The active role played by nonmagnetic Sr in magnetostructural coupling in SrTcO3 from first principles

The total energies and structural parameters of SrTcO₃ are calculated by means of the generalized gradient approximation (GGA) plus on-site Coulomb interaction corrections (GGA+U) method. G-type antiferromagnetic (G-AFM) is found to be ground state, in consistence with the previous experimental result. The distortions around Sr and Tc upon magnetic transition are compared and the change of distortion for SrO bond upon magnetic transition is found to be 25.83 times of the change for TcO bond. Our results point to an active role played by Sr in magnetostructural coupling in SrTcO₃.     

CoO under pressure from first principles

Density functional theory and the generalized gradient approximation with correction for Hubbard energy was used to study the behavior of cobaltous oxide (CoO) under pressure. CoO undergoes an insulator-metal transition which is accompanied by a magnetic collapse. The antiferromagnetic phase of CoO transforms to nonmagnetic phase with the 6-7% reduction in the fractional volume. The magnetic collapse and the energy band gap closure are driven by the lost of correlation which results from the delocalization of 3d electrons. Delocalization process is due to the band broadening with compression. The Hubbard energy influences the transitions pressure. The lower Hubbard terms result in the lower values of transition pressure. The evolution of magnetic moment, energy band gap, and the bandwidth versus increasing pressure is analyzed. The results of calculations are compared to the existing theoretical and experimental data.     

The first measured Mn I Stark widths

The shapes of the astrophysically interesting neutral manganese (Mn I) resonance spectral lines (403.075, 403.306, 403.448, 279.481, 279.826 and 280.108 nm) have been observed together with six other prominent Mn I lines in the laboratory helium plasma at a 47 000 K electron temperature and electron density. With these plasma parameters the Stark broadening has been found to be an important mechanism in the Mn I line shape formation. Our measured Mn I Stark widths (W) are the first data in the literature. Stark widths are compared with line hyperfine structure splittings (Δ_hfs). At above mentioned helium plasma conditions the line broadening due to hyperfine structure splitting of the lines is less than that of the Stark and Doppler broadening for the case of the Mn I lines under investigation. We estimate that at electron densities below and electron temperatures below 4000 K the components in the hyperfine structure play an important role in the mentioned Mn I line shape formation.     

Aharonov-Bohm effect in a draining bathtub vortex

We study planar waves in a circulating, draining fluid flow, which: (i) exhibit an analogue of the Aharonov-Bohm (AB) effect in Quantum Mechanics; (ii) obey a Klein-Gordon equation on an ‘effective spacetime’ which resembles the Kerr spacetime of General Relativity; and (iii) may be observed in the laboratory using gravity waves in a shallow basin. We describe a modified AB effect which depends on two dimensionless parameters, associated with the circulation α and draining β rates; we call this the ‘αβ effect’. We show that the αβ effect is inherently asymmetric even in the low-frequency limit, and that it leads to novel interference patterns which carry the signature of both rotation and absorption.     

Magnetic ground state and spin waves in A(A = K, Rb or Cs)Fe1.5Se2

We present theoretical results on the ground state phase diagram, spin waves and dynamic structure factor on the J₁-J₂ model. In the reasonable physical parameter region corresponding to AFe_1.5Se₂, the A-collinear antiferromagnetic phase is stable. The spin wave spectra have two acoustic branches and four optical branches for this phase on the rhombus-ordered vacancy lattice, and each of them is twofold degenerate. However, they have one nondegenerate acoustic branch and two nondegenerate optical branches on the square-ordered vacancy lattice. To offer the theoretical guidance for the further experiments, we also discuss the magnetic excitation spectra and the inelastic neutron scattering pattern based on linear spin wave theory.     

A generalization of the Virasoro algebra to arbitrary dimensions

Colored tensor models generalize matrix models in higher dimensions. They admit a 1/N expansion dominated by spherical topologies and exhibit a critical behavior strongly reminiscent of matrix models. In this paper we generalize the colored tensor models to colored models with generic interaction, derive the Schwinger Dyson equations in the large N limit and analyze the associated algebra of constraints satisfied at leading order by the partition function. We show that the constraints form a Lie algebra (indexed by trees) yielding a generalization of the Virasoro algebra in arbitrary dimensions.     

Study of classical mechanical systems with complex potentials

We apply the factorization technique developed by Kuru and Negro [Ann. Phys. 323 (2008) 413] to study complex classical systems. As an illustration we apply the technique to study the classical analogue of the exactly solvable PT symmetric Scarf II model, which exhibits the interesting phenomenon of spontaneous breakdown of PT symmetry at some critical point. As the parameters are tuned such that energy switches from real to complex conjugate pairs, the corresponding classical trajectories display a distinct characteristic feature — the closed orbits become open ones.     

Effective action for strongly correlated electron systems

The su(2|1) coherent-state path-integral representation of the partition function of the t-J model of strongly correlated electrons is derived at finite doping. The emergent effective action is compared to the one proposed earlier on phenomenological grounds by Shankar to describe holes in an antiferromagnet [R. Shankar, Nucl. Phys. B 330 (1990) 433]. The t-J model effective action is found to have an important “extra” factor with no analogue in Shankar?s action. It represents the local constraint of no double electron occupancy and reflects the rearrangement of the underlying phase-space manifold due to the presence of strong electron correlation. This important ingredient is shown to be essential to describe the physics of strongly correlated electron systems.     

Kinetic and mass mixing with three abelian groups

We present the possible mixing effects associated with the low-energy limit of a Standard-Model extension by two abelian gauge groups U₁(1)×U₂(1). We derive general formulae and approximate expressions that connect the gauge eigenstates to the mass eigenstates. Applications using the well-studied groups UB(1), U(1)_B−L, U(1)_Lα−Lβ (Lα being lepton flavor numbers), and UDM(1) (a symmetry acting only on the dark matter sector) are discussed briefly.     

Detecting communities in clustered networks based on group action on set

In this paper, we propose a well targeted algorithm (GAS algorithm) for detecting communities in high clustered networks by presenting group action technology on community division. During the processing of this algorithm, the underlying community structure of a clustered network emerges simultaneously as the corresponding partition of orbits by the permutation groups acting on the node set are achieved. As the derivation of the orbit partition, an algebraic structure r-cycle can be considered as the origin of the community. To be a priori estimation for the community structure of the algorithm, the community separability is introduced to indicate whether a network has distinct community structure. By executing the algorithm on several typical networks and the LFR benchmark, it shows that this GAS algorithm can detect communities accurately and effectively in high clustered networks. Furthermore, we compare the GAS algorithm and the clique percolation algorithm on the LFR benchmark. It is shown that the GAS algorithm is more accurate at detecting non-overlapping communities in clustered networks. It is suggested that algebraic techniques can uncover fresh light on detecting communities in complex networks.     

Nuclear spin selection rules in chemical reactions by angular momentum algebra

The detailed selection rules for reactive collisions reported by Quack using molecular symmetry group are derived by using angular momentum algebra. Instead of the representations of the permutation-inversion group for both nuclear spin and rovibronic coordinate wavefunctions, those of the rotation group for nuclear spin wavefunction only are used. The method allows more straightforward derivation of Quack’s results and further extension of the calculation for separating elementary reactions and application to higher proton systems.     

Phase transition in dually weighted colored tensor models

Tensor models are a generalization of matrix models (their graphs being dual to higher-dimensional triangulations) and, in their colored version, admit a 1/N expansion and a continuum limit. We introduce a new class of colored tensor models with a modified propagator which allows us to associate weight factors to the faces of the graphs, i.e. to the bones (or hinges) of the triangulation, where curvature is concentrated. They correspond to dynamical triangulations in three and higher dimensions with generalized amplitudes. We solve analytically the leading order in 1/N of the most general model in arbitrary dimensions. We then show that a particular model, corresponding to dynamical triangulations with a non-trivial measure factor, undergoes a third-order phase transition in the continuum characterized by a jump in the susceptibility exponent.     

Minimum shear viscosity over entropy density at phase transition?—A counterexample

The ratio η/s, shear viscosity (η) to entropy density (s), reaches its local minimum at the (second order) phase transition temperature in a wide class of systems. It was suspected that this behavior might be universal. However, a counterexample is found in a system of two weakly self-interacting real scalar fields with one of them condensing at low temperatures while the other remains in the symmetric phase. There is no interaction between the two fields. In our mean field analysis the resulting η/s is monotonically decreasing in temperature despite the second order phase transition.     

Generalised Tsallis statistics in electron-positron collisions

The scaling of charged hadron fragmentation functions to the Tsallis distribution for the momentum fraction 0.01?x?0.2 is presented for various e⁺e⁻ collision energies. A possible microcanonical generalisation of the Tsallis distribution is proposed, which gives good agreement with measured data up to x≈1. The proposal is based on superstatistics and a Koba-Nielsen-Olesen (KNO) like scaling of multiplicity distributions in e⁺e⁻ experiments.     

Closed form solutions for the self-resonances in a short Josephson junction

We present a closed form solution for the self-resonances in a short Josephson tunnel junction. This solution is alternative to the well-known textbook result (Barone and Paternó (1982) and Kulik (1965)) [1] and [2] based on a series expansion. Results are derived for the up-to-date case of a 0-π junction.     

MOCVD growth of spherical aggregates of SiC nanocrystallites

Spherical aggregates of SiC nanocrystallites can be grown in addition to SiC nanowires via metal organic chemical vapor deposition using methylvinyldichlorosilane as a source gas and Ni catalyst by controlling the growth temperature and the pressure of the source gas. Electron microscopy observations show that the aggregates are typically 300 nm in diameter, which consist of SiC nanocrystallites of about 5 nm in diameter. Electron diffraction reveals that the nanocrystallites have the 3C structure.     

Self-organised criticality in base-pair breathing in DNA with a defect

We analyse base-pair breathing in a DNA sequence of 12 base-pairs with a defective base at its centre. We use both all-atom molecular dynamics (MD) simulations and a system of stochastic differential equations (SDEs). In both cases, Fourier analysis of the trajectories reveals self-organised critical behaviour in the breathing of base-pairs. The Fourier Transforms (FTs) of the inter-base distances show power-law behaviour with gradients close to −1. The scale-invariant behaviour we have found provides evidence for the view that base-pair breathing corresponds to the nucleation stage of large-scale DNA opening (or ‘melting’) and that this process is a (second-order) phase transition. Although the random forces in our SDE system were introduced as white noise, FTs of the displacements exhibit pink noise, as do the displacements in the AMBER/MD simulations.