A metal is a material defined by specific properties, including conducting electricity well. Every metal has a finite conductivity — a measure of how well it conducts — in particular conditions. It changes when the metal is heated or cooled.

For example, at a pleasant 20º C, the electrical conductivity of zinc is roughly 16.9 million siemens per metre. But cool it to a frigid –272.3º C and its conductivity becomes infinite. This is because at this temperature zinc becomes a superconductor: able to conduct an electric current with zero resistance.

Scientists know of many metals that have a finite conductivity at room temperature and infinite conductivity at very low temperatures. This drastic change in behaviour is because of something that happens to the metals’ electrons. At room temperature, the electrons in a grid of zinc atoms move freely throughout the material, transporting electricity if a voltage is applied. Each electron itself repels other electrons and is also acted on by other forces imposed by the 3D grid of atoms around it, including vibrations in the grid, impurities in the material, and attractive forces exerted by protons in the nuclei.

When this grid is cooled to a low temperature, many of the forces weaken. At under a critical temperature, in zinc’s case –272.3º C, the net force on electrons is weakly attractive. That is, the electrons are mildly attracted to each other across large distances (i.e. beyond the short range across which they still repel each other). This net force causes the electrons to ‘pair up’ without getting closer and together behave in a way that individual electrons can’t. These pairs are called Cooper pairs. Thanks to the low temperature, at some point these pairs of electrons undergo a phase transition, forming an exotic state of matter within the zinc grid called a superconductor. It is this superconductor that has infinite conductivity.

Almost a superconductor, yet not

Metals that don’t become superconducting at very low temperatures still become better conductors because the forces that resist the flow of an electric current also weaken at lower temperatures. (‘Current’ here refers only to a direct current. The flow of an alternating current in a superconductor elicits a variety of effects, including those that resist its flow.)

Some metals, or metallic substances, do something weird. Below the critical temperature, their electrons experience the net attractive force and pair up — but then they don’t yet condense to form a superconductor. That is, the material doesn’t become a superconductor but just a better conductor, and it conducts electricity with Cooper pairs, not electrons. In this state, the material is said to have become a Bose metal.

A Bose metal is a kind of anomalous metallic state (AMS). The ‘anomaly’ is that the Cooper pairs are formed but don’t condense into a superconductor. In technical terms, they fail to establish long-range superconducting coherence. Studying AMSs, in a wider field called condensed matter physics, is important to understand disordered metals, where the grid of atoms has an irregular structure or impurities or the material is alloyed in a way that prevents it from behaving like a ‘regular’ metal. Disordered metals thus have deviant properties but we don’t fully understand the different ways in which they can deviate. Studying them helps scientists probe a variety of quantum processes.

For example, traditional theories that describe disordered metals say that at absolute zero temperature, the metals should have either zero conductivity (become an insulator) or infinite conductivity (become a superconductor). A Bose metal challenges this description because its conductivity is between zero and infinity as the temperature tends to absolute zero — or at least it may be if we saw one in action.

So far, Bose metals have only been predicted to exist in specific materials; scientists haven’t synthesised or found them. It’s possible in fact that Bose metals may not exist at all, but that would be useful to know, too, for the implications for physicists’ theories of AMS.

But on February 13, a team of researchers from China and Japan reported in the journal Physical Review Letters that they had found strong signs that niobium diselenide (NbSe₂) can become a Bose metal.

Magnetic field as villain

Like zinc, NbSe₂ also becomes a superconductor at low temperature but with additional ‘abilities’. This is due to a key detail: magnetic fields and a material’s superconducting state never get along. If a zinc sample is placed in an external magnetic field and cooled slowly to under its critical temperature, the moment it becomes a superconductor the sample will expel the magnetic field from within its body.

NbSe₂ goes through the same transition at a particular temperature and magnetic field strength. But when the field strength is slowly increased, NbSe₂ enters a ‘mixed state’: it remains superconducting but also allows the magnetic field to enter its body in small, isolated pockets without spreading through its bulk. If the field continues to strengthen, beyond an upper threshold the superconducting state will collapse and NbSe₂ will revert to its pre-superconducting state.

Materials with this more-dynamic road through superconductivity are called type-II superconductors. The forces that act on electrons in such a material as it is cooled and magnetised become more pronounced if the material is physically thinner. And one theory of Bose metals predicts that if a 2D version of this material — i.e. a single layer of NbSe₂ molecules — is subjected to a magnetic field oriented a certain way, a Bose metal will be created.

The researchers set out to check this and found all the hallmarks of such an AMS, but the study’s lead investigator and Nanjing University professor Xiaoxiang Xi stopped short of calling it a Bose metal, telling Physics magazine the definition of the AMS is “somewhat ambiguous”.

In particular, the team used Raman spectroscopy to find the thin NbSe₂ had Cooper pairs without entering a superconducting state and the material’s Hall resistance vanished as the team increased its thickness. When a piece of regular metal is placed in a magnetic field and a current is passed through it, the piece develops a voltage in the perpendicular direction. The resistance associated with this voltage is called the Hall resistance. The Hall resistance vanishing in NbSe₂ is a sign that its charge-carriers are Cooper pairs rather than electrons.

“Our results suggest that the AMS is characterised by fluctuating local pairing, which fails to condense,” the team wrote in its paper. “Theories focusing on the role of phase fluctuation in disrupting global superconductivity could provide valuable understanding of the phenomena observed.”

They added that the findings impose “limitations on theories centred around” pockets of superconductivity in a non-superconducting material and the coexistence of superconducting and non-superconducting phases in the same material.

Bose metals don’t have concrete applications today but they are a rich playground for physics research that could inform future innovation.