M. Saarela

4He on weakly attractive substrates: Structure, stability, and wetting behavior

Using a microscopic variational approach we examine the structure and the excitation spectrum of layered4He liquids absorbed to alkali metal and graphite substrates. We find that the alkali metal substrates produce a less pronounced layering structure than the shorter-range graphite/solid helium potential. For the excitations, three features are in common to the substrates: First, for coverages of a monolayer or more, a surface mode is present. Second, a bulk mode which gains strength as the coverage is increased, is identifiable for films with sufficiently high coverage. Finally, a two-dimensional mode that propagates within the first layer is observed for the more attractive substrates. We also present results that we obtain by using the nonlocal density functional theory. We document the reliability and shortcomings of this approach by making a detailed comparison of experimental, Monte Carlo, and variational theory results for the structure, energetics, and excitations. We also give a brief discussion on the wetting properties of helium on alkali metal and graphite/solid helium substrates.

Hydrogen isotope and3He impurities in liquid4He

Recently Reynolds, Hayden and Hardy(J. Low Temp. Phys.84,87 (1991)) analyzed atomic deuterium lifetimes in a cell with walls covered by a saturated4He film. They were able to determine the deuterium chemical potential in4He and also discussed the possibility of estimating its effective mass. These measurements inspired our attempt to understand the properties of hydrogen isotope impurities in liquid4He using the extended Jastrow-Feenberg theory based on the correlated wave function. We present results for hydrogen isotope chemical potentials in the bulk4He as a function of density. The accuracy of the method is tested by calculating the chemical potential of the3He impurity. We find a good agreement with existing measurements. We also calculate the effective masses of the hydrogen impurities. For deuterium we predict a value,meff=4mD.

SINGLE-PARTICLE AND FERMI-LIQUID PROPERTIES OF 3HE-4HE MIXTURES : A MICROSCOPIC ANALYSIS

We calculate microscopically the properties of the dilute He-3 component in a He-3/--He-4 mixture. These depend on both, the dominant interaction between the impurity atom and the background, and the Fermi liquid contribution due to the interaction between the constituents of the He-3 component. We first calculate the dynamic structure function of a He-3 impurity atom moving in He-3. From that we obtain the excitation spectrum and the momentum dependent effective mass. The pole strength of this excitation mode is strongly reduced from the free particle value in agreement with experiments; part of the strength is distributed over high frequency excitations. Above k > 1.7

Charged clusters in strongly correlated electron gas

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Excitations and Stability of 2D Liquid 4He

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Dynamic Structure Function of the Quantum Boson Fluid in the Jastrow Theory

the motion of the impurity is damped due to the decay into a roton and a low energy impurity mode. Next we determine the Fermi--Liquid interaction between He-4 atoms and calculate the pressure-- and concentration dependence of the effective mass, magnetic susceptibility, and the He-3--He-3 scattering phase shifts. The calculations are based on a dynamic theory that uses, as input, effective interactions provided by the Fermi hypernetted--chain theory. The relationship between both theories is discussed. Our theoretical effective masses agree well with recent measurements by Yorozu et al. (Phys. Rev. B 48, 9660 (1993)) as well as those by R. Simons and R. M. Mueller (Czekoslowak Journal of Physics Suppl. 46, 201 (1996)), but our analysis suggests a new extrapolation to the zero-concentration limit. With that effective mass we also find a good agreement with the measured Landau parameter F_0^a.

Dipolar clusters and ferroelectricity in high Tc superconductors

We examine a mixture of electrons and holes in semiconductors. It is well known that in such a system the effective Coulomb interaction is perfectly screened at long distances, which leads to Motts metal–insulator phase transition. We show that the screening is not monotonic and at intermediate ranges there are regions of over-screening, which at low densities become strong enough to destroy the homogeneous mixture by clustering charges. We suggest that this phenomenon is responsible for the broad density distribution of the electron–hole liquid and the condensed plasma phase in silicon suggested by Smith and Wolfe (1995 Phys. Rev. B 51 7521). When the hole mass is set equal to the proton mass we can study properties of the metallic hydrogen and show that at high pressures there is a phase transition into a crystal phase of protons.

What is the most important for a nanoscale structure formations in HTSC?, spin, phonon or third way in Coulomb interaction and correlations?

The excitation modes of two-dimensional liquid 4He are approached from two angles: Firstly, the phonon-roton spectrum is calculated and the related transition currents examined to gain insight into the detailed microscopic structure of the excitations and to look for possible evidence of the proposed spontaneous formation of vortex-antivortex pairs at low densities. The roton excitation is interpreted as a resonance effect in which the wavelength of the long-range density fluctuation matches favorably with the short-range oscillations caused by two-particle correlations. Contrary to the 3D case, no backflow rolls are observed in the two-body current at high momenta. The calculations reproduce reasonably the density dependence of the spectrum, and the saturation of the high-momentum part following from the decay processes of excitations is satisfactorily predicted. The stability of the liquid ground state is then studied by searching for soft modes. It is found that at densities near the expected liquid-solid phase transition the energy of the liquid can be lowered by changing the symmetry of the pair distribution function from spherical to non-spherical. This two-body structure indicates the point-group symmetry of the emerging solid phase to be hexagonal.

Soft excitation modes of quantum Bose fluids in the liquid-solid phase transition

The properties of inhomogeneous fluids form a chalenging field of investications in the many-body physics. The increase in the computing power has made it possible to get the Monte Carlo solutions for some ground state properties like a surface profile of a quantum fluid droplet1. At the same time variational techniques based on the Jastrow ansatz for the wave function have been developed to find the optimized solutions for various inhomogeneous systems2,3,4.

Dynamics of3He impurities in4He films

In this paper, we show that doping of hole charge carriers induces formation of resonance plaquettes (RPs) having electric dipolar moments and fluctuating stripes in cuprates. A single RP is created by many-body interactions between the dopant ion or a charge fluctuation outside and holes inside the CuO plane. In such a process, Coulomb interacting holes in the CuO plane are self-organized into four-particles resonance valence bond plaquettes bound with dopants or polarons located in the spacer layer between CuO planes. Such RPs have ordered and disordered phases. They are ordered into charge density waves (CDW) or stripes only at certain conditions. The lowest energy of the ordered phase corresponds to a local antiferroelectric ordering. The RPs mobility is very low at low temperatures and they are bound into dipole–dipole pairs. Electromagnetic radiation interacts strongly with RPs electric dipoles and when the sample is subjected to it, the mobility changes significantly. This leads to a fractal growth of dipolar RP clusters. The existence of electric dipoles and CDW reveal a series of new phenomena such as ferroelectricity, strong light and microwave absorption and the field induced superconductivity.