of the conduction electrons in multidomain ferromagnets

The kinetic properties of conduction electrons in ferromagnets with periodical domain structure are studied. The appearance in the problem of the new parameter of length dimension - the period of domain structure 2d allows one to distinguish three groups of electrons whose trajectories qualitatively differ from those in a case of a homogeneous magnetic field. It has been shown that the absorption of high frequency field is of a resonance type. The resonance frequencies are simply connected with a period of the domain structure (ω_res = π (V_F/d), V_F Fermi velocity). The resonance curve shape is obtained.     

Dependence of electrical and magnetic properties of the AuCu-1 alloy on its atomic ordering

Electrical resistivity and magnetic susceptibility studies have shown that atomic ordering in theα ₁ phase of the AuCu-1 alloy is accompanied by a decrease in the Fermi energy, in the Debye characteristic temperature, and in the Young modulus. The behavior of conduction electrons in the ordered AuCu-1 alloy is appreciably affected by the interaction of such electrons with ions in the crystal lattice.     

Thermal dissociation of intercalated titanium selenides Fe x TiSe2 (x = 0.10, 0.25, 0.50)

Using static tensimetry, the selenium pressure during dissociation of intercalated compounds Fe _x TiSe₂ has been measured in a temperature range of stability of the homogeneous material and in the region of decomposition caused by thermal expansion of the polaron band. It has been shown that covalent centers in the stability region of the homogeneous state, which are stabilized by the polaron state of conduction electrons, behave as the effective oxidants lowering the Fermi level. Broadening of the polaron band in the region of a high iron concentration leads to weakening the oxidizing influence of polarons. Iron selenides are formed during the decomposition of intercalated compounds, which leads to an essential non-quasi-binarity of the Fe-TiSe₂ system.     

High-pressure behaviour and elastic properties of heavy rare-earth Gd monopnictides

Pressure-induced structural phase transition of gadolinium monopnictides GdX (X=As and Sb) has been studied theoretically using an inter-ionic potential theory. This method has been found quite satisfactory in case of the pnictides of rare-earth and describes the crystal properties in the framework of rigid-ion model. We have modified the ionic charge so that it may include the Coulomb-screening effect by the delocalization of f electron of the rare-earth ion. The anomalous structural properties of these compounds with many f electrons have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of Gd ion with the p orbital of neighbouring pnictogen ion. Both the compounds are found to undergo from their initial NaCl (B₁) structure to body centered tetragonal (BCT) structure at high pressure and agree well with the experimental results. The BCT structure is viewed as distorted CsCl structure and is highly anisotropic with c/a=0.82–0.85. The nature of bonds between the ions is predicted by simulating the ion–ion (Gd–Gd and Gd–X) distance at high pressure. Elastic properties of these compounds have also been studied with their second-order elastic constants.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

Anomalous thermoelectric power of rare earth metals and their compounds

We propose a new mechanism for an anomalous thermoelectric power (ATP) in the paramagnetic state of certain rare earth metals and their compounds, in which the ions possess a nonmagnetic ground state in a given crystal field. The ATP is found to be due to higher order inelastic scattering (second Born approximation) of the conduction electrons by the crystal field split rare earth ions. It has a peak at a temperatureΔ/3 ~Δ/2, whereΔ is the splitting energy between the ground state and the first excited state. Our main result is that the appearance of an ATP requires interactions between the conduction electrons and the ions of other than the simple isotropic exchange type. This implies that the ATP may serve as a valuable tool to detect more complicated types of thek-f interaction than the isotropic exchange.     

Fermi surface measurement of ZrB2 by the de Haas-van Alphen effect

The de Haas-van Alphen (dHvA) effect in ZrB₂ single crystal has been studied using the field modulation technique. Observed dHvA frequencies in the (10$\text{1}$0), (11$\text{2}$0) and (0001) planes range from the order of 10⁶-10⁷ G. The dHvA frequencies, together with a recent APW band structure calculation, show that the Fermi surface of ZrB₂ consists of a set of ring-like electron surfaces around the K points on which the nearly ellipsoidal arms are joined together, and a wrinkled dumbbell-like hole surface at the A point. Estimation of the carrier concentration indicates that ZrB₂ is a semi-metal with 0.04 electrons (holes) per unit cell.     

Study of density of localized states in Cd4Se96−xSx (x=0, 4, 8, 12) chalcogenide semiconductor

Thin films of Cd₄Se_96−xS_x (x=0, 4, 8, 12) chalcogenide semiconductor were deposited by the thermal evaporation technique on glass substrates. XRD pattern of CdSeS alloys show that the grain size decreases with the concentration of Sulfur (S). The surface morphology changes due to the addition of sulfur content. The effect of sulfur on the DC conductivity has been investigated, which show that the DC conductivity is a thermally activated process. The Mott parameter shows that dominate conduction is in the localized states, also the addition of sulfur in Cd–Se results an increase in electrical conductivity, which may be due to shift of Fermi level. Current–voltage (I–V) measurements at different fixed temperatures show two regions; Ohmic conduction at low bias having a unit slope, and non ohmic conduction at high bias. Observation of the data shows that conduction is dominated by trap limited space charge limited conduction (SCLC), from where the density of state has been calculated using SCLC measurement data. The increase in the density of states with sulfur concentration may be due to the increase in the defect states.