全文获取类型
收费全文 | 302274篇 |
免费 | 3320篇 |
国内免费 | 1146篇 |
专业分类
化学 | 166030篇 |
晶体学 | 4503篇 |
力学 | 12973篇 |
综合类 | 11篇 |
数学 | 31213篇 |
物理学 | 92010篇 |
出版年
2021年 | 1990篇 |
2020年 | 2298篇 |
2019年 | 2363篇 |
2018年 | 2763篇 |
2017年 | 2812篇 |
2016年 | 4773篇 |
2015年 | 3412篇 |
2014年 | 4705篇 |
2013年 | 13279篇 |
2012年 | 10697篇 |
2011年 | 13221篇 |
2010年 | 8628篇 |
2009年 | 8756篇 |
2008年 | 12367篇 |
2007年 | 12133篇 |
2006年 | 11778篇 |
2005年 | 10849篇 |
2004年 | 9727篇 |
2003年 | 8675篇 |
2002年 | 8485篇 |
2001年 | 10578篇 |
2000年 | 7887篇 |
1999年 | 6059篇 |
1998年 | 4431篇 |
1997年 | 4543篇 |
1996年 | 4391篇 |
1995年 | 3976篇 |
1994年 | 3842篇 |
1993年 | 3566篇 |
1992年 | 4285篇 |
1991年 | 4348篇 |
1990年 | 3965篇 |
1989年 | 3888篇 |
1988年 | 3892篇 |
1987年 | 3822篇 |
1986年 | 3579篇 |
1985年 | 4873篇 |
1984年 | 4862篇 |
1983年 | 3939篇 |
1982年 | 4101篇 |
1981年 | 3857篇 |
1980年 | 3775篇 |
1979年 | 3989篇 |
1978年 | 4145篇 |
1977年 | 4069篇 |
1976年 | 3919篇 |
1975年 | 3790篇 |
1974年 | 3692篇 |
1973年 | 3807篇 |
1972年 | 2352篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
The edge structure and stability of monolayer-high islands fabricated on Si(001) surfaces by scanning tunneling microscopy have been analyzed theoretically. In contrast to the edges of similar islands grown by depositing Si, the properties of edges of fabricated islands are determined by the length of the trench of dimers that are removed to create the island. We demonstrate the possibility of controlling the edge structure, and thus the stability, through a selective process of atom removal. 相似文献
993.
M. Kopcewicz J. Jagielski A. Grabias F. Stobiecki T. Stobiecki 《Hyperfine Interactions》1998,112(1-4):25-28
The Ar-ion-beam mixing of Fe/Zr bi- and multilayers is studied by conversion electron Mössbauer spectroscopy as a function of ion dose ranging from 1×1014 to 1×1017 at./cm2. The mixing leads to amorphization of the Fe/Zr system. It was shown that the mixing process in bilayers depends strongly on the thickness of the Zr substrate. The amorphization is much more effective and occurs at lower ion doses in multilayers as compared to corresponding bilayers. 相似文献
994.
G. T. Adamashvili 《Technical Physics》1998,43(5):600-601
The influence of transverse relaxation on a nonlinear acoustic wave generated in the presence of the acoustic self-induced
transparency (SIT) phenomenon is investigated. An explicit analytical equation for the profile of the nonlinear acoustic wave
is derived by a perturbation theory developed on the basis of the inverse scattering problem. This equation provides a possible
means for investigating experimentally the profile of an acoustic SIT pulse with allowance for relaxation effects, so that
these results can be used to construct new types of acoustoelectronic devices.
Zh. Tekh. Fiz. 68, 130–131 (May 1998) 相似文献
995.
Preparation of Papers 总被引:8,自引:0,他引:8
We motivate the study of a vector variational inequality by a practical flow equilibrium problem on a network, namely a generalization of the well-known Wardrop equilibrium principle. Both weak and strong forms of the vector variational inequality are discussed and their relationships to a vector optimization problem are established under various convexity assumptions. 相似文献
996.
We classify smooth subvarieties of codimension twoX⊂P
n
, 4≤n≤5, which are arithmetically Cohen-Macaulay and of non general type. By the way we exhibit some classes of non extendable
subvarieties. Then we give new proofs of the classification of scrolls inP
4; finally we consider smooth surfaces of non general type inP
4 arising from rank three vector bundles.
Sunto Classifichiamo le sottovarietà lisce di codimensione dueX⊂P n , 4≤n≤5, aritmeticamente Cohen-Macaulay e non di tipo generale. Nel contempo descriviamo alcune classi di sottovarietà non estendibili. Diamo poi due nuove dimostrazioni della classificazione degli scrolls inP 4; infine consideriamo superfici lisce inP 4, non di tipo generale, associate a fibrati di rango tre.相似文献
997.
LetB1:
n×
N1→
m1,B2:
n×
N2→
m2andQ:
m2→
m1be bilinear forms which are related as follows: ifμandνsatisfyB1(ξ, μ)=0 andB2(ξ, ν)=0 for someξ≠0, thenμτQν=0. Supposep−1+q−1=1. Coifman, Lions, Meyer and Semmes proved that, ifuLp(
n) andvLq(
n), and the first order systemsB1(D, u)=0,B2(D, v)=0 hold, thenuτQvbelongs to the Hardy spaceH1(
n), provided that both (i)p=q=2, and (ii) the ranks of the linear mapsBj(ξ, ·) :
Nj→
m1are constant. We apply the theory of paracommutators to show that this result remains valid when only one of the hypotheses (i), (ii) is postulated. The removal of the constant-rank condition whenp=q=2 involves the use of a deep result of Lojasiewicz from singularity theory. 相似文献
998.
999.
We study the spectrum and eigenmodes of the QCD Dirac operator in a gauge background given by an instanton liquid model (ILM) at temperatures around the chiral phase transition. Generically we find the Dirac eigenvectors become more localized as the temperature is increased. At the chiral phase transition, both the low lying eigenmodes and the spectrum of the QCD Dirac operator undergo a transition to localization similar to the one observed in a disordered conductor. This suggests that Anderson localization is the fundamental mechanism driving the chiral phase transition. We also find an additional temperature dependent mobility edge (separating delocalized from localized eigenstates) in the bulk of the spectrum which moves toward lower eigenvalues as the temperature is increased. In both regions, the origin and the bulk, the transition to localization exhibits features of a 3D Anderson transition including multifractal eigenstates and spectral properties that are well described by critical statistics. Similar results are obtained in both the quenched and the unquenched case though the critical temperature in the unquenched case is lower. Finally we argue that our findings are not in principle restricted to the ILM approximation and may also be found in lattice simulations. 相似文献
1000.
Alexis C. Kaporis Lefteris M. Kirousis Yannis C. Stamatiou Malvina Vamvakari Michele Zito 《Discrete Applied Mathematics》2007,155(12):1525-1538
The problem of determining the unsatisfiability threshold for random 3-SAT formulas consists in determining the clause to variable ratio that marks the experimentally observed abrupt change from almost surely satisfiable formulas to almost surely unsatisfiable. Up to now, there have been rigorously established increasingly better lower and upper bounds to the actual threshold value. In this paper, we consider the problem of bounding the threshold value from above using methods that, we believe, are of interest on their own right. More specifically, we show how the method of local maximum satisfying truth assignments can be combined with results for the occupancy problem in schemes of random allocation of balls into bins in order to achieve an upper bound for the unsatisfiability threshold less than 4.571. In order to obtain this value, we establish a bound on the q-binomial coefficients (a generalization of the binomial coefficients). No such bound was previously known, despite the extensive literature on q-binomial coefficients. Finally, to prove our result we had to establish certain relations among the conditional probabilities of an event in various probabilistic models for random formulas. It turned out that these relations were considerably harder to prove than the corresponding ones for unconditional probabilities, which were previously known. 相似文献