Electron-phonon Coupling Strength Research Articles

Tracking the surface atomic motion in a coherent phonon oscillation

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft x-ray pulses from the free electron laser FLASH. We collect Se $3d$ photoelectron diffraction patterns over a wide angular range from optically excited $\mathrm{Bi}{}_{2}\mathrm{Se}{}_{3}$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the ${A}_{1g}$ optical phonons. Using a fluence of $4.2\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 0.01 \AA{}.

Pressure Evolution of Ultrafast Photocarrier Dynamics and Electron-Phonon Coupling in FeTe0.5Se0.5.

Understanding the coupling between electrons and phonons in iron chalcogenides FeTexSe1-x has remained a critical but arduous project in recent decades. The direct observation of the electron-phonon coupling effect through electron dynamics and vibrational properties has been lacking. Here, we report the first pressure-dependent ultrafast photocarrier dynamics and Raman scattering studies on an iron chalcogenide FeTe0.5Se0.5 to explore the interaction between electrons and phonons in this unconventional superconductor. The lifetime of the excited electrons evidently decreases as the pressure increases from 0 to 2.2 GPa, and then increases with further compression. The vibrational properties of the A1g phonon mode exhibit similar behavior, with a pronounced frequency reduction appearing at approximately 2.3 GPa. The dual evidence reveals the enhanced electron-phonon coupling strength with pressure in FeTe0.5Se0.5. Our results give an insight into the role of the electron-phonon coupling effect in iron-based superconductors.

Evolution of Physical Properties of RE3Ni5Al19Family (RE = Y, Nd, Sm, Gd, Tb, Dy, Ho, and Er)

AbstractSingle crystals of RE3Ni5Al19series (RE = Y, Nd, Sm, Gd, Tb, Dy, Ho, and Er) are grown using the Al self‐flux method. The crystal structure is examined by both single crystal and powder X‐ray diffraction. Physical properties are studied for the first time for RE3Ni5Al19(RE = Y, Nd, Gd, Tb, Dy, Ho, and Er) by means of magnetic susceptibility, electrical resistivity, and heat capacity measurements. Complex magnetic behaviors, with up to three transitions present for RE = Sm, Gd, Tb, and Dy, are revealed. Y3Ni5Al19is found to be a nonmagnetic nonsuperconducting metal (aboveT= 1.8 K) with weak electron–phonon coupling strength.

High-Throughput Screening of Strong Electron-Phonon Couplings in Ternary Metal Diborides.

We perform a high-throughput screening on phonon-mediated superconductivity in a ternary metal diboride structure with alkali, alkaline earth, and transition metals. We find 17 ground states and 78 low-energy metastable phases. From fast calculations of zone-center electron-phonon coupling, 43 compounds are revealed to show electron-phonon coupling strength higher than that of MgB2. An anticorrelation between the energetic stability and electron-phonon coupling strength is identified. We suggest two phases, i.e., Li3ZrB8 and Ca3YB8, to be synthesized, which show reasonable energetic stability and superconducting critical temperature.

Oxygen vacancies induced ferromagnetism in RF-sputtered and hydrothermally annealed zinc ferrite (ZnFe2O4) thin films

We report the inception of substantial magnetization of 4107 G in rf-sputtered and annealed hydrothermally at 180 °C zinc ferrite (ZFO) thin films. The value of electron-phonon coupling strength (λ) calculated using Raman A1g and F32g modes contended the shift of inverse spinel structure of the as-prepared thin films to the normal spinel structure with the enhancement in annealing duration. The shift in spinel structure was further affirmed by X-ray photoelectron spectroscopy (XPS) where inversion parameter, i.e., a fraction of the Zn2+ ions occupied by octahedral sites reduced from 0.18 of as-deposited film to 0.13 of air annealed and 0.14 for hydrothermally annealed films. Additionally, the ZFO thin film annealed hydrothermally for 4 h showed lower value of λ of 0.11 for A1g Raman mode and a high value of the ratio of surface hydroxyl (OH− groups) oxygen ions (Oii) to the lattice oxide (Fe/Zn–O) oxygen ions (Oi) in XPS spectra confirmed the presence of enormous oxygen vacancies. The magnetization analysis suggested that the presence of high oxygen vacancies in 4 h of hydrothermally annealed sample promotes the direct ferromagnetic interaction in Fe3+ ions in octahedral sites leading to a high value of saturation magnetization unlike the air annealed samples.

Observation of superconductivity and its enhancement at the charge density wave critical point in LaAgSb2

We discover superconductivity (SC) in LaAgSb$_2$ at ambient pressure and its close correlation with a charge density wave (CDW) under pressure. The superconducting transition temperature ($T_c$) exhibits a sharp peak at the CDW critical pressure of 3.2 GPa. We demonstrate that the carriers inhabiting the Sb-square net is crucial not only in the formation of CDW but also in SC for their relatively strong electron-phonon coupling (EPC). Furthermore, theoretical EPC strength in pristine LaAgSb$_2$ cannot explain the observed peak with $T_c\sim 1$ K, which indicates that an additional mechanism reinforces SC only around the CDW critical pressure.

Tunable Superconductivity in Phosphorus‐Doped Topological Transition‐Metal Germanide Mo5Ge3

AbstractThe effect of phosphorus doping in the transition metal germanide Mo5Ge3, which is predicted to be a nodal line semimetal and exhibits superconductivity below Tc = 0.75 K, is studied. It is found that Mo5GePx retains the tetragonal W5Si3‐type structure up to x = 1.0, and the incorporation of phosphorus leads to a shrinkage of the unit cell volume. Remarkably, Tc of Mo5GePx is enhanced by more than one order of magnitude with increasing x, reaching 8.24 K at x = 1.0. This enhancement is attributed to the increases in both the density of states at the Fermi level and electron–phonon coupling strength. Theoretical calculations show that the phosphorus doping leads to a nearly rigid‐band shift of the Fermi level into a local maximum of the density of states. This study unveils that transition metal germanides offer an emerging platform to study the interplay between superconductivity and nontrivial band topology.

Cross Determination of Exciton Coherence Length in J-Aggregates.

The coherence length of the Frenkel excitons (Ncoh) is one of the most critical parameters governing many key features of supramolecular J-aggregates. Determining experimentally the value of Ncoh is a nontrivial task since it is sensitive to the technique/method applied, causing discrepancies in the literature data even for the same chemical compound and aggregation conditions. By using a combination of different experimental techniques including UV-vis-NIR, fluorescence emission, time-resolved photoluminescence, and transient absorption spectroscopies, we determined Ncoh values for J-aggregates of a cyanine dye. We found that the absorption spectroscopy alone - a widely used technique- fails in determining right value for Ncoh. The correct approach is based on the modification of photoluminescence lifetime and nonlinear response upon aggregation and careful analysis of the Stokes shift and electron-phonon coupling strength. This approach revealed that Ncoh of JC-1 J-aggregates ranges from 3 to 6.

Role of vibrational properties and electron–phonon coupling on thermal transport across metal-dielectric interfaces with ultrathin metallic interlayers

We systematically analyze the influence of 5 nm thick metal interlayers inserted at the interface of several sets of different metal-dielectric systems to determine the parameters that most influence interface transport. Our results show that despite the similar Debye temperatures of Al2O3 and AlN substrates, the thermal boundary conductance measured for the Au/Al2O3 system with Ni and Cr interlayers is ∼2× and >3× higher than the corresponding Au/AlN system, respectively. We also show that for crystalline SiO2 (quartz) and Al2O3 substrates having highly dissimilar Debye temperature, the measured thermal boundary conductance between Al/Al2O3 and Al/SiO2 are similar in the presence of Ni and Cr interlayers. We suggest that comparing the maximum phonon frequency of the acoustic branches is a better parameter than the Debye temperature to predict the change in the thermal boundary conductance. We show that the electron–phonon coupling of the metallic interlayers also alters the heat transport pathways in a metal-dielectric system in a nontrivial way. Typically, interlayers with large electron–phonon coupling strength can increase the thermal boundary conductance by dragging electrons and phonons into equilibrium quickly. However, our results show that a Ta interlayer, having a high electron–phonon coupling, shows a low thermal boundary conductance due to the poor phonon frequency overlap with the top Al layer. Our experimental work can be interpreted in the context of diffuse mismatch theory and can guide the selection of materials for thermal interface engineering.

Kinetics of laser-induced melting of thin gold film: How slow can it get?

Melting is a common and well-studied phenomenon that still reveals new facets when triggered by laser excitation and probed with ultrafast electron diffraction. Recent experimental evidence of anomalously slow nanosecond-scale melting of thin gold films irradiated by femtosecond laser pulses motivates computational efforts aimed at revealing the underlying mechanisms of melting. Atomistic simulations reveal that a combined effect of lattice superheating and relaxation of laser-induced stresses ensures the dominance of the homogeneous melting mechanism at all energies down to the melting threshold and keeps the time scale of melting within ~100 picoseconds. The much longer melting times and the prominent contribution of heterogeneous melting inferred from the experiments cannot be reconciled with the atomistic simulations by any reasonable variation of the electron-phonon coupling strength, thus suggesting the need for further coordinated experimental and theoretical efforts aimed at addressing the mechanisms and kinetics of laser-induced melting in the vicinity of melting threshold.

Electromagnetic field effect on weak-coupling piezoelectric polaron in an asymmetrical Gaussian confinement potential quantum well

The properties of an electron weakly coupled to piezo-acoustic phonon in asymmetrical Gaussian confinement potential quantum well (AGCPQW) subject to external electric field (EF) and magnetic field (MF) has been investigated using the Lee-Low-Pines unitary transformation and linear combination operation methods. We have obtained the ground state energy (GSE) and the ground state binding energy (GSBE) of piezoelectric polaron. The effects of the EF, the MF, the range of the asymmetrical Gaussian confinement potential (RAGCP), Debye cut-off wavenumber (DCOW) and the electron–phonon coupling strength on the GSE and the GSBE are also analyzed. It is found that the GSE is an increasing function of the EF and the cyclotron frequency (CF), whereas it is a decreasing one of the RAGCP, the DCOW and electron–phonon coupling strength. The GSBE is an increasing function of the DCOW and the electron–phonon coupling strength. It is also an aggrandizing function with decreasing of the RAGCP, whereas it is a decayed one of the EF and CF. It is shown that the EF, the RAGCP, the MF, the DCOW and electron–phonon coupling strength are important factors that have great influence on the properties of the piezoelectric polaron in AGCPQW.

First principles investigation of the electronic-thermal transport of ThN, UN, and ThC

The resistivity and electronic-thermal conductivity of ThN, ThC and UN were compared. It was found that the new electron–phonon physics from first principles code Electron-Phonon coupling using Wannier functions (EPW, v. 5.4) code as implemented in Quantum Espresso (QE, v. 6.8) code based on density-functional theory predicts higher electronic-thermal conductivity for thorium and uranium mononitride than for thorium carbide in agreement with experiment. QE combined with EPW code were used to calculate integrated electron–phonon coupling strength, the Eliashberg transport coupling function, and electron–phonon scattering rates. The electronic resistivity was calculated using Ziman’s formula for metals and compared to the values derived from first principles using Boltzmann transport theory. The number of mobility electrons were obtained by equating the calculated resistivity at 1000 K. Estimation showed that in ThN the number of electronic carriers is two times larger and both electron–phonon coupling strength and electron scattering rates are lower, which led to about four times higher electronic thermal conductivity than calculated for ThC. Although a good agreement with experiment was found for ThN and ThC (at high temperatures), the current version of EPW code for non-magnetic solids overestimates the electronic thermal conductivity of UN and underestimates resistivity.

Influence of In-induced resonant level on the normal-state and superconducting properties of Sn1.03Te

Normal-state transport properties (2--300 K) of the polycrystalline series ${\mathrm{Sn}}_{1.03\ensuremath{-}\ensuremath{\delta}\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te} (0\ensuremath{\le}x\ensuremath{\le}0.07$; $\ensuremath{\delta}\ensuremath{\le}0.0025$) were investigated by means of electrical resistivity, thermopower, Hall effect, and thermal conductivity measurements. The distortion of the valence-band structure by the In-induced resonant level (RL) has a profound influence on the evolution of the normal-state properties with $x$ and on the emergence of superconductivity evidenced by specific-heat measurements down to 0.35 K. In addition to a nearly 40-fold increase in the residual electrical resistivity ${\ensuremath{\rho}}_{0}$ on going from $x=0.0$ to 0.05, the thermopower $\ensuremath{\alpha}$ shows a nonlinear, complex behavior as a function of both temperature and $x$. While Hall measurements indicate a dominant holelike response across the entire composition and temperature ranges, $\ensuremath{\alpha}$ changes sign below about 100 K and remains negative down to 5 K for $0.0015\ensuremath{\le}x\ensuremath{\le}0.0045$. Additional measurements under magnetic fields ${\ensuremath{\mu}}_{0}H$ of up to 14 T further shows that $\ensuremath{\alpha}({\ensuremath{\mu}}_{0}H)$ gradually shifts towards positive values, suggestive of a dominant holelike contribution to $\ensuremath{\alpha}$. Superconductivity emerges for $x=0.02$ at a critical temperature ${T}_{\mathrm{c}}=0.67$ K, with ${T}_{\mathrm{c}}$ increasing with $x$ to reach 1.73 K for $x=0.07$. The variations in the superconducting parameters with $x$, notably the specific-heat jump at ${T}_{\mathrm{c}}$, confirm the results reported in prior studies and suggests a nontrivial role of the RL on the electron-phonon coupling strength. The striking similarities between this series and the canonical resonant system ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Tl}}_{x}\mathrm{Te}$ provide an excellent experimental opportunity to gain a deeper understanding of the close interplay between resonant level, anomalous transport properties, and superconductivity.

Robustness of antiferromagnetism in the Su-Schrieffer-Heeger Hubbard model

We recently unveiled that antiferromagnetism (AFM) order can be dominantly induced by the bond Su-Schrieffer-Heeger (SSH) electron-phonon coupling (EPC), which is beyond the conventional wisdom that AFM order is usually driven by strong Coulomb interactions. Nevertheless, many aspects of the interplay between EPC and strong electronic interactions on AFM ordering remain unexplored. Here, we investigate the Su-Schrieffer-Heeger-Hubbard (SSHH) model with bond SSH phonons and on-site Hubbard interactions by large-scale quantum Monte Carlo (QMC) simulations and obtain its ground-state phase diagram for various Hubbard interactions and EPC strength at half filling. Our results show that Hubbard interactions further enhance EPC-induced AFM, especially for small phonon frequency or in adiabatic limit, the regime most relevant to various quantum materials. This could shed further light to understanding the cooperative effect between EPC and electronic correlations in quantum materials.

Intrinsic electron transport in monolayer MoSi2N4 and WSi2N4

We systematically studied the intrinsic electron transport of the recently fabricated two-dimensional (2D) monolayer MoSi2N4 and WSi2N4 (belong to the MA2Z4 family, where M is a transition metal; A is Si or Ge; and Z is N, P, or As) by using the Boltzmann transport theory with the scattering rates calculated from first-principles calculations. It is found that both materials have moderate room temperature electron mobility of 87 cm2/V s for MoSi2N4 and 119 cm2/V s for WSi2N4. Our detailed analysis shows that their electron mobility difference is attributed to the different average effective mass. Moreover, by comparing studies with monolayer MoS2 and WS2, we reveal that the polar optical phonon pattern of MoSi2N4 and WSi2N4 is governed by the antiparallel silicon–nitrogen bond oscillation, different from MoS2 and WS2, in which is governed by the antiparallel transition metal–sulfur bond oscillation. This feature is arising from the large electronegativity difference between N and Si. Nitrogen has the fourth largest electronegativity, so it always tends to attract plenty of electrons from Si and thus Si has a large Born effective charge, thereby resulting in strong Fröhlich interaction. We further found that all the 2D semiconducting MA2Z4 have large Born effective charges, thereby indicating a generally strong electron–phonon coupling strength.

Optical Properties of a CsMnBr3 Single Crystal.

Lead-free perovskite materials with good stability are promising for various applications. In order to explore their application in optoelectronic devices, it is essential to investigate their fundamental optical properties. In this work, we have synthesized a CsMnBr3 single crystal (SC) with red emission at ∼621 nm and studied their optical properties. Through the measurement of temperature-dependent photoluminescence (PL) spectra, it is found that a phase transition occurs at approximately 100 K in the SC, which is absent in the CsMnBr3 nanocrystals (NCs). Furthermore, the SC exhibits stronger electron and longitudinal optical phonon coupling strength than that of the NCs at low temperatures. In addition, under the resonant excitation at 600 nm, the SC possesses strong saturable absorption property, with a modulation depth of ∼27%. Interestingly, the SC also exhibits a large two-photon absorption coefficient of ∼0.035 cm GW–1 at 800 nm and an excellent optical limiting behavior. The experimental results indicate that the CsMnBr3 SC is a class of excellent environmentally friendly optoelectronic materials.

Fröhlich electron–phonon interaction Hamiltonian and potential distribution of a polar optical phonon mode in wurtzite nitride triangular nanowires

Polar optical phonon modes of wurtzite triangular nanowires (NWs) with three different cross sections, including the hemi-equilateral triangle (HET), the isosceles right triangle (IRT), and the equilateral triangle (ET), are deduced and analyzed using the dielectric continuum model. The exact and analytical phonon states of exactly confined (EC) modes in nitride NWs with HET, IRT, and ET cross sections are derived. The characteristic frequency of EC phonon modes in the triangular nitride NW systems is specified. Fröhlich electron–phonon interaction Hamiltonians in wurtzite NWs with three types of triangular cross sections are obtained. It is found from the numerical results that, among the three types of GaN NWs, the electron–phonon coupling of EC modes in NWs with an HET cross section is the weakest one, that in NWs with an ET cross section is the strongest one, and that in NWs with an IRT cross section is in the middle. The electrostatic potentials of EC modes in HET NWs are neither symmetric nor antisymmetric. The potential functions of EC modes in the ET NW structures have one (three) symmetric axis (axes) as the quantum numbers p and q take fractions (integers). The potential functions of EC modes in IRT NWs behave either symmetrically or anti-symmetrically, which are closely dependent on the parities of the quantum numbers p and q. With the increase of order-number of EC modes, the electron–phonon coupling becomes weaker and weaker. This reveals that cross-sectional morphology of quantum structures has an important influence on the symmetries of phonon modes and electron–phonon coupling strengths in low-dimensional quantum systems.

Electron-phonon coupling strength from ab initio frozen-phonon approach

We propose a fast method for high-throughput screening of potential superconducting materials. The method is based on calculating metallic screening of zone-center phonon modes, which provides an accurate estimate for the electron-phonon coupling strength. This method is complementary to the recently proposed Rigid Muffin Tin (RMT) method, which amounts to integrating the electron-phonon coupling over the entire Brillouin zone (as opposed to the zone center), but in a relatively inferior approximation. We illustrate the use of this method by applying it to MgB$_\text{2}$, where the high-temperature superconductivity is known to be driven largely by the zone-center modes, and compare it to a sister compound AlB$_\text{2}$. We further illustrate the usage of this descriptor by screening a large number of binary hydrides, for which accurate first-principle calculations of electron-phonon coupling have been recently published. Together with the RMT descriptor, this method opens a way to perform initial high-throughput screening in search of conventional superconductors via machine learning or data mining.

Decoupling engineering of formamidinium-cesium perovskites for efficient photovoltaics.

Although pure formamidinium iodide perovskite (FAPbI3) possesses an optimal gap for photovoltaics, their poor phase stability limits the long-term operational stability of the devices. A promising approach to enhance their phase stability is to incorporate cesium into FAPbI3. However, state-of-the-art formamidinium–cesium (FA–Cs) iodide perovskites demonstrate much worse efficiency compared with FAPbI3, limited by the different crystallization dynamics of formamidinium and cesium, which result in poor composition homogeneity and high trap densities. We develop a novel strategy of crystallization decoupling processes of formamidinium and cesium via a sequential cesium incorporation approach. As such, we obtain highly reproducible, highly efficient and stable solar cells based on FA1–xCsxPbI3 (x = 0.05–0.16) films with uniform composition distribution in the nanoscale and low defect densities. We also revealed a new stabilization mechanism for Cs doping to stabilize FAPbI3, i.e. the incorporation of Cs into FAPbI3 significantly reduces the electron–phonon coupling strength to suppress ionic migration, thereby improving the stability of FA–Cs-based devices.

On-Site Synthesis and Characterizations of Atomically-Thin Nickel Tellurides with Versatile Stoichiometric Phases through Self-Intercalation.

Self-intercalation of native metal atoms in two-dimensional (2D) transition metal dichalcogenides has received rapidly increasing interest, due to the generation of intriguing structures and exotic physical properties, however, only reported in limited materials systems. An emerging type-II Dirac semimetal, NiTe2, has inspired great attention at the 2D thickness region, but has been rarely achieved so far. Herein, we report the direct synthesis of mono- to few-layer Ni-tellurides including 1T-NiTe2 and Ni-rich stoichiometric phases on graphene/SiC(0001) substrates under ultra-high-vacuum conditions. Differing from previous chemical vapor deposition growth accompanied with transmission electron microscopy imaging, this work combines precisely tailored synthesis with on-site atomic-scale scanning tunneling microscopy observations, offering us visual information about the phase modulations of Ni-tellurides from 1T-phase NiTe2 to self-intercalated Ni3Te4 and Ni5Te6. The synthesis of Ni self-intercalated NixTey compounds is explained to be mediated by the high metal chemical potential under Ni-rich conditions, according to density functional theory calculations. More intriguingly, the emergence of superconductivity in bilayer NiTe2 intercalated with 50% Ni is also predicted, arising from the enhanced electron-phonon coupling strength after the self-intercalation. This work provides insight into the direct synthesis and stoichiometric phase modulation of 2D layered materials, enriching the family of self-intercalated materials and propelling their property explorations.