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## Magnetoplasma excitations of two-dimensional anisotropic heavy fermions in AlAs quantum wells

The spectra of plasma and magnetoplasma excitations in a two-dimensional system of anisotropic heavy fermions are investigated. The spectrum of microwave absorption by disklike samples of stressed AlAs quantum wells at low electron densities shows two plasma resonances separated by a frequency gap. These two plasma resonances correspond to electron mass principle values of $(1.10\pm 0.05)m0\; and$$(0.20\pm 0.01)m0.\; The\; observed\; results\; correspond\; to\; the\; case\; of\; a\; single\; valley\; strongly\; anisotropic\; Fermi\; surface.\; It\; is\; established\; that\; an\; increase\; in\; electron\; density\; results\; in\; the\; population\; of\; the\; second\; valley,\; manifesting\; itself\; as\; a\; drastic\; modification\; of\; the\; plasma\; spectrum.\; We\; directly\; determine\; the\; electron\; densities\; in\; each\; valley\; and\; the\; intervalley\; splitting\; energy\; from\; the\; ratio\; of\; the\; two\; plasma\; frequencies.$