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Yb2Ti2O7 : a quantum material under pressure

New quantum states of matter often escape experimental observations. Our recent study combined neutron diffraction and muon spin relaxation measurements (μSR) 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 for temperatures below 2 K (-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 revolutionize our ways of thinking of the collective behavior of electrons in matter, and further help to consider the future physical properties of quantum materials. Despite the fact that Yb2Ti2O7 has been the subject of intense research since 2000, physicists could not reach 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 ? Experimental results are controversial.

With our colleagues from McMaster University (Hamilton, ON, Canada), we show that there exists a mysterious non-magnetic state at very low temperatures, in agreement with the existence of a quantum spin ice, which turns out to be very sensitive to external perturbations such as the application of a high pressure. Using spectroscopic techniques (neutron diffraction and µSR), we show that pressures between 1.2 and 25 kbar could induce a phase transition from a non-magnetic phase to a ferromagnetic phase, characterized by the appearance of magnetic Bragg peaks under pressure. Besides, previous neutron diffraction measurements evidenced the presence of Yb atoms in excess, localized on the titanium crystallographic site, in real materials with chemical formula Yb2(YbxTi1-x)2O7 (0 ≤ x ≤ 0.02). 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 results resolve the experimental controversy. Defects at the atomic scale could have a significant impact on the magnetic properties of Yb2Ti2O7 at the macroscopic level, explaining the previous experimental findings that seemed controversial so far. Our measurements are a first step to better understand how quantum states can arise in real materials, and justify the need for a close collaboration between solid state physicists and chemists to conceive and understand the next generation of strongly correlated electrons materials.
Yb2Ti2O7 : un matériau quantique sous pression(a) Neutron diffraction measurements performed under hydrostatic pressure (P = 11 kbar) on Yb2Ti2O7 at the D20 beamline of ILL. The evolution of diffraction patterns with temperature show the appearance of magnetic Bragg peaks for T ≤ 400 mK, characteristic feature of a particular ferromagnetic phase with a magnetic moment 0.33(5) µB. (b) Pressure-temperature (P–T) phase diagram established by µSR measurements on Yb2Ti2O7. Empty black circles define the phase transition line between a collective paramagnetic state (PM, orange) and a ferromagnetic-like state (SFM, blue). The green region at low temperatures highlights the existence of an exotic non-magnetic phase (QSL), evidenced for P = 0.

Reference :

Ground state selection under pressure in the quantum pyrochlore magnet Yb2Ti2O7
E. Kermarrec, J. Gaudet, K. Fritsch, R. Khasanov, Z. Guguchia, C. Ritter, K. A. Ross, H. A. Dabkowska and B. D. Gaulin
Nature Communications 8, 14810 (2017), doi:10.1038/NCOMMS14810.

Contact :

Edwin Kermarrec