Nuclear quantum effects in water

A path-integral Car-Parrinello molecular dynamics simulation of liquid water and ice is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path-integral molecular dynamics methodology. It is shown that these results are in good agreement with experimental data.     

Local density variation of gold nanoparticles in aquatic environments

Gold (Au) nanoparticles are widely used in diagnosing cancer, imaging, and identification of therapeutic methods due to their particular quantum characteristics. This research presents different types of aqueous models and potentials used in TIP3P, to study the effect of the particle size and density of Au clusters in aquatic environments; so it can be useful to facilitate future investigation of the interaction of proteins with Au nanoparticles. The EAM potential is used to model the structure of gold clusters. It is observed that in the systems with identical gold/water density and different cluster radii, gold particles are distributed in aqueous environment almost identically. Thus, Au particles have identical local densities, and the root mean square displacement (RMSD) increases with a constant slope. However in systems with constant cluster radii and different gold/water densities, Au particle dispersion increases with density; as a result, the local density decreases and the RMSD increases with a larger slope. In such systems, the larger densities result in more blunted second peaks in gold–gold radial distribution functions, owing to more intermixing of the clusters and less FCC crystalline features at longer range, a mechanism that is mediated by the competing effects of gold–water and gold–gold interactions.     

Classical and path-integral molecular-dynamics study on liquid water and ice melting using non-empirical TTM2.1-F model

The TTMF2.1-F model is a non-empirical intermolecular water potential parametrised from ab-initio calculations of the water dimer with a complete basis set limit including dispersion correction from second-order Moller-Plesset perturbation theory. In this work, using two-phase ice-water NVT molecular-dynamics (MD) simulations, we found the ice melting temperature using the TTM2.1-F potential is close to 273?K when the nuclear quantum effects (NQEs) were included using path-integral centroid MD. Detailed analysis of the radial distribution functions, angle distribution functions, and associated joint probability for both liquid water and the two-phase cases showed that the melting-point-temperature drop when using path-integral simulation is due to the weakening of hydrogen bonds vis-à-vis classically-propagated MD.     

Quantum phase transitions to charge-ordered and Wigner-crystal states under the interplay of lattice commensurability and long-range Coulomb interactions

The relationship among the Wigner crystal, charge ordering, and the Mott insulator is studied by the path-integral renormalization group method in two-dimensional systems with long-range Coulomb interaction. In contrast to the insensitivity of the Hartree-Fock results, the stability of the solid drastically decreases with the decrease in the lattice commensurability. The transition to liquid occurs at the electron gas parameter r(s) approximately 2 for the filling n=1/2, showing a large reduction from r(s) approximately 35 in the continuum limit. A correct account of quantum fluctuations is crucial to understanding the charge-order stability generally observed only at simple fractional fillings and the nature of quantum liquids away from them.     

Structural and response functions at equilibrium in path-integral quantum simple fluids

A discussion on the physical meaning of the r-space structures that can be defined in path-integral quantum simple fluids far from exchange is presented by making the connection with their associated experimentally measurable properties in k-space (response functions). The role played in this issue by weak external fields acting on the fluid is examined by considering both the standard quantum treatment of neutron scattering and the path-integral functional analysis approach. For the sake of completeness, the same discussion is presented for the approximate Gaussian Feynman-Hibbs effective potential picture that can be derived from the path-integral, and also the structural interrelations between both formalisms are stated. To illustrate the points addressed in this paper results for liquid helium-4 at 4.2 K (SVP), obtained with the use of the Aziz-Slaman and the ab initio SAPT2 pair potentials, are reported.     

Ab initio path-integral molecular dynamics and the quantum nature of hydrogen bonds

The hydrogen bond(HB) is an important type of intermolecular interaction, which is generally weak, ubiquitous,and essential to life on earth. The small mass of hydrogen means that many properties of HBs are quantum mechanical in nature. In recent years, because of the development of computer simulation methods and computational power, the influence of nuclear quantum effects(NQEs) on the structural and energetic properties of some hydrogen bonded systems has been intensively studied. Here, we present a review of these studies by focussing on the explanation of the principles underlying the simulation methods, i.e., the ab initio path-integral molecular dynamics. Its extension in combination with the thermodynamic integration method for the calculation of free energies will also be introduced. We use two examples to show how this influence of NQEs in realistic systems is simulated in practice.     

Study of electron solvation in polar solvents using path integral calculations

A path-integral Monte Carlo algorithm for the simulation of electrons solvated in polar liquids is briefly outlined, and results are presented for the electron solvated in liquid water.     

Ellipsometric spectroscopy study of photoluminescent Si/SiO2 systems obtained by magnetron co-sputtering

Si nanograins embedded in silica matrix were obtained by magnetron cosputtering of both Si and SiO₂ at different substrate temperature (200–700°C) and thermal annealing at 1100°C. The samples were characterized by ellipsometric spectroscopy, high-resolution electron microscopy observations and photoluminescence. The highest excess of Si atoms was found to be incorporated for deposition temperature near 400–500°C, giving rise to a maximum PL and a shift of the peak position towards lower energy. These features might be interpreted in terms of quantum size effects and of density of grains, even though the interface states seem to be involved in the improvement of the photoluminescence efficiency.     

Analytical results for the electronic shell structure in mesoscopic systems: Balian-Bloch type theory for spheres,discs, rings and anti-dot lattices

The electronic shell structure resulting from the interference of closed orbital paths is determined for mesoscopic systems like spherical clusters, discs and rings by extending the semiclassical theory of Balian and Bloch. Analytical results for the shell structure in the density of states are obtained. Thus, the dependence of the shell structure on dimension, size and geometry and potential of the mesoscopic system and on an external magnetic field can be studied systematically. Comparison of the semiclassical results and those of quantum mechanical calculations permits analysis of typical quantum mechanical effects and shows the validity of the semiclassical theory. Our results should stimulate new experiments, can be used to calculate oscillations in the binding-energy, ionization-potential, and can be applied to analyze oscillations in the electronic density of states of quantum dot systems like anti-dot lattices.     

Monte Carlo Study of Localization on a One-Dimensional Lattice

We study the equilibrium properties of a single quantum particle interacting with a classical lattice gas. We develop a path-integral formalism in which the quantum particle is represented by a closed, variable-step random walk on the lattice. After demonstrating that a Metropolis algorithm correctly predicts the properties of a free particle, we extend it to investigate the behavior of the quantum particle interacting with the lattice gas. Evidence of weak localization is observed under conditions of quenched disorder, while self-trapping clearly occurs for the fully annealed system. Compared with continuous space systems, convergence of Monte Carlo simulations in this minimum model is orders of magnitude faster in cpu time. Therefore the system behavior can be investigated for a much larger domain of thermodynamic parameters (e.g., density and temperature) in a reasonable time.     

VARIATIONAL CALCULATION ON GROUND-STATE ENERGY OF BOUND POLARONS IN PARABOLIC QUANTUM WIRES

Within the framework of Feynman path-integral variational theory, we calculate the ground-state energy of a polaron in parabolic quantum wires in the presence of a Coulomb potential. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron－phonon coupling constant than the Coulomb binding parameter, and it increases monotonically with decreasing effective wire radius. Moreover, compared to the results obtained by Feynman Haken variational path-integral theory, we obtain better results within the Feynman path-integral variational approach (FV approach). Applying our calculation to several polar semiconductor quantum wires, we find that the polaronic correction can be considerably large.     

Quantum spin dynamics in solution applicable to quantum computing

We investigate quantum spin dynamics of spin network systems in a liquid and in a molecular cluster with external magnetic field and discuss the importance of topological spin structures in relation to the quantum computing and quantum mechanical dimensions of the nuclear and the electron spins.     

Dynamics of structural transformations between lamellar and inverse bicontinuous cubic lyotropic phases

The liquid crystalline lamellar (L(alpha)) to double-diamond inverse bicontinuous cubic (Q(D)(pi)) phase transition for the amphiphile monoelaidin in excess water exhibits a remarkable sequence of structural transformations for pressure or temperature jumps. Our data imply that the transition dynamics depends on a coupling between changes in molecular shape and the geometrical and topological constraints of domain size. We propose a qualitative model for this coupling based on theories of membrane fusion via stalks and existing knowledge of the structure and energetics of bicontinuous cubic phases.     

Study on the coupled multiple nanocrystal quantum-dot system

We have studied excess electron filling rule in the coupled multiple nanocrystal quantum-dot systems, i.e. quantum chain and quantum pattern, by the unrestricted Hartree–Fock–Roothaan method. Assuming each quantum dot of quantum pattern to be confined in a three-dimensional spherical potential well of finite depth, we have studied the intradot and interdot electron Coulomb and exchange interactions. By varying the center distance d between the coupled quantum dots, the transition from the strong- to weak-coupling situation is realized. For the systems in question, our results show that, with the filling of excess electrons into the quantum pattern, the corresponding chemical potentials form quasi-band structure, which is similar to the energy-band structure of crystal material. In each chemical-potential band of quantum pattern, the number of chemical-potential curves is equal to the number of quantum dots, and the distributions of them depend strongly on the quantum-dot arrangement structure of quantum pattern.     

Quantum theory of pure liquid metals as two-component systems

With the ultimate aims of clarifying the interpretation and the utility of effective ion-ion interactions in liquid metals, and of understanding the unusual isotopic mass dependence of the shear viscosity of liquid metal Li, a fully quantum statistical mechanical theory is developed from the many-body Hamiltonian of the conduction electron-positive ion assembly.We have set up quantum equations of motion which are analogs of classical continuity and conservation equations by expanding the equation for the Wigner distribution function about its diagonal. The most important of these equations for our present purposes relates the time derivative of the current density j(r, t) to the flux of current and to density-density correlation functions for electrons, electron-ions, and ions.This theory is then applied to neutron scattering by liquid metals. While the theory is sufficiently general in principle to treat electron-ion interaction of arbitrary strength, it is shown that when the interacion is weak, the usual results are recovered, along with the effective ion-ion interaction. In this latter connection, it is also demonstrated how the effective Ornstein-Zernike function C(q) in a liquid metal is related to bare ion and bare electron direct correlation functions and to the bare electron partial structure factor. Combining C(q) with one of the classical equations of liquid structure such as Born-Green or Percus-Yevick then relates the effective ion-ion interaction to the partial correlation functions of the bare ions and electrons.It is further shown how gradient expansions of the correlation functions lead to equations of motion for the density, current, and energy density which are simply the hydrodynamic equations of the present quantum theory of two-component systems. It is pointed out that the analog of the Navier-Stokes equation for the two-component system may be used to identify the quantity $\text{4}\text{3}\text{η + ζ}$ for the liquid metal, η and ζ being respectively the shear and bulk viscosities. Finally, it is demonstrated that $\text{4}\text{3}\text{η + ζ}$ depends explicitly on functional derivatives of the nonequilibrium pair distribution functions of ion-ion, electron-ion, and electron-electron correlations.     

Effect of interface roughness on the density of states of finite barrier height quantum wells

We calculate the density of states of a 2D electron gas in finite barrier height quantum wells with the explicit inclusion of the interface roughness effect. By using Feynman path-integral method, the analytic expression is derived. The results show that the 2D density of states is dependent on the RMS of the fluctuation potential. The interface roughness causes localized states below the subband edge. We also apply the theory to model the finite barrier height quantum wells in Al_xGa_1?xAs/GaAs.     

Photoinduced charge redistribution and its influence on excitonic states in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures

The photoinduced charge redistribution in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well heterostructures under different conditions of optical excitation has been investigated using scanning probe microscopy and optical spectroscopy in the temperature range from 5 to 300 K. Excitation of the samples by radiation with a photon energy greater than the band gap of Zn(Cd)Se leads to the accumulation of electrons in quantum wells, which is detected using scanning spreading resistance microscopy. For moderate excitation densities (up to 25 W/cm²) and at temperatures ranging from 80 to 100 K, the density of a quasi-two-dimensional electron gas formed in quantum wells is several orders of magnitude higher than the density of electron-hole pairs generated by the excitation radiation. The excess electron concentration in the quantum well leads to a broadening of the exciton resonances and to an increase in the relative intensity of the donor-bound exciton emission line and also determines the increase in the luminescence quantum yield with increasing excitation intensity. An additional illumination with a photon energy less than the band gap of Zn(Cd)Se decreases the concentration of excess electrons in quantum wells. The influence of the additional illumination is observed at a temperature of approximately 100 K and almost completely suppressed at 5 K. The obtained results are explained in terms of the formation of a potential barrier for electrons at the ZnMgSSe/GaAs interface and by the specific features of recombination processes in the electron-hole system containing impurity centers with different charge states.