Spontaneous Magnetization of Non-centrosymmetric Superconductor LaNiC<sub>2</sub>

Sumiyama, Akihiko; Kawakatsu, Daichi; Gouchi, Jun; Yamaguchi, Akira; Motoyama, Gaku; Hirose, Yusuke; Settai, Rikio; Ōnuki, Yoshichika

doi:10.7566/jpsj.84.013702

Cited by 18 publications

(14 citation statements)

References 13 publications

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“…All four instabilities correspond to nonunitary (equal-spin) pairing, for which we expect an additional, subdominant order parameter, in the form of a bulk magnetisation appearing below T c , which may have been observed in LaNiC 2 [41]. However, all of these nonunitary triplet states have nodal gap functions, which is clearly inconsistent with this work and Ref.…”

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confidence: 73%

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

et al. 2016

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The nature of the pairing states of superconducting LaNiC 2 and LaNiGa 2 has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a non-unitary triplet pairing state. However all the allowed non-unitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC 2 . Here we probe the gap symmetry of LaNiGa 2 by measuring the London penetration depth, specific heat and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa 2 , suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wavefunction has a triplet spin component but isotropic even parity gap symmetry, yet the overall wavefunction remains antisymmetric under particle exchange. This model leads to a nodeless twogap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results. The breaking of symmetries in addition to gauge symmetry upon entering the superconducting state usually indicates an unconventional order parameter. Several materials have been found to break time reversal symmetry (TRS) in the superconducting state through the detection of spontaneous magnetic fields below T c using zero-field muon-spin relaxation (µSR [14], where it has been argued that as a result of the low symmetry of the orthorhombic crystal structures of both compounds, broken TRS necessarily implies non-unitary triplet superconductivity and all the TRS breaking states have nodes in the gap function [15]. Although evidence for nodal superconductivity was found from some measurements [16,17], recent specific heat [18,19], nuclear quadrapole relaxation [20] and penetration depth [19] measurements indicate fully gapped behavior in LaNiC 2 . In addition, evidence for two-gap superconductivity was found from the specific heat, superfluid density and upper critical field [19]. There have been fewer measurements of superconductivity in LaNiGa 2 [21], which has an orthorhombic centrosymmetric crystal structure in contrast to noncentrosymmetric LaNiC 2 , although fully gapped behavior was inferred from the specific heat [22].In this Letter, we suggest a solution to this apparent contradiction from measurements of the London penetration depth, specific heat and upper critical field, all of which consistently suggest the presence of two-gap superconductivity in LaNiGa 2 . Along with previous results of LaNiC 2 [19], we establish that nodeless, two-gap superconductivity is a common feature of these compounds. We propose that pairing between electrons with the same spins but on different orbitals gives rise to a triplet superconducting state with even parity pairing in both compounds, where the wave function re...

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confidence: 73%

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

et al. 2016

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“…The muon spin relax-ation µSR or magnetization measurements under pressure could shed more light on this issue and help to verify whether the internal magnetic fields, reported in Refs. [22,24], are enhanced or quenched by the pressure, suggesting whether they compete or cooperate with the superconductivity. Additionally, such measurements could be performed for the electron-doped samples, since, as we mentioned, electron doping and external pressure result in similar changes in the electronic structure and critical temperature of LaNiC 2 .…”

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confidence: 99%

Pressure effects on the unconventional superconductivity of noncentrosymmetric LaNiC2

et al. 2016

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The unconventional superconductivity in the noncentrosymmetric LaNiC2, and its evolution with pressure, is analyzed basing on the ab initio computations and the full Eliashberg formalism. First principles calculations of the electronic structure, phonons and the electron-phonon coupling are reported in the pressure range 0-15 GPa. The thermodynamic properties of the superconducting state were determined numerically solving the Eliashberg equations. We found that already at p = 0 GPa, the superconducting parameters deviate from the BCS-type, and a large value of the Coulomb pseudopotential µ ⋆ = 0.22 is required to get the critical temperature Tc = 2.8 K consistent with experiment. If such µ ⋆ is used, the Eliashberg formalism reproduces also the experimentally observed values of the superconducting order parameter, the electronic specific heat jump at the critical temperature, and the change of the London penetration depth with temperature. This shows, that deviation of the above-mentioned parameters from the BCS predictions do not prejudge on the triplet or multiple gap nature of the superconductivity in this compound. Under the external pressure, calculations predict continuous increase of the electron-phonon coupling constant in the whole pressure range 0-15 GPa, consistent with the experimentally observed increase in Tc for the pressure range 0-4 GPa, but inconsistent with the drop of Tc above 4 GPa and the disappearance of the superconductivity above 7 GPa, reported experimentally. The disappearance of superconductivity may be accounted for by increasing the µ ⋆ to 0.36 at 7 GPa, which supports the hypothesis of the formation of a new high-pressure electronic phase, which competes with the superconductivity.

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“…A comparative study by means of muon-spin relaxation and rotation (µSR) allows us to address the question of TRSB in Re-containing compounds. The choice of µSR as the preferred technique for our study is justified by its key role in detecting TRSB in numerous unconventional superconductors [13][14][15][16][17][18][19][20] (later confirmed by Kerr effect or bulk magnetisation in the cases of Sr 2 RuO 4 , UPt 3 and LaNiC 2 [21][22][23]). We report systematic µSR studies of Re 0.82 Nb 0.18 (T c = 8.8 K) and Re (T c = 2.7 K), whose bulk superconducting properties were characterized by magnetic, transport, and thermodynamic measurements.…”

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Time-Reversal Symmetry Breaking in Re-Based Superconductors

et al. 2018

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To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation (µSR) studies of superconducting noncentrosymmetric Re 0.82 Nb 0.18 (T c = 8.8 K) and centrosymmetric Re (T c = 2.7 K). In Re 0.82 Nb 0.18 , the low-temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re 0.82 Nb 0.18 and pure Re, the spontaneous magnetic fields revealed by zero-field µSR below T c indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric ReT (T = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg 10 Ir 19 B 16 and Nb 0.5 Os 0.5 , strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and ReT . We discuss the superconducting order parameter symmetries that are compatible with the observations. Time reversal and spatial inversion are two key symmetries which influence at a fundamental level the electron pairing in the superconducting state: on the one hand, a number of unconventional superconductors exhibit spontaneous time-reversal symmetry breaking (TRSB) on entering the superconducting state; on the other hand, the absence of inversion symmetry above T c leads to an antisymmetric spin-orbit coupling (SOC), lifting the degeneracy of the conduction-band electrons and potentially giving rise to a mixed-parity superconducting state [1,2]. Some noncentrosymmetric superconductors (NCSC), such as CePt 3 Si [3], CeIrSi 3 [4], Li 2 Pt 3 B [5, 6], and K 2 Cr 3 As 3 [7, 8], exhibit line nodes in the gap, while others such as LaNiC 2 [9] and (La,Y) 2 C 3 [10], show multiple nodeless superconducting gaps. In addition, due to the strong influence of SOC, their upper critical field can greatly exceed the Pauli limit, as has been found in CePt 3 Si [11] and very recently in (Ta,Nb)Rh 2 B 2 [12]. In general, TRSB below T c and a lack of spatial-inversion symmetry of the crystal structure are independent events. Yet, in a few cases, such as in LaNiC 2 [13], La 7 Ir 3 [14], and, in particular, in the Re-based compounds Re 6 Zr [15], Re 6 Hf [16], Re 6 Ti [17], and Re 24 Ti 5 [18], TRSB below T c is concomitant with an existing lack of crystal inversion symmetry. Such an unusually frequent occurrence of TRSB among the superconducting ReT binary alloys (T = transition metal) is rather puzzling. Its persistence independent of the particular transition metal, points to a key role played by Re. To test such a hypothesis, and to ascertain the possible relevance of the noncentrosymmetric structure to TRSB in Re-based NCSC, we proceeded with a twofold study. On one hand we synthesized and investigated an-other Re-based NCSC, Re 0.82 Nb 0.18 . On the other hand, we considered the ...

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Spontaneous Magnetization of Non-centrosymmetric Superconductor LaNiC₂

Cited by 18 publications

References 13 publications

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

Pressure effects on the unconventional superconductivity of noncentrosymmetric LaNiC2

Time-Reversal Symmetry Breaking in Re-Based Superconductors

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Spontaneous Magnetization of Non-centrosymmetric Superconductor LaNiC2

Cited by 18 publications

References 13 publications

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

Two-Gap Superconductivity inLaNiGa2with Nonunitary Triplet Pairing and Even Parity Gap Symmetry

Pressure effects on the unconventional superconductivity of noncentrosymmetric LaNiC2

Time-Reversal Symmetry Breaking in Re-Based Superconductors

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Product

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Spontaneous Magnetization of Non-centrosymmetric Superconductor LaNiC₂