Optical nuclear polarization in heavy-doped silicon

The unusual behaviour of the optical polarization degree of ²⁹Si nuclei as a function of the illumination time has been found in silicon single crystals heavily doped with gadolinium. The effect of an external magnetic field on the process of optical nuclear polarization has been studied. It has been shown that the presence of second phase precipitations in heavily doped silicon leads to the complex unexponential establishment of the optical nuclear polarization degree with illumination time.     

Dynamic nuclear polarization and nuclear orientation in terbium vanadate

Dynamic nuclear polarization is a well established technique, which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests new experiments, involving the nuclear magnetic resonance of two isotopes, one stable, the other radioactive, in an antiferromagnet, terbium vanadate. At temperatures well below the Néel point, the line widths should be comparatively small. Resonance may be detected through changes in the rate of gamma-ray emission observed by a nuclear orientation experiment.     

Dynamic nuclear polarization and nuclear orientation in an antiferromagnet

Dynamic nuclear polarization is a well established technique which has been used to produce polarized targets for experiments in nuclear physics. This paper suggests experiments of a similar type but involving the nuclear magnetic resonance of two isotopes, one stable and the other radioactive. The substance is an antiferromagnet, dysprosium phosphate, at temperatures below the Néel point, where line widths are comparatively small. The effect may be detected through changes in the rate of gamma ray emission observed by a nuclear orientation experiment.     

Dynamic nuclear polarization in electron spin echo

It is shown that under the action of a proper microwave pulse sequence the equilibrium polarization of the electron spin may be transferred dynamically to the longitudinal nuclear magnetization which will oscillate due to the nuclear spin precession around the effective fields relating to differnt electron quantum number manifolds. These oscillations may be measured directly in the radiofrequency band. Analytical formulae are obtained for the case when all the nuclei coupled to an unpaired electron have spins of 1/2.     

Dynamic nuclear polarization in pulsed ENDOR experiments

Properly prepared pulse sequences of microwave and radio frequency have been employed to investigate the effect of polarization transfer from the polarized photo excited triplet state of pentacene in p-terphenyl crystals to the surrounding protons in pulsed ENDOR experiments. The ENDOR signal, measured as the change of electron spin echo (ESE) amplitude, is affected by the mode of RF pulses. When B0 parallelx (the long molecular axis), the ESE amplitude of the high-field transition of the triplet state changes from the maximum positive to zero with a pi RF pulse, and to the maximum negative with a 2pi pulse, while that of the low-field transition changes from nearly zero to the maximum negative as the RF pulse width increases. The effect is attributed to the strong electron spin polarization produced in the creation of the photoexcited triplet state and the subsequent efficient electron- nuclear polarization transfer process.     

Dynamic nuclear polarization of liquid helium-three

At Kazan State University a number of experiments have been proposed using solid paramagnetic compounds to produce nuclear polarization in liquid helium-three, for which the nuclear resonance frequency isf _n=32.4 MHz T^?1. This note suggests the application of magnetic resonance in antiferromagnetic compounds to this problem. In addition to a range of 3d ions, two compounds of lanthanide ions which become antiferromagnetic at liquid helium temperatures are discussed. Also, electron paramagnetic resonance is proposed using trivalent erbium ions at low abundance in metallic silver.     

Dynamic nuclear polarization with single electron spins

We polarize nuclear spins in a GaAs double quantum dot by controlling two-electron spin states near the anticrossing of the singlet (S) and m(S)= +1 triplet (T+) using pulsed gates. An initialized S state is cyclically brought into resonance with the T+ state, where hyperfine fields drive rapid rotations between S and T+, "flipping" an electron spin and "flopping" a nuclear spin. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.     

Dynamic nuclear polarization in irradiated ammonia below 0.5 K

We have reached +90.5% and ?93.6% dynamic proton polarization in solid NH₃ using paramagnetic radicals created by proton irradiation of 40 Mrad total dose. The dynamic polarization experiments were performed at 25 kG field in a dilution refrigerator. These results may indicate a breakthrough in the development of better polarized target materials for high-energy physics experiments.     

Dynamic nuclear polarization by electrical spin injection in ferromagnet-semiconductor heterostructures

Electrical spin injection from Fe into AlxGa1-xAs quantum well heterostructures is demonstrated in small (<500 Oe) in-plane magnetic fields. The measurement is sensitive only to the component of the spin that precesses about the internal magnetic field in the semiconductor. This field is much larger than the applied field and depends strongly on the injection current density. Details of the observed hysteresis in the spin injection signal are reproduced in a model that incorporates the magnetocrystalline anisotropy of the epitaxial Fe film, spin relaxation in the semiconductor, and the dynamic polarization of nuclei by the injected spins.     

Dynamic nuclear polarization system for the SANS-J-II spectrometer at JAEA

We have developed a dynamic nuclear polarization (DNP) system for the SANS-J-II spectrometer at the JRR-3 atomic research reactor of Japan Atomic Energy Agency (JAEA). The DNP system is composed of a split-type horizontal superconducting magnet (3.3 T), a Gunn oscillator as a microwave source (94 GHz), and a cryostat (1.2 K). In particular, a sample cell with 40 in inner diameter and the magnet with a field homogeneity of 5×10⁻⁵ in a volume of 25 mm×8 mm were employed to polarize samples with a diameter of 20 mm for the ultra small-angle scattering experiment using the magnetic lens installed at the SANS-J-II spectrometer [S. Koizumi, H. Iwase, J. Suzuki, T. Oku, R. Motokawa, H. Sasao, H. Tanaka, D. Yamaguchi, H.M. Shimizu, T. Hashimoto, J. Appl. Crystallogr. 40 (2007) s474]. We obtained the proton polarization |P|=32% in the polyethylene doped with 2,2,6,6-tetra-methyl-piperidine-1-oxyl (TEMPO).     

Dynamic nuclear polarization by thermal mixing of cyclohexane in its plastic phase

A new dynamic nuclear polarization effect, probably due to thermal mixing with the electronic spin-spin reservoir, has been observed in plastic cyclohexane doped with the free radical TANO. This probably reflects the existence of regions in the plastic phase where the radical concentration is high.     

Dynamic tunneling polarization as a quantum rotor analogue of dynamic nuclear polarization and the NMR solid effect

The populations of the tunneling states of CH(3) are manipulated by rf irradiation of weakly allowed sideband transitions within the manifold of tunneling-magnetic levels. Substantial positive and negative CH(3) tunneling polarizations are observed, providing a quantum rotor analogue of dynamic nuclear polarization and the solid effect in NMR. The field-cycling NMR technique used in the experiments employs level crossings between tunneling and Zeeman systems to report on the tunneling polarization. The tunneling lifetimes are measured and the field dependence investigated.     

Dynamic polarization detection

The azimuth angle of the plane of polarization of linearly polarized light is measured accurately by an electro-optically modulated opto-electronical device. The two ac output signals show up a phase difference which is linearly dependent on the azimuth angle. Continuously changing azimuth angles can be measured within 1 degree or better; changes in the azimuth angle as fast as 3 × 10⁴ rad/sec are easily followed by the detection electronics.     

High-frequency dynamic nuclear polarization in the nuclear rotating frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.     

Dynamic nuclear polarization at 9T using a novel 250 gyrotron microwave source

In the 1990’s we initiated development of high frequency gyrotron microwave sources with the goal of performing dynamic nuclear polarization at magnetic fields (∼5–23 T) used in contemporary NMR experiments. This article describes the motivation for these efforts and the developments that led to the operation of a gyrotron source for DNP operating at 250 GHz. We also mention results obtained with this instrument that would have been otherwise impossible absent the increased sensitivity. Finally, we describe recent efforts that have extended DNP to 460 GHz and 700 MHz ¹H frequencies.     

Nonequilibrium polarization of triplet centers in silicon

The Liouville equation is considered for the density operator of a triplet center with phenomenological population and decay constants (K₊ and K_-). In the case of anisotropy of K- with respect to the internal magnetic axis of the triplet, the spin polarization and intensities of the EPR lines of the triplet center in a strong magnetic field are calculated. On this basis, an interpretation of experiments on Si-Si centers in irradiated silicon is given. Quantum-chemical calculations of the model of the center in the form of a cluster OSi₄H₁₂ are performed, taking account of the spin-orbital interaction, the anisotropy of which agrees with the EPR spectra of Si-Si centers in silicon.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 97–101, June, 1989.     

Single polarization transmission in pedestal-supported silicon waveguides

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.     

Nuclear relaxation in dynamic nuclear polarization

The behavior of nuclear polarization in a substance, i.e., a solution of the complex HMBA(Cr^V)Na⁺ in 1,2-propylene glycol used in polarized nuclear targets is experimentally investigated by magnetic spectroscopic methods under conditions of dynamic nuclear polarization at hv_S/kT=≈1.5−3.2. Nuclear polarization is measured and analyzed as a function of time at different values of the saturating microwave signal and temperature. It is shown that the process of decreasing the nuclear polarization involving free nuclear relaxation is described by a nonmonoexponential law with two damping decrements, which determine the time of reaching equilibrium between the Zeeman nuclear subsystem, the dipole-dipole pool, and the lattice. Specific features of dynamic processes proceeding in the electronic-nuclear system of the substance investigated are discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 363–366, May–June, 1997.