Electron-optical Phonon Coupling Research Articles

Anomalous Emission Shift of CdSe/CdS/ZnS Quantum Dots at Cryogenic Temperatures.

The band-gap energy of most bulk semiconductors tends to increase as the temperature decreases. However, non-monotonic temperature dependence of the emission energy has been observed in semiconductor quantum dots (QDs) at cryogenic temperatures. Here, using stable and highly efficient CdSe/CdS/ZnS QDs as the model system, we quantitatively reveal the origins of the anomalous emission red-shift (∼8 meV) below 40 K by correlating ensemble and single QD spectroscopy measurements. About one-quarter of the anomalous red-shift (∼2.2 meV) is caused by the temperature-dependent population of the band-edge exciton fine levels. The enhancement of electron-optical phonon coupling caused by the increasing population of dark excitons with temperature decreases contributes an ∼3.4 meV red-shift. The remaining ∼2.4 meV red-shift is attributed to temperature-dependent electron-acoustic phonon coupling.

Hot carrier dynamics and electron-optical phonon coupling in photoexcited graphene via time-resolved ultrabroadband terahertz spectroscopy

Electron-electron (e-e) interaction is known as a source of logarithmic renormalizations for Dirac fermions in quantum field theory. The renormalization of electron-optical phonon coupling (EPC) by e-e interaction, which plays a pivotal role in hot carrier and phonon dynamics, has been discussed since the discovery of graphene. We investigate hot carrier dynamics and EPC strength using time-resolved ultrabroadband terahertz (THz) spectroscopy combined with numerical simulation based on the Boltzmann transport equation and a comprehensive temperature model. The numerical simulation demonstrates that the extrinsic carrier scatterings by the Coulomb potential of the charged impurity and surface polar phonons are significantly suppressed by the carrier screening effect and have negligible contributions to the THz photoconductivity in heavily doped graphene on polyethylene terephthalate (PET) substrate. The large negative photoconductivity and the non-Drude behavior of THz conductivity spectra appear under high pump fluence and can be attributed to the temporal variation of the hot carrier distribution and scattering rate. The transient reflectivity well reflects the EPC strength and temporal evolution of the hot carrier and optical phonon dynamics. We successfully estimate the EPC matrix element of the ${A}_{1}^{\ensuremath{'}}$ optical phonon mode near the $\mathbf{K}$ point as ${\ensuremath{\langle}{D}_{\mathbf{K}}^{2}\ensuremath{\rangle}}_{\mathrm{F}}\ensuremath{\approx}450$ ${(\mathrm{eV}/\AA{})}^{2}$ from the fitting of THz conductivity spectra and temporal evolution of transient THz reflectivity. The corresponding dimensionless EPC constant ${\ensuremath{\lambda}}_{\mathbf{K}}\ensuremath{\approx}0.09$ at Fermi energy ${\ensuremath{\varepsilon}}_{\mathrm{F}}=0.43\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ is slightly larger than the prediction of the renormalization group approach including the dielectric screening effect of the PET substrate. This leads to a significant difference in hot carrier and phonon dynamics compared with those without the renormalization effect by the e-e interaction. This approach can provide a quantitative understanding of hot carrier and optical phonon dynamics and support the development of future graphene optoelectronic devices.

Diffusion properties of electrons in GaN crystals subjected to electric and magnetic fields

We studied the diffusion coefficient of hot electrons of GaN crystals in moderate electric (1...10 kV/cm) and magnetic (1...4 T) fields. Two configurations, parallel and crossed fields, are analysed. The study was carried out for compensated bulk-like GaN samples at different lattice temperatures (30...300 K) and impurity concentrations (10^16..10^17 cm^{-3}). We found that at low lattice temperatures and low impurity concentrations, electric-field dependencies of the transverse-to-current components of the diffusion tensor are non-monotonic for both configurations, while the diffusion processes are greatly controlled by the magnetic field. With an increase of the lattice temperature or the impurity concentration, the behaviour of the diffusion tensor becomes more monotonous and less affected by the magnetic field. We showed that such behaviour of the diffusion processes is due to the distinct kinetics of the hot electrons in polar semiconductors with strong electron-optical phonon coupling. We suggest that measurements of the diffusion coefficient of the electrons subjected to electric and magnetic fields facilitate the identification of features of different electron transport regimes and the development of more efficient devices and practical applications.

Terahertz probe of photoexcited carrier dynamics in the Dirac semimetal Cd3As2

The relaxation dynamics of photoexcited quasiparticles of three-dimensional (3D) Dirac semimetals are vital towards their application in high performance electronic and optoelectronic devices. In this work, the relaxation dynamics of photoexcited carriers of 3D Dirac semimetal Cd3As2 are investigated by transient terahertz spectroscopy. The visible pump-THz probe spectroscopy measurement shows clear biexponential decays with two characteristic time constants. According to the pump-power and temperature dependence, these two characteristic time constants are attributed to the electron phonon coupling (1-4 ps) and anharmonic decay of hot coupled phonons to electronic uncoupled phonons (2-9 ps), respectively. An anomalous electron-optical phonon coupling reduction and a bottleneck slowing of hot optical phonons relaxation are observed with higher excitation intensities similar to that in graphene. On the other hand, the electron-optical phonon coupling can be enhanced due to the phonon frequency broadening and softening at elevated lattice temperature. Furthermore, the transient THz spectrum response is strongly modified by the phonon assisted intraband absorption of hot carriers from a pure electronic Drude model, which is evidenced by a characteristic THz absorption dip in the transient THz absorption spectrum. This absorption dip is pinned by the discrete optical phonon energy that assists the intraband transition enabled by photoexcitation of hot carriers.

Optical absorption of Fröhlich polaron in monolayer transition metal dichalcogenides

We theoretically study the optical absorption of a Fröhlich polaron in monolayer transition metal dichalcogenides (TMDS) on different polar substrates using the Devreese-Huybrechts-Lemmens model, in which both the surface optical phonon modes induced by the polar substrate and the intrinsic longitudinal optical phonon modes have been taken into account. We find that the optical absorption occurs only as the energy of incident photon exceeds the energy of optical phonon. The behaviors of absorption are determined by the interplays between the strength of electron-optical phonon coupling and the energy of optical phonon. The amplitude of absorption can be tuned directly by the internal distance between TMDS and polar substrates. These theoretical results provide significant insight into the infrared absorption in two-dimensional materials.

Transient Carrier Cooling Enhanced by Grain Boundaries in Graphene Monolayer.

Using a high terahertz (THz) electric field (ETHz), the carrier scattering in graphene was studied with an electric field of up to 282 kV/cm. When the grain size of graphene monolayers varies from small (5 μm) and medium (70 μm) to large grains (500 μm), the dominant carrier scattering source in large- and small-grained graphene differs at high THz field, i.e., there is optical phonon scattering for large grains and defect scattering for small grains. Although the electron-optical phonon coupling strength is the same for all grain sizes in our study, the enhanced optical phonon scattering in the high THz field from the large-grained graphene is caused by a higher optical phonon temperature, originating from the slow relaxation of accelerated electrons. Unlike the large-grained graphene, lower electron and optical phonon temperatures are found in the small-grained graphene monolayer, resulting from the effective carrier cooling through the defects, called supercollisions. Our results indicate that the carrier mobility in the high-crystalline graphene is easily vulnerable to scattering by the optical phonons. Thus, controlling the population of defect sites, as a means for carrier cooling, can enhance the carrier mobility at high electric fields in graphene electronics by suppressing the heating of optical phonons.

Coupling between electrons and optical phonons in suspended bilayer graphene

The electron-phonon scattering dominated heat dissipation is difficult to observe in graphene due to the role of impurities in scattering processes. Owing to the suppression of flexural-mode-induced supercollisions in suspended bilayer graphene, the authors of this paper are able to demonstrate that the intrinsic electron-optical phonon coupling governs the heat flow in suspended bilayer graphene samples.

Beyond graphene: stable elemental monolayers of silicene and germanene.

Two-dimensional materials are one of the most active areas of nanomaterials research. Here we report the structural stability, electronic and vibrational properties of different monolayer configurations of the group IV elemental materials silicene and germanene. The structure of the stable configuration is calculated and for planar and low (<1 Å) atomic buckling configurations, analysis of the electronic band structure reveals linear band dispersion giving rise to massless Dirac Fermions with a Fermi velocity about two-thirds that of graphene. Monolayer stability is shown to be directly attributed to the phonons present with the instability being driven by the out-of-plane ZA and ZO phonon modes. Long momentum relaxation lengths and high carrier mobilities are predicted for silicene and germanene based devices as carrier relaxation via phonon scattering is found to be inhibited as the electron-optical phonon coupling matrix elements are calculated to be small, being about a factor of 25 times smaller than in graphene. The consequences for phonon scattering, high energy electrical transport and integration of elemental monolayers into electronic devices are further discussed.

The estimation of current and differential conductance of armchair single-wall carbon nanotubes via dissipative energy method

We are going to apply dissipative energy method, considered as perturbation method, in order to investigate the effects of electron-optical phonon coupling on the electronic transport of armchair single wall carbon nanotubes. This method almost deals with the modeling of the behavior of electrons near ballistic regime. The results of calculations indicate that this model can be applied in estimating the current and the differential conductance of the armchair single-wall carbon nanotubes at low bias; however the perturbation method fails to reproduce the current and differential conductance at high voltages. Furthermore, this approach suggests a method that the observation of phonon energy modes involved in electron-phonon coupling becomes possible experimentally at low temperature.

Electron-phonon bound state in graphene

The fine structure of the Dirac energy spectrum in graphene induced by electron-optical phonon coupling is investigated in the portion of the spectrum near the phonon emission threshold. The derived new dispersion equation in the immediate neighborhood below the threshold corresponds to an electron-phonon bound state. We find that the singular vertex corrections beyond perturbation theory strongly increase the electron-phonon binding energy scale. The predicted enhancement of the effective electron-phonon coupling can be measured using angle-resolved spectroscopy.

Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling

Rights: © 2011 American Physical Society (APS). This is the accepted version of the following article: Fay, A. & Danneau, R. & Viljas, J. K. & Wu, F. & Tomi, M. Y. & Wengler, J. & Wiesner, M. & Hakonen, Pertti J. 2011. Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling. Physical Review B. Volume 84, Issue 24. 245427/1-7. ISSN 1098-0121 (printed). DOI: 10.1103/physrevb.84.245427, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.84.245427.

Geometry, Phase Stability, and Electronic Properties of Isolated Selenium Chains Incorporated in a Nanoporous Matrix

We report the fabrication process of isolated one-dimensional Se chains incorporated in the matrix of AlPO4-5 single crystals and the experimental investigation of the geometry, phase stability, electronic properties, and electron-phonon coupling effect of these Se chains. The structure of the helical Se chains inside the channels is discussed on the basis of X-ray scattering measurements. Thermal analysis and temperature-dependent micro-Raman measurements show that Se single chains are flexible and can convert from a weak distorted phase into another phase with strongly disordered structure ("melting" state) around 340 K. Since the electrons are confined in the one-dimensional channels, the absorption band of the Se chain is obviously blue shifted compared with that of trigonal Se. With increasing temperature, this band shifts linearly to the lower energy side, in sharp contrast to the nonlinear temperature coefficient of trigonal Se, which is attributed to the greatly diminished interchain interaction and the weakening of the electron-optical phonon coupling in a low-dimensional system. In the vicinity of the absorption band, both first-order and second-order Raman signals for the Se chain are enhanced, due to the strong electron-phonon coupling when the excitation laser energy matches the electronic transition in isolated Se chains.

Erratum: Electron optical-phonon coupling inGaAs/AlxGa1−xAsquantum wells due to interface, slab, and half-space modes [Phys. Rev. B48, 4666 (1993)

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Electrical resistivity of zirconium hydrides and deuterides at high temperatures

Electrical resistivity of zirconium hydrides and deuterides has been measured over the range of hydrogen concentration (x: or D/Zr) and temperature . It has been found that increases linearly with x in the phase and exhibits a parabolic change in the phase, shows almost no difference for H and D in the range and deviates from Matthiessen's rule in the phase. To help discuss the findings, we have made energy band calculations of and computed quantities that are closely related to . The observed change in with x can be explained by assuming that H atoms form independent electron scatterers in the phase and H atom vacancies additionally form electron scatterers in the phase. The lack of difference observed in for H and D can be explained by the fact that electron-optical phonon coupling constant obtained from the energy band calculations is much smaller than the electron-acoustic phonon coupling constant. It is difficult to give a reasonable explanation of the deviation from Matthiessen's rule only by the energy band calculation result.

Relativistic band structure calculation of cubic and hexagonal SiC polytypes

A full-potential band structure calculation, within the local density approximation to the density functional theory, has been performed for the polytypes 3C, 2H, 4H, and 6H of SiC. The calculated effective electron masses are found to be in very good agreement with experimental values. The electron-optical phonon coupling has been estimated and the polaron masses are calculated to be 3%–13% larger than the corresponding bare masses. The effective electron masses of the second lowest conduction band minima are also presented and the calculated energy difference between the two lowest minima in 4H–SiC is only 0.12 eV. The lowest conduction band in 6H–SiC is found to be very flat and to have a double-well-like minimum along the ML line. The top of the valence bands has been parametrized according to the k⋅p approximation, whereupon the effective hole masses have been determined. The spin-orbit interaction was found to have a strong influence on the value of the effective hole masses. Furthermore, total and partial densities of states are presented.

Electronic structure and electron-phonon coupling in stoichiometric and defective hydrides MPdH3 (M=Ca, Sr, Eu, Yb).

On the basis of electron transfer from the divalent element to the ${\mathrm{PdH}}_{3}$ entities, the perovskite structure M${\mathrm{PdH}}_{3}$ (M=Sr, Eu, Yb) are expected to have similar electronic states at the Fermi energy as superconducting Pd-${\mathrm{H}}_{\mathit{x}}$ and Pd--noble-metal--${\mathrm{H}}_{\mathit{x}}$ and thus could be potential candidates for superconductivity. With this motivation, we have investigated the electronic structure and some aspects of the electron-phonon coupling in these hydrides using the ab initio augmented plane-wave method, and the results of the energy bands, the total and partial wave analysis of the density of states, and the electronic contribution, ${\mathrm{\ensuremath{\eta}}}_{\mathrm{H}}$ to the electron-phonon coupling are presented. Similar studies for the hydrogen defective material ${\mathrm{CaPdH}}_{2}$ are also included. We have found that all these compounds are metallic with essentially filled Pd-d bands and a small density of states at Fermi level as in PdH. Nevertheless, the calculated values of ${\mathrm{\ensuremath{\eta}}}_{\mathrm{H}}$, which govern the contribution of the hydrogen atoms to the electron-optical phonon coupling parameter \ensuremath{\lambda}, although sizeable, are lower than in PdH. This would indicate that these compounds, if superconducting, would have lower values of the superconducting transition temperatures ${\mathit{T}}_{\mathit{c}}$ than in PdH. \textcopyright{} 1996 The American Physical Society.

Investigation of the electronic structure of the new quaternary hydride PdSr2LiH5

We have investigated the band structure of the new tetragonal quaternary mixed alkali-alkaline earth transition metal hydride PdSr2LiH5 by means of the ab initio augmented plane wave method. We find this material to be metallic. The total density of states as well as its partial wave analysis have been used to investigate the origin of the electronic states. In this hydride, like in superconducting PdH, the palladium d bands are essentially filled and the H-s states have a non-negligible contribution at the Fermi energy. We find the electronic contribution to the electron-optical phonon coupling constant to be sizeable, although smaller than in PdH.

Electron optical-phonon coupling in GaAs/AlxGa1-xAs quantum wells due to interface, slab, and half-space modes.

The electron optical-phonon coupling is studied in GaAs/Al x Ga 1-x As quantum wells as due to the interface modes, the confined slab modes in the well, and the half-space modes in the barriers. The polaron binding energy and effective mass are calculated and the relative importance of the different phonon modes is investigated as a function of the width of the quantum well. The full-energy spectrum, i.e., the discrete energy levels in the well and the continuum energy spectrum above the barrier, are included as intermediate states. The polaron binding energy and effective mass go continuously from the three-dimensional (3D) Al x Ga 1-x As to the 3D GaAs results when the well width varies from zero to infinity

Influence of the electron-phonon interaction on a donor like exciton in a semiconductor microsphere

We study the influence of the electron-optical phonon coupling on a donor like exciton for which the hole is localized at the centre of a spherical microcrystal. This work is performed within the envelope function approximation considering a non degenerated conduction band. The quantum confinement is described by an infinitively deep potential well. The electron-optical phonon coupling is treated within the adiabatic approximation. We determine the Huang-Rhys factor using a one parameter trial wave-function which allows to include the electron-hole correlation. The theoretical model is compared to experiment.

A TWO-BAND SUPERCONDUCTOR WITH A NARROW BAND NEAR THE FERMI LEVEL

The modern theory of conventional superconductivity is based upon the concept of quantum states with broken gauge symmetry providing non-zero values for the so-called "anomalous averages" [Formula: see text], [Formula: see text]. Being applied to the simplest Gorikov-like semiphenomenological electronic scenario for a metal superconductor, this idea (in the mean field approximation) leads towards a complete description of all properties of ordinary superconductors as a manifestation of macroscopic quantum phase coherence. Currently there exists already strong experimental evidence that the newly discovered high Tc superconductors exhibit the same quantum coherence effects including the highly characteristic quantity for these phenomena, namely the doubled electronic charge. On these grounds one can expect that the high Tc superconductors should also be described by "anomalous averages" of the same kind. The only question is whether Gorikov's scenario is still sufficient or we need a new scenario instead in order to grasp the whole situation. In the case of Gorikov's scenario one needs to explain how the strong electron-electron coupling (no matter of what origin) can be consistent with the system stability condition. Yet there is another argument in favour of a new scenario. The large variety of the new SC properties cannot be explained by a model with too limited number of physical parameters. In our dealing with the problem of a possible new scenario we have suggested that in order to understand the high T c superconductors one should not perhaps go so far as to doubt the one-particle approximation and to introduce the Hubbard-like models, RVB and so on. Taking into account the crystalline structure of perfect high T c superconductors we have assumed within the one-particle description the two overlapping bands approach which was proposed by Suhl, Mattias and Walker (USA) and Moskalenko (USSR) as early as in 1959. To obtain qualitatively new results we have suggested a sharp asymmetry between the two bands (one being practically a local energy level close to the Fermi level) and also a special structure of direct electron-electron interaction with dominating interband scattering of singlet electron pairs described by an amplitude g. As the final analysis has shown, an inevitable but comparatively small splitting of the two bands does not invalidate the results of the model. As to the peculiar structure of interaction we believe it might be explained through a strong electron-optical phonon coupling due to the Jahn-Teller-like instability manifested in tetragonal-orthorhombic transitions of HT c samples. The model just outlined can indeed yield high T c . In the mean field approximation (with two superconductivity order parameters) the order of magnitude of T c is given by T c ~ (|g|N(o)/2)2 EF where N(o) is the density of states in the wide band at the Fermi level and EF is a wide band width. There is an exponentially sharp decline of T c while overdoping or underdoping the system and there is also a broad plateau when the narrow band is filled partially (the heavy fermion situation). The other results of calculations including thermodynamics and also kinetic properties for the normal state do not contradict experiments except for the linear temperature dependence of the specific heat observed sometimes. But one must take into account that in reality all high T c superconductors are in fact disordered solutions, and it is necessary to consider the dirty alloy limit with Anderson's theorem being violated because of the narrow band existence. At the same time, as analysis shows, the mean field approximation is not good enough for the model (the small parameter is only ( log 2/|g|N(o))-1) and therefore one has to apply Green's functions technique in order to correct the results. There is also a possibility that the magnetic properties connected with localized spins (if there are any) must be taken into account as well.