Single Crystal Growth And Characterization Of Mercury-based High T_C Cuprate Superconductors And Cuprous Oxide

Since the discovery of high-temperature superconductivity in La2–xBaxCuO4 in 1986, the study of the lamellar copper oxides has remained at the forefront of condensed matter physics. Many new phenomena including'pseudogap'has been found and related to the thermodynamic, magnetic and electronic properties of strong correlation system. The mechanism of high-temperature superconductivity still remain mystery and many scientists including P. W. Anderson try to understand it, but there are too many theories and no one is accepted by all people. The uncertainty of experiments is responsible for the difficulty to explain the mechanism of high temperature superconductivity. Due to the high anisotropy of high temperature superconductor (HTSC), high quality single crystal samples are needed for the measurement. In the HTSC family, the sizable single crystals of La-based, Bi-based, Y-based, Nd, based HTSC have been obtained, but their transition temperature is low or their structure is very complicated, so they are not good candidates for the study the mechanism of high temperature superconductivity. The compounds HgBa₂Can–1CunO2n+2+δ(Hg12(n-1)n )can be viewed as model systems not only because of their record high-Tc values, but also because of their simplest and high-symmetry crystal structures. By now, the sizable crystal of Hg-based HTSC is not available for the measurement because of toxicity and low boiling point of mercury oxide. Due to a lack of sizable single crystals, experimental work on this very attractive system has been significantly limited. Here we report a novel recipe for the growth of Hg1201 and Hg1212 crystals.The methods for the synthesis of precursor and crystal growth of Hg-based HTSC are discussed in this thesis. The sintering of barium, calcium nitrates and copper oxide with oxygen flow were adopted to synthesize the precursor in order to prevent from the pollution of carbonate. The special device was designed to avoid the formation of hydroxides. The traditional encapsulation technique is employed for the crystal growth. The high quality quartz tubes were used to avoid the explosion.By carefully dosing the precursor, controlling the excess mercury oxide and using properly temperature profile, high quality single crystal of Hg1201 and Hg1212 were obtained. The TC of the optimal doping Hg1201 and Hg1212 is 97K and 127K, respectively. The effect of the substitution of magnetic Cu ion with nonmagnetic Zn ion provide a good tool for understanding anomalous physical properties of HTSC. The Hg1201 crystals with different Zn substitution (x = 0.01, 0.03, 0.05) were obtained. The substitution of Zn suppresses the TC significantly. The electronic and magnetic properties of Hg1201 were mainly discussed in this thesis.The magnetometry of Hg1201 crystals were measured with Quantum Design MPMS (SQUID). The results show that the transition of magnetic susceptibly is very sharp (2K), which means the oxygen is very homogeneous in the crystal or the quality of the crystal is very high. The results of magnetometry of zero field cooling (ZFC) and field cooling shows the crystal has very high superconductivity ration, which also prove the high quality of our crystal. The resistivity measurement is very important to understanding the mechanism of high temperature superconductivity. The resistivity along ab plane and c-axis were obtained using Quantum Design PPMS. The connection between the probe and the crystal are improved by evaporation of gold to reduce the contact resistivity. During the measurement of resistivity of ab plane, the four probes were connected to the side of crystal in order to avoid the c-axis component. The results of resistivity measure show that the transition of some crystal is not very sharp, even the high quality was demonstrated by magnetic measurement. Since the magnetic field is screened by superconducting phase on the surface of crystal, the homogeneity cannot be detected by direct magnetic measurement. Therefore, more time are needed to anneal the crystal in order to make the oxygen homogeneous. The reliable data of resistivity measurement was obtained using the crystal with long time annealing. The results show the temperature where the pseudogap open is 250K, which is much higher that that ever reported.The HTSC is type II superconductor, which contain superconducting state and normal state. The remnant field (REM) can be pined into the normal state when the external magnetic field is removed. The REM can be used as'external field'to detect diamagnetic properties of HTSC. Since the REM distribute in the whole crystal, the homogeneity of oxygen can be detected using REM as'external field'.The properties of HTSC are related to the oxygen doping. The as-grown Hg1201 crystals were annealed at the temperature from 300 to 500°C under vacuum, high-oxygen-pressure or in air in order to uniformly tune the oxygen concentration. The high quality crystals with serial doping from very underdoped (TC = 47K) to nearly optimal doped (TC = 95K), then to highly overdoped (TC = 64K) were obtained using the above annealing method.When we studied the synthesis of the precursor for the crystal growth of Hg-based superconductors, Cu2O crystals with truncated octahedron were abtained by high pressure flux method, which have many applications such as pigment, gas sensor, magnetic storage media, solar-energy transformation, electronics, semiconductors, varistors, and catalysis. To date, systematic manipulation of Cu2O has been achieved using solution routes, electrodeposition methods and etc., which requires either very high temperature or the organic surfactants that often cause environment contamination. Due to the need for circumstance protection, new'green'synthesis is developing for Cu2O crystal growth. Compared to above methods, our flux approach is very method for this need, which has many advantages. First of all, the raw materials are stable and facilely obtained from the copper minerals, and there are no other additives to reduce Cu2+ to Cu+, which makes the reduction relatively simple. Secondly, the alkali flux is conventional and environmental compared with the traditional solvents. The substitution of the conventional liquid containing the organic and sulfate solvents avoids the pollution to the environment because the alkali can be easily recycled. Finally, this reaction can be carried out under very low temperature and in extremely short time (even to 10 min) to obtain the Cu2O powder, so the energy can be saved. In general, this approach is novel and'green', and can be applied into the industry production to improve the manipulation of the process and the efficiency of the raw materials, lower the cost of the production and reduce the environmental pollution. Herein, we present a facile, elementary and very'green'approach to synthesize Cu2O at very low temperature and without any assistance of the hazardous organic compounds or surfactants. The reverse dismutation reaction of Cu2O was employed to obtain polycrystalline and grow the crystals with the assistance of the flux of mixed NaOH and KOH.The reaction between CuO and Cu under certain temperature and pressure can be a route for the synthesis of Cu2O, but that normally carries out under extremely high temperature. We take account into the reaction between CuO and Cu in air. The results show that the reaction between CuO and Cu must be at higher temperature than 1050°C in air, and it is actually a two-step reaction: the oxidation of Cu and the decomposition of CuO. Next, we checked this reaction in argon atmosphere and a closed system. In the both of atmosphere, CuO and Cu can react with each other, but not fully. Thus, in argon atmosphere and a closed system, which can be considered as the circumstance lack of oxygen, the reverse dismutation reaction of Cu2O can be carried out. In general, the reaction between CuO and Cu is rather different with or without the existence of oxygen, which means it is quite sensitive to oxygen. The reverse dismutation reaction of Cu2O can be carried out at very low temperature, but more reaction time is required for a full reaction of Cu2O. High pressure normally provides a closed system in which the oxygen is lack, and also increases the reaction rates and improves the crystal growth. The growth of Cu2O crystals in this paper is a perfect example of the combination of high pressure and flux. In this thesis, after optimizing the reaction conditions, we have synthesized Cu2O crystals at 0.2 GPa and 450°C for 24 hours. To the best of our knowledge, these Cu2O crystals with truncated octahedron were artificially synthesized in one step for the first time. We tried to vary the reaction temperature and duration to investigate the influence on the morphology of the products. The SEM images of as-prepared Cu2O crystals at different reaction conditions reveal that the morphology of as-synthesis Cu2O crystals is identical or the temperature and reaction duration didn't affect the growth habit of Cu2O crystal but only on the sizes. Another factor that affects the growth is the kinetics, which is related with its structure. Cu2O crystallizes in the cubic structure, in which each O atom is surrounded by a tetrahedral of copper ions, and each Cu cation has two oxygen neighbors. The atoms constitute results in the differences of the growth rate along different directions. The growth rates along <110> and <111> are almost the same and the growth rate along <100> direction is faster than that <110> and <111>. Thus, the octahedral with truncate edges is formed.