We have investigated the optical properties of unstrained GaAs/ Al x Ga 1−x As quantum dot/well systems with the aim of studying the influence of confinement on the effective exciton mass, as determined from the photoluminescence line shift in high magnetic fields ͑ഛ50 T͒. The effective exciton mass is found to be more than twice the value for bulk GaAs. We attribute this to an enhanced nonparabolicity in the GaAs conduction band at the nanoscale.
The magnetic field dependence of the excitonic states in unstrained GaAs/ Al x Ga 1−x As quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental threedimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fields up to 50 Tesla. Good agreement is found between theory and experiment.
The interaction of externally applied currents with persistent currents induced by magnetic field in a mesoscopic triangle is investigated. As a consequence of the superposition of these currents, clear voltage rectification effects are observed. We demonstrate that the amplitude of the rectified signal strongly depends on the configurations of the current leads with the lowest signal obtained when the contacts are aligned along a median of the triangle. When the contacts are aligned off centered compared to the geometrical center, the voltage response shows oscillations as a function of the applied field, whose sign can be controlled by shifting the contacts. These results are in full agreement with theoretical predictions for an analogous system consisting of a closed loop with a finite number of identical Josephson junctions. DOI: 10.1103/PhysRevB.76.224501 PACS number͑s͒: 74.25.Fy, 74.25.Sv, 74.78.Na, 73.40.Ei In the past few years considerable attention has been paid to the mechanisms responsible for the ratchet effects in a broad variety of physical systems such as colloids, 1 granular materials, 2 fluids, 3 atoms in optical traps, 4 electrons in semiconductor heterostructures, 5 and Josephson systems. [6][7][8] In all cases, a net flux of particles driven by a zero average alternating excitation results from the interaction of the media with an asymmetric potential. This behavior has also been theoretically predicted and experimentally corroborated for the motion of quantum flux lines in superconducting samples with a pinning landscape lacking inversion symmetry.9-15 In superconducting systems, this effect manifests itself as a nonzero dc voltage even when an ac excitation is sent through the superconductor, thus acting as a rectified voltage. Interestingly, it has been recently demonstrated that the presence of this voltage rectification in a superconductor does not necessarily imply the motion of vortices in an asymmetric pinning potential but might also result from nonsymmetric current distributions in the superconducting sample. 16 Indeed, first, Dubonos et al. 17 reported rectification effects in asymmetric superconducting rings. Later on, Morelle et al. 18 showed that similar effects are observed in singly connected structures if the current injection is off centered. In both cases, the effect was attributed to an asymmetry in compensation or reinforcement of an external bias current by the field induced persistent currents, causing a difference in critical current for a positive or negative applied external current. More recently, Van de Vondel et al. 16 showed that both kinds of rectification, due to ratchet vortex motion and due to current compensation effects, can coexist in superconducting samples with periodic arrays of triangular antidots.In this work, we investigate the influence of the position of the current or voltage probes on the resultant rectification effect in microsized superconducting triangles. We show that an ac current injected above the geometrical center of the triangle ...
The magnetization of luminescent Er 3+ -doped PbF 2 nanoparticles ͑formula Er 0.3 Pb 0.7 F 2.3 ͒ has been studied. Despite the high concentration of the doping Er 3+ ions and relatively large size ͑8 nm͒ of these nanoparticles we have found no deviation between field-cooled and zero-field-cooled magnetization curves down to T = 0.35 K, which points out an ultralow blocking temperature for the reversal of magnetization. We also have found strongly deviating magnetization curves M͑H / T͒ for different temperatures T. These results altogether show that the investigated nanoparticles are not superparamagnetic, but rather each Er 3+ ion in these nanoparticles is found in a paramagnetic state down to very low temperatures, which implies the breakdown of the Néel-Brown giant spin model in the case of these nanoparticles. Calculations of magnetization within a paramagnetic model of noninteracting Er 3+ ions completely support this conclusion. Due to the ultralow blocking temperature, these nanoparticles have a potential for magnetic field-induced nanoscale refrigeration with an option of their optical localization and temperature control.
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