No abstract

No abstract

No abstract

No abstract

No abstract

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layersa transverse magnetoresistance which is asymmetric in the field (there can even be a change in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum.

Intersubband scattering processes are investigated in selectively doped asymmetric tunnel-coupled GaAs/AlGaAs quantum wells by infrared pump probe spectroscopy. Various subbands are excited by resonant absorption of a mid-infrared pump pulse and the carrier relaxation is observed via timeand frequency-resolved absorption changes monitored by a subsequent infrared probe pulse. The experimental data show that most of the carriers remain in the quantum well in which the excitation process mainly takes place. Only a minor fraction of the carriers is transferred between excited non-resonant subbands of the neighbouring quantum wells. The situation is different for electrons in the two lowest subbands which are separated only by 10 meV. Here a rapid and efficient transfer is observed. The scattering time is in the order of a picosecond at room temperature; it increases to values between 4 ps and 20 ps at 80 K. The data are interpreted with detailed model calculations taking into account phonon and impurity scattering. Both experimental and calculated data demonstrate the possibility of population inversion between excited states of quantum wells under optical or current electron injection.

Numerical study of momentum-dependent gap function is presented to make clear the origin of superconductivity in copper oxides. We claim that antinodal region with pronounced nesting feature of the Fermi contour gives rise to superconducting pairing with large momentum under screened Coulomb repulsion. Such a pairing results in both spatial checkerboard pattern of the superconducting state below T c and a gapped state of incoherent pairs in a broad temperature range above T c . We explain the momentum dependence of the coherent spectral weight detected in angleresolved photoemission spectroscopy and predict antinodal Bogoliubov quasiparticle interference other than observed in the nodal region.