Superconductivity Reinforced by Magnetic Field and the Magnetic Instability in Uranium Ferromagnets

Aoki, Dai; Matsuda, Tatsuma D.; Hardy, F.; Meingast, C.; Taufour, Valentin; Hassinger, Elena; Sheikin, I.; Paulsen, C.; Knebel, G.; Kotegawa, Hisashi; Flouquet, J.

doi:10.1143/jpsjs.80sa.sa008

Cited by 50 publications

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“…In fact, the resistivity A coefficient and the γ-value are enhanced around 14 T [9,11]. The situation seems to be similar to the case of URhGe, where the spin-reorientation, re-entrant superconductivity, enhancement of γ-value are observed around H m ∼ 12 T [8,[11][12][13]. However, the paradox of UCoGe is that no anomaly is detected around 14 T in the magnetization measurements up to now.…”

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confidence: 65%

“…Since T Curie is suppressed from 2.5 K at zero field to 1 K at 14 T for H baxis, one can consider that the ferromagnetic instabilities induced by the magnetic field reinforce the superconducting state. In fact, the resistivity A coefficient and the γ-value are enhanced around 14 T [9,11]. The situation seems to be similar to the case of URhGe, where the spin-reorientation, re-entrant superconductivity, enhancement of γ-value are observed around H m ∼ 12 T [8,[11][12][13].…”

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Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

et al. 2012

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We review our recent studies on ferromagnetic superconductors, UGe 2 , URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. 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 dH c2 /dT (H → 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. KEYWORDS: superconductivity, ferromagnetism, uranium compound, superconducting upper critical field, AC susceptibility, specific heat, resistivity, magnetization Ferromagnetism and superconductivity had been thought to be competitive phenomena, since the large internal field due to the ferromagnetism easily destroys the superconducting Cooper pairs. In 1980s', some exceptional cases were found for example in ErRh 4 B 4 and HoMo 6 Se 8 [1]. In these materials, the superconductivity is observed in the intermediate temperature range. Further decreasing temperature, the ferromagnetism appears and the superconducting phase collapses at low temperature, meaning that the Curie temperature (T Curie ) is lower than the superconducting transition temperature (T sc ), and there is no microscopic coexistence of superconductivity and ferromagnetism with large ordered moment. The microscopic coexistence was theoretically proposed in the weak ferromagnet ZrZn 2 [2], where the spin-triplet state is formed near the ferromagnetic quantum critical point. There are, however, no experimental evidences.The first microscopic coexistence of ferromagnetism and superconductivity was found in UGe 2 under pressure [3], then in URhGe at ambient pressure [4], and more recently in UCoGe at ambient pressure [5]. All the known ferromagnetic superconductors are uranium compounds, and T sc is lower than T Curie [6,7]. The ordered moments of U atom are substantially reduced, compared to 1 J. Phys. Soc. Jpn.

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confidence: 65%

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confidence: 64%

Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

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“…The direct coupling between a static uniform field and ferromagnetism also leads to fascinating behaviour of the superconducting state under magnetic field. In these systems the pairing mechanism can actually be tuned by magnetic field, either increased [7,8] or suppressed [9], respectively when the magnetic field is applied perpendicular to (transverse field configuration) or along (longitudinal configuration) the easy magnetization axis. The most striking case is URhGe.…”

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“…The first goal is therefore to determine the relations between T Curie , H R , and T SC , the latter both in zero field, and at H R . [7]. It is obtained with a criterion of 50% of the total resistance drop.…”

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confidence: 99%

“…Quantitatively, it was noticed that the rotation occurs when the magnetization in the b-direction approaches the spontaneous zero-field magnetization M c = 0.4 µ B in the easy cdirection [7,17] . This means that if H R occurs at lower fields, the susceptibility along the b- Applying stress along the b-axis will reduce the nearest neighbour distance of U atoms in the b-direction, while the nearest neighbour distance along the c-axis will increase, leading through the variation of the exchange integrals to an increase of the susceptibility along the baxes and a decrease of T Curie .…”

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Dimensionality Driven Enhancement of Ferromagnetic Superconductivity in URhGe

et al. 2018

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In most unconventional superconductors, like the high-T c cuprates, iron pnictides, or heavy fermion systems, superconductivity emerges in the proximity of an electronic instability. Identifying unambiguously the pairing mechanism remains nevertheless an enormous challenge. Among these systems, the orthorhombic uranium ferromagnetic superconductors have a unique position, notably because magnetic fields couple directly to ferromagnetic order, leading to the fascinating discovery of the re-emergence of superconductivity in URhGe at high field. Here we show that uniaxial stress is a remarkable tool allowing fine-tuning of the pairing strength.With a relatively small stress, the superconducting phase diagram is spectacularly modified, with a merging of the low and high field superconducting states and a significant enhancement of superconductivity. The superconducting critical temperature increases both at zero field and under field, reaching 1K, more than twice higher than at ambient pressure. The enhancement of superconductivity is directly related to a change of the magnetic dimensionality with an increase of the transverse magnetic susceptibility, demonstrating that in addition to the Ising-type longitudinal ferromagnetic fluctuations, transverse magnetic fluctuations also play an important role in the superconducting pairing.For usual s-wave superconductors, superconductivity and ferromagnetism are antagonist states as the ferromagnetic exchange field easily destroys the superconducting pairs.Therefore the discovery of the microscopic co-existence of superconductivity and ferromagnetism in three orthorhombic uranium based heavy-fermion systems (UGe 2 , URhGe, UCoGe)[1-6] was one of the most exciting events in recent condensed matter physics. This coexistence strongly suggests a superconducting state with triplet pairing, where the Pauli limiting mechanism is not active and the Cooper pairs can survive in the strong exchange field. The direct coupling between a static uniform field and ferromagnetism also leads to fascinating behaviour of the superconducting state under magnetic field. In these systems the pairing mechanism can actually be tuned by magnetic field, either increased [7,8] or

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Electromagnetic Superconductivity of Vacuum Induced by Strong Magnetic Field

Chernodub

2013

Strongly Interacting Matter in Magnetic Fields

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Superconductivity Reinforced by Magnetic Field and the Magnetic Instability in Uranium Ferromagnets

Cited by 50 publications

References 34 publications

Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

Field-Induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

Dimensionality Driven Enhancement of Ferromagnetic Superconductivity in URhGe

Electromagnetic Superconductivity of Vacuum Induced by Strong Magnetic Field

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