Enhancement of paramagnetic effects during spin alignment in 2D semiconductors

Effects of magnetic ordering associated with the Coulomb exchange interaction of free electrons in a 2D Fermi system are considered. It is shown that the paramagnetic response is substantially enhanced by Fermi-liquid effects. The phase transition to a state with spontaneous polarization of spins can be observed when Heisenberg parameter J is not smaller than μ_F/3.06 (approximately one-third of the Fermi energy) and not larger than half the Fermi energy (J ≤ μ_F/2).     

Selective alignment of molecular spin isomers

We experimentally demonstrate field-free, spin-selective alignment of ortho- and para molecular spin isomers at room temperature. By means of two nonresonant, strong, properly delayed femtosecond pulses within a four wave mixing arrangement, we observed selective alignment for homonuclear diatomics composed of spin 1/2 (15N) or spin 1 (14N) atoms. The achieved selective control of the isomers' angular distribution and rotational excitation may find applications to analysis, enrichment, and actual physical separation of molecular spin modifications.     

Study of φ(1020), D*± and B* spin alignment in hadronic Z0 decays

Measurements of helicity density matrix elements have been made for the φ(1020), D*^± and B* vector mesons in multihadronic Z⁰ decays in the OPAL experiment at LEP. Results for inclusive φ produced with high energy show evidence for production preferentially in the helicity zero state, with ρ ₀₀ = 0.54 ± 0.08, compared to the value of 1/3 expected for no spin alignment. The corresponding element for the D*± has a value of 0.40 ± 0.02, also suggesting a deviation from 1/3. The B* result, with ρ ₀₀ = 0.36 ± 0.09, is consistent with no spin alignment. Off-diagonal elements have been measured for the f and D* mesons; for the D* the element Re ρ _1?1 is non-zero, indicating non-independent fragmentation of the primary quarks.     

Measurement of D*+- diffractive cross sections in photoproduction at HERA

The first measurement of the cross sections for D*^±-meson diffractive photoproduction was performed with the ZEUS detector at the HERA ep collider by using an integrated luminosity of 38 pb^?1. The measurement was performed for photon-proton c.m. energies in the range 130<W<280 GeV and photon virtualities in the region Q ²<1 GeV². The product D*^± mesons were reconstructed for p _T ^D* >2 GeV and ?1.5<η ^D*<1.5 from the decay channel D*⁺ → D ⁰ π _s ⁺ with D ⁰ → K ^? π ⁺ (+ c.c.). The diffractive component was selected for 0.001 <x _P<0.018. The measured cross section in this kinematical range is $\sigma _{ep \to e'D^ * X_{p'} }^{diff} = 0.74 \pm 0.21(stat.)_{ - 0.18}^{ + 0.27} (syst.) \pm 0.16(p.diss.)$ nb (ZEUS preliminary). The measured integrated and differential cross sections are compared with theoretical expectations.     

On the dynamics of a continuum spin system

For a one-dimensional system of classical spins with nearest neighbour Heisenberg interaction we derive the equation of motion for each three-dimensional spin vector. In the continuum limit where the spins lie dense on a line this set of equations reduces to a nonlinear partial differential equation. In addition to spin-wave solutions we obtain some other special solutions of this equation. In particular we find solitary waves having total energy localised in a finite region, with velocity of propagation inversely proportional to the width of this region. Solutions of still another type are shown to have a diffusive character. The stability of such solutions and the possibility of interaction of two or more solitary waves have not yet been studied.     

Search for B+-->D*+pi0 decay

We report on a search for the doubly Cabibbo suppressed decay B+-->D*+pi0, based on a data sample of 657x10(6) BB pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We find no significant signal and set an upper limit of B(B+-->D*+pi0)<3.6x10(-6) at the 90% confidence level. This limit can be used to constrain the ratio between suppressed and favored B-->D*pi decay amplitudes, r<0.051, at the 90% confidence level.     

Calculation of spin alignment of deuterons traveling through matter

Calculations of the spin alignment of the relativistic deuteron beam passing through matter are described. The tensor polarization is calculated within the framework of the Glauber multiple scattering theory. The calculation results are compared with recent experimental data. The text was submitted by the authors in English.     

Molecular bond formation in Na*+N2 energy transfer: Crossed beam study of atomic alignment and orientation

We report the first full analysis of collisionally induced atomic alignment and orientation for a molecular collision process. In an experiment with crossed supersonic beams of N₀ and laser excited Na(3² P _3/2) we have studied the dependence of angular and energy resolved differential quenching cross sections as a function of the linear and circular polarization of the exciting laser light. The anisotropies observed in the linear polarization data range up to 2∶1 when corrected for electron and nuclear spin relaxation. The maximum effect is found at small scattering angles and intermediate energy transfer where the cross section is also largest. The atomic alignment angle most favourable for quenching relates to the scattering angle and can be understood in a model picture in such a way that the (NaN₂)^* molecular system is formed at internuclear distances as low asR=10a ₀. The circular asymmetry is small but with significant structure and is attributed to interaction on different potential surfaces atR>10a ₀. Full analysis of the four measurable parameter is given in terms of the density matrix in a frame withz-axis perpendicular to collision plane which allows a clear understanding of the properties of atomic reflection symmetry and coherence of the scattering process.     

Electronic control of spin alignment in pi-conjugated molecular magnets

Intramolecular spin alignment in pi-conjugated molecules is studied theoretically in a model of a Peierls-Hubbard chain coupled with two localized spins. By means of the exact diagonalization technique, we demonstrate that a spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic doping, depending on the chain length, the positions of the localized spins, and the sign of the electron-spin coupling. The calculated results provide a theoretical basis for understanding the mechanism of spin alignment recently observed in a diradical donor molecule.     

On the evolution of higher dimensional Heisenberg continuum spin systems

We consider the evolution of a classical Heisenberg ferromagnetic spin chain in its continuum limit in higher spatial dimensions. It is shown that the evolution of a radially symmetric chain could be identified with the motion of a helical space curve as in the linear case. The resulting invariant equations for the curvature (radial energy density) and torsion (related to current density) are shown to be equivalent to a generalized nonlinear Schrödinger equation, similar to the one derived by Ruijgrok and Jurkiewicz recently. Equivalent linear equations as well as special static solutions of point singular type are obtained. Similarity solutions, a class of which belong to Riccati type, are discussed in detail. For general higher dimensions, a potentially useful formulation is presented: Under stereographic projection of the unit sphere of spin, the equation of motion takes a neater form even with the inclusion of anisotropic interactions. Classes of explicit solutions are reported in higher dimensions. Propagating spin waves, static spin waves of point singular nature and of finite energy in some cases are also discussed.     

Band splitting and relative spin alignment in bilayer systems

It is shown that one-particle spectra of the lower Hubbard band of bilayer correlated 2D systems with different relative alignments of the spin systems in the layers differ significantly. In particular, the bilayer band splitting differs from zero for identically directed alternating spins of different layers (F _z configuration), but tends to zero for antiparallel alignment (AF _z configuration). It is found that the type of the alignment of the ground state changes upon an increase in the doping δ from the lower AF _z configuration to the F _z configuration of the alignment observed for large values of δ. The behavior of bilayer splitting in Bi₂Sr₂CaCu₂O_8+δ suggests that the configuration of the alignment may change from F _z →AF _z simultaneously with the superconducting transition. The effects associated with the influence of spin alignment on the magnetic excitation spectrum as a method of studying the spin structure of bilayer systems are considered for homogeneous solutions of effective spin models.     

High field Faraday rotation study of spin alignment in EuTe

A striking change of the temperature dependence of one of three Faraday rotation peaks of EuTe is observed as a function of magnetic field (H ⩽ 10 T). The results are in agreement with a simple MFA model giving direct evidence of a superlattice band splitting in the antiferromagnetic phase of EuTe.     

Effect of discreteness on the continuum limit of the Heisenberg spin chain

We discuss the effect of discreteness of the lattice on the classical continuum limit of the isotropic Heisenberg ferromagnetic spin chain, including biquadratic interaction, in the next order. We derive the equivalent higher order nonlinear Schrödinger equation by identifying the underlying geometry of the system and investigate its non-integrability nature. A multiple-scaling method is applied to obtain the perturbed soliton solution.     

Systematic classical continuum limits of integrable spin chains and emerging novel dualities

We examine certain classical continuum long wave-length limits of prototype integrable quantum spin chains. We define the corresponding construction of classical continuum Lax operators. Our discussion starts with the XXX chain, the anisotropic Heisenberg model and their generalizations and extends to the generic isotropic and anisotropic gln$g{l}_n$ magnets. Certain classical and quantum integrable models emerging from special “dualities” of quantum spin chains, parametrized by c-number matrices, are also presented.     

Existence and uniqueness of equilibrium states for some spin and continuum systems

The one to one correspondence between the existence of a unique equilibrium state and the differentiability of the free energy density with respect to the external field previously shown for Ising ferromagnetis is extendend to higher valued spin systems as well as to continuum systems satisfying the Fortuin, Kasteleyn and Ginibre inequalities. In particular this is shown to hold for a mixture ofA –B particles in which there is no interaction between like particles and a repulsion between unlike particles. Where the derivative of the free energy is discontinuous there are at least two equilibrium states.Supported in part by Air Force Grant n. 732430.     

Study ofD *+ and search forD **0 production by neutrinos in BEBC

Data from BEBC experiments are combined to provide large statistics for neutrino interactions. ChargedD ^* mesons are produced in (1.22±0.25)% of neutrino and (1.01±0.31)% of antineutrino charged current interactions. The mean fraction of the hadronic laboratory energy taken by theD ^*+ in these events is 0.59±0.03±0.08. Less than 18% of all chargedD ^* mesons from (anti)neutrino interactions are found to be daughters ofD ^**0 (at the 90% confidence level).