Electron-phonon Interaction Parameter Research Articles

Investigation of superconductivity of A15 type cubic Nb3Os material by using density functional theory

In this study, the structural, elastic, electronic, phonon and electron-phonon interaction properties of A15 type cubic Nb3Os compound were investigated by density functional theory. The lattice constant of this compound was obtained in agreement with experiments, and the investigation of elastic properties revealed its ductile nature. From electronic calculations for Nb3Os, the density of states at Fermi level (N(EF)) was found to be 5.31 states/eV with the largest contribution of Nb 4d orbitals. According to the results obtained from phonon calculations, Nb3Os was found to be dynamically stable in the cubic A15 structure and phonon modes were largely formed by the vibrations of Nb atoms. Using the electronic and phonon properties, we determined the Eliashberg spectral function, which illustrates the electron-phonon interaction. The electron-phonon interaction parameter and average phonon frequency values for Nb3Os were found to be 0.41 and 222.97 K, respectively. The calculated superconducting transition temperature of 1.05 K is in excellent agreement with the experimental value of 1.02 K.

Path for Room-Temperature Superconductivity in Q-Carbon-Related Materials

We present the atomic structures and nonequilibrium synthesis of a new class of materials, where the basic structural unit is a diamond tetrahedron. When units of one, two, and three tetrahedra are randomly packed, we create distinct phases of amorphous Q-carbon. Four tetrahedra in two adjacent layers lead to crystalline diamond lattice, which has four missing tetrahedra alternately. When these four missing tetrahedra are filled, we create subunit cell of crystalline Q-diamond. Theoretical calculations show that the superconducting transition temperature (Tc) in 50 atomic % B-doped Q-diamond can reach room temperature at ambient pressures. This is consistent with our earlier results using low-loss EELS measurements in 50 atomic % B-doped Q-carbon, which had mostly amorphous QB3 phase mixed with some crystalline Q-diamond phase. These EELS results showed that the Tc for these samples was between 90 K and 300 K. Theoretical calculations of density of states, Eliashberg function, electron–phonon interaction parameter, and root-mean-square and logarithmic average of frequency in crystalline Q-diamond show Tc in the range of 268 K to 300 K, which is in complete agreement with our EELS results in QB3.

Synthesis of technetium hydride TcH1.3 at 27 GPa

In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using the synchrotron X-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen content in TcH$_x$ phases increases when the pressure rises, and at 27 GPa we found a new hexagonal (hcp) nonstoichiometric hydride TcH$_{1.3}$. The formation of technetium hydrides is also confirmed by the emergence of a new reflective band at 450-600 nm in the reflectance spectra of TcH$_x$ samples synthesized at 45 GPa. On the basis of the theoretical analysis, we proposed crystal structures for the TcH$_{0.45 \pm 0.05}$ (Tc$_{16}$H$_7$) and TcH$_{0.75 \pm 0.05}$ (Tc$_4$H$_3$) phases previously obtained at 1-2 GPa. The calculations of the electron-phonon interaction show that technetium hydrides TcH$_{1+x}$ do not possess superconducting properties due to the low electron-phonon interaction parameter ($\lambda \sim 0.23$).

Role of anharmonicity in the phonon density of states of high temperature superconductors

Owing to the quantum dynamical approach of double time temperature dependent phonon-Green’s functions, the expression for phonon density of states (PDOS) has been developed via an almost complete Hamiltonian which consists of the contributions due to the unperturbed phonons and electrons, anharmonic phonon fields, localized phonon fields and that of electron–phonon interactions. It has been observed that the anhrmonicity play an vital role in the PDOS through energy line width. Enhancement in the PDOS due to electron–phonon interactions has been mainly discussed in this work, which, in turns, will be capable to explain a large number of dynamical properties of crystals even that of superconductors. The temperature dependence of PDOS has been found as a unique feature of the theory, which certainly becomes the outcome of the anharmonic interactions. The presence of electron–phonon interaction parameters in each term is an additional and new feature of the theory.

Fano-Type Wavelength-Dependent Asymmetric Raman Line Shapes from MoS2 Nanoflakes.

Excitation wavelength-dependent Raman spectroscopy has been carried out to study electron-phonon interaction (Fano resonance) in multi-layered bulk 2H-MoS2 nano-flakes. The electron-phonon coupling is proposed to be caused due to interaction between energy of an excitonic quasi-electronic continuum and the discrete one phonon, first-order Raman modes of MoS2. It is proposed that an asymmetrically broadened Raman line shape obtained by 633 nm laser excitation is due to electron-phonon interaction whose electronic continuum is provided by the well-known A and B excitons. Typical wavelength-dependent Raman line shape has been observed, which validates and quantifies the Fano interaction present in the samples. The experimentally obtained Raman scattering data show very good agreement with the theoretical Fano-Raman line-shape functions and help in estimating the coupling strength. Values of the electron-phonon interaction parameter obtained, through line-shape fitting, for the two excitation wavelengths have been compared and shown to have generic Fano-type dependence on the excitation wavelength.

Optical and Morphological Study of Hydroxyapatite on Titanium Dioxide Particles for Photovoltaic Applications

In this study, a natural mineral termed hydroxyapatite (HA) (Ca10(PO4)6(OH)2) was produced with TiO2 stoichiometrically by normal sol-gel process. An investigation on how the different percentage weights of HA on TiO2 can affect its optical and morphological properties was carried out, while the morphological nature and elemental composition were verified through scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) analysis. Consequently UV–visible spectroscopy was used to measure the absorbance data, where optical constants such as absorption coefficient, extension coefficient, refractive index, transmittance, absorbance, thermal conductivity, the electron-phonon interaction and steepness parameter were evaluated. According to the optical examination, the transmittance in the visible range is between (400 and 800 nm), and the absorption response in the UV region is between (200 and 400 nm) with an absorption edge occurring between (650 and 950 nm). The value of the absorption coefficient (α) and extension coefficient (k) of HA/TiO2 increased with an increase in the percentage weight (wt%) of HA. Equally, the optical conductivity increases with an increase in photon energy. In addition, the band gap energy decreases as the percentage weight of HA increases.

The Electron-Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering.

Atom scattering is becoming recognized as a sensitive probe of the electron-phonon interaction parameter λ at metal and metal-overlayer surfaces. Here, the theory is developed, linking λ to the thermal attenuation of atom scattering spectra (in particular, the Debye-Waller factor), to conducting materials of different dimensions, from quasi-1D systems such as W(110):H(1 × 1) and Bi(114), to quasi-2D layered chalcogenides, and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of λ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition, the number of layers contributing to the electron-phonon interaction, which is measured in an atom surface collision.

Origin of the Electron-Phonon Interaction of Topological Semimetal Surfaces Measured with Helium Atom Scattering.

He atom scattering has been demonstrated to be a sensitive probe of the electron–phonon interaction parameter λ at metal and metal-overlayer surfaces. Here it is shown that the theory linking λ to the thermal attenuation of atom scattering spectra (the Debye–Waller factor) can be applied to topological semimetal surfaces, such as the quasi-one-dimensional charge-density-wave system Bi(114) and the layered pnictogen chalcogenides. The electron−phonon coupling, as determined for several topological insulators belonging to the class of bismuth chalcogenides, suggests a dominant contribution of the surface quantum well states over the Dirac electrons in terms of λ.

Prebreakdown Processes in a Metal Surface Microprotrusion Exposed to an RF Electromagnetic Field

A 2-D two-temperature model has been developed to simulate the heating of a microprotrusion on a metal surface exposed to a radio frequency (~10 GHz) electromagnetic field. The numerical simulation using this model includes a self-consistent calculation of the electric field at the microprotrusion surface and the field emission characteristics of the microprotrusion using the particle-in-cell method, a calculation of the current density distribution in the microprotrusion, and a calculation of the lattice and electron temperatures with the use of the heat equation taking into account the Joule and Nottingham effects, and also the finite rate of the heat exchange between the electrons and the lattice. The microprotrusion heating time has been calculated in relation to the electric field enhancement factor and to the electron–phonon interaction parameters.

The effect of electron-phonon interaction on the formation of reverse currents of p-n-junctions of silicon-based power semiconductor devices

The article represents the physical processes and mechanisms that accompany the work of the real p-n-junction. The nature of the reverse and forward currents of p-n-junctions is studied as the basis for the operation of these devices. The new model of recombination in space charge region of p-n-junction has been developed. This model made it possible to develop a new method for processing current-voltage characteristics and determining the parameters of recombination centers in semiconductor devices including electron-phonon interaction parameters. The probability of electronic transitions is calculated. It takes into account the electron-phonon interaction and explains the rapid build-up of reverse currents as the applied voltage increases. The mechanisms of formation of the reverse current of the p-n-junction were discussed and it was concluded that the current is determined by the generation with the participation of deep centers and electron-phonon interaction.

High-temperature superconductivity in sulfur hydride evidenced by alternating-current magnetic susceptibility.

ABSTRACTThe search for high-temperature superconductivity is one of the research frontiers in physics. In the sulfur hydride system, an extremely high Tc (∼200 K) has been recently developed at pressure. However, the Meissner effect measurement above megabar pressures is still a great challenge. Here, we report the superconductivity identification of sulfur hydride at pressure, employing an in situ alternating-current magnetic susceptibility technique. We determine the superconducting phase diagram, finding that superconductivity suddenly appears at 117 GPa and Tc reaches 183 K at 149 GPa before decreasing monotonically with increasing pressure. By means of theoretical calculations, we elucidate the variation of Tc in the low-pressure region in terms of the changing stoichiometry of sulfur hydride and the further decrease in Tc owing to a drop in the electron–phonon interaction parameter λ. This work provides a new insight into clarifying superconducting phenomena and anchoring the superconducting phase diagram in the hydrides.

The acceptors behavior in the Single and Double Doped Bi12TiO20

Abstract The absorption coefficient of Bi12TiO20:AlI, Bi12TiO20:AlII, Bi12SiO20:P and Bi12SiO20:Al+P single crystals is measured in the spectral region of Urbach’s rule (1.52 – 2.92 eV) at room temperature. The parameters of electron-phonon interaction, Urbach’s energy and the constants of Urbach’s rule are calculated. The behavior of the acceptors Al3+ and P5+ in Urbach’s rule region has been considered.

Polarons on nonlinear lattice in the Su-Schrieffer-Heeger approximation: Exact solution and multipeaked polarons

We investigate the polaron dynamics on the nonlinear lattice with the cubic nonlinearity. The electron-phonon interaction is accounted in the Su-Schrieffer-Heeger approximation. An exact analytical solution is obtained in the continuum approximation at certain relation of parameters. The numerical simulation agrees with analytics very well. Moreover, colliding polarons recover their shapes and velocities after the elastic collision suggesting that the solution belongs to the exactly integrable system. When the continuum approximation is invalid (parameters of nonlinearity and electron-phonon interaction are not small), a new family of stable multipeaked polarons is found. These polarons are formed by the coupled solitons hold together by the electron-phonon interaction.

Nonequilibrium optical conductivity in materials with localized electronic states

A wide range of disordered materials contain electronic states that are spatially well localized. In this work, we investigated the electrical response of such systems in non-equilibrium conditions to external electromagnetic field. We obtained the expression for optical conductivity valid for any non-equilibrium state of electronic subsystem. In the case of incoherent non-equilibrium state, this expression contains only the positions of localized electronic states, Fermi's golden rule transition probabilities between the states and the populations of electronic states. The same form of expression is valid both in the case of weak electron-phonon interaction and weak electron-impurity interaction that act as perturbations of electronic Hamiltonian. The derivation was performed by expanding the general expression for AC conductivity in powers of small electron-phonon interaction or electron-impurity interaction parameter. Applications of the expression to two model systems, a simple one- dimensional Gaussian disorder model and the model of a realistic three-dimensional organic polymer material, were presented, as well.

Electronic Band-Structure Calculations of Ba8Me x Si46-x Clathrates with Me = Mg, Pd, Ni, Au, Ag, Cu, Zn, Al, Sn

Clathrate materials of AlSi, CuSi or NiSi type consisting of abundant elements have a realistic chance of becoming useful thermoelectrics in the near future, because the rattling effect due to their crystal cage structure provides a large figure of merit ZT even in experiments measured under large temperature gradients. In the search for better thermoelectrics, new element combinations in the clathrate type I structure with cubic space group Pm3n were calculated using VASP ab initio software. Predictions of the Seebeck coefficient were made by checking the electronic band structure and density of states for a large variety of input data. For x values around 4 to 6 in the structural formula Ba8MexSi46−x the substituents Cu, Au, and Ag are best for good thermoelectric behavior, which is discussed in this paper as a result of the low electron–phonon interaction parameter.

Optical absorption edge in (Ag3AsS3)x(As2S3)1-x superionic glasses

The spectrometric studies of optical absorption edge in     x 1 3 2 x 3 3 S As AsS Ag  superionic glasses were carried out within the temperature range 77 to 400 K. The influence of temperature and composition on the optical absorption edge, parameters of the Urbach absorption edge, parameters of electron-phonon interaction as well as ordering-disordering processes in     x 1 3 2 x 3 3 S As AsS Ag  superionic glasses are studied.

The calculation of the quadrupole exchange interaction parameter in a system of disordered magnetic moments

The parameters of quadrupole exchange and electron-phonon interaction are calculated within the limits of the exchange interaction model. It is shown that a magnetically disordered system forms an effective exchange interaction between electrons.

Method of calculating the electron-phonon scattering of surface electronic states on the (110) surface of noble metals

A theoretical method of calculating the electron-phonon scattering of electron and hole excitations on the (110) surface of noble metals is described in detail. The method is based on a description of the electronic surface structure by two-dimensional model pseudopotential. As an example, the electron-phonon interaction parameters for the surface states and image potential states localized in the $ \bar{Y} $ -gap are calculated for the Pd(110) surface. The electron-phonon coupling parameter λ calculated in two surface states differs by several times owing to different localizations of the states. The parameter λ for the image potential states is rather small.

Temperature variation of electrical conductivity and absorption edge in Cu 7GeSe 5I advanced superionic conductor

Single crystals of Cu 7GeSe 5I superionic conductor were grown by chemical transport. Their electrical conductivity in the frequency range 1.0×10 6–1.2×10 9 Hz and in the temperature range 196–295 K was measured. Cu 7GeSe 5I crystal is shown to exhibit a rather high electrical conductivity ( σ 295=64.0 S/m at 295 K) and a low activation energy (Δ E a=0.125 eV). Optical absorption edge of Cu 7GeSe 5I crystals in the temperature range 77–300 K was studied, the temperature dependences of the optical pseudogap and Urbach energy being obtained. The effect of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters was investigated.

Electron-phonon interaction and nonlinear transport phenomena in solid Hg point-contacts

At cryogenic temperatures the conductivity of Hg point-contacts was studied in both the superconducting and normal state. An original method of fabricating the Hg-based point-contacts directly in liquid 4He was proposed, which guaranteed creation of small-size high-purity ballistic contacts. The resistance of the contacts as well as the current-voltage characteristics along with the voltage dependence of their first and second derivatives were experimentally investigated at 1.5 K. We analyze such characteristics of the contacts as Josephson critical current, excess current, energy gap and the nonlinear part of conductivity caused by electron-phonon interaction (EPI). The point-contact EPI function gpc(ω) for Hg was reconstructed and integral parameters of EPI were calculated. The gpc(ω) was then used as an approximation to the phonon density of states for estimations of the thermodynamic characteristics in Hg. Finally we discuss the results of our calculations of non-linear conductivity caused by manifestation of the frequency dependence of the energy gap function in the elastic current component through the contact.