Multi-band Conductor Research Articles

Amplitude mode of charge density wave in TTF[Ni(dmit)2]2 observed by electronic Raman scattering

We measured the optical signature of the charge density waves (CDWs) in the multiband conductor TTF[Ni(dmit)2]2 by electronic Raman scattering. At low energies, a hump develops below 60 K. This hump is associated to the amplitude mode of the CDW with an energy around 9 meV. Raman symmetry-resolved measurements show that the CDW amplitude mode is anisotropic and that the CDW can be associated to the band nesting of Ni(dmit)2 chains.

Slow In-Plane Magnetoresistance Oscillations in Multiband Quasi-Two-Dimensional Metals

Slow oscillations (SlO) of magnetoresistance is a convenient tool to measure electronic structure parameters in quasi-two-dimensional metals. We study the possibility to apply this method to multi-band conductors, e.g. to iron-based high-temperature superconducting materials. We show that SlO can be used to measure the interlayer transfer integral in multi-band conductors similar to single-band metals. In addition, the SlO allow to measure and compare the effective masses or the electron scattering rates in various bands.

Charge density wave and metallic state coexistence in the multiband conductorTTF[Ni(dmit)2]2

We have established a pressure-temperature phase diagram of $\mathrm{TTF}{[\mathrm{Ni}{(\mathrm{dmit})}_{2}]}_{2}$ based on spin susceptibility at ambient pressure as well as longitudinal and transverse resistivity measurements under pressure up to 30 kbar. The data were analyzed on the basis of first-principles density functional theory calculations. We were able to find several phase transitions and identify three different charge density wave (CDW) states which all coexist with a metallic state in a wide temperature range and superconductivity at the lowest temperatures. This metallicity arises from the development, upon cooling, of a two-dimensional band associated to the $\mathrm{Ni}{(\mathrm{dmit})}_{2}$ chains. At low pressure, two successive CDW transitions have been clearly identified and are associated to the successive nesting of two strongly one-dimensional bands: The LUMO and ${\mathrm{HOMO}}_{\mathrm{I}}$ of the $\mathrm{Ni}{(\mathrm{dmit})}_{2}$ chains. These two transitions merge into a single one at 12 kbar which probably corresponds to the partial nesting of a bunch of LUMOs into the ${\mathrm{HOMO}}_{\mathrm{I}}$'s. A maximum of this unique CDW transition temperature is observed at 19 kbar. The CDW instability associated to the LUMO band is announced by an important regime of Peierls-like fluctuations in the metallic state which give rise to the progressive development of a pseudogap in the spin susceptibility which has been quantitatively analyzed using the Lee-Rice-Anderson theory.

Dielectric properties of multiband electron systems II. Collective modes

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (q → 0), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collectivemodes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electron-hole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail.In conductors the intra-band charge fluctuations give rise to plasmons. If the atomic dipolar transitions are forbidden, the coupling of inter-band collective modes and plasmons tends to zero as q → 0. On the contrary, in dipolar conductors this coupling is strong and nonperturbative, due to the long range monopole-dipole interactions between intra-band and inter-band charge fluctuations. The resulting collective modes are hybrids of intra-band plasmons and inter-band dipolar oscillations. It is shown that the frequency of the lower hybridized longitudinal mode is proportional to the frequency of the transverse dipolar mode when the latter is small. The dielectric instability in a multi-band conductor is therefore characterized by the simultaneous softening of a transverse and a longitudinal mode, which is an important, directly measurable consequence of the present theory.

Transport properties of hexagonal and tetragonal MoSi2 thin films

The electrical resistivity and the Hall coefficient of thin polycrystalline MoSi2 films were measured as a function of temperature from 4.2 to 300 K. The transverse and longitudinal magnetoresistances have been determined at temperatures between 2 and 50 K at fields up to 10 T. Samples of both the high-temperature tetragonal MoSi2 phase and the low-temperature hexagonal phase have been prepared. The Hall coefficient of tetragonal MoSi2 changes sign with temperature, which means that this compound is a multiband conductor, and thus no reliable value for the carrier concentration can be derived from Hall measurements alone. Magnetoresistance data have therefore been analyzed with a simple two-band model in order to determine the dominant carrier type in both MoSi2 structures. In tetragonal MoSi2, electrons and holes are present in equal concentrations, whereas in hexagonal MoSi2 holes are the dominating type of carrier.

Galvano- and thermo-magnetic coefficients for a multi-band conductor

Galvano- and thermo-magnetic coefficients for a multi-band conductor

Galvano- and thermo-magnetic coefficients for a multi-band conductor

The reasons for the discrepancies between the expressions for the Righi-Leduc effect derived by Putley (1960) and by Honig and Harman (1963) are discussed. To obtain a correct result, the lattice thermal conductivity must be properly included in the calculations. Differences, attributed to sign convections, between the two formulations are discussed. Expressions for the principal transverse galvano- and thermo-magnetic coefficients for an isotropic multi-band conductor are tabulated.

High-Field Hall Coefficient in a Compensated, Multiband Conductor

The theory of the magnetic-field dependence of the Hall coefficient in a simple, multiband model is reviewed. The complete expression rapidly becomes more complicated as the number of bands increases. Simple limiting forms, valid for any number of bands, have been derived earlier for weak magnetic fields, and for the high-field limit when there is no compensation (i.e., when the electron and hole concentrations are not equal). It is shown that a simple result also occurs in the high-field limit, with compensation, for any number of bands.

Interband Electron-Electron Scattering and Transport Phenomena in Semiconductors

The effect of interband electron-electron scattering (electron-hole scattering, light hole-heavy hole scattering, etc.) on the electrical transport phenomena is studied with a variational method obtained by a generalization of Kohler's variation principle to a multiband conductor. To this end we make the following assumptions: (1) The electronic structure is given by parabolic conduction and valence bands, separated from each other by $\ensuremath{\Delta}E\ensuremath{\gg}{k}_{B}T$; the valence band may be twofold degenerate; (2) The average occupation numbers of electronic eigenstates are given by Fermi-Dirac statistics; (3) The dynamical interaction between charge carriers is described by a shielded Coulomb potential.Assuming nondegenerate semiconductors, we consider acoustical and optical phonon scattering and ion scattering, besides electron-electron scattering. Quantitative results are obtained for the electrical conductivity, the heat conductivity, and the Seebeck coefficient, including the ambipolar effect. The results can easily be applied to cases of physical interest; we discuss here hole-hole scattering and mobility of $p$ germanium, intercarrier scattering and mobility of intrinsic germanium, transient conductivity of charge carriers in germanium produced by short pulses of high-energy electrons, intercarrier scattering and its influence on the heat conductivity, and the Wiedemann-Franz ratio of intrinsic semiconductors.