Pseudopin transition of two-dimensional hole systems in hydraulic pressure*

We preform transport measurement of high quality two-dimensional hole systems in a pressure cell at low temperatures. We find that the hydraulic pressure can induce phase transition to fractional quantum Hall effect near $\nu=3/2$. The discovered phenomenon is consistent with spin polarization transitions when the energy separation between the lowest two Landau levels is quenched by the large hydraulic pressure. Our finding reveal subtle dependence of spin degree of freedom on hydraulic pressure in hole systems.