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Increasing Entropy Could Signal When a Mysterious Quantum Flip Is About to Occur

Not everything is as it seems.

FIONA MACDONALD
18 MAY 2017
 

Scientists have shown for the first time that changes in entropy - the measure of disorder in a system - can accurately predict when a mysterious quantum flip, known as a quantum phase transition, is about to happen.

 

That's a big deal, because not only do we know very little about these quantum phase transitions - it's been less than a year since they were observed for the first time - but they could also be the key to unlocking all kinds of crazy properties, such as high-temperature superconductivity.

"This demonstration provides a foundation to better understand how novel behaviours like high-temperature superconductivity are brought about when certain kinds of materials are cooled to a quantum critical point," says Qimiao Si from Rice University.

Si and his colleagues conducted their research using cerium copper gold alloys - quantum materials that are known as 'heavy fermions', which exhibit some incredibly weird electronic properties when they're cooled down.

One of the most desirable of these is high-temperature superconductivity - the ability for materials to conduct electrical current with zero resistance at temperatures well above the traditional superconductor limit of around 30 Kelvin (-243.2 degrees Celsius).

If we could achieve that reliably and efficiently, it would be a huge deal, because right now, electricity grids lose around 7 percent of their energy to friction, and superconductive materials would stop that from happening.

 

But heavy fermions aren't high-temperature superconductors, exactly. Instead, what happens when the materials get very cold is that their electrons appear to effectively get hundreds of times more massive than usual, and - even stranger - this effective electron mass varies hugely as temperature changes.

This behaviour defies traditional physics, and instead occurs at points called quantum phase transitions, where the electrons within a material are changing from one state into another, like atoms within melting ice - but in the quantum realm.

In 2001, Si and colleagues put forward an hypothesis to try to explain this weirdness - they suggested that right at the junction of the quantum phase transition, at a point called the quantum critical point, electrons fluctuate so much that their effective mass becomes infinitely large.

To wrap your head around quantum critical points, Si uses the example of liquid water and ice:

"Ice is a very ordered phase because the H2O molecules are neatly arranged in a crystal lattice. Water is less ordered compared with ice, but flowing water molecules still have underlying order.

The critical point is where things are fluctuating between these two types of order. It's the point where H2O molecules sort of want to go to the pattern according to ice and sort of want to go to the pattern according to water. It's very similar in a quantum phase transition.

 

Even though this transition is driven by quantum mechanics, it is still a critical point where there's maximum fluctuation between two ordered states. In this case, the fluctuations are related to the ordering of the 'spins' of electrons in the material."

Spin is an inbuilt property of electrons that can be classified as either 'up' or 'down'.

In magnets, electron spins are all aligned in the same direction, but many materials exhibit the opposite, where spins alternate in a repeating down, up, down, up pattern that's called antiferromagnetic.

Researchers have shown that this magnetic order flips on either side of a quantum critical point - and the quantum critical point is where fluctuations between those two electron states are at their maximum.

The team has spent the past 15 years testing out their 2001 hypothesis - the big question remaining is whether there are certain tell-tale signs that a quantum critical point is approaching.

In this study, they used a cerium copper gold alloy called cerium copper-six, and tested the idea that entropy would increase right before the quantum critical point, and could even be used to predict the flip.

 

Researchers can't directly measure entropy, but the ratio of entropy changes to stress is known to be directly proportional to another ratio: the amount a material expands or contracts due to changes in temperature.

By chilling the material to ridiculously cold temperatures, the team was able to create a 3D map that showed how entropy at very low temperatures steadily increased as the system approached the quantum critical point.

"We made a detailed map of the entropy landscape in the multidimensional parameter space and verified that the quantum critical point sits on top of the entropy mountain," saus lead researcher Kai Grube from Karlsruhe Institute of Technology in Germany.

In other words, they showed that their predictions were right - entropy does increase right before a quantum critical transition.

"It is quite remarkable that the entropy landscape can connect so well with the detailed profile of the quantum critical fluctuations," says Si.

That might not sound groundbreaking to your everyday life, but this information could be used to better understand and control these flips in future - hopefully allowing researchers to one day be able to figure out how to turn on all the weird and wonderful properties of the quantum realm, such as high-temperature superconductivity.

The research has been published in Nature Physics.

 

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