共查询到20条相似文献,搜索用时 31 毫秒
1.
Bame SJ Anderson RC Asbridge JR Baker DN Feldman WC Fuselier SA Gosling JT McComas DJ Thomsen MF Young DT Zwickl RD 《Science (New York, N.Y.)》1986,232(4748):356-361
A strong interaction between the solar wind and comet Giacobini-Zinner was observed oh 11 September 1985 with the Los Alamos plasma electron experiment on the International Cometary Explorer (ICE) spacecraft. As ICE approached an intercept point 7800 kilometers behind the nucleus from the south and receded to the north, upstream phenomena due to the comet were observed. Periods of enhanced electron heat flux from the comet as well as almost continuous electron density fluctuations were measured. These effects are related to the strong electron heating observed in the cometary interaction region and to cometary ion pickup by the solar wind, respectively. No evidence for a conventional bow shock was found as ICE entered and exited the regions of strongest interaction of the solar wind with the cometary environment. The outer extent of this strong interaction zone was a transition region in which the solar wind plasma was heated, compressed, and slowed. Inside the inner boundary of the transition region was a sheath that enclosed a cold intermediate coma. In the transition region and sheath, small-scale enhancements in density were observed. These density spikes may be due to an instability associated with cometary ion pickup or to the passage of ICE through cometary ray structures. In the center of the cold intermediate coma a narrow, high-density core of plasma, presumably the developing plasma tail was found. In some ways this tail can be compared to the plasma sheet in Earth's magnetotail and to the current sheet in the tail at Venus. This type of configuration is expected in the double-lobe magnetic topology detected at the comet, possibly caused by the theoretically expected draping of the interplanetary magnetic field around its ionosphere. 相似文献
2.
In several metals, including URhGe, superconductivity has recently been observed to appear and coexist with ferromagnetism at temperatures well below that at which the ferromagnetic state forms. However, the material characteristics leading to such a state of coexistence have not yet been fully elucidated. We report that in URhGe there is a magnetic transition where the direction of the spin axis changes when a magnetic field of 12 tesla is applied parallel to the crystal b axis. We also report that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this magnetic transition, well above the field of 2 tesla at which superconductivity is first destroyed. Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the neighborhood of a quantum phase transition under high magnetic field. 相似文献
3.
Hayward MA Cussen EJ Claridge JB Bieringer M Rosseinsky MJ Kiely CJ Blundell SJ Marshall IM Pratt FL 《Science (New York, N.Y.)》2002,295(5561):1882-1884
We present the synthesis and structural characterization of a transition metal oxide hydride, LaSrCoO3H0.7, which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering at temperatures up to at least 350 kelvin. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal--containing electronic and magnetic materials. 相似文献
4.
Rosensweig RE 《Science (New York, N.Y.)》1979,204(4388):57-60
Fluidization of magnetizable particles by a gas stream in the presence of a uniform applied magnetic field oriented parallel to the flow prevents the hydrodynamic instability that otherwise leads to bubbles and turbulent motion within the medium. The fluidized emulsion expands uniformly in response to gas flow speeds in excess of that at the incipient fluidization point, with transition from the quiescent stable state to bubbling occurring suddenly at a characteristic increased rate of flow. Experimental data demonstrate the dependence of this transition velocity on the intensity of the applied magnetic field, length of the bed, and type of magnetic solids. Data illustrate the pressure distribution through the bed medium, the bedflow characteristics, and other phenomena. 相似文献
5.
Direct observation of percolation in a manganite thin film 总被引:1,自引:0,他引:1
Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3 start to grow and merge at the metal-insulator transition temperature TP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2 higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials. 相似文献
6.
Matsumoto Y Murakami M Shono T Hasegawa T Fukumura T Kawasaki M Ahmet P Chikyow T Koshihara S Koinuma H 《Science (New York, N.Y.)》2001,291(5505):854-856
Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with theconcentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin. 相似文献
7.
Okazaki R Shibauchi T Shi HJ Haga Y Matsuda TD Yamamoto E Onuki Y Ikeda H Matsuda Y 《Science (New York, N.Y.)》2011,331(6016):439-442
A second-order phase transition is characterized by spontaneous symmetry breaking. The nature of the broken symmetry in the so-called "hidden-order" phase transition in the heavy-fermion compound URu(2)Si(2), at transition temperature T(h) = 17.5 K, has posed a long-standing mystery. We report the emergence of an in-plane anisotropy of the magnetic susceptibility below T(h), which breaks the four-fold rotational symmetry of the tetragonal URu(2)Si(2). Two-fold oscillations in the magnetic torque under in-plane field rotation were sensitively detected in small pure crystals. Our findings suggest that the hidden-order phase is an electronic "nematic" phase, a translationally invariant metallic phase with spontaneous breaking of rotational symmetry. 相似文献
8.
Kraemer C Nikseresht N Piatek JO Tsyrulin N Dalla Piazza B Kiefer K Klemke B Rosenbaum TF Aeppli G Gannarelli C Prokes K Podlesnyak A Strässle T Keller L Zaharko O Krämer KW Rønnow HM 《Science (New York, N.Y.)》2012,336(6087):1416-1419
Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF(4), establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field H(c|| = 4.0 ± 0.1 kilo-oersteds. We discovered non-mean-field critical scaling for the classical phase transition at the antiferromagnetic transition temperature that is consistent with the two-dimensional XY/h(4) universality class; in accord with this, the quantum phase transition at H(c) exhibits three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic frustrated nature of the dipolar interaction, which strengthens the role of fluctuations. 相似文献
9.
A variety of recent experiments on both the static and the dynamic properties of vortices and flux-line lattices in the mixed state of the copper oxide superconductors are discussed. The experiments are of two basic types: (i) experiments that image the magnetic flux patterns either with magnetic decoration or neutrons and give information about static structures, and (ii) experiments that explore the dynamics of vortices either through the resistivity or other electrodynamic responses of the material. Results of these experiments argue in favor of the existence of a true phase transition in the high-field vortex state from a low-temperature superconducting vortex glass phase into a disordered high-temperature vortex fluid phase. The vortex glass phase transition model does a good job of explaining high-precision measurements of the dynamics at the transition. At low fields and temperatures, very long range hexatic order in the flux-line lattice is observed. 相似文献
10.
P Gilch F Pollinger-Dammer C Musewald ME Michel-Beyerle UE Steiner 《Science (New York, N.Y.)》1998,281(5379):982-984
The recombination dynamics of a transition metal redox system monitored by femtosecond pump-probe spectroscopy are shown to be sensitive to high magnetic fields at times shorter than 10 picoseconds. The effect, based on coherent population beats of different spin states, is quantitatively accounted for and allows direct access to spin relaxation rates far beyond the time resolution of the fastest electron paramagnetic resonance technique. The presence of this ultrafast magnetic field effect helps in understanding complex reaction schemes in transition metal chemistry, which occur in a wide range of fields, such as bioinorganic chemistry and catalysis. 相似文献
11.
Tarascon JM Greene LH McKinnon WR Hull GW Geballe TH 《Science (New York, N.Y.)》1987,235(4794):1373-1376
Structural, magnetic, and electronic properties of compounds in the series La2-xSrx CuO4-y for 0.05 = x = 1.1, with deltax = 0.025, were studied. Resistance, susceptibility, Meissner, and shielding measurements have revealed superconductivity among several members of the series. For x = 0.15, the transition temperature T(c), measured at the midpoint of the resistive transition, is a maximum at 39.3 K with a width of 2 K. At other compositions the transition is broader and occurs at a lower temperature. Annealing the x = 0.15 sample in oxygen at 500 degrees C increases T(c) to 40.3 K, while annealing at the same temperature under vacuum suppresses the superconductivity. These changes in oxygen content and T(c) are reversible. 相似文献
12.
协优57穗部性状及不同抽穗期分蘖成穗规律分析 总被引:1,自引:0,他引:1
(1)协优57穗部性状分析结果表明:穗长与穗粒数呈极显著正相关。1次枝梗数与2次枝粳数、2次枝梗结实率与穗结实率呈显著正相关;2次枝梗着粒密度与1次枝梗着粒密度都很稳定,变幅分别是1.206~1.267粒/cm与0.543~0.743粒/cm。(2)不同时期分蘖穗粒结构:秧田分蘖穗及主茎穗的穗粒数多、结实率高,大田低位蘖次之;大田高位蘖最少。与汕优63相比:协优57秧田分蘖穗、主茎穗、大田低位蘖次之;大田低位蘖穗的穗粒数、结实率相关不大,而大田高位蘖穗的穗粒数、结实率较高。结论:协优57的每穗粒数多、结实率高是其高产的主要原因。 相似文献
13.
The molecular-based magnetic materials Cs(2)Mn(||)[V(||)(CN)(6)] (1) and (Et(4)N)(0.5)Mn(l.25)- [V(CN)(5)].2H(2)O (2) (where Et is ethyl) were prepared by the addition of manganese(II) triflate to aqueous solutions of the hexacyanovanadate(II) ion at 0 degrees C. Whereas 1 crystallizes in a face-centered cubic lattice, 2 crystallizes in a noncubic space group. The cesium salt (1) has features characteristic of a three-dimensional ferrimagnet with a Néel transition at 125 kelvin. The tetraethylammonium salt (2) also behaves as a three-dimensional ferrimagnet with a Néel temperature of 230 kelvin; only two other molecular magnets have higher magnetic ordering temperatures. Saturation magnetization measurements indicate that in both compounds the V(II) and high-spin Mn(II) centers are antiferromagnetically coupled. Both 1 and 2 exhibit hysteresis loops characteristic of soft magnets below their magnetic phase-transition temperatures. The high magnetic ordering temperatures of these cyano-bridged solids confirm that the incorporation of early transition elements into the lattice promotes stronger magnetic coupling by enhancing the backbonding into the cyanide pi* orbitals. 相似文献
14.
Quantum spin hall insulator state in HgTe quantum wells 总被引:1,自引:0,他引:1
König M Wiedmann S Brüne C Roth A Buhmann H Molenkamp LW Qi XL Zhang SC 《Science (New York, N.Y.)》2007,318(5851):766-770
Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e(2)/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field-induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect. 相似文献
15.
为了解大穗型与多穗型小麦基因型的单株生产潜力,以5个大穗型基因型和5个多穗型基因型为试验材料,在株距14cm、8cm和2cm 3种种植密度条件下对单株产量及产量因素进行试验研究。结果表明:两种穗型的基因型都有较高的单株产量潜力,在株距14cm密度处理中单株产量最高,均达到20 g;多穗型基因型单株产量在株距14cm和2cm的密度处理中分别比大穗型基因型多0.8g和0.5g,差异显著,在株距为8cm的密度处理中后者显著高于前者,表明大穗型基因型对生存环境反映敏感,适应范围窄,而多穗型基因型则具有更好的适应能力;多穗型基因型在株距14cm和2cm密度处理条件下穗数高出幅度大于穗粒重高出的幅度,说明穗数是影响其单株产量在这两个密度处理条件下高于大穗型基因型的主要原因。 相似文献
16.
Magnetic x-ray diffraction combined with x-ray focusing optics was used to image individual antiferromagnetic spin density wave domains in a chromium single crystal at the micron scale. The cross section for nonresonant magnetic x-ray scattering depends on the antiferromagnetic modulation vector and spin polarization direction and allows these quantities to be extracted independently. The technique was used to show that the broadening of the nominally first-order "spin-flip" transition at 123 kelvin, at which the spins rotate by 90 degrees C, originates at the walls between domains with orthogonal modulation vectors. During cooling, the transition begins at these walls and progresses inward. The modulation vector domains are themselves unchanged. 相似文献
17.
18.
Kodama K Takigawa M Horvatić M Berthier C Kageyama H Ueda Y Miyahara S Becca F Mila F 《Science (New York, N.Y.)》2002,298(5592):395-399
We report the observation of magnetic superstructure in a magnetization plateau state of SrCu2(BO3)2, a frustrated quasi-two-dimensional quantum spin system. The Cu and B nuclear magnetic resonance (NMR) spectra at 35 millikelvin indicate an apparently discontinuous phase transition from uniform magnetization to a modulated superstructure near 27 tesla, above which a magnetization plateau at 1/8 of the full saturation has been observed. Comparison of the Cu NMR spectrum and the theoretical analysis of a Heisenberg spin model demonstrates the crystallization of itinerant triplets in the plateau phase within a large rhomboid unit cell (16 spins per layer) showing oscillations of the spin polarization. Thus, we are now in possession of an interesting model system to study a localization transition of strongly interacting quantum particles. 相似文献
19.
In the tortoise, Gopherus polyphemus, single unit spikes in the olfactory epithelium in response to amyl acetate were positive relative to the slow potential. The number of spikes in a response train was 4 to 15, the duration 3 to 4 msec, the height 0.5 to 2 mv. The height of successive spikes in a train decreased. The decrement in height, the number, and the frequency of spikes changed with the strength of the odor. 相似文献
20.
Muhammad ZEESHAN Waheed ARSHAD Muhammad Imran KHAN Shiraz ALI Ali NAWAZ Amina BATOOL Muhammad TARIQ Muhammad Imran AKRAM Muhammad Amjad ALI 《农业科学与工程前沿(英文版)》2018,5(2):253
Pre-harvest sprouting in wheat is the germination of seeds within the spikes when rains occur after or during grain ripening, which occurs commonly in the barani tract of Pakistan. Therefore, 10 cultivars and five advanced lines of spring bread wheat were evaluated for pre-harvest sprouting resistance. After natural rainfall, seeds were immediately collected from the wet spikes and tested for germinating ability. Three different germination tests were applied to hand-threshed seed: (1) spikes threshed on the day of sampling and germination tested immediately, (2) spikes threshed on the day of sampling and germination tested 1 week later, and (3) spikes threshed 1 week after sampling and germination test immediately after threshing. Seeds and spikes kept for 1 week were place on blotting paper at room temperature. Cultivars BARS-09, 09FJ17, Doukkala-12, NARC-09 and Ouassou-20 exhibited higher sprouting resistance while other genotypes were susceptible to pre-harvest sprouting in each of the three tests. A diallel crossing was conducted with six susceptible and two resistant genotypes to assess the genetic behavior of pre-harvest sprouting resistance. The combining ability (CA) demonstrated a higher proportion of additive genetic effects for sprouting resistance, because of higher variance of general and specific CA for both parameters under study. Doukkala-12 and BARS-09 showed increased pre-harvest sprouting resistance in their F1 descendants. 相似文献