Scientists have discovered a new state of matter within a quantum material, challenging existing understandings of electron behavior. The discovery, centered around a material called CeRu4Sn6, reveals a sideways flow of electric current without the need for a magnetic field – a phenomenon previously considered impossible under established physics principles.
Voltage in CeRu4Sn6
The research, conducted by an international team and led by physicist Prof. Silke Buhler-Paschen at TU Wien in Vienna, Austria, focused on CeRu4Sn6, a crystal composed of cerium, ruthenium, and tin. This material belongs to a class known as heavy-fermion materials, characterized by strongly interacting electrons. The team observed a sideways voltage within the material at temperatures just below one degree above absolute zero. This unexpected voltage served as the initial clue to the existence of this novel state of matter.
When Particles Vanish
Traditionally, physicists describe electrons as discrete particles carrying charge and momentum. However, in certain heavy-fermion metals, this picture begins to break down. Strong interactions between electrons can spread their energy across multiple possibilities, making it difficult to define a single speed or path for each electron. To manage this complexity, physicists often model these interactions using “quasiparticles” – electron-like packets that retain charge and momentum.
CeRu4Sn6, however, presented a unique challenge. The material exists in a regime of constant fluctuations, which effectively erased these quasiparticles, leaving researchers without their usual building blocks for understanding electron behavior. This is where the discovery becomes particularly significant: a new state of matter emerges *despite* the absence of well-defined particles.
Topology Without Particles
The observed behavior is linked to topological states, which are electronic patterns protected by symmetry and counting rules. These patterns are robust and can persist even in the presence of defects that would normally disrupt electron flow. The 2016 Nobel Prize in Physics recognized the importance of topological states in understanding electron movement within solids.
Traditionally, topological states have been understood within the framework of electrons behaving as particles with definite energies. CeRu4Sn6 defies this expectation, exhibiting topological behavior even as the quasiparticle picture collapses. This suggests a deeper connection between topology and the fundamental properties of the material, independent of particle-like descriptions.
A Hall Signal
The sideways voltage observed in CeRu4Sn6 is a manifestation of the Hall effect, a phenomenon where a voltage develops perpendicular to both the electric current and a magnetic field. Normally, a magnetic field is required to deflect the moving charges and create this voltage. However, in some crystals, a property called Berry curvature can induce this deflection without an external magnetic field.
Previous research had established the possibility of a magnet-free Hall response, and CeRu4Sn6 exhibited this behavior at ultra-low temperatures. This confirms that the observed sideways voltage is not simply a conventional Hall effect, but a more exotic phenomenon driven by the material’s unique quantum properties.
Pressure Draws a Dome
The research team utilized pressure to fine-tune the fluctuations within CeRu4Sn6 and observe how the sideways Hall signal responded. Increasing the pressure weakened the effect and shifted it to lower temperatures, correlating with a reduction in quantum fluctuations. Applying a magnetic field constricted the region where the response appeared, outlining a dome-shaped region in the material’s parameter space.
The fact that this dome is centered on the most fluctuating regime is particularly noteworthy. It challenges the conventional understanding that fluctuations and topology are competing forces. Instead, the results suggest that fluctuations may actually *enable* the emergence of topological states in CeRu4Sn6.
Symmetry Sets the Rules
The sideways motion observed in CeRu4Sn6 is also linked to the material’s lack of inversion symmetry – a property where the material appears identical when flipped. This asymmetry means that the internal forces acting on electrons do not cancel each other out, even in the absence of an external magnetic field. Materials with this characteristic are classified as a special type of metal with protected crossing points in their electronic structure, a description that aligns with the observed behavior of CeRu4Sn6.
The built-in symmetry rules of the material contribute to the detectability of the sideways voltage, as the signal can persist even when the traditional particle picture breaks down.
A Model Catches Up
Existing theoretical models often assume that electrons behave in simple, well-defined ways. To account for the observed behavior in CeRu4Sn6, the research team developed a new model that examined how the material’s internal interactions change at very low temperatures and what happens when those interactions begin to disintegrate. This model moved beyond relying on particle-like behavior and instead focused on identifying deeper, more stable patterns that could emerge even in the most chaotic conditions.
“This was the key insight that allowed us to demonstrate beyond doubt that the prevailing view must be revised,” said Prof. Buhler-Paschen.
A New Search Map
The ability to identify quantum criticality – a low-temperature threshold characterized by constant fluctuations – without needing a complete understanding of electron behavior opens up new avenues for materials discovery. Near this threshold, even small changes in pressure or magnetic field can reshape electron motion throughout the material.
For CeRu4Sn6, the dome-shaped region indicates that the topological semimetal phase emerges directly within the most fluctuating regime, rather than after order is established. This provides a new roadmap for identifying potential candidates, particularly within families of materials where quantum criticality is already known to occur.
CeRu4Sn6 and Future Technology
The robust sideways response observed in CeRu4Sn6 has potential applications beyond fundamental research. The ability to steer currents without relying on bulky magnets could lead to more efficient and compact electronic devices. In strongly interacting metals, electron correlations can amplify subtle quantum forces, transforming abstract band features into measurable voltages.
This control could be valuable in developing sensitive sensors or quantum circuits where even small magnetic fields can be disruptive. However, the current state of matter only appears at extremely low temperatures, so practical applications will depend on finding similar behavior in materials that function at warmer temperatures.
The discovery represents a fundamental shift in our understanding of the relationship between topology and electron behavior. Future research will focus on exploring other quantum-critical metals and investigating whether pressure, strain, or chemical modifications can bring this novel state of matter closer to practical temperatures. The study was published in Nature.
