Electron-phonon Coupling Properties Research Articles

Spectroscopic Study of the Excitonic Structure in Monolayer MoS2 under Multivariate Physical and Chemical Stimuli

Photoluminescence (PL) spectroscopy has proven to provide deep insights into the optoelectronic properties of monolayer . Herein, a corresponding study is conducted on the excitonic properties of mechanically exfoliated monolayer under multivariate physical and chemical stimuli. Specifically, midgap exciton states that originate from lattice defects are characterized and they are compared to existing models. Through statistical data analyses of substrate‐, temperature‐, and laser‐power‐dependent measurements, a PL enhancement is revealed through physisorption of water molecules of the controversially discussed excited‐state A biexciton (). In addition, analyses of monolayer on gold substrates show that surface roughness does not account for changes in doping level within the material. Also, a shift in the electron–phonon coupling properties that arises from thin films of water that are physisorbed on top of the samples is reported.

The optical phonon drag effect on strong-coupling decay magnetopolaron in Gaussian alkali halogen ionic quantum wells

The polaron phenomenon is commonly observed in low-dimensional semiconductor materials and is known to have unique effects on conductive material properties. Furthermore, the phonon dragging effect, which leads to the polaron energy level, is less than the electron energy level. A decay magnetic field also affects the polaron effect, which causes polaron energy level changes. We demonstrate the unique electron–phonon coupling properties of this polaron using numerical calculations. Our findings have strong implications for theories of polaron properties and provide compelling evidence for a semiconductor device that industrial manufacturers use for new low-dimensional materials.

First-principles calculations of electron-phonon coupling in NaSnSb: buckling and pressurization

In this paper, the electron-phonon coupling (EPC) properties of topological material NaSnSb has been explored, whose low phonon branch dominates the EPC constant λ. NaSnSb can remain the strength of its EPC with a slight compression of the distance of the Na and Sb atoms along the z-axis (z Na–Sb), while further compression or stretching would destroy the strength of its EPC. By pressing, in the pressure range of 2 GPa, the superconducting transition temperature (T c) of NaSnSb behaves similar to that of FeSe in the same pressure range, with a local minimum at 1 GPa and a maximum at 1.5 GPa. Both compressing z Na–Sb and pressure lead to phonon softening of NaSnSb. The phonon softening induced by compressing z Na–Sb is three-dimensional, whereas the phonon softening induced by compression is two-dimensional.

Dispersive Spectra of Phonon Modes and Fröhlich Electron–Phonon Coupling Properties in AlGaN Quantum Rings: Wurzite versus Zinc‐Blende Structure

The influence of crystal structure on the phonon modes and electron–phonon coupling properties in nitride quantum ring (QR) structures is investigated based on the macroscopic dielectric continuum model. It is found that three types of phonon modes including the surface optical (SO) phonon modes, the exact confined (EC) modes, and quasiconfined (QC) modes can coexist in the wurtzite nitride QR. However, only the SO modes and EC modes may appear in zinc‐blende nitride QR, and the QC modes cannot exist in zinc‐blende QRs in general. Numerical calculation on AlN QR shows that the SO modes of zinc‐blende QR are more dispersive than those of wurtzite QR. Furthermore, the degenerative behavior of QC modes to SO modes in wurtzite QRs is clearly observed. Moreover, the electron–phonon coupling functions in zinc‐blende QRs are larger than those in wurtzite QRs. These results reveal that the crystal structures greatly affect not only the type of phonon mode, but also the coupling strength of electron–phonon interaction. The present theoretical scheme and the numerical results are quite useful and important for further understanding and investigating the polaronic effect and related physical properties in wurtzite and zinc‐blende nitride QRs.

Electron-phonon coupling and vibrational properties of size-selected linear carbon chains by resonance Raman scattering

UV resonance Raman spectroscopy of size-selected linear sp-carbon chains unveils vibrational overtones and combinations up to the fifth order. Thanks to the tunability of the synchrotron source, we excited each H-terminated polyyne (HCnH with n = 8,10,12) to the maxima of its vibronic absorption spectrum allowing us to precisely determine the electronic and vibrational structure of the ground and excited states for the main observed vibrational mode. Selected transitions are shown to enhance specific overtone orders in the Raman spectrum in a specific way that can be explained by a simple analytical model based on Albrecht’s theory of resonance Raman scattering. The determined Huang–Rhys factors indicate a strong and size-dependent electron-phonon coupling increasing with the sp-carbon chain length.

Superconductivity on ScH3 and YH3 hydrides: Effects of applied pressure in combination with electron- and hole-doping on the electron–phonon coupling properties

The implementation of electron- and hole-doping, in conjunction with applied pressure, is analyzed as a mechanism to induce or enhance the superconducting state on fcc YH3 and ScH3. In particular, the evolution of their structural, electronic, and lattice dynamical properties, including the electron–phonon coupling and superconducting critical temperature (Tc) is presented and discussed, as a function of the electron- and hole-doping content as well as applied pressure. The study was performed within the density functional perturbation theory, taking into account the effects of zero-point energy through the quasi-harmonic approximation, while the doping was implemented by means of the construction of the Sc1−xMxH3 (M = Ca,Ti) and Y1−xMxH3 (M = Sr,Zr) solid solutions modeled with the virtual crystal approximation (VCA). We found that the ScH3 and YH3 hydrides showed a significant improvement of their electron–phonon coupling properties under hole-doping (M = Ca,Sr) and at pressure values close to dynamical instabilities. Instead, by electron-doping (M = Ti,Zr), the systems do not improve such properties, whatever value of applied pressure is considered. Then, as a result, Tc rapidly increases as a function of x on the hole-doping region, reaching its maximum value of 92.7(67.9) K and 84.5(60.2) K at x=0.3 for Sc1−xCaxH3 at 10.8GPa and Y1−xSrxH3 at 5.8GPa respectively, with μ∗=0(0.15), while for both, electron- and hole-doping, Tc decreases as a function of the applied pressure, mainly due to phonon hardening. By the thorough analysis of the electron–phonon properties as a function of doping and pressure, we can conclude that the tuning of the lattice dynamics is a promising path for improving superconductivity on both systems.

Potential distribution of polar optical phonons and their electron–phonon coupling properties of a wurtizite GaN-based trianglar quantum dot

Polar optical phonon modes of a wurtzite nitride trianglar quantum dot (QD) are deduced and analyzed by using the dielectric continuum model. Based on a method of nonseparable variables, the Laplace equation of electristatic potential of the structure is solved, and the exact and analytical phonon states of exact confined (EC) modes are derived. It is found that the frequency of EC modes in the nitride trianglar QD systems can take both the longitudinal optical phonons in [Formula: see text]-direction [Formula: see text] and the longitudinal optical phonons in [Formula: see text]-plane [Formula: see text], which is obviously different from the cases of EC modes in wurtzite GaN quantum wells and quantum wires due to their different confined dimensions. The polarization eigenvectors and their orthogonality relations for the EC phonon modes are derived, which could form a set of orthogonal and completed basis vectors. By using the orthogonality relations of polarization eigenvector and field quantization method, the free phonon field of EC modes and the corresponding Fröhlich electron–phonon interaction Hamiltonian are obtained. Numerical calculations on a wurtztie GaN trianglar QD are performed. The spatial distributions of phonon potentials of EC phonons both in [Formula: see text]-plane and in [Formula: see text]-direction as well as the electron–phonon coupling strength are plotted and discussed. It is observed that the potential of these EC phonon modes is strictly confined within the trianglar QD ranges. The dependence of symmetries and electron–phonon coupling strength on the quantum numbers [Formula: see text] and [Formula: see text] in [Formula: see text]-plane, and the quantum number [Formula: see text] in [Formula: see text]-direction are analyzed in detail. The relationship of the total number of peaks and troughs with these quantum numbers are also discussed and induced.

Temperature-dependent site selection of boron doping in chemically derived graphene

Site dependent light element doping in graphene can lead to exciting phenomenological prospects such as tunable bandgap, enhanced electron phonon coupling and anomalous transport properties for superconductivity, ferromagnetism and catalysis. However, they can lead to more additional defect sites and strain dependent effects as a residual fallout of the process and remains open to interpretations. In this work, we delineate a spectroscopic approach combined with ab-initio results to decipher these factors by using a prototypical in-situ boron doped reduced graphene oxide sample specimens and tuning them from low to moderate hole doping concentrations. The selectivity of doping configurations (BC3, BCO2 and BC2O) as well as their concentration is varied by regulating the annealing temperature (up to 1000 °C). We find a competitive relationship between the dopant and the residual surface oxygen atoms with gradual transformation of favorable doping configuration from out of plane to in-plane (substitutional) with increasing temperature. Furthermore, simultaneous induction of point defects and strain related effects in graphene lattice were also observed with increase in doping concentration. This led to anomalous bandgap crossover at high temperatures in boron doped graphene in comparison to the thermally reduced counterpart which could be important for electronic and transport applications.

Electronic, vibrational, and electron–phonon coupling properties in SnSe2 and SnS2 under pressure

First-principles calculations are used to investigate the effect of pressure on lattice stability, electron–phonon properties, and superconductivity in SnSe2 and SnS2.

Vibrational and electron-phonon coupling properties of β-Ga2O3 from first-principles calculations: Impact on the mobility and breakdown field

The wide band gap semiconductor β-Ga2O3 shows promise for applications in high-power and high-temperature electronics. The phonons of β-Ga2O3 play a crucial role in determining its important material characteristics for these applications such as its thermal transport, carrier mobility, and breakdown voltage. In this work, we apply predictive calculations based on density functional theory and density functional perturbation theory to understand the vibrational properties, phonon-phonon interactions, and electron-phonon coupling of β-Ga2O3. We calculate the directionally dependent phonon dispersion, including the effects of LO-TO splitting and isotope substitution, and quantify the frequencies of the infrared and Raman-active modes, the sound velocities, and the heat capacity of the material. Our calculated optical-mode Grüneisen parameters reflect the anharmonicity of the monoclinic crystal structure of β-Ga2O3 and help explain its low thermal conductivity. We also evaluate the electron-phonon coupling matrix elements for the lowest conduction band to determine the phonon mode that limits the mobility at room temperature, which we identified as a polar-optical mode with a phonon energy of 29 meV. We further apply these matrix elements to estimate the breakdown field of β-Ga2O3. Our theoretical characterization of the vibrational properties of β-Ga2O3 highlights its viability for high-power electronic applications and provides a path for experimental development of materials for improved performance in devices.

Role of compression metallization in UO2 fission-product energy cascade track: Multiscale electron-phonon analyses

While the electronic stoppage of charged fission fragments is relatively well understood, the subsequent energy cascade is not. Recent efforts to investigate this cascade and predict the resulting damage have used a two-temperature model (TTM) of the electronic and phononic systems coupled with a classical molecular dynamics (MD) simulation of the crystal lattice. In order to accurately predict the track radius produced by a fission fragment in UO2, this model (TTM + MD) requires that UO2, an insulator, have metallic properties, e.g., a substantial electron thermal conductivity and heat capacity. However, it has been predicted that UO2 becomes metallic under large pressures, and we perform ab initio (DFT-HSE) simulations to support this prediction. We show that the average U-U bond length decreases within and near the ion track during TTM + MD simulations, supporting the use of volume contraction to model the pressurized UO2 cell. Additionally, we evaluate the electron, phonon, and electron-phonon coupling properties of UO2 for variations in the pressure. In particular, we calculate the electronic heat capacity and thermal conductivity, and the electron-phonon energy coupling for use in subsequent TTM + MD simulations. The ab initio parameterized TTM + MD simulations provide a set of the track radii predictions which bracket and include the experimentally observed radii. The accuracy of the ab initio parameterized TTM + MD simulations depends on the pressure and degree of electron-phonon non-equilibrium assumed during the ab initio calculations. We suggest improvements to the current TTM + MD methodology in light of these results. Still, we show that the pressure-induced transition of UO2 from insulator to metal and subsequent energy transfer from the electronic to phononic systems can accurately explain radiation damage during swift, heavy ion stoppage in UO2. We make some additional observations regarding the accumulation and recombination of damage along the ion track and make comparison to the common SRIM model of ion stoppage and damage accumulation.

Ab initio investigation of electron-phonon interaction in LaSn3 and CaSn3

ABSTRACTWe present results for the structural, electronic, vibrational, and electron-phonon coupling properties of LaSn3 and CaSn3 adopting the simple cubic AuCu3-type structure obtained using the the generalised gradient approximation of the density functional theory and plane wave ab initio pseudopotential method. Our electronic results show that both materials display metallic character with several bands, which have mainly Sn 5p character, crossing the Fermi level. The calculated phonon spectrum of LaSn3 accords very well with reported experimental measurements. The weights of the peaks in the Eliashberg spectral function of both compounds are enhanced with the use of experimental lattice constant in our electron-phonon calculation, increasing the value of average electron phonon coupling parameter from 0.876 to 0.937 for LaSn3 (by 7%) and from 0.642 to 0.725 for CaSn3 (by 13%). The use of experimental lattice constant also improves the agreement between theoretical and experimental values of the superconducting temperature for both compounds.

Energy transfer rate and electron-phonon coupling properties in Eu3+ -doped nanophosphors.

A series of controllable emissions SrWO4 :Eu3+ and charge-compensated SrWO4 : Eum3+ (m=0.01 or 0.20) phosphors was successfully prepared via a simple co-precipitation method. The energy transfer mechanism was studied based on the Huang's theory. A low magnitude of Huang-Rhys factor (10-2 ) was calculated using phonon sideband spectra. The Judd-Ofelt parameters Ωλ (λ=2, 4 and 6) of Eu3+ -activated SrWO4 doped with charge compensation were obtained. The calculated Commission Internationale de l'Eclairage chromaticity coordinates were found to be about (0.67, 0.33) for SrWO4 : Eu0.203+ and charge-compensated SrWO4 : Eu0.203+ phosphors, which coincided with the National Television Standard Committee system standard values for red. A white light emission was obtained under 362nm excitation. The correlated color temperature was computed by a simple equation to characterize light sources. Thus, warm white light-emitting diodes with higher Ra can be constructed by combining as-prepared high efficiency, low correlated color temperature and high color purity phosphor.

Phonons and Electron–Phonon Coupling of Newly Discovered ThCr 2 Si 2 -Type Superconductor BaPd 2 As 2 : A Comparison Study with Sr(Ca)Pd 2 As 2

The phonon dispersion and electron–phonon coupling properties of ThCr2Si2-type superconductor Ba(Sr,Ca)Pd2As2 are revealed for the first time using density functional theory and Eliashberg theory. Non-negative phonons indicate the dynamical stability of the compounds at ambient condition. The calculated superconducting transition temperature T c agrees well with experiments. Vibration of Pd and As atoms contributes predominantly to the electron–phonon coupling. T c is determined fatefully by phonon properties. The increase of T c from CaPd2As2 to BaPd2As2 is attributed to the significant boosting of electron–phonon coupling constant λ, while the decrease of T c from CaPd2As2 to SrPd2As2 is attributed to the decrease of logarithmically averaged frequency.

Real structure influence on the electron–phonon coupling properties of niobium

We performed density functional theory calculations using niobium (Nb) as a model system of a conventional superconductor, to correlate the distortion of twin defects with the electron–phonon coupling properties. Calculations using different settings of the Nb elementary cell (relaxed, distorted, super cell with zig‐zag twin defect) showed that only by including real structure elements into the setting, the Eliashberg spectral function representing the electron–phonon coupling properties was derived convincingly. Based on these density functional theory calculations of the electron–phonon cou‐ pling properties of Nb, we suggest a model for a combined superconducting/charge density wave ground state which uses a lattice distortion induced into the crystal by two‐dimensional defects as modulated background potential of the charge density wave phase. The coexistence of both phases is hereby necessary for a fine‐tuning of the Fermi surface within the small local domain of the defect to match the wavelength of the lattice distortion and the Fermi wavelength by pairing fermions to bosons. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Electron-phonon coupling properties and energy transfer in NaY(WO4)2:Eu3+ phosphor

NaY(WO4)2 phosphors with various Eu3+ concentrations were prepared via a molten salt method. The crystal structure and morphology of the phosphors were characterized by means of XRD and field emission scanning electron microscope. The excitation spectra and emission spectra were measured. The energy transfer type between Eu3+ was proved. What's more, the critical energy transfer distance (Rc) and Huang-Rays factor were calculated. The self-generated quenching process of Eu3+ was explained based on Auzel's model, and the intrinsic radiative transition lifetime for 5D0 level was confirmed. The energy transfer rate between Eu3+ ions was derived on the basis of the fluorescence lifetime. Finally, the refractive index n was calculated.

Luminescence properties of Eu3+ doped CaMoO4 micron phosphors

Eu3+ doped CaMoO4 micron phosphors of different concentrations were prepared by chemical precipitation method. The photoluminescence properties were studied in detail. The X-ray diffraction measurements indicate that the samples are scheelite structure, and doping Eu3+ enlarge the lattice parameter of host material. The scanning electron microscope images show that the particle morphology is near-spherical and the size is 4-5 μm. The excitation and emission spectra of CaMoO4:Eu show that CaMoO4:Eu can be effectively excited by blue light and near UV-light, and the red light emission of high colour purity can be realized. The electron-phonon coupling properties were also studied. The results indicate that the magnitude of Huang-Rhys factor is 10-2, so the CaMoO4:Eu is a weak electron-phonon coupling material. The results also indicate that the Huang-Rhys factor increases with the increase of concentration. It is probably because that increasing the doping concentration enhances the lattice relaxation. The transition intensity parameter Ω2 of Eu3+ enlarges while the concentration is increasing. Because Ω2 and the luminescent center environment are closely related, the numerical value of Ω2 enlarges with the increase of degree of environmental disorder. But Ω4 doesn't have obvious change. It can be explained that the environmental sensitivity level of Eu3+5D0→7F2 transition is higher than that of 5D0→7F4 transition. The quantum efficiency of Eu3+ 5D0 energy level decreases with the increase of doping concentration. This can be attributed that the enhancement of Eu3+ doping concentration enlarges the energy transfer rate between luminescent centers, so the energy of excited electrons can be transferred to quenching center more easily and then the nonradiative relaxation rate of excited electrons increases. The best doping concentration of Eu3+ is 25% by drawing the concentration quenching curve. Furthermore, the energy transfer type of Eu3+ in CaMoO4 host is confirmed to be exchange interaction and the critical distance is calculated to be 8.4 Å. The chromaticity coordinate of CaMoO4:Eu phosphors is (0.654, 0.334), so the samples have high colour purity. The study indicates that CaMoO4:Eu micron phosphor prepared by chemical precipitation method is a red phosphor material with excellent property.

Ab initiolattice dynamics and electron-phonon coupling of Bi(111)

We present a comprehensive ab initio study of structural, electronic, lattice dynamical, and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in superbulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. We give results for the momentum-dependent electron-phonon coupling of electronic states belonging to the two surface electronic bands along $\overline{\ensuremath{\Gamma}M}$ which cross the Fermi energy. For larger momenta, the lower surface band exhibits a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. For momenta close to $\overline{\ensuremath{\Gamma}}$, states of both surface bands show even stronger couplings because of interband transitions to bulk states near $\overline{\ensuremath{\Gamma}}$ around the Fermi level. For these cases, the state-dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-type spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.

Temperature dependence of the dielectric function in the spectral range (0.5–8.5) eV of an In2O3 thin film

We present the dielectric function of a bcc-In2O3 thin film in the wide spectral range from near-infrared to vacuum-ultraviolet and for temperatures 10 K–300 K, determined by spectroscopic ellipsometry. From the temperature dependence of electronic transition energies, we derive electron-phonon coupling properties and found hints that the direct parabolic band-band transitions involve In-d states. Further we discuss possible excitonic contributions to the dielectric function.

Comparing electron-phonon coupling strength in diamond, silicon, and silicon carbide: First-principles study

Renormalization of the electronic band gap due to electron-phonon coupling in the tetrahedral semiconductors diamond, silicon and cubic silicon carbide is studied from first principles. There is a marked difference between the coupling of the vibrational state to the valence band maximum and to the conduction band minimum. The strength of phonon coupling to the valence band maximum is similar between the three systems and is dominated by vibrations that change the bond length. The coupling strength to the conduction band minimum differs significantly in diamond, silicon carbide and silicon. In diamond, the coupling is dominated by six small pockets of vibrational states in the phonon Brillouin zone, that are ultimately responsible for the stronger electron-phonon coupling in this material. Our results represent a first step towards the development of an a priori understanding of electron-phonon coupling in semiconductors and insulators, that should aid the design of materials with tailored electron-phonon coupling properties.