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European scientists have discovered a new state of matter inside a quantum material, and it behaves in a way no one expected.
Electric current suddenly flows sideways without any magnetic field, even though the electrons inside no longer act like normal particles.
Voltage in CeRu4Sn6
Less than one degree above absolute zero, the new study found a sideways voltage in CeRu4Sn6, a crystal made of cerium, ruthenium, and tin that belongs to a class of strongly interacting metals known as heavy-fermion materials.
Using that signal as a guide, physicist Prof. Silke Buhler-Paschen at TU Wien in Vienna, Austria tied it to an unexpected state. The research team described the result as a huge surprise.
That sideways voltage peaked in the same regime where electrons stop acting as tidy carriers, creating a mismatch many physicists had called impossible.
When particles vanish
Some heavy-fermion metals, a class where electrons act unusually massive, make that neat carrier picture start to crumble.
Strong interactions can spread an electron’s energy over many possibilities, so no single speed or path stays meaningful.
That picture turns messy interactions into quasiparticles, electron-like packets that carry charge and momentum, so calculations stay manageable.
CeRu4Sn6 sat in a regime of constant fluctuations, and those fluctuations erased quasiparticles, leaving physicists without their usual building blocks.
Topology without particles
Topological states, electronic patterns protected by symmetry and counting rules, can outlast defects that would normally scramble electrons.
In 2016, the Nobel Prize highlighted how such patterns can appear when electrons move through solids in constrained ways.
Traditional theory still treats the electrons as particles with definite energies, so topology stays tied to clean energy bands.
CeRu4Sn6 breaks that expectation by showing the same sort of protected behavior right where quasiparticles disappear.
A Hall signal
A transverse voltage across a conductor defines the Hall effect, a sideways signal created when moving charges deflect.
Normally, a magnetic field triggers that deflection, which makes the Hall effect a standard tool for measuring charge carriers.
Inside some crystals, Berry curvature, a quantum property that bends electron motion in solids, can replace the magnet entirely.
A paper laid out that magnet-free Hall response, and CeRu4Sn6 matched it at ultra-low temperature.
Pressure draws a dome
Pressure let the team dial down the fluctuations and track how the sideways Hall signal rose and then collapsed.
Squeezing CeRu4Sn6 weakened the effect and pushed it to lower temperatures, which matched a reduction in quantum fluctuations.
Magnetic fields shrank the region where the response appeared, tracing an emergent topological semimetal, a metal with protected crossings, into a dome.
Seeing that dome centered on the most restless regime flips the old expectation that fluctuations and topology must compete.
Symmetry sets the rules
Sideways motion can arise even without magnetism when inversion symmetry, a crystal property where flipped positions look the same, is missing.
In CeRu4Sn6, that missing symmetry means the internal forces on electrons do not cancel, even at zero field.
Physicists classify materials like this as a special kind of metal with protected crossing points in their electronic structure, and CeRu4Sn6 matches that description.
Those built-in symmetry rules make the sideways voltage easier to detect, because the signal can remain even when the usual particle picture breaks down.
A model catches up
Early hesitation made sense, because most existing theories assume electrons behave in simple, well-defined ways.
The team built a new model that examined how the material’s internal interactions change at very low temperatures and what happens when those interactions begin to fall apart.
Rather than relying on tidy particle-like behavior, the theory searched for deeper patterns that could remain stable even in the most chaotic regime.
“This was the key insight that allowed us to demonstrate beyond doubt that the prevailing view must be revised,” said Buhler-Paschen.
A new search map
Labs can spot quantum criticality, a low-temperature threshold with constant fluctuations, without knowing every detail of the electrons.
Near such a threshold, tiny changes in pressure or field can reshape electron motion across the whole material, not just locally.
For CeRu4Sn6, the dome shows the semimetal forms right around the most fluctuating regime, not after order settles.
That link offers a new route to discover candidates, especially in families where quantum criticality already appears under gentle tuning.
CeRu4Sn6 and future technology
A robust sideways response offers more than a curiosity, because it can steer currents without adding bulky magnets.
In strongly interacting metals, electron correlations can amplify subtle quantum forces, turning an abstract band feature into a measurable voltage.
Such control could support sensitive sensors or quantum circuits where tiny fields matter, especially when designers want fewer magnetic parts.
Still, the state appeared only near absolute zero, so practical hardware will depend on finding similar behavior at warmer temperatures.
A single material has forced physicists to separate topology from the particle picture, and that change opens a clearer theory.
Future work will test other quantum-critical metals and determine whether pressure, strain, or chemistry can bring the same response to practical temperatures.
The study is published in Nature.
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