Showing papers on "High-temperature superconductivity published in 2012"

Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO3

Abstract: We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Se-etched SrTiO3 (001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.

Phase Diagram and High Temperature Superconductivity at 65 K in Tuning Carrier Concentration of Single-Layer FeSe Films

Abstract: Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cu- and Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at ~65 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications.

Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor

Abstract: The exact mechanism for superconductivity in iron-based superconductors remains elusive, but is thought to involve complex interactions between many orbitals. Using angle-resolved photoelectron spectroscopy, Liu et al. report the electronic structure of the single-layer parent compound FeSe.

Magnetism and its microscopic origin in iron-based high-temperature superconductors

Abstract: The magnetic states found in iron-based superconductors are more complex than originally thought. This Review argues that the magnetism arises from both itinerant and localized electrons.

Electronic origin of high-temperature superconductivity in single-layer FeSe

Abstract: The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T(c)) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T(c) superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T(c)~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T(c) superconductivity in iron-based superconductors.

Enhancement of superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

Abstract: The recent discovery of high temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer that is inserted between adjacent anionic iron arsenide layers. Until now, superconductivity has only been found in compounds with a cationic spacer layer consisting of metal ions: Li+, Na+, K+, Ba2+ or a PbO-type or perovskite-type oxide layer. Electronic doping is usually necessary to control the fine balance between antiferromagnetism and superconductivity. Superconductivity has also been reported in FeSe, which contains neutral layers similar in structure to those found in the iron arsenides but without the spacer layer. Here we demonstrate the synthesis of Lix(NH2)y(NH3)1-yFe2Se2 (x ~0.6 ; y ~ 0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far and four times higher at ambient pressure than the transition temperature, Tc, of the parent Fe1.01Se. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems in order to greatly optimize the superconducting properties in this family.

Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in high-temperature superconductors

Abstract: It is well known that to reduce dissipation in a superconductor it is necessary to introduce artificial pinning centres, that is, small regions in which superconductivity is suppressed. This is usually achieved by introducing small regions of non-superconducting phases. A new concept of pinning centres, the local suppression of superconductivity induced by strain, is now demonstrated.

Non-centrosymmetric superconductors : introduction and overview

Abstract: Non-Centrosymmetric Superconductors: Strong vs. Weak Electronic Correlations.- Non-Centrosymmetric Heavy-Fermion Superconductors.- Properties of Interfaces and Surfaces.- Kinetic Theory for Response and Transport in Non-Centrosymmetric Superconductors.- Vortex Dynamics in Superconductors Without Inversion Symmetry.- Introduction to Superconductivity in Metals Without Inversion Center.- Effects of Impurities in Non-centrosymmetric Superconductors.- Electronic States and Superconducting Properties of Non-Centrosymmetric Rare Earth Compounds.- Microscopic Theory of Pairing Mechanisms.

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

Abstract: Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals which are crucial for spin and charge ordering and determine the mechanisms of electron–electron interactions, including superconducting pairing. The newly discovered iron-based superconductors (FeSC) promise interesting physics stemming, on one hand, from a coexistence of superconductivity and magnetism and, on the other, from a complex multi-band electronic structure. In this review I want to offer a simple introduction to the physics of FeSC, and to argue that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure has been determined on the basis of numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of magnetic ordering and superconducting pairing in FeSC.

Proximity-induced high-temperature superconductivity in the topological insulators Bi 2 Se 3 and Bi 2 Te 3

Abstract: Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion at the interface between topological insulators and superconductors. Here we produce high-temperature superconductivity in Bi(2)Se(3) and Bi(2)Te(3) via proximity to Bi(2)Sr(2)CaCu(2)O(8+δ), to access higher temperature and energy scales for this phenomenon. This was achieved by a new mechanical bonding technique that we developed, enabling the fabrication of high-quality junctions between materials, unobtainable by conventional approaches. We observe proximity-induced superconductivity in Bi(2)Se(3) and Bi(2)Te(3) persisting up to at least 80 K-a temperature an order of magnitude higher than any previous observations. Moreover, the induced superconducting gap in our devices reaches values of 10 mV, significantly enhancing the relevant energy scales. Our results open new directions for fundamental studies in condensed matter physics and enable a wide range of applications in spintronics and quantum computing.

No-Insulation (NI) HTS Inserts for $>$ 1 GHz LTS/HTS NMR Magnets

Abstract: A no-insulation (NI) technique has been applied to wind and test a NI HTS (YBCO) double-pancake coil at 4.2 K. Having little detrimental effect on field-current relationship, the absence of turn-to-turn insulation enabled the test coil to survive a quench at a coil current density of 1.58 kA/mm2. The NI HTS coil is compact and self-protecting, two features suitable for large high-field magnets. To investigate beneficial impacts of the NI technique on >;1 GHz LTS/HTS NMR magnets, we have designed six new NI HTS inserts for our ongoing 1.3 GHz LTS/HTS NMR magnet, which require less costly LTS background magnets than the original insulated HTS insert. A net result will be a significant reduction in the overall cost of an LTS/HTS NMR magnet, at 1.3 GHz and above.

High-temperature superconductivity at the FeSe/SrTiO 3 interface

Abstract: In several recent experiments the superconducting gap of a single-unit-cell-thick FeSe film on SrTiO${}_{3}$ substrate has been observed by scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. The value of the superconducting gap is about nine times larger than that of the bulk FeSe under ambient pressure, suggesting a much higher pairing energy scale and ${T}_{c}$ than all other iron-based superconductors and thus calling for a better understanding of its superconducting mechanism. In this paper we study the effects of screening due to the SrTiO${}_{3}$ ferroelectric phonons on Cooper pairing in FeSe. We conclude that it can significantly enhance the energy scale of Cooper pairing and even change the pairing symmetry. Our results also raise some concerns on whether phonons can be completely ignored for bulk iron-based superconductors.

Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors

Abstract: Cuprates, ferropnictides and ferrochalcogenides are three classes of unconventional high temperature superconductors, who share similar phase diagrams in which superconductivity develops after a magnetic order is suppressed, suggesting a strong interplay between superconductivity and magnetism, although the exact picture of this interplay remains elusive. Here we show that there is a direct bridge connecting antiferromagnetic exchange interactions determined in the parent compounds of these materials to the superconducting gap functions observed in the corresponding superconducting materials: in all high temperature superconductors, the Fermi surface topology matches the form factor of the pairing symmetry favored by local magnetic exchange interactions. We suggest that this match offers a principle guide to search for new high temperature superconductors.

Electronic and magnetic phase diagram in K(x)Fe(2-y)Se(2) superconductors.

Abstract: The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in high temperature superconductors. Superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator1 while in iron-pnictide superconductors arises from AFM semimetals and can coexist with AFM orders2,3,4,5,6,7,8,9. This difference raises many intriguing debates on the relation between the two classes of high temperature superconductors. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors AxFe2−ySe2 (A = K, Rb and Cs)10,11,12. They have the same structure as that of iron-pnictide 122-system13,14,15. Here, we report electronic and magnetic phase diagram of KxFe2−ySe2 system as a function of Fe valence. We find a superconducting phase sandwiched between two AFM insulating phases. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1 = (1/5, 3/5, 0) and q2 = (1/4, 3/4, 0), respectively.

Can Doping Graphite Trigger Room Temperature Superconductivity? Evidence for Granular High‐Temperature Superconductivity in Water‐Treated Graphite Powder

Abstract: Granular superconductivity in powders of small graphite grains (several tens of micrometers) is demonstrated after treatment with pure water. The temperature, magnetic field and time dependence of the magnetic moment of the treated graphite powder provides evidence for the existence of superconducting vortices with some similarities to high-temperature granular superconducting oxides but even at temperatures above 300 K. Room temperature superconductivity in doped graphite or at its interfaces appears to be possible.

Nature of magnetic excitations in superconducting BaFe1.9Ni0.1As2

Abstract: An outstanding question about the iron-based superconductors has been whether or not their magnetic characteristics are dominated by itinerant or localized magnetic moments. Absolute measurements and calculations of the magnetic response of undoped and Ni-doped BaFe2As2 indicate the latter.

Overview on the physics and materials of the new superconductor KxFe2−ySe2

Abstract: Since the discovery of high temperature superconductivity in iron pnictides in early 2008, many iron-based superconductors with different structures have been discovered, with the highest transition temperature to date being 57 K. By the end of 2010, another kind of new superconductor, the Fe-based chalcogenide K(1-x)Fe(2-y)Se(2) was discovered. A naive counting of the electrons in the system would lead to the conclusion that the system is heavily electron overdoped (~0.4 e/Fe). Band structure calculations further support this speculation and predict that the hole pockets which are found in the iron pnictides may be missing. This greatly challenges the widely perceived picture that the superconducting pairing is established by exchanging antiferromagnetic (AF) spin fluctuations and that the electrons are scattered between the electron and hole pockets. Later, it was found that both potassium and iron might be deficient in K(1-x)Fe(2-y)Se(2), yielding to a picture of phase separation. In this picture the superconducting phase and the AF phase may separate spatially into different regions. This generates further curiosity about what the real superconducting phase is, what the relationship is between the superconducting phase and the AF phase, and what the parent state is for the superconducting phase. We propose a 'spider web' model for the phase separation, which can explain both the transport and magnetic data. In this paper, we review the status of research in this rapidly growing field and list the important and unresolved issues as perspectives for future research.

High, magnetic field independent critical currents in (Ba,K)Fe{2}As{2} crystals

Abstract: Each discovery of a new high temperature superconductor drives the expectation that advanced engineering of materials defect structures will enable effective vortex pinning and high values of the electrical current density. Here, we demonstrate that single crystals of the iron-based superconductor Ba0.6K0.4Fe2As2 with Tc = 37.5 K can accommodate an unprecedented large concentration of strong-pinning defects in the form of discontinuous nm-sized nanorods with no degradation of the superconducting transition temperature. At a temperature of 5 K, we find a critical current density of 5 MA/cm2 that is magnetic field independent in fields up to 7 T.

Local antiferromagnetic exchange and collaborative Fermi surface as key ingredients of high temperature superconductors

Abstract: Cuprates, ferropnictides and ferrochalcogenides are three classes of unconventional high temperature superconductors, who share similar phase diagrams in which superconductivity develops after a magnetic order is suppressed, suggesting a strong interplay between superconductivity and magnetism, although the exact picture of this interplay remains elusive. Here we show that there is a direct bridge connecting antiferromagnetic exchange interactions determined in the parent compounds of these materials to the superconducting gap functions observed in the corresponding superconducting materials: in all high temperature superconductors, the Fermi surface topology matches the form factor of the pairing symmetry favored by local magnetic exchange interactions. We suggest that this match offers a principle guide to search for new high temperature superconductors.

Spectroscopic Imaging Scanning Tunneling Microscopy Studies of Electronic Structure in the Superconducting and Pseudogap Phases of Cuprate High-Tc Superconductors

Abstract: One of the key motivations for the development of atomically resolved spectroscopic imaging scanning tunneling microscopy (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the d -wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d -wave superconductor. These are detected below a lower energy scale | E |=Δ 0 and only upon a momentum space ( k -space) arc which terminates near the lines connecting k =±(π/ a 0 ,0) to k =±(0,π/ a 0 ). Below optimal doping, this “nodal” arc shrinks continuously with decreasing hole density. In both the dSC and PG phases, the only broken symmetries detected in the | E |≤Δ 0 states are those of a d -wave superconductor. The second class of states occurs at energies near the pseudogap energy scale | E |∼Δ 1 which is associated conventionally w...

From antiferromagnetic insulator to correlated metal in pressurized and doped LaMnPO

Abstract: Widespread adoption of superconducting technologies awaits the discovery of new materials with enhanced properties, especially higher superconducting transition temperatures T-c. The unexpected discovery of high T-c superconductivity in cuprates suggests that the highest T(c)s occur when pressure or doping transform the localized and moment-bearing electrons in antiferromagnetic insulators into itinerant carriers in a metal, where magnetism is preserved in the form of strong correlations. The absence of this transition in Fe-based superconductors may limit their T(c)s, but even larger T(c)s may be possible in their isostructural Mn analogs, which are antiferromagnetic insulators like the cuprates. It is generally believed that prohibitively large pressures would be required to suppress the effects of the strong Hund's rule coupling in these Mn-based compounds, collapsing the insulating gap and enabling superconductivity. Indeed, no Mn-based compounds are known to be superconductors. The electronic structure calculations and X-ray diffraction measurements presented here challenge these long held beliefs, finding that only modest pressures are required to transform LaMnPO, isostructural to superconducting host LaFeAsO, from an antiferromagnetic insulator to a metallic antiferromagnet, where the Mn moment vanishes in a second pressure-driven transition. Proximity to these charge and moment delocalization transitions in LaMnPO results in a highly correlated metallic state, the familiar breeding ground of superconductivity.

High-temperature superconductivity in layered nitrides β -Li x M NCl ( M = Ti, Zr, Hf): Insights from density functional theory for superconductors

Abstract: We present an ab initio analysis with density functional theory for superconductors (SCDFT) to understand the superconducting mechanism of doped layered nitrides \beta-Li$_x$MNCl (M=Ti, Zr, and Hf). The current version of SCDFT is based on the Migdal-Eliashberg theory and has been shown to reproduce accurately experimental superconducting-transition temperatures Tc of a wide range of phonon-mediated superconductors. In the present case, however, our calculated Tc$\leq$4.3 K (M=Zr) and $\leq$10.5 K (M=Hf) are found to be less than a half of the experimental Tc. In addition, Tc obtained in the present calculation increases with the doping concentration x, opposite to that observed in the experiment. Our results indicate that we need to consider some elements missing in the present SCDFT based on the Migdal-Eliashberg theory.

Universal behavior of the upper critical field in iron based superconductors

Abstract: The newly discovered iron-based high temperature superconductors have demonstrated rich physical properties. Here we give a brief review on the recent studies of the upper critical field and its anisotropy in a few typical series of the iron-based superconductors (FeSCs). In spite of their characters of a layered crystal structure, all the FeSCs possess an extremely large upper critical field and a weak anisotropy of superconductivity, being unique among the layered superconductors. These particular properties indicate potential applications of the FeSCs in the future. Based on the experimental facts of the FeSCs, we will discuss the possible mechanisms of pair breaking in high magnetic fields and its restrictions on the theoretical analysis of the superconducting pairing mechanisms.

S-4 Symmetric Microscopic Model for Iron-Based Superconductors

Abstract: Although iron-based superconductors are multiorbital systems with complicated band structures, we demonstrate that the low-energy physics which is responsible for their high-T-c superconductivity is essentially governed by an effective two-orbital Hamiltonian near half filling. This underlying electronic structure is protected by the S-4 symmetry. With repulsive or strong next-nearest-neighbor antiferromagnetic exchange interactions, the model results in a robust A(1g) s-wave pairing which can be mapped exactly to the d-wave pairing observed in cuprates. The classification of the superconducting (SC) states according to the S4 symmetry leads to a natural prediction of the existence of two different phases, named the A and B phases. In the B phase, the superconducting order has an overall sign change along the c axis between the top and bottom As (or Se) planes in a single Fe-As (or Fe-Se) trilayer structure, the common building block of iron-based superconductors. The sign change is analogous to the sign change in the d-wave superconducting state of cuprates upon 90 degrees rotation. Our derivation provides a unified understanding of iron pnictides and iron chalcogenides, and suggests that cuprates and iron-based superconductors share an identical high-T-c superconducting mechanism.

Microstructural analysis of phase separation in iron chalcogenide superconductors

Abstract: The interplay between superconductivity, magnetism and crystal structure in iron-based superconductors is a topic of great interest amongst the condensed matter physics community as it is thought to be the key to understanding the mechanisms responsible for high temperature superconductivity. Alkali metal doped iron chalcogenide superconductors exhibit several unique characteristics which are not found in other iron-based superconducting materials such as antiferromagnetic ordering at room temperature, the presence of ordered iron vacancies and high resistivity normal state properties. Detailed microstructural analysis is essential in order to understand the origin of these unusual properties. Here we have used a range of complementary scanning electron microscope based techniques, including high-resolution electron backscatter di raction mapping, to assess local variations in composition and lattice parameter with high precision and sub-micron spatial resolution. Phase separation is observed in the Csx Fe2-ySe2 crystals, with the minor phase distributed in a plate-like morphology throughout the crystal. Our results are consistent with superconductivity occurring only in the minority phase.

Microstructural analysis of phase separation in iron chalcogenide superconductors

Abstract: The interplay between superconductivity, magnetism and crystal structure in iron-based superconductors is a topic of great interest amongst the condensed matter physics community as it is thought to be the key to understanding the mechanisms responsible for high temperature superconductivity. Alkali metal doped iron chalcogenide superconductors exhibit several unique characteristics which are not found in other iron-based superconducting materials such as antiferromagnetic ordering at room temperature, the presence of ordered iron vacancies and high resistivity normal state properties. Detailed microstructural analysis is essential in order to understand the origin of these unusual properties. Here we have used a range of complementary scanning electron microscope based techniques, including high-resolution electron backscatter diffraction mapping, to assess local variations in composition and lattice parameter with high precision and sub-micron spatial resolution. Phase separation is observed in the CsxFe2?ySe2?crystals, with the minor phase distributed in a plate-like morphology throughout the crystal. Our results are consistent with superconductivity occurring only in the minority phase.

Materials process and applications of single grain (RE)–Ba–Cu–O bulk high-temperature superconductors

Abstract: This paper reviews recent advances in the melt process of (RE)–Ba–Cu–O [(RE)BCO, where RE represents a rare earth element] single grain high-temperature superconductors (HTSs), bulks and its applications. The efforts on the improvement of the magnetic flux pinning with employing the top-seeded melt-growth process technique and using a seeded infiltration and growth process are discussed. Which including various chemical doping strategies and controlled pushing effect based on the peritectic reaction of (RE)BCO. The typical experiment results, such as the largest single domain bulk, the clear TEM observations and the significant critical current density, are summarized together with the magnetization techniques. Finally, we highlight the recent prominent progress of HTS bulk applications, including Maglev, flywheel, power device, magnetic drug delivery system and magnetic resonance devices.

Upper critical fields and critical current densities of Fe-based superconductors as compared to those of other technical superconductors

Abstract: Three years since the discovery by the Hosono’s group of Fe-based superconductors, an enormous number of compounds, belonging to several different families have been discovered and fundamental properties have been deeply investigated in order to clarify the interplay between magnetisms and superconductivity in these compounds. Indeed, the actual potential of these compounds for practical applications remains still unclear. Fe-based superconductors are midway between high temperature superconductors (HTSCs) and MgB 2 . In Fe-based superconductors the critical current is rather independent of the field, similarly to HTSCs, as a consequence of the exceptionally high upper critical field and strong pinning associated with nm-scale local modulations of the order parameter. They exhibit low anisotropy of the critical current with respect to the crystalline directions, as in the case of MgB 2 , which allows current flow along the c -axis. However, Fe-based superconductor polycrystalline materials currently available still exhibit electromagnetic granularity, like the HTSCs, which suppresses superconducting current flow over long length. Whether the nature of such granularity is extrinsic, as due to spurious phases or cracks between grains or intrinsic, as related to misalignment of adjacent grains, is under debate. These aspects will be reviewed in the light of the recent literature.