Recent talks given

Why holes are not like electrons, and what that may have to do with superconductivity

USC condensed matter seminar, March 23, 2001
Abstract:

Most Hamiltonians that are used to describe many-body physics in solids are electron-hole symmetric. Yet I argue that nature makes an enormous difference between electrons and holes. In particular, elements with one electron outside a closed shell make good metals, those with one hole in a filled shell are insulators. Why are the holes in Cl less mobile than the electrons in Na? What breaks the symmetry between electrons and holes in nature? I propose that the answer is that hole carriers in solids are heavily dressed, while electron carriers are undressed. Inclusion of this physics leads to a new class of Hamiltonians, and to a theory of 'hole superconductivity' whereby heavily dressed hole carriers pair in order to undress and become more like electrons, hence more mobile. The theory describes a phenomenology of superconductivity that has an eerie resemblance to many aspects of the phenomenology seen in high Tc cuprates. Based on these results I propose that electron-hole asymmetry is the key to superconductivity.

Why can only hole conductors be high temperature superconductors ?

University of Wisconsin Herb Materials Physics Seminar , March 29, 2001
Abstract:
There is strong empirical evidence that superconductivity is favored when the charge carriers in the normal state of a metal are holes rather than electrons. Examples are high T_c cuprates, C_60 (see Batlogg et al, Nature 408, 549 (2000)), and the elements: out of 40 nonmagnetic metallic elements, 16 out of 17 non-superconductors have negative Hall coefficient (indicating dominance of electron carriers), and 18 out of 23 superconductors have positive Hall coefficient (indicating dominance of hole carriers). Why is this so? The conventional theories of superconductivity do not differentiate between electron and hole carriers. We will show that there is a simple and intuitive explanation for it. This explanation leads to a theory of superconductivity(1) that exhibits many aspects of the phenomenology seen in high T_c cuprates. The theory makes predictions for various experimentally observable quantities that will be compared with experimental results where available. (1) References in: http://physics.ucsd.edu/~jorge/hole.html

Why holes are not like electrons, and what that may have to do with superconductivity

UCLA condensed matter seminar, December 5, 2001
Abstract:

Ever since Heisenberg first proposed the 'hole' concept in 1931 based on Pauli's exclusion principe, electrons and holes have been regarded as equivalent quasiparticles in condensed matter. Here we argue instead that electrons and holes are fundamentally different objects. The difference, which arises due to electron-electron interactions and can be understood at the level of a single hydrogen atom, is that holes are heavily dressed while electrons are undressed. In electronic energy bands in solids, as the density of electrons increases the Fermi level rises from the bottom of the band, quasiparticles turn from electrons to holes and become increasingly dressed. Conversely, as the density of holes increases quasiparticles turn from holes to electrons and gradually undress. Pairing of holes increases the local hole density and leads to hole undressing, and to superconductivity. Whether this mechanism of superconductivity may apply to any or all materials will be discussed.

Dynamic Hubbard models: a new class of model Hamiltonians for interacting electrons

Departamento de Teoria de la Materia Condensada, Instituto de Ciencia de Materiales de Madrid, CSIC. September 19, 2002.
Abstract:
The Hubbard model is generally believed to be the simplest model to describe strongly correlated electrons in solids. We argue that it is fundamentally flawed, and in particular cannot be used to describe electrons in energy bands that are more than half-filled. We introduce a new class of model Hamiltonians, 'dynamic Hubbard models', to describe interacting electrons in bands with arbitrary filling. New physics emerging from these models is discussed, and numerical results for small clusters are presented. The results have important implications for the understanding of superconductivity.

Quasiparticle undressing: a new route to collective effects in solids

Concepts in Electron Correlation, September 29th - October 3rd 2002, Hvar, Croatia
Abstract
The carriers of electric current in a metal are quasiparticles dressed by electron-electron interactions, which have a larger effective mass $m^*$ and a smaller quasiparticle weight $z$ than non-interacting carriers. If the momentum dependence of the self-energy can be neglected, the effective mass enhancement and quasiparticle weight of quasiparticles at the Fermi energy are simply related by $z=m/m^*$ ($m$=bare mass). We propose that both superconductivity and ferromagnetism in metals are driven by quasiparticle 'undressing', i.e., that the correlations between quasiparticles that give rise to the collective state are associated with an increase in $z$ and a corresponding decrease in $m^*$ of the carriers. Undressing gives rise to lowering of kinetic energy, which provides the condensation energy for the collective state. In contrast, in conventional descriptions of superconductivity and ferromagnetism the transitions to these collective states result in $increase$ in kinetic energy of the carriers and are driven by lowering of potential energy and exchange energy respectively. Experimentally this undressing physics should manifest itself in transfer of spectral weight from high to low frequencies in one- and two-particle spectral functions as the collective state develops, measured by photoemission and optical absorption, and in violation of low frequency sum rules. The high frequency scale where the spectral weight is transferred from is much higher than and unrelated to the energy scale of the condensation energies and of $T_c$. Experimental evidence for this physics has been seen in high $T_c$ cuprate superconductors[1,2,3,4,5] and in certain ferromagnets[6,7,8]. It is proposed that the dominant quasiparticle dressing originates in Coulomb interaction between the carrier and the neighboring electronic site and bond charge, and that undressing in superconductors and ferromagnets results from the decrease in site and bond charge around the carrier associated with hole pairing and with spin polarization respectively. Model Hamiltonians generically called 'dynamic Hubbard models' will be discussed to describe this physics, and numerical results will be presented. The relation of parameters in the Hamiltonians to the physics of real atoms will be discussed.
[1] H. J. A. Molegraaf et al, Science {\bf 295}, 2239 (2002).
[2] A.F. Santander-Syro et al, cond-mat/0111539 (2001), Europhys.Lett.62, 568 (2003)
[3] D.N. Basov et al, Science {\bf 283}, 49 (1999).
[4] H. Ding et al, Phys. Rev. Lett. {\bf 87}, 227001 (2001).
[5] P. D. Johnson et al, Phys. Rev. Lett. {\bf 87}, 177007 (2001).
[6] L. Degiorgi et al, Phys. Rev. Lett. {\bf 79}, 5134 (1997); Phys.Rev. B{\bf 65}, 121102 (2002).
[7] Y. Okimoto et al, Phys.Rev. B{\bf 57}, 9377 (1998).
[8] E.J. Singley et al, Phys.Rev.Lett. {\bf 89}, 097203 (2002) .

Teoria de superconductividad por huecos

Departamento de Materiales, Universidad Complutense, Madrid, October 18, 2002.
Abstract:
Existe fuerte evidencia empirica que superconductividad en materiales es favorecida cuando los portadores de carga son huecos en lugar de electrones. Ejemplos son los cupratos, diboruro de magnesio, y superconductividad en los elementos. Sin embargo, desde que Heisenberg propuso el concepto de 'hueco' en 1931, electrones y huecos han sido considerados como quasiparticulas equivalentes en materia condensada, y las teorias convencionales de superconductividad no diferencian entre electrones y huecos. Aqui argumento que electrones y huecos en realidad son fundamentalmente diferentes, y esta diferencia se puede entender al nivel de un simple atomo de hidrogeno. Examinando los efectos de esta diferencia para la conduccion en un solido lleva a un nuevo mecanismo para superconductividad, y a la conclusion que solamente huecos pueden dar lugar a superconductividad. La teoria resultante se aplica a cupratos asi como tambien a superconductores convencionales. Discutire algunas consecuencias experimentales de esta teoria, asi como tambien los criterios que se deducen de ella para guiar la busqueda de nuevos materiales superconductores.

Electron-hole asymmetry in solids and its implications for superconductivity

University of Bristol, November 13, 2002
Abstract:
Ever since Heisenberg first introduced the 'hole' concept in 1931 based on Pauli's exclusion principe, electrons and holes have been regarded as equivalent quasiparticles in condensed matter. Instead I argue that electrons and holes are fundamentally different objects. The difference between them arises because of electron-electron interactions and can be understood at the level of a single hydrogen atom. Models generally used to describe electronic correlations in solids, such as the Hubbard model, do not describe this fundamental asymmetry. I propose a new class of model Hamiltonians, bdynamic ubbard modelsb, as the simplest class of models that describe this fundamental aspect of electronic correlation effects. These models predict that only hole carriers can give rise to superconductivity in solids. I review empirical and experimental evidence on superconductors that supports this and other predictions of the models.

What is wrong with the Hubbard model, and how to fix it

Max Planck Institute for the Physics of Complex Systems, Dresden, November 21, 2002.
Abstract:
The Hubbard model is widely accepted as the simplest model describing the essential physics of electronic correlations in solids. However, it has a fundamental flaw: while it accounts for the fact that 2 electrons in the same atomic orbital pay an energy cost of U due to their mutual Coulomb repulsion, it fails to describe the fact that those 2 electrons are described by a correlated wave function rather than a single Slater determinant, precisely because of their Coulomb interaction. This is an elementary fact, well known and studied for the case of atoms and molecules; however, its importance for the physics of the solid state has not been recognized. In particular it implies that there exists a fundamental asymmetry between electrons and holes in electronic energy bands. This leads to the prediction that only hole carriers can give rise to superconductivity in solids. In fact, the empirical observation that hole carriers in solids favor superconductivity was made long ago(1) but never explained. I propose a new class of model Hamiltonians, dynamic Hubbard models, that capture this essential physics that is missing in the conventional Hubbard model. I discuss results and predictions for these models and their relationship with experimental observations on superconducting materials. (1) Kikoin and Lazarev, ~1940; R. Feynman, Rev.Mod.Phys.29, 205 (1957) ; I.M. Chapnik, Sov.Phys.Dokl. 6, 988 (1962).

Asimetria electron-hueco: la clave de la superconductividad?

Departamento de Fisica, UAM, Madrid, Noviembre 26, 2002
Abstract:
Desde que Heisenberg propuso el concepto de "hueco" en 1931 para explicar propiedades de espectros atC3micos basado en el principio de exclusiC3n de Pauli, electrones y huecos han sido considerados como quasiparticulas equivalentes en materia condensada, con la unica diferencia de tener carga negativa y positiva respectivamente. Sin embargo, existe una profunda asimetrC-a entre electrones y huecos, que puede entenderse simplemente en el C!tomo de hidrogeno, y que se aplica a todos los orbitales atC3micos. Analizando las consecuencias de esta asimetria para los solidos, se concluye que superconductividad es favorecida cuando los portadores de carga en la banda de conduccion son huecos y no cuando son electrones. En efecto, empiricamente se encuentra que la gran mayoria de los materiales superconductores tienen coeficiente de Hall positivo en el estado normal, indicando que los portadores de carga son huecos. Este hecho empirico se noto ya hace 40 anos pero nunca fue explicado. La teoria de superconductividad que se deduce de estos principios se aplica a todos los materiales y es independiente de la interaccion electron-fonon. Se discutiran las consecuencias experimentales de esta teoria especialmente para la superconductividad de los cupratos.

Dynamic Hubbard Models: An Ideal Playground for DMFT

KITP Program on Realistic Theories of Correlated Electron Materials , December 4, 2002
Link to talk here

Theory of hole superconductivity

New Theories, Discoveries, and Applications of Superconductors and Related Materials (New3SC-4), San Diego, January 26-21, 2003
Abstract: There exists a fundamental asymmetry between electrons and holes in condensed matter. This can be simply understood at the level of a single He atom , as arising from the difference in the nature of the electronic states of neutral and doubly ionized He. It leads to the conclusion that in electronic energy bands holes are always more bdressedb than electrons. This physics is not described by conventional Hubbard models but by a new class of models recently proposed, bdynamic Hubbard modelsb. These models predict that only hole carriers can give rise to superconductivity, through the process of bhole undressingb. When both electron and hole carriers coexist at the Fermi energy, holes will pair and drive also the electrons superconducting, resulting in pairs with different binding energies, hence multiple gaps. Upon application of a magnetic field electron pairs will break up before hole pairs, resulting in sign reversal of the Hall coefficient. Clear experimental evidence that hole pairing drives superconductivity is seen in cuprates, MgB_2, and many other, including elemental, superconductors. Clear experimental evidence that hole pairing leads to undressing of carriers is seen in the cuprates. Clear experimental evidence that dressed holes turn into undressed electrons upon pairing is seen in both conventional and high Tc superconductors. This universal theory of superconductivity provides well-defined guidelines for the search for new high T_c superconducting compounds and will be reviewed in this talk.

Superconductors as giant atoms

Presented at the meeting 'Highlights in Condensed Matter Physics' in honor of the 60th birthday of Prof. Ferdinando Mancini , May 9-11, 2003, Salerno, Italy
Abstract: In atoms, electrons respond to an applied magnetic field by acquiring a velocity -eA/m_ec, with A the magnetic vector potential, e the electron charge and m_e the free electron mass. The same is true in superconductors according to London's theory and to experiment. Hence we argue that a metal in the superconducting state should be regarded as a 'giant atom' (1), and that superfluid electrons behave as negative bare electrons in atoms, rather than as dressed Bloch-Landau quasiparticles whose charge sign (i.e. electron- or hole-like) depends on the band structure. The correct theory of superconductivity has to describe the 'undressing' of the normal state quasiparticles becoming free-electron-like in the superfluid state. This physics is not part of the standard BCS-Eliashberg theory of superconductivity, but it is described in the theory of hole superconductivity (2). The theory predicts that only hole carriers in the normal state can give rise to superconductivity, and that superconductivity is driven by undressing and kinetic energy lowering, in agreement with old ideas of Schafroth (1954) and Bardeen (1950) and recent experiments. Because superconducting bodies, unlike atoms, can be multiply connected one can observe in superconductors properties that do not exist in atoms such as persistent charge currents. As in atoms, we predict that the electric charge distribution in superconductors is not homogeneous. We also predict that macroscopic spin currents should exist in superconductors arising from macroscopic spin-orbit coupling, and discuss experimental ways to detect them.
(1) J.E. Hirsch, Phys.Lett. A 309, 457 (2003). (2) http://physics.ucsd.edu/~jorge/hole.html

Una nueva concepcion de la superconductividad

Instituto de Investigaciones en Materiales, UNAM, Mexico, Mayo 7, 2004
Abstract:
En sus trabajos originales hace 70 anos, London propuso que superconductores eran como atomos gigantes con respecto a su comportamiento en presencia de campos magneticos, y denomino a su teoria "una nueva concepcion de la superconductividad"(1). La teoria de superconductividad por huecos desarrollada durante los ultimos 15 anos(2) sugiere que la analogia entre superconductores y atomos gigantes es todavia mas cercana que lo que envisionaba London, en cuanto a que se aplica tambien a la distribucion de carga electrica. De alli surge una nueva descripcion de la electrodinamica de superconductores, que reemplaza las ecuaciones de London por otras que tratan campos magneticos y electricos en forma simetrica. De hecho, London mismo habia considerado originalmente una posibilidad analoga, que luego descarto debido al resultado de un experimento. Nuestra teoria predice que superconductores expelen carga negativa de su interior hacia la superficie, que poseen un campo electrico en su interior, y que son incapaces de apantallar campos electricos externos como lo haria un metal normal. La teoria conduce a nuevas y mas simples explicaciones de efectos conocidos, y a predicciones de nuevos efectos medibles en experimentos que aun no se han realizado.
(1)F. London, Nature 140, 793 (1937).
(2)Referencias en: http://physics.ucsd.edu/~jorge/hole.html

Superconductores como atomos gigantes

Centro de Ciencias de la Materia Condensada - UNAM, Ensenada, Mexico, Junio 9, 2004
Abstract:
En sus trabajos originales hace 70 aC1os, London propuso que superconductores eran como atomos gigantes con respecto a su comportamiento en presencia de campos magnC)ticos(1). La teorC-a de superconductividad por huecos(2) sugiere que la analogC-a entre superconductores y atomos gigantes es todavC-a mas cercana que lo que envisionaba London, en cuanto a que se aplica tambien a la distribuciC3n de carga elC)ctrica. La teoria se basa en la asimetria fundamental entre carga positiva y negativa en solidos. Como en atomos, predecimos que la distribucion de carga electrica en superconductores no es homogenea, sino que hay exceso de carga negativa cerca de la superficie. Proponemos una nueva descripciC3n de la electrodinC!mica de superconductores, que reemplaza las ecuaciones de London por otras que tratan campos magnC)ticos y elC)ctricos en forma simC)trica. La teorC-a conduce a nuevas y mas simples explicaciones de efectos conocidos, y a predicciones de nuevos efectos medibles en experimentos que aun no se han realizado.(1)F. London, Nature 140, 793 (1937). (2)Referencias en: http://physics.ucsd.edu/~jorge/hole.html

Electrodinamica de superconductores: una propuesta alternativa

Departamento de Teoria de la Materia Condensada, Instituto de Ciencia de Materiales de Madrid, CSIC. July 9, 2004.
Abstract:
Se asume generalmente que la electrodinamica macroscopica de superconductores esta descripta por las ecuaciones de London. Estas ecuaciones no permiten la presencia de campos electricos en superconductores. Nosotros proponemos una descripcion alternativa de la electrodinamica de superconductores, que surge de la teoria microscopica de superconductividad por huecos. La nueva electrodinamica tiene covariancia relativista y permite la presencia de campos electricos en superconductores. En esta descripcion, superconductores se entienden como 'atomos gigantes'. Discutimos algunas consecuencias experimentales de la teoria que permitirian decidir sobre su validez.

Electron-hole asymmetry and superconductivity

SNS2004, Spectroscopies in Novel Superconductors, Sitges, Spain, July 11-16, 2004
Abstract (pdf)

The fundamental role of charge asymmetry in superconductivity

Temple University, February 7, 2005
Abstract:
Superconductivity occurs predominantly in materials where the charge carriers in the normal state are holes rather than electrons. Examples are high Tc cuprates, magnesium diboride, and the elemental superconductors. Other clear manifestations of charge asymmetry in superconductivity are asymmetric tunneling characteristic in cuprates and properties of rotating superconductors. However the importance of charge asymmetry for superconductivity has not been widely recognized: BCS theory of conventional superconductivity as well as new theories proposed to describe high Tc cuprates do not differentiate between electron and hole carriers. I will discuss an alternative theory of superconductivity that has charge asymmetry as its fundamental ingredient. The theory explains many experimental observations, including the remarkable Tao effect, makes testable predictions, and provides new guidelines for the search for new high Tc superconducting compounds.

Explanation of the Tao effect

2005 APS March Meeting, Thursday, March 24, 2005 LACC - 507, 11:15 AM-11:27 AM
Abstract:
Tao and coworkers discovered that in an applied electric field superconducting microparticles aggregate to form balls of macroscopic dimensions$^{(1)}$. The phenomenon appears to be as general as the Meissner effect. Within the conventional theory of superconductivity electrostatic fields do not penetrate into superconductors and the observed effect would not be expected. We propose an explanation of the effect based on an alternative description of the electrodynamics of superconductors recently proposed$^{(2)}$, that results from the unconventional theory of `hole superconductivity'. In our theory a spontaneous electrostatic field exists inside superconductors and if the sample is not spherical also outside. Experiments to test the theory will be discussed. (1) R. Tao, X. Xu and E. Amr, Physica C 398, 78 (2003) and references therein. (2) J.E. Hirsch, Phys.Rev. B 69, 214515 (2004) and references therein.

Superconductors, Tao balls, and macroscopic atoms

San Diego State University, September 16, 2005
Abstract:
When Rongjia Tao recently applied an electric field to millions of superconducting microparticles in suspension he discovered a surprising new effect(1): they fly towards each other, clumping up into a tightly bound round ball of mm-size radius. Neither London nor BCS, the founders of the currently established understanding of superconductivity, expected this, nor do they have any clue as to why this occurs. To me, Tao balls look like giant atoms, and the phenomenon is a manifestation of the fundamental charge asymmetry of matter that is at the root of the phenomenon of superconductivity according to the unconventional theory of "hole superconductivity"(2). I will present the essential elements of this theory, developed over the past 15 years, describe how it explains the "Tao effect", and discuss other experiments that could be done to decide on its ultimate validity or invalidity.
(1) R. Tao, X. Xu and E. Amr, Physica C 398, 78 (2003) and references therein.
(2) J.E. Hirsch, Phys.Rev. Lett. 94, 187001 (2005) and references therein.

Alternative electrodynamic equations for superconductors: theoretical and experimental implications

Concepts in Electron Correlation, September 30th - October 5th 2005, Hvar, Croatia
Abstract:
The theory of hole superconductivity(1) has been proposed as an alternative to the conventional theory of superconductivity to describe both high Tc and conventional superconductors. It has many elements in common with the conventional London- BCS theory as well as profound differences. In particular, the macroscopic electrodynamic equations governing superconductors are predicted to be different in the new theory, which leads to prediction of unexpected effects: penetration of electric fields into superconductors, spontaneous electric fields around superconductors, spherical aggregation of superconducting microparticles in an electric field (Tao effect), spin currents in the ground state of superconductors, changes in the plasmon dispersion relation. These predictions are experimentally testable. Other new and unexpected effects will be discussed.
(1) References in http://physics.ucsd.edu/ jorge/hole.html

Why are Physicists Silent? The Dangers of New US Nuclear Weapons Policies

94th STATISTICAL MECHANICS CONFERENCE, Rutgers University, December 19, 2005

Electric Fields in Superconductors: an Explanation of the Tao Effect

94th STATISTICAL MECHANICS CONFERENCE, Rutgers University, December 19, 2005
Abstract:
When Rongjia Tao recently applied an electric field to millions of superconducting microparticles in suspension he discovered a surprising new effect(1): they fly towards each other, clumping up into a tightly bound round ball of mm-size radius. Within the conventional theory of superconductivity electrostatic fields do not penetrate into superconductors and the observed effect would not be expected. I propose an explanation of the effect based on an alternative description of the electrodynamics of superconductors that results from the unconventional theory of `hole superconductivity' (2).
(1) R. Tao et al, Phys. Rev. Lett. 83, 5575-5578 (1999)
(2) References in J.E. Hirsch, http://physics.ucsd.edu/~jorge/hole.html

Do superconductors violate basic laws of physics?

San Diego State University, September 15, 2006
Abstract: I will show that superconductors violate at least one basic law of physics: either Lenz's law, or angular momentum conservation, or Newton's second law. For those that have faith in theory I will explain how this conundrum can be resolved with least collateral damage. For those that don't I will discuss a simple experiment that can decide between the different possibilities, that could have been done many years ago but hasn't.

On a new effect observed in the transition to the supraconductive state

Fritz Haber Institute, Berlin, March 23, ~~1937~~ 2007
Abstract: Professor Walther Meissner and Herr Dr. Robert Ochsenfeld in Berlin recently discovered a new effect in supraconductors: the magnetic field intensity in the neighborhood of a supraconducting body changes when the body is cooled in an external magnetic field. This surprising effect appears to violate the Maxwellian theory of electromagnetism as well as the conservation of angular momentum required by Newtonian theory. However I will propose an explanation of Meissner's observation that is consistent with Maxwellian and Newtonian theory. This explanation requires that spontaneous electric fields exist inside supraconductors in the absence of externally applied fields. Theoretical and experimental evidence in favor of this strange hypothesis will be presented, and new experiments will be proposed to test its validity.

The h-index: how useful is it as a measure of scientific achievement?

DPG Meeting, Regensburg, Germany, March 26-30, 2007
Abstract: The h-index was proposed in 2005 as a succinct way to quantify an individual's scientific research output. It has generated considerable interest not only in physics but also in other scientific disciplines, and has recently been implemented in the ISI Web of Science. Several extensions of the original concept have also been proposed. I will discuss various properties of the h-index, what I view as its advantages over other indicators and potential disadvantages, and whether it is a good predictor of future achievement.

How hole conductors become electron superconductors

High-Temperature Superconductivity in Cuprates, Original Concept and New Developments, October 7 - 12, 2007 Tbilisi, Georgia
Abstract: Holes dominate the normal state transport in high Tc hole-doped cuprates, in electron-doped cuprates in the regime where they become superconducting(1), in the relatively high Tc MgB2, and in the vast majority of "conventional" superconductors. Electrons carry the electric current in the superconducting state of those materials (as revealed by London moment measurement and other experiments) and in the normal state of materials that never become superconducting such as alkali and noble metals. The discovery of high Tc superconductivity in cuprates by Bednorz and Muller and subsequent developments shone a bright light into the key role of charge asymmetry in superconductivity generally, which had escaped attention before. The theory of hole superconductivity(2) is proposed to apply to all superconducting materials and explains those as well as many other observations such as asymmetric tunneling spectra(3) and optical spectral weight transfer. It proposes that superconductivity originates in pairing and condensation of electron-hole-asymmetric electronic polarons, driven by kinetic energy lowering. A new class of model Hamiltonians grounded in basic ubiquitous atomic physics, "dynamic Hubbard models", describes the microscopic physics. The theory predicts a novel inhomogeneous charge distribution in superconductors, with excess negative charge near the surface. With respect to "At the extreme forefront of research in superconductivity is the empirical search for new materials" the theory dictates that the search for high Tc superconductivity should be restricted to materials where normal state transport occurs in negatively charged substructures (eg planes) with closely spaced anions and almost filled energy bands.
(1)Y. Dagan and R.L. Greene, " Hole superconductivity in the electron-doped superconductor Pr2-xCexCuO4", Phys.Rev. B76, 024506 (2007). (2)References in: http://physics.ucsd.edu/~jorge/hole.html (3) F. Marsiglio and J.E. Hirsch, "Tunneling asymmetry: A test of superconductivity mechanisms", Physica C 159, 157 (1989); P.W. Anderson and N.P. Ong, "Theory of asymmetric tunneling in the cuprate superconductors", J. Phys. Chem. Solids 67, 1 (2006).

The Meissner Effect, the Tao Effect, and Other Unexplained Riddles of Superconductors

UCSD Physics Colloquium, January 24th, 2008
Abstract: The Meissner effect, discovered in 1933, is the process by which a superconductor expels a magnetic field from its interior as it makes the transition from the normal to the superconducting state. It is a hallmark of superconductivity. It is generally believed that the Meissner effect is throughly explained by theory: phenomenologically by London's 1935 theory and microscopically by BCS (1957) theory. Instead, I will try to convince you that the Meissner effect is a fundamental unexplained riddle within conventional London-BCS theory. Another unexplained effect that occurs when strong electric fields are applied to superconductors was discovered by Rongjia Tao in 1999. Finally, many more unsolved riddles resulted from the 1986 discovery of high temperature superconductivity in cuprate oxides. I will discuss the basic principles of the unconventional theory of hole superconductivity, proposed to describe both high temperature superconductivity in cuprates as well as superconductivity of conventional materials. The theory offers an explanation for the Meissner effect and the Tao effect, and predicts a new as yet unseen physical phenomenon in all superconductors, the "Spin Meissner effect".

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