Superconductivity at 3.85 K in BaPd
 2
 As
 2
 with the ThCr
 2
 Si
 2
 -type structure

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.Energetically low-lying phonons often result in strongcoupling superconductivity with an enhanced superconducting transition temperature T c .1-10) 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. Such a vibration is called a rattling phonon and characterized by a lowfrequency Einstein mode. A remarkable example is the β-pyrochlore 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.10) Such superconductors suggest how low-lying phonons may be produced for enhancing superconductivity.The 12) The ThCr 2 Si 2 -type structure consists of PdAs 4 tetrahedra, while the CeMg 2 Si 2 -type consists of PdAs 4 planar squares, as shown in Figs. 1(a) and 1(b), respectively. The intergrowth structure, shown in Fig. 1(c), consists of Pd 4 As tetrahedra, which are characteristic of the CaBe 2 Ge 2 -type structure such as BaPt 2 As 2 , 13, 14) * E-mail: kudo@science.okayama-u.ac.jp † E-mail: nohara@science.okayama-u.ac.jp as well as PdAs 4 planar squares, which are characteristic of the CeMg 2 Si 2 -type structure. Guo et al. reported superconductivity at T c = 3.85 K for the ThCr 2 Si 2 -type structure. 15)This T c of BaPd 2 As 2 is anomalously high compared with T c = 1.27 and 0.92 K for CaPd 2 As 2 and SrPd 2 As 2 , respectively, 16) which are isoelectronic and isostructural with BaPd 2 As 2 of the ThCr 2 Si 2 -type structure. On the other hand, no bulk superconductiv...

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“…2(a). In a manner consistent with previous reports, 15,16) -type structures. C e was determined from the total specific heat C, as shown in Fig.…”

supporting

confidence: 92%

“…The 15) This T c of BaPd 2 As 2 is anomalously high compared with T c = 1.27 and 0.92 K for CaPd 2 As 2 and SrPd 2 As 2 , respectively, 16) which are isoelectronic and isostructural with BaPd 2 As 2 of the ThCr 2 Si 2 -type structure. On the other hand, no bulk superconductivity was observed for BaPd 2 As 2 when the structure is the CeMg 2 Si 2 -type.…”

mentioning

confidence: 84%

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

Kudo¹,

Yamada

Takeuchi

et al. 2017

J. Phys. Soc. Jpn.

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“…Meanwhile, superconducting evidences around 100 K was reported in single unit cell film of FeSe on SrTiO 3 substrate [12]. Besides, some other layered transition metal pnictide superconductors were also discovered with much lower superconducting T c [13][14][15]. Therefore, it is still challenging to design and explore new iron-based superconductors with higher T c or novel structures for practical applications and physical research.…”

Section: Introductionmentioning

confidence: 99%

Discovery of a novel 112-type iron-pnictide and La-doping induced superconductivity in Eu 1− x La x FeAs 2 ( x = 0–0.15)

Liu

Pan

et al. 2017

Science Bulletin

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“…ThCr 2 Si 2 -type structure has been found in SrNi 2 As 2 , LaRu 2 P 2 , and SrPd 2 As 2 , etc. [6,7,10,11,15,17,23,24,29] [18,19]. Structural phase transition is found in BaNi 2 As 2 from a high-temperature tetragonal phase to a low-temperature triclinic phase at T = 130 K [30].…”

Section: Crystal Structurementioning

confidence: 97%

“…On the other hand, SC is also observed in other 122-type compounds formulated with AT 2 Pn 2 (A = alkaline-earth or rare-earth; T = transition metals; Pn = P, As or Sb) without iron [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. It has been reported that these 122-compounds show a variety of interesting phenomena, such as SC [1], heavy fermion [26], densed Kondo behavior [27], coexistence of SC and anti-ferromagnetism (AFM)/ferromagnetism (FM) /spin-density-wave (SDW) or charge density wave (CDW) [1,13,28].…”

Section: Introductionmentioning

confidence: 98%

Superconductivity in 122-Type Pnictides without Iron

Zhang

Zhai

2017

Condensed Matter

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Abstract:The exploration of superconducting iron-free pnictides with a ThCr 2 Si 2 -type or related structure and the study of their superconducting properties are important in order to get a deeper understanding of the pairing mechanism of 122 iron pnictides. Here we review the properties of 122-type iron-free pnictides superconductors with structures similar to that of BaFe 2 As 2 . Evidence of fully gapped nature of superconducting state has come from the specific heat and thermal conductivity measurements for BaNi 2 As 2 and SrNi 2 P 2 , and nuclear magnetic and quadrupole resonance measurements for CaPd 2 As 2 etc. Combined with the fact that no magnetism is observed in 122-type iron-free pnictides superconductors, the majority of evidence suggests that most of these compounds are conventional electron-phonon-mediated superconductors.

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Superconductivity at 3.85 K in BaPd ₂ As ₂ with the ThCr ₂ Si ₂ -type structure

Cited by 22 publications

References 22 publications

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

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

Discovery of a novel 112-type iron-pnictide and La-doping induced superconductivity in Eu 1− x La x FeAs 2 ( x = 0–0.15)

Superconductivity in 122-Type Pnictides without Iron

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Superconductivity at 3.85 K in BaPd 2 As 2 with the ThCr 2 Si 2 -type structure

Cited by 22 publications

References 22 publications

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

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

Discovery of a novel 112-type iron-pnictide and La-doping induced superconductivity in Eu 1− x La x FeAs 2 ( x = 0–0.15)

Superconductivity in 122-Type Pnictides without Iron

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Superconductivity at 3.85 K in BaPd ₂ As ₂ with the ThCr ₂ Si ₂ -type structure

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

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