Coupling to haloform molecules in intercalated C60?

For field-effect-doped fullerenes it was reported that the superconducting transition temperature Tc is markedly larger for C60.2CHX_3 (X=Cl, Br) crystals, than for pure C60. Initially this was explained by the expansion of the volume per C60-molecule and the corresponding increase in the density of states at the Fermi level in the intercalated crystals. On closer examination it has, however, turned out to be unlikely that this is the mechanism behind the increase in Tc. An alternative explanation of the enhanced transition temperatures assumes that the conduction electrons not only couple to the vibrational modes of the C60-molecule, but also to the modes of the intercalated molecules. We investigate the possibility of such a coupling. We find that, assuming the ideal bulk structure of the intercalated crystal, both a coupling due to hybridization of the molecular levels, and a coupling via dipole moments should be very small. This suggests that the presence of the gate-oxide in the field-effect-devices strongly affects the structure of the fullerene crystal at the interface.Comment: 4 pages, 1 figure, to be published in PRB (rapid communication

A second order minimality condition for the Mumford-Shah functional

A new necessary minimality condition for the Mumford-Shah functional is derived by means of second order variations. It is expressed in terms of a sign condition for a nonlocal quadratic form on $H^1_0(\Gamma)$, $\Gamma$ being a submanifold of the regular part of the discontinuity set of the critical point. Two equivalent formulations are provided: one in terms of the first eigenvalue of a suitable compact operator, the other involving a sort of nonlocal capacity of $\Gamma$. A sufficient condition for minimality is also deduced. Finally, an explicit example is discussed, where a complete characterization of the domains where the second variation is nonnegative can be given.Comment: 30 page

Dwarf galaxies beyond our doorstep: the Centaurus A group

The study of dwarf galaxies in groups is a powerful tool for investigating galaxy evolution, chemical enrichment and environmental effects on these objects. Here we present results obtained for dwarf galaxies in the Centaurus A complex, a dense nearby (~4 Mpc) group that contains two giant galaxies and about 30 dwarf companions of different morphologies and stellar contents. We use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive physical properties and evolutionary histories from the resolved stellar populations of these dwarf galaxies. In particular, for early-type dwarfs we are able to construct metallicity distribution functions, find population gradients and quantify the intermediate-age star formation episodes. For late-type dwarfs, we compute recent (~1 Gyr) star formation histories and study their stellar distribution. We then compare these results with properties of the dwarfs in our Milky Way and in other groups. Our work will ultimately lead to a better understanding of the evolution of dwarf galaxies.Comment: 6 pages, 5 figures; to appear in the proceedings of the conference "A Universe of dwarf galaxies" (Lyon, June 14-18, 2010

Meissner effect, Spin Meissner effect and charge expulsion in superconductors

The Meissner effect and the Spin Meissner effect are the spontaneous generation of charge and spin current respectively near the surface of a metal making a transition to the superconducting state. The Meissner effect is well known but, I argue, not explained by the conventional theory, the Spin Meissner effect has yet to be detected. I propose that both effects take place in all superconductors, the first one in the presence of an applied magnetostatic field, the second one even in the absence of applied external fields. Both effects can be understood under the assumption that electrons expand their orbits and thereby lower their quantum kinetic energy in the transition to superconductivity. Associated with this process, the metal expels negative charge from the interior to the surface and an electric field is generated in the interior. The resulting charge current can be understood as arising from the magnetic Lorentz force on radially outgoing electrons, and the resulting spin current can be understood as arising from a spin Hall effect originating in the Rashba-like coupling of the electron magnetic moment to the internal electric field. The associated electrodynamics is qualitatively different from London electrodynamics, yet can be described by a small modification of the conventional London equations. The stability of the superconducting state and its macroscopic phase coherence hinge on the fact that the orbital angular momentum of the carriers of the spin current is found to be exactly $\hbar/2$, indicating a topological origin. The simplicity and universality of our theory argue for its validity, and the occurrence of superconductivity in many classes of materials can be understood within our theory.Comment: Submitted to SLAFES XX Proceeding

Life after charge noise: recent results with transmon qubits

We review the main theoretical and experimental results for the transmon, a superconducting charge qubit derived from the Cooper pair box. The increased ratio of the Josephson to charging energy results in an exponential suppression of the transmon's sensitivity to 1/f charge noise. This has been observed experimentally and yields homogeneous broadening, negligible pure dephasing, and long coherence times of up to 3 microseconds. Anharmonicity of the energy spectrum is required for qubit operation, and has been proven to be sufficient in transmon devices. Transmons have been implemented in a wide array of experiments, demonstrating consistent and reproducible results in very good agreement with theory.Comment: 6 pages, 4 figures. Review article, accepted for publication in Quantum Inf. Pro

Fluctuating Fuzzy Funnels

It is well known that a D-string ending on a D3, D5 or D7 brane is described in terms of a non-commutative fuzzy funnel geometry. In this article, we give a numerical study of the fluctuations about this leading geometry. This allows us to investigate issues related to the stability and moduli space of these solutions. We comment on the comparison to the linearized fluctuations in supergravity.Comment: 24 pages, 3 figures; v2 references added and correcte

Rotating Strings with Two Unequal Spins in Lunin-Maldacena Background

We study a string motion in the Lunin-Maldacena background, that is, the \beta-deformed AdS_5 \times \tilde{S}^5 background dual to a \beta-deformation of \mathcal{N} = 4 super Yang-Mills theory. For real \beta we construct a rotating and wound string solution which has two unequal spins in \tilde{S}^5. The string energy is expressed in terms of the spins, the winding numbers and the deformation parameter. In the expansion of \lambda/J^2 with the total spin J and the string tension \sqrt{\lambda} we present ``one-loop" and ``two-loop" energy corrections. The ``one-loop" one agrees with the one-loop anomalous dimension of the corresponding gauge-theory scalar operators obtained in hep-th/0503192 from the \beta-deformed Bethe equation as well as the anisotropic Landau-Lifshitz equation.Comment: 13 pages, LaTeX, no figure

Deformed PP-waves from the Lunin-Maldacena Background

In this article we study a pp-wave limit of the Lunin-Maldacena background. We show that the relevant string theory background is a homogeneous pp-wave. We obtain the string spectrum. The dual field theory is a deformation of N=4 super Yang-Mills theory. We have shown that, for a class of operators, at O(g_{YM}^2) and at leading order in N, all contributions to the anomalous dimension come from F-terms. We are able to identify the operator in the deformed super Yang-Mills which is dual to the lowest string mode. By studying the undeformed theory we are able to provide some evidence, directly in the field theory, that a small set of nearly protected operators decouple. We make some comments on operators in the Yang-Mills theory that are dual to excited string modes.Comment: 22 pages, 1 figure; Final version to appear in JHE

Semiclassical Strings, Dipole Deformations of N=1 SYM and Decoupling of KK Modes

In this paper we investigate the recently found $\gamma$-deformed Maldacena-Nunez background by studying the behavior of different semiclassical string configurations. This background is conjectured to be dual to dipole deformations of $\N=1$ SYM. We compare our results to those in the pure Maldacena-Nunez background and show that the energies of our string configurations are higher than in the undeformed background. Thinking in the lines of (hep-th/0505100) we argue that this is an evidence for better decoupling of the Kaluza-Klein modes from the pure SYM theory excitations. Moreover we are able to find a limit of the background in which the string energy is independent of $\gamma$, these strings are interpreted as corresponding to pure gauge theory effects.Comment: 31 pages, references added, new solutions in Section 7 presented, an appendix added, to appear in JHE