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排序方式: 共有149条查询结果,搜索用时 46 毫秒
11.
D. Akitt H. J. J. Seguin M. R. Cervenan S. K. Nikumb 《Applied physics. B, Lasers and optics》1990,51(5):326-328
An on-line, electronic feedback approach for automatic alignment of a laser resonator is described. A measure of the resonator alignment error is derived by partitioning a 1% sample of the laser output into quadrants, and using standard electronic circuits to compare the average power present in each quadrant. A motorized X — Y alignment system is then used to implement the alignment changes required to maintain optimum output quality from the laser. The system is shown capable of stabilizing the output power in each quadrant of the beam to within about 2% of the optimum level. 相似文献
12.
Several empirical rules have been proposed during the past few years to synthesize bulk metallic glasses. But, the real reasons for the improved glass-forming ability of these alloys are still not clear and the ability to design alloy compositions to enable synthesis of larger diameter rods has not improved. The present work conducts a critical analysis of the existing data in terms of the different glass-forming criteria and concludes that the available parameters cannot satisfactorily predict the GFA and explain all the observed data. Reasons for this failure have been suggested. 相似文献
13.
X.-M. Wang H.-P. Zheng C.-J. Li 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(2):255-263
In order to get more reliable electronic structure of
protein in aqueous solution, it is necessary to construct a simple, easy-use
equivalent potential of water molecules for protein's electronic structure
calculation. The first-principles, all-electron, ab initio calculations have been
performed to construct the equivalent potential of water molecules for the
electronic structure of Cys. The process consists of three steps. First, the
electronic structure of the cluster containing Cys and water molecules is
calculated. Then, based on the structure, the electronic structure of Cys
with the potential of water molecules is calculated using the
self-consistent cluster-embedding method. Finally, the electronic structure
of Cys with the potential of dipoles is calculated. The dipoles are adjusted
so the electronic structure of Cys with the potential of dipoles is close to
that of water molecules. The calculations show that the major effect of
water molecules on Cys' electronic structure is lowering the occupied
electronic states by about 0.032 Ry, and broadening energy gap by 16%.
The effect of water molecules on the electronic structure of Cys can be
simulated by dipoles potential. 相似文献
14.
The interaction between particle-like sources of the
nematic director distortions (e.g., colloids, point
defects, macromolecules in nematic emulsions) allows for a useful
analogy with the electrostatic multipole interaction between
charged bodies. In this paper we develop this analogy to the level
corresponding to the charge density and consider the general
status of the pairwise approach to the nematic emulsions with
finite-size colloids. It is shown that the elastic analog of the
surface electric charge density is represented by the two
transverse director components on the surface imposing the
director distortions. The elastic multipoles of a particle are
expressed as integrals over the charge density distribution on
this surface. Because of the difference between the scalar
electrostatics and vector nematostatics, the number of elastic
multipoles of each order is doubled compared to that in the
electrostatics: there are two elastic charges, two vectors of
dipole moments, two quadrupolar tensors, and so on. The
two-component elastic charge is expressed via the vector of
external mechanical torque applied on the particle. As a result,
the elastic Coulomb-like coupling between two particles is found
to be proportional to the scalar product of the two external
torques and does not directly depend on the particles' form and
anchoring. The real-space Green function method is used to develop
the pairwise approach to nematic emulsions and determine its form
and restrictions. The pairwise potentials are obtained in the
familiar form, but, in contrast to the electrostatics, they
describe the interaction between pairs (dyads) of the elastic
multipole moments. The multipole moments are shown to be uniquely
determined by the single-particle director field, unperturbed by
other particles. The pairwise approximation is applicable only in
the leading order in the small ratio particle
size-to-interparticle distance as the next order contains
irreducible three-body terms. 相似文献
15.
A. Saichev D. Sornette 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):443-459
Several recent works point out that the crowd of small unobservable
earthquakes (with magnitudes below the detection threshold md) may
play a significant and perhaps dominant role in triggering future
seismicity. Using the ETAS branching model of triggered seismicity, we
apply the formalism of generating probability functions to investigate
how the statistical properties of observable earthquakes differ from the
statistics of all events. The ETAS (epidemic-type aftershock sequence)
model assumes that each earthquake can trigger other earthquakes
(“aftershocks”). An aftershock sequence results in this model from the
cascade of aftershocks of each past earthquake. The triggering
efficiency of earthquakes is assumed to vanish below a lower magnitude
limit m0, in order to ensure the convergence of the theory and may
reflect the physics of state-and-velocity frictional rupture. We show
that, to a good approximation, the statistical distribution of seismic
rates of events with magnitudes above md generated by an ETAS model
with branching ratio n is the same as that of events generated by
another ETAS model with effective parameter n(md). Our present
analysis thus confirms, for the full statistical (time-independent
or large time-window approximation)
properties, the results obtained previously by one of us and Werner,
based solely on the average seismic rates (the first-order moment of the
statistics). Our analysis also demonstrates that this correspondence is
not exact, as there are small corrections which can be systematically
calculated, in terms of additional contributions that can be mapped onto
a different branching model. We also show that this approximate correspondence
of the ETAS model onto itself obtained by changing m0 into md, and n into
n(md) holds only with respect to its statistical properties
and not for all its space-time properties. 相似文献
16.
We report on the calculations related to the electronic structure of ZnO, CdO, MgO, ZnMgO2 and ZnCdO2 in the wurtzite, rocksalt and chalcopyrite structures. From this study we found that ZnO and MgO are of direct band semiconductor, CdO is of semi metallic in nature. ZnMgO2 and ZnCdO2 are direct band semiconductors. From the energy considerations, we found that ZnMgO2 and ZnCdO2 are more stable in chalcopyrite structure rather than in rocksalt structure. Using the calculated band gap values, the bowing parameter for ZnMgO2 and ZnCdO2 is deduced and found to be in agreement with the reported value. 相似文献
17.
Received: 12 June 1998 相似文献
18.
D. M. Antoniuk H. J. J. Seguin C. E. Capjack 《Applied physics. B, Lasers and optics》1984,35(3):155-162
The operational characteristics of a magnetically stabilized glow discharge are investigated for several different experimental electrode configurations. In this parametric study, successive geometries were designed to accommodate and/or control specific aspects of the plasma glow process. In this manner, an electrode structure which optimizes discharge stability and promotes glow uniformity over an extended active volume, was ultimately achieved. The study teaches a promising new technique for significantly increasing the power density of high power gas lasers.Experimental data is in excellent correlation with predictions provided by a previous numerical study of this magnetic stabilization process. 相似文献
19.
A. Saichev D. Sornette 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,49(3):377-401
We report an empirical determination of the
probability density functions Pdata(r) (and
its cumulative version)
of the number r of earthquakes in finite space-time windows
for the California catalog, over fixed spatial boxes 5 ×5 km2,
20 ×20 km2 and 50 ×50 km2
and time intervals
and 1000 days. The data can
be represented by asymptotic power law tails together with several
cross-overs. These observations are
explained by a simple stochastic branching process previously studied by
many authors, the ETAS (epidemic-type aftershock sequence) model which
assumes that each earthquake can trigger other earthquakes
(“aftershocks”). An aftershock sequence results in this model from the
cascade of aftershocks of each past earthquake. We develop the full
theory in terms of generating functions for describing the space-time
organization of earthquake sequences and develop several approximations
to solve the equations.
The calibration of the theory to the empirical observations
shows that it is essential to augment the ETAS model by taking account
of the pre-existing frozen heterogeneity of spontaneous earthquake
sources. This seems natural in view of the complex multi-scale nature of
fault networks, on which earthquakes nucleate.
Our extended theory is able to account for the empirical
observation but some discrepancies, especially for the shorter time windows,
point to limits of both our theoretical approach and of the ETAS model. 相似文献
20.
This paper presents a new method of forming a Si/SiGe dual channel on a Si0.8Ge0.2 virtual substrate. Generally, in a CMOS process using a Si/SiGe dual channel, due to several processes involving ion-implantation, annealing and dry-etching after the deposition of the Si/SiGe dual channel, the surface can be damaged, leading to reduced electrical properties. However, if the dual channel is formed during a specific stage of the CMOS process, the defects of the dual channel can be reduced and the thermal stability will be excellent. Therefore, in this paper, a method for minimizing the defects of the dual channel is presented. This method uses the segregation of the Ge in the oxidation process of a SiGe. A Si/SiGe dual channel formed using this method achieved results that were identical to a dual channel deposited using the chemical vapor deposition (CVD) method. 相似文献