Electron-phonon Coupling Constant Research Articles

Effect of Y-doped NbB2.5 on structural and superconducting properties

The crystal structure and magnetic properties of Nb1−xYxB2.5 with 0.000 ≤ x ≤ 0.050 compositions are reported. Rietveld refinement of XRD patterns shows that the partial substitution with yttrium atoms in niobium sites increases the unit cell volume. Magnetization measurements show that the critical temperature (T c ) changes from 9.0 K to 9.9 K for x = 0.000 and x = 0.050, respectively. Using the XPS measurements, the Nb 3d and B 1s core levels were identified, and the XPS valence band of XPS was measured. Our results show an increase in the valence band at zero eV as the yttrium content increases, which might be related to the T c change. The electron-phonon coupling constant was calculated from the McMillan equation; our results indicate that the Nb1−xYxB2.5 compounds are weak-coupled superconductors. Our work stablishes the effect of light doping on the structure and physical properties of these superconductors.

First principles study on the lattice dynamics and electron-phonon interaction of HfOs and HfRu compounds

From the first principles calculation based on the plane-wave pseudopotential method of density functional theory, and the linear response technique, we have performed electronic structure, phonon and electron-phonon interaction of superconducting compounds HfOs and HfRu in B2 phase. The distribution of energy with different wave vector in the first Brillouin zone and conductive charge carriers (electrons and holes) at Fermi level are described by the electronic band structure and Fermi surfaces (FS), which reveal that both the compounds are metallic in nature. Phonon dispersion curves along with phonon density of states for both the compounds are calculated. Moreover, for the convincement of phonon calculation, we have described the eigen vector displacement of optical phonon modes at the zone centre which reveals that the computed phonon spectra are real. The interaction of electron with acoustic phonon modes and low-frequency optical phonon modes is responsible for superconductivity for both the compounds. The detailed discussion of superconductivity is given in terms of Eliashberg spectral function (α2F(ω)), electron phonon coupling constants (λ) and superconducting transition temperature (TC). The calculated value of λ is 0.41 and 0.33 for HfOs and HfRu. Using Allen Dynes formula the calculated superconducting transition temperature is 1.16 K and 1.163 K, with good agreement of experimental values.

Nonequilibrium effects on the electron-phonon coupling constant in metals

Understanding of the energy exchange between electrons and phonons in metals is important for micro- and nano-manufacturing and system design. The electron-phonon (e-ph) coupling constant is to describe such exchange strength, yet its variation remains still unclear at micro- and nanoscale where the non-equilibrium effects are significant. In this work, an e-ph coupling model is proposed by transforming the full scattering terms into relaxation time approximation forms in the coupled electron and phonon Boltzmann transport equations. Consequently, the non-equilibrium effects are included in the calculation of e-ph coupling constant. The coupling model is verified by modeling the ultrafast dynamics in femtosecond pump-probe experiments on metal surface, which shows consistent results with the full integral treatment of scattering terms. The e-ph coupling constant is strongly reduced due to both the temporal non-equilibrium between different phonon branches and the spatial non-equilibrium of electrons in confined space. The present work will promote not only a fundamental understanding of the e-ph coupling constant but also the theoretical description of coupled electron and phonon transport at micro- and nanoscale.

A Theoretical Investigation on the Physical Properties of SrPd2Sb2 Superconductor

The high temperature phase of SrPd2Sb2 polymorphs exhibits bulk superconductivity below 0.6 K. The electron phonon coupling constant and density of states are two major factors that control the superconductivity in this intermetallic compound. Here we report the detailed physical properties including structural, elastic, electronic, and optical properties of this superconducting phase by using DFT-based computational method. The calculated lattice parameters and density of states show good agreement with the experimental data. This intermetallic is ductile and comparatively soft with respect to other members of this family. This compound exhibits metallic conductivity mostly emerging from Pd and Sb atoms that is different from other similar type of superconducting materials. The strong covalent interactions in Sb-Sb and Sb-Pd bonds define the electronic characteristics of this superconducting material. The optical properties of this superconductor are fairly comparable with other similar compounds in this family.

Electronic topological transitions and vibrational properties of A-15 type X3Y (X= V, Cr and Mo; Y= Os, Ir and Pt) compounds: A first-principles study

Ab-initio studies of A-15 type X3Y (X = V, Cr and Mo; Y= Os, Ir and Pt) compounds are presented at ambient and high pressures. All the studied compounds satisfy the mechanical stability criteria and are also dynamically stable as evidenced from the positive phonon dispersion curves. Camels back type band structure features are observed at Fermi level in some of the investigated compounds. Electronic topological transitions (ETTs) are the main highlights in all the compounds under pressure at different compressed volumes. The electron-phonon coupling constants λ are calculated on the basis of the Eliashberg theory for the Mo3Y (Y= Os, Ir and Pt) series that, according to the experiments, have the largest transition temperature among the studied compounds. Mo3Os superconductivity is fully understood on the basis of the Eliashberg theory of the phonon-mediated electron pairing and the experimental Tc is recovered using the well known Mc Millan formula. On the contrary, for the other two compounds of the same series Mo3Y (Ir and Pt) the calculated mass enhancement parameters show an unexpected behavior and for Mo3Ir, the experimental transition temperature values cannot be easily recovered through the Mc Millan formula.

Theoretical investigation of superconductivity in diamond: Effects of doping and pressure

The electronic structure, lattice dynamics, and electron–phonon coupling of pure, boron and nitrogen-doped diamond carbon were investigated using first-principle calculations within the generalized-gradient and virtual crystal approximations. To examine the influence of the impurity content and pressure on the superconductivity of these systems, the electron–phonon coupling constant (λ) and the critical temperature (Tc) were calculated as a function of concentrations from 0 to 15% and pressures from 0 to 90 GPa. Regarding the boron-doped diamond, calculations indicated that its electron–phonon coupling strongly relates to the optical phonon modes, and the estimated critical temperatures matched previous theoretical and experimental results. Regarding the nitrogen-doped case, it was observed that both λ and Tc were larger than those obtained for the hole-doped case. The most distinguishing feature of this system was its rising acoustic contribution to the electron–phonon coupling, which led to significant values for λ and Tc. The majority of the scenarios investigated here presented a decreasing critical temperature with increasing pressure. In contrast to the other cases, C0.85N0.15 exhibited a positive dependence between Tc and pressure leading to a superconducting transition temperature of about 122 K at 20 GPa.

The electron-phonon coupling constant, Fermi temperature and unconventional superconductivity in the carbonaceous sulfur hydride 190 K superconductor

Recently, Snider et al (2020 Nature 586 373) reported on the observation of superconductivity in highly compressed carbonaceous sulfur hydride, H x (S,C) y . The highest critical temperature in H x (S,C) y exceeds the previous record of T c = 280 K by 5 K, as reported by Somayazulu et al (2019 Phys. Rev. Lett. 122 027001) for highly compressed LaH10. In this paper, we analyze experimental temperature-dependent magnetoresistance data, R(T,B), reported by Snider et al. The analysis shows that H x (S,C) y compound exhibited T c = 190 K (P = 210 GPa), has the electron–phonon coupling constant λ e−ph = 2.0 and the ratio of critical temperature, T c, to the Fermi temperature, T F, in the range of 0.011 ⩽ T c/T F ⩽ 0.018. These deduced values are very close to the ones reported for H3S at P = 155–165 GPa (Drozdov et al 2015 Nature 525 73). This means that in all considered scenarios the carbonaceous sulfur hydride 190 K superconductor falls into the unconventional superconductor band in the Uemura plot, where all other highly compressed super-hydride/deuterides are located. It should be noted that our analysis shows that all raw R(T,B) data sets for H x (S,C) y samples, for which Snider et al (2020 Nature 586 373) reported T c > 200 K, cannot be characterized as reliable data sources. Thus, independent experimental confirmation/disproof for high-T c values in the carbonaceous sulfur hydride are required.

Decoherence and Relaxation Time of Magnetopolaron in the Presence of Three Dimensional Impurity Under Strong Parabolic Potential

In order to protect coherence of quantum states and reduce the impact of environment on quantum information, we investigate decoherence and relaxation time of magnetopolaron in the presence of three dimensional impurity under strong parabolic potential. The first states energies have been evaluated using the Lee Low Pine transformation and Pekar-type variational method. Parameters such as: decoherence time, transition frequency, spontaneous emission, Shannon entropy, relaxation time and probability density, have been evaluated. It has been seen that the impurity and electron-phonon coupling constant have a considerable effect on formation, protection of quantum qubit and quantum transport. The information exchange measured by the rate of Shannon entropy, has a great dependence on impurity and with its interaction with electrons. The relaxation time τr exhibits increasing behavior as a function of, α, β, and ωc. The electron-phonon coupling constant, impurity and cyclotron frequency are useful parameters to prevent decoherence phenomena. This study paves the way to prolong quantum effect in nanostructure and favor the realization of the future quantum computer.

The electron-phonon coupling constant for single-layer graphene on metal substrates determined from He atom scattering.

Recent theory has demonstrated that the value of the electron-phonon coupling strength λ can be extracted directly from the thermal attenuation (Debye-Waller factor) of helium atom scattering reflectivity. This theory is here extended to multivalley semimetal systems and applied to the case of graphene on different metal substrates and graphite. It is shown that λ rapidly increases for decreasing graphene-substrate binding strength. Two different calculational models are considered which produce qualitatively similar results for the dependence of λ on binding strength. These models predict, respectively, values of λHAS = 0.89 and 0.32 for a hypothetical flat free-standing single-layer graphene with cyclic boundary conditions. The method is suitable for analysis and characterization of not only the graphene overlayers considered here, but also other layered systems such as twisted graphene bilayers.

High-pressure elastic anisotropy and superconductivity of hafnium: A first-principles calculation* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11874247 and U1530258), the National Key R&D Program of China (Grant No. 2017YFA0304500), the 111 Plan of China (Grant No. D18001), the Hundred Talent Program of the Shanxi Province (2018), and the Program of State Key Laboratory of

The elastic anisotropy and superconductivity upon hydrostatic compression of α, ω, and β Hf are investigated using first-principle methods. The results of elastic anisotropies show that they increase with increasing pressure for α and ω phases, while decrease upon compression for β phase. The calculated superconducting transition temperatures are in excellent agreement with experiments. Electron–phonon coupling constants (λ) are increasing with pressure for α and ω phases, while decreasing for β phase. For β phase, the large values of λ are mainly due to the obvious TA1 soft mode. Under further compression, the TA1 soft vibrational mode will disappear gradually.

Synthetic Control of Hot-Electron Thermalization Efficiency in Size-Tunable Au-Pt Hybrid Nanoparticles.

Gold nanoparticles are well-known to exhibit size-dependent properties that are responsible for their unique catalytic, optical, and electronic applications. However, electron-phonon coupling, which is important for photocatalysis and light harvesting, is one of the rare properties of gold that is size-independent above a threshold value, e.g., for nanospheres larger than approximately 5 nm in diameter. Here, we show that when interfaced to a comparably sized Pt nanoparticle, the electron-phonon coupling constant of the hybrid material depends on the diameter of the Au domain. This is important because the electron-phonon coupling constant describes the efficiency by which hot electrons are converted to local heat by the primary electron-phonon scattering thermalization channel. We begin by synthesizing a library of Au-Pt hybrid nanoparticle heterodimers by growing size-tunable Au nanoparticles on Pt nanoparticle seeds. By systematically varying reagent concentration and reaction time, the Au domain diameter of the Au-Pt hybrid nanoparticle heterodimers can be tuned between 4.4 and 16 nm while the size of the Pt domain remains constant. Calibration curves allow us to dial in precise Au domain sizes, and microscopic analysis of the Au-Pt heterodimers provides insights into how they grow and how their morphologies evolve. Femtosecond time-resolved transient absorption spectroscopy reveals that for Au-Pt heterodimers having Au domain diameters of 8.7 to 14 nm, the electron-phonon coupling constant decreases by more than 80%, which is not observed for comparably sized Au nanoparticles. Interfacing smaller Au domains with Pt nanoparticle surfaces causes an increase in the density of states near the Fermi level of Au, which results in accelerated thermalization times through an increased number of electron-phonon interactions. The combination of precision hybrid nanoparticle synthesis and size-dependent electron-phonon coupling may be important for designing composite metals for photocatalytic and light-harvesting applications and for engineering different functions into established materials.

Measuring the Electron–Phonon Interaction in Two-Dimensional Superconductors with He-Atom Scattering

Helium-atom scattering (HAS) spectroscopy from conducting surfaces has been shown to provide direct information on the electron–phonon interaction, more specifically the mass-enhancement factor λ from the temperature dependence of the Debye–Waller exponent, and the mode-selected electron–phonon coupling constants λQν from the inelastic HAS intensities from individual surface phonons. The recent applications of the method to superconducting ultra-thin films, quasi-1D high-index surfaces, and layered transition-metal and topological pnictogen chalcogenides are briefly reviewed.

Calculated elastic moduli and sound velocity changes in YH6 hydride superconductor at the transition temperature

Yttrium hexahydride, YH6 with transition temperature, Tc of 224 K in high pressure of 165 GPa is interesting because it has the highest pressure application efficiency. In this paper, we calculated the discontinuity in the elastic moduli and longitudinal sound velocity at Tc of YH6. The calculated changes at Tc in the bulk modulus, ΔBB is 43 ppm, Young’s modulus, ΔYY is 46 ppm and longitudinal sound velocity, ΔvLvL is 15 ppm. These results showed the role of phonons in this hydride superconductor and the values are within the range of most ultrasonic techniques. The van Hove scenario may be relevant in YH6 due to the large density of states near the Fermi level. In this paper the electron-phonon coupling constant of YH6 in this scenario was calculated to be λvH = 0.040. The elastic anomalies at the transition temperature were compared with other type of superconducting materials.

Estimation of electron-phonon coupling constant in Ba1-xKxBiO3

The Holstein model was used to determine the value of electron-phonon coupling in BaBiO3-based compounds in the framework of the dynamical mean-field theory calculations for various levels of doping with potassium. The results obtained characterize the electron-phonon interaction as strong. The phase diagram of the model was calculated with reference to experimental data.

Electron-phonon coupling superconductivity in two-dimensional orthorhombic MB6 (M=Mg,Ca,Ti,Y) and hexagonal MB6 (M=Mg,Ca,Sc,Ti)

Combining crystal structure search and first-principles calculations, we report a series of two-dimensional (2D) metal borides including orthorhombic (ort-) MB6 (M=Mg, Ca) and hexagonal (hex-) MB6 (M=Mg, Ca, Sc, Ti, Sr, Y). Then, we investigate their geometrical structures, bonding properties, electronic structures, mechanical properties, phonon dispersions, thermal stability, dynamic stability, electron-phonon coupling (EPC), superconducting properties and so on. Our ab initio molecular dynamics simulation results show that these MB6 can maintain their original configurations up to 700/1000 K, indicating their excellent thermal stability. All their elastic constants satisfy the Born mechanically stable criteria and no visible imaginary frequencies are observed in their phonon dispersions. The EPC results show that these 2D MB6 are all intrinsic phonon-mediated superconductors with the superconducting transition temperature (Tc??) in the range of 2.2-21.3 K. Among them, the highest Tc (21.3 K) appears in hex-CaB6, whose EPC constant () is 0.94. By applying tensile/compressive strains on ort-/hex-CaB6, we find that the compressive strain can obviously soften the acoustic phonon branch and enhance the EPC as well as Tc. The Tc of the hex-CaB6 can be increased from 21.3 K to 28 K under compressive strain of 3%. These findings enrich the database of 2D superconductors and should stimulate experimental synthesizing and characterizing of 2D superconducting metal borides.

Sound Velocity and Elastic Moduli of Superconducting and Non-superconducting NdBa2Cu3O7-δ

The mechanical properties of superconducting NdBa2Cu3O7 and non-superconducting NdBa2Cu3O6.60 have been measured between 80 and 300 K at 10 MHz. An anomaly in the longitudinal sound velocity indicating lattice-softening tendency between 160 and 230 K was observed in non-superconducting NdBa2Cu3O6.60 but not in superconducting NdBa2Cu3O7 (critical temperature, Tc = 93 K). The Debye temperature θD increased when the oxygen content was increased where θD = 313 K for NdBa2Cu3O6.6 and θD = 370 K for NdBa2Cu3O7 indicating hardening of the material with the superconducting phase. The electron-phonon coupling constant of the superconducting sample in the conventional three-dimensional BCS theory was λ = 0.67, and in the two-dimensional van Hove scenario, λvH = 0.046. The related elastic moduli of the samples are also reported in this paper.

Theory for the Charge-Density-Wave Mechanism of 3D Quantum Hall Effect.

The charge-density-wave (CDW) mechanism of the 3D quantum Hall effect has been observed recently in ZrTe_{5} [Tang etal., Nature 569, 537 (2019)10.1038/s41586-019-1180-9]. Different from previous cases, the CDW forms on a one-dimensional (1D) band of Landau levels, which strongly depends on the magnetic field. However, its theory is still lacking. We develop a theory for the CDW mechanism of 3D quantum Hall effect. The theory can capture the main features in the experiments. We find a magnetic field induced second-order phase transition to the CDW phase. We find that electron-phonon interactions, rather than electron-electron interactions, dominate the order parameter. We extract the electron-phonon coupling constant from the non-Ohmic I-V relation. We point out a commensurate-incommensurate CDW crossover in the experiment. More importantly, our theory explores a rare case, in which a magnetic field can induce an order-parameter phase transition in one direction but a topological phase transition in other two directions, both depend on one magnetic field.

Superconducting properties of the ternary boride YRh4B4

The low number of physical parameters for superconductivity has motivated researchers to perform bulk measurements in YRh4B4, discovered four decades ago, with superconducting transition temperature () of approximately 11 K. We performed electrical resistivity, magnetic susceptibility, and specific heat measurements to investigate the superconducting properties of the ternary borides YRh4B4 with CeCo4B4-type tetragonal structure. The nearly single-phase YRh4B4 had a moderately high of 10.6 K. The lower and upper critical fields were determined to be 1.5 and 32 kOe, respectively, by magnetic susceptibility and electrical resistivity measurements. The Ginzburg-Landau parameter was approximately 4.35, which is greater than , implying that YRh4B4 is a type-II superconductor. Because of specific heat measurements, the Debye temperature , , and 2 were determined to be 505 K, 1.56, and 3.91, respectively, where the specific heat jump at and the superconducting gap at 0 K are expressed by ΔC and . The electrical resistivity at low temperatures showed quadratic temperature dependence, and its coefficient A was determined to be 0.004426 µΩ· cm K−2, indicating a large density of states at Fermi energy. The electron-phonon coupling constant was derived to be 0.69. These results strongly suggest that YRh4B4 is a conventional s-wave superconductor described by the Bardeen–Cooper–Shriefer theory, and that the moderately high- in YRh4B4 is attributable to a high due to vibrations of boron with a light mass and a large A-value originating from the large density of states at Fermi energy. These results will facilitate microscopic investigations and provide clues to develop a novel high- metal superconductor.

Enhanced Electron–Phonon Coupling and Superconductivity in Two-dimensional BC2N via Lithium Deposition: a First-Principles Investigation

Two-dimensional (2D) superconductors are important both for the basic understanding of pairing mechanism and potential applications in nanosuperconducting quantum interference devices. In this work, we explore the phonon-mediated superconductivity of hole-doped and lithium-deposited monolayer BC2N by first-principles calculations. Our results reveal that the hole-doped planar BC2N cannot superconduct due to weak electron–phonon coupling (EPC) with λ = 0.07. When lithium is deposited on the monolayer, the EPC constant is enhanced significantly to 0.52 and the superconducting transition temperature Tc is determined to be 3.58 K. Because more phonons, in particular the out-of-plane modes, are triggered to participate in the EPC process, the Tc is higher than 1.4 K of Ca-deposited graphene, 1.3 K of Li-deposited stanene, and 3 K of doped WS2 and NbSe2. It is also found that tensile biaxial strain weakens the EPC and leads to a decreased superconducting transition temperature. Our findings will provide a new guideline for designing novel 2D superconductors and enrich the potential application of 2D BC2N.

Excited state geometry of β-carotene influenced by environments: the nature and decisive role of solvent revealing by two-dimensional resonance Raman correlation spectroscopy.

Resonance Raman scattering can be used to investigate the ground and excited state information of carotenoid. It is known that the Dushinsky rotation can significantly influence the resonant Raman intensity of β-carotene (β-car). The excited state geometry revealed by the double components feature of the C = C stretching vibrational modes and the environmental dependence of the Raman intensity for each component remain unknown. We explore the influence of environmental factors on the relative intensity of these two C = C stretching vibration modes and perform two-dimensional resonance Raman correlation analysis to reveal the changes on β-car excited state geometry. The results show that the relative wavelength difference between the 0-0 absorption and the excitation is the key factor that decides the intensity ratio of the two components and that the intensity of each mode is modulated by environmental factors. This modulation is closely related to the excited state geometry and dynamics, effective conjugation length, and electron-phonon coupling constant. It also shows that the asynchronous cross-peaks in the two-dimensional resonance Raman correlation spectrum (2DRRCOS) can effectively characterize the degree of the varied electron-phonon coupling with the changing conditions. These results are not only complementary to the research on the excited states of carotenoids but also applicable to investigate the environmental dependence of Raman intensity for a lot of π-conjugated molecules.