首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   176195篇
  免费   2647篇
  国内免费   1095篇
化学   91830篇
晶体学   3099篇
力学   7456篇
综合类   38篇
数学   20095篇
物理学   57419篇
  2021年   1380篇
  2020年   1594篇
  2019年   1701篇
  2018年   2141篇
  2017年   2206篇
  2016年   3410篇
  2015年   2116篇
  2014年   3420篇
  2013年   8004篇
  2012年   6199篇
  2011年   7338篇
  2010年   5367篇
  2009年   5321篇
  2008年   6849篇
  2007年   6737篇
  2006年   6357篇
  2005年   5746篇
  2004年   5281篇
  2003年   4725篇
  2002年   4601篇
  2001年   5338篇
  2000年   3966篇
  1999年   3085篇
  1998年   2589篇
  1997年   2482篇
  1996年   2331篇
  1995年   2178篇
  1994年   2171篇
  1993年   2055篇
  1992年   2360篇
  1991年   2301篇
  1990年   2228篇
  1989年   2172篇
  1988年   2156篇
  1987年   2157篇
  1986年   2033篇
  1985年   2643篇
  1984年   2692篇
  1983年   2389篇
  1982年   2495篇
  1981年   2315篇
  1980年   2286篇
  1979年   2427篇
  1978年   2462篇
  1977年   2388篇
  1976年   2436篇
  1975年   2360篇
  1974年   2327篇
  1973年   2504篇
  1972年   1623篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In the present work we study the photonic band structure (PBS) and the polarization state of the Bloch eigenmodes of a two-dimensional magnetophotonic crystal (MPC) with square lattice formed from magneto-optically (MO) active cylinders. The refraction of light at the boundary of the MPC is analyzed. We found that both—the PBS and eigenmodes of the MPC—are most significantly altered by the MO activity in the vicinity of the degeneracies. For this case we demonstrated the possibility of an abrupt change in the propagation direction of light by the application of a magnetic field. For the Bloch wave vectors and frequencies corresponding to non-degenerate branches, the alteration of the PBS is shown to be negligible and eigenmodes almost completely coincide with linearly TE- and/or TM-polarized eigenmodes of the non-magnetic photonic crystal.  相似文献   
992.
993.
The goal of this work was to determine whether the frequency dependence of apparent backscatter coefficient (not corrected for attenuation within the myocardium) could differentiate completed, remote infarction from acute myocardial injury in vivo. Myocardial infarcts were produced in six dogs by coronary artery occlusion. One to 12 months later, acute ischemic injury was induced in each dog by ligation of a coronary artery that supplied a region of myocardium adjacent to the established infarct. Infarct, ischemic, and normal regions were interrogated with a 5-MHz, circular, 0.5-in. diam, broadband, focused, piezoelectric transducer mounted in a water-filled stand-off device placed against the exposed, beating heart. Apparent backscatter coefficients were measured over the range of frequencies from 3-7 MHz. The frequency dependence was obtained from the slope of log apparent backscatter coefficient versus log frequency. No significant difference in frequency dependence was found between normal and acutely ischemic myocardium for periods of up to 2 h of ischemia. In contrast, frequency dependence in regions of remote infarct (1.8 +/- 0.1, mean +/- standard error) was significantly lower than that in acutely ischemic or nonischemic regions (2.3 +/- 0.1) (p less than 0.01). These results suggest that remote myocardial infarction can be differentiated from acutely injured but still potentially salvageable myocardium in vivo on the basis of the frequency dependence of backscatter.  相似文献   
994.
Summary A statistical study of precipitation over the Po Valley and the Alps during four winters, based on the application of a principal component (p.c.) method, exhibited a good correlation between the first three rotated eigenvectors of the cross-product matrix (between time series of data recorded at 70 stations) and three typical atmospheric circulation schemes over Northern Italy. The study is here continued, by relating these results to meteorological fields evaluated over a region wider than the area previously considered. Our attention mainly concentrated on the fields of time trend of geopotential height at 850 hPa over Europe, between latitude 34.5 and 51 degrees North and longitudes 0 and 21 degrees East. This was suggested by some peculiar relationships shown by the statistical distributions of geopotential height and wind direction and speed with respect to the first three p.c.’s. These relationships made us think of the possibility to use those trends as predictors of precipitation fields. Our analysis puts into evidence that three spatial patterns of geopotential, height time trend can be safely associated to the first three p.c.’s and, as a consequence, to the three precipitation modes corresponding to the spatial distributions of the eigenvectors, previously identified by us. They can therefore be considered meaningful precursors of precipitation field patterns over Northern Italy and Alpine regions.
Riassunto Uno studio statistico della precipitazione sulla pianura padana e sulle Alpi durante quattro inverni, basato sull’applicazione del metodo delle componenti principali (c.p.), ha mostrato una buona correlazione fra i primi tre autovettori rotati della matrice dei prodotti incrociati (fra le serie temporali dei dati registrati in 70 stazioni) e tre tipici schemi di circolazione atmosferica sull’Italia del Nord. Lo studio viene qui prosequito, mettendo in relazione tali risultati con campi meteorologici valutati su una zona piú ampia di quella precedentemente considerata. La nostra attenzione si è prevalentemente concentrata sui campi di tendenza temporale dell’altezza di geopotenziale a 850 hPa sull’Europa, a latitudini comprese tra 34.5 e 51 gradi Nord e longitudini fra 0 e 21 gradi Est. Ciò è stato suggerito da alcune relazioni peculiari mostrate dalle distribuzioni statistiche di altezza di geopotenziale e di direzione ed intensità del vento rispetto alle prime tre c.p., che ci hanno fatto pensare alla possibilità di usare tali tendenze come predittori del campo di precipitazione. La nostra analisi ha messo in evidenza il fatto che tre distribuzioni spaziali di tendenza temporale del campo di altezza di geopotenziale possono essere con sicurezza associate alle prime tre c.p. e., quindi, ai tre modi di precipitazione corrispondenti alle distribuzioni spaziali degli autovettori precedentemente da noi identificati. Essi possono pertanto essere considerati significativi precursori del campo di precipitazione sull’Italia del Nord e sulle regioni alpine.

Резюме Статистическое исследование выпадения осадков над долиной реки По и Альпами в течение четырех зим, основанное на применении метода главных компонент, обнаруживает хорошую корреляцию между первыми тремя вращающимися собственными векторами матрицы векторного произведения (между временными последовательностями данных, зарегистрированных на 70 станциях) и тремя схемами типичной атмосферной циркуляции над Северной Италией. В этой работе продолжены исследования. Полученные результаты связывайотся с метеорологическими полями, вычисленными над областью, которая шире ранее рассмотренной области. Главное внимание мы обращаем на поля временного хода для геопотенциальной высоты при 850гПа над Европой, между 34.5щ и 51щ северной широты и между 0щ и 21щ восточной долготы. Это следует из специальных соотношений, полученных с помощью статистических распределений геопотенциальной высоты и направления и скорости ветра относительно первых трех главных компонент. Эти соотношения позволяют нам использовать исследованные тенденции, как предсказатели полей вьшадения осадков. Нам анализ подтверждает, что три пространственным распределения временного хода для геопотенциальной высоты могут быть связаны с первыми тремя главными компонентами и, как следствие, с тремя моделями выцадения осадков, которые соответствуют пространственным распределениям собственных векторов, ранее идентифицированных в работе [Ш]. Следовательно, они могут рассматриватяся как предвестники для распределений полей осадков над Северной Италией и над Альпами.
  相似文献   
995.
996.
The rapidity and transverse momentum dependence of the nuclear modification ratio for dilepton production at RHIC and LHC is presented, calculated in the color glass condensate (CGC) framework. The transverse momentum ratio is compared for two distinct dilepton mass values and a suppression of the Cronin peak is verified even for large mass. The nuclear modification ratio suppression in the dilepton rapidity spectra, as obtained experimentally for hadrons at RHIC, is verified for LHC energies at large transverse momentum, although not present at RHIC energies. The ratio between LHC and RHIC nuclear modification ratios is evaluated in the CGC, showing the large saturation effects at LHC compared with the RHIC results. These results consolidate the dilepton as a most suitable observable to investigate the QCD high density approaches.  相似文献   
997.
Crystalline LiFePO4 has been synthesized using solid-state, spray pyrolysis, and wet chemical methods. The crystal parameters were obtained from Rietveld’s refinement methods of the X-ray diffraction patterns. A detailed investigation of the Fe valency carried out using Mössbauer spectroscopy at room temperature indicates that Fe is predominantly present in its bivalent state.  相似文献   
998.
We have studied isospin symmetry violation in nuclear reactions by measuring simultaneously the cross-section of the following two reactionsp +d3H π+ andp +d3He π0. The experiment was perfomed at the cooler synchrotron accelerator COSY, Jülich at several beam energies close to the correspondingη production threshold. We also have ongoing programmes onη-nucleus final-state interaction studies viap+6 Li →7 Be +η reactions, high resolution search for dibaryonic resonances and lambda-proton final state interaction studies. The experimental details and results obtained so far are presented here  相似文献   
999.
This is the report of flavor physics and model building working group at WHEPP-9. While activites in flavor physics have been mainly focused on B-physics, those in model building have been primarily devoted to neutrino physics. We present summary of working group discussions carried out during the workshop in the above fields, and also briefly review the progress made in some projects subsequently.  相似文献   
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号