Electron spin polarisation in the photoreduction of xanthone in alcohol: effect of concentration and temperature

Time-resolved EPR studies of the hydrogen abstraction reaction of photoexcited xanthone in 2-propanol were carried out as a function of the concentration of xanthone and the sample temperature. The temperature was varied from 22°C to about ?30°C, and the concentration from about 0.2 to 4.0?mM. At low temperature or concentration, the observed spectra of the xanthone ketyl radical and the propan-2-olyl radical could be simulated as a superposition of a hyperfine-independent component due to the emissive triplet mechanism and a hyperfine-dependent component due to the S–T₀ radical pair mechanism. However, with an increase in the concentration of xanthone, the relative contribution of TM decreases, and, concomitantly, the net absorptive component of only the xanthone ketyl radical increases. As the spin polarisation mechanisms do not predict any concentration dependence, this unusual behaviour is explained by invoking the enhancement of the spin–lattice relaxation rates due to Heisenberg spin exchange occurring at high local concentrations of the radicals. The net absorptive signal is attributed to thermally equilibrated radicals. The observed temperature dependence of the spin polarisation behaviour is similarly explained. The origin of the net absorptive signal in the TREPR spectra of the acetone?2-propanol system is also attributed to thermally equilibrated radicals. The self-quenching mechanism of xanthone is proposed to be an electron-transfer reaction from an excited xanthone molecule to another xanthone in the ground state.     

Electron spin resonance shift in spin ladder compounds

We analyze the effects of different coupling anisotropies in a spin-1/2 ladder on the electron spin resonance (ESR) shift. Combining a perturbative expression in the anisotropies with density matrix renormalization group computation of the short range correlations at finite temperature, we provide the full temperature and magnetic field evolution of the ESR paramagnetic shift. We show that for well chosen parameters the ESR shift can be in principle used to extract quantitatively the anisotropies and, as an example, discuss the material BPCB.     

Characterisation and application of a SPLEED-based spin polarisation analyser

A commercial electron spin analyser, based on spin-polarised low-energy electron diffraction (SPLEED) from W(1 0 0), has been characterised with incident polarised electron beams from a standard GaAs polarised electron source. The dependence of the Sherman function on the scattering energy and elapse time after CO-flash of the tungsten crystal of the analyser have been measured. The influence of the stray magnetic field on the performance of the analyser has been investigated. The spin analyser has been applied in monitoring the reorientation transition of the easy magnetisation direction of Fe films on W(1 1 0) upon the exposure of CO adsorbent on the surface.     

Electron spin resonance in superconductors

The internal field distribution in type II superconductors substantially alters the electron spin resonance g value and line shape. The resulting changes can be used to determine both the width and approximate shape of the internal field distribution.     

Electron spin torque in atoms

The spin torque and zeta force, which govern spin dynamics, are studied by using monoatoms in their steady states. We find nonzero local spin torque in transition metal atoms, which is in balance with the counter torque, the zeta force. We show that d-orbital electrons have a crucial effect on these torques. Nonzero local chirality density in transition metal atoms is also found, though the electron mass has the effect to wash out nonzero chirality density. Distribution patterns of the chirality density are the same for Sc–Ni atoms, though the electron density distributions are different.     

Real-time control of polarisation in ultra-short-pulse laser micro-machining

The use of a fast-response, transmissive, ferroelectric liquid-crystal device for real-time control of the polarisation direction of a femtosecond laser beam, and the benefits for various aspects of ultra-short pulse micro-machining, are discussed. Several configurations have been used to drive the polarisation in real-time. Our microscopic investigations of the resulting features revealed significant improvements in process efficiency and quality, compared to static linear and circular polarisations. Following our successful micro-machining tests, real-time polarisation control could emerge as a powerful tool in laser engineering.     

Electron spin resonance in polyphenylacetylene

By polymerizing phenylacetylene in the absence of initiator and in vacuo while observing electron spin resonance, it was established that the paramagnetism in the thermal polymer (PPA-T) cannot be due to interaction with oxygen but must be a property of the chemically uncontaminated macromolecule. The ESR behavior of a concentrated solution of PPA(T) in monomer and of solid (amorphous) PPA(T) displays a Curie dependence, whereas the ESR absorption of crystalline polymer made with coordination catalysts increases by one to two orders of magnitude over the temperature of about 60 to 140°C. The major portion of the XRD crystallinity disappears more sharply, at about 120°C. The paramagnetism in PPA must therefore be associated with “conformational defects” in the individual macromolecules.     

Electron spin coherence in Si

We discuss pulsed electron spin resonance measurements of electrons in Si and determine the spin coherence from the decay of the spin echo signals. Tightly bound donor electrons in isotopically enriched ²⁸Si are found to have exceptionally long spin coherence. Placing the donors near a surface or interface is found to decrease the spin coherence time, but it is still in the range of milliseconds. Unbound two-dimensional electrons have shorter coherence times of a few microseconds, though still long compared to the Zeeman frequency or the typical time to manipulate a spin with microwave pulses. Longer spin coherence is expected in two-dimensional systems patterned into quantum dots, but relatively small dots will be required. Data from dots with a lithographic size of 400 nm do not yet show longer spin coherence.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin polarization in field emission

The field emission microscope can be extended to determine the directionally weighted spin-dependent surface density of states of magnetic metals or to obtain information on the electronic structure of metal to magnetic-semiconductor interfaces. We describe the techniques of spin polarization measurements in field emission. The measurement of Kisker et al. on ferromagnetic EuS evaporated on W are discussed along with recent investigations on Ni.     

Electron spin decoherence in isotope-enriched silicon

Silicon is promising for spin-based quantum computation because nuclear spins, a source of magnetic noise, may be eliminated through isotopic enrichment. Long spin decoherence times T2 have been measured in isotope-enriched silicon but come far short of the T2=2T1 limit. The effect of nuclear spins on T2 is well established. However, the effect of background electron spins from ever present residual phosphorus impurities in silicon can also produce significant decoherence. We study spin decoherence decay as a function of donor concentration, 29Si concentration, and temperature using cluster expansion techniques specifically adapted to the problem of a sparse dipolarly coupled electron spin bath. Our results agree with the existing experimental spin echo data in Si:P and establish the importance of background dopants as the ultimate decoherence mechanism in isotope-enriched silicon.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in pyrolyzed lignin

An investigation has been made into electron spin resonance (ESR) occurring in the ranges of carbons prepared from periodate lignin by vacuum pyrolysis. Line shape, line width, g-factors, saturation effects, and free spin concentration have been measured and the effect of oxygen on these parameters has been examined. A g-shift and lines with inhomogeneous broadening have been observed. In the presence of oxygen the lower-temperature carbons showed an unexpected increase in the detectable free spin concentration. The discussion includes factors affecting the nature of the free spins in the pyrolyzed material.     

Electron spin polarization in uranium sulfide

We present spin polarization and energy distribution curves of photo-electrons from US and compare them with a density of states calculation based on a cluster method. For photon energies hv ≤ 11 eV the photo-electron spin polarization is negative. Near threshold a structure is observed in the spectra, which is attributed to emission from the 5f-states.     

Investigation of ultrafast nuclear spin polarization induced by short laser pulses

We theoretically investigate the dynamics of nuclear spin induced by short laser pulses and show that ultrafast nuclear spin polarization can take place. Combined use of the hyperfine interaction together with the static electric field is the key for that. Specifically we apply the idea to unstable isotopes, (27)Mg and (37)Ca, with nuclear spin of 1/2 and 3/2, respectively, and show that 88% and 62% of nuclear spin polarization can be achieved within a few to tens of ns, which is 2-3 orders of magnitude shorter than the time needed for any known optical methods. Because of its ultrafast nature, our scheme would be very effective not only for stable nuclei but also unstable nuclei with a lifetime as short as mus.     

Electron spin resonance-scanning tunneling microscopy

Electron spin resonance-scanning tunneling microscopy (ESR-STM) is a rapidly developing surface-science technique that is sensitive to a single spin existing on or nearby a solid surface. The single spin is detected through elevated noise at the Larmor frequency that appears when the single spin participates in the tunneling process between the tip and the surface. In this review, experimental and theoretical works which have been performed up to date on ESR-STM are reviewed. The remaining experimental problems which have to be solved, possible approaches to differentiate between different mechanisms and the future of ESR-STM are discussed.\ PACS: 72.25.Dc Spin polarized transport in semiconductors, 72.70.+m Noise processes and phenomena, 73.20.Hb Impurity and defect levels; energy states of adsorbed species, 73.40.Gk Tunneling, 75.70.Rf Surface magnetism, 75.76.+j Spin transport effects, 76.30.-v Electron paramagnetic resonance and relaxation, 78.47.-p Spectroscopy of solid state dynamics     

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%.     

Electron spin resonance in sine-gordon spin chains in the perturbative spinon regime

We report the low-temperature multifrequency ESR studies of copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating g tensor and the Dzyaloshinskii-Moriya interaction, allowing us to test a new theoretical concept proposed recently by Oshikawa and Affleck [Phys. Rev. Lett. 82, 5136 (1999)]. Their theory, based on bosonization and self-energy formalism, can be applied for precise calculation of ESR parameters of S=1/2 antiferromagnetic chains in the perturbative spinon regime. Excellent quantitative agreement between the theoretical predictions and experiment is obtained.     

Electron spin resonance in X-irradiated sodium sulphate

Reliable results are reported for the various triple-dipole dispersion energy coefficients needed to evaluate the (exact) triple-dipole dispersion energy for all three-body interactions involving He, Ne, Ar, Kr, Xe, H₂, N₂, CO, O₂ and NO, and at least one molecule. A total of 645 unique coefficients, corresponding to 185 unique interactions, are considered.