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排序方式: 共有65条查询结果,搜索用时 281 毫秒
1.
Battle PD Blundell SJ Brooks ML Hervieu M Kapusta C Lancaster T Nair SP Oates CJ Pratt FL Rosseinsky MJ Ruiz-Bustos R Sikora M Steer CA 《Journal of the American Chemical Society》2004,126(39):12517-12527
The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers. 相似文献
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3.
Zayachuk D Polyhach Y Slynko E Khandozhko O Rudowicz C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2004,60(6):1247-1256
The results of the extensive investigations of the variation of the EPR and NMR spectra of active centers due to the existence of the native defects generated by disorder in the IV-VI semiconductor matrices are presented. Both undoped and doped with Gd impurity powder samples of different grain sizes, made from Pb(1-x)Sn(x)Te crystals with the composition in the range 0 < or = x < or = 0.2 were studied. Impurity Gd ions were used as the paramagnetic EPR probe, whereas the 207 Pb nuclei as the NMR probe. The following aspects have been ascertained. (i) Grinding of the initial single crystals into powders leads to an additional component line appearing in the NMR spectra of the 207 Pb nuclei and also to a significant increase in the intensity of EPR spectra of the impurity Gd ions. (ii) Both the Gd EPR spectra as well as the 207 Pb NMR spectra undergo modifications due to isothermal annealing, whereas the character of these modifications is determined by both the temperature and duration of the thermal treatment applied. (iii) Some characteristic correlation between the variation of the EPR spectra of impurity Gd ions and that of the NMR spectra of 207 Pb nuclei, which results from the annealing of the samples, has been observed. Experimental results are interpreted based on the prevailing models of the behavior of the doped impurities and the native defects in the lead and tin telluride crystals. 相似文献
4.
Czesław Rudowicz Ireneusz Stefaniuk Roman Dziembaj Hitoshi Ohta Marcin Molenda Susumu Okubo Makoto Yoshida 《Research on Chemical Intermediates》2007,33(8):853-862
This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn3+ and Mn4+ ions in the lithium manganese spinel LiMn2O4. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for
this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural
properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried
out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis
of the temperature dependence of the HF-EMR spectra due to Mn3+ and Mn4+ ions in LiMn2O4 is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried
out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental
data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn3+ and Mn4+ ions in accounting for the characteristics most suitable for application of LiMn2O4 as a cathode material may be gained. 相似文献
5.
Subsequent treatment of N-crotoyl-(1S,2R)-bornane-10,2-sultam with EtMgCl, recrystallization of the product and saponification, afforded R-(-)-3-methylpenthanoic acid which was used for acylation of (1R,2S)-bornane-10,2-sultam. The product was converted into N-[(2S,3R)-2-amino-3-methylpentanoyl]-(1R,2S)-bornane-10,2-sultam by hydroxyamination with 1-chloro-1-nitrosocyclohexane, followed by reduction of the hydroxylamine grouping. Saponification of the sultam imide provided (+)-alloisoleucine. 相似文献
6.
This article elucidates the pitfalls identified in the software package SIMPRE recently developed by Baldoví et al. (J. Comput. Chem. 2013, 34, 1961) for modeling the spectroscopic and magnetic properties of single ion magnets as well as single‐molecule magnets. Analysis of the methodology used therein reveals that the crystal field parameters (CFPs), expressed nominally in the Stevens formalism, exhibit features characteristic for the CFPs expressed in the Wybourne notation. The resemblance of the two types of CFPs introduces a serious confusion that may lead to wrong comparisons of the CFPs taken from various sources. To clarify this confusion, the properties of the CFPs ( , ) associated with the Stevens operators ( X = S , J , or L ), which belong to the class of the tesseral‐tensor operators, are contrasted with those of the CFPs Bkq associated with the Wybourne operators , which belong to the class of the spherical‐tensor operators. Importantly, the confused properties of Stevens and Wybourne operators may bear on reliability of SIMPRE calculations. To consider this question independent calculations are carried out using the complete approach and compared with those of the restricted approach utilized earlier. It appears that the numerical results of the package SIMPRE are formally acceptable, however, the meaning of the CFPs must be properly reformulated. Several other conceptual problems arising from misinterpretations of the crucial notions and the CFP notations identified therein are also discussed and clarified. © 2014 Wiley Periodicals, Inc. 相似文献
7.
Baril N Lewa CJ de Certaines JD Canioni P Franconi JM Thiaudière E 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,154(1):22-27
A new method for NMR characterization of mechanical waves, based upon radiofrequency field gradient for motion encoding, is proposed. A binomial B1 gradient excitation scheme was used to visualize the mobile spins undergoing a periodic transverse mechanical excitation. A simple model was designed to simulate the NMR signal as a function of the wave frequency excitation and the periodicity of the NMR pulse sequence. The preliminary results were obtained on a gel phantom at low vibration frequencies (0-200 Hz) by using a ladder-shaped coil generating a nearly constant RF field gradient along a specific known direction. For very small displacements and/or B1 gradients, the NMR signal measured on a gel phantom was proportional to the vibration amplitude and the pulse sequence was shown to be selective with respect to the vibration frequency. A good estimation of the direction of vibrations was obtained by varying the angle between the motion direction and the B1 gradient. The method and its use in parallel to more conventional MR elastography techniques are discussed. The presented approach might be of interest for noninvasive investigation of elastic properties of soft tissues and other materials. 相似文献
8.
A method for determination of magnetocrystalline anisotropy constants of arbitrary order is proposed. The method is based on a least squares fitting of a phenomenological anisotropy energy for a given symmetry truncated at an arbitrary order term to a theoretical anisotropy energy computed exactly for a given energy level model. Several applications of the method to cubic systems are considered. The study reveals that the widely used expressions in the literature for the cubic constants K1 and K2 in terms of free energy for the three symmetry direction are of rather limitedvalidity only. The higher-order cubic constants K3, K4 and K5 are determined besides the usual K1 and K2 in temperature range 0 to 300 K. The importance of the higher- order terms with respect to the first term in the cubic anisotropy energy is discussed. The results show that the cubic constants K3 and K4 cannot be neglected for most of the energy level models studied at certain temperatures. 相似文献
9.
A.?A.?GaleevEmail author N.?M.?Khasanova C.?Rudowicz G.?S.?Shakurov G.?R.?Bulka N.?M.?Nizamutdinov V.?M.?Vinokurov 《Applied magnetic resonance》2004,26(4):533-542
An electron paramagnetic resonance (EPR) study of a synthetic single crystal of LiScGeO4 doped with Cr ions carried out earlier at the X- and Q-bands at 300, K has indicated additional weak lines. A detailed analysis of these EPR lines, which were tentatively attributed to the Fe3+ ions at two different mirror symmetry sites, is presented in this paper. The angular dependences in the three crystallographic planes were resolved by fitting the two distinct spectra denoted Fe3+(I) and Fe3+(II) with a spin Hamiltonian (S=5/2) of monoclinic symmetry. The rank-4 crystal field tensors at tetrahedral and octahedral sites were calculated with the point-charge model to determine the principal axis orientations of their cubic, tetragonal and trigonal components. A comparative analysis of the zero-field splitting tensors and the crystal field ones indicates that Fe3+(I) ions substitute for Sc3+ at octahedral sites and Fe3+(II) ions substitute for Ge4+ at tetrahedral sites with no significant distorition of the coordination polyhedra in the structure of LiScGeO4. 相似文献
10.
Madelin G Baril N Lewa CJ Franconi JM Canioni P Thiaudiére E de Certaines JD 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,161(1):108-111
A B(1) field gradient-based method previously described for the detection of mechanical vibrations has been applied to detect oscillatory motions in condensed matter originated from acoustic waves. A ladder-shaped coil generating a quasi-constant RF-field gradient was associated with a motion-encoding NMR sequence consisting in a repetitive binomial 13;31; RF pulse train (stroboscopic acquisition). The NMR response of a gel phantom subject to acoustic wave excitation in the 20-200 Hz range was investigated. Results showed a linear relationship between the NMR signal and the wave amplitude and a spectroscopic selectivity of the NMR sequence with respect to the input acoustic frequency. Spin displacements as short as a few tens of nanometers were able to be detected with this method. 相似文献