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.     

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.     

Spin Fluctuation Effect on Electron-Electron Coupling in High Temperature Superconductors

The spin fluctuation effect on the electron-phonon coupling factor has been calculated. The system is considered to be two-dimensional and we consider two different models which describe the spin fluctuations. These models correspond to strongly correlated electron systems. The effect of disorder introduced by nonmagnetic impurities is investigated.     

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.     

Lattice,Charge, and Spin Phase Inhomogeneity in Complex-Striped Quantum Matter

The Superstripes 2016 conference, held on June 23–29, 2016, in the island of Ischia in Italy celebrated the 20th anniversary of this series of conferences. For 20 years, structural, electronic, and magnetic phase inhomogeneities in quantum matter have been the scientific focus for a growing physics community interested in complexity in quantum matter. It has been the meeting point for different scientific communities facing the challenging project to unveil the complex space and time landscapes in quantum matter. The interesting spatial inhomogeneity length scale of multiple coexisting phases ranges from atomic to mesoscopic, and the time fluctuations are spread over multiple time scales. The response of these materials changes using different experimental techniques with different spatial and time resolution probing different aspects of the quantum complexity.     

Complex Lattice and Charge Inhomogeneity Favoring Quantum Coherence in High-Temperature Superconductors

The presence of two components in the electron fluid of high-temperature superconductors and the complex charge and lattice inhomogeneity have been the hot topics of the international conference of the superstripes series, SUPERSTRIPES 2015, held in Ischia in 2015. The debate on the mechanisms for reaching room-temperature superconductors has been boosted by the discovery of superconductivity with the highest critical temperature in pressurized sulfur hydride. Different complex electronic and structural landscapes showing up in superconductors which resist to the decoherence effects of high temperature have been discussed. While low-temperature superconductors described by the BCS approximation are made of a single condensate in the weak coupling, the high-temperature superconductors are made of coexisting multiple condensates (in different spots of the k-space and the real space) some in the BCS-BEC crossover regime and others in the BCS regime. The role of “shape resonance” in the exchange interaction between these different condensates, like “the Fano-Feshbach resonance” in ultracold gasses, is emerging as a key term for high-temperature superconductivity.     

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.     

Inhomogeneity of Charge and Spin Distribution in CuO2 Layers of High-Tc Superconductors: NQR/NMR Studies of 1-2-3 Compounds

The copper NQR and ZFNMR and thulium NMR are studied in a series of 123 superconductors. It is found that in the aged 60-K superconductors RBa₂Cu₃O_6+x the well known conducting (nonmagnetic) phase co-exists with the magnetically-ordered phase. Both microphases contain two types of inequivalent Cu(2) sites in CuO₂ planes. In the conducting phase, these sites differ only in the Cu(2) NQR frequency, whereas in the magnetic one they differ also in the value of hyperfine magnetic fields at Cu(2) nuclei. These differences and the occupation numbers of the two sites are found to be independent of x (in the range x=0.5÷0.6) and of the volume fraction of the magnetic phase. At low temperatures, the nuclear spin-lattice relaxation in both microphases proceeds via magnetic centers formed by hole-doped Cu–O–Cu chain fragments.     

Inhomogeneity of Charge and Spin Distribution in CuO2 Layers of High-Tc Superconductors: NQR/NMR Studies of 1-2-3 Compounds

The copper NQR and ZFNMR and thulium NMR are studied in a series of 123 superconductors. It is found that in the aged 60-K superconductors RBa₂Cu₃O_6+x the well known conducting (nonmagnetic) phase co-exists with the magnetically-ordered phase. Both microphases contain two types of inequivalent Cu(2) sites in CuO₂ planes. In the conducting phase, these sites differ only in the Cu(2) NQR frequency, whereas in the magnetic one they differ also in the value of hyperfine magnetic fields at Cu(2) nuclei. These differences and the occupation numbers of the two sites are found to be independent of x (in the range x=0.5÷0.6) and of the volume fraction of the magnetic phase. At low temperatures, the nuclear spin-lattice relaxation in both microphases proceeds via magnetic centers formed by hole-doped Cu–O–Cu chain fragments.     

Static Charge and Spin Inhomogeneity in La2 − x Sr x CuO4 by NMR

NMR experiments on La_{2 ? x} Sr _x CuO₄ are shown that characterize the nature of static spatial inhomogeneities in this material. Large amplitude charge density variations are found that are correlated with spin density variations. While the amplitude of the charge variation is temperature-independent, the spin variation increases Curie-like as the temperature varies and shifts toward local antiferromagnetism up to optimal doping. For x = 0.2 the correlation is independent on temperature and agrees with local incommensurability found in neutron scattering. Relations to possible scenarios are discussed.     

Structure, Superstructure and Charge Order in Bi-Cuprates

Inhomogeneities in any form, affecting either the structural properties as superstructures or the electronic degrees of freedom as stripes, play an essential role in the unusual properties of the cuprates. Bi-cuprates, favored for photoemission studies, are unfortunately prone to structural inhomogeneity and superstructure modulations. We have therefore performed combined STM and high resolution photoemission studies on Bi_2−y Pb _y Sr_2−x La _x CuO_6+δ (Pb-Bi2201) for different compositions and identified the contributions due to superstructures beyond the well-known (1×4.7) reconstruction. Since the inclusion of the related scattering vectors gives a nearly perfect image of the experimental Fermi surface, it can be speculated how far such modeling is necessary to understand the spectral lineshape.     

Resonant Tunneling, Pseudo-Gap, and Charge Dynamics in Cuprate Superconductors

The quasi-two-dimensional, metallic conduction in the CuO₂ planes and activated out-of-plane conduction seem typical for most of the cuprate superconductors. The counteraction of overlap and charging energies U* yield the activated out-of-plane resonant tunneling in the normal state and the Josephson tunneling in the superconducting state which is quantitatively described by accepted parameters for various cuprate superconductors and oxygen contents. In addition, the localized, out-of-plane charge exchange obstructed by U* of states of the blocking layer may explain spectroscopic observations, like the tunnel or the photoelectron pseudo-gap, or may scatter phonons and in-plane quasi-particles. The latter relates U*kT* to the in-plane pseudo-gap temperature T*, where the scattering rate decreases below T* because of the obstructed out-of-plane exchange.     

Spin/Charge Redistributions and Oxygen Atom Displacements Induced by Spin Flip and Hole Doping in the CuO2 Layer of High-Temperature Superconductors

On the basis of first principles density functional calculations, we investigated the effect of spin flip and hole doping on the spin, charge and lattice degrees of freedom in the square planar CuO₂ layers of the high-T _c cuprate superconductors. The local responses of the CuO₂ layers to these point defects appear as spatially extended spin-charge-lattice coupled polarons in ferromagnetic clusters and hence introduce heterogeneity. The results demonstrate the need to treat the spin, charge and lattice degrees of freedom on an equal footing in describing the high-T _c superconductivity.     

Pressure Effect in High-Temperature Superconductors

The model of the pressure effect on the critical temperature T _c in high-temperature superconductors [11], earlier proposed by the present author and consisting in the increase of the pair wave function overlap with increase in pressure p that in its manifestations is equivalent to an increase in doping, is compared with the recent experimental data [1–4]. By making this model more precise for inhomogeneous specimens, we obtained the doping region near the optimal one where the critical temperature is independent of doping, and the pressure region where it is independent of pressure. It is shown that, as observed in [1,2], the dependence of dT _c/dp on doping in HgBa₂CuO₄₊ after excluding the influence of the specimen's inhomogeneous structure is explained satisfactorily by the model of [11]. So the introduction in [1,2] of a term in T _c proportional to p ² is unnecessary. An analogous comparison is made for YBa₂Cu₃O_6+y .     

Pressure Effect in High-Temperature Superconductors and Fullerides

The explanation for the pressure effect in high-temperature superconductors and fullerides is offered. Besides the dependence of the pressure derivative of the critical temperature T _c on doping, the direct dependence of T _c on pressure and the universal dependence of the relative change of T _c ^max–T _c with pressure are obtained for high-temperature superconductors. The unity of the model of the pressure effect in high-temperature superconductors and fullerides is justified. The dependence of T _c on the lattice constant and the connection between the pressure effect and the chemical pressure effect in fullerides are discussed.     

绝缘介质中载流子注入与传导的机理

本文分析了与绝缘介质老化、失效过程密切相关的载流子注入与传导机理,包括具有电极限制特性的肖特基效应、具有体限制特性的普尔-弗兰凯尔效应、隧穿效应、离子跳跃传导以及空间电荷限制电流。研究结果表明:由普尔-弗兰凯尔效应引起的势垒降低的高度是由肖特基效应降低的势垒高度的两倍。如果In(I/E2)-1/E特性关系始终是线性的,说明载流子是由隧穿效应注入介质中的。在高电场下,离子跳跃传导的J-E特性曲线是一条斜率为eλ/2kT的直线,并可由此斜率计算得到离子跳跃的距离。由空间电荷限制电流的J-V特性可以确定临界电压,即电流从陷阱限制值迅速跳高至无陷阱的空间电荷限制电流值的电压。     

The Crucial Interplay Between Spin, Charge and Lattice in Organic Superconducting Charge Transfer Salts

Even though superconductivity in organic charge transfer salts (CTS) is unconventional since evidence for a d-wave order parameter is seen , lattice effect play a crucial role, because isotope effects have been observed as well as substantial influences of the insulating buffer layers on the material properties. Consequently, models for CTS should not only take strong correlations into account but also consider spin, charge lattice interaction effects together with a-axis coupling arising from the anionic buffer layers. Here, we propose a novel model, where these latter effects are incorporated and various experimental findings consistently explained.     

Spin Currents and Intrinsic Spin-Hall Effect in Low Dimensional Systems

We consider the spin-Hall effect in two-dimensional electron systems (2DES) with Rashba and Dresselhaus spin-orbit couplings (SO). We find nonzero diagonal spin conductivity provided that Dresselhaus coupling is finite. In addition we consider the influence of disorder on spin conductivities in these systems. By comparing with the exact diagonalization method the finite quasiparticle lifetime (Born) approximation we argue that the latter one seems to be sufficient to describe the case when spin-orbit coupling is stronger or comparable with disorder strength. Furthermore, in the framework of Landauer–Buttiker formalism we show that the transverse and diagonal spin conductivities in Rashba plus Dresselhaus SO systems are robust against disorder for finite size systems.