High-pressure electron spin resonance (ESR) measurements were performed on tetrakis(dimethylamino) ethylene (TDAE)-C-60 single crystals and stability of the polymeric phase was established in the P-T parameter space. At 7 kbar the system undergoes a ferromagnetic to paramagnetic phase transition due to the pressure-induced polymerization. The polymeric phase remains stable after the pressure release. The depolymerization of the pressure-induced phase was observed at a temperature of 520 K, revealing an unexpectedly high thermal stability of the polymer. Below room temperature, the polymeric phase behaves as a simple Curie-type insulator with one unpaired electron spin per chemical formula. The TDAE(+) donor-related unpaired electron spins, formerly ESR silent, become active above a temperature of 320 K, which demonstrates that the magnetic properties are profoundly defined by miniscule reorientation of TDAE molecules.
en-copyright= kn-copyright= en-aut-name=GarajSlaven en-aut-sei=Garaj en-aut-mei=Slaven kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=Forr?L?szl? en-aut-sei=Forr? en-aut-mei=L?szl? kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=SienkiewiczAndrzej en-aut-sei=Sienkiewicz en-aut-mei=Andrzej kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=FujiwaraMotoyasu en-aut-sei=Fujiwara en-aut-mei=Motoyasu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=OshimaKokichi en-aut-sei=Oshima en-aut-mei=Kokichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= affil-num=1 en-affil= kn-affil=Institute of Physics of Complex Matter, ?cole Polytechnique F?d?rale de Lausanne affil-num=2 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=3 en-affil= kn-affil=Institute of Physics of Complex Matter, ?cole Polytechnique F?d?rale de Lausanne affil-num=4 en-affil= kn-affil=Institute of Physics, Polish Academy of Sciences affil-num=5 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=6 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University END start-ver=1.4 cd-journal=joma no-vol=99 cd-vols= no-issue=17 article-no= start-page= end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2007 dt-pub=200710 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Antiferromagnetic ordering driven by the molecular orbital order of C-60 in alpha '-tetra-kis-(dimethylamino)-ethylene-C-60 en-subtitle= kn-subtitle= en-abstract= kn-abstract=We have studied the ground state of a fullerene-based magnet, the alpha'-phase tetra-kis-(dimethylamino)ethylene-C-60 (alpha'-TDAE-C-60), by electron spin resonance and magnetic torque measurements. Below T-N = 7 K, nonparamagnetic field dependent resonances with a finite excitation gap ( 1.7 GHz) are observed along the a axis. Strong enhancement in their intensity as temperature is decreased is inconsistent with excitation from a singlet state, which had been proposed for the alpha'-phase ground state. Below T-N, nonquadratic field dependence of the magnetic torque signal is also observed in contrast to quadratic field dependence in the paramagnetic phase. The angle-dependent torque signals below T-N indicate the existence of an anisotropy of the bulk magnetization. From both experiments, we propose an antiferro-magnetic ground state driven by the cooperative orientational ordering of C-60 in the alpha'-TDAE-C-60.
en-copyright= kn-copyright= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=KajiyoshiKoichi en-aut-sei=Kajiyoshi en-aut-mei=Koichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=FujiwaraMotoyasu en-aut-sei=Fujiwara en-aut-mei=Motoyasu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=OshimaKokichi en-aut-sei=Oshima en-aut-mei=Kokichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= affil-num=1 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=2 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=3 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=4 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University en-keyword=TDAE-C-60 kn-keyword=TDAE-C-60 en-keyword=Ferromagnetism kn-keyword=Ferromagnetism en-keyword=(NH3)K3C60 kn-keyword=(NH3)K3C60 en-keyword=ESR kn-keyword=ESR END start-ver=1.4 cd-journal=joma no-vol=71 cd-vols= no-issue=17 article-no= start-page= end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2005 dt-pub=20055 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Structural differences in two polymorphs of tetra-kis-(dimethylamino)-ethylene-C-60: An x-ray diffraction study en-subtitle= kn-subtitle= en-abstract= kn-abstract=A type of low-temperature structure for ferromagnetic alpha-tetra-kis (dimethylamino) -ethylene (TDAE)-C-60 is proposed on the basis of low-temperature x-ray analysis. We observed that intense superlattice reflections with odd indices successively appeared below T-s = 170 K. The space group symmetry of the low-temperature phase is determined to be P2(1)/n. Two inequivalent C-60 sites exist in the low-temperature phase, which are indispensable to the orbital ordering model Of C-60. The contact configuration for the neighboring C(60)s along the stacking c direction is uniquely determined. The double bond between the hexagons faces the neighboring pentagon. We found that the surrounding TDAE molecules shift along the c axis (similar to 0.07 angstrom) and that these shifts correlate perfectly to the alignment of C-60. This result indicates that the steric effect betwee n C-60 and TDAE molecules plays an important role in the orientational ordering Of C-60, On the other hand, in the alpha' phase, no structural phase transition was observed below 30 K. This indicates that all the C(60)s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE-C-60.
en-copyright= kn-copyright= en-aut-name=FujiwaraMotoyasu en-aut-sei=Fujiwara en-aut-mei=Motoyasu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=OshimaKokichi en-aut-sei=Oshima en-aut-mei=Kokichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= affil-num=1 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=2 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University affil-num=3 en-affil= kn-affil=Graduate School of Natural Science and Technology, Okayama University en-keyword=Physics kn-keyword=Physics en-keyword=Condensed Matter kn-keyword=Condensed Matter END start-ver=1.4 cd-journal=joma no-vol=61 cd-vols= no-issue=2 article-no= start-page= end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2000 dt-pub=20001 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Annealing effects on the magnetic and structural properties of single-crystal TDAE-C-60 en-subtitle= kn-subtitle= en-abstract= kn-abstract=Annealing effects on the magnetic and structural properties of single-crystal TDAE-C-60 are investigated. When a crystal is well-annealed at 350 K, ferromagnetic ordering takes place below 16 K, though no magnetic phase transition is shown in as-grown crystal. The saturated magnetization was obtained to be 0.9+/-0.1 mu(B) per C-60. It was first found that the well-annealed crystal shows a structural phase transition around 180 K, probably associated with the orientational ordering of C-60 molecules. On the other hand, the as-grown crystal undergoes no structural phase transition at least down to 30 K while the motion of C-60 molecules is restricted below around 150 K. The possible relation between the low-temperature structure and the magnetic ordering is discussed.
en-copyright= kn-copyright= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=NogamiYoshio en-aut-sei=Nogami en-aut-mei=Yoshio kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=OshimaKokichi en-aut-sei=Oshima en-aut-mei=Kokichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= affil-num=1 en-affil= kn-affil=Department of Physics, Faculty of Science, Okayama University affil-num=2 en-affil= kn-affil=Department of Physics, Faculty of Science, Okayama University affil-num=3 en-affil= kn-affil=Department of Physics, Faculty of Science, Okayama University END start-ver=1.4 cd-journal=joma no-vol=71 cd-vols= no-issue=22 article-no= start-page= end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2005 dt-pub=20056 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Metallic phase in the metal-intercalated higher fullerene Rb8.8(7)C84 en-subtitle= kn-subtitle= en-abstract= kn-abstract=A new material of higher fullerene, RbxC84, was synthesized by intercalating Rb metal into C-84 crystals. The RbxC(84) crystals showed a simple cubic (sc) structure with lattice constant, a, of 16.82 (2) angstrom at 6.5 K, and 16.87 (2) angstrom at 295 K. The Rietveld refinements were achieved with the space group, Pa (3) over bar, based on a model that the C-2 axis of D2d-C84 aligned along [111]. The sample composition was determined to be Rb-8.8(7) C-84. The ESR spectrum at 303 K was composed of a broad peak with peak-to-peak linewidth Delta H-pp of 220 G, and a narrow peak with Delta H-pp of 24 G. Temperature dependence of the broad peak clearly showed a metallic behavior. The metallic behavior was discussed based on a theoretical calculation. This finding of new metallic phase in a higher fullerene is the first step for a development of new types of fullerene materials with novel physical properties such as superconductivity.
en-copyright= kn-copyright= en-aut-name=RikiishiYoshie en-aut-sei=Rikiishi en-aut-mei=Yoshie kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=KashinoYoko en-aut-sei=Kashino en-aut-mei=Yoko kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=KusaiHaruka en-aut-sei=Kusai en-aut-mei=Haruka kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=TakabayashiYasuhiro en-aut-sei=Takabayashi en-aut-mei=Yasuhiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=KuwaharaEiji en-aut-sei=Kuwahara en-aut-mei=Eiji kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=KubozonoYoshihiro en-aut-sei=Kubozono en-aut-mei=Yoshihiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=7 ORCID= en-aut-name=TakenobuTaishi en-aut-sei=Takenobu en-aut-mei=Taishi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=8 ORCID= en-aut-name=IwasaYoshihiro en-aut-sei=Iwasa en-aut-mei=Yoshihiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=9 ORCID= en-aut-name=MizorogiNaomi en-aut-sei=Mizorogi en-aut-mei=Naomi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=10 ORCID= en-aut-name=NagaseShigeru en-aut-sei=Nagase en-aut-mei=Shigeru kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=11 ORCID= en-aut-name=OkadaSusumu en-aut-sei=Okada en-aut-mei=Susumu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=12 ORCID= affil-num=1 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=2 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=3 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=4 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=5 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=6 en-affil= kn-affil=Department of Chemistry, Okayama University affil-num=7 en-affil= kn-affil=Department of Physics, Okayama University affil-num=8 en-affil= kn-affil=CREST, Japan Science and Technology Agency affil-num=9 en-affil= kn-affil=CREST, Japan Science and Technology Agency affil-num=10 en-affil= kn-affil=Institute for Molecular Science affil-num=11 en-affil= kn-affil=Institute for Molecular Science affil-num=12 en-affil= kn-affil=Institute of Physics and Center for Computational Science, University of Tsukuba END start-ver=1.4 cd-journal=joma no-vol=11 cd-vols= no-issue= article-no= start-page=235 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2020 dt-pub=20200113 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3 en-subtitle= kn-subtitle= en-abstract= kn-abstract=Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing CuxBi2Se3 single crystals with unprecedented high doping levels. For samples with x = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by the Knight shift, the in-plane upper critical field Hc2 shows a two-fold symmetry. However, the angle at which the Hc2 becomes minimal is different by 90 among them, which indicates that the d-vector direction is different for each crystal likely due to a different local environment. The carrier density for x = 0.46 and 0.54 increases substantially compared to x ? 0.37. Surprisingly, the in-plane Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases. en-copyright= kn-copyright= en-aut-name=KawaiT. en-aut-sei=Kawai en-aut-mei=T. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=WangC. G. en-aut-sei=Wang en-aut-mei=C. G. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=KandoriY. en-aut-sei=Kandori en-aut-mei=Y. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=HonokiY. en-aut-sei=Honoki en-aut-mei=Y. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=MatanoK. en-aut-sei=Matano en-aut-mei=K. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=KambeT. en-aut-sei=Kambe en-aut-mei=T. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= en-aut-name=ZhengGuo-qing en-aut-sei=Zheng en-aut-mei=Guo-qing kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=7 ORCID= affil-num=1 en-affil=Department of Physics, Okayama University kn-affil= affil-num=2 en-affil=Institute of Physics, Chinese Academy of Sciences, and Beijing National Laboratory for Condensed Matter Physics kn-affil= affil-num=3 en-affil=Department of Physics, Okayama University kn-affil= affil-num=4 en-affil=Department of Physics, Okayama University kn-affil= affil-num=5 en-affil=Department of Physics, Okayama University kn-affil= affil-num=6 en-affil=Department of Physics, Okayama University kn-affil= affil-num=7 en-affil=Department of Physics, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=10 cd-vols= no-issue=45 article-no= start-page=26686 end-page=26692 dt-received= dt-revised= dt-accepted= dt-pub-year=2020 dt-pub=20200716 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=A new protocol for the preparation of superconducting KBi2 en-subtitle= kn-subtitle= en-abstract= kn-abstract=A superconducting KBi2 sample was successfully prepared using a liquid ammonia (NH3) technique. The temperature dependence of the magnetic susceptibility (M/H) showed a superconducting transition temperature (Tc) as high as 3.6 K. In addition, the shielding fraction at 2.0 K was evaluated to be 87%, i.e., a bulk superconductor was realized using the above method. The Tc value was the same as that reported for the KBi2 sample prepared using a high-temperature annealing method. An X-ray diffraction pattern measured based on the synchrotron X-ray radiation was analyzed using the Rietveld method, with a lattice constant, a, of 9.5010(1) ? under the space group of Fd[3 with combining macron]m (face-centered cubic, no. 227). The lattice constant and space group found for the KBi2 sample using a liquid NH3 technique were the same as those reported for KBi2 through a high-temperature annealing method. Thus, the superconducting behavior and crystal structure of the KBi2 sample obtained in this study are almost the same as those for the KBi2 sample reported previously. Strictly speaking, the magnetic behavior of the superconductivity was different from that of a KBi2 sample reported previously, i.e., the KBi2 sample prepared using a liquid NH3 technique was a type-II like superconductor, contrary to that prepared using a high-temperature annealing method, the reason for which is fully discussed. These results indicate that the liquid NH3 technique is effective and simple for the preparation of a superconducting KBi2. In addition, the topological nature of the superconductivity for KBi2 was not confirmed. en-copyright= kn-copyright= en-aut-name=LiHuan en-aut-sei=Li en-aut-mei=Huan kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=WangYanan en-aut-sei=Wang en-aut-mei=Yanan kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=NishiyamaSaki en-aut-sei=Nishiyama en-aut-mei=Saki kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=YangXiaofan en-aut-sei=Yang en-aut-mei=Xiaofan kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=TaguchiTomoya en-aut-sei=Taguchi en-aut-mei=Tomoya kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=MiuraAkari en-aut-sei=Miura en-aut-mei=Akari kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= en-aut-name=SuzukiAi en-aut-sei=Suzuki en-aut-mei=Ai kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=7 ORCID= en-aut-name=ZhiLei en-aut-sei=Zhi en-aut-mei=Lei kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=8 ORCID= en-aut-name=GotoHidenori en-aut-sei=Goto en-aut-mei=Hidenori kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=9 ORCID= en-aut-name=EguchiRitsuko en-aut-sei=Eguchi en-aut-mei=Ritsuko kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=10 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=11 ORCID= en-aut-name=LiaoYen-Fa en-aut-sei=Liao en-aut-mei=Yen-Fa kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=12 ORCID= en-aut-name=IshiiHirofumi en-aut-sei=Ishii en-aut-mei=Hirofumi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=13 ORCID= en-aut-name=KubozonoYoshihiro en-aut-sei=Kubozono en-aut-mei=Yoshihiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=14 ORCID= affil-num=1 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=2 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=3 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=4 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=5 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=6 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=7 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=8 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=9 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=10 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= affil-num=11 en-affil=Department of Physics, Okayama University kn-affil= affil-num=12 en-affil=National Synchrotron Radiation Research Center kn-affil= affil-num=13 en-affil=National Synchrotron Radiation Research Center kn-affil= affil-num=14 en-affil=Research Institute for Interdisciplinary Science, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=104 cd-vols= no-issue=14 article-no= start-page=L140402 end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2021 dt-pub=2021107 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Spin-gap formation due to spin-Peierls instability in -orbital-ordered NaO2 en-subtitle= kn-subtitle= en-abstract= kn-abstract=We have investigated the low-temperature magnetism of sodium superoxide (NaO2), in which spin, orbital, and lattice degrees of freedom are closely entangled. The magnetic susceptibility shows anomalies at T1 = 220 K and T2 = 190 K, which correspond well to the structural phase transition temperatures, and a sudden decrease below T3 = 34 K. At 4.2 K, the magnetization shows a clear stepwise anomaly around 30 T with a large hysteresis. In addition, the muon spin relaxation experiments indicate no magnetic phase transition down to T = 0.3 K. The inelastic neutron scattering spectrum exhibits magnetic excitation with a finite energy gap. These results confirm that the ground state of NaO2 is a spin-singlet state. To understand this ground state in NaO2, we performed Raman scattering experiments. All the Raman-active libration modes expected for the marcasite phase below T2 are observed. Furthermore, we find that several new peaks appear below T3. This directly evidences the low crystal symmetry, namely, the presence of the phase transition at T3.We conclude that the singlet ground state of NaO2 is due to the spin-Peierls instability. en-copyright= kn-copyright= en-aut-name=MiyajimaMizuki en-aut-sei=Miyajima en-aut-mei=Mizuki kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=AstutiFahmi en-aut-sei=Astuti en-aut-mei=Fahmi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=FukudaTakahito en-aut-sei=Fukuda en-aut-mei=Takahito kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=KodaniMasashi en-aut-sei=Kodani en-aut-mei=Masashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=IidaShinsuke en-aut-sei=Iida en-aut-mei=Shinsuke kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=AsaiShinichiro en-aut-sei=Asai en-aut-mei=Shinichiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= en-aut-name=MatsuoAkira en-aut-sei=Matsuo en-aut-mei=Akira kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=7 ORCID= en-aut-name=MasudaTakatsugu en-aut-sei=Masuda en-aut-mei=Takatsugu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=8 ORCID= en-aut-name=KindoKoichi en-aut-sei=Kindo en-aut-mei=Koichi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=9 ORCID= en-aut-name=HasegawaTakumi en-aut-sei=Hasegawa en-aut-mei=Takumi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=10 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=11 ORCID= en-aut-name=NakanoTakehito en-aut-sei=Nakano en-aut-mei=Takehito kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=12 ORCID= en-aut-name=WatanabeIsao en-aut-sei=Watanabe en-aut-mei=Isao kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=13 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=14 ORCID= affil-num=1 en-affil=Department of Physics, Okayama University kn-affil= affil-num=2 en-affil=Advanced Meson Science Laboratory, RIKEN Nishina Center kn-affil= affil-num=3 en-affil=Department of Physics, Okayama University kn-affil= affil-num=4 en-affil=Department of Physics, Okayama University kn-affil= affil-num=5 en-affil=Institute for Solid State Physics, University of Tokyo kn-affil= affil-num=6 en-affil=Institute for Solid State Physics, University of Tokyo kn-affil= affil-num=7 en-affil=Institute for Solid State Physics, University of Tokyo kn-affil= affil-num=8 en-affil=Institute for Solid State Physics, University of Tokyo kn-affil= affil-num=9 en-affil=Institute for Solid State Physics, University of Tokyo kn-affil= affil-num=10 en-affil=Graduate School of Advanced Science and Engineering, Hiroshima University kn-affil= affil-num=11 en-affil=Department of Physics, Okayama University kn-affil= affil-num=12 en-affil=Institute of Quantum Beam Science, Ibaraki University kn-affil= affil-num=13 en-affil=Advanced Meson Science Laboratory, RIKEN Nishina Center kn-affil= affil-num=14 en-affil=Department of Physics, Okayama University kn-affil= END start-ver=1.4 cd-journal=joma no-vol=88 cd-vols= no-issue=9 article-no= start-page= end-page= dt-received= dt-revised= dt-accepted= dt-pub-year=2012 dt-pub=20120930 dt-online= en-article= kn-article= en-subject= kn-subject= en-title= kn-title=Superconductivity in (NH3)(y)Cs0.4FeSe en-subtitle= kn-subtitle= en-abstract= kn-abstract=Alkali-metal-intercalated FeSe materials, (NH3)(y)M0.4FeSe (M: K, Rb, and Cs), have been synthesized using the liquid NH3 technique. (NH3)(y)Cs0.4FeSe shows a superconducting transition temperature (T-c) as high as 31.2 K, which is higher by 3.8 K than the T-c of nonammoniated Cs0.4FeSe. The T(c)s of (NH3)(y)K0.4FeSe and (NH3)(y)Rb0.4FeSe are almost the same as those of nonammoniated K0.4FeSe and Rb0.4FeSe. The T-c of (NH3)(y)Cs0.4FeSe shows a negative pressure dependence. A clear correlation between T-c and lattice constant c is found for ammoniated metal-intercalated FeSe materials, suggesting a correlation between Fermi-surface nesting and superconductivity. en-copyright= kn-copyright= en-aut-name=ZhengLu en-aut-sei=Zheng en-aut-mei=Lu kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=1 ORCID= en-aut-name=IzumiMasanari en-aut-sei=Izumi en-aut-mei=Masanari kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=2 ORCID= en-aut-name=SakaiYusuke en-aut-sei=Sakai en-aut-mei=Yusuke kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=3 ORCID= en-aut-name=EguchiRitsuko en-aut-sei=Eguchi en-aut-mei=Ritsuko kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=4 ORCID= en-aut-name=GotoHidenori en-aut-sei=Goto en-aut-mei=Hidenori kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=5 ORCID= en-aut-name=TakabayashiYasuhiro en-aut-sei=Takabayashi en-aut-mei=Yasuhiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=6 ORCID= en-aut-name=KambeTakashi en-aut-sei=Kambe en-aut-mei=Takashi kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=7 ORCID= en-aut-name=OnjiTaiki en-aut-sei=Onji en-aut-mei=Taiki kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=8 ORCID= en-aut-name=ArakiShingo en-aut-sei=Araki en-aut-mei=Shingo kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=9 ORCID= en-aut-name=KobayashiTatsuo C. en-aut-sei=Kobayashi en-aut-mei=Tatsuo C. kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=10 ORCID= en-aut-name=KimJungeun en-aut-sei=Kim en-aut-mei=Jungeun kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=11 ORCID= en-aut-name=FujiwaraAkihiko en-aut-sei=Fujiwara en-aut-mei=Akihiko kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=12 ORCID= en-aut-name=KubozonoYoshihiro en-aut-sei=Kubozono en-aut-mei=Yoshihiro kn-aut-name= kn-aut-sei= kn-aut-mei= aut-affil-num=13 ORCID= affil-num=1 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=2 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=3 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=4 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=5 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=6 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci affil-num=7 en-affil= kn-affil=Okayama Univ, Dept Phys affil-num=8 en-affil= kn-affil=Okayama Univ, Dept Phys affil-num=9 en-affil= kn-affil=Okayama Univ, Dept Phys affil-num=10 en-affil= kn-affil=Okayama Univ, Dept Phys affil-num=11 en-affil= kn-affil=RIKEN, SPring Ctr 8, Japan Synchrotron Radiat Res Inst affil-num=12 en-affil= kn-affil=RIKEN, SPring Ctr 8, Japan Synchrotron Radiat Res Inst affil-num=13 en-affil= kn-affil=Okayama Univ, Res Lab Surface Sci END