Phonon Coupling Strength Research Articles

Influence of Magnetic Field and LO Phonon Effects on the Spin Polarization State Energy of Strong-Coupling Bipolaron in a Quantum Dot

On the basis of Lee–Low–Pines unitary transformation, the influence of magnetic field and LO phonon effects on the energy of spin polarization states of strong-coupling bipolarons in a quantum dot (QD) is studied by using the variational method of Pekar type. The variations of the ground state energy \(E_0\) and the first excited state the energy \(E_1\) of bipolarons in a two-dimensional QD with the confinement strength of QDs \(\omega _0\), dielectric constant ratio \(\eta \), electron–phonon coupling strength \(\alpha \) and cyclotron resonance frequency of the magnetic field \(\omega _{c}\) are derived when the influence of the spin and external magnetic field is taken into account. The results show that both energies of the ground and first excited states (\(E_0\) and \(E_1)\) consist of four parts: the single-particle energy of electrons \(E_\mathrm{e}\), Coulomb interaction energy between two electrons \(E_\mathrm{c}\), interaction energy between the electron spin and magnetic field \(E_\mathrm{S}\) and interaction energy between the electron and phonon \(E_{\mathrm{e-ph}}\); the energy level of the first excited state \(E_1\) splits into two lines as \(E_1^{(1+1)}\) and \(E_1^{(1-1)}\) due to the interaction between the single-particle “orbital” motion and magnetic field, and each energy level of the ground and first excited states splits into three “fine structures” caused by the interaction between the electron spin and magnetic field; the value of \(E_{\mathrm{e-ph}}\) is always less than zero and its absolute value increases with increasing \(\omega _0\), \(\alpha \) and \(\omega _c\); the effect of the interaction between the electron and phonon is favorable to forming the binding bipolaron, but the existence of the confinement potential and Coulomb repulsive energy between electrons goes against that; the bipolaron with energy \(E_1^{(1-1)}\) is easier and more stable in the binding state than that with \(E_1^{(1+1)}\).

Strain-mediated coupling in a quantum dot–mechanical oscillator hybrid system

Recent progress in nanotechnology has allowed the fabrication of new hybrid systems in which a single two-level system is coupled to a mechanical nanoresonator. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies, and for the exploration of the quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain: a quantum dot is embedded within a nanowire that features discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light-extraction efficiency and a large exciton-phonon coupling strength g0. By means of optical and mechanical spectroscopy, we find that g0/2 π is nearly as large as the mechanical frequency, a criterion that defines the ultrastrong coupling regime.

Temperature dependent electron–phonon coupling in chlorin-doped impurity glass and in photosynthetic FMO protein containing bacteriochlorophyll a

Difference fluorescence line-narrowing spectra are studied for evaluating the temperature dependence of the linear electron–phonon and vibronic coupling strengths in the two weakly coupled low-temperature glassy systems: chlorin-doped 1-propanol and the native Fenna–Matthews–Olson (FMO) light-harvesting protein complex folded with bacteriochlophyll a chromophores. Roughly three-fold increases of the electron–phonon coupling strength and constant vibronic couplings were observed in both systems in the available temperature range from 4.5 to 70K. However, while the chlorin system is amenable to simple modeling of impurity spectra, the FMO shows significant deviations, consistent with the complex (exciton) nature of its lowest-energy singlet electronic transition.

CuNNi3: a new nitride superconductor with antiperovskite structure

We have successfully synthesized a new Ni-based antiperovskite nitride, CuNNi3, which exhibits superconductivity with a transition temperature Tc of 3.2 K. This compound is the second nitride superconductor in the Ni-based antiperovskites. We report the synthesis and physical properties of CuNNi3, characterized via x-ray diffraction, magnetization, resistivity, and heat capacity measurements. The temperature dependence of the specific heat is consistent with an isotropic s-wave gap (Δ0 = 0.42 meV). The estimated electron–phonon coupling strength (λep = 0.53) suggests that the superconductivity in CuNNi3 is induced by electron–phonon coupling, and the gap ratio (2Δ0/kBTc) of 3.05 indicates that it is a weak coupling superconductor. Furthermore, the relatively high Ginzburg–Landau parameter (κ = 24.6) signifies that CuNNi3 is a type II superconductor.

Influence of an anisotropic parabolic potential on the quantum dot qubit

To study the influence of an anisotropic parabolic potential (APP) on the properties of a quantum dot (QD) qubit, we obtain the eigenenergies and eigenfunctions of the ground and first excited state of an electron, which is strongly coupled to the bulk longitudinal optical (LO) phonons, in a QD under the influence of an APP by the celebrated Lee—Low—Pines (LLP) unitary transformation and the Pekar type variational (PTV) methods. Then, this kind of two-level quantum system can be excogitated to constitute a single qubit. When the electron locates at the superposition state of its related eigenfunctions, we get the time evolution of the electron's probability density. Finally, the influence of an APP on the QD qubit is investigated. The numerical calculations indicate that the probability density will oscillate periodically and it is a decreasing function of the effective confinement lengths of the APP in different directions. Whereas its oscillatory period is an increasing one and will diminish with enhancing the electron—phonon (EP) coupling strength.

Structure and spectroscopy of rare earth – Doped lead phosphate glasses

Abstract Lead–gallium phosphate glasses doped with rare the earth ions (Eu3+, Dy3+, Tb3+, Er3+) were synthesized. The structure of obtained glasses was examined by means of use: X-ray diffraction (XRD), nuclear magnetic resonance (207Pb and 31P NMR), fourier transform infrared (FT-IR) and Raman spectroscopy. In contrast to fully amorphous Ln-doped samples (Ln = Eu, Dy, Tb), in Er-doped sample the GaPO4 crystalline phase was identified. It was found from the NMR, FT-IR and Raman spectroscopic techniques that, different structural phosphate groups were present in lead phosphate glasses. Based on absorption measurements, the UV–VIS cut-off wavelength for lead phosphate glass was determined and its value is close to 305 nm. Excitation and emission spectra of rare earths were also detected. From excitation spectra of Eu3+ the electron–phonon coupling strength and phonon energy of the glass host were determined. Due to 4f6–4f6 (Eu3+), 4f8–4f8 (Tb3+), 4f9–4f9 (Dy3+) and 4f11–4f11 (Er3+) electronic transitions of trivalent rare earth ions several luminescence bands were stated.

Magnetic field effect on state energies and transition frequency of a strong-coupling polaron in an anisotropic quantum dot

By employing a variational method of the Pekar-type, which has different variational parameters in the x–y plane and the z-direction, we study the ground and the first excited state energies and transition frequency between the ground and the first excited states of a strong-coupling polaron in an anisotropic quantum dot (AQD) under an applied magnetic field along the z-direction. The effects of the magnetic field and the electron–phonon coupling strength are taken into account. It is found that the ground and the first excited state energies and the transition frequency are increasing functions of the external applied magnetic field. The ground state and the first excited state energies are decreasing functions, whereas transition frequency is an increasing function of the electron–phonon coupling strength. We find two ways of tuning the state energies and the transition frequency: by adjusting (1) the magnetic field and (2) the electron–phonon coupling strength.

Specific heat of doped armchair and zigzag carbon nanotubes

We study the temperature behavior of electronic specific heat of metallic carbon nanotubes in the context of Holstein model Hamiltonian. Green's function approach has been implemented in order to calculate the electronic contribution to the specific heat of the compound. Second order perturbation theory has been used to obtain interacting Green's function of model Hamiltonian. The results show a monotonic increasing behavior for specific heat with temperature for both armchair and zigzag carbon nanotubes. Furthermore, the effect of various electronic concentrations and the electron–phonon coupling strengths on the specific heat has been investigated.

The effect of electric field on an asymmetric quantum dot qubit

On the condition of strong electron---LO phonon coupling in an asymmetric quantum dot (QD), we study the eigenenergies and eigenfunctions of the ground and the first excited states under an applied electric field by using variational method of Pekar type. This QD system may be used as a two-level qubit. When the electron is in the superposition state of the ground and the first excited states, we obtain the time evolution of the electron probability density, which oscillates in the QD. It is found that due to the presence of the 3-D anisotropic harmonic potentials in the transverse and longitudinal directions of the QD, the electron probability density shows double-peak configuration, whereas there is only one peak if the confinement is 2-D symmetric in the x- and y-directions. The oscillation period is an increasing function of the transverse and longitudinal effective confinement lengths of the QD, and decreases with respect to the electron---phonon coupling strength and the electric field.

Magnetic field and temperature dependence of the effective mass of strong-coupling bound magnetopolaron in quantum rods with hydrogenic impurity

Magnetic field and temperature dependence of the properties of strong-coupling bound magnetopolarons in quantum rods (QRs) with hydrogenic impurity is studied by means of the Tokuda improved linear combination operator method, Lee-Low-Pines transformation method and quantum statistical theory. The expressions for the effective mass and the vibrational frequency of magnetopolarons are derived. Results of numerical calculations show that the effective mass and the vibrational frequency of magnetopolarons increase with increasing the cyclotron frequency of the magnetic field, the confinement strength of QRs, the electron–phonon coupling strength and the dielectric constant ratio, but decrease with increasing the aspect ratio of QRs and the temperature; the quantum size effects caused by the ellipsoidal structure and the parabolic confinement potential of QRs have significant influence on the changes of the effective mass and the vibrational frequency of magnetopolarons.

EFFECTS OF ANISOTROPIC PARABOLIC POTENTIAL ON THE ENERGY LEVELS AND TRANSITION FREQUENCY OF STRONG-COUPLING POLARON IN A QUANTUM DOT

In the presence of a three-dimensional anisotropic parabolic potential (APP), the energy levels and the transition frequency between relevant levels of the strong-coupling polaron in a quantum dot (QD) are investigated by using the well-known Lee-Low-Pines (LLP) unitary transformation method and the Pekar type variational (PTV) method. The relations of the energy levels and the transition frequency with the electron–phonon (EP) coupling strength and the effective confinement lengths are derived. Numerical calculations show that the energy levels are decreasing functions of the EP coupling strength, whereas the transition frequency is an increasing one of it. And they are all increasing rapidly with decreasing the effective confinement lengths in different directions, which are showing the novel quantum size confining effect of the QD.

Influences of temperature and coulomb bound potential on the properties of quantum rod qubit

We study the eigenenergies and the eigenfunctions of the ground and the first excited states of an electron strongly coupled to LO-phonon in a quantum rod (QR) with a hydrogen-like impurity at the center by using variational method of Pekar type. This QR system may be used as a two-level quantum qubit. When the electron is in the superposition state of the ground and the first excited states, we obtained the time evolution of the electron probability density oscillating in the QR with a certain period. We then investigate the effects of the temperature and the hydrogen-like impurity on the time evolution of the electron probability density and the oscillation period. It is found that the electron probability density and the oscillation period increase (decrease) with increasing temperature in lower (higher) temperature regime. The electron probability density and the oscillation period decrease (increase) with increasing electron–phonon coupling strength when the temperature is lower (higher). Whereas they increase (decrease) with increasing Coulomb bound potential when the temperature is lower (higher).

Magnetic field and temperature dependence of the properties of the ground state of the strong-coupling bound magnetopolaron in quantum rods with hydrogenic impurity

Magnetic field and temperature dependence of the properties of the ground state of the strong-coupling bound magnetopolaron in quantum rods (QRs) with hydrogenic impurity is studied by means of the Huybrechts—Lee—Low—Pines transformation method and the quantum statistical theory. The expressions for the ground-state energy and the mean number of phonons of the magnetopolaron are derived. Results of the numerical calculations show that the bound state of the magnetopolaron cannot be formed when the value of the aspect ratio of the QR, the dielectric constant ratio, the electron—phonon coupling strength or the temperature parameter is small. The larger the deviation of the value of aspect ratio e′ from 1 is, the more it is unfavorable to the stability of the ground state of the magnetopolaron. When the magnetopolaron is in the bound state, the absolute value of its ground-state energy and its mean number of phonons increase with an increase of the dielectric constant ratio and confinement strength of QRs, but decrease with an increase in the cyclotron frequency of the external magnetic field and the temperature. The absolute value of the ground-state energy and the mean number of phonons of the magnetopolaron decrease with decreasing e′ when e′ < 1, but decrease with increasing e′ when e′ > 1. They get the maximum value at e′ = 1.

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0 → 1) to the purely electronic origin transition (0 → 0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous line width small compared with the phonon frequency.

Phonon-assisted thermoelectric effects in a two-level molecule

Thermoelectric properties of a two-level molecule attached to the metallic electrodes are analyzed using the equation of motion technique within the Green function formalism. Results show that the electrical conductance is strongly dependent on the electron and phonon temperatures and the electron–phonon coupling strength. In addition, it is observed that the thermal conductance peaks in the electron–hole symmetry points are vanished in the presence of the strong electron–phonon interaction. It is also found that the figure of merit is strongly suppressed in the strong electron–phonon interaction. The violation of the Wiedemann–Franz law is also observed coming from the Coulomb interactions.

Ultrafast electronic dynamics in laser-excited crystalline bismuth

Femtosecond spectroscopy is applied to study transient electronic and lattice processes in bismuth. Components with relaxation times of 1 ps, 7 ps and ~ 1 ns are detected in the photoinduced reflectivity response of the crystal. To facilitate the assignment of the observed relaxation to the decay of particular excited electronic states we use pump pulses with central wavelengths ranging from 400 nm to 2.3 mum. Additionally, we examine the variation of parameters of coherent A1g phonons upon the change of excitation and probing conditions. Data analysis reveals a significant wavevector dependence of electron-hole and electron- phonon coupling strength along \Gamma--T direction of the Brillouin zone.

KSrScSi2O7:Eu2+: a novel near-UV converting blue-emitting phosphor with high efficiency and excellent thermal stability

A novel blue-emitting phosphor KSrScSi2O7:Eu2+, synthesized from a water-soluble propylene glycol modified silane (PGMS) silicon precursor by a solution approach, exhibits high internal quantum efficiency and remarkable thermal stability, which are attributed to the rigid structural network of the host and weak electron–phonon coupling strength.

Hybrid continuum–atomistic modelling of swift heavy ion radiation damage in germanium

The response of germanium to swift heavy ion irradiation is simulated using a hybrid continuum–atomistic approach. The continuum part of the model, which characterises the electronic excitations is an extension of the inelastic thermal spike based on an approximation to the Boltzmann transport equation; while the atomistic part is represented with molecular dynamics. This integrated method can realistically account for the non-equilibrium carrier dynamics in band-gap materials under irradiation, unlike earlier developments based on the two-temperature approach. The model is used to obtain temporal and spatial evolution of carrier density, electronic temperature and lattice temperature for germanium irradiated with carbon cluster ions. Good agreement with experimental data of amorphised latent track radii for different stopping powers is obtained by fitting a constant value for the electron–phonon coupling strength – the only parameter treated as free in the model.

Superconductivity in β-Tin Germanium

The superconducting properties of the β-tin germanium under pressure have been investigated by the first-principles calculations based on density functional perturbation theory. The electron–phonon coupling strength as a function of pressure was calculated, and it deceases with the application of pressure. Using the calculated electron–phonon coupling strength, the superconducting transition temperature T c has been estimated. Assuming a nominal Coulomb repulsion parameter of 0.1, the calculated T c is in excellent agreement with experimental data. The T c correlates with electron–phonon coupling strength, indicating phonon-mediated superconductivity in β-tin Ge phase. Our findings have great implications to other Group IVa elements.

Excitonic properties of wurtzite InP nanowires grown on silicon substrate

In order to investigate the optical properties of wurtzite (Wz) InP nanowires grown on Si(001) by solid source molecular beam epitaxy with the vapour–liquid–solid method, the growth temperature and V/III pressure ratio have been optimized to remove any zinc-blende insertion. These pure Wz InP nanowires have been investigated by photoluminescence (PL), time-resolved PL and PL excitation. Direct observation of the second and third valence band in Wz InP nanowires using PL spectroscopy at high excitation power have been reported and, from these measurements, a crystal field splitting of 74 meV and a spin–orbit interaction energy of 145 meV were extracted. Based on the study of temperature-dependent optical properties, we have performed an investigation of the thermal escape processes of carriers and the electron–phonon coupling strength.