Dynamic coexistence of various configurations: clusters vs.nuclei

The presence of energy shells in metallic clusters and atomic nuclei leads to a peculiar relation between the number of particles N and the structure, and this leads to a strong correlation between the energy spectrum and N. An analysis of experimental data leads to the conclusion that, in addition to the static Jahn-Teller effect, the dynamic effect leading to the quantum coexistence of different configurations (quantum oscillations) plays an important role. Such suggested coexistence is an essential feature of clusters as well as nuclei, both finite Fermi systems.Comment: 6 pages, 2 figure

Peculiarities in Low Temperature Properties of Doped Manganites A1-xBxMnO3

The phase diagram and low temperature properties of the doped manganites A1-xBxMnO3 are discussed for the concentrations x < 0.4. The transition from insulating antiferromagnetic to metallic ferromagnetic state at x_cr = 0.16 is treated by means of percolation theory. The unifying description of insulating and metallic states is presented. The undoped manganite is a band insulator consisting of ferromagnetic layers, which are coupled antiferromagnetically along the c direction with a low Neel temperature. The metallic phase can be described by the two-band Fermi liquid picture. The behavior of conductivity, spin wave excitations, etc. is analyzed and the comparison with experimental data is carried out.Comment: To appear in EPJ, 35 pages, 5 figure

Isotope Effect for the Penetration Depth in Superconductors

We show that various factors can lead to an isotopic dependence of the penetration depth $\delta$. Non-adiabaticity (Jahn-Teller crossing) leads to the isotope effect of the charge carrier concentration $n$ and, consequently, of $\delta$ in doped superconductors such as the cuprates. A general equation relating the isotope coefficients of $T_c$ and of $\delta$ is presented for London superconductors. We further show that the presence of magnetic impurities or a proximity contact also lead to an isotopic dependence of $\delta$; the isotope coefficient turns out to be temperature dependent, $\beta(T)$, in these cases. The existence of the isotope effect for the penetration depth is predicted for conventional as well as for high-temperature superconductors. Various experiments are proposed and/or discussed.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.

Shell Structure and Strengthening of Superconducting Pair Correlation in Nanoclusters

The existence of shell structure and the accompanying high degeneracy of electronic levels leads to the possibility of strong superconducting pairing in metallic nanoclusters with N~100-1000 delocalized electrons. The most favorable cases correspond to (a) "magic" clusters with strongly degenerate highest occupied and lowest unoccupied shells and a relatively small energy spacing between them as well as to (b) clusters with slightly incomplete shells and small Jahn-Teller splitting. It is shown that realistic sets of parameters lead to very high values of Tc as well as to a strong alteration of the energy spectrum. The impact of fluctuations is analyzed. Spectroscopic experiments aimed at detecting the presence of pair correlations are proposed. The pairing should also manifest itself via odd-even effects in cluster spectra, similar to the case of nuclei

A second phase transition and superconductivity in the beta-pyrochlore oxide KOs2O6

Another phase transition that is probably of first order is found in the beta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition temperature Tc of 9.6 K. It takes place at Tp=7.5 K in the superconducting state in a zero magnetic field. By applying magnetic fields of up to 140 kOe, the Tc gradually decreased to 5.2 K, while Tp changed little, eventually breaking through the Hc2 line at approximately 65 kOe in the H-T diagram. Both the normal-state resistivity and Hc2 change slightly but significantly across the second phase transition. It is suggested that the transition is associated with the rattling of potassium ions located in an oversized cage of osmium and oxide ions.Comment: 10 pages including 6 figures; to be published in the Proceedings of HFM2006(J. Phys.: Condens. Matter

Electron-lattice interaction and its impact on high Tc superconductivity

In this Colloquium, the main features of the electron-lattice interaction are discussed and high values of the critical temperature up to room temperature could be provided. While the issue of the mechanism of superconductivity in the high Tc cuprates continues to be controversial, one can state that there have been many experimental results demonstrating that the lattice makes a strong impact on the pairing of electrons. The polaronic nature of the carriers is also a manifestation of strong electron-lattice interaction. One can propose an experiment that allows an unambiguous determination of the intermediate boson (phonon, magnon, exciton, etc.) which provides the pairing. The electron-lattice interaction increases for nanosystems, and this is due to an effective increase in the density of states

Density of states of helium droplets

Accurate analytical expressions for the state densities of liquid He-4 droplets are derived, incorporating the ripplon and phonon degrees of freedom. The microcanonical temperature and the ripplon angular momentum level density are also evaluated. The approach is based on inversions and systematic expansions of canonical thermodynamic properties.Comment: 20 pages, 5 figure

Evaporative attachment of slow electrons to alkali nanoclusters

The abundance spectrum of Na^-_{n~7-140} anions formed by low energy electron attachment to free nanoclusters is measured to be strongly and nontrivially restructured with respect to the neutral precursor beam. This restructuring is explained in quantitative detail by a general framework of evaporative attachment: an electron is captured by the long-range polarization potential, its energy is transferred into thermal vibrations, and dissipated by evaporative cooling. The data also affirm a formulated relation between the binding energies of cationic, neutral, and anionic clusters, and an adjustment to the prior values of dimer evaporation energies.Comment: 9 pages, 3 figures, revise