Meissner Effect, Spin Meissner Effect and Charge Expulsion in Superconductors

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 Lorenz 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, 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.     

Hall Coefficient of Equilibrium Supercurrents Flowing inside Superconductors

We study augmented quasiclassical equations of superconductivity with the Lorentz force, which is missing from the standard Ginzburg–Landau and Eilenberger equations. It is shown that the magnetic Lorentz force on equilibrium supercurrents induces a finite charge distribution and the resulting electric field to balance the Lorentz force. An analytic expression is obtained for the corresponding Hall coefficient of clean type-II superconductors with simultaneously incorporating the Fermi-surface and gap anisotropies. It has the same sign and magnitude at zero temperature as the normal state for an arbitrary pairing, having no temperature dependence specifically for s-wave pairing. The gap anisotropy may bring about a considerable temperature dependence in the Hall coefficient and can lead to its sign change as a function of temperature, as exemplified for a model d-wave pairing with a two-dimensional Fermi surface. The sign change may be observed in some high-T _c superconductors.     

Paramagnetic Meissner effect of high-temperature granular superconductors: Interpretation by anisotropic and isotropic models

The field-cooled magnetization of high-T _c superconducting ceramics measured in low magnetic fields exhibits the paramagnetic Meissner effect (PME), i.e., the diamagnetic signal initially increases with decrease in temperature but reaches a maximum at temperatureT _d and later decreases with decrease in temperature. Even in some samples the signal is ultimately able to transform inversely into a paramagnetic regime once the sample is cooled below a temperatureT _p as long as the applied field is sufficiently small. This PME has been observed in various high-T _c cuprates and is explained by disparate aspects. An anisotropic model, in which the granular superconductors are assumed to be ideally anisotropic, was first alternatively proposed in the present work so as to theoretically account for this effect. On the other hand, an isotropic model, suitable for granular superconductors with randomly oriented grains, was proposed to deal with the samples prepared by a conventional solid-state reaction method. The anomalous magnetization behavior in the present model was demonstrated to be the superposition of the diamagnetic signal, which occurs as a result of the intragranular shielding currents, over the paramagnetic one due to the induction of the intergranular component induced by these currents where the intergranular one behaved as the efective pinning centers. The PME was demonstrated by this model to exist parasitically in granular superconductors. This intergranular effect is therefore worthy of remark when evaluating the volume fraction of superconductivity for the samples from the Meissner signal, in particular, at a low magnetic field.     

Paramagnetic Meissner Effect

The explanation for the paramagnetic Meissner effect (PME) in high-temperature superconductors (HTSC) and niobium disks, distinct from previous discussions, is proposed. It is based on the impurity mechanism of high-temperature superconductivity and the general localization approach to the superconductivity problem. Turns of the local magnetic moments of the two-level systems are considered to be the cause of this effect in HTSC. The estimations of the magnetic moment magnitude and the magnetic moment concentration are given. In niobium disks, this effect is also explained by turns of the local magnetic moments, but of another nature. The role of the layered structure is demonstrated. It is shown that the proposed models can explain all available experimental facts. The connection between this effect and the electric field effect in HTSC is discussed.     

High temperature superconductivity in orthorhombic LaBa2Cu3O7−δ

High temperature superconducting LaBa₂Cu₃O_7−δ has been prepared by ceramic and nitrate methods to understand the influence of preparation conditions on superconductivity. The characterization was carried out by X-ray diffraction and magnetic susceptibility measurements.T _c onset was observed at 88 K. Meissner effect has been observed above the liquid nitrogen temperature.     

Encounters with Vitaly, Nina, and the Ψ Function

The Ψ function introduced by Ginzburg and Landau has been a turning point in the development of our understanding of superconductivity. Two of its consequences, which have been of particular importance in my work, are mentioned here. One is the Meissner screening of a magnetic field by a normal metal backed by a superconductor, one of the most striking manifestations of the proximity effect. The other one is the occurrence of large thermodynamical fluctuations near the critical temperature, in a range that has become experimentally accessible in the High Temperature superconductors due to their short coherence length, giving access to the pinning potential of these materials.     

Electrons on a dielectric liquid-vapor interface. Stability in presence of normal magnetic field

The capillary wavelength instability of a charged liquid surface due to coupling between surface deformation and charge compression may be modified by altering the transport properties of the charges along the surface. For gas-like electrons, a normal magnetic field lessens the charge density softening of the capillary mode and renders the damped transverse mode unstable at exactly the same charge conditions at which the capillary mode goes unstable in zero field. For solid-like electrons exhibiting shear elasticity, on the other hand, the magnetic field has little effect on the softening of the surface deformation modes. Systems to which these considerations are applicable are electrons on and under a liquid helium surface, and ions at the demixtion interface of ³He-⁴He mixtures.     

On-Demand Local Modification of High-Tc Superconductivity in Few Unit-Cell Thick Bi2Sr2CaCu2O8+δ

High-temperature superconductors (HTSs) are important for potential applications and for understanding the origin of strong correlations. Bi₂Sr₂CaCu₂O_8+δ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, is of immense interest for superconducting electronics. A route to modify superconductivity locally by depositing metal on the surface is reported here by transport studies on few unit-cell thick BSCCO. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the underlying region. Cr locally depletes oxygen in CuO₂ planes and disrupts the superconductivity in the layers below. This technique of modifying superconductivity is suitable for making sub-micrometer superconducting wires and more complex superconducting devices.     

Electronic Damping of Dislocations and Kinetic Phenomena in Superconductors Under Plastic Deformation

Interaction of a moving screw dislocation with conduction electrons is considered in the conventional pure superconductors. At temperaturesT≪T _c , both “slow” dislocations are considered, which interact only with thermally excited quasiparticles, and “fast” dislocations which also break the Cooper pairs. Near the critical temperature, two limiting cases are considered depending on the relation between Meissner penetration depth and anomalous skin-layer depth for the dislocation-induced electromagnetic field. Power dissipation dependence on temperature and dislocation velocity is obtained. Due to low field intensity in the short-wave part of spectrum, the superconducting state is shown not to be destructed by a moving dislocation in a broad range of temperatures. Non-equilibrium states of electronic system created by the dislocation field are analyzed in the paper. With this purpose, Eliashberg’s kinetic equation for a multi-mode excitation source is used. Dislocation field is shown to reduce the order parameter whenT≪T _c , or stimulate superconductivity when(T−T _c )/T _c≪ 1. Damping reduction due to stimulation effect is discussed. Power dissipation dependence on the dislocations concentration in non-equilibrium state is obtained.     

Extended Time-Dependent Ginzburg–Landau Theory

We formulate the gauge invariant Lorentz covariant Ginzburg–Landau theory which describes nonstationary regimes: relaxation of a superconducting system accompanied by eigen oscillations of internal degrees of freedom (Higgs mode and Goldstone mode) and also forced oscillations under the action of an external gauge field. The theory describes Lorentz covariant electrodynamics of superconductors where Anderson–Higgs mechanism occurs, at the same time the dynamics of conduction electrons remains non-relativistic. It is demonstrated that Goldstone oscillations cannot be accompanied by oscillations of charge density and they generate the transverse field only. In addition, we consider Goldstone modes and features of Anderson–Higgs mechanism in two-band superconductors. We study dissipative processes, which are caused by movement of the normal component of electron liquid and violate the Lorentz covariance, on the examples of the damped oscillations of the order parameter and the skin-effect for electromagnetic waves. An experimental consequence of the extended time-dependent Ginzburg–Landau theory regarding the penetration of the electromagnetic field into a superconductor is proposed.

     

Surface Charge of Supramolecular Nanosystems for In Vivo Biodistribution: A MicroSPECT/CT Imaging Study

Bioimaging has revolutionized medicine by providing accurate information for disease diagnosis and treatment. Nanotechnology‐based bioimaging is expected to further improve imaging sensitivity and specificity. In this context, supramolecular nanosystems based on self‐assembly of amphiphilic dendrimers for single photon emission computed tomography (SPECT) bioimaging are developed. These dendrimers bear multiple In³⁺ radionuclides at their terminals as SPECT reporters. By replacing the macrocyclic 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid cage with the smaller 1,4,7‐triazacyclononane‐1,4,7‐triacetic acid scaffold as the In³⁺ chelator, the corresponding dendrimer exhibits neutral In³⁺‐complex terminals in place of negatively charged In³⁺‐complex terminals. This negative‐to‐neutral surface charge alteration completely reverses the zeta‐potential of the nanosystems from negative to positive. As a consequence, the resulting SPECT nanoprobe generates a highly sought‐after biodistribution profile accompanied by a drastically reduced uptake in liver, leading to significantly improved tumor imaging. This finding contrasts with current literature reporting that positively charged nanoparticles have preferential accumulation in the liver. As such, this study provides new perspectives for improving the biodistribution of positively charged nanosystems for biomedical applications.     

Experimental Observation of the Relationship Between the Meissner Effect and the ac Magnetic Susceptibility

Meissner diamagnetic effect is a basic criterion for identifying whether a matter is superconducting or not. However, currently one of main methods to measure magnetic property of a superconductor is ac mutual inductance technique by which ac magnetic susceptibility is measured. To investigate the relation between Meissner diamagnetic effect and the diamagnetism measured by ac susceptibility, we designed and performed an ac susceptibility experiment, that is, ac susceptibility of a superconductor as a function of temperature is measured under a constant dc magnetic field. Our experimental result shows that bulk and surface diamagnetism cannot occur in a superconductor simultaneously. This conclusion further proves that the signal of ac magnetic susceptibility gives no evidence for Meissner effect. Microscopic model and analysis on bulk and surface superconducting diamagnetism was given in the present paper.     

Charge exchange in low-energy ion-surface scattering

Charge exchange between low-energy projectiles and solid materials is explored for systems in which there is overlap between the ionization level and the surface bands. Time-of-flight spectroscopy is used to measure the charge state and kinetic energy distributions of scattered and recoiled atomic particles. Alkali ions scattered from alkali and halogen adatoms have larger neutralization probabilities than for scattering from the substrate, indicating that they both have positively charged regions at the top, which is surprising for the negatively charged halogen adsorbates. Because of strong overlap, Na⁺ ions scattered from Au nanocrystals neutralize by interaction with the quantum-confined states. Charge exchange in symmetric systems was investigated by measurements of the charge state of Al emitted from clean Al(1 0 0) by direct recoil induced by 5 keV Xe⁺ ions. The neutralization of recoiled Al is basically consistent with a resonant process resulting from the overlap of the Al atomic and solid states.     

Characteristic features of the copper-oxygen interaction, electron pairs, and the asymmetric π bond in high-T c superconductors

The experimental data on the structural features and bond character in layers of high-T _c superconductors and related materials are analyzed. Correlations are found between T _c and the orthorhombic distortion parameter and between T _c and the incommensurability of the layers in the structure. It is pointed out that the electric field around the CuO₂ plane is asymmetric. On the basis of this analysis it is proposed that the unusual properties of high-T _c superconductors are due to the mixed covalent-ionic bond character in the CuO₂ layers, a consequence of which is the presence of electron pairs in the layer. It is shown that in the presence of asymmetry of the electric field around the CuO₂ planes these pre-existing electron pairs can give rise to superconductivity via the asymmetric delocalized π bond. Pis’ma Zh. Tekh. Fiz. 23, 57–63 (September 12, 1997)     

Charge redistribution at the phase transitions in solids and charged vortices in type-II superconductors

Different effects connected with the change of the electronic chemical potential at phase transitions in solids are reviewed. It is shown that this chemical potential may have a kink at a 2nd-order phase transition. When a system contains two electronic subsystems, one undergoing transition, another remaining normal, there should be charge redistribution between these subsystems belowT _c . Such charge redistribution (e.g., between planes and chains in YBCO) may explain many anomalies observed in high-T _c superconductors and in other systems. One extra consequence of this effect may be charging of the vortex cores in type II superconductors; this may explain the negative anomaly in the Hall effect, often observed experimentally in a mixed state.     

π-d Interaction in the Field Induced Superconductor λ-(BETS)2FeCl4: Studied by 77Se NMR

⁷⁷Se NMR measurements have been performed in the normal state of the organic, magnetic field induced superconductor λ(BETS)₂FeCl₄, to examine the interaction between Fe local spins and π conduction electrons. Large, negative Knight shift values were observed at fields lower than the one corresponding to onset of field induced superconductivity. This evidences that the conduction electrons feel a negative internal field coming from the Fe spins through the exchange interaction { } · { }. This suggests that the superconductivity induced by high magnetic fields has its origin in the Jaccarino-Peter compensation mechanism. The estimated value of the exchange interaction J, is 30% smaller than that predicted by the resistivity measurements.     

Experiment on Nuclear Ordering and Superconductivity in Lithium

We have investigated lithium metal at ultra-low temperatures in order to study the self-organized ground states of the nuclear spin system and of the conduction electrons. Evidence for nuclear magnetic ordering was found: we observed irreversible behavior with respect to slow changes of the magnetic field at high nuclear polarizations in small fields. The low field NMR spectra also showed a distinguished low frequency anomaly at the same region. We propose a nontrivial phase diagram with an estimated critical spin temperature of (0.3±0.1)~μK at zero field. The lattice and electrons of a lithium sample were refrigerated down to 100 μK in attempts to detect the Meissner effect due to superconductivity, but no indication of such transition was observed.     

Study of Microstructure and Local Electron Structure of Zn-Doping GdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Systems Using Positron Experiment

The Zn-doped GdBa₂Cu_3−x Zn _x O_7−δ (x=0.00–0.40) superconductors have been systemically investigated by structural analysis and positron lifetime. The structural results show that the lattice parameter a and b increase slightly with increasing Zn content x, and the parameter c has a small abrupt decrease. Based on the Jahn–Teller model, we analyze the results of structural distortion. From the positron lifetime data, the local electron density n _e has been calculated and the result implies a redistribution of electrons between the Cu-O chains and the CuO₂ planes, resulting in the carrier localization. Furthermore, combining the results of the electronic density n _e and the transition temperature T _c with Zn doping content, it suggests that the carrier localization could interfere the pairing and the “charge transfer” between Cu-O chains and the CuO₂ planes and then suppresses superconductivity of the systems.     

Hole Doping and Inhomogeneous Charge Distribution in High T c Cuprates Investigated from First Principles

To understand the link between doping and electronic properties in the high-temperature superconductors, we have performed first-principles calculations for several representatives of high T _c compounds. In the single-layer cuprate HgBa₂CuO₄ the excess oxygen attracts electrons from the CuO₂ plane leading to an increase of the hole concentration in this building block, where the maximum amount of holes is reached when the dopant oxygen shell is closed. The usage of supercells allows to study the inhomogeneous charge distribution as a function of doping, i.e. from the underdoped up to the overdoped regime. Comparison is made with other compounds like Ba-doped La₂CuO₄ and oxygen-deficient YBa₂Cu₃O_7?x . The effects of our findings on superconductivity are discussed.