Quantum Dot Superlattice
- Introduction
The electronic properties of solids are closely related to their crystal
structure which is strictly determined by the atomic nature of the individual
elements. However, the imposition of a superstructure on a given lattice
makes the controlled fabrication of structures with chosen properties possible.
Our approach is to place quantum dot (QD), also known as artificial atoms,
on the points of a lattice to form an artificial crystal called a quantum-dot
superlattice (QDSL).
- Flat-band ferromagnetism in quantum dot superlattices
We proposed the theoretical design of quantum dot superlattice exhibiting
ferromagnetism by using a quantum-wire network shown in Fig. 1. The electronic
structure calculations based on a local spin density approximation (LSDA)
show that our designed QD artificial crystal from a structure comprising
the crossing 0.104-micrometer-wide InAs quantum wires forms an effective
Kagome lattice having a flat band. Our examined QD artificial crystal has
the ferromagnetic ground state when the flat band is half-filled as shown
in Fig. 2, even though it contains no magnetic elements. We have also demonstrated
that the ferromagnetic and paramagnetic states can be freely switched by
changing the electron filling.
References
- "Design of a semiconductor ferromagnet in a quantum dot artificial crystal"
K. Shiraishi, H. Tamura, and H. Takayanagi
Applied Physics Letters 78, 3702 (2001).
- "Flat-band ferromagnetism in quantum dot superlattices"
H. Tamura, K. Shiraishi, and H. Takayanagi
Physical Review B 65, 085324 (2002)
- "Magnetic field effects on a two-dimensional Kagome lattice of quantum
dots"
T.Kimura, H.Tamura, K.Shiraishi, and H.Takayanagi
Physical Review B 65, 081307(R)
(2002)
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| Fig. 1: Schematic Kagome wire-network. InAs wire embedded in In0.776Ga0.224As barriers with a barrier height=0.17eV. |
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| Fig. 2: Spin density of Kagome wire network calculated using the local spin density functional method. Polarized spins are induced at the intersetions of crossing wires. |
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- Superconductivity in quantum dot superlattices
we propose a method for forming QDSLs in a quantum wire network of square
and plaquette lattices shown in Fig. 1. The plaquette lattice has a square
plaquette in each unit cell with four lattice-points at the vertices. We
have used the spin dependent local density approximation to obtain the
band structure of the wire network. It can be shown that both QDSLs are
well represented by the Hubbard model. An interesting difference between
the two QDSLs is that the Fermi surface of the plaquette QDSL has disconnected
pieces whereas the square QDSL is formed of one piece. To find a correlation
effect that reflects both the Coulomb interaction and the structures of
the Fermi surface, we studied the existence of superconductivity for both
lattices within the framework of the Hubbard model. We found a superconducting
ground state where the transition temperature Tc of the plaquette lattice
is more than double that of the square lattice as shown in Fig. 2, and
is sufficiently high to allow superconductivity to be observed experimentally.
References
- "Superconductivity in quantum-dot superlattices composed of quantum
wire networks"
T.Kimura, H.Tamura, K.Kuroki, K.Shiraishi, H.Takayanagi, and R.Arita
Physical
Review B 66, 132508 (2002)
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| Fig. 1: Schematic plaquette dot-lattice. |
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| Fig. 2: Superconducting transition temperature for the plaquette lattice as a function of ta calculated using the fluctuation exchange (FLEX) approximation. |
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