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Vanadium dioxide exhibits a first order metal to insulator transition (MIT) at 340 K ($\mathrm{T}_{\mathrm{MI}}$) from a rutile (R) structure to a monoclinic ($\mathrm{M}_{1}$) structure. The mechanism of this transition interpreted as due either to a Peierls instability or to a Mott–Hubbard instability is controversial since half a century. However, in the last twenty years the study of chemical and physical properties of $\mathrm{VO}_{2}$ and of its alloys, benefits of a renewed interest due to possible applications coming from the realization of devices made of thin films. We describe in this review the structural, electronic and magnetic properties of the different metallic (R) and insulating ($\mathrm{M}_{1}$, T, $\mathrm{M}_{2}$) phases of $\mathrm{VO}_{2}$, of its solid solutions and under constraint. We present in a synthetic manner the various phase diagrams and their symmetry analysis. This work allows us to revisit older interpretation and to emphasize in particular the combined role of electron–electron interactions in the various phase of $\mathrm{VO}_{2}$ and of structural fluctuations in the MIT mechanism. In this framework we show that the phase transition is surprisingly announced by anisotropic one-dimensional (1D) structural fluctuations revealing chain like correlations between the V due to an incipient instability of the rutile structure. This leads to an unexpected critical dynamics of the order–disorder (or relaxation) type. We describe how the two-dimensional (2D) coupling between these 1D fluctuations, locally forming uniform $\mathrm{V}^{4+}$ zig-zag chains and V–V pairs, stabilizes the $\mathrm{M}_{2}$ and $\mathrm{M}_{1}$ insulating phases. These phases exhibit a 1D electronic anisotropy where substantial electron–electron correlations conduct to a spin–charge decoupling. The spin-Peierls ground state of $\mathrm{M}_{1}$ is analyzed via a mechanism of dimerization, in the T phase, of the spin 1/2 $\mathrm{V}^{4+}$ zig-zag Heisenberg chains formed in the $\mathrm{M}_{2}$ phase. This review summarizes in a critical manner the main results of the large literature on fundamental aspects of the MIT of $\mathrm{VO}_{2}$. It is completed by unpublished old results. Interpretations are also placed in a large conceptual frame which is also relevant to interpret physical properties of other classes of materials.

In this work, we investigate the complexities (coupling range and magnetic ordering) of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction on the (001) surface of SnTe which is a topological crystalline insulator. We aim to enable the transition between ferromagnetic and antiferromagnetic couplings, as well as between collinear and noncollinear magnetic spins by manipulating doping and the gap structure. The gap is achieved through electron–phonon coupling and gate voltage. The spin susceptibility is calculated by Green's functions in real space. The modulation of RKKY interactions via doping occurs specifically over short (large) impurity separations, with the influence of electron–phonon coupling (gate) only manifesting when the doped Fermi level lies outside the gap. Additionally, electron–phonon coupling leads to weaker adjustments in magnetic orderings, attributed to the renormalized Fermi velocities, while gate voltage induces stronger modifications. This inquiry is of considerable interest due to the potential applications in the realm of spintronics. • Coupling range and magnetic ordering of the RKKY interaction on the (001) surface of SnTe are investigated. • The modulation of RKKY interactions occurs specifically over short impurity separations. • Electron–phonon coupling leads to weaker adjustments in magnetic orderings, attributed to the renormalized Fermi velocities while gate voltage induces stronger modifications. • The results are of considerable interest due to the potential applications in the realm of spintronics. [ABSTRACT FROM AUTHOR]

Recent theoretical prediction of Mg 2 IrH 6 is a significant advance in achieving high-temperature superconductivity under atmospheric pressure, Mg 2 IrH 6 is the hydride superconductor with the highest superconducting transition temperature (T c ∼ 160 K) under ambient pressure so far. In general, T c is usually related to the degree of hydrogen enrichment. As a representative of the hydrogen-poor structures, MgHCu 3 was also predicted to have a T c of 43 K under atmospheric pressure. In this work, we try to replace the Ir atom in Mg 2 IrH 6 with the Cu atom to improve the superconductivity of the system. The phonon dispersion curves and the ab initio molecular dynamics (AIMD) simulation indicate that the novel ternary Mg 2 CuH 6 hydride is dynamically and thermodynamically stable. But, regrettably, the T c and electron-phonon coupling (EPC) parameters λ of Mg 2 CuH 6 are calculated to be 12.5 K and 0.55 at 25 GPa, which is far inferior to the superconductivity of Mg 2 IrH 6. Compared with F m 3 ¯ m Mg 2 CuH 6 , F m 3 ¯ m Mg 2 IrH 6 produces obvious phonon softening in the G point near 30 meV, which significantly enhances the EPC and increases the T c of the system. Moreover, Cu substitution for Ir results in a sharp decrease in the contribution of the electronic states for Mg and H to the total density of states at the Fermi level. The huge difference in predicted T c between Mg 2 CuH 6 and Mg 2 IrH 6 and their causes may provide insights into the design of high-temperature atmospheric superconductors and help to understand the superconducting mechanisms of related systems. [ABSTRACT FROM AUTHOR]

High-throughput a b i n i t i o calculations are the indispensable parts of data-driven discovery of new materials with desirable properties, as reflected in the establishment of several online material databases. The accumulation of extensive theoretical data through computations enables data-driven discovery by constructing machine learning and artificial intelligence models to predict novel compounds and forecast their properties. Efficient usage and extraction of data from these existing online material databases can accelerate the next stage materials discovery that targets different and more advanced properties, such as electron–phonon coupling for phonon-mediated superconductivity. However, extracting data from these databases, generating tailored input files for different a b i n i t i o calculations, performing such calculations, and analyzing new results can be demanding tasks. Here, we introduce a software package named "HTESP" (High-Throughput Electronic Structure Package) written in Python and Bash languages, which automates the entire workflow including data extraction, input file generation, calculation submission, result collection and plotting. Our HTESP will help speed up future computational materials discovery processes. [Display omitted] • HTESP automates high-throughput ab initio calculations using Python and Bash. • Combines data from multiple online materials databases by space groups. • Generates inputs, monitors jobs, extracts results, and create quick plots. • Features: electron-phonon coupling, phase stability, elasticity, magnetism. [ABSTRACT FROM AUTHOR]

• Thermal transport across metal interfaces was measured at the non-equilibrium states. • The measured h ee of Au/Pt matches well with the predicted value by electronic DMM. • Oxide layer significantly hinder electron thermal transport across metal interfaces. Interfaces play a critical role in nanoscale thermal transport, and understanding thermal transport mechanisms across metal/metal interfaces at the non-equilibrium states is urgently needed for highly integrated electronic structures or devices. In this work, the electronic interfacial thermal conductance h ee of two metal interfaces, Au/Pt and Au/Al, was measured by time domain thermoreflectance (TDTR) at the non-equilibrium states and an extended two-temperature model was applied to well describe the interactions of electrons between the metal films. For the Au/Pt structure, the measured h ee is 4.4 ± 1.8 GWm−2K−1, which is consistent with the predicted value by the electronic diffuse mismatch model (DMM). While for the Au/Al structure, the measured h ee (0.13±0.05 GWm−2K−1) is an order of magnitude smaller than that of Au/Pt and the predicted value of the electronic DMM. The results indicate that even the extremely thin oxide layer at the Au/Al interface can significantly hinder the electron thermal transport across the metal interface. Overall, our findings contribute to revealing the non-equilibrium interfacial thermal transport mechanisms and pave the way for designing the highly integrated electronic devices. [ABSTRACT FROM AUTHOR]

Understanding the microscopic dynamics of the fundamental energy carriers in condensed matter under extreme pressure conditions can reveal insights into unique physical phenomena that are otherwise not easily discernible at ambient conditions. Here, by utilizing a combination of machine learning interatomic potential (MLP)-based molecular dynamics simulations to calculate phonon thermal conductivity and parameter-free first-principles calculations of electron–phonon coupling and electronic thermal conductivity, we show that (contrary to typical elemental metals) the phonon contribution to the total thermal conductivity in iridium can be increased from 18% at ambient conditions to 40% at 60 GPa hydrostatic pressure conditions. The strength of electron–phonon scattering, as quantified by the mass enhancement parameter, decreases monotonically with pressure resulting in an increase in the lifetime of electrons around the Fermi energy. This consequently leads to a monotonic increase in the electron thermal conductivity from 134 W m−1 K−1 at ambient to 177 W m−1 K−1 at 60 GPa. Similarly, the phonon thermal conductivity has a relatively higher increase from 30 W m−1 K−1 at ambient to 120 W m−1 K−1 at 60 GPa. This is attributed to considerable phonon hardening and increase in the group velocity of the heat carrying phonons with pressure. Along with the pronounced phonon contributions to the total thermal conductivity, we also show that the temperature dependence of the phonon thermal conductivity at ambient pressure is dictated by higher-order phonon interactions (beyond the typical three-phonon processes) in iridium. • Lattice contribution to thermal conductivity in iridium increases from 18% at 0 GPa to 40% at 60 GPa. • Higher-order phonon scattering (beyond three-phonons) dictates the lattice contribution. • Electron-phonon coupling decreases with increasing pressure in iridium. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Mechanistic studies on lead halide perovskites (LHPs) in recent years have suggested charge carrier screening as partially responsible for long carrier diffusion lengths and lifetimes that are key to superior optoelectronic properties. These findings have led to the ferroelectric large polaron proposal, which attributes efficient charge carrier screening to the extended ordering of dipoles from symmetry-breaking unit cells that undergo local structural distortion and break inversion symmetry. It remains an open question whether this proposal applies in general to semiconductors with LHP-like anharmonic and dynamically disordered phonons. Here, we study electron-phonon coupling in Bi2O2Se, a semiconductor which bears resemblance to LHPs in ionic bonding, spinorbit coupling, band transport with long carrier diffusion lengths and lifetimes, and phonon disorder as revealed by temperature-dependent Raman spectroscopy. Using coherent phonon spectroscopy, we show the strong coupling of an anharmonic phonon mode at 1.50 THz to photo-excited charge carriers, while the Raman excitation of this mode is symmetry-forbidden in the ground-state. Density functional theory calculations show that this mode, originating from the A1g phonon of out-of-plane Bi/Se motion, gains oscillator strength from symmetry-lowering in polaron formation. Specifically, lattice distortion upon ultrafast charge localization results in extended ordering of symmetry-breaking unit cells and a planar polaron wavefunction, namely a two dimensional polaron in a three-dimensional lattice. This study provides experimental and theoretical insights into charge interaction with anharmonic phonons in Bi2O2Se and suggests ferroelectric polaron formation may be a general principle for efficient charge carrier screening and for defect-tolerant semiconductors. [ABSTRACT FROM AUTHOR]

Laser irradiation of metals is widely used in research and applications. In this work, we study how the material geometry affects electron–phonon coupling in nano-sized gold samples: an ultrathin layer, nano-rod, and two types of gold nanoparticles (cubic and octahedral). We use the combined tight-binding molecular dynamics Boltzmann collision integral method implemented within XTANT-3 code to evaluate the coupling parameter in irradiation targets at high electronic temperatures (up to Te~20,000 K). Our results show that the electron–phonon coupling in all objects with the same fcc atomic structure (bulk, layer, rod, cubic and octahedral nanoparticles) is nearly identical at electronic temperatures above Te~7000 K, independently of geometry and dimensionality. At low electronic temperatures, reducing dimensionality reduces the coupling parameter. Additionally, nano-objects under ultrafast energy deposition experience nonthermal damage due to expansion caused by electronic pressure, in contrast to bulk metal. Nano-object ultrafast expansion leads to the ablation/emission of atoms and disorders the inside of the remaining parts. These nonthermal atomic expansion and melting are significantly faster than electron–phonon coupling, forming a dominant effect in nano-sized gold. [ABSTRACT FROM AUTHOR]

Luminescent metal halides doped with ns2-metal ions such as 6s2-metal Bi3+ have aroused reviving interest owing to their outstanding optical properties; however, the origin of the photoluminescence (PL) remains controversial and unclear. Herein, we report a strategy for the controlled synthesis of Bi3+-doped vacancy-ordered double perovskite Cs2SnCl6 nanocrystals (NCs) and unravel the triplet excited-state dynamics of Bi3+ through temperature-dependent PL and ultrafast femtosecond transient absorption spectroscopies. Owing to the aliovalent Bi3+ doping in the spatially confined zero-dimensional (0D) structure of Cs2SnCl6, Bi3+ ions experience an enhancive Jahn-Teller distortion in the excited state, which results in intense broadband blue PL originating from the inter-configurational 3P0,1 → 1S0 transitions of Bi3+ at 450 nm, with a large Stokes shift and a quantum yield of 35.2%. Specifically, an unusual thermal-enhanced Jahn-Teller splitting of the excitation band and a remarkable transition of the PL lifetime from ms at 10 K to µs at 300 K were observed, as solid evidence for the isolated Bi3+ emission. These findings clarify the controversy about the PL origin in ns2-metal ion-doped lead-free luminescent metal halides, thereby paving the way for exploring their optoelectronic applications. [ABSTRACT FROM AUTHOR]

Silicon–germanium (SiGe) alloys have sparked a great deal of attention due to their exceptional high-temperature thermoelectric properties. Significant effort has been expended in the quest for high-temperature thermoelectric materials. Combining density functional theory and electron–phonon coupling theory, it was discovered that silicon–tin (SiSn) alloys have remarkable high-temperature thermoelectric performance. SiSn alloys have a figure of merit above 2.0 at 800 K, resulting from their high conduction band convergence and low lattice thermal conductivity. Further evaluations reveal that Si0.75Sn0.25 is the best choice for developing the optimum ratio as a thermoelectric material. These findings will provide a basis for further studies on SiSn alloys as a potential new class of high-performance thermoelectric materials. [ABSTRACT FROM AUTHOR]

Electron-phonon coupling (EPC) is a key factor for thermoelectric properties of materials. In this paper, the thermoelectric properties of zinc-blende chalcogenides (p-type) ZnS and ZnSe have been studied through full evaluation of EPC from first-principles, including the influences on both electrical and thermal transport. We find that the polar longitudinal optical phonon scattering is the dominant mechanism for electrical transport. Due to the triple degeneracy near the valence band maximum, the inter-band scattering also has detrimental contributions to the electrical conductivities. For phonon transport, it shows that the lattice thermal conductivity can be reduced by the electron-phonon scattering significantly at high carrier concentrations (e.g., at 300 K with 1021 cm−3 of hole, the reduction is ∼24.9% for ZnS and ∼28.4% for ZnSe, respectively). Finally, the p-type thermoelectric figure of merit (ZT) of two systems have been obtained, which are 0.129 for ZnS and 0.141 for ZnSe, at 700 K with their respective optimal hole concentrations. Our work provides a complete and in-depth study of thermoelectric properties in chalcogenides ZnX from the role of EPC. The results suggest EPC plays an important role on the thermoelectric properties and thus full evaluation of EPC is necessary especially for polar materials.

Alloyed lead‐free double perovskites display intense photoluminescence, are environmentally friendly, and their devices show long‐term operation. Thanks to these properties, which make them excellent warm white‐emitting materials, they have recently received great attention in lighting applications. An important factor to tune the optical properties of alloyed lead‐free double perovskites is the presence of self‐trapped excitons. Here, it is demonstrated that in lead‐free double perovskites, the strong electron–phonon coupling plays a crucial role in the generation of self‐trapped excitons. The strong electron–phonon coupling is confirmed by a large Huang–Rhys factor and by the presence of multiphonon transitions. In particular, sharp emission lines superimposed on the broad photoluminescence emission band of one of these samples (Cs2Ag0.6Na0.4InCl6 0.5%Bi) are observed; these are due to the strong coupling of longitudinal‐optical phonons with excited electronic states caused by the tetragonally distorted AgCl6 octahedrons. Such a strong coupling of longitudinal‐optical phonons to electrons can effectively modulate the photophysical properties of alloyed double perovskites, and its understanding is, thus, of paramount importance for the design of future optoelectronic devices. [ABSTRACT FROM AUTHOR]

Electron-phonon interaction and related self-energy are fundamental to both the equilibrium properties and non-equilibrium relaxation dynamics of solids. Although electron-phonon interaction has been suggested by various time-resolved measurements to be important for the relaxation dynamics of graphene, the lack of energy- and momentum-resolved self-energy dynamics prohibits direct identification of the role of specific phonon modes in the relaxation dynamics. Here, by performing time- and angle-resolved photoemission spectroscopy measurements on Kekulé-ordered graphene with folded Dirac cones at the Γ point, we have succeeded in resolving the self-energy effect induced by the coupling of electrons to two phonons at Ω1 = 177 meV and Ω2 = 54 meV, and revealing its dynamical change in the time domain. Moreover, these strongly coupled phonons define energy thresholds, which separate the hierarchical relaxation dynamics from ultrafast, fast to slow, thereby providing direct experimental evidence for the dominant role of mode-specific phonons in the relaxation dynamics. [ABSTRACT FROM AUTHOR]

Electron–phonon coupling plays a key role in affecting the properties of the semiconducting nanostructures, such as providing the possibility for obtaining higher superconducting transition temperatures. Here, using Raman, temperature‐dependent and polarized Raman scattering measurements, ultra‐strong electron–phonon coupling in 1D CdS nanowires and 2D graphene heterostructures is demonstrated. The intensity ratio of 2LO/1LO mode in CdS nanowires provides a spectroscopy‐based method to quantify electron–phonon coupling, the strength of which is temperature and polarization dependent. The intensity ratio mode of 2LO/1LO in heterostructure reached up to 8.95 when the incident laser polarization is parallel to the c‐axis of the nanowire. It is ≈2.37 times higher than in an individual nanowire. In addition, in situ and time‐resolved photoluminescence spectra demonstrate the dynamics of the exciton recombinations, providing a comprehensive understanding of the enhancement of electron–phonon coupling in heterostructures. Via optical waveguiding characterization, the graphene layer is demonstrated to not only be an ultrafast carrier transfer channel but also a low Fermi level channel that induces the formation of the built in electrical field, elevating the electron–phonon coupling. Such new mixed dimensional heterostructures illustrate a straightforward approach to enhance the electron–phonon coupling, which may be applied to many integrated superconducting photonic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

The photoluminescence quantum yield of graphene quantum dots (GQDs) can be tuned by chemical functionalization. A rational design of fluorescent probes based on GQDs requires an understanding of the relationship between the chemical structure and the non-radiative recombination decay of GQDs. The oxygen-containing groups modify the edge states and alter the non-radiative decay of GQDs. In this work, we perform density functional theory (DFT) calculations to investigate the non-radiative decay dynamics of GQDs functionalised with different oxygen-containing groups, e.g. carbonyl, hydroxyl, and carboxyl, based on the principle of Fermi's golden rule. The carbonyl group oxidises the GQD edges, reducing the bandgap and red-shifting the absorption spectra. The carboxyl group increases the strength of electron-vibrational coupling of the high-frequency modes, resulting in faster non-radiative decay. The hydroxyl group, on the other hand, reduces the strength of electron-vibrational coupling in the high-frequency modes, thereby reducing non-radiative decay. Overall, this research extends our current knowledge of the role of individual oxygen-containing groups in the non-radiative decay of GQDs. [ABSTRACT FROM AUTHOR]

Metal halides with self‐trapped exciton (STE‐MHs) featuring broadband emission have manifested great potential in white light‐emitting diodes (WLEDs) with encouraging progress proceeding at an exhilarating pace. In this Perspective article, the focus is on the key issues and challenges of STE‐MH as the single‐layered emitter in WLEDs after providing a brief introduction for STE and its working mechanism of the relevant white‐light emission. Possible approaches and future research directions for efficient STE‐MH‐based WLEDs are also put forward, aiming at promoting their application in the next‐generation high‐quality solid‐state lighting sources. [ABSTRACT FROM AUTHOR]

Structural stabilities of high‐pressure YMgHx phases (x=2−10,12,14$x = 2-10, 12, 14$, and 16) and their superconductivities are investigated by employing evolutionary‐algorithm‐based crystal search combined with first‐principles calculations. For predicted candidate structures of YMgHx, the convex hull and phonon analyses reveal seven stable and two metastable phases. For all the predicted phases, superconducting transition temperatures (Tc) are also predicted by using the McMillan formula. P4/mmm$P4/mmm$‐YMgH6 is found having Tc=76$T_\mathrm{c} = 76$ K at 300 GPa comparable to the boiling temperature of liquid nitrogen, and high‐Tc (≥77 K) being predicted for the H‐richer phases, P4/mmm$P4/mmm$‐YMgH8 (124 K at 300 GPa), Cmmm$Cmmm$‐YMgH12 (152 K at 250 GPa), and Fd3¯m$Fd\bar{3}m$‐YMgH12 (190 K at 200 GPa), which possess clathrate structures composed of H14, H18, H24, and H24 cages, respectively. To elucidate why the H‐rich phases attain high‐Tc, electronic and phonon band structures as well as electron–phonon coupling strength are analyzed based on Eliashberg spectral functions. The clathrate structures exhibit both a larger H‐driven electronic density of states at the Fermi level and a denser H‐driven phonon density of states, correlating with larger EPC constants. These structural and chemical bonding analyses reveal that the highest‐Tc phase Fd3¯m$Fd\bar{3}m$‐YMgH12 has H4 units formed in the sodalite cage. [ABSTRACT FROM AUTHOR]

The temperature dependence of photoluminescence (PL) emission is a valuable tool for investigating carrier localization, recombination, and carrier–phonon interactions. Herein, electron–phonon couplings in lead sulfide (PbS) quantum dots (QDs) and lead sulfide/manganese tellurite (PbS/MnTe) QDs is reported. The effect of temperature on the PL emission of PbS and PbS/MnTe was explored within a temperature range of 10 to 300 K. When temperature increased, PL emission was blue-shifted due to the confinement effect. The gradual broadening of the full width at half maximum (FWHM) with increasing temperature indicates electron–phonon interactions. An analysis based on the Boson model revealed that the values of the exciton acoustic phonon coupling coefficient, σ, and temperature-dependent linewidth, γ, for PbS/MnTe were larger than those for PbS, indicating stronger exciton longitudinal-optical–phonon coupling in the compound structure. [ABSTRACT FROM AUTHOR]

We use ultrafast optical spectroscopy to study the nonequilibrium quasiparticle relaxation dynamics of the iron-based superconductor KCa2Fe4As4F2 with Tc = 33.5 K. Our results reveal a possible pseudogap (ΔPG = (2.4 ± 0.1) meV) below T* ≈ 50 K but prior to the opening of a superconducting gap (ΔSC(0) ≈ (4.3 ± 0.1) meV). Measurements under high pump fluence reveal two distinct, coherent phonon oscillations with 1.95 and 5.51 THz frequencies, respectively. The high-frequency A1g(2) mode corresponds to the c-axis polarized vibrations of FeAs planes with a nominal electron-phonon coupling constant λ A 1 g (2) = 0.194 ± 0.02 . Our findings suggest that the pseudogap is likely a precursor of superconductivity, and the electron-phonon coupling may play an essential role in the superconducting pairing in KCa2Fe4As4F2. [ABSTRACT FROM AUTHOR]

Abstract Electron–phonon coupling plays a key role in affecting the properties of the semiconducting nanostructures, such as providing the possibility for obtaining higher superconducting transition temperatures. Here, using Raman, temperature‐dependent and polarized Raman scattering measurements, ultra‐strong electron–phonon coupling in 1D CdS nanowires and 2D graphene heterostructures is demonstrated. The intensity ratio of 2LO/1LO mode in CdS nanowires provides a spectroscopy‐based method to quantify electron–phonon coupling, the strength of which is temperature and polarization dependent. The intensity ratio mode of 2LO/1LO in heterostructure reached up to 8.95 when the incident laser polarization is parallel to the c‐axis of the nanowire. It is ≈2.37 times higher than in an individual nanowire. In addition, in situ and time‐resolved photoluminescence spectra demonstrate the dynamics of the exciton recombinations, providing a comprehensive understanding of the enhancement of electron–phonon coupling in heterostructures. Via optical waveguiding characterization, the graphene layer is demonstrated to not only be an ultrafast carrier transfer channel but also a low Fermi level channel that induces the formation of the built in electrical field, elevating the electron–phonon coupling. Such new mixed dimensional heterostructures illustrate a straightforward approach to enhance the electron–phonon coupling, which may be applied to many integrated superconducting photonic and optoelectronic devices.

Understanding the thermal transport of various metals is crucial for many energy-transfer applications. However, due to the complex transport mechanisms varying among different metals, current research on metallic thermal transport has been focusing on case studies of specific types of metallic materials. A general understanding of the transport mechanisms across a broad spectrum of metallic materials is still lacking. In this work, we perform first-principles calculations to determine the thermal conductivity of 40 representative metallic materials, within a range of 8-456 W mK -1 . Our predicted values of electrical and thermal conductivity are in good agreement with available experimental results. Based on the data of separated electron and phonon thermal conductivity, we employ a statistical approach to examine nine factors derived from previous understandings and identify the critical factors determining these properties. For electrons, although a high electron density of states around the Fermi level implies more conductive electrons, we find it counterintuitively correlates with low electron thermal conductivity. This is attributed to the enlarged electron-phonon scattering channels induced by substantial electrons around the Fermi level. Regarding phonons, we demonstrate that among all the studied factors, Debye temperature plays the most significant role in determining the phonon thermal conductivity, despite the phonon-electron scattering being non-negligible in some transition metals. Correlation analysis suggests that Debye temperature has the highest positive correlation coefficient with phonon thermal conductivity, as it corresponds to a large phonon group velocity. Additionally, Young's modulus is found to be closely correlated with high phonon thermal conductivity and contribution. Our findings of simple factors that closely correlate with the electron and phonon thermal conductivity provide a general understanding of various metallic materials. They may facilitate the discovery of novel materials with extremely high or low thermal conductivity, or be used as descriptors in machine learning to accurately predict the thermal conductivity of metals in the future., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Single layer magnetic materials can provide a pathway for vdW heterostructures with magnetic properties. While most of the magnetic vdW materials exhibit ordering transitions in the bulk, here we report a successful exfoliation of a triangular lattice S = 1 antiferromagnet NiGa[Formula: see text]S[Formula: see text], which already demonstrates exotic magnetism in the bulk material. We establish the number of layers of the material by atomic force microscopy (AFM) and detail a careful characterization using Raman and optical spectroscopy to demonstrate how the optical, electronic, and structural properties of NiGa[Formula: see text]S[Formula: see text] change as a function of sample thickness. Optical measurements and electronic structure calculations of bulk versus monolayer NiGa[Formula: see text]S[Formula: see text] confirm the material to be a Mott insulator with an electronic gap of about 1.5 eV, which slightly increases for layers below 10 L. We conclude with a theoretical analysis of the possibility of doping monolayer NiGa[Formula: see text]S[Formula: see text] by proximity to a metal., Competing Interests: Declarations Competing interests The authors declare no competing interests., (© 2024. The Author(s).)

Finding high-temperature superconductivity in light-weight element containing compounds at atmosphere pressure is currently a research hotspot but has not been reached yet. Here it is proposed that hard or superhard materials can be promising candidates to possess the desirable high-temperature superconductivity. By studying the electronic structures and superconducting properties of H and Li doped c-BN within the framework of the first-principles, it is demonstrated that the doped c-BN are indeed good superconductors at ambient pressure after undergoing the phase transition from the insulating to metallic behavior, though holding different nature of metallization. Li doped c-BN is predicted to exhibit the superconducting transition temperature of ≈58 K, while H doped c-BN has stronger electron-phonon interaction and possesses a higher transition temperature of 122 K. These results and findings thus point out a new direction for exploring the ambient-pressure higher-temperature superconductivity in hard or superhard materials., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Heterostructure engineering provides versatile platforms for exploring exotic physics and enhancing the device performance through interface coupling. Despite the rich array of physical phenomena presented by heterostructures composed of semiconductor and metal van der Waals materials, significant gaps remain in understanding their optical, thermal, and electronic properties. Here, we demonstrate that the valley pseudospin and electron-phonon coupling in monolayer WSe 2 are significantly influenced by interface coupling with 1T-VSe 2 . The heterointerface alters the relaxation process of valley excitons, leading to a transition in magnetic-field-dependent valley polarization from a linear to a "V" shape. Furthermore, we uncover that enhanced electron-phonon coupling exacerbates variations in exciton and valley exciton behavior with temperature, involving higher phonon energies and a shift from acoustic to optical phonons. These findings highlight a promising pathway to manipulate valley excitons and investigate electron-phonon coupling through van der Waals interface interactions.

Achieving superconductivity at room temperature (RT) is a holy grail in physics. Recent discoveries on high-T c superconductivity in binary hydrides H 3 S and LaH 10 at high pressure have directed the search for RT superconductors to compress hydrides with conventional electron-phonon mechanisms. Here, an exceptional family of superhydrides is predicated under high pressures, MH 12 (M = Mg, Sc, Zr, Hf, Lu), all exhibiting RT superconductivity with calculated T c s ranging from 313 to 398 K. In contrast to H 3 S and LaH 10 , the hydrogen sublattice in MH 12 is arranged as quasi-atomic H 2 units. This unique configuration is closely associated with high T c , attributed to the high electronic density of states derived from H 2 antibonding states at the Fermi level and the strong electron-phonon coupling related to the bending vibration of H 2 and H-M-H. Notably, MgH 12 and ScH 12 remain dynamically stable even at pressure below 100 GPa. The findings offer crucial insights into achieving RT superconductivity and pave the way for innovative directions in experimental research., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

The kinetic aggregation of nonfullerene acceptors under nonequilibrium conditions can induce electron–phonon interaction roll‐off and electronic band structure transition, which represents an important limitation for long‐term operational stability of organic solar cells (OSCs). However, the fundamental underlying mechanisms have received limited attention. Herein, a photophysical correlation picture between intermolecular electron–phonon coupling and trapping of electronic excitation is proposed based on the different aggregation behaviors of BTP‐eC9 in bulk‐heterojunction and layer‐by‐layer processed multicomponent OSCs. Two separate factors rationalize their correlation mechanisms: 1) the local lattice and/or molecular deformation can be regarded as the results of BTP‐eC9 aggregates in binary system under continuous heating, which brings about attenuated intermolecular electron–phonon coupling with intensified photocarrier trapping. 2) The higher density of trap states with more extended tails into the bandgap give rise to the formation of highly localized trapped polarons with a longer lifetime. The stabilized intermolecular electron–phonon coupling through synergistic regulation of donor and acceptor materials effectively suppresses unfavorable photocarrier trapping, delivering the improved device efficiency of 18.10% and enhanced thermal stability in quaternary OSCs. These results provide valuable property–function insights for further boosting photovoltaic stability in view of modulating intermolecular electron–phonon coupling. [ABSTRACT FROM AUTHOR]

Raman spectroscopy has been not only a technique for characterizing the composition and lattice structure of materials, but also a platform to explore the electron-photon and electron-phonon couplings. When excited by circularly polarized light, the Raman scattered light can carry a spin angular momentum ±ħ, which adds a new degree of freedom to study the Raman scattering process. This review explains the principles of the helicity of Raman scattered light excited by circular polarization, and introduces the recent advances in the fundamentals and applications of helicity-resolved Raman scattering in two dimensional (2D) materials, including the assignment of overlapped Raman modes, the characterization of exciton-phonon coupling and the application in chiral optics. We hope that this review will give a comprehensive understanding of the helicity selection rule in the Raman scattering process and inspire more exploration on the applications of the helicity of Raman scattered light. [ABSTRACT FROM AUTHOR]

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

SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers across the bandgap, observing the bimodal and anisotropic lattice distortions that drive carrier localization. Relatively large (18.7 Å), quasi-one-dimensional (1D) polarons are formed on the 300-fs timescale with smaller (4.2 Å) 3D polarons taking an order of magnitude longer (4 ps) to form. This difference appears to be a consequence of the profoundly anisotropic electron-phonon coupling in SnSe, with strong Fröhlich coupling only to zone-center polar optical phonons. These results demonstrate a high density of polarons in SnSe at optimal doping levels. Strong electron-phonon coupling is critical to the thermoelectric performance of this benchmark material and, potentially, high performance thermoelectrics more generally. [ABSTRACT FROM AUTHOR]

The electron–phonon coupling (EPC) and superconductivity in three‐component‐fermion materials WN, WC, and TaN are systematically investigated using first‐principles calculations. The EPC strength in pristine or pressurized WC and TaN are found to be negligibly weak (<0.2), but can be enhanced significantly by electron doping, and the predicted transition temperatures (Tc) reach the values observed in experiments (≈4 K), demonstrating the vital role of charge doping in the formation of superconductivity. Surprisingly, pristine WN has strong EPC due to a synergistic effect of strong Fermi nesting and large deformation potential and behaves as a good superconductor with Tc of 31 K. Going down from 3D to 2D, WN thin film (i.e., monolayer W3N4) is also predicted to be an intrinsic superconductor with Tc of 11 K. Most importantly, monolayer W3N4 hosts nodal lines that are robust against spin–orbit coupling (SOC), close to the Fermi level and apart from other trivial bands, which are scarce in real materials. The coexistence of high‐transition‐temperature superconductivity and topological states in 3D and 2D WN render them promising platforms for realizing topological superconductivity. [ABSTRACT FROM AUTHOR]

Resonance Raman spectra and excitation profiles have been measured and semiquantitatively modeled for core/shell quantum dots consisting of 2.7 nm diameter zincblende CdSe cores and thin (0.5 nm) or thick (1.6 nm) CdS shells. The Raman spectra show previously reported trends of increased peak frequency for both the CdSe and the CdS longitudinal optical (LO) phonons with increasing shell thickness. We also find a strong dependence of the peak CdS frequency on excitation energy and a large discrepancy between the experimental frequency of the CdSe + CdS combination band and the sum of the corresponding fundamental frequencies. This suggests that the dominant transitions at high excitation energies are localized on either the CdSe core or the CdS shell and thereby cannot enhance combination band transitions between core and shell. The CdS to CdSe Raman intensity ratios at high excitation energies further support this picture. The electron-phonon coupling for the CdSe LO phonon in the lowest excitonic transition is slightly weaker in the core/shell structures than in pure CdSe quantum dots, contrary to expectations for the Fröhlich coupling mechanism. Possible explanations for this discrepancy are discussed.

Motivated to understand phonon spectrum renormalization in the ground state of the half-filled Su–Schrieffer–Heeger model, we use the Born–Oppenheimer approximation together with the harmonic approximation to evaluate semi-analytically the all-to-all real-space ionic force constants generated through both linear and quadratic electron-phonon coupling. We then compute the renormalized phonon spectrum and the corresponding lattice zero-point energy (ZPE) as a function of the lattice dimerization. Crucially, the latter is included in the system’s total energy, and thus has a direct effect on the equilibrium dimerization. We find that inclusion of a small quadratic coupling leads to very significant changes in the predicted equilibrium dimerization, calling into question the use of the linear approximation for this model. We also argue that inclusion of the ZPE is key for systems with comparable lattice and electronic energies, and/or for finite size chains. Our method can be straightforwardly generalized to study similar problems in higher dimensions.

Crossed magneto-thermo-electric coefficients are central to novel sensors and spin(calori)tronic devices. Within the framework of Boltzmann’s transport theory, we calculate the resistivity and Seebeck coefficients of the most common 3 d ferromagnetic metals: Fe, Co, and Ni. We use a fully first-principles variational approach, explicitly taking electron-phonon scattering into account. The electronic band structures, phonon dispersion curves, phonon linewidths, and transport spectral functions are reported, comparing with experimental data. Successive levels of approximation are discussed: constant relaxation time approximation, scattering for a non-magnetic configuration, then spin polarized calculations with and without spin–orbit coupling (enabling spin-flips). Spin polarization and explicit electron–phonon coupling are found to be necessary to reach a correct qualitative picture: the effect of spin flipping is substantial for resistivity and very delicate for the Seebeck coefficient. The spin-dependent Seebeck effect is also predicted.

The study of two-dimensional (2D) superconductors is one of the most prominent areas in recent years and has remained a long-standing scientific challenge. Since the introduction of the new member of 2D material family consisting of transition-metal elements with Borides (MBenes), the study of various properties of 2D metal borides has attracted widespread interest. In this work, we systematically investigate the phonon-mediated superconductivity in 2D M B _4 ( M = V, Nb, and Ta), by using the first-principles calculations. Starting from the dynamical stabilities of these structures, we perform a detailed analysis of the electron–phonon coupling and superconductivity of AB-stacked MBenes by solving the anisotropic Eliashberg equation. NbB _4 has the largest electron–phonon coupling and the highest superconducting transition temperature T _c of 35.4 K. Our study broadens the 2D superconducting boride family, which is of great significance for the study of 2D superconductivity.

Dirac semimetal PdTe2 single-crystal temperature-dependent ultrafast carrier and phonon dynamics were studied using ultrafast optical pump-probe spectroscopy. Quantitative analysis revealed a fast relaxation process (τf) occurring at a subpicosecond time scale originating from electron–phonon thermalization. This rapid relaxation was followed by a slower relaxation process (τs) on a time scale of ∼ 7-9.5 ps which originated from phonon-assisted electron–hole recombination. Two significant vibrational modes resolved at all measured temperatures. These modes corresponded to in-plane (Eg), and out-of-plane (A1g), Te atoms motion. Test results suggested that pure dephasing played an important role in the relaxation processes. Analysis of the electron–phonon coupling constant suggested that the A1gmode contributes greatly to the superconductivity, and high-frequency phonons are also involved forming of Cooper pairs. Our observations should improve the understanding of complex superconductivity of PdTe2.

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.