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Journal ArticleDOI

Superconductivity in iron compounds

13 Dec 2011-Reviews of Modern Physics (American Physical Society)-Vol. 83, Iss: 4, pp 1589-1652
TL;DR: A detailed review of the superconductivity of FePnictide and chalcogenide (FePn/Ch) superconductors can be found in this paper.
Abstract: Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled ``conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.
Citations
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Journal ArticleDOI
05 Mar 2018-Nature
TL;DR: The realization of intrinsic unconventional superconductivity is reported—which cannot be explained by weak electron–phonon interactions—in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle.
Abstract: The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1°-the first 'magic' angle-the electronic band structure of this 'twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

5,613 citations

Journal ArticleDOI
TL;DR: In this paper, a comprehensive review of all the important theoretical and experimental advances on silicene to date, from the basic theory of intrinsic properties, experimental synthesis and characterization, modulation of physical properties by modifications, and finally to device explorations is presented.

676 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that the properties of Fe-pnictide and chalcogenide superconductors can be explained by considering spin fluctuation theory and accounting for the peculiar electronic structure of these systems coupled with the likely sign-changing s-wave symmetry.
Abstract: The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.

660 citations

Journal ArticleDOI
TL;DR: The sister compound of W Te2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, is investigated and the observed dome-shaped superconductivity phase diagram provides insights into the interplay between super conductivity and topological physics.
Abstract: Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

653 citations

Journal ArticleDOI
TL;DR: In this article, the authors review the theoretical understanding and physical properties of these Hund's metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3D, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).
Abstract: Strong electronic correlations are often associated with the proximity of a Mott-insulating state. In recent years however, it has become increasingly clear that the Hund’s rule coupling (intra-atomic exchange) is responsible for strong correlations in multiorbital metallic materials that are not close to a Mott insulator. Hund’s coupling has two effects: It influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. We review the theoretical understanding and physical properties of these Hund’s metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3d, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).

583 citations

References
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Journal ArticleDOI
TL;DR: In this paper, Ba−La−Cu−O system, with the composition BaxLa5−xCu5O5(3−y) have been prepared in polycrystalline form, and samples with x=1 and 0.75,y>0, annealed below 900°C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound.
Abstract: Metallic, oxygen-deficient compounds in the Ba−La−Cu−O system, with the composition BaxLa5−xCu5O5(3−y) have been prepared in polycrystalline form. Samples withx=1 and 0.75,y>0, annealed below 900°C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity, then an approximately logarithmic increase, interpreted as a beginning of localization. Finally an abrupt decrease by up to three orders of magnitude occurs, reminiscent of the onset of percolative superconductivity. The highest onset temperature is observed in the 30 K range. It is markedly reduced by high current densities. Thus, it results partially from the percolative nature, bute possibly also from 2D superconducting fluctuations of double perovskite layers of one of the phases present.

10,272 citations


"Superconductivity in iron compounds..." refers background in this paper

  • ...One is that just like the discovery of superconductivity at 35 K in Ba doped La2CuO4 (Bednorz and Müller, 1986) it led to the almost immediate further discovery of even higher Tc materials, with the current record 56 K observed in Gd0:8Th0:2FeAsO (C. Wang et al., 2008), Sr0:5Sm0:5FeAsF (G. Wu et…...

    [...]

  • ...The path to this higher transition temperature was also similar to that in the high Tc cuprates, where pressure experiments (Chu et al., 1987) first increased the Tc in Ba doped La2CuO4 from 35 to 53 K....

    [...]

  • ...One is that just like the discovery of superconductivity at 35 K in Ba doped La2CuO4 (Bednorz and Müller, 1986) it led to the almost immediate further discovery of even higher Tc materials, with the current record 56 K observed in Gd0:8Th0:2FeAsO (C. Wang et al., 2008), Sr0:5Sm0:5FeAsF (G. Wu et al., 2009) and Ca0:4Nd0:6FeAsF (Cheng et al., 2009)....

    [...]

Journal ArticleDOI
TL;DR: It is reported that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site and exhibits a trapezoid shape dependence on the F- content.
Abstract: We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.

6,643 citations


"Superconductivity in iron compounds..." refers background or methods in this paper

  • ...Kamihara et al. (2008) in their discovery of superconductivity in LaFeAsO1 xFx reported that the undoped LaFeAsO resistivity was approximately temperature independent at 5 m cm, with an anomaly at 150 K and an upturn below 100 K....

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  • ...The original discovery of superconductivity at 26 K by Kamihara et al. (2008) was in LaFeO1 xFx, which has the tetragonal, tP8 (‘‘t’’ means tetragonal, ‘‘P’’ means ‘‘primitive’’ or no atoms in either the body or face centers, 8 atoms per unit cell) ZrCuSiAs ( ¼ prototypical compound) structure with…...

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  • ...Both these transitions are suppressed in the discovery compound of Kamihara et al. (2008), LaFeAsO0:92F0:08, Tc ¼ 26 K. TSDW and TS are Rev. Mod....

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  • ...B.2. a. Tc versus doping Electron doping LnFeAsO (Ln ¼ La, Dy, Tb, Gd, Sm, Nd, Pr, Ce), via either the discovery method (F partially replacing O) of Kamihara et al. (2008) where superconductivity starts at 4% F doping or via oxygen deficiency achieved with high 130 K for optimally doped x ¼ 0:2 vs…...

    [...]

  • ...Kamihara et al. (2008) reported that the susceptibility of LaFeAsO is about 0:4 memu=mole and roughly temperature independent below room temperature except for the 150 K anomaly and an upturn below 25 K. McGuire et al. (2008), with an expanded set of data for of LaFeAsO, showed that increases with…...

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Journal ArticleDOI
TL;DR: A stable and reproducible superconductivity transition between 80 and 93 K has been unambiguously observed both resistively and magnetically in a new Y-Ba-Cu-O compound system at ambient pressure.
Abstract: A stable and reproducible superconductivity transition between 80 and 93 K has been unambiguously observed both resistively and magnetically in a new Y-Ba-Cu-O compound system at ambient pressure. An estimated upper critical field H c2(0) between 80 and 180 T was obtained.

5,965 citations


"Superconductivity in iron compounds..." refers methods in this paper

  • ...This was followed by ‘‘chemical pressure’’ experiments where Tc was raised to 93 K (Wu et al., 1987) by replacing La with the smaller Y to make a multiphase sample containing YBa2Cu3O7 ....

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Book
01 Dec 1985
TL;DR: All crystallographic data are now given in the standard setting according to the International Tables for Crystallography, and the following improvements over the original Pearson's.
Abstract: (2 Volume set). The valuable information in Pearson's Handbook is now more affordable in a handy desk reference. 27,686 entries of the highest quality crystal data, representing 27,686 different compounds. Structure type given for all entries. 54 per cent of entries include the coordinates of the atoms. 605 entries are 'filled-up' structure 1,730 structure types have been assigned by the editor 6,426 belong to berthollide compounds. Data included up to 1995 (6-year update to the Second Edition 12-year update to the First Edition). Full 167-page structure-type index (with all its representatives). Entries include full information, as in the Second Edition. Comprises all the international literature from 1913 to 1995. Includes detailed crystallographic data for unary, binary and ternary phases, excluding halides and ternary (or quaternary) oxides. Fully revised and updated. Covers more than 27,000 compounds, with all data critically evaluated. Includes the following improvements over the original Pearson's. Additional literature years between 1989 to 1995 have been covered completely and comprehensively, based on searches of more than 130 journals and more than 10,000 abstract pages per year. Entries contain additional information, such as calculated density, color, more detailed diffraction data, standard deviation of unit cell dimension(s), point-set symmetry, and full reference, including publication title. All entries and structure types have been computer checked for consistency and correctness. All crystallographic data are now given in the standard setting according to the International Tables for Crystallography. Include a Six-Year Update of the Data in The Second Edition.

4,734 citations


"Superconductivity in iron compounds..." refers background in this paper

  • ...The next iron-containing superconductor structure includes members of the MFe2As2 (‘‘122’’) family [of which there are over 450 distinct compounds, (Villars and Calvert, 1985)], where Rotter, Tegel, and Johrendt (2008) discovered Tc ¼ 38 K in K doped BaFe2As2, Ba0:6K0:4Fe2As2....

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Journal ArticleDOI
W. L. McMillan1
TL;DR: In this paper, the superconducting transition temperature is calculated as a function of the electron-phonon and electron-electron coupling constants within the framework of strong coupling theory.
Abstract: The superconducting transition temperature is calculated as a function of the electron-phonon and electron-electron coupling constants within the framework of the strong-coupling theory. Using this theoretical result, we find empirical values of the coupling constants and the "band-structure" density of states for a number of metals and alloys. It is noted that the electron-phonon coupling constant depends primarily on the phonon frequencies rather than on the electronic properties of the metal. Finally, using these results, one can predict a maximum superconducting transition temperature.

3,895 citations