We report a novel GaAs-based device in which I = 3/2 nuclear spins of 69Ga, 71Ga and 75As in a nanometer scale region
can be manipulated by all-electrical means. The device comprises a quantum point contact (QPC), a narrow conduction
channel in a GaAs quantum well defined by split gates, integrated with an additional metal strip on top for applying a
radio-frequency (RF) pulse. With the device set in a special condition characterized by the Landau-level filling factor v =
2/3, nuclear spins in the narrow region near the QPC can be selectively polarized by driving a current through the QPC.
By applying a resonant RF pulse, the polarized nuclei can be coherently manipulated, which we detect through the
electrical resistance of the QPC. Different from the conventional nuclear magnetic resonance measuring the transverse
component of the magnetization, our device measures the longitudinal component, which enables us to observe
otherwise invisible multiple quantum coherences between states with z projection of the angular momentum differing by
more than one. By appropriately tuning the length, intensity, and detuning of the RF pulse, all possible coherent
superposition between two out of the four Zeeman levels can be created for each nuclide.
We analyze generation of maximally entangled states (EPR and W states) of the conduction-band electron spins in systems of an arbitrary number of semiconductor quantum dots under equivalent-neighbor interactions mediated by a single-mode cavity field. We show that the perfect EPR states in bipartite systems and perfect W
states in multipartite ones can only be generatcd in systems of up to six and four dots, respectively, with single or equivalently, all dots except one excited.
The electronic features of semiconductor nanostructures, such as zero-dimensional states, are usually inferred from macroscopic optical and transport experiments. Although, direct probing of electrical features in semiconductor nanostructures looks very attractive, it is very difficult for a conventional semiconductor structure. However, direct probing becomes possible through a combination of low-temperature scanning tunneling microscopy and InAs(111)A surface in an ultra-high vacuum, where conductive electrons automatically accumulate near the clean surface. The clear observation of a Friedel oscillation pattern around a dislocation demonstrates successful mapping of the local-density-of-states (LDOS) of the conductive electrons. Inverted pyramidal defects are naturally formed during molecular beam epitaxial growth of InAs thin films on GaAs(111)A substrates and they operate as well-defined quantum dots. The measured LDOS pattern inside the quantum dots clearly changes as a function of energy, i.e. a sample bias, reflecting the LDOS pattern of each zero-dimensional state. A resonant concentration of the LDOS to the zero-dimensional energy levels is also demonstrated in these experiments. The LDOS measurements of a series of inverted pyramidal quantum dots with different side lengths and their comparison with theoretical calculations suggest a unique feature of the quantum dot system examined in this study.
We have studied new features of semiconductor nano-structures in cyclotron resonance and magneto-optical spectroscopy under very high magnetic fields up to a few megagauss. The new features are attributed to the shrinkage of wave-functions of electrons, holes and excitons. In PbSe/PbEuTe quantum dots that are regularly arranged to form an fcc-like lattice, sharp cyclotron resonance peaks from PbSe quantum dots were observed. It was found that the peaks show a number of anomalous features such as splitting, a remarkable dependence of the intensity on the wavelength, or a peculiar angular dependence of the resonance field. In the photoluminescence spectra of excitons for CdSe/ZnSe quantum dots, GaP/AlP and GaAs/AlAs short period superlattices, a remarkable decrease of the peak intensity and a red shift of the exciton peak were observed with increasing magnetic field; these effects are attributed to the general nature of excitons consisting of spatially separated electrons and holes.