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Register a new userEvidence for Coexistence of Superconductivity and Magnetism in Single Crystals of Co-doped SrFe$_2$As$_2$
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In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe$_2$As$_2$, we have prepared single crystals of SrFe$_{2-x}$Co$_x$As$_2$, $x$ = 0 and 0.4, and characterized them via X-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At $x$ = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while $T_c$ = 19.5 K - indicating coexistent magnetism and superconductivity. A discussion of how these results compare with reported results, both in SrFe$_{2-x}$Co$_x$As$_2$ and in other doped 122 compounds, is given.
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We report anisotropic dc magnetic susceptibility $chi(T)$, electrical resistivity $rho(T)$, and heat capacity $C(T)$ measurements on the single crystals of CaFe$_{2-x}$Co$_x$As$_2$ for $x$ = 0 and 0.06. Large sized single crystals were grown by the high temperature solution method with Sn as the solvent. For the pure compound with $x$ = 0, a high temperature transition at 170 K is observed which is attributed to a combined spin density wave (SDW) ordering and a structural phase transition. On the other hand, for the Co-doped samples for $x$ = 0.06, the SDW transition is suppressed while superconductivity is observed at $simeq$17 K. The superconducting transition has been confirmed from the magnetization and electrical resistivity studies. The $^{57}$Fe Mossbauer spectrum in CaFe$_2$As$_2$ indicates that the SDW ordering is incommensurate. In the Co-doped sample, a prominent paramagnetic line at 4.2 K is observed indicating a weakening of the SDW state.
Among numerous hypotheses, recently proposed to explain superconductivity in iron-based superconductors [1-9], many consider Fermi surface (FS) nesting [2, 4, 8, 10] and dimensionality [4, 9] as important contributors. Precise determination of the electronic spectrum and its modification by superconductivity, crucial for further theoretical advance, were hindered by a rich structure of the FS [11-17]. Here, using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we disentangle the electronic structure of hole-doped BaFe2As2, and show that nesting and dimensionality of FS sheets have no immediate relation to the superconducting pairing. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: the magnitude of the superconducting gap maximizes at 10.5 meV exclusively for iron 3dxz;yz orbitals, while for others drops to 3.5 meV. Presented results reveal similarities of electronic response to superconducting and magneto-structural transitions [18, 19], implying that relation between these two phases is more intimate than just competition for FS, and demonstrate importance of orbital physics in iron superconductors.
The pseudogap, d-wave superconductivity and electron-boson coupling are three intertwined key ingredients in the phase diagram of the cuprates. Sr$_2$IrO$_4$ is a 5d-electron counterpart of the cuprates in which both the pseudogap and a d-wave instability have been observed. Here, we report spectroscopic evidence for the presence of the third key player in electron-doped Sr$_2$IrO$_4$: electron-boson coupling. A kink in nodal dispersion is observed with an energy scale of ~50 meV. The strength of the kink changes with doping, but the energy scale remains the same. These results provide the first noncuprate platform for exploring the relationship between the pseudogap, d-wave instability and electron-boson coupling in doped Mott insulators.
Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band structure calculations suggest that under strain the electron pockets develop a large in-plane anisotropy as compared with the hole pocket. Magnetotransport studies at low temperatures indicate that the mobility of the dominant carriers increases with tensile strain. Close to the critical temperature, all resistivity curves at constant strain cross in a single point, indicating a universal critical exponent linked to a strain-induced phase transition. Our results indicate that the superconducting state is enhanced under compressive strain and suppressed under tensile strain, in agreement with the trends observed in FeSe thin films and overdoped pnictides, whereas the nematic phase seems to be affected in the opposite way by the uniaxial strain. By comparing the enhanced superconductivity under strain of different systems, our results suggest that strain on its own cannot account for the enhanced high $T_c$ superconductivity of FeSe systems.
Recent studies on superconductivity in NbSe$_2$ have demonstrated a large anisotropy in the superconducting critical field when the material is reduced to a single monolayer. Motivated by this recent discovery, we use density functional theory (DFT) calculations to quantitatively address the superconducting properties of bulk and monolayer NbSe$_2$. We demonstrate that NbSe$_2$ is close to a ferromagnetic instability, and analyze our results in the context of experimental measurements of the spin susceptibility in NbSe$_2$. We show how this magnetic instability, which is pronounced in a single monolayer, can enable sizeable singlet-triplet mixing of the superconducting order parameter, contrary to contemporary considerations of the pairing symmetry in monolayer NbSe$_2$, and discuss approaches as to how this degree of mixing can be addressed quantitatively within our DFT framework. Our calculations also enable a quantitative description of the large anisotropy of the superconducting critical field, using DFT calculations of monolayer NbSe$_2$ in the normal state
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