Strangely ’quiet’ current in strange metal

Maxwell Andrews and Silke Bühler-Paschen
Maxwell Andrews and Silke Bühler-Paschen
What happens when electric current flows through a "strange metal"- TU Wien (Vienna) and Rice University (Texas) show: The established picture of electrons and "quasi-electrons" collapses.

At first glance, it all sounds so simple: there are electrons in a cable, and when we apply a voltage, the electrons dash from one side of the cable to the other, and an electric current flows. This picture is not entirely wrong - but it is not right either.

In fact, electrons cannot move freely in a solid. Instead, there are complicated interactions between many different particles. This causes the charge transport in the material to have some inertia - as if the electrons in the material had a greater mass.

This can be treated mathematically by describing the current flow through the material not with ordinary electrons, but with "quasi-electrons", which carry the same electrical charge but have a greater mass. However, new experiments by TU Wien and Rice University in Texas now show that in certain materials, so-called "strange metals", this picture breaks down. The current appears to be transported neither by electrons nor by quasi-electrons, but instead flows as a continuum. The results have now been published in the journal "Science".

Quasiparticles

Quasiparticles - such as quasi-electrons in this case - are not particles in the usual sense. They cannot be extracted from the material and stored separately. They are a concept that can be used to describe the movement of charge through a material in a relatively simple way, even if complicated interactions occur.

"Even if you describe the current flow with quasi-electrons instead of electrons, the basic idea remains the same: the current is transported in discrete portions that all carry exactly one elementary charge and can be detected individually," says Prof. Silke Bühler-Paschen from the Institute of Solid State Physics at TU Wien, one of the authors of the current paper.

Shot noise: The quantized nature of the charge

However, the team has now investigated a very specific material made of ytterbium, rhodium and silicon (YbRh2Si2), a well-known representative of the class of "strange metals". In previous years, Bühler-Paschen and her team have repeatedly shown that this material exhibits astonishing properties, such as a highly unusual relationship between electrical resistance and temperature.

This time, they investigated the question of how electric current flows through such a metal. This was done by measuring fluctuations in the current strength over time - known as shot noise - in the Rice University laboratory of Doug Natelson, the corresponding author of the study.

Shot noise has also analogies in classical physics - for example, the sound of hail pattering on a tin roof. This noise is caused by the hail hitting the roof in discrete portions - in the form of hailstones. If the same amount of precipitation poured onto the roof as a continuum, for example as a completely uniform stream of water, this noise would not be audible.

The same applies to electric current: if it arrives in discrete portions, a certain type of noise is to be expected. "The shot noise is simply due to the granular nature of the current flow. Since a quasielectron has the same discrete charge as a free electron, namely the elementary charge, one expects this shot noise to always be the same. Even in a material in which there are very strong interactions and the quasiparticles have an effective mass that is orders of magnitude higher," explains Silke Bühler-Paschen.

Indeed, the team carried out theoretical investigation to analyze the experimental results. "We performed calculations that allowed us to rule out any quasiparticle description of the measured shot noise in YbRh2Si2," said Rice University’s Qimiao Si, the lead theory co-author on the study and a long-term collaborator of Bühler-Paschen’s in research on strange metals.

Low noise - as if the current did not consist of particles

In order to make the shot noise in the strange metal directly measurable, nanoscopic wires first had to be produced. This was achieved using a molecular beam epitaxy system at the Center for Microand Nanostructures (ZMNS) at TU Wien and by a nanostructuring process developed at Rice University.

The result was surprising: "The shot noise was extremely low," says Silke Bühler-Paschen. "It is as if the electrical charge is not transported by discrete quasi-electrons, as is usually the case, but rather on the verge of flowing continuously."

A model by the theorist Qimiao Si (Rice University), had already predicted the "breaking apart" of Kondo-like quasiparticles in certain strange metals, but reduction in the measured shot noise sheds much new light on the inner workings of the electron system in YbRh2Si2.

A new picture for current flow

"What happens in this strange metal is unlike anything we have ever seen in other materials," says Silke Bühler-Paschen. "We imagine that the particles in this material are in a highly entangled state in which the picture of quasiparticles, which usually works so well with other materials, completely collapses."

The shot noise measurements will help clarify precisely how the particles move in the highly entangled state. "We have to find the right vocabulary to talk about how charge can move collectively through such a material," says Doug Natelson from Rice University.

Original publication

E. Bakali et al., Shot noise in a strange metal, Science (2023) abq6100

More about strange metals:

A new look at strange metals