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81.
Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping
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Subhajit Roychowdhury Dr. U. Sandhya Shenoy Prof. Dr. Umesh V. Waghmare Dr. Kanishka Biswas 《Angewandte Chemie (International ed. in English)》2015,54(50):15241-15245
Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb0.6Sn0.4Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb0.6Sn0.4Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb0.6Sn0.4Te increases p‐type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb0.58Sn0.40Na0.02Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications. 相似文献
82.
H. Nagabhushana B.M. Nagabhushana B. Umesh H.B. Premkumar Nalina Anil T.K. Gundu Rao 《哲学杂志》2013,93(12):1567-1574
Enstatite (MgSiO3) ceramic powders were synthesised by a low-temperature initiated self-propagating, gas-producing solution combustion process. The prepared powders were characterised by powder X-ray diffraction, scanning electron microscopy and Brunauer–Emmer–Teller specific surface area measurements. Defect centres induced by radiation were studied using the techniques of thermoluminescence (TL) and electron spin resonance (ESR). A well-resolved glow with peak at 178°C and a shouldered peak at 120°C were observed. Two defect centres were identified by ESR measurements, which were carried out at room temperature, and these were assigned to an O? ion and F+ centre. The O? ion (hole centre) appears to correlate with the main TL peak at 178°C. 相似文献
83.
Umesh P. Gomes Yiqiao Chen Sanjib Kabi Peter Chow Dhrubes Biswas 《Current Applied Physics》2013,13(3):487-492
The performance of InAlAs/InGaAs quantum well field effect transistors are subject to high impact ionization and band-to-band tunneling (BTBT) due to its narrow bandgap feature. In this work, the energy gap is engineered using strain and quantization techniques to increase the effective energy gap leading to low impact ionization and BTBT leakage current. It is shown that the impact ionization is reduced in 5 nm channel device as compared to 13 nm device with onset at approximately Egeff/q. Also the band-to-band-tunneling current is reduced due to the increase in effective energy gap. We have also investigated the effects of quantum well engineering on the dc performance of InGaAs HEMTs. 相似文献
84.
Recent work in our laboratory has shown that the highly substituted, electronically rich 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (THIQ3CA) scaffold is a key building block for a novel class of promising anticoagulants (Al-Horani et al. J. Med. Chem.2011, 54, 6125-6138). The synthesis of THIQ3CA analogs, especially containing specific, electronically rich substituents, has been a challenge and essentially no efficient methods have been reported in the literature. We describe three complementary, glycine donor-based strategies for high yielding synthesis of highly substituted, electronically rich THIQ3CA esters. Three glycine donors studied herein include hydantoin 1, (±)-Boc-α-phosphonoglycine trimethyl ester 2 and (±)-Z-α-phosphonoglycine trimethyl ester 3. Although the synthesis of THIQ3CA analogs could be achieved using either of the three, an optimal, high yielding approach for the desired THIQ3CA esters was best achieved using 3 in three mild, efficient steps. Using this approach, a focused library of advanced N-arylacyl, N-arylalkyl, and bis-THIQ3CA analogs was synthesized. Variable temperature and solvent-dependent NMR chemical shift studies indicated the presence of two major conformational rotamers in 3:1 proportion for N-arylacyl-THIQ3CA analogs, which were separated by a high kinetic barrier of ~17 kcal/mol. In contrast, N-arylalkyl and bis-THIQ3CA variants displayed no rotamerism, which implicates restricted rotation around the amide bond as the origin for high-barrier conformational interconversion. This phenomenon is of major significance because structure-based drug design typically utilizes only one conformation. Overall, the work presents fundamental studies on the synthesis and conformational properties of highly substituted, electronically rich THIQ3CA analogs. 相似文献
85.
Naik AD Annigeri SM Gangadharmath UB Revankar VK Mahale VB 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(8):1713-1719
A new binucleating ligand containing phenoxide as an endogenous bridging group, 2,6-diformyl-p-cresol bis(2-furanthiocarboxyhydrazone) and its binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes with chloride ion as an exogenous bridge, have been obtained. The complexes were characterized by elemental analysis, molar conductivities, magnetic moment measurements at room temperature, electronic, IR, 1H-NMR, EPR, FAB spectral studies and thermal data. The copper complex assumes a tetranuclear structure composed of two binuclear units related by a center of symmetry. The dimeric nature of copper(II) complex is supported by FAB. This complex is EPR silent. Room temperature magnetic moment reveals the operation of a significant antiferromagnetic spin exchange between the metal centers. Ligand and its copper and zinc complexes exhibit fluorescence at room temperature in DMF. All the compounds show an appreciable antimicrobial activity. 相似文献
86.
In the present work, laser induced breakdown spectroscopy has been employed to detect rare earth elements in archaeological potteries and brick sample collected from different locations. Laser Induced breakdown spectroscopy data has been analyzed to investigate the provenance of ancient clay artifacts which is based on the geochemistry of rare earth elements in the artifacts. The qualitative study of spectra shows the presence of a set of rare earth elements such as La, Ce, Pr, Nd, Sm, Gd, Dy, and Er in archaeological potteries which specifies that these potteries were manufactured from the same clay sources. 相似文献
87.
R.V.K. Mangalam Nirat Ray Umesh V. Waghmare A. Sundaresan C.N.R. Rao 《Solid State Communications》2009,149(1-2):1-5
Some of the Multiferroics [H. Schmid, Ferroelectrics 162 (1994) 317] form a rare class of materials that exhibit magneto–electric coupling arising from the coexistence of ferromagnetism and ferroelectricity, with potential for many technological applications [J.F. Scott, Nat. Mater. 6 (2007) 256; N.A. Spaldin, M. Fiebig, Science 309 (2005) 391]. Over the last decade, an active research on multiferroics has resulted in the identification of a few routes that lead to multiferroicity in bulk materials [C. Ederer, N.A. Spaldin, Nat. Mater. 3 (2004) 849; D.V. Efremov, J. van den Brink, D.I. Khomskii, Nat. Mater. 3 (2004) 853; N. Hur, S. Park, P.A. Sharma, J.S. Ahn, S. Guha, S.W. Cheong, Nature 429 (2004) 392]. While ferroelectricity in a classic ferroelectric such as BaTiO3 is expected to diminish with the reducing particle size, [C.H. Ahn, K.M. Rabe, J.M. Triscone, Science 303 (2004) 488; J. Junquera, P. Ghosez, Nature 422 (2003) 506] ferromagnetism cannot occur in its bulk form [N.A. Hill, J. Phys. Chem. B 104 (2000) 6694]. Here, we use a combination of experiment and first-principles simulations to demonstrate that multiferroic nature emerges in intermediate size nanocrystalline BaTiO3, ferromagnetism arising from the oxygen vacancies at the surface and ferroelectricity from the core. A strong coupling between a surface polar phonon and spin is shown to result in a magnetocapacitance effect observed at room temperature, which can open up possibilities of new electro–magneto-mechanical devices at the nano-scale. 相似文献
88.
P. Poornesh G. Umesh P. K. Hegde M. G. Manjunatha K. B. Manjunatha A. V. Adhikari 《Applied physics. B, Lasers and optics》2009,97(1):117-124
We report here the studies on third-order nonlinear optical properties of two novel polythiophene composite films investigated
using the Z-scan technique. The measurements were carried out using a Q-switched, frequency doubled Nd:YAG laser producing
7 nanosecond laser pulses at 532 nm. Z-scan results reveal that the composite films exhibit self-defocusing nonlinearity.
The real and imaginary parts of the third-order nonlinear optical susceptibility were of the order 10−12 esu. The effective excited-state absorption cross section was found to be larger than the ground state absorption cross section,
indicating that the operating nonlinear mechanism is reverse saturable absorption (RSA). The polythiophene composite films
also exhibit good optical power limiting of the nanosecond laser pulses. The nonlinear optical parameters are found to increase
on increasing the strength of the electron-donor group, indicating the dependence of χ
(3) on the electron-donor/acceptor units of polythiophenes. 相似文献
89.
Anélia N. Gurova Ana P.C. Ribeiro Carlos A. Nieto de Castro Umesh V. Mardolcar 《Fluid Phase Equilibria》2009
The relative permittivity (?r) data of 1,1,1-trifluoroethane (HFC-143a), (CAS N# 420-46-2), a hydrofluorocarbon (HFC) developed as a refrigerant that has zero ozone depletion potential, is reported. The relative permittivity of HFC-143a in the liquid phase was measured using a direct capacitance method at temperatures from T = 218 to 294 K and at pressures up to P = 15 MPa, for a frequency of 10 kHz. The uncertainty of the ?r measurements is estimated to be better than ±1.2 × 10−2. A complete set of tables of experimental data as a function of temperature, pressure and density, is presented that covers the dielectric property needs for most engineering applications. To study the dependence of ?r on density and temperature on a molecular basis, the theory developed by Vedam et al. and adapted by Diguet was applied to analyse the data. The Kirkwood modification of the Onsager equation was used to obtain the value of its dipole moment in the liquid phase (μ*). The apparent dipole moment obtained was μ* = 3.293 D. The effective dipole in the liquid state predicted by the Kirkwood–Frölich theory is 2.530 D. The measured values are compared with density functional and density functional self-consistent calculations (SCIPCM) of the electronic distribution and of the dipole moment of HFC-143a. Finally, the values of the isobaric thermal expansion and isothermal compressibility were estimated from the reported measurements. 相似文献
90.