| Breakthrough in material science played a major role in pushing forward condensed matter physics. The discovery of new materials not only enriched our knowledge enormously, but also challenged traditional theories through their novel properties. As for the unconventional superconductors, an intensively studied family of materials which includes cuprates, iron-based superconductors, ternary metal nitride halides systems etc, though the mechanisms of their superconductivity is yet to be fully understood, some universal features can still be concluded, such as relatively high superconducting critical temperatures, quasi-two-dimensional lattice structures and complex relations with magnetism, etc. Revealing the connections between these features and unconventional superconductivity is extremely crucial for understanding high-temperature superconductivity and the further realization of room-temperature superconductivity. On the other hand, the two-dimensional crystal materials, including graphene, few-layered black phosphorus and phosphorene, which all share similar honeycomb-like structure, novel properties such as Dirac fermion, two-dimentional electron gas, quantum Hall effect, have attracted much attention recently. Distinctive from zero gapped graphene, few-layered black phosphorus has a direct bandgap ranges from far-infrared region to visible light region, which achieve broad prospects in the application fields of electronic technology, solar energy, semiconductor device and so on.Most of my PhD career was contributed on two projects, one is the study of unconventional superconductivity in intercalated (3-MNC1, and the other is the works based on black phosphorus in which I cooperate with my colleague Ziji Xiang and the research group of Prof. Yuanbo Zhang from Fudan University, and the thesis will be launched centering around these two themes. We intercalated magnetic ions Yb into P-HfNCl layers via liquid ammonia method and characterized its superconducting properties, Tc (superconducting transition temperature) of 25.2 K was achieved which is very close to the highest Tc in this system. We successfully synthesized MNC1 (M=Zr, Hf) solid solution through high pressure method and systematically studied the relationship of its superconductivity and lattice mechanism by cointercalation of Ba ions and NH3 molecules. We increased the size of black phosphorus single crystals by improving graphite heater. In order to decrease thermal defect concentration and avoid air contamination, redesigned reaction process was introduced. The thesis is divided into four chapters. The first chapter is introduction, in which background information and current situation of relative studies will be informed. The topic of the second and the third chapter will be the unconventional superconductivity of P-MNX systems. In the fourth chapter, collaborating works and the improving growth method of black phosphorus will be introduced. Detailed description for each chapter is presented below.Chapter 1 IntroductionIn this section, background knowledge, recent achievements, unsolved problems and future tendency of relevant directions will be introduced. It contains three parts: discovery and development of superconductivity, overview of MNX system and the brief introduction of black phosphorus. In the part of discovery and development of superconductivity, a brief historical retrospect will be conducted on superconductivity and BCS theory, unconventional superconductivity of classical iron-based superconductors and cuprates will be mentioned. In the part of overview of MNX system, lattice structure, reaction route, general research method and the current situation of experimental and theoretical studies shall be thoroughly introduced. In the third part, common knowledge of phosphorus allotropes evolution, black phosphorus growth method development and background knowledge will be briefed.Chapter 2 Superconductivity in Rear-earth Intercalated β-MNClThe properties of superconductivity in magnetic rare-earth Yb ion cointercalated electron doped compounds Ybx(M)yHfNCl with M = NH3 and tetrahydrofuran (THF) will be discussed. The first usage of liquid ammonia method is introduced in this compound system. As for Yb0.2(NH3)yHfNCl and Yb0.3(NH3)yHfNCl, the Tc is at about 23 K and 24.6 K. With the replacement of NH3 molecule by a larger THF molecule, the superconducting transition temperature reaches 25.2 K in Yb0.2(THF)yHfNCl, which is very close to the highest Tc of 25.5 K observed in the alkali-metal intercalated β-HfNCl superconductors. The Tc of Yb0.2(THF)yHfNCl is evidently suppressed when pressure goes up to 0.5 GPa, at the same time the pressure effect on Tc becomes subtle above 0.5 GPa. These results indicate that the superconductivity in these layered intercalated β-HfNCl superconductors does not rely on the species of intercalated metal ions, including magnetic ones. The superconductivity in this system can’t be simply concluded with traditional BCS theory.Chapter 3 Substitution Effect on Metal Site of β-MNClSystematically studies will be carried out in barium cointercalated ternary metal nitride chloride compounds of Ba0.8(NH3)yZr1-xHfxNCl (x = 0,0.25,0.5,0.75,1). XRD and Raman Shift indicate a lattice constants and lattice dynamics evolves systematically upon element substitution. Te monotonically increases from 11.7 K to 20.2 K with the increasing hafnium concentration in MN bilayers from zero to one. For Ba0.8(NH3)yHfNCl, the upper critical field is estimated to be around 13.2 T. Accompanied by the remarkably Te increasing, the phonon mode of the highest frequency couples most strongly with electronic system shows a steady hardening with Hf replaced for Zr. The experimental fact indicates the enhancement of electron pairing during Hf substitution arises mainly from the change of electronic structure of metal d-band rather than the frequencies of lattice vibrations.Chapter 4 Single Crystal Growth and Studies of Black PhosphorusIn the last chapter, growth method discussion and the collaborating works in black phosphorus studies will be introduced, including field-effect transistors, quantum oscillation, quantum Hall effect, which were based on few-layered black phosphorus, and pressure induced topological semimetal state which was based on bulk black phosphorus single crystals.At the beginning of this section, single crystal growth method of black phosphorus will be discussed and primary characterization will be conducted. In order to satisfy the stringent requirements of sample quality, such as high carrier mobility and low concentration of lattice defects, a series of techniques were applied in black phosphorus single crystal growth. An annealing step and cooling step is introduced for decreasing thermal defect concentration and avoiding air contamination. Temperature gradient is built in high pressure cube by modifying the graphite tube heater in two approaches. One is to remove part of the graphite heater, the other is to assemble the graphite disks at one end instead of two. The acquired black phosphorus single crystal are of magnitude 4-5 millimeters which almost reaches the maximum of sample chamber. The relation between temperature gradient and crystal growth direction mentioned by Endo is of enlightening significance to the improvement growth method. Based on our results, we can go further with Endo’s conclusion:just like temperature and pressure, temperature gradient is an essential condition for the growth of large black phosphorus single crystal. |
PDF Full Text Request