Quantum Materials: Where Quantum Mechanics Shapes the Macroscopic World

What defines a quantum material

The term "quantum materials" came into wide use in the 2010s as an umbrella for solids whose electronic, magnetic or optical properties cannot be understood by treating electrons as independent particles moving through a fixed lattice. In a conventional metal, the standard band theory of solids - electrons filling energy bands like water filling a glass - works remarkably well. In a quantum material it fails, because quantum-mechanical effects that are normally microscopic instead organise the behaviour of the whole crystal.

Three ingredients tend to signal a quantum material. The first is strong electron-electron correlation: when the repulsion between electrons is comparable to their kinetic energy, they can no longer be treated one at a time, and collective states emerge. The second is topology - global geometric properties of the electronic wavefunction that are protected against small perturbations and give rise to robust conducting edges or surfaces. The third is macroscopic quantum coherence, in which a single quantum wavefunction describes an enormous number of particles at once, as in a superconductor.

What unites these otherwise different systems is that the interesting physics lives at scales you can measure and, in principle, engineer. The challenge - and the reason the field is so active - is that the same complexity that produces useful phenomena also makes these materials extraordinarily hard to predict, synthesise and control.

Superconductivity: the first macroscopic quantum state

Superconductivity is the oldest and best-understood quantum material phenomenon. In 1911 the Dutch physicist Heike Kamerlingh Onnes, having recently learned to liquefy helium, discovered that mercury cooled below about 4 kelvin lost all electrical resistance abruptly and completely. A current started in a superconducting loop will, as far as experiments can tell, circulate for an extraordinarily long time without decaying.

For decades the effect had no theoretical explanation. It arrived in 1957, when John Bardeen, Leon Cooper and Robert Schrieffer published what became known as BCS theory. Their insight was that electrons, despite repelling one another, can form weakly bound "Cooper pairs" through interactions with lattice vibrations. These pairs behave collectively and condense into a single macroscopic quantum state that flows without scattering - quantum mechanics made visible at human scale. BCS earned the trio the 1972 Nobel Prize in Physics.

In 1986 Georg Bednorz and Karl Alex Müller upended the field by discovering superconductivity in a copper-oxide ceramic at temperatures far above what BCS theory seemed to allow, a result recognised with the 1987 Nobel Prize. These "high-temperature" cuprate superconductors, still not fully explained today, are a canonical correlated quantum material. Claims of room-temperature superconductivity have repeatedly drawn headlines - including several high-profile results that were later retracted or failed to reproduce - so the honest current status is that ambient-pressure, room-temperature superconductivity remains an unrealised research goal rather than an established fact.

Topological insulators and the quantum Hall effect

A second pillar of the field is topology. The story begins in 1980, when Klaus von Klitzing discovered the quantum Hall effect: in a two-dimensional electron gas under a strong magnetic field, the transverse (Hall) conductance takes on values that are exact integer multiples of a fundamental constant, reproducible to better than one part in a billion. That precision - indifferent to the sample's shape or impurities - is a hallmark of topological protection, and it now underpins the international standard for electrical resistance. Von Klitzing received the 1985 Nobel Prize; the 1982 discovery of the fractional quantum Hall effect by Daniel Tsui and Horst Störmer, later explained by Robert Laughlin, extended the idea to states with fractionally charged excitations.

Topological insulators, proposed theoretically in 2005 by Charles Kane and Eugene Mele and independently by others, brought this physics to materials that need no external magnetic field. A topological insulator is electrically insulating in its bulk but hosts conducting states on its surface or edges that are protected by the material's electronic topology and by time-reversal symmetry. The first experimental confirmation came in 2007 in mercury telluride quantum wells in Laurens Molenkamp's group, following a prediction by Bernevig, Hughes and Zhang; three-dimensional versions such as bismuth selenide and bismuth telluride followed in 2008–2009.

The appeal is robustness: because the conducting surface states are guaranteed by topology, they resist the disorder and defects that plague ordinary electronics. This makes topological insulators - and their relatives, topological superconductors - promising platforms for low-dissipation devices and, potentially, for fault-tolerant quantum computing.

Correlated electron materials

When electrons interact strongly, the results range from the useful to the deeply puzzling. Correlated electron materials - a group that includes the cuprate and iron-based high-temperature superconductors, heavy-fermion compounds, and Mott insulators - are substances that band theory predicts should be metals but which, because of electron repulsion, become insulators or exhibit exotic ordered phases instead.

A Mott insulator is the clearest example: it has exactly the electron count that should make it conduct, yet the electrons jam one another in place, like cars in a full car park, and no current flows. Tuning such materials with pressure, chemical doping or magnetic field can drive dramatic transitions between insulating, magnetic and superconducting states, often crowded together in the same phase diagram. Understanding why these phases neighbour one another - and whether the correlations that produce magnetism also drive high-temperature superconductivity - is one of the central unsolved problems in condensed-matter physics.

Frontier work continues to enrich this picture. Recent experiments have reported states in which strong correlation, topology and superconductivity appear together rather than in isolation - for example, superconductivity emerging alongside correlation-driven insulating phases in twisted bilayer transition-metal dichalcogenides. Such results suggest that the three organising ingredients of quantum materials are more intertwined than once assumed.

Graphene, moiré systems and the two-dimensional frontier

The isolation of graphene - a single atomic layer of carbon - by Andre Geim and Konstantin Novoselov in 2004 opened an entirely new arena for quantum materials and earned them the 2010 Nobel Prize. In graphene, electrons behave as if they were massless relativistic particles, and the material displays a distinctive form of the quantum Hall effect even at room temperature. It became the archetype of two-dimensional materials, a family that now includes insulators, semiconductors, magnets and superconductors just one or a few atoms thick.

The field took a dramatic turn in 2018, when Pablo Jarillo-Herrero's group at MIT stacked two graphene sheets with a small relative twist of about 1.1 degrees - the so-called magic angle. The resulting moiré pattern flattened the electronic bands so severely that correlation effects dominated, and the material became both an insulator and a superconductor depending on how it was tuned. "Magic-angle" twisted bilayer graphene turned a simple, well-understood material into a highly tunable correlated quantum system, launching the sub-field now called twistronics.

These 2D and moiré platforms are prized because they can be assembled layer by layer and controlled electrically, letting researchers dial through quantum phases in a single device. For a broader treatment of the material class, see graphene and the wider families of advanced materials and nanomaterials.

Quantum spin liquids and other frontier states

Not every quantum material is defined by charge flow; some are defined by magnetism that refuses to settle. In a conventional magnet, spins line up into an ordered pattern as the material cools. In a quantum spin liquid - a state first envisioned by Philip Anderson in 1973 - the spins remain entangled and fluctuating even as the temperature approaches absolute zero, never freezing into order. The concept gained new life in 1987 when Anderson connected it to high-temperature superconductivity.

Quantum spin liquids are experimentally elusive: proving that a material is a genuine spin liquid, rather than simply a magnet that orders at some very low temperature, is notoriously difficult, and few candidate materials are universally accepted. They matter because their entangled excitations may carry fractional quantum numbers and, in some theories, could support the same kind of topologically protected quantum information sought in topological superconductors.

This frontier - spanning quantum spin liquids, topological superconductors and correlated topological phases - is where much of the field's current uncertainty and excitement live. The established phenomena, by contrast, are superconductivity and the quantum Hall effects, which are textbook physics and underpin real metrology. Keeping that distinction clear is part of reading the field honestly. The physics also connects to the broader study of quantum materials as a discipline within solid-state physics.

Why quantum materials matter for technology and energy

The practical stakes are considerable. Superconductors already carry current in MRI magnets, particle accelerators and experimental fusion reactors, and superconducting circuits are one of the leading hardware platforms for quantum computing. Topological materials promise electronics in which information flows along protected channels with far less energy lost to heat - a direct route toward lower-power computing. Correlated materials, meanwhile, are the natural home of new superconductors and of sensors exploiting sharp quantum phase transitions.

In energy, the interest is twofold. Loss-free conduction would transform power transmission and storage, while the quantum properties of engineered materials increasingly define how efficiently devices convert energy from one form to another - the domain of energy conversion and energy harvesting. Two-dimensional and correlated materials are actively studied for thermoelectrics, photovoltaics and other conversion technologies, and feed into wider work on renewable energy innovations and new energy technology.

It is worth stating plainly what is not on offer: quantum materials do not produce energy from nothing, and none of the phenomena described here violate the laws of thermodynamics. A superconductor carries current with little or no loss but does not create that current; a topological device saves energy but does not manufacture it. The genuine promise is efficiency and control, not perpetual motion.

A research bridge: neutrinovoltaic and quantum-material engineering

One example of a research programme built on quantum-material engineering is the neutrinovoltaic work of the Neutrino Energy Group, a research organisation founded in Berlin in 2008. Its investigations centre on a patented multilayer of graphene and doped silicon nanolayers, and draw on studies of atomic-scale graphene motion - work associated with the group of physicist Paul Thibado - as a route to rectifying tiny, ambient mechanical and electromagnetic fluctuations into an electrical current.

This should be understood as in-development research within an open system: the aim is to convert a small fraction of the environmental flux already passing through a device - thermal motion and ambient electromagnetic fields - into usable current, not to generate energy from nothing, and it is framed to respect the laws of thermodynamics. It is not a proven technology and not a purchasable product. It appears here only as an illustration of how the tunable, quantum-mechanical behaviour of two-dimensional materials is being explored across energy research, alongside more established approaches such as the thermoelectric generator and piezoelectricity.