Understanding of the Temperature–Pressure Phase Diagram of β-Pyrochlore Oxides: A Role of Anhamonicity on Superconductivity

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

“…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.…”

“…[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.…”

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

“…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.…”

“…Such a vibration is called a rattling phonon and characterized by a lowfrequency Einstein mode. 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.…”

“…Figure 3 shows the specific heat divided by the temperature C/T as a function of the squared temperature T 2 . The normal-state data could be fitted by C/T = γ + βT 2 + δT 4 , where γ is an electronic specific-heat coefficient, β is the coefficient of phonon contributions from which the Debye temperature Θ D is estimated, and δ is a constant related to phonon contributions. As shown in Fig.…”