Gauge fields with torsion

The Klein-Kaluza theory with a nonvanishing torsion is developed. The torsion is associated with spin and polarization of a gauge field. The electromagnetic polarization is considered as a source of additional components of torsion connected with the fifth dimension. New physical effects obtained due to this torsion are pointed out and some cosmological models are studied. It is proved that new effects are 10³⁶ times bigger than the effects from the Einstein-Cartan theory. The usual Dirac equation is generalized to the Klein-Kaluza theory with and without torsion. The dipole electric moment of a fermion of order 10^–32 cm is obtained. A new generalization of minimal coupling is proposed.Partially supported by Polish Ministry of Science, Higher Education and Technology project No. MR17.     

On a generalization of the Einstein-Cartan theory and the Klein-Kaluza theory

The Klein-Kaluza theory with a nonvanishing torsion is developed. The torsion is associated with the spin and polarization of an electromagnetic field. The electromagnetic polarization is considered as a source of additional components of a torsion connected with a fifth dimension. It is proved that the new effects are 10³⁶ times bigger than the effects from the Einstein-Cartan theory. Partially supported by Polish Ministry of Science, Higher Education and Technology, project No. Mr 17.     

Hole mobility in a homogeneous nucleotide chain

The conductivity of molecular DNA-based conductors has been calculated. Charge motion is described by quantum-mechanical equations, and macromolecular vibrations are described by classical equations of motion with dissipation and a source of temperature fluctuations. In a homogeneous sequence of G-C nucleotide pairs, the calculated hole mobility at T=300 K equals ≈2cm² V^?1 s^?1.     

On the Interplay of Magnetic and Molecular Forces in Curie–Weiss Ferrofluid Models

We consider a mean-field continuum model of classical particles in R ^d with Ising or Heisenberg spins. The interaction has two ingredients, a ferromagnetic spin coupling and a spin-independent molecular force. We show that a feedback between these forces gives rise to a first-order phase transition with simultaneous jumps of particle density and magnetization per particle, either at the threshold of ferromagnetic order or within the ferromagnetic region. If the direct particle interaction alone already implies a phase transition, then the additional spin coupling leads to an even richer phase diagram containing triple (or higher order) points.     

Big bounce from spin and torsion

The Einstein-Cartan-Sciama-Kibble theory of gravity naturally extends general relativity to account for the intrinsic spin of matter. Spacetime torsion, generated by spin of Dirac fields, induces gravitational repulsion in fermionic matter at extremely high densities and prevents the formation of singularities. Accordingly, the big bang is replaced by a bounce that occurred when the energy density e μ gT⁴{\epsilon \propto gT^4} was on the order of n²/m_Pl²{n^2/m_{\rm Pl}^2} (in natural units), where n μ gT³{n \propto gT^3} is the fermion number density and g is the number of thermal degrees of freedom. If the early Universe contained only the known standard-model particles (g ≈ 100), then the energy density at the big bounce was about 15 times larger than the Planck energy. The minimum scale factor of the Universe (at the bounce) was about 10³² times smaller than its present value, giving ≈ 50 μm. If more fermions existed in the early Universe, then the spin-torsion coupling causes a bounce at a lower energy and larger scale factor. Recent observations of high-energy photons from gamma-ray bursts indicate that spacetime may behave classically even at scales below the Planck length, supporting the classical spin-torsion mechanism of the big bounce. Such a classical bounce prevents the matter in the contracting Universe from reaching the conditions at which a quantum bounce could possibly occur.     

Is there evidence for torsion?

Under the assumption of a pulsar's magnetic field originating from the net polarized spin of its neutrons, and by using the post-Newtonian approximated torsion in the fifth order in (v/c), the spin precession arising from torsion is coordinate dependent, which influences the magnetic field of the pulsar and makes the magnetic inclination close to the rotation axis. Assuming the possibility of the magnetic inclination density to be in random alignment at the initial time, our theoretical calculations show that most pulsars should have smaller inclinations at the age of 10⁶-10⁷ yr, and that the inclination decreases with the increasing age of the pulsar. This fits the theoretical calculations and astronomically observed distribution.This essay received an honorable mention in 1990 from the Gravity Research Foundation-Ed.     

Spin gauge fields: From Berry phase to topological spin transport and Hall effects

The paper examines the emergence of gauge fields during the evolution of a particle with a spin that is described by a matrix Hamiltonian with n different eigenvalues. It is shown that by introducing a spin gauge field a particle with a spin can be described as a spin multiplet of scalar particles situated in a non-Abelian pure gauge (forceless) field U (n). As the result, one can create a theory of particle evolution that is gauge-invariant with regards to the group Uⁿ (1). Due to this, in the adiabatic (Abelian) approximation the spin gauge field is an analogue of n electromagnetic fields U (1) on the extended phase space of the particle. These fields are force ones, and the forces of their action enter the particle motion equations that are derived in the paper in the general form. The motion equations describe the topological spin transport, pumping, and splitting. The Berry phase is represented in this theory analogously to the Dirac phase of a particle in an electromagnetic field. Due to the analogy with the electromagnetic field, the theory becomes natural in the four-dimensional form. Besides the general theory, the article considers a number of important particular examples, both known and new.     

Gravitational two-body problem—A source theory viewpoint

An analysis of the semirelativistic gravitational two-body problem based on Schwinger's source theory is given. Our treatment is purely classical but nongeometrical. Only the large distance behavior of the gravitational stress tensor is seen to be relevant for order G² contributions. For a gravitational stress tensor that has a pure Newtonian form, only the traceless choice is seen to be consistent with Einstein's theory. We find results in agreement with the earlier results of Einstein, Infeld, and Hoffmann, and Barker and O'Connell for spin-2 graviton exchanges between Dirac particles, and with Börner, Ehlers, and Rudolph for spin precession in theories with arbitrary post-Newtonian parameter γ. The source approach clearly displays the analogy between gravitational interactions and classical electrodynamics. We also discuss the general relationship between the periastron advance and spin-precession frequencies for a class of gravitation theories. Brief estimates of the various spin-precession effects are given for the Hulse-Taylor pulsar.     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Polarization of baryons and vector mesons in quark jets ine + e −-annihilation

The cross section and density matrix elements fore ⁺ e ^?→V+X ande ⁺ e ^?→B+X are calculated in the γ-Z ⁰ interference region. The Feynman-Field model for the quark fragmentation functions is extended to include baryons and spin of the particles. Possibilities for obtaining more specific information about the spin dependence of the fragmentation mechanism and the weak coupling of quarks are discussed.     

EPR and NMR characterization of theS=9 excited state and spin density distribution in the single-molecule magnet Fe8Br8: Implications to theS=10 model and magnetization tunneling pathways

High-sensitivity, variable-frequency, high-field electron paramagnetic resonance (EPR) measurements and nuclear magnetic resonance (NMR) measurements are reported for theS=10 single-molecule magnet Fe₈Br₈. We find that theS=10 state is nested with its first excited state, withS=9, located at only 24±2 K above. Also reported are some preliminary⁸¹Br NMR measurements of the unpaired electron spin density on the Br^? sites. The results provide new insight and benchmarks for improved electronic and magnetic structural calculations and macroscopic tunneling pathways of this class of single-molecule magnets.     

Electron state density and electron diffusion coefficient in energy space in nonideal nonequilibrium plasmas

We suggest a model for a hydrogenic low-temperature nonequilibrium nonideal plasma that allows the kinetic parameters of the plasma to be calculated by the method of molecular dynamics by taking into account the interparticle interaction. The charges interact according to Coulomb’s law; for unlike charges, the interaction is assumed to be equal to a constant at a distance smaller than several Bohr radii. For a system of particles, we solve the classical equations of motion under periodic boundary conditions. The initial conditions are specified in such a way that the electrons have a positive total energy. We consider the temperatures 1-50 K and densities n = 10⁹?10¹⁰ cm^?3 produced in an experiment through laser cooling and resonant excitation. We calculate the electron state density as a function of the plasma coupling parameter and the electron diffusion coefficient in energy space for highly excited (Rydberg) electron states close to the boundary of the discrete and continuum spectra.     

Phase diagram of uniaxial antiferromagnetic particles: Field perpendicular to the easy axis

We consider a spherical uniaxial antiferromagnetic particle in the presence of an external magnetic field perpendicular to its easy axis. The model is described by a classical Heisenberg Hamiltonian including a single-ion uniaxial anisotropy, where the magnetic moments of the particle are represented by continuous spin vectors. We employ mean-field calculations and Monte Carlo simulations to determine the phase diagram of the system. The phase diagram in the plane field versus temperature is obtained for particles with radii ranging from three up to twelve spacing lattice units. We have seen that a particle with more than nine shells behaves as a true thermodynamic system. We find the explicit dependence of the zero temperature critical field and the Néel temperature on the diameter of the particle. At low temperatures, we have also shown that, for particles with three or more shells, the critical field follows a T² law, which is in agreement with the predictions of the spin-wave theory, when the field is perpendicular to the easy axis.     

Optically induced electron spin resonance in doped amorphous silicon

We report the first observations of optically induced electron spin resonance signals in doped and undoped amorphous silicon. We also report the observation of equilibrium surface or interface spin densities ～ 10¹³cm^?2 and volume spin densities ～ 6 × 10¹⁵ cm^?3. The small number of spins observed in equilibrium compared to the large optically induced spin density shows that most electrons are spin paired in equilibrium. We conclude that this implies a very small mean effective correlation energy, U ～ kT.     

Interactions of μAl acceptor impurity in weakly and heavily doped silicon

The interactions of the aluminum acceptor impurity in silicon are investigated using polarized negative muons. The polarization of negative muons is studied as a function of temperature on crystalline silicon samples with phosphorus (1.6×10¹³ cm^?3) and boron (4.1×10¹⁸ cm^?3) impurities. The measurements are performed in a magnetic field of 4.1 kG perpendicular to the muon spin, in the temperature range from 4 to 300 K. The experimental results show that, in phosphorus-doped n-type silicon, an _μAl acceptor center is ionized in the temperature range T>50 K. For boron-doped silicon, the temperature dependence of the shift of the muon spin precession frequency is found to deviate from the 1/T Curie law in the temperature range T ? 50 K. The interactions of a _μAl acceptor that may be responsible for the effects observed in the experiment are analyzed.     

Colored,spinning classical particle in an external non-abelian gauge field

We derive classical non-relativistic equations of motion for a colored, spinning point-like particle in an external SU(2) gauge field from Dirac equation. We find that in addition to the classical spin and color spin vector, S, I, it is necessary to introduce a new classical dynamical variable [J^ab], a, b = 1, 2, 3, describing a mixing of the spin and color. The constraint relations between [J^ab], S, I are also found.     

Intermediate electronic relaxation betweenS=3/2 andS=1/2 states in Fe(J-mph)NO with a jäger-type ligand J-mph

The incomplete spin transition between the low-spin (LS) (S=1/2) and the intermediate-spin (IS) (S=3/2) states in the iron (III) complex Fe(J-mph)NO (mph=4-methyl-o-phenylene), centered atT _c≈212 K, has been studied with⁵⁷Fe Mössbauer spectroscopy and magnetic susceptibility measurements between 80 K and 320 K. The lineshape of the Mössbauer spectra is well reproduced by a two state stochastic relaxation model resulting in values of about 2·10⁶ s^?1 to 7·10⁶ s^?1 for the IS→LS transition rate constantk _IL. The kinetics of this spin transition can be described by an Arrhenius equation yielding activation energiesE _IL=1.1 (2) kJ/mol andE _LI=6.1 (2) kJ/mol for the IS→LS and LS→IS conversion, respectively.     

Direct observation of superexchange in a disordered molecular solid

A novel observation of magnetic superexchange interactions in a disordered molecular solid is reported. Coordinated electrical conductivity and electron spin resonance measurements are made on controlled, variable concentrations of the molecule trip-p-tolylamine and its paramagnetic radical cation, molecularly dispersed in a polymer matrix. For a given concentration of spins in the concentration range 10¹⁹cm^-3 <N_s <5× 10²⁰ cm^-3, pronounced exchange narrowing is discovered as the density of neutral molecules is systematically increased. The temperature dependences of the ESR linewidth and the conductivity eliminate motional narrowing due to diffusive hopping of spins as the mechanism, since in these systems the paramagnetic ions are also the source of mobile charge.     

Eine empfindliche magnetische Waage zur Bestimmung kleiner Suszeptibilitätsdifferenzen

For a determination of thecarrier susceptibility in a semiconductor sample it is necessary to measure the susceptibilitydifference between different doped samples. The present article describes a susceptibility balance (torsion pendulum) for difference measurements between 140 and 300° K in vacuo. A permanent magnet with cylindrical yoke is used. The achievable accuracy of the susceptibilitydifference amounts to ±0·03% of thewhole susceptibility. The smallest detectable difference in mass susceptibility isΔχ=3 · 10^?11 cgs _m (according to the paramagnetism of 3 · 10¹⁵ electron spins at 140° K).     

Synchrotron radiation studies of mass-selected Fe nanoclusters deposited in situ

A portable UHV-compatible gas aggregation cluster source, capable of depositing clean mass-selected nanoclusters in situ, has been used at synchrotron radiation facilities to study the magnetic behaviour of exposed and Co-coated Fe clusters in the size range 250 to 540 atoms. X-ray magnetic circular dichroism (XMCD) studies of isolated and exposed 250-atom clusters show a 10% enhancement in the spin magnetic moment and a 75% enhancement in the orbital magnetic moment relative to bulk Fe. The spin moment monotonically approaches the bulk value with increasing cluster size but the orbital moment does not measurably decay till the cluster size is above ∼ 400 atoms. The total magnetic moments for the supported particles though higher than the bulk value are less than those measured in free clusters. Coating the deposited particles with Co in situ increases the spin moment by a further 10% producing a total moment per atom close to the free cluster value. At low coverages the deposited clusters are super-paramagnetic at temperatures above 10 K but a magnetic remanence at higher temperature emerges as the cluster density increases and for cluster films with a thickness greater than 50 ?(i.e. 2-3 layers of clusters) the remanence becomes greater than that of an Fe film of the same thickness produced by a conventional deposition source. Thick cluster-assembled film show a strong in-plane anisotropy. Received 14 December 2000