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Register a new userPhonon anomalies in FeS
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Added by
Ana Milosavljevi\\'c
Publication date
2017
fields
Physics
and research's language is
English
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We present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A1g and B1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20K of all observed phonon energies is governed by the lattice contraction. Below 20K the phonon energies increase by 0.5-1 cm$^{-1}$ thus indicating putative short range magnetic order. Along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.
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Low temperature specific heat has been measured in superconductor $beta$-FeS with T$_c$ = 4.55 K. It is found that the low temperature electronic specific heat C$_e$/T can be fitted to a linear relation in the low temperature region, but fails to be described by an exponential relation as expected by an s-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat $Deltagamma$=[C$_e$(H)-C$_e$(0)]/T shows the Volovik relation $Deltagamma_e(H)propto sqrt{H}$, suggesting the presence of nodal gap(s) in this material.
Here we report the electronic structure of FeS, a recently identified iron-based superconductor. Our high-resolution angle-resolved photoemission spectroscopy studies show two hole-like ($alpha$ and $beta$) and two electron-like ($eta$ and $delta$) Fermi pockets around the Brillouin zone center and corner, respectively, all of which exhibit moderate dispersion along $k_z$. However, a third hole-like band ($gamma$) is not observed, which is expected around the zone center from band calculations and is common in iron-based superconductors. Since this band has the highest renormalization factor and is known to be the most vulnerable to defects, its absence in our data is likely due to defect scattering --- and yet superconductivity can exist without coherent quasiparticles in the $gamma$ band. This may help resolve the current controversy on the superconducting gap structure of FeS. Moreover, by comparing the $beta$ bandwidths of various iron chalcogenides, including FeS, FeSe$_{1-x}$S$_x$, FeSe, and FeSe$_{1-x}$ Te$_x$, we find that the $beta$ bandwidth of FeS is the broadest. However, the band renormalization factor of FeS is still quite large, when compared with the band calculations, which indicates sizable electron correlations. This explains why the unconventional superconductivity can persist over such a broad range of isovalent substitution in FeSe$_{1-x}$Te$_{x}$ and FeSe$_{1-x}$S$_{x}$.
We study the temperature dependence of the low energy phonons in the $(H, 0, L)$ reciprocal plane of the highly ordered ortho-II YBa$_2$Cu$_3$O$_{6.55}$ cuprate high temperature superconductor by means of high-resolution inelastic x-ray scattering. Anomalies associated with the emergence of long-range charge density wave (CDW) fluctuations are observed, and are qualitatively similar to those previously observed in the $(0, K, L)$ plane. This confirms the unconventional nature of this bi-dimensional CDW, which is not soft-phonon driven. With the support of first principles calculations, the symmetry of the anomalous phonon is identified and is found to match that of the charge modulation. This suggests in turn that these anomalies originate from a direct coupling between the phonons and the collective CDW excitations.
We report the novel preparation of single crystals of tetragonal iron sulfide, FeS, which exhibits a nearly ideal tetrahedral geometry with S--Fe--S bond angles of 110.2(2) $^circ$ and 108.1(2) $^circ$. Grown via hydrothermal de-intercalation of K${_x}$Fe${_{2-y}}$S${_2}$ crystals under basic and reducing conditions, the silver, plate-like crystals of FeS remain stable up to 200 $^circ$C under air and 250 $^circ$C under inert conditions, even though the mineral mackinawite (FeS) is known to be metastable. FeS single crystals exhibit a superconducting state below $T_c=4$ K as determined by electrical resistivity, magnetic susceptibility, and heat capacity measurements, confirming the presence of a bulk superconducting state. Normal state measurements yield an electronic specific heat of 5~mJ/mol-K$^2$, and paramagnetic, metallic behavior with a low residual resistivity of 250~$muOmegacdot$cm. Magnetoresistance measurements performed as a function of magnetic field angle tilted toward both transverse and longitudinal orientations with respect to the applied current reveal remarkable two-dimensional behavior. This is paralleled in the superconducting state, which exhibits the largest known upper critical field $H_{c2}$ anisotropy of all iron-based superconductors, with $H_{c2}^{||ab}(0) / H_{c2}^{||c}(0)=$(2.75~T)/(0.275~T)=10. Comparisons to theoretical models for 2D and anisotropic-3D superconductors, however, suggest that FeS is the latter case with a large effective mass anisotropy. We place FeS in context to other closely related iron-based superconductors and discuss the role of structural parameters such as anion height on superconductivity.
The magnetoresistance and magnetic torque of FeS are measured in magnetic fields $B$ of up to 18 T down to a temperature of 0.03 K. The superconducting transition temperature is found to be $T_c$ = 4.1 K, and the anisotropy ratio of the upper critical field $B_{c2}$ at $T_c$ is estimated from the initial slopes to be $Gamma(T_c)$ = 6.9. $B_{c2}(0)$ is estimated to be 2.2 and 0.36 T for $B parallel ab$ and $c$, respectively. Quantum oscillations are observed in both the resistance and torque. Two frequencies $F$ = 0.15 and 0.20 kT are resolved and assigned to a quasi-two-dimensional Fermi surface cylinder. The carrier density and Sommerfeld coefficient associated with this cylinder are estimated to be 5.8 $times$ 10$^{-3}$ carriers/Fe and 0.48 mJ/(K$^2$mol), respectively. Other Fermi surface pockets still remain to be found. Band-structure calculations are performed and compared to the experimental results.
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