Subsequent to our recent report of SDW type transition at 190 K and antiferromagnetic order below 20 K in EuFe2As2, we have studied the effect of K-doping on the SDW transition at high temperature and AF order at low temperature. 50% K doping suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below T_c = 32 K, as observed in the electrical resistivity, AC susceptibility as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity.
It is suggested that the appearance of Jordan blocks in the normal form of the recursion (transfer) matrix is responsible for high temperature superconductivity and related phenomena. Several published observations are discussed to support this idea. The draft of a quantum chemical theory is given for a more complete verification. Keywords: Jordan scenario, Van Hove scenario, high temperature superconductivity. Short title: Jordan scenario of high temperature superconductivity.
We theoretically estimate the electron-phonon coupling constant lambda for metallic single-walled carbon nanotubes with a diameter of 1.4 nm. The partial electron-phonon coupling constant for the hardest phonon mode is estimated to be about 0.0036, in good agreement with that deduced from Raman scattering data assuming superconductivity above room temperature. Assuming no superconductivity, we estimate the room-temperature inelastic mean free path l_ep due to electron-phonon scattering to be about 0.46 micrometer, and the total room-temperature inelastic mean free path l_in to be about 0.16 mi…
This is the first book on the subject of room-temperature superconductivity. The main purpose of the book is twofold. First, to show that, under suitable conditions, superconductivity can occur above room temperature. Secondly, to present general guidelines on how to synthesize a room temperature superconductor. The book begins with an introduction into the physics of the superconducting state and superconducting materials. The mechanisms of conventional, half-conventional and unconventional superconductivity are discussed in the following chapters. The last three chapters of the book are devo…
Surface superconductivity in rhombohedral graphite is a robust phenomenon which can exist even when higher order hoppings between the layers lift the topological protection of the surface flat band and introduce a quadratic dispersion of electrons with a heavy effective mass. We show that for weak pairing interaction, the flat band character of the surface superconductivity transforms into a BCS-like relation with high critical temperature characterized by a higher coupling constant due to a much larger density of states than in the bulk. Our results offer an explanation for the recent finding…
Superconductivity is traditionally viewed as a low-temperature phenomenon. Within the BCS theory this is understood to result from the fact that the pairing of electrons takes place only close to the usually two-dimensional Fermi surface residing at a finite chemical potential. Because of this, the critical temperature is exponentially suppressed compared to the microscopic energy scales. On the other hand, pairing electrons around a dispersionless (flat) energy band leads to very strong superconductivity, with a mean-field critical temperature linearly proportional to the microscopic coupling…
Detailed analyses are made on previously published data for multi-walled carbon nanotubes. The field dependence of the Hall voltage, the temperature dependence of the Hall coefficient, and the magnetoresistance effect (Phys. Rev. Lett. 72, 697 (1994)) can all be consistently explained in terms of the coexistence of physically separated tubes and Josephson-coupled superconducting tubes with superconductivity above room temperature. The observed temperature dependencies of the remnant magnetization, the diamagnetic susceptibility, and the conductance are consistent with superconductivity above r…
In this work, we show that the same theoretical tools that successfully explain other hydrides systems under pressure seem to be at odds with the recently claimed conventional room temperature superconductivity of the carbonaceous sulfur hydride. We support our conclusions with I) the absence of a dominant low-enthalpy stoichiometry and crystal structure in the ternary phase diagram. II) Only the thermodynamics of C-doping phases appears to be marginally competing in enthalpy against H$_3$S. III) Accurate results of the transition temperature given by ab initio Migdal-Eliashberg calculations d…
Trying to dope graphite flakes we found that the magnetization of pure, several tens of micrometers grain size graphite powder and after a simple treatment with pure water shows clear and reproducible granular superconducting behavior with a critical temperature above 300K. The observed magnetic characteristics as a function of temperature, magnetic field and time, provide evidence for weakly coupled grains through Josephson interaction, revealing the existence of superconducting vortices.
To synthesize a new superconductor which has a critical temperature, Tc, exceeding the room temperature, one needs to know what chemical components to start with. This chapter presents analysis of experimental data which allow one to draw a conclusion about components and the structure of a potential room-temperature superconductor. The two essential components of a room temperature superconductor are large organic molecules (polymers, tissues) and atoms/molecules which are magnetic in the intercalated state. This conclusion is fully based on experimental facts known today, and does not requir…
We measured the initial M-H curves for a sample of the newly discovered superconductor NdFeAsO0.82Fe0.18, which had a critical temperature, Tc, of 51 K, and was fabricated at the high pressure of 6 GPa. The lower critical field, Hc1, was extracted from the deviation point of the Meissner linearity in the M-H curves, which show linear temperature dependence in the low temperature region down to 5 K. The Hc1(T) indicates no s-wave superconductivity, but rather an unconventional superconductivity with a nodal gap structure. Furthermore, the linearity of Hc1 at low temperature does not hold at hig…
It is well known that copper-based perovskite oxides rightly enjoy consensus as high-temperature superconductors on the basis of two signatures: Meissner effect and zero resistance. In contrast, I provide over twenty signatures for room temperature superconductivity in carbon nanotubes. The one-dimensionality of the nanotubes complicates the right-of-passage for prospective quasi-one-dimensional superconductors. The Meissner effect is less visible because the diameters of nanotubes are much smaller than the penetration depth. Zero resistance is less obvious because of the quantum contact resis…
Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride LaH$_{10}$ has been of particular focus as the first material to exhibit a superconducting critical temperature (T$_c$) near room temperature. Experiments indicate that the use of ammonia borane as the hydrogen source can increase the conductivity onset temperatures of lanthanum superhydride to as high as 290 K. Here we examine the doping effects of B and N atoms on the superc…
We examine the effects of the low-level substitution of S atoms by C and Si atoms on the superconductivity of H$_3$S with the $Im\bar{3}m$ structure at megabar pressure. The hole doping can fine-tune the Fermi energy to reach the electronic density-of-states peak maximizing the electron-phonon coupling. This can boost the critical temperature from the original 203 K to 289 K and 283 K, respectively, for H$_3$S$_{0.962}$C$_{0.038}$ at 260 GPa and H$_3$S$_{0.960}$Si$_{0.040}$ at 230 GPa. The former may provide an explanation for the recent experimental observation of room-temperature superconduc…
A long-standing problem of observing Room Temperature Superconductivity is finally solved by a novel approach. Instead of increasing the critical temperature Tc of a superconductor, the temperature of the room was decreased to an appropriate Tc value. We consider this approach more promising for obtaining a large number of materials possessing Room Temperature Superconductivity in the near future.
Very recent observation of superconductivity in both transport (resistance versus temperature) and magnetization (Zero field cooled) at around 235 K has obviously raised many eyebrows. The experimental fraternity in particular is more thrilled. Although the results presented in the paper are clean and both transport and magnetization measurements do approve the observation of superconductivity to some extent (the shielding fraction is low), the problem is to synthesize or fabricate the material. This I name a goldsmith job, primarily because one has to deal with Gold and Silver nano particles …
we were able to develop a novel method to synthesize Fe-based oxypnictide superconductors. By using LnAs and FeO as the starting materials and a ball-milling process prior to solid-state sintering, Tc as high as 50.7 K was obtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering at temperatures as low as 1173 K for times as short as 20 min.
We introduce below the concept of defecton, and describe briefly the electron polaron effect, the clustering of defectons, and also the defecton mechanism of superconductivity. It is shown that in the case of high-temperature superconducting metal hydrides, which acquire superconducting properties under pressure of hundreds of gigapascals, this mechanism can make a significant contribution to the Cooper pairing.
The current status of basic research on the high temperature cuprate superconductors and prospects for technological applications of these materials is discussed. Recent developments concerning other novel superconductors are also briefly described.
The discovery of charge-density-wave order in the high-temperature superconductor YBa$_2$Cu$_3$O$_{6+y}$ places charge order centre stage with superconductivity, suggesting they they are intertwined rather than competing.
The long-sought goal of room-temperature superconductivity has reportedly recently been realized in a carbonaceous sulfur hydride compound under high pressure, as reported by Snider et al. [1]. The evidence presented in that paper is stronger than in other similar recent reports of high temperature superconductivity in hydrides under high pressure [2-7], and has been received with universal acclaim [8-10]. Here we point out that features of the experimental data shown in Ref. [1] indicate that the phenomenon observed in that material is not superconductivity. This observation calls into questi…
The search for room-temperature superconducting materials has been at the center of modern research for decades. The recent discovery of high-temperature superconductivity, under extreme pressure in hydrogen-rich materials, is a tremendous achievement in this research front. This discovery offers a route in the search for room temperature superconductivity at ambient pressure. The superconductivity of these hydrogen-rich materials was confirmed by the observation of zero-resistance, isotope effects, effect of magnetic field, and other standard properties. However, some of the experimental feat…
We examine the possibility of high temperature superconductivity from two-dimensional semiconductor without antiferromagnetic fluctuations. The weak coupling BCS theory is applied, especially where the Fermi level is near the bottom of the conduction band. Due to screening, the attractive interaction is local in k-space. The density of states(DOS) does not have a peak near the bottom of the band, but k-dependent contribution to DOS has a diverging peak at the bottom. These features lead to high temperature superconductivity which may explain the possible superconductivity of WO_3.
We show that the topologically protected flat band emerging on a surface of a nodal fermionic system promotes the surface superconductivity due to an infinitely large density of states associated with the flat band. The critical temperature depends linearly on the pairing interaction and can be thus considerably higher than the exponentially small bulk critical temperature. We discuss an example of surface superconductivity in multilayered graphene with rhombohedral stacking.
I briefly mention a previous claim of room temperature superconductivity ( arXiv:0905.3524) in Ag-based oxide material and compare their results with the most recent claim of ambient temperature superconductivity in arXiv:1807.08572. In both cases, an electrical transition to low resistance state and diamagnetism are observed. Silver is a common ingredient in both claims. Does it mean that silver holds the key to RT superconductivity or, the missing field cool data (in both reports) hint of some other physical phenomenon than superconductivity.
It is very important to elucidate the mechanism of superconductivity for achieving room temperature superconductivity. This paper is a short review article on the mechanism of high-temperature superconductivity. In the first half of this paper, we give a brief review on mechanisms of superconductivity in many-electron systems. We believe that high-temperature superconductivity may occur in a system with interaction of large-energy scale. Empirically, this is true for superconductors that have been found so far. In the second half of this paper, we discuss cuprate high-temperature superconducto…
Defects in the atomic lattice of solids are sometimes desired. For example, atomic vacancies, single ones or more elaborated defective structures, can generate localized magnetic moments in a non magnetic crystalline lattice. Increasing their density to a few percent magnetic order can appear. Furthermore, certain two dimensional interfaces can give rise to localized superconductivity with a broad range of critical temperatures. Old and new experimental facts emphasize the need to join efforts to start using systematically "ordered defects" in solids to achieve room temperature superconductivi…
We report on the observation of high-temperature ($T_\textrm{c}$) superconductivity and magnetic vortices in single-unit-cell FeSe films on anatase TiO$_2$(001) substrate by using scanning tunneling microscopy. A systematic study and engineering of interfacial properties has clarified the essential roles of substrate in realizing the high-$T_\textrm{c}$ superconductivity, probably via interface-induced electron-phonon coupling enhancement and charge transfer. By visualizing and tuning the oxygen vacancies at the interface, we find their very limited effect on the superconductivity, which exclu…
Cuprate high temperature superconductors consist of two quasi-two-dimensional (2D) substructures: CuO2 superconducting layers and charge reservoir layers. The superconductivity is realized by charge transfer from the charge reservoir layers into the superconducting layers without chemical dopants and defects being introduced into the latter, similar to modulation-doping in semiconductor superlattices of AlGaAs/GaAs. Inspired by this scheme, we have been searching for high temperature superconductivity in ultrathin films of superconductors epitaxially grown on semiconductor/oxide substrates sin…
The realization of high-transition-temperature superconductivity (HTSC) confined to nanometre-sized interfaces has been a long-standing goal because of potential applications and the opportunity to study quantum phenomena in reduced dimensions. Here we discuss HTSC at free surfaces, interfaces, and nanoscale cluster surfaces, and show that the percolative self-organized model of bulk HTSC also gives an excellent description of HTSC in quasi-two-dimensional contexts.
I offer some personal reminiscences from the period of 1976-1983, when I was a M. Sc. and then a Ph.D. student in Vitaly L. Ginzburg's High Temperature Superconductivity group at the P.N. Lebedev Institute in Moscow
We report the observation of an exceptionally large room-temperature electrical conductivity in silver and aluminum layers deposited on a lead zirconate titanate (PZT) substrate. The surface resistance of the silver-coated samples also shows a sharp change near 313 K. The results are strongly suggestive of a superconductive interfacial layer, and have been interpreted in the framework of Bose-Einstein condensation of bipolarons as the suggested mechanism for high-temperature superconductivity in cuprates.
ARPES and wide area, high resolution STM studies of a micaceous cuprate high temperature superconductor have shown strong nanoscale gap disorder, but the question of whether this disorder is intrinsic and necessary for HTSC, or merely incidental to either this material, or even only its surface, appears to be an open one. We present the case that it is merely incidental, and then review the evidence that strong disorder is actually an essential factor in generating this unprecedented phenomenon.
We determine the behavior of the critical temperature of magnetically mediated p-wave superconductivity near a ferromagnetic quantum critical point in three dimensions, distinguishing universal and non-universal aspects of the result. We find that the transition temperature is non-zero at the critical point, raising the possibility of superconductivity in the ferromagnetic phase.
Based on the mechanism described by the authors in cond-mat/0109553, we predict that the compound C60/CHI3 will have Tc~140K, which would be the highest value obtained sofar for intercalated materials.
We analyze a possible superconductivity in the hole-doped system of layered hydrogenized graphene by taking into account thermal fluctuations of the order parameter. In particular, we demonstrate that in the one-layer case the values of the high mean-field (MF) critical temperature $T_{c}^{MF}\sim 80-90K$, predicted recently by Savini et al \cite{Savini}, do not alter significantly due to the fluctuations, and the Berezinskii-Kosterlitz-Thouless (BKT) critical temperature of the vortex superconductivity is almost the same as the MF temperature at doping 0.01-0.1. We show that in the case of mu…
It is the purpose of this paper to explore the theory of high temperature superconductivity. Much of the motivation for this comes from the study of the cuprate high temperature superconductors. However, our primary focus is on the core theoretical issues associated with the mechanism of high temperature superconductivity more generally. We concentrate on physics at intermediate temperature scales of order $T_c$ (as well as the somewhat larger "pseudogap" temperature) and energies of order the gap maximum, $Δ_0$. Prominent themes throughout the article are the need for a kinetic energy driven …
We have studied the magnetic response of two bulk highly oriented pyrolytic graphite (HOPG) samples with different internal microstructure. For the sample with well defined interfaces, parallel to the graphene layers, the temperature and magnetic field hysteresis are similar to those found recently in water-treated graphite powders. The observed behavior indicates the existence of granular superconductivity above room temperature in agreement with previous reports in other graphite samples. The granular superconductivity behavior is observed only for fields normal to the embedded interfaces, w…
We study the transition temperature Tc(H) of a quasi-one-dimensional (Q1D) superconductivity which is derived from the quantum effect of an electron motion in a strong magnetic field. We calculate Tc(H) of both isotropic and anisotropic superconductivity by taking account of the optimal momentum of the Cooper pairs and the effect of higher harmonic terms along second conducting axis in the tight-binding model. We find that, although Tc(H) of the spin-singlet superconductivity is suppressed strongly by the Zeeman effect, the suppression of Tc(H) is not very severe if we take the optimal pair-mo…
Since the discovery of superconductivity at 200 K in H3S [1] similar or higher transition temperatures, Tcs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behaviour of a conventional phonon-mediated superconductor: ze…
We report superconductivity induced in films of the non-superconducting, antiferromagnetic parent material FeTe by low temperature oxygen incorporation in a reversible manner. X-ray absorption shows that oxygen doping changes the nominal Fe valence state from 2+ in the non-superconducting state to mainly 3+ in the superconducting state. Thus superconductivity in O doped FeTe occurs in a quite different charge and strain state than the more common FeTe$_{1-x}$Se$_x$. This work also suggests a convenient path for conducting doping experiments in-situ with many measurement techniques.
Results are presented for the temperature and frequency dependence of the real and imaginary parts of the diagonal self energy for a d-wave superconductor. An Eliashberg analysis, which has been successful in recent fitting of superconductor-insulator-superconductor tunnel junction conductances for BiSrCaCuO (Bi-2212), is extended to finite temperatures. The effect of the temperature dependence of the 40 meV spin resonance mode, measured in inelastic neutron scattering (INS) in Bi-2212, on the finite temperature self energies is investigated.
A scanning micro superconducting quantum interference device (microSQUID) microscope is used to directly image vortices in a superconducting Al thin film. We observe the temperature dependence of the vortex distribution in a regular defect (hole) array patterned into the Al film. The first direct observation of the localized superconducting state around the holes is shown as well as the effect of the hole size on nucleation of the superconducting state.
Here I extend my last work about the origin of the pseudo-gaps in underdoped cuprates (arXiv: cond-mat. 1011.3206), to include the mechanism of superconductivity. This is done by adapting the formalism of the double correlations in systems with nested Fermi surfaces to the semi one dimensional system of strings of holes. It is proposed that magnetic interaction is crucial for the establishment of the pseudogap and the high temperature superconductivity. It is shown that superconductivity disturbs the completeness of the strings of holes, and creates fluctuations in their shapes. This, in turn,…
We determine the magnetic field dependence of the pseudogap closing temperature T* within a precursor superconductivity scenario. Detailed calculations with an anisotropic attractive Hubbard model account for a recently determined experimental relation in BSCCO between the pseudogap closing field and the pseudogap temperature at zero field, as well as for the weak initial dependence of T* at low fields. Our results indicate that the available experimental data are fully compatible with a superconducting origin of the pseudogap in cuprate superconductors.
The cuprates and iron-based high-temperature superconductors share many common features: layered strongly anisotropic crystal structure, strong electronic correlations, interplay between different types of electronic ordering, the intrinsic spatial inhomogeneity due to doping. The understanding of complex interplay between these factors is crucial for a directed search of new high-temperature superconductors. Here we show the appearance of inhomogeneous gossamer superconductivity in bulk FeSe compound at ambient pressure and at temperature 5 times higher than its zero-resistance $T_c$. This di…
The surprising discovery of superconductivity in layered iron-based materials, with transition temperatures climbing as high as 55 K, has lead to thousands of publications on this subject over the past two years. While there is general consensus on the unconventional nature of the Cooper pairing state of these systems, several central questions remain - including the role of magnetism, the nature of chemical and structural tuning, and the resultant pairing symmetry - and the search for universal properties and principles continues. Here we review the progress of research on iron-based supercon…
Unconventional symmetries of the order parameter allowed some researchers to maintain that a purely repulsive interaction between electrons provides superconductivity without phonons in a number of high-temperature superconductors. It is shown that the Cooper pairing in p and d states is not possible with the realistic Coulomb repulsion between fermions at relevant temperatures in any dimension.
The mechanism of superconductivity caused by the electron-vibrational centres and their inherent oscillations in crystals and solid-state structures near room temperature and at higher temperatures - hyperconductivity is discussed and actualized. The experimental data about hiperconductivity in some materials are presented. Hyperconductivity arises and exists from normal-superconducting transition temperature Tc to 800K and probably up to melting of material (probably up to temperature of crystal melting and is higher). Tc in various materials varies from hundred up to several hundreds degrees…
