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Register a new userInfrared Measurement of the Pseudogap in P-Doped and Co-Doped BaFe2As2 High-Temperature Superconductors
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We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe2As2 family. Our experiments have identified hallmarks of the pseudogap state in the BaFe2As2 system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.
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We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, +-e, 0) from the nominal commensurate antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic notation) where e = 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.
Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, voluminous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.
57Fe Mossbauer spectroscopy measurements are presented in the underdoped Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~ 2 and 12K respectively). The spectral shapes in the so-called spin-density wave (SDW) phase are interpreted in terms of incommensurate modulation of the magnetic structure, and allow the shape of the modulation to be determined. In undoped BaFe2As2, the magnetic structure is commensurate, and we find that incommensurability is present at the lowest doping level (x=0.014). As Co doping increases, the low temperature modulation progressively loses its squaredness and tends to a sine-wave. The same trend occurs for a given doping level, as temperature increases. We find that a magnetic hyperfine component persists far above the SDW transition, its intensity being progressively tranferred to a paramagnetic component on heating.
In the last two decades there have been tremendous attempts to built an adequate theory of high-temperature superconductivity. Most studies (including our efforts) used some model Hamiltonians with input parameters not directly related to the material. The dielectric response function of electrons in strongly correlated high-temperature superconductors is apriori unknown. Hence one has to start with the generic Hamiltonian including unscreened Coulomb and Froehlich electron-phonon interactions operating on the same scale since any ad-hoc assumption on their range and relative magnitude might fail. Using such a generic Hamiltonian I have built the analytical theory of high-temperature superconductivity in doped polar insulators predicting the critical temperature in excess of a hundred Kelvin without any adjustable parameters. The many-particle electron system is described by an analytically solvable polaronic t-Jp Hamiltonian with reduced hopping integral, t, allowed double on-site occupancy, large phonon-induced antiferromagnetic exchange, Jp >> t, and a high-temperature superconducting state of small superlight bipolarons protected from clustering.
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