A real „game changer“ – superconductivity at room temperature
In the recent x years, our world has changed more dramatically than in any other comparable long period of human history. Any number greater than 25 for the variable “x” would make this statement true. And we can quickly name why that is: Because of the natural sciences and the technologies that spun out of them. They are the drivers that affect and form our world and the way we live to an ever greater extent. In today’s science high school exams students are often asked far more than what some 50 years ago represented the state of knowledge of the leading scientists of the respective areas. This ongoing acceleration in our knowledge acquisition, in turn, will enable future technologies that seem yet unimaginable to us today (who in the early 20th century could have even remotely imagined today’s world?). In light of these developments, we are likely to repeatedly experience moments in the coming decades when the rules of our living together are changing fundamentally. The most prominent recent year example is certainly the Internet. But transistor effect, laser, GPS, or birth control pills equally qualify. We therefore refer to these technological breakthroughs as “game changers”.
One of the most impactful future technological „game changer“ would result from the ability to conduct electricity without resistance at room temperature. In solid state physics this constitutes something like the equivalent of the Holy Grail. Across many industrial areas the so-called „superconductivity“ for every day temperatures would open doors for entirely new technological applications, such as for energy transport and conversion (superconducting power cables and generators), for the generation of very strong and homogeneous magnetic fields (for example, in magnetic resonance imaging, particle accelerators and nuclear fusion reactors , or at last in traffic engineering), for measuring devices (e.g. for the measurement of brain and heart magnetic fields for medical purposes, in non-destructive material testing, or for geomagnetic prospecting), and last but not least for increasing the efficiency in electronic circuits (e.g. by exchanging single flux quanta between superconducting circuitsone could achieve significantly lowerenergy losses and processing frequencies of more than 100 GHz).
There are numerous technological applications of superconductivityin place already today. The first generator with high-temperature superconductivity was successfully put into operation in Germany ten years ago. And with 1km length the world’s longest superconducting cables (cooled with liquid nitrogen) were used for the first time on a test basis for the operation of a municipal power supply. Currently, scientists within the EU project „Supra Power“ are working on a directly powered, superconducting generator for an offshore wind power plant providing ten megawatts of energy.
One issue however prevents to this day the broad application of superconductivity: For all known materials, the so-called „critical temperature“, i.e. the temperature at which a particular substance becomes superconducting, is stillvery low. For any technological application today’s superconductors must thus be cooled down to around minus 180 degrees Celsius (or even lower for most conventional superconductors), which greatly limits their practicality. A recent announcement from the field of materialresearch therefore catches a particular degree of attention: A research team of the Mainz-based Max Planck Institute for Chemistry published in the journal „Nature“ their success in achieving superconductivity at already minus 70 degrees Celsius, clearly beating the existing record of minus 135 degrees from 1993. Especially interesting about their discovery is the material they obtained this with (a sulfur hydride system), as it is an example of a conventional (metallic) superconductor. For a long time ever higher critical temperatures appeared possible only with ceramic materials, such as copper oxides. The hydrogen sulfide (H2S) however corresponds to the earliest conventional superconductors.
The two different types of superconductors, conventional metallic superconductors and the ceramic, so-called „high-temperature superconductors“ („HTS“ or “high-Tc”), fundamentally differ in how they reach the state of superconductivity. In order for electricity to flow without resistance, so called „Cooper pairs“ have to form within the conductor,i.e. pairs of two electrons coupled together such that they appear and act as a single particle. These coupled particles then have an integer spin and can therefore – at correspondingly low temperatures – emerge ina collective quantum state, the so called „Bose-Einstein condensate“, allowing them to flow without resistance through the conductor (single electrons carry a spin of ½ and thus obey the “Pauli exclusion principle” which excludes collective quantum states). Superconductivity is thus a macroscopic quantum effect. In conventional superconductors this pairing occurswith the help of vibrations of the positive ions in the crystal, the so-called “phonons”. In the HTS, however, the phonon-electron interaction does not suffice as an explanation. Instead physicists assume that the magnetic correlations between the electrons themselves provide the basis for the pairing. However, up to this day, 30 years after its (accidental) discovery, the physicist still have no generally accepted explanatory model for HTS. Thus, it is unclear how far one can getwith the critical temperature for ceramic HTS, whereas in conventional superconductors the critical temperature only depends on the phonons and their interaction with the electron, for which there is no theoretical limit, and thus no upper limit for the critical temperature. This is what fuels hope for a superconductor at room temperature.
The material now found, however, will hardly find its place in technological applications. Besides the fact that it smells like rotten eggs, the enormous pressure of 150 Gigapascal is required to obtain superconductivity upright. Nevertheless, this discovery could pave the way for potential new superconductors that operate without high pressure, so the hope of the researchers. In fact, their experiments suggest that hydrogen-containing molecules such as H2S could generally have favorable properties to be superconducting at higher temperatures. In 1985 the discovery of HTS constituted the start of a rapid succession of materials with ever higher critical temperatures. Such a development based on the latest discovery could actually lift the critical temperature of future materials into the area of everyday usability. This would then be the hoped-for technological „game changer“. It is thus worthwhile to follow closely the developments in the area of solid state physics in the near future.