The Study Of The Thermoelectric Properties On The Electron Doped Superconductor Nd_2-xCe_xCuO₄ And The Organic Molecular Salt (TMTSF)₂ClO₄

Transition metal compounds is a main topic in condensed matter physics for their interesting and abnormal properties,such as high-T_c superconductivity,organic superconductor et al,which have attracted much attention of researching.In this dissertation,we have studied the electron-type superconductor Nd_2-xCe_xCuO₄ single crystal,organic superconductor(TMTSF)₂ClO₄ and thermoelectric materials Nd_0.75Sr_1.25CoO₄ films in detail.The dissertation was arranged as six parts:In chapter 1,the author reviews the structure and phase diagram of the high-T_c superconductors.Their anomalous properties are briefly discussed and the recent development in theoretical and experimental physics about this topic is also mentioned.The theories of resistivity,thermopower and thermal conductivity,are also summarized,and the physical insight of these transport properties is also discussed. Moreover,the applications of thermoelectric effects are recommended.In chapter 2,we have presented the measurements of the in-plane resistivity, thermopower and thermal conductivity of the single crystal Nd_1-xCe_xCuO₄(x=0.14, 0.15),a sign reversal at the temperature about 42K is observed in thermopower of the sample with x=0.14,but it is positive in the whole temperature range for the sample with x=0.15.Moreover,both of the samples exhibit negative slope in the thermopower at high temperatures.All of these behaviors can be well interpreted by the two-band model with one whole band and the other electron band.At the same time,resistivity explained by the two-band model gives convincing evidence that two-band model is valid in electron-doped superconductivity.With x increasing,the thermal conductivity increases at high temperatures but decreases at low temperatures, which indicates that not only the contribution of the carriers on thermal conductivity is increasing,but also the scattering of the phonon is increasing.In chapter 3,the in-plane resistivity(p) and thermopower(S) were measured on Nd_1.86Ce_0.14CuO_4+δ single crystal after different annealing times.Both S and p can be described in terms of a two-band model with an electron broad band and a hole-like narrow band.From p and S data,we found that the hole component increases with the annealing time before the optimal superconductivity is achieved,while S and p reach saturation after further annealing in N₂.So the annealing process has the same effect as Ce doping just before the sample is optimally annealed,while after that they are not equal to each other.Furthermore,we also argue that the holes may be responsible for the superconductivity in the electron-doped cuprates.In chapter 4,the thermopower S(T),Hall coefficient R_H,and resistivity are studied for Nd_1.85-xM_xCe_0.15CuO₄(M=Gd and Sm) single crystals.Disorder is introduced into the cation sites outside the CuO₂ planes in the two systems and its degree is controlled by changing M content.Such doping nominally does not change the doped carrier density,which is confirmed by R_H.S(T) is analyzed in terms of a semiempirical model above 120 K for both doping,which assumes the coexistence of a narrow electron band and a broad one.In this model,both the bandwidth for the density of states and the bandwidth of the effective conductivity broaden with increasing x for both doping,while the tendency for localization is increased for Gd doping but nearly unchanged for Sm doping,which could be understood by stronger electronic disorder for Gd doping than that for Sm doping.This may be resulted from the difference in ionic radius(Gd³⁺＜Sm³⁺＜Nd³⁺).Furthermore,for Gd doping the superconducting T_c is strongly depressed with increasing the doping concentration, while for Sm doping T_c is nearly unchanged with increasing Sm content,which implies that the superconducting T_c may be related to the localization and the band structure of the itinerant carriers.In chapter 5,measurements of thermopower S_α(T) along the highly conducting a axis and specific heat of the Bechgaard salts(TMTSF)₂ClO₄ for various cooling rates through the anion ordering temperature T_α= 24 K were carried out.Sign reversal in S_α(T) is found below T_αand it decreases with increasing cooling rate,which is attributed to the change of a narrow band filling level as the temperature and the cooling rates change.The crossover from 2D to 3D in S_α(T) is observed around 15 K. The onset temperature of anion ordering in S_α(T) decreases from 29.8 to 24.2 K as the cooling rate increases.Meanwhile,the electronic specific heat coefficientγhas a pronounced change within this temperature region,giving strong evidence for a narrow band contribution.The difference in the specific heat between the quenched and relaxed states follows a T-cubic law from 5 to 24 K,implying a lattice distortion by the ordered anion only.The entropy estimated from the specific heat peak between 28 and 15 K is Rln(4/3) lower than the value Rln2,consistent with the thermopower result that some anions have been ordered far above T_αfor the relaxed state.In chapter 6,the experiments of electrical resistivity and thermopower on Nd_0.75Sr_1.25CoO₄ film in the temperature range 90 K to 310 K were carried out.The great difference in the activation energies estimated from thermopower and resistivity, a characteristic of small polarons,is observed,providing strong evidence for polaron-dominated transport mechanism in this material.Furthermore,the activation energy at intermediate-temperature region is larger than that at low-temperature region in resistivity,but it is not observed in thermopower,indicating that the energy for the creation of the carriers is slightly lower at low-temperature region than that at intermediate-temperature region.At the same time,the abrupt drop in the thermopower and the abnormal peak in the differential curve of resistivity indicate that a phase transition between a paramagnetic state and a ferromagnetic state occurs at temperature about 218 K.The positive thermopower in the whole temperature range measured suggests that the carriers are holes in this system.