Scientists in the field of quantum physics have recently stumbled upon a remarkable discovery with the potential to revolutionize quantum technology. Led by the University of Bristol, a team of researchers uncovered an extraordinary phenomenon within purple bronze, a unique one-dimensional metal consisting of individual conducting chains of atoms. Within this material, two opposing electronic states, namely an insulating and a superconducting state, coexist. Even more intriguing, the material displays a polarized versatility that allows for an instant transition between these two states by simple changes in the environment. This finding, known as “emergent symmetry,” holds considerable promise for the development of a perfect On/Off switch for quantum devices of the future.
The Unveiling of Emergent Symmetry
Thirteen years ago, in the laboratories of the University of Bristol, Ph.D. students Xiaofeng Xu and Nick Wakeham initiated the investigation of purple bronze. Their focus was on measuring the magnetoresistance of this material, which refers to changes in resistance caused by a magnetic field. What they discovered was perplexing yet intriguing. In the absence of a magnetic field, the resistance of purple bronze varied significantly depending on the direction of the electrical current. Additionally, its temperature dependence was also intricate, with the material transitioning from a metallic state to an insulating state as the temperature decreased further. Finally, at extremely low temperatures, the resistance plummeted again as purple bronze transformed into a superconductor. However, amidst this complexity, the magnetoresistance proved to be strikingly simple, maintaining a linear temperature dependence and exhibiting consistent behavior irrespective of current or field direction.
The enigmatic behavior of purple bronze persisted for seven years, as the lack of a coherent explanation left the data unpublished and dormant. Quantum research seldom experiences such hiatuses, but in this case, it was not due to a scarcity of statistical evidence. In 2017, Professor Nigel Hussey, then working at Radboud University, attended a seminar by physicist Dr. Piotr Chudzinski regarding purple bronze. These seminars were scarce, which heightened Prof. Hussey’s curiosity. During the seminar, Dr. Chudzinski proposed a theory suggesting that the resistive upturn observed in purple bronze derived from interference between conduction electrons and elusive particles known as dark excitons. Following a discussion with Dr. Chudzinski, they designed an experiment that validated his theory.
Buoyed by this success, Prof. Hussey revisited Xu and Wakeham’s magnetoresistance data and presented it to Dr. Chudzinski. These data exhibited two key features: temperature linearity and independence from current or field direction. These features captivated Dr. Chudzinski, as did the fact that purple bronze could exhibit both insulating and superconducting behavior dependent on the growth method. Dr. Chudzinski speculated that rather than transitioning entirely into an insulator, the interaction between charge carriers and excitons could cause the former to gravitate towards the boundary between the insulating and superconducting states as the temperature decreased. At the border, the system’s probability of being an insulator or a superconductor would be nearly equivalent. This physical symmetry, as the temperature lowered, was unprecedented and dubbed “emergent symmetry.”
Unraveling the Complexity: Emergence of Symmetry
Symmetry breaking, wherein a system’s symmetry is reduced by cooling, is a familiar phenomenon in physics. For example, the intricate arrangement of water molecules in an ice crystal demonstrates a broken symmetry. However, the opposite phenomenon, the emergence of symmetry, is exceptionally rare, if not unique. It is analogous to the complexity of an ice crystal “melting” back into a symmetric water droplet upon further cooling. As Dr. Chudzinski compares it, the material purple bronze, is like a magic trick in which a dull, distorted figure transforms into a beautiful, perfectly symmetrical sphere. Nature itself acts as the magician in this instance.
To test the validity of this theory, another Ph.D. student, Maarten Berben, examined an additional 100 individual crystals of purple bronze, consisting of both insulating and superconducting samples. The exhaustive effort confirmed the theory’s viability and unveiled that the different ground states of the crystals were a consequence of emergent symmetry.
The discovery of emergent symmetry within the unique material of purple bronze opens up new possibilities for quantum devices of the future. With its ability to seamlessly transition between an insulating and superconducting state, purple bronze possesses the potential to become the ideal “perfect switch.” This tantalizing breakthrough highlights that even in the seemingly complex world of quantum physics, unexpected phenomena can emerge, paving the way for transformative advancements in technology.