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排序方式: 共有56条查询结果,搜索用时 15 毫秒
1.
M. Dumm M. Dressel M. Nicklas P. Lunkenheimer A. Loidl M. Weiden F. Steglich B. Assmann H. Homborg P. Fulde 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):317-322
The magnetic susceptibility, using dc and electron spin resonance (ESR) methods, the specific heat, and the infrared properties
of the one-dimensional molecular semiconductors lithium phthalocyanine (LiPc) and the iodinated compound LiPcI have been investigated
for temperatures K. LiPc has a half-filled conduction band and is expected to be an organic metal. However, due to the strong Coulomb repulsion
the system is a one-dimensional Mott-Hubbard insulator with a Hubbard gap of 0.75 eV as inferred from optical measurements.
The localized electrons along the molecular stacks behave like a S = 1/2 antiferromagnetic spin chain. The spin susceptibility, as determined by ESR experiments, and the magnetic contribution
to the heat capacity show a Bonner-Fisher type of behavior with an exchange constant K. LiPcI is an intrinsic narrow-gap semiconductor with an optical gap of 0.43 eV. In ESR experiments it is silent, indicating
that all the unpaired electrons have been removed from the macrocycle via doping with iodine.
Received: 16 June 1998 / Accepted: 14 July 1998 相似文献
2.
Speech can remain intelligible for listeners with normal hearing when processed by narrow bandpass filters that transmit only a small fraction of the audible spectrum. Two experiments investigated the basis for the high intelligibility of narrowband speech. Experiment 1 confirmed reports that everyday English sentences can be recognized accurately (82%-98% words correct) when filtered at center frequencies of 1500, 2100, and 3000 Hz. However, narrowband low predictability (LP) sentences were less accurately recognized than high predictability (HP) sentences (20% lower scores), and excised narrowband words were even less intelligible than LP sentences (a further 23% drop). While experiment 1 revealed similar levels of performance for narrowband and broadband sentences at conversational speech levels, experiment 2 showed that speech reception thresholds were substantially (>30 dB) poorer for narrowband sentences. One explanation for this increased disparity between narrowband and broadband speech at threshold (compared to conversational speech levels) is that spectral components in the sloping transition bands of the filters provide important cues for the recognition of narrowband speech, but these components become inaudible as the signal level is reduced. Experiment 2 also showed that performance was degraded by the introduction of a speech masker (a single competing talker). The elevation in threshold was similar for narrowband and broadband speech (11 dB, on average), but because the narrowband sentences required considerably higher sound levels to reach their thresholds in quiet compared to broadband sentences, their target-to-masker ratios were very different (+23 dB for narrowband sentences and -12 dB for broadband sentences). As in experiment 1, performance was better for HP than LP sentences. The LP-HP difference was larger for narrowband than broadband sentences, suggesting that context provides greater benefits when speech is distorted by narrow bandpass filtering. 相似文献
3.
In [3] Cederbaum proposes the problem of finding a labelling of a connected graph G which minimizes the number of rounds needed to exchange information between all the points, where a round consists of the vertices of G each broadcasting once in order of their labels. In this paper we give a complete solution to the problem. 相似文献
4.
Energized molecules are the essential actors in chemical transformations in solution. As the rearrangement of bonds requires a movement of nuclei, vibrational energy is often the driving force for a reaction. Vibrational energy can be redistributed within the "hot" molecule, or relaxation can occur when molecules interact. Both processes govern the rates, pathways, and quantum yields of chemical transformations in solution. Unfortunately, energy transfer and the breaking, formation, and rearrangement of bonds take place on ultrafast timescales. This Review highlights experimental approaches for the direct, ultrafast measurement of photoinduced femtochemistry and energy flow in solution. In the first part of this Review, we summarize recent experiments on intra- and intermolecular energy transfer. The second part discusses photoinduced decomposition of large organic peroxides, which are used as initiators in free radical polymerization. The mechanisms and timescales of their decarboxylation determine the initial steps of polymerization and the microstructure of the polymer product. 相似文献
5.
6.
D. Evers C. Ley E. Spindler W. Assmann K. Rudolph P. Konrad P. Sperr 《Nuclear Physics A》1977,275(2):363-380
The (3He, n) reaction on 16O and 18O has been used to study low-spin states in 18Ne and 20Ne up to Ex ≈ 8 and 20 MeV, respectively. The measured neutron angular distributions have been analysed using DWBA. By a comparison with shell-model calculations in the (s, d) shell it is found that most of the two-proton transfer strength can be explained within that shell. Important contributions, however, from the (f, p) shell in low-lying negative parity states are also present. 相似文献
7.
P. Sperr D. Evers K. Rudolph W. Assmann E. Spindler P. Konrad G. Denhöfer 《Physics letters. [Part B]》1974,49(4):345-347
A strong selectivity of the 12C(12C,n)23Mg reaction is observed in a resonance at . The neutron spectra and n-γ coincidence measurements show evidence for population of high-spin members of the ground- state rotational band in 23Mg. 相似文献
8.
We present a simple technique for measuring coherence times for stationary light fields using a single detector with tunable time resolution. By measuring the equal-time second-order correlation function at varying instrument response functions it is possible to determine the coherence time and also the shape of the temporal decay without the need to record time-resolved data. The technique is demonstrated for pseudothermal light. Possible applications for dynamic light scattering and photon statistics measurements are discussed. 相似文献
9.
10.
S.A. Karamian J.S. Forster J.U. Andersen W. Assmann C. Broude J. Chevallier J.S. Geiger F. Grüner V.A. Khodyrev F. Malaguti A. Uguzzoni 《The European Physical Journal A - Hadrons and Nuclei》2003,17(1):49-56
Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the
stability line. Thin W crystals were bombarded with 32S ions in the energy range 170-180 MeV and the yield of fission fragments was measured for emission close to a 〈111〉 axis.
The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the 32S beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with
lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison
with a statistical model calculation indicates that the viscosity parameter, η, must be lower than for Th and U nuclei near
β-stability. The effect of the N = 126 magic number is discussed.
Received: 2 October 2002 / Accepted: 16 January 2003 / Published online: 29 April 2003 相似文献