Quantized vortices in mixed 3He-4He drops

Using density functional theory, we investigate the structure of mixed (3)He(N3)-(4)He(N4) droplets with an embedded impurity (Xe atom or HCN molecule) which pins a quantized vortex line. We find that the dopant+vortex+(4)He(N4) complex, which in a previous work [F. Dalfovo et al., Phys. Rev. Lett. 85, 1028 (2000)] was found to be energetically stable below a critical size N(cr), is robust against the addition of 3He. While 3He atoms are distributed along the vortex line and on the surface of the 4He drop, the impurity is mostly coated by 4He atoms. Results for N4 = 500 and a number of 3He atoms ranging from 0 to 100 are presented, and the binding energy of the dopant to the vortex line is determined.     

Probing vortices in 4He nanodroplets

We present static and dynamical properties of linear vortices in 4He droplets obtained from density functional calculations. By comparing the adsorption properties of different atomic impurities embedded in pure droplets and in droplets where a quantized vortex has been created, we suggest that Ca atoms should be the dopant of choice to detect vortices by means of spectroscopic experiments.     

Exotic vortex structures of the dipolar Bose–Einstein condensates trapped in harmonic-like and toroidal potential

Based on the tunable intensity and waist of Gaussian laser, harmonic-like and toroidal potentials can be achieved and the ground-state properties of the dipolar Bose–Einstein condensate (BEC) trapped in such potentials are investigated. It is found that, in the harmonic-like potential, the singly and doubly quantized vortices can exist in the scale condensate and translate respectively into vortex pairs and triangular vortex lattice with increasing dipole–dipole interaction (DDI). Especially, the sandwich-like structure can be observed in the ground-state density profiles by tuning the direction and strength of DDI for some rotating frequency. In the toroidal potential, the competition between the inter-component interaction and DDI can induce the transition between immiscible and miscible states, and results in the structures of a doubly quantized vortex surrounded by a vortex ring. It is worth emphasizing that, with the increasing of DDI, the doubly quantized vortex in the harmonic-like potential becomes two singly quantized vortices, while in the toroidal potential it is no happen due to the presence of Gaussian barrier.     

Staircaselike suppression of supersolidity under rotation

There are a number of distinct signatures of superfluids, one of which is the appearance of quantized vortices. There have been some attempts to understand the putative supersolid 4He in the vortex framework, but no conclusive evidence that supports the existence of the vortices has been reported. Here, we investigate the rotation velocity dependence of the torsional oscillation of solid 4He at various temperatures. The velocity sweep reveals intriguing periodic staircaselike features below about 300 mK. The staircase patterns show remarkable periodicity, and we interpret these patterns as a consequence of vortex injection. However, there are some features that cannot be accounted for with simple injection of vortices into superfluid, and further investigation is required.     

Solvation onset of Ca in mixed helium droplets

Within density functional theory, we have addressed the solvation onset of Ca atoms in small, mixed helium drops. As a case of study, we have considered a Ca@⁴He₅₀ droplet to which we have first added one single ³He atom. Next, the structure of spin-saturated Ca@³He _N3+⁴He₅₀ droplets with N₃=18, 32, 50 and 68 is determined and used to infer the number of ³He atoms needed to solvate the Ca atom for that particular system.     

Quantum and quasiclassical types of superfluid turbulence

By injecting negative ions in superfluid 4He in the zero-temperature limit (T0.7 K, a jet of ions generates quasiclassical tangles identical to those produced by mechanical means.     

Derivation of the transverse force on a moving vortex in a superfluid

We describe an exact derivation of the total nondissipative transverse force acting on a quantized vortex moving in a uniform background. The derivation is valid for neutral boson or fermion superfluids, provided the order parameter is a complex scalar quantity. The force is determined by the one-particle density matrix far away from the vortex core, and is found to be the Magnus force proportional to the superfluid density.     

Dissipation of quantum turbulence in the zero temperature limit

Turbulence, produced by an impulsive spin down from angular velocity Omega to rest of a cube-shaped container, is investigated in superfluid 4He at temperatures 0.08 K-1.6 K. The density of quantized vortex lines L is measured by scattering negative ions. Homogeneous turbulence develops after time t approximately 20/Omega and decays as L proportional, t-3/2. The corresponding energy flux =nu'(kappaL)2 proportional, t-3 is characteristic of quasiclassical turbulence at high Re with a saturated energy-containing length. The effective kinematic viscosity in T=0 limit is nu'=0.003kappa, where kappa=10(-3) cm2 s(-1) is the circulation quantum.     

Splitting times of doubly quantized vortices in dilute bose-einstein condensates

Recently, the splitting of a topologically created doubly quantized vortex into two singly quantized vortices was experimentally investigated in dilute atomic cigar-shaped Bose-Einstein condensates [Y. Shin, Phys. Rev. Lett. 93, 160406 (2004)10.1103/PhysRevLett.93.160406]. In particular, the dependency of the splitting time on the peak particle density was studied. We present results of theoretical simulations which closely mimic the experimental setup. We show that the combination of gravitational sag and time dependency of the trapping potential alone suffices to split the doubly quantized vortex in time scales which are in good agreement with the experiments.     

Investigations of Droplet‐Plasma Interaction using Multi‐Dimensional Coupled Model

Recently developed multi‐dimensional coupled fluid‐droplet model is used to investigate the behavior of complex interaction between the liquid precursor droplets and atmospheric pressure plasma (APP). The significance of this droplet‐plasma interaction is not well understood under diverse realm of working conditions in two‐phase flow. In this study, we explain the implication of vaporization of liquid droplets in APP which are subsequently responsible to control major characteristics of surface coating depositions. Coalescence of water droplets is more dominant than Hexamethyldisiloxane (HMDSO) droplets because of its sluggish rate of evaporation. A disparity in the performance of evaporation is identified in two independent mediums, such as gas mixture and discharge plasma using HMDSO precursor. The length of evaporation of droplets is amplified by an increment of gas flow rate indicating with a reduction in the gas temperature and electron mean energy. In particular, the spatio‐temporal density distributions of charged particles show a clear pattern in which the typical nitrogen impurity ions are primarily effective as compared to other helium ionic species along the pulse of droplets in APP. Finally, we contrast the behavior of discharge species in the pure helium and He‐N2 gas mixtures revealing the importance of stepwise and Penning ionization processes. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Traces of vortices in superfluid helium droplets

We report on the observation of vortices in superfluid 4He droplets produced in the expansion of liquid He. The vortices were traced by introducing Ag atoms, which clustered along the vortex lines, into the droplets. The Ag clusters were subsequently surface-deposited and imaged via electron microscopy. The prevalence of elongated track-shaped deposits shows that vortices are present in droplets larger than about 300 nm and that their lifetime exceeds a few milliseconds. We discuss the possible formation mechanisms and the stability of the vortices.     

Kelvin-wave cascade on a vortex in superfluid 4He at a very low temperature

A study by computer simulation is reported of the behavior of a quantized vortex line at a very low temperature when there is continuous excitation of low-frequency Kelvin waves. There is no dissipation except by phonon radiation at a very high frequency. It is shown that nonlinear coupling leads to a net flow of energy to higher wave numbers and to the development of a simple spectrum of Kelvin waves that is insensitive to the strength and frequency of the exciting drive. The results are likely to be relevant to the decay of turbulence in superfluid 4He at very low temperatures.     

A numerical investigation of vortex-coupled superfluidity

We have performed numerical simulations of quantized vortex lines in a model of normal fluid turbulence. The results are used to discuss the idea, put forward to explain some recent experiments, that in isothermal turbulent helium flow the high density of vortex lines locks the two fluid components together.     

Dirac monopoles on quantized vortices in superfluid3He

Quantized vortices in superfluid³He display a variety of novel structures that have previously not been observed in any other quantum fluids; their basic experimental manifestations and theoretical features have been reviewed by Hakonen, Lounasmaa and Simola [Physica B160 (1989) 1] and by Salomaa and Volovik [Rev. Mod. Phys.59 (1987) 533]. In order not to repeat these reviews in this paper, here we choose to discuss the theme of the possible pointlike orderparameter singularities, monopoles, that can occur on the quantized³He vortices. Such monopoles, mathematical analogues of the magnetic Dirac monopole, may exist in superfluid³He due to the complicated order-parameter structure, which makes it possible to have several different types of quantized vortex lines and phase boundaries between the superfluid states. Analogs of Dirac monopoles, and also monopoles with 1/2 and 1/4 of the magnetic charge of the Dirac monopole, may exist at the points of intersections of quantized vortex lines and phase boundaries — or along vortices if they change their quantum state; several examples are discussed.Invited talk at the International Conference on Macroscopic Quantum Phenomena, Smolenice Castle, Czechoslovakia, September 18–22, 1989.I want to thank G. E. Volovik for a useful discussion and O. V. Lounasmaa for encouragement. This research has been supported through the Award for the Advancement of European Science by the Körber-Stiftung (Hamburg, FRG) and by the Academy of Finland.     

Vortex arrays of coexisting singly and doubly quantized vortex lines in<Superscript>3</Superscript>He-A

In superfluid³He-A singly and doubly quantized vortex lines can coexist in a rotating container. We measure with NMR techniques the radial distribution of the two vortex types in an array of vortex lines. The radial composition is found to depend on the procedure by which the array has been formed. The result shows that in superfluid³He the energy barriers separating different configurations of the vortex array are inpenetrably high for a metastable state to relax.     

Comparative Study of Substitutional N and Substitutional P in Diamond

Based on density functional theory calculations, it is found that for substitutional N in diamond the C_(3v) symmetry structure is more stable, while C_(3v) and D_(2d) symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant(the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.     

B在Hg0.75Cd0.25Te中掺杂效应的第一性原理研究

基于密度泛函理论的第一性原理方法,通过形成能和束缚能的计算研究了B在Hg_0.75Cd_0.25Te 中的掺杂效应.结果表明B在Hg_0.75Cd_0.25Te中存在着两种主要形态:第一种是在完整的 Hg_0.75Cd_0.25Te材料中B稳定存在于六角间隙位置而非替位.此时,B形成容易激活的三级施主使材料表现为n型.另一种是在有Hg空位存在的Hg_0.75Cd_0.25Te中B更容易与Hg空位结合形成缺陷复合体,其束缚能达到了0.96 eV.这种复合体在Hg_0.75Cd_0.25Te材料中形成单施主也使材料表现为n型.考虑到辐照损伤形成的Hg空位受主,这种B与Hg空位的复合体是制约B离子在MCT中注入激活的一个重要因素.     

Vortex pinning: A probe for nanoscale disorder in iron-based superconductors

The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density j_c at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to “weak collective pinning” and a magnetic field-independent contribution j_c^coll. The latter allows one to estimate quasiparticle scattering rates.     

Magnetic properties of nitrogen doped ZnO

Using the first principle method based on density functional theory,this paper studies the electronic structure and the ferromagnetic stability in N-doped ZnO.The calculated results based on local density approximation(LDA) and LDA+U method show that ferromagnetism coupling between N atoms is more energetically favourable for eight geometrically distinct configurations.The dominant ferromagnetic interaction is due to the hybridization between O 2p and N 2p.The origin of the ferromagnetic state in N doped ZnO is discussed by analysing coupling between N 2p levels.We also analyse N dopant concentration and lattice strain effect on ferromagnetism.