Temperature dependence of the spin polarization of composite fermions I. V. Kukushkin* ) Institute of Solid-State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia; Max Planck Institu ¨t, 70569 Stuttgart, Germany K. von Klitzing Max Planck Institute, 70569 Stuttgart, Germany K. G. Levchenko Institute of Solid-State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia Yu. E. Lozovik Institute of Spectroscopy, Russian Academy of Sciences, 142092 Troitsk, Moscow Region, Russia Submitted 10 November 1999 Pis’ma Zh. E ´ ksp. Teor. Fiz. 70, No. 11, 722–726 10 December 1999 It is found that at a critical value of the magnetic field in which a system of composite fermions becomes completely spin-polarized, the temperature dependence of the electronic spin polarization is a linear function at low temperatures. It is shown that the slope of this depen- dence is determined by the Fermi energy of the composite fermions. This made it possible to measure the Fermi energy and the Zeeman splitting of the composite fermions. A large amplification of the spin splitting of composite fermions for complete spin polarization of the system is found. This makes it possible to measure the strength of the interaction between composite fermions. © 1999 American Institute of Physics. S0021-36409900423-5 PACS numbers: 05.30.Pr, 71.10.Pm The concept of new quasiparticles, composite fermions, consisting of an electron with an even number of attached magnetic-flux quanta, has been proposed to explain the fractional quantum Hall effect. 1 New states of compressible quantum liquids, realized with fractional fillings of the Landau levels with even denominators, have been predicted on the basis of the composite-fermion picture. 2 The existence of composite fermions has been demonstrated in various experiments in two-dimensional electronic systems in strong magnetic fields. 3–5 Composite fermions, having the same charge, spin, and statistics as electrons, differ from the latter by the fact that in the mean-field approximation they are influenced by the effective magnetic field B * =B -2 n e  0 , where B is the external field,  0 = hc / e is the magnetic flux quantum, and n e is the electron density. For half-filling of Landau levels JETP LETTERS VOLUME 70, NUMBER 11 10 DEC. 1999 730 0021-3640/99/70(11)/6/$15.00 © 1999 American Institute of Physics