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101.
E. García-Matres J.L Martínez J. Rodríguez-Carvajal 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(1):59-70
A neutron diffraction study, as a function of temperature, of the title compounds is presented. The whole family (space group
Immm, a
≈ 3.8?, b
≈ 5.8?, c
≈ 11.3?) is structurally characterised by the presence of flattened NiO6 octahedra that form chains along the a-axis, giving rise to a strong Ni-O-Ni antiferromagnetic interaction. Whereas for Y-compound only strong 1D correlations exist
above 1.5 K, presenting the Haldane gap characteristic of 1D AF chain with integer spin, 3D AF ordering is established simultaneously
for both R and Ni sublattices at temperatures depending on the rare earth size and magnetic moment. The magnetic structures
of R2BaNiO5 ( R
=
Nd, Tb, Dy, Ho, Er and Tm) have been determined and refined as a function of temperature. The whole family orders with a magnetic
structure characterised by the temperature-independent propagation vector
= (1/2, 0, 1/2). At 1.5 K the directions of the magnetic moments differ because of the different anisotropy of the rare earth ions. Except
for Tm and Yb (which does not order above 1.5 K), the magnetic moment of the R3+ cations are close to the free-ion value. The magnetic moment of Ni2+ is around 1.4
, the strong reduction with respect to the free-ion value is probably due to a combination of low-dimensional quantum effects
and covalency. The thermal evolution of the magnetic structures from T
N
down to 1.5 K is studied in detail. A smooth re-orientation, governed by the magnetic anisotropy of R3+, seems to occur below and very close to T
N
in some of these compounds: the Ni moment rotates from nearly parallel to the a-axis toward the c-axis following the R moments. We demonstrate that for setting up the 3D magnetic ordering the R-R exchange interactions cannot
be neglected.
Received 19 July 2001 相似文献
102.
A versatile and efficient telescoped reaction sequence for the synthesis of tetrahydroisoquinolines(THIQs)is reported that uses TiCl4 to promote cyclization of a benzylaminoacetal derivative and Et3SiH for reduction of the intermediate 4-hydroxy-THIQ.This method is complimentary to the classical Pomeranz-Fritsch and related reactions since it tolerates electron-withdrawing substituents and allows access to 8-substituted THIQs. 相似文献
103.
Lattice-mismatched ZnS1−xTex epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS1−xTex layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A0, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A0, X) peak position of the ZnS1−xTex epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS1−xTex/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions. 相似文献
104.
H.M. Wong Jian-Bai Xia K.W. Cheah 《Applied Physics A: Materials Science & Processing》1997,64(5):507-509
Received: 10 June 1996/Accepted: 20 October 1996 相似文献
105.
Transparent two-dimensional ultralong and ultrabroad single crystal zinc oxide (ZnO) nanosheets were directly synthesized by a simple solid vapor deposition process under lead oxide (PbO) atmosphere. The nanosheets are well grown single crystals with thickness of about 50-70 nm, breadth of 50-100 μm and length of 4-6 mm. The growth mode of the ultrabroad nanosheets displays a unique aspect that (001) planes form the narrowest facets of the nanosheets, which is completely different from other belt-like nanostructures of ZnO. Control experiments show that PbO play an important role in the vapor-solid growth process of ZnO nanosheets. 相似文献
106.
S.L. Young H.Z. Chen C.C. Lin J.B. Shi Lance Horng Y.T. Shih 《Journal of magnetism and magnetic materials》2006
Magnetic and transport properties of (La0.7Pb0.3MnO3)1−xAgx composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La0.7Pb0.3MnO3)1−xAgx composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system. 相似文献
107.
C. Fradin J. Daillant A. Braslau D. Luzet M. Alba M. Goldmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):57-69
The linear compressibility of two-dimensional fatty acid mesophases has been determined by grazing incidence X-ray diffraction.
The unit cell parameters of the , , , S and phases of behenic acid and of the phase of myristic acid were determined as a function of surface pressure and temperature. Surface pressure versus molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion
along a given direction for a two-dimensional isotropic applied stress) was determined both in the sample plane and in a plane
normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude
from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with
different molecular mechanisms. The largest compressibilities (10 m/N) are observed in the tilted phases. They are apparently
independent on the chain length and could be related to the reorganization of the headgroup hydrogen-bounded network, whose
role should be revalued. Intermediate compressibilities are observed in phases with quasi long-range order (directions normal
to the molecular tilt in the or phases, S phase, and could be related to the ordering of these phases. The lowest compressibilities are observed in the solid
untilted phase and for one direction of the S and phases. They are similar to the compressibility of crystalline polymers and correspond to the interactions between methyl
groups in the crystal. Finally, negative compressibilities are observed in the transverse plane for the and phases and can be traced to subtle reorganizations upon untilting.
Received: 29 July 1997 / Revised: 14 October 1997 / Accepted: 23 October 1997 相似文献
108.
In this communication, we report the effect that doping Y2BaCuO5 with Dy has on its two-dimensional (2D) magnetic structure. Pure samples at both ends of the series, as well as samples doped with 1, 5, 10 and 25% dysprosium, have been characterised using X-ray diffraction, and AC susceptibility together with neutron diffraction studies on the 1 and 5% samples, which were used to measure the magnetic ordering at low temperatures. The results show that 1% Dy is enough to disrupt the 2D magnetic ordering turning it into a 3D array. The low dysprosium concentration indicates that the 3D ordering is achieved without the existence of a rare earth magnetic sublattice. The change in the ordering temperature from 27 K for the pure Y2BaCuO5 to 16 K for the 1 and 5% Dy compounds, together with the parameters from the AC susceptibility fittings, reveal that the effect of the Dy doping affects the electronic structure of the Cu ions that become involved in the superexchange pathways. The discrepancy between the parameters obtained by the Curie-Weiss fittings of the real part of the AC susceptibility and the neutron diffraction results, shows that the exchange mechanism deviates from the mean field model for all dysprosium concentrations. 相似文献
109.
P.B FabritchnyiM.V Korolenko M.I AfanasovM Danot E Janod 《Solid State Communications》2003,125(6):341-346
Mössbauer parameters of 119Sn diamagnetic dopant cations in an antiferromagnetic compound having the ilmenite structure are for the first time reported. The spectra reveal a well resolved hyperfine splitting pattern of combined magnetic and quadrupole interactions (at 5 K, δ=0.19 mm/s, H1=52.5 kOe, eVZZQ3/2=−0.80 mm/s, θ≈0°). This spectral component whose contribution (A1=82%) represents more than four fifths of the total amount of the dopant (Sn/(Mn+Ti)=1/200) is assigned to Sn(IV) ions located in the bulk of MnTiO3, on the Mn(II) site, and with a Mn(II) vacancy in their nearest surrounding. Two spectral components with minor contributions are also observed: one of them (H2≈25 kOe, A2=8%) can be assigned to Sn(IV) ions, in the MnTiO3 lattice as well, on a site where they exhibit a weaker spin polarization (this site could be the Ti(IV) one) and the other (H3=0 kOe, A3=10%) to SnO2 or/and Ti1−xSnxO2 clusters. The Néel temperature of MnTiO3 probed by the 119Sn dopant (TN=69±2 K) agrees well with the values previously provided by ESR and antiferromagnetic resonance measurements. Variation of H1 with temperature follows close the Brillouin function for S=5/2. No perturbation appears in the Mössbauer spectra around T=90 K where a broad peak, characteristic of 2D magnetic interactions, is observed on the static magnetic susceptibility curve. 相似文献
110.
Kyungwha Park Mark R Pederson Noam Bernstein 《Journal of Physics and Chemistry of Solids》2004,65(4):805-811
A new type of the single-molecule magnet [Mn4O3Cl4(O2CEt)3(py)3] forms dimers. Recent magnetic hysteresis measurements on this single-molecular magnet revealed interesting phenomena: an absence of quantum tunneling at zero magnetic field and tunneling before magnetic field reversal. This is attributed to a significant antiferromagnetic exchange interaction between different monomers. To investigate this system, we calculate the electronic structure, magnetic properties, intramolecular and intermolecular exchange interactions using density-functional theory within the generalized-gradient approximation. Our calculations agree with experiment. We find that the calculated threefold symmetric structure is vibrationally stable. We also calculate vibrational infrared absorption and Raman scattering intensities for the monomer which can be tested experimentally. 相似文献