Research & Annual Reports

The Theory of Quantum Matter (TQM) Unit carries out research into a wide range of problems in condensed matter theory, with a strong emphasis on the novel phases and excitations found in quantum matter.

This work is described in the Annual Reports listed in the menu on the left side of this page.   These provide details of all of the research carried out by the TQM Unit, publications and presentations by Unit members, outreach activity, and seminars given by visitors to TQM in OIST.  Reports are organised by the Japanese financial year, with FY2018 running from April 1st 2018 until March 31st 2019.

More recent rsearch projects, completed after April 1st 2019, are described below:

1. Putative spin-nematic phase in BaCdVO(PO4)2

M. Skoulatos, F. Rucker, G.J. Nilsen, A. Bertin, E. Pomjakushina, J. Ollivier, A. Schneidewind, R. Georgii, O. Zaharko, L. Keller, Ch. Rüegg, C. Pfleiderer, B. Schmidt, N. Shannon, A. Kriele, A. Senyshyn, and A. Smerald

Like Volborthite, described below, BaCdVO(PO4)2 is a quasi-two dimensional magnet, with competing ferromagnetic and antiferromagnetic interactions.    In such systems, it has been proposed that a new state of matter, known as a "spin nematic", can occur in high magnetic fields.   BaCdVO(PO4)2 has previously been discussed as a candidate for spin-nematic order [A. Smerald, H. T. Ueda, and N. Shannon, Phys. Rev. B 91, 174402 (2015)], but until now, relatively little has been known about the experimental phase diagram of this material.

In this paper, we identify for the first time the nature of the magnetic grounds state of BaCdVO(PO4)2 in the absence of magnetic field, and present evidence for the existence of a novel magnetic phase in  applied magnetic field, consistent with predictions of spin-nematic order.   Evidence in support of this conclusion comes from elastic neutron scattering and dynamical susceptibility measurements on powder samples of BaCdVO(PO4)2.   These results, in combination with recent thermodynamics measurements on single crystals [K. Y. Povarov, V. K. Bhartiya, Z. Yan, and A. Zheludev, Phys. Rev. B 99, 024413 (2019)],  establish BaCdVO(PO4)2, like Volborthite, as a strong candidate for spin-nematic order.

Magnetic phase diagram of BaCdVO(PO4)2, as determined by measurements of magnetic susceptibility, showing how conventional magnetic order vanishes for fields approaching 4T, providing evidence for a new non-magnetic phase where spin-nematic order has been predicted.

 

This work has been accepted for publication in Phys. Rev. B, and is described in the preprint arXiv:1903.12462.

2. Possible observation of quantum spin-nematic phase in the frustrated magnet volborthite

Every child learns that water freezes in the cold of winter, and evaporates quickly in the heat of summer.  Scientifically, these transformations between solid, liquid and gas are called phase transitions, and the fact that the same atoms can exhibit different phases lies at the heart of our understanding of the material world.  A fourth phase of matter was discovered, by chance, late in 19th century, when Freiedrich Reinitzer tried to make crystals from molecules of  cholesterol.   Reinitzer noticed that as he cooled his solution of cholesterol towards its freezing point, it underwent a marked change in its physical properties, while remaining a liquid.    This was the first observation of a "liquid crystal", a phase of matter in which rod- or disk-shaped molecules align like the atoms in a solid, while continuing to flow like a liquid.   

Liquid crystals have since become an important part of every-day life, and are integral to the displays in most electronic devices.   Meanwhile,  the search for new phase of matter, including quantum analogies of liquid crystals, has become an increasingly important field of research.   One long-sought example is the "quantum spin nematic", in which the quantum state of magnetic atoms mimic the rod-like molecules of a nematic liquid crystal.   The possibility of this new phase of matter was first pointed half a century ago  [M. Blume and Y. Y. Hsieh  J Appl Phys 40, 1249 (1969); A. F. Andreev, I. A. Grishchuk, J Exp Theor Phys 97, 467 (1984)].  And it is now well understood how competing, or "frustrated", interactions between the  ions in a magnet can give rise to a spin-nematic phase [A. V. Chubukov, Phys. Rev. B 44, 4693 (1991); N. Shannon et al., Phys. Rev. Lett., 96, 027213 (2006)].  None the less, despite their considerable interest, quantum spin nematics have proved very difficult to observe in experiment.

In this work, we report the possible observation of a quantum spin nematic in the naturally occuring mineral, volborthite.     Volborthite contains copper atoms which are magnetic, with interactions of type needed to promote a spin-nematic state in high magnetic field [O. Janson et al., Phys. Rev. Lett. 117, 037206 (2016)].  We have probed the magnetic behaviour of the copper atoms by carrying out high-precision measurements of the way in which crystals of volborthite absorb heat, in fields of up to 33 Tesla.    The results we find are consistent with the existence of a quantum spin-nematic phase, at temperatures below 1.5 Kelvin, for magnetic fields ranging from 25.5 to 27.5 Tesla.

Magnetic phases found in volborthite in magnetic field of up to 33 Tesla, as determined by measurements of the magnetocaloric effect and specific heat. The phase marked "N2", occurring for temperatures below 1.5 Kelvin, bears the hallmarks of a spin nematic phase.

 

This work was published in the article:  "Possible observation of quantum spin-nematic phase in a frustrated magnet", Yoshimitsu Kohama, Hajime Ishikawa, Akira Matsuo, Koichi Kindo, Nic Shannon, and Zenji Hiroi Proc. Natl. Acad. Sci. 116, 10686 (2019).