Strength Of Electron-phonon Interaction Research Articles

Non-thermal phonon dynamics and a quenched exciton condensate probed by surface-sensitive electron diffraction.

Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron-phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe2. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon-phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material's widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

Nanoscale morphology, optical dynamics and gas sensor of porous silicon

We investigated the multifaceted gas sensing properties of porous silicon thin films electrodeposited onto (100) oriented P-type silicon wafers substrates. Our investigation delves into morphological, optical properties, and sensing capabilities, aiming to optimize their use as efficient gas sensors. Morphological analysis revealed the development of unique surfaces with distinct characteristics compared to untreated sample, yielding substantially rougher yet flat surfaces, corroborated by Minkowski Functionals analysis. Fractal mathematics exploration emphasized that despite increased roughness, HF/ethanol-treated surfaces exhibit flatter attributes compared to untreated Si sample. Optical approaches established a correlation between increased porosity and elevated localized states and defects, influencing the Urbach energy value. This contributed to a reduction in steepness values, attributed to heightened dislocations and structural disturbances, while the transconductance parameter decreases. Simultaneously, porosity enhances the strength of electron‒phonon interaction. The porous silicon thin films were further tested as effective gas sensors for CO2 and O2 vapors at room temperature, displaying notable changes in electrical resistance with varying concentrations. These findings bring a comprehensive exploration of some important characteristics of porous silicon surfaces and established their potential for advanced industrial applications.

Investigation of Optical functions, sub-bandgap transitions, and Urbach tail in the absorption spectra of Ga2O3 thin films deposited using mist-CVD

We report on the investigation of optical properties of epitaxial Ga2O3 thin films deposited at different temperatures using mist-Chemical Vapor Deposition on c-plane sapphire. The deposited films were of highly crystalline quality, with measured full width at half maxima (FWHM) of on-axis rocking curve scans as low as 62 arcsec for α-Ga2O3 sample grown at 500 °C. Pure phase κ-Ga2O3 was found to get stabilized as temperature increased to 650 °C. The optical properties of the as-deposited films were studied using absorbance, reflectance, and transmittance spectra. Various optical functions have been calculated, such as refractive index (n), extinction coefficient (k), real part of dielectric function (ϵreal), imaginary part of dielectric function (ϵimaginary) and high-frequency dielectric constant (ϵ∞). Furthermore, absorption spectra were modeled for sub-bandgap transitions and exponential onset near band-edge using Elliott–Toyozawa and Urbach–Martienssen models, respectively. Codi’s rule was used to estimate the value of the structural disorder parameter (χ) associated with Urbach’s tail. Variation of the extracted parameters, for instance, Urbach’s Energy (EU), phonon energy (hνp), steepness parameter (σ), electron–phonon interaction strength (Ee−phonon), Codi’s structural disorder parameter (χ), bandgap (Eg) and excitonic binding energy (Eexc) were studied against deposition temperature; these parameters were found to correlate with deposition temperature and hence the crystalline quality of the film. In light of the results, it was inferred that the exponential tail is inevitable at finite temperature because of the dominant Eg phonon mode present at ∼430cm−1 in α- or α- dominated Ga2O3 phase.

Tuning the electron–phonon interaction via exploring the interrelation between Urbach energy and fano-type asymmetric raman line shape in GO-hBN nanocomposites

Hexagonal boron nitride (hBN), having an in-plane hexagonal structure in the sp2 arrangement of atoms, proclaims structural similarity with graphene with only a small lattice mismatch. Despite having nearly identical atomic arrangements and exhibiting almost identical properties, the electronic structures of the two materials are fundamentally different. Considering the aforementioned condition, a new hybrid material with enhanced properties can be evolved by combining both materials. This experiment involves liquid phase exfoliation of hBN and two-dimensional nanocomposites of GO-hBN with varying hBN and graphene oxide (GO) ratios. The optical and vibrational studies conducted using UV–vis absorption and Raman spectroscopic analysis report the tuning of electron–phonon interaction (EPI) in the GO-hBN nanocomposite as a function of GO content (%). This interaction depends on disorder-induced electronic and vibrational modifications addressed by Urbach energy (E u ) and asymmetry parameter (q), respectively. The EPI contribution to the induced disorders estimated from UV–vis absorption spectra is represented as EPI strength (E e–p ) and its impact observed in Raman phonon modes is quantified as an asymmetry parameter (q). The inverse of the asymmetry parameter is related to E e–p , as E e–p ∼ 1/|q|. Here in this article, a linear relationship has been established between E u and the proportional parameter (k), where k is determined as the ratio of the intensity of specific Raman mode (I) and q 2, explaining the disorders’ effect on Raman line shape. Thus a correlation between Urbach energy and the asymmetry parameter of Raman mode confirms the tuning of EPI with GO content (%) in GO-hBN nanocomposite.

Determination of optical constants of vacuum annealed ZnO thin films using Wemple Di Domenico model, Sellmier's model and Miller's generalized rules

Thin films of zinc oxide (ZnO) were coated by a spin coater on the glass substrate and are annealed at 300 °C and 500 °C under the reduced atmosphere of 10−3 mbar. The structural properties were examined by X-ray diffractogram (XRD), atomic force microscopy (AFM), and optical study of ZnO thin films by UV–Vis spectrophotometer. The XRD study showed that the films had a preferred orientation towards the c-axis, and the AFM study revealed a columnar growth. Optical parameters such as band gap, Urbach energy, electron-phonon interaction strength, refractive index, carrier concentration to the effective mass (N/m*) ratio, carrier concentration, and plasma frequency were extracted from transmittance and absorption curves by using Wemple Di Domenico and Sellmier's models. The non-linear optical parameters such as susceptibility and refractive index are obtained from Miller's generalized rules. The optical bandgap was found to decrease with an increase in dopant concentrations and also with annealing temperature but for 0.5 M bandgap increase at 300°C. The Urbach energy follows the increasing trend for both increase in concentration and annealing temperature. This indicates vacuum annealing induces defect sites in the material under investigation.

Interplay of charge ordering and superconductivity in two-dimensional 2H group V transition-metal dichalcogenides

Layered $2H\text{\ensuremath{-}}T{X}_{2}$ with $T=\mathrm{Nb}$, Ta and $X=\mathrm{S}$, Se exhibit rich dimensionality effects on charge density wave (CDW) order and superconductivity, but comprehensive understanding of these correlated quantum phases in the two-dimensional limit of $2H\text{\ensuremath{-}}T{X}_{2}$ is still lacking, hindering their practical applications. Here, I calculate from first principles the phonon linewidth and bare susceptibility to study the origin of CDW formation in ${\mathrm{NbSe}}_{2}$, ${\mathrm{TaS}}_{2}$, ${\mathrm{TaSe}}_{2}$, and ${\mathrm{NbS}}_{2}$ monolayers, analyze their relative CDW strength, and evaluate electron-phonon superconductivity within the fully anisotropic Migdal-Eliashberg theory. A peaked linewidth of the longitudinal acoustic branch and no Fermi-surface nesting around ${\mathbf{q}}_{\mathrm{CDW}}$ indicate CDW instability driven by the wave-vector-dependent electron-phonon coupling. The $3\ifmmode\times\else\texttimes\fi{}3$ CDW ground state favors a distinct hollow-centered clustering of transition-metal atoms, with larger distortion amplitude in ${\mathrm{NbSe}}_{2}$ and ${\mathrm{TaSe}}_{2}$ than in ${\mathrm{TaS}}_{2}$ and ${\mathrm{NbS}}_{2}$. Superconducting results of the monolayer CDW phase prove that strong spin-orbit coupling effects are dominant in determining the critical temperature ${T}_{\mathrm{c}}$ of each system via modifying both the strength and anisotropic extent of electron-phonon interactions. I also recapitulate measured opposite thickness dependencies of ${T}_{\mathrm{c}}$ in $\mathrm{Nb}{X}_{2}$ and $\mathrm{Ta}{X}_{2}$, and show that whether superconductivity is weakened or enhanced in monolayer limit relies on specific evolution of CDW-modulated Fermi-level density of states and electron-phonon matrix elements under reduced dimensionality. This work lays the foundation for applications of $2H$ group V transition-metal dichalcogenides in versatile nanoscale quantum devices.

Optical Properties of Cu-Doped Perovskite Nanoplatelets

All inorganic perovskite nanocrystals (NCs) have been widely concerned for their unique optical properties. However, due to the rotation and inclining of the [PbX6]4− octahedron caused by Cs ions, such kind of materials usually have poor stability, which will hinder their commercialization. It is well known that using smaller divalent cations to replace Pb2+ can effectively improve the stability and luminescence properties of CsPbX3 NCs. In this work, Cu-doped CsPbBr3 nanoplatelets (NPs) are prepared, i.e., CsPb0.95Cu0.05Br3 NPs, and their temperature dependent emission properties have been investigated. The experimental results show that Cu-doping method can weaken the thermal expansion effect and electron-phonon interaction strength of perovskite NPs. Although the exciton binding energy of CsPb0.95Cu0.05Br3 NPs is smaller than that of CsPbBr3 NPs, it is still much larger than that of CsPbBr3 cubic NCs. Our experimental results will be of significance for the development of optoelectronic devices based on transition metal doped perovskite NPs.

Influence of low concentration Sm doping on optical and catalytic performance of titania

The work focuses on exploring the effect of the concentration of Sm dopant (0.2–0.6 at.%) on structural, optical and photocatalytic properties of the spin coated titania based thin films annealed at different temperatures. The optical interpretation involves the influence of Sm doping on optical constants and luminescence behaviour of the samples. The comprehensive work on optical bandgap, Urbach energy and electron-phonon interaction strength was conducted for Sm doped samples. The optical band gap was found to increase with the increasing concentration of Sm, but decreased with high temperature annealing. Using ellipsometry measurement, refractive index of the samples was obtained. The orbital level information was gathered using X-Ray Photoelectron Spectroscopy (XPS) study with a special emphasis on the evolution of physico-chemical properties as function of Sm doping. The XPS study confirms the presence of Sm in the titania host material and it helped in estimating defects induced by Sm doping. The photocatalytic study of Sm doped titania thin films was carried out by using methylene blue (MB) and Rhodamine B (RhB) dye and we have found an enhanced photocatalytic activity for the 0.4 at.% Sm doped samples.

Importance of coupling strength in shaping electron energy loss and phonon spectra of phonon-plasmon systems

A systematic analysis of phonon-plasmon coupled excitations in three-dimensional (3D) polar systems is provided through the prism of both raw and integrated electron energy loss spectroscopy (EELS) and phonon spectra in the whole relevant parametric space, spanned by the adiabaticity parameter and the electron-phonon interaction (EPI) strength. We show that the EPI strength plays a prominent role in distributing spectral weights among excitations, providing an experimentally convenient way to estimate it from integrated spectra. By projecting the excitations onto the phonon degree of freedom, we also report for strong couplings large phonon production contributions, which are of very different origins depending on the adiabaticity parameter. In parallel to this thorough spectral weights analysis, excitations' dispersion evolutions, dampings, and various limiting behaviors are qualitatively and quantitatively correctly accounted for in the whole parametric space.

Evidence of phase stability, topological phonon and temperature-induced topological phase transition in rocksalt SnS and SnSe

Both SnS and SnSe have been experimentally and theoretically confirmed as topological crystalline insulators in native rocksalt structure. Here, phononic structure, thermodynamic properties and temperature dependent electron–phonon interaction (EPI) have investigated for both the materials in rocksalt phase. Previously performed theoretical studies have predicted the phase instability of SnS in this crystal structure at ambient condition. But, after a detailed study performing on the phonon calculation of SnS, we have predicted the phase stability of SnS with considering the Sn 4p orbitals as valence states in ab-initio calculation. The importance of long range Coulomb forces along with the themodynamical properties are also described in detailed for both materials. The computed value of Debye temperature () for SnS (SnSe) is ∼315.0 K (∼201.7 K). The preliminary evidence of topological phonon is found along X–W direction, where the linear band touching is observed as compared to type II Weyl phononic material ZnSe (Liu et al 2021 Phys. Rev. B 103 094306). The topological phase transition is seen for these materials due to EPI, where non-linear temperature dependent bandgap is estimated. The predicted value of transition temperature for SnS (SnSe) is found to be ∼700 K, where after this temperature the non-trivial to trivial topological phase is seen. The strength of EPI shows more stronger impact on the electronic structure of SnS than SnSe material. The reason of non-linear behaviour of bandgap with rise in temperature is discussed with the help of temperature dependent linewidths and lineshifts of conduction band and valence band due to EPI. The present study reveals the phase stability of SnS along with the comparative study of thermal effect on EPI of SnS and SnSe. Further, the possibility of temperature induced topological phase transition provides one of important behaviour to apply these two materials for device making application.

Ultrafast Thermalization Pathways of Excited Bulk and Surface States in the Ferroelectric Rashba Semiconductor GeTe.

A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics.

Orientational ordering and electron-phonon interaction in K3C60 superconductor

The normal state and superconducting state properties of a well characterized K3C60 superconductor are studied in terms of the combined measurements of x-ray diffraction, Raman scattering, magnetization, and resistivity by using the same sample synthesized by the wet chemical method. The temperature downshift of about 50 K is observed for the orientational ordering from pristine C60 to the synthesized sample based on the anomalies of both the resistivity and phonon frequency. When entering the superconducting state, the enhanced self-energy effects are reflected from the softening and broadening of the phonon modes. The obtained electron-phonon interaction strength of 0.63 in the superconducting state accounts well for the transition temperature of 18 K in K3C60 and the plausible Coulomb pseudopotential and expected isotope exponent.

Temperature Dependence of the Indirect Gap and the Direct Optical Transitions at the High-Symmetry Point of the Brillouin Zone and Band Nesting in MoS2, MoSe2, MoTe2, WS2, and WSe2 Crystals.

Following the rise of interest in the properties of transition metal dichalcogenides, many experimental techniques were employed to research them. However, the temperature dependencies of optical transitions, especially those related to band nesting, were not analyzed in detail for many of them. Here, we present successful studies utilizing the photoreflectance method, which, due to its derivative and absorption-like character, allows investigating direct optical transitions at the high-symmetry point of the Brillouin zone and band nesting. By studying the mentioned optical transitions with temperature from 20 to 300 K, we tracked changes in the electronic band structure for the common transition metal dichalcogenides (TMDs), namely, MoS2, MoSe2, MoTe2, WS2, and WSe2. Moreover, transmission and photoacoustic spectroscopies were also employed to investigate the indirect gap in these crystals. For all observed optical transitions assigned to specific k-points of the Brillouin zone, their temperature dependencies were analyzed using the Varshni relation and Bose–Einstein expression. It was shown that the temperature energy shift for the transition associated with band nesting is smaller when compared with the one at high-symmetry point, revealing reduced average electron–phonon interaction strength.

Defects and band gap shrinkage in ZnO/rGO composite nano-pebbles prepared by solid–state reaction

In the present investigation, ZnO/rGO nanocomposites were prepared using conventional solid-state reaction method. Raman, XRD, FTIR, UV–Vis, PL and FESEM characterization tools were used to analyze the synthesized materials. Raman analysis reveals the formation of rGO with prominent D and G-band peaks. XRD patterns confirm the crystalline and hexagonal wurtzite structure with P63mc space group of all the samples. Rietveld refinement demonstrates the size variation (33 nm–56 nm) and strain profile. FESEM micrographs of ZnO/rGO composites have established the formation of densely packed hexagonal nano-pebbles. UV–Vis and room temperature PL measurements indicate that the optical properties of ZnO and ZnO/rGO nanocomposites are affected by the rGO concentration. It also confirms a band gap shrinkage during nanocomposite preparation compared to the pure ZnO structure. Urbach energy is found to increase with increase in rGO content and the strength of electron-phonon interaction enhances. It also affects the PL emission in the UV region. FTIR spectra confirm the presence of different functional groups. Present investigation shows that ZnO/rGO materials are potential candidates for design and development of sensors for detecting gases.

Antiresonance and negative differential conductance in parallel-coupled vibrating molecular dots coupled to Luttinger liquid leads

We theoretically study the interplays of electron–phonon interaction, inter-dot coupling tunneling and intralead electron interaction on transport properties of parallel-coupled double quantum dots connected to Luttinger liquid leads. The nonequilibrium Green’s function method is used. Numerical results show several intriguing physic phenomena. In the weak intralead electron interaction, there is a zero-bias antiresonance dip followed by phonon-assisted satellite peaks for strong electron–phonon interaction and inter-dot tunneling. With decreasing inter-dot tunneling, there is an evolution from satellite peaks to dips, which signifies the phase transition from phonon-assisted single-channel to two-channel Kondo physics. When the dot-lead coupling is asymmetric, in the case of moderately strong intralead interaction, strong electron–phonon coupling and weak inter-dot tunneling, there may appear the pronounced negative differential conductance and Fano resonance, while there does not when the coupling is symmetric. These results provide a convenient way to probe destructive quantum interference and extract information of the electron–phonon interaction strength, which is crucial for designing molecular switches and thermoelectric devices. • We study the transport properties of parallel coupled double quantum dots connected to Luttinger liquid leads. • We have theoretically studied a series of problems such as electron phonon interaction . • The method used is nonequilibrium Green’s function. • The phenomena of resonance and peak of satellite are found. • Negative differential conductance and Fano resonance are found.

Effect of electronic concentration on the self-trapping transition of polaron for the weakly correlated system in the adiabatic limit

The self-trapping transition of the polaron is investigated for the two-dimensional extended Holstein-Hubbard model. A series of canonical transformations are performed on the Hamiltonian followed by a many phonon averaging, to eliminate the phonon degrees of freedom from the system. The mean field Hartree-Fock calculation has been done to solve the effective electronic Hamiltonian and the ground state energy is obtained variationally. The effect of electronic concentration on the polaronic hopping parameter is studied with respect to the electron–phonon interaction strengths in the weak Coulomb correlation limit. The transition from a delocalized mobile polaron to a localized self-trapped polaron has been found to be discontinuous for the adiabatic motion of the polaron and the self-trapping transition needs stronger electron–phonon interaction strength as the electronic concentration minimizes.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

Impact of defect sites on the Raman scattering properties of nitrogen doped ZnO thin films

This paper focuses on the influence of nitrogen doping and vacuum annealing on the properties of nitrogen-doped ZnO thin films grown by spin coating technique. The optical absorption study was indirect evidence for the presence of intrinsic defects like oxygen vacancy and Zn interstitials and in line with this, Urbach energy was calculated to confirm the disorder levels. Using Urbach energy, the electron–phonon interaction strength was evaluated and its impact on the line shape of major Raman mode (E2) was studied in detail. Apart from these, possible reasons for peak shift, anisotropic broadening and asymmetry line shape behaviour are detailed here.

Moiré Surface States and Enhanced Superconductivity in Topological Insulators

Recently, moir\'e superlattices have been found on the surface of topological insulators (TI) due to the rotational misalignment of topmost layers. In this work, we study the effects of moir\'e superlattices on the topological surface states using a continuum model of Dirac electrons moving in a periodic potential. Unlike twisted bilayer graphene, moir\'e surface states cannot host isolated bands due to their topological nature. Instead, we find (high-order) van Hove singularities (VHS) in the moir\'e band structure that give rise to divergent density of states (DOS) and enhance interaction effects. Due to spin-momentum locking in moir\'e surface states, possible interaction channels are limited. In the presence of phonon mediated attraction, superconductivity is strongly enhanced by the power-law divergent DOS at high-order VHS. The transition temperature $T_c$ exhibits a power-law dependence on the retarded electron-phonon interaction strength $\lambda^*$. This enhancement is found to be robust under various perturbations from the high-order VHS.

An electronic phase diagram of hole-doped BiCuSeO crystals determined by transport characterization under various growth conditions

Temperature-hole-concentration dependent electronic phase-diagram of BiCuSeO.