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Tokiwa, Yoshifumi; Opletal, P.; Sakai, Hironori; Kubo, Katsunori; Kambe, Shinsaku; Yamamoto, Etsuji; Kimata, Motoi; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; et al.
Physical Review Letters, 135(13), p.136502_1 - 136502_6, 2025/09
Times Cited Count：2 Percentile：69.59(Physics, Multidisciplinary)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.
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：8 Percentile：75.29(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：10 Percentile：65.50(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：33 Percentile：94.96(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：34 Percentile：92.42(Materials Science, Multidisciplinary)
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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：72.97(Physics, Applied)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.
Haga, Yoshinori; Sakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Nakamura, Shintaro*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; et al.
no journal, ,
no abstracts in English
under magnetic field along the easy a-axisTokiwa, Yoshifumi; Opletal, P.; Sakai, Hironori; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; et al.
no journal, ,
We study the interplay between superconductivity and metamagnetism of Ute under magnetic field along the easy a-axis by analyzing the entropy, obtained from measurements of magnetization and magnetocaloric effect. Due to the weak metamagnetism around 8T the entropy in the normal-conducting state at high temperautres above the superconducting transition temperature, Tc, exhibits an anomalous enhancement as a function of the field at the metamagnetism. Suprisingly the anomaly persists even inside the superconducting state and transforms into a sharp anomaly. The anomaly is a step-wise increase of entropy with increasing magnetic field, corresponding to a heat absorbtion. Thus, we found an unprecedented phenomenon of a metamagnetism inducing a first-order phase transition within a superconducting state. We discuss that a transition within the same symmetry of 
with a step-wise increase of 
weight would be consistent with our observations.
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)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.
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)ICl 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, ,
Recently it has been reported that a molecular dimer-Mott insulator -(BEDT-TTF)ICl is a candidate of a new type of ferroelectric system which shows a relaxor-like dielectric response. The system exhibits a glassy ferroelectric state below = 62 K and long-range antiferromagnetic order below = 22 K. We have performed inelastic neutron scattering experiments on this system by using a chopper spectrometer AMATERAS installed at BL14 in MLF. The phonon intensity at 18 and 21 meV decreases with lowering temperature, while that at 13.5 meV increases. We have already found that a period-doubled optical phonon mode at 4.2 meV is enhanced below around and then is suppressed below , indicating significant relationship to both charge and spin dynamics. In sharp contrast to this low-energy mode, the intensity of the optical mode at 18 meV decreases below to reach a constant value at a low temperature.
-(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)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.
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, ,
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. UTe2 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 Tc, while =12T is doubled. In this study, we revealed that this metamagnetism enhances .
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.
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 .
