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Journal Articles

Anomalous vortex dynamics in the spin-triplet superconductor UTe$$_2$$

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

The vortex dynamics in the spin-triplet superconductor, UTe$$_2$$, are studied by measuring the DC electrical resistivity with currents along the $$a$$-axis under magnetic fields along the $$b$$-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.

Journal Articles

Chiral superconductivity in UTe$$_2$$ probed by anisotropic low-energy excitations

Ishihara, Kota*; Roppongi, Masaki*; Kobayashi, Masayuki*; Imamura, Kumpei*; Mizukami, Yuta*; Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Haga, Yoshinori; Hashimoto, Kenichiro*; et al.

Nature Communications (Internet), 14, p.2966_1 - 2966_7, 2023/05

 Times Cited Count:1 Percentile:89.39(Multidisciplinary Sciences)

The superconducting symmetry of the heavy fermion uranium-based superconductor UTe$$_2$$ is investigated using low temperature penetration depth measurements. The anisotropic low-energy quasiparticle excitations indicates multiple superconducting components in a chiral complex form. The most consistent is a chiral non-unitary state.

Journal Articles

Field induced multiple superconducting phases in UTe$$_2$$ 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

 Times Cited Count:0 Percentile:94.54(Physics, Multidisciplinary)

The superconducting (SC) phase diagram in uranium ditelluride is explored under magnetic fields ($$H$$) along the hard magnetic $$b$$-axis using a high-quality single crystal with $$T_{rm c}$$ = 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 $$H^{ast}$$ of $$sim15$$ 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 $$H^{ast}$$, indicating an intermediate SC phase characterized by small flux pinning forces.

Journal Articles

Anisotropic enhancement of lower critical field in ultraclean crystals of spin-triplet superconductor candidate UTe$$_2$$

Ishihara, Kota*; Kobayashi, Masayuki*; Imamura, Kumpei*; Konczykowski, M.*; Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Haga, Yoshinori; Hashimoto, Kenichiro*; Shibauchi, Takasada*

Physical Review Research (Internet), 5(2), p.L022002_1 - L022002_6, 2023/04

Lower superconducting critical fields $$H_{rm c1}$$ of UTe$$_2$$ have been determined. Orthorhombic UTe$$_2$$ has magnetic easy axis along the $$a$$-axis. We found $$H_{rm c1}$$ perpendicular to $$a$$ showed anomalous enhancement. By comparing with anisotropy of upper critical fields, effect of magnetic fluctuations on superconductivity is suggested.

Journal Articles

Ferromagnetic crossover within the ferromagnetic order of U$$_{7}$$Te$$_{12}$$

Opletal, P.; Sakai, Hironori; Haga, Yoshinori; Tokiwa, Yoshifumi; Yamamoto, Etsuji; Kambe, Shinsaku; Tokunaga, Yo

Journal of the Physical Society of Japan, 92(3), p.034704_1 - 034704_5, 2023/03

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

We investigate the physical properties of a single crystal of uranium telluride U$$_{7}$$Te$$_{12}$$. We have confirmed that U$$_{7}$$Te$$_{12}$$ crystallizes in the hexagonal structure with three nonequivalent crystallographic uranium sites. The paramagnetic moments are estimated to be approximately 1 $$mu_{B}$$ per the uranium site, assuming a uniform moment on all the sites. A ferromagnetic phase transition occurs at $$T_{rm C}$$= 48 K, where the in-plane magnetization increases sharply, whereas the out of-plane component does not increase significantly. With decreasing temperature further below $$T_{rm C}$$ under field-cooling conditions, the out-of-plane component increases rapidly around T$$^{star}$$= 26 K. In contrast, the in-plane component hardly changes at T$$^{star}$$. Specific heat measurement indicates no $$lambda$$-type anomaly around T$$^{star}$$, so this is a cross-over suggesting a reorientation of the ordering moments or successive magnetic ordering on the part of the multiple uranium sites.

Journal Articles

First observation of the de Haas-van Alphen effect and Fermi surfaces in the unconventional superconductor UTe$$_2$$

Aoki, Dai*; Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Ishizuka, Jun*; Yanase, Yoichi*; Harima, Hisatomo*; Nakamura, Ai*; Li, D.*; Homma, Yoshiya*; et al.

Journal of the Physical Society of Japan, 91(8), p.083704_1 - 083704_5, 2022/08

 Times Cited Count:15 Percentile:97.16(Physics, Multidisciplinary)

Journal Articles

Single crystal growth of superconducting UTe$$_2$$ by molten salt flux method

Sakai, Hironori; Opletal, P.; Tokiwa, Yoshifumi; Yamamoto, Etsuji; Tokunaga, Yo; Kambe, Shinsaku; Haga, Yoshinori

Physical Review Materials (Internet), 6(7), p.073401_1 - 073401_10, 2022/07

 Times Cited Count:9 Percentile:88.29(Materials Science, Multidisciplinary)

The molten salt flux method is applied as a new synthetic route for the single crystals of the spin-triplet superconductor UTe$$_2$$. The single crystals under an optimized growth condition with excess uranium exhibit a superconducting transition at $$T_{rm c}=2.1$$ K, which is the highest $$T_{rm c}$$ reported for this compound. The obtained crystals show a remarkably large residual resistivity ratio with respect to the room temperature value and a small residual electronic contribution in specific heat well below $$T_{rm c}$$. These results indicate that the increase of $$T_{rm c}$$ in UTe$$_2$$ can be achieved by reducing the disorder associated with uranium vacancies. The excess uranium in the molten salt acts as a reducing agent, preventing tetravalent uranium from becoming pentavalent and suppressing creation of uranium vacancies. At the same time, the relatively low growth temperature can suppress Te volatilization.

Journal Articles

Effect of uranium deficiency on normal and superconducting properties in unconventional superconductor UTe$$_2$$

Haga, Yoshinori; Opletal, P.; Tokiwa, Yoshifumi; Yamamoto, Etsuji; Tokunaga, Yo; Kambe, Shinsaku; Sakai, Hironori

Journal of Physics; Condensed Matter, 34(17), p.175601_1 - 175601_7, 2022/04

 Times Cited Count:14 Percentile:92.72(Physics, Condensed Matter)

Journal Articles

Slow electronic dynamics in the paramagnetic state of UTe$$_2$$

Tokunaga, Yo; Sakai, Hironori; Kambe, Shinsaku; Haga, Yoshinori; Tokiwa, Yoshifumi; Opletal, P.; Fujibayashi, Hiroki*; Kinjo, Katsuki*; Kitagawa, Shunsaku*; Ishida, Kenji*; et al.

Journal of the Physical Society of Japan, 91(2), p.023707_1 - 023707_5, 2022/02

 Times Cited Count:11 Percentile:94.47(Physics, Multidisciplinary)

$$^{125}$$Te NMR experiments in field ($$H$$) applied along the easy magnetization axis (the $$a$$-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe$$_2$$. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30$$-$$40 K, where the spin susceptibility along the hard magnetization axis (the $$b$$-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in $$H|a$$. We suggest that UTe$$_2$$ would be located on the paramagnetic side near an electronic phase boundary, where either the magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations.

Journal Articles

Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures

Tokiwa, Yoshifumi; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

Communications Materials (Internet), 2(1), p.42_1 - 42_6, 2021/04

Generation of very low temperatures has been crucially important for applications and fundamental research, as low-temperature quantum coherence enables operation of quantum computers and formation of exotic quantum states, such as superfluidity/superconductivity. One of the major techniques to reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration. This method uses almost non-interacting magnetic moments of paramagnetic salts where large distances suppress interactions between the moments. The large spatial separations are facilitated by water molecules, with a drawback of reduced stability of the material. Here, we show that the water-free frustrated magnet KBaYb(BO$$_{3}$$)$$_{2}$$ can be ideal for refrigeration, achieving at least 22 mK. Compared to conventional refrigerants, KBaYb(BO$$_{3}$$)$$_{2}$$ does not degrade even under high temperatures and ultra-high vacuum. Further, its magnetic frustration and structural randomness enable cooling to temperatures several times lower than the energy scale of magnetic interactions, which is the main limiting factor for the base temperature of conventional refrigerants.

Journal Articles

Electronic structure of heavy fermion uranium compounds studied by core-level photoelectron spectroscopy

Fujimori, Shinichi; Okochi, Takuo*; Kawasaki, Ikuto; Yasui, Akira; Takeda, Yukiharu; Okane, Tetsuo; Saito, Yuji; Fujimori, Atsushi; Yamagami, Hiroshi; Haga, Yoshinori; et al.

Journal of the Physical Society of Japan, 81(1), p.014703_1 - 014703_9, 2012/01

 Times Cited Count:37 Percentile:82.5(Physics, Multidisciplinary)

High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy Fermion uranium compounds UGe$$_2$$, UCoGe, URhGe, URu$$_2$$Si$$_2$$, UNi$$_2$$Al$$_3$$, UPd$$_2$$Al$$_3$$, and UPt$$_3$$ as well as typical localized and itinerant uranium compounds to understand the relationship between the uranium valence state and their core-level spectral line shapes. The core-level spectra of UGe$$_2$$, UCoGe, URhGe, URu$$_2$$Si$$_2$$, and UNi$$_2$$Al$$_3$$ are rather similar to those of itinerant compounds, suggesting that U 5$$f$$ electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPd$$_2$$Al$$_3$$ and UPt$$_3$$ show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5$$f$$ electrons in UPd$$_2$$Al$$_3$$ and UPt$$_3$$ are less hybridized with ligand states, leading to the correlated nature of U 5$$f$$ electrons in these compounds.

Journal Articles

Localized 5$$f$$ antiferromagnetism in cubic UIn$$_3$$; $$^{115}$$In-NMR/NQR study

Sakai, Hironori; Kambe, Shinsaku; Tokunaga, Yo; Chudo, Hiroyuki; Tokiwa, Yoshifumi*; Aoki, Dai*; Haga, Yoshinori; Onuki, Yoshichika; Yasuoka, Hiroshi

Physical Review B, 79(10), p.104426_1 - 104426_7, 2009/03

 Times Cited Count:6 Percentile:29.7(Materials Science, Multidisciplinary)

$$^{115}$$In nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed on an antiferromagnet UIn$$_3$$ with the cubic AuCu$$_3$$-type structure. Under zero field, the observation of the NQR spectrum has led to an estimated $$nu_{rm Q}$$ of 11.8 MHz at 90 K. The temperature variation of the NQR relaxation rates ($$1/T_1$$) far above the N${'e}$el temperature $$T_{rm N}$$=88 K approaches a constant value, which indicates a localized nature for the $$5f$$-electrons in this system. On the other hand, in the antiferromagnetically ordered state, we propose that the most plausible direction of antiferromagnetic U moments may be $$langle 110rangle$$ among the possible orientations of $$langle 100rangle$$, $$langle 110rangle$$, or $$langle 111rangle$$.

Journal Articles

Direct observation of a quasiparticle band in CeIrIn$$_5$$; An Angle-resolved photoemission spectroscopy study

Fujimori, Shinichi; Fujimori, Atsushi; Shimada, Kenya*; Narimura, Takamasa*; Kobayashi, Kenichi*; Namatame, Hirofumi*; Taniguchi, Masaki*; Harima, Hisatomo*; Shishido, Hiroaki*; Ikeda, Shugo; et al.

Physical Review B, 73(22), p.224517_1 - 224517_5, 2006/06

 Times Cited Count:41 Percentile:82.27(Materials Science, Multidisciplinary)

We have performed a high-resolution angle resolved Ce 4$$d$$-4$$f$$ resonant photoemission experiment on the heavy fermion superconductor CeIrIn$$_5$$. We have observed a quasi-particle band which has a energy disepersion of $$sim$$ 30 meV in the Ce 4$$f$$ on-resonance spectra. The result suggests that although the Ce 4$$f$$ spectra are dominated by the localized/correlated character being dominant, the small itinerant component is responsible for the superconductivity in this compound.

Journal Articles

Soft X-ray synchrotron radiation photoemission study on uranium compounds

Fujimori, Shinichi; Terai, Kota; Takeda, Yukiharu; Okane, Tetsuo; Saito, Yuji; Muramatsu, Yasuji; Fujimori, Atsushi; Yamagami, Hiroshi*; Ikeda, Shugo; Matsuda, Tatsuma; et al.

Physica B; Condensed Matter, 378-380, p.995 - 996, 2006/05

 Times Cited Count:2 Percentile:12.69(Physics, Condensed Matter)

Recently, photoemission experiments in soft X-ray region have attracted much attention due to their high sensitivities for bulk electronic structures of strongly correlated electron system. We have carried out the high-resolution photoemission experiments on some uranium compounds using soft X-ray synchrotron radiation from SPring-8 to understand their electronic structures. Bulk- and U 5$$f$$-sensitive band structure and Fermi surface were obtained for itinerant U 5$$f$$ compounds UFeGa$$_5$$ and USb$$_2$$, and they are compared with the results of the band structure calculations treating U 5$$f$$ states as being itinerant. In addition, we show the results of photoemission study on some heavy fermion compounds such as UPd$$_2$$Al$$_3$$ and UNi$$_2$$Al$$_3$$. The temperature dependences of the U 5$$f$$ electronic structure of UPd$$_2$$Al$$_3$$ will be also reported.

Journal Articles

Itinerant U 5$$f$$ band states in the layered compound UFeGa$$_5$$ observed by soft X-ray angle-resolved photoemission spectroscopy

Fujimori, Shinichi; Terai, Kota; Takeda, Yukiharu; Okane, Tetsuo; Saito, Yuji; Muramatsu, Yasuji; Fujimori, Atsushi; Yamagami, Hiroshi*; Tokiwa, Yoshifumi*; Ikeda, Shugo; et al.

Physical Review B, 73(12), p.125109_1 - 125109_6, 2006/03

 Times Cited Count:22 Percentile:67.29(Materials Science, Multidisciplinary)

We have performed angle-resolved photoemission spectroscopy (ARPES) experiments on paramagnetic UFeGa$$_5$$ using soft X-ray synchrotron radiation ($$hnu$$=500 eV) and derived the bulk- and U 5$$f$$-sensitive electronic structure of UFeGa$$_5$$. Although the agreement between the experimental band strucure and the LDA calculation treating U 5$$f$$ electrons as beeing itinerant is qualitative, the morphology of the Fermi surface is well explained by the calculation. This result claims that the importance of the electron correlation effect even for the itinerant U 5$$f$$ compound.

Journal Articles

Large orbital magnetic moment and its quenching in the itinerant uranium intermetallic compounds U${it T}$Ga$$_5$$ (${it T}$=Ni, Pd, Pt)

Kaneko, Koji; Metoki, Naoto; Bernhoeft, N.*; Lander, G. H.; Ishii, Yoshinobu; Ikeda, Shugo; Tokiwa, Yoshifumi; Haga, Yoshinori; Onuki, Yoshichika

Physical Review B, 68(21), p.214419_1 - 214419_9, 2003/12

 Times Cited Count:59 Percentile:88.9(Materials Science, Multidisciplinary)

The crystal structure, lattice strain due to the antiferromagnetic ordering and magnetic form factor in the itinerant 5$$f$$ compounds UTGa$$_5$$ (T: Ni, Pd, Pt) have been studied by neutron scattering. High-resolution powder diffraction revealed that the tetragonality of the U-Ga layers increases with the ionic radius of the transition metal element T. The observed antiferromagnetic reflections can be well explained with the N$'e$el-type structure for UNiGa$$_5$$, whereas UPtGa$$_5$$ has the antiferromagnetic stacking of the ferromagnetically ordered uranium moments in the $$c$$-plane. The orbital contributions in the magnetic form factor are reduced from the free ion value, especially for UNiGa$$_5$$. This suppression shows a strong correlation with the bulk magnetic susceptibility. The tetragonality of the U-Ga layers is a sensitive measure of the nearest neighbour interaction, the lattice anomaly, and the orbital contribution and suggests that orbital degrees of freedom may play an important role for the magnetic properties in these itinerant 5$$f$$ antiferromagnets.

Journal Articles

NQR measurements of UTGa$$_{5}$$

Kato, Harukazu; Sakai, Hironori; Kambe, Shinsaku; Walstedt, R. E.; Tokiwa, Yoshifumi; Onuki, Yoshichika

Acta Physica Polonica B, 34(2), p.1063 - 1066, 2003/09

We performed $$^{69,71}$$Ga-NMR/NQR studies for UPtGa$$_{5}$$, which is a 5$$f$$-itinerant antiferromagnets with Neel temperature $$T_{N}$$ = 26 K and an electronic specific coefficient $$gamma$$ = 57 mJ/mol K$$^{2}$$. From the NMR spectrum in the paramagnetic temperature region, the hyperfine coupling constants $$A_{hf}^{para}$$ in the paramagnetic state have been evaluated. The magnitude of the transferred field has been evaluated from the zero-applied-field spectrum, yielding a value for the hyperfine coupling constant $$A_{hf}^{AF}$$ in the antiferromagnetic state. The estimated value of $$A_{hf}^{AF}$$ is an order of magnitude larger than that of $$A_{hf}^{para}$$. This implies the existence of a long-ranged hyperfine interaction path. The mechanism of the hyperfine interaction will be discussed in terms of RKKY interactions.

Journal Articles

Ga and Pt NMR study of UPtGa$$_{5}$$ and UNiGa$$_{5}$$

Kato, Harukazu; Sakai, Hironori; Tokunaga, Yo; Tokiwa, Yoshifumi; Ikeda, Shugo; Onuki, Yoshichika; Kambe, Shinsaku; Walstedt, R. E.

Journal of the Physical Society of Japan, 72(9), p.2357 - 2363, 2003/09

 Times Cited Count:17 Percentile:67.12(Physics, Multidisciplinary)

Ga and Pt NMR measurements have been carried out for two isomorphs compounds, UPtGa$$_{5}$$ and UNiGa$$_{5}$$, which indicate different magnetic structures below T$$_{N}$$. Knight shift K measurements in the paramagnetic region are reported here. Transferred hyperfine coupling constants at Ga and Pt sites are determined. The temperature independent part K$$_{0}$$ of K, which probes the conduction electron polarization at the ligand site, has successfully evaluated. An nearly identical conduction electron structure in the paramagnetic region is suggested for the both compounds. The origin of the different magnetic structure is discussed.

Journal Articles

Ga nuclear magnetic resonance study of UTGa$$_{5}$$(T=Ni, Pt)

Kato, Harukazu; Sakai, Hironori; Tokunaga, Yo; Tokiwa, Yoshifumi; Ikeda, Shugo; Onuki, Yoshichika; Kambe, Shinsaku; Walstedt, R. E.

Journal of Physics; Condensed Matter, 15(28), p.S2001 - S2005, 2003/07

 Times Cited Count:4 Percentile:27.06(Physics, Condensed Matter)

UPtGa$$_{5}$$ and UNiGa$$_{5}$$ are 5$$f$$-itinerant antiferromagnets with N$'{e}$el temperature $$T_{N}$$ = 26 K (Pt), 86 K (Ni). Remarkably, although the two compounds show almost identical Fermi surface properties in the paramagnetic state, recent neutron scattering experiments reveal that UPtGa$$_{5}$$ and UNiGa$$_{5}$$ are quite different in magnetic structure below $$T_{N}$$. In the microscopic viewpoint, we have carried out $$^{69,71}$$Ga NMR/NQR measurements for the both coumpounds. We have suceeded in separating the temperature independent term and the Curie-like-temperature-dependent term of the Knight shift $$K$$. This should be a good starting point in considering the magnetism of this system.

Journal Articles

Magnetic properties of U$$_2$$RhGa$$_8$$ and U$$_2$$FeGa$$_8$$

Ikeda, Shugo; Okubo, Tomoyuki*; Inada, Yoshihiko*; Tokiwa, Yoshifumi; Kaneko, Koji; Matsuda, Tatsuma; Yamamoto, Etsuji; Haga, Yoshinori; Onuki, Yoshichika

Journal of Physics; Condensed Matter, 15(28), p.S2015 - S2018, 2003/07

 Times Cited Count:6 Percentile:35.74(Physics, Condensed Matter)

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