One-dimensional and two-dimensional quantum systems on carbon nanotube bundles

We report the first measurement of the structure of 4He atoms adsorbed on bundles of single-walled carbon nanotubes. Neutron diffraction techniques and nanotube samples closed at the end were used. At low coverage, 4He forms a 1D, single line lattice along the grooves between two nanotubes on the surface of the nanotube bundles. As coverage is increased, additional lines of 1D lattices form along the grooves. This is followed by an incommensurate, 2D monolayer covering the whole nanotube bundle surface. The lattice constants of these 1D and 2D systems are largely independent of filling once a single 1D line is formed. No occupation of the interstitial channels between nanotubes is observed in the present sample.     

Evidence of 1D behavior of He4 confined within carbon-nanotube bundles

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He4 gas in carbon single-wall nanotube (SWNT) samples. The vibrational specific heat measured between 100 mK and 6 K demonstrates an extreme sensitivity to outgassing conditions. For bundles with a few number of nanotubes the extra contribution to the specific heat, C(ads), originating from adsorbed He4 at very low density displays 1D behavior, typical for He atoms localized within linear channels as grooves and interstitials, for the first time evidenced. For larger bundles, C(ads) recovers the 2D behavior akin to the case of He4 films on planar substrates (grafoil).     

Anisotropic packing and one-dimensional fluctuations of C60 molecules in carbon nanotubes

The confinement of a C60 molecule encapsulated in a cylindrical nanotube depends on the tube radius. In small tubes with radius RT approximately < 7 A, a fivefold axis of the molecule coincides with the tube axis. The interaction between C60 molecules in the nanotube is then described by a O2-rotor model on a 1D liquid chain with coupling between orientational and displacive correlations. This coupling leads to chain contraction. The structure factor of the 1D liquid is derived. In tubes with a larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of Ag modes of C60 in nanotubes measured by resonant Raman scattering.     

Cs(6D_J)+(H_2,H_e)的反应与非反应碰撞能量转移

在样品池条件下,利用原子荧光光谱方法,测量了Cs(6DJ)与H2,He碰撞中的反应与非反应能量转移截面.利用脉冲激光886nm线双光子激发Cs(6S)到Cs(6D3/2)态,原子荧光中除含有6D3/2→6P的直接荧光外,还含有6D5/2→6P的转移荧光.利用三能级模型的速率方程分析,在不同的He和H2密度下,分别测垦直接荧光与转移荧光的时间积分荧光强度比,得到了6D3/2与H2和He碰撞的精细结构转移截面分别为σ=(55±13)×10-16和(16±4)×10-16 cm2,同时确定了6D5/2与H2和He的碰撞猝灭速率系数.6D5/2态与H2的碰撞猝灭速率系数比6D5/2与He的大,它是反应与非反应速率系数之和,利用实验数据确定非反应速率系数为6.3×10-10 cm3·s-1,得到6D5/2与H2的反应截面为(2.0±0.8)×10-16 cm2.利用不同H2(或He)密度下6D5/2→6P3/2时间积分荧光强度,得到6D3/2与H2反应截面为(4.0±1.6)×10-16 cm2,6D3/2与H2反应的活性大于6D5/2.     

Laser spectroscopic investigation of Ca atoms in liquid helium

Excitation and emission spectra of the 4s21S0 – 4s4p1P1 and 4s4p3PJ(J = 0, 1, 2) – 4s5s3S1 transitions of Ca atoms implanted not only in liquid 4He but also in liquid 3He have been measured. It has been found that the excitation spectra for liquid 3He show considerably smaller widths and peak shifts from transition wavelengths of a free Ca atom, in comparison with those for liquid 4He. These spectral characteristics have been well reproduced by our theoretical calculation based on a vibrating bubble model. This calculation has shown that, due to a significant difference in surface tension between the two liquids, the radius of a bubble formed around Ca in liquid 3He is larger than the one in 4He, and that this fact as well as the smaller number density of He atoms in liquid 3He cause weaker perturbation for Ca, resulting in the smaller peak shift and width for liquid 3He.     

Fully coupled channel approach to doubly strange s-shell hypernuclei

We describe ab initio calculations of doubly strange, S = -2, s-shell hypernuclei (4(LambdaLambda)H, 5LambdaLambda)H, 5(LambdaLambda)He, and 6(LambdaLambda)He) as a first attempt to explore the few-body problem of the full-coupled channel scheme for these systems. The wave function includes LambdaLambda, LambdaSigma, NXi, and SigmaSigma channels. Minnesota NN, D2' YN, and simulated YY potentials based on the Nijmegen hard-core model are used. Bound-state solutions of these systems are obtained. We find that a set of phenomenological B8B8 interactions among the octet baryons in S = 0,-1, and -2 sectors, which is consistent with all of the available experimental binding energies of S = 0,-1, and -2 s-shell (hyper)nuclei, can predict a particle stable bound state of 4(LambdaLambda)H. For 5(LambdaLambda)H and 5(LambdaLambda)He, LambdaN-SigmaN and XiN-LambdaSigma potentials significantly affect the net LambdaLambda-NXi coupling, and a large Xi probability is obtained even for a weaker LambdaLambda-NXi potential.     

液态低沸点气体的表面张力

利用双毛细管法测定了液氮沸点温度下液氮的表面张力。用非线性方程γ＝γ&#176;（１－Ｔ／Ｔｃ）＾ｎ描述了液态Ｎ２、Ｏ２、ＡｒＸｅ、Ｈ２、Ｎｅ、Ｈｅ、Ｆｅ和Ｋｒ的表面张力与温度的关系，并采用一元性回归的方法拟合了上述各流体的参数γ&#176;和ｎ，准确度高于文献值，说明方程能正确表达表面张力与温度的关系。这些数据有利于对上述低沸点气体进行准确的热力学计算。     

Rb(7S-5D)-H2,He能量转移截面与温度的关系

脉冲激光双光子激发Rb(5S)态到Rb(5D)或Rb(7S)态,在样品池条件下,利用原子荧光光谱方法测量了Rb(7S-5D)-H2,He碰撞能量转移截面与池温的关系.利用三能级模型的速率方程分析.通过测量在不同H2或He密度下的直接荧光与转移荧光的时间积分强度比,在353～493 K温度范围内得到了Rb(7S-5D)-H2,He的反应与非反应碰撞能量转移截面.对于Rb(7S)+H2→Rb(5D)+H2,其转移截面随温度的增加而减小,而其逆过程的转移截面则随温度的增加而增加.对于与He的碰撞,在不同温度下7S-5D的转移截面均符合细致平衡原理,7S,5D态与H2的碰撞速率系数是反应与非反应速率系数之和,利用实验数据可以分别确定反应与非反应截面,7S态的平均反应截面与5D态平均反应截面之比约为1.5.Rb(7S)与H2的反应活动性大于Rb(5D).     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

桶式离子源参数计算与实验的比较

在用H_2、D_2和He气体运行的情况下,把桶式离子源的引出电流、放电电流和灯丝电流的计算与TEXTOR装置上的实验结果做了比较。     

Possible one-dimensional 3He quantum fluid formed in nanopores

Heat capacity measurements have been made down to 5 mK for 3He fluid films adsorbed in one-dimensional (1D) nanometer-scale pores, 28 A in diameter, preplated with 4He of 1.47 atomic layers. At low 3He density, the heat capacity shows a density-dependent, Schottky-like peak near 150 mK asymptoting to the value corresponding to a 2D Boltzmann gas at high temperatures. The peak behavior is attributed to the crossover from a 2D gas to a 1D state at low temperatures. The degenerate state of the 1D 3He fluid is indicated by a predominantly linear temperature dependence below about 30 mK.     

Particle spectra from reversed kinematics: Powerful probes of hot rotating nuclei

Inclusive¹H/⁴He emission has been studied for three reactions in “reversed kinematics”,⁴⁰Ar/⁵⁶Fe/⁸⁶Kr (8.5MeV/u)+¹²C. The⁴He spectra include the heretofore unobserved region of low c.m. energies in the direction of the heavy reactant. A test of forward-backward symmetry is made for the hot-spot, moving source model; the data do not support the model. Effective emission barriers for⁴He evaporation are found to be within ～10% of the corresponding fusion barriers, while for¹H evaporation the emission barriers are more than 25% lower. It is possible that deformations predicted by the rotating liquid drop model can account for the⁴He evaporation spectra, but substantial change in the standard evaporation treatment is required to explain the spectra for¹H.     

Investigation of electron localization in harmonic emission from asymmetric molecular ion

We theoretically investigate the electron localization around two nuclei in harmonic emission from asymmetric molecular ion. The results show that the ionization process of electron localized around one nucleus competes with its transfer process to the other nucleus. By increasing the initial vibrational level, more electrons localized around the nucleus D+tend to transfer to the nucleus He2+so that the ionizations of electrons localized around the nucleus He2+increase. In this case, the difference in harmonic efficiency between He H2+and He D2+decreases while the difference in harmonic spectral structure increases. The evident minimum can be observed in the harmonic spectrum of He H2+compared with that in the spectral structure of He D2+, which is due to the strong interference of multiple recombination channels originating from two nuclei. Time-dependent nuclear probability density, electron-nuclear probability density, double-well model, and time-frequency maps are presented to explain the underlying mechanisms.     

Hydrodynamics of superfluid helium in a single nanohole

The flow of liquid helium through a single nanohole with radius smaller than 25 nm was studied. Mass flow was induced by applying a pressure difference of up to 1.4 bar across a 50 nm thick Si(3)N(4) membrane and was measured directly by means of mass spectrometry. In liquid He I, we experimentally show that the fluid is not clamped by the short pipe with diameter-to-length ratio D/L?1, despite the small diameter of the nanohole. This viscous flow is quantitatively understood by making use of a model of flow in short pipes. In liquid He II, a two-fluid model for mass flow is used to extract the superfluid velocity in the nanohole for different pressure heads at temperatures close to the superfluid transition. These velocities compare well to existing data for the critical superflow of liquid helium in other confined systems.     

Phonons, rotons, and layer modes of liquid 4He in aerogel

We report the first observation of two-dimensional layer modes in both fully filled and partially filled aerogel. Using complementary high-energy resolution and high statistical precision neutron scattering instruments, and two different 87% porous aerogel samples, we show that the three-dimensional (3D) phonon-roton excitation energies and lifetimes of liquid 4He in aerogel are the same as in bulk 4He within current precision. The layer modes are the excitations that distinguish aerogel from the bulk rather than a difference in the 3D roton energy.     

Flow of 2D atomic hydrogen over the surface of liquid <Superscript>4</Superscript>He

Experimental evidence has been obtained for the hydrodynamic flow of a 2D gas of hydrogen atoms adsorbed on the surface of liquid helium. The observed flow manifestations are consistent with the concepts of the quantum hydrodynamics of the helium surface. This circumstance allows both investigation of the interaction of 2D hydrogen with ripplons and surface ³He quasiparticles and possible future observation of the superfluidity of the 2D Bose gas of hydrogen atoms. The experimental results on thermal compression make it possible to estimate the characteristic times of the transfer of longitudinal momentum between the subsystems of hydrogen and ³He atoms bound to the surface (τ_H3), as well as from ripplons to the substrate (τ_R). The value τ_H3 ～ 4 × 10^?8 s agrees with a value calculated using the mean-field parameter U₃₀ for the interaction of hydrogen atoms with the ground surface state of ³He. At the same time, τ_R is more than an order of magnitude shorter than the value obtained in experiments by Mantz et al. Phys. Rev. Lett. 44, 66 (1980) [Erratum: Phys. Rev. Lett. 44, 1094 (1980)]. This discrepancy can be attributed to the dependence of the ripplon momentum relaxation rate on the substrate roughness scale.     

Clustering Features of the 7Be Nucleus in Relativistic Fragmentation

Charge topology of fragmentation of 1.2 A GeV ⁷Be nuclei in nuclear track emulsion is presented. The dissociation channels ⁴He + ³He, 2³He+ n, ⁴He + 2¹H are considered in detail. It is established that the events ⁶Be + n amount about to 27 % in the channel ⁴He + 2¹H.     

Low Energy Ion Channelling in Single-Wall Nanotubes

Monte Carlo simulation is used to study the low energy He ion channelling in a （17, 0） single-wall nanotube and its rope. The simulation shows that the channelling critical angle ψc = 48E^-1/2 （E is incident energy） for a 1.31-nm-diameter initial beam into the nanotube, while ψc = 45E^-1/2 for the 1.31-nm-diameter initial beam into its rope.     

烃在四种典型活性炭中吸附状态随温度变化的~1H NMR和~2H NMR的研究

本文观察了吸附在四种孔结构活性炭中苯,环已烷和正已烷的~1H NMR和笊代苯的~2H NMR谱线在297～195 κ温度范围内的变化。在室温下被吸附的上述各种化合物的~1H NMR及~2H NMR谱均为一条线宽不同的单峰,且随温度下降而增宽。在~2H NMR谱中发现,当以对数表示的谱线半高宽和绝对温度倒数的直线关系上出现拐点时,同时可观察到固体粉末谱线。除2~#活性炭外,最初出现固体粉末谱的温度大致相同。这暗示粉末谱是毛细管中三维类液体固化的结果。文内就所观察到的现象进行了初步讨论。     

Quantum frustration in the "spin liquid" phase of two-dimensional 3He

We have measured the ultralow temperature and low field magnetic susceptibility of the 4/7 phase of two-dimensional 3He adsorbed on graphite preplated by one layer of 4He. The experiments are performed by progressively adding 4He to the system, thus suppressing in a controlled way the 3He atoms trapped in substrate heterogeneities. This procedure enables us to determine the intrinsic properties of this spin 1/2 model magnet in the zero field limit. The results show quantitatively that the system is strongly frustrated by multiple spin exchange interactions. A characteristic gapped spin liquid behavior is observed at ultralow temperature.