Showing papers on "High-temperature superconductivity published in 2009"
TL;DR: Comparisons of the normal state and superconducting properties of the two systems reveal the possibility that the emergent electronic state qualitatively depends on the manner in which the CDW state is destabilized, making this a unique example where two different superconductor domes are obtained by two different methods from the same parent compound.
Abstract: The interplay between superconductivity and the charge-density wave (CDW) state in pure 1T-TiSe(2) is examined through a high-pressure study extending up to pressures of 10 GPa between sub-Kelvin and room temperatures At a critical pressure of 2 GPa a superconducting phase sets in and persists up to pressures of 4 GPa The maximum superconducting transition temperature is 18 K These findings complement the recent discovery of superconductivity in copper-intercalated 1T-TiSe(2) The comparisons of the normal state and superconducting properties of the two systems reveal the possibility that the emergent electronic state qualitatively depends on the manner in which the CDW state is destabilized, making this a unique example where two different superconducting domes are obtained by two different methods from the same parent compound
342 citations
TL;DR: In this article, the authors performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2.
Abstract: We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1−xCox)2As2. Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe1−xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field Hc2 both parallel and perpendicular to the c-axis, the irreversibility field Hirrc(T), and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ=Hc2ab/Hc2c deduced from the slopes of dHc2ab/dT=4.9 T/K and dHc2c/dT=2.5 T/K decreases from ∼2 near Tc, to ∼1.5 at lower temperatures, much smaller than γ for 1111pnictides and high-Tc cuprates.
333 citations
TL;DR: In this article, the upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density are compared across the Fe-based superconductor families.
Abstract: Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length, and unconventional pairing, On the other hand the Fe-based superconductors have metallic parent compounds, and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, the supposed order parameter symmetry is s wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviours and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates, while the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.
248 citations
TL;DR: A method for layer-by-layer synthesis of alternating oxides of metal and insulators based on La and Cu is described, which selectively doped layers with isovalent Zn atoms to show that this interface HTS occurs within a single CuO2 plane.
Abstract: The question of how thin cuprate layers can be while still retaining high-temperature superconductivity (HTS) has been challenging to address, in part because experimental studies require the synthesis of near-perfect ultrathin HTS layers and ways to profile the superconducting properties such as the critical temperature and the superfluid density across interfaces with atomic resolution. We used atomic-layer molecular beam epitaxy to synthesize bilayers of a cuprate metal (La1.65Sr0.45CuO4) and a cuprate insulator (La2CuO4) in which each layer is just three unit cells thick. We selectively doped layers with isovalent Zn atoms, which suppress superconductivity and act as markers, to show that this interface HTS occurs within a single CuO2 plane. This approach may also be useful in fabricating HTS devices.
215 citations
TL;DR: In this paper, it was shown that thermal melting of the striped superconducting state can lead to a number of unusual phases, of which the most novel is a charge-4e superconding state, with a corresponding fractional flux quantum h c = 4e.
Abstract: A number of spectacular experimental anomalies have been discovered recently in certain cuprates, notably La2−xBaxCuO4 and La1.6−xNd0.4SrxCuO4, which show unidirectional spin and charge order (known as stripe order). We have recently proposed to interpret these observations as evidence for a new ‘striped’ superconducting state, in which the superconducting order parameter is modulated in space, such that its average is precisely zero. Here, we show that thermal melting of the striped superconducting state can lead to a number of unusual phases, of which the most novel is a charge-4e superconducting state, with a corresponding fractional flux quantum h c=4e. These are never-before-observed states of matter, which, moreover, cannot arise from the conventional Bardeen–Cooper–Schrieffer mechanism. Thus, direct confirmation of their existence, even in a small subset of the cuprates, could have much broader implications for our understanding of high-temperature superconductivity. We propose experiments to observe fractional flux quantization, which could confirm the existence of these states. In a ‘striped’ superconductor, it may be possible to observe a superconducting state that, with increasing temperature, melts into a unique phase with charge-4e superconductivity, instead of the usual charge of 2e from paired electronic excitations.
154 citations
TL;DR: A comprehensive survey of the cuprate, heavy-fermion and iron-based superconductors shows a universal linear relationship between their magnetic resonance energy and superconducting gap.
Abstract: A comprehensive survey of the cuprate, heavy-fermion and iron-based superconductors shows a universal linear relationship between their magnetic resonance energy and superconducting gap. This result suggests that antiferromagnetic fluctuations might have a similar role in the unconventional superconductivity of these seemingly different classes of materials. Superconductivity involves the formation of electron pairs (Cooper pairs) and their condensation into a macroscopic quantum state. In conventional superconductors, such as Nb3Ge and elemental Hg, weakly interacting electrons pair through the electron–phonon interaction. In contrast, unconventional superconductivity occurs in correlated-electron materials in which electronic interactions are significant and the pairing mechanism may not be phononic. In the cuprates, the superconductivity arises on doping charge carriers into the copper–oxygen layers of antiferromagnetic Mott insulators1. Other examples of unconventional superconductors are the heavy-fermion compounds, which are metals with coupled conduction and localized f-shell electrons2, and the recently discovered iron–arsenide superconductors3. These unconventional superconductors show a magnetic resonance, a prominent collective spin-1 excitation mode in the superconducting state4,5,6,7,8. Here we demonstrate the existence of a universal linear relation, Er∝2Δ, between the magnetic resonance energy (Er) and the superconducting pairing gap (Δ), which spans two orders of magnitude in energy. This relationship is valid for the three different classes of unconventional superconductors, which range from being close to the Mott-insulating limit to being on the border of itinerant magnetism. As the common excitonic picture of the resonance has not led to such universality, our observation suggests a much deeper connection between antiferromagnetic fluctuations and unconventional superconductivity.
122 citations
TL;DR: In this paper, the structural and superconducting properties of β-FeSe under pressure up to 26 GPa were studied using synchrotron radiation and diamond anvil cells.
Abstract: We have studied the structural and superconducting properties of β–FeSe under pressures up to 26 GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3) GPa, much smaller than the rest of Fe based superconductors. At 12 GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high-pressure orthorhombic phase has a higher Tc reaching 34 K at 22 GPa.
118 citations
TL;DR: In this article, the electrical resistivity and its behavior under different magnetic fields were measured and it was shown that magnetic field leads the resistivity curve to spread out below transition region, and Tc (ρ = 0) shifts to lower temperatures.
Abstract: In search of finding the dominant mechanism in high temperature superconductivity phenomena, the Y3Ba5Cu8O18 compound was synthesized through the standard solid-state reaction technique. Characteristic XRD experiment was performed on the samples and was analyzed by the MAUD software refinement program. The analysis results indicate a 358 phase structure with the initial nominal stoichiometry. The electrical resistivity and its behavior under different magnetic field were measured. The electrical resistivity indicates the transition temperature T c onset = 102 K with transition width ΔTc = 2.4 K. This is the first observation of such a high transition temperature in the Y-based compound. Application of magnetic field leads the resistivity curve to spread out below transition region, and Tc (ρ = 0) shifts to lower temperatures. Also, a small broadening is observed by the application of high magnetic field in the T c onset region.
107 citations
TL;DR: In this paper, the structural and superconducting properties of tetragonal FeSe under pressure up to 26GPa were studied using synchrotron radiation and diamond anvil cells.
Abstract: We have studied the structural and superconducting properties of tetragonal FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller than the rest of Fe based superconductors. At 12GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high pressure orthorhombic phase has a higher Tc reaching 34K at 22GPa.
104 citations
TL;DR: In this paper, an alternative to conventional BCS theory is proposed to apply to all superconductors, 'conventional' as well as 'unconventional', that offers an explanation for the Meissner effect as well well as for other puzzles and provides clear guidelines in the search for new high temperature supercondors.
Abstract: The time-tested Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity is generally accepted to be the correct theory of conventional superconductivity by physicists and, by extension, by the world at large. There are, however, an increasing number of 'red flags' that strongly suggest the possibility that BCS theory may be fundamentally flawed. An ever-growing number of superconductors are being classified as 'unconventional', not described by the conventional BCS theory and each requiring a different physical mechanism. In addition, I argue that BCS theory is unable to explain the Meissner effect, the most fundamental property of superconductors. There are several other phenomena in superconductors for which BCS theory provides no explanation. Furthermore, BCS theory has proven unable to predict any new superconducting compounds. This paper suggests the possibility that BCS theory itself as the theory of 'conventional' superconductivity may require a fundamental overhaul. I outline an alternative to conventional BCS theory proposed to apply to all superconductors, 'conventional' as well as 'unconventional', that offers an explanation for the Meissner effect as well as for other puzzles and provides clear guidelines in the search for new high temperature superconductors.
88 citations
TL;DR: A neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa2Cu3O6.45 in magnetic fields up to 15 T induces a spectral-weight shift in the magnetic-excitation spectrum and may be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.
Abstract: We present a neutron-scattering study of the static and dynamic spin correlations in the underdoped high-temperature superconductor YBa2Cu3O6.45 in magnetic fields up to 15 T. The field strongly enhances static incommensurate magnetic order at low temperatures and induces a spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of the Fermi surface driven by the field-enhanced magnetic superstructure may thus be responsible for the unusual Fermi surface topology revealed by recent quantum-oscillation experiments.
TL;DR: In this article, the concept of local pairs in systems with low and intermediate charge-carrier density, which can include high-temperature superconductors (HTSCs), is examined.
Abstract: A considerable part of the theoretical and experimental works reflecting the current status of research on high-temperature superconductivity and the unusual phenomenon of a pseudogap in high-temperature superconductors (HTSCs) is reviewed. The concept of local pairs in systems with low and intermediate charge-carrier density, which can include HTSCs, is examined. The experimental part is primarily based on the study of excess and fluctuation conductivity (FC) in YBa2Cu3O7−y (TBCO) and Y1−xPrxBa2Cu3O7−y (YPrBCO) thin epitaxial films. A new approach to analyzing FC and the pseudogap in such high-temperature systems is proposed and checked experimentally. The approach is based on the idea that excess conductivity σ′(T) forms in HTSCs at temperatures substantially above critical temperature Tc as a result of the formation of pair states in the form of noninteracting strongly bound bosons, demonstrating with decreasing temperature a transition from a regime with localized pairs conforming to the Bose–Einstein...
Journal Article•
TL;DR: In this paper, the electronic structure of electron doped iron-arsenide superconductors Ba(Fe{sub 1-x}Co{sub x}){sub 2]As{sub 2} has been measured with Angle Resolved Photoemission Spectroscopy.
Abstract: The electronic structure of electron doped iron-arsenide superconductors Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} has been measured with Angle Resolved Photoemission Spectroscopy. The data reveal a marked photon energy dependence of points in momentum space where the bands cross the Fermi energy, a distinctive and direct signature of three-dimensionality in the Fermi surface topology. By providing a unique example of high temperature superconductivity hosted in layered compounds with three-dimensional electronic structure, these findings suggest that the iron-arsenides are unique materials, quite different from the cuprates high temperature superconductors.
TL;DR: In this article, it was shown that the inclusion of ferromagnetic particles or rods offers a practical means of enhancing the critical currents in oxide high temperature superconductors, and that a magnetic inclusion can also reduce the Lorentz force on a vortex, yielding a substantially enhanced critical current density.
Abstract: Ferromagnetic pinning centres in superconductors form much deeper potential wells than equivalent insulating or metallic non-superconducting inclusions. However, the resultant pinning forces arising from magnetic inclusions are low because the magnetic interaction takes place over the length scale of the magnetic penetration depth which is large in technological superconductors. Nonetheless, we show that a magnetic inclusion can also reduce the Lorentz force on a vortex, yielding a substantially enhanced critical current density for a given pinning force. We calculate this enhancement for a single vortex pinned by a paramagnetic cylinder as well as a vortex lattice interacting with magnetic inclusions, and find that the inclusion of ferromagnetic particles or rods offers a practical means of enhancing the critical currents in oxide high temperature superconductors.
TL;DR: This work finds that key features of the T=0 phase diagram, such as critical doping for SC-AFM coexistence and the maximum value of the SC order parameter, are determined by a single parameter eta which characterizes the topology of the "Fermi surface" at half filling defined by the bare tight-binding parameters.
Abstract: We investigate the asymmetry between electron and hole doping in a 2D Mott insulator and the resulting competition between antiferromagnetism (AFM) and d-wave superconductivity (SC), using variational Monte Carlo calculations for projected wave functions. We find that key features of the T=0 phase diagram, such as critical doping for SC-AFM coexistence and the maximum value of the SC order parameter, are determined by a single parameter eta which characterizes the topology of the "Fermi surface" at half filling defined by the bare tight-binding parameters. Our results give insight into why AFM wins for electron doping, while SC is dominant on the hole-doped side. We also suggest using band structure engineering to control the eta parameter for enhancing SC.
TL;DR: The discovery of iron-based pnictide superconductors may have reinvigorated the field of high-temperature superconductivity, but the cuprate supercondors are still in the game.
Abstract: The discovery of iron-based pnictide superconductors may have reinvigorated the field of high-temperature superconductivity, but the cuprate superconductors are still in the game.
TL;DR: In this article, the authors reported the growth of high-quality c-axis-oriented cobalt (Co)-doped SrFe2As2 thin films with bulk superconductivity using an in situ pulsed laser deposition technique with a 248 nm wavelength KrF excimer laser and an arsenic (As)-rich phase target.
Abstract: The remarkably high superconducting transition temperature and upper critical field of iron (Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented cobalt (Co)-doped SrFe2As2 thin films with bulk superconductivity by using an in situ pulsed laser deposition technique with a 248 nm wavelength KrF excimer laser and an arsenic (As)-rich phase target. The temperature and field dependences of the magnetization showing strong diamagnetism and transport critical current density with superior Jc-H performance are reported. These results provide necessary information for practical applications of Fe-based superconductors.
TL;DR: In this article, the authors presented a model based on superconductivity first principles, accurate and systematic calculations for the susceptibility of a dc metamaterial consisting of parallel square superconducting thin plates.
Abstract: Cloaking a static magnetic field is becoming a real possibility after the recently proposed concept of dc metamaterials. Here, we present, by a model based on superconductivity first principles, accurate and systematic calculations for the susceptibility of a dc metamaterial consisting of parallel square superconducting thin plates. Since both magnetic and kinetic energies are considered, our model takes into account the effect of a penetration depth in the superconductors and therefore can be applied for high-temperature superconductors. Thus, from the calculated results, a device that can cloak a static magnetic field may be constructed based on high-temperature superconducting thin films.
TL;DR: In this paper, two new classes of superconductors have been discovered in the course of material exploration for electronic-active oxides, one is 12CaO·7Al 2 O 3 crystal in which electrons accomodate in the crystallographic sub-nanometer-sized cavities and the other is iron oxypnicitides with a layered structure.
Abstract: We discovered two new classes of superconductors in the course of material exploration for electronic-active oxides. One is 12CaO · 7Al 2 O 3 crystal in which electrons accomodate in the crystallographic sub-nanometer-sized cavities. This material exhibiting metal–superconductor transition at 0.2 K is the first electride superconductor. The other is iron oxypnicitides with a layered structure. This superconductor is rather different from high T c cuprates in several respects. The high T c is emerged by doping carriers to the metallic parent phases which undergo crystallographic transition (tetra to ortho) and Pauli para to antiferromagnetic transition at ∼150 K. The T c is robust to impurity doping to the Fe sites or is induced by partial substitution of the Fe 2+ sites with Co 2+ or Ni 2+ . This article gives a brief summary of these discoveries and recent advances.
TL;DR: In this paper, the effect of partial substitution of Sc3+ by Ti4+ in Sr2ScFeAsO3, Ca2ScEasO3 and Sr3Sc2Fe2As2O5 on their electrical properties was investigated.
Abstract: We report a systematic study on the effect of partial substitution of Sc3+ by Ti4+ in Sr2ScFeAsO3, Ca2ScFeAsO3 and Sr3Sc2Fe2As2O5 on their electrical properties. High levels of doping result in an increased carrier concentration and lead to the appearance of superconductivity with the onset of Tc up to 45 K.
TL;DR: It is found that gamma_{sc} has a humplike anomaly at T_{c} and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near T_{ c}.
Abstract: We have measured the magnetic field and temperature dependence of specific heat on Bi2Sr2� xLaxCuO6þ� single crystals in wide doping and temperature regions. The superconductivity related specific-heat coefficientsc and entropy Ssc are determined. It is found thatsc has a humplike anomaly at Tc and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near Tc. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only when the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above Tc.
TL;DR: In this paper, the role of the electron-phonon coupling in high-temperature superconductivity compounds is discussed, with emphasis on its implications for angle-resolved photoemission and optical conductivity.
Abstract: Results indicating the important role of the electron–phonon coupling in high-temperature superconductivity compounds are presented, with emphasis on its implications for angle-resolved photoemission and optical conductivity. The dimensionless phonon coupling constant λ is determined by comparing the experimental and theoretical results. Although undoped materials are in the strong-coupling (λ ~ 1) regime, hole doping decreases λ, bringing compounds to the intermediate-coupling regime at optimum hole concentrations.
TL;DR: In this paper, the transition temperature of Tl0.6Pb 0.6⋅ Pb0.4Ba1.4Sr 0.5Sr0.5Ca2Cu3O8.76.
Abstract: High temperature superconductors, namely Tl0.6⋅ Pb0.4Ba2−xSrxCa2Cu3O9−δ, have successfully been prepared by the solid state reaction method. The substitutions for this compound were taken as x=0.0, 0.10, 0.20, 0.30, 0.40 and 0.50. The optimum calcination was at 1073 K and for sintering was within 1128–1133 K. Electrical resistivity, using the four-probe technique, was used to find the transition temperature Tc. The highest Tc(offset) was 113 K for Tl0.6Pb0.4Ba1.5Sr0.5Ca2Cu3O8.76. All samples were prepared by using O2 flow. X-ray diffraction (XRD) analysis have shown a pseudotetragonal structure with an increase of the c-axis lattice constant for the samples which were doped with Sr as compared to those which have no Sr content.
TL;DR: In this paper, a detailed review of scaling behavior in the magnetically underdoped cuprate superconductors (hole dopings less than 0.20) is presented, and it reflects the presence of two coupled components throughout this doping regime: a non-Landau Fermi liquid and a spin liquid whose behaviour maps onto the theoretical Monte Carlo calculations of localized Cu spins for most of its temperature domain.
Abstract: We present a detailed review of scaling behaviour in the magnetically underdoped cuprate superconductors (hole dopings less than 0.20) and show that it reflects the presence of two coupled components throughout this doping regime: a non-Landau Fermi liquid and a spin liquid whose behaviour maps onto the theoretical Monte Carlo calculations of the two-dimensional Heisenberg model of localized Cu spins for most of its temperature domain. We use this mapping to extract the doping dependence of the strength, f(x) of the spin liquid component and the effective interaction, J eff(x) between the remnant localized spins that compose it; we find that both decrease linearly with x as the doping level increases. We discuss the physical origin of pseudogap behaviour and conclude that it is consistent with scenarios in which the both the large energy gaps found in the normal state and their subsequent superconductivity are brought about by the coupling between the Fermi liquid quasiparticles and the spin liquid excita...
TL;DR: In this article, the upper Hubbard band and the Zhang-Rice singlet band are two essential components in describing low-energy excitations of electron-doped cuprate superconductors.
Abstract: We propose that the upper Hubbard band (electronlike) and the Zhang-Rice singlet band (holelike) are two essential components in describing low-energy excitations of electron-doped cuprate superconductors. We find that the gap between these two bands is significantly smaller than the charge-transfer gap measured by optics and is further reduced upon doping. This indicates that the charge fluctuation is strong and the system is in the intermediate correlation regime. A two-band model is derived. In the limit that the intraband and interband hopping integrals are equal to each other, this model is equivalent to the unconstrained t-J model with on-site Coulomb repulsions.
TL;DR: In this article, the authors investigated the zero-temperature doping-driven evolution of a superconductor towards the Mott insulator in a two-dimensional electron model, relevant for high temperature superconductivity.
Abstract: High-temperature superconductors at zero doping can be considered strongly correlated two-dimensional Mott insulators. The understanding of the connection between the superconductor and the Mott insulator hits at the heart of the high-temperature superconducting mechanism. In this paper we investigate the zero-temperature doping-driven evolution of a superconductor towards the Mott insulator in a two dimensional electron model, relevant for high temperature superconductivity. To this purpose we use a cluster extension of dynamical mean field theory. Our results show that a standard (BCS) d-wave superconductor, realized at high doping, is driven into the Mott insulator via an intermediate state displaying non-standard physical properties. By restoring the translational invariance of the lattice, we give an interpretation of these findings in momentum space. In particular, we show that at a finite doping a strong momentum-space differentiation takes place: non-Fermi liquid and insulating-like (pseudogap) character rises in some regions (anti-nodes), while Fermi liquid quasiparticles survive in other regions (nodes) of momentum space. We describe the consequence of these happenings on the spectral properties, stressing in particular the behavior of the superconducting gap, which reveals two distinct nodal and antinodal energy scales as a function of doping. We propose a description of the evolution of the electronic structure while approaching the Mott transition and compare our results with tunneling experiments, photoemission and magnetotransport on cuprate materials.
TL;DR: In this article, the same authors reported the observation of the Josephson effect in bicrystal junctions along the c-axis between electron-doped e-drone and hole-dope h-doping pnictide single crystals.
Abstract: SrFe1.74Co0.26As2 and Ba0.23K0.77Fe2As2. The ac Josephson effect was observed under microwave irradiation in the I-V characteristics. By applying an in-plane magnetic field, the critical current is completely modulated and shows a relatively symmetric diffraction pattern consistent with the intermediate junction limit. The observation of the Josephson effect in the p-n bicrystal structure not only has significant implications for designing phase-sensitive junctions to probe the pairing symmetry of iron pnictide superconductors, but also represents an important step in developing all iron pnictide devices for applications. © 2009 American Institute of Physics. DOI: 10.1063/1.3205123 Previously, we have demonstrated Josephson coupling between a single crystal pnictide superconductor Ba1xKxFe2As2 and a conventional superconductor along the c-axis, which indicated the presence of an s-wave order parameter in the iron pnictide superconductor. 1 In this letter, we report the observation of the Josephson effect in bicrystal junctions along the c-axis between electron-doped e-doped and hole-doped h-doped pnictide single crystals. The critical currents are completely modulated by applying an inplane external magnetic field. The obtained magnetic diffraction patterns are Fraunhofer-like with well defined modulation periods. Fabrication of such p-n junctions is motivated by a number of reasons: 1 There have been a number of proposals 2‐4 of definitive phase-sensitive tests for the widely discussed s-wave symmetry 5‐8 in pnictide superconductors which involve transport across pnictide p-n interfaces; 2 as in the case of high-TC cuprates, 9 the presence of both electron and hole doping in the pnictides raises an intriguing question about the possibility of forming rectifying p-n junctions; 3 such junctions represent an important step toward making all pnictide Josephson devices. The e-doped SrFe1.74Co0.26As2 and the h-doped Ba0.23K0.77Fe2As2 iron pnictide single crystals used in this study were grown in FeAs flux 10 and Sn flux, 11 respectively. The single crystals were in platelet shapes with the normal direction along the c-axis. Wavelength dispersive x-ray spectroscopy and energy dispersive x-ray spectroscopy were used to determine the doping concentrations in the e-doped and the h-doped single crystals. Magnetic susceptibility measurements confirmed bulk superconductivity in both types of single crystals. Resistivity measurements Fig. 1a showed that the superconducting transitions were very sharp and the TCs were approximately 17 K for SrFe1.74Co0.26As2 and 22 K for Ba0.23K0.77Fe2As2. The Hall effect measurements Fig. 1b showed that the Hall coefficient of each type of crystal had a clear temperature dependence with no sign change. The negative positive sign of the Hall coefficient of SrFe1.74Co0.26As2 Ba0.23K0.77Fe2As2 single crystals confirms that the transport is dominated by electrons holes, which is consistent with the dopant type of the compound and in qualitative agreement with previous reports. 12,13 In both types of single crystals, all Hall resistivity curves display a strong linear dependence in magnetic fields up to 7 T. As an example, Fig. 1c shows the Hall resistivity curves obtained at 35 K. Scanning electron microscopy of the crystals indicated that their surfaces consisted of large terrace-free areas up to mm 2 in size. 1 Crystals were cut into rectangular pieces with a typical size of approximately 1500300 m 2 along a or b
TL;DR: In this article, a magnetic field concentrator using high T c bulk superconductor (HTS bulk) instead of ferromagnetic materials such as Ho and Dy was proposed.
Abstract: Magnetic flux concentration is a very important technique for the effective generation of high magnetic fields. We propose the use of the diamagnetism of a high T c bulk superconductor (HTS bulk) instead of ferromagnetic materials such as Ho and Dy. We fabricated a magnetic field concentrator using Gd-Ba-Cu-O bulk. The essential point was slits to suppress the current along the circumference. The concentrator was cooled at the center of a superconducting magnet with liquid helium. By increasing the external field to 1.00 T, a magnetic flux density of 3.20 T was obtained at the center of the concentrator. At an external field of 2.00 T, a field of 5.65 T was also obtained. A magnetic lens using HTS bulk was successfully demonstrated. The experimental results were well explained by numerical analyses assuming HTS bulk as a perfect conductor.
TL;DR: In this paper, a simplified analytical model based on a resonating valence bond spin liquid was proposed to understand the optical properties of underdoped cuprates in both the normal and superconducting states.
Abstract: The finite-frequency optical properties of the underdoped cuprates, in both the normal and superconducting state, display features which go beyond a Fermi liquid and a BCS description. We provide an understanding of these properties within a simplified analytical model, which has been evolved out of the Hubbard model and ideas based on a resonating valence bond spin liquid. We find that: 1) in underdoped samples, the missing area integrals reveal a second energy scale due to the pseudogap, not present at optimum or overdoping; 2) the real part of the optical self-energy shows a large sharp peak, that emerges with the opening of the pseudogap which exists within the superconducting state and persists in the normal state; and 3) the amount of optical spectral weight which is transferred to the condensate is greatly reduced by the presence of the pseudogap as compared to the Fermi liquid case. These non-BCS features of the superconducting state are in good qualitative agreement with a body of experimental work on different cuprate systems and provide strong evidence from optical conductivity that they are all a manifestation of the pseudogap energy scale.
TL;DR: In this paper, the authors used the powder in tube method to construct high temperature superconductor tapes with starting compositions (Bi,Pb)2Sr2Ca2Cu3O10-(Fe3O4)0.01, 0.03, and 0.05).
Abstract: The (Bi,Pb)2Sr2Ca2Cu3O10-(Fe3O4)x superconductor for x=0, 0.01, 0.03, and 0.05 were prepared. The average size of Fe3O4 used was 40 nm. The maximum critical current density (Jc) in the bulk form was observed in the x=0.01 sample. Based on this result, Ag sheathed high temperature superconductor tapes with starting compositions (Bi,Pb)2Sr2Ca2Cu3O10-(Fe3O4)0.01 were fabricated using the powder in tube method. The microstructure, phase formation, critical temperature, and transport critical current density were studied. The Jc of the nanosized Fe3O4 added tapes is 5130 A/cm2 at 77 K and 23130 A/cm2 at 30 K in zero fields. The nonadded tapes showed a lower Jc, 3090 A/cm2 at 77 K and 12 400 A/cm2 at 30 K. A sudden decrease in Jc in low magnetic fields (B<0.10 T) when applied parallel and perpendicular to the tapes surface was observed. By adding magnetic nanoparticles, the full vortex magnetic energy can be used to enhance Jc and thus, magnetic nanoparticles such as Fe3O4 can act as an effective flux pinning c...