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

Studies on phonon modes in a molecular dimer-Mott insulator -(BEDT-TTF)ICl 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, , 

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.

Magnetic entropy of 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, , 

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.

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.

Metamagnetism-induced first-order transition within the superconducting state of UTe under magnetic field along the easy a-axis

Tokiwa, 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.

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 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.

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.