実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report
課題番号 Project No.

提出日 Date of Report
装置責任者 Name of responsible person

2016A0035

Yasuhiro Miyake

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Magnetism and superconductivity in hyperexpanded fullerides

D1 Instrument
実施日 Date of Experimentclose to the Mott transition

2017//01/26 – 2017/01/28

実験責任者名 Name of principal investigator
Kosmas Prassides
所属 Affiliation
WPI-Advanced Institute for Materials Research (WPI-AIMR),
Tohoku University

試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.

Rubidium caesium fulleride, Rb0.75Cs2.75C60

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

The electronic phase diagram of cubic alkali fullerides, AxA'3-xC60 (A, A'= alkali metals) exhibits striking
similarities with those of unconventional superconductors such as the cuprates. Superconductivity emerges froman antiferromagnetic strongly correlated Mott insulating state through bandwidth control via outer wavefunction
overlap of the constituent molecules and is accompanied by a dome-shaped dependence of the criticaltemperature, Tc. This is experimentally achieved by tuning a parameter such as physical (through application ofexternal pressure) or chemical (through substitution of ions with varying size) pressure. Moreover, we have alsoestablished that such bandwidth control transforms the Mott insulating state first into an unconventionalcorrelated Jahn-Teller (JT) metal (where localized electrons coexist with metallicity and on-molecule Jahn-teller
distortions persist), and then into a Fermi liquid with a less prominent molecular electronic signature. This
normal state crossover is mirrored in the evolution of the superconducting state, with the highest Tc found at theboundary between unconventional correlated and conventional weak-coupling BCS superconductivity where theinterplay between extended and molecular aspects of the electronic structure is optimized to create thesuperconductivity dome [SCIENCE ADVANCES 1, e1500059 (2015)].

12. 実験方法及び結果（つづき） Experimental method and results (continued)
In this experimental period, we attempted to use the +SR technique in its zero-field (ZF) variant to study theelectronic ground states as we move across the complex bandwidth-controlled phase diagram. We attempted toprobe more deeply the Mott-Jahn-Teller insulator to Jahn-Teller metal crossover, which is controlled by varyingbandwidth (or unit cell volume, V). Both our magnetization and 13C NMR measurements had shown that theJahn-Teller metal exhibits a strongly enhanced spin susceptibility relative to that of a conventional Fermi liquid,characteristic of the importance of strong electron correlations. In the insulating phase, 1/13T1 is governed by
antiferromagnetic spin fluctuations – such fluctuations remain important in the Jahn-Teller metal regime but
gradually diminish with decreasing V as (U/W) decreases and conventional Fermi liquid behavior appears forunderexpanded fullerides.
ZF-+SR data of a representative overexpanded fulleride with composition Rb0.75Cs2.25C60 and asuperconducting Tc = 31.4 K were collected in the temperature range 5 to 240 K. Rb0.75Cs2.25C60 is an insulator athigh temperature but there is a crossover to the Jahn-Teller metal state at ~130 K as established bycomplementary magnetization, NMR and IR measurements. In Fig. 1, we show the ZF time-dependent +SR
spectra of Rb0.75Cs2.25C60 at selected temperatures. At high temperatures, the spectra are characteristic of thepresence of weak static nuclear dipole moments, which result in a small depolarization rate,  ~ 0.04 s1,together with a very slow relaxation, arising from fluctuating electronic moments. Cooling down below 190 K
leads to the emergence of a second relaxing component whose relaxation rate,  increases rapidly and saturates
near the onset of the insulator-to-metal crossover at ~130 K. A second increase in the relaxation rate of thedominant component now also occurs at the metal-to-superconductor transition on further cooling. Fig. 2 shows
the temperature dependence of the relaxation rate as we cross the boundaries between the high-temperatureinsulating state towards the anomalous Jahn-Teller metallic state and finally, the superconducting state. The
significance of these preliminary results are under further consideration but they already provide good evidence
for the emergence of frozen electronic moments as the phase diagram boundary from the Mott-Jahn-Teller
insulator to the Jahn-Teller metal is crossed on cooling.
1.00

P (t)

0.95

0.90

240 K
144 K
51 K
5.3 K

0.85

0.80
02

4

68

Time ( s)Fig. 1: Temperature evolution of the ZF +-spin
polarization, P(t) for Rb0.75Cs2.25C60 at 5.3, 51,
144, and 240 K. The solid lines represent the fits tothe model function employed.

Fig. 2: Temperature dependence of the ZF relaxation
rate,  in Rb0.75Cs2.25C60 showing the emergence ofstatic disordered magnetic component below 190 K
whichcoincides
withtheMott-Jahn-Teller
insulator-to-Jahn-Teller metal crossover established bycomplementary magnetization and 13C NMR
measurements.
2