Physics 838 Seminar
04.02.2018 4:00 pm - 5:00 pm
John S. Toll Rm 1201


Speaker:  Sarkar, Pampa (Greene)

Title:  Evidencefor Fermi surface reconstruction from Thermopower measurement for electrondoped superconductor La2−xCexCuO4

Abstract:  The nature of the normal state and the origin of the high-TCsuperconductor (HTSC) in the cuprates is still a major unsolved problem. The thermopowerS of the cuprate superconductor La2−xCexCuO4 (LCCO)thin films was measured in magnetic fields large enough to access the normalstate at low temperatures, for a range of Ce concentrations from x = 0.11 to x= 0.16. The thermopower in the cupratesuperconductor LCCO suggests that its Fermi surface undergoes a reconstruction forx ≤ 0.14. For 0.15 and 0.16 samplesS/T varies with lnT at low temperature. This analysis indicatesthat low-T thermopower may be influenced by spin fluctuations which areimportant for the superconducting mechanism.

Speaker:  Kohler, Tim (Osborn)

Title:  Possible Quasiparticle transition in superconducting titanium nitride film

Abstract:  High-quality superconducting filmsof TiN and related NbTiN is of recentinterest for astronomy detector and quantum computing applications. We studyTiN films which have an anomolous dependence on the growth conditions.Resonator quality factor (the inverse of single-photonmicrowave loss) can be sensitively destroyed by slightly changing the growthconditions. The films of particular interest are ones grown thin on oxidizedsubstrates,which exhibit[  an order of magnitude decrease ininternal quality factor (Qi) relative to recipes with either thicker films orwithout the oxidation. These films exhitbited smaller grains on average and 111 crystal growth. The average films appear to beonly weakly disordered in some measurements with 13where is the Fermiwave number and 𝑙 is the mean free path. Thetemperature dependence of the conductivity is fit to a model which sometimesincludes inelastic quasiparticle scattering. The difference between filmsappears to come from a thin superconducting layer in one film preparation whichproduces quasiparticles, perhaps fromrare small grains with suppressed superconductivity.

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