This shows the neutron spectrometer used in the study. Photo: EPFL/PSI.
This shows the neutron spectrometer used in the study. Photo: EPFL/PSI.

An international team of scientist has shown experimentally, for the first time, a quantum phase transition in strontium copper borate (Sr2Cu(BO3)2), the only material to date that realizes a famous quantum many-body model.

Many physical phenomena can be modeled with relatively simple math. But in the quantum world there are a vast number of intriguing phenomena that emerge from the interactions of multiple particles – ‘many bodies’ – that are notoriously difficult to model and simulate, even with powerful computers. Examples of quantum many body states with no classical analogue include superconductivity, superfluids, Bose-Einstein condensates and quark-gluon plasmas. As a result, many ‘quantum many-body’ models remain theoretical, with little experimental backing.

Now, scientists from Ecole Polytechnique Federale de Lausanne (EPFL) and the Paul Scherrer Institut (PSI), both in Switzerland, have realized experimentally a new quantum many body state representing a famous theoretical model known as the ‘Shastry-Sutherland’ model. This work, which they conducted with colleagues at many other institutes around the world, is published in Nature Physics.

While there are several one-dimensional many-body models that can be solved exactly, there are but a handful in two-dimensions (and even fewer in three). Such models can be used as lighthouses, guiding and calibrating the development of new theoretical methods.

The Shastry-Sutherland model is one of the few 2D models that have an exact theoretical solution, representing the quantum pairwise entanglement of magnetic moments in a square lattice structure. When conceived, the Shastry-Sutherland model seemed a purely abstract theoretical construct, but remarkably it was discovered that this model is realized experimentally in Sr2Cu(BO3)2.

Mohamed Zayed in the lab of Henrik Rønnow at EPFL and Christian Ruegg at PSI have now discovered that pressure can be used to tune this material away from the Shastry-Sutherland phase in such a way that a so-called quantum phase transition to a completely new quantum many body state is reached.

Unlike classical phase transitions such as ice melting into liquid water and then evaporating as a gas, quantum phase transitions describe changes in quantum phases at absolute zero (-273.15°C). They occur because of quantum fluctuations that are themselves triggered by changes in physical parameters — in this case pressure.

The researchers were able to identify the new quantum state using neutron spectroscopy, a very powerful technique for investigating the magnetic properties of quantum materials and technological materials. Combining neutron spectroscopy with high pressures is very challenging, and this experiment is among the first to do so for a complex quantum state.

In the Shastry-Sutherland model, the atomic magnets — arising from the spins of the atom’s electrons — are quantum-entangled in pairs of two. The researchers found that in the new quantum phase the atomic magnets appear quantum-entangled in sets of four – so-called plaquette singlets.

“This is a new type of quantum phase transition, and while there have been a number of theoretical studies on it, it has never been investigated experimentally,” says Rønnow. “Our system may allow further investigations of this state and the nature of the transition into the state."

The need for high pressure limits what is experimentally feasible at the moment. However, Rønnow and Ruegg are building a new neutron spectrometer (CAMEA) at PSI, which will be ready by the end of 2018, as well as another one at the European Spallation Source in Sweden, which will become operational in 2023. The four-spin state in Sr2Cu(BO3)2 will be among the first physical phenomena studied by these new machines. Experiments combining pressure and magnetic fields may give access to yet undiscovered phases in quantum materials.

“Quantum many-body physics remains a challenge where theory has only scratched the surface of how to deal with it,” says Rønnow. “Better methods to tackle quantum many-body phenomena would have implications from materials science to quantum information technology.”

This story is adapted from material from EPFL, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.