全文获取类型
收费全文 | 7754篇 |
免费 | 606篇 |
国内免费 | 378篇 |
专业分类
化学 | 4820篇 |
晶体学 | 61篇 |
力学 | 328篇 |
综合类 | 40篇 |
数学 | 1073篇 |
物理学 | 2416篇 |
出版年
2023年 | 83篇 |
2022年 | 281篇 |
2021年 | 248篇 |
2020年 | 298篇 |
2019年 | 245篇 |
2018年 | 202篇 |
2017年 | 190篇 |
2016年 | 318篇 |
2015年 | 322篇 |
2014年 | 302篇 |
2013年 | 525篇 |
2012年 | 521篇 |
2011年 | 595篇 |
2010年 | 391篇 |
2009年 | 369篇 |
2008年 | 435篇 |
2007年 | 435篇 |
2006年 | 380篇 |
2005年 | 346篇 |
2004年 | 274篇 |
2003年 | 228篇 |
2002年 | 206篇 |
2001年 | 172篇 |
2000年 | 109篇 |
1999年 | 113篇 |
1998年 | 74篇 |
1997年 | 62篇 |
1996年 | 79篇 |
1995年 | 60篇 |
1994年 | 76篇 |
1993年 | 66篇 |
1992年 | 54篇 |
1991年 | 41篇 |
1990年 | 48篇 |
1989年 | 38篇 |
1988年 | 24篇 |
1987年 | 27篇 |
1986年 | 20篇 |
1985年 | 38篇 |
1984年 | 46篇 |
1983年 | 29篇 |
1982年 | 43篇 |
1981年 | 23篇 |
1980年 | 34篇 |
1979年 | 46篇 |
1978年 | 36篇 |
1976年 | 28篇 |
1975年 | 17篇 |
1974年 | 25篇 |
1973年 | 17篇 |
排序方式: 共有8738条查询结果,搜索用时 15 毫秒
831.
A time-dependent current-density-functional theory for many-particle systems in interaction with arbitrary external baths is developed. We prove that, given the initial quantum state |Psi0> and the particle-bath interaction operator, two external vector potentials A(r,t) and A'(r,t) that produce the same ensemble-averaged current density, j(r,t), must necessarily coincide up to a gauge transformation. This result greatly expands the applicability of time-dependent density-functional theory to open quantum systems, and allows for first-principles calculations of many-particle time evolution beyond Hamiltonian dynamics. 相似文献
832.
Efficient passively Q-switched laser operation of the disordered Yb:NaGd(WO(4))(2) crystal is demonstrated for the first time to our knowledge with a Cr(4+):YAG saturable absorber by diode end pumping. 2.05 W of average output power at a pulse repetition frequency of 13.3 kHz was obtained at an absorbed pump power of 7.7 W, with a slope efficiency of 40%. The energy and duration of the generated laser pulse were 154 microJ and 33 ns, respectively, corresponding to a peak power of 4.67 kW. In continuous-wave operation, the Yb:NaGd(WO(4))(2) laser yielded an output power of 5.5 W with an optical-optical efficiency of 51%. 相似文献
833.
A. Prats Ferrer B. Eynard P. Di Francesco J.-B. Zuber 《Journal of statistical physics》2007,129(5-6):885-935
The Harish-Chandra correlation functions, i.e. integrals over compact groups of invariant monomials
with the weight exp tr (X
Ω
Y
Ω
†
) are computed for the orthogonal and symplectic groups. We proceed in two steps. First, the integral over the compact group
is recast into a Gaussian integral over strictly upper triangular complex matrices (with some additional symmetries), supplemented
by a summation over the Weyl group. This result follows from the study of loop equations in an associated two-matrix integral
and may be viewed as the adequate version of Duistermaat–Heckman’s theorem for our correlation function integrals. Secondly,
the Gaussian integration over triangular matrices is carried out and leads to compact determinantal expressions. 相似文献
834.
H.O. Di Rocco D.I. Iriarte J.A. Pomarico H.F. Ranea Sandoval R. Macdonald 《Journal of Quantitative Spectroscopy & Radiative Transfer》2007,105(1):68-83
In this work we present a method for determining the optical parameters of turbid media, namely its absorption coefficient (μa) and its reduced scattering coefficient . It is based on the measurement of CW transmittance profiles and analysis of the experimental data by a theoretical model based on the diffusion approximation (DA) of the radiative transfer equation (RTE). The method developed has been investigated with solid polymer probes but it could be applied for liquid materials as well. Experimental results are presented and compared to those of other authors together with a discussion about the accuracy of measurements. In addition, measurements using integrating spheres as well as Monte Carlo simulations are also presented to validate these results. 相似文献
835.
We report on a mid-infrared (mid-IR) source consisting of an approximately 10 W average-power, linearly polarized 1.54 microm wavelength pulsed fiber source pumping an optical parametric oscillator. From this source, we obtained average power in excess of 1 W in the 3.8-4.0 microm wavelength range at a pulse repetition frequency of 100 kHz. With a slightly different setup, we also achieved an average power of 0.25 W at 4.5 microm wavelength. To our knowledge, these values represent the highest mid-IR power obtained through wavelength conversion of an eye-safe fiber source. 相似文献
836.
837.
OBJECTIVES: To inform on Teratology Information Services (TIS) in Europe, their history, function and activity in preventing congenital malformations. CONCLUSIONS: Clinical teratology tries to identify human teratogens. TIS play an important role in public health by providing counsel on known reproductive risks and conducting specific research. This has led to a reduction in unnecessary fears of pregnant women and pregnancy terminations, as well as to better, more appropriate drug selection to treat acute or chronic disease. In addition, TIS units also conduct prospective studies and inform the public and physicians of the possible effects of environmental agents on the developing embryo and fetus. 相似文献
838.
We suggest a new method for studying finite dimensional dynamics for evolutionary differential equations. We illustrate this
method for the case of the KdV equation. As a side result we give constructive solutions of the boundary problem for the Schrodinger
equations whose potentials are solutions of stationary KdV equations and their higher generalizations. 相似文献
839.
Vortex generator jets were used to generate large-scale longitudinal vortices embedded in a flat-plate turbulent boundary layer. The investigation was performed in a water tunnel, measuring instantaneous flow fields in planes parallel and normal to the flat plate, using particle image velocimetry. The objective of the research was to observe the response of near-wall turbulence to the imposed perturbing flow. It was shown that a small-amplitude forcing vortical flow had significant influence on the mean and fluctuating velocity profiles. Moreover, particle image velocimetry permitted speculation upon the behaviour of the wall velocity streaks under the action of the perturbing forcing vortical flow. 相似文献
840.
Salvatore Di Pasquale 《Meccanica》1992,27(3):173-184
The modern theory of masonry structures has been set up on the hypothesis of no-tension behaviour, with the aim of offering a reference model, independent of materials and building techniques employed. This hypothesis gives rise to inequalities which have to be satisfied by the stress tensor components and, as a dual aspect, to the kinematic behaviour characteristics of media which can be classified as lying between solids and fluids: the structure of the masonry material consists of particles reacting elastically only when in contact. An examination of the plane-stress problem leads us to define, within the prescribed domain under admissible loads, three different subdomains with null, regular, or non-regular principal stress tensors, respectively. As the boundaries of such subdomains are not known a priori, the problem can be classified as a free boundary value problem. The analysis concerns mainly the subdomains where the stress tensor is non-regular; and a non-regularity condition det =0 is added to the equilibrium equations. This condition makes the stress problem isostatic and leads to a violation of Saint-Venant's compliance conditions on strains. Hence there is a need to introduce a strain tensor, not related to the stress tensor, which can be decomposed into an extensional component and a shearing component; we prove that such strains, of the class c, are similar to those of the theory of plastic flow. From the point of view of computational analysis the anelastic strains are considered as given distortions; they are computed by means of the Haar-Kármán principle, modified for computational purposes by an idea of Prager and Hodge.
Sommario La moderna teoria delle strutture murarie, fondata sulla rigorosa non reagenza a trazione del materiale, ha lo scopo di fornire un modello di riferimento indipendente sia dalle caratteristiche del materiale sia dalle techniche costruttive impiegate. L'ipotesi di non reagenza a trazione si traduce in disuguaglianze che le componenti del tensore di stress devono verificare; dualmente il comportamento caratteristico cinematico può esser classificato di confine, come del resto la stessa statica, tra solidi e fluidi: la struttura ipotizzata del materiale muratura consiste di particelle che reagiscono solo se sono in contatto. L'esame del problema piano porta a definire all'interno del dominio di definizione tre differenti tipi di sub-regioni in cui lo stress è nullo, canonico, o singolare. Poiché le frontiere di queste sub-regioni non sono note a priori il problema può anche essere classificato di frontiera libera. L'analisi concerne fondamentalmente la sub-regione in cui il tensore è non regolare, perché deve verificare anche la condizione det =0. Ciò rende isostatico il problema e conduce anche alla violazione della condizione di integrabilità delle deformazioni. Questo passaggio può essere superato introducendo un tensore di deformazioni a tensioni nulle che si può decomporre in una componente estensionale ed in una componente di scorrimento; si dimostra che queste deformazioni sono equivalenti a quelle che intervengono nella Teoria del flusso plastico. Dal punto di vista computazionale le deformazioni anelastiche sono considerate come distorsioni impresse determinate attraverso il principio di Haar-Kármán modificato, per le techniche computazionali, su idee di Prager e Hodge.相似文献