Effects of Electron-Electron and Electron-Phonon Interactions in Weakly Disordered Conductors and Heterostuctures

We investigate quantum corrections to the conductivity due to the interference of electron-electron (electron-phonon) scattering and elastic electron scattering in weakly disordered conductors. The electron-electron interaction results in a negative $T^2 \ln T$-correction in a 3D conductor. In a quasi-two-dimensional conductor, $d < v_F/T$ ($d$ is the thickness, $v_F$ is the Fermi velocity), with 3D electron spectrum this correction is linear in temperature and differs from that for 2D electrons (G. Zala et. al., Phys. Rev.B {\bf 64}, 214204 (2001)) by a numerical factor. In a quasi-one-dimensional conductor, temperature-dependent correction is proportional to $T^2$. The electron interaction via exchange of virtual phonons also gives $T^2$-correction. The contribution of thermal phonons interacting with electrons via the screened deformation potential results in $T^4$-term and via unscreened deformation potential results in $T^2$-term. The interference contributions dominate over pure electron-phonon scattering in a wide temperature range, which extends with increasing disorder.Comment: 6 pages, 2figure

Gulliver project: performers and visitors

This paper discusses two projects in our research environment. The Gulliver project, an ambitious project conceived by some artists connected to our research efforts, and the Aveiro-project, as well ambitious, but with goals that can be achieved beause of technological developments, rather than be dependent on artistic and 'political' (read: financial) sources. Both projects are on virtual and augmented reality. The main goal is to design inhabited environments, where 'inhabited' refers to autonomous agents and agents that represent humans, realtime or off-line, visiting the virtual environment and interacting with other agents. The Gulliver environment has been designed by two artists: Matjaz Stuk and Alena Hudcovicova. The Aveiro project is a research effort of a group of researchers trying to design models of intelligence and interaction underlying the behavior of (groups of) agents inhabiting virtual worlds. In this paper we survey the current state of both projects and we discuss current and future attempts to have music performances by virtual and real performers in these environments

Virtual Conductor for String Quartet Practice

This paper presents a system that emulates an ensemble conductor for string quartets. This application has been developed as a support tool for individual and group practice, so that users of any age range can use it to further hone their skills, both for regular musicians and students alike. The virtual conductor designed can offer similar indications to those given by a real ensemble conductor to potential users regarding beat times, dynamics, etc. The application developed allows the user to rehearse his/her performance without the need of having an actual conductor present, and also gives access to additional tools to further support the learning/practice process, such as a tuner or a melody evaluator. The system developed also allows for both solo practice and group practice. A set of tests were conducted to check the usefulness of the application as a practice support tool. A group of musicians from the Chamber Orchestra of Malaga including an ensemble conductor tested the system, and reported to have found it a very useful tool within an educational environment and that it helps to address the lack of this kind of educational tools in a self-learning environment.This work has been funded by the Ministerio de Economia y Competitividad of the Spanish Government under Project No. TIN2010-21089-C03- 02 and Project No. IPT-2011-0885-430000 and by the Ministerio de Industria, Turismo y Comercio under Project No. TSI-090100-2011-25

Can nothing be a superconductor and a superfluid?

A superconductor is a material that conducts electric current with no resistance. Superconductivity and magnetism are known to be antagonistic phenomena: superconductors expel weak external magnetic field (the Meissner effect) while a sufficiently strong magnetic field, in general, destroys superconductivity. In a seemingly contradictory statement, we show that a very strong magnetic field can turn an empty space into a superconductor. The external magnetic field required for this effect should be about 10^{16} Tesla (eB ~ 1 GeV^2). The physical mechanism of the exotic vacuum superconductivity is as follows: in strong magnetic field the dynamics of virtual quarks and antiquarks is effectively one-dimensional because these electrically charged particles tend to move along the lines of the magnetic field. In one spatial dimension a gluon-mediated attraction between a quark and an antiquark of different flavors inevitably leads to formation of a colorless spin-triplet bound state (a vector analogue of the Cooper pair) with quantum numbers of an electrically charged rho meson. Such quark-antiquark pairs condense to form an anisotropic inhomogeneous superconducting state similar to the Abrikosov vortex lattice in a type-II superconductor. The onset of the superconductivity of the charged rho mesons should also induce an inhomogeneous superfluidity of the neutral rho mesons. The vacuum superconductivity should survive at very high temperatures of typical Quantum Chromodynamics (QCD) scale of 10^{12} K (T ~ 100 MeV). We propose the phase diagram of QCD in the plane "magnetic field - temperature".Comment: 10 pages, 5 figures, contribution to the proceedings of the workshop "The many faces of QCD", 2-5 November 2010, Gent, Belgiu

Excess Kondo resonance in a quantum dot device with normal and superconducting leads: the physics of Andreev-normal co-tunneling

We report on a novel Kondo phenomenon of interacting quantum dots coupled asymmetrically to a normal and a superconducting lead. The effects of intradot Coulomb interaction and Andreev tunneling give rise to Andreev bound resonances. As a result, a new type of co-tunneling process which we term Andreev-normal co-tunneling, is predicted. At low temperatures, coherent superposition of these co-tunneling processes induces a Kondo effect in which Cooper pairs directly participate formation of a spin singlet, leading to four Kondo resonance peaks in the local density of states, and enhancing the tunneling current.Comment: 4 pages, 2 figures, Late

Many-body effects between unbosonized excitons

We here give a brief survey of our new many-body theory for composite excitons, as well as some of the results we have already obtained using it. In view of them, we conclude that, in order to fully trust the results one finds, interacting excitons should not be bosonized: Indeed, all effective bosonic Hamiltonians (even the hermitian ones !) can miss terms as large as the ones they generate; they can even miss the dominant term, as in problems dealing with optical nonlinearities

Creation of nonlocal spin-entangled electrons via Andreev tunneling, Coulomb blockade and resonant transport

We discuss several scenarios for the creation of nonlocal spin-entangled electrons which provide a source of electronic Einstein-Podolsky-Rosen (EPR) pairs. The central idea is to exploit the spin correlations naturally present in superconductors in form of Cooper pairs. We show that nonlocal spin-entanglement in form of an effective Heisenberg spin interaction is induced between electron spins residing on two quantum dots with no direct coupling between them but each of them being tunnel-coupled to the same superconductor. We then discuss a nonequilibrium setup where mobile and nonlocal spin-entanglement can be created by coherent injection of two electrons in an Andreev tunneling process into two spatially separated quantum dots and subsequently into two Fermi-liquid leads. The current for injecting two spin-entangled electrons into different leads shows a resonance whereas tunneling via the same dot into the same lead is suppressed by the Coulomb blockade effect of the quantum dots. The Aharonov-Bohm oscillations in the current are shown to contain h/e and h/2e periods. Finally we discuss a structure consisting of a superconductor weakly coupled to two separate Luttinger liquid leads. We show that strong correlations again suppress the coherent subsequent tunneling of two electrons into the same lead, thus generating again nonlocal spin-entangled electrons.Comment: 15 pages, 6 figures; proceedings Spintronics conference 2001, Georgetown-University, Washington D

Non-analytic behavior of the Casimir force across a Lifshitz transition in a spin-orbit coupled material

We propose the Casimir effect as a general method to observe Lifshitz transitions in electron systems. The concept is demonstrated with a planar spin-orbit coupled semiconductor in a magnetic field. We calculate the Casimir force between two such semiconductors and between the semiconductor and a metal as a function of the Zeeman splitting in the semiconductor. The Zeeman field causes a Fermi pocket in the semiconductor to form or collapse by tuning the system through a topological Lifshitz transition. We find that the Casimir force experiences a kink at the transition point and noticeably different behaviors on either side of the transition. The simplest experimental realization of the proposed effect would involve a metal-coated sphere suspended from a micro-cantilever above a thin layer of InSb (or another semiconductor with large $g$-factor). Numerical estimates are provided and indicate that the effect is well within experimental reach.Comment: 5 pages + 6 page supplement; 5 figure