Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

Can spin-dependent scattering mimic spin-dependent tunneling in heterogeneous ferromagnets? The discussion is triggered by the surprising similarity in magnetoresistance properties of granular ferromagnets with metallic, insulating or point-contact spacers.     

Interface scattering and spin-dependent transport in granular magnetite

We have prepared nearly monodisperse Fe₃O₄ of ∼50 nm by a chemical route and investigated the electrical and magnetic transports of the composite granular system. A Verwey transition is observed in the vicinity of 113 K. Above and below the Verwey transition, the electrical transport is dominated by electron hopping behavior showing a good linear relation between resistance and T^−1/2. The magnetoresistance (MR) increases with the applied field and does not follow the magnetization to reach the saturation at 10 KOe field. This indicates that the MR is mainly arising from the spin-dependent scattering of electrons through the grain boundaries. The temperature dependence of MR shows it has the highest MR value near the Verwey transition.     

Noneikonal effects in spin-dependent proton-nucleus interactions

Simple expressions are derived for the non-eikonal corrections to the Glauber diffraction approximation for the proton-nucleus scattering amplitudes, with the spin dependence of the proton-nucleon amplitudes taken into account. As an example, we study the numerical importance of these corrections for elastic p ⁵⁸Ni scattering at 800 MeV.     

Time of scattering in the one-dimensional inelastic tunnelling

Summary The various definitions for time of tunnelling across a barrier in one dimension, that have been introduced in the elastic case and discussed by Buttiker and various authors, are here extended allowing for inelastic losses to occur inside the barrier. The framework of a multichannel scattering theory is here used. Their relevance to inelastic tunnelling and resonant tunnelling is critically analysed. Results are reported for the average delay time of eq. (24) in the case of an inelastic point interaction with a harmonic oscillator inside the barrier. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Quantum conditional probability in one-dimensional scattering

A result of Wan and McLean on the asymptotic separation of states in quantum mechanics is used to analyze the motion of a particle across a one-dimensional potential of finite range in terms of quantum conditional probabilities. It is shown that probabilities conditional on transmission or reflection, defined according to the Lüders rule, yield the results to be expected by intuitive argument. The theorem of total probabilities, based on the events of transmission and reflection, is shown to hold for a class of observables, and examples are given both of observables which belong to this class and of observables which do not.     

A precise measurement of the spin-dependent neutron scattering length of 3He

The poor knowledge of the spin-dependent neutron scattering length of ³He has until now handicapped nuclear four body theory and the interpretation of excitations in the quantum liquid. We have measured, for the first time directly, the real part of the bound incoherent neutron scattering length, b_i′ of ³He. A neutron spin echo spectrometer was used to detect pseudomagnetic precession of polarised neutrons passing through polarised ³He gas. Any absolute calibrations of sample and beam parameters were avoided using simple transmission measurements with non-polarised neutrons. The only a priory information required was the spin-dependent neutron absorption cross section of ³He. The result is b_i′ = -2.365(20) fm, which reduces the prior uncertainty by a factor 30. The corresponding new value of the bound incoherent scattering cross section is σ_i = 1.532(12) barn. Including the known value of the coherent neutron scattering length, we obtain new values for the real parts of the free triplet and singlet neutron scattering lengths, a_-′ = 7.370(58) fm and a₊′ = 3.278(53) fm.     

Effects of nuclear structure in the spin-dependent scattering of weakly interacting massive particles

We present calculations of the nuclear from factors for spin-dependent elastic scattering of dark matter WIMPs from¹²³Te and¹³¹Xe isotopes, proposed to be used for dark matter detection. A method based on the theory of finite Fermi systems was used to describe the reduction of the single-particle spin-dependent matrix elements in the nuclear medium. Nucleon single-particle states were calculated in a realistic shell model potential; pairing effects were treated within the BCS model. The coupling of the lowest single-particle levels in¹²³Te to collective 2⁺ excitations of the core was taken into account phenomenologically. The calculated nuclear form factors are considerably less then the single-particle ones for low momentum transfer. At high momentum transfer some dynamical amplification takes place due to the pion exchange term in the effective nuclear interaction. But as the momentum transfer increases, the difference disappears, the momentum transfer increases and the quenching effect disappears. The shape of the nuclear form factor for the¹³¹Xe isotope differs from the one obtained using an oscillator basis.     

Analytical solution for the one-dimensional scattering problem

We introduce the scattering problem in the case of one-dimensional space. The transmission factor is calculated for the whole domain of the potential energy. Connection with a field in interaction with a two-level quantum system is established for which the coupling parameter is considered to be explicitly time-dependent.     

The problem of the 27Al neutron spin-dependent scattering length

Early measurements of the neutron spin-dependent scattering length of ²⁷Al by neutron precession have given two values, consistent within the limits of accuracy, whose mean value is b₊?b_? = + 0.52 ± 0.02 fm. The positive sign was recently put in doubt from known values of neutron resonance parameters. New experiments were undertaken on a sample of ²⁷Al, a compound sample of ²⁷Al and ⁴⁵Sc, together with a redetermination of the sign of the ²⁷Al gyromagnetic ratio. These experiments confirmed in sign and magnitude the b₊?b_? value previously found. We have so far no explanation of the discrepancy.     

Quenching of the spin-dependent scattering of weakly interacting massive particles on heavy nuclei

We present calculations of the quenching of the spin-dependent elastic scattering cross section for dark matter WIMPs on heavy nuclei. The theory of finite Fermi systems was used to describe the behavior of the nuclear spin matrix elements in the nuclear medium. The results of the calculations for planned dark matter detector nuclei are not only always smaller than corresponding single particle estimations but in some cases also differ from the ones obtained by using measured nuclear magnetic moments.One of the authors (M.A.N.) wants to thank the Max-Planck-Institut für Kernphysik (Heidelberg, Germany) for the hospitality during the time in which this work was carried out, and the Deutscher Akademischer Austauschdienst (DAAD) for financial support.     

dHvA observation of spin-dependent scattering by magnetic pairs of Co impurities in Au

We report the first dHvA observations of interaction effects between impurities, via measurements of the spin-dependent scattering of conduction electrons by magnetic pairs of Co impurities in Au. The local moment associated with Co pairs produces the only spin-dependent scattering in these dilute alloys because isolated Co impurities (which are also present) carry no local moment at low temperatures. The results are used to estimate the concentration (or average separation) of magnetic pairs.     

Raman scattering in one-dimensional ionic conductors Rb-priderites

Raman spectra of single crystals of RbAl and RbMg-priderites, which are known to be one-dimensional ionic conductors, have been measured and analyzed. In addition to the spectra due mainly to the vibrations of Ti(Al)-O or Ti(Mg)-O frameworks observed in the frequency region above 100 cm^?1, the Raman bands, which can be assigned to the vibration modes of Rb⁺ ions, have been observed in the frequency region below 100 cm^?1. Raman spectra of the solid solutions (Rb, K)-Al priderites have also been studied.     

Positivity inequalities for spin-dependent lepton-hadron scattering from the light-cone algebra of quark currents

Positivity inequalities restricting spin-dependent electron-nucleon scattering and neutrino-nucleon scattering have been found. Interesting bounds for the difference of the spin asymmetry on proton and neutron have been found from inequalities derived from the light-cone algebra of quark currents. The same results have been derived from the quark-parton model.     

Raman scattering from one-dimensional carbon systems

Although the Raman effect was discovered nearly 80 years ago, it is only recently that the special characteristics of Raman scattering for one-dimensional systems have been seriously considered. This review focuses on the special interest of the Raman effect for one-dimensional systems that is of particular relevance to carbon nanostructures. Two examples of Raman scattering in one-dimensional systems are given. The first illustrates the use of Raman spectroscopy to reveal the remarkable structure and properties of carbon nanotubes arising from their one-dimensionality. Some of the recent advances in using Raman spectroscopy to study doping and intercalation to modify nanotube properties are reviewed, in the context of a one-dimensional system. The second example is the Raman spectra of a linear chain of carbon atoms and the special properties of this interesting system. New approaches toward applying Raman spectroscopy to carbon nanostructures are also emphasized.     

Variable amplitude equations for one-dimensional scattering

Nonlinear first-order equations, similar to Calogero's equations, are derived for the forward and backward one-dimensional scattering amplitudes. In particular, the even potential case yields two uncoupled equations for the even and odd parity phase shifts. The present approach provides a fast and accurate means for the numerical solution of one-dimensional scattering problems. It also has many analytic merits, some of which are discussed. The connection between one-dimensional and three-dimensional high-energy scattering is reviewed. It is demonstrated that in the one-dimensional case, a slightly modified WKB wavefunction provides an excellent approximation to the exact wavefunction in the shortwave limit. In this limit, additivity of phase shifts for nonoverlapping static potentials is satisfied.     

Possible polarized neutron-nucleus scattering search for a new spin-dependent nucleon-nucleon coupling in the fermi range

An experimental opportunity is presented for the future to measure possible P- and T-non-invariant axion-like interaction between nucleons in the Fermi range. This interaction may be searched for in the measurement spin-dependent asymmetry of scattering of polarized neutrons in a keV-MeV energy range by heavy nuclei.     

Time-resolved nuclear spin-dependent small-angle neutron scattering from polarised proton domains in deuterated solutions

We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-Cr^V molecules in glycerol-water mixtures as a function of the concentration of EHBA-Cr^V and for different degrees of deuteration of the solvent. In the EHBA-Cr^V complex, the spins of the 20 protons which surround the paramagnetic Cr^V can be oriented using the method of dynamic nuclear polarisation (DNP), thereby offering the possibility to create locally a nuclear spin-dependent contrast for SANS. The time constants which describe the build-up of polarisation around the paramagnetic centre and the subsequent diffusion of polarisation in the solvent were determined by analysing the temporal evolution of the nuclear polarisation, which in turn was obtained by fitting a core-shell model to the time-dependent SANS curves. The results on the spin dynamics obtained using the scattering function of a core-shell could be independently confirmed by evaluating the integrated SANS intensity. A thermodynamic one-centre model is presented which is able to reproduce the observed dependence of the proton polarisation times on the proton concentration of the solvent.     

Description of spin-dependent observables in elastic proton-deuteron scattering on the basis of a generalized diffraction model

Spin-dependent observables of elastic proton-deuteron (pd) scattering at intermediate energies are considered on the basis of calculations within a generalized diffraction model. The generalization in question takes into account all ten helicity proton-proton (pp) and proton-neutron (pn) amplitudes constructed on the basis of a present-day partial-wave analysis and precise deuteron wave functions derived from two alternative models of nucleon-nucleon (NN) forces. Explicit relations between the invariant amplitudes of pd scattering and input NN helicity amplitudes are found. Among other things, these relations make it possible to reconstruct unknown pn amplitudes at energies above 1 GeV on the basis of data on pd scattering. The predictions of the generalized diffraction model for differential cross sections and analyzing powers are compared with the latest experimental data. Surprisingly good agreement with experimental data over a broad interval of scattering angles is found both for differential cross sections and for vector and tensor analyzing powers.     

Measurement of spin-dependent total cross-section differenceΔσ T in neutron-proton scattering at 16 MeV

A new measurement ofΔσ _T for polarized neutrons transmitted through a polarized proton target at 16.2 MeV has been made. A polarized neutron beam was produced with the³H(d, n)⁴ He reaction; proton polarization over 90% was achieved in a frozen spin target of 20 cm³ volume. The measurement yields the valueΔσ _T=(?126±21±14) mb. The result of a simple phase shift analysis for the³ S ₁?³ D ₁ mixing parameter ε₁ is presented and compared with the theoretical potential model predictions.