Neutron diffraction experiment under pressure at ILL’s D20 beamline enables greater understanding of the quantum material Yb2Ti2O7
New quantum states of matter often escape experimental observations. A recent study combined neutron diffraction and muon spin relaxation measurements to show why in the mineral Yb2Ti2O7: isolated atomic defects (excess of Yb) create a local strain and induce a phase transition to a much more conventional magnetic state.
A new quantum state of matter, dubbed ‘quantum spin ice’, may well exist at temperatures below 2K (-271.15°C) in the rare-earth mineral Yb2Ti2O7. In this state, the orientation of the magnetic moments of the electrons is analogous to the configuration of hydrogen-oxygen chemical bonds in the solid form of water, hence the reference to the ice. By using specific atoms with anisotropic magnetic properties, like the chemical element ytterbium (Yb), this state would acquire fascinating quantum properties such as the coherent superposition of states, depicted by the famous Schrödinger’s cat, or new exotic quasi-particles of electronic character.
The experimental observation of such a quantum state in a large size material could revolutionise our ways of thinking of the collective behavior of electrons in matter, and further help to consider the future physical properties of quantum materials. Although Yb2Ti2O7 has been the subject of intense research since 2000, physicists have not reached a consensus on the nature of its magnetic state at very low temperatures: is it the so-called quantum spin ice, or a more ordinary ferromagnetic phase, similar to the one existing in fridge magnets?
A recent study conducted by an international collaboration of research institutions (Institut Laue-Langevin, ILL, McMaster University, Université Paris-Sud and the Paul Scherrer Institute) has shown that a mysterious non-magnetic state exists at very low temperatures, in agreement with the existence of a quantum spin ice, and that it is very sensitive to external perturbations such as the application of a high pressure.The study used two experimental techniques: muon spin relaxation (µSR) and neutron diffraction. The neutron experiment was conducted at ILL’s D20 beamline, and the µSR measurements were carried out at the Paul Scherrer Institut. D20 is a versatile, two-axis powder diffractometer with a variety of applications, including thermodiffractometry, magnetism, kinetics and physiosorption. The instrument is characterised by its extremely high neutron flux which permitted here the detection of a very small signal inside a heavy high pressure, low temperature sample environment.
The results obtained from neutron scattering indicate that the magnetic state obtained under pressure is long-range ordered, with a static magnetic moment of 0.33(5) µB, as evidenced by the appearance of resolution-limited magnetic Bragg peaks. Previous neutron diffraction measurements have evidenced in real materials with the chemical formula Yb2(YbxTi1-x)2O7-δ the presence of Yb atoms in excess, localised on the titanium crystallographic site. The significant difference between the ionic radius of Yb3+ and Ti4+ creates a strong local strain, in a similar way to an externally applied pressure.
These new measurements will help scientists reach a consensus on the nature of the magnetic state of Yb2Ti2O7 at very low temperatures. Defects at the atomic scale could have a significant impact on the magnetic properties of Yb2Ti2O7 at the macroscopic level, explaining previous findings that seemed controversial. The findings of the neutron diffraction experiment conducted using ILL’s D20 instrument, together with the µSR measurements, provide the first step to better understand how quantum states can arise in real materials. These findings also justify the need for a close collaboration between solid states physicists and chemists to conceive and understand the next generation of strongly correlated electron materials.
Re.: Nature communications. DOI :10.1038/ncomms14810
Dr Clemens Ritter, ILL
Dr Edwin Kermarrec, Laboratoire de Physique des Solides, Université de Paris Sud.