665 research outputs found
Vertical magneto-tunneling through a quantum dot and the density of states of small electronic systems
One-electron tunneling through a quantum dot with a strong magnetic field in
the direction of the current is studied. The linear magneto-conductance is
computed for a model parabolic dot with seven electrons in the intermediate
states and for different values of the magnetic field. It is shown that the dot
density of states at low excitation energies can be extracted from a precise
measurement of the conductance at the upper edge of the Coulomb blockade
diamond. We parametrized the density of states with a single ``temperature''
parameter (in the so called ``constant temperature approximation''), and found
that this parameter depends very weakly on the magnetic field.Comment: Accepted in Physica
Full counting statistics for orbital-degenerate impurity Anderson model with Hund's rule exchange coupling
We study non-equilibrium current fluctuations through a quantum dot, which
includes a ferromagnetic Hund's rule coupling , in the low-energy Fermi
liquid regime using the renormalized perturbation theory. The resulting
cumulant for the current distribution in the particle-hole symmetric case,
shows that spin-triplet and spin-singlet pairs of quasiparticles are formed in
the current due to the Hund's rule coupling and these pairs enhance the current
fluctuations. In the fully screened higher-spin Kondo limit, the Fano factor
takes a value determined by the orbital degeneracy .
We also investigate crossover between the small and large limits in the
two-orbital case M=2, using the numerical renormalization group approach.Comment: 5 pages, 3 figure
Semiconductor quantum dots for electron spin qubits
We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates
Spin configurations in circular and rectangular vertical quantum dots in a magnetic field: Three-dimensional self-consistent simulation
The magnetic field dependence of the electronic properties of \textit{real}
single vertical quantum dots in circular and rectangular mesas is investigated
within a full three-dimensional multiscale self-consistent approach without any
{\it \'a priori} assumptions about the shape and strength of the confinement
potential. The calculated zero field electron addition energies are in good
agreement with available experimental data for both mesa geometries. Charging
diagrams in a magnetic field for number of electrons up to five are also
computed. Consistent with the experimental data, we found that the charging
curves for the rectangular mesa dot in a magnetic field are flatter and exhibit
less features than for a circular mesa dot. Evolution of the singlet-triplet
energy separation in the two electron system for both dot geometries in
magnetic field was also investigated. In the limit of large field, beyond the
singlet-triplet transition, the singlet-triplet energy difference continues to
become more negative in a circular mesa dot without any saturation within the
range of considered magnetic fields whilst it is predicted to asymptotically
approach zero for the rectangular mesa dot. This different behavior is
attributed to the symmetry "breaking" that occurs in the singlet wave-functions
in the rectangular mesa dot but not in the circular one.Comment: 12 pages, 8 gifure
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