The point at which a material changes from one state to another at absolute zero, pushed not by heat but by pressure, a magnetic field or a change in how many electrons it holds. Ordinary transitions, like ice melting, are driven by heat. At absolute zero only quantum fluctuations remain, and tuning a material through the point where one ground state gives way to another leaves its mark far above zero: a wedge of unusual behaviour that widens as it warms, where metals conduct in ways no standard picture fits. Layered crystals such as CeSiI might let gates and do the tuning.
Going deeper
At absolute zero a material can be tuned by pressure, field or doping from one ground state into another. Above the critical point a fan of quantum critical behaviour widens as temperature rises; in metals it is where resistance grows linearly with temperature. A layered heavy-fermion magnet might be tuned there by a gate or by strain.
Transitions without heat
Most are thermal: heat a magnet and its order melts at the Curie or . A quantum phase transition happens at absolute zero instead, between two ground states, as something other than temperature is varied – the pressure on the crystal, a magnetic field, or the density of electrons set by or . Strictly, the quantum critical point exists only at zero temperature; if the transition is continuous, quantum fluctuations there extend over all distances.
The dimer TlCuCl3 shows one cleanly: at low pressure it is a quantum paramagnet, its paired off, and at high pressure a Néel antiferromagnet, with following the change between them (Sachdev and Keimer, 2011).
The fan above it
The critical point lives at zero temperature, but its influence spreads upwards. Above it lies a region shaped like a fan, widening as the temperature rises, in which the material is in neither ground state and its properties follow the critical fluctuations rather than well-defined particles.
In metals this is where strange behaviour turns up. Near the doping or pressure at which antiferromagnetic order disappears in heavy-fermion compounds and copper oxides, neutron scattering finds the spin fluctuations obeying the scaling the critical point predicts, and the electrical resistance rises in proportion to temperature – the hallmark of what is called a strange metal.
Tuning in two dimensions
Bulk heavy-fermion metals are usually driven to their quantum critical points by pressure or magnetic field. A layered heavy-fermion crystal would add the electrostatic gating and strain that allow. CeSiI is the candidate: its cerium are screened into a Kondo lattice near 40 K and order antiferromagnetically at 8 K, in of every thickness down to two layers, so both its heavy electrons and its magnetism are two-dimensional (Torres and colleagues, 2025). Whether a gate or strain can carry it across a quantum critical point is an open question.
For specialists
A continuous phase transition at zero temperature between distinct ground states, tuned by a non-thermal parameter – pressure, magnetic field, carrier density. Its quantum fluctuations govern a finite-temperature quantum critical fan above the critical point that widens with temperature, where the response follows scaling rather than well-defined ; near magnetic quantum critical points in heavy-fermion and copper-oxide metals, linear in temperature marks the strange metal. TlCuCl3 is tuned by pressure from a quantum paramagnet into a Néel state. CeSiI, a heavy-fermion antiferromagnet whose Kondo lattice and order persist down to two layers, is a candidate for reaching a quantum critical point with gates or strain.
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