Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport

Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength, and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover that the dynamics are described by the Fermi's Golden Rule (FGR) and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature the zero-point fluctuations of the bath lead to non-zero transport rates, and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data.Comment: 19 pages, 4 figure

A comparative study of three international construction firms : knowledge management infrastructures for optimising organisations' learning

Sendo a comunicação crescentemente crucial na sociedade contemporânea, e com o aumento do número de plataformas de redes sociais e ferramentas agregadas que vão surgindo, torna-se relevante compreender de que forma estas podem ser aproveitadas e utilizadas enquanto meios de comunicação estratégica para empresas e instituições. A tendência é a de as redes disponibilizarem cada vez mais recursos ao utilizador, fruto da adaptação às necessidades do mesmo, e os baixos custos que possuem são um incentivo à sua exploração. Como tal, através de dois estudos de casos que se tornam aqui relevantes analisar, este trabalho propõe-se compreender de que forma a parceria estratégica entre o YouTube e o Facebook pode funcionar para fortalecer e tornar eficaz a transmissão de mensagens audiovisuais de uma instituição, evidenciando assim a sua presença junto do público-alvo que se encontra cada vez mais presente nestas redes

Quantum Diffusion on Molecular Tubes: Universal Scaling of the 1D to 2D Transition

The transport properties of disordered systems are known to depend critically on dimensionality. We study the diffusion coefficient of a quantum particle confined to a lattice on the surface of a tube, where it scales between the 1D and 2D limits. It is found that the scaling relation is universal and independent of the disorder and noise parameters, and the essential order parameter is the ratio between the localization length in 2D and the circumference of the tube. Phenomenological and quantitative expressions for transport properties as functions of disorder and noise are obtained and applied to real systems: In the natural chlorosomes found in light-harvesting bacteria the exciton transfer dynamics is predicted to be in the 2D limit, whereas a family of synthetic molecular aggregates is found to be in the homogeneous limit and is independent of dimensionality.Comment: 10 pages, 6 figure

Synthesis of TiO2 nanotube arrays and preparation of agar based gel polymer electrolyte for dye-sensitized solar cell.

Dalam kajian ini, katod-udara digunakan untuk menghasilkan tiubnano TiO2. Kesan pH dan voltan dalam pengubahsuaian morfologi tiubnano TiO2 telah dikaji. Tambahan pula, kestabilan katod udara dalam pelbagai keadaan penganodan juga dikaji buat pertama kalinya. In the current study, air-cathode is used to synthesize titanium dioxide (TiO2) nanotube arrays. Effect of pH and voltage in tailoring the morphologies of TiO2 nanotube arrays is investigated

Accuracy of Second Order Perturbation theory in the Polaron and Variational Polaron Frames

In the study of open quantum systems, the polaron transformation has recently attracted a renewed interest as it offers the possibility to explore the strong system-bath coupling regime. Despite this interest, a clear and unambiguous analysis of the regimes of validity of the polaron transformation is still lacking. Here we provide such a benchmark, comparing second order perturbation theory results in the original untransformed frame, the polaron frame and the variational extension with numerically exact path integral calculations of the equilibrium reduced density matrix. Equilibrium quantities allow a direct comparison of the three methods without invoking any further approximations as is usually required in deriving master equations. It is found that the second order results in the original frame are accurate for weak system-bath coupling, the full polaron results are accurate in the opposite regime of strong coupling, and the variational method is capable of interpolating between these two extremes. As the bath becomes more non-Markovian (slow bath), all three approaches become less accurate.Comment: 16 pages, 5 figures, typos correcte

Surprise responses in the human brain demonstrate statistical learning under high concurrent cognitive demand

The ability to learn about regularities in the environment and to make predictions about future events is fundamental for adaptive behaviour. We have previously shown that people can implicitly encode statistical regularities and detect violations therein, as reflected in neuronal responses to unpredictable events that carry a unique prediction error signature. In the real world, however, learning about regularities will often occur in the context of competing cognitive demands. Here we asked whether learning of statistical regularities is modulated by concurrent cognitive load. We compared electroencephalographic metrics associated with responses to pure-tone sounds with frequencies sampled from narrow or wide Gaussian distributions. We showed that outliers evoked a larger response than those in the centre of the stimulus distribution (i.e., an effect of surprise) and that this difference was greater for physically identical outliers in the narrow than in the broad distribution. These results demonstrate an early neurophysiological marker of the brain’s ability to implicitly encode complex statistical structure in the environment. Moreover, we manipulated concurrent cognitive load by having participants perform a visual working memory task while listening to these streams of sounds. We again observed greater prediction error responses in the narrower distribution under both low and high cognitive load. Furthermore, there was no reliable reduction in prediction error magnitude under high-relative to low cognitive load. Our findings suggest that statistical learning is not a capacity limited process, and that it proceeds automatically even when cognitive resources are taxed by concurrent demands