84,192 research outputs found

    Superconductivity from Hole Undressing

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    Photoemission and optical experiments indicate that the transition to superconductivity in cuprates is an 'undressing' transition . In photoemission this is seen as a coherent quasiparticle peak emerging from an incoherent background, in optics as violation of the Ferrell-Glover-Tinkham sum rule indicating effective mass reduction of superconducting carriers. We propose that this is a manifestation of the fundamental electron-hole asymmetry of condensed matter described by the theory of hole superconductivity. The theory asserts that electrons in nearly empty bands and holes in nearly full bands are fundamentally different : the former yield high conductivity and normal metals, the latter yield low normal state conductivity and high temperature superconductivity. This is because the normal state transport of electrons is coherent and that of holes is incoherent. We explain how this asymmetry arises from the Coulomb interaction between electrons in atoms and the nature of atomic orbitals, and propose a simple Hamiltonian to describe it. A universaluniversal mechanism for superconductivity follows from this physics, whereby dressed hole carriers undress by pairing, turning (partially) into electrons and becoming more mobile in the superconducting state.Comment: Presented at the Third International Conference on New Theories, Discoveries, and Applications of Superconductors and Related Materials (New3SC-3), Hawaii, January 2001, to be published in Physica

    Consequences of charge imbalance in superconductors within the theory of hole superconductivity

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    The theory of hole superconductivity proposes that the fundamental asymmetry between electrons and holes in solids is responsible for superconductivity. Here we point out a remarkable consequence of this theory: a tendency for negative charge to be expelled from the bulk of the superconductor towards the surface. Experimentally observable consequences of this physics are discussed

    Hole Superconductivity in MgB2Mg B_2: a high TcT_c cuprate without Cu

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    The theory of hole superconductivity explains high temperature superconductivity in cuprates as driven by pairing of hole carriers in oxygen pπp\pi orbitals in the highly negatively charged CuOCu-O planes. The pairing mechanism is hole undressing and is Coulomb-interaction driven. We propose that the planes of BB atoms in MgB2Mg B_2 are akin to the CuOCu-O planes without CuCu, and that the recently observed high temperature superconductivity in MgB2Mg B_2 arises similarly from undressing of hole carriers in the planar boron px,yp_{x,y} orbitals in the negatively charged BB^- planes. Doping MgB2Mg B_2 with electrons and with holes should mirror the behavior of underdoped and overdoped high TcT_c cuprates respectively. We discuss possible ways to achieve higher transition temperatures in boron compounds based on this theory.Comment: A section on isotope effect has been added, as well as other minor change

    Superconductors as giant atoms predicted by the theory of hole superconductivity

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    The theory of hole superconductivity proposes that superconductivity originates in the fundamental electron-hole asymmetry of condensed matter and that it is an 'undressing' transition. Here we propose that a natural consequence of this theory is that superconductors behave as giant atoms. The model predicts that the charge distribution in superconductors is inhomogeneous, with higher concentration of negative charge near the surface. Some of this negative charge will spill out, giving rise to a negative electron layer right outside the surface of the superconductor, which should be experimentally detectable. Also superconductors should have a tendency to easily lose negative charge and become positively charged. Macroscopic spin currents are predicted to exist in superconducting bodies, giving rise to electric fields near the surface of multiply connected superconductors that should be experimentally detectable.Comment: To be published in Phys.Lett.

    Quantum Monte Carlo and exact diagonalization study of a dynamic Hubbard model

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    A one-dimensional model of electrons locally coupled to spin-1/2 degrees of freedom is studied by numerical techniques. The model is one in the class of dynamicdynamic HubbardHubbard modelsmodels that describe the relaxation of an atomic orbital upon double electron occupancy due to electron-electron interactions. We study the parameter regime where pairing occurs in this model by exact diagonalization of small clusters. World line quantum Monte Carlo simulations support the results of exact diagonalization for larger systems and show that kinetic energy is lowered when pairing occurs. The qualitative physics of this model and others in its class, obtained through approximate analytic calculations, is that superconductivity occurs through hole undressing even in parameter regimes where the effective on-site interaction is strongly repulsive. Our numerical results confirm the expected qualitative behavior, and show that pairing will occur in a substantially larger parameter regime than predicted by the approximate low energy effective Hamiltonian.Comment: Some changes made in response to referees comments. To be published in Phys.Rev.

    A superformula for neutrinoless double beta decay II: The short range part

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    A general Lorentz-invariant parameterization for the short-range part of the 0vBB decay rate is derived. Combined with the long range part already published this general parameterization in terms of effective B-L violating couplings allows one to extract the 0vBB limits on arbitrary lepton number violating theories.Comment: 8 pages, LaTeX, 2 figure

    Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect

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    From the outset of superconductivity research it was assumed that no electrostatic fields could exist inside superconductors, and this assumption was incorporated into conventional London electrodynamics. Yet the London brothers themselves initially (in 1935) had proposed an electrodynamic theory of superconductors that allowed for static electric fields in their interior, which they unfortunately discarded a year later. I argue that the Meissner effect in superconductors necessitates the existence of an electrostatic field in their interior, originating in the expulsion of negative charge from the interior to the surface when a metal becomes superconducting. The theory of hole superconductivity predicts this physics, and associated with it a macroscopic spin current in the ground state of superconductors ("Spin Meissner effect"), qualitatively different from what is predicted by conventional BCS-London theory. A new London-like electrodynamic description of superconductors is proposed to describe this physics. Within this theory superconductivity is driven by lowering of quantum kinetic energy, the fact that the Coulomb repulsion strongly depends on the character of the charge carriers, namely whether electron- or hole-like, and the spin-orbit interaction. The electron-phonon interaction does not play a significant role, yet the existence of an isotope effect in many superconductors is easily understood. In the strong coupling regime the theory appears to favor local charge inhomogeneity. The theory is proposed to apply to all superconducting materials, from the elements to the high TcT_c cuprates and pnictides, is highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter 2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J. Supercond. Nov. Mag

    Why non-superconducting metallic elements become superconducting under high pressure

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    We predict that simple metals and early transition metals that become superconducting under high pressures will show a change in sign of their Hall coefficient from negative to positive under pressure. If verified, this will strongly suggest that hole carriers play a fundamental role in `conventional' superconductivity, as predicted by the theory of hole superconductivity.Comment: Submitted to M2S-IX Tokyo 200

    Meissner effect, Spin Meissner effect and charge expulsion in superconductors

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    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 /2\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

    Towards an understanding of hole superconductivity

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    From the very beginning K. Alex M\"uller emphasized that the materials he and George Bednorz discovered in 1986 were holehole superconductors. Here I would like to share with him and others what I believe to be thethe key reason for why high TcT_c cuprates as well as all other superconductors are hole superconductors, which I only came to understand a few months ago. This paper is dedicated to Alex M\"uller on the occasion of his 90th birthday.Comment: Dedicated to Alex M\"uller on the Occasion of his 90th Birthday. arXiv admin note: text overlap with arXiv:1703.0977