Number Of Electron Spins Research Articles

Proton and Carbon-13 Dynamic Nuclear Polarization of Methylated β-Cyclodextrins.

1H and 13C dynamic nuclear polarizations have been studied in 13C-enriched β-cyclodextrins doped with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl free radical. 1H and 13C polarizations raised above 7.5 and 7%, respectively, and for both nuclear species, the transfer of polarization from the electron spins appears to be consistent with a thermal mixing scenario for a concentration of 9 13C nuclei per molecule. When the concentration is increased to 21 13C nuclei per molecule, a decrease in the spin-lattice relaxation and polarization buildup rates is observed. This reduction is associated with the bottleneck effect induced by the decrease in the number of electron spins per nucleus when both the nuclear spin-lattice relaxation and the polarization occur through the electron non-Zeeman reservoir. 13C nuclear spin-lattice relaxation has been studied in the 1.8-340 K range, and the effects of internal molecular motions and of the free radicals on the relaxation are discussed. 13C hyperpolarization performances and room-temperature spin-lattice relaxation times show that these are promising materials for future biomedical applications.

Manifestation of Strong Coupling Between Microwave Photons and Spins in the epr Spectrum of Anthracite

The change of the anthracite EPR signal shape that was caused by increasing the sample size was observed by continuous-wave EPR with automatic frequency control locked to the measuring cavity. For large samples (>10 mm3), hysteresis (a difference in the resonant magnetic flux densities when the magnetic field was scanned upwards or downwards) and a bistability (a sharp amplitude change) were observed. A two-peak spectrum for a strongly coupled system formed by an electron-spin ensemble and the cavity microwave field was found by changing the frequency of the microwave oscillator during recording of the EPR signals. The difference in the frequencies of these peaks (Rabi splitting) was a measure of the coupling strength between the spins and the microwave field and increased with increasing sample size. The simultaneous increase of the hysteresis and bistability of the EPR signals provided evidence that these peculiarities were caused by the Rabi splitting. Use of the Rabi splitting to determine the number of electron spins in a sample was proposed.

Electron spin resonance on a two-dimensional electron gas

Electron-spin-resonance (ESR) data obtained from modulation-doped ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}y}{\mathrm{C}}_{y}$ heterostructures are reported. The observed signals show that the small number of electron spins in these two-dimensional structures is still sufficient to detect ESR directly, and we have strong indications that the prominent line in the spectra is due to conduction electrons confined in the ${\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{C}}_{y}$ layers, forming a two-dimensional electron gas. We compare our results to the case of confined electrons in III-V heterostructures and quantum wells, where only indirectly detected ESR experiments have been reported so far.

Time‐reversal symmetry in molecular rovibronic states and in second‐order processes

AbstractA unified treatment of time‐reversal symmetry is given for molecular systems. The treatment allows for interconversion of electronic, rotational, and vibrational angular momenta on equal footing because of a coherent phase choice. It also allows for correlation from a continuous group (of spherical or cylindrical symmetry) to lower point groups (such as C3, C4, C6, S4, Sg, T, etc.). General, many‐electron molecular states that form time‐reversal degenerate components are constructed. Attention is called to the Kramers' doublets as the special odd‐electron case of such double degeneracy. Specific examples of a spiropentane and a 5‐azoniaspiro(4.4) nonane with S4 symmetry and possible time‐reversal degeneracy are given. The optical rotation due to a time‐odd polarizability tensor in one of the two time‐degenerate components is shown to be of an opposite sign to that in the other component. The above result is from a second‐order matrix element (over the polarizability tensor) and is proved to be independent of even‐ or odd‐number of electron spins. It is shown to hold also for Kramers' doubly degenerate components. This result is contrasted with that of a first‐order Jahn‐Teller effect which depends on the number of electron spins.

Magnetic Properties of Beta-Uranium Hydride

A thorough magnetic investigation of beta-uranium hydride has been carried out. The curve of reciprocal paramagnetic susceptibility against temperature generally follows the Curie-Weiss law and shows the same curvatures near the Curie point and at high temperatures as do the data for the ferromagnetic elements above their Curie points. A considerable number of magnetization curves have been obtained for a wide range of temperatures. Three methods were used for deducing the spontaneous magnetization for comparison. The curve of reduced spontaneous magnetization against reduced temperature lies very close to the curves of iron, nickel, and cobalt. The paramagnetic Curie temperature, 173\ifmmode^\circ\else\textdegree\fi{}K, is about 8\ifmmode^\circ\else\textdegree\fi{}K lower than the ferromagnetic Curie point. The number of electron spins per molecule obtained from the saturation magnetization and from the temperature dependence of the paramagnetic susceptibility shows the same discrepancy as do the ferromagnetic elements. Three hystereses loops at about 161.1\ifmmode^\circ\else\textdegree\fi{}K, 75\ifmmode^\circ\else\textdegree\fi{}K, and 4.2\ifmmode^\circ\else\textdegree\fi{}K and the coercive forces from about 170\ifmmode^\circ\else\textdegree\fi{}K down to 4.2\ifmmode^\circ\else\textdegree\fi{}K have been obtained. The magnetocrystalline anisotropy constants for the whole low-temperature range calculated from the magnetization curves and from the coercive forces agree in order of magnitude.