Electron-phonon Coupling Parameter Research Articles

Electronic, phononic and superconducting properties of trigonal Li2 MSi2 (M = Ir, Rh)

We have used ab initio density functional theory to study electronic, mechanical, phononic, and superconducting properties of Li2 MSi2 (M = Ir, Rh), which has recently been produced as a new type of transition metal–based ternary compound in the trigonal structure (Horigane et al 2019 New J. Phys. 21 093056). The calculated electronic band structure and the density of states indicate that the Li2IrSi2 and Li2RhSi2 compounds are in metallic character. Mechanical properties such as elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and Debye temperature were calculated for these compounds. The calculated results suggest that the compounds are mechanically stable and behave in a ductile manner. The phonon spectra have no imaginary frequency, which proves that these compounds are dynamically stable. Electron–phonon coupling parameters confirm that they are weak–coupling superconductors. Although the influence of spin–orbit coupling in superconductivity is not significant for these compounds, it has a very small influence on electronic structure for Li2IrSi2. The calculated critical temperature () values of 3.29 K for Li2IrSi2 and 2.82 K for Li2RhSi2 agree well with experimental estimates.

First-principles calculations of physical properties and superconductivity of orthorhombic ScRuSi and ZrRhSi

We have performed ab initio pseudopotential calculations of the structural, electronic, elastic, mechanical, vibrational, electron-phonon interaction, and superconducting properties of ScRuSi and ZrRhSi. An assessment of their elastic and mechanical properties reveals that ScRuSi is a ductile compound, while ZrRhSi is a brittle compound. For both compounds electronic energy bands in the vicinity of the Fermi level originate from the $d$ states of transition-metal atoms. The Eliashberg spectral function reveals that low-frequency phonon modes due to coupled motions of transition-metal atoms are more involved in the process of scattering of electrons than the high-frequency modes due to the vibrations of Si atoms. The average electron-phonon coupling parameter is calculated to be 0.54 for ScRuSi and 0.84 for ZrRhSi from the integration of the Eliashberg spectral function. It is found that the relatively weaker electron-phonon interaction in ScRuSi makes its superconducting transition temperature value $({T}_{c}=3.23 \mathrm{K})$ considerably lower than that of ZrRhSi $({T}_{c}=10.45 \mathrm{K})$.

Temperature-dependent dielectric loss in BaTiO3: Competition between tunnelling probability and electron-phonon interaction

The temperature-dependent impedance spectroscopy, optical absorption spectroscopy, and Raman spectroscopic measurements have been carried out on BaTiO3 in the temperature range from 350 K to 430 K in order to understand the possible correlation between dielectric loss measured in terms of loss tangent (tan(δ)), band gap (Eg) and electron-phonon coupling parameter (λe-ph). It has been observed that in the tetragonal phase tan(δ) increases marginally, whereas in cubic phase tan(δ) shows a decreasing trend with increasing temperature. It has been observed that λe-ph increases with temperature and shows a sudden increase by almost two-fold at tetragonal to cubic transition temperature. The marginal increase in the value of tan(δ) with temperature in tetragonal phase is understood by considering the competition between the tunnelling of electron from valence to the conduction band and electron-phonon coupling, whereas; decrease in the value of tan(δ) in the cubic phase as a function of temperature may be due to the dominating contribution of electron-phonon coupling. Present investigations clearly establish the strong correlation between the temperature dependence of dielectric loss, the value of band gap, and electron-phonon interaction.

Electron-phonon coupling in metals at high electronic temperatures

Electron-phonon coupling, being one of the most important parameters governing the material evolution after ultrafast energy deposition, yet remains the most unexplored one. In this work, we applied the dynamical coupling approach to calculate the nonadiabatic electron-ion energy exchange in nonequilibrium solids with the electronic temperature high above the atomic one. It was implemented into the tight-binding molecular dynamics code, and used to study electron-phonon coupling in various elemental metals. The developed approach is a universal scheme applicable to electronic temperatures up to a few electron-Volts, and to arbitrary atomic configuration and dynamics. We demonstrate that the calculated electron-ion (electron-phonon) coupling parameter agrees well with the available experimental data in high-electronic-temperature regime, validating the model. The following materials are studied here - fcc metals: Al, Ca, Ni, Cu, Sr, Y, Zr, Rh, Pd, Ag, Ir, Pt, Au, Pb; hcp metals: Mg, Sc, Ti, Co, Zn, Tc, Ru, Cd, Hf, Re, Os; bcc metals: V, Cr, Fe, Nb, Mo, Ba, Ta, W; diamond cubic lattice metals: Sn; specific cases of Ga, In, Mn, Te and Se; and additionally semimetal graphite and semiconductors Si and Ge. For many materials, we provide the first and so far the only estimation of the electron-phonon coupling at elevated electron temperatures, which can be used in various models simulating ultrafast energy deposition in matter. We also discuss the dependence of the coupling parameter on the atomic mass, temperature and density.

Probing the physical and superconducting properties of hexagonal ZrRuAs: A first-principles calculation

In the presented work, ab initio pseudopotential calculations have been realized on structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties for ZrRuAs with the hexagonal ZrNiAl-type layer crystal structure. In the vicinity of the Fermi level, Ru 4d and Zr 4d states dominate and mainly contribute to the conduction properties of ZrRuAs. A critical assessment of elastic and mechanical properties for ZrRuAs reveals that, this superconductor is soft (ductile) due to remarkable metallic bonding. Phonon anomalies have been observed in the phonon spectrum of ZrRuAs for the lowest transverse acoustic branch and low-frequency transverse optical branches along the Γ-K and Γ-M-K symmetry directions. Our electron-phonon interaction calculations indicated that ZrRuAs is a conventional phonon-mediated superconductor with strong electron-phonon coupling parameter of 1.32. The strong electron-phonon coupling parameter of ZrRuAs produces its superconducting transition temperature to be 11.12 K which accords nicely with its experimental values of 12 K.

Indirect RKKY interaction in doped armchair nanotube due to electron phonon interaction effects

We study the Ruderman-Kittle-Kasuya-Yosida (RKKY) interaction in doped armchair nanotube in the presence of gap parameter. The effects of both next nearest neighbor hopping parameter and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments have been analyzed. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain the transverse static spin susceptibility of armchair graphene nanoribbon in the presence of electron-phonon coupling and gap parameter. The spin susceptibility components are calculated using Green’s function approach for Holstein model Hamiltonian. The effects of electron doping on dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of next nearest neighbor hopping parameter on the spatial behavior of RKKY interactions are different in the presence of electron phonon coupling.

The effect of spin–orbit interaction on superconductivity in the filled skutterudites MPt4Ge12(M=Ba, Sr and Th)

ABSTRACT Ab initio pseudopotential calculations have been conducted for the superconducting filled skutterudite compounds BaPtGe, SrPtGe, and ThPtGe in order to explore the effect of spin–orbit interaction(SOI) by using the planewave pseudopotential method and the density functional theory. The electronic structure calculations suggest that the density of states at the Fermi level is dominated by Ge atom's p states, with the guest atoms (Ba, Sr, or Th) making very little or no contribution. However, when the SOI is included, the phonon frequencies of BaPtGe and SrPtGe are hardened, which in turn decreases the electron–phonon interaction. By integrating the Eliashberg spectral function F(ω) with SOI included, the average electron–phonon coupling parameters are found to be 0.74 for BaPtGe, 0.79 for SrPtGe, and 0.69 for ThPtGe. Using a reasonable value of = 0.10 for the effective Coulomb repulsion parameter, the superconducting critical temperature is found to be 5.36 K for BaPtGe, 5.43 K for SrPtGe, and 4.45 K for ThPtGe with SOI. These values are in excellent accordance with their reported experimental values of 5.35 K, 5.4 K and 4.62 K.

Superconducting transition temperatures of metallic liquids

We develop a nonperturbative approach for calculating the superconducting transition temperatures (Tc's) of liquids. The electron-electron scattering amplitude induced by electron-phonon coupling (EPC), from which an effective pairing interaction can be inferred, is related to the fluctuation of the T matrix of electron scattering induced by ions. By applying the relation, EPC parameters can be extracted from a path-integral molecular dynamics simulation. For determining Tc, the linearized Eliashberg equations are reestablished nonperturbatively. We apply the approach to estimate Tc's of metallic hydrogen liquids. It indicates that metallic hydrogen liquids in the pressure regime from 0.5 to 1.5TPa have Tc's well above their melting temperatures and therefore are superconducting liquids.1 MoreReceived 29 January 2019Revised 28 January 2020Accepted 30 January 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.013340Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectron-phonon couplingMethods in superconductivityPressure effectsSuperconducting phase transitionTransition temperaturePhysical SystemsHigh-temperature superconductorsLiquidsUnconventional superconductorsTechniquesDensity functional theoryEliashberg theoryGreen's function methodsCondensed Matter, Materials & Applied Physics

Nonequilibrium heat transport in Pt and Ru probed by an ultrathin Co thermometer

Non-equilibrium of electrons, phonons, and magnons in metals is a fundamental phenomenon in condensed matter physics and serves as an important driver in the field of ultrafast magnetism. In this work, we demonstrate that the magnetization of a sub-nm-thick Co layer with perpendicular magnetic anisotropy can effectively serve as a thermometer to monitor non-equilibrium dynamics in adjacent metals, Pt and Ru, via time-resolved magneto-optic Kerr effect. The temperature evolutions of the Co thermometer embedded in Pt layers of different thicknesses, 6-46 nm, are adequately described by a phenomenological three temperature model with a consistent set of materials parameters. We do not observe any systematic deviations between the model and the data that can be caused by a non-thermal distribution of electronic excitations. We attribute the consistently good agreement between the model and the data to strong electron-electron interaction in Pt. By using Pt/Co/Pt and Pt/Co/Pt/Ru structures, we determine the electron-phonon coupling parameters of Pt and Ru, g_ep(Pt)=(6+/-1)x10^17 W m-3 K-1 and g_ep(Ru)=(9+/-2)x10^17 W m-3 K-1. We also find that the length scales of non-equilibrium between electrons and phonons are l_ep=({\Lambda}_e/g_ep)^1/2 =9 nm for Pt and 7 nm for Ru, shorter than their optical absorption depths, 11 and 13 nm, respectively. Therefore, the optically thick Pt and Ru layers show two steps of temperature rise: The initial jump of electron temperature that occurs within 1 ps is caused by direct optical excitation and electronic heat transport within a distance l_ep for the Co layer. The second temperature rise is caused by heat transport by electrons and phonons that are near thermal equilibrium. We contrast two-temperature modeling of heat transport in Pt an Ru films to calculations for Cu, which has a much longer non-equilibrium length scale, l_ep=63 nm.

Probing the electron–phonon interaction in superconductivity for KSn2 using the Migdal-Eliashberg theory and linear-response theory

ABSTRACTWe have studied the structural and electronic properties of potassium distannide KSn crystallizing in the MgZn-type (C14) Laves structure by employing the generalised gradient approximation of the density functional theory and the plane wave ab initio pseudopotential method. It is found that the bonding character in this system is a mixture of covalent, ionic and metallic bonds. The stress–strain method calculations confirm the mechanical stability, and the phonon dispersion results with real frequencies reveal the dynamical stability of the structure. The electron–phonon interaction has been investigated by using the linear response approach and the Migdal-Eliashbergh approach. The integration of the Eliashbergh spectral function gives the value of average electron-phonon coupling parameter to be of the weak strength 0.637. The superconducting temperature is obtained to be 2.7 K, which is in good accordance with its measured value of 3.2 K.

Electron-phonon coupling and hot electron thermalization in titanium nitride

We have studied the thermalization of hot carriers in both pristine and defective titanium nitride (TiN) using a two-temperature model. All parameters of this model, including the electron-phonon coupling parameter, were obtained from first-principles density-functional theory calculations. The virtual-crystal approximation was used to describe defective systems. We find that thermalization of hot carriers occurs on much faster timescales than in gold as a consequence of the significantly stronger electron-phonon coupling in TiN. Specifically, the largest thermalization times, on the order of 200 femtoseconds, are found in TiN with nitrogen vacancies for electron temperatures around 4000 K.

Electron-phonon coupling induced defect recovery and strain relaxation in Ni and equiatomic NiFe alloy

Abstract We study the effects of local heating due to electron-phonon coupling in cumulative 10 MeV Ni ion irradiation on pre-damaged pure Ni and equiatomic NiFe alloy. In particular, we estimate the defect recovery in NiFe compared to pure Ni due to the chemical disorder. The simulations are done using the two-temperature molecular dynamics approach that includes the electronic stopping and electron-phonon interactions. A total of 50% of the damage that consists of isolated point defects and small defect clusters is recovered in NiFe, whereas no noticeable recovery is observed in elemental Ni. We have also demonstrated strain relaxation associated with defect recombination based on the results from both simulation and X-ray diffraction measurements in NiFe. The damage recovery primarily depends on pre-existing damage level, defect configuration, ion energy (stopping powers) and electron-phonon coupling parameter of the materials of interest.

Exploration of the thermal shift parameters for the six luminescence peaks in Nd3+-doped YAlO3

In this paper, we analysis the thermal shifts of six luminescence peaks (R1 → Y2,3,5, R1 → Z3, R2 → Y1, R2 → Z3) in Nd3+-doped YAlO3 crystal with a full expression including both the static effect (characterized by the static parameter A) from lattice thermal expansion and the dynamic effect (characterized by the electron-phonon coupling parameter α′) originating from the electron-phonon interaction. The static effect is evaluated from the pressure shift of the spectral peak. The two thermal shift parameters A and α′ for these peaks are attained by means of an approximate treatment on the full expression. The results indicate that differing from the previous opinion, even in the ‘hard’ crystal YAlO3:Nd3+, the static effects to thermal shifts of most peaks are important and cannot be ignored. The estimated thermal shift parameters A and α′ for the spectral peaks of YAlO3:Nd3+ in order of magnitude are the same as those of various 4fn ions in crystals found previously and can be seen as reasonable. The cause of negative static parameters A (which were not reported previously) for some peaks is discussed.

Investigating the normal state and superconducting state properties of orthorhombic and hexagonal ZrRuP: A first-principles study

We have executed ab initio pseudopotential calculations on the structural, electronic, elastic, mechanical, vibrational, and electron-phonon interaction properties of hexagonal ZrRuP (h-ZrRuP) and orthorhombic ZrRuP (o-ZrRuP). The electronic states of both phases near the Fermi energy are found to be dominated by the $d$ electrons of transition metal atoms, suggesting that they play a more active role in the generation of superconducting state for both phases of ZrRuP. A critical assessment of their elastic and mechanical properties reveals that the lattice of h-ZrRuP is softer than that of o-ZrRuP. A comparison of phonon dispersion curves for both phases indicates that the lower transverse acoustic branch of h-ZrRuP is much softer than that of o-ZrRuP. The soft character of this phonon branch gives rise to strong electron-phonon interaction in h-ZrRuP. Therefore the electron-phonon coupling parameter for h-ZrRuP equals to 1.25 which is considerably larger than the corresponding value of 0.57 for o-ZrRuP. As a consequence, phonon and electron-phonon interaction properties are crucial in making superconducting transition temperature much higher for h-ZrRuP than o-ZrRuP. At the end, the value of this temperature is found to be 12.49 K for h-ZrRuP and 3.89 K for o-ZrRuP which coincide with their experimental values of 12.93 and 3.82 K.

Electronic structure, phonon and superconductivity for WP 5d-transition metal

We have reported the electronic structure, elastic, mechanical, phononic, and superconductivity properties for the orthorhombic WP single crystal, which has very recently been discovered to be the first superconductor among 5d-transition metal pnictides using density functional theory. The calculated electronic band structure and density of states reveal that WP is semimetallic in nature and the bands are mainly strengthened by the d-orbital of W atoms as well as determined several semi-Dirac-like points near the Fermi level. Elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and the brittle/ductile nature were determined. Some of these values were found to be compatible with other theoretical values that we found. The phonon spectrum shows that the orthorhombic MnP-type WP structure is dynamically stable. The calculated Debye temperature is comparable to the fitting experimental measurement of the normal state resistivity into the Bloch–Grüneisen function. The electron-phonon coupling parameter shows that WP is weakly coupled. We have also calculated the critical temperature (Tc) value of ∼0.81 K, which matches the experimental estimated value using electrical resistance, ac magnetic sensitivity, and specific temperature measurements.

Pressure effects on the electronic structure and superconductivity of(TaNb)0.67(HfZrTi)0.33high entropy alloy

Effects of pressure on the electronic structure, electron-phonon interaction, and superconductivity of the high entropy alloy (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa. The electronic structure is calculated using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Effects of pressure on the lattice dynamics are simulated using the Debye-Gr\"{u}neisen model and the Gr\"{u}neisen parameter at ambient conditions. In addition, the Debye temperature and Sommerfeld electronic heat capacity coefficient were experimentally determined. The electron-phonon coupling parameter $\lambda$ is calculated using the McMillan-Hopfield parameters and computed within the rigid muffin tin approximation. We find, that the system undergoes the Lifshitz transition, as one of the bands crosses the Fermi level at elevated pressures. The electron-phonon coupling parameter $\lambda$ decreases above 10 GPa. The calculated superconducting $T_c$ increases up to 40 - 50 GPa and, later, is stabilized at the larger value than for the ambient conditions, in agreement with the experimental findings. Our results show that the experimentally observed evolution of $T_c$ with pressure in (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ can be well explained by the classical electron-phonon mechanism.

Electronic heat transport versus atomic heating in irradiated short metallic nanowires

The two-temperature model (TTM) is commonly used to represent the energy exchange between atoms and electrons in materials under irradiation. In this work we use the TTM coupled to molecular dynamics (TTM-MD) to study swift heavy ion irradiation of Au and W finite nanowires. While no permanent structural modifications are observed in bulk, nanowires behave in a different way depending on thermal conductivity and the electron-phonon coupling parameter. Au is a good heat conductor and it does not transfer energy from electrons to phonons too efficiently. Therefore, energy is quickly carried away from the track so that both electronic and lattice temperatures remain quite uniform across the sample at all times. W has a lower thermal conductivity and a larger electron-phonon coupling, thus supporting an inhomogeneous lattice temperature profile with temperatures well above melting lasting several picoseconds in the irradiated region. Both W and Au nanowires display radiation-induced surface roughening. However, in the case of W there is also sputtering and the formation of a hole in the central part of the wire, purely due to the energy transferred to the atoms by the electrons. The physical mechanisms underlying these findings are rationalized in terms of a combination of sputtering, vacancy formation, and melt flow phenomena. The role of the electron-phonon coupling parameter $g$ is analyzed.

Insight into the thermal shifts of ten fluorescence peaks Y1–Y10 for Sm3+ ions in Y3Al5O12 crystal

The observed linear thermal shifts of ten fluorescence peaks Y1–Y10 from room temperature to about 1000 K for Y3Al5O12:Sm3+ (grown by the Czochralski method) were investigated by considering both the static effect owing to lattice thermal expansion and the dynamic effect stemming from electron–phonon interaction. The static effects were acquired by using the pressure dependences of these fluorescence peak positions. The static effects for the ten fluorescence peaks resulted in thermal blue shift, but the dynamic effects for all peaks except Y7 led to thermal red shift. The observed nature of thermal shift (i.e. red or blue) of a fluorescence peak was due to competition between the static and dynamic effects. Based on an approximate treatment, the static parameter A (characterizing the static effect) and electron–phonon coupling parameter αʹ (characterizing the dynamic effect) for these fluorescence peaks were estimated in the case where thermal shift curves from low temperature to near or higher than room temperature were not measured. Parameters A for all fluorescence peaks and αʹ for all peaks except Y7 were comparable in sign and magnitude with those of the sharp luminescence peaks of other rare earth ions in crystals and so were deemed to be reasonable. The causes of the exception for αʹ of Y7 spectral peak (including the very large observed thermal blue shift of this peak) remain unclear.

Electronic structure and superconductivity in hexagonal Li3B2 and Li2B2H phases under pressure

The relatively low-pressure layered hexagonal Li3B2 (h-Li3B2) phase under 20 GPa was identified on the basis of ab initio calculations. Inspired by the high-temperature superconductivity of metallic hydrogen, this study obtained a novel hexagonal Li2B2H (h-Li2B2H) structure by replacing Li2 with H. Contrast to h-Li3B2, h-Li2B2H may be directly synthesized by α-LiB and H2 under pressure. The two phases exhibited electronic features similar to those in MgB2 and were expected to be good superconductors. Electron–phonon coupling calculations illustrated that the layered h-Li3B2 structure is superconducting at 2 K with λ = 0.39 under 20 GPa. Moreover, h-Li2B2H was predicted to possess a large electron–phonon coupling parameter λ of approximately 0.81 and a resulting superconducting critical temperature about 28 K at 20 GPa. The introduced H in the h-Li2B2H structure had strong vibrational energy due to its light atomic mass and could enhance the energy of σ and π bonds by bonding with Li. Thus, the introduced H in the h-Li2B2H structure is important due to its enhanced electron–phonon coupling and superconducting property.

The effect of strong electron-rattling phonon coupling on some superconducting properties

Using the Eliashberg theory of superconductivity we have examined several properties of a model in which electrons are coupled only to rattling phonon modes represented by a sharp peak in the electron–phonon coupling function. Our choice of parameters was guided by experiments on β-pyrochlore oxide superconductor KOs2Os6. We have calculated the temperature dependence of the superconducting gap edge; the quasi-particle decay rate; the NMR relaxation rate assuming that the coupling between the nuclear spins and the conduction electrons is via a contact hyperfine interaction, which would be appropriate for the O-site in KOs2Os6; and the microwave conductivity. We examined the limit of very strong coupling by considering three values of the electron–phonon coupling parameter λ = 2.38, 3, and 5 and did not assume that the rattler frequency Ω0 is temperature dependent in the superconducting state. We obtained a very unusual temperature dependence of the superconducting gap edge Δ(T), very much like the one extracted from photoemission experiments on KOs2O6.