全文获取类型
收费全文 | 675篇 |
免费 | 29篇 |
国内免费 | 6篇 |
专业分类
化学 | 367篇 |
力学 | 37篇 |
数学 | 205篇 |
物理学 | 101篇 |
出版年
2022年 | 2篇 |
2021年 | 9篇 |
2020年 | 7篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 17篇 |
2015年 | 24篇 |
2014年 | 31篇 |
2013年 | 45篇 |
2012年 | 54篇 |
2011年 | 55篇 |
2010年 | 34篇 |
2009年 | 33篇 |
2008年 | 28篇 |
2007年 | 50篇 |
2006年 | 45篇 |
2005年 | 35篇 |
2004年 | 46篇 |
2003年 | 29篇 |
2002年 | 24篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 5篇 |
1997年 | 5篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 11篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1932年 | 1篇 |
排序方式: 共有710条查询结果,搜索用时 692 毫秒
71.
72.
73.
Banerjee D Paniagua JC Mugnaini V Veciana J Feintuch A Pons M Goldfarb D 《Physical chemistry chemical physics : PCCP》2011,13(41):18626-18637
Water soluble perchlorinated trityl (PTM) radicals were found to be effective 95 GHz DNP (dynamic nuclear polarization) polarizers in ex situ (dissolution) (13)C DNP (Gabellieri et al., Angew Chem., Int. Ed. 2010, 49, 3360). The degree of the nuclear polarization obtained was reported to be dependent on the position of the chlorine substituents on the trityl skeleton. In addition, on the basis of the DNP frequency sweeps it was suggested that the (13)C NMR signal enhancement is mediated by the Cl nuclei. To understand the DNP mechanism of the PTM radicals we have explored the 95 GHz EPR characteristics of these radicals that are relevant to their performance as DNP polarizers. The EPR spectra of the radicals revealed axially symmetric g-tensors. A comparison of the spectra with the (13)C DNP frequency sweeps showed that although the solid effect mechanism is operational the DNP frequency sweeps reveal some extra width suggesting that contributions from EPR forbidden transitions involving (35,37)Cl nuclear flips are likely. This was substantiated experimentally by ELDOR (electron-electron double resonance) detected NMR measurements, which map the EPR forbidden transitions, and ELDOR experiments that follow the depolarization of the electron spin upon irradiation of the forbidden EPR transitions. DFT (density functional theory) calculations helped to assign the observed transitions and provided the relevant spin Hamiltonian parameters. These results show that the (35,37)Cl hyperfine and nuclear quadrupolar interactions cause a considerable nuclear state mixing at 95 GHz thus facilitating the polarization of the Cl nuclei upon microwave irradiation. Overlap of Cl nuclear frequencies and the (13)C Larmor frequency further facilitates the polarization of the (13)C nuclei by spin diffusion. Calculation of the (13)C DNP frequency sweep based on the Cl nuclear polarization showed that it does lead to an increase in the width of the spectra, improving the agreement with the experimental sweeps, thus supporting the existence of a new heteronuclear assisted DNP mechanism. 相似文献
74.
In this paper we classify the centers localized at the origin of coordinates, and their isochronicity for the polynomial differential systems in R2 of degree d that in complex notation z=x+iy can be written as where j is either 0 or 1. If j=0 then d?5 is an odd integer and n is an even integer satisfying 2?n?(d+1)/2. If j=1 then d?3 is an integer and n is an integer with converse parity with d and satisfying 0<n?[(d+1)/3] where [⋅] denotes the integer part function. Furthermore λ∈R and A,B,C,D∈C. Note that if d=3 and j=0, we are obtaining the generalization of the polynomial differential systems with cubic homogeneous nonlinearities studied in K.E. Malkin (1964) [17], N.I. Vulpe and K.S. Sibirskii (1988) [25], J. Llibre and C. Valls (2009) [15], and if d=2, j=1 and C=0, we are also obtaining as a particular case the quadratic polynomial differential systems studied in N.N. Bautin (1952) [2], H. Zoladek (1994) [26]. So the class of polynomial differential systems here studied is very general having arbitrary degree and containing the two more relevant subclasses in the history of the center problem for polynomial differential equations. 相似文献
75.
In this work we study the existence of new periodic solutions for
the well knwon class of Duffing differential equation of the form
$x^{\prime\prime}+ c x^{\prime}+ a(t) x +b(t) x^3 = h(t)$, where $c$
is a real parameter, $a(t)$, $b(t)$ and $h(t)$ are continuous
$T$--periodic functions. Our results are proved using three
different results on the averaging theory of first order. 相似文献
76.
77.
Jaume Masoliver 《Journal of statistical physics》2006,123(3):703-703
78.
79.
Cairó Laurent Giacomini Hector Llibre Jaume 《Rendiconti del Circolo Matematico di Palermo》2003,52(3):389-418
We complete the classication of all Lotka-Volterra systemsx=x(ax+by+c),y=y(Ax+By+C), having a Liouvillian first integral. In our classification we take into account the first integrals coming from the existence
of exponential factors. 相似文献
80.
In this work we study the centers of planar analytic vector fields which are limit of linear type centers. It is proved that all the nilpotent centers are limit of linear type centers and consequently the Poincaré-Liapunov method to find linear type centers can be also used to find the nilpotent centers. Moreover, we show that the degenerate centers which are limit of linear type centers are also detectable with the Poincaré-Liapunov method. 相似文献