Self-reconstruction of order parameter in spin-triplet superconductor UTe

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

https://doi.org/10.1103/z8yx-yzdh

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.

Anomalous vortex dynamics in the spin-triplet superconductor UTe

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

https://doi.org/10.1103/PhysRevB.108.144502

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.

Field induced multiple superconducting phases in UTe along hard magnetic axis

Sakai, 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

https://doi.org/10.1103/PhysRevLett.130.196002

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.

Interface-dependent magnetotransport properties for thin Pt films on ferrimagnetic YFeO

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

https://doi.org/10.1063/1.4883898

Single crystal growth and precise phase diagram of superconducting UTe

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.

Induced first-order transition inside superconducting state by metamagnetism in an easy-axis magnetic field on UTe

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 .

Anomalous vortex state in the spin-triplet superconductor UTe

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.

Phonon modes in a molecular dimer-Mott insulator -(BEDT-TTF)ICl studied by inelastic neutron scattering

Kawamura, 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.

Observation of vortex states in small superconductors with artificial pinning centres

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 (L5) 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.

Magnetic fluctuations in strongly correlated electrons system URuSi under high fields

Sakai, 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.