Binding of a 3He impurity to a screw dislocation in solid 4He

Using first-principles simulations for the probability density of finding a 3He atom in the vicinity of the screw dislocation in solid 4He, we determine the binding energy to the dislocation nucleus E(B)=0.8+/-0.1 K and the density of localized states at larger distances. The specific heat due to 3He features a peak similar to the one observed in recent experiments, and our model can also account for the observed increase in shear modulus at low temperature. We further discuss the role of 3He in the picture of superfluid defects.     

Medium corrections to the reaction cross section for low energy pion4He scattering

We investigate the effects of the Pauli principle and the nucleon binding on the reactive content of the single scattering optical potential. The binding and exclusion principle effects are calculated for low energy pion-⁴He scattering. A careful treatment of the Pauli principle enables us to determine a pion nucleon energy shift representing binding effects from the requirement that the reaction channel opens at the correct threshold energy. We find that binding and Pauli effects lead to a reduction of the reaction cross section by about 30–50% and (expect for very low energies) to a slight increase of the elastic cross section by 5–10%.     

Phenomenological He4-He4 potential and the properties of liquid helium (He4)

The properties of liquid He⁴ whose particles are interacting among themselves through an equivalent two-body Gaussian potential are studied by using the Bogolubov method. It is shown that the value of the hard-core radius 2·2 A°; the most probable values form ^*/m lie between 1·292 and 9 and that for/k between 8 and 16 °K. The results are compared with the experimental and theoretical values known so far. The energy gap and the effective massm ^* increase as the value ofk increases, keeping the density fixed. The resultant two-body interaction between the helium atoms is repulsive in the temperature range T0.On leave fromDepartment of Physics, Dibrugarh University, Dibrugarh, Assam, India.I am grateful to Professors Abdus Salam and P. Budini and to the Internatinoal Atomic Energy Agency for the kind hospitality and help given to me during my stay at the International Centre for Theoretical Physics, Trieste. The financial assistance which enabled me to be at the Centre is gratefully acknowledged. Thanks are also due to Professor B. R. Seth for encouragement.     

Relativistic corrections to electron scattering by 2H, 3He,and 4He

Relativistic corrections to the scattering of electrons by a nonrelativistic nucleus are calculated, using the method of Close and Copley, and Krajcik and Foldy. Particular attention is paid to the deuteron, ³He and ⁴He. It is found that these corrections are substantially the same as those calculated by F. Gross for the deuteron. The corrections to ³He have the effect of moving the diffraction minimum, as calculated in a nonrelativistic model, outward by as much as 0.32 fm^?2. Smaller corrections are found for ⁴He.     

Dimension of an optimum impurity cluster and multiple-occupancy corrections in the theory of scattering of vibrational excitations in a solid solution

A relationship is derived for the correlation length L determining the size of the region in a solid solution in which excitations are scattered coherently. The correlation length depends on the fraction of impurity atoms x in the solid solution and the lattice dimension d. In the physical analysis of single-particle scattering processes in the solid solution and calculations, it is sufficient to take into account clusters with the number of cells n corresponding to the correlation volume L ^d . A theoretical analysis is illustrated by calculations of the spectral functions of the solid solution at different values of x and n. The multiple-occupancy corrections (polynomials in powers of x) to scattering diagrams are calculated using the method of sequential breaking apart of the interaction lines in the diagrams for the self-energy part. The method used was previously applied to the case of scattering by a single impurity. In this paper, the efficiency of the method is checked for scattering by multi-impurity clusters. It is demonstrated that the method can be useful in analyzing and estimating the contributions of scattering diagrams.     

Bound states of an electron in an impurity potential on the surface of liquid helium

The energies and widths of the levels of an electron on impurity centers on the surface of liquid helium are calculated with allowance for the deformation of the surface. The level shift associated with the deformation effects is small and decreases very slowly with increasing level number. However, even a small shift of the energy levels relative to one another affects ripplon scattering, which makes the main contribution to the level width at low temperatures. It is predicted theoretically that this width depends very strongly on the external parameters and on the level number and that a maximum obtains at a clamping field E _⊥=51500 V/cm. The width of the levels of an electron in a bound state is found to be less than for free electrons. This makes it possible to perform a beautiful spectroscopic experiment. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 599–604 (10 November 1997)     

Optical properties of an off-center hydrogenic impurity in a spherical quantum dot with Gaussian potential

Optical absorption coefficients and refractive index changes associated with intersubband transition of an off-center hydrogenic impurity in a spherical quantum dot (QD) with Gaussian confinement potential are theoretically investigated. Our results show that the optical absorption coefficients in a spherical QD are 2–3 orders of magnitude higher than those in quantum wells and are 2–3 orders smaller than those in a disk-like QD. It is found that the optical absorptions and the optical refractive index are strongly affected not only by the confinement barrier height, dot radius but also by the position of the impurity.     

Implantation of impurity ions into He II for optical spectroscopy purposes

An experimental arrangement for implantation and handling of high densities of alkaline earth ions in He II is described. First results, including mobility measurements of Sr⁺, Ba⁺, Zn⁺and Cd⁺ ions as well as optical detection of Ba⁺ ions, are reported.     

Field-induced realignment of a smectic nanodroplet in an external magnetic field: A numerical investigation

The field-induced realignment of a smectic-A phase is in principle a complicated process involving the director rotation via the interaction with the field and the layer rotation via the molecular interactions. Time-resolved X-ray scattering experiments have revealed major phenomena concerning the maintenance of the integrity of the smectic-A layer structure during the alignment process. In order to obtain a deeper insight into this process, we have carried out a dissipative particle dynamics study of the realignment kinetics of a nanodroplet of a smectic-A liquid crystal suspended in an isotropic fluid following a switch in the direction of an applied magnetic field. The strength of the mesogen-field interaction is small compared to the inter-molecular interactions. The reaction of the smectic configuration to the field switch was found to depend on the balance between the inter-molecular interactions stabilising the formation of the smectic layering and the interaction of the mesogens with the external field. It is found that the rotational behaviour of the smectic layers under the influence of an external magnetic field arises from a combination of stochastic translational displacements and rotational motions of the centres of mass of the mesogens in the nanodroplets. The simulations indicate that X-ray scattering and NMR experiments monitoring the orientational order are sensitive to different aspects of the realignment process.     

Medium corrections to the first order optical potential for low-energy pion-4He scattering

We study the Pauli-principle corrections for low energy π-⁴He elastic scattering. In our approach we take into account explicitly the spin and isospin dependence of the Pauli-principle effect. Furthermore we discuss the combined effect of the Pauli-principle correction and the nuclear binding. Contrary to the Pauli principle we treat the binding corrections in an approximate way, using an effective mass for the residual nucleus. In our calculations we use the first-order optical potential of Celenza, Liu and Shakin. The Pauli-principle correction is found to have a considerable effect on the differential cross section. Our results indicate that the Pauli-principle corrections are largely compensated by the nuclear binding.     

Nucleation in a catalytic nanodroplet and growth of nanowires

The kinetics of basic physical processes governing the growth of nanowires by the vapor-liquid-solid mechanism is considered. The role of geometrical, thermodynamic, and kinetic factors early in the growth of the nanowire is studied. Approximate expressions for the nucleation rate and nanowire growth rate are derived, and the influence of transfer processes in the vapor phase is estimated. In terms of the Stefan problem, the effect the release of the latent heat of phase transition has on the nanowire growth is calculated. Numerical estimates are made for carbon nanowires.