Researchers have directly observed fermionic quasiparticles slowly “disappearing” for the first time. This vanishing act took place near a quantum phase transition in a so-called heavy-fermion compound. As well as advancing our understanding of the stability of fermionic quasiparticles, such transitions could have applications in quantum information technology.

The most well-known phase transition occurs when water abruptly transforms into ice as it cools below 0 °C. The characteristics of ice are very different to those of liquid water – the density of ice is much lower, for one, and its structure changes dramatically. In some phase transitions, however, change occurs more gradually. For example, iron goes from being ferromagnetic to paramagnetic when heated to 760 °C, but as the transition progresses, the system takes longer and longer to come to equilibrium, thereby slowing the transition and making it more continuous. This means that the two phases (ferromagnetic and paramagnetic) become closer in energy.

This phenomenon is typical for phase transitions that involve excitations of bosons, which are particles that mediate interactions (including the interactions responsible for magnetism). At a fundamental level, however, matter is not made up of bosons, but of fermions.

“Electrons belong to the family of fermions,” notes study team member Shovon Pal, “and matter made up of these particles cannot usually be destroyed because of the fundamental laws of nature. Fermions therefore cannot disappear and it is for this reason they are normally never involved in phase transitions.”

Superposition of two types of electron states

Using terahertz time-domain spectroscopy measurements, Pal and colleagues in Manfred Fiebig’s group at ETH Zurich, Switzerland observed this critical slowing near a quantum phase transition in YbRh2Si2. The quasiparticles in this material consist of a superposition of two types of electron states: one composed of localized electrons like those found in an insulator and one composed of mobile electrons like in a metal. One striking feature of this superposition is that the electrons are, to a certain extent, spatially bound, which gives them an effective mass 103 to 104 larger than the rest mass of a normal electron. Compounds that support this type of binding are thus known as heavy-fermion compounds.

In another contrast with “normal” electrons, these quasiparticles, which only exist in the quantum regime, can be destroyed during a phase transition. This is the key factor that allows them to undergo a continuous transition comparable to those involving bosons, Pal says.

Critical exponent

In their study, the researchers extracted a parameter known as the critical exponent that relates to a collapse in the probability of forming these exotic states at the phase transition. “Critical exponents can be used to classify phase transitions and this concept can now be extended to classify transitions not only associated with the breakdown of bosonic order parameters, like the magnetization in a ferromagnetic transition, but also to exotic phase transitions with the destruction of fermionic particles,” explains Pal, who is now at NISER in India.

The researchers used terahertz radiation because its energy scales are on a par with the intrinsic energy scales of heavy fermions. “Upon THz excitation, the quasiparticles break down and disappear, taking the system into a non-equilibrium state,” Pal explains. “It naturally strives to return to equilibrium via the re-emergence of quasiparticles and this reconstruction process occurs after a certain time delay that corresponds to the intrinsic energy scales of heavy-fermion systems.”

By measuring this delayed response, the team was able to observe and characterize the evolution – that is, the disappearance and reappearance – of the quasiparticles.

The study, which is detailed in Nature Physics, highlights a new way to investigate many-body correlations in certain exotic quantum materials like heavy-fermion compounds. “It is thus a starting point for many further investigations on different materials unveiling the physics of phase transitions in the quantum world,” Pal tells Physics World.

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