Phase diagram of the Ising antiferromagnet with nearest-neighbor and next-nearest-neighbor interactions on a square lattice

The phase diagram of the Ising model in the presence of nearest-neighbor (J₁) and next-nearest-neighbor (J₂) interactions on a square lattice is studied within the framework of the differential operator technique. The Hamiltonian is solved by effective-field theory in finite cluster (we have chosen N=4 spins). We have proposed a functional for the free energy (similar to Landau expansion) to obtain the phase diagram in the (T,α) space (α=J₂/J₁), where the transition line from the superantiferromagnetic (SAF) to the paramagnetic (P) phase is of first-order in the range 1/2<α<0.95 in contrast to previous study of CVM (Cluster Variational Method) that predict first-order transition for α=1.0. Our results for α=1.0 are in accordance with MC (Monte Carlo) simulations, that predict a second-order transition.     

Rotation-vibration energy cluster formation in XH2D and XHD2 molecules (X = Bi, P, and Sb)

We investigate theoretically the energy cluster formation in highly excited rotational states of several pyramidal XH₂D and XHD₂ molecules (X = Bi, P, and Sb) by calculating, in a variational approach, the rotational energy levels in the vibrational ground states of these species for J?70. We show that at high J the calculated energy levels of the di-deuterated species XHD₂ exhibit distinct fourfold cluster patterns highly similar to those observed for H₂X molecules. We conclude from eigenfunction analysis that in the energy cluster states, the XHD₂ molecule rotates about a so-called localization axis which is approximately parallel to one of the X-D bonds. For the mono-deuterated XH₂D isotopologues, the rotational spectra are found to have a simple rigid-rotor structure with twofold clusters.     

A moment approach to non-Gaussian colored noise

The Langevin system subjected to non-Gaussian colored noise has been discussed, by using the second-order moment approach with two kinds of models for generating the noise. We have derived the effective differential equation (DE) for a variable x, from which the stationary probability distribution P(x) has been calculated with the use of the Fokker-Planck equation. The result of P(x) calculated by the moment method is compared to several expressions obtained by different methods such as the universal colored noise approximation (UCNA) [Jung and Hänggi, Phys. Rev. A 35 (1987) 4464] and the functional-integral method. It has been shown that our P(x) is in good agreement with that of direct simulations (DSs). We have also discussed dynamical properties of the model with an external input, solving DEs in the moment method.     

The controversy in the γγ → ρρ process: potential scattering or qqq̄q̄ resonance?

The γγ → gr⁰ ρ⁰ → 4π reaction shows a broad peak at 1.5 GeV in the (J ^P, J_z)=(2⁺, 2) channel which has no counterpart in the ρ⁺ ρ^? channel. This “resonance” is considered as a candidate for a qqq?q? state in the “s-channel”. We show, however, that it can also be explained by potential scattering of ρ⁰ ρ⁰ via the σ-exchange in the “t-channel”.     

Adsorption in one-dimensional channels arranged in a triangular structure: Theory and Monte Carlo simulations

Adsorption thermodynamics of interacting particles adsorbed on one-dimensional channels arranged in a triangular cross-sectional structure is studied through Bragg-Williams approximation (BWA), Monte Carlo (MC) simulations and the recently reported Effective Substates approximation (ESA) [J.L. Riccardo, G. Zgrablich, W. A. Steele, Appl. Surf. Sci. 196 (2002) 138]. Two kinds of lateral interaction energies have been considered: (1) w_L, interaction energy between nearest-neighbor particles adsorbed along a single channel and (2) w_T, interaction energy between particles adsorbed across nearest-neighbor channels. We focus on the case of repulsive transversal interactions (w_T>0), for which a rich variety of ordered phases are observed in the adlayer, depending on the value of the parameters k_BT/w_T (being k_B the Boltzmann constant) and w_L/w_T. Comparisons between analytical data and MC simulations are performed in order to test the validity of the theoretical models. Appreciable differences can be seen for the different approximations, ESA being the most accurate for all cases.     

A microscopic calculation for n-3H scattering

Starting from a nucleon-nucleon soft core potential, the³H-n scattering states were calculated in the refined cluster model framework. Phase shifts and angular distributions resulting from these calculations are compared with experiments and agree well. In all states except theJ ^π=1^? state the coupling of channels is extremely small, but in this state considerable spin-mixing is found.     

Tempering simulations in the four dimensional ±J Ising spin glass in a magnetic field

We study the four dimensional (4D) ±J Ising spin glass in a magnetic field with the simulated tempering algorithm recently introduced by Marinari and Parisi. We compute numerically the order parameter function P(q) and analyze the temperature dependence of the first four cumulants of the distribution. We discuss the evidence in favor of the existence of a phase transition in a field. Assuming a well defined transition we are able to bound its critical temperature.     

Amino wagging and inversion in hydrazine: K′ = 1 levels in the first excited state of the antisymmetric wagging vibration

The infrared absorption spectrum of NH₂NH₂ vapor has been observed in the region 899–1077 cm^?1, where the antisymmetric wagging band (v_a = 1 ← 0) appears, by the use of a Fourier transform spectrometer with a practical resolution of 0.003 cm^?1. In the region 925.0–925.6 cm^?1, the spectrum was also observed with a tunable diode laser, and a component, β, of the ^pQ₂ cluster has been further resolved. Most of the absorption lines assignable to β-^pP₂(J″), γ-^pP₂(J″), β-^pQ₂(J″), γ-^rQ₀(J″), β-^rR₀(J″), and γ-^rR₀(J″), where J″ = 2 ～ 15, have been identified. From these observed transition frequencies, in combination with the ground state energy levels given by microwave spectroscopy, the energy level structure of the K′ = 1 rotational states was determined. From this, the following molecular parameters for the v_a = 1 state were determined: molecular asymmetry, B′-C″ = 0.00017 cm^?1; a parameter q₅ describing an umbrella motion Coriolis interaction (q₅K) about the a axis, q₅ = ?0.0030 cm^?1; its J(J + 1) variation, q_5j = 0.00014 cm^?1; and a parameter describing an umbrella-motion K-type doubling g₅J(J + 1), g₅ = 0.000021 cm^?1.     

Quantum renormalizations in one dimensional easy plane ferromagnets

We examine the quantum corrections to the spin wave energies of easy plane ferromagnets. We utilize a formalism developed by Villain, and we obtain the quantum corrections of orders $\text{S(S + 1)}{}^{\mathrm{<mtext>-1</mtext><mtext>2</mtext>}}$. The experimentally determined spin wave spectra for CsNiF₃ is used to find the bare exchange J and anisotropy parameter D. We compare our values with those obtained from other theories, which order the corrections in powers of S^-1 and also neglect higher powers of D/J.     

Coherent A2− production of nuclei observed in the K−KS0 decay channel at 17.2 GeV/c

We have studied A₂^? production by π^? on a nuclear target in the K^?K_S⁰ decay channel, where the A₂^? is observed above a small background at 17.2 GeV/c incident momentum. Direct confirmation of coherent A₂ (J^P = 2⁺) production has been found.     

An su (6)w fit to the negative parity baryon decay rates

We present a least squares fit to the experimental data on decays of negative parity baryon resonances into a pseudoscalar meson and either a J^P = 1/2⁺ stable baryon or a J^P = 3/2⁺ decuplet member. We find that the s-waves and d-waves are separately in good agreement with the predictions of SU (6)_w ⊗ O(2)_Lz. Predictions are given regarding several as yet unobserved decay processes, and for those which concern hitherto undetected resonances, their possible detection is discussed.     

A signal of a narrowπNN resonance inpp →ppπ − π +

We report on the first exclusive measurement of the 2π production in NN collisions atT _p=750 MeV using a H₂ cluster jet target and the WASA/PROMICE detector at CELSIUS. In the invariant-mass spectrum $M_{pp\pi ^ - } $ we observe a narrow peak at 2.063(2) GeV with a 4σ deviation relative to the MC simulations for the conventional nonresonant 2π production process. The finding is in agreement with the hypothesis of a narrowπNN resonance withI(J^P)=even (0^?), calledd′, postulated recently to describe the peculiar resonance-like behavior of the pionic double charge exchange on nuclei.     

Quantum transport in randomly diluted quantum percolation clusters in two dimensions

We study the hopping transport of a quantum particle through finite, randomly diluted percolation clusters in two dimensions. We investigate how the transmission coefficient T behaves as a function of the energy E of the particle, the occupation concentration p of the disordered cluster, the size of the underlying lattice, and the type of connection chosen between the cluster and the input and output leads. We investigate both the point-to-point contacts and the busbar type of connection. For highly diluted clusters we find the behavior of the transmission to be independent of the type of connection. As the amount of dilution is decreased we find sharp variations in transmission. These variations are the remnants of the resonances at the ordered, zero-dilution, limit. For particles with energies within 0.25≤E≤1.75 (relative to the hopping integral) and with underlying square lattices of size 20×20, the configurations begin transmitting near p_α=0.60 with T against p curves following a common pattern as the amount of dilution is decreased. Near p_β=0.90 this pattern is broken and the transmission begins to vary with the energy. In the asymptotic limit of very large clusters we find the systems to be totally reflecting in almost all cases. A few clear exceptions we find are when the amount of dilution is very low, when the particle has energy close to a resonance value at the ordered limit, and when the particle has energy at the middle of the band. These three cases, however, may not exhaust all possible exceptions.     

Measurement of the coupling strength distribution in ferromagnetic (F)/antiferromagnetic (AF) exchange bias systems

We propose a method for determination of the distribution function P(j) of the coupling energy density j in polycrystalline textured ferromagnetic (F)/antiferromagnetic (AF) film systems. P(j) governs the entire film coupling J and the exchange bias field H_e and was not measurable until now. The method is verified by torquemetry in a high magnetic field and by reversing its rotation sense. The transition to a new magnetic steady state after rotation reversal is analyzed within a Stoner–Wohlfarth model including thermal relaxation. This transition is completed earlier for strongly coupled grains than for grains with smaller j, which is reflected in the torque curves. We determined P(j) for a sputtered NiFe(16 nm)/IrMn(0.8 nm) film at T=50 K in the hysteretic range of coupling energies and found that P strongly decreases for increasing j.     

Probabilities of rovibronic transitions in the I 1Π g − , J 1Δ g − →C 1Π ± u systems of bands of the deuterium molecule

Probabilities of spontaneous rovibronic transitions I ¹Π _g ^? , v′ J′, J ¹Δ _g ^? , v′, J′→C ¹Π _u ^± , v″, J″ of the D₂ molecule (for vibrational and rotational quantum numbers v′=v″=0–3 and J′=1–9, J″=J′±1) have been obtained for the first time. They were determined using (1) the previously proposed nonadiabatic model, which takes into account the electron-rotational interaction of the upper levels; (2) the coefficients of expansion of wave functions of perturbed states in the Born-Oppenheimer basis, which were found from the experimental data on rovibronic terms; and (3) semiempirical b initio data on electronic transition dipole moments of the 3dπ¹Π_g→2pπ¹Π_u and 3dπδ¹Δ_g→2pπ¹Π_u transitions. The dependences of the transition probabilities on J′ for the same bands of both hydrogen isotopomers H₂ and D₂ were found to be identical. They represent monotone functions for R and P branches and functions with a maximum (minimum) for Q branches. The ratios of transition probabilities of different isotopomers for different branches of the same systems of bands and for the same branches of different systems of bands were found to be correlated. The semiempirical values obtained in the paper agree with the experimental values within the limits of the errors of their determination. The nonempirical values of transition probabilities agree with the experiment considerably worse.     

Doubly Charmed Exotic Mesons

We discuss the mass spectrum of doubly charmed mesons as hadronic molecules composed by D and D^* meson. Considering the heavy quark symmetry and chiral symmetry, we introduce the one-boson ( ${\pi , \rho , \omega}$ ) exchange potential between D and D^* meson. For all possible quantum numbers I(J ^P ) with isospin I, total angular momentum J( ≤ 2) and parity P, we solve the fully coupled channel Schrödinger equation. We find that in many quantum numbers the bound and resonant states composed by D or D^* meson can exist near the DD, DD^* and D^*D^* thresholds.     

Molecular transient effects in K-band spectroscopy

The time dependence of microwave transient absorption and emission was measured forJ=2←1 of the OCS molecule under on- and off-resonant conditions by the technique of Stark switching. The results are analyzed with the theory of a coherent electromagnetic wave interacting with a two-level quantum system through the electric dipole interaction to obtain the pressure dependence of the relaxation timesT ₂ and in consequence the pressure broadening parameterC _w=26.7±0.9 (μs·mTorr)^?1 has been obtained.     

The Rank-One Separable Interaction Kernel for Nucleons with Scalar Propagator

In the paper the covariant kernel of the nucleon-nucleon interaction of particles with scalar propagators is analyzed. The Bethe–Salpeter equation for the T matrix is considered in the rank-one separable kernel. The parameters of the kernel for the specific partial-wave channels explicitly connect with the observables low energy scattering parameters and phase shifts, deuteron binding energy. Covariant separable kernels for the partial-wave channels with total angular momentum J = 0 (¹S₀, ³P₀) and J =1 (³S₁ ? ³D₁, ¹P₁, ³P₁) are constructed.