1,720,973 research outputs found
Switching and memory effects in electron-vibron systems: from single-site junctions to chains and networks
Full text linkThe general framework in which this thesis is embedded is called Molecular Electronics. In this field the dream is to be able to produce stable junctions in which a given molecule is in contact with a certain number of electrodes. Those allow to apply voltages and to perform specific tasks, exploiting the functionality of the molecule itself.
Different kinds of molecules have specific electronic, structural and vibrational properties, but there is something that can be thought as a general property: the typical dimension of a molecule is in general very small (of the order of nanometers or smaller). Molecules can undergo structural changes when additional charges are inserted through electron-tunneling in transport setups. Because of that, the electronic and the vibrational degrees of free- dom are strongly related in molecules and their mutual interaction plays a fundamental role in the investigation of a molecular junction and in view of possible applications.
In general we can consider a molecule as a very tiny object that is flexible and has localized vibrations. This property is peculiar of molecules and is absent in semiconductor devices like quantum-dots, two dimensional electron gases and bulk materials. In those systems the vibrational properties are associated to the phonon structure, i.e. to the lattice structure of the material one considers. The flexibility of the molecules make them interesting and different from semiconductors devices, opening new perspectives and bringing new effects into the game.
The idea of using single molecule junctions in order to obtain functional devices like switches, rectifiers and memory elements, dates back to 1974 when Aviram and Ratner proposed to use a single organic molecule as a rectifier
Pairing of a few Fermi atoms in one dimension
No full textWe study a few Fermi atoms interacting through attractive contact forces in a one-dimensional trap by means of numerical exact diagonalization. From the combined analysis of energies and wave functions of correlated ground and excited states we find evidence of BCS-like pairing even for very few atoms. For moderate interaction strength, we reproduce the even-odd oscillation of the separation energy observed in [G. Zu ̈rn, A. N. Wenz, S. Murmann, A. Bergschneider, T. Lompe, and S. Jochim, Phys. Rev. Lett. 111, 175302 (2013)]. For strong interatomic attraction the arrangement of dimers in the trap differs from the homogeneous case as a consequence of Pauli blockade in real space
Three interacting atoms in a one-dimensional trap: A benchmark system for computational approaches
No full textWe provide an accurate calculation of the energy spectrum of
three atoms interacting through a contact force in a one-dimensional harmonic
trap, considering both spinful fermions and spinless bosons. We use fermionic
energies as a benchmark for exact-diagonalization technique (also known as full
configuration interaction), which is found to slowly converge in the case of strong
interatomic attraction
Experimental defect dynamics driven by phase gradient in liquid-crystal electro-convective patterns
No full textIn this work, we observe experimentally the dynamics of the defects of a periodic convective pattern obtained in a liquid crystal n-(4-methoxybenziliden)-4-butylamilin 98% cell subjected to electro-nematic instability by means of an ac electric field. We perform a check of the validity of the theoretical prescriptions regarding the interaction of these defects with the external phase-field gradient. We found results in a good agreement with the theory
Single-spin polaron memory effect in quantum dots and single molecules
Full text linkWe propose theoretically a spin memory effect in quantum dots and single molecules with strong electron-vibron interaction and coupled to ferromagnetic leads. The controlled electrical switching between spin states is achieved due to an interplay between Franck-Condon blockade of electron transport at low voltages and spin-dependent tunneling at high voltages. Spin lifetimes, currents, and spin polarizations are calculated as a function of the bias voltage by the master-equation method. We further propose to use a third ferromagnetic tunneling contact to probe and readout the spin state
Charge-memory polaron effect in molecular junctions
Full text linkThe charge-memory effect, bistability, and switching between charged and neutral states of a molecular junction, as observed in recent scanning-tunneling microscope (STM) experiments, is considered within a minimal polaron model. We show that in the case of strong electron-vibron interaction, the rate of spontaneous quantum switching between charged and neutral states is exponentially suppressed at zero bias voltage but can be tuned through a wide range of finite switching time scales upon changing the bias. We further find that, while junctions with symmetric voltage drop give rise to random switching at finite bias, asymmetric junctions exhibit hysteretic behavior, enabling controlled switching. Lifetimes and charge-voltage curves are calculated by the master-equation method for weak coupling to the leads and at stronger coupling by the equation-of-motion method for nonequilibrium Green's functions
New energy with ZnS: Novel applications for a standard transparent compound
Full text linkWe revise the electronic and optical properties of ZnS on the basis of first principles simulations, in view of novel routes for optoelectronic and photonic devices, such as transparent conductors and plasmonic applications. In particular, we consider doping effects, as induced by Al and Cu. It is shown that doping ZnS with Al imparts a n-character and allows for a plasmonic activity in the mid-IR that can be exploited for IR metamaterials, while Cu doping induces a spin dependent p-type character to the ZnS host, opening the way to the engineering of transparent p-n junctions, p-type transparent conductive materials and spintronic applications. The possibility of promoting the wurtzite lattice, presenting a different symmetry with respect to the most stable and common zincblende structure, is explored. Homo- and heterojunctions to twin ZnO are discussed as a possible route to transparent metamaterial devices for communications and energy
Charge-memory effect in a polaron model: equation-of-motion method for Green functions
Full text linkWe analyze a single-level quantum system placed between metallic leads and strongly coupled to a localized vibrational mode, which models a single-molecule junction or an STM setup. We consider a polaron model describing the interaction between electronic and vibronic degrees of freedom and develop and examine different truncation schemes in the equation-of-motion method within the framework of nonequilibrium Green functions. We show that upon applying gate or bias voltage, it is possible to observe charge-bistability and hysteretic behavior which can be the basis of a charge-memory element. We further perform a systematic analysis of the bistability behavior of the system for different internal parameters such as the electron-vibron and the lead-molecule coupling strength
Efficient GW calculations in two dimensional materials through a stochastic integration of the screened potential
Full text linkAbstract Many-body perturbation theory methods, such as the G 0 W 0 approximation, are able to accurately predict quasiparticle (QP) properties of several classes of materials. However, the calculation of the QP band structure of two-dimensional (2D) semiconductors is known to require a very dense BZ sampling, due to the sharp q-dependence of the dielectric matrix in the long-wavelength limit (q → 0). In this work, we show how the convergence of the QP corrections of 2D semiconductors with respect to the BZ sampling can be drastically improved, by combining a Monte Carlo integration with an interpolation scheme able to represent the screened potential between the calculated grid points. The method has been validated by computing the band gap of three different prototype monolayer materials: a transition metal dichalcogenide (MoS2), a wide band gap insulator (hBN) and an anisotropic semiconductor (phosphorene). The proposed scheme shows that the convergence of the gap for these three materials up to 50meV is achieved by using k-point grids comparable to those needed by DFT calculations, while keeping the grid uniform
Nanodevices in flatland: Two-dimensional graphene-based transistors with high I on/I off ratio
No full textWe present a multi-scale investigation of graphene-based transistors with a hexagonal boron-carbon-nitride (h-BCN) barrier in the channel. Our approach exploits ab-initio calculations for an accurate extraction of energy bands and tight-binding simulations in order to compute charge transport. We show that the h-BCN barrier inhibits the ambipolar behavior of graphene transistors, leading to a large Ion/Ioff ratio, within the ITRS roadmap specifications for future semiconductor technology nodes
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