Optical properties of a two-dimensional electron gas at even-denominator filling fractions

The optical properties of an electron gas in a magnetic field at filling fractions \nu = {1\over 2m} (m=1,2,3...) are investigated using the composite fermion picture. The response of the system to the presence of valence-band holes is calculated. The shapes of the emission spectra are found to differ qualitatively from the well-known electron-hole results at zero magnetic field. In particular, the asymmetry of the emission lineshape is found to be sensitive to the hole-composite fermion plane separation.Comment: 17 pages, LaTeX, 7 figures. This revised version is to appear in Physical Review

Mixed symmetry superconductivity in two-dimensional Fermi liquids

We consider a 2D isotropic Fermi liquid with attraction in both $s$ and $d$ channels and examine the possibility of a superconducting state with mixed $s$ and $d$ symmetry of the gap function. We show that both in the weak coupling limit and at strong coupling, a mixed $s+id$ symmetry state is realized in a certain range of interaction. Phase transitions between the mixed and the pure symmetry states are second order. We also show that there is no stable mixed $s+d$ symmetry state at any coupling.Comment: 3 figures attached in uuencoded gzipped file

Phase transition from a dx2−y2d_{x^2-y^2} to dx2−y2+dxyd_{x^2-y^2}+d_{xy} superconductor

We study the phase transition from a $d_{x^2-y^2}$ to $d_{x^2-y^2}+d_{xy}$ superconductor using the tight-binding model of two-dimensional cuprates. As the temperature is lowered past the critical temperature $T_c$, first a $d_{x^2-y^2}$ superconducting phase is created. With further reduction of temperature, the $d_{x^2-y^2}+d_{xy}$ phase is created at temperature $T=T_{c1}$. We study the temperature dependencies of the order parameter, specific heat and spin susceptibility in these mixed-angular-momentum states on square lattice and on a lattice with orthorhombic distortion. The above-mentioned phase transitions are identified by two jumps in specific heat at $T_c$ and $T_{c1}$.Comment: Latex file, 5 pages, 6 postscript figures, Accepted in Physical Review

Superconductivity in the SU(N) Anderson Lattice at U=\infty

We present a mean-field study of superconductivity in a generalized N-channel cubic Anderson lattice at U=\infty taking into account the effect of a nearest-neighbor attraction J. The condition U=\infty is implemented within the slave-boson formalism considering the slave bosons to be condensed. We consider the $f$-level occupancy ranging from the mixed valence regime to the Kondo limit and study the dependence of the critical temperature on the various model parameters for each of three possible Cooper pairing symmetries (extended s, d-wave and p-wave pairing) and find interesting crossovers. It is found that the d- and p- wave order parameters have, in general, very similar critical temperatures. The extended s-wave pairing seems to be relatively more stable for electronic densities per channel close to one and for large values of the superconducting interaction J.Comment: Seven Figures; one appendix. Accepted for publication in Phys. Rev.

Spin-triplet superconducting pairing due to local (Hund's rule, Dirac) exchange

We discuss general implications of the local spin-triplet pairing among fermions induced by local ferromagnetic exchange, example of which is the Hund's rule coupling. The quasiparticle energy and their wave function are determined for the three principal phases with the gap, which is momentum independent. We utilize the Bogolyubov-Nambu-De Gennes approach, which in the case of triplet pairing in the two-band case leads to the four-components wave function. Both gapless modes and those with an isotropic gap appear in the quasiparticle spectrum. A striking analogy with the Dirac equation is briefly explored. This type of pairing is relevant to relativistic fermions as well, since it reflects the fundamental discrete symmetry-particle interchange. A comparison with the local interband spin-singlet pairing is also made.Comment: 16 pages, LaTex, submitted to Phys. Rev.

BOSE CONDENSATION IN SPIN POLARIZED ATOMIC HYDROGEN

Nous proposons une description phénoménologique de l'hydrogène polarisé suggérée par l'hamiltonien microscopique et qui inclut deux particules de Bose qui correspondent, à la limite des faibles densités, aux deux états hyperfins les plus bas. On s'attend à ce que l'expérience peuple initialement les deux états et à ce que les temps d'équilibre soient longs. Quand la condensation de Bose a lieu dans chacun des deux états, il apparaît une aimantation cohérente spontanée perpendiculairement au champ de stabilisation ; ceci devrait être facilement observable par une simple expérience de résonance magnétique. Les inhomogénéités de champ ne devraient pas élargir le signal de résonance dans une telle expérience, contrairement à ce qui se passe avec des systèmes non condensés.A phenomenological description of spin-polarized hydrogen, suggested by the microscopic Hamiltonian, is proposed that includes two Bose particles which correspond in the low density limit to the two lowest hyperfine states. Experiments are expected to initially populate both states, and equilibration times will be long. When Bose condensation occurs in both states a spontaneous coherent magnetization perpendicular to the stabilizing field will appear ; that should be readily observable in a simple magnetic resonance experiment, Field inhomogeneities should not broaden the resonance signal in such an experiment, contrary to one's experience with uncondensed systems

Intersubband relaxation of two-dimensional electrons in heterostructures

We calculate the lifetime of a nonequilibrium electron in the first excited subband in the low-density heterostructure where this photocreated carrier occurs at the last stage of its cooling. The electron interaction with acoustic phonons gives the dominant intersubband relaxation mechanism, if the intersubband energy splitting and the Fermi energy splitting are relatively small, 1 > epsilon(f)/DELTA10 > 0.7-0.8. In GaAs-AlxGa1-xAs heterostructures the intersubband relaxation determines the excited-electron lifetime to be of the order of tau(phon) approximately nanoseconds which depends slightly on the value of the two-dimensional electron density. When the ratio epsilon(F)/DELTA10 is smaller, the intersubband relaxation is determined by the Auger-like electron-electron scattering whose rate can increase up to the value tau(Aug)-1 approximately 10(10) sec-1