Carnegie Mellon University
April 16, 2021

New Research Provides Insights on Thermodynamics of Quantum Materials

By Ben Panko

New research from an international collaboration that includes a Carnegie Mellon University in Qatar researcher has shed new light on the thermodynamics of quantum materials by observing a demonstration of critical-point physics in the compound SrCu2(BO3)2, an important model in quantum materials research.

"This is the first time that this has been seen in what's called a 'pure spin system,'" said Associate Teaching Professor of Physics Mohamed Zayed, who is based at Carnegie Mellon University's Qatar campus. The research was published this week in the journal Nature.

One can think of the "spin" of a particle as akin to a microscopic quantum magnet, Zayed said, and much like the magnets we're familiar with, particle spins interact with each other. SrCu2(BO3)2 is an example of a "frustrated" magnet – a material in which particles receive contradictory signals from their neighbors, leading to strange and complex structures. The kinds of calculations normally used in solid state physics often don't work in frustrated magnets, Zayed said, but SrCu2(BO3)2 represents a useful exception to that rule.

"This particular compound is quite important in this subfield of solid-state physics that is magnetism and this subfield of magnetism that is frustrated magnetism," Zayed said. As part of an international collaboration, Zayed helped use this exotic material to look for a physics phenomenon that occurs in a much more familiar compound – water.

Water can exist in three phases (solid, liquid and gas), with the boundaries between those phases depending on the combination of temperature and pressure that the water is exposed to. A simple example of this is the fact that water boils, or transitions from a liquid to a gas, at lower temperatures at higher altitudes, where the air pressure is lower. Conversely, at higher pressures even water that is extremely hot will remain liquid.

However, when water is exposed to enough pressure and heat, it reaches what is known as its critical point, where the phase boundaries cease to exist, and liquid and vapor water become one phase with no transition between the two.

Thermodynamic phases and boundaries aren't limited to substances like water, however – they've long been observed in quantum materials, with the phenomena depending on factors like how much pressure, applied magnetic field and disorder the material is exposed to. However, nobody had discovered whether these materials had thermodynamic pressure-temperature critical points.

In their new study, Zayed and his co-authors took precise specific heat measurements to prove the existence of this phenomena in SrCu2(BO3)2. The results were backed up by newly developed mathematical calculations to help explain how they worked.

Zayed and his team believe that this result will shed far more light on the thermodynamics of quantum magnetic materials, and hopefully inspire more study of them.