Presence Of Electron-phonon Interaction Research Articles

Enhancement of the thermoelectric performance in a nano junction by electron-phonon interaction effects

The thermoelectric properties of a molecular scale junction in the presence of electron-phonon interaction (EPI) that may be used to enhance the thermoelectric performance, are theoretically studied. Such junction is made of trans-PA nanowire connected to similar metallic electrodes. Using the Landauer formalism within the framework of Green's function method and tight-binding (TB) approximation, electronic transport properties including the electronic conductivity (Ge), the thermal conductivity (Ke), the Seebeck coefficient (S), and figure of merit (ZT), are numerically calculated. We used of mapping technique to include the many-body EPI in the calculations. Our results indicate that all thermoelectric characteristics are quite sensitive to EPI and enhance the thermoelectric performance in the trans-PA molecule. Furthermore, it was shown that the figure of merit could be controlled by the coupling strength between the molecule and the electrode.

Quantum transport through a quantum dot side-coupled to a Majorana bound state pair in the presence of electron-phonon interaction

We theoretically study quantum transport through a quantum dot coupled to Majorana bound states confined at the ends of a topological superconducting nanowire. The topological superconductor forms a loop and is threaded by a tunable magnetic flux, which allows one to control the electron transport in the system. In particular, we investigate phonon-assisted transport properties in the device when the central quantum dot interacts with a single long-wave optical phonon mode. We find that when the two Majorana bound states are unhybridized, the zero-temperature linear conductance has a $2\pi$ periodicity as a function of magnetic flux phase, independent of the electron-phonon interaction, the quantum dot energy, or the finite values of dot-Majorana couplings. For a finite overlap between the Majorana bound states, the linear conductance periodicity generally changes to $4\pi$ either due to a finite electron-phonon coupling strength, or a dot energy level that is tuned away from the Fermi level. Additionally, the differential conductance periodicity changes from $2\pi$ to $4\pi$ when the Majorana bound states hybridize and the electron-phonon coupling is finite. Our results provide insight into transport signatures expected in topological quantum computational platforms that integrate quantum dots as a means for Majorana qubit readout. The energy exchange with an environmental bath, here a single phonon mode, significantly alters the current signatures expected from Majorana modes.

Modelling of InAs nanowire and MOSFET under phonon emission and absorption by using NEGF formalism

Applying the nonequilibrium Green's function (NEGF), we have compared the performance of InAs nanowire (NW) and its based superlattice MOSFET under optical phonon absorption and emission. Our results indicate fluctuations in the NW characteristics like density of states (DOS), scattering rate, mobility, effective transmission and current. We applied the NEGF approach to show fluctuations in the NW parameters as a function of the energy. Electron phonon interactions and background effect are also taken into account through scattering self-energies in the self-consistent born approximation. The energy dependence of these oscillations allows us to deduce a coupling between electrons and the phonons. This analysis opens the door for InAs NWs as nano electromechanical devices, superlattice MOSFET and particularly, as phonon detectors. • Semiconductor nanowire based devices and superlattice MOSFET have potential applications in many areas, such as optoelectronics, sensors, and particularly integrated logic circuits. • Identification of InAs nanowire (NW) and its based superlattice MOSFET have been done here. • A nonequilibrium green's function model has been developed here to explore NWs. • The mentioned nonequilibrium green's function can provide accurate and detailed device performances of NWs in the presence of electron phonon interactions.

Nonequilibrium nature of nonlinear optical response: Application to the bulk photovoltaic effect

The bulk photovoltaic effect is an example of a nonlinear optical response that leads to a DC current that is relevant for photovoltaic applications. In this paper, we theoretically study this effect in the presence of electron-phonon interactions. Using the response function formalism, we find that the nonlinear optical response, in general, contains three operator correlation functions, one of which is not ordered in time. This latter correlator cannot be computed from equilibrium field theory. Using a semiclassical approach instead, we show that the bulk photovoltaic effect can be attributed to the dipole moment of the generated excitons. We then confirm the validity of the semiclassical result (which agrees with the noninteracting result) for nonlinear DC response from a quantum master equation approach. From this formalism we find that, in contrast to usual linear response, the scattering rate has a strong implicit effect on the nonlinear DC response. Most interestingly, the semiclassical treatment shows that the nonlinear DC response for spatially inhomogeneous excitation profiles is strongly nonlocal and must involve the aforementioned out-of-time-ordered correlators that cannot be computed by equilibrium field theory.

The effects of electron-phonon coupling and magnetic field on charge structure factors of armchair graphene nanoribbons

We present the behaviors of both dynamical and static charge susceptibilities of undoped armchair graphene nanoribbon using the Green’s function approach in the context of Holstein model Hamiltonian. Specially, the effects of magnetic field and electron-phonon coupling strength on the plasmon modes of armchair graphene nanoribbon are investigated via calculating correlation function of charge density operators. Random phase approximation has been implemented to find the interacting dynamical charge susceptibility. The electrons in this systems interacts with each other by mediation of dispersionless Holstein phonons. Our results show the increase of electron phonon coupling strength leads to decrease intensity of charge collective mode. Also the frequency position of the collective mode tends to higher frequencies with electron phonon coupling. We also show that the frequency positions of plasmon mode of armchair nanoribbon are not affected by the increase of magnetic field. Furthermore the number of collective excitation mode increases with ribbon width in the presence of electron-phonon interaction. Finally the temperature dependence of static charge structure factor of armchair graphene nanoribbon is studied. The effects of magnetic field, ribbon width and electron-phonon interaction on the static structure factor are addressed in details.

Transverse spin susceptibilities of doped armchair graphene nanoribbon due to electron-phonon interaction

We present the behaviors of both dynamical and static charge susceptibilities of undoped armchair graphene nanoribbon using the Green's function approach in the context of Holstein model Hamiltonian. Specially, the effects of magnetic field and electron-phonon coupling strength on the plasmon modes of armchair graphene nanoribbon are investigated via calculating correlation function of charge density operators. Random phase approximation has been implemented to find the interacting dynamical charge susceptibility. The electrons in this systems interacts with each other by mediation of dispersionless Holstein phonons. Our results show the increase of electron phonon coupling strength leads to decrease intensity of charge collective mode Also the frequency position of the collective mode tends to higher frequencies with electron phonon coupling. We also show that the frequency positions of plasmon mode of armchair nanoribbon are not affected by increase of magnetic field. Furthermore the number of collective excitation mode increases with ribbon width in the presence of electron-phonon interaction. Finally the temperature dependence of static charge structure factor of armchair graphene nanoribbon is studied. The effects of both magnetic field, ribbon width and electron-phonon interaction on the static structure factor are addressed in details.

Spectroscopic investigations of Pr3+ ions doped CNGG and CLNGG single crystals

Spectroscopic characteristics of Pr3+ ions doped CNGG and CLNGG single crystals were investigated in order to assess their potential as laser materials for visible emission. The Judd-Ofelt intensity parameters for the f-f transitions of Pr3+ ions were used to determine spectroscopic and laser emission features. The temperature dependence of the absorption spectra for the 3H4 → 3P0 transition was used to highlight the multicenter structure, different Stark levels, hot bands, and also the connection between the vibronic and electronic lines. The presence of electron-phonon interaction was also observed in the emission spectra corresponding to the 3P0 → 3H4 transition under different excitation wavelengths. Based on low temperature absorption and emission spectra, partial energy level diagrams of Pr3+ ions doped in CNGG and CLNGG single crystals were obtained. The emission cross-sections for different transitions of Pr3+ ions were evaluated by the Fuchtbauer-Ladenburg formula. The fluorescence decay curve of the 1D2 level was measured under selective excitation at different concentrations and temperatures. The concentration quenching process for the 1D2 state was also studied.

Effect of phonon induced spin-flip processes on correlated quantum dot kinetics

We present a theoretical analysis of relaxation processes in a correlated quantum dot (QD) coupled to a reservoir in the presence of electron-phonon interaction. The effect of both electron-phonon interaction and Coulomb correlations on system's kinetics is analyzed by means of equations of motion for generalized non-equilibrium Green's functions. It is shown that phonon-assisted tunneling enhances the relaxation processes in a correlated quantum dot. Moreover, electron-phonon interaction changes stationary values and typical relaxation rates of the second-order correlation functions for localized electrons. In spite of the fact that electron-phonon interaction is spin independent, it strongly influences spin correlations in the presence of Coulomb interactions between localized electrons. This effect appears to be due to phonon-induced spin-flip processes of localized electrons.

Wigner Time Delay Induced by a Single Quantum Dot

Resonant scattering of weak coherent laser pulses on a single two-level system realized in a semiconductor quantum dot is investigated with respect to a time delay between incoming and scattered light. This type of time delay was predicted by Wigner in 1955 for purely coherent scattering and was confirmed for an atomic system in 2013 [R. Bourgain etal., Opt. Lett. 38, 1963 (2013)OPLEDP0146-959210.1364/OL.38.001963]. In the presence of electron-phonon interaction, we observe deviations from Wigner's theory related to incoherent and strongly non-Markovian scattering processes which are hard to quantify via a detuning-independent pure dephasing time. We observe detuning-dependent Wigner delays of up to 530ps in our experiments which are supported quantitatively by microscopic theory allowing for pure dephasing times of up to 950ps.

Thermal rectification in a double quantum dots system with a polaron effect

We investigate the rectification of heat current carried by electrons through a double quantum dot (DQD) system under a temperature bias. The DQD can be realized by molecules such as suspended carbon nanotube and be described by the Anderson-Holstein model in presence of electron-phonon interaction. Strong electron-phonon interaction can lead to formation of polaronic states in which electronic states are dressed by phonon cloud. Dressed tunneling approximation (DTA), which is nonperturbative in dealing with strong electron-phonon interaction, is employed to obtain the heat current expression. In DTA, self-energies are dressed by phonon cloud operator and are temperature dependent. The temperature dependency of imaginary part of dressed retarded self-energy gives rise to the asymmetry of the system and is the necessary condition of thermal rectification. On top of this, one can either tune DQD effective energy levels such that $|\bar{\epsilon}_1|\neq |\bar{\epsilon}_2|$ or have asymmetric dot-lead couplings to achieve thermal rectification. We numerically find that increasing electron-phonon coupling and reducing inter dot coupling can both improve thermal rectification effect, while the electronic heat current is reduced.

Structural and electron-phonon interaction effects in optical spectra of Pr 3+ and Sm 3+ in YAG

Abstract Comparative high resolution spectroscopic investigation between 10 and 300 K of the less investigated Pr 3+ and Sm 3+ ions in the YAG crystals and transparent ceramics evidenced that the satellite structures around the main optical lines corresponding to the ions in the dodecahedral c -sites belong either to ions whose crystal field is perturbed by adjacent lattice irregularities or to vibronics. In case of Pr:YAG such irregularities are mainly caused by presence of doping ions in near sites (n.n. and n.n.n. pairs) in both types of materials, while in the melt-grown crystal an additional irregularity is the presence of excess Y 3+ ions that occupy part of the octahedral Al 3+ sites. The new data on perturbed Pr 3+ pair centers remove the ambiguity in identification of several lines that were previously attributed to transitions of the isolated Pr 3+ ions forbidden in the D 2 symmetry of YAG ceramics and rule out the forced assumption of lowering of symmetry. In case of Sm 3+ :YAG ceramic the main perturbed center corresponds to n.n. pairs and plays major role in utilization of this material as ASE suppressor for the Nd:YAG laser. The presence of electron-phonon interaction was evidenced by shift or broadening of lines or by resolved weak vibronic structures. For selected optical transitions, such as the 10 K Sm 3+ 4 G 5/2 → 6 H 5/2,7/2,9/2 emission in YAG the presence of enhanced electron-phonon effects was evidenced by strong alteration of the shape of electronic lines, such as apparition of asymmetries or even doubling of electronic lines whose Stark gap to other transitions is in quasi-resonance with Raman phonons of YAG. zero phonon lines at energies close to Raman phonons of YAG of T 2g symmetry. The shape of the lines is analyzed in terms of the theory of the quasi-resonant electron-phonon coupling between a pure electronic state and vibronics.

Optical properties of gapped graphene structure in the presence of electron-phonon interaction: A full band approach

We compute the optical conductivity of gapped graphene taking into account the effects of interaction between electrons and Einstein phonons beyond the usual Dirac-cone approximation. Our study is based on the Kubo formula that is established upon the retarded self-energy. Specially we report the frequency dependence of optical absorption of the structure. We find numerical results for frequency dependence of optical absorption for different values of gap parameter in the presence of Holstein phonons. We have also studied the effects of electron-phonon coupling strength and electronic concentration on the behavior of optical absorption of the gapped graphene structure. Our results show the increase of electronic chemical potential in doped case leads to appear optical band gap in the structure. This optical gap arises from Pauli blocking of vertical transitions when conduction band is doped. Electron-phonon coupling broadens the peak in the optical spectral and decreases the intensity of optical absorption. However the frequency position of the peak is not considerably affected by electron-phonon coupling.

Persistent current in a correlated quantum ring with electron-phonon interaction in the presence of Rashba interaction and Aharonov-Bohm flux.

Persistent current in a correlated quantum ring threaded by an Aharonov-Bohm flux is studied in the presence of electron-phonon interactions and Rashba spin-orbit coupling. The quantum ring is modeled by the Holstein-Hubbard-Rashba Hamiltonian and the energy is calculated by performing the conventional Lang-Firsov transformation followed by the diagonalization of the effective Hamiltonian within a mean-field approximation. The effects of Aharonov-Bohm flux, temperature, spin-orbit and electron-phonon interactions on the persistent current are investigated. It is shown that the electron-phonon interactions reduce the persistent current, while the Rashba coupling enhances it. It is also shown that temperature smoothens the persistent current curve. The effect of chemical potential on the persistent current is also studied.

Current-conserving and gauge-invariant quantum ac transport theory in the presence of phonon

Using the nonequilibrium Green’s function (NEGF) approach, we develop a microscopic ac transport theory in the presence of electron-phonon interaction. Taking into account the self-consistent Coulomb interaction, the displacement current is included. This ensures that our theory satisfies the current-conserving and gauge-invariant conditions. Importantly, the inclusion of self-consistent Coulomb interaction naturally connects the NEGF formalism to the density functional theory (DFT). This allows us to calculate the self-consistent Hamiltonian using DFT within the NEGF framework, which paves the way for first principles ac transport calculation of nanoelectronic devices in the presence of electron-phonon interaction. It is known that the inelastic electron tunneling spectroscopy (IETS) is a powerful tool in studying the inelastic dc quantum transport in molecular devices. The basic idea of IETS is to obtain the information of vibrational spectrum of molecular devices by measuring the second derivative of the dc current with respect to the bias voltage. In the ac transport, we find that the phonon spectrum and electron-phonon coupling strength can be obtained from the second derivative of the admittance with respect to the frequency which is the working principle of the inelastic electron admittance spectroscopy (IEAS). Hence we propose to use IEAS to probe the effect of the phonon in ac transport. As an example, dynamic conductance of a quantum dot is discussed in detail and the concept of IEAS is demonstrated.

Non-stationary effects in the coupled quantum dots influenced by the electron-phonon interaction

We analyzed time evolution of the localized charge in the system of two interacting single level quantum dots (QDs) coupled with the continuous spectrum states in the presence of electron-phonon interaction. We demonstrated that electron-phonon interaction leads to the increasing of localized charge relaxation rate. We also found that several time scales with different relaxation rates appear in the system in the case of non-resonant tunneling between the dots. We revealed the formation of oscillations in the filling numbers time evolution caused by the emission and adsorption processes of phonons.

Dissipative time-dependent quantum transport theory

A dissipative time-dependent quantum transport theory is developed to treat the transient current through molecular or nanoscopic devices in presence of electron-phonon interaction. The dissipation via phonon is taken into account by introducing a self-energy for the electron-phonon coupling in addition to the self-energy caused by the electrodes. Based on this, a numerical method is proposed. For practical implementation, the lowest order expansion is employed for the weak electron-phonon coupling case and the wide-band limit approximation is adopted for device and electrodes coupling. The corresponding hierarchical equation of motion is derived, which leads to an efficient and accurate time-dependent treatment of inelastic effect on transport for the weak electron-phonon interaction. The resulting method is applied to a one-level model system and a gold wire described by tight-binding model to demonstrate its validity and the importance of electron-phonon interaction for the quantum transport. As it is based on the effective single-electron model, the method can be readily extended to time-dependent density functional theory.

Emission and absorption of optical phonons in Multigate Silicon Nanowire MOSFETs

In this paper we study the effect of emission and absorption processes due to inelastic optical phonons in multigate silicon nanowire transistors. We show that low-energy optical phonons reduce drain current through both phonon emission/absorption processes while high-energy phonons redistribute current spectrum inside the nanowire merely by phonon emission process without reducing the current drive significantly. A three-dimensional quantum mechanical device simulator based on the NEGF formalism in uncoupled-mode space has been developed to model quantities of interest in the presence of electron-phonon interactions. Electron-phonon scattering is accounted for by adopting the self-consistent Born approximation and using the deformation potential theory.

Phonon-assisted spin current in a hybrid system with a single molecular quantum dot system applied with ac magnetic fields

We investigate the spin current through a molecular quantum dot (MQD) ir- radiated with a rotating magnetic field and an oscillating magnetic field by nonequilib- rium Green's function. The rotating magnetic field rotates with the angular frequency ωr around the z-axis with the tilt angle θ, and the time-oscillating magnetic field is lo- cated in the z-axis with the angular frequency ω. Different behaviors have been shown in the presence of electron-phonon interaction(EPI) which plays a significant role in the transport. The spin current displays asymmetric behavior as the source-drain bias eV=0, novel side peaks or shoulders can be found due to the phonon absorption and emission procedure, and the negative spin current becomes stronger as the parameter g increases. However, the spin currents display the same magnitude and the same oscillation behavior in the region m 0B1>3D regardless the parameter g.

Quantum approach to electronic noise calculations in the presence of electron-phonon interactions

A quantum-mechanical approach to the calculation of electronic noise for nanoscale devices is presented. This method is based on the nonequilibrium Green's-function formalism with electron-phonon scattering mechanisms and takes the effects of the Pauli exclusion principle and the long-range Coulomb interactions into account. As examples the drain current noise characteristics of silicon nanowire transistors at room temperature are simulated. The drain current noise in the saturation regime is primarily shot-noise dominant but is suppressed for higher gate biases due to the electron-electron correlation in the channel region. The role of electron-phonon interactions on noise, the transition from thermal to shot noise, and the physical origin of the shot-noise phenomenon are also investigated.

Inelastic scattering in metal-H2-metal junctions

We present first-principles calculations of the $dI/dV$ characteristics of an ${\text{H}}_{2}$ molecule sandwiched between Au and Pt electrodes in the presence of electron-phonon interactions. The conductance is found to decrease by a few percentages at threshold voltages corresponding to the excitation energy of longitudinal vibrations of the ${\text{H}}_{2}$ molecule. In the case of Pt electrodes, the transverse vibrations can mediate transport through otherwise nontransmitting $\text{Pt}\text{ }d$ channels leading to an increase in the differential conductance even though the hydrogen junction is characterized predominately by a single almost fully open transport channel. In the case of Au, the transverse modes do not affect the $dI/dV$ because the $\text{Au}\text{ }d$ states are too far below the Fermi level. A simple explanation of the first-principles results is given using scattering theory. Finally, we compare and discuss our results in relation to experimental data.