{"id":1933,"date":"2015-07-05T23:04:07","date_gmt":"2015-07-06T06:04:07","guid":{"rendered":"http:\/\/physikon.net\/?p=1933"},"modified":"2022-06-22T09:49:37","modified_gmt":"2022-06-22T16:49:37","slug":"1933","status":"publish","type":"post","link":"http:\/\/physikon.net\/?p=1933","title":{"rendered":"Comment on \u201cSuperconductivity in electron-doped layered TiNCl with variable interlayer coupling\u201d"},"content":{"rendered":"

<\/p>\n

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Comment on \u201cSuperconductivity in electron-doped layered TiNCl with variable interlayer coupling\u201d<\/strong>, D. R. Harshman and A. T. Fiory [arXiv<\/a>]\n

In their article, Zhang et al. [Phys. Rev. B 86<\/strong>, 024516 (2012)<\/a>] present a remarkable result for Ax<\/sub>(S)y<\/sub>TiNCl compounds (\u03b1-phase TiNCl partially intercalated with alkali A and optionally co-intercalated molecular species S), finding the superconducting transition temperature T<\/em>C<\/sub> scales with d<\/em>\u20131<\/sup>, where the spacing d<\/em> between TiNCl layered structures depends on intercalant thickness. Recognizing that this behavior indicates interlayer coupling, Zhang et al. cite, among other works, the interlayer Coulombic pairing mechanism picture [Harshman et al., J. Phys.: Condens. Matter. 23<\/strong>, 295701 (2011)<\/a>]. This Comment shows that superconductivity occurs by interactions between the chlorine layers of the TiNCl structure and layers containing Ax<\/sub>, wherein the transverse Ax<\/sub>-Cl separation distance \u03b6 is smaller than d<\/em>. In the absence of pair-breaking interactions, the optimal transition temperature is modeled by T<\/em>C0<\/sub> \u221d (\u03c3\/A<\/em>)1\/2\u00a0<\/sup>\u03b6\u20131<\/sup>, where \u03c3\/A<\/em> is the fractional charge per area per formula unit. Particularly noteworthy are the rather marginally-metallic trends in resistivities of Ax<\/sub>(S)y<\/sub>TiNCl, indicating high scattering rates, which are expected to partially originate from remote Coulomb scattering (RCS) from the Ax<\/sub> ions. By modeling a small fraction of the RCS as inducing pair-breaking, taken to cut off exponentially with \u03b6, observations of TC<\/sub> < TC0<\/sub> are quantitatively described for compounds with \u03b6 < 4 \u00c5, and T<\/em>C<\/sub> \u2248 T<\/em>C0<\/sub> for Na0.16<\/sub>(S)y<\/sub>TiNCl with propylene carbonate and butylene carbonate co-intercalants for which \u03b6 > 7 \u00c5. Since a spatially separated alkali-ion layer is not formed in Li0.13<\/sub>TiNCl, the observed T<\/em>C<\/sub> of 5.9 K is attributed to an intergrowth phase related to TiN (T<\/em>C<\/sub> = 5.6 K).<\/p>\n\n\n\n\n\t
\"Comment_Fig1_A\"<\/a>
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Reduced pair breaking parameter α\/x plotted as a function of the interaction distance ζ for Ax<\/sub>(S)y<\/sub>TiNCl. The fitted curve corresponds to pair breaking due to RCS interactions, attenuating exponentially with ζ.<\/p><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n\n

D. R. Harshman and A. T. Fiory, Phys. Rev. B 90<\/strong>, 186501 (2014)<\/a>.<\/p>\n

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