Interaction Of Conduction Electrons Research Articles

METAL-INSULATOR TRANSITION OF NaxWO3 STUDIED BY ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY

The electronic structure of sodium tungsten bronzes Na x WO 3 is investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES). The ARPES spectra measured in both insulating and metallic phases of Na x WO 3 reveals the origin of metal-insulator transition (MIT) in sodium tungsten bronze system. It is found that in insulating Na x WO 3 the states near the Fermi level (EF) are localized due to the strong disorder caused by the random distribution of Na + ions in WO 3 lattice. Due to the presence of disorder and long-range Coulomb interaction of conduction electrons, a soft Coulomb gap arises, where the density of states vanishes exactly at EF. In the metallic regime the states near EF are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). Volume of electron-like Fermi surface (FS) at the Γ(X) point of the Brillouin zone gradually increases with increasing Na concentration due to W 5d t2g band filling. A rigid shift of the Fermi energy is found to give a qualitatively good description of the Fermi surface evolution. As we move from bulk-sensitive to more surface sensitive photon energy, we found the emergence of Fermi surfaces at X(M) and M(R) points similar to the one at the Γ(X) point in the metallic regime, suggesting that the reconstruction of surface was due to rotation/deformation of WO 6 octahedra.

Effect of localized charge states on the low-frequency part of the tunneling current spectrum (1/f α )

The dependence of the low-frequency part of the tunneling current spectra (1/f α ) above a clean surface and above isolated impurity atoms on the InAs(110) cleaved surface has been investigated by scanning tunneling microscopy/spectroscopy in high vacuum. A theoretical model is proposed to explain the experimental results, which takes into account the many-body interaction of conduction electrons (with suddenly switched on and off Coulomb interaction on an impurity atom and on the scanning tunneling microscope tip) with the continuous-spectrum states in the tunneling contact leads.

Tuning of tunneling current noise spectra singularities by localized states charging

We report the results of theoretical investigations of tunneling current noise spectra in a wide range of applied bias voltage. Localized states of individual impurity atoms play an important role in tunneling current noise formation. It was found that switching “on” and “off” of Coulomb interaction of conduction electrons with two charged localized states results in power law singularity of low-frequency tunneling current noise spectrum (1/f α) and also results on high frequency component of tunneling current spectra (singular peaks appear).

The influence of localized states charging on [formula omitted] tunneling current noise spectrum

We report the results of theoretical investigations of low frequency tunneling current noise spectra component ( 1 / f α ). Localized states of individual impurity atoms play a key role in low frequency tunneling current noise formation. It is found that switching “on” and “off” of Coulomb interaction of conduction electrons with one or two charged localized states results in power law singularity of low-frequency tunneling current noise spectrum 1 / f α . Power law exponent in different low frequency ranges depends on the relative values of Coulomb interaction of conduction electrons with different charged impurities.

微小バンド・ギャップ半導体の相転移 : 格子不安定性と電子-格子相互作用

微小バンド・ギャップ半導体の相転移 : 格子不安定性と電子-格子相互作用

Density of electronic states in gold nanoclusters pulsed laser deposited on HOPG

The results of the experimental investigation of tunneling current–voltage characteristics of Au nanoclusters pulsed laser deposited on highly oriented pyrolytic graphite are presented. It is demonstrated that the observed size dependence of the tunneling conductance of Au nanoclusters can be caused by the change of the density of electronic states at the Fermi energy due to the interaction of conduction electrons with non-regular boundary of a cluster under investigation.

Electron Transport and Spin Scattering in Very Thin Disordered Metallic Films

We consider the electron transport through a very thin disordered metallic film doped with magnetic impurities. We treat the film as a quasi-two-dimensional system with structural disorder where some ions have spins and other are spinless. The interaction of conduction electrons with localized spins is described by means of the exchange term of the Hamiltonian. The scattering is treated in the first Born approximation and the potential is assumed to be the Coulomb screened one. The total effective cross-section is calculated as a sum of the part responsible for the potential scattering and the second part which comes from the spin–spin scattering. The Fermi sphere splits into separate sheets due to the finite size of the system in the z direction, therefore, the cross-section and the relaxation time are calculated for each sheet independently. The total transport relaxation time and the conductivity are obtained as functions of the thickness of the system and the contents of magnetic impurities. Some model calculations have been made for a thin disordered film of copper doped with manganese.

Electronic structure of sodium tungsten bronzesNaxWO3by high-resolution angle-resolved photoemission spectroscopy

The electronic structure of sodium tungsten bronzes, $Na_xWO_3$, for full range of x is investigated by high resolution angle-resolved photoemission spectroscopy (HR-ARPES). The experimentally determined valence band structure has been compared with the results of ab initio band-structure calculation. The HR-ARPES spectra taken in both the insulating and metallic phase of $Na_xWO_3$ reveal the origin of metal-insulator transition (MIT) in the sodium tungsten bronze system. In the insulating $Na_xWO_3$, the near-EF states are localized due to the strong disorder caused by the random distribution of $Na^+$ ions in $WO_3$ lattice. While the presence of an impurity band (level) induced by Na doping is often invoked to explain the insulating state found at low concentrations, there is no signature of impurity band (level) found from our results. Due to disorder and Anderson localization effect, there is a long-range Coulomb interaction of conduction electrons; as a result, the system is insulating. In the metallic regime, the states near $E_F$ are populated and the Fermi level shifts upward rigidly with increasing electron doping (x). The volume of electronlike Fermi surface (FS) at the $\Gamma (X)$ point gradually increases with increasing Na concentration due to $W 5dt_2_g$ band filling. A rigid shift of $E_F$ is found to give a qualitatively good description of the FS evolution.

Interface-phonon-limited two-dimensional mobility in AlGaN∕GaN heterostructures

The room temperature polar-optical-phonon-limited two-dimensional electron mobility in AlxGa1−xN∕GaN heterostructures is calculated taking into account the interaction of conduction electrons and interface-phonon modes. The polar optical oscillations are described via the uniaxial dielectric continuum model. Electron–polar-optical-phonon scattering rates are evaluated from a general expression that is always valid as long as the interaction Hamiltonian matrix elements depend only on the magnitude of the phonon wave vector. Values for the 300K low-field mobility (μ) of a few hundreds cm2∕Vs are obtained within a simplified relaxation time scheme involving electron-phonon absorption scattering rates. It is found that the way of describing the electronic states in the conduction band strongly affects the calculation of μ. The typical triangular well model gives the poorest results compared with a previously proposed analytical approximation of the conduction band potential profile. We present a discussion on the relevance of an appropriate model for long-wavelength polar optical phonons in the obtention of realistic values of the electron mobility in wurtzite heterostructures.

Anderson-Mott transition induced by hole doping inNd1−xTiO3

The insulator/metal transition induced by hole-doping due to neodymium vacancies of the Mott- Hubbard antiferromagnetic insulator, Nd1-xTiO3, is studied over the composition range 0.010(6) < x < 0.243(10). Insulating p-types conduction is found for x < 0.071(10). Anderson localization in the presence of a Mott-Hubbard gap, is the dominant localization mechanism for the range of 0.074(10) < x < 0.089(1) samples. For x < 0.089(1), n-type conduction is observed and the activation energy extrapolates to zero by x < 0.1. The 0.095(8) < x < 0.203(10) samples are Fermi-liquid metals and the effects of strong electronic correlations are evident near the metal-to-insulator boundaries in features such as large Fermi liquid T2 coefficients. For 0.074(9) < x < 0.112(4), a weak negative magnetoresistance is found below ~ 15 K and it is attributed to the interaction of conduction electrons with Nd3+ magnetic moments. Combining information from our companion study of the magnetic properties of Nd1-xTiO3 solid solution, a phase diagram is proposed. The main conclusions are that long range antiferromagnetic order disappears before the onset of metallic behavior and that the Anderson-Mott transition occurs over a finite range of doping levels. Our results differ from conclusions drawn from a similar study on the hole doped Nd1-xCaxTiO3 system which found the co-existence of antiferromagnetic order and metallic behavior and that the Mott transition occurs at a discrete doping level.

Conductance of metallic single-wall nanotubes with single magnetic impurities

Using a model of conducting cylinder with a few number of impurities on its surface, we investigate the effects of magnetic impurity scattering on the conductance of metallic single-wall carbon nanotubes. The nonlinear part of conductance, which is due to the interaction of conduction electrons with impurities, is obtained. The signature of Kondo anomaly is found in the nonlinear conductance and it is shown that its amplitude strongly depends on the position of impurities and diameter of nanotube.

Kondo effect in a metal with correlated conduction electrons: Diagrammatic approach

We study the low-temperature behavior of a magnetic impurity which is weakly coupled to correlated conduction electrons. To account for conduction electron interactions a diagrammatic approach in the frame of the 1/N expansion is developed. The method allows us to study various consequences of the conduction electron correlations for the ground state and the low-energy excitations. We analyse the characteristic energy scale in the limit of weak conduction electron interactions. Results are reported for static properties (impurity valence, charge susceptibility, magnetic susceptibility, and specific heat) in the low-temperature limit.

Ground-state of the single impurity Anderson model with correlated conduction electrons

Utilizing the nonperturbative Lanczos procedure, we study the ground-state spectrum of the Single-Impurity Anderson Model. An additional term is added to the Hamiltonian which represents a Coulomb-repulsion between the conduction electrons. The ground-state energy as well as the singlet-triplet energy is calculated in this strongly interacting system. Our results are consistent with those of Fermi-liquid theory which predicts a renormalization of the low energy properties of the system upon introduction of local, repulsive conduction electron interactions.

The effects of charge carriers on the magnetic behavior of ferromagnetic semiconductors within single-site approximation

Coherent potential approximation (CPA) is used to study the effects of charge carriers on the magnetic properties of ferromagnetic semiconductors. We have used a generalized mean-field theory for fluctuating f spins, and Vonsovskii model for interaction of conduction electrons with localized spins to calculate the edge of conduction band, the magnetization, the electron-spin polarization and the magnetic transition temperature. The results of our calculations are compared with the RKKY theory and the experimental results for Gd-doped EuO.

Deuterium NMR inYMn2Dxcompounds

The NMR investigations of ${\mathrm{YMn}}_{2}\mathrm{D}x (x=0.65,1.0,1.5,2.0,2.5)$ deuterides are presented and discussed. The observations of ${}^{2}\mathrm{D}$ spin echo signals were made for the samples in the paramagnetic state in the temperature ranges close above their magnetic ordering temperatures. The temperature dependencies of ${}^{2}\mathrm{D}$ resonance frequency, ${T}_{1}$ and ${T}_{2}$ relaxation times at the ordering temperature show a divergence which is dependent on the deuterium concentration. It is shown that the mechanisms of conduction electron interaction and of dipolar or orbital interactions are responsible for the observed relations between Knight shift and the susceptibility. The observed concentration dependent relaxation rates are explained on the basis of extended theory of spin fluctuations.

Electron spectrum, thermodynamics, and transport in antiferromagnetic metals at low temperatures

Electron spectrum of 2D and 3D antiferromagnetic metals is calculated with account of spin-fluctuation corrections within perturbation theory in the s-f exchange model. Effects of the interaction of conduction electrons with spin waves in thermodynamic and transport properties are investigated. At lowest temperatures $T<T^{*}\sim (\Delta /E_F)T_N$ ($\Delta $ is the AFM splitting of the electron spectrum) a Fermi-liquid behavior takes place, and non-analytic T^3lnT-contributions to specific heat are present for D=3. At the same time, for T>T^{*}, in 2D and ``nested'' 3D systems the picture corresponds to a marginal Fermi liquid (TlnT-contributions to specific heat and nearly T-linear dependence of resistivity). Frustrations in the spin system in the 3D case are demonstrated to lead to similar results. The Kondo contributions to electronic properties are analyzed and demonstrated to be strongly suppressed. The incoherent contributions to transport properties in the presence of impurity scattering are considered. In particular, in the 2D case T-linear terms in resistivity are present up to T=0, and thermoelectric power demonstrates the anomalous T\lnT-dependence.

Electron–quasiparticle interaction in DyNi 2B 2C measured by point-contact spectroscopy

The electron–quasiparticle interaction (EQI) spectral function has been measured for DyNi 2B 2C in the normal state at low temperatures by means of point-contact spectroscopy (PCS). A low-frequency peak is found around eV∼5 meV. It becomes measurable at T m*≃15 K and grows in intensity with constant width as the temperature is lowered. We argue that this peak arises from the strong interaction of conduction electron with coupled crystal–electric-field–phonon excitations whose branches cross at low energy. The comparison with PC spectra for HoNi 2B 2C suggests that a similar peak also exists for this compound. The magnitude of the point-contact EQI parameter λ PC for DyNi 2B 2C is estimated.

Phenomenological model for Casimir attraction of a metal film

The force of surface interaction (Casimir effect) between a bulk conductor and a metal film deposited on a dielectric substrate is studied by the method of quantum field theory. The film thickness is assumed to be much smaller than the skin depth at characteristic frequencies of fluctuation fields. The equations for one-particle Green’s function of the electromagnetic field in the metal film are solved on the basis of a simple phenomenological model. Namely, complex macroscopic electrodynamic properties of the film are described by introducing the surface permittivity determined by the conductivity 〈σ〉 averaged over sample thickness. The dependence of the Casimir attractive force on the specularity parameter ρ characterizing the interaction of conduction electrons with the film surface is predicted. The results of investigations demonstrate that the electronic and surface properties of metal films can in principle be studied experimentally by measuring the force of their Casimir attraction.

Magnetocaloric properties of the La 0.7− xY xSr 0.3MnO 3 giant magnetoresistance ceramics

Giant magnetoresistance (GMR), observed in several La 0.7 x A 0.3MnO 3 ceramics, where A is a two-valent addition, is the result of an interaction of conduction electrons and intrinsic magnetization, which is likely to result in unusual magnetocaloric effects. On this basis, magnetization, magnetic entropy change and adiabatic temperature change were measured as a function of Y content in La 0.7− x Y x Sr 0.3MnO 3 ceramics. It is found that doping influences only the transition temperature, while the magnetic entropy change remains constant and displays a saturation tendency in high fields.

Magnetic impurity in a metal with correlated conduction electrons: An infinite-dimensions approach

We consider the Hubbard model with a magnetic Anderson impurity coupled to a lattice site. In the case of infinite dimensions, one-particle correlations of the impurity electron are described by the effective Hamiltonian of the two-impurity system. One of the impurities interacts with a bath of free electrons and represents the Hubbard lattice, and the other is coupled to the first impurity by the bare hybridization interaction. A study of the effective two-impurity Hamiltonian in the frame of the 1/N expansion and for the case of a weak conduction-electron interaction (small U) reveals an enhancement of the usual exponential Kondo scale. However, an intermediate interaction (U/D = 1 - 3), treated by the variational principle, leads to the loss of the exponential scale. The Kondo temperature T_K of the effective two-impurity system is calculated as a function of the hybridization parameter and it is shown that T_K decreases with an increase of U. The non-Fermi-liquid character of the Kondo effect in the intermediate regime at the half filling is discussed.