Motion of the spin of particles and nuclei of arbitrary spin in the electric field of crystals

The possibility of verifying the conclusions of quantum theory on the motion of the spin of particles and nuclei of arbitrary spin in an electric field is analyzed. The theory predicts that the planar channeling of particles and nuclei with higher spins (one or more) through straight crystals is accompanied by the rotation of their spin. For some nuclei, the spin-rotation angle per unit length is about 10^?1 rad/cm. For ultrarelativistic nuclei undergoing planar channeling through bent crystals, the spin-rotation angle per unit length is on the order of 1 rad/cm, which makes it possible to verify the validity of the Bargmann-Michel-Telegdi equation for particles and nuclei of higher spins.     

Reentrant spin glass behaviour in Nd0.84K0.12MnO3

Polycrystalline Nd_0.84K_0.12MnO₃ was prepared in single phase form with Pbnm space group. The magnetic properties are studied from magnetization, linear and non-linear susceptibility, and thermoremanent magnetization measurements. The sample exhibits paramagnetic to ferromagnetic transition followed by low temperature spin glass like transition. From frequency variation of ac susceptibility measurements, the spin glass transition temperature is found to be 97.6±0.1 K with critical exponents zυ=1.13±0.06. The critical exponent γ corresponding to spin glass transition has been determined from the third harmonic susceptibility analysis and it is found to be 3.09±0.05. The effective number of spins blocked under frustration and their correlation length are determined from the analysis of thermoremanent magnetization.     

The 4d auger,coster-kronig,and recombination spectra of the lanthanides

A complete set of electron-excited 4d-basedAuger spectra of the lanthanide metals from lanthanum to lutetium (except the unstable promethium) is presented, in both differential and integral forms. It is believed that the set is more representative of clean surfaces of the lanthanides than any published hitherto. With the help of binding energy and electron loss measurements made in this laboratory and elsewhere, values of the various possible Auger, Coster—Kronig, and direct recombination transition energies are calculated, and for each transition a “centre of gravity” is derived based on relative intensities of final state multiplets, electron occupation of core levels, etc. By using arguments based on trends in the spectra across the series, on theoretically and experimentally derived values of U_eff for the difference 4fⁿ⁺¹→4f^n?1, and on plausibility, values of U_eff for 4fⁿ→ 4f^n?2 as well as for the other final state hole pair configurations are allocated. The relaxed transition energies so calculated are then compared with the experimental energies, from which it is possible in most cases to make assignment of the spectral features to the various transitions. As a result it is found that there are some significant disagreements with the theoretical rates of McGuire for lanthanide free atoms. The reasons for these disagreements are discussed, and an empirical model based on effective 4f and 5d populations and on the restrictions imposed by spin alignment is used to resolve the differences qualitatively.     

Helical instability of a straight Abrikosov vortex in an anisotropic superconductor

Because of attraction of the parallel currents forming an Abrikosov vortex, the vortex energy per unit length decreases, under bending of the vortex, by a quantity proportional to the square of the curvature. Solving the London equation in an approximation allowing for this effect makes it possible to calculate the energy of an Abrikosov vortex in the form of a helix whose length and pitch are much larger than the correlation length, whose curvature is small compared to the reciprocal London length, and whose slope in relation to an axis coinciding with the direction in which the vortex energy is the highest is also small. When the anisotropy is large, which is characteristic of high-T _c superconductors, the energy of such an Abrikosov vortex is lower than that of a straight Abrikosov vortex. Certain consequences of the fact that the Abrikosov vortices in a high-T _c superconductor are helical are discussed. Among these is a phase transition that breaks the symmetry between Abrikosov vortices shaped like right-and left-hand helixes in relation to the magnetic field. Zh. éksp. Teor. Fiz. 111, 1869–1878 (May 1997)     

Analytic theory of correlation energy and spin polarization in the 2D electron gas

We present an analytic theory of the pair distribution function and the ground-state energy in a two-dimensional (2D) electron gas with an arbitrary degree of spin polarization. Our approach involves the solution of a zero-energy scattering Schrödinger equation with an effective potential which includes a Fermi term from exchange and kinetic energy and a Bose-like term from Jastrow-Feenberg correlations. The form of the latter is assessed from an analysis of data on a 2D gas of charged bosons. We obtain excellent agreement with data from quantum Monte Carlo studies of the 2D electron gas. In particular, our results for the correlation energy show a quantum phase transition occurring at coupling strength r_s≈24 from the paramagnetic to the fully spin-polarized fluid.     

Theoretical calculation of transition probabilities and lifetimes of levels of the 4d 95p configuration of CdIII

The configuration superposition method realized in the basis of Hartree-Fock functions and transformed radial orbitals with variable parameters is applied to ab initio calculation of electronic transition probabilities from levels of the 4d ⁹5p configuration of the CdIII ion. Relativistic corrections in the energy spectra are calculated within the framework of the Hartree-Fock-Pauli approximation, i.e., in the second order in the fine structure constant. The inclusion of correlation and relativistic corrections makes it possible to achieve a good coincidence of the values of electric dipole transition probabilities calculated with the use of two forms of the operator—the form of length and the form of velocity—for both strong and weak transitions. Using the transition probabilities obtained, radiative lifetimes of levels of the configuration indicated are determined. They are in a good agreement with the currently available experimental data.     

Computer search for defects in a D = 3 Heisenberg spin glass

A computer study of T = 0 metastable states (local minima) of a Heisenberg spin glass (1728 fixed-length spins on a simple cubic latice with random nearest-neighbor bonds) is reported, in which an attempt is made to describe the relation of nearby minima in terms of defects and to characterize those defects. The local rotation matrix relating one equilibrium configuration to another is well defined, and has a (relatively long) correlation length of 3–4 lattice units. The size of the clusters of spins which tend to be locked together in rigid rotations is 100–200 spins by several criteria. It is found that disclination lines and continuous twists of 360° across the sample exist as stable minima of the energy. However, the domain walls, across which the rotation matrix undergoes a reflection, appear to be more “fundamental” defects. The excitation energy of a defect is of the order of one exchange coupling or less. It does not seem possible to resolve the differences between two metastable states as a simple combination of spatially isolated defects.     

Ground state and low-lying excitations in the Heisenberg XXZ chain of arbitrary spin S

The ground state energy and spin wave modes are obtained for an anisotropic Heisenberg chain of arbitrary spin. The spin wave excitation has a gap for the Ising-like anisotropy, while for the XY-like case there is no gap.     

Fermi surface evolution in the ensemble of spin-polarized quasiparticles in La2 − x Sr x CuO4

For the spin-fermion model, it has been shown that the concept of spin polarons makes it possible to reproduce fine details of the Fermi surface evolution in the nodal direction of La_{2 ? x} Sr _x CuO₄ occurring with changes in the doping level x. The physics here is determined by the spin-correlated hopping of charge carriers and by the doping dependence of the inverse magnetic correlation length.     

Spin dynamics of qqq wave function on light front in high momentum limit of QCD: Role of qqq force

The contribution of a spin-rich qqq force (in conjunction with pairwise qq forces) to the analytical structure of the qqq wave function is worked out in the high momentum regime of QCD where the confining interaction may be ignored, so that the dominant effect is Coulombic. A distinctive feature of this study is that the spin-rich qqq force is generated by a ggg vertex (a genuine part of the QCD Lagrangian) wherein the 3 radiating gluon lines end on as many quark lines, giving rise to a (Mercedes-Benz type) Y-shaped diagram. The dynamics is that of a Salpeter-like equation (3D support for the kernel) formulated covariantly on the light front, a la Markov-Yukawa Transversality Principle (MYTP) which warrants a 2-way interconnection between the 3D and 4D Bethe-Salpeter (BSE) forms for 2 as well as 3 fermion quarks. With these ingredients, the differential equation for the 3D wave function ? receives well-defined contributions from the qq and qqq forces. In particular a negative eigenvalue of the spin operator iσ₁ · σ₂ × σ₃ which is an integral part of the qqq force, causes a characteristic singularity in the differential equation, signalling the dynamical effect of a spin-rich qqq force not yet considered in the literature. The potentially crucial role of this interesting effect vis-a-vis the so-called ‘spin anomaly’ of the proton, is a subject of considerable physical interest.     

Investigation of three-particle spin correlations during critical scattering of polarized neutrons from a nickel single crystal

The critical small-angle scattering of polarized neutrons from spin fluctuations in a nickel single crystal with a special inclined geometry of the magnetic field has been studied. The method of inclined geometry makes it possible to investigate not only two-particle spin correlations but also three-particle spin correlations that determine the polarization-dependent contribution to scattering, which is asymmetric with respect to the momentum transfer q. This contribution depends on the momentum transfer q as 1/(q ² + ξ^?2)^5/2, where f is the neutron scattering correlation length; it linearly increases with an increase in the magnetic field H in the low-field range and then reaches saturation. The results obtained are in good agreement with the similarity theory.     

Correlation functions in the multiple Ising model coupled to gravity

The model of p Ising spins coupled to 2d gravity, in the form of a sum over planar φ³ graphs, is studied and in particular the two-point and spin-spin correlation functions are considered. We first solve a toy model in which only a partial summation over spin configurations is performed and, using a modified geodesic distance, various correlation functions are determined. The two-point function has a diverging length scale associated with it. The critical exponents are calculated and it is shown that all the standard scaling relations apply. Next the full model is studied, in which all spin configurations are included. Many of the considerations for the toy model apply for the full model, which also has a diverging geometric correlation length associated with the transition to a branched polymer phase. Using a transfer function we show that the two-point and spin-spin correlation functions decay exponentially with distance. Finally, by assuming various scaling relations, we make a prediction for the critical exponents at the transition between the magnetized and branched polymer phases in the full model.     

Magnetopolaron states of a spin-correlated antiferromagnet in the neighborhood of the spin-glass transition

The problem of the spectrum of magnetopolaron states of a strongly correlated conducting canted antiferromagnet is solved. The approach used to study the spectrum is based on an atomic representation and a diagram technique for Hubbard operators. This approach makes it possible to include strong intra-ion interactions in a first-principles way, and to obtain the dispersion equation for the magnetopolaron spectrum for arbitrary values of the magnitude of the spin, temperature, and magnetic field. In the vicinity of the spin-flip transition an analytic expression is obtained for the spectrum of magnetopolaron states that goes beyond the framework of the quasiclassical approximation. Fiz. Tverd. Tela (St. Petersburg) 40, 310–314 (February 1998)     

The 3H(t, α)2n reaction at 40 keV

The two-dimensional energy spectrum of the α-particle and the neutron of the ³H(t, α)2n reaction was measured with a small single-gap accelerator at 90 keV. The mean energy of the ³H ions initiating the reaction was estimated to be about 40 keV, i.e. small against the Q-value of 11.3 Me V. It was found that the n-α interaction plays a strong part in the reaction so that no unambiguous determination of a_n-n, the n-n scattering length, was possible. The addition of an n-α final-state interaction to the n-n Jost-function enhancement factor in the analysis greatly improves the agreement with the experimental results, but it was not possible to distinguish between various forms of the n-α enhancement factors nor between different combinations of the n-α and the n-n interactions. The failure to obtain a well established n-n interaction peak is attributed to the absence of a direct reaction mechanism.     

Interacting ghost dark energy in Brans-Dicke theory

We investigate the QCD ghost model of dark energy in the framework of Brans-Dicke cosmology. First, we study the non-interacting ghost dark energy in a flat Brans-Dicke theory. In this case we obtain the equation of state and the deceleration parameters and a differential equation governing the evolution of ghost energy density. Interestingly enough, we find that the equation of state parameter of the non-interacting ghost dark energy can cross the phantom line (wD=−1) provided the parameters of the model are chosen suitably. Then, we generalize the study to the interacting ghost dark energy in both flat and non-flat Brans-Dicke framework and find out that the transition of wD to phantom regime can be more easily achieved for than when resort to the Einstein field equations is made.     

A comparative study of a Heusler alloy Co2FeGe using LSDA and LSDA+U

We have calculated the on-site Coulomb repulsion (U) for the transition elements Co and Fe. To study the impact of Hubbard potential or on-site Coulomb repulsion (U) on structural and electronic properties the calculated values of U were added on GGA and LSDA. We performed the structure optimization of Co₂FeGe based on the generalized gradient approximation (GGA and GGA+U). The calculation of electronic structure was based on the full potential linear augmented plane wave (FP-LAPW) method and local spin density approximation (LSDA) as well as exchange correlation LSDA+U. The Heusler alloy Co₂FeGe fails to give the half-metallic ferromagnetism (HMF) when treated with LSDA. The LSDA+U gives a good result to prove that Co₂FeGe is a HMF with a large gap of 1.10 eV and the Fermi energy (E_F) lies at the middle of the gap of minority spin. The calculated density of states (DOS) and band structure show that Co₂FeGe is a HMF when treated with LSDA+U.     

The energy of the spin-glass state of a binary mixture at T = 0 and its variational properties

In the random-bond model of Ising spins, the concept of a multiple-bond distribution of effective field was introduced in the pair approximation. The integral equation for a single-bond distribution was derived intuitively. The variational energy at T = 0 is expressed in terms of two parameters μ and η where μ is the probability of zero effective field in the single-bond distribution and η is the magnetization per spin. For η = 0, the energy of the spin-glass state corresponds to a local minimum as a function of μ, for an even z (number of the nearest neighbours) and to an inflection point for an odd z. It was shown that the spin-glass state corresponds to a local minimum with respect to μ and η for z = 4, to an inflection point with respect to μ and a local minimum with respect to η for z = 3. It is conjectured that the maximum of the energy of the spin-glass state of Sherrington and Kirkpatrick is attributed not to the replica method, but to the mean field approximation. Stationary properties of the energy as a function of both μ and η were examined in detail.     

Self-consistent approximations for itinerant ferromagnets above the phase transition point

The “conserving and self-consistent” approximation scheme of Kadanoff and Baym is generalized to systems of particles of non-zero spin. The additional conservation law of the total spin of the system restricts the possible approximations for the self-energy part, and thus the approximations for the irreducible vertex part occuring in the equation for the correlation function. This guarantees, for instance, the correct behavior of the dynamical susceptibility in the long wave length limit. Two examples are discussed under these aspects: the Hartree-Fock and theT-matrix approximation for the self-energy part and the resulting susceptibility.     

Electronic structure and magnetism in (CoPt)n(n⩽5) clusters

The geometrical and magnetic properties of bimetallic clusters (CoPt)_n(1?n?5) have been studied by using the generalized gradient correction spin density formalisms. In general, the ground state structures of (CoPt)_n clusters are the three-dimension structures. We found that both the binding energy and magnetism per (CoPt) unit are increasing consistently with the size of the Co–Pt cluster (n). However, as the n increases, the magnetism shows a trace of convergence while the binding energy shows a linearly increasing pattern. Generally, Co average magnetic moment is enhanced when alloyed with Pt atoms than that in pure Co clusters.