Electron-phonon Coupling Strength Research Articles

Ultrafast dynamics evidence of strong coupling superconductivity in LaH10±δ

Recently, tremendous research interest has been aroused in clathrate superhydrides. However, their microscopic properties, especially the superconducting (SC) gap and electron-phonon coupling (EPC) strength, are largely unexplored experimentally. Here, we investigate the time-resolved ultrafast spectroscopy of a superconductor LaH10±δ under an ultrahigh pressure of 165 GPa. By analyzing the ultrafast dynamics of the quasiparticles, we experimentally obtain the SC gap Δ(0) = 53 ± 5 meV, revealing a gap ratio 2Δ(0)/kBTc = 5.6 and a gap parameter ϑ = 1.95. Significantly, we experimentally estimate λ⟨Ω2⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda \\langle {\\Omega }^{2}\\rangle$$\\end{document} = (2.4 ± 0.1) × 104 (meV)2, which corresponds to an EPC strength λ = 2.58 ± 0.11. These results together provide direct experimental evidence that strong EPC is responsible for the near-room-temperature superconductivity in clathrate superhydrides. Our investigation significantly advances the experimental exploration of superhydrides, and contributes to the ultrafast dynamics investigations of quantum materials under high pressures.

Phonon modes and electron-phonon coupling at the FeSe/SrTiO3 interface.

The remarkable increase in superconducting transition temperature (Tc) observed at the interface of one-unit-cell FeSe films on SrTiO3 substrates (1 uc FeSe/STO)1 has attracted considerable research into the interface effects2-6. Although this high Tc is thought to be associated with electron-phonon coupling (EPC)2, the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75-99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiOx layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiOx terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent Tc enhancement in FeSe/STO and potentially other superconducting systems.

Band order reversal and valley engineering driving to dual high electron and hole mobility in strained monolayer BS

Strain engineering is an effective method to tune the physical properties of materials. In this work, we found that applying 5% biaxial compression to monolayer boron sulfur can simultaneously induce a band order reversal of valence bands and valley engineering of conduction bands. This change significantly enhances the room temperature intrinsic mobility, boosting it from 2 to 260 cm2 V−1 s−1 for holes and 63 to 543 cm2 V−1 s−1 for electrons. The high hole mobility surpasses that of bulk GaN and silicon. We further demonstrate that the valence band order reversal not only lifts the px/y state above the pz states, giving to a more dispersive highest valence band, and thus smaller electron scattering channels and hole effective mass, but also shifts the stronger σz bonding to weaker π-bonding characteristics, resulting in smaller electron–phonon coupling strength. Those combined effects results in a substantial increase in hole mobility.

Effect of magnetic and electric field on the ground state properties of weak coupling piezoelectric polaron in a quantum well with asymmetric Gaussian confinement

In this study, we employed the linear combination operation and a modified Lee-Low-Pines unitary transformation method to explore the ground state properties of an interacting electron coupled with piezo-acoustic phonons within a quantum well featuring asymmetric Gaussian confinement potential well under the influence of magnetic and electric fields. We calculated various properties, including the ground state energy, ground state binding energy, magnetic moment, magnetic susceptibility, and polarizability of the weakly coupled piezoelectric polaron. The effects of the magnetic and electric fields, electron-phonon weak coupling strength, Debye cut-off wavenumber, and confinement depth and range on these properties were thoroughly analyzed. Our findings reveal that the ground state energy decreases with increasing the Debye wavenumber, confinement range, and electron-phonon weak coupling strength. In contrast, the ground state binding energy shows the opposite trend, increasing with these parameters. Furthermore, the magnetic and electric fields, confinement depth and range, electron-phonon coupling strength, and Debye wavenumber are critical factors that significantly impact the magnetic and optical properties of the piezoelectric polaron.

Toward an Ideal Light Source for Indoor Photosynthesis: Broadband Red Emission in Zero-Dimensional Hafnium-Based Metal Halide (TPP)2HfCl6·4C2H3N:Sb3.

With suitable electron-phonon coupling strength, a near-unity broadband photoluminescence quantum yield (PLQY) can be achieved in organic-inorganic hybrid metal halides (OIHMHs) via self-trapped exciton (STE) emission. However, it is still challenging to obtain high-quality red emission from OIHMHs with a desirable emission wavelength and high chemical stability, which hinders their practical application in high-performance displays, plant-growth lighting, and biomedical imaging. Herein, a series of hafnium-based zero-dimensional (TPP)2HfCl6·4C2H3N (TPP: tetraphenylphosphonium) single crystals with different Sb3+ doping levels are synthesized. The Sb3+-doped (TPP)2HfCl6·4C2H3N shows dual-band red emission with a full width at half-maximum of 178 nm and a high PLQY of 91.09%. This broad dual-band emission originates from dopant-induced extrinsic free excitons and STEs. Furthermore, (TPP)2HfCl6·4C2H3N:Sb3+ was employed as a luminescence converter in a light-emitting diode (LED) for plant growth regulation. A correlated color temperature of 4055 K and a color rendering index of 82.13 were achieved upon excitation of the LED at 365 nm. These results provide fundamental perspectives on the emission behavior of Sb3+-doped OIHMHs and illustrate their promise for use in plant-growth lighting.

Realization of hydrogenation-induced superconductivity in two-dimensional Ti2N MXene.

Two-dimensional (2D) MXene superconductors have been currently attracting considerable interest due to their unique electronic properties and diverse applicability. Utilizing first-principles computational methods, we have designed two distinct configurations of hydrogenated 2D Ti2N MXene materials, namely Ti2NH2 and Ti2NH4, and have conducted an exhaustive analysis of their structural stability, electronic characteristics, and superconductivity. Hydrogenation endows monolayer Ti2N with inherent metallic characteristics, as evidenced by an elevated density of states (DOS) at the Fermi level (Ef). Notably, Ti2NH4 exhibits a superconducting critical temperature (Tc) of 15.8 K, which is predominantly ascribed to the electronic contributions stemming from the Ti 3d orbitals. Analysis of phonon dispersion underscores the pivotal role that diverse lattice vibrational modes play in electron-phonon coupling (EPC), particularly the significance of low-frequency vibrations for facilitating electron pairing and the emergence of superconductivity. Furthermore, strain engineering can effectively modulate the superconducting properties of Ti2NH4, with a 2% tensile strain enhancing the EPC strength (λ) to 0.857 and increasing Tc to 18.7 K. This research elucidates the superconducting mechanisms of hydrogenated Ti2N structures, offering valuable insights for the development of novel 2D superconducting materials.

Orientation-dependent electronic structure in interfacial superconductors LaAlO3/KTaO3

Emergent superconductivity at the LaAlO3/KTaO3 interfaces exhibits a mysterious dependence on the KTaO3 crystallographic orientations. Here by soft X-ray angle-resolved photoemission spectroscopy, we directly resolve the electronic structure of the LaAlO3/KTaO3 interfacial superconductors and the non-superconducting counterpart. We find that the mobile electrons that contribute to the interfacial superconductivity show strong k⊥ dispersion. Comparing the superconducting and non-superconducting interfaces, the quasi-three-dimensional electron gas with over 5.5 nm spatial distribution ubiquitously exists and shows similar orbital occupations. The signature of electron-phonon coupling is observed and intriguingly dependent on the interfacial orientations. Remarkably, the stronger electron-phonon coupling signature correlates with the higher superconducting transition temperature. Our observations help scrutinize the theories on the orientation-dependent superconductivity and offer a plausible and straightforward explanation. The interfacial orientation effect that can modify the electron-phonon coupling strength over several nanometers sheds light on the applications of oxide interfaces in general.

Tuning electronic and thermoelectric properties of armchair graphene nanoribbon in the presence of electron phonon coupling

Electronic and thermoelectric properties of armchair graphene nanoribbon taking into account the effects of interaction between electrons and Einstein phonons have been addressed. Specially we study the temperature dependence of thermal conductivity, density of states and specific heat of the structure. The effects of electron phonon coupling strength on thermal conductivity and thermoelectric electronic of the system have been studied. Green’s function method has been implemented to obtain electronic properties of the system in the context of Holstein Hamiltonian. One loop electronic self-energy of the Hamiltonian has been obtained in order to find interacting electronic Green’s function. The transport and thermoelectric properties of armchair graphene nanoribbon in the presence of electron phonon coupling can be readily found using interacting Green’s function based on Kubo formula. We find numerical results for chemical potential dependence of thermal conductivity and thermoelectric properties in the presence of Holstein phonons. Specially the behaviors of Seebeck coefficient, power factor function, figure of merit and Lorenz number of the system have been analyzed. Our results show turning on electron phonon coupling leads to reduction of band gap in density of states of the armchair nanoribbon.

Mechanism of Pressure-Modulated Self-Trapped Exciton Emission in Cs2TeCl6 Double Perovskite.

Pressure-modulated self-trapped exciton (STE) emission mechanism in all-inorganic lead-free metal halide double perovskites characterized by large Stokes-shifted broadband emission, has attracted much attention across various fields such as optics, optoelectronics, and biomedical sciences. Here, by employing the all-inorganic lead-free metal halide double perovskite Cs2TeCl6 as a paradigm, the authors elucidate that the performance of STE emission can be modulated by pressure, attributable to the pressure-induced evolution of the electronic state (ES). Two ES transitions happen at pressures of 1.6 and 5.8GPa, sequentially. The electronic behaviors of Cs2TeCl6 can be jointly modulated by both pressure and ES transitions. When the pressure reaches 1.6GPa, the Huang-Rhys factor S, indicative of the strength of electron-phonon coupling, attains an optimum value of ≈12.0, correlating with the pressure-induced photoluminescence (PL) intensity of Cs2TeCl6 is 4.8-fold that of its PL intensity under ambient pressure. Through analyzing the pressure-dependent STE dynamic behavioral changes, the authors have revealed the microphysical mechanism underlying the pressure-modulated enhancement and quenching of STE emission in Cs2TeCl6.

Prediction of novel layered indium halide superconductors

Abstract We design two new layered indium halide compounds LaOInF2 and LaOInCl2 by means of first-principles calculations and evolutionary crystal structure prediction. We find both compounds crystallize in a tetragonal structure with P4/nmm space group and have indirect band gaps of 2.58 eV and 3.21 eV, respectively. By substituting O with F, both of them become metallic and superconducting at low temperature. The F-doping leads to strong electron–phonon coupling in the low-energy acoustic phonon modes which is mainly responsible for the induced superconductivity. The total electron–phonon coupling strength are 1.86 and 1.48, while the superconducting transition temperature (T c) are about 7.2 K and 6.5 K with 10% and 5% F doping for LaOInF2 and LaOInCl2, respectively.

Accelerating the calculation of electron-phonon coupling strength with machine learning.

The calculation of electron-phonon couplings (EPCs) is essential for understanding various fundamental physical properties, including electrical transport, optical and superconducting behaviors in materials. However, obtaining EPCs through fully first-principles methods is notably challenging, particularly for large systems or when employing advanced functionals. Here we introduce a machine learning framework to accelerate EPC calculations by utilizing atomic orbital-based Hamiltonian matrices and gradients predicted by an equivariant graph neural network. We demonstrate that our method not only yields EPC values in close agreement with first-principles results but also enhances calculation efficiency by several orders of magnitude. Application to GaAs using the Heyd-Scuseria-Ernzerhof functional reveals the necessity of advanced functionals for accurate carrier mobility predictions, while for the large Kagome crystal CsV3Sb5, our framework reproduces the experimentally observed double domes in pressure-induced superconducting phase diagrams. This machine learning framework offers a powerful and efficient tool for the investigation of diverse EPC-related phenomena in complex materials.

High-Order Harmonic Generation in Photoexcited Three-Dimensional Dirac Semimetals.

High-order harmonic generation (HHG) in condensed matter is highly important for potential applications in various fields, such as materials characterization, all-optical switches, and coherent light source generation. Linking HHG to the properties or dynamic processes of materials is essential for realizing these applications. Here, a bridge has been built between HHG and the transient properties of materials through the engineering of interband polarization in a photoexcited three-dimensional Dirac semimetal (3D-DSM). It has been found that HHG can be efficiently manipulated by the electronic relaxation dynamics of 3D-DSM on an ultrafast time scale of several hundred femtoseconds. Furthermore, time-resolved HHG (tr-HHG) has been demonstrated to be a powerful spectroscopy method for tracking electron relaxation dynamics, enabling the identification of electron thermalization and electron-phonon coupling processes and the quantitative extraction of electron-phonon coupling strength. This demonstration provides insights into the active control of HHG and measurements of the electron dynamics.

Regulation of chiral structure and spin polarization by circularly polarized light and electric field in copolymers

Combining chirality with ferromagnetism is challenging in organic materials and may bring potential application for the cross-integration of multiple disciplines. In this work, we achieved the intrinsic coexistence of chirality and spontaneous spin polarization in the diblock copolymers, where left-handed and right-handed circularly polarized lights present tunability on both the circular dichroism and spin polarization. Without electron dependence of transport in chiral copolymer, chirality is still coupled with spin. The phenomenon observed here is not the traditional chirality induced spin selectivity effect. Moreover, the electric field could also effectively tune the chirality dependence of circular dichroism to further affect the magnitude of spin polarization. Thus, a strong relationship between the chirality and spin polarization is formed in chiral copolymers. In addition, because of the different molecular packing for chiral enantiomers, electron–phonon coupling strengths are different to lead to a difference in spin polarization. Overall, the properties of chirality, spontaneous spin polarization, and photon-chirality-spin coupling are developed and studied, which effectively promotes the ability of potential applications of chiral copolymers.

Enhanced cation ordering, electron-spin-phonon interactions and Fano resonance in half-metallic Sr2FeMoO6 thin films

We report the presence of electron-spin-phonon interactions in half-metallic ferromagnetic Sr2FeMoO6 (SFMO) double perovskite thin films using temperature-dependent Raman spectroscopy. A series of SFMO thin films have been prepared on LaAlO3 (001) single-crystal substrate using the Pulsed Laser Deposition technique. These depositions have been made in different gas-conditions such as in vacuum, and under nitrogen and oxygen gas pressures. At room temperature, Raman spectra manifest a Fano feature which indicates the presence of electron-phonon coupling in the films. The electron-phonon coupling strength further changes with a change in deposition conditions. Magnetization results show that the SFMO film grown in vacuum has the highest saturation magnetization which suggests better cation ordering as compared to the other films. For enhanced understanding, Raman spectra were recorded at varied temperatures and the data were analyzed by theoretical model fittings. A parameter quantifying temperature-dependent anharmonic nature of phonons has been derived using Balkanski model fits. This parameter shows a drastic deviation in the vicinity of Curie temperatures, manifesting a spin-phonon coupling in SFMO films. We further show that the spin-phonon coupling strengthens with improved Fe–Mo ordering. Any experimental observation of spin-phonon coupling has not been reported for SFMO systems till date. The magnetization data corroborate well with these observations made by Raman measurements. Our results of Raman spectroscopy, magnetization and resistivity collectively suggest that the SFMO films exhibit electron-spin-phonon interactions, which are influenced by the cation ordering. We also devised out the method of relating the anharmonic nature of Raman modes with the degree of Fe–Mo ordering and spin-phonon coupling in double-perovskite materials.

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical AMX3 halide perovskite, we model the case of n=1 and n=2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. Published by the American Physical Society 2024

Enhancement of superconductivity in Zr2S2C arising from phonon softening on transition from bulk to monolayer

We report a new compound, Zr2S2C, belonging to the transition metal carbo-chalcogenide (TMCC) family. Through first-principles calculations, our analysis of phonon dispersion spectra indicates that the compound is dynamically stable in both bulk and monolayer forms. We systematically investigated the electronic structure, phonon dispersion, and electron–phonon coupling (EPC) driven superconducting properties in bulk and monolayer Zr2S2C. The results demonstrate the metallic character of bulk Zr2S2C, with a weak EPC strength (λ) of 0.41 and superconducting critical temperature ( Tc ) of ∼3 K. The monolayer Zr2S2C has an enhanced λ of 0.62 and Tc of ∼6.4 K. The increased λ value in the monolayer results from the softening of the acoustic phonon mode. We found that when biaxial strain is applied, the low energy acoustic phonon mode in monolayer becomes even softer. This softening leads to a transformation of the Zr2S2C monolayer from its initial weak coupling state (λ = 0.62) to a strongly coupled state, resulting in an increased λ value of 1.33. Consequently, the superconducting critical temperature experiences a twofold increase. These findings provide a theoretical framework for further exploration of the layered two-dimensional TMCC family, in addition to offering valuable insights.

Tuning Electron–Phonon Coupling Interaction for the Efficient Wide Blue Emission of Pb2+‐Doped Cs2InCl5·H2O

AbstractLead‐free perovskites exhibit unique crystal structures and optical properties due to their low‐dimensional structure. However, the 0D structure of Cs2InCl5·H2O usually demonstrates poor optical properties at room temperature due to their extremely strong electron–phonon coupling interaction. In this study, a doping strategy is employed by introducing Pb2+ into Cs2InCl5·H2O, resulting in the achievement of an efficient broadband blue emission, with a photoluminescence quantum yield (PLQY) of 58%. The temperature‐dependent PL spectra reveal that the efficient emission is attributed to the weakening of the electron–phonon coupling strength in the lattice and the inhibition of the phonon‐assisted non‐radiative recombination process through Pb2+ doping. The validity of this deduction is further corroborated by the results of the Raman spectra. The results of the first‐principle calculation demonstrate that the broadband emission originates from multiple emission paths in In3+ and Pb2+ polyhedrons. In addition, Pb2+‐doped Cs2InCl5·H2O can realize the transformation from sky‐blue emission to deep‐blue emission by absorbing water in the air. Remarkably, this change is reversible during the heating–cooling cycles, demonstrating excellent optical stability with repeatable recyclability.

Electron–Phonon Coupling in Copper-Substituted Lead Phosphate Apatite

Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron–phonon interaction is a common mechanism for superconductivity, the electron–phonon coupling strength is an important quantity to compute for LK99. In this work, we compare the electron–phonon coupling strength among the proposed compositions of LK99. The results of our study are in alignment with the conclusion that LK99 is a candidate for low-temperature, not room-temperature, superconductivity if electron–phonon interaction is to serve as the mechanism.

Dual‐Doping Strategy of Metal Chlorides in Ambient Air with High Humidity for Achieving Highly Air‐Stable All‐Inorganic Perovskite Solar Cells

It is fundamentally challenging to achieve stable and efficient all‐inorganic perovskite solar cells (PSCs) in ambient air conditions for low‐cost commercial manufacturing, as the crystallinity, surface morphology, and optical activity of all‐inorganic perovskites exhibit high sensitivity to environmental factors such as moisture and illumination. Herein, a dual‐doping strategy is presented to prepare high‐quality dual‐doping CsPbI2Br films under ambient air with relative humidity of 70% (70% RH) by introducing CaCl2 and InCl3 additives into precursor solution. The results from the experiments and calculations reveal that the CaCl2 additive can isolate moisture by forming hydrates at the surface and grain boundary of perovskites. Meanwhile, the addition of InCl3 can significantly improve the optoelectronic properties via the heterovalent substitution of Pb2+ by a small amount of In3+. Moreover, the phase segregation can be significantly suppressed in the dual‐doping CsPbI2Br films owing to decreasing the electron–phonon coupling strength and increasing the activation energy of ion migration. The unencapsulated dual‐doping CsPbI2Br PSC with the power conversion efficiency (PCE) of 15.51% (70% RH) demonstrates high humidity storage and long‐term optical stability, remaining 90% of the original PCE after aging 2400 h under ambient air (50% RH) and 1500 h under continuous illumination, respectively.

Unveiling defect-related Raman modes of topological phononic β−MoB2 single crystal via angle-resolved polarized Raman spectra

As a rare topological phonon material, β−MoB2 exhibits superconductivity (SC) and undergoes structural transitions when subjected to pressure or doping. This unique property makes it an intriguing platform for investigating the interplay of diverse quantum order. However, the current understanding of phonons in this material remains limited. Therefore, in this study, we employ a combination of Raman scattering and first-principle calculations to investigate the phonon properties of β−MoB2. Several noteworthy observations were made during the study. (1) We identified five Raman-active modes (3A1g+2Eg) and two abnormal modes in β−MoB2. The authenticity of the five Raman-active modes was further confirmed through angle-resolved polarized Raman spectra (ARPRS) measurements and theoretical calculations. (2) We found that all Raman spectra obtained at various temperatures exhibited symmetric Lorentzian profiles. This finding implies a weak electron-phonon coupling (EPC) strength in β−MoB2. We calculated the EPC strength λ of bulk β−MoB2 to be 0.37, which is significantly lower than 1.71 of superconducting α−MoB2. (3) Based on ARPRS measurements and the phonon dispersion, we attributed the origin of the two abnormal modes to structural distortions with regard to B atom layers of β−MoB2. This research provides a foundation for exploring the relationship between defect structures and various intriguing physical phenomena observed in β−MoB2. Published by the American Physical Society 2024