Structure,rotational dynamics,and superfluidity of small OCS-doped He clusters

The structural and dynamical properties of carbonyl sulfide (OCS) molecules solvated in helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system and provide a more complex and detailed microscopic picture of this system than inferred from experiments.     

Experiments on small (H2)N clusters

The nucleation process of small hydrogen clusters has been studied in detail with space, time, and number size resolution on the basis of supersonic expansions of the gas through a cryogenic nozzle operating in the thermal range 24 < T <110 K. The diagnostic of the jet medium is based on high sensitivity (few photons/sec) Raman spectroscopy with very high spatial resolution ( $\approx 2~\mu$ m), enabling for quantitative measurements with time resolution in the domain of nanoseconds. Temperatures down to 0.1 K have been attained in the jet at a collisional rate low enough to avoid sudden freezing of the gas. Clusters between two and eight molecular units are observed size-resolved. Features of the ortho-ortho and ortho-para hydrogen dimers, and the role of three-body collisions for nascent dimers and trimers are reported.     

Stability of (3)He(2)(4)He(n) and (3)He(3)(4)He(n) L=0 clusters

We have studied the stability of mixed (3)He/(4)He clusters in L=0 states by the diffusion Monte Carlo method, employing the Tang-Toennies-Yiu He-He potential. The clusters (3)He(4)He(N) and (3)He(2)(4)He(N) are stable for N>1. The lighter atoms tend to move to the surface of the cluster. The minimum number of 4He atoms able to bind three 3He atoms in a L=0 state is nine. Two of three fermionic helium atoms stay on the surface, while the third one penetrates into the cluster.     

Stability of small mixed clusters of 4He and 3He atoms

The existence of small helium clusters containing a variable number of 4He and 3He atoms is studied within a variational Monte Carlo calculation employing the Aziz HFD-B(HE) pair interaction. The clusters 4He(2) support one and two 3He atoms; however, the system with three 3He atoms is metastable, and the next bound system requires at least 18 fermions. All clusters obtained by adding 3He atoms to the trimer 4He(3) and the tetramer 4He(4) are bound, but the clusters 4He(3)-3He(3,4,5) and 4He(3,4)-3He(9) are metastable. All remaining clusters with three or more bosons and any number of fermions are stable.     

Onset temperature of Bose-Einstein condensation in incommensurate solid (4)He

The temperature dependence of the one-body density matrix in (4)He crystals presenting vacancies is computed with path integral Monte Carlo simulations. The main purpose of this study is to estimate the onset temperature T(0) of Bose-Einstein condensation in these systems. We see that T(0) depends on the vacancy concentration X(v) of the simulated system, but not following the law T(0) ~ X(v)(2/3) obtained assuming noninteracting vacancies. For the lowest X(v) we study, that is X(v)= 1/256, we get T(0) = 0.15 ± 0.05 K, close to the temperatures at which a finite fraction of nonclassical rotational inertia is experimentally observed. Below T(0), vacancies do not act as classical point defects becoming completely delocalized entities.     

Onset of non-collinear magnetism in small Fe clusters

The onset of noncollinear magnetism in small Fe_N clusters is investigated by using a rotational invariant tight-binding Hamiltonian. The ground-state local magnetic moments, magnetic order and average magnetic moments are calculated as a function of the Coulomb exchange integral J. Representative results for Fe₃ and Fe₅ show that the noncollinear magnetic solutions are the most stable. A variety of qualitatively different self-consistent solutions is obtained as a function of J. This includes magnetic solutions with collinear and noncollinear spin arrangements. Our calculations are compared with previous density-functional results. Extensions and limitations of our work are also pointed out.     

Interplay between magic number stabilities and superfluidity of small parahydrogen clusters

The interplay between magic number stabilities and superfluidity of small parahydrogen clusters with sizes N=5 to 40 and temperatures 0.5 K相似文献   

Local superfluidity of parahydrogen clusters

We study by quantum Monte Carlo simulations the local superfluid response of small (up to 27 molecules) parahydrogen clusters, down to temperatures as low as 0.05 K. We show that at low temperature superfluidity is not confined at the surface of the clusters, as recently claimed by Khairallah et al. [Phys. Rev. Lett. 98, 183401 (2007)10.1103/PhysRevLett.98.183401]. Rather, even clusters with a pronounced shell structure are essentially uniformly superfluid. Superfluidity occurs as a result of long exchange cycles involving all molecules.     

Anomalous suppression of superfluidity in 4He confined in a nanoporous glass

We explore the superfluidity of 4He confined in a porous glass, which has nanopores of 2.5 nm in diameter, at pressures up to 5 MPa. With increasing pressure, the superfluidity is drastically suppressed, and the superfluid transition temperature approaches 0 K at some critical pressure, Pc approximately 3.4 MPa. The feature suggests that the extreme confinement of 4He into the nanopores induces a quantum phase transition from a superfluid to a nonsuperfluid at 0 K and at Pc.     

Zeeman relaxation of N2 + (2Σ+) in collisions with 3He and 4He

We compare the cross sections for the transitions changing the projection of the total angular momentum of N₂ ⁺(²Σ) in collisions with ³He and ⁴He at very low collision energy. The fundamental states of the two nuclear spin isomers of N₂ ⁺ are considered as well as the two fine structure levels of the first excited para level N=2. It is shown that the two fundamental states of the two nuclear spin isomers behave differently. For the fundamental para level N=0 of N₂ ⁺, the projection changing cross section is always negligible compared to the elastic one for both He isotopes. For the fundamental ortho level N=1 of N₂ ⁺, the spin-rotation interaction couples the different spin levels directly so the spin relaxation becomes a first order process. The associated resonances increase the projection changing cross section which remains smaller but becomes comparable with the elastic one. This is in contrast with the excited rotational levels of N₂ ⁺, which for the rotational deactivation and elastic channels are found to be equal around the resonances for the collisions involving ³He. These two channels are always larger than the projection changing one. We also find that, for transitions involving the fundamental rotational state, the domain of validity of the threshold laws discussed by Krems and Dalgarno [Phys. Rev. A 67, 050704 (2003)] for a potential decreasing faster than 1/r2 is shortened, due to the long range charge induced dipole potential. This effect is illustrated for the collisions of ³He with the fundamental para state of N₂ ⁺.     

He superfluidity in the presence of aerogel

Aerogels introduce disorder into the p-wave-paired superfluid ³He and suppress T_c. Quantifiable (by small angle X-ray scattering) differences in the long-range structure of two identical density aerogels are primarily responsible for their different transition temperatures. We also demonstrate that alteration of the short-range correlations by the addition of ⁴He does not strongly affect T_c. Acoustic measurements of the fast and slow modes of ³He in aerogel are described. These can be used to explore the superfluid component. We also outline future prospects.     

显现超流体最初端倪的分子--氦团簇

最近 ,文章作者所在的研究小组发表了数篇关于氦原子数多达二十的气相分子团簇的振动转动光谱的实验观测 .研究结果表明 ,N2 O分子和CO2 分子的氦团簇在氦原子数为 7个和 6个时就分别显示出分子超流体现象的开端 .文章简要地介绍了这些实验光谱观测的最新进展及其背景和物理意义     

Quantum-electrodynamical coherence and superfluidity in3He

Summary We explore the possibility that the superfluid³He transition is related to the quantum-electrodynamical coherent interaction, occurring in the system at low temperatures and producing coherent oscillation in phase with particular classical modes of the electromagnetic field. We give a microscopic explanation for the BCS pairing potential and calculate the phase diagram for³He finding a transition toA- andB-phases with the correct temperatures.     

Columnar-to-disk structural transition in nanoscale (SiO2)N clusters

Extensive large-scale global optimizations refined by ab initio calculations are used to propose (SiO2)N N = 14-27 ground states. For N < 23 clusters are columnar and show N-odd-N-even stability, energetically and electronically. At N = 23 a columnar-to-disk structural transition occurs reminiscent of that observed for SiN. These transitions differ in nature but have the same basis, linking the nanostructural behavior of an element (Si) and its oxide (SiO2). Considering the impact of devices based on the nanoscale manipulation of the result is of potential technological importance.     

Bubble formation and decay in 3He and 4He clusters

The energy transfer in 3He and 4He clusters electronically excited by monochromatic synchrotron radiation is investigated by luminescence spectroscopy. Depending on the cluster size and the isotopic constitution, either sharp, broadened, or shifted emission bands of single He molecules are observed. The spectral features show that He molecules emit light either within a bubble inside the cluster or in the vacuum after desorption from the cluster. From the luminescence intensity, the cluster diameter, and the radiative lifetime, an average velocity of approximately 7 m/s of bubbles in 4He clusters could be deduced. In the nonsuperfluid 3He clusters this velocity was observed to be significantly lower.