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J. E. Hirsch

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

At least 55 records · Page 3Linked to original sources
arXiv2019

Defying inertia: how rotating superconductors generate magnetic fields

I discuss the process of magnetic field generation in rotating superconductors in simply connected and multiply connected geometries. In cooling a normal metal into the superconducting state while it is rotating, electrons slow down or speed up depending on the geometry and their location in the sample, apparently defying inertia. I argue that the conventional theory of superconductivity does not explain these processes. Instead, the theory of hole superconductivity does. Its predictions agree with experimental observations of Hendricks, King and Rohrschach for solid and hollow cylinders.

cond-mat.supr-con↗
arXiv2019

Moment of inertia of superconductors

We find that the bulk moment of inertia per unit volume of a metal becoming superconducting increases by the amount $m_e/(πr_c)$, with $m_e$ the bare electron mass and $r_c=e^2/m_e c^2$ the classical electron radius. This is because superfluid electrons acquire an intrinsic moment of inertia $m_e (2λ_L)^2$, with $λ_L$ the London penetration depth. As a consequence, we predict that when a rotating long cylinder becomes superconducting its angular velocity does not change, contrary to the prediction of conventional BCS-London theory that it will rotate faster. We explain the dynamics of magnetic field generation when a rotating normal metal becomes superconducting.

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arXiv2018

$\mathbf{h_α}$: An index to quantify an individual's scientific leadership

The $α$ person is the dominant person in a group. We define the $α$-author of a paper as the author of the paper with the highest $h$-index among all the coauthors, and an $α$-paper of a scientist as a paper authored or coauthored by the scientist where he/she is the $α$-author. For most but not all papers in the literature there is only one $α$-author. We define the $h_α$ index of a scientist as the number of papers in the $h$-core of the scientist (i.e. the set of papers that contribute to the $h$-index of the scientist) where this scientist is the $α$-author. We also define the $h'_α$ index of a scientist as the number of $α$-papers of this scientist that have $\geq$ $h'_α$ citations. $h_α$ and $h'_α$ contain similar information, while $h'_α$ is conceptually more appealing it is harder to obtain from existing databases, hence of less current practical interest. We propose that the $h_α$ and/or $h'_α$ indices, or other variants discussed in the paper, are useful complements to the $h$-index of a scientist to quantify his/her scientific achievement, that rectify an inherent drawback of the $h$-index, its inability to distinguish between authors with different coauthorships patterns. A high $h$ index in conjunction with a high $h_α/h$ ratio is a hallmark of scientific leadership.

cs.DL↗
arXiv2018

Spinning superconductors and ferromagnets

When a magnetic field is applied to a ferromagnetic body it starts to spin (Einstein-de Haas effect). This demonstrates the intimate connection between the electron's magnetic moment $μ_B=e\hbar/2m_ec$, associated with its spin angular momentum $S=\hbar/2$, and ferromagnetism. When a magnetic field is applied to a superconducting body it also starts to spin (gyromagnetic effect), and when a normal metal in a magnetic field becomes superconducting and expels the magnetic field (Meissner effect) the body also starts to spin. Yet according to the conventional theory of superconductivity the electron's spin only role is to label states, and the electron's magnetic moment plays no role in superconductivity. Instead, within the unconventional theory of hole superconductivity, the electron's spin and associated magnetic moment play a fundamental role in superconductivity. Just like in ferromagnets the magnetization of superconductors is predicted to result from an aggregation of magnetic moments with angular momenta $\hbar/2$. This gives rise to a "Spin Meissner effect", the existence of a spin current in the ground state of superconductors. The theory explains how a superconducting body starts spinning when it expels magnetic fields, it provides a dynamical explanation for the Meissner effect, and it explains how supercurrents stop without dissipation, all of which we argue the conventional theory fails to explain. Essential elements of the theory of hole superconductivity are that superconductivity is driven by lowering of kinetic energy, which we have also proposed is true for ferromagnets], that the normal state charge carriers in superconducting materials are holes, and that the spin-orbit interaction plays a key role in superconductivity. The theory is proposed to apply to all superconductors.

cond-mat.supr-con↗
arXiv2018

Comment on arXiv:1810.04634v1 [cond-mat.str-el], "Hole-pocket-driven superconductivity and its universal features in the electron-doped cuprates"

The following letter, titled "Electron-Superconductivity in Oxides?", was submitted to Nature [Scientific Correspondence Section] on March 2, 1989, shortly after the experimental discovery of electron-doped cuprates announced in Nature on January 26, 1989. It predicted the findings recently reported in arXiv:1810.04634 by Yangmu Li, W. Tabis, Y. Tang, G. Yu, J. Jaroszynski, N. Barisic and M. Greven that "clearly point to hole-pocket-driven superconductivity in these nominally electron-doped materials", "reveal hole superconductivity in the electron-doped high-temperature superconductors" "after three decades of enormous scientific inquiry", and point to "a single underlying hole-related mechanism of superconductivity in the cuprates regardless of nominal carrier type". Many more papers on this topic were published by F. Marsiglio and the author in the ensuing 30 years from 1989 to the present. In omitting to refer to any of these theoretical works, arXiv:1810.04634 presents a misleading picture.

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arXiv2018

Enhancement of Superconducting $T_c$ due to the Spin-orbit Interaction

We calculate the superconducting $T_c$ for a system which experiences Rashba spin-orbit interactions. Contrary to the usual case where the electron-electron interaction is assumed to be wave vector-independent, where superconductivity is suppressed by the spin-orbit interaction (except for a small region at low electron or hole densities), we find an enhancement of the superconducting transition temperature when we include a correlated hopping interaction between electrons. This interaction originates in the expansion of atomic orbitals due to electron-electron repulsion and gives rise to superconductivity only at high electron (low hole) densities. When superconductivity results from this interaction it is enhanced by spin-orbit coupling, in spite of a suppression of the density of states. The degree of electron-hole asymmetry about the Fermi surface is also enhanced.

cond-mat.supr-con↗
arXiv2018

Entropy generation and momentum transfer in the superconductor-normal and normal-superconductor phase transformations and the consistency of the conventional theory of superconductivity

Since the discovery of the Meissner effect the superconductor to normal (S-N) phase transition in the presence of a magnetic field is understood to be a first order phase transformation that is reversible under ideal conditions and obeys the laws of thermodynamics. The reverse (N-S) transition is the Meissner effect. This implies in particular that the kinetic energy of the supercurrent is not dissipated as Joule heat in the process where the superconductor becomes normal and the supercurrent stops. In this paper we analyze the entropy generation and the momentum transfer between the supercurrent and the body in the S-N transition and the N-S transition as described by the conventional theory of superconductivity. We find that it is impossible to explain the transition in a way that is consistent with the laws of thermodynamics unless the momentum transfer between the supercurrent and the body occurs with zero entropy generation, for which the conventional theory of superconductivity provides no mechanism. Instead, we point out that the alternative theory of hole superconductivity does not encounter such difficulties.

cond-mat.supr-con↗
arXiv2017

Towards an understanding of hole superconductivity

From the very beginning K. Alex Müller emphasized that the materials he and George Bednorz discovered in 1986 were $hole$ superconductors. Here I would like to share with him and others what I believe to be $the$ key reason for why high $T_c$ 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 Müller on the occasion of his 90th birthday.

cond-mat.supr-con↗
arXiv2017

Why only hole conductors can be superconductors

The conventional theory of superconductivity says that charge carriers in a metal that becomes superconducting can be either electrons or holes. I argue that this is incorrect. In order to satisfy conservation of mechanical momentum and of entropy of the universe in the superconductor to normal transition in the presence of a magnetic field it is necessary that the normal state charge carriers are holes. I will also review the empirical evidence in favor of the hypothesis that all superconductors are hole superconductors, and discuss the implications of this for the search for higher $T_c$ superconductors.

cond-mat.supr-con↗
arXiv2016

Momentum of superconducting electrons and the explanation of the Meissner effect

Momentum and energy conservation are fundamental tenets of physics, that valid physical theories have to satisfy. In the reversible transformation between superconducting and normal phases in the presence of a magnetic field, the mechanical momentum of the supercurrent has to be transferred to the body as a whole and vice versa, the kinetic energy of the supercurrent stays in the electronic degrees of freedom, and no energy is dissipated nor entropy is generated in the process. We argue on general grounds that to explain these processes it is necessary that the electromagnetic field mediates the transfer of momentum between electrons and the body as a whole, and this requires that when the phase boundary between normal and superconducting phases is displaced, a flow and counterflow of charge occurs in direction perpendicular to the phase boundary. This flow and counterflow does not occur according to the conventional BCS-London theory of superconductivity, therefore we argue that within BCS-London theory the Meissner transition is a `forbidden transition'. Furthermore, to explain the phase transformation in a way that is consistent with the experimental observations requires that (i) the wavefunction $and$ charge distribution of superconducting electrons near the phase boundary extend into the normal phase, and (ii) that the charge carriers in the normal state have hole-like character. The conventional theory of superconductivity does not have these physical elements, the theory of hole superconductivity does.

cond-mat.supr-con↗
arXiv2016

What is the speed of the supercurrent in superconductors?

Within the conventional theory of superconductivity superfluid carriers respond to an applied magnetic field and acquire a speed according to their effective (band) mass. On the other hand it can be shown theoretically and is confirmed experimentally that the mechanical momentum of the supercurrent carriers is given by the product of the supercurrent speed and the $bare$ electron mass. By combining these two well-established facts we show that the conventional BCS-London theory of superconductivity applied to Bloch electrons is $internally$ $inconsistent$. Furthermore, we argue that BCS-London theory with Bloch electrons does not describe the phase rigidity and macroscopic quantum behavior exhibited by superconductors. Experimentally the speed of the supercurrent in superconductors has never been measured and has been argued to be non-measurable, however we point out that it is in principle measurable by a Compton scattering experiment. We predict that such experiments will show that superfluid carriers respond to an applied magnetic field according to their bare mass, in other words, that they respond as $free$ $electrons$, undressed from the electron-ion interaction, rather than as Bloch electrons. This is inconsistent with the conventional theory of superconductivity and consistent with the alternative theory of hole superconductivity. Furthermore we point out that in principle Compton scattering experiments can also detect the presence of a spin current in the ground state of superconductors predicted by the theory of hole superconductivity.

cond-mat.supr-con↗
arXiv2016

On the reversibitity of the Meissner effect and the angular momentum puzzle

It is generally believed that the laws of thermodynamics govern superconductivity as an equilibrium state of matter. Here we point out that within the conventional BCS-London description of the normal-superconductor transition in the presence of a magnetic field, the transition cannot be reversible, in contradiction with the thermodynamic description and with experiments. This indicates that the conventional theory of superconductivity is internally inconsistent. We argue that to describe a reversible transition it is necessary to assume that charge transfer occurs across the normal-superconductor phase boundary, as proposed in the theory of hole superconductiviy. This provides also a solution to the angular momentum puzzle pointed out in previous work. Our explanation can only apply if the current carriers in the normal state are holes. An experimental test of these ideas is proposed.

cond-mat.supr-con↗
arXiv2016

The disappearing momentum of the supercurrent in the superconductor to normal phase transformation

A superconductor in a magnetic field has surface currents that prevent the magnetic field from penetrating its interior. These currents carry kinetic energy and mechanical momentum. When the temperature is raised and the system becomes normal the currents disappear. Where do the kinetic energy and mechanical momentum of the currents go, and how? Here we propose that the answer to this question reveals a key necessary condition for materials to be superconductors, that is not part of conventional BCS-London theory: superconducting materials need to have hole carriers.

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arXiv2015

The Bohr superconductor

Superconductors have often been described as `giant atoms'. The simplest description of atoms that heralded their quantum understanding was proposed by Bohr in 1913. The Bohr atom starts from some simple assumptions and deduces that the angular momentum of the electron in Bohr orbits is quantized in integer units of $\hbar$. This remarkable result, which does not appear to be implicit in the assumptions of the model, can be interpreted as a `theoretical proof' of the model's validity to describe physical reality at some level. Similarly we point out here that from some simple assumptions it can be deduced that electrons in superconductors reside in mesoscopic orbits with orbital angular momentum $\hbar/2$. This implies that both in superconductors and in ferromagnets the long-range order results from elementary units of identical angular momentum. Similarly to the case of the Bohr atom we propose that this remarkable result is compelling evidence that this physics, which is not part of conventional BCS theory, describes physical reality at some level and heralds a qualitatively new understanding of superconductors.

cond-mat.supr-con↗
arXiv2015

On the dynamics of the Meissner effect

The question of how a metal becoming superconducting expels a magnetic field is addressed. It is argued that the conventional theory of superconductivity has not answered this question despite its obvious importance. We argue that the growth of the superconducting into the normal region and associated expulsion of magnetic field from the superconducting region can only be understood if it is accompanied by motion of charge from the superconducting into the normal region. From a microscopic point of view it is shown that the perfect diamagnetism of superconductors requires that superconducting electrons reside in orbits of spatial extent $2λ_L$, with $λ_L$ the London penetration depth. Associated with this physics, the spin-orbit interaction of the electron magnetic moment and the positively charged ionic background gives rise to a "Spin Meissner" effect, the generation of a macroscopic spin current near the surface of superconductors. We point out that both the Meissner and the Spin Meissner effect can be understood dynamically under the assumption that the superfluid condensate wavefunction $Ψ(\vec{r})$ does not screen itself, just like the $Ψ(\vec{r})$ for an electron in a hydrogen atom. We argue that the conventional theory of superconductivity cannot explain the Meissner effect because it does not contain the physical elements discussed here.

cond-mat.supr-con↗
arXiv2015

Dynamics of the normal-superconductor phase transition and the puzzle of the Meissner effect

The analisis of Pippard \cite{pip} for the growth of the normal phase into the superconducting phase in the presence of a magnetic field $H>H_c$ is applied in reverse to the case $H<H_c$ ($H_c=$critical magnetic field). We carry out the analysis both for a planar and a cylindrical geometry. As the superconducting phase grows into the normal phase, a supercurrent is generated at the superconductor-normal phase boundary that flows in direction opposite to the Faraday electric field resulting from the moving phase boundary. This supercurrent motion is in direction opposite to what is dictated by the Lorentz force on the current carriers, and in addition requires that mechanical momentum of opposite sign be tranferred to the system as a whole to ensure momentum conservation. In the cylindrical geometry case, a macroscopic torque of unknown origin acts on the body as a whole as the magnetic field is expelled. We argue that the conventional BCS-London theory of superconductivity cannot explain these facts, and that as a consequence the Meissner effect remains unexplained within the conventional theory of superconductivity. We propose that the Meissner effect can only be understood by assuming that there is motion of charge in direction perpendicular to the normal-superconductor phase boundary and point out that the unconventional theory of hole superconductivity describes this physics

cond-mat.supr-con↗
arXiv2015

Superconducting materials classes: introduction and overview

An introduction to and overview of the contents of this Special Issue are given. 32 classes of superconducting materials are discussed, grouped under the three categories "conventional", "possibly unconventional" and "unconventional" according to the mechanism believed to give rise to superconductivity.

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