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Register a new userQ Dependence of Magnetic Resonance Mode on FeTe$_{0.5}$Se$_{0.5}$ Studied by Inelastic Neutron Scattering
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Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe$_{0.5}$Se$_{0.5}$ to examine the ${bf Q}$-dependent enhancement of the dynamical structure factor, $S({bf Q},E)$, from ${bf Q}$ = (0, 0) to ($pi$, $pi$), including ($pi$, 0) in the superconducting state. In most of iron-based superconductors, $S({bf Q},E)$ is enhanced at ${bf Q}$ = ($pi$, 0), where the magnetic resonance mode is commonly observed in the unfolded Brillouin zone. Constant-$E$ cuts of $S({bf Q},E)$ suggest that the enhancement is not uniform in the magnetic excitation, and limited around ${bf Q}$ = ($pi$, 0). This result is consistent with the theoretical simulation of the magnetic resonance mode due to the Bardeen$-$Cooper$-$Schrieffer coherence factor with the sign-reversing order parameter of s$_{pm}$ wave.
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We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.
Mechanism of unconventional superconductivity in FeSe has been intensely scrutinized recently because of a variety of exotic properties unprecedented for other iron-based superconductors. A central unanswered question concerns the origin of the interaction that causes the nematic transition at $T_s=90,K$ without accompanying magnetic order. Elucidating the nature of spin excitations in the normal state is a key to addressing this issue. Here we report, from inelastic neutron-scattering measurements in FeSe single crystals, that high-energy spin excitation spectra of FeSe exhibit characteristic energy dependence with missing intensity at around 70-80$,$meV, which are very different from other iron-based superconductors. Despite of the strongest electron correlations among the iron-based superconductor family, the spectra are qualitatively at variance with the local moment model and can be essentially described by the itinerant electron picture. Moreover, the dynamical spin susceptibility above $T_s$ is only weakly temperature dependent, which is in stark contrast to the Curie-Weiss behavior of the electronic nematic susceptibility, suggesting that the nematic transition is not likely driven by spin but by orbital degrees of freedom.
NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ is a new layered superconductor. We have studied the low-lying electronic structure of a single crystalline NdO$_{0.5}$F$_{0.5}$BiS$_{2}$ superconductor, whose superconducting transition temperature is 4.87K, with angle-resolved photoemission spectroscopy. The Fermi surface consists of two small electron pockets around the X point and shows little warping along the $k_z$ direction. Our results demonstrate the multi-band and two-dimensional nature of the electronic structure. The good agreement between the photoemission data and the band calculations gives the renormalization factor of 1, indicating the rather weak electron correlations in this material. Moreover, we found that the actual electron doping level and Fermi surface size are much smaller than what are expected from the nominal composition, which could be largely explained by the bismuth dificiency. The small Fermi pocket size and the weak electron correlations found here put strong constraints on theory, and suggest that the BiS$_2$-based superconductors could be conventional BCS superconductors mediated by the electron-phonon coupling.
We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H|c$ and $H|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c sim 0.99$ (for $H|ab$) which is the consequence of a very small value of the coherence length $xi_c(0) sim 4 AA$ (and $xi_{ab}(0) sim 15 AA$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $lambda_{ab}(0) = 430 pm 50$ nm and $lambda_c(0) = 1600 pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($sim 4$), being close to the anisotropy of $H_{c2}$ for $Trightarrow T_c$. The temperature dependence of both $lambda$ ($propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.
We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at $mathbf{Q}=(0.5pmdelta, 0.5mpdelta, q_l)$. Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor eight weaker than the resonance signal in nearly optimally Co-doped BaFe$_2$As$_2$, although a factor two is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin space anisotropy with slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.
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