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Tokiwa, Yoshifumi; Opletal, P.; Sakai, Hironori; Kambe, Shinsaku; Yamamoto, Etsuji; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Haga, Yoshinori; et al.
Physical Review B, 109(14), p.L140502_1 - L140502_6, 2024/04
Times Cited Count：6 Percentile：81.78(Materials Science, Multidisciplinary)The normal-conducting state of the superconductor UTe is studied by entropy analysis for magnetic fields along the 
-axis, obtained from magnetization using the relation 
. We observe a strong increase in entropy with magnetic field due to metamagnetic fluctuations (spatially uniform, 
). The field dependence is well described by the Hertz-Millis-Moriya theory for quantum criticality of itinerant metamagnetism. Notably, the lower bound of the quantum-critical region coincides well with the position of the minimum in the superconducting transition temperature 
. Hence, our results suggest that fluctuations reinforce the superconductivity.
Tokiwa, Yoshifumi; Sakai, Hironori; Kambe, Shinsaku; Opletal, P.; Yamamoto, Etsuji; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Yanase, Yoichi*; Haga, Yoshinori; et al.
Physical Review B, 108(14), p.144502_1 - 144502_5, 2023/10
Times Cited Count：8 Percentile：64.82(Materials Science, Multidisciplinary)The vortex dynamics in the spin-triplet superconductor, UTe, are studied by measuring the DC electrical resistivity with currents along the 
-axis under magnetic fields along the -axis. Surprisingly, we have discovered an island region of low critical current deep inside the superconducting (SC) state, well below the SC upper critical field, attributed to a weakening of vortex pinning. Notably, this region coincides with the recently proposed intermediate-field SC state. We discuss the possibility of nonsingular vortices in the intermediate state, where SC order parameter does not vanish entirely in the vortex cores due to the mixing of multiple SC components.
Yoshitomi, Hiroshi; Manabe, Kentaro; Ochi, Kotaro; Kono, Takahiko; Sasaki, Michiya*; Yoshida, Hiroko*
Hoken Butsuri (Internet), 58(2), p.105 - 111, 2023/08
This article gives the report on participation in the 6th Asian and Oceanic Congress on Radiation Protection (AOCRP6), which was held in Mumbai, India on between February 7th and 11th, 2023.
along hard magnetic axisSakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori
Physical Review Letters, 130(19), p.196002_1 - 196002_6, 2023/05
Times Cited Count：26 Percentile：94.91(Physics, Multidisciplinary)The superconducting (SC) phase diagram in uranium ditelluride is explored under magnetic fields (
) along the hard magnetic -axis using a high-quality single crystal with 
= 2.1 K. Simultaneous electrical resistivity and AC magnetic susceptibility measurements discern low- and high-field SC (LFSC and HFSC, respectively) phases with contrasting field-angular dependence. Crystal quality increases the upper critical field of the LFSC phase, but the 
of 
T, at which the HFSC phase appears, is always the same through the various crystals. A phase boundary signature is also observed inside the LFSC phase near , indicating an intermediate SC phase characterized by small flux pinning forces.
induced by magnetic fieldKinjo, Katsuki*; Fujibayashi, Hiroki*; Kitagawa, Shunsaku*; Ishida, Kenji*; Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Nakamura, Ai*; Shimizu, Yusei*; Homma, Yoshiya*; et al.
Physical Review B, 107(6), p.L060502_1 - L060502_5, 2023/02
Times Cited Count：27 Percentile：92.34(Materials Science, Multidisciplinary)
Fe
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Shiomi, Yuki*; Otani, Takashi*; Iguchi, Satoshi*; Sasaki, Takahiko*; Qiu, Z.*; Nakayama, Hiroyasu*; Uchida, Kenichi*; Saito, Eiji
Applied Physics Letters, 104(24), p.242406_1 - 242406_5, 2014/06
Times Cited Count：31 Percentile：73.97(Physics, Applied)Kitano, Kohei*; Okayasu, Satoru; Nojima, Tsutomu*; Sasaki, Takahiko*; Kokubo, Nobuhito*
no journal, ,
We try to observe a new vortex state including anti-vortex by introducing small bores or hollows in microscopic superconductors. We made direct observations of vortex states of microscopic square superconducting samples with 5 bores, located the center of the sample and four corners, by using a scanning SQUID microscope. A unique vortex alignment at low vorticity (L
5) is observed in comparison with the previous measurements on the samples with no bore. It becomes clear that the vortex alignment is determined by the distance of bores and the interaction among the vortices.
electrons system URuSi under high fieldsSakai, Hironori; Higa, Nonoka; Hattori, Taisuke; Tokunaga, Yo; Haga, Yoshinori; Kambe, Shinsaku; Goto, Takayuki*; Hirata, Michihiro*; Awaji, Satoshi*; Sasaki, Takahiko*
no journal, ,
We have performed 
Si nuclear magnetic resonance (NMR) experiments using single crystal of strongly correlated 
electrons system URuSi under high external field of about 24 T. The temperature dependence of NMR shift in the paramagnetic state varies with external fields. The temperature and field dependences of NMR shifts resembles those of magnetic susceptibility.
Tokiwa, Yoshifumi; Opletal, P.; Sakai, Hironori; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; et al.
no journal, ,
UTe has attracted much attention due to the possibility of reentrant superconductivity and spin triplet superconductivity in a magnetic field. In this material, when a magnetic field is applied in the b-axis direction, Tc decreases once due to the magnetic field, but Tc increases under a magnetic field of about 16T or more. In this field direction, there is a metamagnetic transition at 34T. The relationship between this metamagnetism and the anomalous enhancement of Tc has been discussed. Many unconventional superconductors exhibit an unusual metallic state in the normal phase. It is believed that magnetic fluctuations, which are the cause of this phenomenon, generate superconducting electron pairs. Entropy is a direct measure of fluctuations, as entropy exhibits a maximum at the quantum critical point. Therefore, the relationship between fluctuations and superconductivity can be clarified by entropy measurements in the normal state. In this study, we performed precise magnetization measurements at temperatures up to 2 K and magnetic fields up to 24 T using cryogen-free superconducting magnets at the Research Center for High Magnetic Field Superconducting Materials, IMR, Tohoku University. We used a thermodynamic relation to map the entropy in the magnetic field-temperature parameter space. In this talk, we discuss the relationship between the entropy increase due to metamagnetic fluctuations and superconductivity.
Haga, Yoshinori; Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo; Kimata, Motoi*; Nakamura, Shintaro*; Awaji, Satoshi*; et al.
no journal, ,
no abstracts in English
Tokiwa, Yoshifumi; Opletal, P.; Sakai, Hironori; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; et al.
no journal, ,
In unconventional superconductivity, magnetism is considered to be the origin of the superconducting interaction, and the relationship between magnetism and superconductivity is a central theme of research. UTe has attracted much attention due to its potential for reentrant superconductivity and spin-triplet superconductivity in magnetic fields. In this material, when a magnetic field is applied along the a-axis, a weak metamagnetism occurs near 6T due to Fermi surface instability. In the initial crystals synthesized by the vapor phase epitaxy, superconductivity disappeared in the magnetic field where metamagnetism was generated because 
= 1.6K and upper critical magnetic field 
= 6T, and superconductivity was not affected. In the pure single crystal synthesized by the molten salt flux method, =2.1K is 25% stronger than the initial while =12T is doubled. In this study, we revealed that this metamagnetism enhances .
-(BEDT-TTF)ICl by inelastic neutron scatteringKawamura, Seiko; Matsuura, Masato*; Iguchi, Satoshi*; Sasaki, Takahiko*; Taniguchi, Hiromi*; Kubota, Aiko*; Sato, Kazuhiko*; Inamura, Yasuhiro; Kikuchi, Tatsuya*; Nakajima, Kenji
no journal, ,
It has been reported that in a molecular dimer-Mott insulator -(BEDT-TTF)ICl shows a relaxor-like dielectric response. The system exhibits a glassy ferroelectric state associated with charge disproportionation within the dimer, where a hole (
= 1/2) is localized, below = 62K, and a long-range antiferromagnetic order below 
= 22K. For such molecule-based materials, it is expected that low-energy phonon modes are coupled with spin/charge degrees of freedom. In order to observe the phonon modes in this system, we have performed inelastic neutron scattering measurements in a wide wave-vector and energy region using a chopper spectrometer AMATERAS at J-PARC MLF. The lowest energy mode shows remarkable temperature dependence of its intensity, while any softening is not observed. The result suggests that this phonon mode reflects changes in the spin/charge dynamics.
'-(BEDT-TTF)IClKawamura, Seiko; Taniguchi, Hiromi*; Matsuura, Masato*; Kofu, Maiko*; Murai, Naoki; Sasaki, Takahiko*
no journal, ,
Sakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori
no journal, ,
High-quality UTe single crystals were grown using a molten salt flux method with the highest reported superconducting (SC) transition temperature. The crystals had a small residual electronic contribution in specific heat and a large residual resistivity ratio. The SC phase diagram of UTe was studied, revealing contrasting field-angular dependence of low and high-field SC phases. An intermediate SC state was also found. The high-field SC phase appeared consistently at about 15 T, while crystal quality increased the upper critical field of the low-field superconducting phase. The MSF method facilitated the observation of quantum oscillations and progress in Fermi surface research for this spin-triplet superconductor.
V nuclear magnetic resonance study of the triangular Kondo lattice system YbV
SnSakai, Hironori; Park, S.*; Dioguardi, A. P.*; Furukawa, Tetsuya*; Sasaki, Takahiko*; Kimata, Motoi; Kambe, Shinsaku; Tokunaga, Yo; Thomas, S. M.*; Thompson, J. D.*; et al.
no journal, ,
YbVSn has a HfFeGe-type structure, in which V atoms form a Kagome lattice and Yb atoms form a triangular lattice. This rare Yb-based Kondo triangular lattice system has attracted much attention as an essential model to explore the relationship between quantum magnetism and topological properties. To microscopically clarify the quantum criticality of this system, we have performed V nuclear magnetic resonance (NMR) studies using a single crystal of YbVSn. In this talk, I will report on the spectral assignments of V nuclei revealed from V NMR measurements in the paramagnetic state.
Sakai, Hironori; Tokiwa, Yoshifumi; Kimata, Motoi; Awaji, Satoshi*; Sasaki, Takahiko*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori
no journal, ,
We investigated the angular dependence of the superconducting phase diagram in UTe using magnetic fields up to 25 T. Precise measurements of ac susceptibility and resistivity were performed with a two-axis rotator. The results show that the low-field (LFSC) and high-field (HFSC) superconducting phases respond differently to field orientation. Notably, the HFSC phase emerges only within a narrow angle near the axis and disappears rapidly with tilting, suggesting strong anisotropy. We will discuss how the HFSC phase vanishes as the field tilts.
Tokiwa, Yoshifumi; Opletal, P.*; Sakai, Hironori; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori; Kubo, Katsunori; Kimata, Motoi; Awaji, Satoshi*; et al.
no journal, ,
We investigate the effect of easy-axis metamagnetic crossover on superconductivity in UTe along the -axis through measurements of AC susceptibility, magnetization, and the magnetocaloric effect. In ultra-clean single crystals, we identify a field-induced phase transition within the superconducting state at 5.6 T, driven by metamagnetism. This transition leads to a high-field superconducting state, significantly increasing the upper critical field to 12 T. A sudden increase in entropy at the transition suggests a self-reconstruction of the order parameter, enabling multi-component superconducting states to adapt to external perturbations.
-(BEDT-TTF)IClKawamura, Seiko; Matsuura, Masato*; Iguchi, Satoshi*; Sasaki, Takahiko*; Taniguchi, Hiromi*; Kubota, Aiko*; Sato, Kazuhiko*; Inamura, Yasuhiro; Kikuchi, Tatsuya; Kawakita, Yukinobu; et al.
no journal, ,
A molecular dimer-Mott insulator -(BEDT-TTF)ICl is a candidate for a new type of ferroelectric system which shows a relaxor-like dielectric response. We performed inelastic neutron scattering measurements on this system by using a chopper specrometer, and observed phonon spectra in a wide wave vector (
)-energy (
) region for the first time. Clear optical modes with the energies up to 
36 meV were observed together with an acoustic phonon dispersion. The intensity of the optical mode with the lowest energy = 4.2 meV increases at a zone boundary along the * axis below 
= 62 K, at which a glassy ferroelectricity appears, and it further increases all over the measured range below the antiferromangetic transition temperature = 22 K. The results suggest that the lattice dynamics characterized by this optical phonon mode is significantly coupled with strongly correlated charge and spin degrees of freedom.
-(BEDT-TTF)ICl coupled with charge and spin studied by inelastic neutron scatteringKawamura, Seiko; Matsuura, Masato*; Iguchi, Satoshi*; Sasaki, Takahiko*; Taniguchi, Hiromi*; Kubota, Aiko*; Sato, Kazuhiko*; Inamura, Yasuhiro; Kikuchi, Tatsuya*; Nakajima, Kenji
no journal, ,
-(BEDT-TTF)ICl is an organic dimer-Mott insulator which exhibits a glassy ferroelectric state below = 62K and an antiferromagnetic order below = 22K. It is expected in such molecule-based materials that low-energy phonon modes lie in the energy region of charge and spin, and then the lattice is expected to couple with charge and spin degrees of freedom. To observe such phonon modes in this system, we have performed inelastic neutron scattering study using a chopper spectrometer AMATERAS installed at J-PARC MLF. The system shows the lowest-energy optical phonon mode at = 4.2 meV. which is significantly related to the charge degree of freedom. Analyzing the data more in detail, the intensity of the 4.2 meV-mode monotonically increases with lowering temperature toward , becomes constant below and drastically increases below . The behavior of the optical phonons at some representative positions are reported.
Sakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori
no journal, ,
For a paramagnetic superconductor UTe, a "field-reionforced superconducting state" in which increases with magnetic field above 15 T emerges when a magnetic field is applied "exactly" in the direction of the magnetically hard axis. In order to investigate this high-field superconducting phase, we have attempted to determine the magnetic field-temperature phase diagram of ultra-clean single crystals using a cryogen-free 25 T superconducting magnet (25T-CSM) at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In this presentation, we will report the obtained results.
