Conductance and shot noise of inelastic tunneling through a quantum dot in the Kondo regime

We investigate the inelastic transport properties of a quantum dot connected to two leads, based on the combination of a recently developed nonperturbative technique and slave-boson methods involving the approximate mapping of the many-body electron–phonon coupling problem onto a multichannel scattering problem in the Kondo regime. The nonequilibrium Green's function method is adopted in calculations for the inelastic transport processes of electrons in the limit of large Coulomb interaction U→∞$U\to \infty$ under nonequilibrium conditions. The electron–phonon interactions, which are the main source of the inelasticity, are taken into account. For a single quantum dot, we find that the differential conductance and the shot noise exhibit new structures of peaks and dips which are absent in the case without electron–phonon interactions.     

Electron mobility variance in the presence of an electric field: Electron–phonon field-induced tunnel scattering

The problem of electron mobility variance is discussed. It is established that in equilibrium semiconductors the mobility variance is infinite. It is revealed that the cause of the mobility variance infinity is the threshold of phonon emission. The electron–phonon interaction theory in the presence of an electric field is developed. A new mechanism of electron scattering, called electron–phonon field-induced tunnel (FIT) scattering, is observed. The effect of the electron–phonon FIT scattering is explained in terms of penetration of the electron wave function into the semiconductor band gap in the presence of an electric field. New and more general expressions for the electron–non-polar optical phonon scattering probability and relaxation time are obtained. The results show that FIT transitions have principle meaning for the mobility fluctuation theory: mobility variance becomes finite.     

Polaron effects on the third-harmonic generations in a two-dimensional quantum pseudodot system

The third-harmonic generations (THG) considering polaron effects in the two-dimensional quantum pseudodot system with a uniform magnetic field are theoretically studied. It is found that the THG coefficient is strongly affected not only by an external magnetic field, but also by the geometrical size of the pseudodot system. Moreover, the theoretical values of the THG is obviously increased when considering the electron–LO–phonon interaction.     

Size dependence of surface optical mode and electron–phonon coupling in ZnO nanocombs

ZnO nanocombs with different sizes are synthesized by simple thermal evaporation methods. Scanning electron microscopy and transmission election microscopy testify the growth of single crystal ZnO nanocombs along [0 0 0 2] direction. The temperature-dependent Raman spectra show that the intensity of surface optical (SO) modes in ZnO nanocombs obviously increases with declining measure temperatures. With the decrease of diameters, the frequency of SO modes shows a blue shift due to the passivation of surface states. The resonant Raman scattering shows that the strength of electron–phonon coupling increases with decreasing size. Calculated on size-dependent electron–phonon interaction energy agrees well with measured values for a large size range. The origin of electron–phonon coupling in ZnO nanocombs is also discussed.     

Temperature dependence of electron transport in GaN/AlGaN quantum cascade detectors

In this work we investigate the influence of extractor design and temperature on transport properties of quantum cascade detector. For this purpose we realize numerical calculation of electron lifetimes considering electron–phonon and electron impurities scattering. Electron–phonon interactions are treated using Fermi Golden Rule which allows to calculate lifetime of carriers with temperature and structure design taking into account. Transport characteristics of the quantum cascade detectors have been computed using density matrix theory. As a result, we have obtained the system of ordinary differential equations describing dynamics of electron distribution functions and intersubband correlations. Managing carrier lifetime in quantum wells gives us possibility to make device response faster. Also carrier lifetime is the relevant characteristic, allows us to calculate a lot of parameters such as quantum efficiency and photocurrent.     

Polaronic effects due to surface optical vibration modes in a freestanding cylindrical wurtzite nanowire

The ground-state polaron self-trapped energy and effective mass due to the surface optical (SO) phonon modes in a freestanding wurtzite GaN nanowire (NW) were studied by means of the Lee–Low–Pines variational approach. Based on the dielectric continuum and Loudon’s uniaxial crystal models, the polar optical phonon modes in the one-dimensional (1D) systems are analyzed, and the vibrating spectra of SO modes and electron–SO phonon coupling functions are discussed and analyzed. The calculations on the ground-state polaron self-trapped energy and correction of effective mass due to the SO phonon modes in the 1D GaN NWs reveal that the polaron self-trapped energy and correction of effective mass are far larger than those in 1D GaAs NW systems. The reasons resulting in this obvious difference in the two 1D structures are mainly due to the different electron–phonon coupling constants and electron effective masses of bulk materials constituting the two types of 1D confined system. Finally, the polaronic properties of the wurtzite 1D GaN NWs have been compared with those of the wurtzite GaN-based two-dimensional quantum wells. The physical origination of these characteristics and their distinction in the different-dimensionality systems has been analyzed in depth.     

First-order Raman scattering in cylindrical wurtzite nanowire

We have presented a theoretical study on electron resonant Raman scattering (ERRS) process associated with the bulk longitudinal optical (LO), surface optical (SO) and quasi-confined (QC) phonon modes in a free-standing wurtzite nanowire (NW). We consider the Fröhlich electron–phonon interaction in the framework of the dielectric continuum model. Numerical calculations on the GaN material reveal that differential cross-section (DCS) is sensitive to the wire size. The bulk LO and high-frequency quasi-confined (QC₊) phonons make main contributions to the DCS and the impact of the SO phonon can be negligible in the ERRS process. Moreover, scattering intensity of the bulk LO phonon is strongly enhanced as the incident photon energy approaches the energy band-gap of the GaN.     

Influence of the quantum dot shape on the determination of the electronic structure and electron decoherence

Theoretical calculations of electron–phonon scattering rates in AlGaN/GaN quantum dots (QDs) have been performed by means of effective mass approximation in the frame of finite element method. The influence of a symmetry breaking of the carrier's wave function on the electron dephasing time is investigated for various QDs shapes. In a QD system the electron energy increases when the QD shape changes from a spherical to a non-spherical form. In addition, the influence of the QD shape upon the electronic structure can be modulated by external magnetic fields. We also show that the electron–acoustic phonon scattering rates strongly depend upon both the QD shape and the applied magnetic field. As an additional parameter, the QD shape can be used to modify the electron–acoustic phonon interaction in a wide range. Moreover, the scattering rate of different transitions, such as Δm=0(1), presents distinct magnetic field dependency.     

Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic perovskites Ba_0.6K_0.4BiO₃using the strong coupling theory. An interaction potential which includes the Coulomb, electron–optical phonon and electron–plasmon interactions is developed to elucidate the superconducting state. A model dielectric function is constructed with these interactions fulfilling thef-sum rule. The screening parameter (μ^* = 0.26) infers the poor screening of charge carriers. The electron–optical phonon strength (λ) estimated as 0.98 is consistent with an attractive electron–electron interaction and supports the moderate to strong coupling theory. The superconducting transition temperature of Ba_0.6K_0.4BiO₃is then estimated as 32 K. Ziman's formula of resistivity is employed to analyse and compare this with the temperature-dependent resistivity of a single crystal. The estimated contribution from the electron–optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental data. The subtracted data infer a quadratic temperature dependence in the temperature domain (30 ≤ T ≤ 200 K). The quadratic temperature dependence of ρ [ = ρ_exp − (ρ₀ + ρ_e–ph)] is understood in terms of 3D electron–electron inelastic scattering. The presence of these el–el and el–ph interactions allows a coherent interpretation of the physical properties. Analysis reveals that a moderate to strong coupling exists in the Ba_0.6K_0.4BiO₃system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed.     

Influence of Ce doping on electrical and thermal properties of La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La_0.7−xCe_xCa_0.3MnO₃ (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T_C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (T_MS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.     

Anomalous dephasing of two-dimensional electrons in an AlGaAs/GaAs parabolic quantum well structure

Magnetoconductivity measurements are performed on a parabolic quantum well structure. The weak localization effect is observed at a low magnetic field for both single-subband and double-subband occupation regimes. Applying weak-localization theory, we have extracted the dephasing rate. The extracted dephasing rate increases with increasing conductivity in the small-energy-transfer regime and shows a similar trend as the electron density is increased in the large-energy-transfer regime. This is in conflict with Fermi-liquid theory, and cannot be attributed to electron–phonon scattering.     

Phonon coupling effect upon transport in nanoscale polymers

In an environmental coupled polymer, a variation of the conductivity is evaluated, which results from the external electron–phonon interaction coupling with the internal one. A quantized current appears under the external phonon coupling. The resonant tunnelling in the nanoscale polymer driven by the internal electron–phonon interaction is enhanced by the external phonon coupling. In addition, the external electron–phonon interaction softens the stiffness of the polymer.     

E–mv coupling of vibrational overtone in organic conductors: Relationship to optical nonlinearities and ferroelectricity

A strong electron–phonon coupling effect, referred to as electron–molecular vibration (e–mv) coupling, induces characteristic vibronic signals in the vibrational spectra of organic conductors. This paper discusses a new spectroscopic signal induced by the e–mv coupling effect, and the physical implications of its emergence. This vibronic signal, attributable to an overtone of a molecular vibration, appears with an anti-resonance form in the infrared spectrum of some mixed-valency complexes when the compounds undergo a charge-ordering transition. Based on the results of cluster model calculations, the activation of the overtone signal is interpreted as an indication of the generation of anharmonicity in the electronic potential. This analysis suggests that the series of the complexes should possess potential nonlinear optical abilities, though these compounds have long been viewed as electric conductors having the opposite characteristics of dielectrics that show nonlinear optical properties.     

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

Intervalley scattering in GaAs: ab initio calculation of the effective parameters for Monte Carlo simulations

Interactions between excited electrons and short-wavelength (intervalley) phonons in GaAs are studied using density functional theory for the conduction bands, and density functional perturbation theory for phonon frequencies and matrix elements of the electron–phonon interaction. We have calculated the deformation potentials (DPs) and the average intervalley scattering time 〈τ〉. The integration of the scattering probabilities over all possible final states in the Brillouin zone has been performed without any ad hoc assumption about the behavior of the electron–phonon matrix elements nor the topology of the conduction band. For transitions from the L point to Γ valley (within the first conduction band), we find 〈τ〉_L to be 1.5 ps at 300 K, in good agreement with time-resolved photoluminescence experiment. We discuss the difference between our calculated DPs, and effective parameters used in Monte Carlo simulations of optical and transport properties of semiconductors. The latter are based on Conwell’s model, in which electron–phonon interaction is described by one single constant and a parabolic model is used for conduction bands. We deduce the effective DP from our 〈τ〉, and compare it to our calculated DPs. We conclude that only effective DPs obtained from a full calculation of 〈τ〉 are relevant parameters for Monte Carlo simulations. PACS 71.10-w; 72.10.Di; 71.55.Eq     

Decoherence in elastic and polaronic transport via discrete quantum states

In this work we study the effect of decoherence on elastic and polaronic transport via discrete quantum states. Calculations are performed with the help of a nonperturbative computational scheme, based on Green’s function theory within the framework of polaron transformation (GFT-PT), where the many-body electron-phonon interaction problem is mapped exactly into a single-electron multi-channel scattering problem. In particular, the influence of dephasing and relaxation processes on the shape of the electrical current and shot noise curves is discussed in detail under linear and nonlinear transport conditions.     

Phonon heat transport in gallium arsenide

The lifetimes of quantum excitations are directly related to the electron and phonon energy linewidths of a particular scattering event. Using the versatile double time thermodynamic Green’s function approach based on many-body theory, an ab-initio formulation of relaxation times of various contributing processes has been investigated with newer understanding in terms of the linewidths of electrons and phonons. The energy linewidth is found to be an extremely sensitive quantity in the transport phenomena of crystalline solids as a collection of large number of scattering processes, namely, boundary scattering, impurity scattering, multiphonon scattering, interference scattering, electron–phonon processes and resonance scattering. The lattice thermal conductivities of three samples of GaAs have been analysed on the basis of modified Callaway model and a fairly good agreement between theory and experimental observations has been reported.     

Electron–phonon interaction in fan-shaped quantum dot and quantum wire

The optical phonon modes and electron–optical-phonon interaction in fan-shaped quantum dot and quantum wire are studied with the dielectric continuum (DC) model and separation of variables. The explicit expressions for the longitudinal optical (LO) and interface optical (IO) phonon eigenmodes are deduced. It is found that there exist two types of IO phonon modes: top interface optical (TIO) phonon mode and arc interface optical (AIO) phonon mode, in a fan-shaped quantum dot. After having quantized the eigenmodes, we derive the Hamiltonian operators describing the LO and IO phonon modes as well as the corresponding Fröhlich electron–phonon interaction. The potential applications of these results are also discussed.     

Charge-carrier dynamics in single-wall carbon nanotube bundles: a time-domain study

We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron–electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level – the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron–phonon interaction occurs on the picosecond time scale and allows us to determine the electron–phonon mass-enhancement parameter λ. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper. Received: 4 March 2002 / Accepted: 7 March 2002 / Published online: 3 June 2002