Electron-phonon Coupling Parameter Research Articles

Luminescence properties and phase transformation of broadband NIR emitting A2(WO4)3:Cr3+ (A=Al3+, Sc3+) phosphors toward NIR spectroscopy applications

The synthesis, structural, and luminescence properties have been carried out for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for application in pc-NIR LED. Upon blue excitation, these compounds are capable of exhibiting broadband NIR emission stems primarily from 4T2→4A2 transition in the range of 670–1200 nm (maxima ∼808 nm, FWHM ∼140 nm) for AWO:Cr and of 700–1300 nm (maxima ∼870 nm, FWHM ∼164 nm) for SWO:Cr. The significant shift of NIR emission is attributed to the substitution of AlO6 with larger ScO6 octahedrons. To gain insight into the luminescence the crystal field strength, Racah parameters, nephelauxetic effect, and electron-phonon coupling have been analyzed based on spectroscopic results. The electron-phonon coupling parameter S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr, which is in accordance with its strong thermal quenching. The abrupt changes occurring at 275 K in temperature-dependent luminescence spectra and decay lifetime of AWO:Cr is associated with temperature-driven phase transformation from low-temperature monoclinic to high-temperature orthorhombic phase. Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was confirmed by employing high pressure evolution of Raman spectra. A high-power NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip, shows good performance with an output power of 17.1 mW driven by a current of 320 mA, revealing potential application of studied materials for NIR light source.

Relaxation of photoexcited hot carriers beyond multitemperature models: General theory description verified by experiments on Pb/Si(111)

The equilibration of electronic carriers in metals after excitation by an ultra-short laser pulse provides an important class of non-equilibrium phenomena in metals and allows measuring the effective electron-phonon coupling parameter. Since the observed decay of the electronic distribution is governed by the interplay of both electron-electron and electron-phonon scattering, the interpretation of experimental data must rely on models that ideally should be easy to handle, yet accurate. In this work, an extended rate-equation model is proposed that explicitly includes non-thermal electronic carriers while at the same time incorporating data from first-principles calculations of the electron-phonon coupling via Eliashberg-Migdal theory. The model is verified against experimental data for thin Pb films grown on Si(111). Improved agreement between theory and experiment at short times (<0.3ps) due to non-thermal electron contributions is found. Moreover, the rate equations allow for widely different coupling strength to different phonon subsystems. Consequently, an indirect, electron-mediated energy transfer between strongly and weakly coupled groups of phonons can be observed in the simulations that leads to a retarded equilibration of the subsystems only after several picoseconds.

Temperature-dependent band modification and energy dependence of the electron-phonon interaction in the topological surface state on Bi2Te3

We report the temperature-dependent band structure and energy dependence of the electron-phonon interaction of the topological surface state (TSS) on the three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ using angle-resolved photoemission spectroscopy. Despite the nonrigid-band-like band modification as the temperature increases, the degeneracy at the Dirac point (DP) was robust, indicating topological protection. The electron-phonon coupling parameter depends on the initial-state electron energy, varying from 0.13 (at the DP) to 0.02 (100 meV above the DP), in agreement with reported first-principles calculations. Our findings provide insight into the physical properties derived from TSSs at finite temperatures.

Multi-temperature modeling of femtosecond laser pulse on metallic nanoparticles accounting for the temperature dependences of the parameters

This review considers the fundamental dynamical processes of metal nanoparticles during and after the impact of a femtosecond laser pulse on a nanoparticle, including the absorption of photons. Understanding the sequence of events after photon absorption and their timescales is important for many applications of nanoparticles. Various processes are discussed, starting with optical absorption by electrons, proceeding through the relaxation of the electrons due to electron–electron scattering and electron–phonon coupling, and ending with the dissipation of the nanoparticle energy into the environment. The goal is to consider the timescales, values, and temperature dependences of the electron heat capacity and the electron–phonon coupling parameter that describe these processes and how these dependences affect the electron energy relaxation. Two- and four-temperature models for describing electron–phonon relaxation are discussed. Significant emphasis is paid to the proposed analytical approach to modeling processes during the action of a femtosecond laser pulse on a metal nanoparticle. These consider the temperature dependences of the electron heat capacity and the electron–phonon coupling factor of the metal. The entire process is divided into four stages: (1) the heating of the electron system by a pulse, (2) electron thermalization, (3) electron–phonon energy exchange and the equalization of the temperature of the electrons with the lattice, and (4) cooling of the nanoparticle. There is an appropriate analytical description of each stage. The four-temperature model can estimate the parameters of the laser and nanoparticles needed for applications of femtosecond laser pulses and nanoparticles.

A complete study on the temperature dependences of zero-field splitting b4 0 for Eu2+ and Gd3+ ions in CdF2 crystal

ABSTRACT This paper gives for the first time a complete equation to analyze the temperature dependence (or thermal variation) of spin-Hamiltonian parameter from low to high temperatures for transition metal and rare-earth ions in crystals. This equation includes not only the dynamic effect (only considered in previous papers) caused by electron–phonon interaction but also the static effect owing to lattice thermal expansion. Based on the complete equation, the thermal variations of zero-field splitting b4 0 from low to high temperatures for cubic Eu2+ and Gd3+ centers in CdF2 crystal are studied. The contributions due to static effect are estimated from the pressure dependence of zero-field splitting b4 0. The static parameters B (charactering the static effect) and the electron–phonon coupling parameters K (charactering the dynamic effect) for both systems are determined and the coefficients (B + K) in the complete equation are obtained. The results are discussed.

Prediction of phonon-mediated superconductivity in new Ti-based M_2AX phases

A high-throughput computational method is used to predict 39 new superconductors in the Ti-based M_2AX phases, and the best candidates are then studied in more detail using density functional theory electron–phonon coupling calculations. The detailed calculations agree with the simple predictions, and Ti_2AlX (X: B, C and N) materials are predicted to have higher values of T_c than any currently known hexagonal M_2AX phases. The electronic states at the Fermi level are dominated by the Ti 3d states. The choice of X (X: B, C and N) has a significant impact on the electronic density of states but not on the phonon characteristics. The electron–phonon coupling parameter for Ti_2AlX (X: B, C and N) was determined to be 0.685, 0.743 and 0.775 with a predicted T_c of 7.8 K, 10.8 K and 13.0 K, respectively.

Effect of Atomic-Temperature Dependence of the Electron-Phonon Coupling in Two-Temperature Model.

Ultrafast laser irradiation of metals can often be described theoretically with the two-temperature model. The energy exchange between the excited electronic system and the atomic one is governed by the electron–phonon coupling parameter. The electron–phonon coupling depends on both, the electronic and the atomic temperature. We analyze the effect of the dependence of the electron–phonon coupling parameter on the atomic temperature in ruthenium, gold, and palladium. It is shown that the dependence on the atomic temperature induces nonlinear behavior, in which a higher initial electronic temperature leads to faster electron–phonon equilibration. Analysis of the experimental measurements of the transient thermoreflectance of the laser-irradiated ruthenium thin film allows us to draw some, albeit indirect, conclusions about the limits of the applicability of the different coupling parametrizations.

Physical properties and superconductivity in the cubic A15-type Ta[formula omitted]Ge compound: A first principles study

In this theoretical study, we have targeted to present the characteristic features of the superconductivity in the cubic A15-type compound Ta3Ge by investigating its structural, electronic, elastic, mechanical, phonon, and electron–phonon interaction properties in detail. A critical examination of elastic moduli of Ta3Ge reveals that it is soft (ductile) due to significant metallic bonding. A critical surveying of electronic density of states reveals that Ge electronic states make negligible additive to the main valence band region of Ta3Ge while its transition metal d states dominate this region. An examination of our phonon results indicate that the low-frequency optical phonon branches and three acoustic phonon branches of Ta3Ge are mainly dominated by the vibrations of Ta atoms. Thereby, their d states and their vibrations become the main actors of the transition from the normal state to the superconducting state for Ta3Ge. From the integration of the Eliashberg spectral function of Ta3Ge, the values of average electron–phonon coupling parameter and logarithmically averaged phonon frequency are found to be 1.08 and 123.05 K, respectively, producing its superconducting temperature Tc = 8.3 K, being in good accordance with the experimental value of 8.0 K.

Single-crystalline transition metal phosphide superconductor WP studied by Raman spectroscopy and first-principles calculations

We present a study of phonon modes and electron-phonon coupling in superconductor WP using angle-resolved polarized Raman spectroscopy. Seven out of twelve Raman-active modes are detected, with frequencies in good accordance by the first-principles calculations. We analyze parallel- and cross-polarized configurations of ${\mathrm{A}}_{\mathrm{g}}$ and ${\mathrm{B}}_{3\mathrm{g}}$ phonon modes with polarization- and angle-resolved Raman measurements. For the observed seven modes, the total electron-phonon coupling parameter $\ensuremath{\lambda}$ is only about 0.07, which is too small to achieve superconductivity. Our results elucidate that the low-frequency phonon modes play a key role in the superconductivity within the Bardeen-Cooper-Schrieffer theory framework.

Nonperturbative ab initio approach for calculating the electrical conductivity of a liquid metal

We propose a nonperturbative ab initio approach to calculate the electrical conductivity of a liquid metal. Our approach is based on the Kubo formula and the theory of electron-phonon coupling (EPC) and, unlike the conventional empirical approach based on the Kubo-Greenwood formula, fully takes into account the effect of coupling between electrons and moving ions. We show that the electrical conductivity at high temperature is determined by an EPC parameter ${\ensuremath{\lambda}}_{\mathrm{tr}}$, which can be inferred, nonperturbatively, from the correlation of electron scattering matrices induced by ions. The latter can be evaluated in a molecular dynamics simulation. Based on the density-functional theory and pseudopotential methods, we implement the approach in an ab initio manner. We apply it to liquid sodium and obtain results in good agreement with experiments. This approach is efficient and based on a rigorous theory suitable for applying to general metallic liquid systems.

Electron-phonon interaction in the dynamics of trap filling in quantum dots

We analyze theoretically the effects of electron-phonon interaction in the dynamics of an electron that can be trapped to a localized state and detrapped to an extended band state of a small quantum dot (QD) using a simple model system. It consists of a one-dimensional tight-binding linear chain of a few sites, having a discrete set of energy levels mimicking the discrete levels of the conduction band of the QD that is connected at its end to another site, the trap, having a single energy level well below the conduction band, where the electron is allowed to interact with a local phonon of a single frequency. In spite of its simplicity, the time-dependent model has no analytical solution but a numerically exact one can be found, producing a rich dynamics. The electronic motion is quasiperiodic in time, with oscillations around a mean value that are basic characteristics of the weak and strong coupling regimes of electron-phonon interaction and set the timescales of the system. Using values of the parameters appropriate for defects in semiconductor QDs, we find these timescales to range typically from tenths of picoseconds to a few picoseconds. The values of the time-averaged trap occupancy strongly depend on the the strength of the electron-phonon interaction and can be as large as $40%$ when the coupling is most efficient, independently of other parameters. An interesting result of the present paper is the formation of resonances at specific values of the electron-phonon coupling parameter that only exist when several levels are allowed to coherently cooperate in the filling of the trap. They are characterized by a trap occupancy that is a periodic function of time with large amplitude and period picturing an electron that is periodically trapped and detrapped. We conclude that the formation of these resonances is a robust consequence of electron-phonon interaction in small systems. Electron-phonon interaction is an efficient mechanism that can provide ca. $50%$ filling of a deep trap state on a subpicosecond to picosecond timescale, much faster than radiative decay occurring in timescales of tens of picoseconds to nanoseconds, while the occupancy of this state will be smaller than ca. $1%$ in the absence of electron-phonon coupling.

Scanning tunneling spectroscopy of subsurface Ag and Ge impurities in copper

We investigate single Ge and Ag impurities buried below a Cu(100) surface using low temperature scanning tunneling microscopy. The interference patterns in the local density of states are surface scattering signatures of the bulk impurities, which result from 3D Friedel oscillations and the electron focusing effect. Comparing the isoelectronic d scatterer Ag and the sp scatterer Ge allows to distinguish contributions from impurity scattering and the host. Energy-independent effective scattering phase shifts are extracted using a plane wave tight-binding model and reveal similar values for both species. A comparison with ab initio calculations suggests incoherent sp scattering processes at the Ge impurity. As both scatterers are spectrally homogeneous, scanning tunneling spectroscopy of the interference patterns yields real-space signatures of the bulk electronic structure. We find a kink around zero bias for both species that we assign to a renormalization of the band structure due to many-body effects, which can be described with a Debye self-energy and a surprisingly high electron–phonon coupling parameter λ. We propose that this might originate from bulk propagation in the vicinity of the surface.

Ultrafast lattice dynamics and electron-phonon coupling in platinum extracted with a global fitting approach for time-resolved polycrystalline diffraction data.

Quantitative knowledge of electron–phonon coupling is important for many applications as well as for the fundamental understanding of nonequilibrium relaxation processes. Time-resolved diffraction provides direct access to this knowledge through its sensitivity to laser-induced lattice dynamics. Here, we present an approach for analyzing time-resolved polycrystalline diffraction data. A two-step routine is used to minimize the number of time-dependent fit parameters. The lattice dynamics are extracted by finding the best fit to the full transient diffraction pattern rather than by analyzing transient changes of individual Debye–Scherrer rings. We apply this approach to platinum, an important component of novel photocatalytic and spintronic applications, for which a large variation of literature values exists for the electron–phonon coupling parameter . Based on the extracted evolution of the atomic mean squared displacement and using a two-temperature model, we obtain (statistical error). We find that at least up to an absorbed energy density of 124 J/cm3, is not fluence-dependent. Our results for the lattice dynamics of platinum provide insights into electron–phonon coupling and phonon thermalization and constitute a basis for quantitative descriptions of platinum-based heterostructures in nonequilibrium conditions.

Electron–phonon interaction and superconductivity in hexagonal ternary carbides Nb2 AC (A: Al, S, Ge, As and Sn)

The superconducting transition temperatures T c of hexagonal Nb2 AC (A: Al, S, Ge, As and Sn) are investigated using density functional perturbation theory to model the electron–phonon interaction. A critical assessment of the calculated electronic structure and density of states revealed that the electronic states near to the Fermi level are mostly composed of the Nb 4d states, which are responsible for the electrical conductivity. The theoretical T c data from electron–phonon calculations are in excellent agreement with the Fröhlich model, and this model was used as a computationally efficient screening method to identify promising Nb–C M 2 AX phase materials. For Nb2 AC (A: Zn, Cd, Al, Ga, In, Tl, Si, Pb and P), the model indicated that Nb2AlC should have the highest T c of this set, a little lower than Nb2GeC and comparable to Nb2SC and Nb2SnC. Superconductivity in Nb2AlC has not been studied experimentally, but this result was confirmed by full electron–phonon calculations, which also revealed that the mechanism for superconductivity is the interactions of Nb 4d-state electrons with low-frequency phonons (in particular, acoustic phonon and low-frequency optical phonons dominated by Nb and the A element). The average electron–phonon coupling parameter was found to be λ ∼ 0.646, 0.739, 0.685, 0.440 and 0.614 for Nb2 AC (A: Al, S, Ge, As and Sn), respectively, with a corresponding superconducting critical temperature T c ∼ 6.7 K, 7.7 K, 9.8 K, 2.1 K and 6.3 K, respectively.

Structural stability and electron‐phonon coupling in two‐dimensional carbon allotropes at high electronic and atomic temperatures

Several two-dimensional carbon allotropes have been recently fabricated experimentally, triggering various innovative research and technological applications. The properties of such 2d materials under extreme conditions are to a large extent unknown. In this work, we study theoretically the effects of high electronic and atomic temperatures on free-standing graphene, pentaheptite, and biphenylene network. We analyse the limits of their structural stability with respect to thermal and nonthermal damage. With a state-of-the-art approach, we calculate the electronic heat capacity and the electron-phonon coupling parameter dependent on both electronic and atomic temperatures. The knowledge of these parameters may facilitate modelling of various two-dimensional carbon allotropes under laser pulse irradiation, improving understanding of their behaviour in experimental applications.

Elucidating the underlying mechanism of relatively high T c value of the orthorhombic MoRuP: a first-principles study

ABSTRACT In this paper, the structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties of orthorhombic MoRuP have been analysed by using the generalised gradient approximation of the density functional theory and the plane-wave pseudopotential method. An examination of elastic constants and elastic moduli indicates that the orthorhombic MoRuP is mechanical stable and ductile. A critical assessment of electronic structure and electronic density of states implies that the electronic states in the vicinity of the Fermi level are mostly composed of the 4d states of the molybdenum atoms in MoRuP. A comparison of Eliashberg spectral function with electronic and phonon density of states reveals that the mechanism behind the superconductivity for the orthorhombic MoRuP is the coupling of Mo 4d electrons with Mo-related lattice vibrations. The integration of Eliashberg spectral function gives the electron-phonon coupling parameter of MoRuP to be 0.98, implying that the orthorhombic MoRuP belongs to the conventional BCS-type superconductors with strong coupling. Finally, the value of superconducting transition temperature for MoRuP amounts to 16.5 K, harmonising with the experimental value of 15.5 K.

Estimations of the electron-phonon coupling parameters of the spectral lines in GaAs: Er3+ and Y3Al5O12:Yb3+

The temperature dependences (or shifts) of spectral lines 4I13/2→4I15/2 in GaAs: Er3+ and v5→v1 in Y3Al5O12:Yb3+ crystals are studied by using an approximate processing based on a full expression. This expression contains both the static effect owing to lattice thermal expansion and the dynamic effect coming from electron-phonon interaction. The static effect is evaluated from the pressure shift of spectral line position. The studies manifest that for both the pressure and temperature shifts of a spectral line, even if one shift is zero (e.g., the zero pressure shift in GaAs: Er3+ and zero temperature shift in Y3Al5O12:Yb3+), the electron-phonon coupling parameter (charactering the dynamic effect) can be obtained from another shift. The obtained magnitudes of electron-phonon coupling parameters for 4fn ions Er3+ in GaAs and Yb3+ in Y3Al5O12 in order of magnitude are close to those of other 4fn ions in crystals and hence are reasonable.

Optoelectronic Properties of Chalcogenide Perovskites by Many-Body Perturbation Theory.

Chalcogenide perovskites have emerged as lead-free, stable photovoltaic materials, having promising optoelectronic properties. However, a detailed theoretical study on excitonic properties is rather demanding task due to the huge computational cost and, therefore, is hitherto unknown. Here, we report the excitonic properties of chalcogenide perovskites AZrS3 (A = Ca, Sr, Ba) using state-of-the-art hybrid density functional theory and many-body perturbation theory (within the framework of GW and BSE). We find the exciton binding energy (EB) is larger than that of conventional halide perovskites. We also observe, by computing the electron-phonon coupling parameters, a more stable charge-separated polaronic state as compared to that of the bound exciton. The ionic contribution to dielectric screening is found to be negligible in this class of materials. On the basis of the direct band gap and the absorption coefficient, the estimated spectroscopic limited maximum efficiency is quite good when these materials are considered as promising environmentally friendly perovskites suitable for photovoltaics.

Electron-Phonon Coupling Parameter of Ferromagnetic Metal Fe and Co.

The electron-phonon coupling (g) parameter plays a critical role in the ultrafast transport of heat, charge, and spin in metallic materials. However, the exact determination of the g parameter is challenging because of the complicated process during the non-equilibrium state. In this study, we investigate the g parameters of ferromagnetic 3d transition metal (FM) layers, Fe and Co, using time-domain thermoreflectance. We measure a transient increase in temperature of Au in an FM/Au bilayer; the Au layer efficiently detects the strong heat flow during the non-equilibrium between electrons and phonons in FM. The g parameter of the FM is determined by analyzing the temperature dynamics using thermal circuit modeling. The determined g values are 8.8–9.4 × 1017 W m−3 K−1 for Fe and 9.6–12.2 × 1017 W m−3 K−1 for Co. Our results demonstrate that all 3d transition FMs have a similar g value, in the order of 1018 W m−3 K−1.

Thermal conductivity and electrical resistivity of single copper nanowires.

Copper nano-interconnects are ubiquitous in semiconductor devices. The electrical and thermal properties of copper nanowires (CuNWs) profoundly affect the performance of electronics. In contrast to the intensively studied electrical properties of CuNWs, the thermal conductivities of CuNWs have seldom been examined. In this study, the electrical resistivity and thermal conductivity of single CuNWs were investigated. The Bloch-Grüneisen formula was introduced to determine the mechanisms responsible for the obtained electrical resistivity of the CuNWs. High residual resistivity was found, which indicated strong structural scattering on the electron transport resulting from defect scattering and boundary scatterings at the copper-copper oxide interface and grain boundaries. The mean structural scattering distance was employed to appreciate the degree of structural scattering in the CuNWs. The residual resistivity and electron-phonon coupling parameter were found to increase with the degree of structural scattering. Moreover, the unified thermal resistivity was introduced to illustrate the mechanisms responsible for the CuNWs' thermal conductivities. Similarly, large values of residual unified thermal resistivity and electron-phonon-induced unified thermal resistivity were found. The obtained unified thermal resistivities of the CuNWs could also be qualitatively explained by the degree of structural scattering in the CuNWs. The results suggested that structural scattering was predominant in the electrical current transport and heat transfer in the nanowires. This study revealed the mechanisms of electrical resistivity and thermal conductivity of CuNWs, and the insights could assist in improving the design of semiconductor architectures.