MuSR studies of Mott transitions in RENiO3, V2O3 and Ba(Co,Ni)S2:
Progress report from the 2016 Reimei project:
Yasutomo J. Uemura1, Benjamin A. Frandsen1, Zurab Guguchia1, Sky Cheung1, Zizhou Gong1,Lian Liu1, Shinichi Shamoto2, Wataru Higemoto2, Takashi Ito2, Elvezio Morenzoni3,
Thomas Prokscha3, Hubertus Luetkens3, Rustem Khasanov3, Bijuan Chen4, Wenmin Li4,Changqing Jin4, Timothy J.S. Munsie5, Alannah M. Hallas5, Murray N. Wilson5, Yipeng Cai5,Graeme M. Luke5, Cui Ding6, Shengli Guo6, Fanlong Ning6, Simon J.L. Billinge7,8,Masatoshi Imada9, Shoya Sakamoto10, Atsushi Fujimori10, Taito Murakami11,Hiroshi Kageyama11, Jose Antonio Alonso12, Gabriel Kotliar7,13, Roberto de Renzi14,Andrea Gauzzi15, Yannick Klein15, David Santos-Cottin16, Ilya Valmianski,17Alexander S. McLeod17, Dimitri N. Basov1,17, Ivan K. Schuller17
1 Department of Physics, Columbia University, New York, New York 10027, USA.
2 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195,

Japan.
3 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen,Switzerland.
4 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
5 Department of Physics and Astronomy, McMaster University, Hamilton, Ontario,

Canada L8S 4M1.
6 Department of Physics, Zhejiang University, Hangzhou 310027, China.
7 Condensed Matter Physics and Materials Science Department, Brookhaven National

Laboratory, Upton, New York 11973, USA.
8 Department of Applied Physics and Applied Mathematics, Columbia University, New York,New York 10027, USA.
9 Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan.
10 Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
11 Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto

University, Nishikyo, Kyoto 615-8510, Japan.
12 Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Madrid E-28049, Spain.
13 Department of Physics & Astronomy, Rutgers University, Piscataway, New Jersey

08854-8019, USA.
14 Dipartimento di Fisica e Scienze della Terra "Macedonio Melloni", 43124 Parma, Italy
15 IMPMC, Université Pierre et Marie Curie - Sorbonne Universités 4, place Jussieu, 75005

Paris, France
16 LPEM, École Supérieure de Physique et de Chimie Industrielles, 10, rue Vauquelin, 75005

Paris, France
17 Department of Physics, University of California San Diego, La Jolla, California 92093, USA.

Abstract
With the financial support from the JAEA Reimei Project, Muon Spin Relaxation (MuSR)
measurements of Mott transition systems RENiO3 (RE = Rare Earth), V2O3 and Ba(Co,Ni)S2
have been performed to study evolution from the antiferromagnetic insulator to paramagnetic
metal state via quantum tunings with hydrostatic pressure in V2O3 and BaCoS2, and withchemical substitutions in RENiO3 and Ba(Co,Ni)S2. In all these measurements, first-order
quantum transition, associated with phase separation, has been found at the quantum phaseboundary. In the quantum tuning of RENiO3, V2O3 and BaCoS2, static magnetic order isdestroyed via the reduction of the ordered volume fraction while the ordered moment size in themagnetically ordered volume shows little change. The results on bulk specimens of RENiO3and V2O3 have been published in Nature Communications in 2016, while those on bulk
Ba(Co,Ni)S2 and a thin film of V2O3 are under preparation for submittal.
1. Research Objectives.
Mott transition from paramagnetic metal to antiferromagnetic (AF) insulator is a remarkablephenomenon caused by strong correlations among electrons. It has been known for some years
that in thermal phase transitions of Mott transition systems, such as V2O3 and RENiO3, themagnetic transition occurs as first-order phase transition, often associated with thermal
hysteresis and accompanied by metal-insulator (M-I) charge transition and structural phasetransition [1-3]. Evolution of magnetic quantum phase transition, however, has not yet beenstudied in details. The present study aims to elucidate details of magnetic order in thesesystems with quantum tuning via hydrostatic pressure and chemical substitutions. For thispurpose, Muon Spin Relaxation (MuSR) is an ideal method, because MuSR can determine thevolume fraction of the magnetically ordered regions and the local ordered moment size
separately. In V2O3 thin film specimens, MuSR results can be compared with those fromnano-scale real-space mapping of optical conductivity (nano-optics) and x-ray diffraction. This
comparison is expected to shed new light in understanding interplays of charge (M-I), magnetic(AF-Para) and structural phase transitions.
2. Research Contents
The present project has been performed in the following subjects and steps:(A) Bulk RENiO3 and V2O3: MuSR measurements at TRIUMF in RENiO3 at ambientpressure and at PSI in V2O3 under applied pressure in 2014-15; data analysis and preparationof publication in 2015 -16; publication accepted and printed in 2016 [4]. (B) Thin film V2O3:
Low energy MuSR measurements at PSI on a 100 nm film in 2015-16; data analyses andpreparation of publication: 2016 [2]; Low energy MuSR at PSI on a 300 nm film being plannedin 2017 summer. Optical conductivity measurements by Basov and collaborators on a 300 nm
film: performed in 2014-15; published in 2016 [5]. Nano optics studies on a 100 nm film:
performed in 2016.

(C) Bulk BaCoS2: MuSR measurements under applied pressure in 2016

at PSI; Ba(Co,Ni)S2 MuSR measurements at TRIUMF at ambient pressure in 2016: dataanalyses and preparation of publication in 2017. Neutron scattering beamtime proposal atORNL accepted in May 2017; neutron experiments planned in the fall of 2017.

3. Research Results
Figure 1(a) and 1(b) show the phase diagrams of V2O3 and BaCoS2 with pressure tuning. The
metal insulator transition in V2O3 is accompanied by a structural phase transition, while there
is no structural transition in BaCoS2. MuSR results in Figs. 1(c) and 1(d) demonstrate that inboth of these systems the volume fraction of the magnetically ordered region decreases withincreasing pressure and becomes clearly smaller than even a half of the total volume near thequantum critical region. We also observed that the frequency of Zero-field MuSR signal
exhibits almost no dependence on pressure at low temperatures near the disappearance of thestatic magnetic order, and there was no signature of critical slowing down of spin fluctuationsin the paramagnetic state. There features clearly demonstrate that the quantum phasetransition is first order in nature. Similar results were obtained in RENiO3 with composition
tuning [4], while the local ordered moment size shows significant composition dependence inBa(Co,Ni)S2 in ambient pressure, due possibly to the effect of disorder and/or charge doping.
Figure 1(f) shows the comparison of the temperature evolution of the volume fraction of themagnetically ordered region in V2O3 100 nm film with those for insulating volume fraction fromnano-optics measurements and for the monoclinic structure derived from x-ray Bragg-peakintensity. The comparison indicates that these three orders follow nearly the same
temperature evolution, but there may be subtle differences. Since the 100 nm film exhibits
more disordered pattern in nano-optics as compared to 300 nm film (Fig. 1(e)), we plan toperform low-energy MuSR measurements of a 300 nm film at PSI in the summer of 2017.

Fig. 1: (a, top left) Phase diagram of V2O3. PS shows the phase separation region [4]; (b, topcenter) Phase diagram of BaCoS2; (c, top right) Magnetic ordered volume fraction of BaCoS2(unpublished); (d, bottom left) AF ordered volume fraction in V2O3 [4]; (e, bottom middle)
Nano-optics imaging for metallic and insulating regions in V2O3; (f, bottom right)
Magnetic, structural and insulating volume in 100 nm V2O3 film (unpublished).

4. ConclusionsThese results clearly indicate that quantum phase evolution in can be regarded as first ordertransition, associated with phase separation, in prototypical Mott transition systems V2O3,RENiO3 and Ba(Co,Ni)S2, in both composition tuning and pressure tuning, regardless ofwhether the transition is associated with structural transition or not. We are presently
preparing a manuscript to report MuSR results on Ba(Co,Ni)S2, and planning to performinelastic neutron scattering studies on that system at ORNL to seek for analogue of themagnetic resonance mode found in unconventional superconductors.
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