Under certain conditions, such as temperatures lower than deep space, which weaken bones, superconductors can conduct electricity without resistance and without the resulting loss of energy. Although they vary in their chemical composition, operating temperature ranges, and magnetic properties, superconductors are divided into two groups. First, conventional and unconventional superconductors differ in the way their electrons are entangled in tandems called Cooper pairs, a state of shared identity that allows them to glide gracefully through the atomic mixture of their materials to facilitate uninterrupted electrical flow. Second, type I and type II superconductors respond differently to magnetic fields, with different types of thresholds at which they abruptly stop functioning as superconductors and begin acting as regular conductors. Unconventional and type II varieties are generally considered more exotic; the Lamborghinis of their domain, surpassing physical limits because they require custom engineering. They may also be better suited as workhorses used in technologies like MRIs, in the same way that Lamborghinis used to be tractors, but maybe not anymore. Because, for the first time, physicists have created a type I superconducting material that can apparently break the mathematical symmetry of time, a hallmark of unconventional type II superconductors. Known as time reversal symmetry, this comprises a curious observation that most laws of physics work equally well whether time flows mathematically forward or backward. In a study recently published in Physical Review Letters, an international team of physicists led by the Indian Institute of Science Education and Research (IISER) in Bhopal analyzed and described the unique electrical and quantum qualities of this material, suggesting that it may eventually improve quantum computing designs. The researchers synthesized single crystals of a material called ytterbium diantimonide (YbSb2) and used X-rays to determine their chemical purity and complex crystal structure, a structure common in conventional and unconventional superconductors because it allows quantum behaviors to emerge. A graphic summary of this work. In the center, the teal spheres represent the element ytterbium (Yb) and the bronze spheres represent antimony (Sb). (IISER Bhopal) They determined that its electrical resistance dropped to zero and it abruptly became superconducting at a temperature of around -272 degrees Celsius (-457.6 degrees Fahrenheit), which is just a smidge above absolute zero (-273.15 degrees Celsius). Combined with specific heat measurements of how its electrons respond to heat, the researchers confirmed that their material exhibited type I superconducting properties. This measurement also helped confirm its “completely spaced” state, meaning that it presents an energy barrier that makes it more difficult to untangle its paired electrons and therefore take it out of a superconducting state. The researchers also used two types of muon spin spectroscopy, a quantum probing technique that reveals magnetic fields within a material by attacking it with muons, or fundamental subatomic particles that act like small bar magnets. When they did so without applying an external magnetic field, they revealed that, once the material entered the superconducting phase, small internal magnetic fields spontaneously appeared within it. (Oak Ridge National Laboratory). This provided key evidence that YbSb2 could break time reversal symmetry, as magnetic fields reverse their direction when time is mathematically reversed; if they did not, they would preserve, rather than break, this curious property of time. In another version of this quantum probing technique, the researchers applied external magnetic fields while irradiating muons to the material. Additionally, when in a bulk superconducting state, the electrons in YbSb2 do not pair in the traditional way. Instead, they join together in an unconventional “spin triplet,” which is a Cooper pair, but called a triplet because of the possible combinations of the electron spins. Importantly, this is more twistedly called the “internally antisymmetric non-unitary triplet (INT) state”, so it shows a net magnetic moment: its magnetic forces do not cancel. This pairing allows YbSb2 to break time-reversal symmetry on its own, without needing an external magnetic field to do so. To discover how triplets form, the researchers modeled their material based on fundamental physical qualities, revealing that each of the paired electrons comes from different energy orbitals. As a result, their combined properties may make YbSb2 capable of hosting “gapless Majorana surface modes”, which is not one of the best-selling Zelda games. It means that, at low temperatures, while its mass acts as a superconductor through which electrons flow unhindered, its surface can manifest Majorana modes, or quantum excitations that act as its own antiparticles. (Possibly comparable to a delicious Red Vine, which transmits air through its mass and sugar through its edges.) This could represent more than a theoretical advance. These “topological quantum materials” can better protect quantum information by making it less vulnerable to external conditions such as heat or electromagnetic noise: sources of interference that knock quantum systems out of their entangled (and therefore useful) states and relegate them to the computing powers of, well, regular computers. The research has been published in Physical Review Letters. This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we take pride in our process, we are human. If you spot an error, please let us know.
