Challenges in uncovering the origin of the proton's spin

One of the most fascinating challenges facing modern strong interaction physics is to understand the origin of the spin of the nucleon in terms of the spin and orbital angular momentum of the quarks and gluons.We review recent progress on this problem as well as some of the uncertainties associated with state of the art lattice QCD simulations.In particular,we explain the importance of the corrections associated with chiral extrapolation and finite volume corrections,especially for the term B（0） extracted from the appropriate low moment of the deeply virtual Compton scattering amplitude.     

The NMR indirect nuclear spin–spin coupling constant of the HD molecule

We present new calculated and experimental values of the NMR indirect nuclear spin–spin coupling constant in HD. In the quantum-chemical ab initio calculations, the full configuration-interaction (FCI) method is used, yielding an equilibrium value of 41.22?Hz in the basis-set limit. Adding a calculated zero-point vibrational correction of 1.89?Hz and a temperature correction of 0.20?Hz at 300?K, we obtain a total calculated spin–spin coupling constant of J _FCI(HD)?=?43.31(5)?Hz at 300?K. This result is within the error bars of the experimental gas-phase NMR value, J _exp(HD)?=?43.26(6)?Hz, obtained by extrapolating values measured in HD–He mixtures to zero density.     

Effects of impurity on the entanglement of the three-qubit Heisenberg XXX spin chain

We investigate the entanglement of the three-qubit Heisenberg XXX chain in the presence of impurity and obtain the analytical expressions of the concurrence C. It is found that for impurity entanglement, C appears only when J1 > J for J > 0, and J1 > 0 for J < 0, and in these two regions C increases with the increase of J1, so is the critical temperature Tc. When J1 >>|J| , C reaches its maximum value 0.5 and Tc reaches the asymptotic value Tc = 3.41448J1. For entanglement between the normal lattices, C appears only when J > 0 and 2J < J1 < J, and initially increases with the increase of J1 and arrives at the maximum value Cmax = (e4JIT-3)/(e4JIT+3) before it decays to zero gradually, so is the critical temperature Tc with, however, the maximum value Tcmax = 4J/ln3.     

Characteristic features of the singlet–triplet mechanism of the electron spin polarization

Characteristic features of net chemically induced dynamic electron spin polarization (CIDEP) P _n in triplet–radical (TR) quenching are analyzed in detail within the framework of a general model that enables one to analyze CIDEP both numerically and analytically. This model also makes it possible to accurately describe the nonadiabatic transitions between the terms of the TR-pair spin Hamiltonian that lead to CIDEP generation. The proposed theory yields a simple analytical dependence of P _n on the parameters of the model. In particular, it is shown that, within a wide region of parameters, the dependence of P _n on the coefficient of relative TR diffusion D _r is described by a simple linear relation: \(P_n^{ - 1}\left( {{D_r}} \right) = {Q_0} + \overline {{q_n}} {D_r}\) (with Q ₀ and \({{q_n}}\) independent of D _r ). It is also demonstrated that the numerical and analytical results obtained are very useful in analysis of experimental data, as demonstrated by analyzing the experimental dependence of P _n on D _r .     

Influence of the spin–orbit interaction in the impurity-band states of n-doped semiconductors

We study numerically the effects of an extrinsic spin–orbit interaction on the model of electrons in n-doped semiconductors of Matsubara and Toyozawa (MT). We focus on the analysis of the density of states (DOS) and the inverse participation ratio (IPR) of the spin–orbit perturbed states in the MT set of energy eigenstates in order to characterize the eigenstates with respect to their extended or localized nature. The finite sizes that we are able to consider necessitate an enhancement of the spin–orbit coupling strength in order to obtain a meaningful perturbation. The IPR and DOS are then studied as a function of the enhancement parameter.     

A simulation of the mixed spin 3–spin 3/2 ferrimagnetic Ising model

The mixed spin 3–spin 3/2 ferrimagnetic Ising model was simulated using cooling algorithm on cellular automaton (CA). The simulations were carried out in the intervals ?4 ≤ D_A/J ≤ 8 and ?4 ≤ D_B/J ≤ 8 for the square lattices with periodic boundary conditions. The ground-state phase diagram of the model has different types of ferrimagnetic phases. Although only the antiferromagnetic nearest-neighbor interaction was contained in the Hamiltonian, the compensation points emerged through D_A/J = 2 at kT/J = 0. The values of the critical exponents (ν, α , β and γ) were estimated within the framework of the finite-size scaling theory and power-law relations for the selected D_A/J values (?2, 0, 1, 2, and 4). The estimated critical exponent values were in good agreement with the universal values of the two-dimensional Ising model (ν = 1, α = α′ = 0, β = 0.125, β′ = 0.875 and γ = γ′ = 1.75).     

RVB description of the low-energy singlets of the spin 1/2 kagomé antiferromagnet

Extensive calculations in the short-range RVB (Resonating valence bond) subspace on both the trimerized and the regular (non-trimerized) Heisenberg model on the kagomé lattice show that short-range dimer singlets capture the specific low-energy features of both models. In the trimerized case the singlet spectrum splits into bands in which the average number of dimers lying on one type of bonds is fixed. These results are in good agreement with the mean field solution of an effective model recently introduced. For the regular model one gets a continuous, gapless spectrum, in qualitative agreement with exact diagonalization results. Received 7 March 2000     

Density-functional calculations of the nuclear magnetic shielding and indirect nuclear spin–spin coupling constants of three isomers of C20

The parameters of the nuclear magnetic resonance (NMR) spectrum – shielding constants and indirect spin–spin coupling constants – of three isomers of C₂₀ are studied using density-functional theory. The performance of different exchange–correlation functionals is analysed by optimising the geometry for the ring, bowl and cage isomers, followed by a computation of the NMR constants at the optimised structure. The results are analysed and rationalised by performing comparisons of the three isomers with one another and with related systems such as polyynes (for the ring), o-benzyne (for the bowl) and C₆₀ (for the cage). The shielding and spin–spin parameters calculated using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional are sufficiently reliable to assist in future experimental NMR studies of C₂₀ and, in particular, the identification of its isomers.     

NMR probing of the spin polarization of the ν=5/2 quantum Hall state

Resistively detected nuclear magnetic resonance is used to measure the Knight shift of the 75As nuclei and determine the electron spin polarization of the fractional quantum Hall states of the second Landau level. We show that the 5/2 state is fully polarized within experimental error, thus confirming a fundamental assumption of the Moore-Read theory. We measure the electron heating under radio frequency excitation and show that we are able to detect NMR at electron temperatures down to 30 mK.     

Effects of the spin–orbit coupling on the vacancy-induced magnetism on the honeycomb lattice

The local magnetism induced by vacancies in the presence of the spin–orbit interaction is investigated based on the half-filled Kane–Mele–Hubbard model on the honeycomb lattice. Using a self-consistent mean-field theory, we find that the spin–orbit coupling will enhance the localization of the spin moments near a single vacancy. We further study the magnetic structures along the zigzag edges formed by a chain of vacancies. We find that the spin–orbit coupling tends to suppress the counter-polarized ferrimagnetic order on the upper and lower edges, because of the open of the spin–orbit gap. As a result, in the case of the balance number of sublattices, it will suppress completely this kind of ferrimagnetic order. But, for the imbalance case, a ferrimagnetic order along both edges exists because additional zero modes will not be affected by the spin–orbit coupling.     

Prediction of the 1H NMR spectra of epoxy-fused cyclopentane derivatives by calculations of chemical shifts and spin–spin coupling constants

¹H NMR spectra of epoxy-fused cyclopentane derivatives have been computationally investigated with density functional calculations in order to unravel the shielding effect of the epoxy ring on the ¹H NMR chemical shifts of N-substituted epoxy-fused cyclopentane-3, 5-diol derivatives. Both ¹H NMR chemical shifts and spin–spin coupling constants have been calculated with the WP04/cc-pVTZ level of theory in solution. The WP04/cc-pVTZ// B3LYP/6-31+G(d) methodology has been found to reproduce the best experimental results on epoxy-fused cyclopentane derivatives. This study is expected to lead experimentalists in their endeavour to characterize epoxy-fused cyclic systems with ease.     

Measurement of the state of an individual spin using a “turnstile”

A scheme is proposed for measuring the state of an individual spin (system of spins). The scheme is based on the idea of a single-electron “turnstile” and the injection of single spin-polarized electrons from the magnetic metallic borders. Applications to the recently proposed scheme of quantum spin gates based on a silicon matrix are discussed (B. E. Kane, Nature 393, 133 (1998)). Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 141–147 (25 July 1999)     

Acoustic phonons mediated non-equilibrium spin current in the presence of Rashba and Dresselhaus spin–orbit couplings

Influence of electrons interaction with longitudinal acoustic phonons on magnetoelectric and spin-related transport effects are investigated. The considered system is a two-dimensional electron gas system with both Rashba and Dresselhaus spin–orbit couplings. The works which have previously been performed in this field, have revealed that the Rashba and Dresselhaus couplings cannot be responsible for spin current in the non-equilibrium regime. In the current Letter, a semiclassical method was employed using the Boltzmann approach and it was shown that the spin current of the system, in general, does not go all the way to zero when the electron–phonon coupling is taken into account. It was also shown that spin accumulation of the system could be influenced by electron–phonon coupling.     

Effects of charge on the structures and spin moments of Nil3 cluster

The structural stability and magnetic properties of the icosahedral Ni13, Ni13^＋1 and Ni13^-1 clusters have been obtained by utilizing all-electron density functional theory with the generalized gradient approximations for the exchange-correlation energy. The calculated results show that the ground states of neutral and charged clusters all favour a D3d structure, a distorted icosahedron, due to the Jahn-Teller effect. The radial distortions caused by doping one electron and by doping one hole are opposite to each other. Doping one electron will result in a 1/2 decrease and doping one hole will result in a 1/2 increase of the total spin. Both increasing interatomic spacing and decreasing coordination will lead to an enhancement of the spin magnetic moments for Nil3 clusters.     

Singular temperature dependence of the equation of state of superconductors with spin–orbit interaction in the low-temperature region

The equation of state is investigated for a thin superconducting film in a longitudinal magnetic field and with strong spin-orbit interaction at the critical point. As a first step, the state with the maximal value of the magnetic field for a given value of spin–orbit interaction at T = 0 is chosen. This state is investigated in the low-temperature region. The temperature contribution to the equation of state is weakly singular.     

Effect of Rashba spin–orbit coupling on the spin polarized transport in diluted magnetic semiconductor barrier structures

We investigate theoretically the effects of Rashba spin–orbit coupling on the spin dependent transport through diluted magnetic semiconductor single and double barrier structures in the presence of a magnetic field. We find that the Rashba spin–orbit coupling gives rise to an enhancement of the negative tunnelling magnetoresistance of the diluted magnetic semiconductor single barrier structure and a pronounced beating pattern in the tunnelling magnetoresistance and spin polarization of the diluted magnetic semiconductor double barrier structure.