Superconductive Organic Materials

You have probably heard of the word “Superconductivity.” One of its characteristics is that the electric resistance becomes zero (0). The percentage of the loss in Japan’s energy transmission and distribution is said to be 4.8 % (fiscal year 2010), and superconductivity at room temperature can help eliminate this loss in energy transmission. So, what do you imagine when thinking of the superconductors? Many people might expect metals or copper oxides. However, many superconductive materials have been found among the organic materials people once thought as insulators. Many people, who hear the idea of passing electricity through organic materials, may think of a “conducting polymer” discovered by Prof. Hideki Shirakawa, who is a Nobel laureate in chemistry.

 The research on conductive organic materials has evolved into the development of organic materials through which not only electricity passes but also electric resistance becomes zero. Or a fancy way of saying it is: the development of the lightweight superconductive materials derived from oil.
The figure on the left shows the crystalline structure of (TMTSF)₂PF₆ [TMTSF: tetramethyltetraselenafulvalenium] which is the first organic superconductive material discovered among organic materials. It has a superconductivity transition temperature Tc around 1 K, however, it has characteristics in which the conductivity is higher only in a one-dimensional direction indicated by the arrow, which is a characteristic of an organic superconductor.
In addition, the Tc of (BEDT-TTF)₂Cu(NCS)₂ which is a material that expands two-dimensionally was successfully raised to around 10 K. Apart from its low-dimensional characteristics, this material was also found to have a strong correlation between electrons and is called a strongly-correlated electron system. The relationship among the low-dimensional electron system, strongly-correlated electron system and superconductivity is very interesting when considering a superconductivity formation mechanism.

 

Approaching a Mechanism for Superconductivity Using Nuclear Magnetic Resonance

Many organic materials have been developed which show superconductivity in the low-dimensional electron system having this type of a correlation. We have been engaged in research to investigate their mechanisms using the method of nuclear magnetic resonance (NMR). The nuclear magnetic resonance is a phenomenon in which when the specific atomic nuclei placed in a magnetic field are irradiated with electromagnetic waves, the nuclei absorb (resonate with) and re-emit the electromagnetic waves. This is the principle used in MRI (magnetic resonance imaging) found in medical care facilities.

Original In-house Broad-Line Nuclear Magnetic Resonance Spectrometer

The MRI conducts a check using the magnetic resonance for the ¹H nucleus of the H₂O in your body. We investigate the states of the electrons which generate superconductivity inside materials using the magnetic resonance for the ¹³C, the isotope of the C which constitutes organic materials. (¹²C which accounts for the majority of the natural world cannot be used for our experiments because it does not show the magnetic resonance phenomenon.) Thus, we synthesize the molecules of which only the specific positions are substituted with ¹³C using the method of organic synthesis using the isotope reagents of ¹³C. In other words, probes for precise measurement are artificially installed at the specific molecular positions using the method of organic synthesis. Using various positions where the carbon atoms are substituted enables the observation of the states of electrons at different positions, which can be comprehensively analyzed to investigate the states of electrons.

The characteristics of electrons can be studied by varying the exterior conditions (pressure, temperature) too. The pressures (atmospheric pressure) and temperatures, however, are not the environments we experience in our daily life, but are the ultra-high pressure which can be found only in the interior of the earth, extremely-low temperature around 0 K, and the combination of ultra-high pressure and extremely-low temperature. Those environments are generated in our laboratory and used for the experiments in association with the nuclear magnetic resonance (NMR). 

Such nuclear magnetic resonance (NMR) devices which can work under multiple extreme conditions are not commercially available. Thus, such original devices are manufactured in-house using electric circuit designs based on instruction and training given by the Department of Physics.

 

Is it Physics or Chemistry?

While the number of elements is roughly 100, the number of molecules is infinite, and therefore we can design and create new molecules. The characteristics of the aggregation for such molecules include not only superconductivity but also magnetism, dielectric properties, and other concepts which are essential for important science and technology that support our modern life. One of our important research subjects includes explaining and investigating the formation mechanisms of the magnetism and dielectric properties shown by molecules.

Whether our research can be classified into either physics or chemistry seems to be a tough question to answer if judged by the targets and methods of our experiments. Much of the current research is in the interdisciplinary field and may be appropriate to be classified as material science rather than physics or chemistry. On the other hand, among the various material sciences, the development of those materials actually achieving the highest superconductivity temperature belongs to a type of chemistry field, and the manufacturing of such materials is included in the engineering field if anything. 

The goal of physics is to explain and investigate the universal principles and mechanisms which govern the natural phenomena as represented by elementary particle physics and cosmic physics, and the goal of material science is to offer the principles for actual material development by explaining and investigating the mechanisms of superconductivity formation.

 

References

Takehiko Mori, “Basic Molecular Electronics (Bunshi Electronics no Kiso)”, Kagaku Dojin, 2013.