Phonon Coupling Research Articles

Theoretical Predictions on Superconducting Phase above Room Temperature in Lutetium-Beryllium Hydrides at High Pressures

High-pressure structural search was performed on the hydrogen-rich compound LuBeH8 at pressures up to 200 GPa. We found an structure that exhibits stability and superconductivity above 100 GPa. Our phonon dispersion, electronic band structure, and superconductivity analyses in the 100–200 GPa pressure range reveal a strong electron–phonon coupling in LuBeH8, while the superconducting critical temperature T c shows a decreasing trend as the pressure increases, with T c = 255 K at 200 GPa and maximal T c = 355 K at 100 GPa. This study demonstrated the room-temperature superconductivity in -LuBeH8, thus enriching the family of ternary hydrides. These findings provide valuable guidance for identifying new high-temperature superconducting hydrides.

DFT approach into the physical properties of MTe3 (M = Hf, Zr) superconductors: A comprehensive study

In this article, we investigated the structural, electronic, mechanical, optical, and superconducting state properties of the trichalcogenides, MTe3(M = Hf, Zr) compounds using the density functional theory. Electronic energy dispersion curves demonstrate that the title compounds are metallic in nature, with a significant contribution from the Te atom. The technologically important mechanical properties (stiffness constant, elastic moduli, brittle/ductile behavior, Poisson’s ratio, elastic anisotropy, machinability index, and hardness) are thoroughly examined and addressed. The value of Pugh’s ratio indicates the ductility (brittleness) of ZrTe3 (HfTe3). The Vickers hardness value is 0.86 and 0.54 GPa for MTe3 (M = Hf, Zr), respectively, which confirms their softness. The value of lattice thermal conductivity (in W m−1 K−1) for HfTe3 (3.64) and ZrTe3 (2.36) is low due to significant phonon scattering as confirmed by the Grüneisen parameter study. The optical constants were computed, which confirmed the strong optical anisotropy of MTe3 (M = Hf, Zr). For ZrTe3, with the electric field polarization along the [100] direction, the highest reflectivity (51.36%) is obtained compared to HfTe3 (45.21%). This shows promise for application as a radiative heat reflector of these two compounds. The superconducting state properties, such as London penetration depth, coherence length, Ginzburg–Landau parameter, and electron–phonon coupling parameters are estimated and discussed. The value of electron–phonon coupling parameters suggests that both compounds are moderately coupled superconductors.

The coupling between the interlayer magnetic order and Davydov splitting modes in few-layer CrI3

The magnetic order in 2D material CrI3 has a significant impact on the Raman polarization selection rules of phonons, as evidenced by magneto-optical Raman spectroscopy. In a monolayer, the forbidden Raman scattering of A1g mode can be detected in the XY channel at low temperatures. However, in the bilayer, the Ag mode splits into two modes, and the selection rules of the splitting modes are determined by the magnetic order and symmetry. In contrast, the inversion symmetry of the spin structure is maintained in both antiferromagnetic (AFM) and ferromagnetic (FM) states in the trilayer (3L) CrI3, and the evolution of its Raman scattering with magnetic field is not yet clear. In this work, we use magneto-optical polarized Raman spectroscopy and reflective magnetic circular dichroism to investigate the Davydov splitting of the Ag phonon mode and its coupling to layered magnetism in 2–5L CrI3. Our results show that the Raman scattering of the Ag mode is strongly coupled to the layered magnetic order, indicating strong spin–phonon coupling in CrI3. Our study might shed light on the research on interaction between magnetic and vibrational properties of 2D magnetic materials and provide important implications for developing novel 2D spintronic devices.

Lattice Dynamics of Quinacridone Polymorphs: A Combined Raman and Computational Approach.

Polarized low-frequency Raman microscopy and a posteriori dispersion-corrected density functional simulations are combined to investigate the lattice vibrations of the αI, β, and γ polymorphs of the model organic semiconductor quinacridone, which are known to display different optical and electronic properties. The comparison between experiments and calculations allows for unambiguous mode assignment and identification of the scattering crystal faces. Conversely, the agreement between simulations and experiments validates the adopted computational methods, which correctly describe the intermolecular interaction of the molecular material. The acquired knowledge of quinacridone lattice dynamics is used to describe the αI to β thermal transition and, most consequentially, to reliably characterize the electron-lattice phonon coupling strength of the three polymorphs, obtaining hints about the electrical transport mechanism of the material.

Toward first-principles approaches for mechanistic study of self-trapped exciton luminescence

In recent years, broadband photo-luminescence phenomena arising from self-trapped exciton (STE) in metal halides, including perovskites and various low-dimensional derivatives and variants, have attracted increasing attention for their potential diverse optoelectronic applications like lighting, display, radiation detection, and sensing. Despite great success in experimental discovery of many efficient STE emitters, the current understanding of the STE emission mechanism in metal halides is still immature, and often controversial, which calls for help urgently from predictive first-principles theoretical calculation. Although density-functional theory (DFT) based calculations are routinely used to provide electronic band structure of materials and have contributed greatly to qualitative analysis of luminescence mechanism, more in-depth and quantitative information is highly needed to provide guidelines for rational design of new luminescent materials with desirable features. However, due to the complicated nature of STE emission, involving in particular electron–phonon coupling in both ground and excited states, the usage of DFT is no longer a routine job as for ground state properties. While more sophisticated methods formulated in the framework of many-body perturbation theory like GW-Bethe–Salpeter equation are available and provide theoretically rigorous and accurate description of electronic transitions in extended systems, their application to real STE systems is still severely limited due to highly demanding computational cost. In practice, approximated DFT methods are employed, which have their own strengths and limitations. In this review, we focus on the theoretical approaches that have been heavily used in interpreting STE luminescence mechanism, with a particular emphasis on theoretical methods for exciton self-trapping structural optimization. It is hoped that this review, by summarizing the current status and limitations of theoretical research in the STE emission, will motivate more methodological development efforts in this important field, and push forward the frontiers of excited state electronic structure theory of materials in general.

Numerical investigation of thermal conductivity of Cu/graphene/Cu interface

The purpose of this work is to investigate the contribution of in-plane and out-of-plane phonon modes to interface thermal conductivities (ITC) of the Cu/graphene/Cu interface through nonequilibrium molecular dynamics simulations. The proportions of the ITC of the in-plane and out-of-plane phonon modes in the pristine ITC are 1.1% and 99.3%, respectively. Defect engineering can change the coupling strength between in-plane and out-of-plane phonon modes. There is a strong coupling between the in-plane and the out-of-plane phonon mode when the defect concentration is lower than 3%. Phonon coupling has been transformed into weak interaction when the defect concentration is higher than 3%. The high defect concentration can suppress the coupling between in-plane and out-of-plane phonon modes. The results of the phonon density of states show that the out-of-plane phonons are mainly concentrated at low frequencies, and the in-plane phonons are mainly concentrated at high frequencies. This work helps to understand the mechanism of heat transfer of the graphene-based interface and provides theoretical guidance for the application of graphene-based interface nanodevices.

Effects of electron–phonon interactions on electrical transport properties of GaAs/GaAlAs semiconductor heterostructure

The effects of electron–phonon coupling, and band bending are studied in a quasi-one-dimensional (1D) heterostructure consisting of two different doping concentration layers attached between two ideal semi-infinite electrodes, theoretically. The layers are composed two types of material Ga1−c1Alc1As (with Al doping concentration c1) and Ga1−c2Alc2As (with Al doping concentration c2), which c1≤c2. We use the Green’s function method within the framework of the tight-binding approximation to calculate the electrical transport properties in the coherent regime. The transmission spectrum and also the electrical transport properties are investigated for incident electrons by varying the doping concentration and structural parameters. By modifying these parameters, we found the bandgaps, saddle-like behavior and resonance energies of the system in the transmission spectrum. This causes to allow or forbid the transmission of electrons in certain energy ranges. The numerical results can support the choice of suitable parameters to design and fabricate nano-energy filter devices for specific purposes.

Annealing-induced one order of magnitude enhancement in heat dissipation at graphene/SiO2 interface

The potential applications of two-dimensional (2D) materials-based devices in various fields have garnered significant interest. However, the limited heat transfer efficiency across the interface between 2D materials and the substrate has impeded their widespread application. This study presents a novel approach to enhancing the interface thermal transport of a widely used graphene/substrate structure through repeated annealing. Using Raman thermometry, we measured the interface thermal resistance (R) at 9.51 × 10−5 m2 K/W, which was subsequently reduced by an order of magnitude to 5.00 × 10−6 m2 K/W after repeated annealing. Our experimental results, supported by atomic modeling, revealed the significant influence of wrinkles on the interface thermal transport. Atomic force microscopy (AFM) images vividly illustrate the modulation of the surface structure due to repeated annealing, with the initial loose structure becoming tighter and the wrinkle flattened. Further phonon transport analysis revealed a significantly reduced phonon coupling rate between graphene and SiO2 due to the presence of wrinkles in the graphene. Our findings highlight the feasibility of annealing as a means of manipulating interface thermal transport and provide insights into the underlying mechanism, which may inspire further studies on enhancing thermal interface properties.

Quantum Light from Lossy Semiconductor Rydberg Excitons.

The emergence of photonic quantum correlations is typically associated with emitters strongly coupled to a photonic mode. Here, we show that semiconductor Rydberg excitons, which are only weakly coupled to a free-space light mode can produce strongly antibunched fields, i.e., quantum light. This effect is fueled by a micron-scale excitation blockade between Rydberg excitons inducing pair-wise polariton scattering events. Photons incident on an exciton resonance are scattered into blue- and red-detuned pairs, which enjoy relative protection from absorption and thus dominate the transmitted light. We demonstrate that this effect persists in the presence of additional phonon coupling, strong nonradiative decay, and across a wide range of experimental parameters. Our results pave the way for the observation of quantum statistics from weakly coupled semiconductor excitons.

Proximity effects of 2D antiferromagnets on superconductivity in exfoliated niobium disulfide

The conventional theory of superconductivity holds that Cooper pairs form due to electron–phonon coupling; however, this description may not be adequate to describe certain unconventional superconductors such as cuprates and iron chalcogenides. In these unconventional superconductors, it has been proposed that spin fluctuations may be responsible for the formation of Cooper pairs. In this study, we explore spin interactions in the transition metal, dichalcogenide niobium disulfide, induced through proximity effects by fabricating antiferromagnet/NbS2 heterostructures. We tested three different 2D antiferromagnetic materials, each with different spin textures: anganese phosphorus trisulfide, manganese phosphorus triselenide, and chromium trichloride. Our results showed a substantial reduction in the critical temperature in the case of NbS2/MnPSe3. We hypothesize that this could be due to spin fluctuations in MnPSe3 inducing proximity effects in NbS2.

Dispersive response of polar optical phonon modes on the dielectric and size effects in a nitride nanostructure

Within the framework of the macroscopic dielectric continuum model, the dispersive spectra of polar optical phonon modes of III-nitride based nanostructures with different inner and outer dielectric media are investigated in this paper. The dielectric and quantum size effects on the dispersive frequencies of phonon modes are focused and emphasized. In the case of GaN quantum rings (QRs), we found that the dielectric constant of the inner/outer medium could significantly influence the dispersive spectra of the surface optical (SO) phonon mode as the high/low-frequency SO phonon modes are mainly localized at the inner/outer surface of QRs, respectively. Moreover, such effects associated with different crystallographic structures of GaN, i.e., the wurtzite (WZ) and zinc blende (ZB) crystal structures, are also studied in detail, showing the different degenerating behavior and electron–phonon coupling strength. The results obtained in this study provide a beneficial opportunity to independently adjust dispersive spectra of the two branches of SO modes in GaN nanostructures; thus, it could be used to design and develop phonon-based detectors and sensors.

Phonon Anharmonicity and Spin-Phonon Coupling in CrI3.

We report on the far-infrared, temperature-dependent optical properties of a CrI3 transition metal halide single crystal, a van der Waals ferromagnet (FM) with a Curie temperature of 61 K. In addition to the expected phonon modes determined by the crystalline symmetry, the optical reflectance and transmittance spectra of CrI3 single crystals show many other excitations as a function of temperature as a consequence of the combination of a strong lattice anharmonicity and spin-phonon coupling. This complex vibrational spectrum highlights the presence of entangled interactions among the different degrees of freedom in CrI3.

Reducing Emission Linewidth of Pure-Blue ZnSeTe Quantum Dots through Shell Engineering toward High Color Purity Light-Emitting Diodes.

High color purity blue quantum dot light-emitting diodes (QLEDs) have great potential applications in the field of ultra-high-definition display. However, the realization of eco-friendly pure-blue QLEDs with a narrow emission linewidth for high color purity remains a significant challenge. Herein, a strategy for fabricating high color purity and efficient pure-blue QLEDs based on ZnSeTe/ZnSe/ZnS quantum dots (QDs) is presented. It is found that by finely controlling the internal ZnSe shell thickness of the QDs, the emission linewidth can be narrowed by reducing the exciton-longitudinal optical phonon coupling and trap states in the QDs. Additionally, the regulation of the QD shell thickness can suppress the Förster energy transfer between QDs in the QLED emission layer, which will help to reduce the emission linewidth of the device. As a result, the fabricated pure-blue (452 nm) ZnSeTe QLED with ultra-narrow electroluminescence linewidth (22 nm) exhibit high color purity with the Commission Internationale de l'Eclairage chromatic coordinates of (0.148, 0.042) and considerable external quantum efficiency (18%). This work provides a demonstration of the preparation of pure-blue eco-friendly QLEDs with both high color purity and efficiency, and it is believed that it will accelerate the application process of eco-friendly QLEDs in ultra-high-definition displays.

A‐site coordinating cation engineering in zero‐dimensional antimony halide perovskites for strong self‐trapped exciton emission

AbstractLow‐dimensional hybrid halide perovskites represent a promising class of materials in optoelectronic applications because of strong broad self‐trapped exciton (STE) emissions. However, there exists a limitation in designing the ideal A‐site cation that makes the material satisfy the structure tolerance and exhibit STE emission raised by the appropriate electron–phonon coupling effect. To overcome this dilemma, we developed an inorganic metal‐organic dimethyl sulfoxide (DMSO) coordinating strategy to synthesize a series of zero‐dimensional (0D) Sb‐based halide perovskites including Na3SbBr6·DMSO6 (1), AlSbBr6·DMSO6 (2), AlSbCl6·DMSO6 (3), GaSbCl6·DMSO6 (4), Mn2Sb2Br10·DMSO13 (5) and MgSbBr5·DMSO7 (6), in which the distinctive coordinating A‐site cation [Am‐DMSO6]n+ efficiently separate the [SbXz] polyhedrons. Advantageously, these materials all exhibit broadband‐emissions with full widths at half maxima (FWHM) of 95–184 nm, and the highest photoluminescent quantum yield (PLQY) of 3 reaches 92%. Notably, compounds 2–4 are able to remain stable after storage of more than 120 d. First‐principles calculations indicate that the origin of the efficient STE emission can be attributed to the localized distortion in [SbXz] polyhedron upon optical excitation. Experimental and calculational results demonstrate that the proposed coordinating strategy provides a way to efficiently expand the variety of novel high‐performance STE emitters and continuously regulate their emission behaviors.

Exceptionally high hole mobilities in monolayer group-IV monochalcogenides GeTe and SnTe

Two-dimensional semiconductors are considered as promising channel materials for next-generation nanoelectronics devices, while their practical applications are typically limited by their low mobilities. In this work, using first-principles calculations combined with the Boltzmann transport formalism involving electron–phonon coupling, we study the transport properties of monolayer group-IV monochalcogenides (MX, M = Ge, Sn; X = S, Se, and Te). We find that the GeTe and SnTe possess exceptionally high hole mobilities, which even reach 835 and 1383 cm2/V s, respectively, at room temperature. More interestingly, the hole mobilities increase with the increase in the atomic number of “X” in MXs when “M” remains the same. Such a trend is mainly due to the increased group velocity and decreased density of states, and the latter plays a significant role in determining the carrier scattering space and relaxation time. Meanwhile, different from the acoustic deformation potential theory, we find that the high-energy optical phonons contribute a lot to the scattering. Our work shows that the monolayer GeTe and SnTe are promising p-type semiconductors in nanoelectronics and reveals the intrinsic connection between phonons, charge density of states, and mobility, which would shed light on exploring the two-dimensional materials with high mobility.

Influence of CoFeB layer thickness on elastic parameters in CoFeB/MgO heterostructures

The surface acoustic waves, i.e., surface phonons may have huge potential for future spintronic devices, if coupled to other waves (e.g., spin waves) or quasiparticles. In order to understand the coupling of acoustic phonons with the spin degree of freedom, especially in magnetic thin film-based heterostructures, one needs to investigate the properties of phonons in those heterostructures. This also allows us to determine the elastic properties of individual magnetic layers and the effective elastic parameters of the whole stacks. Here, we study frequency versus wavevector dispersion of thermally excited SAWs in CoFeB/MgO heterostructures with varying CoFeB thickness by employing Brillouin light spectroscopy. The experimental results are corroborated by finite element method-based simulations. From the best agreement of simulation results with the experiments, we find out the elastic tensor parameters for CoFeB layer. Additionally, we estimate the effective elastic parameters (elastic tensors, Young’s modulus, Poisson’s ratio) of the whole stacks for varying CoFeB thickness. Interestingly, the simulation results, either considering elastic parameters of individual layers or considering effective elastic parameters of whole stacks, show good agreement with the experimental results. These extracted elastic parameters will be very useful to understand the interaction of phonons with other quasiparticles.

Structural modification and conductivity enhancement of NiCl2 incorporated P3HT: PC60BM based organic photoactive layer

High planar defect and poor crystalline structure in P3HT has limited the device performance of organic photovoltaic. To date, surface modification on the P3HT molecular structure has been introduced to improve its intrinsic properties. In this project, different volume percent of NiCl2 were incorporated into the P3HT:PC60BM film using a wet chemical approach. The effects on the structural, electrical, and optical properties of the active material after the metal halide incorporation were elucidated. Among the samples, 20 vol% NiCl2 sample presented the highest C–C intra-ring in raman analysis, which signifies the formation of long conjugate P3HT chain that contributes to a stronger electron phonon coupling and promotes a charge transfer from CC bonds. After the active layer was incorporated with 20 vol% NiCl2, the surface roughness of the film decreased from (13.8 ± 2.6) nm to (4.2 ± 1.5) nm as compared to pristine P3HT:PC60BM film. The photoabsorbance and electrical conductivity of 20 vol% NiCl2 sample also increased by 1.8 fold and 47%, respectively. The presence of NiCl2 can serve as a surface passivator in P3HT interface which eventually reduces the electron organisation energy and increases the charge transport ability in active layer. This novel finding can serve as an alternative to improve the device performance of P3HT-based active material organic solar cells.

RTS Noise Characterization of Trap Properties in InGaAs nFinFETs

In order to extract and characterize the trap properties in In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\textit{x}}$</tex-math> </inline-formula> Ga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text{1}-\textit{x}}$</tex-math> </inline-formula> As nFinFET sensitively and effectively, the time-domain and frequency-domain characteristics of low-frequency noise are studied by the random telegraph signal (RTS) method. It is found that the RTS noise in this study originates from the strong electron–phonon coupling mechanism. The activation energies of traps are extracted from the Arrhenius curves, and the results show that these traps mainly come from extended defects, such as dislocation, etc. Furthermore, the relationship between noise characteristics and gate length is explored, it is found that with the reduction of the gate length, the noise contains more RTS components. The noise behavior of devices with different In content is also investigated, it shows that the average of activation energies, as well as the capture cross section of bulk traps, decrease with the increasing In content in the channel. Overall, it can be concluded that the RTS noise is an effective tool to reveal the defect properties of nano-scaled devices. It can be used to provide theoretical guidance in improving the performance and reliability of devices.

Laser‐induced Fano interference subsumed by electron–phonon coupling in orthorhombic KNbO3nano‐bricks: An ab initio vibrational and Raman spectroscopic investigation

AbstractHydrothermally 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 KNbO3system, 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 and phonon modes.

Process-Gas-Influenced Anti-Site Disorder and Its Effects on Magnetic and Electronic Properties of Half-Metallic Sr2FeMoO6 Thin Films

Anti-site disorder, arising due to the similar size of Fe and Mo ions in Sr2FeMoO6 (SFMO) double perovskites, hampers spintronic applicability by deteriorating the magnetic response of this double perovskite system. A higher degree of anti-site disorder can also completely destroy the half-metallicity of the SFMO system. To study the effects of different process gas conditions on the anti-site disorder, we have prepared a series of SFMO thin films on SrTiO3 (001) single-crystal substrate using a pulsed laser deposition (PLD) technique. The films are grown either under vacuum or under N2/O2 partial gas pressures. The vacuum-grown SFMO film shows the maximum value of saturation magnetization (MS) and Curie temperature (TC), signaling the lowest anti-site disorder in this series. In other words, there is a long-range Fe/Mo-O-Mo/Fe ferrimagnetic exchange in the vacuum-grown thin film, thereby enhancing the magnetization. Further, all the SFMO films show a semiconducting state with a systematic increase in overall resistivity with the increased anti-site disorder. The electrical conduction mechanism is defined by the variable-range hopping model at low temperatures. Raman spectra show a weak Fano peak, suggesting the presence of electron–phonon coupling in SFMO thin films. These results show the significance of the process gas in causing anti-site disorder, tuning the degree of this disorder and therefore its influence on the structural, magnetic, electrical, and vibrational properties of SFMO thin films.