Probing microscopic properties of low-dimensional inorganic materials using scanning probe microscopes

Low-temperature ultra high vacuum scanning tunneling microscopy (STM) has been used to study charge density wave (CDW) systems in layered transition metal dichalcogenides including 1T-TaSe{dollar}sb2{dollar}, 1T-TaS{dollar}sb2{dollar}, 2H-TaSe{dollar}sb2{dollar} and 4Hb-TaSe{dollar}sb2{dollar}. We have discovered a new experimental approach to reproducibly make nanometer sized defects on the sample surface. In 1T-TaSe{dollar}sb2{dollar} system, we observed lines in the CDW lattice around the defects. CDW lattices are commensurate on both sides of the lines, but they change their phases across the lines. The behavior of these lines is similar to discommensuration lines predicted by McMillan in his theory. However, the termination of the lines can not be readily explained by discommensuration alone. We developed a theoretical model that takes into account of the effects of atomic dislocations on discommensuration to explain our experimental observations.; In 1T-TaS{dollar}sb2{dollar} system, the CDW lattice forms irregular domain structure around the defects. Inside each domain, the CDW lattices are commensurate to the underlying atomic lattice, and CDW change phases across the domain walls. The phase shifting across the domain walls is not limited to only one atomic vector, shifting with two atomic vectors were observed in our experiment.; In 4Hb-TaSe{dollar}sb2{dollar} and 2H-TaSe{dollar}sb2{dollar} systems, we observed a totally different and unexpected phenomenon, a STM tip induced local H to T solid phase transformation at low temperature. This phase transformation is induced by the same technique we used to create nano-defects in 1T-TaSe{dollar}sb2{dollar} and 1T-TaS{dollar}sb2{dollar}, but instead of a CDW lattice change around the defect as in the other two systems, the H phase atomic layer around the defect changed to T phase. An atomic structure model was developed to simulate the phase change. This discovery is very important for both technological applications and fundamental researches in nanostructures.; The effect of oxygen and lead doping in high temperature superconducting materials Bi{dollar}rmsb2Srsb2CuOsb{lcub}6+delta{rcub}{dollar} and Bi{dollar}rmsb2Srsb2CaCusb2Osb{lcub}8+d{rcub}{dollar} has been systematically studied by STM, scanning tunneling spectroscopy (STS) and other techniques. We were able to prepare high quality Bi-2212 single crystals with different superconducting temperatures (T{dollar}sb{lcub}rm c{rcub}{dollar}'s) by varying the oxygen concentration. Systematic STS measurements of these high-quality Bi-2212 single crystals with T{dollar}sb{lcub}rm c{rcub}{dollar}'s ranging from 79 to 92K have been made at liquid helium temperature. From these studies it has been possible to delineate intrinsic features of the quasiparticle spectrum in Bi-2212 materials. Specifically, we find that the magnitude of 2{dollar}Delta{dollar} at 4.2K is nearly constant in the crystals with T{dollar}sb{lcub}rm c{rcub}{dollar}'s varying nearly 15K. Hence, in these oxygen-doped Bi-2212 crystals, the reduced gap 2{dollar}Delta{dollar}/kT{dollar}sb{lcub}rm c{rcub}{dollar} is not constant, but increases as T{dollar}sb{lcub}rm c{rcub}{dollar} decreases. In addition, we find that there are intrinsic quasiparticel excitations within the gap region irrespective of the T{dollar}sb{lcub}rm c{rcub}{dollar}'s of the samples. Analyses of these data using a variety of pairing models suggest that highly anisotropic gap functions provide a good fit to the experimental data.