The Effect of Roton Backflow on Quantum Evaporation from Superfluid 4He

We investigate the effect of roton backflow on the scattering of atoms, rotons and phonons at the free surface of superfluid ⁴ He at T=0 K by including backflow semi-phenomenologically in the form of a backflow potential in the theory of Sobnack et al. [M. B. Sobnack, J. C. Inkson, and J. C. H. Fung, Phys. Rev. B 60, 3465 (1999)]. We assume that all the surface scattering processes are elastic and that the quasiparticles and atoms are incident obliquely to the free surface. We calculate probabilities for the various one-to-one surface scattering processes allowed for a range of energies and compare the scattering rates with those obtained when backflow is neglected.     

The Energy Dependence of Roton Quantum Evaporation from Superfluid 4He

We have measured the energy dependence of roton quantum evaporation. A transiently heated cavity filled with superfluid ⁴ He generates R⁺ and R^– rotons. The emitted rotons are collimated and arrive at the free liquid surface with a narrow range of incident angles. We detect two beams of evaporated atoms, one due to R⁺ rotons and the other due to R^– rotons. Our numerical simulation of roton and atom trajectories, using an evaporation probability of 1, yields two angular distributions of atom flux which are similar to our experimental results, but do have systematic differences which we attribute to the evaporation probability. The ratio of the observed signal to the computed value at each bolometer position gives the relative roton quantum evaporation probability as a function of roton energy. We find that this probability increases with roton energy, except perhaps for low energy R^– rotons. We compare these results with theoretical predictions.     

A Beliaev Theory Incorporating a Semi-Phenomenological Roton Backflow Effect

We have developed a Beliaev theory incorporating a semi-phenomenological roton backflow effect in order to examine the effect of the backflow on the Quantum Evaporation of atoms from the free surface of superfluid ⁴ He. A theory of Quantum Evaporation based on a real-space Beliaev theory neglecting roton backflow was recently developed by Sobnack et al. [Phys. Rev. B 60, 3465 (1999)], and in this paper we discuss the extension of the theory to include the backflow physics from the Aldrich-Pines polarization potential theory of the 1970's. The calculation of the effect of the backflow on Quantum Evaporation is presented elsewhere.     

Quantum Evaporation from Superfluid 4He

The quantum evaporation experiments of Brown and Wyatt ² have been re-analysed in the light of a recent measurement of the high-energy phonon spectrum created by a pulse-heated thin film ¹⁰ . Two sources of systematic error become significant at the level of the precision required by this new analysis: firstly, in the detector position which is recalibrated by using large-angle roton evaporation; and secondly, in the liquid height due to capillary action affecting the level-detectors. These effects have been included in an improved simulation of the experiment which has brought the angular dependence of the measured and theoretical phonon-atom evaporation results into agreement within the mechanical tolerances of the apparatus. The reanalysis suggests that the roton-atom evaporation probability increases with wave vector.     

The Wave-Vector Dependence of Quantum Evaporation from Superfluid 4He

Absolute measurements of the probability of quantum evaporation of atoms by rotons from the surface of superfluid ⁴ He are still problematic. However, it is possible to obtain information about the wave-vector dependence of the evaporation process by using a refined simulation ¹⁴ to interpret the experiments of Brown and Wyatt ¹² . Two theories (Guilleumas et al. ⁹ and Sobnack et al. ¹⁰ ) are compared with these experiments by incorporating their predictions for the quantum evaporation probability into a numerical simulation. Both theories over-estimate the probability of phonon-atom evaporation. For roton (R ⁺ –atom) evaporation, compared with a simulation that assumes all kinetically allowed events are equally probable, the theory of Guilleumas et al. does not significantly improve the agreement with experiment, and the theory of Sobnack et al. increases the discrepancy.     

油墨回流对供墨调节的影响

输出墨量的正确控制是影响印刷质量的最重要因素之一.分析油墨转移过程中,输出墨量、回流墨量和消耗墨量之间的关系,提出了输出墨量并不等于承印物消耗的墨量,墨键开度的设置不应该正比例于版面覆盖率分布,橡皮布的油墨传递率的变化需要等量地增减输出墨量来适应.     

Quantum Reflection, Evaporation, and Transport Currents in 4He

We study transport currents induced by excited states in the surface and in bulk ⁴He. This requires the calculation of particle currents to second order in the excitation amplitudes. For that purpose, we extend a well-tested microscopic theory of inhomogeneous quantum liquids to find the currents created when either a quasiparticle propagates through the liquid, when atoms scatter off its surface, or when excitations evaporate atoms. Specifically, we look at the phonon lifetime and quantum reflection. We show that quantum reflection is sensitively affected both by details of the liquid surface profile and by many-body effects.     

Roton growth resistance of helium-4 solid surfaces

Experimental data available on roton contribution to the growth resistance of helium-4 surfaces from the work of Amrit, Legros and Poitrenaud (ref. 4) are analysed using the quantum kind model of Edwards, Mukherjee and Pettersen (ref. 5). The results are in strong favour of dissipation being due to kink-roton interactions. Further, they also support the idea that the angular anisotropy of the roton growth resistance of rough crystral surfaces is related to the density of steps.     

Scintillation and Quantum Evaporation Generated by Single Monoenergetic Electrons Stopped in Superfluid Helium

An electron stopped in superfluid helium generates phonons and rotons in the liquid as well as uv photons via scintillation. We report recent measurements with single 364 ke V electrons. A sapphire wafer with a superconducting transition-edge sensor is mounted above the liquid and can measure energy and timing information of individual events. We observe both uv photons and the quantum evaporation of helium atoms resulting from phonons and rotons generated by the ionizing particle in the liquid. The production of photons and rotons is strikingly different for an electron and for an alpha particle. The origin of the differences is associated with the different density of excitations along the tracks of an alpha particle and an electron.     

Excitations Beyond the Roton of Liquid 4He in Aerogel

The dynamic structure factor, S(Q, ), for wavevectors, 2.0Q3.6 Å ^–1 of liquid ⁴ He in 95% porous aerogel has been measured by inelastic neutron scattering methods. The aerogel was grown with deuterated materials and the multiple scattering involving the aerogel was negligible. S(Q, ) in the superfluid phase consists of a single peak plus broad intensity at higher energy , as in bulk superfluid ⁴ He. The single peak is identified with the phonon-roton excitation at higher Q. The weight in the peak, Z_Q , and the excitation energy dispersion curve, _Q , has the same basic wavevector dependence as in the bulk. The energy _Q is 2–3% below the bulk value at the end point and the peak is unobservable beyond Q=3 Å ^–1 within the present statistical precision. No peak is observed at T=2.3 K in normal ⁴ He suggesting, as in bulk ⁴ He, that the characteristic excitation at higher Q is associated with the superfluid phase.     

The Dynamic Structure Factor of 4He beyond the Roton Minimum

The dynamic structure factor S( , ) of ⁴ He is studied at zero temperature in the momentum region at and above the roton minimum by field-theoretical methods. The model is derived from the Gavoret-Nozières (GN) two particle propagator by introducing the concept of quasiparticles. In this way a connection between the field theory of GN and the phenomenological models of Zawadowski-Ruvalds-Solana (ZRS) type is obtained. An improved expression for the dynamic structure factor within ZRS-like models is found. Numerical results for S( , ) are presented for momentum and energy independent effective interactions between quasiparticles.     

Microwave Spectroscopy of Condensed Helium at the Roton Frequency

A spectral absorption line of electromagnetic radiation in the frequency range 40–200 GHz and at temperatures 1.4–2.75 K is measured in liquid helium. It is found that the narrow line of resonance absorption near the roton frequency does exist against a wide pedestal. The results obtained are compared with the data on the roton spectrum obtained in neutron scattering experiments. The possible reason for narrow absorption peak appearance is analyzed and the analogy between the observed phenomenon and Mössbauer effect is considered.     

Layer- and Bulk Roton Excitations of 4He in Porous Media

Layer-roton excitations of ⁴He in porous media, such as aerogel, Gegtech or Vycor, depend on the number of atoms in the active liquid layer, which, in turn, depends on the substrate potential. Hence, by measuring or calculating the energetics of the layer modes, one can get first-hand information about the substrate potential. Layer rotons can have energies as high as 8 K, while calculated two-dimensional rotons never seem to reach 6 K. Further indication that layer modes are actually an intermediate case between two and three dimensions is given by the way layer modes show up in neutron scattering data of helium in porous materials. We argue that layer rotons should be regarded as a third, completely independent kind of excitation.     

快速热退火对多层Ge量子点晶体质量的影响

使用超高真空化学气相淀积(UHV/CVD)设备在Si衬底上生长多层Ge量子点,用双晶X射线衍射(DCXRD)、拉曼光谱(Raman)等手段表征在不同条件下快速热退火的Ge量子点材料的组分、应力等特性,研究了快速热退火对多层Ge量子点晶体质量的影响.结果表明:随着退火温度的升高,量子点中Ge的组分下降,量子点应变的弛豫程...     

Temperature Dependence of S(Q, E) in Liquid 4He Beyond the Roton

The temperature dependence of the dynamic structure factor S(Q, E) for superfluid and normal liquid ⁴ He at saturated vapor pressure has been measured by inelastic neutron scattering for wave vectors Q between 2.3 and 3.6 Å ^–1 in the temperature range 0.65 S(Q, E) has three components: A sharp peak at low energies whose dispersion flattens out and whose strength decreases with increasing Q, a broader peak at somewhat higher energies with a stronger dispersion, and a third broad and structureless contribution with a long high-energy tail. The main result is that the first two components disappear at T , while the high-energy contribution broadens toward lower energies. The two-peak structure can be interpreted in terms of a Bose-condensate induced coupling of the two-particle spectrum to the one-particle response, which appears in S(Q, E) in the presence of the Bose condensate. This coupling is analyzed using the hybridization theory of Zawadowski, Ruvalds, and Solana.     

蒸发冷凝法中气体压强和蒸发温度对粒子大小的影响

在Ｎ＿２气氛中，用蒸发冷凝法，制备了Ｓｎ细微粒和Ｆｅ及Ｆｅ－４７（ｗｔ）％Ｎｉ合金超细微粒。分别用ＳＥＭ和ＴＥＭ，直接观测粒子大小及其分布。结果表明，平均粒径不是随Ｎ＿２气压强增加单调上升，而是在某一压强Ｐ＿（ｍａｘ）下，出现最大值。随蒸发温度提高，Ｐ＿（ｍａｘ）值迅速增大。粒子大小及其分布与粒子生长机制密切相关。对粒子凝聚生长条件进行了简要讨论。     

基板温度对真空热蒸发沉积纯锌镀层组织性能的影响

通过真空热蒸发技术沉积得到一种耗锌量少且性能良好的锌镀层,研究了不同基板温度对所得锌镀层组织形貌、微观结构、腐蚀行为和附着力的影响。结果表明,当基底温度低于100℃时,镀层中存在排列紧密的柱状晶,当基板温度超过150℃后,镀层的微观结构发生明显变化。随着基板温度的升高,镀层表面电子转移的阻抗降低,基板温度为100℃时所得镀层的自腐蚀电位较正。随着基板温度的升高,镀层的附着力呈现先降低后增强的趋势,在25℃和50℃下沉积的镀层试样以及GI镀层试样中出现较为明显的镀层剥离现象。     

Influence of Surface Modifications on the Spatiotemporal Microdistribution of Quantum Dots In Vivo

For biomedical applications of nanoconstructs, it is a general prerequisite to efficiently reach the desired target site. In this regard, it is crucial to determine the spatiotemporal distribution of nanomaterials at the microscopic tissue level. Therefore, the effect of different surface modifications on the distribution of microinjected quantum dots (QDs) in mouse skeletal muscle tissue has been investigated. In vivo real‐time fluorescence microscopy and particle tracking reveal that carboxyl QDs preferentially attach to components of the extracellular matrix (ECM), whereas QDs coated with polyethylene glycol (PEG) show little interaction with tissue constituents. Transmission electron microscopy elucidates that carboxyl QDs adhere to collagen fibers as well as basement membranes, a type of ECM located on the basolateral side of blood vessel walls. Moreover, carboxyl QDs have been found in endothelial junctions as well as in caveolae of endothelial cells, enabling them to translocate into the vessel lumen. The in vivo QD distribution is confirmed by in vitro experiments. The data suggest that ECM components act as a selective barrier depending on QD surface modification. For future biomedical applications, such as targeting of blood vessel walls, the findings of this study offer design criteria for nanoconstructs that meet the requirements of the respective application.     

Gravitational Effects on Multi-component Droplet Evaporation

This paper focuses on the analysis of multi-component droplet heating and evaporation under microgravity and normal gravity conditions. This analysis is based on the conventional conservation equations of species and energy for the gas phase, and the energy balance equation at the liquid?Cgas interface. The species diffusion is based on the Hirschfelder law, rather than on the less general Fick??s equation. Moreover, the heat flux due to species diffusion is taken into account in addition to the classical conduction heat flux between the gas and the liquid droplets. The liquid phase analysis is based on the infinite thermal conductivity liquid phase model, which has been justified by a reasonably good agreement between the predicted and experimental results. Indeed, the developed evaporation model has been validated against experimental data reported by Chauveau et al. (2008), where the droplets evaporation has been observed in microgravity and normal gravity conditions. The effects of gravity have been taken into account by introducing the Grashof number in the expressions of the Sherwood and Nusselt numbers. This model has been implemented in the multidimensional IFP-C3D industrial software. The modeling and experimental results have been shown to be reasonably close and the gravitational effects have been revealed to be significant especially for multi-component liquids including heavy components.