Observation of a strongly interacting degenerate Fermi gas of atoms

We report on the observation of a highly degenerate, strongly interacting Fermi gas of atoms. Fermionic lithium-6 atoms in an optical trap are evaporatively cooled to degeneracy using a magnetic field to induce strong, resonant interactions. Upon abruptly releasing the cloud from the trap, the gas is observed to expand rapidly in the transverse direction while remaining nearly stationary in the axial direction. We interpret the expansion dynamics in terms of collisionless superfluid and collisional hydrodynamics. For the data taken at the longest evaporation times, we find that collisional hydrodynamics does not provide a satisfactory explanation, whereas superfluidity is plausible.     

Heat capacity of a strongly interacting Fermi gas

We have measured the heat capacity of an optically trapped, strongly interacting Fermi gas of atoms. A precise addition of energy to the gas is followed by single-parameter thermometry, which determines the empirical temperature parameter of the gas cloud. Our measurements reveal a clear transition in the heat capacity. The energy and the spatial profile of the gas are computed using a theory of the crossover from Fermi to Bose superfluids at finite temperatures. The theory calibrates the empirical temperature parameter, yields excellent agreement with the data, and predicts the onset of superfluidity at the observed transition point.     

Universal quantum viscosity in a unitary Fermi gas

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.     

Pairing and phase separation in a polarized Fermi gas

We report the observation of pairing in a gas of atomic fermions with unequal numbers of two components. Beyond a critical polarization, the gas separates into a phase that is consistent with a superfluid paired core surrounded by a shell of normal unpaired fermions. The critical polarization diminishes with decreasing attractive interaction. For near-zero polarization, we measured the parameter beta = -0.54 +/- 0.05, describing the universal energy of a strongly interacting paired Fermi gas, and found good agreement with recent theory. These results are relevant to predictions of exotic new phases of quark matter and of strongly magnetized superconductors.     

Observation of Fermi pressure in a gas of trapped atoms

We report the attainment of simultaneous quantum degeneracy in a mixed gas of bosons (lithium-7) and fermions (lithium-6). The Fermi gas has been cooled to a temperature of 0.25 times the Fermi temperature by thermal collisions with the evaporatively cooled bosons. At this temperature, the spatial size of the gas is strongly affected by the Fermi pressure resulting from the Pauli exclusion principle and gives clear experimental evidence for quantum degeneracy.     

Observation of the pairing gap in a strongly interacting Fermi gas

We studied fermionic pairing in an ultracold two-component gas of 6Li atoms by observing an energy gap in the radio-frequency excitation spectra. With control of the two-body interactions through a Feshbach resonance, we demonstrated the dependence of the pairing gap on coupling strength, temperature, and Fermi energy. The appearance of an energy gap with moderate evaporative cooling suggests that our full evaporation brought the strongly interacting system deep into a superfluid state.     

Comment on "Pairing and phase separation in a polarized Fermi gas"

Partridge et al. (Reports, 27 January 2006, p. 503) reported pairing and phase separation in a polarized Fermi gas. We argue that it is not possible to distinguish the superfluid from the normal regimes in the presented data, or to discern which clouds were phase-separated. Some of the reported conclusions are inconsistent with recent experiments.     

Revealing the superfluid lambda transition in the universal thermodynamics of a unitary Fermi gas

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.     

蜕化p-Laplace方程无穷多解的存在性

研究了一类在无穷远处具超线性条件的蜕化p-Laplace方程无穷多解的存在性.由于没有假设A-R条件,不能像通常那样得到(PS)c条件,为此证明其满足Cerami条件,再利用对称山路引理,得到了无穷多解的存在性.     

Phase diagram of degenerate exciton systems

Degenerate exciton systems have been produced in quasi-two-dimensional confined areas in semiconductor coupled quantum well structures. We observed contractions of clouds containing tens of thousands of excitons within areas as small as (10 micron)2 near 10 kelvin. The spatial and energy distributions of optically active excitons were determined by measuring photoluminescence as a function of temperature and laser excitation and were used as thermodynamic quantities to construct the phase diagram of the exciton system, which demonstrates the existence of distinct phases. Understanding the formation mechanisms of these degenerate exciton systems can open new opportunities for the realization of Bose-Einstein condensation in the solid state.     

黄河某水库塌岸预测研究

水库塌岸导致沿岸植被破坏,水土流失加剧,对当地的经济及环境产生影响,因此必须对其作出预测,定量估计塌岸宽度。通过对某水库详细的地质调查,总结出该区存在冲蚀磨蚀型、坍（崩）塌型、滑移型3种主要的塌岸模式,并对塌岸发生的影响因素进行了分析。通过采用卡丘金塌岸预测方法,对该库区进行塌岸预测,其结果是可靠的,为选定工程处理措施提供了科学的依据。     

Pressure-Tuned Fermi Resonance in Ice VII

Fermi resonance was observed between the OH stretch and the overtone of the OH bending modes of HDO molecules contaminated in phase VII of D(2)O ice over the pressure range from 17 to 30 gigapascals. An anharmonic coupling constant, which is related to the potential energy surface on which hydrogen-bonded protons oscillate, was found to range around 50 wave numbers through the resonant pressure range. Its experimentally obtained magnitude and pressure-insensitive behavior will be useful for theoretical studies of the potential energy surface and hence of the nature of hydrogen bonding in ice.     

鲫鱼基因组DNA的扩增及在SSR分析中的验证

目的优化鲫鱼全基因组DNA扩增方法。方法使用内切酶酶切初孵仔鱼的基因组DNA,酶切片段连接寡核苷酸接头,根据酶切位点序列与接头序列设计引物,采用PCR法扩增酶切片段,增加基因组DNA的数量。结果使用Taq I内切酶酶切鲫鱼基因组DNA,酶切片段连接寡核苷酸接头后PCR扩增,获得扩增产物集中于250~1 500 bp。以扩增的滇池高背鲫鱼基因组DNA为模板,PCR扩增滇池高背鲫鱼的微卫星分子标记（SSR）位点,获得预期的扩增片段,电泳图谱与对照组（未扩增的基因组DNA为PCR模板）无差异。结论本研究优化了鲫鱼基因组DNA的扩增方法,并可用于SSR分析中,为鱼类大规模遗传分析提供了技术支撑。     

油罐凹瘪原因分析

分析了油罐在运行中发生凹瘪的原因，结果表明，油罐负压超限是导致其凹瘪的主要原因，列举了引起油罐负压超限的各种因素，其中机械呼吸阀反映的问题有，安装精度未达到要求，日常维护方式不当，被空气和油蒸气腐蚀后运动阻力增大等。