Magnetism: the Driving Force of Order in CoPt. A First-Principles Study

CoPt or FePt equiatomic alloys order according to the tetragonal L10 structure which favors their strong magnetic anisotropy. Conversely magnetism can influence chemical ordering. We present here {\it ab initio} calculations of the stability of the L10 and L12 structures of Co-Pt alloys in their paramagnetic and ferromagnetic states. They show that magnetism strongly reinforces the ordering tendencies in this system. A simple tight-binding analysis allows us to account for this behavior in terms of some pertinent parameters

Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects

Building effective models from sparse but precise data

A common approach in computational science is to use a set of of highly precise but expensive calculations to parameterize a model that allows less precise, but more rapid calculations on larger scale systems. Least-squares fitting on a model that underfits the data is generally used for this purpose. For arbitrarily precise data free from statistic noise, e.g. ab initio calculations, we argue that it is more appropriate to begin with a ensemble of models that overfit the data. Within a Bayesian framework, a most likely model can be defined that incorporates physical knowledge, provides error estimates for systems not included in the fit, and reproduces the original data exactly. We apply this approach to obtain a cluster expansion model for the Ca[Zr,Ti]O3 solid solution.Comment: 10 pages, 3 figures, submitted to Physical Review Letter

Imaging the symmetry breaking of molecular orbitals in carbon nanotubes

Carbon nanotubes have attracted considerable interest for their unique electronic properties. They are fascinating candidates for fundamental studies of one dimensional materials as well as for future molecular electronics applications. The molecular orbitals of nanotubes are of particular importance as they govern the transport properties and the chemical reactivity of the system. Here we show for the first time a complete experimental investigation of molecular orbitals of single wall carbon nanotubes using atomically resolved scanning tunneling spectroscopy. Local conductance measurements show spectacular carbon-carbon bond asymmetry at the Van Hove singularities for both semiconducting and metallic tubes, demonstrating the symmetry breaking of molecular orbitals in nanotubes. Whatever the tube, only two types of complementary orbitals are alternatively observed. An analytical tight-binding model describing the interference patterns of ? orbitals confirmed by ab initio calculations, perfectly reproduces the experimental results

First-principles study of phase stability of Gd-doped EuO and EuS

Phase diagrams of isoelectronic Eu$_{1-x}$Gd$_x$O and Eu$_{1-x}$Gd$_{x}$S quasi-binary alloy systems are constructed using first-principles calculations combined with the standard cluster expansion approach and Monte-Carlo simulations. The oxide system has a wide miscibility gap on the Gd-rich side but forms ordered compounds on the Eu-rich side, exhibiting a deep asymmetric convex hull in the formation enthalpy diagram. The sulfide system has no stable compounds. The large difference in the formation enthalpies of the oxide and sulfide compounds is due to the contribution of local lattice relaxation, which is sensitive to the anion size. The solubility of Gd in both EuO and EuS is in the range of 10-20% at room temperature and quickly increases at higher temperatures, indicating that highly doped disordered solid solutions can be produced without the precipitation of secondary phases. We also predict that rocksalt GdO can be stabilized under appropriate experimental conditions.Comment: 14 pages, 6 figures (some with multiple panels), revtex4 with embedded ep

Transfer matrix solution of the Wako-Sait\^o-Mu\~noz-Eaton model augmented by arbitrary short range interactions

The Wako-Sait{\^o}-Mu\~noz-Eaton (WSME) model, initially introduced in the theory of protein folding, has also been used in modeling the RNA folding and some epitaxial phenomena. The advantage of this model is that it admits exact solution in the general inhomogeneous case (Bruscolini and Pelizzola, 2002) which facilitates the study of realistic systems. However, a shortcoming of the model is that it accounts only for interactions within continuous stretches of native bonds or atomic chains while neglecting interstretch (interchain) interactions. But due to the biopolymer (atomic chain) flexibility, the monomers (atoms) separated by several non-native bonds along the sequence can become closely spaced. This produces their strong interaction. The inclusion of non-WSME interactions into the model makes the model more realistic and improves its performance. In this study we add arbitrary interactions of finite range and solve the new model by means of the transfer matrix technique. We can therefore exactly account for the interactions which in proteomics are classified as medium- and moderately long-range ones.Comment: 15 pages, 2 figure

Cellular Dynamical Mean Field Approach to Strongly Correlated Systems

We propose a cellular version of dynamical-mean field theory which gives a natural generalization of its original single-site construction and is formulated in different sets of variables. We show how non-orthogonality of the tight-binding basis sets enters the problem and prove that the resulting equations lead to manifestly causal self energies.Comment: RevTex, 4 pages, 1 embedded figur

Ordering in ternary nitride semiconducting alloys

We present a thorough theoretical study of ordering phenomena in nitride ternary alloys GaInN, AlInN, and AlGaN. Using the Monte Carlo approach and energetics based on the Keating model we analyze the influence of various factors on ordering in bulk crystals and epitaxial layers. We characterize the degree of both short range order (SRO) and long ranger order (LRO) for different compositions, temperatures and for substrates associated with different epitaxial strain. For the description of the SRO the Warren-Cowley parameters related to the first four coordination shells are used. The LRO is detected by means of the introduced sim-LRO parameter, based on the Bragg-Williams approach. The description of the observed long-range ordering patterns and conditions for their occurrence follows

Fictive Impurity Models: an Alternative Formulation of the Cluster Dynamical Mean Field Method

"Cluster" extensions of the dynamical mean field method to include longer range correlations are discussed. It is argued that the clusters arising in these methods are naturally interpreted not as actual subunits of a physical lattice but as algorithms for computing coefficients in an orthogonal function expansion of the momentum dependence of the electronic self-energy. The difficulties with causality which have been found to plague cluster dynamical mean field methods are shown to be related to the "ringing" phenomenon familiar from Fourier analysis. The analogy is used to motivate proposals for simple filtering methods to circumvent them. The formalism is tested by comparison to low order perturbative calculations and self consistent solutions