A. Subedi, D. J. Singh, M. H. Du
We report first principles calculations of the electronic structure, phonon dispersions and electron phonon coupling of LaNiPO. These calculations show that this material can be explained as a conventional electron phonon superconductor in contrast to the FeAs based high temperature superconductors.
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In this chapter we review the essentials of conventional electron-phonon superconductivity, as provided through the Migdal-Eliashberg formalism. Signatures of the electron-phonon interaction, as they appear in conventional superconductors, are described in detail. An appendix briefly describes more recent attempts to understand the electron-phonon interaction for all coupling strengths.
Breaking down the traditional perception on phonons which are achiral, the recent discovery of chiral phonon carrying angular momentum provides possible ways to couple electron, photon, spin, magnon and exciton, etc. We theoretically proposed an electron-chiral phonon interaction with two-phonon process, in contrast to conventional electron-phonon interaction, and a kind of effective Hubbard U through exchanging two chiral phonons is proposed. Taking two-dimensional diatomic honeycomb lattice as an example, we found this repulsive Hubbard U mediated by chiral phonons induces unconventional and…
I present the results of first principles calculations of the phonon dispersions and electron-phonon coupling for BaTi$_2$Sb$_2$O. The phonon dispersions show a weak lattice instability near the zone corners that leads to a charge-density wave phase. The calculations of the electron-phonon coupling reveal strong coupling, especially to the in-plane Ti modes. The total coupling is large enough to readily explain the superconductivity in this compound. As the Fermi surfaces are disconnected with different orbital character weights, this compound is likely to host a multiband superconductivity.
To study the possible mechanism of superconductivity in Na$_{0.35}$CoO$_2$, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and $t_{2g}(a_{1g}+e_g')$-electrons of Co-ions, and study the transition temperature for a s-wave superconductivity. The obtained $T_{\rm c}$ is very low when the $e_g'$-valence-bands are far below the Fermi level. However, $T_{\rm c}$ is strongly enhanced when the top of the $e_g'$-valence-bands is close to the Fermi level (say -50meV), thanks to interband hopping of Cooper pairs caused by shear phonons. This ``valence-ba…
Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be…
We investigate the influence of a d-density wave (DDW) gap on the superconductivity-induced renormalization of phonon frequency and linewidth. The results are discussed with respect to Raman and inelastic neutron scattering experiments. It turns out that the DDW gap can enhance the range of frequencies for $q=0$ phonon softening depending on the underlying band structure. Moreover we show that an anisotropic 'd-wave' pseudogap can also contribute to the q-dependent linewidth broadening of the 340cm$^{-1}$ phonon in YBa$_2$Cu$_3$O$_7$.
We predict that electron-doped silicene is a good two-dimensional electron-phonon superconductor under biaxial tensile strain by first-principles calculations within rigid band approximation. Superconductivity transition temperature of electron-doped silicene can be increased to be above 10 K by 5% tensile strain. Band structures, phonon dispersive relations, and Eliashberg functions are calculated for detailed analysis. The strong interaction between acoustic phonon modes normal to the silicene plane and the increasing electronic states around the Fermi level induced by tensile strain is main…
K$_3$-picene is a superconducting molecular crystal with critical temperature T$_c=7$K or 18K, depending on preparation conditions. Using density functional theory we show that electron-phonon interaction accounts for T$_c ~ 3-8$ K. The average electron-phonon coupling, calculated by including the phonon energy scale in the electron-phonon scattering, is $λ=0.73$ and $ω_{\rm log}=18.0\,$ meV. Intercalant and intermolecular phonon-modes contribute substantially (40%) to $λ$ as also shown by the isotope exponents of potassium (0.19) and carbon (0.31). The relevance of these modes makes supercond…
Large impact of phonon lineshapes on the superconductivity of solid hydrogen
本研究探讨了非洛伦兹声子线型对高压固态氢超导性的影响，通过从头计算完整声子谱函数发现声子软化与线展宽对超导临界温度T_C分别产生增强和降低作用，总体提升了T_C估计值，并揭示了450-500 GPa下氢分子相VI的超导性，解释了理论与实验的分歧。
We investigate the role of specific phonon mode symmetries for the room temperature superconductivity in atomic hydrogen under large pressure. Using anisotropic Migdal-Eliashberg theory with ab initio input from density functional theory, we show that the $E_u$ phonon modes are the dominant driving force for obtaining such high critical temperatures. When going from 400 to 600 GPa, we find an increased transition temperature, however, the total electron-phonon coupling strength is counterintuitively reduced. Our analysis reveals that this is due to an enhanced contribution to the coupling stre…
The out-of-plane intercalate phonons of superconducting YbC6 have been measured with inelastic x-ray scattering. Model fits to this data, and previously measured out-of-plane intercalate phonons in graphite intercalation compounds (GICs), reveal surprising trends with the superconducting transition temperature. These trends suggest that superconducting GICs should be viewed as electron-doped graphite.
We argue that Raman scattering in $B_{1g}$ symmetry allows one to distinguish between phonon-mediated and magnetically-mediated $d-$wave superconductivity. In spin mediated superconductors, $B_{1g}$ Raman intensity develops a resonance at a frequency $ω_{res} < 2Δ_{max}$, whose origin is similar to a neutron resonance. In phonon-mediated $d-$wave superconductors, such a resonance does not develop. Several extensions of the argument are presented.
We observe a characteristic energy $\hbarω_{c}\approx 2.1$ eV which separates regions of different behavior of the phonon intensities in the Raman spectra of the $R\rm Ba_2Cu_3O_7$ system. A superconductivity-induced drop of phonon intensities is found for the oxygen modes O(4) and O(2)-O(3) only for excitation energies below $\hbarω_c$. This intensity drop indicates an order parameter which affects energies in the vicinity of $\hbarω_c$.
In strontium titanate, the Froehlich electron - LO-phonon interaction dominates the electron response and can also provide superconductivity. Because of high LO-phonon frequencies in SrTiO3, the superconducting system is non-adiabatic. We demonstrate that the dielectric function approach is an adequate theoretical method for superconductivity in SrTiO3 and on the SrTiO3-LaAlO3 interface. The critical temperatures are calculated using realistic material parameters. The obtained critical temperatures are in line with experimental data both for bulk and interface superconductivity. The present me…
Electron pairing by dispersive phonons in altermagnets: re-entrant superconductivity and continuous transition to finite momentum superconducting state
研究交错磁体中色散声子介导的电子配对，发现热声子散射抑制交错磁性，导致超导电性随温度重入，并在低温稳定 FFLO 有限动量超导态，无需 d 波配对。
We report a detailed study of specific heat, electrical resistivity and thermal expansion in combination with inelastic neutron and inelastic X-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 K and 1.5 K respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function and phonon anharmonicity from these…
The effective attractive interaction between electrons, mediated by electron-phonon coupling, is a well-established mechanism of conventional superconductivity. In metals exhibiting a Fermi surface, the critical temperature of superconductivity is exponentially smaller than the characteristic phonon energy. Therefore such superconductors are found only at temperatures below a few Kelvin. Systems with flat energy bands have been suggested to cure the problem and provide a route to room-temperature superconductivity, but previous studies are limited to only BCS models with an effective attractiv…
Theory of superconductivity generated in one atomic layer thick two dimensional electron gas by a single flat band of high energy longitudinal optical phonons is considered. The polar dielectric $SrTiO_{3}$ (STO) exhibits such an energetic phonon mode and the 2DEG is created both when one unit cell $FeSe$ layer is grown on its $\left( 100\right) $ surface and on the interface with another dielectric like $LaAlO_{3}$ (LAO). We obtain a quantitative description of both systems solving the gap equation for $T_{c}$ without making use of approximations like the Kirzhnits Ansatz for arbitrary chemic…
Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.1Bi2Se3, a prime candidate for realizing topological superconductivity by doping the topological insulator Bi2Se3, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only …
Eliashberg's foundational theory of superconductivity is based on the application of Migdal's approximation, which states that vertex corrections to first order electron-phonon scattering are negligible if the ratio between phonon and electron energy scales is small. The resulting theory incorporates the first Feynman diagrams for electron and phonon self-energies. However, the latter is most commonly neglected in numerical analyses. Here we provide an extensive study of full-bandwidth Eliashberg theory in two and three dimensions, where we include the full back reaction of electrons onto the …
We suggest that the high-temperature superconductivity is attributed to the director-roles of van Hove singularity between electron-electron interaction and electron-phonon interaction. Difference between the critical temperature and pairing temperature is presented, and the Fermi arc, d-wave symmetry and poor conductor etc are discussed. Particularly, non-s-wave symmetry is predicted to have the highest Tc for superconductors.
Drosophila of phonon-mediated superconductivity: Full Eliashberg theory of the jellium model
该研究针对声子介导超导的均匀电子气模型给出Migdal-Eliashberg方程完整数值解，发现临界温度很低，低于1K，与早期观点不同，强调估计临界温度时要考虑能隙函数和正常自能的动量及频率依赖性。
Density functional theory (DFT) based ab-initio calculations of electronic, phononic, and superconducting properties of 1T MoS2 are reported. The phonon dispersions are computed within density-functional-perturbation-theory (DFPT). We have also computed Eliashberg function alpha2Fomega and electron-phonon coupling constant lambda from the same. The superconducting transition temperature (Tc) computed within the McMillan-Allen-Dynes formula is found in good agreement with the recent experimental report. We have also evaluated the effect of pressure on the superconducting behavior of this system…
"Quantum bipolaron" superconductivity from quadratic electron-phonon coupling
本研究探索二次电子-声子耦合下的量子双极化子超导机制，发现强耦合区临界温度仅轻微抑制、最优值更高，验证该耦合可在钙钛矿中实现并提出超晶格实现方案。
Emergence of superconductivity in a two-band system coupled with breathing and Jahn-Teller phonons is discussed in a weak-coupling limit. With the use of a standard quantum mechanical procedure, the phonon-mediated attraction is derived. From the analysis of the model including such attraction, a BCS-like formula for a superconducting transition temperature $T_{\rm c}$ is obtained. When only the breathing phonon is considered, $T_{\rm c}$ is the same as that of the one-band model. On the other hand, when Jahn-Teller phonons are active, $T_{\rm c}$ is significantly enhanced by the interband att…
We predict a strong enhancement of the critical temperature in a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor in the presence of a bosonic condensate of exciton-polaritons. The effect depends strongly on the ratio of the cutoff frequencies for phonon and exciton-polariton mediated BCS superconductivity, respectively. We also discuss a possible design of hybrid semiconductor-superconductor structures suitable for the experimental observation of such an effect.
Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB$_2$ are presented. The $E_{2g}$ phonons, which involve in-plane B displacements, couple strongly to the $p_{x,y}$ electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting $λ$ in the clean limit is calculated to be significantly larger. The $E_{2g}$ phonons are strongly anharmonic, and the non-linear contribution to the coupling between the $E_{2g}$ modes and the p$_{x,y}$ bands is signi…
Superconductivity-induced changes of phonon lineshapes in niobium have been re-investigated by high-resolution inelastic neutron scattering. We show that the changes go beyond a simple change in lifetime and frequency when the phonon frequency is close to the superconducting energy gap $2Δ$. The observed lineshapes in elemental niobium are qualitatively similar to those found previously in borocarbide superconductors and agree very well with those predicted by the theory of Allen et al. [PRB 56, 5552 (1997)]. Our results indicate that the peculiar phonon lineshapes in the superconducting state…
Inelastic x-ray scattering and $ab$-$initio$ calculation are applied to investigate the lattice dynamics and electron-phonon coupling of the ternary silicide superconductor CaAlSi ($P/bar{6}m2$). A soft c-axis polarized mode is clearly observed along the $/Gamma$-$A$-$L$ symmetry directions. The soft mode is strongly anharmonically broadened at room temperature, but, at 10 K, its linewidth narrows and becomes in good agreement with calculations of linear electron-phonon coupling. This establishes a coherent description of the detailed phonon properties in this system and links them clearly and…
Recent experiments have tuned the monolayer 1T'-WTe2 to be superconducting by electrostatic gating. Here, we theoretically study the phonon-mediated superconductivity in monolayer 1T'-WTe2 via charge doping. We reveal that the emergence of soft-mode phonons with specific momentum is crucial to give rise to the superconductivity in electron-doping regime, whereas no such soft-mode phonons and no superconductivity emerge in hole-doping regime. We also find a superconducting dome, which can be attributed to the change of Fermi surface nesting condition as electron doping. By taking into account t…
Effect of tetragonal distortion on the electronic structure, dynamical properties and superconductivity in Mo$_3$Sb$_7$ is analyzed using first principles electronic structure and phonon calculations. Rigid muffin tin approximation (RMTA) and McMillan formulas are used to calculate the electron-phonon coupling constant $λ$ and superconducting critical temperature. Our results show, that tetragonal distortion has small, but beneficial effect on superconductivity, slightly increasing $λ$, and the conclusion that the electron-phonon mechanism is responsible for the superconductivity in Mo$_3$Sb$_…
We report first principles calculations of the electronic structure and electron-phonon coupling in the non-centrosymmetric superconductor LaNiC$_2$. These show that the material is a conventional electron-phonon superconductor with intermediate coupling. There are large contributions to the coupling by two low frequency C non-bond-stretching modes, one of which has strong Kohn anomalies. Since LaNiC$_2$ lacks inversion symmetry, the pairing is of dominant s-wave type with some mixture of p-wave character. This will give exponential decay in the specific heat.
Superconductivity in 2D systems enhanced by nonadiabatic phonon-production effects
本研究通过动态重整化声子计算Eliashberg函数，发现非绝热声子产生效应可增强二维系统超导电性，且该效应随电子密度和有效质量增大而增强。
The influence of antiadiabatic phonons on the temperature of superconducting transition is considered within Eliashberg - McMillan approach in the model of discrete set of (optical) phonon frequencies. A general expression for superconducting transition temperature $T_c$ is proposed, which is valid in situation, when one (or several) of such phonons becomes antiadiabatic. We study the contribution of such phonons into the Coulomb pseudopotential $μ^{\star}$. It is shown, that antiadiabatic phonons do not contribute to Tolmachev's logarithm and its value is determined by partial contributions f…
Phonon-mediated spin-polarized superconductivity in altermagnets
本文在弱耦合框架下用最小Lieb晶格模型研究交变磁体，发现声子可介导一种动量奇宇称、自旋偶宇称的自旋极化Cooper对超导不稳定性，并归因于费米面自旋结构与有效相互作用的动量结构。
We applied $ab$ $initio$ phonon analysis to layered titanium-oxypnictides, Na$_2$Ti$_2Pn_2$O ($Pn$ = As, Sb), and found a clear contrast between the cases with lighter/heavier pnictogen in comparisons with experiments. The result completely explains the experimental structure, $C$2/$m$ for $Pn$ = As, within the conventional electron-phonon framework, while there arise discrepancies when the pnictogen gets heavier, being in the same trend for the BaTi$_2Pn_2$O ($Pn$ = As, Sb, Bi) case. The fact implies a systematic dependence on pnictogen to tune the mechanism of the phase transition and superc…
We propose a theory of strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular to the atomic volume of Fe defined by the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner crite…
Unconventional superconductivity is commonly linked to electronic pairing mechanisms, since it is believed that the conventional electron-phonon interaction (EPI) cannot cause sign-changing superconducting gap symmetries. Here, we show that this common understanding needs to be revised when one considers a more elaborate theory of electron-phonon superconductivity beyond standard approximations. We selfconsistently solve the full-bandwidth, anisotropic Eliashberg equations including vertex corrections beyond Migdal's approximation assuming the usual isotropic EPI for cuprate, Fe-based and heav…
We study the impact of a time-dependent external driving of the lattice phonons in a minimal model of a BCS superconductor. Upon evaluating the driving-induced vertex corrections of the phonon-mediated electron-electron interaction, we show that parametric phonon driving can be used to elevate the critical temperature $T_c$, while a dipolar phonon drive has no effect. We provide simple analytic expressions for the enhancement factor of $T_c$. Furthermore, a mean-field analysis of a nonlinear phonon-phonon interaction also shows that phonon anharmonicities further amplify $T_c$. Our results hol…
We present a theory of phonon-mediated superconductivity in near magic angle twisted bilayer graphene. Using a microscopic model for phonon coupling to moiré band electrons, we find that phonons generate attractive interactions in both $s$ and $d$ wave pairing channels and that the attraction is strong enough to explain the experimental superconducting transition temperatures. Before including Coulomb repulsion, the $s$-wave channel is more favorable; however, on-site Coulomb repulsion can suppress $s$-wave pairing relative to $d$-wave. The pair amplitude varies spatially with the moiré period…
The specific heat of two polymorphs of BaPd2As2 was measured. The ThCr2Si2-type polymorph (space group I4/mmm, $D_{4h}^{17}$, No. 139) is a previously reported superconductor with a transition temperature Tc $\simeq$ 3.5 K, while the CeMg2Si2-type polymorph (P4/mmm, $D_{4h}^{1}$, No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr2Si2-type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg2Si2-type. Moreover, a large specific-heat jump at Tc indicated that the superconductivity of ThCr2Si2-t…
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c \approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulate…
We investigate the dynamics of a phonon-mediated superconductor driven out of equilibrium. The electronic hopping amplitude is ramped down in time, resulting in an increased electronic density of states. The dynamics of the coupled electron-phonon model is investigated by solving Migdal-Eliashberg equations for the double-time Keldysh Green's functions. The increase of the density of states near the Fermi level leads to an enhancement of superconductivity when the system thermalizes to the new state at the same temperature. We provide a time- and momentum-resolved view on this thermalization p…
I study the lattice dynamics and electron-phonon coupling in non-centrosymmetric quasi-one-dimensional K$_2$Cr$_3$As$_3$ using density functional theory based first principles calculations. The phonon dispersions show stable phonons without any soft-mode behavior. They also exhibit features that point to a strong interaction of K atoms with the lattice. I find that the calculated Eliashberg spectral function shows a large enhancement around 50 cm$^{-1}$. The phonon modes that show large coupling involve in-plane motions of all three species of atoms. The $\mathbf{q}$ dependent electron-phonon …
We have evaluated the total carrier mass enhancement factor f_{t} for MgB_{2} from two independent experiments (specific heat and upper critical field). These experiments consistently show that f_{t} = 3.1\pm0.1. The unusually large f_{t} is incompatible with the measured reduced gap (2Δ(0)/k_{B}T_{c} = 4.1) and the total isotope-effect exponent (α= 0.28\pm0.04) within the conventional phonon-mediated model. We propose an unconventional phonon-mediated mechanism, which is able to quantitatively explain the values of T_{c}, f_{t}, α, and the reduced energy gap in a consistent way.
Computationally efficient method for calculating electron-phonon coupling for high-throughput superconductivity search
本文提出一种基于冻结声子和DFT能带结构的低成本电子-声子耦合计算方法，用于高通量筛选大晶胞材料中的高超导临界温度候选体系，并在MgB$_2$、H$_{3}$S等常规超导体上验证了有效性。
We investigate three dimensional anharmonic phonons in tetrahedral symmetry and superconductivity mediated by these phonons. Three dimensional anharmonic phonon spectra are calculated directly by solving Schrödinger equation and the superconducting transition temperature is determined by using the theory of strong coupling superconductivity assuming an isotropic gap function. With increasing the third order anharmonicity $b$ of the tetrahedral ion potential, we find a crossover in the energy spectrum to a quantum tunneling regime. We obtain strongly enhanced transition temperatures around the …
We calculate the critical temperature $T_c$ and the superconducting energy gaps $Δ_n$ of a thin film superconductor system, where $Δ_n$ is the superconducting energy gap of the $n$-th subband. Since the quantization of both the electron energy and phonon spectrum arises due to dimensional confinement in one direction, the effective electron-electron interaction mediated by the quantized confined phonons is different from that mediated by the bulk phonon, leading to the modification of $T_c$ in the thin film system. We investigate the dependence of $T_c$ and $Δ_n$ on the film thickness $d$ with…
We study the newly discovered Pt phosphides $A$Pt$_3$P ($A$=Sr, Ca, La) [ T. Takayama et al. Phys. Rev. Lett. 108, 237001 (2012)] using first-principles calculations and Migdal-Eliashberg theory. Given the remarkable agreement with the experiment, we exclude the charge-density wave scenario proposed by previous first-principles calculations, and give conclusive answers concerning the superconducting state in these materials. The pairing increases from La to Ca and Sr due to changes in the electron-phonon matrix elements and low-frequency phonons. Although we find that all three compounds are w…
The recently discovered APt$_3$P (A=Sr,Ca,La) family of superconductors offers a platform to study frequency dependent superconducting phenomena as the electron-phonon coupling varies from weak to strong. Here we perform ab initio Eliashberg theory calculations to investigate two such phenomena, the occurrence of dip-hump structures in the tunneling spectra and the magnetic field induced coexistence of even and odd frequency superconductivity in these compounds. By calculating the superfluid density, we make predictions for the occurrence of the paramagnetic Meissner effect as a hallmark of od…
Anisotropic pairing interactions mediated by phonons are examined in layer systems. It is shown that the screening effects become weaker when the layer spacing increases. Then the anisotropic components of the pairing interactions increase with the screening length since the momentum dependence changes. As a result, various types of anisotropic superconductivity occur depending on the parameter region. For example, p-wave superconductivity occurs when the short-range part of Coulomb repulsion is strong and the layer spacing is large. Two kinds of inter-layer pairing may occur when the layer sp…
The dispersion of the in-plane Cu-O bond-stretching LO phonon mode in the high-$T_{C}$ superconducting cuprates shows strong softening with doping near the zone boundary. We suggest that it can be described with a negative electronic dielectric function that results in overscreening of inter-site Coulomb interaction due to phonon-induced charge transfer and vibronic electron-phonon resonance. We propose that such a strong electron-phonon coupling of specific modes can form a basis for the phonon mechanism of high-temperature superconductivity. With the Eliashberg theory using the experimentall…
Phonon-Mediated Superconductivity near Phase Boundaries in Rhombohedral Multilayer Graphene
本研究通过求解耦合自洽Migdal-Eliashberg方程，结合第一性原理计算与电子基态对称性破缺，重现菱面体多层石墨烯的超导区域并预测新相，证实其超导可由声子介导配对解释。
We study the depairing effect due to Coulomb interactions in Na$_x$CoO$_2$. We consider the electron-phonon coupling and the Coulomb interactions, and determine $T_c$ for s-wave superconductivity by solving the linearized Eliashberg equation. When we consider shear phonons as well as breathing phonons, $T_c$ is enhanced by Suhl-Kondo (SK) mechanism. Since SK mechanism is strong against Coulomb interactions, $T_c$ remains finite even if strong Coulomb interactions cancel out the attractive force due to breathing phonons. The orbital degree of freedom is important to understand the mechanism of …
We report $^{11}$B-NMR study on Al-doped MgB$_2$ that addresses a possible mechanism for a high superconducting (SC) transition temperature ($T_c$) of $\sim 40$ K in recently discovered MgB$_2$. The result of nuclear spin lattice relaxation rate $1/T_1$ in the SC state revealed that the size in the SC gap is not changed by substituting Al for Mg. The reduction on $T_c$ by Al-doping is shown to be due to the decrease of $N(E_F)$. According to the McMillan equation, the experimental relation between $T_c$ and the relative change in $N(E_F)$ allowed us to estimate a characteristic phonon frequenc…
We propose that screened pairing interactions mediated by phonons could give rise to a spin triplet superconductivity in ferromagnetic compounds such as UGe_2. It is pointed out that the pairing interactions include anisotropic components such as those of p, d, f-waves in addition to dominant s-wave component due to the momentum dependence. Since the ferromagnetic long-range order coexists, there is a large splitting of the Fermi surfaces of up and down spin electrons, which suppresses singlet pairing. Therefore, triplet pairing occurs at last due to the sub-dominant anisotropic interactions, …
We investigate the spin-triplet superconductivity mediated by phonons in quasi-one-dimensional (Q1D) systems with open Fermi surfaces. We obtain the ground state phase diagrams. It is found that spin-triplet superconductivity occurs for weak screening and strong on-site Coulomb interaction, even in the absence of any additional nonphonon pairing interactions. We find that the nodeless spin-triplet state is more favorable than the spin-triplet state with line nodes, for the parameter values of the Q1D superconductors (TMTSF)_2X. We also find that Q1D open Fermi surface, which is the specific fe…
The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K phonon is ~0.25 in the single-layer FeSe on SrTiO3. With increasing …
Using a microscopic model of lattice vibrations in the STO(001) substrate, an additional 50 mev longitudinal optical (LO) interface mode is identified. The soft mode propagating mainly in the first TiO2 layer ("O chains") has stronger electron - phonon coupling to electron gas in FeSe than a well known 100 mev hard mode. The coupling constant, critical temperature, replica band are calculated. Although there exists a forward in the electron - phonon scattering peak, it is clearly not as sharp as assumed in recent theories (delta function - like). The satellite is broad and its peak appears at …
We present neutron scattering measurements on low-energy phonons from a superconducting ($T_{c} = 2.7$~K) Sn$_{0.8}$In$_{0.2}$Te single crystal sample. The longitudinal acoustic phonon mode and one transverse acoustic branch have been mapped out around the (002) Bragg peak for temperatures of 1.7 K and 4.2 K. We observe a substantial energy width of the transverse phonons at energies comparable to twice the superconducting gap; however, there is no change in this width between the superconducting and normal states. We also confirm that the compound is well ordered, with no indications of struc…
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