全文获取类型
收费全文 | 309315篇 |
免费 | 4146篇 |
国内免费 | 644篇 |
专业分类
化学 | 169336篇 |
晶体学 | 4366篇 |
力学 | 13275篇 |
综合类 | 9篇 |
数学 | 36340篇 |
物理学 | 90779篇 |
出版年
2020年 | 2662篇 |
2019年 | 2875篇 |
2018年 | 4028篇 |
2017年 | 3835篇 |
2016年 | 5849篇 |
2015年 | 3897篇 |
2014年 | 5519篇 |
2013年 | 13218篇 |
2012年 | 11025篇 |
2011年 | 12983篇 |
2010年 | 8894篇 |
2009年 | 8335篇 |
2008年 | 11906篇 |
2007年 | 12103篇 |
2006年 | 11160篇 |
2005年 | 10285篇 |
2004年 | 9330篇 |
2003年 | 8265篇 |
2002年 | 7986篇 |
2001年 | 8037篇 |
2000年 | 6274篇 |
1999年 | 4807篇 |
1998年 | 4190篇 |
1997年 | 4152篇 |
1996年 | 3989篇 |
1995年 | 3598篇 |
1994年 | 3777篇 |
1993年 | 3500篇 |
1992年 | 3982篇 |
1991年 | 3938篇 |
1990年 | 3856篇 |
1989年 | 3702篇 |
1988年 | 3765篇 |
1987年 | 3647篇 |
1986年 | 3444篇 |
1985年 | 4654篇 |
1984年 | 4890篇 |
1983年 | 4054篇 |
1982年 | 4253篇 |
1981年 | 4166篇 |
1980年 | 4111篇 |
1979年 | 4087篇 |
1978年 | 4376篇 |
1977年 | 4276篇 |
1976年 | 4368篇 |
1975年 | 4035篇 |
1974年 | 4083篇 |
1973年 | 4240篇 |
1972年 | 2931篇 |
1971年 | 2440篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
F. Klappenberger K. N. Alekseev K. F. Renk R. Scheuerer E. Schomburg S. J. Allen G. R. Ramian J. S. S. Scott A. Kovsh V. Ustinov A. Zhukov 《The European Physical Journal B - Condensed Matter and Complex Systems》2004,39(4):483-489
We report an experimental study indicating ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice under the action of a THz field. Our experiment was performed for an InGaAs/InAlAs superlattice with the conduction electrons undergoing miniband transport. We applied to a superlattice a dc bias that was slightly smaller than a critical bias necessary for the formation of space-charge domains caused by a static negative differential conductivity. Additionally subjecting the superlattice to a strong THz field, resulted in a dc transport governed by the formation of domains if the frequency of the field was smaller than an upper frequency limit (~3 THz). From this frequency limit for the creation and annihilation of domains we determined the characteristic time of the domain buildup. Our analysis shows that the buildup time of domains in a wide miniband and heavily doped superlattice is limited by the relaxation time due to scattering of the miniband electrons at polar optic phonons. Our results are of importance for both an understanding of ultrafast dynamics of pattern formation in nanostructures and the development of THz electronic devices.Received: 25 March 2004, Published online: 23 July 2004PACS:
72.20.Ht High-field and nonlinear effects - 72.30. + q High-frequency effects; plasma effects - 73.21.Cd SuperlatticesK.N. Alekseev: Permanent address: Department of Physical Sciences, P.O. Box 3000, University of Oulu FIN-90014, Finland. 相似文献
992.
L. G. Smirnova Yu. B. Grunin S. V. Krasil'nikova M. A. Zaverkina D. R. Bakieva E. V. Smirnov 《Colloid Journal》2003,65(6):778-781
The pulsed NMR, isopiestic and pycnometric methods are applied to investigate the physicochemical properties of softwood [GOST (State Standard) 9571-89] and hardwood [GOST 28172-89] celluloses, as well as flax cellulose [GOST 10330-76, 9394-76], both in the initial state and subjected to chemical treatment. Possible changes occurring in the supramolecular structure of a polymer are analyzed. The structural and sorption characteristics of the investigated samples are calculated. Their water absorption capacities are determined. 相似文献
993.
Within the framework of free interaction theory numerical methods are used to investigate the occurrence of supersonic zones with shocks in the outer inviscid region for flow past roughness in the lower viscous sublayer, with and without the formation of local separation zones. 相似文献
994.
This paper presents some properties of two restricted classes of multi-degree-of-freedom potential systems subjected to Gaussian white-noise excitations. Specifically, potential systems which exhibit damping terms with energy-dependent polynomial form are referred to. In this context, first systems with coupled stiffness terms and damping terms depending on the total energy are investigated. Then, systems with uncoupled stiffness terms and damping terms depending on the total energy in each degree-of-freedom are considered. For these two classes, it is found that algebraic relations among the stationary statistical moments of the energy functions can be derived by applying standard tools of Itô calculus. Further, it is noted that these relations are very useful within the framework of an equivalent statistical non-linearization technique to build approximate solutions for arbitrary non-linear systems. 相似文献
995.
996.
W. G. Habashi G. Baruzzi M. F. Peeters M. M. Hafez 《Numerical Methods for Partial Differential Equations》1991,7(2):193-207
Finite element solutions of the Euler and Navier-Stokes equations are presented, using a simple dissipation model. The discretization is based on the weak-Galerkin weighted residual method and equal interpolation functions for all the unknowns are permitted. The nonlinearity is iterated upon using a Newton method and at each iteration the linear algebraic system is solved by a direct solver with all unknowns fully coupled. Results are presented for two-dimensional transonic inviscid flows and two- and three-dimensional incompressible viscous flows. Convergence of the algorithm is shown to be quadratic, reaching machine accuracy in very few iterations. The inviscid results demonstrate the existence of nonunique numerical solutions to the steady Euler equations. 相似文献
997.
A. G. Khovanskii 《Proceedings of the Steklov Institute of Mathematics》2007,259(1):82-100
We discuss the problem of representability and nonrepresentability of algebraic functions by radicals. We show that the Riemann
surfaces of functions that are the inverses of Chebyshev polynomials are determined by their local behavior near branch points.
We find lower bounds on the degrees of equations to which sufficiently general algebraic functions can be reduced by radicals.
We also begin to classify rational functions of prime degree whose inverses are representable by radicals. 相似文献
998.
In the present work we consider the problem of interpolating scattered data using radial basis functions (RBF). In general, it is well known that this leads to a discrete linear inverse problem that needs to be regularized in order to provide a meaningful solution. The work focuses on a metric-regularization approach, based on a new class of RBF, called anisotropic RBF. The work provides theoretical justifications for the regularization approach and it considers a suitable proposal for the metric, supporting it by numerical examples. 相似文献
999.
G. I. Petrashen V. V. Reshetnikov Yu. A. Surkov 《Journal of Mathematical Sciences》2007,142(6):2645-2654
A nonstationary problem concerning the propagation of low-frequency waves for a model that consists of a packet of elastic
layers lying on an elastic half-space is considered, and exact mathematical solutions in the form of repeated integrals are
constructed in three basic forms. It is proved that these solutions satisfy initial conditions. The representation of solutions
as a superposition of multiple waves is treated. Bibliography: 6 titles.
__________
Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 332, 2006, pp. 220–239. 相似文献
1000.
The photogyrotropic properties of several mordant yellow azo dyes in gelatin and polymer matrices that are in the normal state or exposed to circularly polarized light are investigated. A model explanation for the experimental data is given. 相似文献