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1.
Pohmann R Rommel E von Kienlin M 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,141(2):197-206
Chemical shift imaging (CSI) often suffers from the inconvenient shape of its spatial response function (SRF), which affects both localization and signal-to-noise ratio. Replacing the magnetic field gradients for phase encoding by higher order magnetic fields allows a better adjustment of the SRF to the structures in the sample. We combined this principle with the SLOOP (spectral localization with optimal pointspread function) technique to simultaneously obtain spectra from several arbitrarily shaped compartments within a sample. Linear combinations of the fields of the shim coils are used to generate the pulsed fields for phase encoding. Their shapes are matched to the given sample geometry by numerical optimization. Using this method, spectra from a phantom were obtained that show a higher signal-to-noise ratio and a strongly reduced contamination compared to an equivalent CSI experiment. 相似文献
2.
Algirdas Audzijonis Gediminas Gaigalas Leonardas Žigas Audrius Pauliukas Raimundas Žaltauskas Aurimas Čerškus Jurgis Narusis 《Central European Journal of Physics》2005,3(3):382-394
This paper presents the theoretical investigation of energy levels of valence bands (VB) and core levels (CL) of the ferroelectric
SbSl single crystals in antiferroelectric and ferroelectric phases. Since the best approximation for the deep VB levels is
a calculation by the Hartree-Fock method, the molecular model of a SbSI crystal was used for calculations. This model of the
crystal was also used for calculations of the total density of states. It was found that the VB and CL of this ferroelectric
semiconductor are sensitive to the small lattice distortion at the phase transition, and that an average of the total density
of states, when all atoms participate in oscillations of all normal modes, are more similar to the experimental X-ray photoelectron
spectra (XPS). The experimental splitting of CL obtained by XPS was compared with the theoretically calculated one by two
different methods. The cluster model calculations showed that the splitting of the CL in SbSI might be caused by photoelectron
emission from the atoms, which have different valence state, at the surface. 相似文献
3.
We consider a method for deriving relativistic two-body wave equations for fermions in the coordinate representation. The
Lagrangian of the theory is reformulated by eliminating the mediating fields by means of covariant Green's functions. Then,
the nonlocal interaction terms in the Lagrangian are reduced to local expressions which take into account retardation effects
approximately. We construct the Hamiltonian and two-fermion states of the quantized theory, employing an unconventional “empty”
vacuum state, and derive relativistic two-fermion wave equations. These equations are a generalization of the Breit equation
for systems with scalar, pseudoscalar, vector, pseudovector and tensor coupling. 相似文献
4.
V. P. Karassiov 《Journal of Russian Laser Research》2005,26(6):484-513
The notion and main features of polarization states of light are discussed within the framework of classical and quantum optics.
This notion is shown to be correctly defined for arbitrary light beams only within quantum optics by using the P-quasispin formalism developed earlier. Polarization states of quantum light are shown to be fully described by a polarization
density operator (PDO) obtained via reducing the total field density operator. Theoretical foundations are given for quantum
tomography of polarization states of light fields considered as a way of measuring PDO. Herewith, the main attention is paid
to a method where proper polarization tomographic observables (PDO “measurers”) are used. The method is shown to be adequately
formulated by means of quasi-spectral tomographic expansions of PDO in special operator bases (given by finite sums of partially
orthogonal projectors), which determine probability distributions of tomographic observables as expansion coefficients. Matrix
versions of such “tomographic” PDO representations are obtained. In particular, projections of these expansions on quasiclassical
operator bases, determining polarization quasiprobability functions, are given. An example of experimental implementation
of polarization tomography of unpolarized light (biphoton radiation with hidden polarization) is analyzed. 相似文献
5.
Electron spectral density of the half-filled Hubbard model in the atomic limit at finite temperature
We have calculated the spectral function and density of states of halffilled two-dimensional Hubbard model in the Hubbard-I
approximation assuming an antiferromagnetic long range order at low temperature and compared results to the QMC data. It occurs
that calculated functions are in a qualitative agreement with the QMC one. We have also shown that Neel ordered state dispersion
has the similar form to the spin density wave one. 相似文献
6.
Lin L Ding WX Brower DL Bergerson WF Carter TA Yates TF Almagri AF Chapman BE Sarff JS 《Physical review letters》2012,108(17):175001
Three-wave nonlinear coupling among spatial Fourier modes of density and magnetic fluctuations is directly measured in a magnetically confined toroidal plasma. Density fluctuations are observed to gain (lose) energy from (to) either equilibrium or fluctuating fields depending on the mode number. Experiments indicate that nonlinear interactions alter the phase relation between density and magnetic fluctuations, leading to strong particle transport. 相似文献
7.
求出了γ维空间中理想玻色气体的态密度,采用Thomas-Fermi近似,导出了γ维广义幂律势阱中粒子数密度的空间分布.在此基础上,求出了原子激射器的空间有效增益范围(即γ维势阱中玻色-爱因斯坦凝聚的空间有效范围),并对其产生影响的相关因素进行了讨论. 相似文献
8.
Xiaomin Wang Hong Zhou Kangsheng Chen 《International Journal of Infrared and Millimeter Waves》1993,14(6):1229-1236
A new kind of mm-wave cusptron tube with anti-coaxial-magnetron-type wall structure is proposed. The dispersion equation of its RF circuit and the coupled resonant interaction equations of electrons with the waveguide mode fields are derived. A numerical simulation of the non-linear interaction process is carried out. It is found that in the cusptron tube proposed here, there may exists a new energy conversion mechanism with higher efficiency than that of conventional cusptron. Numerical results also show that the saturation efficiency for this mechanism is not sensitive to the applied dc magnetic field Bo. The saturation efficiency as the function of operation parameters, such as magnetic field Bo and electron voltage V, and the geometry of waveguide circuit is numerically illustrated. From that, the optimized design parameters for anti-coaxial-magnetron-type cusptron can be determined. 相似文献
9.
R.I. Chelcea R. Fechete E. Culea D.E. Demco B. Blümich 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2009,196(2):178-190
The single-sided NMR-MOUSE sensor that operates in highly inhomogeneous magnetic fields is used to record a CPMG 1H transverse relaxation decay by CPMG echo trains for a series of cross-linked natural rubber samples. Effective transverse relaxation rates 1/T2,short and 1/T2,long were determined by a bi-exponential fit. A linear dependence of transverse relaxation rates on cross-link density is observed for medium to large values of cross-link density. As an alternative to multi-exponential fits the possibility to analyze the dynamics of soft polymer network in terms of multi-exponential decays via the inverse Laplace transformation was studied. The transient regime and the effect of the T1/T2 ratio in inhomogeneous static and radiofrequency magnetic fields on the CPMG decays were studied numerically using a dedicated C++ program to simulate the temporal and spatial dependence of the CPMG response. A correction factor T2/T2,eff is derived as a function of the T1/T2 ratio from numerical simulations and compared with earlier results from two different well logging devices. High-resolution T1–T2 correlations maps are obtained by two-dimensional Laplace inversion of CPMG detected saturation recovery curves. The T1–T2 experimental correlations maps were corrected for the T1/T2 effect using the derived T2/T2,eff correction factor. 相似文献
10.
I.?G.?Iashina L.?I.?Shchepina 《Bulletin of the Russian Academy of Sciences: Physics》2015,79(2):259-262
Various spatial and charge configurations of precolloidal centers in lithium fluoride crystals are explored. Theoretical investigations are based on density functional theory and pseudopotential and supercell geometry. Band gap structure is analyzed from the energy distribution of the electronic density of states. 相似文献
11.
Myron W. Evans 《Foundations of Physics Letters》2004,17(1):25-47
The first and second Maurer-Cartan structure relations are combined with the Evans field equation [1] for differential forms to build a grand unified field theory based on differential geometry. The tetrad or vielbein plays a central role in this theory, and all four fields currently thought to exist in nature can be described by the same equations, the tangent space index of the tetrad in general relativity being identified with the tetrad's internal (gauge group) index guage theory. 相似文献
12.
采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be1-xCdxO合金进行电子结构与能带特性研究. 结果表明: Be1-xCdxO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be1-xCdxO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be1-xCdxO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be1-xCdxO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系. 相似文献
13.
14.
The B-spline expansion technique and the time-dependent multilevel approach (TDMA) are used to study the interaction between microwave field and sodium atoms. The Rydberg sodium atom energy levels of p states in zero field are calculated, and the results are in good agreement with the other theoretical ones. The time evolutions during the population transfers of the five states from n=75 to n=79 in different microwave fields are obtained. The results show that the coherent control of the population transfer from the lower states to the higher ones can be accomplished by optimizing the microwave pulse parameters. 相似文献
15.
We apply density functional theory and the augmented spherical wave method to analyze the electronic structure of V2O3 in the vicinity of an interface to Al2O3. The interface is modeled by a heterostructure setup of alternating vanadate and aluminate slabs. We focus on the possible modifications of the V2O3 electronic states in this geometry, induced by the presence of the aluminate layers. In particular, we find that the tendency of the V 3d states to localize is enhanced and may even cause a metal-insulator transition. 相似文献
16.
We report on studies of stationary states and their transient dynamic for an incommensurate charge density wave (ICDW) in a restricted geometry of two spatial dimensions. The model takes into account multiple fields in mutual nonlinear interactions: the amplitude and the phase of the complex order parameter, and distributions of the electric and chemical potentials, of the density and the current of normal carriers. We observed spontaneous formation of vortices (the ICDW dislocations), and followed events of their creation and the subsequent evolution. The vortices appear when the voltage across, or the current through, the sample exceed a threshold. The number of vortices remnant in the reconstructed stationary state increases stepwise – in agreement with experiments, while a much greater number of vortices appears during the intermediate transient states. The vortex core concentrates the electric dipole leading to sharp drops of the electric and chemical potentials across the core. That can lead to enhanced inter-layer tunneling making the core to be a self-tuned microscopic tunneling junction. The results are applied to experiments on nano-fabricated mesa-junctions. They also appeal to modern efforts of the field-effect transformations in correlated electronic systems. 相似文献
17.
M. W. Evans 《Foundations of Physics Letters》1996,9(2):191-204
The B cyclics of electrodynamics, which relate transverse and longitudinal fields in vacuo, are one photon relations which are also valid on a macroscopic scale. In the same way, the Maxwell equations in the received view were originally phenomenological relations between electric and magnetic fields, but, in the received view are also written down for one photon. Point by point replies to van Enk are given. 相似文献
18.
Spherically symmetric solution in higher-dimensional teleparallel equivalent of general relativity 下载免费PDF全文
Gamal G.L.Nashed 《中国物理 B》2013,22(2):20401-020401
A theory of(N+1)-dimensional gravity is developed on the basis of the teleparallel equivalent of general relativity(TEGR).The fundamental gravitational field variables are the(N+1)-dimensional vector fields,defined globally on a manifold M,and the gravitational field is attributed to the torsion.The form of Lagrangian density is quadratic in torsion tensor.We then give an exact five-dimensional spherically symmetric solution(Schwarzschild(4+1)-dimensions).Finally,we calculate energy and spatial momentum using gravitational energy-momentum tensor and superpotential 2-form. 相似文献
19.
This paper is devoted to the asymptotics of the density of surfacic states near the spectral edges for a discrete surfacic Anderson model. Two types of spectral edges have to be considered: fluctuating edges and stable edges. Each type has its own type of asymptotics. In the case of fluctuating edges, one obtains Lifshitz tails the parameters of which are given by the initial operator suitably 'reduced' to the surface. For stable edges, the surface density of states behaves like the surface density of states of a constant (equal to the expectation of the random potential) surface potential. Among the tools used to establish this are the asymptotics of the surface density of states for constant surface potentials.Mathematics Subject Classifications (2000) 35P20, 46N50, 47B80. 相似文献
20.
二氧化钚分子激发态结构的外场效应 总被引:2,自引:0,他引:2
在相对论有效原子实势近似下,用B3LYP密度泛函方法计算优化得到分子轴方向不同电偶极场(-0.005~0.005 a.u.)作用下,二氧化钚的基态几何结构、电偶极矩和分子总能量.在优化构型下,用同样的基组,采用含时密度泛函(TDDFT)方法(TD-B3LYP),研究同样外电场条件下,对二氧化钚的激发能的影响.计算结果表明,在外场作用下,对PuO2的前5个激发态电子跃迁光谱属于可见-红外-远红外光谱,波长为501.47~10291.5 nm,这是钚原子的奇异特征;激发能与外电场的关系近似满足Grozema等人提出的关系. 相似文献