Experimental Study Of Spin Properties In The Fractional Quantum Hall States

The discoveries of the integer and fractional quantum hall effects opened a new research field in condensed matter physics. Great progress has been made in the last 30 years. Landau quantization, disorder, electron-electron interaction and spin interactions varies the research of the Quantum Hall Effects.In this thesis, we mainly study the spin properties in the fractional quantum hall states. The spin degree of freedom brings various physics into the fractional quantum hall states and has led to a remarkable research field. Many interesting phenomena, such as spin transition between different ground states, nuclear and electron interaction, are relevant to the spin degree of freedom. In the two-Dimensional Electron System (2DES), electron and nuclear are coupled with each other via the hyperfine interaction. This couple enables us to study the spin properties by the technique of nuclear magnetic resonance (NMR).This thesis mainly describes two works. First, we reduced the electron temperature to about 10 mK through our improvements of the dilution refrigerator. Ultra-low temperature is a necessary condition for the fractional quantum hall effects. All the studies of the fractional quantum hall states are carried out at ultra-low temperature. Second, we use the technique of RDNMR to study the spin properties in the fractional quantum hall states.Our thesis is divided into five chapters.In chapter 1, we briefly introduce the background of 2DES, the basic concepts and the principles of the Quantum Hall Effects.In chapter 2, we briefly introduce the fundamental knowledge of the low temperature physics and the principle of the dilution refrigerator used in our experiments.In chapter 3, we describe the improvements of the dilution refrigerator through which we successfully reduce the electron temperature of the dilution refrigerator to about 10 mK. In chapter 4, we introduce the basic principle of resistance detected nuclear magnetic resonance (RDNMR). We use the technique of RDNMR to study the spin transition at 2/3 state and the nuclear spin relaxation at 1/2 state. And we have proved that the nuclear spin relaxation time T1 at 1/2 state do not follow the Korringa law.In chapter 5, we summarize the above experimental works.