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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：0The 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.
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：5 Percentile：92.42(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：7 Percentile：93.16(Materials Science, Multidisciplinary)
anisotropic triangular antiferromagnet Ca
ReO
ClNawa, Kazuhiro*; Hirai, Daigoro*; Kofu, Maiko; Nakajima, Kenji; Murasaki, Ryo*; Kogane, Satoshi*; Kimata, Motoi*; Nojiri, Hiroyuki*; Hiroi, Zenji*; Sato, Taku*
Physical Review Research (Internet), 2(4), p.043121_1 - 043121_11, 2020/12
The spin excitations of the 
= anisotropic triangular antiferromagnet CaReOCl were investigated by inelastic neutron-scattering experiments. The spin excitation spectrum exhibits sharp dispersive modes in addition to a spinonlike continuum. The consistency with the simulated spectrum based on the random-phase approximation is better for CaReOCl than for CsCuCl
, indicating that the spin system in the former remains closer to a Tomonaga-Luttinger liquidlike disordered state.
Niimi, Yasuhiro*; Kimata, Motoi*; Omori, Yasutomo*; Gu, B.; Ziman, T.*; Maekawa, Sadamichi; Fert, A.*; Otani, Yoshichika*
Physical Review Letters, 115(19), p.196602_1 - 196602_5, 2015/11
Times Cited Count：12 Percentile：63.05(Physics, Multidisciplinary)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
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
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 .
Sakai, Hironori; Tokiwa, Yoshifumi; Opletal, P.; Kimata, Motoi*; Awaji, Satoshi*; Sasaki, Takahiko*; Aoki, Dai*; Kambe, Shinsaku; Tokunaga, Yo; Haga, Yoshinori
no journal, ,
We have recently developed and optimized a molten salt flux method for growing high-purity single crystals of the superconductor UTe. The obtained crystals show a remarkably large residual resistivity ratio to the room temperature value and a small residual electronic contribution in specific heat well below . Using the high-quality single crystal of UTe, we have explored the superconducting phase diagram under magnetic fields along the hard magnetic -axis. 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 characteristic field of 
15 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 the characteristic field of 15 T, indicating an intermediate SC phase characterized by small flux pinning forces.
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 .
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 .
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.
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 superconductors, there are cases of vortex liquid-like state. When the pinning is strong, vortices do not move, and thus, the formation of such abnormal states requires pure and pristine crystals. Recently, the growth of pure single crystals of the spin-triplet superconductor UTe using a molten salt flux has been reported. In this study, we report research on vortices in UTe using such pure single crystals. As a result, in the magnetic field range of 15 T to 22 T, the critical current showed a maximum in temperature dependence, and a low-temperature region with a low critical current was observed deep in the superconducting state. This low critical current region matches the recently reported intermediate field superconducting phase. This suggests a weakening of the pinning force and a decrease in the critical current in the intermediate field superconducting phase.
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 superconductors, there are cases of vortex liquid-like state. When the pinning is strong, vortices do not move, and thus, the formation of such abnormal states requires pure and pristine crystals. Recently, the growth of pure single crystals of the spin-triplet superconductor UTe using a molten salt flux has been reported. In this study, we report research on vortices in UTe using such pure single crystals. As a result, in the magnetic field range of 15 T to 22 T, the critical current showed a maximum in temperature dependence, and a low-temperature region with a low critical current was observed deep in the superconducting state. This low critical current region matches the recently reported intermediate field superconducting phase. This suggests a weakening of the pinning force and a decrease in the critical current in the intermediate field superconducting phase.
