We study electronic and optical properties in nanoscale structures and
develop technologies to fabricate these structures for functional nanoscale
devices. Those devices would have operation principles completely different
from conventional devices such as transistors, and are expected to show
very low power consumption. An example to realize such devices would be
to take advantages of coherent control of electrons, spins, excitons and
superconducting states in nanoscale structures. So, single electron devices
and quantum information devices would be possible solutions we are working
on.
To use quantum effects is a possibility to realize functional nanoscale
devices. Since the quantum effects manifest themselves in smaller structures,
it would be attractive to use bottom-up nanomaterials such as semiconductor
nanowires and carbon nanotubes which have a size that cannot be realized
with conventional lithography techniques. These bottom-up materials are
used as building blocks of nanoscale devices in combination with top-down
techniques. We also develop atom manipulation techniques with scanning
probes for the ultimately small structures. Fabricated devices are measured
in low temperatures in a dilution refrigerator.
Electrons are controlled one by one in quantum dots. The coherent control
of electron or spin leads to qubits in quantum computing. Especially, to
use the spin for quantum information is attractive because it has a long
coherence time, and could be used to store the quantum information (quantum
memory). We fabricate quantum dots with group four materials such as Si
nanotransisitors and carbon nanotubes, which are supposed to have long
spin coherence. To manipulate spins with electric field, we also fabricate
quantum dots with InAs, InSb or Ge/Si in which the spin orbit interaction
is large.
2. Quantum dots in a microwave circuit cavity (c-QED)
An atom in a cavity is a subject of quantum electrodynamics (QED) in quantum
optics. Quantum dots are an artificial atom with two levels, a system that
has a resonant frequency in a microwave range. We study it in a microwave
circuit cavity made of superconductor embedded with the quantum dots, where
an electron or a spin would couple quantum mechanically with photons in
a cavity (c-QED). This technique can be applied to various qyantum states
that interact with a cavity, and is used to study the spectroscopic structures
of the system.
3. Nanostructures/Superconductor hybrid nanodevices
Semiconductor nanowires with
superconducting contacts are an interesting SNS (Super/Normal/Super) structure
with a low dimensional normal channel. It is considered that the Andreev bound
states are formed in the normal channel and to study the physics associated
with them as well as superconducting properties of the structures are the
subject of research. They could be also used for the Andreev qubit. When the
nanowires have the strong spin-orbit interaction (InAs and InSb), they can be
studied to search for Majorana Fermion. We also study a topological insulator
with superconducting contacts.
4. Fabrication of atomic and molecular scale nanostructures
One of the unique features of the carbon
nanotubes is a possible chemical modification of surface and edge of carbon
nanotubes. With this advantage, we fabricate molecular scale nanostructures using
the single-wall carbon nanotube (SWCNT)/molecule heterojunctions. The
structures are characterized with a scanning tunnel microscope in combination
with optical spectroscopy. Electrical and optical properties of the fabricated
unique nanostructures are studied.
We also fabricate artificial atomic structures with the home-made atom
manipulation apparatus based on STM.
(April 4, 2014)