Electron-phonon Coupling Strength Research Articles

Tuning superconductivity in highly crystalline Pb1−xBix alloy ultrathin films at atomic level

Achieving highly crystalline superconductors with reduced dimensionality and fine-tuning their superconductivity have been daunting challenges in condensed matter physics. Here, we have successfully grown two-monolayer highly crystalline ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$(111) films with controllable Bi concentrations $x$ by molecular beam epitaxy and systematically studied their structural and superconducting properties by scanning tunneling microscopy. We first show that the superconducting transition temperatures of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ films exhibit a domelike behavior with increasing Bi concentration $x$. Our first-principles calculations reveal that Bi doping can promote the electronic states and the electron-phonon coupling strength at lower $x$ and suppress the electron-phonon coupling strength and superconductivity with largely Bi electronic states when Bi doping increases over a critical ratio. Our findings not only demonstrate a quantum phenomenon of superconductivity in highly crystalline alloyed films but also provide a practical pathway to tune the superconductivity at the atomic level.

Topological quantum transition driven by charge-phonon coupling in higher-order topological insulators

We investigate a second-order topological quantum transition of a modified Kane-Mele model driven by electron-phonon interaction. The results show that the system parameters of the bare modified Kane-Mele model are renormalized by the electron-phonon interaction. Starting from the second-order topological phase for the bare model, the increasing electron-phonon coupling strength can drive the second-order topological insulator into a semimetal phase. Such a secondorder topological phase transition is characterized by the band-gap closing, discontinuity of averaged ferminoic number and topological invariant.

Dynamical renormalization of electron-phonon coupling in conventional superconductors

The adiabatic Born-Oppenheimer approximation is considered to be a robust approach that very rarely breaks down. Consequently, it is predominantly utilized to address various electron-phonon properties in condensed matter physics. By combining many-body perturbation and density functional theories we demonstrate the importance of dynamical (nonadiabatic) effects in estimating superconducting properties in various bulk and two-dimensional materials. Apart from the expected long-wavelength nonadiabatic effects, we found sizable nonadiabatic Kohn anomalies away from the Brillouin zone center for materials with strong intervalley electron-phonon scatterings. Compared to the adiabatic result, these dynamical phonon anomalies can significantly modify electron-phonon coupling strength $\ensuremath{\lambda}$ and superconducting transition temperature ${T}_{c}$. Further, the dynamically induced modifications of $\ensuremath{\lambda}$ have a strong impact on transport properties, where probably the most interesting is the rescaling of the low-temperature and low-frequency regime of the scattering time $1/\ensuremath{\tau}$ from about ${T}^{3}$ to about ${T}^{2}$, resembling the Fermi liquid result for electron-electron scattering. Our goal is to point out the potential implications of these nonadiabatic effects and reestablish their pivotal role in computational estimations of electron-phonon properties.

Origin of contrasting trends of intrinsic electron mobility with tensile strain in hexagonal MoS2 and triangular PdSe2

Strain engineering is an effective method to extend the properties of two-dimensional (2D) materials. In this paper, we theoretically studied the electron mobility of 2D hexagonal (H) ${\mathrm{MoS}}_{2}$ and triangular (T) ${\mathrm{PdSe}}_{2}$ under different strains. We observe contrary trends of the electron transport with strain. Our study shows it inherently comes from the contrary response of chemical bonding to strain. The covalency of both $\mathrm{H}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ and $\mathrm{T}\text{\ensuremath{-}}\mathrm{PdS}{\mathrm{e}}_{2}$ decreases with increasing strain due to the elongated bond length. The ionicity of $\mathrm{H}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{S}}_{2}$ is also decreased, resulting in decreased electron-phonon coupling strength. On the contrary, the ionicity of $\mathrm{T}\text{\ensuremath{-}}\mathrm{PdS}{\mathrm{e}}_{2}$ is increased, which overwhelms the decrease of the covalency, resulting in enhanced electron-phonon coupling strength. Our work not only uncovers the underlying physics governing the contrary trends of electron mobility with strain in hexagonal and triangular $M{X}_{2}$, but also offers a paradigm analysis methodology that is applicable to other research areas.

Synergetic optimization of thermoelectric properties in SnSe film via manipulating Se vacancies

Thin film thermoelectric materials have attracted much attention due to the application prospect in nanoscale devices. However, the commonly used optimization strategy in bulk materials, such as defect engineering, is hard to be realized in films. In this work, we synthesized SnSe films by the magnetron sputtering method and manipulated the Se vacancy concentration simply by controlling the sputtering time. The carrier transport properties were systematically investigated by combining theoretical simulation and experimental measurement. In the 130 nm-thick ultrathin film, Se vacancies were formed due to the difference in atomic mass between Sn and Se. The electrical conductivity and carrier mobility are improved by the weak electron-phonon coupling strength and small indirect band gap. Meanwhile, the thermal conductivity of the vacancy structure has an average reduction of 33.8% compared with the pristine SnSe films due to the strong vacancy scattering. The Seebeck coefficient maintains above 300 μV K-1 due to the relatively large direct band gap. A high ZT of 0.6 was finally achieved at 700 K in the film sample with Sn:Se ratio of 1.08, 50% improvement over the pristine SnSe film. However, a transition to the metallic characteristic occurs when Se vacancy concentration further increases, resulting in deterioration of ZT at high temperature. The present work demonstrates a guiding principle of manipulating vacancy concentration for high thermoelectric performance and reveals how vacancies influence energy transfer.

Steady-state Peierls transition in nanotube quantum simulator

Quantum dots placed along a vibrating nanotube provide a quantum simulation platform that can directly address the electron-phonon interaction. This offers promising prospects for the search of new quantum materials and the study of strong correlation effects. As this platform is naturally operated by coupling the dots to an electronic reservoir, state preparation is straightforwardly achieved by driving into the steady state. Here we show that for intermediate electron-phonon coupling strength, the system with spin-polarized quantum dots undergoes a Peierls transition into an insulating regime which exhibits charge-density wave order in the steady state as a consequence of the competition between electronic Coulomb repulsive interactions and phonon-induced attractive interactions. The transport phenomena can be directly observed as fingerprints of electronic correlations. We also present powerful methods to numerically capture the physics of such an open electron-phonon system at large numbers of phonons. Our work paves the way to study and detect correlated electron-phonon physics in the nanotube quantum simulator with current experimentally accessible techniques.

Revealing the decisive factors of the lattice thermal conductivity reduction by electron-phonon interactions in half-Heusler semiconductors

The role of electron-phonon (EP) scattering in lattice thermal conductivity (κL) of thermoelectric material has not been fully studied until now, especially for the decisive factors that influence the reduction of κL. To address this issue, we report the effect of EP interactions on κL at a series of temperatures and carrier concentrations for 18 half-Heusler compounds. Among all the compounds investigated, the hole-doped TiCoSb and the electron-doped ZrIrSb have the largest κL reductions (32% & 20%) by EP interactions at 300 K, under the carrier concentration of 1021 cm−3. Detailed analyses reveal that the system with strong EP coupling strength and high electronic density of states at the Fermi level (N (EF)) favor the κL reduction, because these two factors are beneficial for EP scattering rates. And a high N (EF) can be caused by a high carrier concentration and/or a large effective mass. Temperature is another factor that affects the reduction of κL by EP interactions due to its imbalance influence on EP and phonon-phonon (PP) scatterings. Furthermore, after considering the influences from the aliovalent doping and grain boundary, the EP interactions still play a non-negligible role on κL reduction, especially at low temperatures and high carrier concentrations. Our work provides a complete picture for understanding the mechanism of EP interactions in material's thermal transport.

Symmetry-Guaranteed High Carrier Mobility in Quasi-2D Thermoelectric Semiconductors.

Quasi-2D semiconductors have garnered immense research interest for next-generation electronics and thermoelectrics due to their unique structural, mechanical, and transport properties. However, most quasi-2D semiconductors experimentally synthesized so far have relatively low carrier mobility, preventing the achievement of exceptional power output. To break through this obstacle, a route is proposed based on the crystal symmetry arguments to facilitate the charge transport of quasi-2D semiconductors, in which the horizontal mirror symmetry is found to vanish the electron-phonon coupling strength mediated by phonons with purely out-of-plane vibrational vectors. This is demonstrated in ZrBeSi-type quasi-2D systems, where the representative sample Ba1.01 AgSb shows a high room-temperature hole mobility of 344 cm2 V-1 S-1 , a record value among quasi-2D polycrystalline thermoelectrics. Accompanied by intrinsically low thermal conductivity, an excellent p-type zT of ≈1.3 is reached at 1012 K, which is the highest value in ZrBeSi-type compounds. This work uncovers the relation between electron-phonon coupling and crystal symmetry in quasi-2D systems, which broadens the horizon to develop high mobility semiconductors for electronic and energy conversion applications.

Two-photon absorption of FAPbBr3 perovskite nanocrystals

Perovskite nanocrystals (NCs) with high two-photon absorption (TPA) cross-section are of great interest due to their potential applications in three-dimensional optical data storage and multiphoton fluorescence microscopy. Among various perovskite materials, FAPbBr3 NCs show a better development prospect due to their excellent stability. However, there are few reports on their nonlinear optical properties. In this work, the nonlinear optical behavior of FAPbBr3 NCs is studied. The methods of multiphoton absorption photoluminescence saturation and open aperture Z-scan technique were applied to determine the TPA cross-section of FAPbBr3 NCs, which was around 2.76 × 10−45 cm4⋅s⋅photon−1 at 800 nm. In addition, temperature-dependent photoluminescence induced by TPA was investigated, and the small longitudinal optical phonon energy and electron–phonon coupling strength was obtained, which confirm the weak Pb–Br interaction. Meanwhile, it is found that the exciton binding energy in FAPbBr3 NCs was 69.668 meV, which may be ascribed to the strong hydrogen bond interaction. It is expected that our findings will promote the application of FAPbBr3 NCs in optoelectronic devices.

Hydrogenation induced high-temperature superconductivity in two-dimensional W2C3.

The discovery of highly crystalline two-dimensional (2D) superconductors provides a new alluring branch for exploring the fundamental significances. Based on first-principles calculations, we predict a new kind of 2D stable material W2C3, which is a semimetal but not a superconductor because of the weak electron-phonon coupling (EPC) strength. After hydrogenation, W2C3H2 possesses the intrinsic metallic properties with a large density of states (DOS) at the Fermi energy (EF). More interestingly, the EPC strength is greatly enhanced after hydrogenation and the calculated critical temperature (Tc) is 40.5 K. Furthermore, the compressive strain can obviously soften the low-frequency phonons and enhance the EPC strength. Then, the Tc of W2C3H2 can be increased from 40.5 K to 49.1 K with -4% compressive strain. This work paves the way for providing a new platform for 2D superconductivity.

First-principles study of the temperature-induced band renormalization in thermoelectric filled skutterudites.

Band structure characteristics, such as band gap and band dispersion, are fundamental properties of materials. Temperature can affect them because of lattice expansion and phonon-induced atomic vibrations. Here, we apply the recently developed electron-phonon renormalization method to study the temperature effect on the band structures of thermoelectric (TE) filled skutterudites BaCo4Sb12, BaFe4Sb12, and YbFe4Sb12 from first-principles. The results reveal that the band gap in BaCo4Sb12 drops slower with temperature compared with our previous study on CoSb3, where it considerably reduces from 0 K to 800 K for BaFe4Sb12 (∼0.222 eV) and YbFe4Sb12 (∼0.201 eV). Furthermore, the band dispersions near the band edges at the Γ-point in the three systems at high temperatures are similar to those at 0 K, and the electron energies have small linewidths, whereas the linewidths for energies near the Fermi level are large. The different phenomena are due to the different phonon vibration-induced electronic structure disorders, reflecting the strength of electron-phonon coupling. Band renormalization would further affect the TE properties of these filled skutterudites. Our work provides a deeper understanding of the temperature-dependent band structure in skutterudites.

Intervalley scattering induced significant reduction in lattice thermal conductivities for phosphorene.

The thermal transport properties of buckled phosphorene (β-P) and antimonene (β-Sb) are investigated using first-principles methods. The large acoustic-optical phonon gaps of 3.8 THz and 2.2 THz enable the four-phonon interaction to play an important role in phonon scattering for both β-P and β-Sb. Considering the electron-phonon coupling, the lattice thermal conductivity can further undergo 84% decrease to 4.9 W mK-1 for p-type β-P at n = 5 × 1013 cm-2. By quantitatively describing the scattering probability of electrons in different paths combined with electron-phonon coupling matrix element analysis, it is found that multi-valley features of electronic band structure and strong electron-phonon coupling strength make electrons have strong intervalley scattering behavior in β-P. The former plays an important role in the energy conservation condition of the scattering process, and the latter determines the selection rule. Our work elucidates the contribution of higher-order phonon interactions as well as electron-phonon coupling effects to lattice thermal conductivity, and provides a new idea for finding materials with low lattice thermal conductivity induced by intervalley scattering.

Influence of Cu induced crystallographic disorder on the optical and lattice vibrational properties of ZnO nanoparticles.

In this study, the vibrational and optical responses of 0-10% excess Cu incorporated ZnO nanoparticles (NPs) prepared by the low temperature (∼400 °C) wet chemical route were investigated experimentally and were found to be predominantly linked to the formation of both intrinsic and Cu induced crystallographic defects instead of substitutional Cu itself for the first time. For low temperature chemical synthesis, the effective band gap (Eg) of pristine ZnO NPs was found to be as low as 2.84 eV, which was followed by a further reduction by as high as ∼24.4% with gradual Cu inclusion. Excess Cu incorporation was found to drastically restrict the particle growth phenomenon by ∼61% and alter the shape morphology from anisotropic to irregular isotropic. Although all NPs showed a single phase structure with hexagonal P63mc symmetry, the X-ray peak profile analysis along with the characteristic E2(High) Raman peak and electron-phonon coupling strength obtained from the 2nd order Raman mode suggested the presence of appreciable crystallographic disorders pertaining to point defects like Cu induced Zn and O interstitials and vacancies. Optical analysis revealed a gradual increase in Urbach tailing (Eu) from 0.35 to 0.96 eV directly associated with both intrinsic and Cu induced disorders arising from successive Cu incorporation at low processing temperature. A correlation between Eg and Eu predicted a direct-type band-to-band transition energy of ∼3.2 eV for the pristine ZnO crystal, suggesting both intrinsic and excess Cu induced extrinsic disorders to be the primary source of optical modulation of the NPs for the low temperature processing condition.

Mixed Sulfur/Selenium Anions Weaken Electron-Vibrational Interaction in Cu2ZnSn(S,Se)4 Photoabsorber

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar absorbers have attracted intensive investigations for next-generation photovoltaic applications. Here, by using ab initio static and molecular dynamics simulations, we investigated the anion compositional dependence of electron-vibration interaction in CZTSSe materials. We found that the conduction band fluctuates more than the valence band, and as a result, the band gap variation is more sensitive to the change of the former, which can be understood in terms of p-d hybridization in the valence bands. Electron-phonon coupling is smaller in CZTSSe alloy compared to pure S- or Se-containing structures, as evidenced by the smaller fluctuation of excitation energy, and can be attributed to the weaker structural dynamics of the metal-anion bond. Small electron-phonon coupling strength may lead to better charge transport in these materials. We also elucidated the interplay between disordered structures and S/Se stoichiometry through analysis of optical line width. The results highlight the importance of anion composition engineering and provide new insights into the rational design of high-performing kesterite absorbers for solar cells.

Functional renormalization group study of the two-dimensional Su-Schrieffer-Heeger-Hubbard model

We study the Hubbard model on the square lattice coupled in addition to the optical Su-Schrieffer-Heeger (SSH) phonons, using the singular-mode functional renormalization group method. At half-filling and in the absence of the Hubbard interaction $U$, we find the degenerate spin-density-wave (SDW)/charge-density-wave (CDW)/s-wave superconductivity (sSC) state at smaller electron-phonon coupling strength $\lambda$ and higher phonon frequency $\omega$, and the valence bond solid (VBS) state at larger $\lambda$ and lower $\omega$. After switching on a positive $U$, the VBS state is suppressed, while the SDW state is enhanced. At finite doping, the SSH phonon is found to favor sSC at $U=0$. With increasing positive $U$, we find d-wave superconductivity (dSC) and incommensurate SDW states. In a narrow window of moderate $U$ and $\lambda$, we also find the incommensurate VBS state. The sSC and dSC here can be naturally related to the CDW and SDW fluctuations, both of which can be triggered by the SSH phonons. In contrast, the repulsive interaction $U$ enhances SDW but suppresses CDW fluctuations. Our results at half filling are consistent with quantum Monte Carlo (QMC), and provide insights at finite doping where QMC may suffer from the minus sign problem.

Effects of electron-phonon coupling on the phonon transport properties of the Weyl semimetals NbAs and TaAs: A comparative study

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.

Stimulated emission cross sections and temperature‐dependent spectral shifts of neutral nitrogen‐vacancy centers in diamonds

AbstractThe neutral nitrogen‐vacancy (NV0) defects in diamond are photostable color centers, suitable for a wide range of applications in science and engineering. However, the photophysical properties of the centers have not yet been fully characterized. This work measured the stimulated emission cross sections of NV0 in a single‐crystal diamond by two‐photon excitation of its matrix at 344 nm. From the measured photoluminescence spectrum and the fluorescence lifetime of 20 ± 1 ns, we determined a peak stimulated emission cross section of 1.43 ± 0.07 × 10−17 cm2 at 650 nm for the NV0 centers. In addition, we have also examined the thermal shifts of the zero‐phonon line of NV0 centers in nanoscale diamonds (~100 nm in diameter) over the temperature range of 30–120°C. A temperature measurement sensitivity of 0.2 K·Hz−1/2 was achieved, which is about two‐fold better than that of NV−, despite that the fluorescence intensity of NV0 is about six‐fold lower than that of NV− in the same nanoparticles. The result is attributed to the smaller electron–phonon coupling strength of the neutral center, compared with its negatively charged counterpart.

Phonon-induced breakdown of Thouless pumping in the Rice-Mele-Holstein model

Adiabatic and periodic variation of the lattice parameters can make it possible to transport charge through a system even without net external electric or magnetic fields, known as Thouless charge pumping. The amount of charge pumped in a cycle is quantized and entirely determined by the system's topology, which is robust against perturbations such as disorder and interactions. However, coupling to the environment may play a vital role in topological transport in many-body systems. We study the topological Thouless pumping, where the charge carriers interact with local optical phonons. The semi-classical multi-trajectory Ehrenfest method is employed to treat the phonon trajectories classically and charge carriers quantum mechanically. We find a breakdown of the quantized charge transport in the presence of phonons. It happens for any finite electron-phonon coupling strength at the resonance condition when the pumping frequency matches the phonon frequency, and it takes finite phonon coupling strength away from the resonance. Moreover, there exist parameter regimes with non-quantized negative and positive charge transport. The modified effective pumping path due to electron-phonon coupling accurately explains the underlying physics. In the large coupling regime where the pumping disappears, the phonons are found to eliminate the staggering of the onsite potentials, which is necessary for the pumping protocol. Finally, we present a stability diagram of quantized pumping as a function of time period of pumping and phonon coupling strength.

Effect of quartic anharmonicity on the carrier transport of cubic halide perovskites CsSnI3 and CsPbI3

Recent years have witnessed the rising of halide perovskites for potential applications in photovoltaic and optoelectric devices. Due to the tilting motions of the octahedrons and rattling of the filling atoms, cubic halide perovskites show strong anharmonicity and imaginary frequencies in the phonon dispersion, which brings a great challenge to the prediction of carrier transports based on many-body theory. We have investigated the effect of quartic lattice anharmonicity on the carrier transports of cubic ${\mathrm{CsSnI}}_{3}$ and ${\mathrm{CsPbI}}_{3}$ in a comparative perspective by first-principles calculations. The hybrid functional of HSE06 was employed to get accurate band gaps and the self-consistent phonon method was used to renormalize interatomic force constants. Based on the electron-phonon Wannier interpolation and Boltzmann transport equation, the carrier mobilities and mode-resolved scattering rates were calculated and the dominant scattering channels were analyzed. Our results reveal that the mobility takes 595.9 and 84.5 ${\mathrm{cm}}^{2}/\mathrm{Vs}$ at room temperature for cubic ${\mathrm{CsSnI}}_{3}$ and ${\mathrm{CsPbI}}_{3}$, respectively, in good agreement with the experiment results. The longitudinal stretching mode of the Sn(Pb)-I bond with frequency of 120 ${\mathrm{cm}}^{\ensuremath{-}1}$ plays a dominant role in carrier scattering. In comparison, the acoustic modes and the rattling modes have a negligible effect on carrier scattering. Calculations of the band scattering rates show that deep valleys emerge at the band edges for both the highest valence band and the lowest conduction band, which is responsible for the slow relaxation of hot carriers. The strength of intraband electron-phonon coupling (EPC) between the band edge and the Sn(Pb)-I bond stretching mode is far larger than those of other modes, which leads to the dominant scattering in carrier transport. Compared with the harmonic approximation of atom interactions, the inclusion of quartic anharmonicity leads to the enhancement of atom interactions, which decreases the EPC strength and consequently increases the carrier mobilities.

First-principle study on electronic structures and optical properties of Au-Cu intermetallic compounds

The optical properties of Au-Cu intermetallic compounds can be tailored by varying the components, which advances their promising applications in plasmonics and optical metamaterials. However, physical insight on how components tailor the electronic structures and optical properties of those Au-Cu intermetallic compounds still lacks. In this work, first-principle study on the electronic structures and optical properties of three intermetallic compounds Au 3 Cu, AuCu and AuCu 3 is presented. The influence of components on the electronic structure, electron-phonon coupling, dielectric function and electrical resistivity are fully investigated. The calculation of dielectric function is divided into interband and intraband part. The calculated interband dielectric function show large differences compared with that of pure Au and pure Cu, with an absorption peak emerging at low energy of around 0.3 eV. The temperature effect of intraband dielectric function is investigated by considering electron-phonon interaction. By superimposing the interband and intraband contribution, the total dielectric function of AuCu 3 and AuCu shows an overall good agreement with available experimental data in literature. The calculated electron-phonon coupling strength of Au 3 Cu is higher than that of pure Au and Cu, whereas, the results of AuCu and AuCu 3 are lower than them. In addition, the calculated electrical resistivity of all the three Au-Cu compounds are larger than that of pure Au and Cu. • The changes of electronic structure, electron-phonon coupling strength, and optical properties for different components are presented. • The electron-phonon coupling strength of AuCu and AuCu 3 is lower than that of pure Au and Cu. • The electrical resistivity of Au-Cu compound is higher than pure Au and Cu.