Gadolinium oxysulfide powders doped with different Tb
3+ concentrations were prepared from sulfur vaporization on rare earths’ basic carbonate precursors. Single-phase Gd
2O
2S samples were obtained, ...with Tb
3+ doping up to 9
at%. The study of the excitation mechanisms revealed that the Tb
3+ emission might occur after the direct Tb
3+ excitation either by energy transfer from Gd
3+ or from the phosphor host. The characteristic terbium emission lines were observed, resulting from the radiative decay from
5D
3 or
5D
4 to
7F
j levels. The cross-relaxation phenomenon was observed and its effects on the materials emission color were discussed based on the CIE diagram. By using time-resolved spectroscopy,
5D
3→
7F
J and
5D
4→
7F
J transitions were separated.
Using ideas from Landau's Fermi-liquid theory, we apply irreversible thermodynamics to conducting and insulating ferromagnets with magnetic variables M for the quantization axis and for the spin ...accumulation m of the nonequilibrium excitations; thus the total magnetization is taken to be M = M + m. The resulting theory closely corresponds to the theory of Silsbee et al. Silsbee, Janossy, and Monod, Phys. Rev. B 19, 4382 (1979). For the bulk, in addition to confirming the usual Landau-Lifshitz equation for M and a Bloch-like equation for → m (with a nonuniform precession term), there are two related cross-relaxation terms between the transverse parts of the nonequilibrium m and M. Unlike the s-d model, where in a field H the equilibrium magnetizations Ms and Md are both nonzero, for this m-M model in a field H, only the equilibrium magnetization M is nonzero. For the interface, the boundary condition for M is given by micromagnetics, and that for m is given by irreversible thermodynamics, where the current of transverse spins crossing the interface is proportional to the discontinuity in the transverse part of the vector spin chemical potential. M, m, and H are coupled; in the decoupled approximation, we find the wave vectors for the modes of M and the transverse m. We discuss reciprocity between spin pumping (M driven out of the ferromagnet) and spin transfer torque (M driven into the ferromagnet).
Trivalent thulium-doped K sub(5Bi(MoO) sub(4)) sub(4 single crystals were grown by the Czochralski method. Its polarized absorption and fluorescence spectra and fluorescence decay curves were ...recorded at room temperature. On the basis of the Judd-Ofelt theory, the spectral parameters of the Tm) super(3)+:K sub(5Bi(MoO) sub(4)) sub(4 crystal were calculated. The cross relaxations between Tm) super(3)+ ions were analyzed. The emission cross sections of the super(3F) sub(4) right arrow super(3H) sub(6) transition were obtained by the Fuchtbauer-Ladenburg formula and then the gain cross sections around 1.9 mu m were calculated. The peak emission cross section and width of emission band around 1.9 mu m are comparable to those for Tm super(3+:YAG and the tunable range is about 280 nm for the potential approx1.9 mu m laser operation via the ) super(3)F sub(4 right arrow ) super(3)H sub(6 transition.)
Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired ...electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40 mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.
A series of novel emission-tunable ScPO sub(4):xTb super(3+), yEu super(3+) phosphors were prepared by a high temperature solid-state reaction. The phase purity was examined using X-ray diffraction ...refinement. X-ray photoelectron spectroscopy (XPS) and the crystal information, luminescence properties and energy transfer between Tb super(3+) and Eu super(3+) are analyzed systematically. The cross relaxation from super(5)D sub(3) to super(5)D sub(4) of single doped Tb super(3+) in the host is investigated. The energy transfer between Tb super(3+) and Eu super(3+) has been demonstrated by the decay times, which are ascribed to the dipole-dipole (d-d) mechanism, and the eta sub(T) reaches 54.4%. Additionally, the energy transfer critical distance between Tb super(3+) and Eu super(3+) was calculated to be about 12.95 Aa. The emission color can be adjusted from green to yellow to orange-red by tuning the ratio of Tb super(3+)/Eu super(3+). The ScPO sub(4):0.03Tb super(3+), 0.025Eu super(3+) exhibits good thermal stability, indicating its great potential in w-LED applications.
One of the most remarkable properties of the nitrogen-vacancy (NV) center in diamond is that optical illumination initializes its electronic spin almost completely, a feature that can be exploited to ...polarize other spin species in their proximity. Here we use field-cycled nuclear magnetic resonance to investigate the mechanisms of spin-polarization transfer from NVs to C13 spins in diamond at room temperature. We focus on the dynamics near 51 mT, where a fortuitous combination of energy-matching conditions between electron and nuclear spin levels gives rise to alternative polarization transfer channels. By monitoring the C13 spin polarization as a function of the applied magnetic field, we show C13 spin pumping takes place via a multispin cross-relaxation process involving the NV− spin and the electronic and nuclear spins of neighboring P1 centers. Further, we find that this mechanism is insensitive to the crystal orientation relative to the magnetic field, although the absolute level of C13 polarization-reaching up to ∼3% under optimal conditions-can vary substantially depending on the interplay between optical pumping efficiency, photogenerated carriers, and laser-induced heating.
Atomic-scale colour centres in bulk and nanocrystalline SiC are promising systems for quantum photonics compatible with fiber optics, quantum information processing and sensing at ambient conditions. ...Colour centres which acts as stable single photon sources in SiC can be key elements for quantum photonics and communications. It has been shown that there are at least two families of colour centres in SiC with S = 1 and S = 3/2, which have the property of optical alignment of the spin levels even at room temperature and above. The spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance (ODMR), level anticrossing and cross-relaxation. Recently, we observed the effects of “hole burning” in the ODMR spectra, which made it possible to narrow the ODMR line by approximately an order of magnitude, which substantially increases the possibilities of technological applications of spin centres.
There is a close correlation between the photoluminescence and photocatalysis of a semiconductor because the two processes involve the same electronic transition. Eu3+ is one of the most popular ...rare-earth ions; it presents efficient luminescence and improved photocatalysis. Usually, the luminescence from the higher states (5D1,2) of Eu3+ can be hardly observed due to cross-relaxation processes. This work demonstrates a particularly developed transition from the 5D1 level of Eu3+ and its significant contribution to the improved photocatalysis of (Bi3Li)O4Cl2via prolonging the decay time of the excited state. Single-phase (Bi3−3xEu3xLi)O4Cl2 (x = 0, 0.01, 0.03, 0.05) was synthesized by the sol–gel method in combination with a solid-state reaction. This is a bismuth layer structure with (Bi/Li)O22+ layers formed by alternative Bi3+ and Li+ stripes. The intrinsic luminescence of (Bi3Li)O4Cl2 (λem = 500 nm) with a decay time of 0.26 μs was detected even at 300 K. Unusually, the phosphor shows a prominent transition from 5D1 in (Bi3−3xEu3xLi)O4Cl2. Moreover, in addition to 5D0 → 7F3, 5D0 → 7FJ (J = 0, 1, 2, 4) is characterized by intense transitions with comparable intensities. The intrinsic emission of (Bi3Li)O4Cl2 has a decay time which involves the band transition of an electron from the valence band to the conduction band. In Eu3+-doped (Bi3Li)O4Cl2, there are midgap states formed by the 5D1 level with a longer lifetime of about 60 μs. The D1 levels of Eu3+ significantly contribute to the separation of light-induced charges by prolonging the decay time of the excited states. This work demonstrates a simple strategy to develop optical materials with simultaneous luminescence and has improved the photocatalysis in bismuth oxychlorides featuring strong polarization and rigid phonons.
Cross-relaxation imaging is a new quantitative MRI modality, which allows mapping of fundamental parameters determining the magnetization transfer (MT) effect in tissues, cross-relaxation rate ...constant (
k) and bound pool fraction (
f). This study introduces a new time-efficient technique for cross-relaxation imaging, which obtains three-dimensional (3D) whole-brain
k and
f maps with scan time of <30 min and isotropic spatial resolution of 1.4 mm. The technical principle of the method is based on four-point fit of a matrix model of pulsed MT to imaging data obtained with variable offset frequency saturation while using a complimentary
R
1 (=1 /
T
1) map. Anatomical correlations of in vivo cross-relaxation parametric maps were evaluated in three healthy subjects. The
f maps revealed correspondence of areas with highly elevated
f = 12–15% to major fiber tracts such as corpus callosum, anterior commissure, optic radiations, and major brain fasciculi. The rest of white matter (WM) demonstrated lower
f = 9–11%, resulting in clear visual contrast of fiber tracts. Even lower
f = 6.5–8.5% were found in gray matter (GM) with the highest
f = 8.5% in the anterior thalamus. Distribution of
k was relatively uniform in WM and produced sharp contrast between GM and WM (
k = 1.6 and 3.3 s
−1, respectively). The most marked feature of
k maps was their ability to visualize the corticospinal tract, which had elevated
k = 3.4–3.8 s
−1 but appeared invisible on
f maps. The observed patterns on
f maps can be explained by variations in the density of myelinated fibers, while the trends of
k may reflect regional differences in axonal organization. Cross-relaxation imaging can be used in various clinical studies focused on brain development and white matter diseases.
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