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Tesi etd-01092006-110946

Thesis type
Tesi di laurea specialistica
Deon, Fabio
email address
Damping Mechanisms of Intersubband Spin Plasmons in GaAs-AlGaAs Quantum Heterostructures
Corso di studi
relatore Dott. Pellegrini, Vittorio
relatore Prof. Beltram, Fabio
Parole chiave
  • Double Quantum Well
  • SDE
  • Intersubband Spin Plasmons
  • Quantum Heterostructures
Data inizio appello
Data di rilascio
Riassunto analitico
This thesis focuses on the study of the damping mechanisms of spindensity excitations (SDEs) of the quasi two-dimensional electron gas confined
in n-type quantum heterostructures. SDEs are intersubband collective modes that consist in an out-of-phase oscillation along the growth direction of the heterostructure of the two components of the electron gas with opposite spin. Our analysis is based on the observation of SDEs by resonant inelastic light (Raman) scattering. Besides the SDEs, inelastic light scattering allows for the observation of charge-density excitations (CDE) and single-particle excitations
(SPEs). The first are ordinary charge plasmons, involving oscillation along the growth direction of the charge density, while SPEs are generally
interpreted as the incoherent excitation of single electrons from the first to the second subband. The energy of the collective charge and spin modes is shifted from the single-particle tunneling gap by corrections due to dynamic screening (depolarization effect) and to exchange-correlation effects.
The main experimental result of this thesis is the measurement of the Lorentzian lineshape of the SDE in a wide range of temperatures down to 50mK. To this end, the samples are placed in a dilution fridge equipped with windows for optical access. Our results indicate that scattering by low energy, thermally-excited acoustic phonons represents a major contribution to the damping of this excitation, even at the few-K temperatures at which most of the experiments reported in the literature are usually performed.
However, the behavior of the linewidth saturates at low temperatures to a value of ∼ 0.08meV corresponding to a damping time of approximately
8ps. We argue that in the high mobility samples studied in this thesis the measured "zero-temperature" value of the SDE linewidth is determined to a considerable extent by the spin-coulomb drag (SCD) effect, a mechanism
that introduces friction-like damping between spin-polarized currents.