On the Mass Eigenstate Composition of the 8B Neutrinos from the Sun

The present data of gallium experiments provide indirectly the only experimental limit on the fraction of $\nu_2$ mass eigenstate for the $^8$B neutrinos from the Sun. However, if to use the experimental data alone, the fraction of $\nu_2$ and, consequently, $sin^2\theta_{sol}$ still is allowed to be varied within a rather broad range. The further experimental efforts are needed to clear this point.Comment: 13 pages, 1 figure, 1 table. Corrected version, published in JCAP04(2007)00

Thermal suppression of surface barrier in ultrasmall superconducting structures

In the recent experiment by Cren \textit{et al.} [Phys. Rev. Lett. \textbf{102}, 127005 (2009)], no hysteresis for vortex penetration and expulsion from the nano-island of Pb was observed. In the present paper, we argue that this effect can be associated with the thermoactivated surmounting of the surface barrier by a vortex. The typical entrance (exit) time is found analytically from the Fokker-Planck equation, written in the form suitable for the extreme vortex confinement. We show that this time is several orders of magnitude smaller than 1 second under the conditions of the experiment considered. Our results thus demonstrate a possibility for the thermal suppression of the surface barrier in nanosized low-$T_{c}$ superconductors. We also briefly discuss other recent experiments on vortices in related structures.Comment: 12 pages, 2 figure

Magnetism in Closed-shell Quantum Dots: Emergence of Magnetic Bipolarons

Similar to atoms and nuclei, semiconductor quantum dots exhibit formation of shells. Predictions of magnetic behavior of the dots are often based on the shell occupancies. Thus, closed-shell quantum dots are assumed to be inherently nonmagnetic. Here, we propose a possibility of magnetism in such dots doped with magnetic impurities. On the example of the system of two interacting fermions, the simplest embodiment of the closed-shell structure, we demonstrate the emergence of a novel broken-symmetry ground state that is neither spin-singlet nor spin-triplet. We propose experimental tests of our predictions and the magnetic-dot structures to perform them.Comment: 4 pages, 4 figures; http://link.aps.org/doi/10.1103/PhysRevLett.106.177201; minor change

Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation

We present measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time resolved magnetization experiments using a Hall probe magnetometer. We investigate the relaxation behavior of magnetization after the microwave pulse. The analysis of the experimental data is performed in terms of different contributions to the magnetization after-pulse relaxation. We find that the phonon bottleneck with a characteristic relaxation time of 10 to 100 ms strongly affects the magnetization dynamics. In addition, the spatial effect of spin diffusion is evidenced by using samples of different sizes and different ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure

Stress induced dislocation roughening -- phase transition in 1d at finite temperature

We present an example of a generically forbidden phase transition in 1d at finite temperature -- stress induced and thermally assisted roughening of a superclimbing dislocation in a Peierls potential. We also argue that such roughening is behind the strong suppression of the superflow through solid \he4 in a narrow temperature range recently observed by Ray and Hallock (Phys.Rev. Lett. {\bf 105}, 145301 (2010)).Comment: 4 revtex pages, 5 figures. Replaced with the published versio

The role of Helium-3 impurities in the stress induced roughening of superclimbing dislocations in solid Helium-4

We analyze the stress induced and thermally assisted roughening of a forest of superclimbing dislocations in a Peierls potential in the presence of Helium-3 impurities and randomly frozen in static stresses. It is shown that the temperature of the dip $T_d$ in the flow rate observed by Ray and Hallock (Phys.Rev. Lett. {\bf 105}, 145301 (2010)) is determined by the energy of the impurity activation from dislocation core. However, it is suppressed by, essentially, the logarithm of the impurity fraction. The width of the dip is determined by inhomogeneous fluctuations of the stresses and is shown to be much smaller than $T_d$.Comment: Submitted to the LT26-conference proceeding