Anisotropic thermal expansion of ${\text{Fe}}_{1.06}\text{Te}$ and ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ single crystals

Heat capacity and anisotropic thermal expansion was measured for ${\text{Fe}}_{1.06}\text{Te}$ and ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ single crystals. Previously reported phase transitions are clearly seen in both measurements. In both cases the thermal expansion is anisotropic. The uniaxial pressure derivatives of the superconducting transition temperature in ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ inferred from the Ehrenfest relation have opposite signs for in-plane and $c$-axis pressures. The Grüneisen parameters for both materials are similar and only weakly temperature dependent above $\sim 80\text{K}$.

Figure 1

Temperature-dependent heat capacity of ${\text{Fe}}_{1.06}\text{Te}$ single crystal. Left inset: enlarged region near the structural/magnetic phase transition; right inset: low-temperature part of the heat capacity plotted as $C_p/T$ vs $T^2$.

Figure 2

(Color online) Anisotropic thermal expansivities (lower panel) and thermal expansion coefficients (upper panel) of ${\text{Fe}}_{1.06}\text{Te}$ single crystal. Inset to the upper panel: enlarged region near the structural/magnetic phase transition. Symbols: normalized at $T=150\text{K}$ data for ${\text{Fe}}_{1.076}\text{Te}$ taken from Ref. 15.

Figure 3

Temperature-dependent heat capacity of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ single crystal. Inset: enlarged region near the superconducting transition plotted as $C_p/T$ vs $T$. Lines show how $\Delta C_p/T_c$ value is defined.

Figure 4

(Color online) Anisotropic thermal expansion coefficients of ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ single crystal. Inset: enlarged region near the superconducting transition. Lines show how $\Delta {\alpha }_i$ values are defined.

Figure 5

(Color online) Grüneisen parameters, $\beta /C_p$, of ${\text{Fe}}_{1.06}\text{Te}$ and ${\text{FeTe}}_{0.5}{\text{Se}}_{0.5}$ single crystals.