Two-dimensional silica: Crystalline and vitreous

Two-dimensional SiO₂ films may be grown on metal single crystal surfaces. It is possible to grow crystalline and vitreous (glassy) films and study their structural, vibrational, and electronic properties. In particular, the structures of a crystalline and a vitreous film may be imaged with atomic resolution side by side which opens avenues to study long standing problems of real space imaging of a crystal to glass transition

What can be learned from binding energy differences about nuclear structure: the example of delta V_{pn}

We perform an analysis of a binding energy difference called delta V_{pn}(N,Z) =- 1/4(E(Z,N)-E(Z,N-2)-E(Z-2,N)+ E(Z-2,N-2) in the framework of a realistic nuclear model. Using the angular-momentum and particle-number projected generator coordinate method and the Skyrme interaction SLy4, we analyze the contribution brought to delta V_{pn} by static deformation and dynamic fluctuations around the mean-field ground state. Our method gives a good overall description of delta V_{pn} throughout the chart of nuclei with the exception of the anomaly related to the Wigner energy along the N=Z line. The main conclusions of our analysis are that (i) the structures seen in the systematics of delta V_{pn} throughout the chart of nuclei can be easily explained combining a smooth background related to the symmetry energy and correlation energies due to deformation and collective fluctuations; (ii) the characteristic pattern of delta V_{pn} around a doubly-magic nucleus is a trivial consequence of the asymmetric definition of delta V_{pn}, and not due to a the different structure of these nuclei; (iii) delta V_{pn} does not provide a very reliable indicator for structural changes; (iv) \delta V_{pn} does not provide a reliable measure of the proton-neutron interaction in the nuclear EDF, neither of that between the last filled orbits, nor of the one summed over all orbits; (v) delta V_{pn} does not provide a conclusive benchmark for nuclear EDF methods that is superior or complementary to other mass filters such as two-nucleon separation energies or Q values.Comment: 19 pages and 12 figure

Pairing-excitation versus intruder states in 68Ni and 90Zr

A discussion on the nature of the 0+ states in 68Ni (Z=28, N=40) is presented and a comparison is made with its valence counterpart 90Zr (Z=40, N=50). Evidence is given for a 0+ proton intruder state at only ~2.2 MeV excitation energy in 68Ni, while the analogous neutron intruder states in 90Zr reside at 4126 keV and 5441 keV. The application of a shell-model description of 0+ intruder states reveals that many pair-scattered neutrons across N=40 have to be involved to explain the low excitation energy of the proton-intruder configuration in 68Ni.Comment: 10 pages, 2 figures, 1 tabl

From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images

N_pN_n dependence of empirical formula for the lowest excitation energy of the 2^+ states in even-even nuclei

We examine the effects of the additional term of the type $\sim e^{- \lambda' N_pN_n}$ on the recently proposed empirical formula for the lowest excitation energy of the $2^+$ states in even-even nuclei. This study is motivated by the fact that this term carries the favorable dependence of the valence nucleon numbers dictated by the $N_pN_n$ scheme. We show explicitly that there is not any improvement in reproducing $E_x(2_1^+)$ by including the extra $N_pN_n$ term. However, our study also reveals that the excitation energies $E_x(2_1^+)$, when calculated by the $N_pN_n$ term alone (with the mass number $A$ dependent term), are quite comparable to those calculated by the original empirical formula.Comment: 14 pages, 5 figure

From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images

STM studies of ordered (√31x √31)R9° CO islands on Ag(111)

The adsorption behavior of CO on Ag(111) is studied using low-temperature scanning tunneling microscopy. At submonolayer coverage, only single CO molecules are observed upon adsorption at 5 K. A further dosage leads to the formation of islands with various shapes and sizes. In addition, clusters with a diameter of about 11 Å are found that are mobile on the surface at 5 K. Though the position of the CO molecules within these clusters cannot be resolved, their size points to CO hexamers or heptamers. Well-ordered CO islands are observed only after heating the sample to 17 K, whereby two rotational domains with hexagonal (√root 31x √root 31)R9° structure are formed. A structural model is proposed in which CO hexamers represent the fundamental building blocks. The existence of two domains is explained with the alternate CO adsorption on the fcc and hcp places within the hexamers. The (√31 x √31)R9° superlattice is the only well-ordered CO structure found in the temperature range between 5 K and 35 K

Coframe teleparallel models of gravity. Exact solutions

The superstring and superbrane theories which include gravity as a necessary and fundamental part renew an interest to alternative representations of general relativity as well as the alternative models of gravity. We study the coframe teleparallel theory of gravity with a most general quadratic Lagrangian. The coframe field on a differentiable manifold is a basic dynamical variable. A metric tensor as well as a metric compatible connection is generated by a coframe in a unique manner. The Lagrangian is a general linear combination of Weitzenb\"{o}ck's quadratic invariants with free dimensionless parameters \r_1,\r_2,\r_3. Every independent term of the Lagrangian is a global SO(1,3)-invariant 4-form. For a special choice of parameters which confirms with the local SO(1,3) invariance this theory gives an alternative description of Einsteinian gravity - teleparallel equivalent of GR. We prove that the sign of the scalar curvature of a metric generated by a static spherical-symmetric solution depends only on a relation between the free parameters. The scalar curvature vanishes only for a subclass of models with \r_1=0. This subclass includes the teleparallel equivalent of GR. We obtain the explicit form of all spherically symmetric static solutions of the ``diagonal'' type to the field equations for an arbitrary choice of free parameters. We prove that the unique asymptotic-flat solution with Newtonian limit is the Schwarzschild solution that holds for a subclass of teleparallel models with \r_1=0. Thus the Yang-Mills-type term of the general quadratic coframe Lagrangian should be rejected.Comment: 28 pages, Latex error is fixe