共查询到20条相似文献，搜索用时 641 毫秒
1.
We consider trapped atomic Fermi gases with Feshbach-resonance enhanced interactions in pseudogap and superfluid temperatures. We calculate the spectrum of radio-frequency (or laser) excitations for transitions that transfer atoms out of the superfluid state. The spectrum displays the pairing gap and also the contribution of unpaired atoms, that is, in-gap excitations. The results support the conclusion that a superfluid, in which pairing is a manybody effect, was observed in recent experiments on radio-frequency spectroscopy of the pairing gap. 相似文献
2.
Fermi gases, collections of fermions such as neutrons and electrons, are found throughout nature, from solids to neutron stars. Interacting Fermi gases can form a superfluid or, for charged fermions, a superconductor. We have observed the superfluid phase transition in a strongly interacting Fermi gas by high-precision measurements of the local compressibility, density, and pressure. Our data completely determine the universal thermodynamics of these gases without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity, which displays a characteristic lambda-like feature at the critical temperature T(c)/T(F) = 0.167(13). The ground-state energy is 3/5ξN E(F) with ξ = 0.376(4). Our measurements provide a benchmark for many-body theories of strongly interacting fermions. 相似文献
3.
Wang Y Ono S Onose Y Gu G Ando Y Tokura Y Uchida S Ong NP 《Science (New York, N.Y.)》2003,299(5603):86-89
We have determined the upper critical field Hc2 as a function of hole concentration in bismuth-based cuprates by measuring the voltage induced by vortex flow in a driving temperature gradient (the Nernst effect), in magnetic fields up to 45 tesla. We found that Hc2 decreased steeply as doping increased, in both single and bilayer cuprates. This relationship implies that the Cooper pairing potential displays a trend opposite to that of the superfluid density versus doping. The coherence length of the pairs xi(0) closely tracks the gap measured by photoemission. We discuss implications for understanding the doping dependence of the critical temperature Tc0. 相似文献
4.
Cohen EG 《Science (New York, N.Y.)》1977,197(4298):11-16
I have argued, following Einstein and London, that Bose-Einstein statistics is important for understanding the behavior of superfluid (4)He, while Fermi-Dirac statistics is important for understanding that of (3)He. In order to understand qualitatively the general behavior of (3)He-(4)He mixtures at constant pressure, the interaction between the helium atoms cannot be neglected. A very simple model of (3)He-(4)He mixtures is then a binary mixture of two kinds of hard spheres that follow Bose-Einstein and Fermi-Dirac statistics, respectively. This model correctly predicts the most striking features of the phase diagrams of helium mixtures in the temperature-concentration plane. In particular, the Bose-Einstein statistics of (4)He is responsible for the occurrence of a phase separation of the mixture at low temperatures that starts at an unusual type of critical point, while the Fermi-Dirac statistics of (3)He leads to an incomplete phase separation near the absolute zero of temperature, which makes possible the successful operation of a powerful cooling device, the helium dilution refrigerator. 相似文献
5.
6.
7.
小麦淀粉的退火改性及其性质表征 总被引：2，自引：0，他引：2
【目的】对小麦淀粉进行退火（annealing）处理能够使其发生物理改性,以处理后淀粉的糊化温度（起始温度To、峰温Tp、终止温度Tc）为依据优化退火条件,并研究其颗粒形态与退火的相关性。【方法】利用差示扫描量热法（differential scanning calorimetry,DSC）并结合DPS数据分析软件对小麦淀粉退火过程中的水分含量、退火时间、退火温度3个因子进行优化;采用扫描电子显微镜（SEM）和激光粒度分析仪（LPA）研究退火处理对小麦淀粉颗粒形貌和大小的影响。【结果】根据DSC测得的热力学参数分析得出小麦淀粉退火的最优条件为：80%水分含量的条件下,45℃退火3 d。退火处理会改变小麦淀粉颗粒的外貌而不破坏其形状,且对A-淀粉的影响较B-淀粉明显,淀粉颗粒粒径也因退火而增大。【结论】经退火处理的小麦淀粉的糊化温度显著高于原淀粉,且淀粉糊化温度范围变窄,同时小麦淀粉颗粒外貌和粒径的变化与退火温度呈正相关。 相似文献
8.
T Kimura Y Tomioka H Kuwahara A Asamitsu M Tamura Y Tokura 《Science (New York, N.Y.)》1996,274(5293):1698-1701
The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2-2xSr1+2xMn2O7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers. The CPP-MR is extremely large (10(4) percent at 50 kilo-oersted) at temperatures near above the three-dimensional ordering temperature (Tc approximately 90 kelvin) because of the field-induced coherent motion between planes of the spin-polarized electrons. Below Tc, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field, which gives rise to the low-field tunneling MR as large as 240 percent. 相似文献
9.
The percolation phase transition in sea Ice 总被引：1，自引：0，他引：1
Sea ice exhibits a marked transition in its fluid transport properties at a critical brine volume fraction pc of about 5 percent, or temperature Tc of about -5 degreesC for salinity of 5 parts per thousand. For temperatures warmer than Tc, brine carrying heat and nutrients can move through the ice, whereas for colder temperatures the ice is impermeable. This transition plays a key role in the geophysics, biology, and remote sensing of sea ice. Percolation theory can be used to understand this critical behavior of transport in sea ice. The similarity of sea ice microstructure to compressed powders is used to theoretically predict pc of about 5 percent. 相似文献
10.
A sharp peak of the zero-temperature penetration depth at optimal composition in BaFe2(As(1-x)P(x))2
Hashimoto K Cho K Shibauchi T Kasahara S Mizukami Y Katsumata R Tsuruhara Y Terashima T Ikeda H Tanatar MA Kitano H Salovich N Giannetta RW Walmsley P Carrington A Prozorov R Matsuda Y 《Science (New York, N.Y.)》2012,336(6088):1554-1557
In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λ(L). In two-dimensional systems, this ratio n/m* (~1/λ(L)(2)) determines the effective Fermi temperature, T(F). We report a sharp peak in the x-dependence of λ(L) at zero temperature in clean samples of BaFe(2)(As(1)(-x)P(x))(2) at the optimum composition x = 0.30, where the superconducting transition temperature T(c) reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T(c)/T(F) at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality. 相似文献
11.
Tenne DA Bruchhausen A Lanzillotti-Kimura ND Fainstein A Katiyar RS Cantarero A Soukiassian A Vaithyanathan V Haeni JH Tian W Schlom DG Choi KJ Kim DM Eom CB Sun HP Pan XQ Li YL Chen LQ Jia QX Nakhmanson SM Rabe KM Xi XX 《Science (New York, N.Y.)》2006,313(5793):1614-1616
We demonstrated that ultraviolet Raman spectroscopy is an effective technique to measure the transition temperature (Tc) in ferroelectric ultrathin films and superlattices. We showed that one-unit-cell-thick BaTiO3 layers in BaTiO3/SrTiO3 superlattices are not only ferroelectric (with Tc as high as 250 kelvin) but also polarize the quantum paraelectric SrTiO3 layers adjacent to them. Tc was tuned by approximately 500 kelvin by varying the thicknesses of the BaTiO3 and SrTiO3 layers, revealing the essential roles of electrical and mechanical boundary conditions for nanoscale ferroelectricity. 相似文献
12.
Dinnebier RE Gunnarsson O Brumm H Koch E Stephens PW Huq A Jansen M 《Science (New York, N.Y.)》2002,296(5565):109-113
CHCl3 and CHBr3 intercalated C60 have attracted particular interest after a superconductivity transition temperature (Tc) of up to 117 K was discovered. We have determined the structure using synchrotron x-ray powder-diffraction and found that the expansion of the lattice mainly takes place in one dimension (triclinic b axis), leaving planes of C60 molecules on an approximately hexagonal, slightly expanded lattice. We have performed tight-binding band structure calculations for the surface layer. In spite of the slight expansion of the layers, for the range of dopings where a large Tc has been observed, the density of states at the Fermi energy is smaller for C60.2CHCl3 and C60.2CHBr3 than for C60. This suggests that the lattice expansion alone cannot explain the increase of Tc. 相似文献
13.
14.
15.
Akimitsu J Takenawa K Suzuki K Harima H Kuramoto Y 《Science (New York, N.Y.)》2001,293(5532):1125-1127
We report a high Curie-temperature ferromagnet, CaB2C2. Although the compound has neither transition metal nor rare earth ions, the ferromagnetic transition temperature Tc is about 770 Kelvin. Despite this high T(c), the magnitude of the ordered moment at room temperatures is on the order of 10(-4) Bohr magneton per formula unit. These properties are rather similar to those of doped divalent hexaborides, such as Ca(1-x)La(x)B6. The calculated electronic states also show similarity near the Fermi level between CaB2C2 and divalent hexaborides. However, there is an important difference: CaB2C2 crystallizes in a tetragonal structure, and there are no equivalent pockets in the energy bands for electrons and holes-in contrast with CaB6. Thus, the disputed threefold degeneracy, specific to the cubic structure, in the energy bands of divalent hexaborides turns out not to be essential for high-temperature ferromagnetism. It is the peculiar molecular orbitals near the Fermi level that appear to be crucial to the high-Tc ferromagnetism. 相似文献
16.
Quantum confinement of itinerant electrons in atomically smooth ultrathin lead films produces strong oscillations in the thickness-dependent film energy. By adding extra electrons via bismuth alloying, we showed that both the structural stability and the superconducting properties of such films can be tuned. The phase boundary (upper critical field) between the superconducting vortex state and the normal state indicates an anomalous suppression of superconducting order just below the critical temperature, Tc. This suppression varies systematically with the film thickness and the bismuth content and can be parametrized in terms of a characteristic temperature, Tc* (less than Tc), that is inversely proportional to the scattering mean free path. The results indicate that the isotropic nature of the superconductive pairing in bulk lead-bismuth alloys is altered in the quantum regime. 相似文献
17.
Cao C Elliott E Joseph J Wu H Petricka J Schäfer T Thomas JE 《Science (New York, N.Y.)》2011,331(6013):58-61
A Fermi gas of atoms with resonant interactions is predicted to obey universal hydrodynamics, in which the shear viscosity and other transport coefficients are universal functions of the density and temperature. At low temperatures, the viscosity has a universal quantum scale ? n, where n is the density and ? is Planck's constant h divided by 2π, whereas at high temperatures the natural scale is p(T)(3)/?(2), where p(T) is the thermal momentum. We used breathing mode damping to measure the shear viscosity at low temperature. At high temperature T, we used anisotropic expansion of the cloud to find the viscosity, which exhibits precise T(3/2) scaling. In both experiments, universal hydrodynamic equations including friction and heating were used to extract the viscosity. We estimate the ratio of the shear viscosity to the entropy density and compare it with that of a perfect fluid. 相似文献
18.
北京11种宿根地被植物对镍的耐性和富集转运特征 总被引：1，自引：0，他引：1
为研究北京常用宿根地被植物对Ni胁迫的耐受和富集能力,对绢毛匍匐委陵菜、匍匐委陵菜、青绿苔草、涝峪苔草、荆芥、射干、马蔺、麦冬、大花萱草、‘金娃娃’萱草、玉簪11种宿根地被植物展开为期3个月的不同浓度含Ni土壤栽培试验,4种处理CK、T1、T2、T3土壤重金属Ni含量分别为:26.99、539.78、1 028.83、1 545.58 mg·kg-1。结果表明,本试验条件下未出现Ni超富集植物,但几乎所有处理组植物Ni转运系数均>1,具备较强的Ni转运能力。耐性方面:青绿苔草、大花萱草、麦冬、射干和马蔺Ni耐性较强,匍匐委陵菜、玉簪、‘金娃娃’萱草和涝峪苔草Ni耐性较弱。富集方面:匍匐委陵菜和青绿苔草在T1和T2浓度下具有较强Ni富集能力;马蔺和麦冬在Ni浓度达到T3时表现出相对较强的Ni富集能力。而绢毛匍匐委陵菜和荆芥在本试验条件下对Ni胁迫不具有耐性和富集性。青绿苔草、马蔺和麦冬兼具较强耐性和富集性,可作为净化北京园林绿地Ni污染的理想地被植物。 相似文献
19.
负压供水下土壤水分对樱桃萝卜生长及水分利用效率的影响 总被引：1，自引：0，他引：1
为探究负压供水下土壤水分对樱桃萝卜生长及水分利用效率的影响，采用盆栽试验，设置-5（T1）、-9（T2）、-14 kPa（T3）三个负压供水压力处理和一个常规浇灌（CK）处理，分析不同水分处理对樱桃萝卜土壤含水量、农艺性状、光合特性、水分利用效率等指标的影响。结果表明：①整个水分处理期间，CK、T1、T2、T3的土壤含水量分别平均为12.60%、12.63%、10.07%和8.47%，其中T1和T2为弱时间变异土壤水分，而T3和CK为中等时间变异土壤水分；全生育期累计灌水量和蒸散量均表现为CK>T1>T2>T3，各处理间均差异显著。②成熟期时，T1、T2、T3三个处理的樱桃萝卜株高、最大叶长、根径、干物质量、根鲜重等指标整体上均随土壤含水量的下降而下降；土壤含水量基本相同的T1和CK处理，前者的最大叶长、根鲜重显著高于后者，但株高、根径、干物质量等无明显差异。③不同处理樱桃萝卜叶片的SPAD值整体上无明显差异，生长前期净光合速率在处理之间没有表现出明显差异，但生长后期T1和T2显著高于CK，T3显著低于CK。④采收后不同处理的根鲜重、干物质量均表现为T1>T2>CK>T3，其中T1、T2显著高于T3，T1的根鲜重较CK显著提高36.09%，T3的根鲜重较CK显著降低42.36%；不同处理的根冠比没有显著差异，在0.95~1.05之间；产量水分利用效率(WUEY)和生物量水分利用效率(WUEB)均表现为T2>T1>T3>CK，T1、T2的WUEY和WUEB较CK分别显著提高了43.42%、57.89%和52.67%、62.00%。综上所述，土壤含水量和土壤水分时间变异性共同影响了樱桃萝卜的生长发育和产量形成，平均含水量基本相同时弱时间变异的土壤水分条件能够促进植株生长发育，显著提高樱桃萝卜的产量及水分利用效率。以上研究结果对进一步明确土壤水分与作物关系具有重要的科学指导意义。 相似文献
20.
为探究负压供水下土壤水分对樱桃萝卜生长及水分利用效率的影响,采用盆栽试验,设置-5（T1）、-9（T2）、-14 kPa（T3）三个负压供水压力处理和一个常规浇灌（CK）处理,分析不同水分处理对樱桃萝卜土壤含水量、农艺性状、光合特性、水分利用效率等指标的影响。结果表明：①整个水分处理期间,CK、T1、T2、T3的土壤含水量分别平均为12.60%、12.63%、10.07%和8.47%,其中T1和T2为弱时间变异土壤水分,而T3和CK为中等时间变异土壤水分;全生育期累计灌水量和蒸散量均表现为CK>T1>T2>T3,各处理间均差异显著。②成熟期时,T1、T2、T3三个处理的樱桃萝卜株高、最大叶长、根径、干物质量、根鲜重等指标整体上均随土壤含水量的下降而下降;土壤含水量基本相同的T1和CK处理,前者的最大叶长、根鲜重显著高于后者,但株高、根径、干物质量等无明显差异。③不同处理樱桃萝卜叶片的SPAD值整体上无明显差异,生长前期净光合速率在处理之间没有表现出明显差异,但生长后期T1和T2显著高于CK,T3显著低于CK。④采收后不同处理的根鲜重、干物质量均表现为T1>T2>CK>T3,其中T1、T2显著高于T3,T1的根鲜重较CK显著提高36.09%,T3的根鲜重较CK显著降低42.36%;不同处理的根冠比没有显著差异,在0.95~1.05之间;产量水分利用效率(WUEY)和生物量水分利用效率(WUEB)均表现为T2>T1>T3>CK,T1、T2的WUEY和WUEB较CK分别显著提高了43.42%、57.89%和52.67%、62.00%。综上所述,土壤含水量和土壤水分时间变异性共同影响了樱桃萝卜的生长发育和产量形成,平均含水量基本相同时弱时间变异的土壤水分条件能够促进植株生长发育,显著提高樱桃萝卜的产量及水分利用效率。以上研究结果对进一步明确土壤水分与作物关系具有重要的科学指导意义。 相似文献