Topological Macroscopic Quantum Devices

Application of topology in devices produces advancement of specification of ordinary devices and creates new functions that have never been realized yet

1) Room-temperature working quantum interference devices

In ring-shaped crystals of charge density wave (CDW) materials, there is a circulating current as the result of the closed topology. Since the origin of this current is CDW collective motion, it is affected sensitively by changing of magnetic flux penetrating the ring. Hence, the ring-shaped crystals carry out a function, which is essentially the same with superconducting quantum interference devices.

The most high transition temperature of superconductivity is about 160 K. On the contrary, transition temperatures of some CDW materials are higher than the room temperature. Therefore, it is expected that realization of the CDW quantum interference devices working at the room temperature. Conventional medical SQUIDs have been used in large hospitals because they have to cool down at low temperatures. The development of the room-temperature working devices will provide more facile and more reasonable applications.

To prove this principle, our group investigate magnetic-field-dependence of conductivity at a charge density wave state experimentally. The left figure shows the scanning electron microscopy image of a ring-shaped crystal used for experiment. The ring of about 20 mm diameter is attached electrical contacts. The right figure indicates that the magnetic field dependence of current as applied voltage is constant. We discovered the conductivity periodically oscillates as a function of magnetic field the first in the world. (M. Tsubota et al., Reported at the 62nd Annual Meeting of Japan Physical Society.)

2) Topological Soliton Transistors

The phase of charge density wave is the function of position. The local phase change of 2p (360o) is known as the stable topological soliton. The creation probability of a soliton pair is described as a periodic function of electric field with a period determined by an electric charge. It is the same with the single electron transistors.

The single electron transistors needs ultra micro-fabrication and low temperatures because the Coulomb energy to excite one electron is enough higher than thermal fluctuation. (Or basically different approaches such as super-microparticle- or single-molecular-devices have to be carried out.) On the other hand, since the Coulomb energy is relatively large in the CDW systems, it is possible to realize such devices working at higher temperatures.

The single electron transistors have been expected for applications due to their super sensitive property. However they have worked only in laboratory yet. The topological soliton transistors will make a break-through.

3) Charge Density Wave High-Speed Devices

Since electron transport phenomena in normal metals and semiconductors are caused by diffusive motion of electrons, the response speed becomes slower than the expected speed of each electron.
The basic excitations of CDW are phason mode and amplitudon mode. Especially, long wavelength limit of the former mode is corresponding to that the phase of all system moves coherently. Then the CDW per one wavelength carries 2e charge (Frohlich conduction). Due to the collective motion, the response speed of the CDW must be determined by the phase velocity of the CDW. Furthermore, there is no shot noise fundamentally due to there is no fluctuation around the average.
On the background, we investigate new devices that control the collective motion of the CDW using gate voltage.

4) Nanomaterial-Devices

gNanotubeh is now a common term referring to single-atom-thick crystalline sheets rolled into a cylinder of diameter 1-100 nm in a self-organizing manner. Carbon nanotube was first discovered in 1994, and instantly spotlighted as its cylindrical topology had bestowed distinct physical properties such as;

These unique properties have provided a broad range of applications e.g.; constituent of highly strong composite material, making carbon nanotube representative of nanotechnology.Not only in practical applications, this material also has played a huge role in the development of the study of quantum phenomena inherent to one-dimensional system. We have been working on the experimental observation of quantum conduction phenomena in CNT as well as the development of nano-fabrication technique in corporation with distinguished international collaborators, such as those in Helsinki University of Technology. Figure 3 exemplifies our achievements, showing a SEM image of a nanotube sample fabricated by a dielectrophoretic method. In making this sample, an AC electric field was applied between metallic electrodes to attract nanotubes dispersed in organic solvent toward the exact position between the electrodes. This kind of sample is essential for investigations of:

Besides carbon nanotubes, our group have been employing ourselves in synthesizing novel nanotube material with undiscovered properties. So far we have succeeded in synthesizing inorganic nanotube of layered materials having superconducting or charge-density-wave property, such as niobium diselenide (NbSe2) and magnesium diboride (MgB2). All our synthesis is based on gas-phase growth at rather low temperature up to 800 oC without the use of any catalyst. In contrast to high-energy conditions under the presence of catalyst particles, the mechanism of nanotube formation under these conditions is still unexplored and of wide scientific interest. One of our findings is that a ribbon-shaped nanofiber of NbSe2 voluntarily rolls itself into a tube when exposed to an electron beam (*). This useful behavior is attributed to the interaction between accumulated charge and the electron beam, and has the potential to develop into a technique to manufacture nano-structures with arbitrary shape. MgB2, whose superconductivity was discovered as recently as in 1999, are now considered promising material to form superconducting wire owing to its moderately high transition temperature of 39 K and good crystallinity. Undoubtedly MgB2 nanotubes, once a sufficient yield is attained, would be incorporated in a SQUID to fulfill spatial resolution unparalleled among conventional film-based SQUIDs.

Figure 3: the nanotube sample controlled by dielectrophoresis method.

* Formation of metallic NbSe2 nanotubes and nanofibers;
T. Tsuneta, T. Toshima, K. Inagaki, T. Shibayama, S. Tanda, S. Uji, M. Ahlskog, P. Hakonen and M. Paalanen, Current Applied Physics 3 (2003) p.473-476