Showing papers on "High-temperature superconductivity published in 2008"

Structural and magnetic phase diagram of CeFeAsO 1− x F x and its relation to high-temperature superconductivity

Abstract: According to a neutron-scattering study of the structural and magnetic properties of the pnictide CeFeAsO1−xFx, the phase diagram of this material shows considerable similarities with the high-Tc cuprate superconductors. These results are an important addition to the effort to find out where superconductivity in these iron–arsenic alloys arises.

Structural and magnetic phase diagram of CeFeAsO1-xFx and its relationship to high-temperature superconductivity

Abstract: We use neutron scattering to study the structural and magnetic phase transitions in the iron pnictides CeFeAsO1-xFx as the system is tuned from a semimetal to a high-transition-temperature (high-Tc) superconductor through Fluorine (F) doping x. In the undoped state, CeFeAsO develops a structural lattice distortion followed by a stripe like commensurate antiferromagnetic order with decreasing temperature. With increasing Fluorine doping, the structural phase transition decreases gradually while the antiferromagnetic order is suppressed before the appearance of superconductivity, resulting an electronic phase diagram remarkably similar to that of the high-Tc copper oxides. Comparison of the structural evolution of CeFeAsO1-xFx with other Fe-based superconductors reveals that the effective electronic band width decreases systematically for materials with higher Tc. The results suggest that electron correlation effects are important for the mechanism of high-Tc superconductivity in these Fe pnictides.

Magnetism, superconductivity, and pairing symmetry in iron-based superconductors

Abstract: We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the itinerant model of small electron and hole pockets near 0,0 and ,. We argue that the effective interactions in both channels logarithmically flow toward the same values at low energies; i.e., antiferromagnetism and superconductivity must be treated on equal footing. The magnetic instability comes first for equal sizes of the two pockets, but loses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces extended s-wave symmetry. We argue that the T dependencies of the spin susceptibility and NMR relaxation rate for such a state are exponential only at very low T, and can be well fitted by power laws over a wide T range below Tc.

Unconventional superconductivity in Ba 0.6 K 0.4 Fe 2 As 2 from inelastic neutron scattering

Abstract: The discovery of a new family of superconductors containing layers of iron arsenide, the iron oxypnictides, has stimulated much interest, largely because they combine promisingly high transition temperatures with a mechanism of superconductivity rather similar to that of that the high-temperature copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase is suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation that is predicted to occur when the superconducting energy gap has opposite sign on different parts of the Fermi surface. Now inelastic neutron scattering observations of the iron arsenide Ba0.6K0.4Fe2As2 reveal a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, demonstrating that the superconducting energy gap in these materials has unconventional symmetry. A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures and similarities with the high-Tc copper oxide superconductors. This paper reports inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, demonstrating that the superconducting energy gap has unconventional symmetry. A new family of superconductors containing layers of iron arsenide1,2,3 has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping4. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase5,6,7,8,9. This resonance, which has also been observed in several heavy-fermion superconductors10,11,12, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface13, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range14. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface15. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors.

Electronic liquid crystal state in the high-temperature superconductor YBa2Cu3O6.45.

Abstract: Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one-dimensional, incommensurate modulation of the spin system in the high-transition-temperature superconductor YBa2Cu3O6.45 upon cooling below ∼150 kelvin, whereas static magnetic order is absent above 2 kelvin. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route toward electronic liquid crystals and that nematic order can coexist with high-temperature superconductivity in underdoped cuprates.

Resonant Spin Excitation in the High Temperature Superconductor Ba0.6K0.4Fe2As2

Abstract: A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (T{sub c}), some of which are >50 K, and because of similarities with the high-{sub c} copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below T{sub c} in Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry inmore » the iron arsenide superconductors.« less

The t–J model for the oxide high-Tc superconductors

Abstract: A theoretical review is given on high temperature superconductivity in copper oxides (cuprates) by focusing on the hole doping cases based on the view that it is realized in carrier doped Mott insulators, as noted by Anderson in the initial stage. From the detailed knowledge of electronic states deduced from experiments that showed the undoped parent case is Mott insulators (charge transfer type insulators, to be precise) and that the hole doping is mainly on oxygen sites, the t?J model, as derived by Zhang and Rice, is shown to be a canonical model for hole doped cuprates and values of various parameters of the model have been assessed. Results of many different numerical methods so far obtained for this t?J model, especially the variational Monte Carlo method, have clearly indicated the stability of the -wave superconductivity at absolute zero for the parameter region of actual experimental interest and the particular doping dependences of the condensation energy of superconductivity reflecting particular features of doped Mott insulators. For finite temperatures, on the other hand, the field theoretical slave-boson approximation based on the spin (spinons) and charge (holons) separations and the gauge fields as a glue combining them predicts qualitatively particular features of the existence of characteristic crossover temperatures of the spin singlet of the resonating valence bond (RVB) state, TRVB and the onset of Bose condensation of holons, TB, triggering coherent motion of electrons as convoluted particles of spinons and holons. The considerations based on the gauge field indicate that the onset temperature of superconductivity, Tc, is the lower one of these two, i.e. either TB (overdoped cases) or TRVB (underdoped cases), respectively. These characteristic features of the 'phase diagram' at finite temperatures are in overall agreement with various experimental observations, especially with the existence of spin-gap or pseudo-gap phases. In more detailed examinations of the underdoped region, the antiferromagnetic long-range order and superconductivity show a very intricate relationship at low temperatures depending on the system; they coexist as clarified in the inner layer of Hg-1245 but spin glass states intervene between them in La2?xSrxCuO4 (LSCO). It is argued that these differences can be attributed to the different degrees of disorder. Actually, theories based on the t?J model have also predicted the coexistence of antiferromagnetism and superconductivity in the ground state of clean systems. On the other hand, interesting experimental findings of large Nernst effect and 'Fermi arc' in LSCO and impurity effects in YBCO have prompted the necessity of theoretical investigations of electronic states of lightly doped Mott insulators in the presence of strong disorder.

Electron-hole symmetry and magnetic coupling in antiferromagnetic LaFeAsO.

Abstract: When either electron or hole doped at concentrations x approximately 0.1, the LaFeAsO family displays remarkably high temperature superconductivity with Tc up to 55 K. In the most energetically stable Q-->M=(pi,pi,0) antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy arising from light carriers (m* approximately 0.25-0.33), and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond x approximately 0.08 results in heavy carriers per Fe (by roughly an order of magnitude) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings Jij(R) reveal that exchange coupling is strongly dependent on both the AFM symmetry and on the Fe-As distance.

High-temperature superconductivity in iron-based layered compounds

Abstract: Basic experimental data are presented for a new class of high-temperature superconductors — iron-based layered compounds of the types REOFeAs (RE = La, Ce, Nd, Pr, Sm, ...), AFe2As2 (A = Ba, Sr, ...), AFeAs (A = Li, ...), and FeSe(Te). The structure of electronic spectra in these compounds is discussed, including the correlation effects, as is the spectrum and role of collective excitations (phonons and spin waves). Basic models for describing various types of magnetic ordering and Cooper pairing are reviewed.

Ab Initio Description of High-Temperature Superconductivity in Dense Molecular Hydrogen

Abstract: We present a first-principles study of the electron-phonon interaction and the prediction of the superconducting critical temperature in molecular metallic hydrogen. Our study is able to single out the features which drive the system towards superconductivity: mainly, a rich and complex Fermi surface and strongly coupled phonon modes driving the intra- or intermolecular charge transfer. We demonstrate that in this simple system, a very high superconducting critical temperature can be reached via electron-phonon and Coulomb electron-electron interactions.

In situ doping control of the surface of high-temperature superconductors

Abstract: In copper-oxide superconductors, charge carriers must be added to the insulating ‘parent’ compound before superconductivity appears. Exactly how the dopants affect the crystalline surface and evolving Fermi surface is now clear.

New high-temperature superconductors based on rare-earth and transition metal oxyarsenides and related phases: synthesis, properties and simulations

Abstract: The discovery in February 2008 of superconductivity with the transition temperature about 26 K in fluorine-doped oxyarsenide LaO1–xFxFeAs stimulated numerous studies of superconducting and other physical properties of this and related materials, resulting in a new family of high-temperature (Tc ~ 26–55 K) superconductors.The experimental and theoretical state of the art is reviewed concerning the synthesis, properties and simulation of the new family and related systems.

High-temperature superconductivity in the iron pnictides

Abstract: A new class of high-temperature superconductors has been discovered in layered iron arsenic compounds. Results in this rapidly moving field may shed light on the still unsolved problem of high-temperature cuprate superconductivity.

Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor

Abstract: We report measurements of quantum oscillations in SrFe2As2—which is an antiferromagnetic parent of the iron arsenide family of superconductors—known to become superconducting under doping and the application of pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55 T in the liquid helium temperature range suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high Tc superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state.

Evidence for pairing above the transition temperature of cuprate superconductors from the electronic dispersion in the pseudogap phase.

Abstract: In the underdoped high temperature superconductors, instead of a complete Fermi surface above Tc, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy gap. We show that in this pseudogap phase, the energy-momentum relation of electronic excitations near EF behaves like the dispersion of a normal metal on the Fermi arcs, but like that of a superconductor in the gapped regions. We argue that this dichotomy in the dispersion is difficult to reconcile with a competing order parameter, but is consistent with pairing without condensation.

Fermi surface of superconducting LaFePO determined by quantum oscillations

Abstract: The recent discovery of superconductivity in ferrooxypnictides, which have a maximum transition temperature intermediate between the two other known high temperature superconductors MgB{sub 2} and the cuprate family, has generated huge interest and excitement. The most critical issue is the origin of the pairing mechanism. Whereas superconductivity in MgB{sub 2} has been shown to arise from strong electron-phonon coupling, the pairing glue in cuprate superconductors is thought by many to have a magnetic origin. The oxypnictides are highly susceptible to magnetic instabilities, prompting analogies with cuprate superconductivity. Progress on formulating the correct theory of superconductivity in these materials will be greatly aided by a detailed knowledge of the Fermi surface parameters. Here we report for the first time extensive measurements of quantum oscillations in a Fe-based superconductor, LaFePO, that provide a precise calliper of the size and shape of the Fermi surface and the effective masses of the relevant charge carriers. Our results show that the Fermi surface is composed of nearly-nested electron and hole pockets in broad agreement with the band-structure predictions but with significant enhancement of the quasiparticle masses. The correspondence in the electron and hole Fermi surface areas provides firm experimental evidence that LaFePO, whilst unreconstructed, liesmore » extremely close to a spin-density-wave instability, thus favoring models that invoke such a magnetic origin for high-temperature superconductivity in oxypnictides.« less

Electronic structure of prototype AFe2As2 and ReOFeAs high-temperature superconductors: A comparison

Abstract: We have performed ab initio LDA calculations of the electronic structure of newly discovered prototype high-temperature superconductors AFe2As2 (A = Ba, Sr) and compared it with the previously calculated electronic spectra of ReOFeAs (Re = La, Ce, Pr, Nd, Sm). In all cases, we obtain almost identical densities of states in a rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two dimensional in this energy interval, leading to the same basic (minimal) model of the electronic spectra, determined mainly by Fe d orbitals of the FeAs layers. The other constituents, such as A ions or rare-earth Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe2As2 are also very similar to that of ReOFeAs. This makes the more simple AFe2As2 a generic system to study the high-temperature superconductivity in FeAs-layered compounds.

Electronic Structure of Prototype AFe_2As_2 and ReOFeAs High-Temperature Superconductors: a Comparison

Abstract: We have performed ab initio LDA calculations of electronic structure of newly discovered prototype high-temperature superconductors AFe_2As_2 (A=Ba,Sr) and compared it with previously calculated electronic spectra of ReOFeAs (Re=La,Ce,Pr,Nd,Sm). In all cases we obtain almost identical densities of states in rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two-dimensional in this energy interval, leading to the same basic (minimal) model of electronic spectra, determined mainly by Fe d-orbitals of FeAs layers. The other constituents, such as A ions or rare earths Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe_2As_2 are also very similar to that of ReOFeAs. This makes the more simple AFe_2As_2 a generic system to study high-temperature superconductivity in FeAs - layered compounds.

Quantum oscillations in the parent magnetic phase of an iron arsenide high temperature superconductor

Abstract: We report quantum oscillation measurements in SrFe2As2 - which is an antiferromagnetic parent of the iron-arsenide family of superconductors - known to become superconducting under doping and the application of pressure. The magnetic field and temperature dependences of the oscillations between 20 and 55 T in the liquid helium temperature range suggest that the electronic excitations are those of a Fermi liquid. We show that the observed Fermi surface comprising small pockets is consistent with the formation of a spin-density wave. Our measurements thus demonstrate that high Tc superconductivity can occur on doping or pressurizing a conventional metallic spin-density wave state.

Thick YBa2Cu3O7−x+BaSnO3 films with enhanced critical current density at high magnetic fields

Abstract: The thickness dependence was studied for the critical current density (Jc) of YBa2Cu3O7−x(YBCO)+BaSnO3 (BSO) nanocomposite films. These films showed a significantly reduced decline of the Jc with thickness, especially at high magnetic fields. For example, a 2 μm thick YBCO+BSO film had a Jc∼3×105 A/cm2 at 5 T as compared to a typical Jc of 2.4×103 A/cm2 at 5 T for a 300 nm thick YBCO film. The thick YBCO+BSO films maintained high Tc (>88 K) and had a high density (2.5×1011/cm2) of continuous BSO nanocolumns that likely contributed for the observed Jc enhancements.

Fermi pockets and quantum oscillations of the Hall coefficient in high-temperature superconductors

Abstract: Recent quantum oscillation measurements in high-temperature superconductors in high magnetic fields and low temperatures have ushered in a new era. These experiments explore the normal state from which superconductivity arises and provide evidence of a reconstructed Fermi surface consisting of electron and hole pockets in a regime in which such a possibility was previously considered to be remote. More specifically, the Hall coefficient has been found to oscillate according to the Onsager quantization condition, involving only fundamental constants and the areas of the pockets, but with a sign that is negative. Here, we explain the observations with the theory that the alleged normal state exhibits a hidden order, the d-density wave, which breaks symmetries signifying time reversal, translation by a lattice spacing, and a rotation by an angle π/2, while the product of any two symmetry operations is preserved. The success of our analysis underscores the importance of spontaneous breaking of symmetries, Fermi surface reconstruction, and conventional quasiparticles. We primarily focus on the version of the order that is commensurate with the underlying crystalline lattice, but we also touch on the consequences if the order were to incommensurate. It is shown that whereas commensurate order results in two independent oscillation frequencies as a function of the inverse of the applied magnetic field, incommensurate order leads to three independent frequencies. The oscillation amplitudes, however, are determined by the mobilities of the charge carriers comprising the Fermi pockets.

FeAs systems: a new class of high-temperature superconductors

Abstract: This is the first systematic review of a new class of high-Tc superconductors which includes iron-based layered compounds such as REOFeAs (RE is a rare-earth element), AFe2As2 (A= Ba, Sr, Ca), and LiFeAs, all of which are antiferromagnetic normal metals while being stoichiometric and becoming superconducting (with the current maximum Tc given by 55 K) when doped with an element of a different valence. The common structural element of all these compounds is layers formed by FeAs4 complexes. Electron states near the Fermi level are formed by Fe 3d states. As was shown theoretically by LDA calculations and experimentally by ARPES, the electronic structure of all compounds of the FeAs class is similar; their Fermi surface is multi-sheeted, consisting of two hole pockets at the center and two electron pockets at the corners of the Brillouin zone. In this paper, the superconducting properties of such systems are reviewed in detail, including the dependence of Tc on the doping level, external pressure, superconducting critical field, and superconducting order parameter. The controversy over the order parameter symmetry determined from different measurements is discussed. The transport, magnetic, and superconducting properties of FeAs systems are analyzed in comparison with those of cuprates. Basic electronic models of FeAs compounds, with their electronic structure and the proximity of the state of doped compounds to the antiferromagnetic ordering taken into account, are described to explain the specific features of electron pairing in them. It is shown that unlike the cuprates, superconducting FeAs systems are weakly (or moderately) correlated materials that are far from the Mott – Hubbard transition. A conclusion is made that the physical properties of FeAs compounds have mainly been well understood, except for the symmetry of the superconducting order parameter.

Identification of a New Form of Electron Coupling in the Bi2Sr2CaCu2O8 Superconductor by Laser-Based Angle-Resolved Photoemission Spectroscopy

Abstract: Laser-based angle-resolved photoemission measurements with superhigh resolution have been carried out on an optimally doped Bi(2)Sr(2)CaCu(2)O(8) high temperature superconductor. New high energy features at approximately 115 meV and approximately 150 meV, in addition to the prominent approximately 70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which cannot be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.

59Co and 75As NMR Investigation of Electron-Doped High Tc Superconductor BaFe1.8Co0.2As2 (Tc = 22 K)

Abstract: We report an NMR investigation of the superconductivity in BaFe 2 As 2 induced by Co doping ( T c = 22 K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron–pnictide superconductors is quite different from the widely accepted physical picture of high T c cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Δ PG / k B ∼560 K, where χ spin ∼constant and resisitivty ρ∝ T . The NMR Knight shift below T c decreases for both along the c -axis and a b -plane, and is consistent with the singlet pairing scenario.

High pressure synthesis of late rare earth RFeAs(O,F) superconductors; R = Tb and Dy

Abstract: New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures Tc= 46 and 45 K and very high critical fields over 100 T have been prepared at 1100- 1150C and 10-12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La - Nd, Sm, Gd) high temperature superconductors.

Magnetization relaxation and collective vortex pinning in the Fe-based superconductor SmFeAsO 0.9 F 0.1

Abstract: By measuring the dynamic and traditional magnetization relaxations we investigate the vortex dynamics of the recently discovered superconductor SmFeAsO(0.9)F(0.1) with T(c) = 55 K. It is found that the relaxation rate is rather large reflecting a small characteristic pinning energy. Moreover it shows a weak temperature dependence in wide temperature region, which resembles the behavior of the cuprate superconductors. Combined with the resistivity data under different magnetic fields, a vortex phase diagram is obtained. Our results strongly suggest that the model of collective vortex pinning applies to this superconductor very well.

Charge dynamics of doped holes in high Tc cuprate superconductors: a clue from optical conductivity.

Abstract: The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the midinfrared band.