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1.
iScience ; 23(3): 100926, 2020 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-32146324

RESUMO

Long decoherence time is a key consideration for molecular magnets in the application of the quantum computation. Although previous studies have shown that the local symmetry of spin carriers plays a crucial part in the spin-lattice relaxation process, its role in the spin decoherence is still unclear. Herein, two nine-coordinated capped square antiprism neodymium moieties [Nd(CO3)4H2O]5- with slightly different local symmetries, C1 versus C4 (1 and 2), are reported, which feature in the easy-plane magnetic anisotropy as shown by the high-frequency electron paramagnetic resonance (HF-EPR) studies. Detailed analysis of the relaxation time suggests that the phonon bottleneck effect is essential to the magnetic relaxation in the crystalline samples of 1 and 2. The 240 GHz Pulsed EPR studies show that the higher symmetry results in longer decoherence times, which is supported by the first principle calculations.

2.
Chemphyschem ; 15(13): 2859-70, 2014 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-25044624

RESUMO

The energy transport process in natural-light-harvesting systems is investigated by solving the time-dependent Schrödinger equation for a source-network-drain model incorporating the effects of dephasing and dissipation, owing to coupling with the environment. In this model, the network consists of electronically coupled chromophores, which can host energy excitations (excitons) and are connected to source channels, from which the excitons are generated, thereby simulating exciton creation from sunlight. After passing through the network, excitons are captured by the reaction centers and converted into chemical energy. In addition, excitons can reradiate in green plants as photoluminescent light or be destroyed by nonphotochemical quenching (NPQ). These annihilation processes are described in the model by outgoing channels, which allow the excitons to spread to infinity. Besides the photoluminescent reflection, the NPQ processes are the main outgoing channels accompanied by energy dissipation and dephasing. From the simulation of wave-packet dynamics in a one-dimensional chain, it is found that, without dephasing, the motion remains superdiffusive or ballistic, despite the strong energy dissipation. At an increased dephasing rate, the wave-packet motion is found to switch from superdiffusive to diffusive in nature. When a steady energy flow is injected into a site of a linear chain, exciton dissipation along the chain, owing to photoluminescence and NPQ processes, is examined by using a model with coherent and incoherent outgoing channels. It is found that channel coherence leads to suppression of dissipation and multiexciton super-radiance. With this method, the effects of NPQ and dephasing on energy transfer in the Fenna-Matthews-Olson complex are investigated. The NPQ process and the photochemical reflection are found to significantly reduce the energy-transfer efficiency in the complex, whereas the dephasing process slightly enhances the efficiency. The calculated absorption spectrum reproduces the main features of the measured counterpart. As a comparison, the exciton dynamics are also studied in a linear chain of pigments and in a multiple-ring system of light-harvesting complexes II (LH2) from purple bacteria by using the Davydov D1 ansatz. It is found that the exciton transport shows superdiffusion characteristics in both the chain and the LH2 rings.

3.
J Phys Condens Matter ; 25(41): 415501, 2013 Oct 16.
Artigo em Inglês | MEDLINE | ID: mdl-24047725

RESUMO

We propose a scheme to generate metal-insulator transition in the random binary layer (RBL) model, which is constructed by randomly assigning two types of layers along the longitudinal direction. Based on a tight-binding Hamiltonian, the localization length is calculated for a variety of RBLs with different cross section geometries by using the transfer-matrix method. Both analytical and numerical results show that a band of extended states could appear in the quasi-one-dimensional RBLs and the systems behave as metals by properly tuning the model parameters, due to the existence of a completely ordered subband, leading to a metal-insulator transition in parameter space. Furthermore, the extended states are irrespective of the diagonal and off-diagonal disorder strengths. Our results can be generalized to two- and three-dimensional disordered systems with arbitrary layer structures, and may be realized in Bose-Einstein condensates.


Assuntos
Condutividade Elétrica , Metais/química , Modelos Químicos , Modelos Moleculares , Transição de Fase , Simulação por Computador
4.
J Chem Phys ; 137(9): 094107, 2012 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-22957555

RESUMO

We investigate the influence of static disorder and thermal excitations on excitonic energy transport in the light-harvesting apparatus of photosynthetic systems by solving the Schrödinger equation and taking into account the coherent hoppings of excitons, the rates of exciton creation and annihilation in antennas and reaction centers, and the coupling to thermally excited phonons. The antennas and reaction centers are modeled, respectively, as the sources and drains which provide the channels for creation and annihilation of excitons. Phonon modes below a maximum frequency are coupled to the excitons that are continuously created in the antennas and depleted in the reaction centers, and the phonon population in these modes obeys the Bose-Einstein distribution at a given temperature. It is found that the energy transport is not only robust against the static disorder and the thermal noise, but it can also be enhanced by increasing the randomness and temperature in most parameter regimes. Relevance of our work to the highly efficient energy transport in photosynthetic systems is discussed.

5.
J Phys Condens Matter ; 22(6): 065102, 2010 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-21389362

RESUMO

We put forward a model Hamiltonian to describe the influence of backbone energetics on charge transport through guanine-quadruplex DNA (G4-DNA) molecules. Our analytical results show that an energy gap can be produced in the energy spectrum of G4-DNA by hybridization effects between the backbone and the base and by on-site energy difference of the backbone from the base. The environmental effects are investigated by introducing different types of disorder into the backbone sites. Our numerical results suggest that the localization length of G4-DNA can be significantly enhanced by increasing the backbone disorder degree when the environment-induced disorder is sufficiently large. There exists a backbone disorder-induced semiconducting-metallic transition in short G4-DNA molecules, where G4-DNA behaves as a semiconductor if the backbone disorder is weak and behaves as a conductor if the backbone disorder degree surpasses a critical value.


Assuntos
DNA/química , Quadruplex G , Guanina/química , Transporte Biológico , Biofísica/métodos , Cátions , Metais , Modelos Estatísticos , Conformação Molecular , Conformação de Ácido Nucleico , Hibridização de Ácido Nucleico , Semicondutores
6.
Phys Rev E Stat Nonlin Soft Matter Phys ; 79(4 Pt 1): 041924, 2009 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-19518273

RESUMO

We investigate transport properties of DNA sequences in human chromosome 22 and compare the results with those of a random artificial DNA sequence based on the single- and double-stranded charge transfer models. The statistical quantities, including the Hurst exponent, the distribution of Lyapunov exponent (LE), the central moments, and the scaling parameter, are numerically calculated by using the transfer-matrix approach. It is found that the existence of satellite DNA segments in human chromosome 22 could result in deviations from usual Gaussian distribution of LE. Our results suggest that the presence of the satellite DNA segments, together with the long-range correlations and the base-pairing correlations could lead to the violation of single-parameter scaling hypothesis which holds for the random artificial DNA sequence although the behaviors of the averaged LEs for both DNA sequences are similar. This provides a viewpoint to analyze differences between the genomic DNA sequences and the nonliving random ones on the basis of localization properties of wave functions in the sequences.


Assuntos
Cromossomos Humanos Par 22 , DNA de Cadeia Simples/química , DNA/química , Modelos Genéticos , Algoritmos , Sequência de Bases , DNA Satélite , Humanos , Distribuição Normal
7.
J Chem Phys ; 129(12): 124308, 2008 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-19045024

RESUMO

The geometries stability, electronic properties, and magnetism of Y(n)O clusters up to n=14 are systematically studied with density functional theory. In the lowest-energy structures of Y(n)O clusters, the equilibrium site of the oxygen atom gradually moves from an outer site of the cluster, via a surface site, and finally, to an interior site as the number of the Y atoms increases from 2 to 14. Starting from n=12, the O atom falls into the center of the cluster with the Y atoms forming the outer frame. The results show that clusters with n=2, 4, 8, and 12 are more stable than their respective neighbors, and that the total magnetic moments of Y(n)O clusters are all quite small except Y(12)O cluster. The lowest-energy structure of Y(12)O cluster is a perfect icosahedron with a large magnetic moment 6mu(B). In addition, we find that the total magnetic moments are quenched for n=2, 6, and 8 due to the closed-shell electronic configuration. The calculated ionization potentials and electron affinities are in good agreement with the experimental results, which imply that the present theoretical treatments are satisfactory.

8.
J Chem Phys ; 128(18): 184310, 2008 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-18532815

RESUMO

The geometries, stabilities, electronic properties, and magnetism of FeB(n) clusters up to n=10 are systematically studied with density functional theory. We find that our optimized structures of FeB(2), FeB(3), FeB(4), and FeB(5) clusters are more stable than those proposed in previous literature. The results show that it is favorable for the Fe atom to locate at the surface, not at the center of the cluster, and that FeB(4) and FeB(9) clusters exhibit high stability. For all the FeB(n) clusters studied, we find the charge transfer from Fe to B site and the coexistence of ionic and covalent bonding characteristics. The computed total magnetic moments of the lowest-energy structures oscillate with the cluster size and are quenched at n=4, 6, 8, and 10.

9.
Phys Rev E Stat Nonlin Soft Matter Phys ; 78(6 Pt 1): 061922, 2008 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-19256883

RESUMO

We report a theoretical study highlighting the fundamental effects of backbone disorder which simulates the environmental complications on charge transport properties of biological and synthetic DNA molecules. Based on effective tight-binding models of duplex DNA, the Lyapunov coefficient and current-voltage characteristics are numerically calculated by varying the backbone disorder degree. In contrast to the localization picture that the conduction of duplex DNA becomes poorer when the backbone disorder degree is increased, we find that the backbone disorder can enhance the charge transport ability of the DNA molecules when the environment-induced disorder surpasses a critical value, giving rise to a semiconducting-metallic transition. The physical origin for this is traced back to the antiresonant effects. These results provide a scenario to interpret a variety of transport behaviors observed in DNA molecules and suggest perspectives for future experiments intending to control the charge transport through DNA-based nanodevices.


Assuntos
DNA/química , DNA/metabolismo , Modelos Biológicos , Bacteriófago lambda , Transporte Biológico Ativo , Fenômenos Biofísicos , Cromossomos Humanos Par 22/química , Cromossomos Humanos Par 22/metabolismo , DNA Viral/química , DNA Viral/metabolismo , Humanos , Estrutura Molecular , Nanoestruturas , Poli A/química , Poli C/química , Poli G/química , Poli T/química , Eletricidade Estática , Termodinâmica
10.
J Chem Phys ; 123(22): 221105, 2005 Dec 08.
Artigo em Inglês | MEDLINE | ID: mdl-16375462

RESUMO

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.

11.
Phys Rev E Stat Nonlin Soft Matter Phys ; 69(6 Pt 2): 066102, 2004 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-15244662

RESUMO

We construct differential-integrative equations to investigate the effects of different distributions for the incubation period, defined as the period between receiving of the message and the beginning of the active state, and for the active period, the length of the active state, on the spreading dynamics in a closed system where one member can be dynamically linked to any other with given probability. The evolution of the ensemble-averaged infected rate gamma(t) is calculated by solving the equations for various distribution functions. Both the short-term oscillations and long-term saturation crucially depend on the form and parameters of the distribution functions. The obtained results may provide insights into the characteristics of oscillations and a prognosis of a spreading process in closed system.

12.
Phys Rev Lett ; 92(21): 218702, 2004 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-15245324

RESUMO

A dynamical model of small-world networks, with directed links which describe various correlations in social and natural phenomena, is presented. Random responses of sites to the input message are introduced to simulate real systems. The interplay of these ingredients results in the collective dynamical evolution of a spinlike variable S(t) of the whole network. The global average spreading length (s) and average spreading time (s) are found to scale as p(-alpha)ln(N with different exponents. Meanwhile, S(t) behaves in a duple scaling form for N>>N(*): S approximately f(p(-beta)q(gamma)t), where p and q are rewiring and external parameters, alpha, beta, and gamma are scaling exponents, and f(t) is a universal function. Possible applications of the model are discussed.


Assuntos
Modelos Teóricos , Transmissão de Doença Infecciosa , Economia , Humanos , Apoio Social
13.
Phys Rev E Stat Nonlin Soft Matter Phys ; 66(2 Pt 1): 021908, 2002 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-12241215

RESUMO

We propose a model to interpret the large-bandgap behavior in transport of electrons through an individual DNA molecule where the tunneling electrons are coupled with two-level systems (TLS). The TLS can be regarded as the simplest way to describe vibrations and inelastic scattering in the molecules if the two levels represent the low-lying phonon states. The nonlinear current-voltage curves can be derived by the use of the transfer matrices in an equivalent single-particle multichannel network. At low temperatures, the gap of the conduction band is sensitive to the strength of the coupling between the TLS and the conduction electrons. It is shown that the large-bandgap behavior similar to that of semiconductors stems from the inelastic scattering by the TLS.


Assuntos
Biofísica/métodos , DNA/química , Transporte de Elétrons , Modelos Estatísticos
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