摘要 :
Fundamental and higher harmonics of the ac magnetic susceptibility have been measured on LaFeAsO_(0.92)F_(0.08) samples as a function of the temperature, at various amplitudes and frequencies of the ac magnetic field, with a small...
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Fundamental and higher harmonics of the ac magnetic susceptibility have been measured on LaFeAsO_(0.92)F_(0.08) samples as a function of the temperature, at various amplitudes and frequencies of the ac magnetic field, with a small superimposed dc field parallel to the ac field. The granularity of the samples has been investigated and the intergrain and intragrain contributions have been clearly individuated looking at both the first and the third harmonics. The vortex dynamics has been also analyzed, and a comparison with the magnetic behavior of both the MgB_2 and the cuprate superconductors has been performed. Some vortex dissipative phenomena, i.e., the thermally activated flux flow and the flux creep, have been detected in the presented experimental data, similar to what have been obtained on YBCO. Nevertheless, although the general behavior is similar, several differences have been also evidenced between these different classes of superconductors, mainly in the third harmonics. We infer that different vortex dynamics has to be included into the analysis of the magnetic response in this iron-based material.
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High-resolution angle-resolved photoemission measurements have been carried out on the (Sr,K)Fe_2As_2 superconductor (T_c=21 K). Three holelike Fermi-surface sheets are resolved around the Γ point. One electronlike Fermi surface ...
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High-resolution angle-resolved photoemission measurements have been carried out on the (Sr,K)Fe_2As_2 superconductor (T_c=21 K). Three holelike Fermi-surface sheets are resolved around the Γ point. One electronlike Fermi surface and strong Fermi spots are observed near the M(π,π) point which are different from a round electron pocket expected from band calculations. This discrepancy and the observed band renormalization ask for further theoretical effort in understanding the electronic structure of the Fe-based compounds.
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We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO_(1-x)F_x as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a stru...
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We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO_(1-x)F_x as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group PAInmm) to orthorhombic (space group Cmma) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.
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Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below T_N～ 135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed simi...
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Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below T_N～ 135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed similar behavior in BaFe_2As_2, In single crystals of superconducting Ba_(0.55)K_(0.45)Fe_2As_2, μSR spectra indicate static magnetism with incommensurate or short-ranged spin structure in ～70% of volume below T_N～80 K, coexisting with the remaining volume which exhibits a superfluid response below T_c～30 K.
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摘要 :
We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class (Sr/Ba)_(1-x)K_xFe_2As_2. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi su...
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We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class (Sr/Ba)_(1-x)K_xFe_2As_2. By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, Δ(k~→). A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 A, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40 ± 15 meV kink likely reflects contributions from the frustrated spin excitations in a J_1-J_2 magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting.
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摘要 :
Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the str...
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Polarized and unpolarized neutron-diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdFeAsO. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c axis are antiparallel like LaFeAsO. The ordered moment is 0.25(7) μ_B, which is the smallest moment found so far in these systems.
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The structural properties of the AFe_2As_2 (A = Sr,Ca) compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20 K. The experimental results demonstrate that the SrFe_2As_2...
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The structural properties of the AFe_2As_2 (A = Sr,Ca) compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20 K. The experimental results demonstrate that the SrFe_2As_2 compound, consistent with the previous x-ray data, undergoes a visible tetragonal-to-orthorhombic (T-O) phase transition at low temperature. Moreover, twinning lamellae arising from T-O transition clearly appear in the orthorhombic phase. On the other hand, TEM observations of CaFe_2As_2 reveal the presence of a pseudoperiodic structural modulation with a periodicity of around 40 nm at room temperature. This modulation is likely related to the local structural distortions within the Ca layers. In situ cooling TEM observations of CaFe_2As_2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.
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摘要 :
Muon spin relaxation measurements in iron-oxypnictide systems have revealed: (1) commensurate long-range order in undoped LaFeAsO; (2) a Bessel function line shape in LaFeAs(O_(0.97)F_(0.03)) which indicates possible incommensurat...
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Muon spin relaxation measurements in iron-oxypnictide systems have revealed: (1) commensurate long-range order in undoped LaFeAsO; (2) a Bessel function line shape in LaFeAs(O_(0.97)F_(0.03)) which indicates possible incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in superconducting LaFePO, CeFeAs(O_(0.084)F_(0.16)), and NdFeAs(O_(0.88)F_(0.12)) but absent in superconducting LaFeAs(O_(0.92)F_(0.08)); and (4) scaling of the superfluid density with T_c in the Ce-, La-, and Nd-FeAs superconductors following a nearly linear relationship found in cuprates.
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摘要 :
We report ~(75)As nuclear quadrupole resonance studies on superconducting oxypnictide LaFeAsO_(0.92)F_(0.08) (T_c=23 K). The temperature dependence of the spin-lattice relaxation rate (1 /T_1) decreases below T_c without a coheren...
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We report ~(75)As nuclear quadrupole resonance studies on superconducting oxypnictide LaFeAsO_(0.92)F_(0.08) (T_c=23 K). The temperature dependence of the spin-lattice relaxation rate (1 /T_1) decreases below T_c without a coherence (Hebel-Slichter) peak and shows a temperature dependence that is not simple power law nor exponential. We show that the result can be understood in terms of two superconducting gaps of either d- or ±s-wave symmetry, with the larger gap △_1 ～4kBT_c and the smaller one △_2 ～ 1.5k_BT_c. Our result suggests that the multiple-gap feature is universal in the oxypnictide superconductors, which is probably associated with the multiple electronic band structures in this class of materials. We also find that 1/T_1T above T_c. increases with decreasing temperature, which suggests weak magnetic fluctuations in the normal state.
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We performed optical spectroscopy measurement on single crystals of BaFe_2As_2 and SrFe_2As_2, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high te...
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We performed optical spectroscopy measurement on single crystals of BaFe_2As_2 and SrFe_2As_2, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave state, the formation of partial energy gaps was clearly observed with the surprising presence of two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the spin-density-wave instability is more likely to be driven by the Fermi surface nesting of itinerant electrons than a local-exchange mechanism.
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