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Structural mosaic of rare-earth tri-tellurides (RTe3) inlaid with non-classical structural motifs like the 2D-polytelluride square nets has attracted immense attention owing to their enigmatic chemical bonding, unconventional structure, and harboring charge density wave (CDW) ground states. GdTe3, an archetypal RTe3, is a natural heterostructure of charged and van der Waals (vdW) layers, formed by intercalating vdW gap separated 2D square telluride nets … between the charged double corrugated slabs of n[GdTe]+. Here, we have investigated the evolution of structural distortions along with the electrical and thermal transport properties of GdTe3 across its CDW transition through X-ray pair distribution function analysis, thermal conductivity measurements, Raman spectroscopy and first principles theoretical calculations. The results reveal that the unusual structure of GdTe3 engenders a large anisotropic lattice thermal conductivity by concomitantly hampering the phonon propagation along parallel to the spark plasma sintering (SPS) pressing direction via chemical bonding hierarchy while facilitating phonon propagation along perpendicular to the SPS pressing direction through the metallic Te sheets and phason channel. The low lattice thermal conductivity is attributed to the strong vibrational anharmonicity caused by CDW-induced concerted local lattice distortions of both Gd-Te slab and Te square net, and the robust electron-phonon coupling. [ABSTRACT FROM AUTHOR]

Highlights: The soft 2D material reduces the coupling strength between carriers and longitudinal optical phonons, releasing the mechanical stress of lattice vibration. The power conversion efficiency of rigid devices and flexible devices reaches 25.5% and 23.4%, respectively. This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron–phonon coupling of perovskite in perovskite solar cells (PSCs). Via A-site cation engineering, a weaker electron–phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine (CMA+) cation, which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations, compared to the rigid phenethyl methylamine (PEA+) analog. It demonstrates a significantly lower non-radiative recombination rate, even though the two types of bulky cations have similar chemical passivation effects on perovskite, which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation. The resulting PSCs achieve an exceptional power conversion efficiency (PCE) of 25.5% with a record-high open-circuit voltage (VOC) of 1.20 V for narrow bandgap perovskite (FAPbI3). The established correlations between electron–phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley–Queisser limit. [ABSTRACT FROM AUTHOR]

Single‐component white‐light emitters are ideal for solid‐state lighting applications. Here, the dual‐emission white light in the 3D Cs2NaInCl6:0.1 Sb3+ crystal is successfully achieved by the high‐pressure treatment strategy. Upon compression, the remarkable self‐trapped exciton (STE) emission enhancement is observed at 20.6 GPa due to the tilting and twisting of inorganic octahedron caused by the phase transition of Cs2NaInCl6:0.1 Sb3+ crystal. Intriguingly, after undergoing the high‐pressure treatment, in addition to the blue high‐energy emission from the STEs, [SbCl6]3− exhibits the orange low‐energy emission from STEs, which is ascribed to the remaining local distortion of inorganic octahedron resulting from the presence of residual amorphous phase in the local region upon decompression. Furthermore, coherent phonon generation and density functional theory calculations indicate that the electrons exhibit a strong coupling to A1g stretch mode and T2gIn bending mode in Cs2NaInCl6:0.1 Sb3+. After high‐pressure treatment, the coupling between the electrons and the T2gIn bending mode disappears, suggesting local octahedral symmetry disruption due to residual amorphous structures induced by high‐pressure treatment. This work unveils the relationship between the electron‐phonon coupling and the STE emission and offers a new strategy to design and synthesize the new single‐component white‐light metal halide perovskites. [ABSTRACT FROM AUTHOR]

Lead‐free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light‐yellow to black over a temperature range of 10 to 423 K is investigated. First‐principles, density functional theory (DFT)‐based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group‐IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials. [ABSTRACT FROM AUTHOR]

To assess the potential optoelectronic applications of metal‐halide perovskites, it is critical to have a detailed understanding of the nature and dynamics of interactions between carriers and the polar lattices. Here, the electronic and structural dynamics of bismuth‐based perovskite Cs3Bi2I9 are revealed by transient reflectivity and ultrafast electron diffraction. A cross‐examination of these results combined with theoretical analyses allows the identification of the major carrier–phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into Cs3Bi2I9 form self‐trapped excitons on an ultrafast time scale. However, they retain most of their energy, and their coupling to Fröhlich‐type optical phonons is limited at early times. Instead, the long‐lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub‐ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye–Waller random atomic motions on longer times. The current results suggest the potential use of bismuth‐based perovskites for applications other than photovoltaics to take advantage of the carriers' stronger self‐trapping and long lifetimes. [ABSTRACT FROM AUTHOR]

To assess the potential optoelectronic applications of metal‐halide perovskites, it is critical to have a detailed understanding of the nature and dynamics of interactions between carriers and the polar lattices. Here, the electronic and structural dynamics of bismuth‐based perovskite Cs3Bi2I9 are revealed by transient reflectivity and ultrafast electron diffraction. A cross‐examination of these results combined with theoretical analyses allows the identification of the major carrier–phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into Cs3Bi2I9 form self‐trapped excitons on an ultrafast time scale. However, they retain most of their energy, and their coupling to Fröhlich‐type optical phonons is limited at early times. Instead, the long‐lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub‐ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye–Waller random atomic motions on longer times. The current results suggest the potential use of bismuth‐based perovskites for applications other than photovoltaics to take advantage of the carriers' stronger self‐trapping and long lifetimes. [ABSTRACT FROM AUTHOR]

Lead‐free halide double perovskites (HDPs) have emerged as a new generation of thermochromic materials. However, further materials development and mechanistic understanding are required. Here, a highly stable HDP Cs2NaFeCl6 single crystal is synthesized, and its remarkable and fully reversible thermochromism with a wide color variation from light‐yellow to black over a temperature range of 10 to 423 K is investigated. First‐principles, density functional theory (DFT)‐based calculations indicate that the thermochromism in Cs2NaFeCl6 is an effect of electron–phonon coupling. The temperature sensitivity of the bandgap in Cs2NaFeCl6 is up to 2.52 meVK−1 based on the Varshni equation, which is significantly higher than that of lead halide perovskites and many conventional group‐IV, III–V semiconductors. Meanwhile, this material shows excellent environmental, thermal, and thermochromic cycle stability. This work provides valuable insights into HDPs' thermochromism and sheds new light on developing efficient thermochromic materials. [ABSTRACT FROM AUTHOR]

Electron‐phonon interactions, crucial in condensed matter, are rarely seen in Metal–Organic Frameworks (MOFs). Detecting these interactions typically involves analyzing luminescence in lanthanide‐ or actinide‐based compounds. Prior studies on Ln‐ and Ac‐based MOFs at high temperatures revealed additional peaks, but these were too faint for thorough analysis. In our research, we fabricated a high‐quality, crystalline uranium‐based MOF (KIT‐U‐1) thin film using a layer‐by‐layer method. Under UV light, this film showed two distinct "hot bands," indicating a strong electron‐phonon interaction. At 77 K, these bands were absent, but at 300 K, a new emission band appeared with half the intensity of the main luminescence. Surprisingly, a second hot band emerged above 320 K, deviating from previous findings in rare‐earth compounds. We conducted a detailed ab‐initio analysis employing time‐dependent density functional theory to understand this unusual behaviour and to identify the lattice vibration responsible for the strong electron‐phonon coupling. The KIT‐U‐1 film's hot‐band emission was then utilized to create a highly sensitive, single‐compound optical thermometer. This underscores the potential of high‐quality MOF thin films in exploiting the unique luminescence of lanthanides and actinides for advanced applications. [ABSTRACT FROM AUTHOR]

Excited-states play a crucial role in the optical absorption and luminescence of solids and hence their accurate information is highly desired. Herein, we attempt to seize the excited-states information of Mn4+ ions in K2SiF6 microcrystals via measuring and calculating their variable-temperature photoluminescence excitation (PLE) spectra. At cryogenic temperatures, an unpredicted splitting of the high-excited-state is observed. Moreover, the two-split high-excited-state levels are further revealed to primarily couple with the two hyperfine split modes of quasi-localized v2 vibration in the distorted Mn-F6 octahedral configuration, whereas the coupling strengths are found to be substantially different from each other. The slightly split vibrational mode is firmly supported by the low-temperature Raman spectra. Jahn-Teller lattice distortion is believed to be responsible for the observed splitting of the electronic high-excited-state and the quasi-localized vibrational mode. [ABSTRACT FROM AUTHOR]

Gallium antimonide (GaSb) is a III-V semiconductor of technological interest for low-power, high-mobility field-effect transistors, as well as for mid-wave infrared detectors. In such devices, GaSb interfaces with other III–V semiconductors with different lattice constants that can induce strain in the GaSb layers. Two dominant limiting factors in hot carrier relaxation are the intra-valley and the inter-valley electron–phonon (e-ph) scattering. In GaSb, these are sensitive to the Γ –L energy ordering, which depend intimately on the strain. Here, we report ab initio calculations of electronic structure, phonon dispersion, e-ph scattering and relaxation times for GaSb as a function of strain. As observed previously for other group IV and III-V semiconductors, our results show strong anisotropy, a strong contribution from LO phonons, and the need to go beyond the deformation potential scattering. For GaSb, the main finding is that a compressive strain between 0.4% and 0.6% converts GaSb from a direct-bandgap semiconductor to an indirect-bandgap semiconductor, with dramatic changes in the competing scattering rates and carrier relaxation times. [ABSTRACT FROM AUTHOR]

Bismuth halide perovskites are highly promising for various applications owing to their lower toxicity and greater environmental stability. The temperature‐dependent color tunability of these materials renders them a strong contender for use in smart window applications and temperature sensors. To delve deeper into their properties, the thermochromism and fundamental lattice dynamics are investigated in bismuth halide perovskites through the transmutation of cation and anions, specifically focusing on Cs3Bi2Br9, MA3Bi2Br9, Cs2AgBiBr6, and Cs2AgBiCl6 perovskites. Absorption spectroscopy and powder‐XRD patterns as a function of temperature reveal that 2D‐layered Cs3Bi2Br9 and MA3Bi2Br9 perovskites exhibit significantly greater bandgap changes and thermal expansion compared to 3D connected Cs2AgBiCl6 and Cs2AgBiBr6 perovskites attributed to more space in the 2D connected lattice. The reduction in bandgap and color variation in these perovskites stems from the intricate interplay between electron‐phonon interactions and thermal expansion. Notably, it is found that the type of cation and anion play a crucial role in influencing the degree of thermochromism, as the electron‐phonon coupling depends on this transformation. This study sheds new light on the fundamental mechanisms underlying the thermochromic behavior of bismuth halide perovskites and paves the way for the rational design and engineering of novel thermochromic materials with tailored properties. [ABSTRACT FROM AUTHOR]

The charge density wave (CDW) instability, usually occurring in low-dimensional metals, has been a topic of interest for longtime. However, some very fundamental aspects of the mechanism remain unclear. Recently, a plethora of new CDW materials, a substantial fraction of which is two-dimensional or even three-dimensional, has been prepared and characterised as bulk and/or single-layers. As a result, the need for revisiting the primary mechanism of the instability, based on the electron–hole instability established more than 50 years ago for quasi-one-dimensional (quasi-1D) conductors, has clearly emerged. In this work, we consider a large number of CDW materials to revisit the main concepts used in understanding the CDW instability, and emphasise the key role of the momentum dependent electron–phonon coupling in linking electronic and structural degrees of freedom. We argue that for quasi-1D systems, earlier weak coupling theories work appropriately and the energy gain due to the CDW and the concomitant periodic lattice distortion (PLD) remains primarily due to a Fermi surface nesting mechanism. However, for materials with higher dimensionality, intermediate and strong coupling regimes are generally at work and the modification of the chemical bonding network by the PLD is at the heart of the instability. We emphasise the need for a microscopic approach blending condensed matter physics concepts and state-of-the-art first-principles calculations with quite fundamental chemical bonding ideas in understanding the CDW phenomenon in these materials. [ABSTRACT FROM AUTHOR]

Many issues concerning the origin of high-temperature superconductivity (HTS) are still under debate. For example, how the magnetic order varies with doping and its relationship with the superconducting temperature (Tc); and why Tc always peaks near the quantum critical point. In this paper, taking hole-doped La2CuO4 as a classical example, we employ the first-principles band structure and total energy calculations with Monte Carlo simulations to explore how the symmetry-breaking magnetic ground state evolves with hole doping and the origin of a dome-shaped superconductivity region in the phase diagram. We demonstrate that the local antiferromagnetic order and doping play key roles in determining the electron-phonon coupling, thus Tc. Initially, the La2CuO4 possesses a checkerboard local antiferromagnetic ground state. As the hole doping increases, Tc increases with the enhanced electron-phonon coupling strength. But as the doping increases further, the strength of the antiferromagnetic interaction weakens and spin fluctuation increases. At the critical doping level, a magnetic phase transition occurs that reduces the local antiferromagnetism-assisted electron-phonon coupling, thus diminishing the Tc. The superconductivity disappears in the heavily overdoped region when the ferromagnetic order dominates. These observations could account for why cuprates have a dome-shaped superconductivity region in the phase diagram. Our study, thus, contributes to a fundamental understanding of the correlation between doping, local magnetic order, and superconductivity of HTS. [ABSTRACT FROM AUTHOR]

Electron–phonon coupling (EPC) and phonon–phonon scattering (PPS) are at the core of the microscopic physics mechanisms of vast quantum materials. However, to date, there are rarely reports that these two processes can be spatially separated, although they are usually temporally detached with different characteristic lifetimes. Here, by employing ultrafast spectroscopy to investigate the photo‐carrier ultrafast dynamics in a LaCoO3 thin film on a (100) SrTiO3 substrate, intriguing evidence is found that the two interactions are indeed spatially separated. The EPC mainly occurs in the thin film, whereas PPS is largely in the substrate, especially at the several atomic layers near the interface. Across‐interface penetration and decay of optical phonons into acoustic phonons thus naturally occur. An EPC strength λEg = 0.30 is also obtained and an acoustic phonon mode at 45.3 GHz is observed. The finding lays out a cornerstone for future quantum nano device designs. [ABSTRACT FROM AUTHOR]

Electron–phonon coupling (EPC) and phonon–phonon scattering (PPS) are at the core of the microscopic physics mechanisms of vast quantum materials. However, to date, there are rarely reports that these two processes can be spatially separated, although they are usually temporally detached with different characteristic lifetimes. Here, by employing ultrafast spectroscopy to investigate the photo‐carrier ultrafast dynamics in a LaCoO3 thin film on a (100) SrTiO3 substrate, intriguing evidence is found that the two interactions are indeed spatially separated. The EPC mainly occurs in the thin film, whereas PPS is largely in the substrate, especially at the several atomic layers near the interface. Across‐interface penetration and decay of optical phonons into acoustic phonons thus naturally occur. An EPC strength λEg = 0.30 is also obtained and an acoustic phonon mode at 45.3 GHz is observed. The finding lays out a cornerstone for future quantum nano device designs. [ABSTRACT FROM AUTHOR]

Hybrid inorganic/organic systems (HIOS) open new avenues for tailoring them with respect to desired features and functions by exploiting the respective advantages of their components. Therefore, these materials are actively explored in many experimental studies and devices. On the theory side, similar investigations are rather scarce as such interfaces, in addition to exhibiting large unit cells, require highest‐level theories to be described reliably. Consequently, hybrid materials pose a challenge for electronic structure theory, starting from density‐functional theory to methods beyond, particularly many‐body perturbation theory. This concerns both conceptual aspects and computational bottlenecks. In this perspective, the performance of state‐of‐the‐art theoretical approaches applied to HIOS is summarized, mainly focusing on optoelectronic properties. Recent achievements, open challenges, and urgent needs are addressed. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we predict a new type of two-dimensional (2D) beryllium(Be)-decorated T-graphene named BeC2, where Be atoms are inserted into C–C bonds linking the carbon tetrarings of T-graphene. The band structure shows that BeC2 is metallic, thus, the possible phonon-mediated superconductivity is explored based on the Eliashberg equation. The calculated electron-phonon coupling (EPC) constant λ is up to 4.07, and the corresponding superconducting critical temperature (Tc) is 72.1 K, approaching the liquid nitrogen temperature. The reason for the high Tc is the strong EPC. And it is proved to be an anisotropic single-gap superconductor by analyzing the superconducting gap Δk of BeC2. The electronic susceptibility calculation shows strong nesting effect in BeC2. Since rare 2D superconductors show such a strong EPC constant λ which originates from the coupling between electrons in C–pz orbital and in-plane vibrations of Be and C atoms, the predicted BeC2 provides a new platform for investigating strong EPC 2D superconductor. [ABSTRACT FROM AUTHOR]

Perovskites have been recognized as a class of promising materials for optoelectronic devices. We intentionally include excessive Cs+ cations in precursors in the synthesis of perovskite CsPbBr3 nanocrystals and investigate how the Cs+ cations influence the lattice strain in these perovskite nanocrystals. Upon light illumination, the lattice strain due to the addition of alkali metal Cs+ cations can be compensated by light–induced lattice expansion. When the Cs+ cation in precursors is about 10% excessive, the electron–phonon coupling strength can be reduced by about 70%, and the carrier cooling can be slowed down about 3.5 times in lead halide perovskite CsPbBr3 nanocrystals. This work reveals a new understanding of the role of Cs+ cations, which take the A–site in ABX3 perovskite and provide a new way to improve the performance of perovskites and their practical devices further. [ABSTRACT FROM AUTHOR]

The cooling mechanism of hot carriers (HC) in metal halide perovskites is a topic of debate which gathered huge attention due to its critical role in the performance of perovskite‐based optoelectronics. HC cooling in 2D perovskites is faster than in its 3D counterpart, whereas in 2D/3D perovskites cooling becomes faster with decreasing the thickness of the inorganic quantum wells. Using state‐of‐the art first principles calculations it is showed that the modulation of electron–phonon (e‐ph) coupling strength between bending and stretching phonon branches can explain this observation. Starting from the prototype BA2PbI4 and PEA2PbI4 2D perovskites, e‐ph coupling of individual phonon modes is investigated for 2D/3D perovskites with n = 1 and 3, along with a vis‐à‐vis comparison with the prototypical 3D MAPbI3 system. This study shows that e‐ph coupling with high‐frequency stretching phonon modes in the 60–120 cm−1 range is highest for n = 1 while it decreases with increasing the quantum well layers, by approaching the 3D bulk limit where e‐ph coupling with low‐frequency bending phonon modes (<60 cm−1) is dominant. Longer spacer cations with identical quantum well structures have a limited impact on the e‐ph coupling, highlighting that the primary factor governing HC cooling is the quantum confinement within the inorganic sublattice. This study provides an advancement in the understanding of the mode‐specific e‐ph mediated HC cooling mechanism in metal‐halide perovskites and can provide a route map toward tuning the e‐ph interaction, which is instrumental for effectively gathering HC in solar cell devices. [ABSTRACT FROM AUTHOR]

Recently, it was demonstrated that inelastic helium atom scattering from conducting surfaces provides a direct measurement of the surface electron–phonon coupling constant (mass enhancement factor λ) via the temperature or the incident wave vector dependence of the Debye–Waller exponent. Here, previous published as well as unpublished helium atom scattering diffraction data from the vicinal surfaces of copper (Cu(11α), with α = 3, 5, 7) and aluminum (Al(221) and Al(332)) were analyzed to determine λ. The results suggested an enhancement with respect to the corresponding data for the low-index surfaces (111) and (001) above the roughening transition temperature. The specific role of steps compared to that of terraces is briefly discussed. [ABSTRACT FROM AUTHOR]

Understanding the full electron–phonon and phonon–phonon interaction effects in semiconductors is crucial for tuning electronic and phonon transport properties with the purpose to the enhancement of thermoelectric performances. In this work, based on the detailed investigations on 2D lead phosphorene (PbP) by using the first‐principle calculations, it is demonstrated that, in α−$\alpha -$, β−$\beta -$, and γ−$\gamma -$PbP, the electron–phonon interactions are significantly stronger than conventional group‐IV monolayers such as silicene by five orders in magnitude, and intravalley scatterings dominate over the intervalley scattering due to less degenerate band extrema near Fermi level. Based on the selection‐rule analysis, it is found that the symmetry breaking in the janus γ phase compared to the higher‐symmetry α phase allows fewer symmetry operations, leading to more allowed scattering channels for electrons and phonons, subsequently suppressing the electron and phonon transport in γ−$\gamma -$PbP. Due to the low Debye temperatures, and strong four‐phonon interactions, the lattice thermal conductivities at room temperature in these monolayers are sharply decreased. Finally, it is found that the maximum thermoelectric figure of merits for these three monolayers reach 0.90/0.24/1.25 at room temperature, respectively. This work not only paves the way for the promising applications of PbP monolayers in thermoelectric devices, but also suggests that symmetry breaking and strong high‐order phonon–phonon interactions are crucial to identifying excellent thermoelectric materials. [ABSTRACT FROM AUTHOR]

Broadband emission induced by self‐trapped excitons (STEs) in double halide perovskites (DHPs) has received continuous attention in recent years. However, the comprehensive understanding of the STEs formation mechanism is still in its early stage. The corresponding roles of different B‐site cations also remain unclear in these advanced materials. The lack of an effective STEs database for DHPs hinders the efficient discovery of potential optoelectronic materials with strong STEs. Herein, a systematic STEs database is built for DHPs through density functional theory (DFT) calculations and proposed a highly efficient predictive machine learning (ML) model of the Huang–Rhys factor S for the first time. Results reveal the different contributions of two B‐site metal cations to the formation of STEs in DHPs, which helps to understand the in‐depth nature of STEs. Based on the accurate predictions of the effective phonon frequency ωLO$\left(\omega\right)_{\text{LO}}$ , it is further realized that the prediction of S without conducting the time‐consuming phonon property calculations of DHPs offers new opportunities for exploring the STEs. Combining DFT calculations and ML techniques, this study supplies an effective approach to efficiently discover the potential novel optoelectronic materials, which provides important guidance for the future exploration of promising solid‐state phosphors. [ABSTRACT FROM AUTHOR]

The influence of external pressure on the properties of Sn2P2S6 has been systematically studied using density-functional theory calculations. It has been found that the ambient-pressure polymorph (space group P21/c) is most stable up to 100 GPa than the other two polymorphs reported in the literature. We have also found that the electronic band gap closes under compression, being metallization achieved at 35 GPa. In addition, electron–phonon coupling calculations show that the studied material is a superconductor at pressures of 35 GPa and higher. The critical temperature increases under compression. We will also present results on phonon-dispersion calculations, showing that Sn2P2S6 becomes dynamically unstable near 60 GPa. Finally results on the linear and volumetric compressibility are reported showing that Sn2P2S6 is highly compressible with a bulk modulus of 34(1) GPa.

Blue-emission (∼480 ​nm) CsPbBr3 nanoparticles with ultra-small size (∼2.1 ​nm) are synthesized using the liquid nitrogen freezing with the ligand of dodecylbenzene sulfonic acid (DBSA). Asymmetric narrow emissions at the low energy side, with the full width at half-maximum of ∼20 ​nm, are observed in solution and film at room temperature. The spectral asymmetry is mainly ascribed to phonon vibronic replica with averaged phonon energy of ∼40 ​meV. Moreover, exciting this CsPbBr3 nanoparticles solution using linearly polarized 6 ns pulsed laser at 355 ​nm, we observe polarized emission with polarization degree (PPL) of ∼7%, and PPL decreases more than 20% in the vibronic progression. However, the PPL goes to zero in frozen solutions as well as in films. Thus we speculate the polarized emission is due to the photoinduced re-alignment of nanoparticles, and the diminished PPL at the phonon side band may be due to the non-adiabatic electronic-to-vibronic transitions. The novel phenomena from the ultra-small CsPbBr3 nanoparticle demonstrated in this work may provide fundamental insights into its photophysics with direct implications for optoelectronics.

Broadband emission induced by self‐trapped excitons (STEs) in double halide perovskites (DHPs) has received continuous attention in recent years. However, the comprehensive understanding of the STEs formation mechanism is still in its early stage. The corresponding roles of different B‐site cations also remain unclear in these advanced materials. The lack of an effective STEs database for DHPs hinders the efficient discovery of potential optoelectronic materials with strong STEs. Herein, a systematic STEs database is built for DHPs through density functional theory (DFT) calculations and proposed a highly efficient predictive machine learning (ML) model of the Huang–Rhys factor S for the first time. Results reveal the different contributions of two B‐site metal cations to the formation of STEs in DHPs, which helps to understand the in‐depth nature of STEs. Based on the accurate predictions of the effective phonon frequency ω LO , it is further realized that the prediction of S without conducting the time‐consuming phonon property calculations of DHPs offers new opportunities for exploring the STEs. Combining DFT calculations and ML techniques, this study supplies an effective approach to efficiently discover the potential novel optoelectronic materials, which provides important guidance for the future exploration of promising solid‐state phosphors.

To explore carbide superconductors with higher transition temperature, two novel carbon structures of cage‐network are designed and their superconductivity is studied by doping metals. MC6 and MC10 are respectively identified as C24 and C32 cage‐network structures. This study finds that both carbon structures drive strong electron–phonon interaction and can exhibit superconductivity above liquid nitrogen temperature. Importantly, the superconducting transition temperatures above 100 K are predicted to be achieved in C24‐cage‐network systems doped by Na, Mg, Al, In, and Tl at ambient pressure, which is far higher than those in graphite, fullerene, and other carbides. Meanwhile, the superconductivity of cage‐network carbides is also found to be sensitive to the electronegativity and concentration of dopant M. The result indicates that the higher transition temperatures can be obtained by optimizing the carbon‐cage‐network structures and the doping conditions. The study suggests that the carbon‐cage‐network structure is a direction to explore high‐temperature superconducting carbides. [ABSTRACT FROM AUTHOR]

In this study, the superconducting properties of MgB 2 bulk samples fabricated at different conditions were investigated. Ex situ MgB 2 samples were further mixed with Mg and B at different amounts and sintered at different times and temperatures. Using Raman spectroscopy, the bearing of electron-phonon (e-ph) coupling (EPC) has been investigated. The dependence of temperature on magnetization, M(T), along with the dependence of magnetic field on magnetization, M(H), were evaluated to analyze the superconducting characteristics. Using the McMillan formular, which is modified by Allen–Dynes, and the frequency of phonon located in the mid peak centre ω 2 (E 2g), the contribution of EPC, whose value is closely proportional to the variation of critical temperature was reckoned. The lowest EPC value was determined by Raman spectrum analysis to be 1.094 for the MgB 2 sample added with 0.5 Mg and sintered at 1000 °C for 1 h. In addition, the sample showed the lowest critical temperature along with the highest critical current density. The small bundle field (B sb) as well as the large bundle field (B lb) were obtained using the collective pinning theory, demonstrating the expansion of the small also the large bundle regimes under suitable sample fabrication conditions. Based on the dependence of temperature on normalized critical current density (j) and normalized small bundle field (B sb), δT c pinning is the predominant flux pinning mechanism across totally MgB 2 bulk samples. The model of Dew–Hughes confirmed that the addition of Mg along with B creates surface pinning in the matrix by analyzing the pinning properties. [ABSTRACT FROM AUTHOR]

The electron-phonon interaction can reveal the microscopic mechanism of heat transfer in metals. The two-step heat conduction considering electron-phonon interaction has become an effective theoretical model for extreme environments, such as micro-scale and ultrafast processes. In this work, the two-step heat transfer model is further extended by considering the Burgers heat conduction model with the second-order heat flux rate for electrons. Then, a novel generalized electron-phonon coupling thermoelasticity is proposed with the Burgers electronic heat transfer. Then, the problem of one-dimensional semi-infinite copper strip subject to a thermal shock at one side is studied by the Burgers two-step (BTS) model. The thermoelastic analytical solutions are systematically derived in the Laplace domain, and the numerical Laplace inversion method is adopted to obtain the transient responses. The new model is compared with the parabolic two-step (PTS) model and the hyperbolic two-step (HTS) model. The results show that in ultrafast heating, the BTS model has the same wave front jump as the HTS model. The present model has the faster wave speed, and predicts the bigger disturbed regions than the HTS model. More deeply, all two-step models also have the faster wave speeds than one-step models. This work may benefit the theoretical modeling of ultrafast heating of metals. [ABSTRACT FROM AUTHOR]

Superconductivity in compressed sulfur hydride (H3S) at above 200 K has attracted great interest in the study of hydrogen-based superconductors. However, the pressure required to stabilize H3S is 150 GPa, posing significant challenges for experiments. Therefore, it is essential to find a strategy to reduce this pressure. In this study, by introducing halogen atoms into the H-S system, we discovered that hydrogen-based superconductors of H6SX (X = Cl and Br) can be dynamically stable at mild pressures (5 GPa for H6SCl and H6SBr), as confirmed by first-principles calculations. Through the analysis of the bond properties, we revealed that introducing halogen elements would strengthen the H-S covalent bonds to reduce the dynamically stable pressure of H3S. Our study provides a scheme to reduce the superconducting pressure of hydrogen-based superconductors. [ABSTRACT FROM AUTHOR]

In conventional superconductors, electron-phonon coupling plays a dominant role in generating superconductivity. In high-temperature cuprate superconductors, the existence of electron coupling with phonons and other boson modes and its role in producing high-temperature superconductivity remain unclear. The evidence of electron-boson coupling mainly comes from angle-resolved photoemission (ARPES) observations of ∼70-meV nodal dispersion kink and ∼40-meV antinodal kink. However, the reported results are sporadic and the nature of the involved bosons is still under debate. Here we report findings of ubiquitous two coexisting electronmode couplings in cuprate superconductors. By taking ultrahigh-resolution laser-based ARPES measurements, we found that the electrons are coupled simultaneously with two sharp modes at ∼70meV and ∼40meV in different superconductors with different dopings, over the entire momentum space and at different temperatures above and below the superconducting transition temperature. These observations favor phonons as the origin of the modes coupled with electrons and the observed electron-mode couplings are unusual because the associated energy scales do not exhibit an obvious energy shift across the superconducting transition. We further find that the wellknown “peak-dip-hump” structure, which has long been considered a hallmark of superconductivity, is also omnipresent and consists of “peak-double dip-double hump” finer structures that originate from electron coupling with two sharp modes. These results provide a unified picture for the ∼70-meV and ∼40-meV energy scales and their evolutions with momentum, doping and temperature. They provide key information to understand the origin of these energy scales and their role in generating anomalous normal state and high-temperature superconductivity. [ABSTRACT FROM AUTHOR]

Organic semiconductors with distinct molecular properties and large carrier mobilities are constantly developed in attempt to produce highly‐efficient electronic materials. Recently, designer molecules with unique structural modifications have been expressly developed to suppress molecular motions in the solid state that arise from low‐energy phonon modes, which uniquely limit carrier mobilities through electron–phonon coupling. However, such low‐frequency vibrational dynamics often involve complex molecular dynamics, making comprehension of the underlying electronic origins of electron–phonon coupling difficult. In this study, first a mode‐resolved picture of electron–phonon coupling in a series of materials that are specifically designed to suppress detrimental vibrational effects, is generated. From this foundation, a method is developed based on the crystalline orbital Hamiltonian population (COHP) analyses to resolve the origins—down to the single atomic‐orbital scale—of surprisingly large electron–phonon coupling constants of particular vibrations, explicitly detailing the manner in which the intermolecular wavefunction overlap is perturbed. Overall, this approach provides a comprehensive explanation into the unexpected effects of less‐commonly studied molecular vibrations, revealing new aspects of molecular design that should be considered for creating improved organic semiconducting materials. [ABSTRACT FROM AUTHOR]

This paper makes an improvement on the determination of the temperature, bowing parameter, and antimony (Sb) composition effects on the fundamental band gap energy of InAs1−xSbx ternary alloys for infrared (IR) photodetection. The present study was carried out in a broader temperature range from cryogenic to room temperature for the entire alloy range, and a reasonable analysis has also been carried out. Meanwhile, a proposed expression for the band gap bowing parameter temperature dependence has been established. The optical cut-off wavelength can be further tuned over a significant range up to 14 μm or more by altering the stoichiometry of InAs1−xSbx alloys. The three Varshni and Bose–Einstein thermodynamic parameters were found to exhibit a parabolic trend with Sb composition. Consequently, the quadratic equations relating these parameters to Sb composition were derived. The relatively weak band gap temperature coefficient reported here is hoped to offer an improvement on infrared photodetector (IR-PD) stability. Also, the average phonon energy and the corresponding electron–phonon coupling interaction strength were investigated versus Sb composition. The available experimental results were used, where possible, to confirm our theoretical estimates, and the agreement is satisfactory. We expect that the present work will be helpful in designing and improving the photodetection properties of IR optoelectronics devices. Additionally, it provides a firm basis for our forthcoming determination of the electronic band structure properties of InAs1−xSbx based superlattices. [ABSTRACT FROM AUTHOR]

Double halide perovskites exhibit impressive potential for the self‐trapped exciton (STEs) luminescence. However, the detailed mechanism of the physical nature during the formation process of STEs in double perovskites is still ambiguous. Herein, theoretical research on a series of double halide perovskites Cs2B1B2Cl6 (B1 = Na+, K+; B2 = Al3+, Ga3+, In3+) regarding their electronic structures, exciton characteristics, electron–phonon coupling performances, and geometrical configuration is conducted. These materials have flat valence band edges and thus possess localized heavy holes. They also show high exciton binding energies, and their short exciton Bohr radius indicates that the spatial size of their excitons is comparable to the dimension of their single lattice. Based on the Fröhlich coupling constant and Feynman polaron radius, the stronger electron–phonon coupling strength in Ga‐series double halide perovskites is revealed. In particular, Cs2NaGaCl6 shows a high and effective Huang–Rhys factor of 36.21. The phonon characteristics and vibration modes of Cs2NaGaCl6 are further analyzed, and the Jahn–Teller distortion of the metal–halogen octahedron induced by hole‐trapping after excitation is responsible for the existence of STEs. This study strengthens the physical understanding of STEs and provides effective guidance for the design of advanced solid‐state phosphors. [ABSTRACT FROM AUTHOR]

Charge and thermal transport in a crystal is carried by free electrons and phonons (quantized lattice vibration), the two most fundamental quasiparticles. Above the Debye temperature of the crystal, phonon-mediated thermal conductivity (κ L) is typically limited by mutual scattering of phonons, which results in κ L decreasing with inverse temperature, whereas free electrons play a negligible role in κ L. Here, an unusual case in charge-density-wave tantalum disulfide (1T-TaS2) is reported, in which κ L is limited instead by phonon scattering with free electrons, resulting in a temperature-independent κ L. In this system, the conventional phonon-phonon scattering is alleviated by its uniquely structured phonon dispersions, while unusually strong electron-phonon (e-ph) coupling arises from its Fermi surface strongly nested at wavevectors in which phonons exhibit Kohn anomalies. The unusual temperature dependence of thermal conduction is found as a consequence of these effects. The finding reveals new physics of thermal conduction, offers a unique platform to probe e-ph interactions, and provides potential ways to control heat flow in materials with free charge carriers. The temperature-independent thermal conductivity may also find thermal management application as a special thermal interface material between two systems when the heat conduction between them needs to be maintained at a constant level.

Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron-phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron-phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.

It has now become recognized that the electron-phonon coupling (EPC) may play an important role in governing the phonon transport, especially for metallic and semiconducting systems at high carrier concentration. Here we focus on the Weyl semimetals TaAs and NbAs and give a comparative study on their phonon transport properties by explicitly including the EPC in first-principles calculations. It is found that the lattice thermal conductivities of both systems are significantly reduced by the EPC, which is more pronounced for the TaAs compared with the NbAs at the same carrier concentration. Detailed analysis indicates that the TaAs exhibits smaller EPC phonon relaxation time, as characterized by stronger EPC strength which is associated with larger deformation potential constant and Born effective charge. Moreover, we see that the TaAs exhibits obviously larger overlap between the EPC relaxation time and that from intrinsic phonon-phonon scattering, which could further reduce the lattice thermal conductivity. Our work not only highlights the vital importance of EPC in accurately predicting the phonon transport behaviors, but also offers a simple alternative to evaluate the EPC strength of various material systems.

Metallic nanostructures exhibit localized surface plasmons (LSPs), which offer unprecedented opportunities for advanced photonic materials and devices. Following resonant photoexcitation, LSPs quickly dephase, giving rise to a distribution of energetic ‘hot’ electrons in the metal. These out-of-equilibrium carriers undergo ultrafast internal relaxation processes, nowadays pivotal in a variety of applications, from photodetection and sensing to the driving of photochemical reactions and ultrafast all-optical modulation of light. Despite the intense research activity, exploitation of hot carriers for real-world nanophotonic devices remains extremely challenging. This is due to the complexity inherent to hot carrier relaxation phenomena at the nanoscale, involving short-lived out-of-equilibrium electronic states over a very broad range of energies, in interaction with thermal electronic and phononic baths. These issues call for a comprehensive understanding of ultrafast hot electron dynamics in plasmonic nanostructures. This paper aims to review our contribution to the field: starting from the fundamental physics of plasmonic nanostructures, we first describe the experimental techniques used to probe hot electrons; we then introduce a numerical model of ultrafast nanoscale relaxation processes, and present examples in which experiments and modelling are combined, with the aim of designing novel optical functionalities enabled by ultrafast hot-electron dynamics.

We present a detailed helium atom scattering study of the charge-density wave (CDW) system and transition metal dichalcogenide 1T-TaS2. In terms of energy dissipation, we determine the electron-phonon (e-ph) coupling, a quantity that is at the heart of conventional superconductivity and may even “drive” phase transitions such as CDWs. The e-ph coupling of TaS2 in the commensurate CDW phase (λ = 0.59 ± 0.12) is compared with measurements of the topo-logical insulator TlBiTe2 (λ = 0.09 ± 0.01). Furthermore, by means of elastic He diffraction and resonance/interference effects in He scattering, the thermal expansion of the surface lattice, the surface step height, and the three-dimensional atom-surface interaction potential are determined including the electronic corrugation of 1T-TaS2. The linear thermal expansion coefficient is similar to that of other transition-metal dichalcogenides. The He−TaS2 interaction is best described by a corrugated Morse potential with a relatively large well depth and supports a large number of bound states, comparable to the surface of Bi2Se3, and the surface electronic corrugation of 1T-TaS2 is similar to the ones found for semimetal surfaces.

The electron‐phonon interaction is known as one of the major mechanisms determining electrical and thermal properties. In particular, it alters the carrier transport behaviors and sets fundamental limits to carrier mobility. Establishing how electrons interact with phonons and the resulting impact on the carrier transport property is significant for the development of high‐efficiency electronic devices. Here, carrier transport behavior mediated by the electron‐phonon coupling in BiFeO3 epitaxial thin films is directly observed. Acoustic phonons are generated by the inverse piezoelectric effect and coupled with photocarriers. Via the electron‐phonon coupling, doughnut shape carrier distribution has been observed due to the coupling between hot carriers and phonons. The hot carrier quasi‐ballistic transport length can reach 340 nm within 1 ps. The results suggest an effective approach to investigating the effects of electron‐phonon interactions with temporal and spatial resolutions, which is of great importance for designing and improving electronic devices. [ABSTRACT FROM AUTHOR]

The electron‐phonon interaction is known as one of the major mechanisms determining electrical and thermal properties. In particular, it alters the carrier transport behaviors and sets fundamental limits to carrier mobility. Establishing how electrons interact with phonons and the resulting impact on the carrier transport property is significant for the development of high‐efficiency electronic devices. Here, carrier transport behavior mediated by the electron‐phonon coupling in BiFeO3 epitaxial thin films is directly observed. Acoustic phonons are generated by the inverse piezoelectric effect and coupled with photocarriers. Via the electron‐phonon coupling, doughnut shape carrier distribution has been observed due to the coupling between hot carriers and phonons. The hot carrier quasi‐ballistic transport length can reach 340 nm within 1 ps. The results suggest an effective approach to investigating the effects of electron‐phonon interactions with temporal and spatial resolutions, which is of great importance for designing and improving electronic devices. [ABSTRACT FROM AUTHOR]

Hydrothermally prepared potassium niobate (KNbO3) nanoparticles are characterized meticulously to explore various crystallographic, microstructural, morphological, optical, compositional, and lattice‐vibrational properties. Laser power‐dependent Raman spectroscopy is employed to study the Fano effect in asymmetrically broadened optic (LO/TO) phonon modes in the orthorhombic KNbO3 system, considering interference with the interband electronic continuum. The phonon softening, alterations in charge‐phonon coupling constant, linewidth and lifetime of phonon modes, and so on are explained in the light of theories formulated by Fano, Allen, and Hui, accompanying factor group analysis of the Raman‐active modes. Under local heating via focused laser irradiation, highly sensitive Raman spectra introspect perturbations in the generic vibrations at the first Brillouin zone center and moderations in the constructive/destructive interference conditions of resonant Fano scattering. Density functional theory (DFT)‐based calculations on phonon dispersion, density of phonon states, and relevant thermodynamic attributes decipher the theoretical background. Amid photoexcited electron plasma, the charge‐phonon coupling for distinct modes is strengthened considerably against strong excitation intensity without any unwanted anharmonicity, extensive lattice thermal expansion, phonon decay, or microscopic phase transformation. Finally, the direction of asymmetry, particle–particle, and particle–quasiparticle interactions are illustrated using Feynman diagrams for the associated A1 and B1 phonon modes. [ABSTRACT FROM AUTHOR]

Background: Motivated by the recent experimental discovery of strongly surface-plane-dependent superconductivity at surfaces of KTaO3 single crystals, we calculate the electron-phonon coupling strength, λ, of doped KTaO3 along the reciprocal space high-symmetry directions. Methods: Using the Wannier-function approach implemented in the EPW package, we calculate λ across the experimentally covered doping range and compare its mode-resolved distribution along the [001], [110] and [111] reciprocal-space directions. Results: We find that the electron-phonon coupling is strongest in the optical modes around the Γ point, with some distribution to higher k values in the [001] direction. The electron-phonon coupling strength as a function of doping has a dome-like shape in all three directions and its integrated total is largest in the [001] direction and smallest in the [111] direction, in contrast to the experimentally measured trends in critical temperatures. Conclusions: This disagreement points to a non-BCS character of the superconductivity. Instead, the strong localization of λ in the soft optical modes around Γ suggests an importance of ferroelectric soft-mode fluctuations, which is supported by our findings that the mode-resolved λ values are strongly enhanced in polar structures. The inclusion of spin-orbit coupling has negligible influence on our calculated mode-resolved λ values.

The anisotropic transport properties of gallium telluride (GaTe) have been reported by several experiments, giving rise to many debates recently. The anisotropic electronic band structure of GaTe shows the extreme difference between the flat band and tilted band in two distinct directions, Γ ¯ - X ¯ and Γ ¯ - Y ¯ , and which we called as the mixed flat-tilted band (MFTB). Focusing on such two directions, the relaxation of photo-generated carriers has been studied using the non-adiabatic molecular dynamics (NAMD) method to investigate the anisotropic behavior of ultrafast dynamics. The results show that the relaxation lifetime is different in flat band direction and tilted band direction, which is evidence for the existence of anisotropic behavior of the ultrafast dynamic, and such anisotropic behavior comes from the different intensities of electron–phonon coupling of the flat band and tilted band. Furthermore, the ultrafast dynamic behavior is found to be affected strongly by spin–orbit coupling (SOC) and such anisotropic behavior of the ultrafast dynamic can be reversed by SOC. The tunable anisotropic ultrafast dynamic behavior of GaTe is expected to be detected in ultrafast spectroscopy experiments and it may provide a tunable application in nanodevice design. The results may also provide a reference for the investigation of MFTB semiconductors. [ABSTRACT FROM AUTHOR]