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1.
T. V. Ramakrishnan H. R. Krishnamurthy S. R. Hassan G. Venketeswara Pai 《Journal of Chemical Sciences》2003,115(5-6):767-774
Manganites of the LA1−x
Ca
x
MnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach
recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degeneratee
g
states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment
of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and
CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and
spatial correlation is the key to manganite physics.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
2.
The spherical-model limitn of then-vector model in a random field, with either a statistically independent distribution or with long-range correlated random fields, is studied to demonstrate the correctness of the replica method in which then and replica limits limits are interchanged, provided the replica and thermodynamic limits are taken in the right order, in the case of long-range correlated random fields. A scaling form for the two-point correlation function relevant to the first-order phase transition below the lower critical dimensionality of the random system is also obtained. 相似文献
3.
A. K. Zhuravlev V. Yu. Irkhin M. I. Katsnelson 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):377-382
Using the numerical renormalization group method, the dependences on
temperature of the magnetic susceptibility χ(T) and specific heat C(T)
are obtained for the single-impurity Anderson model with inclusion of
d-f the Coulomb interaction. It is shown that the exciton effects
caused by this effect (charge fluctuations) can significantly change the
behaviour of C(T) in comparison with the standard Anderson model at
moderately low temperatures, whereas the behaviour of χ(T) remains nearly
universal. The ground-state and temperature-dependent renormalizations of the
effective hybridization parameter and f-level position caused by the
d-f interaction are calculated, and satisfactory agreement with the
Hartree-Fock approximation is derived. 相似文献
4.
Jean Richert 《Physica B: Condensed Matter》2012,407(4):729-734
The propagation of an external transverse magnetic signal acting locally on a 1d chain of spins generates a disturbance which runs through the system. This quantum effect can be interpreted as a classical travelling wave which contains a superposition of a large set of frequencies depending on the size of the chain. Its local amplitude fixes the size of the z-component of the spins at any location in the chain. The average and maximum value of the group velocity are determined and compared with the transmission velocity fixed by the Lieb-Robinson upper bound inequality. 相似文献
5.
6.
A ferromagnetic ground state was identified for the compounds CeCuGe (TC=10 K), CeCuSi (TC=15 K) [F. Yang, et al., J. Appl. Phys. 69 (1991) 4705] and CeAuGe (TC=10 K) [R. Pottgen, J. Magn. Magn. Mater. 152 (1996) 196]. The observed saturation magnetic moment values at low temperatures for all three compounds are considerably less than the theoretically expected value gJJ=2.14μB for the free Ce3+ ion involving the entire six-fold J=5/2 multiplet, and thus provide a first indication of partial lifting of the f-electron level degeneracy in these compounds. Specific heat data yield crystal electric field (CEF) excitation energies (ΔSch) equivalent to 140 K for CeCuGe, 110 K for CeCuSi and 280 K for CeAuGe. To confirm the presence of CEF excitations directly, we have carried out inelastic neutron scattering (INS) measurements on all three compounds, using the HET spectrometer at ISIS Facility. Here, we present a detailed analysis of the INS spectra of CeCuSi on the basis of a CEF model and the detailed analysis of the INS of the other two compounds will be reported elsewhere. 相似文献
7.
8.
J.-P. Pouget 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(3):321-333
CuGeO3 exhibits a Spin-Peierls (SP) transition, at T
SP
= 14.3 K, which is announced above 19 K by an important regime of one-dimensional (1D) pretransitional lattice fluctuations which
can be detected until about 40 K using X-ray diffuse scattering investigations. A quantitative analysis of this scattering
shows that in this 1D direction the correlation length follows the “universal” behaviour expected for the thermal fluctuations
of a real order parameter which characterizes the lattice dimerization. This allows to define a 1D mean-field temperature,
T
SP
MF
, of about 60 K and invalidates any mean field scenario for the SP transition of CuGeO3. As T
SP
MF
is as high as 4
T
SP
we propose that the 3D-SP order is achieved by the interchain coupling between 1D solitons which form below about 16-20 K.
CuGeO3 being in the non-adiabatic regime, it is also suggested that the observed pretransitional fluctuations of CuGeO3 originate from the X-ray scattering on a very broad damped critical response of lower frequency than the “critical” phonon
modes. From the quantitative analysis of the 1D fluctuations we also estimate the microscopic parameters of the SP chain.
These parameters allow to locate CuGeO3 close to the quantum critical boundary separating the gapped SP ground state to the ungapped anti-ferromagnetic ground state.
The vicinity of a quantum critical point emphasizes the role of the quantum and non-adiabatic fluctuations and the importance
of the interchain coupling in the physics of CuGeO3. Finally we compare these findings with those obtained for the organic SP systems (BCPTTF)2PF6, (TMTTF)2PF6 and MEM(TCNQ)2. From a similar analysis of the pretransitional lattice fluctuations it is found that (BCPTTF)2PF6 and (TMTTF)2PF6 are located on the SP gapped classical-quantum boundary and are in the adiabatic regime where the fluctuations lead to the
formation of a pseudo-gap in the spin degrees of freedom. Differently, we place MEM(TCNQ)2 inside the SP quantum phase around the crossover line between the adiabatic and non-adiabatic regimes.
Received 13 September 2000 and Received in final form 6 February 2001 相似文献
9.
We present the Fermi surface properties in strongly correlated electron systems of rare earth and uranium compounds via de Haas–van Alphen experiments. The conduction electrons with large cyclotron effective masses over 100m0 (m0: rest mass of an electron) are detected in CeRu2Si2, CeCoIn5 and UPt3. These electrons move slowly in the crystal. The topology of the Fermi surface and the cyclotron mass are compared to those of energy band calculations. 相似文献
10.
S.G. Mishra P.A. Sreeram 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(2):287-291
Specific heat versus temperature curves for various pressures, or magnetic fields (or some other external control parameter) have been seen to
cross at a point or in a very small range of temperatures in many correlated fermion systems. We show that this behavior is
related to the possibility of existence of a quantum critical point. Vicinity to a quantum critical point in these systems
leads to a crossover from quantum to classical fluctuation regime at some temperature . The temperature at which the curves cross turns out to be near this crossover temperature. We have discussed the case of
the normal phase of liquid Helium three and the heavy fermion systems CeAl3 and UBe13 in detail within the spin fluctuation theory, a theory which inherently contains a low energy scale which can be identified
with . When the crossover scale is a homogeneous function of these control parameters there is always crossing at a point. We also
mention other theories exhibiting a low energy scale near a quantum critical point and discuss this phenomenon in those theories.
Received 25 June 1999 相似文献