Analytical description of spin-Rabi oscillation controlled electronic transitions rates between weakly coupled pairs of paramagnetic states with S=1/2

We report on an analytical description of spin-dependent electronic transition rates which are controlled by a radiation induced spin-Rabi oscillation of weakly spin-exchange and spin-dipolar coupled paramagnetic states (S=1/2). The oscillation components (the Fourier content) of the net transition rates within spin-pair ensembles are derived for randomly distributed spin resonances with account of a possible correlation between the two distributions that correspond to the two individual pair partners. The results presented here show that when electrically or optically detected Rabi spectroscopy is conducted under an increasing driving field B_ 1, the Rabi spectrum evolves from a single resonance peak at s=\Omega_R, where \Omega_R=\gamma B_1 is the Rabi frequency (\gamma is the gyromagnetic ratio), to three peaks at s= \Omega_R, s=2\Omega_R, and at low s<< \Omega_R. The crossover between the two regimes takes place when \Omega_R exceeds the expectation value \delta_0 of the difference of the Zeeman energies within the pairs, which corresponds to the broadening of the magnetic resonance lines in the presence of disorder caused by hyperfine field or distributions of Lande g-factors. We capture this crossover by analytically calculating the shapes of all three peaks at arbitrary relation between \Omega_R and \delta_0. When the peaks are well-developed their widths are \Delta s ~ \delta_0^2/\Omega_R.Comment: 10 page, 5 figure

Electrical Detection and Magnetic-Field Control of Spin States in Phosphorus-Doped Silicon

Electron paramagnetic resonance of ensembles of phosphorus donors in silicon has been detected electrically with externally applied magnetic fields lower than 200 G. Because the spin Hamiltonian was dominated by the contact hyperfine term rather than by the Zeeman terms at such low magnetic fields, superposition states $\alpha{}| \uparrow \downarrow >+\beta{}| \downarrow \uparrow >$ and $-\beta{}| \uparrow \downarrow > + \alpha{}| \downarrow \uparrow >$ were formed between phosphorus electron and nuclear spins, and electron paramagnetic resonance transitions between these superposition states and $| \uparrow \uparrow >$ or $| \downarrow \downarrow >$ states are observed clearly. A continuous change of $\alpha{}$ and $\beta{}$ with the magnetic field was observed with a behavior fully consistent with theory of phosphorus donors in silicon.Comment: 6 pages, 5 figure

Transport and recombination through weakly coupled localized spin pairs in semiconductors during coherent spin excitation

Semi-analytical predictions for the transients of spin-dependent transport and recombination rates through localized states in semiconductors during coherent electron spin excitation are made for the case of weakly spin-coupled charge carrier ensembles. The results show that the on-resonant Rabi frequency of electrically or optically detected spin-oscillation doubles abruptly as the strength of the resonant microwave field gamma B_1 exceeds the Larmor frequency separation within the pair of charge carrier states between which the transport or recombination transition takes place. For the case of a Larmor frequency separation of the order of gamma B_1 and arbitrary excitation frequencies, the charge carrier pairs exhibit four different nutation frequencies. From the calculations, a simple set of equations for the prediction of these frequencies is derived

Magnetization Switching of Single Magnetite Nanoparticles Monitored Optically

Magnetic nanomaterials record information as fast as picoseconds in computer memories but retain it for millions of years in ancient rocks. This exceedingly broad range of times is covered by hopping over a potential energy barrier through temperature, ultrafast optical excitation for demagnetization or magnetization manipulation, mechanical stress, or microwaves. As switching depends on nanoparticle size, shape, orientation, and material properties, only single-nanoparticle studies can eliminate ensemble heterogeneity. Here, we push the sensitivity of photothermal magnetic circular dichroism down to individual 20-nm magnetite nanoparticles. Single-particle magnetization curves display superparamagnetic to ferromagnetic behaviors, depending on size, shape, and orientation. Some nanoparticles undergo thermally activated switching on time scales of milliseconds to minutes. Surprisingly, the switching barrier appears to vary in time, leading to dynamical heterogeneity. Our observations will help to identify and eventually control the nanoscale parameters influencing the switching of magnetic nanoparticles, an important step for applications in many fields

X-ray diffraction studies of the effects of N incorporation in amorphous CNx, materials

The effects of nitrogen incorporation on the atomic-scale structure of amorphous CNx samples have been studied for 0, 5, 20, and 30 at. % N concentration, by x-ray diffraction. Significant differences in the structure are observed on the incorporation of only 5 at. % N, and the changes in structure continue as further N is added. From the experimental data, we are able to obtain directly the average bond distances and then calculate the average bond angles for each of the samples. The average first neighbor distance shows a gradual decrease from 1.55 Angstrom for 0 at. % N, to 1.44 Angstrom for 30 at. % N, and a similar trend is observed in the position of the second neighbor peak. This gives a corresponding increase in the average bond angle from 108 degrees to 114 degrees. The results show an increase in the fraction of sp(2) bonded carbon atoms with increasing N concentration, and there is evidence for the presence of significant numbers of C=N and C=N bonds. These results are also consistent with stress, hardness, and optical gap measurements for these samples. (C) 1998 American Institute of Physics. [S0021-8979(98)03907-3]

The effect of lattice temperature on surface damage in fused silica optics

We examine the effect of lattice temperature on the probability of surface damage initiation for 355nm, 7ns laser pulses for surface temperatures below the melting point to temperatures well above the melting point of fused silica. At sufficiently high surface temperatures, damage thresholds are dramatically reduced. Our results indicate a temperature activated absorption and support the idea of a lattice temperature threshold of surface damage. From these measurements, we estimate the temperature dependent absorption coefficient for intrinsic silica

The effect of temporal pulse shape on optical damage

The conditions under which optical materials are susceptible to laser-induced damage is a topic which has been the subject of considerable study. Laser parameters such as wavelength and temporal pulse duration have been studied extensively. Until this work the effect of temporal pulse shape has not been considered. We present here data from a simple single-parameter model and a supporting experiment which predicts that a Flat-In-Time-pulse will produce damage at approximately 80% of the fluence of a Gaussian pulse of the same FWHM duration

Spectral Analysis of the Supreme Court

The focus of this paper is the linear algebraic framework in which the spectral analysis of voting data like that above is carried out. As we will show, this framework can be used to pinpoint voting coalitions in small voting bodies like the United States Supreme Court. Our goal is to show how simple ideas from linear algebra can come together to say something interesting about voting. And what could be more simple than where our story begins— with counting