Superconductivity and the electronic phase diagram of LaPt$_{2-x}$Ge$_{2+x}$

TL;DR

This study investigates how structural phase transitions and charge density wave fluctuations influence superconductivity in LaPt$_{2-x}$Ge$_{2+x}$, revealing the suppression of structural transition enhances superconducting temperature and features an isotropic gap.

Contribution

It demonstrates the suppression of the structural phase transition and the enhancement of superconductivity through stoichiometric tuning in LaPt$_{2-x}$Ge$_{2+x}$, linking CDW fluctuations to superconductivity.

Findings

Suppression of structural phase transition increases $T_c$.

Moderate fluctuations are associated with the phase transition.

An isotropic superconducting gap is observed in the modified compound.

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. LaPt$_2$Ge$_2$ undergoes a tetragonal-to-monoclinic structural phase transition (SPT) at $T_{\rm s}$ = 394 K, accompanying a double period modulation in the $a$-axis direction, and superconducts at $T_{\rm c}$ = 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 $T_{\rm s}$ and increasing $T_{\rm c}$ in LaPt$_{2-x}$Ge$_{2+x}$. 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 LaPt$_{2-x}$Ge$_{2+x}$ ($x$ = 0.20). We discuss the relationship between superconductivity and the SPT order/fluctuations.