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Register a new userStrong spin resonance on BaFe2(As0.65P0.35)2 with Tc = 30 K
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We performed inelastic neutron scattering on powder sample of the P-doped iron-based superconductor BaFe2(As0.65P0.35)2 with Tc = 30K, whose superconducting (SC) order parameter is expected to have line nodes. We have observed spin resonance at Q $sim$ 1.2{AA}^{-1} and E=12 meV in the SC state. The resonance enhancement, which can be a measure of the area of sign reversal between the hole and electron Fermi surfaces (FSs), is comparable to those of other iron-based superconductors without line nodes. This fact indicates that the sign reversal between the FSs is still dominant in this system, and the line nodes should create only limited area of sign-reversal on a single FS. Hence the system can hold relatively high-Tc. Comparison with theoretical calculation indicates horizontal line nodes may be a candidate to reproduce the observation.
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We report specific heat measurements on the Fe-based superconductor BaFe2(As0.7P0.3)2, a material on which previous penetration depth, NMR, and thermal conductivity measurements have observed a high density of low-energy excitations, which have been interpreted in terms of order parameter nodes. Within the resolution of our measurements, the low temperature limiting C/T is found to be linear in field, i.e. we find no evidence for a Volovik effect associated with nodal quasiparticles in either the clean or dirty limit. We discuss possible reasons for this apparent contradiction.
The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x = 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K, respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus temperature using 57Fe Mossbauer spectroscopy. Special attention was paid to regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound exhibits a reduced amplitude of SDW as compared to the parent compound and preserved universality class of two-dimensional magnetic planes with one-dimensional spins. The spin-nematic phase region for x = 0.10 is characterized by an incoherent magnetic order. BaFe2(As0.69P0.31)2 shows coexistence of a weak magnetic order and superconductivity due to the vicinity of the quantum critical point. The charge density modulations in the BaFe2(As0.67P0.33)2 and BaFe2(As0.47P0.53)2 superconductors are perturbed near Tc. Pronounced hump of the average quadrupole splitting across superconducting transition is observed for the system with x = 0.33. The phosphorus substitution increases the Debye temperature of the BaFe2(As1-xPx)2 compound. Moreover, experimental electron charge densities at Fe nuclei in this material conclusively show that it should be recognized as a hole-doped system. The measured Mossbauer spectral shift and spectral area are not affected by transition to the superconducting state. This indicates that neither the average electron density at Fe nuclei nor the dynamical properties of the Fe-sublattice in BaFe2(As1-xPx)2 are sensitive to the superconducting transition. Theoretical calculations of hyperfine parameters determining the patterns of Mossbauer spectra of BaFe2(As1-xPx)2 with x = 0, 0.31, 0.5, and 1.0 are performed within the framework of the density functional theory.
We demonstrate that the anisotropy R of the paramagnetic spin fluctuations grows toward Tc at 75As sites in the optimally electron-doped superconductor Ba[(Fe0.92Co0.08)2]2As2, with stronger spin fluctuations along the c-axis. Our finding is in remarkable contrast with the case of high T$_c$ cuprates, where R is independent of temperature above Tc.
A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 10 K), enables investigation of the vortex structure within the mixed state of this highly anisotropic, type-II, superconductor. Contrary to previous assumptions, frequency dependent studies (28 - 153 GHz) indicate that the squared plasma frequency depends exponentially on the magnetic field strength. Such behavior has been predicted for a weakly pinned quasi-two-dimensional vortex lattice [Bulaevskii et al. Phys. Rev. Lett. 74, 801 (1995)], but has not so far been observed experimentally. Our data also suggests a transition in the vortex structure near the irreversibility line not previously reported for an organic superconductor using this technique.
We use inelastic neutron scattering to study temperature and energy dependence of spin excitations in optimally P-doped BaFe2(As0.7P0.3)2 superconductor (Tc = 30 K) throughout the Brillouin zone. In the undoped state, spin waves and paramagnetic spin excitations of BaFe2As2 stem from antiferromagnetic (AF) ordering wave vector QAF= (1/-1,0) and peaks near zone boundary at (1/-1,1/-1) around 180 meV. Replacing 30% As by smaller P to induce superconductivity, low-energy spin excitations of BaFe2(As0.7P0.3)2form a resonance in the superconducting state and high-energy spin excitations now peaks around 220 meV near (1/-1,1/-1). These results are consistent with calculations from a combined density functional theory and dynamical mean field theory, and suggest that the decreased average pnictogen height in BaFe2(As0.7P0.3)2 reduces the strength of electron correlations and increases the effective bandwidth of magnetic excitations.
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