Phonon scattering limited performance of monolayer MoS2 and WSe2 n-MOSFET

In this paper we show the effect of electron-phonon scattering on the performance of monolayer (1L) MoS2 and WSe2 channel based n-MOSFETs. Electronic properties of the channel materials are evaluated using the local density approximation (LDA) in density functional theory (DFT). For phonon dispersion we employ the small displacement / frozen phonon calculations in DFT. Thereafter using the non-equilibrium Green's function (NEGF) formalism, we study the effect of electron-phonon scattering and the contribution of various phonon modes on the performance of such devices. It is found that the performance of the WSe2 device is less impacted by phonon scattering, showing a ballisticity of 83% for 1L-WSe2 FET for channel length of 10 nm. Though 1L-MoS2 FET of similar dimension shows a lesser ballisticity of 75%. Also in the presence of scattering there exist a a 21-36% increase in the intrinsic delay time (tau) and a 10-18% reduction in peak transconductance (g(m)) for WSe2 and MoS2 devices respectively. (C) 2015 Author(s).

Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).

Phonon-limited mobility in two-dimensional semiconductors with spin-orbit coupling

The authors demonstrate that the Rashba spin-orbit interaction in low-dimensional semiconductors can enhance or reduce the electron-phonon scattering rate by as much as 25%. The underlying mechanism is that the electron-phonon scattering phase space for the upper (lower) Rashba band is significantly enhanced (suppressed) by the spin-orbit interaction. While the scattering time decreases for the upper level, the mobility of the level increases due to an additional term in the electron velocity. (C) 2007 American Institute of Physics.

Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).

Correlated electron-ion dynamics: the excitation of atomic motion by energetic electrons

Correlated electron-ion dynamics (CEID) is an extension of molecular dynamics that allows us to introduce in a correct manner the exchange of energy between electrons and ions. The formalism is based on a systematic approximation: small amplitude moment expansion. This formalism is extended here to include the explicit quantum spread of the ions and a generalization of the Hartree-Fock approximation for incoherent sums of Slater determinants. We demonstrate that the resultant dynamical equations reproduce analytically the selection rules for inelastic electron-phonon scattering from perturbation theory, which control the mutually driven excitations of the two interacting subsystems. We then use CEID to make direct numerical simulations of inelastic current-voltage spectroscopy in atomic wires, and to exhibit the crossover from ionic cooling to heating as a function of the relative degree of excitation of the electronic and ionic subsystems.

Inelastic quantum transport in nanostructures: The self-consistent Born approximation and correlated electron-ion dynamics

A dynamical method for inelastic transport simulations in nanostructures is compared to a steady-state method based on nonequilibrium Green's functions. A simplified form of the dynamical method produces, in the steady state in the weak-coupling limit, effective self-energies analogous to those in the Born approximation due to electron-phonon coupling. The two methods are then compared numerically on a resonant system consisting of a linear trimer weakly embedded between metal electrodes. This system exhibits an enhanced heating at high biases and long phonon equilibration times. Despite the differences in their formulation, the static and dynamical methods capture local current-induced heating and inelastic corrections to the current with good agreement over a wide range of conditions, except in the limit of very high vibrational excitations where differences begin to emerge.

Extracting a more realistic pseudopotential for aluminum, lead, niobium and tantalum from superconductor electron tunnelling spectroscopy data

Electron tunnelling spectroscopy, developed to extract superconductive metals the electron-phonon spectral density, $\alpha^2F(\nu)$, is found to be a powerful tool also for extracting a more realistic pseudopotential from such metals. The pseudopotential so extracted has a range of surprising but physically reasonable properties and regenerates $\alpha^2F(\nu)$ accurately. Free from most of its long-standing uncertainties, thie pseudopotential may be useful in a number of active fields.

Méthode de calcul à N-corps basée sur la G0W0 : étude du couplage électron-phonon dans le C60 et développement d’une approche accélérée pour matériaux organiques

La présente thèse porte sur les limites de la théorie de la fonctionnelle de la densité et les moyens de surmonter celles-ci. Ces limites sont explorées dans le contexte d'une implémentation traditionnelle utilisant une base d'ondes planes. Dans un premier temps, les limites dans la taille des systèmes pouvant être simulés sont observées. Des méthodes de pointe pour surmonter ces dernières sont ensuite utilisées pour simuler des systèmes de taille nanométrique. En particulier, le greffage de molécules de bromophényle sur les nanotubes de carbone est étudié avec ces méthodes, étant donné l'impact substantiel que pourrait avoir une meilleure compréhension de ce procédé sur l'industrie de l'électronique. Dans un deuxième temps, les limites de précision de la théorie de la fonctionnelle de la densité sont explorées. Tout d'abord, une étude quantitative de l'incertitude de cette méthode pour le couplage électron-phonon est effectuée et révèle que celle-ci est substantiellement plus élevée que celle présumée dans la littérature. L'incertitude sur le couplage électron-phonon est ensuite explorée dans le cadre de la méthode G0W0 et cette dernière se révèle être une alternative substantiellement plus précise. Cette méthode présentant toutefois de sévères limitations dans la taille des systèmes traitables, différents moyens théoriques pour surmonter ces dernières sont développés et présentés dans cette thèse. La performance et la précision accrues de l'implémentation résultante laissent présager de nouvelles possibilités dans l'étude et la conception de certaines catégories de matériaux, dont les supraconducteurs, les polymères utiles en photovoltaïque organique, les semi-conducteurs, etc.

Intersubband plasmon-phonon modes in quantum wires at high temperatures

Coupled intersubband plasmon-phonon modes are studied in a multisubband parabolic quantum wire at room temperatures. These modes are found by calculating the spectral weight function which is related to the inelastic Raman spectra. We use a 13 subband model. The plasmon-phonon coupling strongly modifies the dispersion relation of the intersubband modes in the vicinity of the optical phonon frequency omega(LO). Extra modes show up as a result of the electron-phonon interaction. We carefully study the density and temperature dependence of these extra modes. We also show that coupled intersubband plasmon-phonon modes should be observed for temperatures as high as 300 K.

Two-phase separation observed in thermally treated pellets of ClO4- doped poly(3-methylthiophene) from electron spin resonance measurements

A pressed pellet of CIO (-)(4) poly (3-methylthiophene) (P3MT) was heated for two hours at 85 degrees C and suddenly dropped in liquid nitrogen. A change was observed around 220 K in the Electron Spin Resonance (ESR) spectra when the sample was slowly cooled from room temperature. ESR line asymmetry parameter (A/B) showed two spatially separated phases. One was identified as a small metallic-like phase. The other phase, the larger one, makes a transition to a semiconducting Charge-Density Wave (CDW) state.

Localized heating or cooling in mesoscopic devices by electron transport

We describe a method to produce local heating or cooling (depending on how the system is tuned) in a mesoscopic device by transport of electrons. The mechanism can operate on molecules or quantum dots, or any system where the local modes are coupled to vibrations. We believe this will be of future interest in micro electro mechanical systems (MEMS). The amount of heating/cooling obtained depends on the details of the device. We also perform a numerical calculation to display the effect. (C) 2004 Elsevier B.V. All rights reserved.

Electronic structure of La1-xSrxCrO3

LaCrO3 is a wide-band-gap insulator which does not evolve to a metallic state even after hole doping. We report electronic structure of this compound and its Sr substituents investigated by photoemission and inverse photoemission spectroscopies in conjunction with various calculations. The results show that LaCrO 3 is close to the Mott-Hubbard insulating regime with a gap of about 2.8 eV. Analysis of Cr 2p core-level spectrum suggests that the intra-atomic Coulomb interaction strength and the charge-transfer energy to be 5.0 and 5.5 eV, respectively, We also estimate the intra-atomic exchange interaction strength and a crystal-field splitting of about 0.7 and 2.0 eV, respectively. Sr substitution leading to hole doping in this system decreases the charge-excitation gap, but never collapses it to give a metallic behavior. The changes in the occupied as well as unoccupied spectral features are discussed in terms of the formation of local Cr4+ configurations arising from strong electron-phonon interactions.

Enhancing field emission from a carbon nanotube array by lateral control of electrodynamic force field

Fluctuation of field emission current from carbon nanotubes (CNTs) poses certain difficulties for their use in nanobiomedical X-ray devices and imaging probes. This problem arises due to deformation of the CNTs due to electrodynamic force field and electron-phonon interaction. It is of great importance to have precise control of emitted electron beams very near the CNT tips. In this paper, a new array configuration with stacked array of CNTs is analysed and it is shown that the current density distribution is greatly localised at the middle of the array, that the scatter due to electrodynamic force field is minimised and that the temperature transients are much smaller compared to those in an array with random height distribution.

Universal jellium BCS picture for Peierls and spin-Peierls phase transitions

Both metal-insulator Peierls and antiferromagnetic spin-Peierls dimerized phase transitions are observed to have a BCS electron-phonon interaction parameter which is compatible with the jellium value λ = 2/3π ≈ 0.21.

Radiation induced enhancement of superconductivity

The effect of microwave radiation on the electron-phonon vertex in superconductors is taken into account. This leads to an enhancement of effective pairing interaction and hence to the transition temperature (Tc) which depends on the photon density and the frequency. This prediction is in agreement with recent experimental results.