Anisotropic electron-phonon scattering in metals

A previously published general theory for the effects of electron-phonon scattering in metals involves all relevant anisotropies and permits a virtually exact solution of the Boltzmann equation. The electron-phonon interaction is represented by a few Wannier matrix elements that are adjusted to experimental data. Calculations of the electron lifetime and the mass enhancement in copper show good average agreement with the experiments. For reproducing the exact anisotropies further non-diagonal elements have to be considered. The calculated temperature dependence of the Hall effect agrees qualitatively with the observations and reveals the local maximum found for both pure copper and dilute alloys.     

Ultrasonic attenuation in copper at low temperatures

Ultrasonic attenuation in copper at low temperatures has been analysed in terms of electron-electron and electron-phonon interactions. The contribution due to electron-electron interactions has been evaluated using a simplified spherical Fermi-surface model with an isotropic Umklapp transition probability. We observe that the contribution to the attenuation constant (α) due to electron-electron scattering in copper at 3 K is about 10% of the electron-phonon contribution. We have also considered the viscous attenuation and the induction attenuation mechanisms simultaneously in copper as suggested by Orlov and find that our results improve the existing theoretical data considerably.     

Electron-phonon interaction in the quantum well state of the 1 ML Na/Cu(111) system

The electron-phonon interaction in the quantum well state formed by a Na monolayer coating on Cu(111) is investigated theoretically. The calculations show that the electron-phonon coupling constant γ in this state decreases insignificantly (≈1%) compared to the value of γ for a clean copper surface. The corresponding electron-phonon contribution to the lifetime τ of the quantum well state increases by a factor of 1.5 compared to τ for the clean Cu(111) surface.     

Diagrammatic weak-coupling expansion for the magneto-polaron energy

The problem of a Pekar-Fröhlich polaron in a magnetic field is considered for weak electron-phonon interaction. A diagrammatic technique for zero temperature is developed. Previously obtained results for two dimensions are reproduced. For three dimensions the polaron ground-state energy and the longitudinal effective mass are derived in the first orders of the electron-phonon coupling constant for non-zero values of a magnetic field. In a strong magnetic field the bulk polaron is shown to be equivalent to a one-dimensional polaron with a renormalized electron-phonon coupling constant. This leads to exact results in the strong-coupling limit.     

Theory of impurity scattering: Interstitial impurities

A framework for studying impurity scattering in dilute, non-magnetic, metal alloys can be developed from a knowledge of the exact electronic eigenstates of a single impurity in an otherwise perfect lattice of muffin tin potentials. Such an approach has been developed for systems in which the impurity occupies a substitutional site of the lattice, as will be discussed by Coleridge, Lee, Harris, and other speakers of this conference. In this paper, motivated by recent experimental studies of Dingle temperature anisotropies induced by hydrogen impurities in copper, we will discuss the analogous treatment of scattering by interstitial impurities. In contrast to a substitutional impurity, an interstitial impurity introduces an additional scattering site into the lattice. Whereas the substitutional impurity wavefunctions can be described in terms of the same structure factors as can the Bloch wavefunctions for the pure host lattice, the interstitial impurity wavefunctions depend upon additional structure factors appropriate to the new scattering geometry. These additional structure factors appear in the transition matrix for impurity-induced scattering between Bloch states of the host lattice, and consequently in the weight factors involved in a partial wave analysis of the Dingle temperature anisotropies.     

Point-contact spectroscopy of electron-phonon interaction in alloys

The point contact spectroscopy is used for the investigation of the electron-phonon interation in alloys. The possibilities offered by the method are studied for the case when the impurity electron mean free path is comparable with or much smaller than the point contact diameter. The changes of the electron-phonon interaction spectra of CuNi and CuFe alloys caused by those in impurity concentrations are studied and compared with the PC spectra of pure copper.     

W(110)基底上的铁纳米岛初始自发磁化态的微磁学模拟

用微磁学模拟研究W(110)基底上铁纳米岛的初始自发磁化态的磁畴结构,确定了不规则形状、椭圆形和矩形岛中不同磁畴态之间的各向异性常数的临界点,得到了纳米岛的磁化态作为各向异性常数和厚度函数的完整相图,相图中存在一较宽的过渡区,把双畴态与涡旋态和菱形态分开,过渡区两侧的边界是不确定的.计算结果表明,初始自发磁化态的磁畴结构主要由各向异性及岛的厚度决定,而且岛的边沿形状对涡旋态和菱形态的磁畴结构有重要影响.准确的铁纳米岛的各向异性常数仍有待于进一步确定. 关键词： 初始自发磁化磁畴结构 铁纳米岛 微磁学模拟 各向异性     

Theory of impurity scattering: Interstitial impurities

A framework for studying impurity scattering in dilute, non-magnetic, metal alloys can be developed from a knowledge of the exact electronic eigenstates of a single impurity in an otherwise perfect lattice of muffin tin potentials. Such an approach has been developed for systems in which the impurity occupies a substitutional site of the lattice, as will be discussed by Coleridge, Lee, Harris, and other speakers of this conference. In this paper, motivated by recent experimental studies of Dingle temperature anisotropies induced by hydrogen impurities in copper, we will discuss the analogous treatment of scattering by interstitial impurities. In contrast to a substitutional impurity, an interstitial impurity introduces an additional scattering site into the lattice. Whereas the substitutional impurity wavefunctions can be described in terms of the same structure factors as can the Bloch wavefunctions for the pure host lattice, the interstitial impurity wavefunctions depend upon additional structure factors appropriate to the new scattering geometry. These additional structure factors appear in the transition matrix for impurity-induced scattering between Bloch states of the host lattice, and consequently in the weight factors involved in a partial wave analysis of the Dingle temperature anisotropies.This work was supported by the National Science Foundation through the Materials Research Laboratory at the University of Chicago.     

Temperature dependence of the angle resolved photoemission spectra in the undoped cuprates: self-consistent approach to the t-J Holstein model

We develop a novel self-consistent approach for studying the angle resolved photoemission spectra (ARPES) of a hole in the t-J Holstein model giving perfect agreement with numerically exact diagrammatic Monte Carlo (DMC) data at zero temperature for all regimes of electron-phonon coupling. Generalizing the approach to finite temperatures, we find that the anomalous temperature dependence of the ARPES in undoped cuprates is explained by cooperative interplay of coupling of the hole to magnetic fluctuations and strong electron-phonon interaction.     

铜薄膜电子-声子耦合系数的实验研究

电子元件的超微型化和高集成化,对金属互连线的导热性能提出极高的要求;同时,新型高性能光电材料的开发,又需要尽可能降低受光激发电子与声子之间的耦合。电子、声子的相互作用,对于能量转换与传递过程微观机理的研究至关重要。这些载能粒子的作用时间都在飞秒-皮秒量级,本文采用飞秒激光瞬态热反射方法,对铜薄膜中微观粒子作用过程进行了飞秒量级的实验研究。通过对实验系统精细调节,得到了理想的信号曲线,在此基础上测量了不同泵浦光功率下沉积在硅基底上厚度为60 nm铜薄膜的电子-声子耦合系数,测量结果为(11～12)×10~(16)W/(m~3K),和理论值14×10~(16)W/(m~3K)较为接近,并且不随泵浦光强发生变化。     

Non-conservation of k in photoemission into electrolytes

Recent measurements of the photoemission yield from copper single crystals into electrolyte revealed unexplained polarization and crystallographic anisotropies. It is shown that these anisotropies indicate a breakdown of parallel momentum conservation at the interface due to scattering by water molecules. The similarity between the experimental results above and below the interband transition threshold suggests the involvement of evanescent states in photoemission below threshold.     

Superconductivity in monolayer Pb on Si(111) from first principles

Observation of superconductivity in a single layer of Pb on the (111) surface of bulk silicon has renewed interest in a longstanding question; can superconductivity persist to the ultimate atomic limit? Using first-principles techniques, we investigate the total electron-phonon coupling in monolayer Pb supported by a Si(111) substrate. Our ultra-fine sampling of the electronic structure, lattice dynamics and electron-phonon matrix elements in the nearly two-dimensional Brillouin zone yields a total electron-phonon coupling parameter which explains the experimentally observed superconducting transition temperature of 1.83 K [T. Zhang, et al., Nat. Phys. 6 (2010) 104]. The observed suppression of the superconducting transition temperature from the bulk value of 7.2 K is found to arise from the interplay of reduced electron-phonon matrix elements and a modification of the lattice dynamics resulting from the Pb-Si bonding.     

On the Fröhlich charge density wave and Peierls transition

The exact solution of the system of nonlinear equations for Fröhlich charge density wave (CDW) in the continuum approximation is discussed, and the temperature behaviour of the CDW condensate is investigated in a framework of the mean-field approximation. It is shown that the CDW representation in the form of a conventional harmonic wave is only correct under certain conditions, namely, not too strong an electron-phonon interaction or not too low an electron density. According to the exact solution, the CDW is essentially a nonlinear wave represented by a harmonic series. This is confirmed by the experimental data on the CDW current oscillation spectra in which the overtones are present together with the principal harmonic (the so called “narrow band noise”). At strong enough electron-phonon coupling or at low enough electron density the CDW wave vector is temperature dependent, and the BCS relation between Peierls transition temperature and energetic gap in the electron spectra at zero temperature is violated.     

Ab initio method for calculating electron-phonon scattering times in semiconductors: application to GaAs and GaP

We propose a fully ab initio approach to calculate electron-phonon scattering times for excited electrons interacting with short-wavelength (intervalley) phonons in semiconductors. Our approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states with no ad hoc assumptions. We apply it to the deexcitation of hot electrons in GaAs, and calculate the lifetime of the direct exciton in GaP, both in excellent agreement with experiments. Matrix elements of the electron-phonon coupling, and their dependence on the wave vector of the final state and on the phonon modes, are shown to be crucial ingredients of the evaluation of electron-phonon scattering times.     

Phonon effects in tunnelling through a double quantum dot molecule

Phonon effects in tunnelling through a double quantum dot molecule are investigated by use of a recently developed technique, which is based on an exact mapping of a many-body electron-phonon interaction problem onto a multichannel one-body problem. The molecule is sandwiched between two ideal electrodes and the electron at each dot of the molecule interacts independently with Einstein phonons. Single-electron transmission rates through the molecule are computed and the nonlinear spectrum obtained shows a structure with many more satellite peaks due to the excitations of phonons. The strength of resonant peaks is found to be strongly dependent on the number of excited phonons. The effects of electron-phonon interaction on the current and shot noise, depending on the voltage bias applied at the two electrodes as well as the potential energy of the molecule, are discussed.     

Polaron effect and the dc phonon-assisted hopping conductivity in amorphous semiconductors

A new small parameterW of off-diagonal electron-phonon matrix elements with respect to diagonal ones is introduced. Limiting ourselves to terms of the second order inW, the dc phonon assisted hopping conductivity is calculated to the infinite order in the electron-phonon coupling constant. Result clearly exhibits the polaron effect which is discussed in detail.     

Superconductivity-induced phonon self-energy effects in high-T c superconductors

The superconductivity-induced self-energy of phonons has been calculated in the lowest-order conserving approximation using conventional strong-coupling theory and also including scattering at nonmagnetic impurities. Results for the dependence of the shift and the width of phonons on temperature and scattering rates are presented and their sensitivity on the strength of the electron-phonon coupling and on impurities is pointed out. The theory is applied to optical data in YBa₂Cu₃O₇ using the experimental one-phonon density, the value ofT _c and [^*=0.25 as inputs. Neglecting anisotropies and the momentum dependence of the electron-phonon coupling the resulting strong-coupling model with 2.9 yields results which are in good agreement with the data, in particular, if impurity scattering is taken into account.     

Density-functional theory of the Compton profile anisotropy of copper metal

Band structure calculations of the Compton profiles of copper are compared with the experiment in the light of a rigorous interpretation of momentum densities in the Hohenberg-Kohn-Sham ground state density-functional formalism. There are strong evidences that the significant discrepancies for the Compton profile anisotropies do not originate from inaccurate solutions of the Kohn-Sham self-consistent equations, but from a non-local momentum density correlation correction functional.     

Electronic structure of warm dense copper studied by ultrafast x-ray absorption spectroscopy

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10?000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.     

Theorie der Elektron-Phonon-Wechselwirkung in paramagnetischen Salzen

The electron-phonon hamiltonian of paramagnetic ions, derived from the assumption of a deformable potential of the surrounding ligands, is given as a function of the static crystalline field. With that it is possible to reduce the spin-lattice relaxation time to the coefficients of the expansion of the crystalline potential in terms of spherical harmonics, present in the treatment of energy splitting by the crystalline field. The anisotropy of the matrix elements of the electron-phonon hamiltonian relative to the propagation vector of lattice vibrations is discussed.