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81.
Heinrich Puff Willi Schuh Rolf Sievers Wolfgang Wald Ren Zimmer 《Journal of organometallic chemistry》1984,260(3):271-280
Di-t-butyltin oxide (t-Bu2SnO)3 and di-t-amyltin oxide (t-Am2SnO)3 were synthesized by hydrolysis of the diorganotin dichlorides. Their molecular and crystal structures were investigated by X-ray diffraction techniques and vibrational spectroscopy. Both compounds form isolated molecules with planar six-membered tin-oxygen rings. 相似文献
82.
It is commonly assumed that quantum field theory arises by applying ordinary quantum mechanics to the low energy effective degrees of freedom of a more fundamental theory defined at ultra-high-energy/short-wavelength scales. We shall argue here that, even for free quantum fields, there are holistic aspects of quantum field theory that cannot be properly understood in this manner. Specifically, the subtractions needed to define nonlinear polynomial functions of a free quantum field in curved spacetime are quite simple and natural from the quantum field theoretic point of view, but are at best extremely ad hoc and unnatural if viewed as independent renormalizations of individual modes of the field. We illustrate this point by contrasting the analysis of the Casimir effect, the renormalization of the stress-energy tensor in time-dependent spacetimes, and anomalies from the point of quantum field theory and from the point of view of quantum mechanics applied to the independent low energy modes of the field. Some implications for the cosmological constant problem are discussed. 相似文献
83.
Helmut Friedrich István Rácz Robert M. Wald 《Communications in Mathematical Physics》1999,204(3):691-707
We consider smooth electrovac spacetimes which represent either (A) an asymptotically flat, stationary black hole or (B) a
cosmological spacetime with a compact Cauchy horizon ruled by closed null geodesics. The black hole event horizon or, respectively,
the compact Cauchy horizon of these spacetimes is assumed to be a smooth null hypersurface which is non-degenerate in the
sense that its null geodesic generators are geodesically incomplete in one direction. In both cases, it is shown that there
exists a Killing vector field in a one-sided neighborhood of the horizon which is normal to the horizon. We thereby generalize
theorems of Hawking (for case (A)) and Isenberg and Moncrief (for case (B)) to the non-analytic case.
Received: 4 November 1998 / Accepted: 13 February 1999 相似文献
84.
KATJA KRüGER 《Pramana》2012,79(4):563-578
Recent QCD results from electron?Cproton interactions at HERA and JLAB are presented. Inclusive cross-section measurements as well as studies of the hadronic final state like jet production or the production of heavy quarks are discussed. The results are compared with perturbative QCD predictions and their impact on the determination of the parton density functions of the proton as well as of the strong coupling ?? s is discussed. 相似文献
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87.
L. L. Wald G. C. Wiggins A. Potthast C. J. Wiggins C. Triantafyllou 《Applied magnetic resonance》2005,29(1):19-37
The development of 300 MHz radio-frequency (RF) head coils analogous to those used at field strengths of 1.5 and 3 T is complicated
by increased dissipative losses in conductive tissue, effects arising from the short RF wavelength in biological tissue (about
13 cm at 300 MHz), and the constraints imposed by the use of head gradient sets desirable for mitigating increased static
field susceptibility effects. In this study, five RF head coils were constructed and tested on a 7 T scanner including 2 TEM
designs, 2 birdcage designs and a local receive-only array. Signal-to-noise ratio, coil reception profiles and interactions
between the coil and dielectric head were examined. Particular attention was placed on the coil’s reception in the neck and
shoulders, where the head gradient is unable to spatially encode the image. With the use of conductive shields and distributed
capacitance, all of the coil designs could be made to image effectively at high field, but each design was found to have subtle
differences in field distribution, interaction with the dielectric boundary conditions of the head and fringe fields in the
neck and shoulders. In particular, the birdcage and array coils were found to have reducedB
1 reception field profiles in the neck and shoulders which helped reduce signal detection outside the linear region of the
head gradient coil. Although the TEM coils exhibited higher signal detection in the neck and shoulders, all the coils picked
up enough signal from these regions to produce artifacts in the brain. These artifacts could be mitigated through use of a
conductive shield or by small local dephasing shims sewn into the shoulders of a jacket worn by the subject. Although homogeneous
in low-dielectric-constant phantoms, the volume coil’sB
1 profile was strongly peaked in the center of the head, rendering them spatially complementary to that observed in the surface
coil array. The image profile of the surface coil was found to be less dramatically changed from patterns observed at lower
field strength. Its dielectric brightening pattern was found to depend on the orientation of the coil with respect to the
head. 相似文献
88.
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90.
A single-parameter form approximating the transition potential in muonic atoms is presented. The single-parameter form yields comparable results with that obtained by the Barrett form in radial moment analysis. The single parameter,β, is isotopically independent and is uniquely defined by the particular transition of a given element. 相似文献