Copper Doping of BaNi$_{2}$As$_{2}$: Giant Phonon Softening and Superconductivity Enhancement

Kudo, Kazutaka; Takasuga, Masaya; Nohara, Minoru

doi:10.48550/arxiv.1704.04854

Cited by 3 publications

(5 citation statements)

References 32 publications

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“…Consistent with prior studies [28][29][30], the Ba 1−x Sr x Ni 2 As 2 system exhibits a discontinuity in the Debye temperature Θ D at x c . However, Θ D remains approximately constant between x = 0.71 (Θ D =198 K) and x = 0.86 (Θ D =188 K), despite a nearly two-fold difference in the superconducting T c .…”

Section: Isupporting

confidence: 86%

Sixfold enhancement of superconductivity in a tunable electronic nematic system

et al. 2019

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The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba1−xSrxNi2As2 substitution series to show that strontium substitution promotes an electronically driven B1g nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity. I.High-temperature superconductivity in both cuprate [1, 2] and iron-based materials [3-5] emerges from a notably complex normal state. Though magnetic spin fluctuations are commonly believed to drive Cooper pairing in both of these families, the common occurrence of a rotational symmetry-breaking nematic phase has captured increasing attention in recent years [6,7]. In contrast to a conventional structural transition, overwhelming evidence suggests that the nematic phase in these compounds is promoted by an electronic instability rather than lattice softening [8,9].Theoretical analyses have shown that fluctuations associated with such an electronic nematic phase, particularly near a putative quantum critical point, can significantly enhance superconductivity [10][11][12][13][14]. Being peaked at zero wave-vector, nematic fluctuations favor pairing instabilities in several symmetry channels, in contrast to the case of magnetic fluctuations. Experiments have indeed shown a striking enhancement of nematic fluctuations centered at optimal tuning of superconductivity in a number of iron-based superconductors [8,9], and a strong tendency towards nematicity in high T c cuprate materials [15][16][17]. However, the overarching presence of magnetic fluctuations emanating from proximate antiferromagnetic instabilities complicates drawing any isolated relation between enhanced pairing and nematicity in most nematic materials. The FeSe 1−x S x substitution series is one exception, where the system exhibits both superconductivity and nematicity in the absence of magnetic order [18]. However, in this series, the superconducting transition temperature T c is virtually unaffected by tuning through the nematic quantum critical point [18,19], leaving ...

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Section: Isupporting

confidence: 86%

Sixfold enhancement of superconductivity in a tunable electronic nematic system

et al. 2019

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“…The modeled heat capacity jump ∆C e /γT is ∼2.2, well above the BCS limit of 1.43, indicating strongly coupled superconductivity at this Co concentration. This value is consistent with previous reports of enhanced normalized heat capacity jumps of approximately 1.9 in both Ba(Ni 1−x Cu x ) 2 As 2 and BaNi 2 (As 1−x P x ) 2 [21,22] and greatly exceeds the near-BCS value observed in pure BaNi 2 As 2 [16]. While previous work on Cu-and Psubstituted BaNi 2 As 2 suggested that the enhancement in the tetragonal phase was consistent with a phonon softening picture, this is not the case here, as the enhancement occurs in the triclinic phase and the Debye frequency exhibits little change through the entire Co substitution range as noted above.…”

Section: Resultssupporting

confidence: 93%

“…Electronic structure calculations suggest BaNi 2 As 2 should not host an s ± state, as any nodal planes would necessarily intersect the Fermi Surface due to its complexity [20], and the heat capacity and thermal conductivity data of BaNi 2 As 2 has been well fit to a BCS s-wave model [16]. Despite the distinctions from its iron-based counterpart, previous substitutional studies in Ba(Ni 1−x Cu x ) 2 As 2 [21] and BaNi 2 (As 1−x P x ) 2 [22] have found an abrupt, strong enhancement of T c from 0.7 K to 3.3 K upon suppression of the triclinic phase [21,22], with strengthened pairing attributed to a soft phonon mode at the first-order structural phase boundary. The enhanced T c value in the tetragonal phase of BaNi 2 (As 1−x P x ) 2 extends to the x = 1 end-member BaNi 2 P 2 [23], suggesting the enhancement is rooted in the tetragonal structure itself.…”

Section: Introductionmentioning

confidence: 95%

Evolution of Structure and Superconductivity in Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$

Eckberg,

Wang,

Hodovanets

et al. 2018

Preprint

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The effects of Co-substitution on Ba(Ni1−xCox)2As2 (0 ≤ x ≤ 0.251) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi2As2 exhibits a first order tetragonal to triclinic structural phase transition at Ts = 137 K upon cooling, and enters a superconducting phase below Tc = 0.7 K. The structural phase transition is sensitive to cobalt content and is suppressed completely by x ≥ 0.133. The superconducting critical temperature, Tc, increases continuously with x, reaching a maximum of Tc = 2.3 K at the structural critical point x = 0.083 and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi2As2 substitutional studies, which show an abrupt change in Tc at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni1−xCox)2As2 exhibits a dome-like phase diagram centered around the first-order critical point. Together with an anomalously large heat capacity jump ∆Ce/γT ∼ 2.2 at optimal doping, the smooth evolution of Tc in the Ba(Ni1−xCox)2As2 system suggests a mechanism for pairing enhancement other than phonon softening.

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“…Energetically low-lying phonons often result in strongcoupling superconductivity with an enhanced superconducting transition temperature T c . [1][2][3][4][5][6][7][8][9][10] Thus, engineering materials to produce low-lying phonons has become an important issue in superconductivity. A promising route to producing low-lying phonons is a local anharmonic vibration of the ion that is weakly bound in a cage-like structure.…”

mentioning

confidence: 99%

“…A remarkable example is the βpyrochlore oxide KOs 2 O 6 with T c = 9.6 K, [1][2][3][4] as well as Ba 3 Ir 4 Ge 16 with T c ≃ 6 K. 5,6) Another route to producing low-lying phonons is structural instability that becomes evident from the occurrence of a structural phase transition due to applied pressure or chemical doping. Prominent examples of such superconductors include the 122-type pnictides BaNi 2 (As 1−x P x ) 2 with T c = 3.3 K, 7) BaNi 2 (Ge 1−x P x ) 2 with T c = 2.9 K, 8) and Ba(Ni 1−x Cu x ) 2 As 2 with T c = 3.2 K, 9) all of which exhibit strong-coupling superconductivity because of a structural phase transition and the subsequent phonon softening characterized by an anomalously low Debye frequency. The MnP-type compound IrGe with T c = 4.7 K is a rare example that exhibits strong-coupling superconductivity with lowlying phonons but does not exhibit a structural phase transition.…”

mentioning

confidence: 99%

Strong-Coupling Superconductivity in BaPd2As2 Induced by Soft Phonons in the ThCr2Si2-type Polymorph

Kudo,

Yamada,

Takeuchi

et al. 2017

Preprint

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The specific heat of two polymorphs of BaPd 2 As 2 was measured. The ThCr 2 Si 2 -type polymorph (space group I4/mmm, D 17 4h , No. 139) is a previously reported superconductor with a transition temperature T c ≃ 3.5 K, while the CeMg 2 Si 2 -type polymorph (P4/mmm, D 1 4h , No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr 2 Si 2 -type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg 2 Si 2 -type. Moreover, a large specific-heat jump at T c indicated that the superconductivity of ThCr 2 Si 2 -type is a strong-coupling type, which is likely derived from soft phonons.

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Copper Doping of BaNi$_{2}$As$_{2}$: Giant Phonon Softening and Superconductivity Enhancement

Cited by 3 publications

References 32 publications

Sixfold enhancement of superconductivity in a tunable electronic nematic system

Sixfold enhancement of superconductivity in a tunable electronic nematic system

Evolution of Structure and Superconductivity in Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$

Strong-Coupling Superconductivity in BaPd2As2 Induced by Soft Phonons in the ThCr2Si2-type Polymorph

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