Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

Visser, A. de; Huy, Nguyễn Thanh; Gasparini, Alessia; Nijs, D.E. de; Andreica, Daniel; Baines, C.; Amato, A.

doi:10.1103/physrevlett.102.167003

Cited by 59 publications

(70 citation statements)

References 25 publications

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“…The FM transition temperature was evaluated to be 2.55 K from the Arrot plots, and the midpoint SC transition temperatures were determined from ac susceptibility as 0.57 K. Clear anomalies in the specific heat were observed at T Curie and T SC , indicating that two anomalies are the bulk transitions. Microscopic measurements have shown the occurrence of superconductivity in the FM region of the sample, indicating homogeneous coexistence of ferromagnetism and superconductivity 9,10 . The NQR measurements on the present single-crystal sample indicates that nearly half region of the sample is in the SC state but remaining region is non-SC, although the whole region is in the FM state above T SC 9 .…”

Section: Methodsmentioning

confidence: 99%

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

et al. 2013

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In order to determine the superconducting paring state in the ferromagnetic superconductor UCoGe, 59 Co NMR Knight-shift, which is directly related to the microscopic spin susceptibility, was measured in the superconducting state under magnetic fields perpendicular to spontaneous magnetization axis: 59 K a,b . 59 K a,b shows to be constant, but does not decrease below a superconducting transition. These behaviors as well as the invariance of the internal field at the Co site in the superconducting state exclude the spin-singlet pairing, and can be interpreted with the equal-spin pairing state with a large exchange field along the c axis, which was studied by Mineev [Phys. Rev. B 81, 180504 (2010)].

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Section: Methodsmentioning

confidence: 99%

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

et al. 2013

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“…Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing.…”

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Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

et al. 2012

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From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (TCurie ∼ 2.5 K and TSC ∼ 0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results combined with model calculations strongly suggest that the longitudinal FM spin fluctuations tuned by H c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.PACS numbers: 71.27.+a 74.25.nj, 75.30.Gw The discovery of superconductivity in ferromagnetic (FM) UGe 2 opened up a new paradigm of superconductivity [1,2], since most unconventional superconductivity has been discovered in the vicinity of an antiferromagnetic (AFM) phase [3]. From the theoretical point of view, in an itinerant FM superconductor with the presence of a large energy splitting between the majority and minority spin Fermi surfaces, exotic spintriplet superconductivity is anticipated, in which pairing is between parallel spins within each spin Fermi surface. In addition, it has been argued that critical FM fluctuations near a quantum phase transition could mediate spin-triplet superconductivity [4]. However, there have been no experimental results indicating a relationship between FM fluctuations and superconductivity.Among the FM superconductors discovered so far, UCoGe is one of the most readily explored experimentally, because of its high superconducting (SC) transition temperature (T SC ) and low Curie temperature (T Curie ) at ambient pressure [5]. Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing. In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9]. The observed characteristic H c2 behavior is one of mysterious features of SC UCoGe and its origin can be related to the mechanism of the superconductivity.Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-...

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“…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][2][3][4][5][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.…”

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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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Muon Spin Rotation and Relaxation in the Superconducting Ferromagnet UCoGe

Cited by 59 publications

References 25 publications

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

Spin susceptibility in the superconducting state of the ferromagnetic superconductor UCoGe

Superconductivity Induced by Longitudinal Ferromagnetic Fluctuations in UCoGe

Dimensionality Driven Enhancement of Ferromagnetic Superconductivity in URhGe

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