טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYaish Yuval
SubjectMagneto-Transport and Thermodynamic Properties of a Two
Dimensional Hole Layer
DepartmentDepartment of Physics
Supervisor Professor Uri Sivan


Abstract

''Is there a metal and a metal insulator transition (MIT) in two dimensions?'' Numerous experiments in recent years find metallic characteristics, resistance increase with increased temperature, in various 2D systems. The same systems turn insulating at low densities. The crossover from metallic to insulating behavior was identified by some authors as a MIT The metallic characteristics are in sharp contrast  with the prevailing dogma that 2D systems are insulating and should hence display  diverging resistance at vanishingly low temperatures. This conflict motivated   extensive experimental and theoretical efforts. None of the explanations accounts for  all features in all materials. It would thus be fair to state that the metallic behavior, as  well as the MIT remain unexplained as universal phenomena. The situation is further complicated by recent thermodynamic measurements on 2D holes. In those experiments, the expected negative in-plane inverse compressibility was found to turn

positive when the carrier density was reduced below the MIT. Such positive values contrast all present theories for uniform, strongly correlated fermi liquid.

Today, there is no accepted theory, nor benchmark experiments, that map the free energy of strongly correlated 2D fermions in wide temperature and density ranges. The RPA is accurate at high densities where the Fermi energy is much larger than the Coulomb interaction between particles. However, in real metals and even more so in 2DHG, the opposite is correct, namely, the interaction energy is considerably larger than the Fermi energy. Various approximation schemes that go beyond RPA and take short range correlations into account have been invented. A particularly successful procedure was introduced by Singwi {\it et al.} We find that its generalization to finite temperatures accounts remarkably well for the measured inverse compressibility as long as the 2DHG is homogenous, even when the interaction energy is considerably larger than the kinetic one.