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

Abstract

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...

Structural, elastic, electronic, thermodynamic, and optical properties of layered BaPd2As2 pnictide superconductor: A first principles investigation

“…Debye temperature extracted from the QHDA and elastic constants are almost identical. It should be noted that both Kudo et al [11] and Abdel-Hafiez et al [15] have obtained θ D from the analysis of experimental heat capacity data which turns out to be ~ 144 K. This value is significantly lower than the value obtained in this study. Chen et al [14], on the other hand, has calculated the Debye temperature from the analysis of phonon dispersion and found a value of 213 K. Johnston et al [46] have found θ D = 273 K from low-T fit to the heat capacity data.…”

“…Chen [14] carried out lattice dynamical calculations to study the electron-phonon coupling and its relevance to superconductivity in BaPd 2 As 2 at relatively higher temperature. Kudo et al [11] found a high electron phonon coupling constant (λ e-ph ), dominated by soft phonon modes in this SC compound. Very recently Abdel-Hafiez et al [15] studied the physical properties of BaPd 2 As 2 under pressure both experimentally and theoretically.…”

“…139), CeMg 2 Si 2 -type structure (space group P4/mmm, No. 123), and the intergrowth structure of the CaBe 2 Ge 2 -typeand CeMg2Si 2 -type (I4/mmm) [11]. Superconductivity is seen in the ThCr 2 Si 2 -type structure which consists of PdAs 4 tetrahedra, while the non superconductingCeMg 2 Si 2 -type structure consists of PdAs 4 planar squares.…”

Temperature-dependent local structure and superconductivity of BaPd2As2 and SrPd2As2

“…The compounds with lower Debye temperature tend to show higher T c . For instance, BaPd 2 As 2 shows superconductivity below ∼3.5 K with the Debye temperature being ∼140 K while T c of SrPd 2 As 2 is ∼1 K with the Debye temperature of ∼300 K. The estimated specific heat γ values of these compounds are very close to each other [23], suggesting that the density of states of the two compounds can be comparable. Hence it has been discussed that the presence of soft phonon may enhance electron-phonon interaction (λ) in the phonon-mediated superconductivity.…”

“…As a matter of fact, superconductivity also appears in the nonmagnetic CT phase of 122 materials when Fe atoms are completely substituted by other metals, although the superconducting transition temperature (T c ) remains relatively low [14][15][16][17][18][19][20][21][22][23]. Recently, an interesting relationship between the Debye temperature and T c has been found in Pd-based 122 type of CT arsenide [23]. The compounds with lower Debye temperature tend to show higher T c .…”