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論文

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

青木 大*; 酒井 宏典; Opletal, P.; 常盤 欣文; 石塚 淳*; 柳瀬 陽一*; 播磨 尚朝*; 仲村 愛*; Li, D.*; 本間 佳哉*; et al.

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

https://doi.org/10.7566/JPSJ.91.083704
 被引用回数：44 パーセンタイル：97.97(Physics, Multidisciplinary)

de Haas-van Alphen oscillation was successfully observed in an unconventional superconductor UTe$$_2$$ using high-quality single crystals. From this observation, Fermi surfaces of this compound were determined to be cylinders elongated along the c-direction, in consistent with theoretical expectation. However, the extremely large cyclotron effective masses ranging 32 to 57 m$$_0$$, in consistent with the electronic specific heat, far exceed calculated values.

論文

Unconventional superconductivity in UTe$$_2$$

青木 大*; Brison, J.-P.*; Flouquet, J.*; 石田 憲二*; Knebel, G.*; 徳永 陽; 柳瀬 陽一*

Journal of Physics; Condensed Matter, 34(24), p.243002_1 - 243002_41, 2022/06

https://doi.org/10.1088/1361-648X/ac5863
 被引用回数：95 パーセンタイル：91.08(Physics, Condensed Matter)

The novel spin-triplet superconductor candidate UTe$$_2$$ attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe$$_2$$ has been expected to be close to a ferromagnetic instability, showing many similarities to the U-based ferromagnetic superconductors, URhGe and UCoGe. The competition between different types of magnetic interactions and the duality between the local and itinerant character of the 5 f Uranium electrons, as well as the shift of the U valence appear as key parameters in the rich phase diagrams discovered recently under extreme conditions like low temperature, high magnetic field, and pressure. We discuss macroscopic and microscopic experiments at low temperature to clarify the normal phase properties at ambient pressure for field applied along the three axis of this orthorhombic structure.

論文

Feedback of superconductivity on the magnetic excitation spectrum of UTe$$_{2}$$

Raymond, S.*; Knafo, W.*; Knebel, G.*; 金子 耕士; Brison, J.-P.*; Flouquet, J.*; 青木 大*; Lapertot, G.*

Journal of the Physical Society of Japan, 90(11), p.113706_1 - 113706_5, 2021/11

https://doi.org/10.7566/JPSJ.90.113706
 被引用回数：17 パーセンタイル：81.74(Physics, Multidisciplinary)

We investigate the spin dynamics in the superconducting phase of UTe$$_2$$ by triple-axis inelastic neutron scattering on a single crystal sample. At the wave-vector $$k_1$$ = (0, 0.57, 0), where the normal state antiferromagnetic correlations are peaked, a modification of the excitation spectrum is evidenced, on crossing the superconducting transition, with a reduction of the relaxation rate together with the development of an inelastic peak at $${Omega} approx$$ 1 meV. The low dimensional nature and the a-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below Tsc. The high ratio of $${Omega}$$ $$k_{rm B}$$$$T$$$$_{sc}$$ $$approx$$ 7.2 contrasts with the most common behaviour in heavy fermion superconductors.

論文

Low-dimensional antiferromagnetic fluctuations in the heavy-fermion paramagnetic ladder compound UTe$$_{2}$$

Knafo, W.*; Knebel, G.*; Steffens, P.*; 金子 耕士; Rosuel, A.*; Brison, J.-P.*; Flouquet, J.*; 青木 大*; Lapertot, G.*; Raymond, S.*

Physical Review B, 104(10), p.L100409_1 - L100409_6, 2021/09

https://doi.org/10.1103/PhysRevB.104.L100409
 被引用回数：48 パーセンタイル：94.81(Materials Science, Multidisciplinary)

Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$$_{2}$$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wave-vector $${bf K}_{1}$$ = (0, 0.57, 0). A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $$mu$$ $$||$$ ${bf a}$, with a sine-wave modulation of wave-vector $${bf K}_{1}$$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction ${bf c}$. These results constitute constraints for models of magnetically-mediated superconductivity in UTe$$_{2}$$.

論文

$$T$$ dependence of nuclear spin-echo decay at low temperatures in YbRh$$_2$$Si$$_2$$

神戸 振作; 酒井 宏典; 徳永 陽; 服部 泰佑; Lapertot, G.*; 松田 達磨*; Knebel, G.*; Flouquet, J.*; Walstedt, R. E.*

Physical Review B, 95(19), p.195121_1 - 195121_8, 2017/05

AA2017-0083.pdf:0.74MB

https://doi.org/10.1103/PhysRevB.95.195121
 被引用回数：3 パーセンタイル：16.15(Materials Science, Multidisciplinary)

YbRh$$_2$$Si$$_2$$の単結晶について、$$^{29}$$Si核スピンエコー振動と減衰を測定した。これらの量は、300Kから20Kまで温度依存がないが、温度依存が20K以下であることを示している。これらの振動と減衰は、最近傍Si核間のRuderman-Kittelおよび擬似双極子相互作用によるものである。したがって、フェルミ準位近傍の電子状態は、20K以下で変化していることが結論できる。低温におけるフェルミ面の可能な改変について議論する。

論文

$$^{29}$$Si NMR spin-echo decay in YbRh$$_2$$Si$$_2$$

神戸 振作; 酒井 宏典; 徳永 陽; 服部 泰佑; Lapertot, G.*; 松田 達磨*; Knebel, G.*; Flouquet, J.*; Walstedt, R. E.*

Journal of Physics; Conference Series, 683(1), p.012006_1 - 012006_7, 2016/02

BB2015-1546.pdf:0.87MB

https://doi.org/10.1088/1742-6596/683/1/012006
 被引用回数：1 パーセンタイル：54.30(Materials Science, Multidisciplinary)

単結晶試料YbRh$$_2$$Si$$_2$$の核磁気共鳴(NMR)のスピン-格子緩和時間($$T_1$$)の測定によって、量子臨界相転移の近傍の磁気励起を調べ、量子臨界相転移の近くではフェルミと非フェルミ状態の共存状態であることを明らかにした。このようなまだらな電子状態の起源を議論する。

論文

Anisotropic magnetic fluctuations in YbRh$$_2$$Si$$_2$$

神戸 振作; 酒井 宏典; 徳永 陽; Lapertot, G.*; 松田 達磨*; Knebel, G.*; Flouquet, J.*; Walstedt, R. E.*

Journal of Physics; Conference Series, 592(1), p.012085_1 - 012085_5, 2015/03

https://doi.org/10.1088/1742-6596/592/1/012085
 被引用回数：2 パーセンタイル：59.95(Physics, Atomic, Molecular & Chemical)

最近、強相関電子系金属システムにおけるT=0Kでの量子相転移(QCPT)に新たな興味がもたれている。初期の実験結果から近藤格子YbRh$$_2$$Si$$_2$$のQCPTは、Ce系での通常のスピン密度波(SDW)不安定とは異なる局所QCPTであると考えられている。本研究では、共存した静的なフェルミ液体(FL)および非フェルミ液体(NFL)状態がYbRh$$_2$$Si$$_2$$のQCPTの重要な特徴であることを明らかにした。そのような共存状態は、Ce系では短い時間スケールのため観測できなかったと考えられる。2相共存に関する新しいスケーリング則についても議論する。

論文

Degenerate Fermi and non-Fermi liquids near a quantum critical phase transition

神戸 振作; 酒井 宏典; 徳永 陽; Lapertot, G.*; 松田 達磨*; Knebel, G.*; Flouquet, J.*; Walstedt, R. E.*

Nature Physics, 10(11), p.840 - 844, 2014/11

https://doi.org/10.1038/nphys3101
 被引用回数：18 パーセンタイル：70.09(Physics, Multidisciplinary)

共存した静的なフェルミ液体および非フェルミ液体状態がYbRh$$_{2}$$Si$$_{2}$$の中の量子相転移の重要な特徴であることを解明した。単結晶サンプルの核磁気共鳴(NMR)スピン格子緩和時間測定し、フェルミの液体および非フェルミ液体状態の共存比が磁場により変化することを突き止めた。またこの比がもつスケール則も見いだした。

論文

Fermi surface in the hidden-order state of URu$$_2$$Si$$_2$$ under intense pulsed magnetic fields up to 81T

Scheerer, G. W.*; Knafo, W.*; 青木 大*; Nardone, M.*; Zitouni, A.*; B$'e$ard, J.*; Billette, J.*; Barata, J.*; Jaudet, C.*; Suleiman, M.*; et al.

Physical Review B, 89(16), p.165107_1 - 165107_12, 2014/04

https://doi.org/10.1103/PhysRevB.89.165107
 被引用回数：10 パーセンタイル：40.72(Materials Science, Multidisciplinary)

Fermi surface of URu$$_2$$Si$$_2$$ has been studied by measuring the quantum oscillation at low temperature and high magnetic field up to 81T. The data suggest interplay between Fermi surface, magnetic properties and the hidden order.

論文

Anisotropic energy scale for degenerate Fermi and non-Fermi liquids near a quantum critical phase transition in YbRh$$_2$$Si$$_2$$

神戸 振作; 酒井 宏典; 徳永 陽; Lapertot, G.*; 松田 達磨*; Knebel, G.*; Flouquet, J.*; Walstedt, R. E.*

Physical Review B, 91(16), p.161110_1 - 161110_5, 2014/04

https://doi.org/10.1103/PhysRevB.91.161110
 被引用回数：3 パーセンタイル：14.21(Materials Science, Multidisciplinary)

単結晶YbRh$$_2$$Si$$_2$$の$$^{29}$$Si核スピン-格子緩和時間データに基づいて、量子臨界相転移周りのフェルミ液体と非フェルミ液体の共存状態は、印加磁場の方向に対して敏感であることが見出された。スケーリング解析により、異方的な特性温度と特性磁場は以前に得られた異方性相図と一致してH//a軸方向よりH//c軸方向が10倍大きいことがわかった。

論文

Pressure evolution of the metamagnetic transition in UCoAl As measured using $$^{59}$$Co NMR

小手川 恒*; 野原 弘貴*; 藤 秀樹*; 松田 達磨; 山本 悦嗣; 芳賀 芳範; Fisk, Z.; 大貫 惇睦; 青木 大*; Flouquet, J.*

Journal of the Korean Physical Society, 63(3), p.341 - 344, 2013/08

https://doi.org/10.3938/jkps.63.341
 被引用回数：1 パーセンタイル：11.16(Physics, Multidisciplinary)

NMR measurements under pressure were preformed on the quantum critical endpoint (QCEP) of the metamagnetic transition in UCoAl. $$^{59}$$Co-NMR sensitively detects the evolution of the internal field caused by applying the external field. The loss of the NMR signal on approaching the QCEP indicates the development of spin fluctuations.

論文

Magnetic phase diagram of UCoAl

松田 達磨; 立岩 尚之; 山本 悦嗣; 芳賀 芳範; 大貫 惇睦; 青木 大*; Flouquet, J.*; Fisk, Z.

Journal of the Korean Physical Society, 63(3), p.575 - 578, 2013/08

https://doi.org/10.3938/jkps.63.575
 被引用回数：4 パーセンタイル：33.02(Physics, Multidisciplinary)

Heavy fermion UCoAl is known to be a unique system with metamagnetic transition from paramagnetic to ferromagnetic ground state in uranium-based compounds. These kinds of metamagnetism and quantum criticality in strongly correlated electron systems have attracted much attention because of expectation of new quantum phases. Recently, we succeeded to grow a high quality single crystal of UCoAl by the Czochralski pulling method and performed the magnetic measurements. We present the magnetic phase diagram of UCoAl in detail.

論文

Magnetic polarization and Fermi surface instability; Case of YbRh$$_2$$Si$$_2$$

Pourret, A.*; Knebel, G.*; 松田 達磨; Lapertot, G.*; Flouquet, J.*

Journal of the Physical Society of Japan, 82(5), p.053704_1 - 053704_5, 2013/05

https://doi.org/10.7566/JPSJ.82.053704
 被引用回数：30 パーセンタイル：78.69(Physics, Multidisciplinary)

Thermoelectric and resistivity measurements in the antiferromagnetic heavy fermion compound YbRh$$_2$$Si$$_2$$ under high magnetic field are reported. At low temperature, the thermoelectric power and resistivity present distinct anomalies as a function of magnetic field. They are resulting from a Lifshitz-type topological transition of the Fermi surface.

論文

Thermal expansion under uniaxial pressure in URu$$_2$$Si$$_2$$

神戸 振作; 青木 大*; Salce, B.*; Bourdarot, F.*; Braithwaite, D.*; Flouquet, J.*; Brison, J.-P.*

Physical Review B, 87(11), p.115123_1 - 115123_6, 2013/03

https://doi.org/10.1103/PhysRevB.87.115123
 被引用回数：9 パーセンタイル：39.24(Materials Science, Multidisciplinary)

URu$$_2$$Si$$_2$$の隠れた秩序転移温度T$$_0$$、及び反強磁性秩序T$$_N$$の一軸圧力依存性について実験を行った。高品質の単結晶2$$times$$2$$times$$4mmを用いた。隠れた秩序及び反強磁性秩序転移を検出するため、ストリングゲージを用いて熱膨張係数を測定した。一軸圧$$sigma$$は、Heガスを用いて低温で連続的に圧力を変化できるシステムを用いて[100]と[110]方向、0.6GPaまでの測定を行った。静水圧下では、T$$_N$$は0.5GPa程度から現れるが、[100]方向一軸圧では、0.25GPa程度から現れることがわかった。また$$T$$$$_0$$も上昇している。これは中性子散乱の結果と整合している。一方、[110]の場合、0.6GPaまでT$$_N$$は観測できなかった。T$$_N$$の[100]方向と[110]方向による違いは、U-U間の結合距離で整理できることがわかった。また、$$d$$$$T$$$$_0$$/$$d$$$$sigma$$に対するEhrenfestの式に基づく考察により、隠れた秩序相は、小さい一軸圧力により状態が敏感に変化することが示唆された。

論文

Field-induced phenomena in ferromagnetic superconductors UCoGe and URhGe

青木 大*; Taupin, M.*; Paulsen, C.*; Hardy, F.*; Taufour, V.*; 小手川 恒*; Hassinger, E.*; Malone, L.*; 松田 達磨; 三宅 厚志*; et al.

Journal of the Physical Society of Japan, 81(Suppl.B), p.SB002_1 - SB002_6, 2012/12

https://doi.org/10.1143/JPSJS.81SB.SB002
 被引用回数：3 パーセンタイル：26.20(Physics, Multidisciplinary)

We review our recent studies on ferromagnetic superconductors, UGe2, URhGe, and UCoGe. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field $$H$$$$_{c2}$$ on UCoGe for the field along the hard magnetization axis ($$b$$-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge $$H$$$$_{c2}$$ along the hard-magnetization axis, $$H$$$$_{c2}$$ along the easy-magnetization axis ($$c$$-axis) is relatively small in value. However, the initial slope of $$H$$$$_{c2}$$, namely $$d$$$$H$$$$_{c2}$$/$$dT$$ ($$H$$$$rightarrow$$0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along $$c$$-axis.

論文

High-field Fermi surface properties in the low-carrier heavy-fermion compound URu$$_2$$Si$$_2$$

青木 大*; Knebel, G.*; Sheikin, I.*; Hassinger, E.*; Malone, L.*; 松田 達磨; Flouquet, J.*

Journal of the Physical Society of Japan, 81(7), p.074715_1 - 074715_6, 2012/07

https://doi.org/10.1143/JPSJ.81.074715
 被引用回数：24 パーセンタイル：73.76(Physics, Multidisciplinary)

We performed the Shubnikov-de Haas (SdH) experiments of the low-carrier heavy-fermion compound URu$$_2$$Si$$_2$$ at high fields up to 34T and at low temperatures down to 30 mK. All main SdH branches named $$alpha, beta$$ and $$gamma$$ and were observed for all the measured field-directions, indicating that they are attributed to the closed Fermi surfaces with nearly spherical shapes. An anomalous split of branch $$alpha$$ was detected for the field along the basal plane; the split immediately disappeared by tilting the field to [001] direction, implying a fingerprint of the hidden order state. High-field experiments reveal the complicated field-dependence of the SdH frequencies and the cyclotron masses due to the Zeeman spin-splitting associated with the Fermi surface reconstruction in the hidden order state with small carrier numbers. A new SdH branch named $$omega$$ with a large cyclotron mass of 25 $$m_0$$ was detected at high fields above 23T close to hidden order instabilities.

論文

Reply to "Comment on 'Details of sample dependence and transport properties of URu$$_2$$Si$$_2$$'"

松田 達磨; 青木 大*; Knebel, G.*; 芳賀 芳範; Flouquet, J.*

Journal of the Physical Society of Japan, 81(5), p.056002_1 - 056002_2, 2012/05

https://doi.org/10.1143/JPSJ.81.056002
 被引用回数：0 パーセンタイル：0.00(Physics, Multidisciplinary)

2011年11月にJournal of the Physical Society of Japanに掲載された論文"Details of sample dependence and transport properties of URu$$_2$$Si$$_2$$"に関して寄せられたコメント論文に対する回答である。コメント論文によって指摘された参照データの間違いを一部修正した。またコメント論文の著者らによって行われた加圧実験の条件に関する議論に対し、本論文におけるわれわれの主張点とは論点が異なることを述べたうえで、それらの実験に関する問題点について議論を行った。

論文

High-field moment polarization in the ferromagnetic superconductor UCoGe

Knafo, W.*; 松田 達磨; 青木 大*; Hardy, F.*; Scheerer, G. W.*; Ballon, G.*; Nardone, M.*; Zitouni, A.*; Meingast, C.*; Flouquet, J.*

Physical Review B, 86(18), p.184416_1 - 184416_4, 2012/00

https://doi.org/10.1103/PhysRevB.86.184416
 被引用回数：42 パーセンタイル：81.61(Materials Science, Multidisciplinary)

We report magnetization and magnetoresistivity measurements on the isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields up to 60 T and temperatures from 1.5 to 80 K. At low temperature, a moment polarization in UCoGe in a field $$mu_0$$H $$||$$ b of around 50T leads to well-defined anomalies in both magnetization and magnetoresistivity. These anomalies vanish in temperatures higher than 30-40 K, where maxima in the magnetic susceptibility and the field-induced variation of the magnetoresistivity are found. A comparison is made between UCoGe and URhGe, where a moment reorientation in a magnetic field $$mu_0$$H $$||$$ b of 12T leads to field-induced reentrant superconductivity.

論文

Details of sample dependence and transport properties of URu$$_2$$Si$$_2$$

松田 達磨; Hassinger, E.*; 青木 大*; Taufour, V.*; Knebel, G.*; 立岩 尚之; 山本 悦嗣; 芳賀 芳範; 大貫 惇睦; Fisk, Z.; et al.

Journal of the Physical Society of Japan, 80(11), p.114710_1 - 114710_10, 2011/11

https://doi.org/10.1143/JPSJ.80.114710
 被引用回数：43 パーセンタイル：84.26(Physics, Multidisciplinary)

Resistivity and specific heat measurements were performed in the low carrier unconventional superconductor URu$$_2$$Si$$_2$$ on various samples with very different qualities. The superconducting transition temperature ($$T_{rm SC}$$) and the hidden order transition temperature of these crystals were evaluated as a function of the residual resistivity ratio. We discuss a possible scenario concerning the distribution of $$T_{rm SC}$$ related with the fact that the hidden order phase is very sensitive to the pressure inhomogeneity.

論文

Ferromagnetic quantum critical endpoint in UCoAl

青木 大*; Combier, T.*; Taufour, V.*; 松田 達磨; Knebel, G.*; 小手川 恒*; Flouquet, J.*

Journal of the Physical Society of Japan, 80(9), p.094711_1 - 094711_10, 2011/09

https://doi.org/10.1143/JPSJ.80.094711
 被引用回数：85 パーセンタイル：92.26(Physics, Multidisciplinary)

Resistivity and magnetostriction measurements were performed at high magnetic fields and under pressure on UCoAl. At ambient pressure, the 1st order metamagnetic transition at $$H$$$$rm_m$$ $$sim$$ 0.7T from the paramagnetic ground state to the field-induced ferromagnetic state changes to a crossover at finite temperature $$T$$$$rm_0$$ $$sim$$ 11K. With increasing pressure, $$H$$$$rm_m$$ linearly increases, while $$T$$$$rm_0$$ decreases and is suppressed at the quantum critical endpoint (QCEP). We compare our results on UCoAl with those of UGe$$_2$$ and Sr$$_3$$Ru$$_2$$O$$_7$$.

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