Showing papers on "High-temperature superconductivity published in 2017"

Evolution of High-Temperature Superconductivity from a Low-Tc Phase Tuned by Carrier Concentration in FeSe Thin Flakes

Abstract: We report the evolution of superconductivity in an FeSe thin flake with systematically regulated carrier concentrations by the liquid-gating technique. With electron doping tuned by the gate voltage, high-temperature superconductivity with an onset at 48 K can be achieved in an FeSe thin flake with T_{c} less than 10 K. This is the first time such high temperature superconductivity in FeSe is achieved without either an epitaxial interface or external pressure, and it definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with T_{c}^{onset} as high as 48 K in bulk FeSe. Intriguingly, our data also indicate that the superconductivity is suddenly changed from a low-T_{c} phase to a high-T_{c} phase with a Lifshitz transition at a certain carrier concentration. These results help to build a unified picture to understand the high-temperature superconductivity among all FeSe-derived superconductors and shed light on the further pursuit of a higher T_{c} in these materials.

Unconventional Superconductivity

Abstract: Conventional superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron-pairs described by BCS theory Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon exchange but instead by exchange of some other kind, e g spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 06 K in 1979 Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994 Further progress in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing This review compares the fundamental properties of 9 unconventional superconducting classes of materials - from 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples, with the hope that common features will emerge to help theory explain (and predict!) these phenomena In addition, three new emerging classes of superconductors (topological, interfacial [e g FeSe on SrTiO3], and H2S under high pressure) are briefly covered, even though their conventionality is not yet fully determined

Origin of charge transfer and enhanced electron–phonon coupling in single unit-cell FeSe films on SrTiO 3

Abstract: Interface charge transfer and electron–phonon coupling have been suggested to play a crucial role in the recently discovered high-temperature superconductivity of single unit-cell FeSe films on SrTiO3. However, their origin remains elusive. Here, using ultraviolet photoemission spectroscopy and element-sensitive X-ray photoemission spectroscopy, we identify the strengthened Ti–O bond that contributes to the interface enhanced electron–phonon coupling and unveil the band bending at the FeSe/SrTiO3 interface that leads to the charge transfer from SrTiO3 to FeSe films. We also observe band renormalization that accompanies the onset of superconductivity. Our results not only provide valuable insights into the mechanism of the interface-enhanced superconductivity, but also point out a promising route toward designing novel superconductors in heterostructures with band bending-induced charge transfer and interfacial enhanced electron–phonon coupling. The origin of interface charge transfer and electron-phonon coupling in single unit-cell FeSe on SrTiO3 remains elusive. Here, Zhang et al. report strengthened Ti-O bond and band bending at the FeSe/SrTiO3 interface, which leads to several important processes.

Discovery of High-Temperature Superconductivity (Tc = 55 K) in B-Doped Q-Carbon

Abstract: We have achieved a superconducting transition temperature (Tc) of 55 K in 27 at% B-doped Q-carbon. This value represents a significant improvement over previously reported Tc of 36 K in B-doped Q-carbon and is the highest Tc for conventional BCS (Bardeen–Cooper–Schrieffer) superconductivity in bulk carbon-based materials. The B-doped Q-carbon exhibits type-II superconducting characteristics with Hc2(0) ∼ 10.4 T, consistent with the BCS formalism. The B-doped Q-carbon is formed by nanosecond laser melting of B/C multilayered films in a super undercooled state and subsequent quenching. It is determined that ∼67% of the total boron exists with carbon in a sp3 hybridized state, which is responsible for the substantially enhanced Tc. Through the study of the vibrational modes, we deduce that higher density of states near the Fermi level and moderate to strong electron–phonon coupling lead to a high Tc of 55 K. With these results, we establish that heavy B doping in Q-carbon is the pathway for achieving high-te...

High-temperature superconductivity in one-unit-cell FeSe films

Abstract: Since the dramatic enhancement of the superconducting transition temperature (T c) was reported in a one-unit-cell FeSe film grown on a SrTiO3 substrate (1-UC FeSe/STO) by molecular beam epitaxy (MBE), related research on this system has become a new frontier in condensed matter physics. In this paper, we present a brief review on this rapidly developing field, mainly focusing on the superconducting properties of 1-UC FeSe/STO. Experimental evidence for high-temperature superconductivity in 1-UC FeSe/STO, including direct evidence revealed by transport and diamagnetic measurements, as well as other evidence from scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), are overviewed. The potential mechanisms of the enhanced superconductivity are also discussed. There are accumulating arguments to suggest that the strengthened Cooper pairing in 1-UC FeSe/STO originates from the interface effects, specifically the charge transfer and coupling to phonon modes in the TiO2 plane. The study of superconductivity in 1-UC FeSe/STO not only sheds new light on the mechanism of high-temperature superconductors with layered structures, but also provides an insight into the exploration of new superconductors by interface engineering.

Phonon-enhanced superconductivity at the FeSe/SrTiO3 interface

Abstract: The dream of room temperature superconductors has inspired intense research effort to find routes for enhancing the superconducting transition temperature (Tc). Therefore, single-layer FeSe on a SrTiO3 substrate, with its extraordinarily high Tc amongst all interfacial superconductors and iron based superconductors, is particularly interesting, but the mechanism underlying its high Tc has remained mysterious. Here we show through isotope effects that electrons in FeSe couple with the oxygen phonons in the substrate, and the superconductivity is enhanced linearly with the coupling strength atop the intrinsic superconductivity of heavily-electron-doped FeSe. Our observations solve the enigma of FeSe/SrTiO3, and experimentally establish the critical role and unique behavior of electron-phonon forward scattering in a correlated high-Tc superconductor. The effective cooperation between interlayer electron-phonon interactions and correlations suggests a path forward in developing more high-Tc interfacial superconductors, and may shed light on understanding the high Tc of bulk high temperature superconductors with layered structures.

High-Temperature Superconductivity in Boron-Doped Q-Carbon

Abstract: We report high-temperature superconductivity in B-doped amorphous quenched carbon (Q-carbon). This phase is formed after nanosecond laser melting of B-doped amorphous carbon films in a super-undercooled state and followed by rapid quenching. Magnetic susceptibility measurements show the characteristics of type-II Bardeen–Cooper–Schrieffer superconductivity with a superconducting transition temperature (Tc) of 36.0 ± 0.5 K for 17.0 ± 1.0 atom % boron concentration. This value is significantly higher than the best experimentally reported Tc of 11 K for crystalline B-doped diamond. We argue that the quenching from metallic carbon liquid leads to a stronger electron–phonon coupling due to close packing of carbon atoms with higher density of states at the Fermi level. With these results, we propose that the non-equilibrium undercooling-assisted synthesis method can be used to fabricate highly doped materials that provide greatly enhanced superconducting properties.

Orbital-selective pairing and superconductivity in iron selenides

Abstract: An important challenge in condensed matter physics is understanding iron-based superconductors. Among these systems, the iron selenides hold the record for highest superconducting transition temperature and pose especially striking puzzles regarding the nature of superconductivity. The pairing state of the alkaline iron selenides appears to be of d-wave type based on the observation of a resonance mode in neutron scattering, while it seems to be of s-wave type from the nodeless gaps observed everywhere on the Fermi surface. Here we propose an orbital-selective pairing state, dubbed sτ 3, as a natural explanation of these disparate properties. The pairing function, containing a matrix τ 3 in the basis of 3d-electron orbitals, does not commute with the kinetic part of the Hamiltonian. This dictates the existence of both intraband and interband pairing terms in the band basis. A spin resonance arises from a d-wave-type sign change in the intraband pairing component, whereas the quasiparticle excitation is fully gapped on the FS due to an s-wave-like form factor associated with the addition in quadrature of the intraband and interband pairing terms. We demonstrate that this pairing state is energetically favored when the electron correlation effects are orbitally selective. More generally, our results illustrate how the multiband nature of correlated electrons affords unusual types of superconducting states, thereby shedding new light not only on the iron-based materials but also on a broad range of other unconventional superconductors such as heavy fermion and organic systems. Orbital-selective pairing could explain the unusual properties observed in the unconventional superconductor iron selenide. Conventional superconductivity arises when electrons form Cooper pairs due to electron-phonon coupling. In some materials, however, unconventional superconductivity can arise, which is driven by electron-electron rather than electron-phonon couplings. The detailed mechanism that facilitates electron pairing in unconventional systems remains elusive but iron selenide systems could help to provide insights as they exhibit both relatively high temperature superconductivity, and also strong electron correlations. With different experiments suggesting different pairing mechanisms, however, these systems are somewhat puzzling. An international team of researchers led by Qimiao Si from Rice University now theoretically demonstrate that an orbital-selective pairing state could explain this unusual behaviour, which may also be at play in other unconventional superconductors such as heavy fermion and organic systems.

How resistive must grain boundaries in polycrystalline superconductors be, to limit J c?

Abstract: Although we can use misorientation angle to distinguish the grain boundaries that can carry high critical current density (J_c) in high temperature superconductors (HTS) from those that cannot, there is no established normal state property equivalent. In this paper, we explore the superconducting and normal state properties of the grains and grain boundaries of polycrystalline YBa2Cu3O7–x (YBCO) using complementary magnetisation and transport measurements, and calculate how resistive grain boundaries must be to limit J_c in polycrystalline superconductors. The average resistivity of the grain boundaries, ρ_GB, in our micro and nanocrystalline YBCO are 0.12 Ωm and 8.2 Ωm, values which are much higher than that of the grains (ρ_G) and leads to huge ρ_GB/ρ_G values of 2 × 103 and 1.6 × 105 respectively. We find that the grain boundaries in our polycrystalline YBCO are sufficiently resistive that we can expect the transport J_c to be several tens of orders of magnitude below the potential current density of the grains in our YBCO samples, as is found experimentally. Calculations presented show that increasing J_c values by ~ 2 orders of magnitude in high fields is still possible in all state-of-the-art technological high-field superconductors. We conclude: grain boundary engineering is unlikely to improve J_c sufficiently in randomly aligned polycrystalline YBCO, to make it technologically useful for high-field applications; in low temperature superconducting intermetallics, such as Nb3Sn, large increases in J_c may be achieved by completely removing the grain boundaries from these materials and, as is the case for thin films of Nb, Ba(FeCo)2As2 and HTS materials, by incorporating additional artificial pinning.

Ultrafast Gap Dynamics and Electronic Interactions in a Photoexcited Cuprate Superconductor

Abstract: New investigations of a type of high-temperature superconductor using time- and angle-resolved photoemission spectroscopy reveal new details about the nature of photoexcitation and its relation to superconductivity.

Superconductivity above 120 kelvin in a chain link molecule

Abstract: The search for new superconducting compounds with higher critical temperatures $T_{c}^{\prime}$s has long been the very heart of scientific research on superconductivity. It took 75 years for scientists to push the $T_{c}$ above liquid nitrogen boiling temperature since the discovery of superconductivity. So far, the record high $T_{c}$ of about 130 K at atmosphere pressure was reported in some multilayer Hg(Tl)-Ba-Ca-Cu-O compounds. Meanwhile, sulfur hydride system holds the highest $T_{c}$ of around 200 K at high pressure of about 150 GPa. While keeping these records for superconductivity, either the toxicity of these superconductors or the requirement of extreme pressure condition for superconductivity limits their technology applications. Here we show that doping a chain link molecule $-$ $p$-terphenyl by potassium can bring about superconductivity at 123 K at atmosphere pressure, which is comparable to the highest $T_{c}$ in cuprates. The easy processability, light weight, durability of plastics, and environmental friendliness of this kind of new superconductor have great potential for the fine-tuning of electrical properties. This study opens a window for exploring high temperature superconductivity in chain link organic molecules.

Imaging the real space structure of the spin fluctuations in an iron-based superconductor

Abstract: Spin fluctuations are a leading candidate for the pairing mechanism in high temperature superconductors, supported by the common appearance of a distinct resonance in the spin susceptibility across the cuprates, iron-based superconductors and many heavy fermion materials. The information we have about the spin resonance comes almost exclusively from neutron scattering. Here we demonstrate that by using low-temperature scanning tunnelling microscopy and spectroscopy we can characterize the spin resonance in real space. We show that inelastic tunnelling leads to the characteristic dip-hump feature seen in tunnelling spectra in high temperature superconductors and that this feature arises from excitations of the spin fluctuations. Spatial mapping of this feature near defects allows us to probe non-local properties of the spin susceptibility and to image its real space structure. The knowledge of how spin fluctuations affect high-Tc superconductivity comes exclusively from neutron scattering. Here, Chi et al. establish characteristic excitation features of the spin fluctuations in real space from the scanning tunnelling spectra in an iron-based superconductor.

Modulated Structure Calculated for Superconducting Hydrogen Sulfide.

Abstract: Compression of hydrogen sulfide using first principles metadynamics and molecular dynamics calculations revealed a modulated structure with high proton mobility which exhibits a diffraction pattern matching well with experiment. The structure consists of a sublattice of rectangular meandering SH- chains and molecular-like H3 S+ stacked alternately in tetragonal and cubic slabs forming a long-period modulation. The novel structure offers a new perspective on the possible origin of the superconductivity at very high temperatures in which the conducting electrons in the SH chains are perturbed by the fluxional motions of the H3 S resulting in strong electron-phonon coupling.

A novel high-temperature carbon-based superconductor: B-doped Q-carbon

Abstract: Following a brief report on high-temperature superconductivity in B-doped Q-carbon [Bhaumik et al., ACS Nano 11(6), 5351–5357 (2017)], we present detailed structure-property correlations to understand the origin of superconductivity in strongly bonded lightweight materials and methods to further enhance the superconducting transition temperature (Tc). Nanosecond melting of carbon in a super undercooled state and rapid quenching result in a strongly bonded unique phase of B-doped Q-carbon. The temperature-dependent resistivity and magnetic susceptibility measurements demonstrate type II superconductivity in this material with a transition temperature of 36.0 ± 0.5 K and an upper critical field of 5.4 T at ∼0 K. It has also been shown that in B-doped Q-carbon, the upper critical magnetic field (Hc2(T)) follows Hc2(0) [1-(T/Tc)2.1] temperature dependence and is consistent with the Bardeen–Cooper–Schrieffer formalism. In the present study, B-doped Q-carbon thin films are formed on sapphire substrates by emplo...

Competition between Superconductor - Ferromagnetic stray magnetic fields in YBa2Cu3O7-x films pierced with Co nano-rods.

Abstract: Superconductivity and ferromagnetism are two antagonistic phenomena that combined can lead to a rich phenomenology of interactions, resulting in novel physical properties and unique functionalities. Here we propose an original hybrid system formed by a high-temperature superconducting film, patterned with antidots, and with ferromagnetic nano-rods grown inside them. This particular structure exhibits the synergic influence of superconductor (SC) - ferromagnetic (FM) stray fields, in both the superconducting behaviour of the film and the three-dimensional (3D) magnetic structure of nano-rods. We show that FM stray fields directly influence the critical current density of the superconducting film. Additional functionalities appear due to the interaction of SC stray fields, associated to supercurrent loops, with the non-trivial 3D remanent magnetic structure of FM nano-rods. This work unravels the importance of addressing quantitatively the effect of stray magnetic fields from both, the superconductor and the ferromagnet in hybrid magnetic nano-devices based on high temperature superconductors.

Superconductivity at 43 K in a single C-C bond linked terphenyl

Abstract: Organic compounds are promising candidates to exhibit high temperature or room temperature superconductivity. However, the critical temperatures of organic superconductors are bounded to 38 K. By doping potassium into $p$-terphenyl consisting of C and H elements with three phenyl rings connected by single C-C bond in para position, we find that this material can have a superconducting phase with the critical temperature of 43 K. The superconducting parameters such as the critical fields, coherent length, and penetration depth are obtained for this superconductor. These findings open an encouraging window for the search of high temperature superconductors in chain link organic molecules.

Growth and physical properties of top-seeded infiltration growth processed large grain (Gd, Dy)BCO bulk superconductors

Abstract: The primary prerequisite for successful single grain growth of a REBa2Cu3Oy bulk superconductor is the determination of an optimized growth temperature window of the given system. In our recent work, addition of 20 wt. % of Dy2BaCuO5 (Dy-211) to the GdBa2Cu3O7-δ bulk sample [(Gd,Dy)BCO] enhanced superconducting properties. Then, different isothermal dwell times were tested at various undercooling temperatures to find the temperature window appropriate for the single grain growth by a top-seeded infiltration growth process performed in air. Nearly perfect single-grain (Gd,Dy)BCO crystals were grown. A systematic and detailed investigation of the microstructure, composition, and physical properties of the single-grain (Gd,Dy)BCO superconductors was carried out. The superconducting transition was sharp with the onset at 92.9 K. The field dependence of the critical current density (Jc) was determined at several temperatures. At 77 K, a self-field Jc of 55 kA/cm2 and a trapped field of 0.19 T were achieved. The possibilities for further improvement of the microstructure and superconducting performance are discussed.

A review of thickness-induced evolutions of microstructure and superconducting performance of REBa 2 Cu 3 O 7−δ coated conductor

Abstract: The research and development of high temperature superconducting (HTS) films, especially ReBa2Cu3O7− δ (REBCO or RE123; RE=Y, Gd, or other rare earths) yttrium-based coated conductors, has generated widespread interest for the potential applications of the second generation superconducting films. In view of commercialization, however, the maximum superconducting currents for coated conductors should be increased further. Unfortunately, it has been frequently observed that the average critical current density J c decreases with an increase in film thickness. The thickness effect is still a hurdle for large-scale production, especially in pulsed laser deposition and metal organic deposition processes. An engineering current of more than 1 000 A/cm is desired owing to the high cost of 2G superconducting materials. The present work attempts to review the evolution of various issues subject to the thickness effect, including the microstructure, epitaxial texture, surface roughness, pinning force, oxygen deficiency, residual stress, copper-rich layers, and segregation of elements. Furthermore, recent progress in enhancing the performance of superconductors especially in terms of critical current density is illustrated, such as the use of heavy doping. Further understanding of the thickness effect is extremely important for large-scale commercial development of the second generation high temperature superconductors.

Proximity and Josephson effects in microstructures based on multiband superconductors

Abstract: Emerging in the 1950s, the multiband superconductivity has been considered for a long time as an approximate model in the form of a generalization of the BCS theory to the case of two bands for a more accurate quantitative description of the properties and characteristics of such superconductors as cuprates, heavy fermions compounds, metal boron carbides, fullerides, strontium ruthenate etc. due to their complex piecewise-continuous Fermi surfaces. However the discovery of the multiband structure of the superconducting state in magnesium diboride in 2001 and iron oxypnictides and chalcogenides in 2008 led to the appearance of many papers in which effects and different dependences well known for conventional single-band s-wave superconductors were reexamined. The main purpose of these studies was to reveal the symmetry type of the order parameter, which provides an important information about the mechanism of Cooper pairing in these superconductors. One of the most effective methods of obtaining information on the symmetry properties of the order parameter in the multiband superconductors is phase-sensitive techniques. This review summarizes the results of theoretical and experimental studies of the proximity and Josephson effects in systems based on multiband superconductors in contact with normal metals, insulators and other superconductors.

High-Temperature Superconductors

Abstract: The discovery by J. G. Bednorz and K. A. Muller in 1986 that the superconducting state can exist in oxides at temperatures above 30 K stimulated research in the field of superconductivity and opened a new field of research. Within a few years a large number of cuprate superconductors with transition temperatures well above the boiling point of liquid nitrogen have been found. In this chapter, an overview of the major families of high-temperature superconductors and their physical properties is presented.

A possible new family of unconventional high temperature superconductors

Abstract: We suggest a new family of Co/Ni-based materials that may host unconventional high temperature superconductivity (high- Tc T c ). These materials carry layered square lattices with each layer being formed by vertex-shared transition metal tetrahedra cation-anion complexes. The electronic physics in these materials is determined by the two dimensional layer and is fully attributed to the three near degenerated t2g t 2 g d -orbitals close to a d7 d 7 filling configuration in the d -shell of Co/Ni atoms. The electronic structure meets the necessary criteria for unconventional high Tc T c materials proposed recently by us to unify the two known high- Tc T c families, cuprates and iron-based superconductors. We predict that they host superconducting states with a d -wave pairing symmetry with Tc T c potentially higher than those of iron-based superconductors. These materials, if realized, can be a fertile new ground to study strongly correlated electronic physics and provide decisive evidence for superconducting pairing mechanism.

Full Magnetization of Bulk (RE)Ba2Cu3O7-δ Magnets With Various Rare-Earth Elements Using Pulsed Fields at 77 K

Abstract: REBa 2 Cu 3 O 7-δ bulk superconductors are able to trap large magnetic fields and as a result they have potential to work as strong magnets. In this study, highly textured yttrium, gadolinium, and samarium based cuprate bulk superconductors were prepared using conventional top-seeded melt-textured growth, and magnetized by employing both field-cooling magnetization (FCM) and pulsed-field magnetization (PFM) techniques at 77 K. The same peak value of a trapped field was achieved using FCM and PFM, which means all the bulk samples were fully magnetized by using the PFM method. In addition, the magnitude of the pulsed field required to fully magnetize the bulk samples varied with rare-earth element in the (RE)BCO formulation.

A theoretical quest for high temperature superconductivity on the example of low-dimensional carbon structures.

Abstract: High temperature superconductivity does not necessarily require correlated electron systems with complex competing or coexisting orders. Instead, it may be achieved in a phonon-mediated classical superconductor having a high Debye temperature and large electronic density of states at the Fermi level in a material with light atoms and strong covalent bonds. Quasi-1D conductors seem promising due to the Van Hove singularities in their electronic density of states. In this sense, quasi-1D carbon structures are good candidates. In thin carbon nanotubes, superconductivity at ~15 K has been reported, and it is likely the strong curvature of the graphene sheet which enhances the electron-phonon coupling. We use an ab-initio approach to optimize superconducting quasi-1D carbon structures. We start by calculating a T c of 13.9 K for (4.2) carbon nanotubes (CNT) that agrees well with experiments. Then we reduce the CNT to a ring, open the ring to form chains, optimize bond length and kink structure, and finally form a new type of carbon ring that reaches a T c value of 115 K.

Two-year progress in experimental investigation on high-temperature superconductivity of sulfur hydride

Abstract: The cooperation between theoretical and experimental investigations broke the record for the superconducting critical temperature T c in hydrogen sulfide at a high pressure at the end of 2014. Surprisingly, the material improved its highest T c by more than 30 K and showed conventional superconductivity, which can be explained by the Bardeen–Cooper–Schrieffer theory. Recent experimental works have gradually clarified the instability of the H2S molecule and the pathway to the high-T c phase with a three-dimensional conductive structure unlike high-T c superconductors thus far. In this review, the present progress on a sulfur hydride system is reported.

Metal-insulator transition in copper oxides induced by apex displacements

Abstract: High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole-doped, such as {La$_{2}$CuO$_4$} (LCO) with Sr (LSCO), or electron doped, such as {Nd$_{2}$CuO$_4$} (NCO) with Ce (NCCO). In the electron doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygens dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first principles method to explore how displacement of the apical oxygen (A-O) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small displacements of the apical oxygens drive the compound to a metallic state through a localization/delocalization transition, with a concomitant maximum $d$-wave order parameter at the transition. We address the question whether NCO can be seen as the limit of LCO with large apical displacements, and elucidate the deep physical reasons why the behaviour of NCO is so different than the hole doped materials. We shed new light on the recent correlation observed between T$_c$ and the charge transfer gap, while also providing a guide towards the design of optimized high-Tc superconductors. Further our results suggest that strong correlation, enough to induce Mott gap, may not be a prerequisite for high-Tc superconductivity.

High Temperature Superconductivity in Strongly Correlated Electronic Systems

Abstract: In this paper we give a selective review of our work on the role of electron correlation in the theory of high temperature superconductivity. The question of how electronic repulsions might give rise to off-diagonal long range order (ODLRO) in high temperature superconductors is currently one of the key questions in the theory of condensed matter. This paper argues that the key to understanding the occurrence of high temperature superconductivity (HTSC) in cuprates is to be found in the Bohm-Pines Hamiltonian modified to include a polarisable dielectric background. The approach uses reduced electronic density matrices and discusses how these can be used to understand whether ODLRO giving rise to superconductivity might arise from a Bohm-Pines type potential which is comprised of a weak long-range attractive tail and a much stronger short-range repulsive Coulomb interaction. This allows time-reversed electron pairs to undergo a superconducting condensation on alternant Cuprate lattices. Thus, a detailed summary is given of the arguments that such interacting electrons can cooperate to produce a superconducting state in which time-reversed pairs of electrons effectively avoid the repulsive hard-core of the inter-electronic Coulomb interaction but reside on average in the attractive well of the effective potential. In a superconductor the plasma wave function becomes the longitudinal component of a massive photon by the Anderson-Higgs mechanism. The alternant cuprate lattice structure is the key to achieving HTSC in cuprates with dx2-y2 symmetry condensate symmetry.

Why only hole conductors can be superconductors

Abstract: 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 Tc superconductors.