Theory of Spin-polarized Superconductors —An Analogue of Superfluid <sup>3</sup>He A-phase—

Machida, Kazushige

doi:10.7566/jpsj.89.033702

Cited by 31 publications

(26 citation statements)

References 49 publications

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“…25) These experimentally observed superconducting phases imply different order parameters. 12,27,28) This is also consistent with a spin-triplet state, which could have multiple phases owing to the spin degree of freedom.…”

Section: Introductionsupporting

confidence: 77%

“…The large residual γ-value even in the high quality samples is one of the enigma, suggesting a partially gapped superconductivity, such as the A 1 state in superfluid 3 He. 12) However, it should be stressed that the direct 2 nd order transition from the paramagnetic state to a non-unitary state as the A 1 state is forbidden, according to symmetry arguments. 13) Thus, the possibility of a double transition at T c , or the development of short range magnetic order at higher temperature are still under debate.…”

Section: Introductionmentioning

confidence: 99%

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Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Aoki,

Kimata,

Sato

et al. 2021

Preprint

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We report the magnetoresistance in the novel spin-triplet superconductor UTe 2 under pressure close to the critical pressure P c , where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H a-axis just below P c shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field H c2 for H c-axis shows a quasi-vertical increase in the H-T phase diagram just below P c , indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20 T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge H c2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a-and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, H m for the field direction tilted by ∼ 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe.

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Section: Introductionsupporting

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Aoki,

Kimata,

Sato

et al. 2021

Preprint

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“…Regarding a possible mechanism of anomalous behaviors in H c2 (T ) and C e (H ) for H a, we here consider a phenomenological model based on the Ginzburg-Landau framework in which the degeneracy of nonunitary order parameters d ∝ (b ± ic) with equal spin pairing (i.e., ↑↑ and ↓↓ ) is lifted by the easy-axis magnetization M a ; one of the order parameters ( ↑↑ ) arises at T c and the other ( ↓↓ ) appears at a lower temperature. In this model, T c of ↑↑ is written as T c (M ) = T c0 + ηM a [34,38,39]. Here, η is a positive constant coefficient.…”

mentioning

confidence: 99%

Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

Kittaka

Shimizu

Sakakibara

et al. 2020

Phys. Rev. Research

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The gap structure of a novel uranium-based superconductor UTe 2 , situated in the vicinity of ferromagnetic quantum criticality, has been investigated via specific-heat C(T, H,) measurements in various field orientations. Its angular (φ, θ) variation shows a characteristic shoulder anomaly with a local minimum in H a at moderate fields rotated within the ab and ac planes. Based on theoretical calculations, these features can be attributed to the presence of point nodes in the superconducting gap along the a direction. Under the field orientation along the easy-magnetization a axis, an unusual temperature dependence of the upper critical field at low fields together with a convex downward curvature in C(H) were observed. These anomalous behaviors can be explained on the basis of a nonunitary triplet state model with equal-spin pairing whose T c is tuned by the magnetization along the a axis. From these results, the gap symmetry of UTe 2 is most likely described by a vector order parameter of d(k) = (b + ic)(k b + ik c).

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“…Experimentally, not much can be said yet on the value and on the effects of this spin-orbit interaction in UTe 2 . Theoretically both the weak and strong spin-orbit limit are considered regarding the orientation of the d-vector, and some models [106,107] explicitly predict a strongly reduced paramagnetic limit in the transverse (b and c) directions due to a very anisotropic spin-orbit interaction. Indeed, it is quite puzzling that H c2 so strongly violates the paramagnetic limit for field along c or b, as most models of the order parameter assume, at least in zero field, that the dvector has no component along the a axis but non-zero components along both the c and b axes.…”

Section: 41mentioning

confidence: 99%

“…Most of them assume purely odd-parity spintriplet superconductivity, such as the B 3u + iB 2u state [108] and the B 3u + iB 1u state [19]. Assuming that long-lived ferromagnetic fluctuation plays the same role as the ferromagnetic long-range order and ignoring the spin-orbit coupling, a spin-triplet pairing state with d(k) = (k b + ik c )( b + iĉ) was also proposed [109,106,197]. This state is classified into a B 3u + iA u state in the terminology of IR.…”

Section: Theories For Superconductivity In Utementioning

confidence: 99%

Unconventional Superconductivity in UTe2

Aoki,

Brison,

Flouquet

et al. 2021

Preprint

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The novel spin-triplet superconductor candidate UTe 2 was discovered only recently at the end of 2018 and attracted enormous attention. We review key experimental and theoretical progress which has been achieved in different laboratories. UTe 2 is a heavy-fermion paramagnet, but right after the discovery of superconductivity it 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 5f 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. Special attention will be given to the occurrence of a metamagnetic transition at Hm = 35 T for a magnetic field applied along the hard magnetic axis b. Adding external pressure leads to strong changes in the magnetic and electronic properties with a direct feedback on superconductivity. Attention will be given on the possible evolution of the Fermi surface as a function of magnetic field and pressure.Superconductivity in UTe 2 is extremely rich exhibiting various unconventional behaviors which will be highlighted.It shows an exceptionally huge superconducting upper critical field with a re-entrant behavior under magnetic field and the occurrence of multiple superconducting phases in the temperature field, pressure phase diagram.There is evidence for spin-triplet pairing. Experimental indications exist for chiral superconductivity and spontaneous time reversal symmetry breaking in the superconducting state. The different theoretical approaches will be described. Notably we discuss that UTe 2 is a possible example for the realization of a fascinating topological superconductor. The recent discovery of superconductivity in UTe 2 reemphasizes that U-based heavy fermion compounds give unique examples to study and understand the strong interplay between the normal and superconducting properties in strongly correlated electron systems.

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Theory of Spin-polarized Superconductors —An Analogue of Superfluid ³He A-phase—

Cited by 31 publications

References 49 publications

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

Unconventional Superconductivity in UTe2

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Theory of Spin-polarized Superconductors —An Analogue of Superfluid 3He A-phase—

Cited by 31 publications

References 49 publications

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Field-Induced Superconductivity near the Superconducting Critical Pressure in UTe2

Orientation of point nodes and nonunitary triplet pairing tuned by the easy-axis magnetization in UTe2

Unconventional Superconductivity in UTe2

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Theory of Spin-polarized Superconductors —An Analogue of Superfluid ³He A-phase—