Bias drop and phonon emission in molecular wires

Using the recursion-transfer-matrix (RTM) method combined with the non-equilibrium Green's function (NEGF) method and density-functional theory, we perform ab initio calculations for the electron transport of molecular wires bridged between electrodes. We present an effective potential of molecular wire under a finite bias voltage and discuss the phonon emission and local heating due to inelastic electron-phonon coupling effects. We find that it is strongly dependent on contact conditions. When the contacts to electrodes are bad, excitation phonon modes at contacts become dominant for the energy dissipation.     

Electron-phonon coupling spectrum in photodoped pentacene crystals

The coupling between conduction charges and the vibrational modes of the molecular lattice plays a defining role in the transport characteristics of organic semiconductors. Using electron tunneling spectroscopy, we obtain the electron--optical-phonon coupling spectrum in photodoped pentacene crystals at energies <30 meV. Comparison of the tunneling spectrum to infrared absorption data on the optical phonon density of states yields the energy dependence of the electron-phonon scattering matrix element. The integrated spectral weight of the electron-phonon coupling shows that superconductivity in pentacene is likely of electron-phonon origin.     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声.我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响.数值结果表明:随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感.我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离.在高偏压区,Fano因子随着电-声子耦合强度的增大而增大.     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声。我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响。数值结果表明：随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感。我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离。在高偏压区,Fano因子随着电-声子耦合强度的增大而增大。     

Mode excitation induced by the scanning tunnelling microscope

The scanning tunnelling microscope (STM) can induce molecular vibrations. This is detected by changes in the tunnelling conductance as the bias voltage matches a vibrational excitation threshold. Vibrational spectroscopy is available for a unique molecule. Nevertheless, the experimental results present several challenges: few modes are detected, certain symmetries are dominant, and site and conformation properties affect the detection. Despite these difficulties, this technique is proving to be of the uttermost importance in the analysis of molecular adsorbates and in their manipulation. We present a general theory that can predict the outcome of STM induced vibrational spectroscopy. The above challenges are analysed and quantitative results are then shown. PACS 68.37.Ef; 63.22.+m; 68.35.Ja; 34.50.Ez     

β-胡萝卜素分子的黄琨因子的温度特性

β-胡萝卜素具有光采集、光防护功能, 又是重要的光电材料, 它在外场下的分子结构和性能变化既有理论意义也有应用价值。测量了β-胡萝卜素在环己醇中68～26 ℃温度范围内的紫外-可见吸收、拉曼光谱。实验结果表明随着温度的降低, 黄琨因子和碳碳键每个振动模的电子-声子耦合常数减小, 紫外-可见吸收光谱红移, 碳碳键拉曼散射截面增加。用线性链状多烯分子的“相干弱阻尼电子-晶格振动模型”、“有效共轭长度模型”等理论给予了解释。随着温度的降低,β-胡萝卜素分子的热无序减小,分子结构有序性增加,π电子离域扩展,有效共轭长度增加,导致紫外-可见吸收光谱红移和强的拉曼活性。相干弱阻尼电子-晶格振动增强,使碳碳键拉曼散射截面增加。引用带有量纲的电子-声子相互作用常数,既可以与黄昆因子建立关系式,计算出碳碳键每个振动模的数值,也可以表征分子的有效共轭长度,π电子离域程度及拉曼散射截面的大小等。     

The effect of polarons on the conductivity of the narrow-band Hubbard chain

We consider a half-filled, narrow band Hubbard chain with Holstein-type electron-phonon coupling to a set of intramolecular vibrational modes. We allow for dispersion in the vibrational spectrum and show, in the limit of zero temperature, that contrary to some recent results the one-electron density of states vanishes at the Fermi energy and the d.c. conductivity does not diverge as the inverse temperature.     

Electronic transport in fullerene C20 bridge assisted by molecular vibrations

The effect of molecular vibrations on electronic transport is investigated with the smallest fullerene C20 bridge, utilizing the Keldysh nonequilibrium Green's function techniques combined with the tight-binding molecular-dynamics method. Large discontinuous steps appear in the differential conductance when the applied bias voltage matches particular vibrational energies. The magnitude of the step is found to vary considerably with the vibrational mode and to depend on the local electronic states besides the strength of electron-vibration coupling. On the basis of this finding, a novel way to control the molecular motion by adjusting the gate voltage is proposed.     

IR excitation spectra of low dimensional CT crystals: Multidimensional linear response theory approach

The effects of electron-phonon (e-ph) coupling in the infrared (IR) excitation spectra of low-dimensional charge transfer crystals are investigated through multidimensional, mean-field linear response theory. This very general approach enables one to take into account simultaneously different types of e-ph coupling, showing how the vibrational modes are indirectly mixed through their common interaction with charge-transfer and/or localized electrons. The somewhat unexpected consequences of this mixing are illustrated by the spectral simulation of two simple model systems.     

Role of the dopant in the superconductivity of diamond

We present an ab initio study of the recently discovered superconductivity of boron doped diamond within the framework of a phonon-mediated pairing mechanism. The role of the dopant, in substitutional position, is unconventional in that half of the coupling parameter lambda originates in strongly localized defect-related vibrational modes, yielding a very peaked Eliashberg alpha2F(omega) function. The electron-phonon coupling potential is found to be extremely large, and T(C) is limited by the low value of the density of states at the Fermi level. The effect of boron isotope substitution is explored.     

Nonlinear spin current and magnetoresistance of molecular tunnel junctions

We report on a theoretical study of spin-polarized quantum transport through a Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction (MTJ). Our study is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism, so that microscopic details of the molecular MTJ are taken into account from first principles. A magnetoresistance ratio of approximately 27% is found for the Ni-BDT-Ni MTJ which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states and magnetic leads.     

Phase transitions and electron-phonon coupling in platinum hydride

Structural phase transitions and superconducting properties of platinum hydride under pressure are explored through the first-principles calculations based on the density functional theory. Three new low-pressure phases (Pm3m, Cmmm and P4/nmm) are predicted, and all of them are metallic and stable relative to decomposed cases. The superconducting critical temperature of two high-pressure phases correlates with the electron-phonon coupling. The presence of soft modes induced by Kohn anomalies and the hybridization between H and Pt atoms result in the strong electron-phonon coupling. Our results have major implications for other transition metal hydrides under pressure.     

Negative differential resistance in a one-dimensional molecular wire with odd number of atoms

We have investigated the effects of electron-phonon coupling on the current-voltage characteristics of a one-dimensional molecular wire with odd number of atoms. The wire has been modelled using the Su-Schreiffer-Heeger (SSH) Hamiltonian and the current-voltage characteristics have been obtained using the Landauer’s formalism. In the presence of strong electron-lattice coupling, we find that there are regions of negative differential resistance (NDR) at some critical bias, due to the degeneracy in the energies of the frontier molecular orbitals. The presence of the applied bias and the electron-lattice coupling results in the delocalization of these low-lying molecular states leading to the NDR behaviour.     

Coupling between adsorbate vibrations and an electronic surface state

We report direct angle-resolved photoemission measurements of the coupling between the symmetric stretch vibrational mode of adsorbed hydrogen and a surface band on W(110). This coupling is manifested by the surface band being split into two branches at a binding energy comparable to the vibrational mode energy, as confirmed by observation of a dramatic hydrogen/deuterium isotope effect. The electron-phonon coupling parameter lambda is found to be significantly larger than that for bulk W, and to be closely related to the degree of surface localization of the surface state wave function.     

Intercalant and intermolecular phonon assisted superconductivity in K-doped picene

K(3) picene is a superconducting molecular crystal with a critical temperature of T(c) = 7 or 18 K, depending on the 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(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 superconductivity in K-doped picene peculiar and different from that of fullerenes.     

Phonon-induced gaps in graphene and graphite observed by angle-resolved photoemission

Mapping by angle-resolved photoemission spectroscopy of the spectral functions of graphite and graphene layers at low temperatures reveals a heretofore unreported gap of ~ 67 meV at normal emission. This gap persists to room temperature and beyond, and diminishes for increasing emission angles. We show that this gap arises from electronic coupling to out-of-plane vibrational modes at the K(ˉ) point in the surface Brillouin zone in accordance with conservation laws and selection rules governed by quantum mechanics. Our study suggests a new approach for characterizing phonons and electron-phonon coupling in solids.     

Interplay of electron-electron and electron-phonon interactions in molecular junctions

In this work we consider a current carrying molecular junction with both electron-phonon and electron-electron interactions taken into account. After performing Lang-Firsov transformation and considering Markov approximations in accordance to weak coupling to the electronic leads, we obtain the master equation governing the time evolution of the reduced density matrix of the junction. The steady state of the density matrix can be used to obtain I-V characteristic of the junction in several regimes of strengths of the interactions. Our results indicate that the system can show negative differential conductance (that is, the current decreases by increasing the applied bias voltage) in some regimes as an interplay between the electron-phonon and Coulomb interactions.     

Many-body effects on the transport properties of single-molecule devices

The conductance through a molecular device including electron-electron and electron-phonon interactions is calculated using the numerical renormalization group method. At low temperatures and weak electron-phonon coupling the properties of the conductance can be explained in terms of the standard Kondo model with renormalized parameters. At large electron-phonon coupling a charge analog of the Kondo effect takes place that can be mapped into an anisotropic Kondo model. In this regime the molecule is strongly polarized by a gate voltage which leads to rectification in the current-voltage characteristics of the molecular junction.     

Electron-magnon coupling and nonlinear tunneling transport in magnetic nanoparticles

We present a theory of single-electron tunneling transport through a ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. The model employed to describe the interaction between quasiparticles and collective excitations captures the salient features of a recent microscopic study. Our analysis of nonlinear quantum transport in the regime of weak coupling to the external electrodes is based on a rate-equation formalism for the nonequilibrium occupation probability of the nanoparticle many-body states. For strong electron-boson coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with recent tunneling experiments.     

Quantum dephasing in carbon nanotubes due to electron-phonon coupling

We report on the effect of electron-phonon coupling on quantum transport in carbon nanotubes. The vibrational atomic displacements as well as the electron-phonon coupling strength are introduced through a time-dependent perturbation of the pi-electron Hamiltonian. The effect of dephasing on the Kubo conductance is studied for metallic and semiconducting nanotubes, and from a phenomenological law, coherence length (time) scales are found to fluctuate within the range 10 to 150 nm (0.01 to 4 ps) depending on the energy of charge carriers and phonon amplitude.