Photon-electron correlations in double Compton scattering

The exact matrix element for the production of two circularly polarised photons in a double Compton scattering when the initial photon is also circularly polarised is calculated. The differential cross-sections for the various photon-electron polarisation correlations are obtained for high energy photons (i.e.) taking only near forward emission of photons. It is found that irrespective of the spin of the initial electron, the cross-section for either right or left circular polarisation of all three photons is zero under this approximation.     

Coupling-probe laser spectroscopy of degenerate two-level systems: An experimental survey of various polarisation combinations

Several absorption and dispersion spectra of a probe and a coupling laser of different polarisations generating electromagnetically induced absorption in atomic caesium were measured and compared. In the case of at least one linearly polarised laser, electromagnetically induced absorption and absorption within transparency were observed for the probe and coupling laser, respectively. For laser beams of counter-rotating circular polarisation the coupling absorption spectra changed from absorption within transparency to transparency within transparency once the medium was saturated. However, the corresponding parametric dispersion spectra remained unchanged in structure.     

Dynamic nuclear polarisation in metallic silver

Dynamic nuclear polarisation is a well established technique, which has been used to produce polarised targets for experiments in nuclear physics. This paper suggests that the nuclei spins in metallic silver may be polarised by inclusion of erbium as a small impurity with an electronic spin.     

Measurement of the gluon polarisation with COMPASS

The COMPASS experiment at CERN is investigating the nucleon spin structure in polarised deep inelastic muon scattering, mainly the gluon polarisation .     

Spin dynamic simulations of solid effect DNP: the role of the relaxation superoperator

Relaxation plays a crucial role in the spin dynamics of dynamic nuclear polarisation. We review here two different strategies that have recently been used to incorporate relaxation in models to predict the spin dynamics of solid effect dynamic nuclear polarisation. A detailed explanation is provided on how the Lindblad–Kossakowski form of the master equation can be used to describe relaxation in a spin system. Fluctuations of the spin interactions with the environment as a cause of relaxation are discussed and it is demonstrated how the relaxation superoperator acting in Liouville space on the density operator can be derived in the Lindblad–Kossakowski form by averaging out non-secular terms in an appropriate interaction frame. Furthermore we provide a formalism for the derivation of the relaxation superoperator starting with a choice of a basis set in Hilbert space. We show that the differences in the prediction of the nuclear polarisation dynamics that are found for certain parameter choices arise from the use of different interaction frames in the two different strategies. In addition, we provide a summary of different relaxation mechanisms that need to be considered to obtain more realistic spin dynamic simulations of solid effect dynamic nuclear polarisation.     

In situ SEOP polarised He neutron spin filter for incident beam polarisation and polarisation analysis on neutron scattering instruments

We discuss the development and characterisation of a new in situ spin exchange optical pumping (SEOP) based ³He neutron spin filter polarisation device. We present results from a recent test of the prototype system developed with the Institut Laue-Langevin. The polariser was installed on the polarised reflectometer CRISP at ISIS in the analyser position. The ³He was pumped continuously in situ on the beamline. The system also integrated a ³He adiabatic fast passage spin flipper that allowed reversal of the ³He and therefore neutron polarisation state, allowing for measurement of all four polarisation cross-sections. The system was run for a number of days reaching a ³He polarisation of 63%.     

A surprising method for polarising antiprotons

We propose a method for polarising antiprotons in a storage ring by means of a polarised positron beam moving parallel to the antiprotons. If the relative velocity is adjusted to v/c ≈ 0.002 the cross-section for spin-flip is as large as about 2 ^. 10¹³ barn as shown by new QED calculations of the triple spin cross-sections. Two possibilities for providing a positron source with sufficient flux density are presented. A polarised positron beam with a polarisation of 0.70 and a flux density of approximately 1.5 ^. 10¹⁰ /(mm² s) appears to be feasible by means of a radioactive ¹¹C dc-source. A more involved proposal is the production of polarised positrons by pair production with circularly polarised photons. It yields a polarisation of 0.76 and requires the injection into a small storage ring. Such polariser sources can be used at low (100MeV) as well as at high (1GeV) energy storage rings providing a time of about one hour for polarisation build-up of about 10¹⁰ antiprotons to a polarisation of about 0.18. A comparison with other proposals show a gain in the figure of merit by a factor of about ten.     

How could the proton transversity be measured

The perspectives of two new nonstandard methods of transverse quark polarisation measurement are considered: the jet handedness and the so-called “Collins effect” due to spin dependent T-odd fragmentation function responsible for the left-right asymmetry in fragmenting of transversely polarised quarks. Recent experimental indications in favour of the latter is observed. This makes us hope to use these effects in polarised DIS experiments for transversity measurement. The first estimation of transversity was done by using the azimuthal asymmetry in semi-inclusive DIS recently measured by HERMES and SMC.     

Polarised targets for 4<Emphasis Type="Bold">π</Emphasis> detectors at MAMI

The spin, as a fundamental property of a particle, has been a main object of investigation in particle and nuclear physics research in recent decades. Consequently, the investigation of polarisation observables has become more and more important. Nowadays cw beams with high duty cycles are standard technology. In parallel, the development of detectors with full angular acceptance, so called 4π detectors, has kept pace with the accelerator technology to allow for precise determination of the final state of the reaction under investigation. The availability of high speed data acquisition systems has enabled an increase in the luminosities by orders of magnitude. For complete spin investigation, one requires a polarised target in additon to polarised beams and recoil polarimeters. The advent of advanced beam and large acceptance detector technologies has driven the development of highly polarised, full angular acceptance targets.     

β-NMR

The β-NMR facility at ISAC is constructed specifically for experiments in condensed matter physics with radioactive ion beams. Using co-linear optical pumping, a ⁸Li^?+? ion beam having a large nuclear spin polarisation and low energy (nominally 30 keV) can be generated. When implanted into materials these ions penetrate to shallow depths comparable to length scales of interest in the physics of surfaces and interfaces between materials. Such low-energy ions can be decelerated with simple electrostatic optics to enable depth-resolved studies of near-surface phenomena over the range of about 2–200 nm. Since the β-NMR signal is extracted from the asymmetry intrinsic to beta-decay and therefore monitors the polarisation of the radioactive probe nuclear magnetic moments, this technique is fundamentally a probe of local magnetism. More generally though, any phenomena which affects the polarisation of the implanted spins by, for example, a change in resonance frequency, line width or relaxation rate can be studied. The β-NMR program at ISAC currently supports a number of experiments in magnetism and superconductivity as well as novel ultra-thin heterostructures exhibiting properties that cannot occur in bulk materials. The general purpose zero/low field and high field spectrometers are configured to perform CW and pulsed RF nuclear magnetic resonance and spin relaxation experiments over a range of temperatures (3–300 K) and magnetic fields (0–9 T).     

NMR with Hyperpolarised Protons in Metals

Proton pulse NMR, established as a versatile method in Solid State Physics, Chemistry, Biology and Medical Science, requires on the order of 10¹⁸ nuclei to detect an electromagnetic signal in a free induction decay (FID). The main cause for this small sensitivity is the low polarisation in the order of a few ppm due to the Boltzmann distribution in the magnetic field. Thus, NMR experiments on hydrogen are limited to metals with extremely high hydrogen solubility like Pd near room temperature. Using a polarised proton beam, a NMR signal is possible with as few as 10¹³ implanted nuclei. For the first time spin–spin and spin–lattice relaxation times were measured in Au and W with this technique at the Bonn cyclotron.     

Influence of spin–orbit interaction,Zeeman effect and light polarisation on the electronic and optical properties of pseudo-elliptic quantum rings under magnetic field

ABSTRACT

We theoretically investigated the influences of the magnetic field and light polarisation on the electronic and optical properties of a GaAs/GaAlAs pseudo-elliptic quantum ring, modelled by an outer ellipsis and an inner circle, in the presence of the Rashba and Dresselhaus spin–orbit interactions and Zeeman effect. We show that Aharonov-Bohm oscillations of the energy spectrum are not affected by the presence of the Zeeman effect alone but, in the presence of Rashba and Dresselhaus spin–orbit couplings, the periodicity of certain levels becomes hardly definite. The Zeeman effect generally enhances/diminishes the separation levels produced by Rashba/Dresselhaus interactions (SOI) and when both types of SOI are considered, the effect depends on their relative strength. The magnetic field can trigger spin-flip for each type of spin–orbit interaction and Zeeman effect or their combination through anticrossings in the energy spectra. Our results reveal that the absorption spectra are very sensitive to the magnetic field and light polarisation. For all polarisations considered, the magnetic field increment leads to the redshift or blueshift of some particular peaks (an effect of this ring geometry) and a better separation of the peaks. The x-polarised light determines spectra with many small, but separated peaks while the circular polarised light leads to spectra with large peaks of high amplitude.     

Transparent manganite films as hole injectors for organic light emitting diodes

Organic light-emitting diodes (OLEDs) are nowadays one of the most attractive devices based on organic semiconductors due to their successful application in the display technology. Electroluminescence in OLEDs is mainly governed by the fluorescence from excited singlet states, which have large transition probabilities providing the major radiative pathway. The “forbidden” triplet state emission can be activated by increasing spin–orbit coupling via dye doping. The singlet–triplet exciton formation statistics is usually given by 1:3 partition due to the quantum constrains.

Injection of carriers with finite spin polarisation should influence and modify the recombination statistics and can be used for tuning of the device efficiency. In this context, the development of a new class of electrodes able to guarantee both efficient charge and spin injection becomes of paramount importance. We show that strongly spin polarised colossal magnetoresistance manganite La_0.7Sr_0.3MnO₃ (LSMO) can successfully substitute conventional ITO electrodes in OLEDs. Highly transparent, metallic and ferromagnetic LSMO layers were used in combination with standard Al and spin polarised Co top electrodes. Electrical and optical characterisations of the OLEDs with spin polarised electrodes indicate the applicability of the new manganite electrodes for organic light-emitting devices.     

The Goos-Hänchen shift on a layer

A beam of linearly polarised light suffering total internal reflection is shifted longitudinally. Two different shifts are found, belonging to the eigenstates of TE and TM polarisation. A much smaller transverse shift occurs when left and right circular polarisation are considered as eigenstates. The measured shifts agree well with theoretical predictions and are referred to as Goos-Hänchen effect. In this contribution shifts are investigated for the case of frustrated total internal reflection on a thin optical barrier. As well known, there appears, in addition to the reflected beam, a transmitted beam for which hitherto the shifts were not reported.     

Transport in magnetic graphene superlattice with Rashba spin-orbit interaction

Based on the transfer-matrix method, we have investigated the spin-dependent transport properties of magnetic graphene superlattice in the presence of Rashba spin-orbit interaction (RSOI). It is shown that the angular range of the spin transmission probability through magnetic graphene superlattice can be efficiently controlled by the number of barriers. As the number of magnetic barriers increases, the angular range of the transmission through the magnetic superlattice decreases, the gaps in the transmission and conductivity versus energy become wider. It is also found that the spin conductivities oscillate with the Fermi energy and RSOI strength. Specifically, when a magnetic field is present, the spin polarisation can be observed, whereas for the RSOI alone it is zero. Application of such a phenomenon to design a spin polarised electron device based on the graphene material is anticipated.     

High intensity Compton scattering in a strong plane-wave field of general form

Photon emission by an electron embedded in a strong external field of general form is studied theoretically. The external field considered is a plane-wave electromagnetic field of any number of components, period and polarisation. Exact, Volkov solutions of the Dirac equation with the 4-potential of the general external field are obtained. The photon emission is considered in the usual perturbation theory using the Volkov solutions to represent the electron. An expression for the transition probability of this process is obtained after the usual spin and polarisation sums, trace calculation and phase space integration. The final transition probability in the general case contains a single sum over contributions from external field photons, an integration over one of the phase space components and the Fourier transforms of the Volkov phases. The validity of the general expression is established by considering specific external fields. Known specific analytic forms of the transition probability are obtained after substitution of the 4-potential for a circularly polarised and constant crossed external field. As an example usage of the general result for the transition probability, the case of two circularly polarised external fields separated by a phase difference is studied both analytically and numerically.     

Annihilation-in-flight of polarised positrons with polarised electrons as an analyser of the positron polarisation from muon decay

The use of annihilation-in-flight of arbitrarily polarised positrons with arbitrarily polarised electrons as an analyser for the positron polarisation from muon decay is discussed. Analysing powers for the longitudinal and the two transverse positron polarisation components are derived and algorithms for the simulation of polarised muon decay and of annihilation-in-flight are given. PACS 13.88.+e; 13.35.Bv; 12.60.-i; 11.30.Er; 02.70.Uu