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Register a new userSuperconducting Gap and Symmetry in FeSe_1-x_Te_x_ Studied by Specific Heat in Magnetic Fields
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In order to investigate details of the superconducting (SC) gap in the iron-chalcogenide superconductors, the specific heat, C, of FeSe_1-x_Te_x_ with x=0.6-1 has been measured in magnetic fields. Using the two-gap model, it has been found that the smaller SC gap is significantly depressed by the application of magnetic field, resulting in the increase of the slope of the C/T vs T^2^ plot at low temperatures. From the specific-heat measurements at very low temperatures down to 0.4 K, it has been found that the enhancement of the residual electronic-specific-heat-coefficient in the ground state, gamma_0_, by the application of magnetic field is much smaller than that expected for superconductors with the typical s-wave or d-wave SC paring symmetry, which is in sharp contrast to the significant enhancement of gamma_0 observed in the iron-pnictide superconductors. These results are discussed in relation to the multi-band effect in the iron-based superconductors.
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We report on specific heat measurements in Ba$_{1-x}$K$_x$Fe$_{2}$As$_{2}$ ($xle 0.6$). For the underdoped sample with $x=0.2$ both the spin-density-wave transition at $T = 100$ K and the superconducting transition at 23 K can be identified. The electronic contribution to the specific heat in the superconducting state for concentrations in the vicinity of optimal doping $x=0.4$ can be well described by a full single-gap within the BCS limit.
We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ compound and determined the accurate momentum dependence of the superconducting (SC) gap in four Fermi-surface sheets including a newly discovered outer electron pocket at the M point. The SC gap on this pocket is nearly isotropic and its magnitude is comparable ($Delta$ $sim$ 11 meV) to that of the inner electron and hole pockets ($sim$12 meV), although it is substantially larger than that of the outer hole pocket ($sim$6 meV). The Fermi-surface dependence of the SC gap value is basically consistent with $Delta$($k$) = $Delta$$_0$cos$k_x$cos$k_y$ formula expected for the extended s-wave symmetry. The observed finite deviation from the simple formula suggests the importance of multi-orbital effects.
We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc = 8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional and/or multiband superconductivity in PrPt4Ge12.
The heat capacity of a 2H-NbS2 single crystal has been measured by a highly sensitive ac technique down to 0.6 K and in magnetic fields up to 14 T. At very low temperatures data show excitations over an energy gap (2DS/kBTc approx 2.1) much smaller than the BCS value. The overall temperature dependence of the electronic specific heat Ce can be explained either by the existence of a strongly anisotropic single-energy gap or within a two-gap scenario with the large gap about twice bigger than the small one. The field dependence of the Sommerfeld coefficient shows a strong curvature for both principal-field orientations, parallel and perpendicular to the c axis of the crystal, resulting in a magnetic field dependence of the superconducting anisotropy. These features are discussed in comparison to the case of MgB2 and to the data obtained by scanning-tunneling spectroscopy. We conclude that the two-gap scenario better describes the gap structure of NbS2 than the anisotropic s-wave model.
The specific heat of the superconducting cuprates is calculated over the entire phase diagram. A d-wave BCS approach based on the large Fermi surface of Fermi liquid and band structure theory provides a good description of the overdoped region. At underdoping it is essential to include the emergence of a second energy scale, the pseudogap and its associated Gutzwiller factor, which accounts for a reduction in the coherent piece of the electronic Greens function due to increased correlations as the Mott insulating state is approached. In agreement with experiment, we find that the slope of the linear in T dependence of the low temperature specific heat rapidly increases above optimum doping while it is nearly constant below optimum. Our theoretical calculations also agree with recent data on Bi$_2$Sr$_{2-rm x}$La$_{rm x}$CuO$_{6+delta}$ for which the normal state is accessed through the application of a large magnetic field. A quantum critical point is located at a doping slightly below optimum.
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