The smooth structural change in mesoscopic Peierls chains

We investigate the Peierls transition in finite chains by exact (Lanczos) diagonalization and within a seminumerical method based on the factorization of the electron-phonon wave function (Adiabatic Ansatz, AA). AA can be applied for mesoscopic chains up to micrometer sizes and its reliability can be checked self-consistently. Our study demonstrates the important role played for finite systems by the tunneling in the double well potential. The chains are dimerized only if their size N exceeds a critical value N_c which increases with increasing phonon frequency. Quantum phonon fluctuations yield a broad transition region. This smooth Peierls transition contrasts not only to the sharp mean field transition, but also with the sharp RPA soft mode instability, although RPA partially accounts for quantum phonon fluctuations. For weak coupling the dimerization disappears below micrometer sizes; therefore, this effect could be detected experimentally in mesoscopic systems. Received: 3 January 1998 / Revised: 13 March 1998 / Accepted: 3 April 1998     

Period doubling in quasi-one-dimensional systems : A nonadiabatic approach

In this article we propose a new approach to the electron-phonon problem in partially filled bands. With the help of the ‘rotating wave approximation’ we derive a reduced hamiltonian that can partially be diagonalized analytically in a many-particle basis. Structurally different many-particle states show up. While in the conventional adiabatic treatment of quasi-one-dimensional systems already an arbitrary small electron-phonon coupling can lead to a Peierls-type distortion, within the here proposed formalism we may conclude that a critical coupling strength has to be overcome for an energy gap above the highest occupied state to occur.     

Atomic motion in Se nanoparticles embedded into a porous glass matrix

By neutron diffraction it was shown that nanostructured Se confined within a porous glass matrix exists in a crystalline as well as in an amorphous state. The spontaneous crystallization of crystalline Se from confined amorphous phase was observed. The root-mean-square amplitudes of the atomic motions in the bulk as well as in confinement are found to be essentially different in a basal plane and in the perpendicular direction along the hexagonal axis. The atomic motions in the confined Se differ from the atomic motions in the bulk at low temperatures. The results shows an unusual “freezing" of the atomic motion along the chains, while the atomic motions in the perpendicular plane still keep. This “freezing" is accompanied by the deformation of nanoparticles and the appearance of inner stresses. This effect is attributed to the interaction of confined nanoparticle with the cavity walls.     

Enhanced light emission in Si-nanoclusters arrays

An array of silicon nanoclusters aimed at producing light emission upon injection of electrons and holes from external sources is studied by Monte Carlo simulations. The conditions for obtaining a significant charge accumulation in the emitting nanoclusters are investigated as a function of array geometry and applied electric fields. It is found that if a stationary state, reached for an applied field F₀, is suddenly perturbed by a field F₁≫F₀, a significant increase in electron-hole pairs population can be obtained with respect to the case of a single field of constant intensity F₁, leading to enhanced light emission when the conductivity of the array is above 6×10^-10 [ Ω cm] ^-1. The excess population thus created gets fully recombined on the time scale of milliseconds, suggesting a device that can produce enhanced light emission in the range of kilohertz.     

Oxygen-mediated suppression of twinning in gas-phase prepared FePt nanoparticles

We report on the influence of oxygen on the morphology and crystal structure of gas-phase prepared FePt nanoparticles. The particles are prepared by DC-sputtering in an Ar/He gas mixture. Without any oxygen, the obtained particles are predominantly icosahedra. The additional supply of oxygen leads to significant changes in both the crystal structure and morphology of the FePt nanoparticles. With increasing oxygen concentration, we observe the onset of particle agglomeration and a drop of the particle size. In addition, the crystal structure changes from icosahedral to fcc. These results are ascribed to oxygen mediated changes of the surface properties of the FePt nanoparticles such as the surface diffusivity and the surface free energy.     

Recursive approach to study transport properties of atomic wire

In this study, we propose a recursive approach to study the transport properties of atomic wires. It is based upon a real-space block-recursion technique with Landauer's formula being used to express the conductance as a scattering problem. To illustrate the method, we have applied it on a model system described by a single band tight-binding Hamiltonian. Results of our calculation therefore may be compared with the reported results on Na-atom wire. Upon tuning the tight-binding parameters, we can distinctly identify the controlling parameters responsible to decide the width as well as the phase of odd-even oscillations in the conductance.     

Control of extraordinary light transmission through perforated metal films using liquid crystals

We calculate the effective dielectric tensor of a metal film penetrated by cylindrical holes filled with a nematic liquid crystal (NLC). We assume that the director of the NLC is parallel to the film, and that its direction within the plane can be controlled by a static magnetic field, via the Freedericksz effect. To calculate the effective dielectric tensor, we consider both randomly distributed holes (using a Maxwell-Garnett approximation) and a square lattice of holes (using a Fourier technique). Both the holes and the lattice constant of the square lattice are assumed small compared to the wavelength. The films are found to exhibit extraordinary light transmission at special frequencies related to the surface plasmon resonances of the composite film. Furthermore, the frequencies of peak transmission are found to be substantially split when the dielectric in the holes is anisotropic. For typical NLC parameters, the splitting is of order 5–10% of the metal plasma frequency. Thus, the extraordinary transmission can be controlled by a static magnetic or electric field whose direction can be rotated to orient the director of the NLC. Finally, as a practical means of producing the NLC-filled holes, we consider the case where the entire perforated metal film is dipped into a pool of NLC, so that all the holes are filled with the NLC, and there are also homogeneous slabs of NLC on both sides of the film. The transmission in this geometry is shown to have similar characteristics to that in which the NLC-filled screen is placed in air.     

Quantum transport in honeycomb lattice ribbons with armchair and zigzag edges coupled to semi-infinite linear chain leads

We study quantum transport in honeycomb lattice ribbons with either armchair or zigzag edges. The ribbons are coupled to semi-infinite linear chains serving as the input and output leads and we use a tight-binding Hamiltonian with nearest-neighbor hops. The input and output leads are coupled to the ribbons through bar contacts. In narrow ribbons we find transmission gaps for both types of edges. The appearance of this gap is due to the enhanced quantum interference coming from the multiple channels in bar contacts. The center of the gap is at the middle of the band in ribbons with armchair edges. This particle-hole symmetry is because bar contacts do not mix the two sublattices of the underlying bipartite honeycomb lattice when the ribbon has armchair edges. In ribbons with zigzag edges the gap center is displaced to the right of the band center. This breakdown of particle-hole symmetry is the result of bar contacts now mixing the two sublattices. We also find transmission oscillations and resonances within the transmitting region of the band for both types of edges. Extending the length of a ribbon does not affect the width of the transmission gap, as long as the ribbon’s length is longer than a critical value when the gap can form. Increasing the width of the ribbon, however, changes the width of the gap. In ribbons with zigzag edges the gap width systematically shrinks as the width of the ribbon is increased. In ribbons with armchair edges the gap is not well-defined because of the appearance of transmission resonances. We also find only evanescent waves within the gap and both evanescent and propagating waves in the transmitting regions.     

Transport properties of a single-quantum dot Aharonov-Bohm interferometer

We consider a two-terminal Aharonov-Bohm (AB) interferometer with a quantum dot inserted in one path of the AB ring. We investigate the transport properties of this system in and out of the Kondo regime. We utilize perturbation theory to calculate the electron self-energy of the quantum dot with respect to the intradot Coulomb interaction. We show the expression of the Kondo temperature as a function of the AB phase together with its dependence on other characteristics such as the linewidth of the ring and the finite Coulomb interaction and the energy levels of the quantum dot. The current oscillates periodically as a function of the AB phase. The amplitude of the current oscillation decreases with increasing Coulomb interaction. For a given temperature, the electron transport through the AB interferometer can be selected to be in or out of the Kondo regime by changing the magnetic flux threading perpendicular to the AB ring of the system.     

Thermionic electron emission in the 1D edge-to-edge limit

Thermionic emission is a tunneling phenomenon, which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot enough to overtake their work functions. This principle has led to the great success of the so-called vacuum tubes in the early 20th century. To date, major challenges still remain in the miniaturization of a vacuum channel transistor for on-chip integration in modern solid-state integrated circuits. Here, by introducing nano-sized vacuum gaps (～ 200 nm) in a van der Waals heterostructure, we successfully fabricated a one-dimensional (1D) edge-to-edge thermionic emission vacuum tube using graphene as the filament. With the increasing collector voltage, the emitted current exhibits a typical rectifying behavior, with the maximum emission current reaching 200 pA and an ON-OFF ratio of 10³. In addition, it is found that the maximum emission current is proportional to the number of the layers of graphene. Our results expand the research of nano-sized vacuum tubes to an unexplored physical limit of 1D edge-to-edge emission, and hold great promise for future nano-electronic systems based on it.     

Magnetic field dependence of the domain wall resistance in a quantum wire

For an ideal one-dimensional ferromagnetic wire with a magnetic domain wall (DW), contribution of the DW to the resistivity of the system has been investigated. We have studied the resistance due to the magnetic impurities in the domain wall which was suspended in a weak magnetic field for two types of chiralities. The analysis has been based on Boltzmann transport equation, within the relaxation time approximation. Through this formalism, both increasing and decreasing of the resistance due to the DW have been predicted in presence of Zeeman interaction as an extrinsic mechanism.     

Mo6S6 nanowires: structural, mechanical and electronic properties

The properties of nanowires were investigated with ab initio calculations based on the density-functional theory. The molecules build weakly coupled one-dimensional chains, like and Mo₆S_9-xI_x, and the crystals are strongly uniaxial in their mechanical and electronic properties. The calculated moduli of elasticity and resilience along the chain axis are c₁₁ = 320 GPa and E_R = 0.53 GPa, respectively. The electronic band structure and optical conductivity indicate that the crystals are good quasi-one-dimensional conductors. The frequency-dependent complex dielectric tensor ε, calculated in the random-phase approximation, shows a strong Drude peak in ε_∥, i.e., for the electric field polarised parallel to the chain axis, and several peaks related to interband transitions. The electron energy loss spectrum is weakly anisotropic and has a strong peak at the plasma frequency ħω_p ≈20 eV. The stability analysis shows that is metastable against the formation of the layered .     

Polaron effects and boundary conditions in cylindrical wires

In this work we show that the polaron effects in cylindrical quantum wires are function of the cylinder radius R₀ through the boundary conditions for both the ionic and the electronic motion and through the size dependence of the static and high frequency dielectric constants. We find that the dielectric constants are increasing functions of R₀. This fact and the different boundary conditions for the ions and the electrons have the final consequence that polaron self-energy can either be an increasing or a decreasing function of R₀.     

Tuneable micro- and nano-periodic structures in a free-standing flexible urethane/urea elastomer film

We have studied the control and manipulation of tuneable equilibrium structures in a free-standing urethane/urea elastomer film by means of atomic force microscopy, small-angle light scattering and polarising optical microscopy. The urethane/urea elastomer was prepared by reacting a poly(propyleneoxide)-based triisocyanate-terminated prepolymer (PU) with poly(butadienediol) (PBDO), with a weight ratio of 60% PU/40% PBDO. An elastomer film was shear-cast onto a glass plate and allowed to cure, first in an oven, then in air. Latent micro- and nano-periodic patterns are induced by ultra-violet (UV) irradiation of the film and can be “developed” by applying a plane uniaxial stress or by immersing the elastomer in an appropriate solvent and then drying it. For this elastomer we describe six pattern states, how they are related and how they can be manipulated. The morphological features of the UV-exposed film surface can be tuned, reproducibly and reversibly, by switching the direction of the applied mechanical field. Elastomers extracted in toluene exhibit different surface patterns depending upon the state in which they were developed. Stress-strain data collected for the films before and after UV irradiation reveal anisotropy induced by the shear-casting conditions and enhanced by the mechanical field. We have interpreted our results by assuming the film to consist of a thin, stiff surface layer (“skin”) lying atop a thicker, softer substrate (“bulk”). The skin's higher stiffness is hypothesised to be due to the more extensive cross-linking of chains located near the surface by the UV radiation. Patterns would thus arise as a competition between the effects of bending the skin and stretching/compressing the bulk, as in the work of Cerda and Mahadevan (Phys. Rev. Lett. 90, 074302 (2003)). We present some preliminary results of a simulation of this model using the Finite Element package ABAQUS.     

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.     

Quantum transport in armchair graphene ribbons: analytical tight-binding solutions for propagation through step-like and barrier-like potentials

Based on the nearest-neighbor tight-binding approximation, we present exact analytical expressions for transmission coefficients through piecewise constant step-like and barrier-like electrostatic potentials. In the case of single mode propagation through semiconducting ribbon families our analytical solutions predict a new kind of resonances. Its features substantially change the behavior of the transmission coefficients in the range of moderate potentials, which become family-dependent. For semimetal ribbons our approach predicts no unit propagation. The non-zero backscattering is derived to be proportional as the square of the potential amplitude applied.     

Transport through quantum dots: a combined DMRG and embedded-cluster approximation study

The numerical analysis of strongly interacting nanostructures requires powerful techniques. Recently developed methods, such as the time-dependent density matrix renormalization group (tDMRG) approach or the embedded-cluster approximation (ECA), rely on the numerical solution of clusters of finite size. For the interpretation of numerical results, it is therefore crucial to understand finite-size effects in detail. In this work, we present a careful finite-size analysis for the examples of one quantum dot, as well as three serially connected quantum dots. Depending on “odd-even” effects, physically quite different results may emerge from clusters that do not differ much in their size. We provide a solution to a recent controversy over results obtained with ECA for three quantum dots. In particular, using the optimum clusters discussed in this paper, the parameter range in which ECA can reliably be applied is increased, as we show for the case of three quantum dots. As a practical procedure, we propose that a comparison of results for static quantities against those of quasi-exact methods, such as the ground-state density matrix renormalization group (DMRG) method or exact diagonalization, serves to identify the optimum cluster type. In the examples studied here, we find that to observe signatures of the Kondo effect in finite systems, the best clusters involving dots and leads must have a total z-component of the spin equal to zero.     

X-ray edge spectra — a sea-boson perspective

The well-studied X-ray-edge problem is revisited using the sea-boson method. This approach is contrasted with the well-known theories of Mahan, Nozières and De Dominicis (MND). The present approach does not use the sudden approximation and the holes carry a momentum label unlike in the MND theory. We focus on the case of doped semiconductors rather than metals. The problem of electrons in a partially filled conduction band and holes in the initially hole-depleted valence band is recast in the sea-boson language. The resulting hamiltonian is shown to be equivalent to the electron-phonon hamiltonian with the excitons taking on the role of electrons and intra-conduction band particle-hole excitations known as ‘conductrons’ taking on the role of phonons. It is shown that the excitonic pole in the computed absorption spectra is replaced by a branch cut with a simple radical leading to a broadening of the exicton line due to these many-body effects. A critical comparison is made with the MND theory as well as with relevant experiments.     

Tip-induced distortions in STM imaging of carbon nanotubes

By means of STM measurements and fully self-consistent transport calculations we analyze how STM trajectories for the mapping of nanostructures on surfaces are affected by the atomic structure of the tip. For the particular case of carbon nanotubes we show that considerable distortions of the STM trajectory with respect to the actual structure, position and diameter of the nanotube can occur for certain tip geometries. Comparison between theory and experiment can allow to characterize and correct these distortions.