Thermal Transport Properties Of Electron-doped Cuprates La_2-xCe_xCuO_4±? And Probe Design

The thermal transport can provide useful information on Fermi-surface structure,quantum critical points,dynamic of vortex motion and superconducting fluctuations of cuprates,yet the lack of high accuracy for measurement has hindered the development of thermal transport.In this thesis,we designed a thermal transport probe based on PPMS and systematically studied the thermal transport properties of electron-doped cuprate superconductor La_2-xCe_xCuO₄?LCCO?films.The main results are listed as follows:?1?We designed a thermal transport probe with double shield and heat sink,which effectively reduce the temperature drift on the cold end and the temperature fluctuation on the hot end.When establishing maximum permissible temperature gradient,the temperature fluctuation is less than 2%.The integrated design for measuring wires was adopted to minimize the number of wire nodes and to smooth the measuring curve.Therefore,the small signals of Seebeck and Nernst could be accurately achieved by this thermal transport system.?2?We verified the evolution of Fermi-surface structure from under-doped to over-doped regime on the LCCO films by Seebeck measurement.The dramatic change in temperature dependence of S has been also used to catch the quantum critical point near the optimal-doped.Since the extrapolation of nomal state S-T curve at low temperature does not pass the origin,an electron band may exist in the over-doped regime?at least x ? 0.15?.Meanwhile,phonon drag effects seem to play an important role for the thermoelectric effects in LCCO as is suggested by characteristic point?T = 80K?of the S-T curve.With applied external magnetic field,it's surprising that S-B curves of different doped samples are independent of magnetic field.?3?We achieved three conclusions by measuring Nernst signal on LCCO film at x = 0.11 and 0.15.a)Nernst signal has a linear relationship with the magnetic field in the normal state;b)The Gaussian fluctuations exists on LCCO at T > Tc,in which Nernst signal reveals a field scale in its field dependence that increases with increasing temperature;c)In case of weak fields?d Bxl?,USH calculation has verified that /SCN B is independence of B in agreement with our experiment data.