arXiv subjects

J. E. Hirsch

Publications and source records attributed to J. E. Hirsch.

At least 73 records · Page 4Linked to original sources

arXiv2015

Absence of Josephson coupling between certain superconductors

It is generally believed that superconductivity can occur in materials irrespective of whether the charge carriers in the material are electrons or holes. Here we point out that Josephson tunneling would not occur between superconductors with charge carriers of opposite sign. Consequently, observation of Josephson tunneling between two superconductors implies that their charge carriers have the same sign. We propose that this has profound implications for the understanding of superconductivity and in particular is consistent with the theory of hole superconductivity.

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arXiv2015

Superconductivity in the elements, alloys and simple compounds

We give a brief review of superconductivity at ambient pressure in elements, alloys, and simple three-dimensional compounds. Historically these were the first superconducting materials studied, and based on the experimental knowledge gained from them the BCS theory of superconductivity was developed in 1957. Extended to include the effect of phonon retardation, the theory is believed to describe the subset of superconducting materials known as `conventional superconductors', where superconductivity is caused by the electron-phonon interaction. These include the elements, alloys and simple compounds discussed in this article and several other classes of materials discussed in other articles in this Special Issue.

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arXiv2014

Hole superconductivity in $H_2S$ and other sulfides under high pressure

Superconductivity at temperatures up to 190 K at high pressures has recently been observed in $H_2S$ and interpreted as conventional BCS-electron-phonon-driven superconductivity.\cite{h2s} Instead we propose that it is another example of the mechanism of hole superconductivity at work. Within this mechanism high temperature superconductivity arises when holes conduct through negatively charged anions in close proximity. We propose that electron transfer from $H$ to $S$ leads to conduction by holes in a nearly full band arising from direct overlap of $S^=$ $p$ orbitals in a planar structure. The superconductivity is non-phononic and is driven by pairing of heavily dressed hole carriers to lower their kinetic energy. Possible explanations for the observed lower critical temperature of $D_2S$ are discussed. We predict that high temperature superconductivity will also be found in other sulfides under high pressure such as $Li_2S$, $Na_2S$ and $K_2S$.

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arXiv2014

Proposed experimental test of an alternative electrodynamic theory of superconductors

An alternative form of London's electrodynamic theory of superconductors predicts that the electrostatic screening length is the same as the magnetic penetration depth. We argue that experiments performed to date do not rule out this alternative formulation and propose an experiment to test it. Experimental evidence in its favor would have fundamental implications for the understanding of superconductivity.

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arXiv2014

Dynamic Hubbard model for solids with hydrogen-like atoms

We discuss how to construct a tight binding model Hamiltonan for the simplest possible solid, composed of hydrogen-like atoms. A single orbital per atom is not sufficient because the on-site electron-electron repulsion mixes in higher energy orbitals. The essential physics is captured by a dynamic Hubbard model with one electronic orbital and an auxiliary spin degree of freedom per site. We point out that this physics can lead to a substantial shift in the position and width of electronic energy bands relative to what is predicted by conventional band structure calculations.

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arXiv2014

Effect of orbital relaxation on the band structure of cuprate superconductors and implications for the superconductivity mechanism

Where the doped holes reside in cuprate superconductors has crucial implications for the understanding of the mechanism responsible for their high temperature superconductivity. It has been generally assumed that doped holes reside in hybridized Cu $d_{x^2-y^2}$ - O $pσ$ orbitals in the $CuO_2$ planes, based on results of density functional band structure calculations. Instead, we propose that doped holes in the cuprates reside in O $pπ$ orbitals in the plane, perpendicular to the $Cu-O$ bond, that are raised to the Fermi energy through local orbital relaxation, that is not taken into account in band structure calculations that place the bands associated with these orbitals well below the Fermi energy. We use a dynamic Hubbard model to incorporate the orbital relaxation degree of freedom and find in exact diagonalization of a small $Cu_4O_4$ cluster that holes will go to the O $pπ$ orbitals for relaxation energies comparable to what is expected from atomic properties of oxygen anions. The bandwidth of this band becomes significantly smaller than predicted by band structure calculations due to the orbital relaxation effect. Within the theory of hole superconductivity the heavy hole carriers in this almost full band will pair and drive the system superconducting through lowering of their quantum kinetic energy.

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arXiv2013

The London moment: what a rotating superconductor reveals about superconductivity

The London moment is the magnetic moment acquired by a rotating superconductor. We propose that the London moment reveals the following fundamental properties of the superconducting state: (i) superconductors (unlike normal metals) know the $sign$ of the charge carriers, (ii) the superconducting charge carriers are $free$ electrons, (iii) electrons are expelled from the interior to the surface in the transition to the superconducting state, (iv) superfluid electrons occupy orbits of radius $2λ_L$ ($λ_L=$London penetration depth), and (v) a spin current exists in the ground state of superconductors. These properties are consistent with the Meissner effect, however the Meissner effect does not $directly$ reveal the sign of the charge carriers nor the fact that the carrier's mass is the free electron mass nor the fact that a spin current exists in superconductors. Note also that within the BCS theory of superconductivity none of the key properties of superconductors listed above are predicted. Instead, these properties are predicted by the theory of hole superconductivity.

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arXiv2013

Superconductivity, diamagnetism, and the mean inner potential of solids

The mean inner potential of a solid is known to be proportional to its diamagnetic susceptibility. Superconductors exhibit giant diamagnetism. What does this say about the connection between superconductivity and mean inner potential? Nothing, according to the conventional theory of superconductivity. Instead, we propose that a deep connection exists between the mean inner potential, diamagnetism, and superconductivity: that they are all intimately linked to the fundamental charge asymmetry of matter. We discuss how this physics can be probed experimentally and what the implications of different experimental findings would be for the understanding of superconductivity.

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arXiv2013

Charge expulsion, charge inhomogeneity and phase separation in dynamic Hubbard models

Dynamic Hubbard models are extensions of the conventional Hubbard model that take into account the fact that atomic orbitals expand upon double occupancy. It is shown here that systems described by dynamic Hubbard models have a tendency to expel negative charge from their interior to the surface, and to develop charge inhomogeneity and even phase separation in the bulk. These effects are associated with lowering of electronic kinetic energy. We propose that these models may explain the charge inhomogeneity and negatively charged grain boundaries observed in cuprate oxides and other materials.

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arXiv2013

Dynamic Hubbard model: kinetic energy driven charge expulsion, charge inhomogeneity, hole superconductivity, and Meissner effect

Dynamic Hubbard models describe the fact that the wavefunction of an electron in an atomic orbital expands when a second electron occupies the orbital. These models give rise to superconductivity driven by lowering of kinetic energy when the electronic energy band is almost full, with higher transition temperatures resulting when the ions are negatively charged. We show that the charge distribution in dynamic Hubbard models can be highly inhomogeneous in the presence of disorder, and that a finite system will expel {\it negative charge} from the interior to the surface, and that these tendencies are largest in the parameter regime where the models give rise to highest superconducting transition temperatures. High $T_c$ cuprate materials exhibit charge inhomogeneity and they exhibit tunneling asymmetry, a larger tendency to emit electrons rather than holes in NIS tunnel junctions. We propose that these properties, as well as their high $T_c$'s, are evidence that they are governed by the physics described by dynamic Hubbard models. Below the superconducting transition temperature the models considered here describe a negatively charged superfluid and positively charged quasiparticles, unlike the situation in conventional BCS superconductors where quasiparticles are charge neutral on average. We examine the temperature dependence of the superfluid and quasiparticle charges and conclude that spontaneous electric fields should be observable in the interior and in the vicinity of superconducting materials described by these models at sufficiently low temperatures. We furthermore suggest that the dynamics of the negatively charged superfluid and positively charged quasiparticles in dynamic Hubbard models can provide an explanation for the Meissner effect observed in superconducting materials.

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arXiv2013

Prediction of unexpected behavior of the mean inner potential of superconductors

Off-axis electron holography can measure the mean inner electric potential of materials. The theory of hole superconductivity predicts that when a material is cooled into the superconducting state it expels electrons from its interior to the surface, giving rise to a mean inner potential that increases with sample thickness. Instead, in a normal metal and in a conventional BCS superconductor the mean inner potential is expected to be independent of sample thickness and temperature. Thus, this experiment can provide a definitive test of the validity of the theory of hole superconductivity.

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arXiv2013

Apparent increase in the thickness of superconducting particles at low temperatures measured by electron holography

We predict that superconducting particles will show an apparent increase in thickness at low temperatures when measured by electron holography. This will result not from a real thickness increase, rather from an increase in the mean inner potential sensed by the electron wave traveling through the particle, originating in expansion of the electronic wavefunction and resulting negative charge expulsion from the interior to the surface of the superconductor, giving rise to an increase in the phase shift of the electron wavefront going through the sample relative to the wavefront going through vacuum. The temperature dependence of the observed phase shifts will yield valuable new information on the physics of the superconducting state of metals.

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arXiv2012

Kinetic energy driven superfluidity and superconductivity and the origin of the Meissner effect

Superfluidity and superconductivity have many elements in common. However, I argue that their most important commonality has been overlooked: that both are kinetic energy driven. Clear evidence that superfluidity in $^4He$ is kinetic energy driven is the shape of the $λ$ transition and the negative thermal expansion coefficient below $T_λ$. Clear evidence that superconductivity is kinetic energy driven is the Meissner effect: I argue that otherwise the Meissner effect would not take place. Associated with this physics I predict that superconductors expel negative charge from the interior to the surface and that a spin current exists in the ground state of superconductors (spin Meissner effect). I propose that this common physics of superconductors and superfluids originates in rotational zero point motion. This view of superconductivity and superfluidity implies that rotational zero-point motion is a fundamental property of the quantum world that is missed in the current understanding.

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arXiv2012

Spherical agglomeration of superconducting and normal microparticles with and without applied electric field

It was reported by R. Tao and coworkers that in the presence of a strong electric field superconducting microparticles assemble into balls of macroscopic dimensions. Such a finding has potentially important implications for the understanding of the fundamental physics of superconductors. However, we report here the results of experimental studies showing that (i) ball formation also occurs in the absence of an applied electric field, (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature, and (iii) it can also occur for non-superconducting materials. Possible origins of the phenomenon are discussed.

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arXiv2012

Experimental consequences of predicted charge rigidity of superconductors

The theory of hole superconductivity predicts that in superconductors the charged superfluid is about a million times more rigid than the normal electron fluid. We point out that this physics should give rise to large changes in the bulk and surface plasmon dispersion relations of metals entering the superconducting state, that have not yet been experimentally detected and would be in stark contradiction with the expected behavior within conventional BCS-London theory. We also propose that this explains the puzzling experimental observations of Avramenko et al\cite{sound} on electron sound propagation in superconductors and the puzzling experiments of W. de Heer et al\cite{clusters} detecting large electric dipole moments in small metal clusters, as well as the Tao effect\cite{tao} on aggregation of superconducting microparticles in an electric field. Associated with the enhanced charge rigidity is a large increase in the electric screening length of superconductors at low temperatures that has not yet been experimentally detected. The physical origin of the enhanced charge rigidity and its relation to other aspects of the theory of hole superconductivity is discussed.

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arXiv2012

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

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 $T_c$ cuprates and pnictides, is highly falsifiable, and explains a wide variety of experimental observations.

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arXiv2012

Materials and mechanisms of hole superconductivity

The theory of hole superconductivity proposes that there is a single mechanism of superconductivity that applies to all superconducting materials. This paper discusses several material families where superconductivity occurs and how they can be understood within this theory. Materials discussed include the elements, transition metal alloys, high $T_c$ cuprates both hole-doped and electron-doped, $MgB_2$, iron pnictides, doped semiconductors, and elements under high pressure.

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arXiv2011

The origin of the Meissner effect in new and old superconductors

It is generally believed that superconducting materials are divided into two classes: `conventional' and `unconventional'. Conventional superconductors (the elements and thousands of compounds including $MgB_2$) are described by conventional London-BCS-Eliashberg electron-phonon theory. There is no general agreement as to what mechanism or mechanisms describe `unconventional' superconductors such as the heavy fermions, organics, cuprate and pnictide families. However all superconductors, whether `conventional' or `unconventional', exhibit the Meissner effect. I argue that there is a single mechanism of superconductivity for all materials, that explains the Meissner effect and differs from the conventional mechanism in several fundamental aspects: it says that superconductivity is driven by lowering of kinetic rather than potential energy of the charge carriers, it requires conduction by holes rather than electrons in the normal state, and it predicts a non-homogeneous rigid charge distribution and an electric field in the interior of superconductors. Furthermore I argue that neither the conventional mechanism nor any of the other proposed unconventional mechanisms can explain the Meissner effect. Superconductivity in materials is discussed in the light of these concepts, some experimental predictions, connections to Dirac's theory, and connections to the superfluidity of $^4He$.

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