New research in the journal Nature has identified so-called 'strange metals', which refer to a class of materials that share fundamental quantum attributes with black holes.
Study: Signatures of a strange metal in a bosonic system. Image Credit: denis kalinichenko/Shutterstock.com
A recent discovery made by a collaborative team of researchers from Brown University, USA, and the University of Electronic Science and Technology of China (UESTC) has helped scientists understand strange metals in new ways.
Published in the journal Nature, the research team, led by Professor of Physics James M. Valles and Professor of Materials Science Jie Xiong, of Brown University and UESTC respectively, explains how they discovered strange metal behavior in a material in which electrical charge is carried by “wave-like” entities electrons called Cooper pairs.
Understanding the behavior of these so-called strange metals could offer fundamental insights into the quantum world and assist scientists in their understanding of curious phenomena such as high-temperature superconductivity.
Understanding Strange Metal Behavior
Scientists already understand the effect temperature has on electrical conductance in common everyday metals such as silver or copper, but recent research has turned researchers’ gaze towards a class of materials that bend or even break traditional electrical rules.
Fermi liquid theory underscores the basis for understanding the majority of metals, however a number of quantum materials – in particular high-temperature superconductors – exhibit strange-metallic behavior with a linear scattering rate in temperature, which contravenes this central paradigm.
Typically, electrical charge is carried electrons belong to a class of particles known as fermions, however in this new discovery, the charge was carried by cooper pairs which act as bosons, which have very different principles than electrons.
This is the first time researchers have been able to observe strange metal behavior in a bosonic system and could provide insight into how strange metals function – a feature that has evaded researchers for some time.
We have these two fundamentally different types of particles whose behaviors converge around a mystery. What this says is that any theory to explain strange metal behavior can’t be specific to either type of particle. It needs to be more fundamental than that.
James Valles, Professor of Materials Science Jie Xiong, of Brown University
Cooper Pairs & Bosons
In recent years, Brown and UESTC researchers have been investigating electrical activity in which the charge carriers are not electrons. Since Nobel Laureate Leon Cooper discovered in 1952 that in conventional superconductors (not high-temperature), electrons tend to come together and form Cooper pairs that can pass through an atomic lattice with no resistance.
In spite of being comprised of two fermions (electrons), Cooper pairs act as bosons: “Fermion and boson systems usually behave very differently,” says Valles. “Unlike individual fermions, bosons are allowed to share the same quantum state, which means they can move collectively like water molecules in the ripples of a wave.”
While the researchers have previously established that Cooper pair bosons can demonstrate metallic behavior, that is, they show some levels of resistance when conducting electricity, they wanted to see if this also made them strange metals.
To conduct their tests, the team made use of a cuprate material called yttrium barium copper oxide that induced the Cooper-pair metallic state via a pattern of tiny holes. To observe any transformations in its conductance, the material was cooled down almost to its superconducting temperature.
They discovered a bosonic Cooper-pair metal conductance that is linear with temperature as with fermionic strange metals. This exciting breakthrough will allow others to develop new theories as work continues in the field to better understand the behavior of strange metals.
Traditionally, it has been extremely challenging for theoretical physicists and the science community at large to fully explain the behavior that strange metals demonstrate. “Our work shows that if you’re going to model charge transport in strange metals, that model must apply to both fermions and bosons — even though these types of particles follow fundamentally different rules,” says Valles.
In the long term, developing a solid and sound theory of strange metals could have bold implications across various fields of research. Understanding strange metal behavior could generate shockwaves of understanding in relation to how high-temperature superconductivity functions, which has immense scope for things like quantum computers and lossless power grids.
Ultimately, as strange metal behavior seems to have a constant relationship with some of the fundamental quantum attributes of the universe, this discovery could also lead to a better understanding of the basic principles of the known physical world.
Chao Yang, Haiwen Liu, Yi Liu, Jiandong Wang, Dong Qiu, Sishuang Wang, Yang Wang, Qianmei He, Xiuli Li, Peng Li, Yue Tang, Jian Wang, X. C. Xie, James M. Valles Jr, Jie Xiong and Yanrong Li, “Signatures of a strange metal in a bosonic system” 12 January 2022, Nature. https://www.nature.com/articles/s41586-021-04239-y