Enhanced Kadowaki-Woods Ratio and Weak-Coupling Superconductivity in Noncentrosymmetric YPt$_2$Si$_2$ Single Crystals

Abstract

Superconductivity in noncentrosymmetric RPt2Si2 (R = rare earth) compounds exhibit a rich playground to explore the competition between different ground states, such as unconventional superconductivity, antiferromagnetism and charge density wave. Here, we report the successful single crystal synthesis of noncentrosymmetric YPt2Si2 superconductor, with a transition temperature Tc = 1.67 K, via Sn flux method. The high quality of the prepared single crystals was confirmed using powder and Laue XRD measurements. The superconducting and normal state properties are investigated using electrical transport and heat capacity measurements down to 0.5 K. In the normal state, unlike LaPt2Si2, no charge density wave transition is observed in YPt2Si2, as evidenced by electrical transport and specific heat measurements. A relatively large Kadowaki-Woods ratio and a linear temperature variation of the electrical resistivity in an extended temperature range of 50-300 K suggest an unconventional normal-state in YPt2Si2. The estimated superconducting parameters indicate that YPt2Si2 is a type-II superconductor with weak electron-phonon coupling. The temperature dependence of specific heat in the superconducting state can be explained reasonably well using an isotropic two-gap model. A positive curvature near Tc in the temperature variation of upper critical field also supports the two-gap superconductivity. First-principles DFT calculations suggest a BCS-like superconducting state driven primarily by d-electron contributions. The calculated electron-phonon coupling constant identifies the material as a weak-coupling superconductor, with the McMillan-Allen-Dynes formula yielding a Tc of 1.8 K. Additionally, we provide a comparative analysis of the superconducting and normal-state properties of YPt2Si2 and compositionally similar LaPt2Si2.

Figures (11)

• Figure 1: Two dominant structure types for RT$_2$X$_2$ compounds: (a) the centrosymmetric ThCr$_2$Si$_2$-type structure (I4/mmm) and (b) the noncentrosymmetric CaBe$_2$Si$_2$-type structure (P4/nmm). Atomic representations: Y (dark gold), Si (black), and Pt (blue).
• Figure 2: Upper) Laue photograph along the (001) plane, and bottom) powder XRD pattern at room temperature. The solid black lines represent the Rietveld refinement fit, the solid blue line represents the difference between the observed and calculated profile and the vertical green lines shows the Bragg positions.
• Figure 3: (a) Temperature dependence of specific heat, $C(T)$, at zero applied magnetic field in the range 2-100 K. Inset shows a linear fit of $C_p/T$ vs. $T^2$ in the $T$ range of 2-10 K (see details in the text). (b) Temperature dependence of electrical resistivity, $\rho(T)$, in the $T$ range of 2-300 K. Solid line represents the fit of the experimental data using the Bloch-Grüneisen (BG) model. Inset shows that the BG model can not explain the data satisfactorily below $\approx$ 75 K (see main text for details). (c) $\rho(T)$ in the $T$ range of 2-300 K. The solid line represents the combined fit using a linear fit at high temperature (50 - 300 K) and a $T^2$ dependence in the range 2 - 50 K. Inset (i) and (ii) show the $T^2$ and linear temperature dependence of $\rho(T)$ in the respective temperature ranges.
• Figure 4: The coefficient of the $T^2$ term, $A$, in $\rho(T)$ below 50 K is plotted as a function of the Sommerfeld coefficient, $\gamma$, obtained from the specific heat for different materials. This representation is widely known as the Kadowaki-Woods (KW) plot KWR_Jacko2009. The two dotted straight lines indicate the characteristic trends observed for transition metals and for heavy-fermion compounds respectively. YPt$_2$Si$_2$ falls in the region associated with materials exhibiting strong electronic correlations
• Figure 5: Temperature dependence of electrical resistivity, $\rho(T)$, in zero applied magnetic field showing the superconducting transition. $T_c^{\rho}$ is defined as the temperature at which $\rho$ decreases to 50 $\%$ of its normal-state residual value $\rho_0$ (b) Temperature dependence of the ac susceptibility depicting the superconducting transition. $T_c^{\chi}$ is defined as the mid-point of the transition. (c) Temperature dependence of the specific heat, $C(T)$, at $H = 0$, showing a bulk superconducting transition at $T_c \approx 1.67$ K. The method used to estimate the specific heat jump, $\Delta C$ at $T_c$ is indicated in the figure.