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Register a new user17O-NMR Knight shift study of the interplay between superconductivity and pseudogap in (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y
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We report systematic 17O-NMR measurements on the high-Tc cuprate (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y, for four different families (different x). Using Knight shift data, we show that the pseudogap lines for all families are inconsistent with a quantum critical point inside the superconducting dome. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. We therefore argue that pseudogap and superconductivity are separate phenomena in these cuprates. Using Knight shift data, we show that the pseudogap opening temperature T* is much higher than Tc near optimal doping, unlike structurally similar YBCO. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. This puts constraints on the nature of the pseudogap and position of the quantum critical point inside the superconducting dome.
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We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting CaxLa(1-x)Ba(1.75-x)La(0.25+x)Cu3Oy (0<x<0.4, 6.4<y<7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO2 plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (Tcmax) in different cuprate families at optimum hole density.
We performed $^{31}$P-NMR measurements on LaFe(As$_{1-x}$P$_{x}$)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature $T_{rm N}$ and the moment $mu_{rm ord}$ are continuously suppressed with increasing P content $x$ and disappear at $x = 0.3$ where bulk superconductivity appears. At this superconducting $x = 0.3$, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As$_{1-x}$P$_{x}$)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ and SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, with the 122 structure. Moreover, the AFM order reappears with further P substitution ($x > 0.4$). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3$d$ orbitals around the Fermi level.
Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor $Ba_{1-x}K_{x}BiO_{3}$ near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature $T_{c}$, and near an insulating phase with long-range lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and tails of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature $T_{p} > T_{c}$, which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near $E_{F}$ into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.
We report the results of a ^63Cu and ^17O NMR study of the nuclear quadrupole interaction tensor, ^(17,63)nu_{Q,alpha}, in the hole doped spin ladder system Sr_(14-x)Ca_xCu_24O_41 (x = 0 and 12) performed under ambient and high pressures. NMR data show that the hole density in the Cu_2O_3 ladder layer grows with temperature, Ca content and an applied pressure. We have derived the hole occupation of Cu 3d and O 2p orbitals at the different ion sites in the Cu_2O_3 ladder as a function of the temperature, Ca substitution and pressure. We also suggest that the most important role of high pressure for the stabilization of a superconducting ground state in Ca-rich two-leg ladders is an increase of the hole concentration in the conducting Cu_2O_3 planes. We have obtained an estimate of 0.10 hole per Cu1 for the hole concentration at low temperature in Ca12 under 32 kbar when this compound undergoes a superconducting transition at 5K. Such a value fits fairly well with the doping phase diagram of cuprate superconductors.
The magnetic order in CaK(Fe[1-x]Ni[x])4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and non-collinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.
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