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Register a new userAnharmonicity and asymmetry of Landau levels for a two-dimensional electron gas
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Added by
Chaubet Christophe Pr
Publication date
2005
fields
Physics
and research's language is
English
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We calculate the density of states of a two dimensional electron gas located at the interface of a GaAlAs/GaAs heterojunction. The disorder potential which is generally created by a single doping layer behind a spacer, is here enhanced by the presence of a second delta doped layer of scatterers which can be repulsive or attractive impurities. We have calculated the density of states by means of the Klauders approximation, in the presence of a magnetic field of arbitrary strength. At low field either band tails or impurity bands are observed for attractive potentials, depending on the impurity concentration. At higher field, impurity bands are observed for both repulsive and attractive potentials. We discuss the effect of such an asymmetrical density of states on the transport properties in the quantum Hall effect regime.
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We demonstrate coherent control of cyclotron resonance (CR) in a two-dimensional electron gas (2DEG). We use a sequence of terahertz pulses to control the amplitude of CR oscillations in an arbitrary fashion via phase-dependent coherent interactions. We observe a self-interaction effect, where the 2DEG interacts with the terahertz field emitted by itself within the decoherence time, resulting in a revival and collapse of quantum coherence. These observations are accurately describable using {em single-particle} optical Bloch equations, showing no signatures of electron-electron interactions, which verifies the validity of Kohns theorem for CR in the coherent regime.
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs-AlGa$_{0.3}$As$_{0.7}$ heterostructures, and can be well explained by the Zener mechanism.
We have performed conductivity measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities n_1 and n_2 across the slot. Dynamic longitudinal resistance was measured by a standard lock-in technique, while maintaining a large DC current through the source-drain channel. We find that in a parallel magnetic field, the resistance of the sample, R(I_DC), is asymmetric with respect to the direction of the DC current. The asymmetry becomes stronger with an increase of either the magnetic field or the difference between n_1 and n_2. These observations are interpreted in terms of the effective spin injection: the degree of spin polarisation is different in the two parts of the sample, implying different magnitudes of spin current away from the slot. The carriers thus leave the excess spin (of the appropriate sign) in the region around the slot, leading to spin accumulation (or depletion) and to the spin drift-diffusion phenomena. Due to the positive magnetoresistance of the two-dimensional electron gas, this change in a local magnetisation affects the resistivity near the slot and the measured net resistance, giving rise to an asymmetric contribution. We further observe that the value of R(I_DC) saturates at large I_DC; we suggest that this is due to electron tunnelling from the two-dimensional n-type layer into the p-type silicon (or into another spin reservoir) at the slot.
In a recent letter M. Lilly et al [PRL 82, 394 (1999)] have shown that a highly anisotropic state can arise in certain two dimensional electron systems. In the large square samples studied, resistances measured in the two perpendicular directions are found to have a ratio that may be 60 or larger at low temperature and at certain magnetic fields. In Hall bar measurements, the anisotropy ratio is found to be much smaller (roughly 5). In this comment we resolve this discrepancy by noting that the anisotropy of the underlying sheet resistivities is correctly represented by Hall bar resistance measurements but shows up exponentially enhanced in resistance measurements on square samples due to simple geometric effects. We note, however, that the origin of this underlying resistivity anisotropy remains unknown, and is not addressed here.
A giant asymmetry in the magnetoresistance was revealed in high-mobility, two-dimensional electron gas on a cylindrical surface. The longitudinal resistance along the magnetic-field gradient impressed by the surface curvature was found to vanish if measured along one of the edges of the curved Hall bar. If the external magnetic field is reversed, then the longitudinal resistance vanishes at the opposite edge of the Hall bar. This asymmetry is analyzed quantitatively in terms of the Landauer-Buettiker formalism.
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