Superconductivity and the electronic phase diagram ofLaPt2−xGe2+x

Abstract

In many cases, unconventional superconductivity are realized by suppressing another order parameter, such as charge density wave (CDW) or spin density wave (SDW). This suggests that the fluctuations of these order parameters play an important role in producing superconductivity. LaPt2Ge2 undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at Ts = 394 K, accompanying a double period modulation in the a-axis direction, and superconducts at Tc = 0.41 K. We performed band calculations and found 2D (two dimensional)-like Fermi surfaces with partial nesting. A reduction in the density of states in the monoclinic phase was found in the calculation and confirmed by 195 Pt-NMR. We suggest a CDW as a possible cause for the SPT. By changing the stoichiometry between Pt and Ge, we succeeded in suppressing Ts and increasing Tc in LaPt2−xGe2+x. Comparison of 139 La-and 195 Pt-NMR data reveals moderate fluctuations associated with SPT. From 139 La-NQR measurements at zero field, we found that an isotropic superconducting gap is realized in LaPt2−xGe2+x (x = 0.20). We discuss the relationship between superconductivity and the SPT order/fluctuations.

Structural phase transition, precursory electronic anomaly, and strong-coupling superconductivity in quasi-skutterudite (Sr 1− x Ca x ) 3 Ir 4 Sn 13

“…As the pressure increases, T s and T * are suppressed while T c increases slowly, which means that there exists a competition between the structural phase transition and superconductivity. A similar phase diagram has been seen in other systems such as LaPt 2−x Ge 2+x , [37] where T c increases from 0.41 K to 1.95 K and T s decreases from 394 K to 50 K. Note that the Knight shift and thus the electronic DOS remain Tindependent at low temperatures, while its absolute value increases from Sr 3 Ir 4 Sn 13 to Ca 3 Rh 4 Sn 13 . Therefore, the in-crease of DOS may be partly responsible for the increase of T c .…”

Evolution of charge density wave order and superconductivity under pressure in LaPt2Si2

“…Moreover, the Fermi surface of LaPt 2 Ge 2 in the tetragonal phase resembles that of LaPt 2 Si 2 [24,35]. Upon varying the ratio of Pt and Ge, it was suggested from NMR measurements that enhanced structural fluctuations in LaPt 2−x Ge 2+x can possibly give rise to the increase of T c [35]. As such, it would also be of interest to look for the role played by structural fluctuations in influencing the superconductivity of LaPt 2 Si 2 , which may be addressed by NMR measurements under pressure.…”

“…It is noted that LaPt 2 Ge 2 is also a CDW superconductor, with a lower T c = 0.4 K and a higher T CDW = 385 K compared to LaPt 2 Si 2 . The latter is accompanied by a structural phase transition from the tetragonal CaBe 2 Ge 2 -type to a monoclinic structure [34,35]. Moreover, the Fermi surface of LaPt 2 Ge 2 in the tetragonal phase resembles that of LaPt 2 Si 2 [24,35].…”

“…The latter is accompanied by a structural phase transition from the tetragonal CaBe 2 Ge 2 -type to a monoclinic structure [34,35]. Moreover, the Fermi surface of LaPt 2 Ge 2 in the tetragonal phase resembles that of LaPt 2 Si 2 [24,35]. Upon varying the ratio of Pt and Ge, it was suggested from NMR measurements that enhanced structural fluctuations in LaPt 2−x Ge 2+x can possibly give rise to the increase of T c [35].…”

NMR studies of coexistence of superconductivity and CDW in LaPt2Si2

“…Though the coexistence of SC and CDW was reported in various compounds such as transition metal dichalcogenides (2H-NbSe 2 [3], 2H-TaSe 2 [4], etc.) and APt 2 X 2 (A=La, Sr; X=As, Ge) compounds (SrPt 2 As 2 [5], LaPt 2 Ge 2 [6], etc. ), the origin of the coexistence has been still unrevealed.…”