Finite-temperature thermophysical properties of fcc-Ca

At an extreme environment, such as high-temperature and high-pressure, harmonic theory has obvious limitations, where the anharmonic effects are influential in determining bulk properties of the materials. In this regard, necessity for incorporating anharmonicity through vibrational contribution and thermally excited electrons to the total free energy at finite temperatures is illustrated taking an example of divalent fcc-Ca. In this regard, we have employed a coupling scheme of combining recently proposed mean-field potential (MFP) with the local pseudopotential to obtain vibrational contribution to the total free energy. To access the applicability of the present coupling scheme, we have calculated temperature variation of several thermodynamical properties. Static EOS, shock Hugoniot and temperature along principal Hugoniot are also estimated. Results are satisfactorily compared with the other theoretical and experimental data and the use of local pseudopotential in conjunction with the MFP approach is justified.     

Structural and energetic properties of sodium clusters

In this work we present results from a theoretical study on the properties of sodium clusters. The structures of the global total-energy minima have been determined using two different methods. With the parameterized density-functional tight-binding method (DFTB) combined with a genetic-algorithm we investigated the properties of Na_N clusters with cluster size up to 20 atoms, and with our own Aufbau/Abbau algorithm together with the embedded-atom method (EAM) up to 60 atoms. The two sets of results from the independent calculations are compared and a stability function is studied as function of the cluster size. Due to the electronic effects included in the DFTB method and the packing effects included in the EAM we have obtained different global-minima structures and different stability functions.     

Shape dependence of optical properties of sodium clusters

We develop an analytical approach to calculate the aspect ratios of free alkali metal clusters using measured surface plasmon frequencies. The method is based on the concept of small deviations from a spherical shape and can be applied to clusters with spheroidal, icosahedral and other shapes. Results of experimental data as well as of numerical calculations for the surface plasmon resonance frequencies in small spheroidal sodium clusters containing less than 50 atoms are reproduced accurately through a quite simple formula which links the aspect ratio of the cluster to the observable surface plasmon frequencies. The developed approach allows also to reveal the dependence of the dielectric function of alkali metal on the number of atoms in the cluster.     

Finite-temperature properties of multiferroic BiFeO3

An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i) the symmetry of the ground state, (ii) the Néel and Curie temperatures, and (iii) the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a) an overlooked phase above Tc approximately 1100 K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b) improperlike dielectric features above Tc, and (c) that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases.     

Influence of electronic and geometric properties on melting of sodium clusters

Systematics of the melting transition for sodium clusters with 40-355 atoms has been studied with both ab initio and semiclassical molecular dynamics simulations. The melting temperatures obtained with an ab initio method for Na₅₅ ⁺ and Na₉₃ ⁺ correlate well with the experimental results. The semiclassically determined melting temperatures show similarities with the experimentally determined ones in the size region from 55 to 93 and near size 142, and the latent heat in the size region from 55 to 139, but not elsewhere in the size region studied. This indicates that the nonmonotonical melting behavior observed experimentally cannot be fully explained by geometrical effects. The semiclassically determined melting temperature and the latent heat correlate quite well, indicating that they respond similarly to changes in cluster geometry and size. Similarly, the binding energy per atom seems to correlate with the melting temperature and the latent heat of fusion.Received: 30 October 2003, Published online: 20 January 2004PACS: 36.40.Ei Phase transitions in clusters     

Magic numbers of sodium clusters

The cohesive energy E_c of crystalline-like b.c.c. and f.c.c. sodium clusters is calculated as a function of the number of atoms in the cluster using the density functional formalism. The maxima of E_c define the theoretical magic numbers for crystalline clusters. A comparison with theoretical results using the jellium background model and with experimental magic numbers obtained by Knight and co-workers suggests that sodium clusters prepared in those expeiments are in a disordered liquid-like or amorphous state.     

Charge-induced fragmentation of sodium clusters

The fission of highly charged sodium clusters with fissilities X>1 is studied by ab initio molecular dynamics. Na4+24 is found to undergo predominantly sequential Na+3 emission on a time scale of 1 ps, while Na(Q+)(24) ( 5< or =Q< or =8) undergoes multifragmentation on a time scale > or =0.1 ps, with Na+ increasingly the dominant fragment as Q increases. All singly charged fragments Na(+)(n) up to size n = 6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T< or =1500 K. These findings are consistent with and explain recent trends observed experimentally.     

Temperature-dependent ionization potential of sodium clusters

The ionization potential of sodium clusters () at a finite temperature is studied using density functional theory and ab initio molecular dynamics. The threshold regions of the photoionization efficiency curves are deduced from the integrated IP distributions, which are obtained from the energy eigenvalues of the highest occupied Kohn-Sham states during molecular dynamics by applying a theoretically well-defined shift. The calculated ionization potentials are directly compared to the experimental values. The energetically best geometry of Na₅₅ is found to be a slightly distorted icosahedron. Received 16 April 1999 and Received in final form 6 July 1999     

General axial shapes of sodium clusters

The shell correction method is applied to calculate the shapes and binding energies of Na clusters. The axial equilibrium shapes are calculated by minimizing five deformation parameters simultaneously. Strong deviations from spheroidal shapes including reflection asymmetric ones are found. The influence of cluster deformation on the separation energies and ionization potentials and on the splitting of the photo - resonance are compared with the available experimental data. The N-dependence of the cluster shape and its relation to the N- and Z-dependences of the nuclear shape is discussed.     

Structure determination of medium-sized sodium clusters

Sodium cluster anions Na(n)(-) with n = 39-350 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT). The highly structured experimental spectra are in excellent agreement with the electronic density of states (DOS) of the DFT lowest energy structures. Even for the largest sizes, a pronounced sensitivity of the DOS on fine geometric details could be observed, allowing for a reliable identification of a specific icosahedral growth motif. The intermediate sizes between the closed-shell Mackay clusters with 55, 147, and 309 atoms form by growth of overlayers, which often exhibit a twist deformation with respect to regular (Mackay-type) ones.     

Finite-temperature HFB theory

The finite-temperature Hartree-Fock-Bogoliubov (FTHFB) equations are derived. For the pairing hamiltonian FTHFB simplifies to finite-temperature BCS (FTBCS). The solution of the FTBCS equations for the degenerate model displays a temperature-dependent pairing “phase transition”.     

Finite-temperature correlation functions

Lattice measurements of the Pisa group are analyzed numerically, and the parameters of correlation functions are extracted fromthe data—both below and above the deconfinement temperature T _c. The gluon condensate is found for six temperatures in the interval (0.956–1.131)T _c, and field distributions in the deconfined phase are obtained.     

碱金属钠原子修饰硅原子团簇的结构及储氢性能研究

提出碱金属钠原子修饰笼形Si_6团簇的结构模型,采用密度泛函理论(DFT)研究钠原子修饰笼形Si_6团簇的结构及储氢性能.研究结果表明,氢分子与笼形Si_6团簇表面相互作用很弱,氢分子在其表面容易脱附.采用钠原子修饰笼形Si_6团簇后可有效避免氢分子的脱附,并且钠原子在笼形Si_6团簇的表面不发生团聚,有利于氢分子在其表面吸附和循环利用.研究发现在两个钠原子修饰笼形Si_6团簇的结构中,每个钠原子可以有效吸附六个氢分子.计算得到Na2Si_6团簇结构储氢的质量分数高达10.08 wt%,且氢分子的平均吸附能约为0.837 kcal/mol.可见,实现钠原子修饰笼形Si_6团簇结构在常温常压条件下储氢是有可能的.